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  • Biotechnol Rep (Amst)
  • v.27; 2020 Sep

A review on prospective production of biofuel from microalgae

Ramya ganesan.

a Department of Chemistry, St. Joseph’s Institute of Technology, Chennai 600 119, India

S. Manigandan

b Department of Aeronautical Engineering, Sathyabama Institute of Science and Technology, Chennai 600 119, India

Melvin S. Samuel

c Department of Materials Science and Engineering, CEAS, University of Wisconsin-Milwaukee, Milwaukee, WI, 53211, United States

Rajasree Shanmuganathan

d Institute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Nam

Kathirvel Brindhadevi

e Innovative Green Product Synthesis and Renewable Environment Development Research Group, Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Viet Nam

Nguyen Thuy Lan Chi

Pham anh duc.

f Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Viet Nam

Arivalagan Pugazhendhi

Graphical abstract.

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  • • Challenges faced in I generation biofuels and transition to II generation biofuels.
  • • Microalgae as a feedstock for biofuel production has been discussed in this review.
  • • Two stage cultivation strategies and extraction techniques were discussed.
  • • HTL, fermentation, transesterification and pyrolysis of III generation biomass in briefly analyzed.

This critical review summarizes the utilization of algae as the resilient source for biofuel. The paper validates the different stages in generation of biofuels and provides a clarity on III generation biofuels. The microalgae is focused as an incredible source and a detailed discussion has been carried out from the cultivation, extraction and conversion to the final product. An elaborate view on conversion methodologies and troubles involved in the respective techniques are presented. The efficiency of the algal fuel performing in I/C engines derived from major techniques is considered. There exist new challenging barriers in the implementation of microalgae as prospective source in the energy market. In addition, types of pyrolysis for the production of main product from microalgae had been discussed in detail. Besides, some microalgae grow easily from fresh to waste water, make it more feasible source. Although the microalgae are a best alternative, cost of production and the yield of biofuel are still challenging. Further, cultivation of microalgae is very effective by applying two stage cultivation strategies. This comprehensive review provides the useful tool to identify, innovate and operate microalgae as the potential based biofuel.

1. Introduction

Over the past few decades the world has witnessed the energy thirst and exploitation of fuels by developing countries mainly India and Africa to satisfy the raising standards [ 1 ]. The major concerns associated with the existing energy insecurity are rapid industrialization consuming fossil fuels, record high gasoline process, scaling the dependency on middle east oil sources, negative impact of fossil resources on greenhouse gas emissions forcing the pressure on society, increased levels of NOX, SOX and metal particles in atmosphere due to the usage of present day fuels [ 2 , 3 ]. Beginning from the individual country’s thirst, changes in environmental scenario and friction between nations with respect to fuel consumption, the drives are challenging and hence has to be answered. Once, the world’s energy search was indolent without the track of source. But, today in 20th century it is really indeed close to loop with the aid of propitious choices in the direction of biofuels. The endless expectations of the society for a fuel which can be the savior of present day problems points at limited sources. Within a decade span, developing countries with the large interest towards economy growth have consumed 2/3rd of energy than previous years ( Fig. 1 ). International Energy Agency (IEA) has summarized the global energy demand to be faced in near future along with and Organization for economic co-operation and development (OECD). According to late economist Angus Maddison, the developing countries would surpass the developed countries by 2030 in economic weight. India and China have all the key sources for the improvement of the poor country involving investment and trading [ 4 ]. “South – South links” is important sources for development. Though this has improved the overall world economy, significant decline in reserves is also a known fact.

Fig. 1

Energy consumption by Region. [ 5 ] Source: https://www.eia.gov/todayinenergy/detail.php?id=41433 .

The global oil need is constrained with limited number of countries compared to the large energy seeking countries. Treasures of 260 billion barrels and 115 billion barrels of oil makes Saudi Arabia as the king in the market followed by Iraq [ 6 ]. For energy security reasons, it is important to search for other oil suppliers including Nigeria and Angola where the oil production may rise to nearly 10 million barrels/ day by 2030.With the sense of urgency and insecurity in energy supply, China is receiving its load from more than 20 countries around the world. Initiatives have been taken by Putin’s government to choose route of pacific coast to Siberia field for Russia’s energy demand. This shows globally the hunt for new sources is also a key concern for the search of novel fuel. The Earth Summit held by UN conference in Rio de Janeiro during 3rd to 14th June in 1992 paved the path for environmental concerns including greenhouse effect and acid rain due to the GHG (Greenhouse gas) emissions. Kyoto protocol, a committee of Europe, Japan and Russia aims at CO 2 reduction levels [ 7 ]. Developing countries on the surge to industrialization rely on the large quantity of coal and other fossils which results in enhanced release of CO 2 gas.

Petro- fuels satisfies almost 94% of energy needs in road transportation division. Almost nine countries accounts for consumption of 75% liquid fuels and interestingly these are developing landscapes ( Fig. 2 ).

Fig. 2

Country wise consumption of road transportation fuels.

The perpetual increase in CO 2 will definitely end with significant global warming effect. The prediction indicated that by 2030, the levels of CO 2 may almost triple by the spurring motor vehicle population. These critical issues poses strong emphasizes on the present generations fuel and also drives us towards the new alternative fuel ( Fig. 3 ). Research is pioneered in replicating this type of liquid fuels from renewable sources and surprisingly biomass is the only source that yields all the 3 phases of fuel. This review features the diverse plants, seeds and crops in the league of biofuel and classified as generations of biofuel. The stages of biofuel has raised from 1st generation to 3rd generation which decides the future of energy harnessing. The 1st generation biofuels is known for their potential of reducing CO 2 emissions while they are criticized a lot for the land usage and food shortage. The lignocellulosic biomass and waste animal oils constitute 2nd generation biofuels. The literature survey gives the difficulties with the 1st and 2nd generation biofuels. With the classy results in the production of bio-oils they can make a good choice but the problems faced with the techniques and cost ineffectiveness lags the chance in the race. The 3rd generation spots the viability of algae among the single celled organism (SCO) as an excellent alternative in fuel market. The article ventures the positive aspects of these microbodies with elaboration about the methods involved in growth and extraction of lipids from the oil rich species. Various techniques developed to convert the lipids or the whole algae into fuels and engine aspects of fuel derived from different methods are also discussed.

Fig. 3

Present and future energy scenario.

2. Biofuels

Fossil fuels are formed beneath the earth’s crust over the period of years by the process called as fossilization. The fuel which is obtained from a chemical method from biomass rather than a slow geological process is known as Biofuel. U.S. Energy Information Administration (EIA) names the fuel and the same is followed in most of the countries. The liquid and gaseous form of fuels are generally termed as biofuels and have high utility in transportation sector. These fuels are easy to blend with the existing liquid fuels namely gasoline and diesel. The various forms of liquid biofuels are bio-alcohol, bio-diesel, bio-oil and group of gasoline, kerosene & diesel. The mixture of gasoline and alcohol derived from saccharification reactions is known as “Gasohol”. The gaseous biofuel is known as biogas and is rich in methane gas. Bio-alcohol may comprise bio methanol, bioethanol whereas bio-gas has major products. The biofuels are expected to satisfy at least one-fourth of the globe’s energy demand in the mid of 20th century. Three generations of biomass are under focus for the delivery of alternative fuels.

2.1. I generation (G1) biomass derived biofuels

In the debate between biofuels, it is necessary to distinguish the generation technologies as I and II generation. 1st generation biofuels comprises of both liquid and gaseous types of fuel. Biodiesel, vegetable oil, bio-ethers and bio-alcohol are the liquid forms while forms of biofuels. The I generation (G1) biomass are the oil bearing seeds and edible crops namely starch, sugarcane, animal fats, sunflower, rapeseed and palm. The complex triglycerides molecules with different alkyl chain lengths make each oil molecule unique. Triglycerides are three fatty acid units which are linked to an ester molecule and these triglycerides can be converted to biofuels with the type of methodology followed. The techniques used for the generation of fuels are transesterification, anerobic decomposition, fermentation and pyrolysis. The I generation biofuels wither can be used without blending or can be improvised by blending. Table 1 lists the utilization of edible (G1) crop oils such as mustard, rice bran, lemon seed, wheat germ, coconut, soyabean, rapeseed, sunflower, almond, walnut and pistachio in the biofuel production [ [8] , [9] , [10] , [11] , [12] , [13] , [14] , [15] , [16] , [17] , [18] ]. Europe, N. America, S. America and Asian countries function biodiesel plants with castor, sunflower, palm, and jatropha oil sources. Corn and sugarcane are considered to be fortune makers in India, US, France, China, Germany and Australia. According to Lowa State University, 2013 the expenditure of ethanol producing corn plant is nearly 0.75 US$ per litre annually which includes installation, feedstock availability, production and transportation cost. Despite of reduction GHG emissions and atmospheric pollution; they interfere with food security and social conflicts. Hence, with the incorporation of the views of stakeholders in ranking & evaluating the specific generation, it raises question on the sustainability of G1 fuels for the future.

Data of varieties of feedstocks for biofuels.

2.2. II generation biofuels (G2)

G2 fuels or advanced fuels are an assured alternative to GI fuels because of the source being non-food biomass. The by-products of food processing industry and wooden factories include inedible parts ex. dry wood, stalks of corn and wheat constitute 2nd generation biomass. Used oil products from restaurants, animal wastes and oil crops likely jojoba, jatropha and sea mango are also available materials as this next generation resource [ [19] , [20] , [21] , [22] , [23] , [24] , [25] , [26] , [27] , [28] ]. Types of nonedible converted to biofuels and the process used for production is given in Table 1 . The products caters from ethanol derived from cellulose to bio syngas (BioSNG); a mixture of hydrogen and carbon mono oxides with different technical approaches. Fermentation of saccharides, gasification of dried biomass, BtL (biomass to liquid) technology and HTL (hydrothermal liquification) of vegetable oils are the major process involved in G2. The gasification of dried biomass yields the lighter fuel biohydrogen, fermentation of cellulose & syngas delivers ethanol, butanol (C 4 H 9 OH) & methanol, FT synthesis followed by BtL provides C5-C18 hydrocarbon fuels. Comparatively, these fuels are in line with EU-RED (EU- Renewable Energy Directive), eco-friendly, clean burning, non-corrosive, does not lead to deforestation and also it is not used as fodder for animals due to its trace level of toxicity [ [29] , [30] , [31] , [32] ]. This perspective of advanced energy has accumulated investors around the world for the synthesis of biodiesel and bioethanol. Biodiesel using re used oils (cooking, tallow and vegetable oils) by Australian Renewable fuels Limited and bioethanol from wheat straws, agricultural residues, wood chips and sugarcane bagasse by BlueFire Ethanol Fuels, Inc, US, Cosan, Brazil and Coskata, US are to be mentioned. However, the outcome of G2 is still meagre compared to G1 with former producing five hundred million gallons while the latter outshines with fifteen billion gallons. In addition, the lack of proper technique and rich content of saturated fatty acid for biofuel production from the second generation feedstocks propels this choice as temporary. The search for the renewable and clean fuel has paved its path to III generation biofuels (G3).

2.3. A bloom in biofuel market – Algae – III generation biofuels (G3)

The bright or dark green patches found in wet regions are algae, the non-flowering plants like species containing chlorophyll yet distinct from floras ranging from micro to macro sizes. The processing of these microorganism diversifies the scope in nutritional industry, bioplastics, pharmaceuticals, special chemicals manufacture, organic fertilizer and the flourishing biofuel industry. The distinctive properties of algae such as : a) CO 2 absorbance for the growth helps in reduction of green-house effect, b) they do not require large area for development compared to other food crops, c) can adjust to brine water and d) also their lipid content is found high [ 33 , 34 ]. In early 19th century methane production from algae won a big momentum during energy crisis. Harder and Von Wiltsch proposed the algae as source of food and energy half a century ago. World’s first biodiesel plant suing algae proposed the algae as source of food and energy in mid 19th century. Japan, England and Israel began the cultivation of Chlorella algae on large scale during II world war. The abundance of fossil fuels diverted the idea of using these algae in Energy production to food commodities. The official Program “The aquatic species” was initiated with 25 million $ by US 18 years ago [ 34 ]. In recent years, the objective to be benefited by algae in synthesizing alternate fuel has been of great interest and they can significantly replace the G1 & G2 biomass. The lipids in algae can be converted to biodiesel by the generalized method used for conversion of vegetable oil into biodiesel. Whereas bioethanol and biobutanol are prepared from carbohydrates of algae. The market value of other biofuels is just half of the algal biofuel (420 US million $). This Fig. 3 would soar in the near future with proper technology practice. Few algae that are of research interest are Chlamydomonas reinhardtii (21% lipids, 48% sugars), Spirulina platensis (8% lipids, 60% sugars) and Chlorella sp. (19% lipids, 56% sugars) [ 35 , 36 ]. Today nearly 10 countries are keen in biofuel production from the algae biomass. Various techniques applied for the processing of microalgal biofuels is given in Fig. 4 .

Fig. 4

Steps and techniques involved in biofuel production using microalgae.

3. Algae – Growth, extraction and conversion

During the last decade considerable attention is drawn by algae for the economic possibilities in their mass growth. The biofuel synthesis from algal biomass proceeds through the following steps.

  • • Culturing of algae
  • • Harvesting of algae (or) Dewatering of algae
  • • Extraction of oil from algae
  • • Purification of algal oil
  • • Processing of oil into biofuels

3.1. Culturing of algae or algaculture

Algaculture refers to the growth of algae similar to that of aquaculture. The growth expectation of algae is very simple and affordable i.e. sufficient light, naturally available dissolved nutrients and CO 2 [ 37 , 38 ]. Unialgal growth without contamination of other eukaryotic or prokaryotic organisms and axenic culture (bacteria free) is challenging. The growth rate of algae is spectacularly high (doubling by 24 h) unlike plants as their energy is not spent on the growth of their parts. The two kinds of algae culture classified based on the growth characteristics are:

  • a) Batch culture (BC): The inoculation of algal cells in a container when the abundant resource is available follows the sigmoidal curve. The loss of medium slays the culture and this can be subdued by introducing small volumes of fresh medium into existing culture.
  • b) Continuous flow culture (CFC): The regulated addition of adequate volume of fresh medium rich in nutrients (infinite source) to the culture medium to attain “steady state” is performed in CFC method. A steady state is the uniform cell density where birth rate is equivalent to death rate. This is done proportional to the growth of algae in a special culture technique known as “Turbidostat culture or Chemostat culture”. Turbidostat culture is the fresh addition of medium to the culture when the growth reaches a certain limit while Chemostat culture is the introduction of fresh medium to the culture at a predetermined rate.

Major physical parameters that affect the growth of algae are:

  • (I) pH: The total collapse of algal cell wall occurs with the unoptimized pH level. The proper cell growth happens in the pH range of 8.2–8.7 and supplement of CO 2 into the medium enables the attainment of optimized pH.
  • (a) Fluoroscent lamps: The radiation in 380–500 nm (blue light) and 600–700 nm (red light) is preferred for algal growth.
  • (b) Photoperiod: The illumination period is expected to be around 16–18 h for the appropriate culture maintenance
  • (c) Light intensity: Algae growth differs with the intensity of light ranging from 5% to 10%. Mostly, the light/ dark cycles are followed as the cells do not grow in continuous illumination.
  • (III) Temperature: The temperature of the culture medium varies with respect to the temperature zones of regions. Algaculture in countries like India and US (temperate zones) operates at 10 °C–25 °C and in tropical countries (ex., Brazil and Singapore) the temperature of action is below 20 °C. The temperature beyond 35 °C leads to destructive algal growth.
  • (IV) Medium of culture: The medium is responsible for the contamination of culture and this creates hindrance in sterilization of culture. The quality of water used in media has significance and sea water with unpredictable contaminants is a serious issue in culturing medium. The sea water may contain vitamins, chelating agents, buffers, soil extract etc.,

Three major types of culturing are practiced worldwide and are discussed in this section.

3.1.1. Open pond system

Algae usually grow in lakes and copying this similar pattern known as open pond system (OPS) is used for algae culturing. The ponds are of one-foot depth and algal cultivation could be from one acre to several acres. OPS is the most common system used in the algal growth. The types of OPS are: Raceway ponds, natural ponds (shallow lagoons and shallow ponds), mixed ponds, circular open ponds mixed with center pivot mixer. The lack of agitation and low sunlight penetration in the open pond system results in inadequate mass and heat transfer. Whereas the raceway pond system (RPS) is a cost cutting mechanism made with concrete earth. In RPS, a closed loop with recirculation channel (03.m deep) is designed with paddles for better mixing, laminar flow and circulation of CO 2 . But it also has high peril of contamination with low rates of production due to its sensitivity to environmental fluctuation. Perhaps, they are easy to maintain and operate [ 39 ]. Narala et al. [ 40 ] studied the biofuel extraction from Tetraselmis sp. M8 by OPS and by 32nd day about 2.8 × 10 6 cells/mL was harvested.

3.1.2. Photobioreactors (PBR) -Closed loop culturing

Intensive research on algae production compelled the idea of closed reactor systems. Photobioreactor (PBR) is a worth substitute for OPS for its massive productivity rate and high quality of algae. Researchers have created many versions like tubular, bubble, Christmas, plate, horizontal, foil and porous PBRs. The TBR is the common PBR type used in the algaculture and it comprises of tubular solar arrays, biomass unit, exchange column to exchange gas and pump. The vertical column TPBR (VTPBR) offers good gas exchange while the horizontal column TPBR (HTPBR) gets better access of light and also possess high surface area. However, the negatives of the VTPBR is low surface to volume ratio and HTPBR are low mass transfer leading to difficulty in CO 2 elimination, and excessive heat generation. Disappointingly, in general PBRs suffer from their high capital cost which exceeds the output due to its complexity and exclusive erection materials [ 41 , 42 ]. Further, it suffers from a) improper CO 2 and O 2 balance b) Control in temperature and c) biofilm formation (Fouling).

3.1.3. Hybrid systems

Phototrophic system is considered to be economical as the alga grows using CO 2 and sunlight. On the other hand, a very slow rate of growth is witnessed. The energy source is replaced by a carbon rich compound or biosugars like glucose, dairy products and waste food items in hetrotrophic systems. This receives bright focus in biofuel industry as the algae grown in this environment yields better biodiesel. Adhering to the cost and energy constraints, the proposals of hetrotrophic mode is restricted. Screening among the two methods becomes a difficult task and hence a hybrid cultivation needs to be developed [ 43 ]. Synergizing the effectiveness of OPS and PBR can be achieved by the hybridization of both systems. The two stage hybrid cultivation system is the advanced version in the algaculture where the cell medium is transferred from OPS to RPS when the nutrients are found to decline. The feasible separation of biomass from the lipid accumulation and least possibility of contamination strikes the positive note. The hybrid system can be of a) small PBRs with big ponds and b) ponds with large PBRs. The hybrid system comprises of two stages:

  • a) I stage:

PBRs are chosen as I phase to reduce the lipid accumulation and contamination in the culture. The density of biomass can be increased with the closed PBRs.

  • b) II stage:

The selection of OPS in the II stage increases the economic compatibility of the

process. The two most significant at the II phase after the completion of I phase helps to promote rich carbohydrate and lipids in algae. Increase of lipid content by alternative nitrogen supply

The trial of reducing the nitrogen environment retards the growth of culture. The latest research shows the supply of nitrogen at the beginning of the culture growth and deprivation of the same after the considerable dense biomass raises the lipid production [ 44 ]. The nitrogen starvation disrupts the cell and directs the carbon towards carbohydrate and lipid production. Nannochloropsis gaditana and Chlorella protothecoides are the few algae that produced good results in the switch over of nitrogen source. Brine condition

This methodology is reverse to the nitrogen supply mechanism. Better products were produced only with two stage cultivation process compared to single stage cultivation. Enhancement of algal growth was observed with low saline condition and the gradual increase of salinity lowered the metabolism. The lower salinity level improved the lipid and carbohydrate generation in algae. Chlorophycae species indicated the effect of salinity in their growth in II stage cultivation [ 45 , 46 ].

3.2. Harvesting or de-watering or algae

The cultured algae needs to be dewatered in order to access the lipid profile. The dewatered algae looks like an interim of solid-liquid medium instead of a liquid which flows easily. The experiments prove that only 0.1% of dry matter is available in 1 L of cultured media. Filtration and centrifugation are the processes involved in removing water from algae. Many advanced mechanics are explored under these categories. Flocculation and membrane filtration is effective in drying algae [ [47] , [48] , [49] ]. Methods involved in filtration: Pressure, Vacuum, deepbed sand, cross flow and magnetic filtration.

3.3. Algal oil trough lipid extraction from dry algal mass

The biological micro species has multilayered cell wall made of polysaccharides and cellulose synthesized from silicic acid. The cell wall envelops the lipids or fatty acids and the removal of algal oil is known as lipid extraction. The specific extraction of lipids is also performed by solvent extraction using methanol and chloroform. Interest is on microwave, grinding, bead beating and ultrasound mechanical methods for extraction. This method does not require extra chemicals and the subsequent extraction step becomes easier. Mostly, bead beating is done to disturb the cell walls of microbes in small scale level with beads made up of ceramics or glass. The beads of large surface area are mixed with the cell suspension and their vigorous shaking collapses the walls to release the lipids. Pulverizing the dried algal biomass is analogous to wheat milling which protects the maximum nutrient content of the material. In recent years, research is directed towards extraction free from solvents. The super critical fluid technique (SCF) accomplishes the demand by producing safe and good quality end products. The efficacy of this method in extracting specific components from a complex biological species is worth enough.

The oil extracted using n-heptane by Soxhlet extraction method is much lower than SCF method. About 39.4% of oil was extracted from algae using Cyclopentyl Methyl Ether (CPME) and ethanol (EtOH) as Super critical fluids against 32.8% of usually used CO 2 [ 50 ]. The obtained algal oil is a pure triglyceride and has to be upgraded by reduction catalysts to liquid fuels. Algal oil was extracted from the algae collected from Simlapuri Nahar, Ludhiana, Punjab. 9 wt% and 8 wt% of oil was gained using costly hexane and inexpensive acetone respectively [ 51 ]. Lipids extracted from various microalgae and the method of application is given in Table 2 . Supercritical Water Reactor (SCWR) for extraction of algal oil. The research article concentrates on the demand for liquid transportation fuels and the further sections will elaborate on the routes to synthesize liquid fuels in specific.

Lipid extraction from algae [ [52] , [53] , [54] , [55] ].

4. Unprocessed algal oils as biofuels

The concept of using oil of vegetables without any type of conversion and processing as fuel in the engine emerged in Paris by 19th century when Rudolph Diesel made use of groundnut oil to operate the engine [ 56 ]. However, the application of raw oils as such in engines is technically unfeasible and poses lot of problems in the long run because a) it is highly viscous, possess low cetane value and low flash point, b) it causes plugging and gumming of filters, c) it causes engine knocking and d) it deposits carbon on piston. Hence a suitable technique is required to convert the vegetable oil into a sustainable biofuel with good fuel characteristics and only such fuel can be an alternative to fossil fuels. Engine performance was evaluated using unprocessed raw algal oil in a digital software Diesel RK which was analogous to Yanmar diesel engine by Tsaousis et al. [ 57 ]. The power generation of the fuel which magnifies the consumption of fuel, spikes the CO 2 emissions but indeed relates to lower emissions of NOx.

4.1. Techniques towards novel fuels

The journey of making biofuel is not a recent work perhaps was kicked on by mid of 18th century itself. Although the biofuel was introduced during II world war for the production of glycerol and its application as explosives, its first commercial patent was filed by late 19th century by a Brazilian researcher, Expedito Parente. This section will discuss the procedures followed by researchers to convert biomass into alternative fuels ( Fig. 5 ). The productive value of oil from biomass grown in lands interprets the superiority of algae over the other food crops. Algae yields liberal quantity of oil compared to the common oil supplying crops and a better aspect of alga in biofuel generation can be understood. ( Fig. 6 ).

Fig. 5

Techniques involved in the production of biofuel.

Fig. 6

Oil yielding capacities of different biomass.

4.1.1. Fermentation

Ethanol derived from vegetables is considered as the cleanest liquid fuel [58, 59 ]. The conversion of biomass to ethanol is done through a technique called fermentation. Sugar cane and starch containing materials are used for the production of ethanol on commercial scale. This process involves a) solublization of starch (liquification step), b) conversion of soluble starch into glucose (hydrolysis step) and c) conversion of glucose into ethanol (fermentation step). The process involves crushing of biomass, addition of water and yeast and fermenting them in the large tanks called fermentors. Fermentation in synthesis of algae biofuel

Each cell of the algae is a rich source of ethanol factory. The outer cell walls of algae constitute of pectin and alginate while the nucleus of the green algae is made of hemi cellulose or polyose with nearly 3000 sugar units and cellulose (polysaccharide) made by 15,000 sugar molecules. The different species of algae has potential carbohydrate content which on cell lysis turns to be feasible feedstock as bioethanol. Chlamydomonas reinhardtii and Chlorella vulgaris are spotted to have surplus carbohydrate and the estimated bio ethanol production from algae by US Renewable Fuels Standard is 36 billion gallons [ 60 , 61 ]. The complex form of carbohydrate in algae has to be broken to monomers prior to fermentation step and is followed by fermenting by using specific microorganisms like bacteria or yeast at approximately 38 °C [ 62 ]. Scenedesmus dimorphus is an apt alga to produce ethanaol with 53 W/W of carbohydrate. Scientist reported sugars stemmed from Scenedesmus sp. resulted in 93% yield of bioethanol. In the heterolactic fermentation, choice of fermenter microbes facilitates the fermentation of algae ex. Saccharomyces cerevisiae [ 63 ]. The energy derived from bio alcohol by fermentation was observed to be as low as 1 because half of the cost of production of bio alcohol is spent in the distillation of bio alcohol from the fermentation mixture. Fermentation faces the challenge during scale up and also commercialization of this fuel is solely dependent on the source being used for the purpose. The process is also claimed to be lengthy and is found to be time consuming extending from several hours to days. Although bioethanol is clean, produces less CO, hydrocarbons and oxides of N 2 , it is an oxygenated fuel containing 35% of oxygen which on combustion often produces aldehyde which is the culprit for photochemical smog. In addition, the production cost of enzyme makes it uneconomical.

4.1.2. Transesterification

The front runners in synthesizing biodiesel by tranesterification process under EPA Renewable Fuel Standard are Biodiesel International, Biosource Fuels and Crown Iron Works Company located in US. The lipids of microalgae ranging from 20 to 50% and the conversion of this into C18 range carbon fuels by the process called transesterification is a desirable task. Tranesterification is the reaction between one mole of triglyceride molecule which is a complex ester and 3–4 moles of alcohol to produce simple esters (Biodiesel). The transesterification technique is often catalyzed by several acid catalysts namely sulphonic acid and sulphuric acid and base catalysts such as NaOH, KOH, sodium methoxide, sodium ethoxide and K 2 CO 3 . The base catalyzed reactions are favoured industrially because the base catalyzed process is less corrosive than acid catalyzed process. The acid catalyst add H + to carbonyl group producing a stronger electrophile whereas the base catalyst eliminates a proton from the alcohol rendering a stronger nucleophilic. Methyl and ethyl esters (biodiesel) are obtained by using methanol and ethanol respectively. To overcome the problems posed by homogenous catalysts, transesterification is prompted through heterogeneous catalysis path. The silica alumina framework zeolites (MOR, HY, HZSM-5, Hβ, materials with large pore size like mesoporous materials (MCM-41, SBA-15, MCM-48), metal oxides (ZrO 2 , WO 3 , CaO, ZnO, SrCO 3 ) and hetropoly acids (H 3 PW 12 O 40 , H 4 SiW 12 O 40 ) have been in use over a decade [ 56 , 64 , 65 ]. Food crops, vegetable oils, animal fats and several other sources were the targets for conversion into biodiesel. Biofuel synthesis from algae by transesterification process

Algae competes the fellow contestants in the biodiesel synthesis market which gives a insight for future oil demand. The conversion of various feedstocks to biodiesel is compared in Fig. 7 . It has to be noted that, catalysts plays a vital role in the biodiesel forming reaction irrespective of the type of oil examined. But, algal oil displays a remarkable tendency to get converted into diesel range esters. With several classes of catalysts in action, porous catalyst Hβ and mixed oxide of Nickel and Molybdenum turns to be veracious materials. The biodiesel yield in the presence on these catalysts almost reaches 100%. Liu et al. [ 66 ] extracted the lipids from microalgal strain by cautious engulfing of them by newly synthesized nanoparticles in mesoporous range. The transfer of lipids present within the cell was enabled by in vitro method with SrO 2 /CaO intrusion into the cells. Potential of nano carbon particles is also being used to convert lipids onto biodiesel. The conventional transesterification uses inorganic catalysts and has a demerit of polluting the environment due to its disposal hitches. Therefore, green substitutes like enzymes can act as better auxiliary. Biological catalysts are also treated for the biodiesel formation and out of them lipases have created a niche in the industry [ 67 , 68 ]. Immobilized lipases on metal oxide nanoparticles have fine thermal stability, corresponds to good selectivity and also can be easily separated. Biodiesel yield was high as 90% with enzyme concentration (1%–3.5%). Croto megalocarpus when catalyzed by sulphated tin oxide over silica yielded 95 % of biodiesel [ 69 ]. Lipase R oryzae immobilized on exchangeable resins transesterified pistachia chnesisBge into 94% biodiesel [ 70 ]. Many countries have begun to fuel their future with biofuels and their native plants were chosen as major feeds. In 2018, United States (380888 TMT) is in the spotlight followed by Brazil (21375 TMT) and Indonesia (4849 TMT) ( Fig. 8 ). To list a few companies working on the algal diesel are Terravia Holdings (Formerly called as Solazyme), Algenol, Blue Marble Production, Culture Biosystem Organization, Oil Inc., Proviron industries, Solix Biofuel, Reliance Life Science.Algenol is a massive firm located in Florida with a bouncing stock value of $3.1 million produces 8000 gallons of biodiesel for every acre of algal harvest annually. Reliance from India operates to deliver 100 barrels of biodiesel per day. Works in biofuels are established in developing countries like India and a joint venture is commenced by Williamson Magor Bio Fuel Limited (North East India) and Oils of U.K. The composition and characteristics of biodiesel from jatropha oil through transesterification process with the correlated values of different standards [ 71 ] ( Table 3 ).

Fig. 7

Transesterification of feedstocks to biodiesel.

Fig. 8

Leading countries in Biodiesel production.

Properties of Jatropha FAME (fatty acid methyl ester or Biodiesel).

The process produces large quantity of fuel within a short time and the quality of the product is also impeccable. Despite of few merits, to obtain a maximum yield of biodiesel, the alcohol has to be used in large quantity. Literature study reveals that the oil to alcohol ratio varied from 1:3 to 1:9 [ 72 ] The excess of alcohol favours the formation of the mixture of fatty acid alkyl esters (biodiesel) but the excessive amount of alcohol makes the revival of glycerol tricky. Among the base catalysts, sodium methoxide was found to be the most active catalyst (98% yield, reaction time – 30 min) even if its molar ratio is as low as 0.5 mol%. The base catalyst requires absence of water and hence it is inappropriate for typical industrial processes. The transesterification process is also catalyzed by Bronsted acids such as RSO 3 H and H 2 SO 4 . This process produces high yield of alkyl esters but the reaction is very slow. Both the acid and base catalyzed transesterification process requires longer time, consumes larger quantity of methanol or ethanol and hence they are not cost effective. The usage of biodiesel in the engines is restricted and hence has to be blended with the diesel or petrol [ 73 ] and it also burns emitting smoke as gums are formed on engines [ 74 ]. Engine performance using algal biodiesel

The importance of running towards these alternative fuels lies in its application in the available design of engine. The algal biodiesel’s application in single cylinder engine at 1500 rpm provides the lucidity it against diesel fuel. The properties like fuel usage, thermal capacity, emission profile were much similar [ 75 ]. Algal oil blended with diesel fuel in 20% ratio completely reduced hydrocarbon exhaust and was claimed to be perfect alternative for diesel engine [ 76 ]. Naresh and Prabhakar [ 77 ], also reported the exact 20% blending of algal oil ended with better emission characteristics strikingly it has to be noted most of the research papers defends blending of diesel with 20% algal oil [ 78 ]. Biodiesel yield from N. cinta with B10 and B7 blending pattern was of good cetane value (50–51) [ 79 ].

4.1.3. HTL of algal biomass

Hydro thermal liquification (HTL) of wet biomass is a promising way to thick and viscous bio crude rich in energy density compared to combustion process. HTL converts entire algae nutrients including proteins and carbohydrates and not only lipids. Therefore, the lipids augmentation step for this technique is bypassed. The operational cost of this technique is expected to be more and this reduces the upgradation of this method. Besides, the possibility of processing the algae with high water content (90%) without any pre-treatment should definitely make the expenditure for this method low than the drying cost of algae. HTL in the absence of a catalyst has a low conversion rate while the addition of catalyst creates a hype in conversion rate. A whole wet microalgae can be liquified in a pressurized condition of 25 MPa and at a temperature below 375 °C. Vinod kumar et al. compared HTL of macroalgal blooms with and without catalyst. Catalytic HTL (Na 2 CO 3, TiO 2 ) showed double conversion (20%) compared to noncatalytic HTL. HTL of Nannochloropsis (NAS) at a 300 °C with Ni doped TiO 2 yielded 48 % of biocrude at 90 % conversion order. The complete combustion of algae released higher % of NO x into atmosphere due to the significant presence of nitrogen in algae (5−8 wt%) [ 80 ].

Biocrude from HTL is found to be low in HHV (32−35 MJ/Kg) and high in oxygen content (10–11 wt%). The viscosity of the fuel obtained by this technique has low viscosity and also had high concentration of lower hydrocarbons [ 81 ]. The engine run with this biocrude will result in NO x and SO X emissions from the exhaust [ 82 ]. Apart from that, the engine performance also will be low owing to their low calorific value. However, HTL biocrude used in the diesel run marine engines with low speed will have trace emissions.

4.1.4. Pyrolysis

Pyrolysis involves the direct thermal decomposition of the biomass at high temperature (400 °C – 1000 °C) in the absence of catalyst and oxygen to produce bio-oil into solid (char and coke), liquid (bio oil) or gaseous fuels (methane and higher gaseous hydrocarbons) in a limited supply of oxygen [ 83 ]. Two types of pyrolysis are reported in the literature: slow (low process temperatures and longer time) and fast pyrolysis (higher process temperatures and shorter time) [ 84 ]. The product produced through the pyrolysis technique is called bio oil. An aqueous phase comprising of varied low m.w compounds (primary alcohol, acid and ketone) and non-aqueous phase of oxygen containing compounds (aliphatics, alcohols, carbonyls, acids, phenols and cresols etc.) and aromatic hydrocarbons [ 85 , 86 ]. Several research works were reported on the thermal treatment of different vegetable oils like sunflower, safflower, palm, castor, tung oils [ [87] , [88] , [89] ]. The comparison between the characteristics of pure soyabean and pyrolyzed soyabean oil is given

in Table 4 [ 90 ].

Comparison of fuel properties of pure soyabean oil with pyrolyzed soyabean oil

The table reveals that there is a drastic reduction in viscosity and pour point of the bio oil derived in the pyrolysis of soyabean oil. However, no significant difference in the heating value between raw and pyrolysis oils were noticed. The different types of biomass cracked thermally in fluidized bed reactor is given in Table 5 . The products derived the biomass are distinct from other sources.

Thermochemical conversion of crop oil and lignocellulosic biomass. Pyrolysis of algal biomass to produce biofuel

The combustion of biomass emits CO 2 which in turn 183 tonnes of CO 2 is absorbed by 100 tonns of algae for its growth and thus self-sustaining the CO 2 cycle. Thus, algae helps in greenhouse gas sequestration [ 96 , 97 ]. Innumerous articles published on pyrolysis of algae is a supplement for aspirants on algal fuel [ [98] , [99] , [100] , [101] ]. Yanik and Sinag pyrolyzed algae Laminaria digitate and fucus serratus at 500 °C in fluid reactor [ 102 ]. Products from the process were solids, liquid and gaseous in nature. Bio oil extracted from the algae was 17%. Pyrolysis of algae yield lower hydrocarbons range of liquids, syngas, bio oil and bio char. 44% of bio oil was reported in the pyrolysis of algal biomass from waste water treatment plant [ 103 ]. Nannochloropis gaditana , a macro algal biomass produced bio oil with highest calorific value of 12.6 MJ/Kg. Majority of the liquid product were decanes and gas products restricted to methane [ 104 ]. About 78% of bio oil was a promising result in flash pyrolysis of macroalgae [ 105 ]. Open pond lake system grown algae on cracking at 500 °C delivered 59% of bio oil with 21 MJ/Kg of heating value [ 106 ]. The slow pyrolysis process always lead to low bio oil contrast to the fast pyrolysis [ 107 ]. The pyrolysis done at higher temperature lacks the efficiency to achieve the products specifically.

The various strains of algae produced different results and the temperature at which pyrolysis was performed also played a vital role ( Table 6 ). Catalytic hydrocracking is another technique in which the compounds with high molecular weight (biomass) are broken down to form low molecular weight compounds using suitable catalysts and stream of hydrogen (1−10 MPa) in the temperature range of 200 °C–450 °C [ 125 ]. Cracking and hydrogenation are complementary, where the cracking of biomass yields olefins by absorbing heat energy (endothermic) and hydrogenation provides heat for cracking (exothermic). The extracted algal oil can be converted into useful fuels within a prescribed range of hydrocarbons and this conversion was mediated by both homogeneous and heterogeneous catalytic way. The homogeneous catalysts used in transesterification cracked the oil but the catalysts impacts the acidity of the medium enhancing the chance of engine corrosion. Catalytic cracking of algal oil and vegetable oils are intensively studied in the production of modest fuels. HZSM-5, a zig zag channeled microporous catalyst produced bio oil yield of 52.7 wt% with microalgae. The aromatic content in the biofuel was nearly 26 wt% [ 126 ]. Upgrading of oil catalytic cracking of 100 Kg of algae performed in ASPEN plus was 95% successful with 41% kerosene yield. Coprocessing of biofuel derived from HTL (hydro thermal liquification) of algae combined with HVGO (Heavy vehicle gas oil) decreased the conversion rate and also increased coking of catalyst. This also further confines the economic suitability of algal oil blended with HVGO [ 127 ]. Fuels in used for aviation or avgas, a highly refined version of gasoline was obtained cracking of alga at H 2 atmosphere in presence of Pt-Re catalyst. About 50% of Bot-oil (Botryococcus braunni ) cracking yielded 16.7% of C10 -C15 range products which could be termed as diesel [ 128 ]. Zhao et al. [ 129 ] catalytically deoxygenized microalgae with non-sulfide metal as catalysts into hydrocarbons.

Pyrolysis of Micro algae into bio oil.

The main advantages of this method are (a) it is inexpensive [ 130 ] (b) the bio oil is easy to store, transport and (c) fuels of high demand can be easily prepared by upgrading the oil [ 131 ]. While the drawbacks of bio oil are (a) highly viscid, harsh and lacks thermal stability [ 132 ], (b) it exhibits low calorific value and (c) resemblance to the reactant oil as it has predominant oxygenated molecules [ 133 ].

4.2. Algae- the new hope of future energy but a long way to go

The microorganism have the potential to yield higher quantity of biofuel compared to other general biomass. Interestingly, algae is a treasure hunt for the society to develop butanol, ethanol and jet fuels. The algal bloom is definitely easy to grow in an open pond or closed loo systems in unfertile lands. The algal oil is inedible and therefore do not have to answer for food availability controversies. The higher concentration of lipids/ fatty acids present in algae enables its successful conversion into biofuel. The hypes for algal biofuel is quite acceptable and but also has to accept few of the discontents. Companies like Algenol, Solazyme and Sapphire Energy attracted 100’s of millions of dollars from private sectors. With a promise to produce millions if gallons of fuel within short span. The large span invested were in vain as the industrial goals were not attained. In 2017, Professor Kevin Flynn, Swansea University reported that to equalize 10% of fuels utilized in European transportation, the algae should be cultivated in ponds thrice the size of Belgium. Furthermore, the fertilizers should be required in anomalous level approximately 50% of European crop plant needs. As a result, the companies shifted their target to prepare cosmetics and animal fodder regarding the huge investments and less fruitfulness of the project. Irrespective of failures in the research, commendable achievements has to be remembered.

5. Conclusion

Conventional fossil fuel usage makes the society to contemplate on producing the renewable and sustainable sources of energy. Exploitation of biomass is the right choice to produce biofuels with ultimate requisites of alternative fuels. Sources used in I and II generation biofuel are unviable and are criticized for the economic, social and food insecurity. Many countries have pinned their hope in bio III generation biofuel constituted of whole alga and oil derived from alga after conversion processes. Though the thought of alga cultivation is simple, however, the existence of difficulties in feedstock production with high lipid content and harvesting needs to be addressed. Challenges is really enthralling in the area of conversion of alga into perfect fuel. The algal biofuel works wonders in engines and yet a detailed study on the parameters for fuel compatibility is required. However, also, we have a long way to go for making algal biofuel a commercially viable alternate in place of fossil fuel.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  • Open access
  • Published: 20 May 2019

Latest development in microalgae-biofuel production with nano-additives

  • Nazia Hossain   ORCID: orcid.org/0000-0001-7925-0894 1 ,
  • T. M. I. Mahlia 2 &
  • R. Saidur 3 , 4  

Biotechnology for Biofuels volume  12 , Article number:  125 ( 2019 ) Cite this article

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Microalgae have been experimented as a potential feedstock for biofuel generation in current era owing to its’ rich energy content, inflated growth rate, inexpensive culture approaches, the notable capacity of CO 2 fixation, and O 2 addition to the environment. Currently, research is ongoing towards the advancement of microalgal-biofuel technologies. The nano-additive application has been appeared as a prominent innovation to meet this phenomenon.

The main objective of this study was to delineate the synergistic impact of microalgal biofuel integrated with nano-additive applications. Numerous nano-additives such as nano-fibres, nano-particles, nano-tubes, nano-sheets, nano-droplets, and other nano-structures’ applications have been reviewed in this study to facilitate microalgae growth to biofuel utilization. The present paper was intended to comprehensively review the nano-particles preparing techniques for microalgae cultivation and harvesting, biofuel extraction, and application of microalgae-biofuel nano-particles blends. Prospects of solid nano-additives and nano-fluid applications in the future on microalgae production, microalgae biomass conversion to biofuels as well as enhancement of biofuel combustion for revolutionary advancement in biofuel technology have been demonstrated elaborately by this review. This study also highlighted the potential biofuels from microalgae, numerous technologies, and conversion processes. Along with that, the study recounted suitability of potential microalgae candidates with an integrated design generating value-added co-products besides biofuel production.


Nano-additive applications at different stages from microalgae culture to end-product utilization presented strong possibility in mercantile approach as well as positive impact on the environment along with valuable co-products generation into the near future.

research paper on biofuel from algae

Biofuel has caught substantial attention worldwide nowadays as an alternative fuel due to its capability to adapt with gasoline for a maximum 85% blend without any engine modification. Subsequently, the suitability of various candidates for biofuel is being continuously quested by the researchers and environmentalists [ 1 , 2 , 3 , 4 , 5 ]. In this recent era, one of the most sophisticated technologies, nano-technology integration with bioenergy application by the nano-energy sector has brought a revolutionary impact on biofuel conversion processes and enhancement of engine performances. Nano-technology is defined as designing a device or material in nano-scale (10 −9 m). To accelerate the biofuel yield and improve the efficiency of biofuel utilization in petrol and diesel, nano-technology has been initiated via nano-additives such as nano-magnets, nano-crystals, nano-fibres, nano-droplets, and others [ 6 , 7 , 8 ]. Figure  1 presents the perspectives of nano-additives on microalgae cultivation to microalgal-biofuel implementation.

figure 1

Nano-additive applications for the enhancement of microalgae cultivation to biofuel implementation

On this eve of the quest for suitable biomass for biofuel, the concept of microalgae cultivation appeared to the spotlight for biofuel manufacturing due to several positive perspectives such as (i) they do not clash with human or animal food chains, (ii) very rich with carbohydrate, protein, and oil content, (iii) can grow in aqueous media such as wastewater, freshwater, saline water, and assimilate nutrients from brackish water, salt water, or highly polluted water, (iv) demand low water, (v) sustain capability to grow whole year naturally with sunlight presence, (vi) can be cultivated in the waste dump area, sea, ponds, rivers, industrial, and municipal waste drainage, wet bare lands especially in cold regions, (vii) develop sustainable O 2 generation system, and (viii) diminish CO 2 by up taking it for photosynthesis respiration [ 9 , 10 , 11 , 12 , 13 , 14 ]. In addition, microalgae contain very short harvesting life cycle and yield nascent biomass that drives higher productivity of the desired biofuel. Interestingly, microalgae carry a prodigious amount of carbohydrates, protein and lipid, the sole components of biofuel conversion [ 13 , 15 , 16 , 17 ]. Nano-technology applications have been implemented to biofuel industries, since the existing controversial approaches of traditional microalgae culture-biofuel production contain a number of limitations such as inconsistent industrial-scale microalgae production, high microalgae production and harvesting cost, energy consumption for biofuel production from microalgae, and the increase of greenhouse gas intensity in environmental [ 18 ]. Nano-technology applications can be entailed in different stages from microalgae cultivation to microalgae-biofuel application in fuel engines due to durability, recyclability, adsorption efficiency, catalytic performance, stability, crystallinity, economical advantage, high storage capacity, excellent biofuel yield, and environment-friendly characteristics. According to the previous studies, nano-technology application enhanced microalgae cultivation, the maximum yield of numerous microalgae biofuels as well as microalgae-biofuel implications in petrol and diesel engines. Various nano-materials, e.g., nano-fibres, nano-particles, nano-tubes, nano-sheets, and other nano-structures, have been investigated as effective nano-catalysts in direct and indirect approaches in biofuel (e.g., bioethanol, biodiesel, biomethane, and others) yield enhancement [ 19 , 20 , 21 , 22 ]. For instance, magnetic nano-particles were used as a carrier for enzyme immobilization for bioethanol and biodiesel generation effectively. Owing to high coercivity and powerful paramagnetic characteristics, magnetic nano-particles were also preferred for methanogenesis to produce biomethane [ 21 ].

To authors’ best knowledge, no review study has been performed on numerous biofuel productions from microalgae integrated with the nano-additive application so far. The closest review with this study was conducted on the bioenergy production from lignocellulosic biomass (agricultural residues), industrial waste (sludge) as well as algae (microalgae and macroalgae) with the nano-scale optimization which has merely emphasized on the mechanism of nano-particles, biomass characteristics, and nano-particle application on biomass growth [ 23 ]. Compared with that, the current review contextualized the numerous biofuel productions from pure microalgae and optimization with nano-additive application on biomass growth to end-product application. Therefore, the major objectives of this review work are (i) to determine the array of the techniques and methods associated with nano-particles incorporation with microalgae culture as well as microalgal biofuel, (ii) to demonstrate divergent nano-additive applications on microalgae cultivation, biomass conversion to biofuels, and biofuel combustion, (iii) to identify the potential sources of microalgae, especially the carbohydrate, protein, and lipid-enriched microalgae types for biofuel production and determine the possible microalgae biofuels, biomass conversion technologies, and processes to biofuels, and (iv) to assess the future prospects of the process development planning along with integrated design of some other value-added products besides biofuel.

General perspective of microalgae

Microalgae are referred as photosynthetically driven single or multi-cellular living being, the habitat of moist environment either on the solid mud or float on various water types, e.g., fresh water, marine water, wastewater with the presence of sunlight, or artificial light. The scientific consensus is that through photosynthesis respiration, they convert CO 2 to O 2 and generate large amounts of cellular energy content embedded with sugar, protein, and lipid [ 24 , 25 , 26 , 27 ].

Nowadays, industrialization and urbanization threaten the existing ecosystem severely by dumping heavy metal waste containing phosphorus, nitrogen, sulfur and others as well as exhaling high amount CO 2 to the free air. Another knocking threat to the energy sector is rapid depletion of fossil fuel worldwide due to excessive energy uses [ 28 , 29 , 30 ]. With this circumstance, microalgae cultivation in the wastewater, unused fresh, and saline water, drainage is considered as suitable scientific solution for green energy due to some favorable aspects such as multi-functionality, genuine conversion competency biologically and flexibility with growth system, wastewater accumulation, CO 2 sequestration, and large amount of carbohydrate–lipid–protein content. To note, carbohydrate–lipid–protein are the main components to generate divergent biofuels (e.g., bio-oil, biodiesel, biobutanol, and others) and biogas (e.g., bio-hydrogen) [ 31 , 32 ]. The cellular components of microalgae are composed of huge fraction of lipid, protein, and carbohydrates resulting in the driving factors of biofuel production. Table  1 presents some well-known potential microalgae candidates for biofuels. These species were extensively researched in the laboratory and large-scale applications so far. Type and description of these species have been tabulated to present a detailed view of selected species, suitable growth conditions (such as water type and region for cultivation), availability, and cellular specifications. Table  2 represents prime microalgae cellular component composition of several well-known microalgae species for biofuel production [ 24 , 33 , 34 ].

Biofuels from microalgae

Numerous biofuels, e.g., bioethanol, biodiesel, bio-oil, biomethane, bio-hydrogen, and others, have been extracted from microalgae [ 46 , 47 ]. Nano-particles’ incorporation with microalgae cultivation (e.g., cell suspension, cell separation, and cell harvesting), biofuel conversion technologies, and biofuel application have amplified the overall yield in every stage [ 22 ]. According to the previous studies, a very small amount of colloidal hydrous iron(III) oxide particles boosted almost 100% microalgae cell suspension; magnetic particles incorporated with aluminum sulfate were very effective for cell separation from the mixed culture of Anabaena and Aphanizomenon microalgae species; silver nano-particles application on Chlamydomonas reinhardtii and Cyanothece 51142 microalgae harvesting increased 30% higher biomass productivity; and calcium-oxide nano-particles escalated the large-scale biodiesel conversion yield up to 91% via catalytic transesterification [ 18 , 22 , 48 ]. This study summarized the overall microalgae cultivation integrated with nano-particles until biofuel production in Fig.  2 . Different biofuels from microalgae and conversion processes are diagrammed in Table  3 .

figure 2

Process flow diagram of carbon capture, water treatment and biofuels production from microalgae incorporated with nano-particles [ 18 , 25 , 49 , 50 , 51 , 52 , 53 , 54 ]

Preparing techniques of nano-additives for microalgae biofuel

Magnetic nano-particle (NP) powder has been enumerated to the microalgae cell suspension in the photobioreactor cultivation process to flocculate cells for uniform distribution of nutrients and light all over the reactor. Immunomagnetic detection and modification of microalgae cell by NPs are another well-practiced method for cell suspension enhancement. Nano-liquid has been injected to the cell culture for microalgae harvesting and bio-separation through this technique. Silver nano-materials have also been implemented on the photobioreactor surface coating for higher light accessibility [ 22 ]. Along with microalgae culture and harvesting, sphere nano-particles have been enacted during hydrolysis, lipid extraction, transesterification, and biofuel purification from microalgae via irradiation and ultra-sonication methods and much higher biofuel yield have been obtained. Another established method of nano-materials application includes enzymatic nano-catalyst, lipase carrier. The reactant diffusion rate enhancement by the NPs to the active side of lipase has been determined by Eq.  1 [ 55 ]:

where R df  = diffusion rate to the active sides of lipase and D  = diffusion path diameter of reactant to the access of lipase active side.

NPs for biofuel doping can be formulated by either physical or chemical methods. For instance, plasma-arcing, sol–gel method has been presented as chemical method and ball mill process (agitation rate: 450 rpm) was presented as a physical method for NPs’ preparation in the previous studies [ 56 , 57 , 58 ]. Subsequently, NPs were doped with microalgal biofuel (e.g., microalgae oil, biodiesel, bioethanol, and others) with different doses (e.g., 25 ppm, 50 ppm, 100 ppm, and others) via ultra-sonication processing by the presence of magnetic stirrer and implemented on compression ignition (CI), direct ignition (DI) engines without any engine modification. NPs are dispersed in a base fuel and smoothen potential agglomerate into nano-scale due to its’ larger surface area and surface energy [ 58 , 59 , 60 , 61 ]. The ultra-sonication method was conducted with various parameters such as frequency (e.g., 20 kHz, 40 kHz, and 45 kHz), power (e.g., 120 W and 220 W), and time (30 min and 60 min) [ 60 , 62 , 63 ]. Cationic surfactants, e.g., tetra methyl ammonium hydroxide, cetyl trimethyl ammonium bromide, have been incorporated on the nano-particle surface for a negative-charge envelope formation to resist NPs’ sedimentation [ 56 , 64 ]. After biofuel-NPs’ doping, the NP-blended biofuel was preserved under the static condition to stabilize for fuel purpose [ 59 ]. Several potential NPs–microalgae-biofuel blends are tabulated in Table  4 . The morphology and crystalline phases after NP-doping were analysed through a scanning electron microscope and X-ray diffraction, respectively [ 61 ]. Botryococcus braunii oil was doped with almost 50 nm sized titanium dioxide (TiO 2 ) and silicon dioxide (SiO 2 ) incorporated with biodiesel (B20) of different doses for enhanced fuel efficiency in CI engine [ 60 ]. Caulerpa racemosa green algae biodiesel (B20) was doped with 50 nm sized zirconium dioxide (ZrO 2 ) by the different doses for CI engine [ 59 ].

Future applications of nano-additives for microalgae-biofuel

Nano-additive application on microalgae-biofuel enhancement has been categorized into several stages from raw material production to end-product implications. The stages are:

nano-additives for microalgae cultivation;

nano-additives for microalgae biomass conversion to biofuels;

nano-additives for microalgae-biofuel applications.

Nano-additives for microalgae cultivation

Improvement of the microalgae biomass productivity with the minimum area requirement is considered as the main purpose of nano-additive application in the microalgae culture. Nano-technology is being applied for enzyme immobilization, since nano-structures broaden the immobilization surface area causing high loading power of enzymes and stability of immobilized enzymes. Enzyme immobilization can be performed in different approaches such as electrospun nanofibers, covalently attached enzymes into nano-fibres, and enzyme aggregate coatings on nanofibers. The enzyme immobilization was investigated on various carbon nano-particles, e.g., graphene oxide (GO), multi-walled carbon nano-tubes (MWNTs), oxidized-MWNTs (O-MWNTs), and fullerene (C60). Among these nano-structures, O-MWNTs yielded the highest, and C60 yielded the lowest [ 21 , 22 ]. Nano-particles’ (NPs’) application was implemented on several microalgae species harvesting and yielded outstanding in each phase of the application. Application of nano-particles on microalgae harvesting claimed 20–30% microalgae production cost in large-scale application [ 22 ]. Table  5 presents the harvesting efficiency of several microalgae species cultivated with various nano-particles. Nano-particles also boosted the light conversion efficiency in photobioreactor (PBR) during the microalgae culture period. It is also worth mentioning that PBR is run by artificial light sources consuming additional energy and cost. However, during biomass growth, light sources do not reach in culture broth inadequately due to self-shading and biofilm formation on the PBR surface. To achieve desired illumination properties and photo-conversion efficiency in the PBR, various light-emitting diodes (LEDs) equipped with nano-materials fabrication are being implemented recently. Gallium aluminum arsenide (GAA)-fabricated LEDs have been experimented on laboratory scale algae culture so far. It was evident that the application of optical fibres in algal culture saves much energy, additional light cost, and increase efficiency [ 18 ]. Another latest development of nano-particle, integration of metallic nano-particles (MNPs) with localized surface plasmon resonance (LSPRs) amplifies the light scattering at certain wavelength [ 65 ]. An experimental study revealed that silver nano-particles’ (Ag-NPs’) suspension in plasmonic mini-PBRs backscatter blue light strongly. The blue light increased the photosynthetic efficiency significantly for green microalgae, Chlamydomonas reinhardtii, and blue–green microalgae, Cyanothece 51142, and 30% higher microalgae biomass have been obtained [ 48 ]. Nano-particles addition in microalgae cultivation can also improve the yield of the CO 2 absorption from the atmosphere and CO 2 sequestration that can boost the biomass growth. For instance, nano-bubbles in microalgae culture remained stable for a longer time. Nano-bubbles also floated algae biomass into the culture, ensured high mass transfer efficiency, and improved biomass density by sufficient accumulation of CO 2 , O 2 stripping, and minor buoyancy. Moreover, nano-bubbles suspended the biomass around airlift-loop bioreactor (ALB) and required less energy than micro-bubbles. Uniform nonporous membrane of ALB was also capable of producing 100 nm sized bubbles for this purpose [ 18 , 66 , 67 ]. The previous studies also delineated that nano-additives played a significant role in flocculation and separation process before biofuel production besides microalgae harvesting [ 22 ].

Nano-additives for microalgae biomass conversion to biofuels

Among microalgae biofuel, biodiesel has been appeared as the most popular and commercial biofuel in the mobile fuel market. For the case of biodiesel production, applications of acidic and basic nano-catalyst spheres can substitute the chemical compounds such as sodium methoxide by reacting with the free fatty acids and oils. Additional advantages of these nano-catalysts are recyclability and positive economical impact. Moreover, reactions can take place with low temperature and pressure as well as this approach reduces the contaminant release to the environment borne by sodium methoxide [ 6 ]. An industrial biodiesel study demonstrated that commercial CaO-NPs presented 91% biodiesel conversion efficiency during scaled-up catalytic transesterification [ 18 ]. Experimental study of microalgae cultivation with spherical nano-particles composed with sand (silica) and calcium compounds revealed that microalgae cellular growth increased drastically without harming harvesting as well as biofuel production from vegetable oil. The best way to address this issue was described as one of the major driving factors for commercial biofuel, biofuel production cost dropped effectively [ 6 , 7 , 8 , 68 ]. The experimental study mentioned that mesoporous silica nano-catalyst, Ti-loaded SBA-15 presented ten times higher free fatty acids (FFA) and water tolerance level than any other catalysts for biodiesel production from vegetable oil as well as this nano-catalyst performed three times better than other effective nano-catalysts titanium silicalite-1 (TS-1) and titanium dioxide silicate (TiO 2 -S) [ 69 ].

Moreover, Ti-loaded Santa Barbara Amorphous-15 (SBA-15) nano-catalyst application reduced the chemical (alkaline catalyst, NaOH) cost of transesterification process for biodiesel production by recycling the nano-catalyst as well as this process is more environment-friendly [ 6 , 69 ]. On the other hand, sulfate incorporated Ti-SBA-15 also performed as biocatalyst to convert vegetable oil to 100% esterified bio-lubricant. In consequences, this nano-particle can be expected to produce bio-lubricant from bio-oil of microalgae [ 70 ]. Other study showed that Niobia (N 2 O 5 ) incorporated with SBA-15 application on biodiesel production from biomass through esterification presented a significant scenario for microalgae-biodiesel yield [ 71 ]. Another study delineated that the enzyme extracted from Pseudomonas cepacia conjuncts with the nano-particles such as polyacrylonitrile (PAN) nanofibre, Fe 3 O 4, and nanoporous gold; silica nano-particles with lipase enzyme from Rhizopus miehei ; ferric silica and magnetic nano-particles with lipase from Burkholderia sp., polyacrylonitrile nano-fibre bound with lipase from Thermomyces lanuginosa has performed very effectively to produce biodiesel from various bio-oil by the transesterification process [ 7 ]. Furthermore, nano-magnetic biocatalyst of KF/CaO–Fe 3 O 4 , Li(lithium)-doped CaO, Fe 2 O 3 –CaO, sulfate (SO 4 − ) incorporated Zi (zirconium), sodium titanate and carbon-based nano-tubes and nano-particles reached up to 95% or above biodiesel yield from diverse types of biomass and biodegradable waste [ 7 , 72 ]. Besides enhancement of biodiesel-yield efficiency, a type of NP, zeolite (an alumina silicate mineral), has been used as commercial absorbent during the transesterification process. Zeolites absorbed the undesirable moisture content (4–6%) and produced pure glycerine as co-product besides biodiesel. Mesoporous nano-particles also presented a vital capability for continuous microalgal-biofuel generation from biomass without cell lysis. Zeolites also removed lipids from the microalgae cell membrane [ 18 , 73 ]. Table 6 presented the applications of nano-additives for biodiesel-yield enhancements during microalgae to biofuel conversion, suitable conversion processes, and efficiencies.

Nano-particles were efficiently capable to perform as immobilizing beds for valuable enzymes due to their large surface area-to-volume ratio. This capability of NPs broke down the long chains of complex sugar of microalgae, converted it to simple sugar, and consequently turned into bioethanol via the fermentation process. Due to the large surface area, the interaction between the surface of the nano-particles and fuel surrounded by them achieved adequate stability to overcome density variations. Nano-particles prepared by carbon nano-tubes doped with iron-oxide nano-particles presented excellent biocatalytic efficiency in a bioreactor, recyclable option enzyme applications, less capital cost as well as better enzyme loading for this purpose [ 6 , 7 , 74 ]. A catalytic study mentioned that mesoporous niobium oxide (N 2 O 5 ) application on complex sugar (sucrose) possessed both Lewis acid (LA) and Bronsted acid (BA) sites to convert fructose to 5-hydroxymethylfurfural (HMF) with the highest yield so far. The synergistic catalytic effect from a large amount of both LA and BA acid site quantities and surface areas played a positive impact on the reaction rate with a few times faster conversions [ 75 ]. Functionalised multiwall carbon-nano-tube (MWCNT) immobilization presented more than 55% initial activity of microalgal hydrolysis for Candida Antarctica. Nano-catalysts such as cobalt–molybdenum fabricated with Si/Al have been experimented on Botryococcus braunii and presented stable hydrocarbons. Another nano-catalyst, mobil composition of matter No. 41 (MCM-41), mesoporous material effectively reduced oxygenated fractions of bio-oil through catalytic pyrolysis [ 18 ].

Nano-catalysts can synthesize biomethane produced from microalgae from wastewater into pure hydrogen and carbon content. In a further step, this methane can produce biogas through anaerobic digestion. Biogas could be used as raw material of bio-fuelled electricity generation further. The elemental carbon can also be utilized as pure nano-graphite for the applications on batteries, aerospace, automobiles, and others [ 6 ]. The latest development conducted by quantum sphere on marine microalgae species evinced biogasification from wet microalgae biomass by metal nano-catalysts [ 18 ]. Besides that, nano-particles such as TiO 2 , CeO 2 were manifested to improve 10–11% of the biogas yield from wastewater treatment. Therefore, these nano-particles can be projected further for the biomethane production from microalgae grown in wastewater [ 7 ]. Apart from that, nano-substances with SiO 2 , nano-particles of platinum (Pt), nickel (Ni), cobalt (Co), and iron (Fe) can increase methane production from biomass up to 70%. Nano-fly ash and nano-bottom ash were proved to increase biomethane yield up to 3.5 times more. Nano-metal oxides, e.g., MgO, CaO, and SrO, nano-materials such as silica, single-walled nano-tubes of carbon-based materials, nano-clay, and nano-zero valence metal applications in biodiesel, bio-hydrogen, and biomethane production from microalgae and other biomass presented outstanding yield. These nano-particles can be projected for large-scale microalgal-biofuel production in the future to obtain revolutionary yield [ 7 , 8 ]. In addition, nano-hybrid catalysts are being commercialized as emulsion stabilizers in industrial applications. For instance, quaternary ammonium salts have been documented as an emulsifying surfactant for separation, extraction, isolation, and purification of biofuels. Carbon nano-tubes with silica fusion, SiO 2 –MgO nanohybrids have been performed as stabilizers on bio-oil in water emulsion due to its inherent hydrophobicity and resulted in full conversion in different emulsion phases [ 18 ].

Nano-additives for microalgae-biofuel applications

Solid nano-particles, nano-fluids, or nano-droplets with biofuel and fossil fuel were proved to improve the fuel lubricity, cetane number, burning rate, chemical reaction, catalytic performance, fire/flash point, heat and mass transfer efficiency and water co-solvency as well as decrease delay period [ 76 , 77 ]. That resulted in more complete and cleaner combustion of microalgae biofuel mixed with fossil fuel in compression ignition (CI), spark ignition (SI), and direct ignition (DI) engines. In line with that, nano-technology applications showed the capability of amplifying microalgal-biofuel combustion efficiency and reduced soot, NO x , smoke, HC, CO 2, and CO emission to the environment up to 72% [ 6 , 76 , 78 ]. Application of solid nano-additives such as alumina (Al 2 O 3 ), CERIA, carbon nano-tubes (CNT), Co 3 O 4 , ZrO 2 , La 2 O 3 , CeO 2 , SiO 2 , Ni 2 O, TiO 2 , ZnO, Fe 2 O 3 , CuO, Ce x Zr (1– x ) O 2 , and amide-doped MWCNTs-CeO 2 boosted the engine power, torque, and brake thermal performance of biodiesel (extracted from microalgae and other biomass) in CI and DI engines up to 11% [ 59 , 76 , 79 , 80 ]. The experimental study of nano-particles on DI engines demonstrated that nano-particles blended with biodiesel as well as diesel–biodiesel mixture performed outstanding. The effectiveness was higher compared to usual catalysts [ 61 , 81 ]. Another study presented that nano-particles of CeO 2 incorporated with an emulsion of biofuel with sol–gel combustion technology performed excellent mono-cylinder 4 stroke direct CI and DI engines without any hardware modification. Nano-particles addition with biofuels escalated the fuel calorific value, fastened evaporation rate, improved brake-specific fuel consumptions and thermal efficiency, reduced greenhouse gases (GHGs) such as CO, NO x , and smoke, and unburned HCs. Chemical reactions between CeO 2 and GHGs gases are presented in Rc. 2 , Rc. 3 , and Rc. 4 [ 82 ]:

In contrast, liquid nano-additive, nano-Al-droplet application (nano-suspension) on biofuel mixture has been manifested more efficient than even micro-suspension. Liquid nano-additives also presented outstanding performance by achieving better suspension than n -decane-based fuels. Nano-Al suspension with ethanol was strong enough for a longer period than other particles, because ethanol tended to form a gel around the nano-particles due to higher viscosity [ 74 ]. Nano-droplets coated a monolayer on the mechanical parts of the engine touched with liquid fuel and improved fuel efficiency [ 18 ]. In addition, NPs such as nano-Al, Al 2 O 3 , CuO, MgO, MnO, and ZnO incorporated with water–diesel–biodiesel (E10) emulsion and bioethanol performed remarkably. Among these NPs, Al 2 O 3 performed the best because of mandate disabling, consistent torque boosting, higher heat of combustion, super-high DTG max value, tiniest size of water droplets, the minimum value of brake-specific fuel consumption, and lowest values of Soot, NO x , CO, and HC [ 19 , 83 ].

Challenges and future prospects

Although nano-additive applications played significant role in microalgae cultivation, harvesting, conversion to biofuel and biofuel applications to enhance the efficiency, yet some challenges remained before the implementation of nano-additives for the mercantile approach. Most of the nano-additives from experimental research were not well-characterized in terms of particle size, shape, and size distribution as well as clustering [ 84 ]. Before large-scale application, well characterization of nano-particles and nano-fluids must be studied comprehensively. Appropriate nano-additive selection, preparation methods, and time for the selected application should be emphasized for optimum productivity. The effect of nano-catalyst implementation for microalgae-biofuel combustion quality, engine performance, and gas emission should be well studied and well-understood before implementation. In line with that, the availability of appropriate nano-additives with large amount might be a challenge for mass application though for laboratory scale, nano-additives are adequately available. Another constraint is cost-effectiveness of nano-catalysts for an industrial application which may hinder the commercial perspective, since many nano-catalysts are quite expensive.

Along with the potential microalgae determination and biofuel generation, integration of a plant design of value-added co-products will be the predominant advantage of the overall project with the economical aspect. This review encouraged biofuel research and development (R&D) sector worldwide to convert their unused, abandoned and wastewater sources, wet, and barren lands into microalgae farm as an eminent source of biofuel production. However, it should be highlighted that based on the existing research experiments, microalgae fuel production still stands at initial stage due to downward economic profile worldwide. Nano-additive applications on microalgal biofuel are yet confined into laboratory and pilot scale which can be counted as a significant limitation. Hence, it is strongly recommended to figure out large-scale process development with nano-additive applications for enhancement of microalgal growth, biofuel transformation processes, and fuel utilization in CI and DI engines. Nano-additive applications at different stages from microalgae culture to end-product utilization have a strong possibility to gain economical feasibility. Therefore, the detailed techno-economic analysis must be commanded to determine whether NP applications on microalgae biofuel are economically favorable or not, since economical issue is one of the most effective factors behind large-scale plant setup. Besides, these applications also have positive impact on the environment with value-added co-product generation into near further. Since the nano-additive utilization manifested itself environment-friendly, still a comprehensive life cycle assessment should be conducted to present the environmental positivities transparently. Besides all these factors, public safety, impact on flora and fauna, and the possibility of bio-hazards are also needed to be analysed extensively before commercialization.

Microalgae utilization for biofuel production is undoubtedly desirable all over the world. Though this approach is energy-efficient and environment-friendly, experts are still looking for an innovation that can boost the microalgae-biofuel yield from primary stage to end product as well as shift the whole process towards a cost-effective fuel solution. Hence, this review was emphasized on the synergistic effect of nano-additive-enhanced microalgal biofuel for mercantile approach and fuel-yield extension. Application of various forms of nano-additives in different phases on microalgae growth to biofuel demonstrated an excellent outcome that may project revolutionary improvement of commercial microalgae biofuel. However, the sustainability analysis of stepwise production rounds for microalgae biofuel still presented a bare need of further research and innovative concepts. These concepts may determine the most appropriate nano-additive for the desired type of biofuel in the context of economical aspect. Since nano-additive application on microalgae is quite new research concept, policy making and implementation of nano-additives will remain as the most vital issues for commercial output especially in developing countries. Therefore, managerial insights are needed to be emphasized further on proper policy, socio-economic impact, advantages and limitations for the overall system to attract the government and non-government fuel industries.

Enhancement of microalgae cultivation and harvesting by nano-bubbles and nano-particles application.

Identification of suitable microalgae species, possible biofuels from microalgae, latest conversion technologies, processes, and required equipments.

Excellent microalgal-biofuel yield by nano-droplet and nano-additives.

Complete and cleaner combustion in fuel engines by nano-emulsion and nano-stabilizers.

Availability of data and materials

Not applicable.



silver nano-particles

silver oxide

airlift-loop bioreactor

aluminum oxide/alumina

aluminum droplet

bronsted acid

calcium carbonate

cerium oxide

calcium oxide

calcium oxide nano-particles blends

compression ignition


carbon mono oxide

cobalt oxide

carbon nano-tubes

copper oxide

cerium–zirconium oxide

chitosan/magnetic nano-particles

direct ignition

deep eutectic solvent

maximum derivative thermogravimetry

free fatty acids

ferric oxide

ferric chloride

gallium aluminum arsenide

greenhouse gases

graphene oxide


5-hydroxymethyl furfural

lanthanum oxide

light-emitting diodes

localized surface plasmon resonance

multiwall carbon nano-tubes

mobil composition of matter No 41

magnesium oxide

manganese oxide

metal nano-particles

niobia/niobium oxide

nickel oxide

nitrogen oxide

oxidized MWNTs



poly dimethylammonium chloride

research and development

rhodium oxide

santa barbara amorphous-15

supercritical water gasification (SCWG)

surface-functionalized iron-oxide nano-particles

spark ignition

silicon dioxide

simultaneous saccharification and co-fermentation

separate hydrolysis and co-fermentation

separate hydrolysis and fermentation

simultaneous saccharification and fermentation

strontium oxide

titanium dioxide

titanium silicalite-1

titanium dioxide silicate

alumino silicate mineral

zirconium dioxide

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Hossain, N., Mahlia, T.M.I. & Saidur, R. Latest development in microalgae-biofuel production with nano-additives. Biotechnol Biofuels 12 , 125 (2019). https://doi.org/10.1186/s13068-019-1465-0

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Environmental impact of algae-based biofuel production: A review

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Saleh M A Mobin , Firoz Alam , Harun Chowdhury; Environmental impact of algae-based biofuel production: A review. AIP Conf. Proc. 17 November 2022; 2681 (1): 020084. https://doi.org/10.1063/5.0117093

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Concerns about the rapid depletion of fossil fuels, energy security, climate change due to global warming, environmental pollution, and faster increase of fossil fuel prices have drawn attention to researchers, the scientific community, and government policymakers to develop alternative energy sources for reducing dependence on fossil fuel. In recent years, microalgae culture has received significant attention due to its potential application for bioenergy production, wastewater treatment, industrial CO 2 removal, and production of biochemical compounds that can be used for human and animal health and other benefits. However, large-scale microalgae production and their processing for producing various products and by-products could have environmental impacts beyond energy consumption in the microalgal production process. This article has reviewed the environmental effects of microalgae-based biofuel production on water resources and quality, eutrophication, biodiversity, waterborne toxicant, algal toxicity, wastewater remediation or treatment, waste generation, and greenhouse gas land-use changes, and genetically engineered microalgae.

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Review article, scope of algae as third generation biofuels.


  • Biochemical Conversion Division, Sardar Swaran Singh National Institute of Renewable Energy, Kapurthala, Punjab, India

An initiative has been taken to develop different solid, liquid, and gaseous biofuels as the alternative energy resources. The current research and technology based on the third generation biofuels derived from algal biomass have been considered as the best alternative bioresource that avoids the disadvantages of first and second generation biofuels. Algal biomass has been investigated for the implementation of economic conversion processes producing different biofuels such as biodiesel, bioethanol, biogas, biohydrogen, and other valuable co-products. In the present review, the recent findings and advance developments in algal biomass for improved biofuel production have been explored. This review discusses about the importance of the algal cell contents, various strategies for product formation through various conversion technologies, and its future scope as an energy security.


The requirement of energy for the mankind is increasing day by day. The major source of energy is based on fossil fuels only. Thus, the scarcity of fossil fuels, rising price of petroleum based fuels, energy protection, and increased global warming resulted in focusing on renewable energy sources such as solar, wind, hydro, tidal, and biomass worldwide ( Goldemberg and Guardabassi, 2009 ; Dragone et al., 2010 ; Rajkumar et al., 2014 ).

Different biomass from various sources like agricultural, forestry, and aquatic have been taken into consideration as the feedstocks for the production of several biofuels such as biodiesel ( Boyce et al., 2008 ; Yanqun et al., 2008 ), bioethanol ( Behera et al., 2014 ), biohydrogen ( Marques et al., 2011 ), bio-oil ( Shuping et al., 2010 ), and biogas ( Hughes et al., 2012 ; Singh et al., 2014 ). However, the environmental impact raised from burning of fuels has a great impact on carbon cycle (carbon balance), which is related to the combustion of fossil fuels. Besides, exhaustion of different existing biomass without appropriate compensation resulted in huge biomass scarcity, emerging environmental problems such as deforestation and loss of biodiversity ( Goldemberg, 2007 ; Li et al., 2008 ; Saqib et al., 2013 ).

Recently, researchers and entrepreneurs have focused their interest, especially on the algal biomass as the alternative feedstock for the production of biofuels. Moreover, algal biomass has no competition with agricultural food and feed production ( Demirbas, 2007 ). The photosynthetic microorganisms like microalgae require mainly light, carbon dioxide, and some nutrients (nitrogen, phosphorus, and potassium) for its growth, and to produce large amount of lipids and carbohydrates, which can be further processed into different biofuels and other valuable co-products ( Brennan and Owende, 2010 ; Nigam and Singh, 2011 ). Interestingly, the low content of hemicelluloses and about zero content of lignin in algal biomass results in an increased hydrolysis and/or fermentation efficiency ( Saqib et al., 2013 ). Other than biofuels, algae have applications in human nutrition, animal feed, pollution control, biofertilizer, and waste water treatment ( Thomas, 2002 ; Tamer et al., 2006 ; Crutzen et al., 2007 ; Hsueh et al., 2007 ; Choi et al., 2012 ). Therefore, the aim of the current review is to explore the scope of algae for the production of different biofuels and evaluation of its potential as an alternative feedstock.

Algae: Source of Biofuels

Generally, algae are a diverse group of prokaryotic and eukaryotic organisms ranging from unicellular genera such as Chlorella and diatoms to multicellular forms such as the giant kelp, a large brown alga that may grow up to 50 m in length ( Li et al., 2008 ). Algae can either be autotrophic or heterotrophic. The autotrophic algae require only inorganic compounds such as CO 2 , salts, and a light energy source for their growth, while the heterotrophs are non-photosynthetic, which require an external source of organic compounds as well as nutrients as energy sources ( Brennan and Owende, 2010 ). Microalgae are very small in sizes usually measured in micrometers, which normally grow in water bodies or ponds. Microalgae contain more lipids than macroalgae and have the faster growth in nature ( Lee et al., 2014a ). There are about more than 50,000 microalgal species out of which only about 30,000 species have been taken for the research study ( Surendhiran and Vijay, 2012 ; Richmond and Qiang, 2013 ; Rajkumar et al., 2014 ). The short harvesting cycle of algae is the key advantage for its importance, which is better than other conventional crops having harvesting cycle of once or twice in a year ( Chisti, 2007 ; Schenk et al., 2008 ). Therefore, the main focus has been carried out on algal biomass for its application in biofuel area.

There are several advantages of algal biomass for biofuels production: (a) ability to grow throughout the year, therefore, algal oil productivity is higher in comparison to the conventional oil seed crops; (b) higher tolerance to high carbon dioxide content; (c) the consumption rate of water is very less in algae cultivation; (d) no requirement of herbicides or pesticides in algal cultivation; (e) the growth potential of algal species is very high in comparison to others; (f) different sources of wastewater containing nutrients like nitrogen and phosphorus can be utilized for algal cultivation apart from providing any additional nutrient; and (g) the ability to grow under harsh conditions like saline, brackish water, coastal seawater, which does not affect any conventional agriculture ( Spolaore et al., 2006 ; Dismukes et al., 2008 ; Dragone et al., 2010 ). However, there are several disadvantages of algal biomass as feedstock such as the higher cultivation cost as compared to conventional crops. Similarly, harvesting of algae require high energy input, which is approximately about 20–30% of the total cost of production. Several techniques such as centrifugation, flocculation, floatation, sedimentation, and filtration are usually used for harvesting and concentrating the algal biomass ( Demirbas, 2010 ; Ho et al., 2011 ).

The algae can be converted into various types of renewable biofuels including bioethanol, biodiesel, biogas, photobiologically produced biohydrogen, and further processing for bio-oil and syngas production through liquefaction and gasification, respectively ( Kraan, 2013 ). The conversion technologies for utilizing algal biomass to energy sources can be categorized into three different ways, i.e., biochemical, chemical, and thermochemical conversion and make an algal biorefinery, which has been depicted in Figure 1 . The biofuel products derived from algal biomass using these conversion routes have been explored in detail in the subsequent sections.


Figure 1. Algal biomass conversion process for biofuel production .

Biodiesel Production

Biodiesel is a mixture of monoalkyl esters of long chain fatty acids [fatty acid methyl esters (FAME)], which can be obtained from different renewable lipid feedstocks and biomass. It can be directly used in different diesel engines ( Clark and Deswarte, 2008 ; Demirbas, 2009 ). Studies to explore the microalgae as feedstock for the production of liquid fuels had been started for the mid-1980s. In order to solve the energy crisis, the extraction of lipids from diatoms was attempted by some German scientists during the period of World War-II ( Cohen et al., 1995 ). The higher oil yield in algal biomass as compared to oil seed crops makes the possibility to convert into the biodiesel economically using different technologies. A comparative study between algal biomass and terrestrial plants for the production of biodiesel has been depicted in Table 1 . The oil productivity (mass of oil produced per unit volume of the microalgal broth per day) depends on the algal growth rate and the biomass content of the species. The species of microalgae such as Kirchneriella lunaris , Ankistrodesmus fusiformis , Chlamydocapsa bacillus , and Ankistrodesmus falcatus with high levels of polyunsaturated FAME are generally preferred for the production of biodiesel ( Nascimento et al., 2013 ). They commonly multiply their biomass with doubling time of 24 h during exponential growth. Oil content of microalgae is generally found to be very high, which exceed up to 80% by weight of its dry biomass. About 5,000–15,000 gal of biodiesel can be produced from algal biomass per acre per year, which reflects its potentiality ( Spolaore et al., 2006 ; Chisti, 2007 ).


Table 1 . Comparative study between algal biomass and terrestrial plants for biodiesel production .

However, there are some standards such as International Biodiesel Standard for Vehicles (EN14214) and American Society for Testing and Materials (ASTM), which are required to comply with the algal based biodiesel on the physical and chemical properties for its acceptance as substitute to fossil fuels ( Brennan and Owende, 2010 ). The higher degree of polyunsaturated fatty acids of algal oils as compared to vegetable oils make susceptible for oxidation in the storage and further limits its utilization ( Chisti, 2007 ). Some researchers have reported the different advantages of the algal biomass for the biodiesel production due to its high biomass growth and oil productivity in comparison to best oil crops ( Chisti, 2007 ; Hossain et al., 2008 ; Hu et al., 2008 ; Rosenberg et al., 2008 ; Schenk et al., 2008 ; Rodolfi et al., 2009 ; Mutanda et al., 2011 ).

Algal biodiesel production involves biomass harvesting, drying, oil extraction, and further transesterification of oil, which have been described as below.

Harvesting and Drying of Algal Biomass

Unicellular microalgae produce a cell wall containing lipids and fatty acids, which differ them from higher animals and plants. Harvesting of algal biomass and further drying is important prior to mechanical and solvent extraction for the recovery of oil. Macroalgae can be harvested using nets, which require less energy while microalgae can be harvested by some conventional processes, which include filtration ( Rossignol et al., 1999 ) flocculation ( Liu et al., 2013 ; Prochazkova et al., 2013 ), centrifugation ( Heasman et al., 2008 ), foam fractionation ( Csordas and Wang, 2004 ), sedimentation, froth floatation, and ultrasonic separation ( Bosma et al., 2003 ). Selection of harvesting method depends on the type of algal species.

Drying is an important method to extend shelf-life of algal biomass before storage, which avoids post-harvest spoilage ( Munir et al., 2013 ). Most of the efficient drying methods like spray-drying, drum-drying, freeze drying or lyophilization, and sun-drying have been applied on microalgal biomass ( Leach et al., 1998 ; Richmond, 2004 ; Williams and Laurens, 2010 ). Sun-drying is not considered as a very effective method due to presence of high water content in the biomass ( Mata et al., 2010 ). However, Prakash et al. (2007) used simple solar drying device and succeed in drying Spirulina and Scenedesmus having 90% of moisture content. Widjaja et al. (2009) showed the effectiveness of drying temperature during lipid extraction of algal biomass, which affects both concentration of triglycerides and lipid yield. Further, all these processes possess safety and health issues ( Singh and Gu, 2010 ).

Extraction of Oil from Algal Biomass

Unicellular microalgae produce a cell wall containing lipids and fatty acids, which differ them from higher animals and plants. In the literature, there are different methods of oil extraction from algae, such as mechanical and solvent extraction ( Li et al., 2014 ). However, the extraction of lipids from microalgae is costly and energy intensive process.

Mechanical oil extraction

The oil from nuts and seeds is extracted mechanically using presses or expellers, which can also be used for microalgae. The algal biomass should be dried prior to this process. The cells are just broken down with a press to leach out the oil. About 75% of oil can be recovered through this method and no special skill is required ( Munir et al., 2013 ). Topare et al. (2011) extracted oil through screw expeller by mechanical pressing (by piston) and osmotic shock method and recovered about 75% of oil from the algae. However, more extraction time is required as compared to other methods, which make the process unfavorable and less effective ( Popoola and Yangomodou, 2006 ).

Solvent based oil extraction

Oil extraction using solvent usually recovers almost all the oil leaving only 0.5–0.7% residual oil in the biomass. Therefore, the solvent extraction method has been found to be suitable method rather than the mechanical extraction of oil and fats ( Topare et al., 2011 ). Solvent extraction is another method of lipid extraction from microalgae, which involves two stage solvent extraction systems. The amount of lipid extracted from microalgal biomass and further yield of highest biodiesel depends mainly on the solvent used. Several organic solvents such as chloroform, hexane, cyclo-hexane, acetone, and benzene are used either solely or in mixed form ( Afify et al., 2010 ). The solvent reacts on algal cells releasing oil, which is recovered from the aqueous medium. This occurs due to the nature of higher solubility of oil in organic solvents rather than water. Further, the oil can be separated from the solvent extract. The solvent can be recycled for next extraction. Out of different organic solvents, hexane is found to be most effective due to its low toxicity and cost ( Rajvanshi and Sharma, 2012 ; Ryckebosch et al., 2012 ).

In case of using mixed solvents for oil extraction, a known quantity of the solvent mixture is used, for example, chloroform/methanol in the ratio 2:1 (v/v) for 20 min using a shaker and followed by the addition of mixture, i.e., chloroform/water in the ratio of 1:1 (v/v) for 10 min ( Shalaby, 2011 ). Similarly, Pratoomyot et al. (2005) extracted oil from different algal species using the solvent system chloroform/methanol in the ratio of 2:1 (v/v) and found different fatty acid content. Ryckebosch et al. (2012) optimized an analytical procedure and found chloroform/methanol in the ratio 1:1 as the best solvent mixture for the extraction of total lipids. Similarly, Lee et al. (1998) extracted lipid from the green alga Botryococcus braunii using different solvent system and obtained the maximum lipid content with chloroform/methanol in the ratio of 2:1. Hossain et al., 2008 used hexane/ether in the ratio 1:1 (v/v) for oil extraction and allowed to settle for 24 h. Using a two-step process, Fajardo et al. (2007) reported about 80% of lipid recovery using ethanol and hexane in the two steps for the extraction and purification of lipids. Therefore, a selection of a most suitable solvent system is required for the maximum extraction of oil for an economically viable process.

Lee et al. (2009) compared the performance of various disruption methods, including autoclaving, bead-beating, microwaves, sonication, and using 10% NaCl solution in the extraction of Botryococcus sp., Chlorella vulgaris , and Scenedesmus sp, using a mixture of chloroform and methanol (1:1).


This is a process to convert algal oil to biodiesel, which involves multiple steps of reactions between triglycerides or fatty acids and alcohol. Different alcohols such as ethanol, butanol, methanol, propanol, and amyl alcohol can be used for this reaction. However, ethanol and methanol are used frequently for the commercial development due to its low cost and its physical and chemical advantages ( Bisen et al., 2010 ; Surendhiran and Vijay, 2012 ). The reaction can be performed in the presence of an inorganic catalyst (acids and alkalies) or lipase enzyme. In this method, about 3 mol of alcohol are required for each mole of triglyceride to produce 3 mol of methyl esters (biodiesel) and 1 mol of glycerol (by-product) ( Meher et al., 2006 ; Chisti, 2007 ; Sharma and Singh, 2009 ; Surendhiran and Vijay, 2012 ; Stergiou et al., 2013 ) (Figure 2 ). Glycerol is denser than biodiesel and can be periodically or continuously removed from the reactor in order to drive the equilibrium reaction. The presence of methanol, the co-solvent that keeps glycerol and soap suspended in the oil, is known to cause engine failure ( Munir et al., 2013 ). Thus, the biodiesel is recovered by repeated washing with water to remove glycerol and methanol ( Chisti, 2007 ).


Figure 2. Transesterification of oil to biodiesel . R 1–3 are hydrocarbon groups.

The reaction rate is very slow by using the acid catalysts for the conversion of triglycerides to methyl esters, whereas the alkali-catalyzed transesterification reaction has been reported to be 4000 times faster than the acid-catalyzed reaction ( Mazubert et al., 2013 ). Sodium and potassium hydroxides are the two commercial alkali catalysts used at a concentration of about 1% of oil. However, sodium methoxide has become the better catalyst rather than sodium hydroxide ( Singh et al., 2006 ).

Kim et al. (2014) used Scenedesmus sp. for the biodiesel production through acid and alkali transesterification process. They reported 55.07 ± 2.18%, based on lipid by wt of biodiesel conversion using NaOH as an alkaline catalyst than using H 2 SO 4 as 48.41 ± 0.21% of biodiesel production. In comparison to acid and alkalies, lipases as biocatalyst have different advantages as the catalysts due to its versatility, substrate selectivity, regioselectivity, enantioselectivity, and high catalytic activity at ambient temperature and pressure ( Knezevic et al., 2004 ). It is not possible by some lipases to hydrolyze ester bonds at secondary positions, while some other group of enzymes hydrolyzes both primary and secondary esters. Another group of lipases exhibits fatty acids selectivity, and allow to cleave ester bonds at particular type of fatty acids. Luo et al. (2006) cloned the lipase gene lipB68 and expressed in Escherichia coli BL21 and further used it as a catalyst for biodiesel production. LipB68 could catalyze the transesterification reaction and produce biodiesel with a yield of 92% after 12 h, at a temperature of 20°C. The activity of the lipase enzyme with such a low temperature could provide substantial savings in energy consumption. However, it is rarely used due to its high cost ( Sharma et al., 2001 ).

Extractive transesterification

It involves several steps to produce biodiesel such as drying, cell disruption, oils extraction, transesterification, and biodiesel refining ( Hidalgo et al., 2013 ). The main problems are related with the high water content of the biomass (over 80%), which overall increases the cost of whole process.

In situ transesterification

This method skips the oil extraction step. The alcohol acts as an extraction solvent and an esterification reagent as well, which enhances the porosity of the cell membrane. Yields found are higher than via the conventional route, and waste is also reduced. Industrial biodiesel production involves release of extraction solvent, which contributes to the production of atmospheric smog and to global warming. Thus, simplification of the esterification processes can reduce the disadvantages of this attractive bio-based fuel. The single-step methods can be attractive solutions to reduce chemical and energy consumption in the overall biodiesel production process ( Patil et al., 2012 ). A comparison of direct and extractive transesterification is given in Table 2 .


Table 2 . Comparison of extractive transesterification and in situ methods ( Haas and Wagner, 2011 ) .

Bioethanol Production

Several researchers have been reported bioethanol production from certain species of algae, which produce high levels of carbohydrates as reserve polymers. Owing to the presence of low lignin and hemicelluloses content in algae in comparison to lignocellulosic biomass, the algal biomass have been considered more suitable for the bioethanol production ( Chen et al., 2013 ). Recently, attempts have been made (for the bioethanol production) through the fermentation process using algae as the feedstocks to make it as an alternative to conventional crops such as corn and soyabean ( Singh et al., 2011 ; Nguyen and Vu, 2012 ; Chaudhary et al., 2014 ). A comparative study of algal biomass and terrestrial plants for the production of bioethanol has been given in Table 3 . There are different micro and macroalgae such as Chlorococcum sp., Prymnesium parvum , Gelidium amansii , Gracilaria sp., Laminaria sp., Sargassum sp., and Spirogyra sp., which have been used for the bioethanol production ( Eshaq et al., 2011 ; Rajkumar et al., 2014 ). These algae usually require light, nutrients, and carbon dioxide, to produce high levels of polysaccharides such as starch and cellulose. These polysaccharides can be extracted to fermentable sugars through hydrolysis and further fermentation to bioethanol and separated through distillation as shown in Figure 3 .


Table 3 . Comparative study between algal biomass and terrestrial plants for bioethanol production .


Figure 3. Process for bioethanol production from microalgae .

Pre-Treatment and Saccharification

It has been reported that, the cell wall of some species of green algae like Spirogyra and Chlorococcum contain high level of polysaccharides. Microalgae such as C. vulgaris contains about 37% of starch on dry weight basis, which is the best source for bioethanol with 65% conversion efficiency ( Eshaq et al., 2010 ; Lam and Lee, 2012 ). Such polysaccharide based biomass requires additional processing like pre-treatment and saccharification before fermentation ( Harun et al., 2010 ). Saccharification and fermentation can also be carried out simultaneously using an amylase enzyme producing strain for the production of ethanol in a single step. Bioethanol from microalgae can be produced through the process, which is similar to the first generation technologies involving corn based feedstocks. However, there is limited literature available on the fermentation process of microalgae biomass for the production of bioethanol ( Schenk et al., 2008 ; John et al., 2011 ).

The pre-treatment is an important process, which facilitates accessibility of biomass to enzymes to release the monosaccharides. Acid pre-treatment is widely used for the conversion of polymers present in the cell wall to simple forms. The energy consumption in the pre-treatment is very low and also it is an efficient process ( Harun and Danquah, 2011a , b ). Yazdani et al. (2011) found 7% (w/w) H 2 SO 4 as the promising concentration for the pre-treatment of the brown macroalgae Nizimuddinia zanardini to obtain high yield of sugars without formation of any inhibitors. Candra and Sarinah (2011) studied the bioethanol production using red seaweed Eucheuma cottonii through acid hydrolysis. In this study, 5% H 2 SO 4 concentration was used for 2 h at 100°C, which yielded 15.8 g/L of sugars. However, there are other alternatives to chemical hydrolysis such as enzymatic digestion and gamma radiation to make it more sustainable ( Chen et al., 2012 ; Yoon et al., 2012 ; Schneider et al., 2013 ).

Similar to starch, there are certain polymers such as alginate, mannitol, and fucoidan present in the cell wall of various algae, which requires additional processing like pre-treatment and saccharification before fermentation. Another form of storage carbohydrate found in various brown seaweeds and microalgae is laminarin, which can be hydrolyzed by β-1,3-glucanases or laminarinases ( Kumagai and Ojima, 2010 ). Laminarinases can be categorized into two groups such as exo- and endo-glucanases based on the mode of hydrolysis, which usually produces glucose and smaller oligosaccharides as the end product. Both the enzymes are necessary for the complete digestion of laminarin polymer ( Lee et al., 2014b ).

Markou et al. (2013) saccharified the biomass of Spirulina ( Arthrospira platensis ), fermented the hydrolyzate and obtained the maximum ethanol yield of 16.32 and 16.27% (g ethanol /g biomass ) produced after pre-treatment with 0.5 N HNO 3 and H 2 SO 4 , respectively. Yanagisawa et al. (2011) investigated the content of polysaccharide materials present in three types of seaweeds such as sea lettuce ( Ulva pertusa ), chigaiso ( Alaria crassifolia ), and agar weed ( Gelidium elegans ). These seaweeds contain no lignin, which is a positive signal for the hydrolysis of polysaccharides without any pre-treatment. Singh and Trivedi (2013) used Spirogyra biomass for the production of bioethanol using Saccharomyces cerevisiae and Zymomonas mobilis . In a method, they followed acid pre-treatment of algal biomass and further saccharified using α-amylase producing Aspergillus niger . In another method, they directly saccharified the biomass without any pre-treatment. The direct saccharification process resulted in 2% (w/w) more alcohol in comparison to pretreated and saccharified algal biomass. This study revealed that the pre-treatment with different chemicals are not required in case of Spyrogyra , which reflects its economic importance for the production of ethanol. Also, cellulase enzyme has been used for the saccharification of algal biomass containing cellulose. However, this enzyme system is more expensive than amylases and glucoamylases, and doses required for effective cellulose saccharification are usually very high. Trivedi et al. (2013) applied different cellulases on green alga Ulva for saccharification and found highest conversion efficiency of biomass into reducing sugars by using cellulase 22119 rather than viscozyme L, cellulase 22086 and 22128. In this experiment, they found a maximum yield of sugar 206.82 ± 14.96 mg/g with 2% (v/v) enzyme loading for 36 h at a temperature of 45°C.


There are different groups of microorganisms like yeast, bacteria, and fungi, which can be used for the fermentation of the pretreated and saccharified algal biomass under anaerobic process for the production of bioethanol ( Nguyen and Vu, 2012 ). Nowadays, S. cerevisiae and Z. mobilis have been considered as the bioethanol fermenting microorganisms. However, fermentation of mannitol, a polymer present in certain algae is not possible in anaerobic condition using these well known microorganisms and requires supply of oxygen during fermentation, which is possible only by Zymobacter palmae ( Horn et al., 2000 ).

Certain marine red algae contain agar, a polymer of galactose and galactopyranose, which can be used for the production of bioethanol ( Yoon et al., 2010 ). The biomass of red algae can be depolymerized into different monomeric sugars like glucose and galactose. In addition to mannitol and glucose, brown seaweeds contain about 14% of extra carbohydrates in the form of alginate ( Wargacki et al., 2012 ). Horn et al. (2000) reported the presence of alginate, laminaran, mannitol, fucoidan, and cellulose in some brown seaweeds, which are good source of sugars. They fermented brown seaweed extract having mannitol using bacteria Z. palmae and obtained an ethanol yield of about 0.38 g ethanol/g mannitol.

In the literature, there are many advantages supporting microalgae as the promising substrate for bioethanol production. Hon-Nami (2006) used Chlamydomonas perigranulata algal culture and obtained different by-products such as ethanol and butanediol. Similarly, Yanagisawa et al. (2011) obtained glucose and galactose through the saccharification of agar weed (red seaweed) containing glucan and galactan and obtained 5.5% of ethanol concentration through fermentation using S. cerevisiae IAM 4178. Harun et al. (2010) obtained 60% more ethanol in case of lipid extracted microalgal biomass rather than intact algal biomass of Chlorococcum sp. This shows the importance of algal biomass for the production of both biodiesel and bioethanol.

Biogas Production

Recently, biogas production from algae through anaerobic digestion has received a remarkable attention due to the presence of high polysaccharides (agar, alginate, carrageenan, laminaran, and mannitol) with zero lignin and low cellulose content. Mostly, seaweeds are considered as the excellent feedstock for the production of biogas. Several workers have demonstrated the fermentation of various species of algae like Scenedesmus , Spirulina , Euglena , and Ulva for biogas production ( Samson and Leduy, 1986 ; Yen and Brune, 2007 ; Ras et al., 2011 ; Zhong et al., 2012 ; Saqib et al., 2013 ). The production of biogas using algal biomass in comparison to some terrestrial plants is shown in Table 4 .


Table 4 . Comparative study between algal biomass and terrestrial plants for biogas production .

Biogas is produced through the anaerobic transformation of organic matter present in the biodegradable feedstock such as marine algae, which releases certain gases like methane, carbon dioxide, and traces of hydrogen sulfide. The anaerobic conversion process involves basically four main steps. In the first step, the insoluble organic material and higher molecular mass compounds such as lipids, carbohydrates, and proteins are hydrolyzed into soluble organic material with the help of enzyme released by some obligate anaerobes such as Clostridia and Streptococci . The second step is called as acidogenesis, which releases volatile fatty acids (VFAs) and alcohols through the conversion of soluble organics with the involvement of enzymes secreted by the acidogenic bacteria. Further, these VFAs and alcohols are converted into acetic acid and hydrogen using acetogenic bacteria through the process of acetogenesis, which finally metabolize to methane and carbon dioxide by the methanogens ( Cantrell et al., 2008 ; Vergara-Fernandez et al., 2008 ; Brennan and Owende, 2010 ; Romagnoli et al., 2011 ).

Sangeetha et al. (2011) reported the anaerobic digestion of green alga Chaetomorpha litorea with generation of 80.5 L of biogas/kg of dry biomass under 299 psi pressure. Vergara-Fernandez et al. (2008) evaluated digestion of the marine algae Macrocystis pyrifera and Durvillaea antarctica marine algae in a two-phase anaerobic digestion system and reported similar biogas productions of 180.4 (±1.5) mL/g dry algae/day with a methane concentration around 65%. However, in case of algae blend, same methane content was observed with low biogas yield. Mussgnug et al. (2010) reported biogas production from some selected green algal species like Chlamydomonas reinhardtii and Scenedesmus obliquus and obtained 587 and 287 mL biogas/g of volatile solids, respectively. Further, there are few studies, which have been conducted with microalgae showing the effect of different pre-treatment like thermal, ultrasound, and microwave for the high production of biogas ( Gonzalez-Fernandez et al., 2012a , b ; Passos et al., 2013 ).

However, there are different factors, which limit the biogas production such as requirement of larger land area, infrastructure, and heat for the digesters ( Collet et al., 2011 ; Jones and Mayfield, 2012 ). The proteins present in algal cells increases the ammonium production resulting in low carbon to nitrogen ratio, which affects biogas production through the inhibition of growth of anaerobic microorganisms. Also, anaerobic microorganisms are inhibited by the sodium ions. Therefore, it is recommended to use the salt tolerating microorganisms for the anaerobic digestion of algal biomass ( Yen and Brune, 2007 ; Brennan and Owende, 2010 ; Jones and Mayfield, 2012 ).

Biohydrogen Production

Recently, algal biohydrogen production has been considered to be a common commodity to be used as the gaseous fuels or electricity generation. Biohydrogen can be produced through different processes like biophotolysis and photo fermentation ( Shaishav et al., 2013 ). Biohydrogen production using algal biomass is comparative to that of terrestrial plants (Table 5 ). Park et al. (2011) found Gelidium amansii (red alga) as the potential source of biomass for the production of biohydrogen through anaerobic fermentation. Nevertheless, they found 53.5 mL of H 2 from 1 g of dry algae with a hydrogen production rate of 0.518 L H 2 /g VSS/day. The authors found an inhibitor, namely, 5-hydroxymethylfurfural (HMF) produced through the acid hydrolysis of G. amansii that decreases about 50% of hydrogen production due to the inhibition. Thus, optimization of the pre-treatment method is an important step to maximize biohydrogen production, which will be useful for the future direction ( Park et al., 2011 ; Shi et al., 2011 ). Saleem et al. (2012) reduced the lag time for hydrogen production using microalgae Chlamydomonas reinhardtii by the use of optical fiber as an internal light source. In this study, the maximum rate of hydrogen production in the presence of exogenic glucose and optical fiber was reported to be 6 mL/L culture/h, which is higher than other reported values.


Table 5 . Comparative study between algal biomass and terrestrial plants for biohydrogen production .

Some of microalgae like blue green algae have glycogen instead of starch in their cells. This is an exception, which involves oxidation of ferrodoxin by the hydrogenase enzyme activity for the production of hydrogen in anaerobic condition. However, another function of this enzyme is to be involved in the detachment of electrons. Therefore, different researchers have focused for the identification of these enzyme activities having interactions with ferrodoxin and the other metabolic functions for microalgal photobiohydrogen production. They are also involved with the change of these interactions genetically to enhance the biohydrogen production ( Gavrilescu and Chisti, 2005 ; Hankamer et al., 2007 ; Wecker et al., 2011 ; Yacoby et al., 2011 ; Rajkumar et al., 2014 ).

Bio-Oil and Syngas Production

Bio-oil is formed in the liquid phase from algal biomass in anaerobic condition at high temperature. The composition of bio-oil varies according to different feedstocks and processing conditions, which is called as pyrolysis ( Iliopoulou et al., 2007 ; Yanqun et al., 2008 ). There are several parameters such as water, ash content, biomass composition, pyrolysis temperature, and vapor residence time, which affect the bio-oil productivity ( Fahmi et al., 2008 ). However, due to the presence of water, oxygen content, unsaturated and phenolic moieties, crude bio-oil cannot be used as fuel. Therefore, certain treatments are required to improve its quality ( Bae et al., 2011 ). Bio-oils can be processed for power generation with the help of external combustion through steam and organic rankine cycles, and stirling engines. However, power can also be generated through internal combustion using diesel and gas-turbine engines ( Chiaramonti et al., 2007 ). In literature, there are limited studies on algae pyrolysis compared to lignocellulosic biomass. Although, high yields of bio-oil occur through fluidized-bed fast pyrolysis processes, there are several other pyrolysis modes, which have been introduced to overcome their inherent disadvantages of a high level of carrier gas flow and excessive energy inputs ( Oyedun et al., 2012 ). Demirbas (2006) investigated suitability of the microalgal biomass for bio-oil production and found the superior quality than the wood. Porphy and Farid (2012) produced bio-oil from pyrolysis of algae ( Nannochloropsis sp.) at 300°C after lipid extraction, which composed of 50 wt% acetone, 30 wt% methyl ethyl ketone, and 19 wt% aromatics such as pyrazine and pyrrole. Similarly, Choi et al. (2014) carried out pyrolysis study on a species of brown algae Saccharina japonica at a temperature of 450°C and obtained about 47% of bio-oil yield.

Gasification is usually performed at high temperatures (800–1000°C), which converts biomass into the combustible gas mixture through partial oxidation process, called syngas or producer gas. Syngas is a mixture of different gases like CO, CO 2 , CH 4 , H 2 , and N 2 , which can also be produced through normal gasification of woody biomass. In this process, biomass reacts with oxygen and water (steam) to generate syngas. It is a low calorific gas, which can be utilized in the gas turbines or used directly as fuel. Different variety of biomass feedstocks can be utilized for the production of energy through the gasification process, which is an added advantage ( Carvalho et al., 2006 ; Prins et al., 2006 ; Lv et al., 2007 ).

Conclusion and Future Perspectives

Recently, it is a challenge for finding different alternative resources, which can replace fossil fuels. Due to presence of several advantages in algal biofuels like low land requirement for biomass production and high oil content with high productivity, it has been considered as the best resource, which can replace the liquid petroleum fuel. However, one of its bottlenecks is the low biomass production, which is a barrier for industrial production. Also, another disadvantage includes harvesting of biomass, which possesses high energy inputs. For an economic process development in comparison to others, a cost-effective and energy efficient harvesting methods are required with low energy input. Producing low-cost microalgal biofuels requires better biomass harvesting methods, high biomass production with high oil productivity through genetic modification, which will be the future of algal biology. Therefore, use of the standard algal harvesting technique, biorefinery concept, advances in photobioreactor design and other downstream technologies will further reduce the cost of algal biofuel production, which will be a competitive resource in the near future.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


The authors are thankful to Prof. Y. K. Yadav, Director, NIRE, Kapurthala for his consistent support to write this review paper. The authors greatly acknowledge the Ministry of New and Renewable Energy, New Delhi, Govt. of India, for providing funds to carry out research work.

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Keywords: algae, microalgae, biofuels, bioethanol, biogas, biodiesel, biohydrogen

Citation: Behera S, Singh R, Arora R, Sharma NK, Shukla M and Kumar S (2015) Scope of algae as third generation biofuels. Front. Bioeng. Biotechnol. 2 :90. doi: 10.3389/fbioe.2014.00090

Received: 31 July 2014; Accepted: 29 December 2014; Published online: 11 February 2015.

Reviewed by:

Copyright: © 2015 Behera, Singh, Arora, Sharma, Shukla and Kumar. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Sachin Kumar, Biochemical Conversion Division, Sardar Swaran Singh National Institute of Renewable Energy, Jalandhar-Kapurthala Road, Wadala Kalan, Kapurthala 144601, Punjab, India e-mail: sachin.biotech@gmail.com

This article is part of the Research Topic

Marine biomolecules

Algae: Biomass to Biofuel


  • 1 Sustainable Materials and Catalysis Research Laboratory (SMCRL), Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, India.
  • 2 Sustainable Materials and Catalysis Research Laboratory (SMCRL), Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, India. [email protected].
  • PMID: 34009581
  • DOI: 10.1007/978-1-0716-1323-8_3

Worldwide demand for ethanol alternative fuel has been emerging day by day owing to the rapid population growth and industrialization. Culturing microalgae as an alternative feedstock is anticipated to be a potentially significant approach for sustainable bioethanol biofuel production. Microalgae are abundant in nature, which grow at faster rates with a capability of storing high lipid and starch/cellulose contents inside their cells. This process offers several environmental advantages, including the effective utilization of land, good CO2 sequestration without entering into "food against fuel" dispute. This chapter focuses on the methods and processes used for the production of bioethanol biofuels from algae. Thus, it also covers significant achievements in the research and developments on algae bioethanol production, mainly including pretreatment, hydrolysis, and fermentation of algae biomass. The processes of producing biodiesel, biogas, and hydrogen have also been discussed.

Keywords: Bioethanol; Biofuel; Biogas; Biohydrogen; Biomass; Microalgae.

Publication types

  • Biotechnology / methods*
  • Ethanol / metabolism*
  • Fermentation
  • Hydrogen / metabolism
  • Microalgae* / growth & development
  • Microalgae* / metabolism
  • Phaeophyceae / classification
  • Phaeophyceae / metabolism
  • Phytoplankton
  • Rhodophyta / classification
  • Rhodophyta / metabolism
  • Ultrasonics
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UB, partners work to make algal fuel more efficient, affordable

research news

Close up of hands working with test tubes in Ian Bradley's lab.

Inside the lab of Ian Bradley, where researchers are finding innovative ways to improve algae cultivation for biofuels. Photo: Douglas Levere


Published March 12, 2024

Ian Bradley.

Harvesting biofuel from algae is effective, but not yet practical.

A UB-led research project — funded by a $2 million U.S. Department of Energy grant — is tackling this problem using polyculture farming, artificial intelligence, microscopy and other techniques.

Algae are microorganisms that live in aquatic environments. They behave like plants and use photosynthesis to produce energy from sunlight.

Algal cultivation is an effective way to produce biomass, a renewable energy source that can be directly converted into biofuel — a fuel source that would reduce greenhouse gas emissions. The current process takes months and must restart whenever algae is attacked by micropests, like fungus.

“The algal cultures are always growing. When the system gets contaminated, the algae get completely wiped out,” says the grant’s principal investigator Ian Bradley, assistant professor in the Department of Civil, Structural and Environmental Engineering. “You miss growing biomass for a few weeks or a few months and lose between 30-50% of the product.”

Bradley, who is also a core faculty member in UB's RENEW Institute, will lead the team in addressing this problem.

Tracking changes in algal DNA

Many biomass harvesters wait until an infiltration from pests occurs and add chemicals or use other methods to remedy the infection. Bradley’s team will examine environmental conditions like temperature, sunlight and wastewater treatment, and track changes in the algae’s metagenomes and transcriptomes — DNA and RNA — before and after the organisms are infected.

Collaborators at the Georgia Institute of Technology will monitor these organisms using low-cost microscopy and provide updates about algae and pests in real time. The team at Georgia Tech will also use artificial intelligence and deep learning to analyze data and try to develop predictive correlations between algal responses and environmental conditions.

“We want to make algae a viable producer for biofuel applications. Right now, it’s expensive and not consistent,” Bradley says. “Our goal is to predict the infections before they occur.”

Viable production

In addition to monitoring and understanding factors that lead to infection in algal cultures, Bradley and team will use polyculture farming — growing more than one crop species in the same space — to protect biomass production. Polyculture farming mimics natural ecosystems and can increase crop diversity, enhance productivity and help protect against common pests.

“Pure cultures, or monocultures — made up of the same types of algae — are the most common for biomass production. When a fungus comes into a monoculture, it completely wipes it out and you get culture crash,” Bradley explains. “We grow polycultures to make the process more sustainable. If fungus attacks and wipes out one type of algae, there are others to continue the process.”

Using polycultures also increases productivity because it prevents a complete restart of the biomass production process. Bradley believes using polycultures instead of monocultures could double productivity.

“We’re in phase one of this project. Phase two is to progress the state of this technology and then scale it up for implementation,” Bradley says.

Additional collaborators include researchers at the University of Illinois Urbana-Champaign and Montana-based Clearas Water Recovery Inc.



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  6. Appendix B: Statement of Task

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