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PerspectiveReceived: 28 February 2011 Revised: 21 April 2011 Accepted: 2 May 2011 Published online in Wiley Online Library: 14 June 2011

(wileyonlinelibrary.com) DOI 10.1002/jctb.2666

A viable technology to generatethird-generation biofuelAnoop Singh,a∗ Stig Irving Olsena and Poonam Singh Nigamb

Abstract

First generation biofuels are widely available because the production technologies are well developed. However, growth ofthe raw materials conflicts with food security, so that first- generation biofuels are not so promising. The second generation ofbiofuels will not compete directly with food but requires several energy intensive processes to produce them, and also increasesland-use change, which reduces its environmental and economic feasibility. The production of third-generation biofuels avoidsthe issues met with first- and second- generation biofuels, namely food–fuel competition, land-use change, etc., and is thusconsidered a viable alternative energy resource. On all dimensions of sustainability (environmental, social and economical),a life cycle assessment approach is most relevant to avoid issues in problem shifting. The utilization of organic waste andcarbon dioxide in flue gases for the production of biomass further increases the sustainability of third generation biofuels, as itminimizes greenhouse gas emissions and disposal problems.c© 2011 Society of Chemical Industry

Keywords: third-generation biofuel; algae; technology; sustainability; life cycle assessment

INTRODUCTIONThe already evident difficulty of reducing CO2 emissions is com-pounded by the growth in global population (from 6.6 billionin 2008 to 9.2 billion by 20501), which will result in increasedfuel consumption and increased energy demands.2 The in-creasing energy demand and oil price, depletion of fossil re-serves and concern about greenhouse gases (GHG) emissionhave led to a move towards alternative, renewable, sustain-able, efficient and cost-effective energy sources with loweremissions.3 – 9 Among many energy alternatives, biofuels, hydro-gen, natural gas and syngas are likely to emerge as the fourstrategically important sustainable fuel sources in the foresee-able future. Within these four, biofuels have emerged as oneof the most strategically important sustainable fuel sourcesdue to their renewability, biodegradability and emitting loweremissions of exhaust gases. Biofuels refer to liquid, gas andsolid fuels predominantly produced from biomass, for exampleethanol, methanol, biodiesel, Fischer-Tropsch diesel, hydrogen,methane, etc.3 Renewability and carbon neutrality are essentialrequirements for environmental and economic sustainability ofbiofuels.

The main attention of biofuel production has been focusedon lowering production costs, GHG emissions, and land andwater resource needs, and on improving compatibility with fueldistribution systems and vehicle engines.10 In recent years, mucheffort has been applied to the search for potential feedstocks(biomass) from different sources that can be used for theproduction of biofuels. Biomass has been identified as sustainablefeedstock on the basis of input required and emissions in theirproduction. In recent years the possibilities of using algae as asource of bio-oil and biogas for energy applications has beeninvestigated.11

CLASSIFICATION OF BIOFUELSBiofuels are broadly classified as primary and secondary biofuels.Primary biofuels are used in an unprocessed form, primarily forheating, cooking or electricity production, and include fuel wood,wood chips and pellets, etc. Secondary biofuels are produced byprocessing biomass, and include ethanol, biodiesel, etc., that canbe used in vehicles and industrial processes. Secondary biofuels arefurther divided into first-, second- and third-generation biofuelson the basis of raw material and the technology used for theirproduction (Fig. 1).

BENEFITS OVER FIRST- AND SECOND-GENE-RATION BIOFUELSThe first-generation biofuels which have attained commercial-level production in several countries, have generally beenproduced from food and oil crops, as well as animal fatsusing conventional technology.3,12 The most well-known first-generation biofuels are ethanol made by fermenting sugarextracted from crop plants and starch contained in maizekernels or other starchy crops, and biodiesel produced fromvegetable oils (SVO) of oleaginous plants by transesterificationprocesses or cracking. However, the viability and sustainabilityof first-generation biofuels production is questionable due to

∗ Correspondence to: Anoop Singh, Quantitative Sustainability Assessment,Department of Management Engineering, Technical University of Denmark,Lyngby, Denmark. E-mail: apsinghenv@gmail.com

a Quantitative Sustainability Assessment, Department of Management Engi-neering, Technical University of Denmark, Lyngby, Denmark

b Faculty of Life and Health Sciences, University of Ulster, Coleraine BT52 1SA,Northern Ireland, United Kingdom

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www.soci.org A Singh, SI Olsen, PS Nigam

Firewood

Wood chips

Pellets

Animal waste

Forest and cropresidues

Landfill gas

Biofuels

Primary Secondary

1stgenerationSubstrate: Seeds, grains orsugarsBioethanol or butanol byfermentation of starch (wheat,barley, corn, potato) or sugars(sugar cane, sugar beet, etc.)

Biodiesel by transesterification ofplant oils (rapeseed, soybean,sunflower, palm, coconut,jatropha, used cooking oil,animal fats, etc.)

2nd generationSubstrate: lignocellulosic biomass

Bioethanol or butanol by enzymatichydrolysis

Methanol, Fischer-Tropsch gasolineand disesel, mixed alcohol,dimethyl ether and green diesel bythermochemical processes

Biomethane by anaerobic digestion

3rd generationSubstrate: Algae, sea weeds

Biodiesel from algae

Bioethanol from algae and seaweeds

Hydrogen from green algae andmicrobes

Figure 1. Classification of biofuels (adapted from Nigam and Singh1).

the conflict with food and fibre production for the use ofarable land, high water and fertiliser requirements, lack ofwell managed agricultural practices in emerging economies,biodiversity conservation and regionally constrained marketstructures.12 Consequently the search for non-edible biomass (e.g.agricultural residues, industrial and municipal organic wastes)for the production of biofuels is favoured. Second-generationbiofuels are generally produced by two fundamentally differentapproaches, i.e. biological or thermochemical processing, fromagricultural lignocellulosic biomass (non-edible crop residues orwhole plant biomass) and industrial or municipal organic waste.The main advantage of second-generation biofuels productionfrom non-edible feed-stocks or from wastes is that it limits thedirect food versus fuel competition; however, it increases land-use change and land-use efficiency, and requires sophisticatedprocessing production equipment, more investment per unit ofproduction and larger-scale facilities to confine and curtail capitalcost,13 which limits its environmental and economic feasibility.

Third-generation biofuels production is focused on the useof microscopic organisms. Therefore, on the basis of currentscientific knowledge and technology projections, third-generationbiofuels specifically derived from microbes and microalgae areconsidered to be a viable alternative energy resource devoid ofthe major drawbacks (food–fuel competition, land-use change,etc.) associated with first- and second-generation biofuels3.

THIRD GENERATION BIOFUELSMicrobial species such as yeast, fungi and algae can be usedas a potential source for the production of biofuels. Huanget al.14 produced microbial oil from sulphuric acid-treated ricestraw hydrolysate by cultivation of a microorganism, Trichosporonfermentans, and on the basis of results they conclude that thisorganism is capable of growing and utilizing rice straw hydrolysateto accumulate lipid within its cell biomass with a high yield(minimum of 10.4 g L−1). Zhu et al.15 also reported the productionof microbial biofuel from waste molasses. The ability of yeast togrow well on pretreated lignocellulosic biomass could efficientlyenhance lipid accumulation, thus providing a promising optionfor the production of economically and environmentally soundmicrobial oil from agricultural residues.3

Algae are distinguished as one of the oldest life forms,and are present in all existing ecosystems and represent a

prominent diversity in a wide range of environmental conditions.Algae utilize enormous amounts of CO2 for their growth andremove CO2 from power plant emissions, convert biomass viaphotosynthesis and liberate more oxygen to the atmosphere.Algae also remove N and P from wastewater thus reducing thepotential for pollution. Algal biomass can be converted into variousbiofuels using liquefaction, pyrolysis, gasification, extractionand transesterification, fermentation, and aneorobic digestionprocesses.10,16 – 20 Microalgae can produce lipids, proteins andcarbohydrates in large amounts over short periods of time andthese products can be used for the production of biofuels.Microalgae have the ability to take up CO2 from the atmosphere,discharge gases and soluble carbonates21 and can tolerate andutilize substantially higher levels of CO2 (up to 150 000 ppmv (partsper million by volume))22 compared with higher plants. Algal cellsare veritable miniature biochemical factories, and appear morephoto-synthetically efficient than terrestrial plants due to theirvery efficient CO2 fixation. The ability of algae to fix CO2 has beenproposed as a method of removing CO2 from flue gases frompower plants, and thus can be used to reduce emission of GHG.

TECHNOLOGYThree distinct algae production mechanisms, photoautotrophic,heterotrophic and mixotrophic can be used, all of which follownatural growth processes. Many microalgae strains have high lipidcontent (up to 80% of dry weight).12 Nitrogen starvation is the mosteffective method for improving microalgae lipid accumulation,which also gradually changes the lipid composition from free fattyacids to triacylglycerol (TAG),23 a more useful product for biodieselproduction.

The whole algal biomass or algal oil extracts can be convertedinto different fuel forms, such as biogas, liquid and gaseoustransportation fuel, kerosene, ethanol, aviation fuel, and biohy-drogen through the implementation of appropriate processingtechnologies.24 The biofuels conversion technologies for algalbiomass can be separated into four basic categories: biochemicalconversion; thermochemical conversion; chemical reaction; anddirect combustion (Fig. 2). The biological process of energy con-version of biomass into other fuels includes anaerobic digestion,alcoholic fermentation and photobiological hydrogen production.Thermochemical conversion covers the thermal decomposition oforganic components to fuel products, such as gasification, liquefac-

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AlgalBiomass

BiochemicalConversion

ThermochemicalConversion

ChemicalReaction

DirectCombustion

Transesterification

PowerGeneration

Biodiesel

Heat andElectricity

Chisti35; Huang et al.36

Bruhn et al.37

Pyrolysis

Liquefaction

Bio-oil, Charcoal

Bio-oil

Miao and Wu32;Chiaramonti et al.33

Minowaand Sawayama34

Anaerobic digestion

Gasification

CH4, H2

Fuel gas

Sialve et al.27; Yang et al.28

Hirano et al.31

FermentationEthanol,

Acetone, Butanol Harun et al.25; Choi et al.26

Photobiological H2production

H2Burgess and Fernández –

Velasco29; Amutha andMurugesan30

Figure 2. Different energy production routes for algal biomass (adapted from Tsukahara and Sawayama16, Wang et al.38 and Brennan and Owende39).

tion and pyrolysis. Chemical reactions involve transesterificationof extracted oil from algal biomass, and in direct combustion,the dried algal biomass can be burnt directly to provide heatand power. The first three conversion technologies are more ac-ceptable and many companies are involved in commercializingthem.

Manipulation of metabolic pathways can redirect cellularfunction towards the synthesis of specific products and evenexpand the processing capabilities of microalgae. One methodof controlling microalgae employs specific environmental factors,such as nutrient regimens, to induce desired fluxes in metabolismand the development of a number of transgenic algal strainswith recombinant protein expression, engineered photosynthesis,and enhanced metabolism encourage the prospects of designermicroalgae.

SUSTAINABILITYSustainability assessment of products or technologies is normallyseen as encompassing impacts in three dimensions, i.e. social,environmental, and economic.40 For all three a life cycleperspective is relevant to avoid problem shifting in the productsystem.41

Algae, especially microalgae can tolerate high levels of CO2 andutilize much of the carbon. They can also utilize CO2 from industrialemissions and can play a role in minimizing GHG emissions. Inthis way algal biofuels can couple CO2-neutral fuel productionwith CO2 sequestration from other power industries, in turngenerating carbon credits.42 Algae can utilize nutrients (N andP) from a variety of wastewater sources (e.g. agricultural run-off, concentrated animal feed operations, industrial effluent andmunicipal wastewater), thus providing sustainable bioremediationof these wastewaters for environmental and economic benefits.43

A number of algal characteristics make them attractive comparedto terrestrial feedstock crops. One advantage is that they use

resources very efficiently and therefore have high productivitywith comparatively lower water usage.44

A widely stated claim is that microalgae are capable of producingseveral times more oil per unit land area than terrestrial oilseedcrops. The actual global oil production from oilseed crop was0.592 t ha−1 in 2007–2008.45 If one assumes an oil concentration inalgae of ∼42%46 and productivity 365 t ha−1 year−1 in AlgaeLinkbioreactors, this equates to 153.3 t ha−1 year−1 oil production,which is about 259 times higher than the oilseed crops.47 Even ifthese estimates are optimistic the potential benefits are obvious.The cost of algal biofuels can be reduced by using cheap sourcesof CO2 (flue gas), nutrient-rich wastewaters, inexpensive fertilizers,cheaper design culture systems with automated process control,greenhouses and heated effluents to increase algal yields. Thesemeasures will also help to reduce GHG emissions and wastedisposal problems.

Singh et al.18 concluded, in a review on mechanisms and chal-lenges in commercilization of algal biofuels, that the integrationof microalgae cultivation with fish-farms, food processing facilitiesand wastewater treatment plants, etc., will offer the possibilityfor waste remediation through recycling of organic matter and atthe same time low-cost nutrient supply required for algal biomasscultivation. These options could all be explored as part of anintegrated biorefinery concept.

The major factors that will determine the impacts of biofuelsinclude their contribution to land-use change, the feedstock used,and issues of technology and scale. Biofuels offer economic bene-fits, and in the right circumstances can reduce emissions and makea small contribution to energy security. The complexity during thewhole biofuel production chain generates significantly different re-sults due to the differences in input data, methodologies applied,and local geographical conditions. A useful tool for addressingenvironmental sustainability issues is life cycle assessment.19

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Simultaneoussaccharification andfermentation (SSF)Anaerobic

digestion

Bioethanol

BiomethaneCarbon dioxide

Biofertilizer for agricultural fields /Nutrients for culture production

Glycerol

Distillation

Solid separation

Algal cultureproduction

Algal cultureharvesting

Algal culturedewatering

Extractionof fattyacids

Trans-esterification

Biodiesel

NutrientWater

Recycling of water

Discharged water

Steam and power generation

Residualsolids

Figure 3. Life cycle stages of biofuels production from algal biomass.

LIFE CYCLE ASSESSMENT (LCA)LCA is a tool used to assess the environmental impacts andresources used throughout a product’s life cycle, and considersall attributes or aspects of natural environment, human health,and resources.41,48 Clarens et al.49 reported in a study on thelife cycle model for algae production that only in total landuse and eutrophication potential do algae perform favorably.The large environmental footprint of algae cultivation is drivenpredominantly by upstream impacts, mainly the CO2 demand andfertilizer. They also suggested that these impacts can be reducedby using flue gas and wastewater/sea water, to offset most ofthe environmental burdens associated with cultivation of algalbiomass. Various life cycle stages associated with algal biofuelsproduction are depicted in Fig. 3.

Evans and Wilkie50 calculated a range of net energy andeconomic benefits associated with hydrilla harvests and theutilization of biomass for biogas and compost production usinga LCA approach. With moderate data assumptions they foundnet energy benefit ratios (NEBRs) and a monetary benefit costratio (BCR) of 1.54 and 1.79 for biogas production and 1.32 and1.83 for compost production pathways, respectively. On the otherhand very high NEBRs (3.94 for biogas and 6.37 for compost)and BCRs (>11 for both) were found for optimistic scenarios,assuming high hydrilla biomass density, high nutrient contentin biomass, and high priority for nutrient remediation. Based onthe results, they concluded that energy and economic returnswere largely decoupled, with biogas and fertilizer providing thebulk of output energy, while nutrient remediation and herbicideavoidance dominated the economic output calculations, whichmakes hydrilla harvest a suitable and cost effective managementprogram for many nutrient-impaired waters.

Lardon et al.51 conducted a comparative LCA study of avirtual facility to assess the energetic balance and the potentialenvironmental impacts of the whole process chain, from biomassproduction to biodiesel combustion. The findings of the studyconfirm the potential of microalgae as an energy source buthighlights the imperative necessity of decreasing energy andfertilizer consumption. They suggested that control of nitrogenstress during the culture and optimization of wet extraction seemsto be a valuable option and also emphasize the potential of

anaerobic digestion of oilcakes to reduce external energy demandand to recycle part of the mineral fertilizers.

In an LCA study of biogas production from the microalgaeChlorella vulgaris Collet et al.52 compared algal biodiesel withfirst-generation biodiesels and suggested on the basis of resultsobtained that the impacts generated by the production ofmethane are strongly correlated with the electricity consumptionand better results can be achieved by decreasing the mixingcosts and circulation between different production steps and byimproving the efficiency of the anaerobic process.

The LCA of third-generation biofuels is very important beforetaking them into consideration for commercial scale productionand forming a policy for that purpose.

CONCLUSIONThe cells of microalgae are naturally able to bio-synthesize andstore lipids similar to those types present in vegetable oils,however, research is required to achieve commercially viablelevels of fuels. This could be done by genetic modifications ofalgal strains to get more efficient forms, accumulating higherquantities of lipids/carbohydrates and able to grow on organicwastes. The utilization of organic waste, flue gases and industrialeffluents for the production of algal biomass will also reduce GHGemissions and waste disposal problems, and will contribute tothe sustainability and market competitiveness of the microalgalbiofuel industry. An LCA will help in assessing the sustainability ofthird generation biofuels and adopting the appropriate policiesfor that.

ACKNOWLEDGEMENTAnoop Singh and Stig Irving Olsen would like to acknowledge thefunding from DTU Climate Center.

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