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  • Review Article

    Biofuels: Engineering and Biological ChallengesPURNENDU GHOSH*Birla Institute of Scientific Research, Statue Circle, Jaipur 302 001, India

    (Received on 30 March 2014; Accepted on 02 August 2015)

    The second generation biofuel technologies are evolving rapidly to provide solutions for the partial replacement of fossil

    fuels. Both bioethanol and biodiesel have great potential in India. Both the technologies, however, have to overcome various

    bottlenecks before they become commercial technologies. In this regard, several critical questions, besides science and

    technology, need to be resolved. This will require new ways of thinking about agriculture, energy infrastructure and rural

    economic development.

    Keywords: Biofuels Technology; Bioethanol; Biomass; Algal Biofuel; Bioenergy

    *Author for Correspondence: E-mail: ghoshiitbisr@gmail.com; Phone: 0141-2385283

    Proc Indian Natn Sci Acad 81 No. 4 September 2015 pp. 765-773 Printed in India. DOI: 10.16943/ptinsa/2015/v81i4/48295

    Introduction

    In recent times, a great concern about fossil fuelssupplies, their non-renewable nature andenvironmental consequences of their use has driveninterest in biofuel programmes all over the world.There is no doubt that the best substitute forpetroleum is petroleum and, as one analyst puts it,replacement of fossil fuel by biofuel is not possible,butaugmentation of fuel supply probably is. As Churchand Regis (2012) write in their book Regenesis,Were now in a transitional period, caught betweenthe age of fossil fuels and the age of biofuels.

    It is believed that a partial transition from oil tobiofuels can stabilize the energy market significantly.To be a viable alternative, a biofuel should provide anet energy gain, have environmental benefits, beeconomically competitive and be producible in largequantities without affecting food security of thecountry.

    The National Policy on Biofuels of India (2009)proposes a target of 20% blending of bioethanol by2017. A target of 10% petrol blending seem morerealistic for 2017. Even this seems a difficult

    proposition keeping in view the present supply anddemand situation. The intermediate target of 5% and10% blending by 2007-2008 has not been achieved.The government is unable to implement compulsoryblending of 5% ethanol in petrol.

    First and Second Generation Biofuels

    The basic routes for converting biomass to biofuelare biochemical and thermochemical. The two classicthermochemical options, namely, gasification andpyrolysis produce different intermediates. Gasificationinvolves rapid heating and partial oxidation to producesyngas, which is largely carbon monoxide andhydrogen. The high oxygen content of biomass resultsin the production of significant quantities of carbondioxide, which reduces carbon efficiency. Also thesulphur, nitrogen, phosphorous, potassium, and mineralcontent of biomass complicates matters further. Inpyrolysis, lower temperatures are used to break downbiomass into smaller molecules such as oxygenatedaromatics, ketones, organic acids, and otheroxygenates, as well as light hydrocarbon gases. Inaddition to the lower energy input to achieve biomassdeconstruction, pyrolysis has a high theoretical yieldfor liquid products.

    Published Online on 13 October 2015

  • 766 Purnendu Ghosh

    In biochemical processing, biomass is typicallyprocessed to yield monosacchrides, which are thenconverted by microbes to produce fuels. Thoughbioethanol is an established biofuel, there is an alternateview that it would be a good idea to look at theconversion of lignocellulose into organic acids ratherthan sugars.

    Another biochemical route uses anaerobicdigestion to produce biogas, a mix of methane andcarbon dioxide. Here, natural consortia of bacteriadecompose organic matter into methane in theabsence of oxygen. Although much of the biomassresource might be dedicated to biofuel production (thusdiminishing its role in electricity generation), biogastechnologies could provide a small but nontrivial partof a renewable electricity portfolio, particularly giventheir flexibility and potential for distributed generation.

    The feedstock for first generation biofuelsproduced through biochemical routes are primarily foodcrops, such as sugar cane, grain (corn), oil seeds andvegetable oils. Their limited contribution to meet theenergy demands of the future has raised questionsabout their role in the transport fuel mix of the future.This makes the need for second generation biofueltechnologies inevitable and desirable.

    The feedstock for second generation biofuels isnon-food biomass, such as lignocellulosic materials(bagasse, cereal straw, forest residues, and short-rotation energy crops). The second-generation biofuelproduction has the potential to provide benefits suchas consuming waste residues and making use ofabandoned land. Job creation and regional growthare probably the most important drivers for theimplementation of second-generation biofuel projectsin major economies and developing countries.

    According to the estimates of InternationalEnergy Agency (2010) biofuels are expected toprovide 9% (11.7 EJ) of the total transport fuel demand(126 EJ) in 2030, 26% (29 EJ) of total transportationfuel (112 EJ) in 2050, with second-generation biofuelsaccounting for roughly 90% of all biofuels .

    Biofuels derived from lignocellulosic biomass andalgae are promising additional sources to meet energy

    demands of the country. Both can play a significantpart to solve energy supply picture in the futureprovided key obstacles are overcome. Both, however,are future technologies as there are no commercialplants, but a considerable number of pilot anddemonstration plants have been planned or set up inrecent years, mainly in North America, Europe, Brazil,China, India and Thailand.

    In India, the commercial viability of both theoptions is highly dependent on the future price of oiland the government policy. There is thus a promiseas well as an uncertainty. The promise is tosignificantly reduce our dependence on imported oil,create new jobs, improve rural economies, reducegreenhouse gas emissions, and enhance national fuelsecurity. The major uncertainties are feedstockavailability and cost, conversion technologies and cost,and the impact of technologies on the environment.The milestones (USDOE, 2006) that are suggestedfor the development of biofuels are provided in Fig.1.

    Fig. 1: Biofuels development milestones (USNAS, 2012)

    A brief overview on the future of cellulose andalgae-based biofuels is given here.

    Cellulose-Based Biofuel

    Production of ethanol from biomass follows variousconversion routes (Fig. 2). Ethanol is produced in India

  • Biofuel Challenges 767

    from cane molasses. Efforts to produce ethanol fromother sugar-based feedstock such as sweet sorghum,sugar beets, and sweet potatoes are at present in theexperimental stage. Lower molasses availability andconsequent higher prices impact ethanols cost ofproduction, thereby causing a disruption in the supplyof ethanol at pre-negotiated fixed ethanol prices.

    All the countries in the world are looking forsolutions for their growing energy needs usingsustainable and renewable resources. The first-generation technologies for bioethanol productionbased on sugars and starches cannot provide long-term solution. They compete for land with food crops,resulting in misleading cost-benefit analysis. What weneed is a cheap, abundant and renewable raw materialthat does not interfere with food production.Lignocellulosic biomass (LCB) is such a feedstockfor the production of second-generation bioethanol.

    Supply of Biomass

    The global supply of cellulosic biomass is estimatedto contain energy that is equivalent to much morethan the worlds current annual consumption oftransportation fuel. The sources of cellulosic biomassinclude crop wastes, forest residues, and dedicatedenergy crops. Lignocellulosic biomass (LCB) is lessexpensive than sugar or starch-based feedstock, butits conversion to ethanol at present is more costly.The commercialization of this technology thus has toovercome various bottlenecks. These includefeedstock availability, scale of operation, cheaper andeffective pretreatment technologies, efficienthydrolytic agents, availability of recombinantorganisms capable of co-fermenting the whole rangeof sugars at a temperature compatible to optimum

    hydrolysis, and better co-product value (Ghosh andGhose, 2003).

    Supply of biomass is one of the most criticalfactors for the development of a viable bioethanoltechnology. Three distinct goals need to be met forthe development of biomass-based biofuels, namely,maximizing the total amount of biomass produced perhectare per year, maintaining sustainability whileminimizing inputs, and maximizing the amount of fuelthat can be produced per unit of biomass. Exact valuesfor each of these parameters would vary, dependingupon the type of energy crop and the growing zone.Logistics of raw material supply (availability,collection, storage and handling) to meet largedemands of biofuels is a major issue of concern. Inaddition, the availability of the feedstock on asustainable basis would need either large storagefacilities or availability of plants to operate on multiplefeedstock for their continued operation throughout theyear.

    Ideal Pretreatment Technology

    Pretreatment of LCB continues to be a major barrierfor the development of a viable technology. In theLCB-based bioethanol technology, cellulose andhemicellulose present in the lignocellulose arehydrolysed to sugars (hexoses and pentoses) usingacids or enzymes. Lignin is the major interference inthe hydrolysis of native lignocellulose. In the enzymaticprocess, the LCB is pretreated in order to increasethe accessibility of cellulolytic enzymes (cellulases)to the substrate. Typically, hydrolysis yields in theabsence of pretreatment are less than 20% oftheoretical yields, whereas yields after pretreatmentoften exceed 90%. The rationale for pretreatmenthas thus been to separate individual components ofLCB with minimum component losses, concomitantwith an increase in surface area and a decrease incrystallinity.

    An ideal technology is expected to produce areactive fibre that will require little or no size reduction,and can be operated at a high solid/liquid ratio. Oneneeds to ascertain what is more important forenzymatic hydrolysis the extent of delignificationthat requires harsher conditions for complete lignin

    Fig. 2: Biomass-based bioethanol conversion routes

  • 768 Purnendu Ghosh

    separation or loosening of cellulose hemicellulose-lignin bonds under milder conditions. The benefits oflignin solubilization need to be weighed against thepotential for fermentation inhibition by soluble ligninderivatives.

    Various mechanical, physical, chemical, andbiological approaches, either singly or in combinationhave been attempted to meet these objectives, butnone has shown the promise expected from an idealpretreatment technology. Development of LCB-basedenergy plants with traits such as increased celluloseand hemicellulose and less lignin not only has thepotential to improve ethanol yields, but also thepossibility of application of much simpler pretreatmenttechnologies. Metabolic engineering of the ligninbiosynthetic pathway has been suggested as a methodfor modifying lignin content in the feedstock.

    Enzyme and Enzymatic Hydrolysis

    The important parameters of enzymatic hydrolysis aresugar yield, duration of hydrolysis, enzyme loading,characteristics of substrate cellulose, and enzymecellulases.

    The most desirable attributes of the enzymecellulases include the ability to produce a completecellulase system with high catalytic activity againstcrystalline cellulose, thermal stability, decreasedsusceptibility to enzyme inhibition by the products ofhydrolysis (glucose, and cellobiose), selectiveadsorption of the enzyme on cellulose, and the abilityto withstand shear forces. Strategies to improvecellulases include discovering new enzymes throughbioprospecting, creating new/better mixtures ofenzymes, and developing improved expression systemsthrough protein engineering. De-novo and in-silicodesigning of improved cellulases are also beingattempted. Creating a more effective cellulose bindingdomain in the enzyme molecule is another approachto increase enzyme efficiency.

    A critical element for the success of bioethanoltechnology is the availability of cheap cellulases.Industrial enzyme producers are trying to achievereduction in enzyme cost in order to support aneconomical and robust cellulose biorefinery. Cellulase

    enzymes are too expensive for bioethanol. Forexample, costs of amylase enzymes for convertinggrain starch to ethanol are about ten times cheaperthan the most optimistic cost estimates for cellulasepreparations. There is, however, a good possibility ofproducing effective cellulases at a much reduced cost.For the hydrolysis of pretreated biomass, extremelycomplex cellulases may not be required; simplercellulase systems may serve the purpose. The majormarket for cellulase enzymes is the textile industry,and the enzymes produced are tailored to meet therequirement of this industry. It is important to recognizethat biofuels application needs are significantlydifferent from textile applications.

    An Ideal Ethanol Producing Organism

    The bioethanol process needs an efficient organismwith capability to convert sugars (both hexoses andpentoses) to ethanol. An ideal ethanol producingorganism should have characteristics such as highethanol tolerance, capacity to withstand high osmoticpressure, high temperature, and low pH, high cellviability, appropriate flocculation and sedimentationcharacteristics, capability to ferment broad range ofsugars mainly to ethanol and possibly negligible levelsof by-products (such as acids and glycerol), andresistance to inhibitory compounds present in thepretreatment/hydrolysis stream. A strategy forincreasing ethanol tolerance or other traits usesevolutionary engineering concepts and methods. Thisstrategy allows the microbial process to evolve underproper selective pressure (in this case, higher ethanolconcentrations) to increasingly higher ethanoltolerances.

    Conversion of cellulose and hemicellulose toethanol comprises hydrolysis followed by fermentationof hexoses and pentoses by ethanol producingorganisms. Simultaneous saccharification andfermentation (SSF) integrates the processes ofhydrolysis with fermentation. The development ofthermophilic ethanol-producing organisms for use inSSF could allow the consolidated process to run athigher temperatures, thus realizing significant savingsby reducing cellulase enzyme requirements.Combining cellulase production, cellulose hydrolysis,

  • Biofuel Challenges 769

    and co-fermentation of hexose and pentose sugars ina single step, called consolidated bioprocessing, isconsidered the ultimate low-cost configuration forcellulose hydrolysis and fermentation.

    Companies are engaged in creating syntheticmicrobes to accelerate the conversion of agriculturalwaste to ethanol. Di...

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