Baixe Bio-ethanol ? the fuel of tomorrow e outras Notas de estudo em PDF para Engenharia de Produção, somente na Docsity! Bio-ethanol – the fuel of tomorrow from the residues of today B. Hahn-Hägerdal, M. Galbe, M.F. Gorwa-Grauslund, G. Lidén and G. Zacchi Lund University, PO Box 124, S-221 00 Lund, Sweden, Getingevägen 60 Review TRENDS in Biotechnology Vol.24 No.12The increased concern for the security of the oil supply and the negative impact of fossil fuels on the environ- ment, particularly greenhouse gas emissions, has put pressure on society to find renewable fuel alternatives. The most common renewable fuel today is ethanol produced from sugar or grain (starch); however, this raw material base will not be sufficient. Consequently, future large-scale use of ethanol will most certainly have to be based on production from lignocellulosic materi- als. This review gives an overview of the new technol- ogies required and the advances achieved in recent years to bring lignocellulosic ethanol towards industrial pro- duction. One of the major challenges is to optimize the integration of process engineering, fermentation tech- nology, enzyme engineering and metabolic engineering. Introduction One of the greatest challenges for society in the 21st century is to meet the growing demand for energy for transportation, heating and industrial processes, and to provide rawmaterial for the industry in a sustainable way. An increasing concern for the security of the oil supply has been evidenced by increasing oil prices, which during 2006 approached US$80 per barrel. More importantly, the future energy supply must be met with a simultaneous substantial reduction of green house gas emissions. Actions towards this aim have been initiated. The Eur- opeanCommission plans to substitute progressively 20% of conventional fossil fuels with alternative fuels in the trans- port sector by 2020, with an intermittent goal set at 5.75% in 2010. In the USA, the Energy Policy Act of 2005 requires blending of 7.5 billion gallons of alternative fuels by 2012 [1], and recently the US President, in his state of the union address, set the goal to replace more than 75% of imported oil with alternative fuels by the year 2025 [2]. Liquid biofuels from renewable resources, particularly from lig- nocellulose materials, will have a substantial role in meet- ing these goals (Box 1). Ethanol has already been introduced on a large scale in Brazil, the US and some European countries, and we expect it to be one of the dominating renewable biofuels in the transport sector within the coming 20 years. Ethanol can be blended with petrol or used as neat alcohol in dedicated engines, taking advantage of the higher octane number and higher heat of vaporization; furthermore, it is an excellent fuel for future advanced flexi-fuel hybridCorresponding author: Zacchi, G. (Guido.Zacchi@chemeng.lth.se). Available online 16 October 2006. www.sciencedirect.com 0167-7799/$ – see front matter 2006 Elsevier Ltd. All rights reservevehicles. Currently, ethanol for the fuelmarket is produced from sugar (Brazil) or starch (USA) at competitive prices. However, this rawmaterial base, which also has to be used for animal feed and human needs, will not be sufficient to meet the increasing demand for fuel ethanol; and the reduction of greenhouse gases resulting from use of sugar- or starch-based ethanol is not as high as desirable [3]. Both these factors call for the exploitation of lignocellulose feed- stocks, such as agricultural and forest residues as well as dedicated crops, for the production of ethanol. This review summarizes recent developments in the bioconversion processes aimed at fuel ethanol production, with emphasis on process integration. In particular, the concept that each individual unit operation has to be developed and optimized in relation to the preceding and subsequent process steps will be discussed. Overview of the conversion process With so many advantages, why are there still no production facilities using lignocellulosic materials? Ethanol is currently produced from sugar cane and starch-containing materials, where the conversion of starch to ethanol includes a liquefaction step (to make the starch soluble) and a hydrolysis step (to produce glu- cose). The resulting glucose is then readily fermented. Although there are similarities between the lignocellulosic and the starch process, the techno-economic challenges facing the former are large. There are several options for a lignocellulose-to-ethanol process but, regardless of which is chosen, the following features must be assessed in comparison with established sugar- or starch-based ethanol production. (i) Ed. doi:1fficient de-polymerization of cellulose and hemi- cellulose to soluble sugars.(ii) Efficient fermentation of a mixed-sugar hydrolysate containing six-carbon (hexoses) and five-carbon (pen- toses) sugars as well as fermentation inhibitory compounds.(iii) Advanced process integration to minimize process energy demand.(iv) Cost-efficient use of lignin. The first step in the conversion of biomass to ethanol issize reduction and pretreatment. In this review, only the enzymatic process (Figure 1) will be discussed because it is considered to be the most promising technology [4–6]. The hemicellulose and cellulose polymers are hydrolyzed with enzymes or acids to release monomeric sugars. The sugars from the pretreatment and enzymatic hydrolysis steps are fermented by bacteria, yeast or filamentous fungi,0.1016/j.tibtech.2006.10.004 Box 1. Advantages of lignocellulose-based liquid biofuels Biofuel sources are geographically more evenly distributed than the fossil fuels; thus, the sources of energy will, to a larger extent, be domestic and provide security of supply. Lignocellulosic raw materials minimize the potential conflict between land use for food (and feed) production and energy feedstock production. The raw material is less expensive than conventional agricultural feedstock and can be produced with lower input of fertilizers, pesticides, and energy. Biofuels from lignocellulose generate low net greenhouse gas emissions, reducing environmental impacts, particularly climate change. Biofuels might also provide employment in rural areas 550 Review TRENDS in Biotechnology Vol.24 No.12although the enzymatic hydrolysis and fermentation can also be performed in a combined step – a so-called simul- taneous saccharification and fermentation (SSF). After final purification (by distillation and molecular sieves or other separation techniques), the ethanol is ready to be used as a fuel, either neat or blended with petrol. A part of the lignin, the principal solid part of the biomass remain- ing, can be burnt to provide heat and electricity for the process, whereas the rest is retained as a valuable co- product. The most probable use today would be as an ash-free solid fuel, but various technologies are under development to convert it to a higher-value product, which could form the basis for a new branch of industrial chem- istry [7]. First, the biomass is converted to sugars. . . It is only 40 years ago that the biodegradation of lignocellulosics was first discussed [8]. Enzyme conversion is substrate-specific without by-product formation, which reduces inhibition of the following process steps. However, enzyme-catalysed conversion of cellulose to glucose is slow unless the biomass has been subjected to pretreatment, which is also required to reach high yields and to make the process commercially successful [9]: the pretreatment aimsFigure 1. Schematic flowsheet for the conversion of biomass to ethanol. www.sciencedirect.comto increase pore size and reduce cellulose crystallinity. In acid-catalyzed pre-treatment, the hemicellulose layer is hydrolyzed, whereas in alkali-catalyzed pretreatment, mainly, a part of the lignin is removed and hemicellulose has to be hydrolysed by the use of hemicellulases. Hence, pretreatment is necessary to expose the cellulose fibres to the enzymes or to at least make the cellulose more acces- sible to the enzymes. An efficient pretreatment can sub- stantially reduce the enzyme requirements, which make up a large part of the production cost. Pretreatment is usually assessed in a number of ways: by enzymatic hydrolysis (EH) of the solid material to determine the digestibility; by fermentation of the liquid to assess the effect of potential inhibitors towards the fermenting microorganism; and/or by simultaneous sac- charification and fermentation (SSF) of the pretreated material. The conditions for the assessment can vary, particularly in terms of washed or non-washed solids and the concentration of solids and enzymes in the EH and SSF, making comparison of results from different investigations difficult. In an extensive study undertaken in theUSA, where the same batch of corn stover was pretreated using various methods (e.g. dilute acid, AFEX, hot water treatment) and then subjected to standard evaluation techniques, the yields of sugars were found to be more or less the same [10]. Total sugar yields – after pretreatment followed by enzymatic hydrolysis – of around 90% or more were reached, demonstrating that corn stover is an easily degradable material. When corn stover was steam pre- treated with small amounts of SO2, overall sugar yields close to the theoretical value were obtained; steam pre- treatment without a catalyst also resulted in90% glucose yield [11]. In countries such as Sweden, Canada and the USA, much of the available biomass is softwood, which is more difficult to hydrolyse than corn stover. For softwood, steam pretreatment with the addition of an acid catalyst such as Review TRENDS in Biotechnology Vol.24 No.12 553vastly different raw materials, and pretreatment and hydrolysis conditions [29]. Table 1 summarizes the most recently published results on the fermentation of lignocel- lulose hydrolysate with natural and recombinant bacteria and yeast. In essence, so far only recombinant S. cerevisiae strains have been able to ferment xylose in non-detoxified hydrolysates, where the fed-batch technology also permits the fermentation of extremely inhibitory softwood hydro- lysates. Moving on from research to a commercial process The estimated cost of producing ethanol from cellulosic materials varies widely between investigations, as shown both in some earlier review papers [57,58], with production costs in the range of 0.28 to 1.0 US$/l ethanol, and in more recent techno-economic evaluations [13,59,60]. However, most cost estimations are based on laboratory-scale and, to some extent, pilot-scale data for individual process steps and should be treated with caution. The cost of raw mate- rial, which varies considerably between different studies (US$22–US$61 per metric ton dry matter), and the capital costs, which makes the total cost dependent on plant capacity, contribute most to the total production cost. The cost for hydrolysis, particularly for the enzymatic process, is also a major cost contributor. Rawmaterial cost is reduced by using the whole crop for products and co-products. High ethanol yield requires complete hydrolysis of both cellulose and hemicellulose with aminimum of sugar degradation, followed by efficient fermentation of all sugars in the biomass. In the short- term, co-products are likely to be used for the production of fuel, heat and electricity; however, in the long term, bioethanol technology will form the basis for the sustain- able production of commodity chemicals and materials in future biorefineries. High co-product yield requires reduced energy demand for ethanol production. This is achieved when high solids concentrations (Figure 3) are combined with integration of energy-intensive process steps (e.g. pretreatment, distillation, evaporation andFigure 3. Ethanol production cost as a function of water-insoluble solids (WIS) in the SSF step (expressed as weight-% of total material) for production of ethanol from spruce, based on a production capacity of 200 000 DM raw material per year. Broken line: use of meachanical vapour recompression in evaporation (1 SEK = $0.13). This figure clearly shows the importance of working at high WIS. www.sciencedirect.comdrying). In SSF, 12 % WIS (water-insoluble solids) result in an ethanol concentration>4 wt-% (weight-%; kg ethanol per 100 kg solution), which is necessary to reduce the energy demand in the distillation steps (A. Wingren, PhD thesis, Lund University, 2005). Further reductions in the energy demand can be obtained by recycling certain process streams, to minimise the amount of fresh water used [61]. However, high solids concentrations and recy- cling of process streams increase the concentration of compounds that are inhibitory to enzymatic hydrolysis and fermentation, necessitating detoxification or fed-batch technology, as described previously. It also results in high viscosity, which limits mixing and pumping. Process integration reduces the capital costs. In the separate hydrolysis and fermentation (SHF) process, cel- lulose is first hydrolyzed to glucose and then glucose is fermented to ethanol. The primary advantage of SHF is that hydrolysis and fermentation occur at optimum condi- tions; the disadvantage is that cellulolytic enzymes are end-product inhibited so that the rate of hydrolysis is progressively reduced when glucose and cellobiose accu- mulate [24]. Product inhibition was the rationale for the first report on simultaneous saccharification and fermen- tation (SSF) of cellulose [62]: in SSF, hydrolysis and fer- mentation occur simultaneously in the same vessel, and the end-product inhibition of the enzymes is relieved because the fermenting organism immediately consumes the released sugars. Furthermore, the fermentation seems to decrease the inhibition of the enzymes by converting some of the toxic compounds present in the hydrolysate [24]. This increases the overall ethanol productivity, the ethanol concentration and the final ethanol yield [12,63] (Figure 4). More recently, the SSF technology has proved advanta- geous for the simultaneous fermentation of hexose and pentose sugars (so called SSCF). In SSCF, the enzymatic hydrolysis continuously releases hexose sugars, which increases the rate of glycolysis such that the pentose sugars are fermented faster and with higher yield [37].Figure 4. Overall yields of fermentable sugars and ethanol from steam-pretreated spruce. Abbreviations: EH, sugar yield after separate enzymatic hydrolysis; SSF, ethanol yield after SSF; SO2, after impregnation with sulfur dioxide; H2SO4, after impregnation with sulfuric acid. Adapted from data presented in [12]. Figure 5. Biorefinery – integration of a combined heat and power plant with an ethanol production plant. 554 Review TRENDS in Biotechnology Vol.24 No.12Further process integration can be achieved by performing both hydrolysis and fermentation in a single reactor, using one or a mixture of microorganisms that produce all the required enzymes and ferment all sugars – so-called con- solidated bioprocessing (CBP) [64]. However, no such microorganisms are currently available, and the concept is subject to further research. The economic analysis [60] of the cellulosic bioethanol process shows that reliable cost estimations require laboratory results are verified in pilot and demonstration plants, where all steps are integrated into a continuous process. This also provides the possibility to explore the benefits of process integration to reduce the number of process steps and the energy demand, and to recirculate process streams to eliminate the use of fresh water and to reduce the amount of waste streams. Currently, the Iogen Corp. (http://www.iogen.ca/) demo-plant is the only operat- ing plant for the production of bioethanol from lignocellu- lose using the enzymatic hydrolysis process. The plant can handle up to 40 tonnes per day of wheat, oat and barley straw and is designed to produce up to 3 million litres of cellulose ethanol per year. Abengoa Bioenergy (http:// www.abengoabioenergy.com) is also constructing a pilot plant in York, USA, to convert residual starch, cellulose and hemicellulose – mainly corn stover – to bioethanol and high-protein feed. In Salamanca, the same company con- structed a demonstration plant integrated with a fuel- ethanol-from-grain plant, producing 195 million liters. In the demonstration plant, an additional 5 million liters of ethanol per year will be produced from cellulose, mainlyBox 2. Major research challenges Improving the enzymatic hydrolysis with efficient enzymes, reduced enzyme production cost and novel technology for high solids handling. Developing robust fermenting organisms, which are more tolerant to inhibitors and ferment all sugars in the raw material in concentrated hydrolysates at high productivity and with high ethanol concentration. Extending process integration to reduce the number of process steps and the energy demand and to re-use process streams to eliminate the use of fresh water and to reduce the amount of waste streams. www.sciencedirect.comfrom wheat staw. In Sweden, a fully integrated pilot plant for ethanol production from softwood, comprising both two- stage dilute acid hydrolysis and the enzymatic process, was taken into operation inmid 2004. The pilot has amaximum capacity of 2 ton (DM) wood per day (http://www.etek.se). The step from pilot- and demo-scale production of lig- nocellulosic ethanol to competitive full-scale production requires further reduction of the production cost. One approach to this is the integration of ethanol production with a combined heat and power plant (Figure 5) or with a pulp and paper mill. 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