New improvements for lignocellulosic ethanol
Introduction
Liquid transport fuels derived from renewable lignocellulosic resources offer unique and desirable features: a secure source of supply, limited conflict with land use for food and feed production, and lower fossil fuel inputs. The biological production of ethanol from forest and agricultural residues, or dedicated lignocellulosic crops, offers these benefits but its development is still hampered by economic and technical obstacles [1••, 2].
The ‘conventional’ process for producing ethanol from lignocellulosic biomass includes four main steps (Figure 1):
- (1)
Pretreatment—breaking down the structure of the lignocellulosic matrix.
- (2)
Enzymatic hydrolysis—depolymerizing cellulose to glucose by means of cellulolytic enzymes.
- (3)
Fermentation—metabolizing the glucose to ethanol, generally by yeast strains.
- (4)
Distillation-rectification-dehydration—separating and purifying the ethanol to meet fuel specifications.
Around the world there are numerous R&D projects seeking to overcome the remaining obstacles to commercialization. Some of the projects, principally those in USA, include pilot and demonstration facilities. The key obstacles being tackled are: pretreatment selection and optimization; decreasing the cost of enzymatic hydrolysis; maximizing the conversion of sugars (including pentoses) to ethanol; process scale-up and integration to minimize energy and water demand; characterization and valuation of the lignin co-product; and lastly, the use of representative and reliable data for cost estimation, and the determination of environmental and socio-economic impacts. Besides seeking to improve the conventional process, which utilizes Trichoderma reesei cellulolytic enzymes and Saccharomyces cerevisiae yeast strains, alternative and novel schemes are also being investigated, for example, the use of thermophilic enzymes [3••], recombinant ethanol-producing strains [4, 5] and consolidated bioprocessing [6]. This review focuses on recent advances in the four-step process, underlined by efforts performed within the framework of a European research project: the NILE (New Improvements for Lignocellulosic Ethanol) project. It should be noted, however, that major breakthroughs here could also benefit these other production pathways.
Section snippets
Pretreatment of lignocellulosic materials
Pretreatment of lignocellulosic biomass aims at rendering cellulose accessible to the action of hydrolytic enzymes by altering the lignocellulosic cell wall [7, 8]. Pretreatment effects include: an increase of the accessible surface area, cellulose decrystallization, partial cellulose depolymerization, hemicellulose and/or lignin solubilization, and the modification of the lignin structure. Many pretreatment technologies have been proposed generally on the basis of combined physical and
Overcoming the recalcitrance of lignocellulosic biomass
The goal of enzymatic hydrolysis is to depolymerize the polysaccharides in the water insoluble solid fraction that remains after pretreatment. After most pretreatments, the bulk of these remaining polysaccharides are cellulose. Three classes of enzymes act synergistically to hydrolyse cellulose: endo-β-1,4-glucanases (EG, EC 3.1.2.4) attack the endogenous part of cellulose chain, cellobiohydrolases (CBH, EC 3.2.1.91) attack the ends of the polymer, releasing cellobiose that is ultimately
Ethanolic fermentation of lignocellulose
Ethanolic fermentation of lignocellulose hydrolysates requires that the organism ferments both the hexose sugars glucose, mannose, and galactose, and the pentose sugars, xylose and arabinose in the presence of inhibitory compounds including weak acids, furaldehydes and phenolics. Baker's yeast Saccharomyces cerevisiae, which has been the preferred organism for fermentative ethanol production throughout recorded human history is also tolerant toward lignocellulose derived metabolic inhibitors [34
Process integration
Increasing production capacity to commercial scale can only be done with confidence when a process is shown to be robust at an intermediate, pilot scale. An ideal pilot plant needs to be fully integrated, able to evaluate the complete system (e.g. enzymes and yeasts) while having sufficient flexibility to investigate alternative process configurations and test options for better heat integration and the recycling of process streams. Recently there has been a significant effort for building
Economics and environmental impacts
The markets for biofuels in North America and the EU are almost entirely dependent on policy mandates and fiscal incentives, predicated on the contribution of ethanol to greenhouse gas saving, security of supply, and employment policy objectives [63]. For example, the latest EU policy (10% biofuels by 2020) makes access to subsidized markets contingent on a minimum 35% GHG saving (increasing to 50% from 2017). Currently, it is uncertain which competing technology pathways will become dominant,
Conclusions
Advances in the cost-effective conversion of lignocellulosic biomass are often difficult to assess accurately because of the lack of integrated testing, for example, lab and pilot scale trials, and the lack of appropriate tools, for example, process, cost, and environmental impact models. Integrated projects such as NILE are required because of the high level of interdependence between process steps and the necessity to give a global standpoint on the whole chain. All results generated in
References and recommended reading
Paper of particular interests, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The authors acknowledge the support provided by the European Commission Framework Programme 6 (NILE project—Contract Number 019882). We are also thank the members of the 22 partners of the NILE consortium involved in this project for their constant effort.
References (74)
- et al.
Thermostable enzymes in lignocellulose hydrolysis
Adv Biochem Eng Biotechnol
(2007) - et al.
Fractionating recalcitrant lignocellulose at modest reaction conditions
Biotechnol Bioeng
(2007) - et al.
Dissolution of cellulose with ionic liquids and its application: a mini-review
Green Chem
(2006) - et al.
Role of oxidative enzymatic treatments on enzymatic hydrolysis of softwood
Biotechnol Bioeng
(2004) - et al.
Physiological evaluation of the filamentous fungus Trichoderma reesei in production processes by marker gene expression analysis
BMC Biotechnol
(2007) - et al.
Lack of aldose 1-epimerase in Hypocrea jecorina (anamorph Trichoderma reesei): a key to cellulase gene expression on lactose
Proc Natl Acad Sci U S A
(2008) - et al.
Transcriptional regulation of xyr1, encoding the main regulator of the xylanolytic and cellulolytic enzyme system in Hypocrea jecorina
Appl Environ Microbiol
(2008) - et al.
The Hypocrea jecorina (Trichoderma reesei) hypercellulolytic mutant RUT C30 lacks a 85 kb (29 gene-encoding) region of the wild-type genome
BMC Genomics
(2008) - et al.
Towards industrial pentose-fermenting yeast strains
Appl Microbiol Biotechnol
(2007) - et al.
Co-utilization of L-arabinose and D-xylose by laboratory and industrial Saccharomyces cerevisiae strains
Microb Cell Factories
(2006)
Novel evolutionary engineering approach for accelerated utilization of glucose, xylose and arabinose mixtures by engineered Saccharomyces cerevisiae strains
Appl Environ Microbiol
Identification of furfural as a key toxin in lignocellulosic hydrolysates and evolution of a tolerant yeast strain
Microb Biotechnol
Identification of an NADH-dependent 5-hydroxymethylfurfural-reducing alcohol dehydrogenase in Saccharomyces cerevisiae
Yeast
NADH- vs NADPH-coupled reduction of 5-hydroxymethyl furfural (HMF) and its implications on product distribution in Saccharomyces cerevisiae
Appl Microb Biotechnol
A short review on SSF—an interesting process option for ethanol production from lignocellulosic feedstocks
Biotechnol Biofuels
Land clearing and the biofuel carbon debt
Science
Biomass recalcitrance: engineering plants and enzymes for biofuels production
Science
Trends in biotechnological production of fuel ethanol from different feedstocks
Bioresour Technol
Minimal Escherichia coli cell for the most efficient production of ethanol from hexoses and pentoses
Appl Environ Microbiol
Genetic engineering of Zymobacter palmae for production of ethanol from xylose
Appl Environ Microbiol
Metabolic engineering of a thermophilic bacterium to produce ethanol at high yield
Proc Natl Acad Sci U S A
Pretreatment: the key to unlocking low-cost cellulosic ethanol
Biofuels Bioproducts Bioref
Pretreatments to enhance the digestibility of lignocellulosic biomass
Bioresour Technol
Recent process improvements for the ammonia fiber expansion (AFEX) process and resulting reductions in minimum ethanol selling price
Bioresour Technol
Bioethanol production from barley hull using SAA (soaking in aqueous ammonia) pretreatment
Bioresour Technol
Regenerating cellulose from ionic liquids for an accelerated enzymatic hydrolysis
J Biotechnol
Process and economic analysis of pretreatment technologies
Bioresour Technol
Microbial cellulose utilization: fundamentals and biotechnology
Microbiol Mol Biol Rev
Evaluation of novel fungal cellulase preparations for ability to hydrolyze softwood substrates—evidence for the role of accessory enzymes
Enzyme Microb Technol
Genetic improvement of Trichoderma reesei for large scale cellulase production
Enzyme Microb Technol
Comparative enzymatic hydrolysis of pretreated spruce by supernatants, whole fermentation broths and washed mycelia of Trichoderma reesei and Trichoderma atroviride
Bioresour Technol
Regulation of transcription of cellulases- and hemicellulases-encoding genes in Aspergillus niger and Hypocrea jecorina (Trichoderma reesei)
Appl Microbiol Biotechnol
Genome sequencing and analysis of the biomass-degrading fungus Trichoderma reesei (syn. Hypocrea jecorina)
Nat Biotechnol
Comparative secretome analyses of two Trichoderma reesei RUT-C30 and CL847 hypersecretory strains
Biotechnol Biofuels
Evaluation of minimal Trichoderma reesei cellulase mixtures on differently pretreated Barley straw substrates
Biotechnol Prog
Transcriptional regulation of biomass-degrading enzymes in the filamentous fungus Trichoderma reesei
J Biol Chem
The carbohydrate-active enzymes database (CAZy): an expert resource for glycogenomics
Nucleic Acids Res
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