New improvements for lignocellulosic ethanol

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The use of lignocellulosic biomass for the production of biofuels will be unavoidable if liquid fossil fuels are to be replaced by renewable and sustainable alternatives. Ethanol accounts for the majority of biofuel use worldwide, and the prospect of its biological production from abundant lignocellulosic feedstocks is attractive. The recalcitrance of these raw materials still renders proposed processes complex and costly, but there are grounds for optimism. The application of new, engineered enzyme systems for cellulose hydrolysis, the construction of inhibitor-tolerant pentose-fermenting industrial yeast strains, combined with optimized process integration promise significant improvements. The opportunity to test these advances in pilot plants paves the way for large-scale units. This review summarizes recent progress in this field, including the validation at pilot scale, and the economic and environmental impacts of this production pathway.

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.

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