Pretreatment of lignocellulosic biomass through removal of inhibitory compounds

Abstract

A process for the pretreatment of lignocellulosic biomass is disclosed. The process is intended for use in connection with biomass to ethanol processes and is directed in particular to an economical removal of inhibitory compounds generated in biomass pretreatment, which are inhibitory to downstream hydrolysis and fermentation steps. The process includes the steps of heating the lignocellulosic biomass with steam to a preselected temperature, at a preselected pressure and for a preselected time to hydrolyze and solubilize hemicelluloses in the biomass; explosively decomposing the biomass into fibers; and extracting from the resulting reaction mixture a liquefied portion of the lignocellulosic biomass before or after explosive decomposition. The liquefied portion is extracted to remove compounds from the lignocellulosic biomass which are inhibitory to enzymatic cellulose hydrolysis and sugar fermentation to ethanol. For improved efficiency and economy, the inhibitory compounds are not completely removed. Furthermore, xylose has been found to be a good indicator compound for the general level of inhibitory compounds in the reaction mixture and the extraction step is therefore controlled on the basis of the xylose content in the reaction mixture. In particular, the extracting step is discontinued once a dry matter (dm) content of xylose, as monomer or oligomer, in the reaction mixture of 4% to 8% (w / w dm) is achieved. This most economically balances the practical need for inhibitory compound removal with the economical need to control and preferably minimize the costs of the overall ethanol production process.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application claims the benefit of priority of U.S. Provisional Patent Application No. 61 / 170,805 filed Apr. 20, 2009, the contents of which is incorporated herein by reference.FIELD OF THE INVENTION[0002]The present invention generally relates to the production of ethanol from biomass and in particular to a process for the pretreatment of lignocellulosic biomass.BACKGROUND OF THE INVENTION[0003]World energy consumption is predicted to increase 54% between 2001 and 2025. Considerable research effort is being directed towards the development of sustainable and carbon neutral energy sources to meet future needs.[0004]Biofuels are an attractive alternative to current petroleum-based fuels, as they can be utilized in transportation with little change to current technologies and have significant potential to improve sustainability and reduce greenhouse gas emissions.[0005]Biofuels include fuel ethanol. Fuel ethanol is produced from biomass...

AI Technical Summary

Benefits of technology

[0020]It is now an object of the present invention to provide a process which overcomes at least one of the above disadvantages by reducing the inhibition impact on the rate of hydrolysis and fermentation of pretreated biomass by degradation and hydrolysis products and other inhibitory compounds produced during pretreatment of lignocellulosic biomass.

[0021]It is a further object of the invention to provide a lignocellulosic biomass pretreatment process wherein hemicellulose, hemicellulose degradation and hydrolysis products, cellulose degradation products and other inhibitory compounds typically present in biomass such as fatty acids, sterols, esters, ethers etc. are removed in a commercially viable, economical manner prior to the enzymatic hydrolysis step to achieve the most economical maximization of hydrolysis and fermentation yields.

[0022]As is apparent from the above discussion, known approaches to improve the overall ethanol yield by successfully reducing the amount of inhibitory compounds in the pretreated biomass are generally linked to increased cost for operating the respective method. As a result, increased yields are only obtainable at significantly increased costs which are higher overall than the value of the increased ethanol yield or decreased hydrolysis or fermentation times and reduced enzyme costs, rendering existing methods economically unacceptable.

[0023]The inventors of the present application have now surprisingly discovered that complete removal of the inhibitory compounds is neither required nor desirable for the achievement of the most economically viable pretreatment process. The inventors have discovered a narrow range of extraction and inhibitory compounds removal conditions at which hemicelluloses and hemicellulose hydrolysis and degradation products and other inhibitors are still present, but reduced to a level where they have a much reduced inhibitory effect on the enzymes. The extraction is achieved with the use of a lower volume of diluent and level of dilution and at equipment cost which requires sufficiently lower additional extraction and compound removal cost to render the process much more cost effective, practical and commercially viable. In effect, the additional extraction cost is thereby significantly less than the value of any increased ethanol yield, enzyme cost reduction or reduced processing time achieved.

[0024]The removal of inhibitory compounds can be carried out through many different methods, typically a combination of mechanical pressing and draining, aqueous extraction, solvent extraction, filtering, centrifuging, venting, purging, draining, or the like, with or without the addition of eluents. These removal steps can occur during and / or after the pretreatment process. The removal of inhibitory compounds improves the economics of the process by reducing enzyme load and improving enzyme efficiency and fermentation performance. The term washing used throughout this specification defines removal of inhibitory compounds using water as the eluent.

Problems solved by technology

As is apparent from the above discussion, known approaches to improve the overall ethanol yield by successfully reducing the amount of inhibitory compounds in the pretreated biomass are generally linked to increased cost for operating the respective method.

As a result, increased yields are only obtainable at significantly increased costs which are higher overall than the value of the increased ethanol yield or decreased hydrolysis or fermentation times and reduced enzyme costs, rendering existing methods economically unacceptable.

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