Pretreatments for Enhanced Digestibility of Feedstocks

David K. Johnson, and Richard T. Elander

This chapter reviews how pretreatment is used to decrease the recalcitrance of biomass, and make the cellulose in the feedstock more susceptible to digestion by cellulase enzymes. Pretreatments have been designed to operate under very different chemistries, at widely varying temperatures, and for very different reaction times. With some pretreatments there is almost no change in the composition of the feedstock whereas in others the composition is changed considerably by dissolving the hemicelluloses, the lignin, or both. Pretreatment can also cause changes in the physicochemical structure of the biomass with changes in cellulose crystallinity, molecular weight, and accessibility, plus changes in biomass porosity and particle size. This chapter will focus on more recent developments in pretreatment and describe how these pretreatment processes attempt to overcome the natural recalcitrance of biomass to digestion by enzymes.

14.1 Introduction

Prior to the discovery of cellulolytic enzyme systems, processes that could thermochem­ically hydrolyze lignocellulose to produce soluble sugars were investigated and developed with some commercial use during wartime periods. Most of these processes were operated under fairly severe conditions and typically utilized concentrated acids (primarily sulfuric or hydrochloric) at relatively low temperatures (under 100°C) or dilute acids (again, typically sulfuric of hydrochloric) at much higher temperatures (often above 200°C). Such harsh conditions generally resulted in relatively low recoverable sugar yields due to sugar degra­dation reactions that produced primarily aldehydes and organic acids, along with other undesirable compounds. Also, the harsh conditions and, in the case of concentrated acid processes, the large amounts of catalyst used, caused such processes to be highly capital intensive, either from the requirements for pressurized corrosion resistant reactors capable of processing solid materials or the economic requirement to recover and recycle the acid catalysts. Advances have been made in thermochemical acid hydrolysis processes to improve their commercialization potential in some niche opportunities, especially for concentrated acid processes. These improvements are primarily related to acid catalyst recovery processes that allow for the cost-effective recycling of the large quantities of required acid catalysts (1).

Biomass Recalcitrance: Deconstructing the Plant Cell Wall for Bioenergy. Edited by Michael. E. Himmel © 2008 Blackwell Publishing Ltd. ISBN: 978-1-405-16360-6

The discovery of cellulase enzymes and the commercial availability of such enzyme sys­tems have dramatically changed the context of the thermochemical step in the conversion of lignocellulosic biomass to sugars. Rather than requiring this thermochemical step to directly produce all resulting sugars from biomass, this step can now be viewed as a true pre­treatment step, whose purpose is to prepare the biomass for rapid and complete enzymatic hydrolysis to produce a monomeric sugar stream. Conceptually, the enzymatic hydrolysis approach has several inherent advantages over thermochemical hydrolysis, including low — temperature, mild-pH conditions leading to less expensive and less complex reactor systems and significantly lower or no losses of resulting sugars to degradation products.