Comparative Performance of Leading Pretreatment Technologies for Biological Conversion of Corn Stover, Poplar Wood, and Switchgrass to Sugars

Abstract

Cellulosic biomass is the only resource from which liquid fuels so vital to transportation can be made at alarge scale sustainably and also minimize the conflict with food production [1]. Furthermore, cellulosic bio-mass costing about $60/dry ton is competitive with petroleum at about $20/barrel on an equivalent energycontent basis [2]. Therefore, the primary challenge to competitiveness is low-cost processing of cellulosicbiomass to fuels, and biological routes can take advantage of the rapidly evolving tools of biotechnology toradically reduce costs [1,3]. The economic challenge for biological options is to overcome the natural resist-ance of plants to release of sugars that are the building blocks of cellulose and hemicellulose and typicallycomprise two-thirds to three-quarters of cellulosic materials. Although modification of plants to facilitatesugar release and better enzyme/organism combinations that can attack cellulosic materials more effectivelyare often favored to improve performance [1], to date, biomass pretreatment has been essential to achievingthe high yields of sugars vital to economic success and is likely to remain an essential step in the overallconversion system [4]. At this point, we are faced with the conundrum that pretreatment is among the mostexpensive single steps in biological processing [5], but unit production costs are even higher if pretreatment iseliminated, due to the resulting low product yields [6]. It has therefore been stated that the “only step moreexpensive than pretreatment is no pretreatment” [4]. Full attention must be focused on developing lower-costpretreatment technologies that can integrate with advanced biological conversion systems and perhaps takeadvantage of plants that have been modified to facilitate sugar release.The Biomass Refining Consortium for Applied Fundamentals and Innovation (CAFI) was originally con-ceived in late 1999 in a meeting in Dallas, Texas among pretreatment leaders interested in working collabo-ratively. It was formally organized in early 2000 in a Chicago meeting among this team [7]. During theentire CAFI lifetime, the following goals were set: Develop data on leading pretreatments using common feedstocks, enzymes, analytical methods, materialand energy balance methods and costing methods. Seek to understand mechanisms that influence performance and differentiate pretreatments by providinga technology base to facilitate commercial use and identifying promising paths to advance pretreatmenttechnologies that achieve lower costs. Train and educate students in biomass conversion technologies.A key objective was to provide information to help industry select technologies fo r commercial applica-tions and not to “downselect” pretreatments. Rather, it was vital to provide extensive data on promisingoptions so that others could decide which technologies t o employ and avoid a lack of data clouding decisions.The CAFI team was fortunate to be selected for funding by a new United States Department of Agricul-ture (USDA) Program called the Initiative for Future Agricultural and Food Systems (IFAFS) through acompetitive solicitation released in the spring of 2000. Although the program unfortunately had a short life,the IFAFS approach was unique in that it funded large collaborative projects focused on advancing andapplying biomass conversion technologies, consistent with the CAFI spirit. In this inaugural CAFI projectthat ran from 2000 to 2004, now designated CAFI 1, the emphasis was on comparative data from applica-tion of leading pretreatments to a shared source of corn stover, with most of the work focused on perform-ance from just the pretreatment and enzymatic hydrolysis steps [7]. In 2004, the Office of the BiomassProgram (OBP) of the US Department of Energy selected the CAFI team for a second project as a result ofa competitive solicitation. This project, now known to our team as CAFI 2, applied most of the pretreatmenttechnologies employed in CAFI 1 to poplar wood but with more data developed on enzymatic hydrolysisand fermentation [8]. Following completion of the CAFI 2 work, OBP funded the team to apply the samestable of pretreatment technologies to switchgrass. This latter CAFI 3 project was somewhat broader inscope than prior projects in that, in addition to determining yields from pretreatment and enzymatic

Keywords

Identifiers

DOI

10.1002/9780470975831.CH12

Journal

Aqueous Pretreatment of Plant Biomass for Biological and Chemical Conversion to Fuels and Chemicals

Year

2013