Response of Thermochemical and Biochemical Conversion Processes to Lignin Concentration in Alfalfa Stems † A. A. Boateng, ‡, * P. J. Weimer, § H. G. Jung, ⊥ and J. F. S. Lamb ⊥ USDA-ARS, Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, PennsylVania 19038, USDA-ARS, U.S. Dairy Forage Research Center, 1925 Linden DriVe W., Madison, Wisconsin 53706, and USDA-ARS, Plant Science Research Unit, 411 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, Minnesota 55108 ReceiVed March 12, 2008. ReVised Manuscript ReceiVed May 2, 2008 The technologies currently in place to convert lignocellulosic biomass to energy are either biochemical or thermochemical, the efficiencies of which may vary depending on the composition of the feedstock. One variable that conversion technologists have wrestled with, particularly in the simultaneous saccharification and fermentation process, is biomass lignin content. While lignin is considered a recalcitrant to biochemical conversion, it can be a good source of combustion fuel, but the true effect of composition on thermochemical conversion has not been well quantified. In this study we examined the effect of lignin content of alfalfa stems on two biofuel conversion methodologies: (i) biochemical conversion using in-vitro ruminal fermentation as a surrogate for fermentability to ethanol and (ii) thermochemical conversion using pyrolysis. Lignin was found to account for little of the variation in pyrolysis product yield compared to biochemical conversion. Linear regression of lignin concentration on pyrolysis product yields resulted in few significant relationships whereas in-vitro gas production exhibited a strong negative response to lignin content. For alfalfa stems, lignin had a much larger effect on biological conversion potential than it did on thermochemical conversion potential. The results suggest that genetic modification or agronomic management of lignocellulosic biomass for bioenergy feedstock composition should be based on the intended energy conversion platform. Introduction The main technological question confronting the biorefinery investor is the choice of the conversion technology. Often the decision is based on the availability of the biomass, the logistics of handling it, and its processing characteristics. The technolo- gies that are currently available for lignocellulosic conversion include both biochemical- or thermochemical-based platforms. 1 Biochemical conversion, often called the sugar platform, involves depolymerization of polysaccharides and fermentation of the resulting sugars. This is the technology of choice for the conversion of starch and simple sugars to fuel ethanol and related alcohols. Thermochemical conversion involves medium or high temperature degradation of biomass in an oxidized or reduced atmosphere to release the inherent energy (combustion) or to produce fuel intermediates (energy carriers) such as synthesis gas (syngas) and pyrolysis liquids. Both technologies can result in the production of transportation fuel from cellulosic biomass at varying economic penalties. 2 Through plant breeding and advances in genomics, plants could be genetically engineered to tailor their composition to the desired conversion technology. Yields, conversion efficien- cies, and ultimately the economics of biochemical-based biofuel production are all greatly impacted by feedstock composition. 3,4 For example, enzymatic hydrolysis of lignocellulose to free sugars is hindered by the presence of lignin because the latter acts as a physical barrier to hydrolytic enzymes, and because enzymes reversibly bind to lignin, resulting in inefficient use of the polysaccharide-degrading enzymes. When poplar (Popu- lus) wood was subjected to acid hydrolysis, Chang and Holtzapple 5 found a strong negative correlation between lignin content and degradability of the biomass. Dien et al. 6 found that lignin content negatively influenced total glucose yield from dilute-acid pretreatment and enzymatic saccharification for several perennial herbaceous species including alfalfa (Medicago satiVa), reed canarygrass (Phalaris arundinacea), and switch- grass (Panicum Virgatum). While there appears to be a negative impact of lignin on biochemical conversion, such may not be the case for thermo- † Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. * To whom correspondence should be addressed. E-mail address: akwasi.boateng@ars.usda.gov. ‡ Eastern Regional Research Center. § U.S. Dairy Forage Research Center. ⊥ Plant Science Research Unit. (1) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering. Chem. ReV. 2006, 106, 4044–4098. (2) Roadmap for biomass technologies in the United States, 2002. http:// www.brdisolutions.com/pdfs/FinalBiomassRoadmap.pdf (accessed March 1, 2008). (3) Lynd, L. R.; Weimer, P. J.; Pretorius, I. S.; van Zyl, W. H. Microbial cellulose utilization: fundamentals and biotechnology. Microbial Mol Biol. ReV. 2002, 66, 506–577. (4) Weimer, P. J.; Dien, B. S.; Springer, T. L.; Vogel, K. P. In-Vitro gas production as a surrogate measure of the fermentability of cellulosic biomass to ethanol. Appl. Microbiol. Biotechnol. 2005, 67, 52–58. (5) Chang, V. S.; Holtzapple, M. T. Fundamental factors affecting biomass enzymatic reactivity. J. Appl. Biochem. Biotechnol. 2000, 84, 5– 38. (6) Dien, B. S.; Jung, H. G.; Vogel, K. P.; Casler, M. D.; Lamb, J. F. S.; Iten, L.; Mitchell, R. B.; Sarath, G. Chemical composition and response to dilute-acid pretreatment and enzymatic saccharification of alfalfa, reed canarygrass, and switchgrass. Biomass Bioenergy 2006, 30, 880–891. Energy & Fuels 2008, 22, 2810–2815 2810 10.1021/ef800176x This article not subject to U.S. Copyright. Published 2008 by the American Chemical Society Published on Web 06/24/2008