Hot Water Flowthrough Pretreatment of Cellulosic Biomass
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Transportation is a key energy use sector which will likely need liquid fuels for a long time if not indefinitely. Cellulosic biomass is a promising source of liquid fuels for very large scale sustainable energy production. Agricultural residues , particularly sugarcane bagasse , are an important low hanging fruit source of cellulosic biomass worldwide. The main obstacle to achieve commercial , cost competitive biological conversion of cellulose to fuel is biomass recalcitrance , referring to the incomplete accessibility of attack by microbes and their saccharolytic enzymes. As employed in the biological conversion field , pretreatment refers to the key limiting process of preparing the biomass to overcome biomass recalcitrance. There are a wide variety of pretreatment processes , generally involving exposing biomass to a solvent at elevated temperature and pressure. Pretreatment decisively improves the subsequent yield of sugar hydrolysis , but accounts for a substantial fraction of the cost of overall conversion. The conundrum of conventional pretreatment is that conditions severe enough to yield reactive fiber are accompanied by sugar degradation. We report that a compressed hot water flowthrough (FT) pretreatment allows 85% sugar recoveries compared to 40% with batch pretreatment at conditions severe enough to provide 90% glucan conversion. FT solubilizes all of the hemicelluloses and 75% of the lignin , while reducing the capital cost and use of chemicals compared with other pretreatment methods . The improved hemicelluloses and lignin removal in a FT configuration is a result of liquid being removed from the reactor , allowing less time for solubilized sugars to degrade and minimizing recondensation of solubilized lignin and xylan on cellulose fibers upon cooling  . However , FT pretreatment is challenging to implement because it involves liquid flowing through solids at controlled flow rates , elevated pressure (250 PSI) and high solids concentration (~25-40% solids). The purpose of this study is to propose a hot water pretreatment configuration for sugarcane bagasse that realizes the advantages of FT pretreatment at scale , addresses the mechanical complexities of arranging a bed of biomass for FT configuration and avoids unacceptably high energy requirements and sugar dilution. We evaluated the time and temperature effect on key performance metrics and measured key physical properties—pressure drop , viscous compaction and water retention. We carefully analyze the fluid dynamics of fluid flow through biomass and we use kinetic and process modeling to investigate several configurations—batch , co- , counter- and cross-flow. Compared to batch pretreatment , FT pretreatment consistently resulted in higher hemicelluloses recovery , higher removal of non-carbohydrate carbon and higher glucan solubilization by simultaneous saccharification and fermentation (SSF). Hemicelluloses removal trends were similar between feedstocks whereas glucan conversion trends were significantly different , suggesting that factors in addition to hemicelluloses removal impact amenability of glucan to enzymatic attack. Corn stover exhibited higher hydrolysis yields than bagasse or poplar , which could be due to higher removal of non-carbohydrate carbon. Hemicelluloses were more easily degraded in bagasse than in corn stover and poplar. The observed pressure drop using biomass knife-milled to pass through a 2 mm sieve was highest for bagasse , intermediate for switchgrass , and lowest for poplar. The highest pressure drop correlated with the presence of more fine particles , greater viscous compaction of the biomass and the degree of water absorption. Grasses and agricultural residues are mechanically more challenging than wood , absorbing more water , compressing more easily and containing smaller particle sizes. The process and kinetic model indicate what solids concentration and configurations will yield acceptable energy consumption and sugar dilution. 1. Mosier N , Wyman CE , Dale BE , Elander R , Lee YY , Holtzapple M , Ladisch M: Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresource Technology 2005 , 96:673-686. 2. Liu C , Wyman CE: The effect of flow rate of very dilute sulfuric acid on xylan , lignin , and total mass removal from corn stover. Ind Eng Chem Res 2004 , 43:2781-2788.
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