Enhancement in Enzymatic Hydrolysis by Mechanical Refining of Pretreated Biomass

Cellulose is the most abundant natural polymer.  Therefore, cellulosic bioethanol has been identified as a fuel source that has tremendous potential to supplement the United States consumption of non-renewable transportation fuel sources.  Although cellulosic bioethanol is poised for market entry, several processing problems and high processing costs have has been identified as barriers to commercialization.  In order to make cellulosic bioethanol economically viable, these barriers (including high enzyme and raw material costs, and the natural resistance of biological degradation of a plant; “biomass recalcitrance”) need to be overcome.  This study looks at the effectiveness of refining (a mechanical post-treatment) in addition to the traditional pretreatment step to assist in overcoming the biomass recalcitrance barrier.  Laboratory scale refining was conducted using a disk refiner, a PFI mill, and a valley beater at different refining severities to determine which refining mechanisms allowed for the most substantial gains in enzymatic hydrolysis conversion.  Studies were conducted using green liquor pretreated hardwood, and also various lignin content kraft (NaOH + Na₂S) pretreated hardwood.  Initial results show that refining allows for significant improvements in enzymatic hydrolysis yield.  Enzymatic sugar recovery of alkaline pretreated hardwood increased from 55.9% to 81.9% with 5 FPU/g after mechanical refining.  Enzyme reduction of more than 50% was achieved at the same level of sugar recovery.  To investigate the reason for increased digestibility, pore volume and surface area were evaluated in a wet state using three independent analyses; Water retention value, Simons’ staining, and DSC thermoporometry methods. A strong correlation was found between sugar recovery and measured properties.