Dilute sulfuric acid catalyzed hydrolysis of biomass such as wood chips often involves pressing the wood particles in a dewatering step (e.g., after acid impregnation) or in compression screw feeders commonly used in continuous hydrolysis reactors. This study addresses the effects of pressing biomass feedstocks using a hydraulic press on soluble sugar yield obtained from two-stage dilute-acid hydrolysis of softwood. The pressed acid-impregnated feedstock gave significantly lower soluble sugar yields than the never-pressed (i.e., partially air-dried or filtered) feedstock. Pressing acid-impregnated feedstocks before pretreatment resulted in a soluble hemicellulosic sugar yield of 76.9% from first-stage hydrolysis and a soluble glucose yield of 33.7% from second-stage hydrolysis. The dilute-acid hydrolysis of partially air-dried feedstocks having total solids and acid concentrations similar to those of pressed feedstocks gave yields of 87.0% hemicellulosic sugar and 46.9% glucose in the first and second stages, respectively. Microscopic examination of wood structures showed that pressing acid-impregnated wood chips from 34 to 54% total solids (TS) did not cause the wood structure to collapse. However, pressing first-stage pretreated wood chips (i.e., feedstock for second-stage hydrolysis) from approximately 30 to 43% TS caused the porous wood matrix to almost completely collapse. It is hypothesized that pressing alters the wood structure and distribution of acid within the cell cavities, leading to uneven heat and mass transfer during pretreatment using direct steam injection. Consequently, lower hydrolysis yield of soluble sugars results. Dewatering of corn stover by pressing did not impact negatively on the sugar yield from single-stage dilute-acid pretreatment.
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