Comparison of Detailed Characteristics of Substrates Produced By Pretreatment of Lignocellulosic Biomass in Flowthrough and Batch Reactors with Liquid Hot Water and Extremely Dilute Acid

Flowthrough pretreatment, unlike batch operations, continuously removes hemicellulose and lignin fractions solubilized in liquor during pretreatment, thereby preventing re-association on the lignocellulosic surface. Its application provides valuable insights into the behavior of hemicellulose and lignin fractionation in aqueous pretreatments because the deconstruction products from the pretreatment liquor in flowthrough pretreatment can be tracked over time. Thus, to understand the impact of batch vs. flowthrough pretreatment on the components solubilized, resulting biomass solids features, and their biological conversion, pretreatment of Populus hardwood was performed at 140 °C for 192 minutes and 180 °C for 12 minutes in liquid hot water (LHW) and extremely dilute sulfuric acid (DA, 0.05 wt%) in batch and flowthrough reactors. DA enhanced sugar and lignin recovery compared to LHW at all pretreatment conditions. Maximum xylan recovery was 88% for DA flowthrough pretreatment at 180 °C for 12 minutes. Flowthrough pretreatment recovered approximately 64% of the lignin from biomass solids in LHW and DA conditions whereas batch pretreatment recovered only 13% and 18% lignin in LHW and DA, respectively, at 180 °C. 2D HSQC NMR and GPC of the resulting pretreatment liquor showed that guaiacyl (G) lignin in Populus was released in the early stages, while syringyl (S) lignin release appeared in liquor throughout flowthrough pretreatment. On the other hand, batch pretreatment released only traces of G lignin in the liquor, suggesting re-condensation of hydrolyzed G lignin onto biomass. Enzymatic hydrolysis at low and moderate enzyme loadings presented 20% and higher digestibility of cellulose in flowthrough pretreated biomass compared to batch pretreated biomass. We also found a strong correlation between biomass accessibility measured through Simons’ staining and lignin removal for both of the two pretreatment configurations. The findings in this study help in better understanding of the changes occurring during aqueous pretreatment of lignocellulosic biomass.