(390d) Significant Increase in the Production of Sugars from Pyrolysis of Agricultural Residues By Application of Iron Sulfate Pretreatment

Fast pyrolysis of lignocellulosic biomass produces very little sugar or anhydrosugars compared to pyrolysis of pure polysaccharides because naturally occurring alkali and alkaline earth metals (AAEMs) in biomass catalyze the fragmentation of pyranose and furanose rings. Sugar yields can be dramatically increased by pretreating the biomass with sulfuric acid prior to pyrolysis, which passivates the catalytic activity of the metals by converting them into thermally stable salts. However, these AAEMs are also important to the depolymerization of lignin. Their passivation has the unintended consequence of slowing the rate of volatiles release from lignin, which results in a transient melt phase that agglomerates and fouls fluidized bed pyrolyzers. To overcome this problem, an alternative catalyst that is active toward lignin depolymerization but not pyranose ring fragmentation is needed. Pretreatment of herbaceous biomass with ferrous sulfate was found to have the desired effects of both passivating AAEMs and introducing iron that is catalytic toward lignin depolymerization. Conventional nitrogen-blown pyrolysis of pretreated corn stover increased the reactor weight hourly space velocity (WHSV), defined as biomass feed rate (kg/hr) per fluidized bed mass (kg), from 1 hr^-1 with acid treatment to 4 hr^-1 with iron sulfate. Substituting ferrous sulfate for sulfuric acid as pretreatment of corn stover had an even more dramatic increase in WHSV when operating the pyrolyzer under air-blown (autothermal) conditions, increasing from 3 hr^-1to 13 hr^-1. Sugar yields from the pyrolysis of corn stover increased from 0.5 wt% to 13 wt% on a biomass basis – a 26-fold increase through the use of the ferrous sulfate pretreatment. These advantages were combined to increase volumetric sugar productivity by two orders of magnitude, from 30 g/L/hr for conventional untreated corn stover to 3000 g/L/hr for autothermal ferrous sulfate treated corn stover.