Chapter 4: Dilute Acid and Hydrothermal Pretreatment of Cellulosic Biomass

Biomass pretreatment remains one of the most pressing challenges in terms of cost-effective production of biofuels. We present a short summary of pretreatments that re based on dilute acid and water. Water-only and dilute acid pretreatments can be effective in producing sugars from hemicellulose along with a solid residue enriched in cellulose that can be digested to glucose with high yields. Generally speaking, dilute acid is often favored because it realizes higher yields than water alone and produces mostly monomeric sugars, but water-only pretreatment can reduce the consequences of hydrolyzate conditioning to remove inhibitors, employ lower cost reaction vessels, and avoid the challenges of adding acid. The performance of water-only systems correlates well with the severity parameter, while the modified severity parameter is an effective tool in analyzing dilute acid performance. Kinetic models have also been applied to describe sugar release profiles from dilute acid pretreatments, with the parameters fit to match the data, but the models are not robust in terms of a’ priori predictions of performance. Feedstock features can have a significant effect on performance, with lignin amounts and makeup and mineral contents having potentially large effects. Economic studies clearly show that pretreatment is an expensive operation with pervasive impacts on the costs of other steps. Thus, although dilute acid and water-only pretreatments appear to be frontrunners currently, much more must be done to understand and advance pretreatment technologies to realize really low costs and high yields that are essential to production of commodity products.