Diffusion of sulfuric acid within lignocellulosic biomass particles and its impact on dilute-acid pretreatment

Intra-particle diffusion of sulfuric acid into sugarcane bagasse, corn stover, rice straw and yellow poplar was investigated to determine the effective diffusivity of sulfuric acid within the porous biomass structure. Diffusion experiments were conducted over 25–75 °C for two different biomass sizes using dynamic diffusion test cells. Diffusivities of sulfuric acid in agricultural residues were significantly higher than those of hard wood. Diffusivity data for each biomass were fitted into the Arrhenius equation for extrapolation to higher temperatures. The diffusivity data were subsequently incorporated into a theoretical model to determine acid profile within the biomass matrix. The modeling results indicate that intra-particle diffusion of acid influences the rate of dilute-acid pretreatment if unground biomass feedstock is used under normal pretreatment conditions. A criterion was set up to determine the critical biomass size at which the intra-particle acid diffusion becomes a rate-influencing factor for a given pretreatment condition.