Chemical–mechanical pretreatment of wood: Reducing downsizing energy and increasing enzymatic digestibility

Abstract

A chemimechanical (CM) pretreatment method was devised, wherein wood chips are acid-treated to weaken the physical structure then disc-refined to produce a fibrous substrate. CM pretreatment was directly compared with a common dilute acid (DA) pretreatment method, wherein wood is mechanically downsized to a powder or fiber substrate and then acid-treated. It was hypothesized that the CM pretreatment sequence would reduce the energy required for size reduction and increase enzymatic digestibility of the pretreated substrate. By treating wood chips in a dilute sulfuric acid liquor before the mechanical downsizing step, the net specific energy (NSE) of disc-refining was reduced by up to 95%. At the optimal acid treatment and disc-refining conditions determined within this work, CM pretreatment could produce a highly digestible lignocellulose substrate (95% cellulose conversion) while requiring less than 100 kWh/tonne od NSE for mechanical downsizing. A comparison of CM and DA pretreated hardwood revealed that CM pretreatment produced a significantly more digestible substrate than DA pretreatment. Differences in the digestibility of CM and DA pretreated substrates were attributed to differences in physical structure. CM pretreatment produced a substrate that consisted primarily of single fibers and small fiber bundles, while DA pretreatment produced larger fiber bundles. Furthermore, the CM pretreated substrate had a more accessible pore structure, and an altered distribution of surface lignin.