Characterization of Mechanically Pretreated Wheat Straw for Biogas Production

Biogas production from wheat straw is one strategy of converting biomass to bioenergy, but pretreatment of the lignocellulosic material is necessary in order to render the substrate biodegradable. In this study, four different mechanical pretreatments of wheat straw were investigated: roll milling, extrusion, pelletization, and hammer milling. The effects of the pretreatments on anaerobic digestion (AD) at 37 °C and on enzymatic hydrolysis (EH) with Cellic CTec2 at 50 °C were analyzed in terms of biochemical methane potential (BMP), maximum daily methane production (DMPmax), and EH yield as amount of released glucan/initial glucan content. Roll milling generated the highest BMP of 287 NmL CH4 gVS−1, an increase of 21% compared with untreated wheat straw. Extrusion provided the highest methane production rate (52 NmL CH4 gVS−1 day−1), the lowest floating capacity, and a high bulk density. It was further concluded that a linear relationship exists between the DMPmax and the EH yield (R2 = 0.768) which for roll-milled size fractioned wheat straw was even stronger (R2 = 0.900) but weaker for mechanically pretreated non-size fractioned straw (R2 = 0.625). Specific surface analysis and EH of 0.25–1-mm particles revealed that mechanical pretreatment most likely affect other physiochemical properties in wheat straw, such as the crystallinity or melting of the outer wax layer, which in turn affects AD and EH processes differently.