Mechanical pretreatment of lignocellulosic biomass to improve biogas production: Comparison of results for giant reed and wheat straw

The effects of a mechanical pretreatment were determined on the methane yield of giant reed stems (Arundo Donax) and wheat straw. The feedstocks were pretreated using a two stages dry milling device, whose working capacity - up to 1,2 t h−1- can meet the requirements of medium and large-scale biogas plants. Untreated and pretreated materials were anaerobically digested in batch reactors under mesophilic conditions for 28 days. The cumulative biogas production from the pretreated giant reed stems exceeded 212 Nm3 t−1 of volatile solids, showing a 137% gain as compared to the raw material; furthermore, a significant decrease in the acid detergent fiber content was observed in the processed material. Pretreated straw reached a cumulative methane yield of 250.3 Nm3 t−1 of volatile solids, gaining a 49.1% as compared to the feedstock; no statistically significant differences were observed in physicochemical composition following pretreatment. The aforementioned gains resulted far above the known range for mechanical pretreatments. The specific electric energy requirement for pretreatment decreased from 76.5 kWh t−1 of processed giant reed stems to 66.0 kWh t−1 for straw. With reference to the use of biogas in CHP plants, the net electric energy output from pretreated materials was enhanced by about 111.7% and 38.3% as compared to the feedstocks for reed and straw, respectively. The cost for the extra electric energy produced by pretreatment was estimated at 0.034 and 0.048 € kWh−1 for giant reed and straw, respectively, resulting fully compatible with the selling price for the electric energy produced from biogas in force in Italy.