Torrefaction at low temperature as a promising pretreatment of lignocellulosic biomass in anaerobic digestion

The lignocellulosic structure of agricultural biomass, which makes it resistant to microbial attack, is the main obstacle in its anaerobic digestion. However, not all biomasses behave the same in the face of anaerobic digestion. Instead, depending on their composition in terms of lignin, cellulose and hemicellulose content, the anaerobic digestion process may be more or less favoured, although a pretreatment stage is usually necessary. This paper presents the results of a laboratory scale batch experiment to study the effect of thermal pretreatment (torrefaction) on the anaerobic digestion of two biomass materials: barley straw and vine shoot. For this, thermal pretreatment temperatures of 100, 130, 180 and 210 degrees C, with a residence time of 24 h, were studied for both biomasses and the results were compared with the material without pretreatment. Significant differences in biogas production were observed for both biomasses depending on the pretreatment temperature. The highest biogas yields of 458 and 213 mL/gVS for straw and vine shoot, respectively, were observed from biomass pretreated at 100 degrees C. Barley straw pretreated at 100 degrees C showed 275% higher methane yield compared to untreated straw. In the case of the vine shoot, the increase was 210%. Furthermore, FTIR-ATR analysis and thermogravimetric analysis coupled with a pseudocomponent kinetic model revealed changes in the lignocellulosic composition of both biomasses while SEM analysis revealed also structural changes. The modified Gompertz model fitted the production data well and predicted shorter lag time and higher biogas production at pretreatment temperatures from 100 to 180 degrees C compared with the untreated materials. From there, a reduction in methanogenic activity is observed when the pretreatment temperature increases. In general, the change in the lignocellulosic structure and the decrease in the hemicellulose content could be considered as the main reasons to improve the biogas production.