Techno-economic Feasibility of Extrusion as a Pretreatment Step for Biogas Production from Grass

Grass clippings have a good biomethane potential and, if acquired from roadside verges, nature management or natural grasslands do not compete for arable land, avoiding the food versus fuel debate. However, before the grass is processed in a wet anaerobic digester, a pretreatment step is advisable to minimize the problems associated with its fibrous nature. In this study, the effects of a semi-industrial extrusion pretreatment on fresh and ensiled grass were investigated through an energetic and economic assessment. Extrusion improved the mixing properties of the grass feedstock and reduced the formation of a floating layer even at a solid concentration of 10% (w/v). This pretreatment also enhanced the biomethane potential of ensiled grass and fresh grass by, respectively, 18 and 11% on a fresh matter basis, while shredding reduced this value by 14% when compared to fresh grass. This was attributed to changes in the volatile solids (VS) content of the treated samples, as all conditions resulted in similar biomethane yields when calculated per ton of VS, ranging from 325.5 to 337.6 Nm3 CH4/ton VS. However, ensiling resulted in a longer lag phase during biogas production attributed to the leaching of readily available sugars from the ruptured plant cells; nevertheless, this is not expected to be significant in a buffered industrial system. The revenue resulting from the extrusion treatment, between €6 and €17 per tonne of FM, compensated the cost of this additional step, indicating that extrusion would be a techno-economically sound process for the anaerobic digestion of grass.