The required characteristics of ensiled crops used as a feedstock for biogas production: a review

Maize and grass silages are the main feedstock for anaerobic digestion in agricultural biogas plants. High-quality silage is necessary for high methane yields. Grasses should be cut and ensiled at leafy stages, until full heading, prior to an extensive lignification. Late ripening maize varieties should be harvested towards full ripening due to the increasing starch content in grains, and early to medium ripening varieties at the end of waxy ripeness. The substrate availability for methanogens is improved by fine chopping. Pretreatment processes of a thermal, chemical or biological nature attempting to disrupt lignocellulosic matter are economically demanding, including the application of enzyme hydrolysing structural polysaccharides. Application of lactic acid bacteria inoculants at ensiling seems to have an insignificant effect on methane yields. Some micronutrients necessary for methanogens growth are often deficient in the silages and particularly cobalt, nickel and iron should be supplemented. Maize silage has too low nitrogen content for methanogens growth. The high acidity of silage needs to be partially neutralised prior to anaerobic digestion.