**8.5 Biogas production**

Anaerobic digestion of biomass produces biogas, a renewable energy containing methane as primary constituent and a final solid nutrient rich residue. Stages of anaerobic digestion include hydrolysis, acidogenesis, acetogenesis and methanogenesis. In hydrolysis, the water splits into H<sup>+</sup> and OH− . Larger polymers such as proteins, fats and carbohydrates breakdown to smaller monomers such as amino acids, simple sugars, and fatty acids in presence of an acid catalyst. In acidogenesis, acidogenic bacteria further break down organic matter still too large for methane production. Acetogenesis is the formation of acetate by acetogens for further breaking down of the biomass to a point from where methanogens can further act and degrade the remaining material to generate methane as biofuel [64]. Dried cereal crop residues should not be directly injected into the biogas unit rather mixing of animal dung in partial combination is preferable to increase the biogas efficiency. However, maize silage can be directly used for biogas production [65]. Biogas generation technology is older than biofuel production technology. The methane production potential of wheat straw is of 0.145 to 0.39 m3 kg−1 and rice straw of 0.241 to 0.367 m3 kg−1 [15]. By 2030, grasses and cereals could be the primary source of biomass for the biogas plants across the globe [66]. **Table 4** enlists the major composition of bio-wastes from major crops.
