Enhanced biogas production from rice straw, triticale straw and softwood spruce by NMMO pretreatment

Softwood spruce (chips and milled), rice straw and triticale (a hybrid of rye and wheat) straw, were pretreated with N-methylmorpholine-N-oxide (NMMO or NMO) prior to anaerobic digestion to produce biogas. The pretreatments were performed at 130 °C for 1–15 h, and the digestions continued for six weeks. The digestions of untreated chips (10 mm) and milled (<1 mm) spruce, rice straw and triticale straw resulted in 11, 66, 22 and 30 Nml CH4/g raw material. However, the pretreatments have improved these methane yields by 400–1200%. The best digestion results of the pretreated chips and milled spruce, rice straw and triticale straw were 125, 245, 157 and 203 Nml CH4/g raw material (or 202, 395, 328 and 362 Nml CH4/g carbohydrates) respectively, which correspond to 49, 95, 79 and 87% of the theoretical yield of 415 Nml CH4/g carbohydrates. Although the experiments were carried out for six weeks, one and a half weeks was enough to digest the materials.