Straw to biogas

Straw, wooden chips, harvest remains, sunflower husk, yellow old grass, tree branches, chicken dung with bedding, everything is suitable for biogas. To enable conversion, ligno-cellulosic material like straw and wooden chips needs to be pre-treated before being fed into a biogas plant. This is achieved by first cooking and then applying steam explosion.

At high temperatures, lignin effectively melts and is freed from the cellulose/hemicellulose framework. It can no longer hold the cellulose fibres together and shield the cellulose/hemicellulose from chemical or bacterial digestion. During the cooking process at high temperature 180 C and pressure (between 5 and 8 bar) the feedstock becomes fully saturated with water, filling the spaces between the fibres. After the feedstock heating process has been completed the mixture is released from the system and the pressure is spontaneously reduced from high pressure to atmospheric conditions in a matter of seconds. This causes the water trapped in the fibres to turn into steam with an explosion effect, breaking apart the fibres.

A typical straw treatment plant has the capacity 20 ton straw/day. It produces enough substrate for 800 kW biogas plant (19200 kWh electric energy per day). 1 straw gives 425 m3 of biogas. Parasitic power consumption is 45 kW electric and 240 kW heat.