How Energy Is Produced - The Renaissance of Renewable Energy - page 132

why biogas plants currently rely on wet biomass from agriculture (e.g., maize, sorghum,
potatoes), municipal and industrial organic wastes, sewage, slurry, and slaughterhouse
waste.
Artificially produced biogas can be used in much the same way as natural gas. A biogas
power plant consists of four sections. The raw fuel (known as feedstock) is stored in
trenches and silos. This is fed into a digester, a sealed tank where bacteria break down the
organic matter in a fermentation reaction to produce biogas. This biogas is first treated to
reduce humidity, and to remove sulphur and other trace pollutants, then piped to the power
station. Here thebiogas fuels an internal combustion engine. Obviously, the energy output
of biogas is much lower than that of natural gas since half of it comprises carbon dioxide,
which doesn'tburn.The engine shaft isconnected toagenerator which produces electricity
(see Figure 4.44 ). Some of the heat produced through combustion is pumped back into the
digestion tank to maintain the warm temperature. Digested material (known as digestate) is
regularly removed from the tank. Properly treated, this is a valuable agricultural fertilizer
(Al Seadi and Lukehurst 2012 ; Braun et al. 2009 ; Murphy et al. 2011 ; Petersson and
Wellinger 2009 ) .
Figure 4.42. Maize and food waste such as potatoes and tomato skins for anaerobic
digestion. In most biogas power plants slurry or sewage is added to aid digestion. Source:
Gian Andrea Pagnoni.