Page 276 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

252 8 Pre-combustion Air Emission Control

CO þ H 2 O ! H 2 þ CO 2 ð8:23Þ

The resultant hydrogen with other oxygen-containing functional groups, elimi-
nating the oxygen element and yielding hydrocarbon-like compounds.

8.4.5 Biogas

In addition to the thermochemical conversion approach, biomass can also be
converted into gaseous fuels by a biological approach called anaerobic digestion.
And the resultant gas is sometimes called biogas. It is well known that typical
biogas from a well-controlled anaerobic process contains 60 % CH 4 and 40 % CO 2
and other trace compounds.
Anaerobic digestion is a complex biochemical reaction that involves the fol-
lowing four steps.
• Hydrolysis:
It is the process where complex organic matter is decomposed into simple
organic molecules. It is done with the existence of water that splits the chemical
bonds of the organic matter.
• Fermentation:
It is also called acidogenesis, the process where carbohydrates are decomposed
by bacteria, enzymes, molds, or yeasts in the absence of oxygen.
• Acetogenesis:
It is the process where the products of the fermentation process are converted
into H 2 ,CO 2 , and acetate by acetogenic bacteria.
• Methanogenesis:
It involves the formation of CH 4 from acetate, H 2 , and CO 2 by methanogenic
bacteria.

In the anaerobic digestion process, the methanogenic bacteria are very sensitive
and are easily upset by sudden changes in temperature or pH, and toxic substances
such as arsenic, copper, and antibiotics.
There are mainly two types of anaerobic digesters:
• Mixed digester
• Plug-flow digester
Mixed digesters are usually employed for liquid feedstock and the latter for
semi-solid feedstock (about 13 % solids). In a mixed digester solids are kept in
contact with the bacteria for reaction and the mixing is usually maintained either
mechanically or by bubbling. Plug-flow digesters eliminate the need for mixing by
slowly moving the waste through a tube-shaped vessel. A lagoon is another com-
monly used method for waste treatment, where aerobic bacteria use oxygen to