Biomedical Engineering Reference
In-Depth Information
total electricity output [
94
]. In Sweden, natural gas is partially replaced by biogas
and then used as vehicle fuel gas, in which the biogas proportion is about 45 %.
There are more than 4,000 biogas-powered cars now, and the first biogas-powered
train has already been developed [
95
]. In Britain, biogas can replace 25 % of
the country's gas consumption. The United States established a large number of
methane automated factories utilizing livestock manure after the energy crisis in
1973. There are also 31 biogas plants running successfully, such as the biogas
project in Iowa State, which can treat about 1,500 livestock's waste per day [
96
].
China has been testing applications of biogas since the 1950s. Rural household
biogas digesters are common in China. The volume of these digesters is 5-12 m
3
,
and the rate of gas yield per unit volume of digester reaches 0.1-0.4 m
3
day
1
.
There are also about 2,000 medium-size biogas projects, which can produce 5.5-
10
9
m
3
biogas annually. Since 2000, considering the situations and experiences
in different places, the Ministry of Agriculture has developed several projects
throughout the country [
97
,
98
].
m
3
6.4.2
Physiological and Biochemical Processes of Biogas
Fermentation
Biogas fermentation is a complex process. It can be artificially divided into three
basic stages. Initially, the hydrolytic and fermenting bacteria convert organic
materials to small molecule water-soluble compounds, such as monosaccharides,
amino acids, glycerin and fat. This stage, hydrolysis, is usually slow and is
considered to be the rate-limiting stage of solid organic matter degradation and
anaerobic fermentation of suspended solids in wastewater. Second, the hydrogen-
producing acetogenic bacteria and the homoacetogenic bacteria convert the products
of the first stage to acetic acid, hydrogen, carbon dioxide, and so on. Finally, the
methanogenic bacteria convert acetic acid, CO
2
, and H
2
into methane. More than
70 kinds of methanogenic bacteria have been studied; they can take advantage of
small molecules, such as formic acid, acetic acid, hydrogen and carbon dioxide,
generated by the other stages to produce methane gas.
The reaction rate is different depending on the substrate. For example, for
lignocellulose, hydrolysis is usually the rate-limiting stage. To sugars, starch,
proteins, and other soluble substances, because they can be rapidly decomposed by
microorganisms, the third stage is often the rate-limiting step. The anaerobic diges-
tion process can be divided into three stages, which are carried out simultaneously
in the anaerobic reactor system and maintain a dynamic equilibrium. If the dynamic
equilibrium is broken by the external factors, such as pH value, temperature, or
organic load, the fragile methanogenic bacteria tend to be affected first. Thus, fatty
acids accumulate gradually and finally lead to acidification and stagnation of the
system.
