Page 203 - Advances in Eco-Fuels for a Sustainable Environment

Anaerobic digestion of various feedstocks for second-generation biofuel production 167

13.19, 19.56, 24.53, 31.76, and 64.58) for wastewater, and bagasse feedstock pro-
duced a better biogas yield than other C/N values. Achmad et al. [54] examined beef
cattle feces and water hyacinth as a feedstock for three different C/N ratios. Out of
these three, the highest methane content was noted by a C/N ratio of 30.

6.5 Need of biogas up gradation

For upgradation of biogas and to convert biogas to biofuel, contaminants such as
moisture, hydrogen sulfide, and carbon dioxide need to be removed as much as
possible.

6.5.1 Need for removing H 2 S from biogas
In the fermented slurry, the composition of biogas might change during the biogas
production process due to the presence of hydrogen sulfide (H 2 S), which is present
as a dissolved form. The use of the material having less sulfur, diluting the mixture
with water, and stirring the slurry vigorously are possible remedies to reduce
H 2 S. Vijay et al. [55] indicated that if the concentration of H 2 S is higher than 1%
in the composition of biogas used in the engines, the metal components will corrode,
and that the concentration of H 2 S in biogas lower than 1000ppm could be used in the
boilers to produce heat by Weiland et al. [56]. Zicari et al. [57] mentioned that the H 2 S
concentration limit is 100ppm for electric power production by IC engines. Com-
pressed natural gas (CNG) equivalent compressed biomethane (CBM) should have
a concentration of H 2 S below 16ppm, as it’s used as fuel for an automobile engine.
If natural biogas is needed, the H 2 S content must be reduced to a level of less than
0.006g/m.

6.5.2 Need for removing CO 2 from biogas
The power output from the IC engine is affected by the presence of the CO 2 content in
the biogas. The carbon dioxide present in the biogas can result in freezing problems at
valves and metering points due to the formation of ice. CO 2 removal from biogas may
result in improvement in the heating value of the gas by about 225% on a weight basis
and around 80% on a volume basis [52].

6.5.3 The need for removing moisture from biogas

Biogas with moisture may corrode the fuel supply system and the metallic parts of the
engine. Carbon dioxide chemically reacts with moisture to form weak acid, which
might corrode metal parts.