Page 256 - Synthetic Fuels Handbook

242 CHAPTER EIGHT

Peteves, 2005). Biofuels are generally more environmentally benign than traditional fuels
and are further defined as being renewable, meaning that the feedstock used to make a
particular biofuel can be replenished at a rate equal to or faster than the rate at which the
biofuel is consumed.

8.4.1 Gaseous Fuels
Gaseous fuels are those fuels that are in the gaseous state under ambient conditions. In some
circumstances, the definition of gaseous fuels may also include the low-boiling hydrocar-
bons such as pentane but, for the purpose of this text, such fuels are considered to be liquid
fuels.
Biogas is a clean and renewable form of energy and the most important biogas compo-
nents are methane (CH ), carbon dioxide (CO ), and sulfuric components (H S) (Coelho
4
2
2
et al., 2006). The gas generally composes of methane (55–65 percent), carbon dioxide
(35–45 percent), nitrogen (0–3 percent), hydrogen (0-1 percent), and hydrogen sulfide
(0–1 percent) (Table 8.2) (Anunputtikul and Rodtong, 2004).
TABLE 8.2 Composition of Biogas
% v/v

) 55–65
Methane (CH 4
Carbon dioxide (CO ) 35–45
2
Hydrogen sulphide (H S) 0–1
2
Nitrogen (N ) 0–3
2
Hydrogen (H ) 0–1
2
Oxygen (O ) 0–2
2
Ammonia (NH ) 0–1
3
Biogas could very well substitute for conventional sources of energy (i.e., fossil fuels)
which are causing ecological-environmental problems and at the same time depleting at
a faster rate (Santosh et al., 2004). Due to its elevated methane content, resultant of the
organic degradation in the absence of molecular oxygen, biogas is an attractive source of
energy. The physical, chemical, and biological characteristics of the manure are related
to diet composition, which can influence the biogas composition (Mogami et al., 2006).
Natural gas is about 90 to 95 percent methane, but biogas is about 55 to 65 percent meth-
ane. So biogas is basically low-grade natural gas (Speight, 2007). The biogas composition
is an essential parameter, because it allows identifying the appropriate purification sys-
tem, which aims to remove sulfuric gases and decrease the water volume, contributing to
improve the combustion fuel conditions (Coelho et al., 2006). Biogas has a heat value of
3
approximately 5.0 to 7.5 kWh/m (Table 8.3).
TABLE 8.3 Typical Properties of Biogas

Density (dry basis) 1.2 kg/m 3
Heat value 5.0–7.5 kWh/m 3
Ignition point 700°C
Ignition concentration gas content 6–12%