Environmental Engineering Reference
In-Depth Information
17.3.1 Biogas as fuel for boilers
The production of heat (thermal energy) in boilers is the most common and
simple way of using biogas. The conversion efficiencies for heat production
from biogas are typically 75-85% (Krich et al., 2005). Conventional boilers
that are used for natural gas can be adjusted for biogas use by changing the
air-to-gas ratio and enlargement of the fuel orifice or burner jets. In
addition, the combustor needs to be modified in order to handle the high
flow rate of biogas, which is needed as it has lower energy content than
natural gas.
Boilers can use low-quality biogas for combustion. For successful
operation, the operating temperatures should be above the dew point in
order to prevent condensation. To prevent corrosion from H 2 S and water
vapour, all metal surfaces of the housing should be coated.
17.3.2 Biogas as an engine fuel
Use of biogas as fuel for electricity generation in gas engines is a
commercially available and proven technology all around the world
(Chambers and Potter, 2002; Krich et al., 2005; Deublein and
Steinhauser, 2008; US EPA CHPP, 2008). Most of the biogas installations
use spark-ignited natural gas or propane engines that have been modified to
operate on biogas. Diesel and four-stoke gasoline engines have also been
modified to use biogas. Waste heat
from engine operations is used
frequently in CHP applications.
17.3.3 Four-stroke engines
The present-day four-stroke biogas engines were originally developed for
natural gas and therefore can be used for biogas with little or no engine
modification. Four-stroke engines are available with capacity ranging from
a few kilowatts to 10MW with an approximate life span of 60 000 h and an
electrical efficiency of 35 - 40% (Deublein and Steinhauser, 2008). Four-
stroke engines with electrical efficiencies above 40% are generally equipped
with a recuperator (US EPA CHPP, 2008). The amount of fuel energy
converted to electricity generally increases with size, ranging from 30% for
small units to 40% for large engines. The amount of fuel converted to
thermal energy is 40-50%, resulting in overall efficiencies of 80-85%
(Chambers and Potter, 2002). In general, electrical conversion efficiency
decreases with an increase in the CO 2 concentration in the biogas and,
consequently, increased anti-knock properties. Classification of reciprocat-
ing engine types based on available rating is presented in Table 17.3.
Both spark ignition and diesel four-stroke engines complete a power cycle
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