Environmental Engineering Reference
In-Depth Information
avoided in the fuel delivery system. They also have better resistance against
sulphur compounds and lower exhaust emissions.
17.4 Conclusion and future trends
Biogas is produced during the anaerobic digestion of organic materials from
industry, municipalities and agriculture. The produced biogas is considered
a versatile renewable energy source and can be converted to heat and/or
electricity. However, biogas needs pre-treatment depending on the biogas
composition and equipment used. There are several commercially available
systems for treating biogas and utilising the cleaned biogas to produce heat
and/or electricity in stationary applications. Typically, biogas pre-treatment
involves the removal of foam, particulate matter and water vapour along
with H
2
S(
100 ppmv) and siloxanes.
Biogas power generation in stationary applications includes internal
combustion (IC) engines such as four-stroke spark ignition and diesel
engines, gas turbines, micro turbines, Stirling engines and fuel cells.
However, the total efficiency of equipment is dependent on the fuel
conversion capability. The conversion efficiency for IC engines is 25-42%,
for a Stirling engine, 25-30% and for a micro turbine 15-30%. However,
the most prevalent on-site simultaneous generation of power and heat for
biogas has traditionally been CHP plants. In CHP mode, there is not a great
difference in the total energy conversion efficiency (85-90%) for the range of
conversion options. However, the total CHP efficiency to electricity is
dependent on size and power-to-heat ratio and varies from one type of
equipment to another. IC engines (both four-stroke spark ignition and diesel
engines) can be coupled with a generator to produce electricity. The overall
efficiencies of 80-90% are achieved as heat can also be recovered, both from
the exhaust gas and from the engine cooling system. Similarly, micro turbine
or Stirling engine based CHP units have efficiencies of 85-90%. Fuel cell
technologies may achieve total CHP efficiency in the 65-75% range
depending upon the technology.
Other factors influence the choice of equipment, particularly investment
and maintenance costs, reliability and exhaust emissions. Total installed
costs for gas turbines, micro turbines, reciprocating engines and Stirling
engine are comparable. The total installed cost for a typical gas turbine (5-
40MW) ranges from
<
1500/kW and micro turbines in grid-
interconnected CHP applications range from
900/kW to
€
€
€
600/kW to
€
1200/kW.
Similarly, commercially available gas engines have total
installed costs
of
1100/kW. Fuel cells are currently the most expensive of
the existing CHP technologies with total installed costs ranging between
€
€
400/kW to
€
4000/kW. Operation and maintenance (O&M) costs
typically include routine inspections,
3000/kW and
€
scheduled overhauls, preventive
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