Environmental Engineering Reference
In-Depth Information
However, the most prevalent on-site simultaneous generation of power and
heat for biogas has traditionally been CHP plants and this trend is likely to
grow. Approximately 50% of the CHP plants installed in biogas plants in
Europe are operated with four-stroke engines and about 50% with ignition
oil diesel engines (Deublein and Steinhause, 2008; Kallio, 2010). Most of the
engines can be operated without biogas purification if the level of sulphur
compounds is low enough (100 ppmv). On the other hand, some gas motors
require more or less upgraded biogas. The total efficiency of modern CHP
plants (i.e. the sum of electrical and thermal efficiencies) is within the range
85-90% (Kallio, 2010). Only 10-15% of the energy of the biogas is lost.
However, the electrical efficiency (maximum 40%) is still low, and only
2.4 kWh of electric power can be produced from 1m
3
of biogas (Deublein
and Steinhause, 2008). It is thus important that a thermal demand is in close
proximity to the facility.
The economics of on-site CHP applications are enhanced by effective use
of recovered heat generated by the engine jacket and exhaust gas.
Approximately 60-70% of the engine fuel energy can be recovered through
this waste heat, for example by recovering hot water for process heat, pre-
heating boiler feed water, space heating, anaerobic digester heating and
digestate processing. The heat generated in a CHP plant is usually recovered
through the exhaust heat exchanger. However, only a part of the heat
generated is regained due to high exhaust gas temperatures of 120-180
C
(Deublein and Steinhause, 2008). However, some heat loss in the form of
radiation cannot be avoided. Engines with a turbocharger are generally
equipped with an intercooler, while gas engines are equipped with a mixture
radiator.
Depending on the design, the heat generated in these engines is
transferred to the cooling water or to a separate water cycle. The water in
the cycle, which transports the heat from the biogas burner to consumers, is
normally heated to 90-130
8
8
C and flows back to the burner at a temperature
of 70-110
C (Deublein and Steinhause, 2008). One drawback of gas-driven
systems is that the engines are said to require much more maintenance than
an electric motor. Figure 17.2 shows a CHP system that uses a diesel engine
for combustion of the biogas with recovery of heat from the engine coolant,
engine oil circulating system and exhaust manifold.
8
17.3.5 Generation of electricity in a Stirling engine
The principle of a Stirling engine is based on the conversion of thermal
energy into mechanical energy (Chambers and Potter, 2002). The Stirling
engine is a closed system in which gas or fluid moves inside the engine
between cold and heat exchangers. As a result, the engine medium is heated
and cooled alternately, resulting in cyclic compression and expansion and,
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