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In a reciprocating engine power plant heat can be recovered from lubrication oil and engine cooling water as well as from exhaust gases as shown in the following figure. The electricity generating efficiencies of reciprocating engines vary from 35 to 42 %. In cases, where low nitrogen oxide (NOx) emissions are required by legal standards, the efficiencies that can be reached are about one percentage lower. |
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Figure
7.
Reciprocating Engine Power Plant.
Some of the waste heat from the engine, such as waste heat from generator cooler, can normally not be used for useful heat generation due to low temperature level.
Because modern engines have rather cool exhaust gases (approximately 400 oC), only part of the heat can be recovered in the form of steam. For example, a diesel engine generating 4,2 MW of electricity can generate about 1,5 MW of steam and about 3,1 MW of warm and hot water. Since the fuel consumption is about 10,0 MW, the total efficiency of the plant in this example is 88 %.
When using supplementary firing, about 9,5 MW of steam or district heat can additionally be generated with the remaining oxygen in flue gases. This steam or district heat, however, cannot be considered as CHP steam.