Overall Efficiency of a Combination Boiler

The combined cycle is also appHcable to dedicated power production. When the steam from the waste heat boiler is fed to a condensing turbine, overall conversion efficiencies of fuel to electricity in excess of 50% can be achieved. A few pubHc utihty power plants use this cycle, but in general utihties have been slow to convert to gas turbines. Most electricity is generated by the cycle shown in Figures 5d and 6d. 

Smaller fuel cells (<20 kW, but more typically <5 kW) are being developed for residential applications, particularly in Japan and Europe. These fuel cells use natural gas as fuel (therefore they are equipped with a reformer). Most often they also enable capture and utilization of waste heat and use it for space and/or hot water heating in combination with a natural gas boiler, resulting in high overall efficiency. 

Frequently it will be appropriate to oversize the condensation stages of machines which have to be started up at a time when steam generated from process heat is not yet available, i.e. when the entire steam output has to be provided by fired steam boilers. This reduces the steam rate required to start up the plant and ensures that the steam boiler plant can be dimensioned so that it meets the normal continuous steam demand of the plant. However, it will be desirable as a rule to use bleeding and backpressure turbines which are not only much more cost-effective than condensation turbines, but which also improve the overall energy efficiency considerably owing to the combination of energy, process steam and heating steam. 

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