3.4 The simplest kind of process, a plant that provides heat output only, is shown schematically in Figure 3-I. The fuel is burned with air in a combustion chamberiv. The hot gases produced by combustion pass into a heat exchanger, where they cool and transfer heat to another fluid. In the case of a heating plant, such as for district heating, this fluid is water that is pumped through the heat exchanger and circulated to distribute the heat. The cooled gases are then cleaned to remove particulates and other pollutants before being emitted to the atmosphere through a chimney or stack. Typically up to 90% of the Net Calorific Value of the fuel can be recovered as heat; the proportion is higher (and can exceed 100%!) if a condensing heat exchanger is used.

Figure 3-1 'Heat only' combustion plant

To stack

Bottom ash

3.5 Most of the non-combustible part of the fuel - primarily minerals - leaves the combustion chamber as bottom ash. Finer particles are conveyed out of the combustor and removed in the gas cleaning stage, along with any material injected to clean the gases, as fly ash. Bottom ash and fly ash are commonly handled and disposed of separately.

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