The term is generally used to describe Boilers in which steam is generated by firing coal, oil or gas in a furnace whose envelope consists of water-cooled tubes. Combustion products may be ducted over the tube banks providing superheat, reheat, economizing and combustion air-heating duties. This type of equipment is also known as a furnace-fired boiler to distinguish it from ones in which solid fuel is burned on a grate or stoker or in a fluidized bed, these being the other major classes of fossil fuel fired boiler.
The boilers can be classified in a variety of ways depending on the firing scheme, as above; the duty, whether for electricity generation (utility boiler) or industrial heat and/or power purposes; the geometrical arrangement, tower, two-pass. Coiled Tube, Water-tube, Shell and many others; or by the type of generation, recirculation, natural circulation, Once-through etc.
Combustion systems and the furnace geometry are chosen to suit the type of fuel being burnt. Coal composition varies so widely that different concepts need to be adopted dependent on the carbon/volatile ratio, the ash content and the calorific value (CV). For example, most bituminous and sub-bituminous coals mined in the northern hemisphere have high enough CV and low enough ash contents to be suitable for wall firing, whereas many southern hemisphere bituminous coals from India and South Africa have ash contents up to 45% and may require fluidized beds to sustain combustion.
In contrast, anthracite and some bituminous coals have such a low volatile content that downshot firing has to be adopted (as shown for example in Figure 1).