FURNACE SIZING OF COAL POWER PLANT BASIC INFORMATION

The most important step in coal-fired unit design is to properly size the furnace. Furnace size has a first-order influence on the size of the structural-steel framing, the boiler building and its foundations, as well as on the sootblowers, platforms, stairways, steam piping, and duct work.

The fuel-ash properties that are particularly important when designing and establishing the size of coal-fired furnaces include

The ash fusibility temperatures (both in terms of their absolute values and the spread or difference between initial deformation temperature and fluid temperature)

The ratio of basic to acidic ash constituents

The iron/calcium ratio

The fuel-ash content in terms of pounds of ash per million British thermal units

The ash friability

These characteristics and others translate into the furnace sizes in Fig. 5-5, which are based on the six coal ranks shown in Table 5-2. This size comparison illustrates the philosophy of increasing the furnace plan area, volume, and the fuel burnout zone (the distance from the top fuel nozzle to the furnace arch), as lower-grade coals with poorer ash characteristics are fired.

 Figure 5-5 is a simplified characterization of actual furnaces built to burn the fuels listed in Table 5-2.

Wide variations exist in fuel properties within coal ranks, as well as within several subclassifications (e.g., subbituminous A, B, C), each of which may require a different size furnace.

Among the most important design criteria in large pulverized-fuel furnaces are net heat input in British thermal units per hour per square foot of furnace plan area (NHI/PA) and the vertical distance from the top fuel nozzle to the furnace arch.

Furnace dimensions must be adequate to establish the necessary furnace retention time to properly burn the fuel as well as to cool the gaseous combustion products. This is to ensure that the gas temperature at the entrance to the closely spaced convection surface is well below the ash-softening temperature of the lowest-quality coal burned.

Heat-absorption characteristics of the walls are maintained using properly placed wall blowers to control the furnace outlet gas temperature by removing ash deposited on the furnace walls below the furnace outlet plane.