5.4 Pulverized Coal

Coal is an abundant and low-cost fuel for boilers. It can be burned in a number of ways depending on the characteristics of the coal and the particular boiler application. Various firing methods for the burning of coal are used: cyclone furnaces, stokers, pulverized coal, and fluidized bed. Of these applications, pulverized-coal firing is the dominant method in use today in terms of the quantity of coal burned. Pulverized-coal (PC) firing is predominant on the large, highly efficient utility boilers that are used to provide the base-loaded electric capacity in utilities throughout the world.

Pulverized-coal firing uses much smaller particle sizes, which allows high combustion rates, and, therefore, it is significantly different from the other coal combustion technologies. The combustion rate of coal is controlled by the total particle surface area. By pulverizing coal, the coal can be burned completely in 1 or 2 seconds. This is similar to burning oil and natural gas. By comparison, the other coal-firing technologies use crushed coal of various sizes and provide longer combustion times, up to 1 minute.

The pulverization of coal exposes a large surface area to the action of oxygen and consequently accelerates combustion. The increase in surface area, for a given volume, can be expressed by reference to an inch cube of coal. The cube has six faces, each having an area of 1 in ² a total of 6 in ² of surface. Now, if this 1-in cube of coal is cut into two equal...