Ash, Slag, and Clinker Formation
Ash is the noncombustible mineral residue that remains in a furnace or boiler after the fuel has been thoroughly burned. During the combustion process, the combustible portion of the fuel is consumed and the noncombustible ash remains in place. As the process continues, the ash residue is subjected to a certain degree of shrink- age. This shrinkage results in portions of the ash fusing together and forming more or less fluid globules, or slag. This process is referred to as slagging.
Under the proper temperature conditions, the fluid slag globules can solidify and form clinkers in the fuel bed or deposits on the heating surfaces of the furnace or boiler. Clinkers can obstruct the necessary air flow to the fire and result in its reduced efficiency or extinction. These deposits on the heating surfaces will reduce their heating capacity and must be removed. Suggestions for removing clinkers are given in Chapter 3 of Volume 2 (“Coal-Firing Methods”).
Soot
Soot is a fine powder consisting primarily of carbon produced by the combustion process. Because of its extreme light weight, it frequently rises with the smoke from the fire and coats the interior walls of the chimney and flue. Although the heat loss from the insulating effect of a soot layer is small (generally under 6 percent), it can cause a considerable rise in the stack temperature. Soot accumulation can also clog the flues, thereby reducing the draft and resulting in improper combustion. Soot may be blasted loose from the walls of the chimney or flue with a jet of compressed air, or it may be sucked out with a vacuum cleaner. Another method is to use a brush to remove the accumulated soot layer from the walls.