A kiln is a device to which heat is applied to bring about physical and chemical changes in materials. It is, therefore, a type of Furnace and the two terms are sometimes used to describe very similar pieces of equipment. In general, the word "kiln" is applied to devices for thermal processing of nonmetallic solids, and is particularly associated with the ceramic, cement and lime industries. The three principle types are vertical shaft kilns, rotary kilns and periodic kilns.
As shown in Figure 1, a vertical shaft kiln is a refractory-lined tower fitted with peripheral fuel burners and air inlets in the lower section, and gas exhaust ports in the upper. Material to be processed is introduced at the top and discharged through the base so that gases and solids are in countercurrent flow and some preheating occurs at the top section. Kilns of this type have been used for centuries for calcining limestone and in the manufacture of cement. They are still employed for the same purpose although in some modern plants, they have been replaced by fluidized-bed calcining plant.
A rotary kiln, as shown diagramatically in Figure 2, is a long refractory-lined inclined tube slowly rotated to ensure the steady flow of solid material from the feeder at the higher end to the collector at the lower. A burner at the lower end produces hot gases which pass in countercurrent flow to the moving solids. The fuel may be pulverized coal, gas or oil, or a combination of these.
The tube is fitted externally with steel hoops or "tires" supported by rollers or "trunnions," and is rotated by an electric motor geared to a toothed ring. The rotational speed of cement or lime kilns is usually about 1 rpm and the inclination of the tube about 1 in 20. Improved heat transfer and longer residence time — which may be required for calcining some ores — can be obtained by reducing the inclination and increasing the rotational speed, but at the cost of lower throughput.
Rotary kilns are widely used for thermal processing of bulk materials such as cement and lime, for calcining or agglomeration of minerals, rocks and ores, clays and shales, and for incineration or pyrolysis of combustible wastes. They are produced in a range of sizes and units with diameters up to 6 m and lengths up to 200 m have been made.
Because of their poor thermal efficiency rotary kilns, particularly large ones, are often fitted with energy-saving devices. For example, the combustion air may be preheated by channeling it through the hot material leaving the kiln and the hot gases leaving the kiln may be diverted to preheated feedstock, or to raise steam in a waste heat boiler.
This title covers a wide range of batch-type thermal processing kilns, all having the characteristic that the material passes through a cycle of heating, soaking and cooling. Basically there are two categories: one in which the material is stationary and the kiln temperature varies with time; the other in which the material passes through the kiln where temperature varies with position. An example of the first category is the small muffle furnace, often electrically heated, used by potters. An example of the second category is the tunnel kiln. The latter is a long tunnel with cold air entering at one end, being heated by burners in the central region, and leaving as hot gas at the other end. The material to be processed, usually ceramics, is carried by moving refractory-based cars going in the opposite direction to the gas and in their passage through the tunnel their contents are subjected to the required three thermal phases, preheating, soaking and cooling.