STOKERS

A stoker should not only be designed from the combustion point of view, but it must be mechanically strong to withstand all working stresses due to high temperature, etc. A simple design will ensure low first cost, minimum maintenance and operation for long periods without failure. Some of the factors to be aimed at in stoker design are: maximum rates of burning, highest continuous efficiency and the unlimited choice of fuels.

Any study of the use of stokers must begin with an analysis of the four principal constituents of coal, namely, moisture, volatiles, mixed carbon and ash, or more generally, water, tar, coke and dirt. These determine the features which should be embodied in the stoker and furnace equipments

so that proper treatment of the coal at the correct time is effected on its passage, through the furnace. Whichever of the two types be used the coal has to be taken from the bunkers to the feeding hoppers on the boilers. The coal falls by gravity from the bunkers through a valve into feeding chutes. In some installations automatic weighers are included in the downspouts between the cut-off valves and the boiler feed hoppers. The cut-off valves may be operated from the firing floor by means of chains. The chutes are one of two types namely, traversing and fixed.

There are usually two or three chutes for large boilers. The traversing chutes travel the full width of the feeding hopper, the motion being affected by means, of a continuously rotating screwed shaft which engages with a special nut attached to the chute. The operating shaft has right- and left-hand helical grooves and the nut is designed so that at the end of its travel it reverses automatically.

The chutes are operated from the stoker drive, there being two or four chutes for large boiler units. Coal chutes are of; welded mild steel plates, wearing plates also being included.

The spreader stoker is designed to throw coal continuously onto a stationary or moving grate. A spreader stoker is equipped with a moving grate which travels toward the feed­er mechanism and discharges the refuse continuously. Coal is fed from the hopper by means of a reciprocating feeder plate having a variable-speed drive which for best performance should be regulated automatically to feed coal in accordance with the demand for energy.

The coal is delivered by the feeder to a rapidly revolving drum or rotor on which are fastened specially shaped blades which throw the fuel into the furnace and distribute it uniformly over the grate. Coal can be distributed thus for a total distance of about 22 ft. The feeder mechanism is built in standardized widths, and several units may be installed across the front of the larger furnaces. Air is supplied by means of a blower to the space under the moving grate through an adjustable damper. The active fuel bed is normally not over 1,5 in. deep so that an adequate supply of air can penetrate the fuel bed and enter the furnace. Active fuel beds much thicker than 1,5 in. will produce excessive amounts of smoke. Much of the volatile matter is distilled from the coal before it strikes the fuel bed, and the caking properties of the fuel are thus destroyed, thereby making it possible to burn[9] eyen the strongly caking bituminous coals. Since the fuel bed is thin and undisturbed and the ash is cooled by the flow of air through it, trouble with clinkering or fusing of the ash is uncommon, and this stoker can burn almost any kind of bituminous coal. Since the finer sizes of coal are burned in suspension, large furnaces are required, and objectionable quantities of dust may be discharged from the installation it is not designed correctly and if dust collectors are not installed to clean the gases leaving the steam-generating unit. Also, it is standard practice to install high-velocity steam jets in the furnace to promote turbulence, improve combustion, and reduce smoke.

Large units provided with continuous ash-discharge grates are capable of burning 12 to 15 tons of coal per hr. Small units may, have stationary grates with clean-out doors through which the ashes may be removed manually with a hoe, or they may have dump grates operated by a power cylinder in which grate sections may be tilted periodically to dump the ashes.

The spreader stoker is simple in construction and reliable in operation. It can burn a wider variety of coal successfully than any other type of stoker. Maximum continuous combustion rates of 45 to 60 psf of grate area per hr are normally used. When provided with automatic regulation of fuel and air in accordance with the demand for energy, this stoker is very responsive to rapidly fluctuating loads.

However, it is not so adaptable to light-load operation as other types of stokers because of the difficulty of maintaining ignition and combustion in the very thin fuel bed with a cold furnace. It is because of the thin fuel bed and the continuous, uniform firing of coal that the spreader stoker overcomes the smoke-producing problem associated with the thick intermittently hand-fired fuel bed.