Spreader stokers

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 1/2 in. deep so that an adequate supply of air can penetrate the fuel bed and enter the furnace. Active fuel beds much thicker than 1112 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

even 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 if it is not designed correctly and if dust collectors are not installed lo 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. 1t 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.