Power Plant Engineering

FUELS AND COMBUSTION 223

The spreader stoker is the most widely used for steam capacities of 75,000 to 400,000 lbm/h
(9.5 to 50 kg/s). It can burn a wide variety of coals from high-rank bituminous to lignite and even some
by-product waste fuels such as wood wastes. pulpwood, bark, and others and is responsive to rapid load
changes. In the spreader stoker coal is fed from a hopper to a number of feeder-distributor units, each of
which has a reciprocating feed plate that transports the coal from the hopper over an adjustable spill
plate to an overthrow rotor equipped with curved blades. There are a number of such feeder-distributor
mechanisms that inject the coal into the furnace in a wide uniform projectile over the stoker grate
(Fig. 7.2). Air is primarily fed upward through the grate from an air plenum below it. This is called
undergrate air. The finer coal particles, between 25 and 50 percent of the injected coal, are supported by
the upward airflow and are burned while in suspension. The larger ones fall to the grate and burn in a
relatively thin layer. Some air, called overfire air, is blown into the furnace just above the coal projectile.
Forced-draft fans are used for both undergrate and overfire air. The unit has equipment for collecting
and reinjecting dust and controls for coal low and airflow to suit load demand on the steam generator.

The problem with stationary spreader stokers was the removal of ash, which was first done manually
and then by shutting off individual sections of grates and their air supply for ash removal without affect-
ing other sections of the stoker. The spreader stoker became widely accepted only after the introduction
of the continuous-ash-discharge traveling-grate stoker in the late 1930s. Traveling-grate stokers, as a
class, also include the so-called chain-grate stoker. They have grates, links, or keys joined n an endless
belt that is driven by a motorized sprocket drive at one end and over an idle shaft sprocket mechanism at
the other. Coal may be injected in the above manner or fed directly from a hopper onto the moving grate
through an adjustable gate that -regulates the thickness of the coal layer. Ash is discharged into an ash
pit at either end depending upon the direction of motion of the traveling grate.

Continuous-cleaning grates that use reciprocating or vibrating designs have also been devel-
oped, as have underfeed stokers that are suitable for burning special types of coals. The continuous-ash-
removal traveling-grate stoker, however, has high burning rates and remains the preferred type of stoker.

Coal Hopper

Feeder

Stroke

Chain

Ash Hopper

Air Seal

Overthrow

Rotor

Overfire

Air

Overfire

Air

Air Seal

Fig. 7.2. Feeder Distributor Mechanism.