Power Plant Engineering

7.5 MECHANICAL STOKERS
Almost all kinds of coal can be fired on stokers. Stoker firing, however, is the least efficient
of all types of firing except hand firing. Partly because of the low efficiency. stoker firing is limited to
relatively low capacities, usually for boilers producing less than 400,000 lbm/h (50 kg/s) of steam,
though designers are limiting stoker use to around 100,000 lbm/h (12.6 kg/s). These capacities are the
result of the practical limitations of stoker physical sizes and relatively low burning rates which require
a large furnace width for a given steam output. Pulverized and cyclone firing, on the other hand, have
higher burning rates and are flexible enough in design to meet the millions of pounds per hour of steam
requirements of modern steam generators with narrower and higher furnaces. Stokers, however, remain
an important part of steam generator systems in their size range.
Mechanical stokers are usually classified into four major groups, depending upon the method of
introducing the coal into the furnace. These are spreader stokers underfed stokers, vibrating-grate stok-
ers, and traveling-grate stokers.
* There are some seven screen, or sieve, standards in the United States and Europe. The one used here : the
U.S. Standard Sieve, in which the number of openings per linear inch designates the mesh. A 100 mesh screen has
100 openings to the inch, or 10,000 openings per square inch. The higher the mesh, the finer the screen. The
diameter of the wire determines the opening size.

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FUELS AND COMBUSTION
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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