222 POWER PLANT ENGINEERINGPulverized-coal firing was introduced in the 1920s and represented a major increase in combus-
tion rates over mechanical stokers. It is widely used today. To prepare the coal for use in pulverized
firing, it is crushed and then ground to such a fine powder that approximately 70 percent of it will pass
a 200 mesh sieve* (Fig. 7.1). It is suitable for a wide variety of coal, particularly the higher-grade ones.
Advantages of pulverized coal firing are the ability to use any size coal; good variable-load response; a
lower requirement for excess air for combustion, resulting in lower fan power consumption; !lower
carbon loss; higher combustion temperatures and improved thermal efficiency; lower operation and
maintenance costs; and the possibility of design for multiple-fuel combustion (oil, gas, and coal).
In the late 1930s cyclone furnace firing was introduced and became the third major advance in
coal firing. It is now also widely used though for a lesser variety of uses than is pulverized coal. In
addition to those advantages already mentioned for pulverized-coal firing, cyclone firing provides sev-
eral other advantages. These are the obvious savings in pulverizing equipment because coal need only
be crushed, reduction in furnace size, and reduction in fly ash content of the flue gases. Coal size for
cyclone furnace firing is accomplished in a simple crusher and covers a wide band, with approximately
95 percent of it passing a 4-mesh sieve (Fig. 7.1).
Most recently, fluidized-bed combustion has been introduced. In this type of firing, crushed par-
ticles of coal are injected into the fluidized bed so that they spread across an air distribution grid. The
combustion air, blown through the grid, has an upward velocity sufficient to cause the coal particles to
become fluidized, i.e. held in suspension as they burn. Unburned carbon leaving the bed is collected in
a cyclone separator and returned back to the bed for another go at combustion. The main advantage of
fluidized-bed combustion is the ability to desulfurize the fuel during combustion in order to meet air
quality standards for sulfur dioxide emissions. (Other methods are the use of low-sulfur coal,
desulfurization of coal before it is burned, and removal of SO 2 from the flue gases by the use of scrub-
bers). Desulfurization is accomplished by the addition of limestone directly to the bed. Fluidized-bed
combustion is still undergoing development and has other attractive features.
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.