Power Plant Engineering

224 POWER PLANT ENGINEERING

Ignition of the fresh coal in stokers, as well as its combustible volatile matter, driven off by

distillation, is started by radiation heat transfer from the burning gases above. The fuel bed continues to

burn and grows thinner as the stoker travels to the far end over the bend, where ash is discharged to the

ash pit. Arches are sometimes built into the furnace to improve combustion by reflecting heat onto the

coal bed.

7.6 Pulverized-Coal Firing

The commercial development of methods for firing coal in pulverized form is a landmark in
the history of steam generation. It made possible the construction of large, efficient, and reliable steam
generators and power plants. The concept of firing “powdered” coal, as it was called in earlier times,
dates back to Carnot , whose idea envisaged its use for the Carnot cycle; to Diesel, who used it in his first
experiments on the engine that now bears his name; to Thomas Edison, who improved its firing in
cement kilns, thus improving their efficiency and production; and to many others. It was not, however,
until the pioneering efforts of John Anderson and his associates and the forerunner of the present Wis-
consin Electric Power Company that pulverized coal was used successfully in electric generating power
plants at their Oneida Street and Lakeside Stations, Milwaukee, Wisconsin.
The impetus for the early work on coal pulverization stemmed from the belief that, if coal were
made fine enough, it would burn as easily and efficiently as a gas. Further inducements came from an
increase in oil prices and the wide availability of coal, which makes the present situation sound rather
like history repeating itself. Much theoretical work on the mechanism of pulverized-coal combustion
began in the early 1920s. The mechanism of crushing and pulverizing has not been well under-stood
theoretically and remains a matter of controversy even today. Probably the most accepted law is one
published in 1867 in Germany, called Rittinger’s law, that states that the work needed to reduce a mate-
rial of a given size to a smaller size is proportional to the surface area of the reduced size. This, and other
laws, however, do not take into account many of the processes involved in coal pulverization, and much
of the progress in developing pulverized-coal furnaces relies heavily on empirical correlations and de-
signs.
To burn pulverized coal successfully in a furnace, two requirements must be met: (1) the exist-
ence of large quantities of very fine particles of coal, usually those that would pass a 200-mesh screen,
to ensure ready ignition because of their large surface- to-volume ratios and (2) the existence of a mini-
mum quantity of coarser particles to ensure high combustion efficiency. These larger coarse particles
should contain a very small amount larger than a given size, usually that which would be retained on a
50 mesh screen, because they cause slagging and loss of combustion efficiency. Line A in Fig. 7.1
represents a typical range for pulverized coal. It shows about 80 percent of the coal passing a 200 mesh
screen that corresponds to a 0.074 mm opening and about 99.99 percent passing a 50 mesh screen that
corresponds to a 0.297 mm opening, i.e. only 0.1 percent larger than 0.297 mm.
The size of bituminous coal that is shipped as it comes from the mine, called run of mine coal, is
about 8 in. Oversized lumps are broken up but the coal is not screened. Other sizes are given names like
lump, which is used in hand firing and domestic applications, egg, nut, stoker, and slack. [Anthracite
coal has similar designations, ranging from broken to buckwheat and rice, ASTM D 310].

Coal is usually delivered to a plant site already sized to meet the feed size required by the pulver-
izing mill or the cyclone furnace. If the coal is too large, however it must go through crushers, which are