(WITH PARTICULAR REFERENCE TO COAL)
Fuels for steam boilers may be classified as solid, liquid or gaseous.
Of the solid fuels, anthracite and bituminous coals are the most common,
but in this class must also be included lignite, peat, wood, bagasse and
the refuse from certain industrial processes such as sawdust, shavings,
tan bark and the like. Straw, corn and coffee husks are utilized in
isolated cases.
The class of liquid fuels is represented chiefly by petroleum, though
coal tar and water-gas tar are used to a limited extent.
Gaseous fuels are limited to natural gas, blast furnace gas and coke
oven gas, the first being a natural product and the two latter
by-products from industrial processes. Though waste gases from certain
processes may be considered as gaseous fuels, inasmuch as the question
of combustion does not enter, the methods of utilizing them differ from
that for combustible gaseous fuel, and the question will be dealt with
separately.
Since coal is by far the most generally used of all fuels, this chapter
will be devoted entirely to the formation, composition and distribution
of the various grades, from anthracite to peat. The other fuels will be
discussed in succeeding chapters and their combustion dealt with in
connection with their composition.
Formation of Coal--All coals are of vegetable origin and are the remains
of prehistoric forests. Destructive distillation due to great pressures
and temperatures, has resolved the organic matter into its invariable
ultimate constituents, carbon, hydrogen, oxygen and other substances, in
varying proportions. The factors of time, depth of beds, disturbance of
beds and the intrusion of mineral matter resulting from such
disturbances have produced the variation in the degree of evolution from
vegetable fiber to hard coal. This variation is shown chiefly in the
content of carbon, and Table 35 shows the steps of such variation.