Under this head come petroleum and coal gas, which are obtained in
great variety and varying value from coal and coal oil. The heating
power of these fuels stands in the front rank, as will be seen by the
table annexed.
There are kinds of fuel other than coal, such as wood, coke, sawdust,
tan bark, peat and petroleum oil and the refuse from oil. These are all
burned with atmospheric air of which the oxygen _combines_ with the
combustible part of the fuel while the nitrogen passes off into the
chimney as waste.
The combustible parts of coal are carbon, hydrogen and sulphur and
the unburnable parts are nitrogen, water and the incombustible solid
matters such as ashes and cinder. In the operation of firing under a
boiler the three first elements are totally consumed and form heat; the
nitrogen, and water in the form of steam, escapes to the flue, and the
ashes and cinders fall under the grates.
The anthracite coal retain their shape while burning, though if too
rapidly heated they fall to pieces. The flame is generally short, of a
blue color. The coal is ignited with difficulty; it yields an intense
local or concentrated heat; and the combustion generally becomes
extinct while yet a considerable quantity of the fuel remains on the
grate.
The dry or free burning bituminous coals are rather lighter than
the anthracites, and they soon and easily arrive at the burning
temperature. They swell considerably in coking, and thus is facilitated
the access of air and the rapid and complete combustion of their fixed
carbon.
The method of firing with different sorts of fuel will be treated
elsewhere.