The composition of coal varies over such a wide range, and the methods
of firing have to be altered so greatly to suit the various coals and
the innumerable types of furnaces in which they are burned, that any
instructions given for the handling of different fuels must of necessity
be of the most general character. For each kind of coal there is some
method of firing which will give the best results for each individual
set of conditions. General rules can be suggested, but the best results
can be obtained only by following such methods as experience and
practice show to be the best suited to the specific conditions.
The question of draft is an all important factor. If this be
insufficient, proper combustion is impossible, as the suction in the
furnace will not be great enough to draw the necessary amount of air
through the fuel bed, and the gases may pass off only partially
consumed. On the other hand, an excessive draft may cause losses due to
the excess quantities of air drawn through holes in the fire. Where coal
is burned however, there are rarely complaints from excessive draft, as
this can be and should be regulated by the boiler damper to give only
the draft necessary for the particular rate of combustion desired. The
draft required for various kinds of fuel is treated in detail in the
chapter on "Chimneys and Draft". In this chapter it will be assumed that
the draft is at all times ample and that it is regulated to give the
best results for each kind of coal.