Region
require fires from 4 to 6 inches thick, and should be fired often in
comparatively small charges. Kentucky, Tennessee, Ohio and Illinois
coals require a thickness from 4 to 6 inches. Free burning coals from
Rock Springs, Wyoming, require from 6 to 8 inches, while the poorer
grades of Montana, Utah and Washington bituminous coals require a depth
of about 4 inches.

In general as thin fires are found necessary, the intervals of firing
should be made more frequent and the quantity of coal fired at each
interval smaller. As thin fires become necessary due to the character of
the coal, the tendency to clinker will increase if the thickness be
increased over that found to give the best results.

There are two general methods of hand firing: 1st, the spreading method;
and 2nd, the coking method.

[Illustration: Babcock & Wilcox Chain Grate Stoker]

In the spreading method but little fuel is fired at one time, and is
spread evenly over the fuel bed from front to rear. Where there is more
than one firing door the doors should be fired alternately. The
advantage of alternate firing is the whole surface of the fire is not
blanketed with green coal, and steam is generated more uniformly than if
all doors were fired at one time. Again, a better combustion results due
to the burning of more of the volatile matter directly after firing than
where all doors are fired at one time.

In the coking method, fresh coal is fired at considerable depth at the
front of the grate and after it is partially coked it is pushed back
into the furnace. The object of such a method is the preserving of a bed
of carbon at the rear of the grate, in passing over which the volatile
gases driven off from the green coal will be burned. This method is
particularly adaptable to a grate in which the gases are made to pass
horizontally over the fire. Modern practice for hand firing leans more
and more toward the spread firing method. Again the tendency is to work
bituminous coal fires less than formerly. A certain amount of slicing
and raking may be necessary with either method of firing, but in
general, the less the fire is worked the better the results.

Lignites--As the content of volatile matter and moisture in lignite is
higher than in bituminous coal, the difficulties encountered in burning
them are greater. A large combustion space is required and the best
results are obtained where a furnace of the reverberatory type is used,
giving the gases