The source of all the power in the steam engine is stored up in coal in
the form of heat.
And this heat becomes effective by burning it, that is, by its
combustion.
Coal consists of carbon, hydrogen, nitrogen, sulphur, oxygen and ash.
These elements exist in all coals but in varying quantities.
These are the common proportions of the best sorts:
=========+============+============+===========+========+========
| | | WOOD | | PEAT
| ANTHRACITE | BITUMINOUS | (AVERAGE) | PEAT | 1/4
| | | DRY. | | WATER
---------+------------+------------+-----------+--------+--------
Carbon | 90-1/2 | 81 | 50 | 59 | 44
Hydrogen | 2-1/2 | 5-1/4 | 6 | 6 | 4-1/2
Nitrogen | 0-1/4 | 1 | 1 | 1-1/4 | 1
Sulphur | 00 | 1-1/2 | 0 | ? | (25)
Oxygen | 2-1/2 | 6-1/2 | 41 | 30 | 22-1/2
Ash | 4-1/4 | 4-3/4 | 2 | 3-3/4 | 3
| ----- | ------ | ------ | ------ |--------
| 100 | 100 | 100 | 100 | 100
In burning coal or other fuel atmospheric air must be introduced before
it will burn; the air furnishes the oxygen, without which combustion
cannot take place.
It is found that in burning one lb. of coal one hundred and fifty cubic
feet of air must be used and in every day practice it is necessary to
supply twice as much; this is supplied to the coal partly through the
grate bars, partly through the perforated doors, and the different
devices for applying it already heated to the furnace.