Nitrogen.

The nitrogen in coal is very small and plays no part in combustion; it passes up the smoke stack when the coal is burnt carrying with it some heat.

Of great importance is the large volume of nitrogen which has to pass through the fire-box in the air required to burn the coal. This will be understood when you remember that to burn 1lb of coal requires 12 lbs of air of which 9 lbs is nitrogen.

This nitrogen does not itself burn, but it reduces the rate of combustion and having to be heated up with the other gases in the firebox it absorbs a lot of heat in doing it, and of course this is waste.

The temperature at which the gases leave the smoke stack is often over 800 deg. F. and to heat 9lbs of nitrogen to this temperature will take about 1–9th of a lb of coal.

You will see then how necessary it is that only the amount of air required for burning the fuel is allowed to pass through the firehole and damper doors owing to the loss arising from the heat carried away by any superfluous air which may be drawn in.