Now we can obtain the CO2 by supplying more air to the fuel as it is burning. But, we do
not want to supply too much air because if we do, we will be supplying more oxygen than
is actually needed to combine with carbon, and this excess oxygen will play no part in the
combustion process. Not only will it play no part but it will actually detract from the
efficiency by absorbing heat that otherwise could be used to heat the water in the boiler.
In actual practice it is not possible to supply exactly the required amount of air, so somewhat
more than enough is supplied. This is commonly referred to, as excess air.