When does maximising CO2 minimise CO2?

Answer: when you are tuning a burner. The efficiency of a burner (how much useful heat you get out per unit of fuel put in) is largely determined by how much heat is lost in the exhaust gases. Keeping the air:fuel ratio just right prevents the situation where excess air (which is of no use for combustion) just carries away heat and wastes it.

CO2, of course, is one of the main products of combustion, and is produced in proportion to the amount of fuel burned. Excess air dilutes the exhaust, so the percentage of CO2 in the exhaust goes down as the amount of excess air goes up. When the air:fuel ratio is exactly right:

  • the percentage of CO2 is at a maximum, and
  • the fuel required for a given heat output is at a minimum.
Hence the CO2 emissions are minimised when the percentage of CO2 in the exhaust is maximised.

(Read extended article).

Example: a heating boiler

F: fuel energy in
S: exhaust loss
C: casing losses
U: useful heat out

U = F - S - C