Flue gas carbon dioxide and oxygen

Flue gas analysers produce a set of figures for carbon dioxide, carbon monoxide, and excess oxygen. In the case of oxygen and carbon dioxide, there’s a quite simple relation between the two, which it can be instructive to work out. Indeed, at least some analysers rely on this. While some measure carbon dioxide directly, others, including mine, actually measure oxygen and derive the carbon dioxide figure from this relationship.

 

Air, consisting pretty much of around 78% nitrogen, 1% argon and 21% oxygen, is drawn into the gas boiler along with natural gas which is largely methane. The nitrogen and argon (“inert gases”) emerge almost entirely unscathed, while the methane reacts with oxygen, giving off heat, in the

process producing carbon dioxide and water vapour.

 

The water vapour is encouraged to condense into liquid water in a modern boiler, releasing heat which is recovered, and in any case it is kept out of the innards of the flue gas analyser, so it isn’t counted in the analysis of flue gases.

 

The chemical formula for the reaction giving complete combustion of methane is:

 

CH4 + 2O2 >>>> CO2 + 2H2O + energy

 

Focusing on the oxygen and carbon dioxide, this tells us that two “units” of oxygen are consumed in producing one “unit” of carbon dioxide. We can now work through an example: 79 units of inert gases (let’s just call it nitrogen) and 21 units of oxygen enter the boiler through the air intake. In the

combustion chamber, let’s say the mixture is set so that 16 units of oxygen are used to burn methane. That means 8 units of carbon dioxide are produced, and 5 units (21 – 16) of unused oxygen exit the flue.

 

The total number of units of flue gas is 79 + 8 + 5 = 92. This allows us to work out the percentages of gases as displayed by the flue gas analyser.

 

Carbon dioxide: 100 x 8/92 = 8.7%

 

Oxygen: 100 x 5/92 = 5.4%

 

So, my flue gas analyser would actually measure 5.4% oxygen and display this and 8.7% carbon dioxide.

 

No more calculations will be done here: an exercise for the enthusiastic student is to put the numbers into a spreadsheet and produce a graph like the one shown here.

 

Modern boilers have their combustion settings adjusted for particular values of carbon dioxide. Picking two values from manuals, 8.8% CO2 will have 5.2% excess oxygen; 10.3% CO2 will have 2.5% O2.

 

The weakness of the graph is at its right hand end – further to the right than we’ve been considering. It shows all the oxygen being used up at 11.7%  CO2 but that’s not the way things work – the calculation is an over-simplification. As the oxygen is consumed, incomplete combustion occurs and the boiler produces rapidly increasing amounts of toxic carbon monoxide, CO. I don’t know how to work out the amount, so there’s a challenge for you!