Carbon monoxide measurement in Boiler

In a perfect world the maximum efficiency would be achieved with 0% oxygen in the flue and the lowest flue gas temperature. In the real world allowance must be made for variations and uncertainties and so 0% oxygen is not practical. The settings on a boiler must allow for differences in fuel composition, atmospheric pressure, wind direction, boiler demands etc. maximum efficiency can usually be maintained when the Carbon monoxide is between 100 and 400 ppm.

If the oxygen level is set too low and something changes the combustion process can become ‘fuel rich’ as there is insufficient oxygen for all the fuel to burn. This can cause high levels of CO to be generated and in the extreme enough fuel to enter the boilers flue and ignite (explode) outside the combustion chamber. CO is very sensitive indicator of improperly adjusted burners; if its concentration rises to 1000 ppm, that is reliable indication of unsafe conditions. Because CO is a direct measure of the completeness of combustion and nothing else, it is also unaffected by air infiltration, other than the dilution effect.

A Flue gas analyzer measures flue gases by probe in the chimney or appliance exhaust and see what levels of Oxygen (O2), Carbon Monoxide (CO), Carbon Dioxide (CO2) and flue gas temperatures are present. CO is usually the first combustible gas component to appear when combustion fuel/air ratios start becoming too rich. Desired CO levels in combustion flue gases are typically less than 200 PPM, and infrared spectroscopy is well suited to measuring at these low levels. The CO analyzers cannot operate at high temperatures and therefore are usually located downstream of the last heat exchangers or economizer.

Flue gas analyzers usually display a CO/CO2 ratio. This shows the ratio of carbon monoxide and carbon dioxide in the exit flue (chimney) of the appliance. Generally when this level is below 0.004 the boiler is considered to be running efficiently. When between 0.004 and 0.008 this indicates a potential problem and investigation and retesting must take place. When above 0.008 the appliance must be fully investigated, cleaned and retested.

Typically for a natural gas fired boiler, oxygen readings may be in the range 3% to 5%, for an oil fired boiler 5% to 8% and for a coal fired boiler 8% to 10%.