Excess Air

While supplying just the correct amount of oxygen to assure complete combustion would appear
from a chemical standpoint, as indicated by the above table, to be a simple and straight forward matter, it is the difficulty of supplying sufficient oxygen for complete combustion, and maintaining the such supply with its accompanying nitrogen at a point as nearly as possible at the theoretical In other words, it is the relation of the amount of air actually supplied to that theoretically required for combustion that is the measure of the efficiency of such combustion.


Nitrogen, which almost of necessity must be introduced into the boiler furnace with the oxygen required for the atmospheric combustion of any fuel, is, as previously stated, an inert gas which performs no function combustion. It passes through the furnace and boiler without change amount required, that is the primary problem of combustion.
except in temperature and volume, dilutes the air, absorbs heat and reduces the temperature of the products of combustion. For this reason, and because of the high proportion of nitrogen to oxygen in atmospheric air, it is the principal source of heat loss in combustion.



Any oxygen supplied to the furnace in excess of that required for combustion results in the same losses as in the case for nitrogen, and furthermore, such excess oxygen is accompanied by additional nitrogen which accentuates the combustion losses. On the other hand, when there is insufficient oxygen for complete combustion, the nitrogen losses become inappreciable, when compared to the losses caused by the incomplete combustion of the carbon fuel. As previously explained, if insufficient oxygen is present, carbon will not combust to CO2 (carbon dioxide) but to CO (carbon monoxide). From data previously presented, burning one pound of carbon to CO2 will release approximately 14,540
BTU's, while burning the same amount of carbon to CO will only release approximately 4,380 BTU's.
Insufficient oxygen will result in incomplete combustion of the fuel, some of the fuel will be completely burnt to CO2, and some will be partially burnt to CO. The loss due to such incomplete combustion is very appreciable. Such losses can be computed from the equation below, where:
CO and CO2 are the percentage by volume of these constituents revealed during flue gas analysis.
C is the constituent weight of carbon per pound of fuel, obtained from the fuel specification sheet (chemical composition).

10,160[CO/(CO+CO2)]C Btu;s

It is very clear that controlling the amount of oxygen required for combustion is critical. Just how practical this is, and the financial impact of getting it wrong, or not attending to it at all, is covered in our next section.
Sometimes a simple concept such as perfect combustion, where just the right amount of oxygen is supplied for the complete combustion of the fuel, with no excess oxygen present in the flue gasses, can be obscured, or lost by the use of inappropriate technical verbiage. Such is the case of the simple English term "perfect combustion." Which carries the meaningless technical name of, Stoichiometric Combustion
If an insufficient amount of air is supplied to the burners, unburned fuel, soot and smoke, and carbon monoxide (the incomplete conversion to carbon dioxide) appear in the exhaust from the boiler stack. These can result in heat transfer surface fouling, pollution, lower combustion efficiency, flame instability (i.e., the flame blows out), and the potential for an explosion. To avoid these costly and potentially unsafe conditions, boilers are normally operated at excess air levels. This excess air level also provides operating protection from an insufficient oxygen condition caused by variations in fuel composition, and "operating slop" in the fuel/air control system on the boiler.



It is important to understand that "excess air" and "excess oxygen" are not the same. The air we breathe is roughly 21% oxygen by volume. A 50% excess air condition implies approximately 10.5% oxygen remains in the boiler exhaust stack.
While insufficient air to the burners can be dangerous, air flows in excess of those needed for stable flame propagation and complete fuel
combustion needlessly increase flue gas flow and consequent heat losses, thereby lowering boiler efficiency. Minimizing these losses requires monitoring and periodic tuning. Ideally, the fuel/air ratio is automatically controlled based on the percentage of O2 in the stack, and an unburned hydrocarbons indication. These automated systems are called O2 trim packages.