What is it?
Combustion, or burning, is a rapid combination of oxygen with a fuel, resulting in a release of heat and light.
The oxygen comes from the combustion air, which is approximately 21% oxygen (O2) and 79% Nitrogen (N2). Hydrocarbon fuels contain carbon, hydrogen and in some cases, sulfur and nitrogen. Natural and LP gas contain no fuel bound nitrogen. Nos. 2 through 6 fuel oil contain high levels of fuel bound nitrogen.
The combustion process will combine the carbon and the oxygen to produce carbon dioxide, heat and light. It will combine the hydrogen and the oxygen to produce water vapor, heat and light. Sulfur, when present, will combine with oxygen to produce sulfur dioxide, heat and light. It is preferable to burn fuels with little or no sulfur content. Some fuel bound nitrogen will combine with oxygen to form NOx (oxides of nitrogen).
Stoichiometric combustion would be attained by mixing and burning exact portions of fuel and oxygen required – in order to completely consume both. In reality, to make certain that all fuel is burned an extra quantity of air is required. This is referred to as “excess air”. Modern commercial and industrial standard burners typically operate between 5% and 20% excess air, most at 20%. Many older units will run as high as 50% or more excess air – due to poor fuel/air mixing. If there is a deficiency of air in the combustion process, carbon monoxide (CO) and/or smoke will be generated, representing a safety hazard and reduced efficiencies.
High levels of excess air also mean high levels of nitrogen (79% approx. of the combustion air) in the flue gases. Nitrogen does not take part in the combustion process. It is a negative constituent, as it absorbs heat from the combustion process, raising stack temperatures (excess O2 also absorbs heat from the combustion process). Both high CO and excess air result in reduced combustion efficiencies. In addition, the nitrogen content in the high excess combustion air is subject to ultimately forming NOx.
A simple method to determine burner/boiler system efficiencies measures the percentage of carbon dioxide (CO2) or the oxygen (O2) in the flue gases. These values are then correlated to flue gas temperature, the combination of values providing the efficiency percentage. (Note that “dirty” water side or fire side surfaces would reduce heat transfer capability which would raise flue gas temperatures resulting in lowered “system” efficiencies).
For more information on how Poor Combustion Wastes Money, please see our previous article of the same name – LINK HERE .
Our boiler experts can help define the scope and best apply the boiler room services and equipment that will help you realize the highest efficiency possible, for your boiler plant. If you would like more information, or help, in establishing a Quarterly Preventative Maintenance Plan or reviewing efficiency improving upgrades for your boiler room (with payback estimates), or with any other boiler room needs – we can help!
 The material referenced above are from Power Flame’s white paper titled “Combustion”. For the latest in combustion technology, please also visit their website powerflame.com