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What Is Boiler Combustion?

By Paul Reed
Updated May 21, 2024
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Boiler combustion is the study of how fuels are burned in boilers that heat water for steam. There are many applications for steam boilers, including chemical process heating, steam heat for buildings and hot water, and steam to drive electrical turbine generators. Combustion is the reaction of fuels with oxygen in air to create heat that is used for steam production.

A variety of fuels can be used for boiler combustion, including natural gas, fuel oil, and biofuels produced from plants or animal wastes. When fuel is sprayed or atomized into a boiler with air, an ignition coil or small pilot flame can ignite the mixture. Combustion releases a great deal of heat, some of which heats water to steam, and some is lost due to radiation and flue losses. Radiation is infrared heat loss that occurs from a hot boiler into a cooler room. Flue losses are heated gases that are vented from the boiler through its flue or vent.

Owners and operators are interested in maximizing efficiency of boiler combustion. The main issues to consider are combustion efficiency, or how well fuel and air mixtures burn, and how to minimize heat losses. Radiant heat loss can be minimized with proper insulation of boiler and steam piping. Boiler design and controls can be used to maximize combustion efficiency.

The combustion area of a boiler normally has tubes containing water and steam passing through an open box that may contain burners and controls. Tube design can improve efficiency, by using multi-pass systems. Water tubes entering the boiler may first pass through the flue gas zone, which takes some of waste heat and preheats the water. Tubes can then pass through the combustion zone more than once to fully utilize combustion heat, which also improves efficiency.

Boiler combustion efficiency for air and fuel mixtures is critical to proper boiler operation. A molecule of fuel requires a theoretical amount of oxygen to burn completely, but in reality excess oxygen is needed due to various losses in the combustion zone. Air is about 21 percent oxygen, so unburned nitrogen in air must also be heated in the boiler and vented by the flue. This further affects boiler efficiency and produces nitrogen compounds that have been connected to acid rain and smog formation.

Too much oxygen reduces the boiler combustion temperature, can create some undesirable pollutants, and requires fuel to heat oxygen and nitrogen that are not used. Lack of oxygen can reduce boiler efficiency and create soot and other byproducts that can damage the boiler over time. Research has found that monitoring oxygen and combustion gas concentrations in the flue gas, and maintaining a proper flue temperature, can optimize boiler performance.

Smaller boilers can be adjusted manually using flue gas sensors and flue gas thermometers, but many boilers can benefit from automatic controls. Boilers may not operate at a single operating point, but will have varying steam demands or operating conditions, which makes manual efficiency settings impractical. Older boilers can be retrofitted with electronic controls that provide feedback to air and fuel input pumps to give the best ratio for combustion.

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