Boiler combustion is the process of burning fuels to produce steam for various applications. Different fuels can be used, and maximizing combustion efficiency is important for proper operation. Proper insulation, pipe design, and controls can improve efficiency, and monitoring oxygen and flue gas concentrations can optimize performance. Automatic controls are beneficial for boilers with varying steam demands.
Boiler burning 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 electric turbine generators. Combustion is the reaction of fuels with oxygen in the air to create heat which is used to produce steam.
A variety of fuels can be used for boiler combustion, including natural gas, fuel oil, and biofuels produced from plant or animal waste. When fuel is sprayed or atomized in a boiler with air, an ignition coil or small pilot light can ignite the mixture. Combustion releases a large amount of heat, part of which heats the water into steam, and part is lost due to radiation and losses in the fumes. Radiation is the infrared heat loss that occurs from a hot boiler to a cooler room. Flue gas leaks are heated gases that are discharged from the boiler through its flue or vent.
Owners and operators are interested in maximizing boiler combustion efficiency. The main issues to consider are combustion efficiency, i.e. how well fuel-air mixtures burn and how to minimize heat losses. Radiant heat loss can be minimized with proper insulation of the boiler and steam pipes. Boiler design and controls can be used to maximize combustion efficiency.
The combustion zone of a boiler normally has pipes containing water and steam which pass through an open box which may contain burners and controls. Pipe design can improve efficiency, using multi-pass systems. The water pipes entering the boiler may first pass through the flue gas zone, which absorbs some of the waste heat and pre-heats the water. The pipes can then pass through the combustion zone more than once to fully utilize the heat of combustion, which also improves efficiency.
Boiler combustion efficiency for fuel-air mixtures is critical to proper boiler operation. A fuel molecule requires a theoretical amount of oxygen to burn completely, but in reality an excess of oxygen is required due to various losses in the combustion zone. The air contains approximately 21% oxygen, so any unburned nitrogen in the air must also be heated in the boiler and vented through the flue. This further affects boiler efficiency and produces nitrogenous compounds that have been linked to acid rain and smog formation.
Too much oxygen lowers the combustion temperature of the boiler, can create some unwanted 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 your boiler over time. Research has found that monitoring oxygen and flue gas concentrations in flue gases and maintaining proper flue gas temperature can optimize boiler performance.
Smaller boilers can be manually adjusted 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 may have electronic controls that provide feedback to the air and fuel inlet pumps to provide the best ratio for combustion.
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