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The burner-out-of-service (BOOS) technique terminates the fuel flow to selected burners while leaving the air registers open. The remaining burners operate fuel rich, thereby limiting oxygen availability, lowering peak flame temperatures, and reducing NOx formation. The unreacted products combine with the air from the terminated fuel burners to complete burnout before exiting the furnace.

Installing air-only (OFA) ports above the burner zone also achieves staged combustion. This technique redirects a portion of the air from the burners to the OFA ports. A variation of this concept, lance air, has air tubes installed around the periphery of each burner to supply staged air.

Combustion Temperature Reduction

Reducing the combustion temperature effectively reduces thermal NOx but not fuel NOx. One way to reduce the temperature further is to introduce a diluent, as in flue gas recirculation (FGR). FGR recirculates a portion of the flue gas back into the windbox. The recirculated flue gas, usually 10-20% of the combustion air, provides sufficient dilution to decrease NOx emissions.

An advantage of FGR is that it can be used with most other combustion control methods. However, in retrofit applications, FGR can be expensive. In addition to requiring new large ducts, FGR may require major modifications to fans, dampers, and controls.

Water or steam injection (W/SI) is another method that works on the principle of combustion dilution, similar to FGR. In addition, W/SI reduces the combustion air temperature. In some cases, W/SI is a viable option when moderate NOx reductions are required for compliance.

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