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NOx Control Technologies

ETEC Swoosh

Burners-Out-Of-Service (BOOS) Background

In many projects, ETEC has combined FGR operation with combustion modifications, viz., Burners-Out-Of-Service (BOOS) to achieve dramatic NOx emissions reductions. Implementation of BOOS operation is a practical and effective means of achieving staged combustion on an existing boiler. The technique of BOOS operation involves terminating 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 limited availability of oxygen in the fuel-rich region of the flame also results in a reduction in NOX emissions from the conversion of fuel-bound nitrogen. The unreacted products combine with the air from the terminated-fuel burners to complete burnout before exiting the furnace.

For utility boilers, several alternate BOOS arrangements or patterns can be used, but not all burner patterns have the same effectiveness. The most effective BOOS pattern is determined through field tests on the particular boiler. Generally, removing burners from service in the upper portion of the furnace results in the lowest NOX emission levels. Air-only burners at the top of the furnace permit the most extensive staging and generate an air blanket through which the fuel-rich combustion products from the lower burner levels must pass prior to exiting the furnace. Although this approach is desirable for preserving fuel-rich operation, it occasionally results in higher than normal overall excess air levels to achieve complete combustion. Sometimes an effective compromise is to select the next to the top row for burners-out-of-service operation. Often, the blanket of air from the out-of-service burners sandwiched under the top row of in-service burners will provide sufficient air for smoke and carbon burn-out without a significant increase in NOX or excessive overall air levels.