The use of combustion modification has enabled many utility boilers to meet the emission standards for NOx. Its usefulness in reducing NOx emissions from industrial boilers (ranging from 11 GJ/hr to 528 GJ/hr) has been investigated during a recently completed field test program. The gaseous and particulate emissions from coal, oil, and natural-gas fuels were measured both before and after the combustion modification. Data were taken on particulate size as well as concentration. The principal combustion modification methods that were investigated included reduced excess combustion air, staged combustion air, recirculated flue gas, tuned burners, and reset burner registers. Staging was implemented by the use of overfire air ports or by turning off the fuel to some burners and increasing the fuel to others, thus creating zones of fuel-rich combustion. All of the combustion modification methods were effective to varying degrees in reducing the nitrogen oxides emissions, and reductions of as much as 50 percent were obtained with several of the modifications. In most instances the boiler efficiency was not degraded, although the particulate emissions increased by up to 50 percent in some cases. There was no substantive effect on the other pollutant emissions that were measured.

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