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  Summary Report, Control of NOx Emissions by Reburning (79 pp, 5.7 MB) February 1996

Nitrogen oxide (NOx) emission control technologies that are capable of achieving NOx emission reductions from a coal-fired boiler can be classified as either combustion modifications or post-com-bustion flue gas treatment. Combustion modification techniques prevent the formation of NOx during combustion or destroy the NOx formed during primary combustion. These techniques include the use of low-NOx burners, overfire air, and boiler combustion optimization. Post-combustion flue gas treatment reduces the NOx content of the flue gas through techniques such as selective catalytic reduction and selective non-catalytic reduction. Reburning, as described in this report, is a combustion modification since the formation of NOx is minimized in one portion of the boiler and a portion of the NOx that does form is destroyed in another.

Unlike some other NOx control approaches, reburning technology is applicable to a wide variety of the boilers and, in many cases, can be implemented within a relatively short period of time. Reburning is ideal for wet-bottom (i.e., slagging) boilers. The only other commercially available NOx control alternative for this type of boiler is flue gas treatment, which is more costly per ton of NOx reduction achieved. Because of reburnings applicability to a wide variety of coal-fired combustion sources, several demonstration projects have been undertaken to gather data on reburning. As a result of such projects, reburning technology is now offered commercially by several firms.

Reburning reduces NOx emissions by completing combustion in three stages. In the first stage, NOx formation due to interactions between the fuel and combustion air at high temperatures is controlled by reducing the burner heat release rate and the amount of oxygen present. In the second stage, additional fuel is added under reducing (oxygen-deficient) conditions to produce hydrocarbon radicals that react with the NOx formed in the first stage to produce nitrogen gas. Additional combustion air is added in the lower-temperature third stage and combustion is completed. In retrofit applications such as discussed in Section 3, reburning has achieved up to 60% reduction from baseline NOx emissions.

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