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Burnout Zone

• Normal Excess Air Reburn Zone

• Slightly Fuel Rich

Primary Combustion Zone

• Reduced Firing Rate

FIGURE 6-13. Schematic of the reburn process. (From EPA, Control of NOx Emissions by Reburning, Office of Research and Development, U.S. Environmental Protection Agency, U.S. Government Printing Office, Washington, D.C., February 1996.)

NOx Reduced to N2

in the reburn zone reacting with the NOz to form and water vapor. The temperature in this zone must be greater than 1800°F. The remaining combustion air is injected above the reburn zone to produce a fuel-lean burnout zone.

Reburn technology is considered relatively new, but numerous pilot-scale tests and full-scale demonstrations have been conducted and the concept was proposed in the late 1960s [33]. The concept was based on the principle of Myerson et al. [34] that CH fragments can react with NO. The major chemical reactions for the reburn process are [32]:

Hydrocarbon fuel Heat and O2 deficiency > ^H2 (6-60)

where hydrocarbon radicals are produced due to the pyrolysis of the fuel in the oxygen-deficient, high-temperature reburn zone. The hydrocarbon radicals then mix with the combustion gases from the primary combustion zone:

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