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Ox (NO and NO^sub 2^) emissions have local (air quality), regional (acid rain), and global (ozone production) consequences. The "Nitrogen Oxides Emission Reduction Program" and "Ozone Non-Attainment Program" in the 1990 Clean Air Act provide guidelines for controlling NOx emissions in new and existing stationary sources.
Three types of NOx can be formed in a com
Three types of NOx can be formed in process: thermal, fuel, and prompt. bustion process: thermal, fuel, and prompt. Thermal NOx is created by high temperatures in the presence of free oxygen. The reactions are reversible and controlled by reaction kinetics; therefore, less heat and less oxygen generate less NOx
Reaction (1) is the hydrocracking reaction, followed by the oxidation reaction (2). Prompt NOx contributes 5-10 ppm of total NOx in natural gas combustion.
Technologies currently available to reduce the amount of NOx emissions fall into four categories: precombustion, combustion modifications, postcombustion, and alternative or emerging methods. The technology chosen depends on the amount of NOx to be reduced and budgetary constraints imposed.
Precombustion
Fuel type directly affects the formation of thermal, fuel, and prompt NOx. For example, oxygenated fuels with low hydrocarbon content, such as methanol or ethanol, reduce prompt NOx contributions.
The following list rates various fuels from least to greatest NOx production potential.
Methanol
Ethanol
Natural gas
Propane
Butane
Ultra-low nitrogen fuel oil
No. 2 fuel oil
No. 6 fuel oil
Coal
Numerous possibilities exist to reduce NOx formation after the fuel is selected and equipment installed.
Burners out-of-service. This method is applicable to multiple-burner equipment. One burner is taken out of service, but its air register remains open. Under a constant load, the remaining burners fire in a fuel-rich environment as the air is distributed as if all the burners were operating. Local oxygen concentration and peak flame temperatures are lower at the burners, thus reducing thermal NOx.
Derating. At low loads, boilers and furnaces operate at cooler temperatures. Lowered flame temperatures reduce the potential for thermal NOx formation. If excess capacity is available, combustion equipment should be operated at partial loads, rather than with a few at full load. Overall NOx production is lowered.
Burner system modification. Conventional burners can be modified to delay fuel-air mixing. Burner modifications result in lower oxygen concentrations and...