Nox Emission Calculator

Emission factor estimation multiplies an activity rate by a representative mass of pollutant released per unit of activity, then reduces the result by the fractional efficiency of any control device. Here the activity is fuel throughput per hour, the factor is kilograms of NOx per fuel unit, and operating hours convert the rate into an annual mass. NOx is reported as equivalent nitrogen dioxide even though most of what leaves the stack is nitric oxide, since NO oxidizes in the atmosphere within minutes to hours. Three formation routes set the factor: thermal NOx from atmospheric nitrogen splitting above roughly 1,300 to 1,500 degrees C, fuel NOx from nitrogen chemically bound in coal or residual oil, and prompt NOx from hydrocarbon radicals attacking N2 in the flame front. Typical control efficiencies are 30 to 60 percent for low-NOx burners, 30 to 50 percent for selective non-catalytic reduction, and 70 to 90 percent for selective catalytic reduction. The main limitation is that published factors are population averages carrying EPA quality ratings from A to E; for one specific unit they can be wrong by a factor of two, which is why continuous emission monitoring rather than calculation governs compliance.

The chemistry behind the factors was set out by Yakov Zeldovich, who in 1946 published the chain mechanism showing that nitric oxide formation from atmospheric nitrogen depends exponentially on flame temperature; Charles Fenimore added the prompt NO pathway in 1971. Because that temperature sensitivity makes output depend more on burner design than on fuel chemistry, engineers needed tabulated technology-specific factors rather than the simple stoichiometric balance available for sulfur dioxide. Those tables became AP-42, the Compilation of Air Pollutant Emission Factors, first issued in 1968 by the National Air Pollution Control Administration and inherited by the Environmental Protection Agency when it was created in 1970; it has since run through five editions and hundreds of supplements. Control efficiency entered the calculation as a separate term as abatement spread. Selective catalytic reduction was patented in the United States in 1957 but first deployed commercially on Japanese utility boilers in the late 1970s under that country's post-oil-crisis air rules, while low-NOx burners became widespread in America after the 1990 Clean Air Act Amendments and the 1998 NOx SIP Call forced eastern power plants to cut summertime emissions.