Quantifying NOx for industrial combustion processes

The objectives of this paper are to (1) identify the problems with many of the units that are used to report and regulate NOx, (2) show how to properly correct NOx measurements for oxygen-enhanced combustion, and (3) recommend a preferred type of NOx unit. The current variety of NOx units make comparisons difficult and can cause considerable confusion. NOx may be measured on a wet or dry basis, but it is commonly reported on a dry basis. The reported NOx may differ from the actual measurements, which may be converted to a specific O2 basis level. Nearly all of the measured NOx from industrial combustion systems is in the form of NO, which is converted to NO2 in the atmosphere. However, when given on a mass basis, the measured NO is commonly reported as NO2 for regulatory purposes, but may be reported as NO, NO2, or simply NOx in technical papers. Some existing regulations may penalize combustion technologies with higher efficiencies and lower flue gas volumes, such as oxygen-enhanced combustion. Confusion may occur when applying some of the "conventional" NOx units to oxygen-enhanced processes. A better unit is the mass of NOx generated per unit of production, which also incorporates the overall process efficiency into the emissions. That unit does not penalize more efficient processes that may generate more NOx on a volume basis, but less NOx on a production basis.