The NOx emission index from the combustion of distallate type fuels is a function of the amount of water that is present in the primary zone. A simple analysis based on modified Zeldovich kinetics was used to predict the magnitude of the NOx reduction due to water. All empirical and theoretical data that were evaluated fit the expression
Percent NOx reduction = (1 – exp (CHH)) 100
where CH = 22 ± 8 and H is the absolute humidity. This expression can be used to predict the quantity of water that is required to achieve a desired level of NOx reduction. Emission data that are collected at varying ambient humidities can thus be corrected to a common reference level. Methanol combustion was used to illustrate a particular application of the semiempirical calculational technique. It was shown that the NOx emissions from methanol combustion are equivalent to those obtained by adding 8.7 percent water to the combustion air of a kerosine type fuel. The NOx emissions from methanol combustion, on an equal space rate basis, are a factor of 4 lower than from kerosine type fuels. Corrections for ambient temperature and pressure are also required. Thus, in addition to the humidity correction, a technique for correcting NOx measurements to commonly accepted reference ambient conditions of temperature and pressure is presented in this paper.