Catalytic stabilization of lean premixed combustion: Method for improving NOx emissions

An experimental and numerical investigation is reported of NOx emissions from both catalytically stabilized and noncatalytic, lean premixed, atmospheric combustion of methane. Experiments were in a tubular, adiabatic flow reactor with catalytically active or inactive honeycomb structures and with adiabatic flame temperatures from 1300 degrees to 1500 degrees C. In order to study the effect of catalytic conversion on NOx formation, the fraction of fuel converted within the catalyst was varied within the range of 0% (corresponding to noncatalytic combustion) to 100%. In all cases, complete burnout is accomplished in a subsequent, homogeneous combustion zone. NOx emissions were computed with combinations of ideal reactor models such as one-dimensional flames, perfectly stirred and plug flow reactors, and catalytic oxidation reactors (Cat). In the Cat model, it is assumed that no intermediates and no NOx are formed, whereas for the homogeneous combustion models a detailed chemical reaction mechanism including NOx formation is employed. The NOx emissions of catalytically stabilized, premixed combustion are remarkably lower than those of noncatalytic, premixed combustion and depend on the fraction of catalytically converted fuel. The higher this fraction, the lower are the NOx emissions. The calculated results compare well with the experimental data, hence validating the modeling assumptions.