Comparison of the influence of fuel/air unmixedness on NO_x emissions in lean premixed, non-catalytic and catalytically stabilized combustion

Experimental results on the influence of temporal unmixedness on NO_x emissions are presented for both non-catalytic and catalytically stabilized, lean premixed combustion. The test rig used for the experiments consists of a fuel/air mixing section which allows variation of the degree of temporal unmixedness while maintaining a uniform `average over time' concentration profile over the cross section at the inlet to the combustion chamber. The unmixedness is measured as `rms fluctuations in fuel concentration' by an optical probe using laser absorption at 3.39 μm over a 9 mm gap. `Average over time' measurements are taken with `conventional' suction probe analyzers. The combustion chamber is an insulated, tubular reactor (i.d. 26.4 mm). At the inlet to the combustion chamber a honeycomb monolith section is inserted. This monolith is either catalytically active or inactive for catalytically stabilized or non-catalytic combustion respectively. For both modes, the exact same inlet conditions are applied. In catalytically stabilized combustion a fraction of the fuel is consumed within the catalyst and the remaining fuel is burnt in the subsequent homogeneous combustion zone. It is shown that catalytically stabilized combustion yields lower NO_x emissions and, more important, that the effect of temporal fuel/air unmixedness on NO_x emissions is much smaller than with non-catalytic combustion under identical inlet conditions. Experimental evidence leads to the conclusion, that the catalyst is capable of reducing temporal fluctuations in fuel concentration and/or temperature in the combustion process, thereby preventing excess NO_x formation. As a result, the requirements on mixing quality are less stringent when using catalytically stabilized combustion instead of conventional, non-catalytic combustion.