Nitrogen oxides reduction in a porous burner

Abstract

Different aspects of porous burners have been studied in the past in terms of the bed material, design, heat transfer modes and flame characteristics. However, the application of porous burners to NOx reduction and the effect of the bed surface on the chemical reactions have not yet been explored. Hence, the objective of this study is to investigate the effect of the design and operating parameters on NOx reduction inside a porous burner. To achieve this objective, a variety of flames, stabilised inside porous burners, were investigated experimentally, utilizing thermocouples, gas sampling and chromatography. Numerical tools were also used to understand the chemical pathways under different operating conditions better. Premixed CNG-air and LPG-air flames at very low equivalence ratios were stabilised inside the porous bed. The relationship between the volumetric flow rate of the mixture and the minimum equivalence ratio was studied (experimentally and numerically) for equivalence ratios as low as φ=0.35 (equivalent to thermal power of 2kW). The maximum temperature observed to be consistent with super-adiabatic flame temperatures. The maximum measured NOX and CO mole fractions at the burner exit were found to be in the order of few PPMs. The conversion of NOx was then assessed. A mixture of CNG-air doped with NO was introduced into the burner inlet and the effects of the operating parameters on NOx reduction were assessed. It was found that NOx reduction is a function of the equivalence ratio, total flow rate and NO mole fraction at the inlet. Higher flow rates led to an increase in the conversion rate at higher equivalence ratios, due to shorter residence times, and the greater need for more flame radicals in the flame. The numerical study revealed that different chemical pathways dominate at different equivalence ratios, which led to the production of other intermediates and stable radicals. The study showed that the Total Fixed Nitrogen, TFN, reduction followed a similar trend to the NOx reduction for moderately fuel-rich conditions (φ ≤ 1.2) and opposite trends for higher equivalence ratios. For φ>1.2, most of the NO is converted to N-containing species such as N₂O, NH₃ and HCN and not to N₂. Analysis of the chemical pathways showed that the formation of nitrogen-containing species under very fuel rich conditions is due to the increased importance of the HCNO path, as compared with the HNO path. The best TFN conversion efficiency, 65%, was found at φ=1.1. Intermediate radicals have different rates of destruction and production on the porous bed surface, especially for mixtures close to stoichiometric conditions. Under these conditions, the conversion of NOX is strongly influenced by the concentration of H radicals. A collision probability of η = 8 x 10⁻⁴ was found to represent this radical loss effect and to help predict the destruction and production of intermediate terminals with a good level of accuracy. This study also found that NOx reductions using porous burners are technically feasible and that the resulting CO in the exhaust, derived from the rich mixtures, can be burned outside the porous bed.