Numerical study of fuel-NO formation and reduction in a reversed flow MILD combustion furnace firing ammonia-doped methane

Abstract

Moderate or intense low-oxygen dilution (MILD) combustion is well-known for its high potential of reducing thermal-NO formation during combustion, but its capability of mitigating fuel-NO formation is still elusive. To provide new insights into this aspect, this paper numerically studies the fuel-NO formation characteristics under CH4/NH3 MILD combustion in a lab-scale reversed flow furnace by computational fluid dynamics (CFD) modeling with NH3 volume fraction varying from 0% to 4%. It is found that, the overall NO emission at the furnace outlet increases as the initial NH3 concentration rises under both MILD combustion and traditional bluff-body stabilized combustion mode, however, fuel-NO conversion ratio gradually drops. In the present reversed flow furnace, there exists a critical value (1.4%) for the initial NH3 volume fraction, above which MILD combustion loses the advantage of generating lower NO emission in comparison with the traditional combustion operation. However, this critical value would be case-dependent and should be further re-evaluated for different combustors. In the current combustion system, the lower NO emission under traditional combustion when firing higher NH3 doping fuels is caused by the enhanced NO reduction via both CHi reburning and NO2 interconversion routes, which occurs on the outer and inner sides of the flame front, respectively. For MILD combustion, the weakening of CHi reburning in the outer side of reaction zone causes the higher NO emission when initial NH3 concentration exceeds the critical value.