Effect of ammonia addition on suppressing soot formation in methane co-flow diffusion flames

Abstract

Due to issues surrounding carbon dioxide emissions from carbon-containing fuels, there is growing interest in ammonia (NH3) as an alternative combustion fuel. One attractive method of burning NH3 is to co-fire it with hydrocarbons, such as natural gas, and in this case soot formation is possible. To begin understanding the influence of NH3 on soot formation when co-fired with hydrocarbons, soot volume fractions and mole fractions of gas-phase species were computationally and experimentally interrogated for CH4 flames with up to 40% NH3 by volumetric fuel fraction. Mole fractions of gas-phase species, including C2H2 and C6H6, were measured with on-line electron impact mass spectrometry, and soot volume fractions were obtained via color-ratio pyrometry. The simulations employed a detailed chemical mechanism developed for capturing nitrogen interactions with hydrocarbons during combustion. The results are compared to findings in N2CH4 flames, in order to separate thermal and dilution effects from the chemical influence of NH3 on soot formation. Experimentally, C2H2 concentrations were found to decrease slightly for the NH3CH4 flames relative to N2CH4 flames, and a stronger suppression of C6H6 was found for NH3 relative to N2 additions. The measured results show a strong suppression of soot with the addition of NH3, with soot concentrations reduced by over a factor of 10 with addition of up to 20% or more NH3 by mole fraction. The model satisfactorily captured relative differences in maximum centerline C2H2, C6H6, and soot concentrations with addition of N2, but was unable to match measured differences in NH3CH4 flames. These results highlight the need for an improved understanding of fuel-nitrogen interactions with higher hydrocarbons to enable accurate models for predicting particulate emissions from NH3/hydrocarbon combustion.