(364a) The Influence of Fuel-Nitrogen on Soot Formation

In order to overcome the negative impacts related to combustion emissions, alternative fuels which mitigate soot, CO₂, and other emissions are being developed. Biomass-derived fuels and ammonia (NH₃)-hydrocarbon blends have emerged as possible solutions to these problems, as they have the potential to be made renewably while providing lower CO₂ emissions. However, biomass and NH₃ itself contains nitrogen, and the influence of nitrogen on soot formation has not been studied. Soot formation becomes relevant for biomass-derived hydrocarbons which contain nitrogen, as well as systems where NH₃ is co-fired with carbon-containing fuels.

In order to close these knowledge gaps, the influence of fuel-nitrogen on soot formation was experimentally and computationally studied. To analyze the influence of fuel-nitrogen on soot formation for compounds containing carbon, the sooting tendencies of a large set (~70) of nitrogen-containing compounds were quantified using the Yield Sooting Index (YSI) and compared to sooting propensities of pure hydrocarbons and oxygenates. Sooting tendencies and decomposition pathways of select nitrogen-containing compounds were simulated using reactive molecular dynamics in order to understand the experimental trends in sooting tendencies. The influence of NH₃ on soot formation was also analyzed by measuring flame characteristics and species/soot concentrations in nonpremixed laminar flames with varying ratios of NH₃ and CH₄ in the fuel. The NH₃/CH₄ flames were simulated using a detailed nitrogen-chemistry mechanism, from which important reaction pathways in these flames were examined. The results from both sets of experiments demonstrate that nitrogen has an inhibitory effect on soot formation. Nitrogen-containing hydrocarbons had an overall lower propensity to form soot than structurally analogous hydrocarbons and oxygenates. Ammonia was found to have a strong chemical effect on suppressing soot formation in CH₄ flames, in addition to having a dilution and thermal effect. Soot-relevant chemical pathways which are influenced by fuel-nitrogen are discussed.