Analyzing the Effect of Fuel Nitrogen on Soot Formation
Abstract
In modern-day life, energy is primarily supplied by the combustion of carbon-containing fuels. As global economic and population growth occurs, the demand for energy, and consequently the amount of energy supplied by carbon-containing fuels, is predicted to increase. Despite overwhelming use of carbon-containing fuels, there are various human health, climate, and environmental issues related to combustion emissions like carbon dioxide (CO2) and soot. To alleviate our dependence on fossil fuels while concomitantly minimizing the negative impacts of these pollutants, research into renewable fuels, engine geometries, and burning strategies is needed alongside advancements in renewable energy technologies. One strategy to reduce emissions is to change the fuel-type, which requires understanding the relationship between fuel structure and pollutant formation pathways. The dominance of combustion in providing energy in the modern age, coupled with the drive to control harmful emissions from these systems, motivates the work presented in this dissertation. This thesis aims to elucidate the effect of fuel-nitrogen on soot formation, a subject receiving comparatively less attention than regular and oxygenated fuels. The effect of nitrogen on soot formation becomes relevant for diesel fuels with nitrogen-containing additives, as well as biomass or biomass-derived fuels, which can contain up to 30% nitrogen-containing compounds by dry weight. In addition, nations such as Korea, Japan, and Australia are exploring ammonia (NH3) as a CO2-neutral fuel. Due to issues in stabilizing NH3-combustion, initial efforts look to enhance the stability of NH3-combustion by co-firing it with hydrocarbons. This doesn’t completely eliminate CO2 emissions, but it reduces them and serves as a stepping stone to burning pure NH3/hydrogen. In these scenarios, soot formation can occur, and the influence of NH3 on soot emissions becomes relevant. In this work, various experimental and computational techniques were employed to study the influence of fuel-nitrogen on soot formation. To understand the chemical influence of fuel-nitrogen on soot formation, the sooting tendencies of 14 amines were measured. Sooting tendencies were quantified by re-scaling relative soot concentrations measured in fuel-doped methane flames into Yield Sooting Indices (YSIs). All amines had lower sooting tendencies than structurally analogous hydrocarbons, and the sooting tendencies of amines with the same chemical formula varied significantly. Calculations were performed to analyze decomposition pathways for three of the amines, revealing that trends in sooting tendency correlate with predicted primary decomposition products. The results suggest the suppressive effect of amines on soot formation may be due to carbon-nitrogen interactions which interfere with aromatic growth pathways. While 2D simulations have been implemented to understand NH3 oxidation and NOx emissions from NH3-seeded hydrocarbon mixtures, few studies have analyzed the ability of chemical mechanisms to capture flame characteristics and soot formation in 2D atmospheric nonpremixed NH3-CH4 flames with large ratios of NH3. To fill this gap, experiments were performed in nonpremixed NH3-CH4 and N2-CH4 co-flow flames with varying ratios of NH3/N2 to CH4, and compared to simulations. Experimentally, NH3 had a strong chemical effect on suppressing soot formation, which is attributed to NH3-hydrocarbon interactions which reduce the formation of aromatics. While the model was able to capture the physical flame characteristics, it was unable to capture the inhibitive effect of NH3 on soot. This highlights the need to identify and include nitrogen-hydrocarbon reactions relevant to soot formation in the underlying chemical mechanism. For the first time, synchrotron X-ray fluorescence (XRF) and X-ray scattering (XRS) were employed to measure spatially-resolved temperatures and mixture fractions in sooting methane/air flames. Both techniques provide evidence that the flame physics are well-captured by the model, and suggest that issues in capturing the suppressive effect of NH3 on soot is related to deficiencies in the kinetic mechanism. The XRF technique was shown to be insensitive to soot and compositional variations in the flame, and measured temperatures displayed excellent agreement with simulated temperatures. Simulated mixture fractions showed satisfactory agreement with mixture fractions determined by XRS, demonstrating the potential of this technique for probing mixing characteristics in sooting flames. Lastly, YSIs were measured in partially-premixed flames, demonstrating that sooting tendency trends hold across a range of temperatures and air-to-fuel ratios relevant to soot formation. This suggests that the sooting tendency trends reported for the amines may also hold across these conditions. While a wide range of studies are reported in this thesis, they overlap, and help to strengthen our understanding of fuel-chemistry and soot formation. The work presented here is expected to aid in the development of models which describe nitrogen-hydrocarbon interactions, ultimately enabling the rational design of fuel-types and combustion geometries that mitigate pollutant formation.