Investigation of Non-premixed Laminar Counterflow Ammonia Flames

  • Wenkai Yang

Student thesis: Master's Thesis

Abstract

Energy is indispensable to our economic and social lives. Fossil fuels provide for 80% of global mainstream energy supply in 2020. Increased emissions of air pollutants and greenhouse gases (GHGs) including CO2 and NOx have aroused significant environmental concern. Appropriate energy supply management while tackling climate change is thus a hot topic nowadays. Several renewable energy sources have been proposed. However, they are frequently intermittent depending largely on external geological and meteorological variables such as altitude, wind speed, and UV exposure. In order to overcome this intermittency, long term storage is needed and, in this context, synthetic fuels seem to emerge as an indispensable component of a sustainable, carbon-neutral, power-generation portfolio. Even though hydrogen has been considered a potential alternative fuel, implementing a worldwide hydrogen economy is now unfeasible without a proper storage medium. Thus, ammonia can be an answer solving these issues. Since hydrogen is required to obtain significant amounts of ammonia, it also can be seen as H2 carrier. The main objective of this thesis is to study the flame structure of NH3/O2 counterflow diffusion flames and compare it with widely studied CH4/O2 counterflow flames. Furthermore, a recent finding indicates that for homogeneous, isochoric, and adiabatic auto-ignition of ammonia, the addition of H2O2 brings about a significant reduction in the ignition time in the ammonia/air combustion. Therefore, two dimensional NH3-H2O2/O2 counterflow flames are investigated numerically. The structure of non-premixed, laminar, counterflow NH3/O2 flames was studied and compared with the structure of CH4/O2 flames. A commercially available computational tool was utilized through the introduction of ammonia and methane chemistry in order to compute the flow fields of strained flames. The tool was validated by comparing with previously published results for CH4 flames and by employing two different mechanisms for NH3 oxidation kinetics. It was shown that NH3 flames achieve lower maximum temperature and narrower high-temperature area compared to CH4 flames, which was attributed to much less heat release from the NH3 oxidization process. This is due to the fact that NH3 oxidation proceeds through a chemical path drastically different than the one of CH4 and is completed with the formation of N2 as an equilibrium product, without substantial formation of nitrogen oxides. The structure of the CH4 and NH3 flames were compared for mildly strained flames and it was shown that, despite its much slower kinetics, ammonia can sustain near-equilibrium flames, even for relatively small values of the Damkohler number. Five NH3-H2O2/O2 counterflow flames with varying H2O2 addition were performed by validated computational tool. It is shown that the gradual increase of H2O2 content has a spectacular effect on NH3 combustion. The NH3/H2O2 premixed reaction only occurred near the fuel nozzle when H2O2 addition increases to specific value, as well as equivalence ratio drops to certain low level. After completion of the first NH3/H2O2 premixed reaction stage, mixture of remaining NH3 and H2 formed in the first stage reacts with O2 in the second non-premixed reaction stage. It is noted that the fuel of the non-premixed flame is a mixture of NH3 and H2, which is often referred to as 'dissociated' ammonia. Formation of NO and formation of NO2 happens in both reaction zones. Specifically, for NO formation, it is dominant in the second non-premixed reaction stage, in which temperature peaks. This proves that the formed NO is mainly thermal. H2O2 addition has limited impact on NO production amount. On the other hand, formation of NO2 seems to be dependent on the H2O2 addition in the NH3/H2O2 premixed reaction stage. As more H2O2 in the stream, more NO2 is formed. Similar to formation of NO, increase H2O2 addition has minimum effect on formation of NOx in non-premixed reaction, which is favorable for the application of H2O2 as additive to ammonia combustion.
Date of AwardDec 2021
Original languageAmerican English

Keywords

  • Ammonia
  • Combustion
  • Energy
  • Counterflow diffusion flame
  • Computational fluid dynamics
  • NOx.

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