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Climate change is strongly linked to the quantity of carbon dioxide (CO2) released by burning fossil fuels, which contain carbon. To address the problem, ammonia (NH3) has recently gained significant attention as a potential alternative fuel due to its carbon-free nature, ease of storage, and good energy density. However, ammonia combustion in practical devices presents challenges, particularly its high nitrogen oxides (NOâ‚“) emissions and combustion behavior that differs significantly from conventional fuels.
Besides ammonia, nitrogen atoms are also commonly found in other fuels, such as coal, coal-derived oils, biomass, bio-oils, and wastes, predominantly incorporated in the form of pyrrolic and pyridinic heterocyclic functional groups. These nitrogen-containing compounds serve as precursors to fuel-NOâ‚“ during combustion.
Developing accurate chemical kinetic models is essential for understanding the reaction mechanisms of the combustion of nitrogen-containing fuels, as well as for predicting and optimizing fuel performance and reducing emissions. However, even for ammonia, a relatively light N-containing species, there is a lack of consensus regarding its reaction mechanisms and the associated kinetic parameters across existing models. To achieve a reliable and unified chemical kinetic model for these fuels, validations through fundamental combustion experiments are necessary. These experiments are specifically used to isolate and scrutinize detailed chemistry problems by minimizing the influence of complex flow and mixing effects. Besides, they provide valuable fundamental-level combustion data that characterizes the behavior of each fuel.
Therefore, this study conducted fundamental combustion experiments on nitrogen-containing fuels to provide two types of key fundamental combustion data—laminar burning velocity (LBV) and flame structure. LBVs were measured using the heat flux method, while flame structure was measured using a flat-flame configuration with the aid of Raman spectroscopy. In addition, related fundamental combustion data from the literature, obtained using methods such as spherical flame experiments, shock tubes, and flow reactors, were also used to support a more comprehensive model validation. All the modeling work was performed using ANSYS Chemkin-Pro software.
The research is based on five experimental and modeling studies, encompassing various fuel compositions, oxidizer environments, and temperature conditions. Each study explored a specifically designed fuel mixture aimed at addressing existing knowledge gaps. Variations in the fuel compositions alter the flame radical pools, allowing for the investigation of different dominant chemistries within nitrogen combustion chemistry.
In the first study, pyrrole (C4H5N), the simplest five-membered nitrogen-containing aromatic molecule, was used as a model compound to investigate the fuel-bound nitrogen conversion mechanisms in the pyrrolic functional group. LBVs of pyrrole+air flames were measured, and the comprehensive kinetic model from our group was updated by incorporating reactions involving pyrrole and its intermediates.
Subsequent investigations focus on ammonia flames. Argon (Ar) was used as a diluent instead of conventional nitrogen (N2) to elucidate specific ammonia flame chemistry. LBVs of ammonia+oxygen+argon mixtures were measured, and nine literature kinetic models were evaluated. The rate constants of three key reactions in ammonia chemistry were reviewed and updated in our model.
Further experiments were carried out to explore the LBVs of oxygen-enriched ammonia flames, which revealed discrepancies with previously reported LBV data, prompting model re-evaluations.
A separate study employs Raman spectroscopy for non-intrusive detection and quantification of nitric oxide (NO) and nitrous oxide (N2O) in ammonia+oxygen+argon flames, providing valuable data for model validations.
Finally, ammonia was blended with nitromethane (CH3NO2), a precursor to NO, to elucidate the impact of NH3–NOx interactions. Interestingly, the results show that adding NH3 has a non-monotonic effect on the LBV of the fuel blend, increasing the LBV only when the NH3 fraction was below 70%. Kinetic analysis tools were used to explain this behavior, and our model was updated with a focus on CH3NO2 chemistry.
These findings collectively advance the understanding of the combustion characteristics of nitrogen-containing fuels and support the validation and refinement of existing chemical kinetic models.