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The present worldwide concern with global warming has stimulated the development of carbon-neutral energy technologies in order to mitigate the need of fossil fuels and the emission of greenhouse gases. In this sense, ammonia (NH3) is regarded as one of the most viable alternatives to produce carbon-free energy, presenting high energy density and ease of storage and handling . Furthermore, due to the long-lasting use of ammonia in the fertilizer and refrigerant industries, its possible implementation as a fuel presents an unmatched economic feasibility, when compared to other carbon-free alternatives . However, ammonia has proven to be more resilient to combustion than common hydrocarbons , which is why it is often coupled with other fuels, such as methane, to enhance its flammability properties. Nevertheless, information on the combustion of these fuel mixtures (NH3/CH4) is still scarce, especially at high pressures, which are particularly relevant for modern combustion systems. Therefore, the objective of this work is to study the ignition characteristics of diluted NH3/CH4/O2 mixtures at elevated pressures, through measurements of ignition delay times at pressures of 20 and 40 bar, temperatures between 920 and 1100 K, and equivalence ratios ranging from 0.5 to 2.
The experiments were conducted in a rapid compression machine (RCM), which uses a single creviced piston to compress the gases. A pressure and a temperature sensor are positioned into the ports surrounding the reaction chamber. The pressure history allows for the observation of the mechanical compression and expansion of the gases due to ignition. The ignition delay time (IDT) is defined as the time interval between the end of the mechanical compression (EOC) and the ignition point. The sample mixtures were prepared manometrically in 10 L stainless steel tanks at room temperature, with argon and nitrogen being used as diluent gases. Different ignition temperatures were achieved by either increasing the initial pre-compression temperature or increasing the amount of N2 present in the dilution of the gas mixtures. The mixtures contained a molar fraction of ammonia in the fuel mixture of either 0.8 or 0.9, with a molar fraction of methane in the fuel mixture of 0.2 or 0.1, respectively. The equivalence ratios employed were 0.5, 1 and 2. For all cases, the dilution applied was of 70% (either Ar only or a Ar/N2 blend), and all measurements were conducted for pressures of 20 and 40 bar. The experiments were compared with previous results for pure ammonia , measured in the same RCM setup.
Figure 1 displays the experimental results of IDT measurements for NH3/CH4 fuel mixtures and pure ammonia  at 20 bar and 40 bar. It is seen that an increase in the content of ammonia in the fuel mixture delays the ignition onset, hindering the flammability of the mixture. Another relevant aspect is the range of temperatures for which IDT results were measured: the interval is shorter for pure ammonia (1060-1130 K), when compared with mixtures with methane (920-1030 K). This indicates a narrower range of conditions for which pure NH3 ignites. Therefore, the presence of CH4 appears to enhance the flammability properties of ammonia. It can also be observed that, regardless of the fuel mixture composition and pressure, a decrease in equivalence ratio shortens the ignition delay, thereby increasing the mixture reactivity. Moreover, the results show that higher pressures lead to lower ignition delay times, when comparing for the same temperature.
The experimental results were compared with 3 chemical kinetics mechanisms, by Glarborg et al. , Konnov , and Mendiara et al. . Figure 2 shows the experimental results and numerical simulations of IDT for NH3/CH4 fuel mixtures at 20 and 40 bar. Overall, it can be seen that the mechanism by Konnov  under-predicted the measurements, while the mechanisms by Glarborg et al.  and Mendiara et al.  were capable of matching more closely the experimental results. Nevertheless, the trends were captured for all conditions by the three mechanisms. A closer examination of Figure 2 reveals that the difference between numerical and experimental values is larger for the case with higher content of ammonia in the fuel mixture. In addition, for the same content of ammonia, the numerical results were closer to the experimental values for higher pressures. This is especially noticeable in Figure 2b, where the simulation results reflected the small variance of the experimental values with respect to the equivalence ratio.
Figure 2 – Measurements and numerical results of ignition delay times of NH3/CH4 fuel mixtures, using chemical kinetics mechanisms from Glarborg et al. , Konnov , and Mendiara et al. .
This work was partially supported by Fundação para a Ciência e a Tecnologia (FCT), through IDMEC, under LAETA, project UID/EMS/50022/2019. C.F. Ramos acknowledges the COST Action CM1404 “Chemistry of Smart Energy Carriers and Technologies (SMARTCATS)” for the financial support.
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