Experimental and Numerical Investigation of NH3/CH4 Mixture Combustion Properties under Elevated Initial Pressure and Temperature

Toward a carbon-free economy, ammonia is proposed as an alternative with high capabilities to replace hydrocarbon fuels. In the present study, the fundamental combustion properties of the ammonia/methane mixture were investigated experimentally and numerically. The experimental procedure was conducted via a constant volume chamber and Schlieren optical method, and CHEMKIN Pro software was employed for the numerical part. An extensive number of study conditions were considered: equivalence ratios: 0.7 to 1.6, ammonia mole fraction: 0.0 to 1.0, initial pressure: 1.0 to 5.0 atm, and initial temperature: 298 to 473 K. Results show that laminar burning velocity decreases non-linearly with ammonia addition. Interestingly, it was found that the decreasing (increasing) effect of pressure (temperature) is diminished (augmented) by adding ammonia. Ammonia’s laminar burning velocity is about 80% lower than methane’s; nonetheless, with moderate ammonia content, e.g., xNH3 = 0.4, and preheating conditions, e.g., Ti = 373–473 K, an ammonia/methane mixture can have the same laminar burning velocity as the standard industrial hydrocarbon fuels. In terms of flame chemistry, the interaction between ammonia and methane predominantly occurs through their competition for O/H radicals, with no significant direct interaction. NO emission has a rising-falling trend with ammonia concentration, reaching the maximum at xNH3 = 0.4. Additionally, the Markstein length strongly correlated with flame thickness and is increased with the ammonia addition. The comprehensive data set of measured laminar burning velocities and Markstein lengths obtained from this study serves as a valuable resource for mechanism validation and enhances our understanding of NH3/CH4 mixture combustion.