Flammability limits of NH3–H2–N2–air mixtures at elevated initial temperatures

Experiments investigating the flammability of hydrogen and ammonia in air and of mixtures of these two fuels in air were performed in a 12.6-liter combustion vessel at an initial pressure of 1 atm and initial temperatures up to 600 °C. Flammability maps based on the limiting fuel concentration as a function of the initial temperature for the various mixtures were constructed. The flammability limit data obtained for both hydrogen and ammonia agreed well with data in the literature. The flammability limits for both hydrogen–air and ammonia–air were found to widen linearly with increased initial temperature. The lower flammability limits of hydrogen–ammonia–air mixtures at various initial temperatures were found to follow closely the Le Chatelier flammability limit mixing rule. Ammonia readily dissociates to hydrogen and nitrogen. The flammability limits of ammonia dissociation products mixed with pure ammonia and air were also measured at initial temperatures of 400, 500, and 600 °C. At each temperature it was found that as the mixture fraction consisting of dissociated ammonia increases the flammability envelope based on the volume fraction of air in the mixture also increases. The flammability limits for these mixtures were largely unchanged for temperatures between 400 and 600 °C except for the fuel-rich flammability limit which decreased significantly at 600 °C. This can be attributed to the slow reaction of the mixture during the vessel fill process before ignition. For experiments at 500 and 600 °C it was found that mixtures with a large fraction of dissociated ammonia autoignited upon injection into the test vessel. The range of mixtures that autoignited was larger at 600 °C than at 500 °C. For mixtures with a large fraction of dissociated ammonia, autoignition of the mixture prevented the measurement of the fuel-rich flammability limit.