Flammability Limits of Ammonia in Air from 298 to 423 K at Elevated Pressures

Abstract

The flammability limit (FL) data of ammonia at elevated temperatures and pressures are useful for fire prevention and control of the storage, transportation, and bunkering process of a large amount of ammonia, as well as for the detailed study of the reaction kinetics of ammonia-containing alternative fuels. In this work, the FLs of ammonia were measured in the temperature range from 298 to 423 K and in the pressure range from 0.05 to 0.5 MPa based on the ASTM E918 and EN 1839 standards, and the ammonia concentrations at the FLs were characterized using the mole fraction and volume fraction, respectively. The FLs of ammonia varied linearly with the temperature and logarithmically with the pressure. The combined effect of the temperature and pressure expanded the FLs of ammonia and increased the likelihood of an explosion. By correlating the FLs of ammonia, the correlated results of ammonia FLs varying with the temperature and pressure were obtained. The mean absolute deviation between the fitted values and the experimental values of ammonia at the lower FLs (LFLs) was 0.22%, while at the upper FLs (UFLs) it was 0.37%. The influence of the temperature and pressure on the ammonia FLs was explained by using the reaction kinetics of ammonia combustion. The presence of OH radicals was crucial for the LFLs in the ammonia/air mixture, followed by NH2 radicals. The UFLs in the ammonia/air mixture were highly dependent on the change of NH2 radicals. The formation of NH2 radicals primarily relied on the elementary reaction NH3 + OH⇔NH2 + H2O at the lower and upper FLs. The peak value of the free radical mole fraction gradually decreased with increasing experimental pressure. The increase in pressure resulted in a significant increase in the change rate of radical ROP, while the increase in temperature resulted in a slight decrease in the rate of change.