High-pressure study of the conversion of NH3/H2 mixtures in a flow reactor

Carbon free fuels such as hydrogen (H2) and ammonia (NH3) will probably play a fundamental role owing to their potential for clean combustion without CO2 emissions. To contribute to the knowledge and development of technologies based on carbon free combustion, the present work deals with an experimental and kinetic analysis of ammonia oxidation in its mixtures with hydrogen at high pressure (up to 40 bar), in the 250-1250 K temperature range using a quartz tubular reactor and argon as bath gas. The impact of temperature, pressure, stoichiometry and H2/NH3 ratio on the concentrations of NH3, NO, NO2, N2O, and N2 obtained as main products of oxidation has been analyzed. The main results indicate that either increasing pressure, H2/NH3 ratio or oxygen availability results in a shift of NH3 and H2 conversion to lower temperatures. The NH3 and H2 consumptions are promoted by the reactions that produce NH2, H, OH, O, HO2 and HNO species and inhibited by reactions that form H2O, NH3, NO, H2NO and O2 species. The pressure effect is particularly significant in the low range of pressures studied. The main ammonia oxidation nitrogen products are N2 and N2O, while NO and NO2 concentrations are below the detection limit under all considered conditions. N2O formation is favored by increasing stoichiometry, pressure and H2/NH3 ratio. The experimental results are simulated and interpreted in terms of a literature detailed chemical kinetic mechanism, which, in general, is able to describe quite well both the H2 and NH3 conversion profiles under the studied conditions.