Study of mechanism of ammonia decomposition and oxidation: From NOx reduction to ammonia auto-ignition problem

A chemical kinetic mechanism for ammonia decomposition and combustion from the CRECK modeling group is analyzed in this study with respect to the fast and slow chemical processes and the results are used to generate skeletal mechanisms for different applications. As examples two regimes of system parameters are chosen: values typical for engine exhaust gas systems (low temperature ammonia decomposition and NO_X reduction) and a high temperature ignition problem studied by flow reactors and shock tube experiments. The mechanism is first tested and compared to existing experimental results. A time scale analysis is implemented to study the mechanism. A combination of local and global analysis based on the Global Quasi-Linearisation (GQL) concept is suggested and employed to estimate the number of relatively fast and slow processes governing the system dynamics. A GQL decomposition into invariant sub-spaces is used to determine the complimentary sub-sets of relatively fast and slow reactions. These are used to devise specific skeletal mechanisms for the chosen applications, which are verified in the considered parametric ranges. The suggested approach can be used to identify rate limiting processes, study how the coupling of relatively fast and slow (slaved) reactions proceed. The latter is extremely important for mechanism development and model reduction. The results show a very good agreement not only with respect to the ignition delay times, but also with the results from flow reactor experiments.