Combustion characteristics of ammonia as a renewable energy source and development of reduced chemical mechanisms

Abstract

In the first section of this numerical study we investigate the combustion characteristics of ammonia-air mixtures at elevated pressure and lean conditions which are encountered in gas turbine combustors. Laminar premixed freely propagating flame and homogenous reactor models are used to calculate the combustion properties. The improvement by hydrogen addition to the fuel mixture in combustion characteristics such as laminar flame speed and ignition delay time is noticeable. Based on ammonia decomposition sensitivity analysis, it is found that the OH radicals have a leading role in controlling the fuel mole conversion and the laminar flame speed. The results of sensitivity study of total NOx formation with respect to the equivalence ratio reveal the possibility of localized rich combustion as an effective way to reduce the NOx concentration down to levels that are the same order as the modern gas turbine engines. In the second part of the study, by considering a wide range of conditions in terms of pressure, fuel mixture, and equivalence ratio we develop two reduced mechanisms based on the Konnov mechanism. The reduced mechanisms are capable of predicting total NOx emission level and laminar flame speed in an acceptable accuracy under wide range of conditions. Evaluating performance of the reduced mechanisms with respect to the full mechanism and experimental data shows that the mechanisms are able to predict the combustion properties with almost the same accuracy as the full Konnov mechanism and with nearly five times less CPU time expense.