Numerical study of combustion characteristics of ammonia as a renewable fuel and establishment of reduced reaction mechanisms

Abstract

With its high hydrogen density and already existing infrastructure, ammonia (NH3) is believed to be an excellent green fuel that can be used in energy generation and transportation systems. Combustion of ammonia has certain challenges (associated with its low flame speed and fuel bond NOx emissions) that need to be addressed before its widespread use in practical systems. The primary objective of this study is to develop a reduced reaction mechanism for the combustion of ammonia which can be used to expedite the design of effective ammonia combustors through numerical simulations of realistic combustor geometries with accurate kinetics models. First we have investigated the combustion characteristics of NH3/H2/air mixtures at elevated pressure and lean conditions which are encountered in practical systems such as gas turbine combustors. Laminar premixed freely propagating flame model is used to calculate the combustion properties. The results of sensitivity study of total NOx formation with respect to the equivalence ratio indicates 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 have developed two reduced mechanisms based on the Konnov mechanism. The reduced mechanisms are capable of predicting the total NOx emission level and the laminar flame speed at an acceptable accuracy over a wide range of conditions. Evaluating the 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 almost at the same accuracy level as the Konnov mechanism, but at a nearly five times less CPU time expense.