Prediction of MILD Combustion Characteristics and NOx Formation for Ammonia Conversion and Flue Gas Recirculation

Many researchers are encouraging efforts to reduce greenhouse gas emissions. To achieve this, it is necessary to reduce the use of hydrocarbon-based fuels or switch to carbon-free fuels. Ammonia can be one of transport methods of hydrogen. Transported ammonia can be used to extract hydrogen through decomposition or directly as a fuel. The purpose of this study is to nummerically analyze the MILD combustion characteristics and NOx emission of hydrogen-ammonia-nitrogen mixtures. Numerical simulations were performed by 0D Homogeneous model and 1D OPPDIF model of Chemkin. The target fuel was assumed to be the tail gas generated by the ammonia decomposition, and the composition of fuels was defined through the ammonia conversion rate(CR) of 0-100%. The results showed that MILD combustion was achieved when the flue gas recirculation rate was increased or the ammonia content was high. As the recirculation rate increased, the region of maximum heat release rate merged into one. Moreover, the NO emission decreased while the N2O emission increased. This means that NO and N2O are formed by the routes of Thermal NOx and Fuel NOx, respectively. A future work will include experimental studies of MILD combustion characteristics in terms of ammonia content in a lab-scale MILD combustion system.