Autoignition of Hydrogen/Ammonia Blends at Elevated Pressures and Temperatures

Hydrogen stored, or transported, as ammonia has been proposed as a sustainable, carbon-free alternative for fossil-fuels in high-temperature industrial processes, including power generation. Although ammonia itself is toxic and exhibits both a low flame speed and calorific value, it rapidly decomposes to hydrogen in high temperature environments suggesting the potential use in applications which incorporate fuel preheating. In this work, the rate of ammonia-to-hydrogen decomposition is initially simulated at elevated temperatures to indicate the proportion of fuel conversion in conditions similar to gas pipelines, gas-turbines or furnaces with exhaust-gas recirculation. Following this, different proportions of hydrogen and ammonia are numerically simulated in independent, zero-dimensional plug-flow-reactors at pressures ranging from atmospheric to 50 MPa, and pre-heating temperatures from 600 K to 1600 K. Deflagration of very-lean-to-fuel-rich mixtures was investigated, employing air as the oxidant stream. Analyses of these reactors provide estimates of autoignition thresholds of the hydrogen/ammonia blends which are relevant for the safe implementation and operation of hydrogen/ammonia blends, or pure ammonia, as a fuel source. Further operational considerations are subsequently identified for using ammonia or hydrogen/ammonia blends as a hydrogen fuel carrier, by quantifying residual concentrations of hydrogen and ammonia fuel products, as well as other toxic emissions, within the hot exhaust products.