An experimental and modeling study on the oxidation of ammonia and n-heptane with JSR

Abstract

The oxidation of neat ammonia, n-heptane, and ammonia/n-heptane blends was investigated both experimentally and numerically. Experiments were carried out in a fused silica jet-stirred reactor (JSR) under atmospheric pressure and lean conditions covering a temperature range of T=500–1200 K. Mole fraction profiles of 28 species were obtained with online SVUV-PIMS and GC techniques. A detailed analysis of reaction products and intermediates was performed, and the results were interpreted with an improved kinetic model, describing the oxidation of ammonia and n-heptane, as well as fuel interaction and NOx formation. The measurements indicate that ammonia reactivity is promoted by n-heptane addition at wide-ranging temperatures, while ammonia addition can also accelerate the oxidation process of n-heptane, especially at intermediate temperatures, manifesting mutual reinforcement in oxidation reactivity. Kinetic analysis shows that the low-temperature reaction of n-heptane generates reactive species that initiate and interact with the oxidation of ammonia along the pathway of NH2→H2NO/HNO→NO→NO2 at low temperatures. As the reaction pathway changes with increasing temperature, n-heptane oxidation likely provides the radical pool necessary to trigger ammonia oxidation through H2O2 decomposition at an intermediate temperature; while ammonia oxidation in turn accelerates the generation of hydroxyl radicals via NOx chemistry as a more direct pathway. As such, a self-reinforcing looping cycle could be formed, leading to the rapid oxidation of ammonia/n-heptane blends at intermediate temperatures. It is noted that despite the good overall performance, the kinetic model under-predicts NOx concentration and only qualitatively captures the rapid oxidation behavior. More kinetic modeling work is necessary in order to better understand the co-oxidation mechanism of ammonia and large hydrocarbons at wide-ranging operating conditions.