The oxidation of NH3/CO/O2/H2O system in a plug flow reactor: Experimental and kinetic modeling study

Abstract

Ammonia, as a carbon-free fuel, is easier to store and transport than hydrogen. Due to the high ignition energy and low reactivity of ammonia, adding hydrogen or carbon-based fuels as combustion aids may improve the ignition and burnout of ammonia. CO is an important intermediate product in the co-combustion process of ammonia and carbon-based fuels. Research on NH₃/CO co-combustion will further promote the application of such co-fuel in propulsion systems and power generation. In this work, experimental results were supplemented with novel flow reactor results on the effect of NH₃ on CO oxidation in the absence of NO, and explained based on a detailed chemical kinetic model. The effects of temperature (1023–1223 K), NH₃ concentration (250–1500 ppm), and water content (1 %-10 %) on CO oxidation, NH₃ conversion, and NO generation were analyzed. In the NH₃/CO system, the properties of CO always dominate. As the NH₃ content increases, NH₃ gradually inhibits the oxidation of CO by seizing free radicals (O, H, OH) and converting into NH₂. NH₂ further interacts with free radicals to convert into NH or HNO, and ultimately into NO. An increase in temperature will decrease the release of NO and CO and gradually decrease the conversion of NO from NH₃. However, ammonia concentration had little effect on the ratio of ammonia conversion to NO. When H₂O increases from 1 % to 2 %, it has a significant inhibitory effect on the production of NO and promotes the oxidation of CO. When the water concentration increases from 5 % to 10 %, the inhibitory effect reaches saturation. The present work evaluates the amine subset of the reaction mechanism under the studied conditions and provides experimental data under different NH₃/CO ratios, which can be used to construct and verify the reaction mechanism of mixed fuels of carbon-based fuels and ammonia.