Molecular mechanism of N oxidation in ammonia-coal co-firing

In order to achieve the dual carbon goals, it is imperative to reduce the carbon emissions in coal-fired power plants. Carbon-free fuel ammonia and coal co-combustion is considered to be one of the effective ways to reduce carbon emissions from thermal power plants. Ammonia as an N source increases the possibility of increased NOx emissions from ammonia-coal co-combustion. Therefore, an in-depth study of the NO formation mechanism of ammonia-coal co-firing is very important to realize low-carbon and low-nitrogen emission of ammonia-coal co-firing. In this study, a quantum chemical method was used to explore the oxidation mechanism of N in ammonia-coal co-combustion when NH3 exists in the form of NH, and the adsorption behavior of NH and O2 on the coal surface was analyzed by wave function. The calculation results showed that the adsorption process of NH on the C5 surface to form the intermediate IM1 is a highly exothermic process with 754.79 kJ/mol. Moreover, the C atom is an electron donor and lost electrons, and the NH is an electron acceptor and obtained electrons, which promotes the C—N bond. The reaction mechanism of the NH/char/O2 system when O2 is adsorbed in different ways was further explored. It was concluded that in the NH/char/O2 co-combustion system the oxidation of NH first occurs on the char surface, and then the residual oxygen on the char surface or O2 in the system oxidizes the char-N. In the NH/coal/O2 heterogeneous system, the NH generates oxidation products NO, NO2 and HNO through different reaction paths, and the corresponding rate-determining step energy barriers are 120.67, 323.37 and 193.50 kJ/mol, respectively. It was concluded that the process of the NO formation from ammonia-N/coal-N is easier to carry out. The kinetic results showed that the rate-determining step rate of NO formation is significantly higher than that of NO2 at all temperatures, and with the temperature increase, the rate-determining step rate of NO formation is gradually close to that of HNO. After the release of NH oxidation products, the residual oxygen on the char surface is further combined with C to form CO, which realizes the oxidation of N in ammonia fuel and C in pulverized coal in co-firing system. Then the char-N and O2 in the system undergoes a heterogeneous oxidation to realize the conversion of char-N to NO. This study revealed the molecular mechanism of the important transition product HNO formation in the process of ammonia combustion to NO at the microscopic level, and clarified the formation path of another oxidation product NO2, which provides theoretical support and data reference for the development of ammonia-coal co-combustion mechanism. © 2023 China Coal Society. All rights reserved.