The effect of temperature on the co-combustion of hydrochar and NH3 using the ReaxFF method

Abstract

The utilization of NH3, a carbon-free hydrogen carrier, can be optimized through its co-combustion with solids. However, prior studies have primarily focused on coal as the solid fuel, which entails its consumption. Considering that hydrochar possesses abundant oxygen-containing functional groups, as well as high mass and energy density, it presents itself as a promising alternative solid fuel to coal, thereby decreasing dependence on fossil fuels. Therefore, this study investigates the co-combustion of different proportions of hydrochar, NH3, and O2. Molecular dynamic simulations using the ReaxFF MD method were conducted to investigate the impact of temperature on combustion by analyzing product distribution, evolution of bonds, and gas characteristic curves. Additionally, N migration was examined to elucidate the processes leading to the generation of N2 and NOx from NH3. The results showed that temperature facilitated the progression of combustion as the total number of molecules increased (from 2614 at 2000 K to 3137 at 2500 K) and variety of products expanded (from 48 at 2000 K to 69 at 2500 K) with increasing temperature. With increasing temperature, the descent rate of NH3 and oxygen accelerated, whereas the quantities of H2 and H2O increased. Furthermore, during combustion, hydrochar underwent three stages: initial coke formation, subsequent tar generation, and organic gas production. Second, during N migration, hydrochar directly or indirectly participated in the formation of N2. In this process, hydrochar indirectly contributed to the formation of NH and NH2 intermediates, leading to the formation of N2. In contrast, the majority of NO was formed through the direct decomposition of hydrochar. Finally, because of the limited quantity of generated NO2, N existed primarily in the form of N2 and NO. These findings provide theoretical guidance for the combustion of NH3 using hydrochar.