NH3/coal co-firing: Effects on N transfer, char combustion, and ash transformation via TG-FTIR experiments

Abstract

The innovative approach of using carbon-free NH₃ co-firing with pulverized coal to reduce the CO₂ emission of thermal power plants has drawn increasing attention. Understanding the complex gas–solid interactions among NH₃, volatiles and char combustion is crucial for developing the NH₃/coal co-firing technology. In this study, the TG-FTIR technique was used to systematically reveal the influence of NH₃ existence on the entire process of coal pyrolysis-combustion, where the co-pyrolysis behavior, the evolution of N-containing species, the kinetic analysis of char combustion, and the physical and chemical properties of coal ash were investigated. The results show that NH₃ mixing can promote the volatile release and char refinement, enhance the NH₃ adsorption capacity on the char surface, and significantly improve the physicochemical properties of char. Meanwhile, it is found that the quaternary-N content of char increases by up to 31.30% as the NH₃ mixing ratio increases from 5% to 50%, while the pyridine-N exhibits the opposite trend, with a maximum reduction of 48.20%. These changes effectively lower the combustion characteristic temperature and activation energy, thereby improving combustion efficiency. Furthermore, NH₃ existence changes the composition and structure of coal ash, facilitates the deconstruction of inorganic functional groups, increases the reactive sites of Al-OH, and optimizes the surface pore structure and iron enrichment of coal particles. This mixed NH₃ pre-pyrolysis and char combustion process provides novel insights for NH₃/coal co-firing technology.