Combustion characteristics and nitrogen conversion mechanism in ammonia/coal Co-firing process

Abstract

The coupled combustion of coal with the carbon-free fuel NH3 is an effective technology for reducing CO2 emissions. Based on elemental analysis, proximate analysis, Fourier Transform infrared spectroscopy (FTIR), Carbon-13 Nuclear Magnetic Resonance (13C NMR), and X-ray photoelectron spectroscopy (XPS) characterizations, a coal macromolecular model was constructed. The combustion characteristics of coal/NH3 co-firing were investigated by reactive force field molecular dynamics simulations (ReaxFF MD). The transformation mechanism of elemental N was elucidated. The influence of temperature, oxygen concentration, and NH3 co-firing ratio (CR) on products was investigated. The results showed that during the non-isothermal combustion phase, the coal macromolecules decomposed more rapidly when the heating rate was low. In the isothermal phase, high temperatures promoted secondary reactions. The production of inorganic gases increased with increasing temperature. As the molar fraction of NH3 increased, the production of CO2 and CO was effectively reduced. There were three main pathways for NO formation. As NH3 decomposed to form NH2, NH2 was subsequently converted to NH, and NH reacted with O2 to form NO. The NH2 could also react with O radicals to form HNO, which was then converted to NO. Besides, both thermal NOx and prompt NOx were generated since the N2 was involved in the reaction. In addition, the production of NOx increased significantly at oxygen concentrations greater than 1.4.