Fully-resolved simulations of volatile combustion and NO_x formation from single coal particles in recycled flue gas environments

The interaction of coal particles and recycled/recirculated flue gas (RFG) with elevated temperatures and low levels of oxygen occurs in various pulverised coal combustion scenarios. In this work, the effect of oxygen level and temperature on single coal particle combustion characteristics and NO_x formation in N₂ diluent is studied by means of fully-resolved particle simulations. Comprehensive gas-phase kinetics are utilised to consider the critical pathways of NO_x formation including tar-N. Results show that higher RFG temperatures decrease the time to reach the peaks of temperature and species profiles and increase the corresponding peak values. When decreasing O₂, irrespective of the RFG temperature, the fuel release period is prolonged, the volatile combustion time increases and the combustion process becomes overall less intense. The reduction of O₂ in RFG results in a significant decrease of NO production, while the reduction of the RFG temperature has a smaller effect. The analysis of the key reactions that contribute to NO production in the region around stoichiometry shows that fuel-NO_x is the major contributor. Both NH₃ and HCN in fuel-N play a major role, while tar-N only contributes in the case with the lowest temperature and O₂ concentration. The classical NO_x formation pathways are negligible and the initiation reaction of the Zeldovich mechanism is even reversed, i.e. NO⟶+NN₂ is dominant and contributes to NO destruction. The destruction of NO mainly occurs in a rich region close to the particle surface where abundant tar species and their derivatives play a major role for NO destruction via the re-burn mechanism. The prompt mechanism is also active in this region and eventually contributes to NO reduction via production of HCN which is the feed to the re-burn mechanism.