NO emissions under pulverized char combustion in O2/CO2/H2O preheated by a circulating fluidized bed

•Newly-preheated system of pulverized char oxy-fuel combustion for low NOx emissions.•Steam hinders nitrogen reduction process during preheating.•NO emissionss gradually increase as steam concentration increases.•Combustion efficiencies are above 99.0%. The char combustion characteristics and NO emission characteristics of O2/CO2/H2O preheated by a circulating fluidized bed were studied; such analysis can simulate the actual wet flue gas recirculation process in the preheating combustion process. The study was divided into two parts: the preheating characteristics and the combustion characteristics. During preheating, the preheating temperature decreased as the steam concentration increased. In addition, both CO2 and H2O reacted with char in the gasification reactions, and the steam shift reaction occurred, so the CO concentration decreased and the H2 concentration increased. As the steam concentration increased, the reduction ratio of fuel-nitrogen to N2 decreased, indicating that steam hindered the nitrogen reduction process during preheating. During combustion, as the steam concentration increased, the combustion temperature variation in the region near the down-fired combustion chamber (DFC) inlet decreased, while the combustion temperature variation in the region away from the DFC inlet increased. The steam shift reaction was weakened along the path of the DFC. The NO emissions gradually increased as the steam concentration increased because the content of unstable nitrogenous compounds from the pre-circulating fluidized bed (PCFB) participating in the chemical reactions in the DFC increased. Under these conditions, the combustion efficiencies were all above 99.0%.