Numerical Study of MILD Combustion for Pulverized Coal in O2/N2, O2/CO2, and O2/H2O Atmospheres

MILD (moderate or intense low-oxygen dilution) combustion is a promising technology for high efficiency and extremely low-NO X emission during industrial heating, and it is expected to be combined with oxy-fuel combustion for CCS (carbon capture and storage). However, studies related to coal MILD combustion are quite sparse. In this paper, the coal MILD combustion characteristics in O2/N2, O2/CO2, and O2/H2O atmospheres were compared numerically, to offer guidance for combining MILD combustion with oxy-fuel combustion for pulverized coal in our future work. First, a validation of the predictions against the measurements was performed to assess the reliability of the numerical models. Then, coal MILD combustion was modeled in O2/N2, O2/CO2, and O2/H2O atmospheres, respectively, with the validated models. The main emphasis was laid on the temperature distribution, intermediate species conversion, and NO X formation. The results reveal that the present combustion mechanism is capable of modeling coal MILD combustion. Moreover, CO2 is superior to both N2 and H2O in homogenizing the temperature field, except for delayed ignition. Due to the enhanced char gasification reactions by CO2 and H2O, the formations of the intermediates (CO and H2) are promoted. As a consequence, the CO2- and H2O-moderated MILD combustion shows a greater potential on the final NO emission than the N2 moderated case. Because of the differences in NO reduction ability between CO and H2, NO emission in O2/H2O is further lower than that in O2/CO2 atmosphere.