Effect of H2O/CO2 mixture on heat transfer characteristics of pulverized coal MILD-oxy combustion

Abstract

The integration of moderate or intense low-oxygen dilution (MILD) combustion technology with oxy-fuel combustion, that is, MILD-oxy combustion, has a favorable compatibility to obtain high thermal efficiency and extra low CO2 emissions for pulverized coal combustion. MILD-oxy combustion is characterized by high levels of H2O/CO2 dilution, whose heat transfer characteristics are affected by the physical and chemical properties (i.e., gasification reactivity, heat capacity, and heat radiation) of H2O/CO2 mixture. In this work, dependences of the heat transfer characteristics of MILD oxy-coal combustion on the physical and chemical properties are qualitatively analyzed. Results show that high CO2 levels is favorable for the establishment of pulverized coal MILD combustion for a given 30 vol% O2. An increase in H2O concentration reduces transferred heat while the enhancing the share of heat radiation. In addition, the effect of gasification reactivity on flame temperature is remarkable at a narrow region and gradually weakens with the increase in H2O concentration. The heat capacity makes a slight reduction on overall furnace temperature. As the H2O is substitute for CO2, the influence of heat capacity progressively declines. Moreover, the effect of heat capacity on strengthening convective heat transfer or weakening radiative heat transfer gradually decreases. As the H2O concentration increases, the reduction of flame temperature affected by heat radiation ranges from 13 K to 34 K. Notably, 20% and 30% H2O, and 40% and 50% steam cases have similar radiative emissivity, but the intensity of radiation of the steam cases is increasingly intense, thereby resulting in a significant reduction on flame temperature.