Numerical study of H2O addition effects on pulverized coal oxy-MILD combustion

Abstract

This paper numerically investigates the effects of H2O addition on pulverized coal oxy-MILD (moderate or intense low oxygen dilution) combustion based on our previous study, in which only O2/CO2 was treated as oxidizer. While in the present study, the initial H2O mole fraction in oxidizer is varied at a constant oxygen level (30% by volume) to represent four typical oxy-fuel operations, i.e. ideal dry recycle (0% H2O+70% CO2), practical dry recycle (5% H2O+65% CO2), wet recycle (15% H2O+55% CO2) and oxy-steam (70% H2O+0% CO2). It is revealed that, with the addition of H2O, the internal recirculation rate (KV) as well as the coal ignition is improved, however, due to the difference in physical effects between CO2 and H2O, the temperature peak is likewise promoted. CO2 is preferred than H2O to moderate the flame temperature and realize uniform temperature distribution under MILD combustion. Higher H2 and lower CO formations are obtained by adding H2O because of the competing char gasification reactions between CO2 and H2O under in-furnace low oxygen condition of MILD combustion. H2O addition tends to suppress the volatiles-N conversion while accelerate the char-N conversion in the upper furnace, respectively, and reduce the final NO emission by enhanced H2 formation. Moreover, H2O addition is beneficial to increasing heat transfer in both radiative and convective forms.