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

This paper numerically investigates the effects of H 2 O addition on pulverized coal oxy-MILD (moderate or intense low oxygen dilution) combustion based on our previous study, in which only O 2 /CO 2 was treated as oxidizer. While in the present study, the initial H 2 O 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% H 2 O + 70% CO 2 ), practical dry recycle (5% H 2 O + 65% CO 2 ), wet recycle (15% H 2 O + 55% CO 2 ) and oxy-steam (70% H 2 O + 0% CO 2 ). It is revealed that, with the addition of H 2 O, the internal recirculation rate (K V ) as well as the coal ignition is improved, however, due to the difference in physical effects between CO 2 and H 2 O, the temperature peak is likewise promoted. CO 2 is preferred than H 2 O to moderate the flame temperature and realize uniform temperature distribution under MILD combustion. Higher H 2 and lower CO formations are obtained by adding H 2 O because of the competing char gasification reactions between CO 2 and H 2 O under in-furnace low oxygen condition of MILD combustion. H 2 O 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 H 2 formation. Moreover, H 2 O addition is beneficial to increasing heat transfer in both radiative and convective forms. [ABSTRACT FROM AUTHOR]