Moderate or intense low oxygen dilution (mild) combustion characteristics of pulverized coal in a self-recuperative furnace

Abstract

Moderate or Intense Low oxygen Dilution (MILD) combustion is a promising technology that offers high thermal efficiency and low pollutant emissions. This study investigates the MILD combustion characteristics of pulverized coal in a laboratory-scale self-recuperative furnace. High-volatile Kingston brown coal and low-volatile Bowen basin black coal with particle sizes in the range of 38−180 μm were injected into the furnace using either CO₂ or N₂ as a carrier gas. A water-cooled sampling probe was used to conduct in-furnace gas sampling. Measurements of in-furnace gas concentration of O₂, CO, and NO, as well as exhaust gas emissions and in-furnace temperatures, are presented. The results suggest major differences between the two coals and minor differences associated with the carrier gas. It was found that the measured CO level of brown coal cases was 10 times higher than that of black coal cases. However, NO emission for brown coal was only 37% of that measured for black coal at an equivalence ratio of Φ = 0.88. Ash content analysis showed that black coal was not burnt effectively, which is thought to be due to the particle residence times being insufficient for complete combustion in the furnace. To augment the experimental measurements, computational fluid dynamic modeling was used to investigate the effects of coal particle size and inlet air momentum on furnace dynamics and global CO emissions. It is found that coal particle size affects the coal penetration depth within the furnace and the location of the particle’s stagnation point. The effects of air inlet momentum are tested in two ways: first, by raising the inlet temperature at a constant mass flow rate, and, second, by increasing the mass flow rate at a constant temperature. In both cases, increasing the air jet momentum broadens the reaction zone and facilitates MILD combustion, but also lowers reaction rates and increases CO emissions.