Evaluation of ignition process and NOx reduction of coal under moderate and intensive low-oxygen dilution combustion by implementing fuel-rich/lean technology

Abstract

This paper numerically investigated the coal ignition, burnout, char consumption, and NOx conversion behaviors under moderate and intensive low-oxygen dilution (MILD) combustion with the implementation of fuel-rich/lean technology in a semi-industrial scale furnace. For the ignition behavior, the results show that the fuel-rich/lean technology can reduce the lift-off distance compared to the original MILD combustion burner due to the higher equivalence ratio in the fuel-rich jet. Moreover, the improvement of the coal ignition is more pronounced by adjusting primary air supply between the lean and rich jets. For coal burnout at the furnace exit, the impact of fuel-rich/lean technology is negligible, but it significantly affects the coal consumption rate near the burner exit region (x ≤ 1 m), which subsequently results in different devolatilization, char combustion, and char gasification rates. The adjustment of primary air supply has a much more significant impact on the burnout profile than the adjustment of fuel supply due to the different devolatilization processes and flue gas recirculation intensity. Moreover, the coal consumption rate as well as ignition process are highly dependent on the relative mixing sequences among the inner stream, the outer stream, the secondary air stream and the recirculated flue gas. In addition, the NOx formation and reduction mechanisms have been revealed with the help of a user defined function (UDF). It is found that the implementation of fuel-rich/lean technology can further mitigate NOx emissions mainly from the hindered thermal NOx route.