Effects of secondary and tertiary air on reducing fine mode particles and NO_x during gasification-combustion of coal in a self-sustained furnace

Gasification-combustion can decrease pollutants formation in coal-fired boilers during flexible peak-shaving. This study investigates the impact of the ratio of secondary air (α_s) to tertiary air (α_t) and the ratio of inner secondary air (α_i) to outer secondary air (α_o) on the formation of fine mode particles. Particulate matters (PM) were sampled at various positions along the self-sustained gasification-combustion furnace, and the size distributions and compositions of PM were analyzed. The results revealed that α_s:α_t influenced the temperature profile and the oxidizing/reducing atmosphere of the combustion furnace, consequently affecting the formation of PM and NO_x. Temperature was found to has a greater effect on elements vaporization compared to the reducing atmosphere. The key strategy to inhibit the generation of fine mode PM is to reduce the local high temperature in the combustion furnace. Notably, the optimal α_s:α_t of 4:4 was found to promote the synergistic reduction of fine mode PM and NO_x. In addition, α_i:α_o can affect the peak temperature of the combustion furnace upstream the tertiary air. Increasing the outer secondary air can decrease the peak furnace temperature and the formation of fine mode PM. However, decrease the temperature in reducing atmosphere is not favorable for reducing NO_x formation. Consequently, fine mode PM and NO_x can't be synergistically reduced with the optimization of α_i:α_o.