Effect of coaxial secondary-air distribution on thermal modification, combustion and NOx emissions of pulverized coal using purifying-combustion technology

To meet increasingly stringent NO_x emission standards for coal-fired boilers, purification-combustion technology had garnered attention due to its great potential for NO_x reduction. Central to this technology was high-temperature reduction unit (HTRU). Previous studies focused on the influence of operating parameters in the HTRU. Nevertheless, the optimization of its structural design was often neglected. This deficiency limited the improvement of its performances, including fuel activation and fuel-N removal, and thus hindered NO_x emission control. Addressing this gap, this study systematically investigated the impact of coaxial air nozzle designs in the HTRU, including single-channel and dual-channel configurations, on thermal modification, combustion, and NO_x emission characteristics in a 30 kW purification-combustion test rig. Experimental variables for these designs were air jet velocity (v_se) and staged air ratio (c₁₂). The findings revealed that the purifying burner effectively converted pulverized coal into high-temperature coal gas and highly reactive coal char. CO and NO₂ served as the predominant combustible gas and nitrogen-containing oxide respectively in the coal gas, and their concentrations increased as v_se and c₁₂ increased. Additionally, these conditions improved the particle structure, reactivity, and organic component conversion rates of the coal char. The purifying modification positively impacted combustion efficiency improvement and NO_x emission control, resulting in NO_x emission reduction while improving combustion efficiency under these conditions. Notably, NO_x emissions dropped to a minimum of 54.03 mg/m³ (@6% O₂) with combustion efficiency of 99.43 % when v_se and c₁₂ increased to 11.44 m/s and ∞ respectively.