Experimental study on the effect of pre-oxidation regulation on coal devolatilization and NO_x emission during char combustion in O₂/CO₂/H₂O atmosphere under pressurized conditions

Pressurized oxy-fuel combustion is a potential carbon capture technology in coal-fired power generation. Pre-oxidation is an effective means to reduce NO_x emissions during coal combustion. In this study, a pressurized horizontal furnace experimental system was used to explore the effects of pre-oxidation temperature (750–1000 °C), O₂ concentration (0.5–10 %), gasifying agent and residence time (1–7 min) on the generation of CO, CO₂, CH₄ and NO_x in the devolatilization stage of pulverized coal and the generation of NO_x when char was burned in an O₂/CO₂/H₂O atmosphere at 1.3 MPa. The contribution of the devolatilization the char combustion stage to NO_x generation was comprehensively analyzed, and a pre-oxidation strategy suitable for pressurized oxy-fuel combustion NO_x reduction was proposed. The results showed that pre-oxidation of pulverized coal at low oxygen concentration in the devolatilization stage can promote the generation of reductive CO and inhibit the generation of NO_x. The NO_x generation was the lowest at 2 % O₂ concentration at 1.3 MPa pressure and 800 °C. In the combustion stage of the char obtained by pre-oxidation, the modified char obtained by 2 % O₂ concentration has the least NO_x generation. The NO_x emission level in the whole combustion process can be controlled by increasing the reaction temperature in the pre-oxidation stage, extending the residence time as much as possible within the devolatilization degree of 80 %, and reasonably regulating the O₂ in a low concentration range to increase the devolatilization degree in the pre-oxidation stage. This work provided evidence that targeted pre-oxidation under pressurized conditions synergistically suppresses NO_x formation across both devolatilization and char combustion stages, advancing the design of low-NO_x pressurized oxy-fuel systems.