A research team from the Institute of Engineering Thermophysics, Chinese Academy of Sciences has reported a novel coal purification-combustion technology that enables stable and efficient ultra-low nitrogen oxide combustion under low-load conditions, as published in Engineering. The work was carried out on a 200 kW integrated test system, with a focus on purification performance and nitrogen migration mechanisms at approximately 55% load.

The technology integrates medium-temperature activation, high-temperature purification, and moderate or intense low-oxygen dilution combustion into a continuous process. Pulverized coal first undergoes activation in a circulating fluidized bed at around 850 °C, during which volatile nitrogen is released and partially reduced to N₂, with coal nitrogen conversion to N₂ ranging from 43.8% to 53.1%. This step significantly increases the specific surface area and pore development of char particles, providing more active sites for subsequent high-temperature reactions.

In the high-temperature purification stage operated at more than 150 °C above the ash flow temperature of each coal type, inorganic components are effectively separated with removal rates between 62% and 85%. Coal is converted into high-temperature gaseous fuel dominated by CO and H₂, while char nitrogen is extensively released and reduced. The conversion of coal nitrogen to N₂ reaches 93.6%–96.6% in this section, creating favorable conditions for low-pollutant combustion.

The high-temperature gas–solid fuel then enters the combustion chamber and realizes MILD combustion through multi-stage staged air distribution. In the reduction zone, NH₃ is completely converted to N₂, and the remaining char nitrogen is gradually released and reduced, lifting coal nitrogen conversion to N₂ above 99.6%. The oxidation zone completes char burnout with controlled temperature and mixing intensity to limit NOₓ formation.

Tests using three coal types show that raw NOₓ emissions at the furnace outlet are 34.6 mg·m⁻³ for Shenmu bituminous coal, 42.1 mg·m⁻³ for Beishan lignite, and 28.4 mg·m⁻³ for Yihua coal. Combustion efficiency remains above 99.6% across all tested fuels under the low-load condition. The system demonstrates stable operation and consistent performance, supporting the feasibility of this approach for clean coal utilization in power systems with high renewable penetration.

The study clarifies the transformation pathways of fuel-nitrogen throughout activation, purification, and MILD combustion, providing a technical route for improving operational flexibility and reducing pollutant emissions in coal-fired units under low-load conditions.

The paper “A Novel Coal Purification-Combustion Technology: Purification Characteristics and Ultra-Low Nitrogen Combustion at Low Load,” is authored by Shaobo Yang, Shaobo Han, Ruifang Cui, Linxuan Li, Chen Liang, Shuai Guo, Neng Fang, Wei Li, Qiangqiang Ren. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.09.026. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.