A Novel Coal Purification-Combustion Technology: Purification Characteristics and Ultra-Low Nitrogen Combustion at Low Load

Abstract

To meet the demand for clean and efficient coal utilization under low-load conditions and new power systems, an innovative coal purification-combustion technology is proposed in this study. The feasibility and fuel adaptability were verified using a 200 kW coal purification-combustion system. The high-temperature purification characteristics of three types of coal under a low load of 55 % and the nitrogen migration and transformation mechanism during the purification-combustion process were studied. The results show that the medium-temperature activation process mainly involves the release and reduction of volatile nitrogen to N₂, with a nitrogen conversion rate of 43.8%–53.1%. During this process, coal powder activation is achieved, which significantly increases the specific surface area of the char, develops a pore structure, and increases the number of active sites, which are beneficial for high-temperature gasification reactions under low loads. During high-temperature purification, 62%–85% of the inorganic components were separated, achieving the separation of carbon and inorganic components. Coal powder is converted into high-temperature gaseous fuel, mainly composed of CO and H₂, and the pore structure of char is further developed, which is conducive to stable combustion under low loads. The high-temperature purification process mainly involves the release and reduction of char nitrogen to N₂, with a nitrogen conversion rate of 93.6%–96.6%. The fuel, mainly composed of high-temperature CO and H₂, achieved a moderate or intense low-oxygen dilution (MILD) combustion process. In the reduction zone of the combustion furnace, NH₃ was completely converted to N₂ and char nitrogen was gradually released and reduced to N₂, with a nitrogen conversion rate of 99.6% in the reduction zone. The oxidation zone involves the burnout of char, which mainly releases char nitrogen and oxidizes it to NO_x. Ultimately, only 0.2%–0.9% of the coal nitrogen is converted to NO_x. The minimum original NO_x emissions of the three types of coal at low loads were 28 mg·Nm⁻³ (@6% O₂), and the combustion efficiency exceeded 99.6%.