Study on microwave ignition behavior of inferior bituminous coal assisted by coke under dynamic particle spacing

In China’s power system, low-volatile coals represented by inferior bituminous coal and coke exhibit significant production output, yet their application in coal-fired power generation is constrained by their low-volatile characteristics. Microwave technology has been demonstrated to efficiently convert electromagnetic energy to thermal energy through its unique non-contact heating method and plasma effect, facilitating in-situ generation of additional volatiles. However, there is limited research applying this microwave enhancement effect to the ignition process of low-volatile carbonaceous fuels such as inferior bituminous coal and coke. Therefore, this study focuses on inferior bituminous coal and its coke, conducting microwave-assisted ignition experiments in a single-mode microwave reactor. Characterization and analysis of microwave absorption, temperature variations, discharge phenomena during reaction stages, and the structure and physicochemical properties of residual char are performed to investigate the enhancement mechanisms of microwave interactions with carbonaceous particles in the microwave ignition process. The study finds that during the early ignition stage (coal sample heating phase), coal temperature rise depends mainly on heat accumulation under low flow rates (0.25 L/min). In the mid-to-late stages (co-combustion of volatiles and coke, coke combustion stage), significant interparticle discharge phenomena are observed, leading to localized multi-point combustion enhancing effects, particularly noticeable at higher flow rates (0.75 L/min). Under combined microwave thermal and electric effects, the graphitization degree of the coal sample initially increases and then decreases, while its absorption and thermal conversion capabilities first rise and then decline. Surface cracking of the coal sample intensifies, with initial ash formation creating a hindering ash shell to impede oxygen diffusion, followed by island-like aggregation exposing the coal surface again. Overall, a small amount of coke blending (25% coke blending ratio) significantly enhances microwave absorption capability and ignition efficiency of inferior bituminous coal. The coal sample exhibits optimal combustion reactivity during the mid-stage of volatile-coke co-combustion, enabling sustained combustion independent of the microwave action zone.