动态颗粒间距下焦炭辅助劣质烟煤微波点火行为

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

  • 摘要: 在我国电力系统中,以劣质烟煤、焦炭为代表的低挥发分煤产量巨大,但低挥发分特性限制了其在燃煤发电中的应用。微波技术已被证明能够通过其独特的非接触式加热方式以及等离子体效应高效快速地实现电磁能向热能转化,并实现额外挥发分的原位生成。然而,目前很少有研究将这种微波增强效应应用于劣质烟煤、焦炭等低挥发分碳基燃料的点火过程。为此,以劣质烟煤及其煤焦为代表,在单模态微波反应器中进行焦炭辅助劣质烟煤微波点火试验。对样品在反应过程中的微波吸收情况、温度变化情况、放电情况,以及反应各阶段残炭的结构、理化性质进行表征和分析,以探究微波与炭颗粒间的相互作用对焦/煤微波点火过程的强化规律。研究发现点火前期(煤样加热阶段)煤样温升主要依赖于样品在小流量(0.25 L/min)下的热量累积;中后期(挥发分−焦炭共燃阶段、焦炭燃烧阶段)可以观察到明显的炭颗粒间放电现象,由此引发的多点局域燃烧会形成多个“火核”强化燃烧,大流量(0.75 L/min)下强化效果更明显。在微波热效应及电效应共同作用下,煤样的石墨化度先上升后下降,吸波及热转化能力随之先升高后降低;煤样表面裂纹不断增多加深,析出的灰分先形成一层灰壳阻碍氧分子扩散,后发生岛状聚集使炭表面重新暴露。综合来看,少量的焦炭掺混(焦炭掺混比为25%)对劣质烟煤的吸波能力及点火效果具有显著提升,煤样在挥发分−焦炭共燃阶段中期时具有最佳的燃烧反应性,可以脱离微波作用区域,进行自持燃烧。

     

    Abstract: 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.

     

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