沉积煤尘卷扬诱发燃爆过程实验研究

Experimental study on the combustion and explosion process induced by the entrainment of deposited coal dust

  • 摘要: 电厂制粉系统运行时具有环境温度高、设备表面的沉积煤尘易出现热失控的特点,自燃煤尘被卷扬后可能会诱发燃爆. 为揭示煤尘在气流卷扬作用下诱发粉尘云燃爆过程,本研究搭建煤尘燃爆实验装置,从自燃过程及特征参数、卷扬燃爆临界条件、燃爆行为及机制等方面进行了研究. 结果表明:热传导和氧化放热是沉积煤尘自燃过程高温点迁移的主因,高温点由热表面向上再向下移动;最高温度、燃烧蔓延和衰减阶段时间随堆积厚度增加而增加,8 mm、10 mm厚度下最高温度和燃烧蔓延阶段时间分别为538 ℃、510 ℃、810 s、1520 s. 煤尘卷扬诱发燃爆包含颗粒分散、喷射传播、燃爆蔓延、熄灭四个阶段,离散火焰形成并引燃可燃气体形成连续性火焰是喷射传播阶段的重要特征. 中心温度为280 ~ 420 ℃的煤尘层卷扬后可诱发燃爆;随着中心温度的增加,火焰传播速度先增加后减小,固体残余物粒径、表面平滑度降低;爆炸强度随煤尘质量增加先增加后减小. 沉积煤尘燃爆是由碳颗粒非均相燃烧和CO、H2等挥发分均相燃烧共同作用导致,低浓度、低自燃程度的煤尘燃爆由非均相燃烧主导;两种点火机制的耦合作用也会造成燃烧不完全、二次引燃和多点火源等现象. 本研究对工业制粉系统的自燃诱发爆炸灾害防治提供了理论依据.

     

    Abstract: During the operation of pulverizing systems in power plants, high ambient temperatures create conditions in which coal dust deposited on equipment surfaces is prone to thermal runaway. When these self-ignited coal dust particles are lifted by airflow, there is a significant risk of combustion and explosion. To reveal the processes and mechanisms of dust cloud explosions induced by airflow entrainment, we constructed an experimental setup to monitor coal dust self-ignition, dispersion in a dust cloud, and the subsequent ignition and explosion. Next, we investigated the self-ignition process, characteristic parameters, critical conditions for entrainment-induced explosions, explosion behavior, and underlying mechanisms. The experimental procedure involved first placing the coal powder on a high-temperature flat plate and then using a high-pressure airflow to entrain the coal powder into the air under various spontaneous combustion conditions. Subsequently, the phenomena of spontaneous combustion, explosion, and their transitions were observed. The results indicated that thermal conduction and oxidative heat release were the primary causes of high-temperature spot migration during the self-ignition process of the deposited coal dust, with the high-temperature point moving upward from the hot surface and then downward. As the deposition thickness increased, both the peak temperature and the duration of the combustion propagation and decay stages increased, reaching 538 ℃, 510 ℃, 810 s, and 1520 s for thicknesses of 8 mm and 10 mm, respectively. The degree of self-ignition and the mass of coal dust significantly influenced the occurrence of explosions, with the central temperature representing the self-ignition level. When the central temperature of the coal dust layer ranged from 280 ℃ to 420 ℃, the entrained coal dust could trigger an explosion. As the central temperature increased, the flame propagation speed first increased and then decreased, whereas the particle size and surface smoothness of the solid residue decreases. The flame propagation speed of the explosion was the largest (4.76 m·s−1) at a mass of 6.0 g and central temperature of 340 ℃. Additionally, the explosion intensity initially increased and then decreased with an increasing coal dust mass. Moreover, the maximum flame length and flame area occurred at 6.0 g, measuring 26.81 cm and 301.4 cm2, respectively. A lower dust mass resulted in insufficient combustible particles, leading to a decrease in flame intensity, whereas a higher dust mass limited combustion owing to an inadequate oxygen supply. Furthermore, the combined effects of the heterogeneous combustion of carbon particles and homogeneous combustion of volatiles, such as CO and H2, are the primary trigger mechanisms driving the explosion of deposited coal dust. The homogeneous combustion of combustible gases ignited the coal dust particles, thus further promoting the pyrolysis and combustion of coal dust, and producing more combustible gases and strengthening the explosion process. When coal dust has a low concentration and a low degree of spontaneous combustion, the combustion and explosions are dominated by heterogeneous combustion. Additionally, the coupling of these two ignition mechanisms leads to incomplete combustion, secondary ignition, and multiple ignition sources. This study provides a theoretical basis for the prevention and control of spontaneous combustion-induced explosion hazards in pulverized industrial systems.

     

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