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沉積煤塵卷揚誘發燃爆過程實驗研究

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

  • 摘要: 電廠制粉系統運行時具有環境溫度高、設備表面的沉積煤塵易出現熱失控的特點,自燃煤塵被卷揚后可能會誘發燃爆. 為揭示煤塵在氣流卷揚作用下誘發粉塵云燃爆過程,本研究搭建煤塵燃爆實驗裝置,從自燃過程及特征參數、卷揚燃爆臨界條件、燃爆行為及機制等方面進行了研究. 結果表明:熱傳導和氧化放熱是沉積煤塵自燃過程高溫點遷移的主因,高溫點由熱表面向上再向下移動;最高溫度、燃燒蔓延和衰減階段時間隨堆積厚度增加而增加,8 mm、10 mm厚度下最高溫度和燃燒蔓延階段時間分別為538 ℃、510 ℃、810 s、1520 s. 煤塵卷揚誘發燃爆包含顆粒分散、噴射傳播、燃爆蔓延、熄滅四個階段,離散火焰形成并引燃可燃氣體形成連續性火焰是噴射傳播階段的重要特征. 中心溫度為280 ~ 420 ℃的煤塵層卷揚后可誘發燃爆;隨著中心溫度的增加,火焰傳播速度先增加后減小,固體殘余物粒徑、表面平滑度降低;爆炸強度隨煤塵質量增加先增加后減小. 沉積煤塵燃爆是由碳顆粒非均相燃燒和CO、H2等揮發分均相燃燒共同作用導致,低濃度、低自燃程度的煤塵燃爆由非均相燃燒主導;兩種點火機制的耦合作用也會造成燃燒不完全、二次引燃和多點火源等現象. 本研究對工業制粉系統的自燃誘發爆炸災害防治提供了理論依據.

     

    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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