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地下煤层强制燃烧注气流量对含水煤燃烧动力学的影响

Effect of gas injection in underground coal combustion on kinetic of water-containing coal

  • 摘要: 煤层地下原位燃烧取能是未来煤炭资源绿色安全开采的潜在路径。为探究原位含水煤层在不同注气流量下的燃烧特性及动力学特征,以典型含水率(11%~32%)褐煤为研究对象,利用热重–差热扫描量热(TG–DSC)联用技术,分析煤样在燃烧过程中特征温度、综合燃烧性能指数、最大热流等参数,同时采用Coats-Redfern积分法对煤燃烧过程进行动力学计算,得到煤样的活化能E。结果表明:水分对煤燃烧及放热特性存在显著影响,随着含水率的增加,煤样着火温度及最大燃烧速率温度先降低后增加,而综合燃烧性能指数及最大热流先增加后降低,当含水率为22.8%时,煤样的着火温度最低,综合燃烧性能指数和放热强度最大,表明适量含水率有利于煤炭的快速着火及燃烧。注气流量显著影响煤的燃烧速率及放热强度,注气流量的增加提高了煤的燃烧速率,但当注气流量升高至80 mL/min时,会显著降低放热强度,其原因是流量过大会带走更多煤燃烧释放的热量,不利于煤自身氧化升温。根据燃烧动力学参数计算结果得出,随着煤中含水率增加,其反应活化能先降低后增加,3种注气流量下,22.8%含水率煤的活化能最低,分别为54.55、54.18、57.97 kJ/mol,含水率11.6%和22.8%煤在50 mL/min时活化能最小,其余含水率煤在20 mL/min时活化能最小,而当注气流量为80 mL/min时,煤样的活化能显著提高,进一步证实了含水率22.8%煤在注气流量50 mL/min时容易着火,较高的注气流量不利于含水煤的点火。研究结果丰富了煤炭燃烧机理的理论基础,同时为地下煤层原位点火及稳定燃烧提供理论指导。

     

    Abstract: In-situ combustion for energy recovery from underground coal seams is a potential way for green and safe mining of coal resources in the future. In order to explore the combustion and kinetic characteristics of water-bearing coal under different gas injection rates, lignite with typical moisture content (11%–32%) was taken as the research object. The parameters such as characteristic temperatures, comprehensive combustion performance indices, and maximum exothermic rate of the coal samples were analyzed by the thermogravimetric analyzer and differential Scanning Calorimetry(TG–DSC), and the Coats-Redfern method was used to calculate the activation energy of the coal samples. The results show that the moisture content has a significant impact on the combustion and exothermic properties of coal. With the increase of moisture content, the ignition temperature and the maximum combustion rate temperature of the experimental coal samples first decrease and then increase, while the comprehensive combustion performance index and the maximum exothermic rate first increase and then decrease. When the moisture content is 22.8%, the ignition temperature of the coal is the lowest, and the comprehensive combustion performance index and the exothermic intensity are the largest, which shows that the moderate water content is conducive to the rapid ignition and combustion of the coal. The air flow rate exerts a significant influence on the combustion rate and the exothermic strength of coal. The combustion rate will be improved by increasing air flow rate, but when the air flow rate increases by 80 mL/min, the exothermic intensity will be significantly reduced, because the flow rate is too large to take away more heat, which is not conducive to the oxidation of the coal itself. The kinetic analysis shows that with the increase of moisture content, the activation energy decreases first and then increases. The lowest values of activation energy are 54.55, 54.18 and 57.97 kJ/mol for the coal with 22.8% moisture content under the three air flow rates, The activation energy of coal with 11.6% and 22.8% moisture content is the smallest at 50 mL/min, and the activation energy of coal with remaining moisture content is the smallest at 20 mL/min. When the air flow rate is 80 mL/min, the activation energy value of coal sample is significantly increased, which further confirms that coal with 22.8% moisture content is easy to fire at 50 mL/min. High air flow is not conducive to the ignition of water-containing coal. The research results can lay a theoretical foundation for coal combustion mechanism, and provide theoretical guidance for in-situ ignition and stable combustion of underground coal seams.

     

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