基于铁系活性炭的电厂烟气催化脱氧防灭火动力学行为与机制

Kinetic behavior and mechanism of catalytic deoxygenation for fire prevention in power plant flue gas using iron-based activated carbon

  • 摘要: 将电厂烟气注入煤矿采空区可实现煤矿灾害防治与烟气减排的统一,契合国家新型绿色技术发展主题。然而现场实测大量的烟气氧含量较高,易引发采空区遗煤自燃,亟需研发适合电厂烟气环境的高效经济脱氧理论、技术与装备。通过硝酸铁对煤制活性炭进行改性以提高其在烟气氛围下的脱氧性能,利用扫描电镜−能量色散X射线光谱联用(SEM-EDS)、X射线衍射分析(XRD)、氢气程序升温还原(H2-TPR)、傅里叶红外光谱(FTIR)表征了铁系活性炭的结构。在此基础上,采用同步热分析(STA)结合非等温动力学理论,揭示了铁系活性炭在烟气环境中的氧化动力学行为与机制。最终在固定床色谱联用平台验证了铁系活性炭在烟气环境中的催化脱氧性能。结果表明:硝酸铁改性可在活性炭表面形成以Fe3O4为代表的直径约1 μm的Fe氧化物团簇,并提高活性炭表面的含氧官能团占比,且含氧官能团占比随着Fe元素质量分数的增加而升高。Fe氧化物负载可降低活性炭的燃点,并提高综合燃烧特性指数。其中,当Fe元素质量分数为9%时,活性炭燃点下降可达21 ℃,综合燃烧特性指数增加10.6%。此外,Fe系氧化物负载量与活性炭在烟气环境的最大失重速率温度、最大放热温度和燃尽温度均存在线性相关,所有特征温度均随着负载量的提高而下降。动力学分析指出,Fe系氧化物的引入显著降低了活性炭在烟气环境的氧化活化能,最大减少了52.75 kJ/mol,降低幅度接近50%。此外,热力学分析指出,Fe系氧化物的催化作用为降低活性炭氧化过程熵值,进而加速反应中气体的吸附过程。固定床脱氧测试验证了Fe系活性炭的性能优势,在实现尾气中更低O2浓度和更高CO2浓度的同时,不仅降低了活性炭的脱氧反应温度,还使CO释放量最高减少了81.5%。研究可为烟气防灭火技术提供理论指导。

     

    Abstract: Integration of mine disaster prevention with flue gas emission reduction can be achieved by injecting power plant flue gas into coal mine goafs, which is aligned with the national theme of green technology development. However, it is shown by field measurements that large-volume flue gas is often found to contain high oxygen levels, whereby the risk of spontaneous combustion of residual coal in the goafs is increased. Therefore, efficient and economical deoxygenation theories, technologies, and equipment tailored to the characteristics of power plant flue gas are urgently needed. Coal-based activated carbon is modified with iron nitrate to enhance its deoxygenation performance in a flue gas atmosphere. "The structures of the iron-based activated carbons are characterized by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), and Fourier transform infrared spectroscopy (FTIR). On this basis, simultaneous thermal analysis (STA) combined with non-isothermal kinetic theory is employed to reveal the oxidation kinetic behavior and mechanism of the iron-based activated carbons in a flue gas environment. Finally, the catalytic deoxygenation performance is verified on a fixed-bed reactor coupled with gas chromatography. The results indicate that Fe oxide clusters, typified by Fe3O4, with a diameter of approximately 1 μm, are formed on the surfaces of activated carbons through ferric nitrate modification, and the proportion of oxygen-containing functional groups on the activated carbon surfaces is increased; this proportion rises as the Fe loading is increased. The ignition temperatures of the activated carbons are reduced and the comprehensive combustion characteristic indices is enhanced by Fe oxide loading. At a 9% Fe loading, the ignition temperature is decreased by up to 21 ℃, and the comprehensive combustion characteristic index is increased by 10.6%. Furthermore, a linear correlation is observed between the amount of Fe-based oxide loading and the characteristic temperatures of the activated carbons in flue gas, including the maximum weight loss temperature, maximum heat release temperature, and burnout temperature. All these characteristic temperatures are decreased as the Fe oxide loading is increased. It is revealed by kinetic analysis that the oxidation activation energies of the activated carbons in a flue gas environment are significantly reduced, achieving a maximum reduction of 52.75 kJ/mol, a decrease of nearly 50%. Moreover, it is indicated by thermodynamic analysis that the entropy changes of the activated carbons oxidation process are reduced by the catalytic effect of Fe-based oxides, thereby accelerating gas adsorption during the reaction. Fixed-bed deoxygenation tests verify the performance advantages of the Fe-based activated carbons: a lower O2 concentration and a higher CO2 concentration in the exhaust gas are achieved; meanwhile, the deoxygenation reaction temperatures are reduced and CO emission is decreased by up to 81.5%. Theoretical guidance is thus provided for flue-gas-based fire prevention and extinguishing technologies.

     

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