Effect of moisture adsorption and air pre-oxidation on spontaneous combustion liability of upgraded lignite
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摘要: 为探究提质褐煤露天储存仍易自燃的规律和机制，选用自燃倾向性测定装置测定交叉点温度（CPT）等参数求得FCC，判定105-900℃提质褐煤在空气中吸湿预氧化前后的自燃倾向性变化；采用红外光谱仪、氮吸附仪、微量热仪等测定了各样品的化学结构变化、孔结构及吸湿性差异。结果表明，随提质温度的升高，提质新鲜样的自燃倾向性降低，但吸湿预氧化使200-500℃提质煤的自燃倾向性比其新鲜样显著增加，甚至接近原煤。预氧化增加了表面氧元素含量和氧化活性基团数量；脂肪侧链、含氧官能团和（提质后产生的）新自由基等氧化活性基团的存在及比表面积的增大，共同强化了提质样品的预氧化和自燃。吸湿不仅产生润湿热进一步加深预氧化，而且吸湿程度越大的提质样品，其自燃倾向性增强越显著。Abstract: The effect of moisture adsorption and air pre-oxidation on the spontaneous combustion liability of lignite upgraded was investigated for the purposes of obtaining the characteristics and mechanism of the spontaneous combustion liability during storage outside. By a testing apparatus for spontaneous combustion liability, the crossing-point temperature (CPT) for calculating the aggregative indicator FCC was measured for the fresh samples upgraded at the temperature ranging from 105℃ to 900℃ and for the upgraded samples suffering moisture adsorption and pre-oxidization. The changes of chemical structure were analyzed by Fourier transform infrared spectroscopy and energy dispersive spectrometer, and the evolution of pore structure was identified by physical adsorption experiments, and the difference of wetting heat for moisture absorption was determined by the micro calorimeter. The results show that the spontaneous combustion liability of fresh upgraded lignite is reduced with increasing upgrading temperature. But the adsorbing moisture and pre-oxidation make the spontaneous combustion liability of samples upgraded at 200-500℃ increase obviously comparing with the corresponding fresh upgraded samples. Pre-oxidation can increase the elemental oxygen content and the amount of oxidation active groups on the surface of samples. The oxidation active groups of aliphatic side chains, oxygen-containing functional groups, new free radicals from upgrading process, and the increase of specific surface area after deep upgrading all intensify the pre-oxidation and spontaneous combustion of the upgraded samples. The heat of wetting from adsorbing moisture not only further deepens the pre-oxidation, but also enhances the spontaneous combustion liability remarkably for the upgraded samples adsorbing more moisture.
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图 1 自燃倾向性测定的实验装置示意图
Figure 1 Schematic diagram of measuring instrument of spontaneous combustion liability
图 2 提质褐煤在吸湿预氧化后的自燃倾向性(由FCC指标判定)变化
Figure 2 Change of liability of spontaneous combustion for upgraded samples
SA: moisture adsorption and pre-oxidation samples； SB: fresh samples High: be prone to spontaneous combustion；Medium: spontaneous combustion；Low: be difficult of spontaneous combustion
图 3 表面氧元素含量(a)及分布(b)
Figure 3 Normalization content (a) and distribution (b) of elemental oxygen on sample surface from EDS spectrum
图 5 提质样品的比表面积对FCC的影响
Figure 5 Effect of specific surface area of upgraded samples on FCC
图 6 不同提质温度新鲜样的润湿热和其水含量的关系
Figure 6 Relationship between the heat of wetting and the moisture content of fresh sample upgraded at different temperatures
表 1 原煤的工业分析和元素分析
Table 1 Proximate and ultimate analyses of raw coal
Sample Proximate analysis w/% Ultimate analysis wdaf /% Mad Ad Vdaf FCdaf N C S H O* Lignite 24.24 13.23 46.81 53.19 1.22 71.20 0.40 6.54 20.64 *： by difference 
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表 2 提质样品的工业分析
Table 2 Proximate analysis of upgraded samples
Upgrading temperature t/℃ SB w/% SA w/% Md Ad Vdaf FCdaf Md 25* - - - - 6.80 105 5.49 12.86 46.88 53.12 5.53 200 1.15 13.01 46.10 53.90 3.09 300 1.07 13.69 43.69 56.31 2.66 400 0.48 15.87 32.07 67.93 2.74 500 0.76 17.18 21.67 78.33 2.51 600 0.44 18.74 12.77 87.23 1.11 700 0.32 19.50 6.81 93.19 0.72 800 0.37 20.21 4.62 95.38 0.51 900 0.26 20.77 3.44 96.56 0.31 * raw coal is regarded as the sample at 25 ℃
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