Low-temperature pyrolysis of coal is a crucial step in the coal thermal conversion process and involves very complex physical and chemical reactions that can have different effects on the coal's structure. The thermal evolution behavior and transformation mechanism of the coal microstructure are not yet fully understood, which also limits the efficient utilization of coal to a certain extent. The aromatic structural features (including size, molecular ordering, nematic symmetry, stacking, and curvature) of the char produced from low-temperature pyrolysis of high volatile A bituminous coal (hvAb) from the Xutuan coal mine, China, were quantitatively assessed via high-resolution transmission electron microscopy (HRTEM) image processing and analytical techniques. The thermal transformation process and the mechanisms controlling it were explored. The results show that, except for the unexpected slight growth of aromatic sheets at 440 °C, the lower pyrolysis temperature (< 521 °C) contributed weakly to their size growth, whereas at higher temperatures (561–600 °C), it significantly increased their size. The aromatic molecular ordering tended to gradually change in three stages: increasing between 340 and 440 °C, decreasing between 440 and 521 °C and increasing again between 521 and 600 °C. The nematic symmetry strength of aromatic fringes also followed a similar pattern with temperature at different scales. Additionally, in addition to a very minor development trend at 440 °C, the stacking did not significantly change at temperatures below 521 °C but developed appreciably further with increasing temperatures at 561–600 °C; however, the average spacing of the stacks did not appear to be significantly reduced at all temperatures. The curvature of the aromatic sheets also varied in different temperature stages, i.e., initially slightly increasing (340–380 °C), then gradually decreasing (380–480 °C), later increasing again (480–521 °C), and eventually decreasing (521–600 °C). The properties of the chemical composition and structure of the initial coal play important roles in the thermal reaction behavior, and the physical and chemical reactions that dominate at the different temperature stages may be responsible for such wiggly trends in the evolution of the aromatic structure. Notably, the properties of the mesophase (approximately 440 °C) strongly influence the subsequent structural transformation. These findings could provide useful information for the microstructure–property relationships and preparation of coal-based carbon materials.

中文翻译：

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金管热解实验中高挥发性 A 烟煤 （hvAb） 芳烃结构的阶段性演化特征


煤的低温热解是煤热转化过程中的关键步骤，涉及非常复杂的物理和化学反应，可能对煤的结构产生不同的影响。煤微观结构的热演化行为和转化机理尚不完全清楚，这也在一定程度上限制了煤的高效利用。采用高分辨率透射电子显微镜 （HRTEM） 图像处理和分析技术，定量评价了徐团煤矿高温热解高挥发性 A 烟煤 （hvAb） 产生的焦炭的芳香族结构特征（包括大小、分子排序、向列对称性、堆叠和曲率）。探讨了热转化过程及其控制机制。结果表明，除了芳香片在 440 °C 时意外的轻微生长外，较低的热解温度 （< 521 °C） 对它们的尺寸增长贡献很小，而在较高温度 （561–600 °C） 下，它显着增加了它们的尺寸。芳香族分子排序趋于三个阶段逐渐变化：在 340 至 440 °C 之间增加，在 440 至 521 °C 之间降低，在 521 至 600 °C 之间再次增加。 芳香族边缘的向列对称强度也遵循类似的模式，在不同尺度上随温度的变化。此外，除了在 440 °C 时非常微小的发展趋势外，堆积在低于 521 °C 的温度下没有显着变化，而是随着 561-600 °C 温度的升高而进一步明显发展;然而，堆栈的平均间距在所有温度下似乎都没有显着减小。 芳香片的曲率在不同的温度阶段也不同，即最初略微升高 （340–380 °C），然后逐渐降低 （380–480 °C），然后再次升高 （480–521 °C），最后降低 （521–600 °C）。初始煤的化学成分和结构性质在热反应行为中起着重要作用，在不同温度阶段占主导地位的物理和化学反应可能是造成芳烃结构演变中这种摆动趋势的原因。值得注意的是，中间相的性质（约 440 °C）强烈影响随后的结构转变。这些发现可为煤基碳材料的微观结构-性质关系和制备提供有用的信息。