干燥过程中褐煤非均质多孔液固结构应力响应与碎裂损伤机制

Drying was an effective way to improve the energy utilization efficiency of lignite with high moisture content. Nevertheless, for lignite, due to the thermal stability problem, large amounts of fine particles would be generated in drying, which limited its safety and continuous production, and it was the major barrier of industrial application of lignite drying. Thus, for the anisotropic physical properties of lignite, characteristics of lignite, stress response, interaction, mechanism of fragment and process control would be the major study method to solve the two critical scientific problems, namely, the non-linear coupling between thermal/ mass transfer and drying stress; interaction of multi-factors of the damage of porous liquid-solid structure of lignite under variable temperature field. The research contents will be involved in the mechanism of transfer of liquid- vapor-gas as well as the damage of porous structure in drying, the response mechanism of drying stress of anisotropy structure under variable temperatures and the mechanism of damage and fragment of liquid-solid structure. Illustrate the interaction of different factors that resulted in the overloading stress during drying; investigate the critical factor that lead to the fragment at different drying stages; build the mechanism of damage and fragment in drying; propose the methods of process control of fragment and pulverization; make a breakthrough in the theory of fragment and pulverization during heating.

褐煤高含水造成利用效率低，干燥是提高能效的重要途径。由于热稳定性较差，褐煤干燥生成大量细粉，难以保证生产安全性和连续性，是遏制褐煤干燥技术发展的难点之一。本项目针对褐煤非均质多孔液固结构的物性特征，以物性认知-力学响应-交互作用-碎裂机制-过程调控为思路，围绕受热过程褐煤非均质结构诱导热湿-应力双向耦合非线性演化和可变温度场褐煤多孔液固结构损伤力学响应及多因素交互作用机制两个关键科学问题开展研究。研究干燥过程褐煤复杂多孔结构水-汽-气传递机制，阐明多相传递的孔结构破坏行为；探析非稳态干燥过程非均质结构热湿应力模型及演化特征，构建干燥过程非均质结构应力响应机制；揭示褐煤非均质多孔液固结构的受热碎裂多因素交互作用，探寻不同干燥阶段褐煤受热碎裂应力临界值及关键控制因素；建立干燥过程中褐煤非均质多孔液固结构碎裂损伤机制，提出控制褐煤干燥碎裂粉化方法，实现控制褐煤受热碎裂粉化的理论突破和方法创新。

褐煤的高含水特性限制了其大规模高效利用，通过干燥提质技术来提高褐煤的能量密度是实现褐煤高效利用的关键。由于热稳定性差，干燥过程易发生严重碎裂和粉化，影响生产安全和连续性。本项目围绕褐煤碎裂粉化热质传递同干燥应力应变间的非线性耦合作用和可变温度场褐煤多孔液固结构损伤力学响应及多因素交互作用机制两个关键科学问题开展研究。首先研究了褐煤多孔结构内的水分运移与孔结构的破坏。发现快速干燥阶段自由水以液体形式在大孔隙中快速运移，该阶段受干燥条件的控制；之后干燥速率减小，干燥过程受内部孔隙结构和水分扩散过程的影响。昭通褐煤的总孔容和各孔径（<10 nm、10-100 nm、100-1000 nm、>1000 nm）的孔容随着干燥时间延长而逐渐减少。同时，研究了干燥过程中褐煤热物理特性的变化规律，基于构建的褐煤干燥损伤因素模型分析了褐煤颗粒内部热湿应力的时空演化。在30-250 ℃的温度区间内褐煤热扩散系数为0.12-0.23 mm2/s；在30-180 ℃，褐煤比热为1.10-1.60 J/K/g。水分是比热和导热系数的主要影响因素。褐煤颗粒的径向应力和切向应力各自的内外应力差在600 s达到最大。径向应力在整个干燥过程中始终为拉应力，而切向应力在内层为拉应力外层为压应力。其次，探究了褐煤颗粒干燥结构损伤特性，研究了褐煤干缩过程与内部高压蒸汽演化，构建了褐煤干燥结构损伤机制。不同煤种碎裂率为ZT＞XLT＞MD，随着干燥时间和温度的增加，颗粒抗压强度逐渐降低，褐煤抗压强度与碎裂率负相关。干缩过程仅与水分含量有关，温度对结构产生的损伤体现在内部高压蒸汽的破坏行为。当温度较低时，干缩过程对结构的损伤占据主导；干燥温度升高后，高压蒸汽影响显著增强。最后提出了褐煤干燥损伤预防策略。采用分阶段干燥操作在提高干燥速率的同时尽可能防止颗粒发生破碎，保证干燥过程的高效、安全运行。本成果以褐煤多孔液固结构干燥过程中的气-液运移为基础，构建了褐煤干燥损伤机制，为实现干燥过程中对碎裂行为的预防与控制提供理论支撑。