多尺度褐煤颗粒内部水分迁移机制及脱水过程能量优化的研究

The lignite is restricted to efficiently and clean use by the characteristics of high water content and low calorific value. So the safe and high-efficient technique that is low-energy dehydration and upgrading is necessary for the lignite efficient utilization. Not only does it conform to China's growing energy demand, but also it is a response to a national strategy of energy conservation and emissions reduction. The project focuses on inefficient dehydration for Lignite complex physical characteristic and the uneven distribution of energy, and particle breakage because of internal stress changes by heat and mass transfer. Based on the lignite pore network model of the of fractal theory, the interaction between water and lignite molecules was explored based on molecular simulation. The field of temperature and moisture distribution is calculated by finite element method, and the moisture migration rule is studied by field effect of hydro-thermal coupling in the lignite particle. For different temperature and humidity gradient, the Lignite internal porosity shrinkage and deformation are studied to reveal lignite internal stress generation mechanism. And the lignite internal moisture igration rate is regulated and controlled by the way of energy transmission and distribution. Associated water-holding, pore structure, skeleton shrinkage parameters and the effective diffusion coefficient of the lignite, the interior moisture transfer model is built between the gas phase and the liquid phase in the the multi-scale lignite particle. And the energy transmission and distribution with the mechanism of lignite dehydration process are elucidated. The theoretical basis and technical support are provided for process intensification, energy optimization and process security in the lignite dehydration process.

褐煤的高水含量和低热值严重制约了其高效清洁利用，因此开发安全高效的低能耗脱水技术是褐煤高效利用的关键，既符合我国日益增长的能源需求，也是响应国家节能减排的战略部署。本项目拟针对褐煤复杂的物性特征与能量输配不均的低效脱水过程和颗粒内部应力变化而产生的颗粒破碎问题，基于褐煤颗粒孔隙分形理论的空间网络模型，采用理论计算与分子模拟方法，研究褐煤分子与水分子的相互作用机制；采用有限元法计算褐煤颗粒内部的温度场和水分分布场，探究水热耦合的场效应下褐煤颗粒内部水分迁移规律；研究温度和湿度梯度下褐煤颗粒内部孔隙的收缩和变形，揭示褐煤颗粒内部应力产生机理，调控能量输配方式控制颗粒内部水分迁移速率；关联褐煤持水特性、孔隙结构、颗粒骨架收缩参数和有效扩散系数，构建两相流下的多尺度褐煤颗粒内部水分迁移模型，阐明褐煤脱水过程能量输配方式和作用机制，为褐煤脱水过程强化、能量优化和过程安全提供理论基础和技术支持。

褐煤的高含水性限制了其大规模高值化利用。因此，在利用之前必须先对褐煤进行干燥脱水。由于褐煤热稳定向较差，导致干燥后的褐煤强度降低，容易发生收缩和破裂现象，进而形成潜在的自燃和爆炸问题。本项目首先研究了褐煤热风干燥条件下的干燥特性和干燥动力学，得到褐煤热风干燥的干燥曲线、干燥速率曲线以及最佳动力学模型，并拟合出褐煤有效水分扩散率与温度的关系。利用量子化学理论计算，模拟计算褐煤模型化合物与水分子的相互作用，研究了褐煤中不同赋存状态水分脱除所需的能量。针对褐煤热风干燥过程中的破碎开裂问题，研究了褐煤的表面损伤和粉碎行为。用裂纹率(CR)定量描述了褐煤的表面损伤，分析了脱水过程对开裂的影响。最后基于有限元分析，从实际工况出发，建立褐煤多孔介质传热、传质以及应力的物理模型和数学模型。计算出褐煤干燥过程的温度场和湿度场，进而探索褐煤温度-水分协同效应对颗粒内部应力的作用机制。得出以下结论：logarithmic模型更加适合描述昭通褐煤等温热风干燥过程，并总结出含水率关于干燥温度的表达式，可用于计算任意时刻样品的含水率，为工艺的选择和优化提供有效帮助。褐煤表面裂纹生长发育共分为快速发育、稳定、收缩三个阶段；增大干燥温度，裂纹率也随之增大；但最后裂纹率都稳定在10%左右。粉碎率和含水量进的关系为：β=97.39-190.64X+98.78X2;粒度变化率和含水量的关系为：βs=0.31-68.75X+26.32X2。有限元计算结果较直观的展现了褐煤干燥过程的温度场、湿度场和应力-应变变化结果:褐煤的湿度分布呈现外侧含水率低，中间高的特点；温度分布呈现外侧温度高，内测温度低的特点。并且随着干燥过程的进行，温湿度梯度先增大，后减小。干燥过程中外侧应力较大，内部应力较小。并且产生的干燥湿应力和湿应变远大于热应力和热应变，说明热应力是导致褐煤开裂收缩的主要因素。本研究可为进一步研究褐煤干燥收缩破碎提供理论依据。