深入理解水在褐煤上的吸附是褐煤干燥提质技术的理论基础之一。采用傅里叶变换红外光谱(FTIR)、扫描电子显微镜(SEM)和低温氮吸附/脱附试验分析了褐煤的含氧官能团和孔隙结构,利用水蒸气吸附/脱附试验和分子动力学(MD)模拟探究了水蒸气在褐煤孔隙中的吸附行为。研究结果表明:褐煤样品丰富的含氧官能团和较多的微孔、中孔(孔径约2nm),为水蒸气提供了吸附位点和吸附场所。水蒸气在煤样上的吸附过程可分为3个阶段。第1阶段,相对蒸气压(P/P0)<0.21,水分子直接吸附于含氧官能团上,此时的吸附速度最大;第2阶段(P/P0=0.21~<0.71),水分子与已吸附的水分子相互作用,促使水团簇逐渐生长;第3阶段(P/P0≥0.71),水团簇填充孔隙,并发生毛细凝聚,此时的吸附速度略大于第2阶段。根据Dent模型对水蒸气吸附等温线的拟合结果,吸附类型属于多级吸附,包括初级吸附(第1阶段)和次级吸附(第2、3阶段)。初级吸附能(-48.77kJ/mol)显著大于水的液化热(EL=-43.99kJ/mol),而次级吸附能(-42.28kJ/mol)略小于EL,表明吸附于褐煤的水呈液态。水蒸气脱附过程存在明显的脱附迟滞现象,表明水蒸气吸附稳定,较难去除,其中高压迟滞发生在P/P0≈0.4~0.9,主要由毛细凝聚和“墨水瓶”效应引起;而低压迟滞发生在P/P0<0.4,由水分子与含氧官能团间强相互作用引起。MD模拟结果与水蒸气吸附/脱附等温线分析结果一致,水分子优先吸附在孔隙中,与孔隙壁的含氧官能团形成氢键,且其扩散系数(2.98×10-5cm2/s)与液态水的分子扩散系数相近。

Deeply understanding the adsorption of water on lignite is one of the theoretical foundations of lignite drying and upgrading technology. The oxygen-containing functional groups and pore structure of lignite were analyzed using Fourier Transform Infrared Spectroscopy(FTIR), Scanning Electron Microscopy (SEM), and low-temperature nitrogen adsorption/ desorption experiments. The adsorption behavior of water vapor in the pores of lignite was investigated using water vapor adsorption/ desorption experiments and Molecular Dynamics(MD) simulations. Results show that the rich oxygen-containing functional groups and numerous micropores and mesopores (with the poresize mainly around 2 nm) in lignite samples provide adsorption sites and places for water vapor. The adsorption process of water vapor onthe coal sample can be divided into three stages. In the first stage, with the relative vapor pressure (P/ P0)<0.21, water molecules directlyadsorbe on oxygen-containing functional groups, and the adsorption speed is the highest. In the second stage (P/ P0 = 0.21-<0.71),water molecules interact with adsorbed water molecules, promoting the gradual growth of water clusters. In the third stage (P/ P0≥0.71),water clusters fill the pores and the capillary condensation appeared, the adsorption speed in this stage is slightly higher than in the secondstage. According to the fitting results of the Dent model on the water vapor adsorption isotherm, the adsorption type belonged to multi-stage adsorption, including primary adsorption (first stage) and secondary adsorption (second and third stages). The primary adsorptionenergy (-48.77 kJ/ mol) is significantly greater than the liquefaction heat of water (EL = -43.99 kJ/ mol), while the secondary adsorptionenergy (-42.28 kJ/ mol) is only slightly less than EL, indicating that the water adsorbed on lignite is liquid. There is a significant desorption hysteresis phenomenon during the water vapor desorption process, indicating that the water vapor adsorbes stably and is difficult to remove. High pressure hysteresis occurr in the range of P/ P0 ≈0.4-0.9, mainly caused by capillary condensation and the " ink bottle"effect. Low pressure hysteresis occurr in the range of P/ P0<0.4, caused by strong interactions between water molecules and oxygen-containing functional groups. The MD simulation results are consistent with the analysis of water vapor adsorption/ desorption isotherm. Water molecules preferentially adsorb in the pore and form hydrogen bonds with oxygen-containing functional groups on the pore walls, with adiffusion coefficient (2.98×10-5 cm2 / s) similar to that of liquid water.

0 引言

1 煤样与试验方法

   1.1 试验煤样

   1.2 SEM测试

   1.3 低温氮吸附/脱附试验

   1.4 水蒸气吸附/脱附试验

2 模拟方法

3 结果与讨论

   3.1 煤样孔结构特征

   3.2 水蒸气吸附/脱附行为

   3.3 孔隙中水分子动力学特性

4 结论