Abstract:

The decomposition process of methane hydrate was measured by laser Raman and X-ray powder diffraction (PXRD) at 253 K under normal pressure. The results showed that the methane hydrates at the surface dissociated into Ⅰh ice in the initial 30—50 min and then the ice film covered the hydrate phase which triggered the “self-preservation” effect and finally led to a dramatic decrease in dissociation rate of methane hydrate. During the dissociation, the ratio of methane content in large and small hydrate cages obtained from Raman spectra remained stable at around 3.2 which was generally in accord with the ratio of large to small cages in methane hydrate, while the characteristic peaks of hydrate lattice planes in the powder X-ray diffraction patterns decreased in the same profile, suggesting that the methane hydrate dissociated as a whole crystal unit. However, the characteristic peaks of Ⅰh ice lattice planes increased in different ways. The intensity of (002) plane was found to increase linearly up to 370% of its original value in 60 min while the intensity of (100) plane kept steady at about 200% of its original value after the first 20 min, indicating that the ice inclined to grow into horizontal plates instead of columnar growth. Combining with the transport characteristics of water molecules on the ice surface under low temperature, the growth of ice film covered on the inner hydrate cores was suggested to cultivate the “self-preservation” effect.

Key words: methane, hydrate, kinetics, microscale

摘要：

实验采用激光拉曼和X射线粉末衍射(PXRD) 在253 K，常压条件下对甲烷水合物的分解过程分别进行了原位测量。研究发现，位于表层的甲烷水合物在前30～50 min内发生分解并生成Ⅰh冰相，随后表层冰相对内层水合物相的包覆引起了“自保护”效应的产生并导致甲烷水合物分解速率显著降低。分解过程中，甲烷在水合物大小笼中的含量之比始终保持在3.2左右，同时水合物晶面特征峰峰面积也按照相同的曲线下降，表明甲烷水合物以晶胞为单位进行整体分解。Ⅰh冰的各个晶面特征峰峰面积差异化的增长曲线表明形成的Ⅰh冰相倾向于片状生长，有助于在水合物表面生成一层冰膜，进而产生“自保护”效应。

关键词: 甲烷, 水合物, 动力学, 微尺度