受限空间煤粒瓦斯吸附扩散特性及模型适配差异分析

Analysis of gas adsorption and diffusion characteristics of coal particles in confined space and model adaptation differences

瓦斯吸附扩散系数与解吸扩散系数存在差异,而直接采用定压边界的单孔扩散模型拟含吸附数据从而获得定容条件下的吸附扩散系数有所欠妥。为了对比定压扩散模型和定容扩散模型的拟合差异,正确评价不同边界条件下的瓦斯扩散过程,采用工业分析、等温吸附及吸附扩散动力学试验的方法,对不同初始压力下煤粒的吸附平衡过程及扩散系数变化进行了试验研究。研究发现,定容条件下试验煤样吸附压力随吸附时间呈对数曲线逐渐降低,而吸附平衡时间的长短由煤体的孔隙结构决定。另外,根据真实气体状态方程,可将压降规律转换为吸附规律。利用定容和定压2种扩散模型对瓦斯吸附扩散规律进行拟合,可以发现定容扩散模型要比定压扩散模型拟合效果好,且定压模型测得的扩散系数较之真正的定容扩散系数要大约2倍,并随着初始压力的增加其倍数逐渐增大。恒定压力的边界相当于给了瓦斯分子一个容易逸散的空间,相较于浓度逐渐升高的定容边界来说,扩散难度更小,扩散系数更大。最后,根据定压扩散系数以及定容扩散系数的差异,确定了试验中以及工程中2种扩散系数的应用范围:定压扩散模型适合描述试验中的定压吸附或者定压解吸试验,以及工程中煤层卸压区等高渗区的瓦斯运移规律。相对地,定容扩散模型则适合描述试验中的定容吸附或定容解吸试验,以及工程中被煤层低渗区隔绝的瓦斯运移规律,如应力集中区隔绝的区域或断层、褶曲等构造煤发育的部位。

There is a difference between the gas adsorption diffusion coefficient and the desorption diffusion coefficient,and the adsorption diffusion coefficient under constant volume conditions obtained by the single-hole diffusion model is not appropriate.In order to compare the fitting difference of the two models and correctly evaluate the gas diffusion process under different boundary conditions,the experiments of industrial analysis,isothermal adsorption and dynamic adsorption diffusion kinetics experiments were used to study the gas adsorption equilibrium process in coal particles and the induced variation of diffusion coefficients under different initial pressures.The changes were studied experimentally.The results show that the adsorption pressure of the test coal sample under constant volume conditions gradually decreases with the adsorption time in a logarithmic curve,and the length of the adsorption equilibrium time is determined by the pore structure in coal.In addition,based on the real gas state equation,the pressure drop law can be converted into the adsorption law,and the constant volume and constant pressure diffusion models are used to fit the gas adsorption and diffusion law.It can be found that the diffusion coefficients obtained from the constant-pressure diffusion model is two times larger than that obtained from the constant-volume diffusion model,and this magnification increases with the gas pressure increases.The constant-pressure boundary gives the gas molecules a free space to diffuse,while in the constant-volume boundary,the gas concentration gradually increases.The boundary of constant pressure is equivalent to a space where gas molecules can easily escape.Compared with the boundary of constant volume with gradually increasing concentration,the diffusion is less difficult and the diffusion coefficient is greater.Finally,according to the difference between the constant pressure diffusion coefficient and the constant volume diffusion coefficient,the application range of the two diffusion coefficients in the experiment and in the project is determined:the common constant-pressure diffusion model is appropriate for describing adsorption or desorption laboratory experiments in a fixed pressure environment,and gas emission law in high-permeability zone of coal seams.In contrast,the constant-volume diffusion model is appropriate for describing adsorption or desorption laboratory experiments in a fixed volume environment,as well as the gas emission law isolated by low-permeability coal,such as the region isolated by high stress concentration area,or faults and folds where tectonically deformed coal commonly exists.