Pore characterization of different types of coal from coal and gas outburst disaster sites using low temperature nitrogen adsorption approach

To characterize the pore features of outburst coal samples and investigate whether outburst coal has some unique features or not, one of the authors, working as the member of the State Coal Mine Safety Committee of China, sampled nine outburst coal samples(coal powder and block) from outburst disaster sites in underground coal mines in China, and then analyzed the pore and surface features of these samples using low temperature nitrogen adsorption tests. Test data show that outburst powder and block coal samples have similar properties in both pore size distribution and surface area. With increasing coal rank, the proportion of micropores increases, which results in a higher surface area. The Jiulishan samples are rich in micropores, and other tested samples contain mainly mesopores, macropores and fewer micropores. Both the unclosed hysteresis loop and force closed desorption phenomena are observed in all tested samples. The former can be attributed to the instability of the meniscus condensation in pores,interconnected pore features of coal and the potential existence of ink-bottle pores, and the latter can be attributed to the non-rigid structure of coal and the gas affinity of coal.

Adsorption behavior of carbon dioxide and methane in bituminous coal:A molecular simulation study

The adsorption behavior of CO_2, CH_4 and their mixtures in bituminous coal was investigated in this study. First, a bituminous coal model was built through molecular dynamic(MD) simulations, and it was confirmed to be reasonable by comparing the simulated results with the experimental data. Grand Canonical Monte Carlo(GCMC)simulations were then carried out to investigate the single and binary component adsorption of CO_2 and CH_4with the built bituminous coal model. For the single component adsorption, the isosteric heat of CO_2 adsorption is greater than that of CH_4 adsorption. CO_2 also exhibits stronger electrostatic interactions with the heteroatom groups in the bituminous coal model compared with CH_4, which can account for the larger adsorption capacity of CO_2 in the bituminous coal model. In the case of binary adsorption of CO_2 and CH_4mixtures, CO_2 exhibits the preferential adsorption compared with CH_4 under the studied conditions. The adsorption selectivity of CO_2 exhibited obvious change with increasing pressure. At lower pressure, the adsorption selectivity of CO_2 shows a rapid decrease with increasing the temperature, whereas it becomes insensitive to temperature at higher pressure. Additionally, the adsorption selectivity of CO_2 decreases gradually with the increase of the bulk CO_2 mole fraction and the depth of CO_2 injection site.

Preferential adsorption behaviour of CH_4 and CO_2 on high-rank coal from Qinshui Basin,China

In order to better understand the prevailing mechanism of CO2 storage in coal and estimate CO2 sequestration capacity of a coal seam and enhanced coalbed methane recovery （ECBM） with CO2 injection into coal, we investigated the preferential adsorption of CH4 and CO2 on coals. Adsorption of pure CO2, CH4 and their binary mixtures on high-rank coals from Qinshui Basin in China were employed to study the preferential adsorption behaviour. Multiple regression equations were presented to predict CH4 equi- librium concentration from equilibrium pressure and its initial-composition in feed gas. The results show that preferential adsorption of CO2 on coals over the entire pressure range under competitive sorption conditions was observed, however, preferential adsorption of CH4 over CO2 on low-volatile bituminous coal from higher CH4-compostion in source gas was found at up to 1O MPa pressure. Preferential adsorp- tion of CO2 increases with increase of CH4 concentration in source gas, and decreases with increasing pressure. Although there was no systematic investigation of the effect of coal rank on preferential adsorp- tion, there are obvious differences in preferential adsorption of gas between low-volatile bituminous coal and anthracite. The obtained preferential adsorption gives rise to the assumption that CO2 sequestration in coal beds with subsequent CO2-ECBM might be an ootion in Qinshui Basins, China.

Influence of Coal Fly Ash Particle Size on Structure and Adsorption Properties of Forming Adsorbents for Cr^6＋

Forming adsorbents FFA-R,FFA-A and FFA-B were prepared from different particle size coal fly ashes FA-R,FA-A and FA-B,their average particle sizes（d_（50）） were 15.75,3.61 and 1.73 μm respectively.The structure and adsorption properties for Cr^（6＋） of forming adsorbents from aqueous solution were studied.The results show that forming adsorbent prepared from the coal fly ash with smaller particle size exhibits higher specific surface area,higher pore volume and better adsorption properties for Cr^（6＋）.The adsorption kinetics of Cr^（6＋） on FFA-R,FFA-A and FFA-B fitts the second order kinetic model and the second adsorption rate constants are 7.523,3.197 and 2.187 mg·g^（-1）·min^（-1/2）,respectively.The adsorption of Cr^（6＋） on FFA-R,FFA-A and FFA-B can be described in terms of Langmuir isotherms better,and the adsorption processes are spontaneous and exothermic.

Analysis of irrationality of coal susceptibility to spontaneous combustion determination method with fluid oxygen adsorption

Based on experiment results and theoretical analysis,pointed out that the method of coal susceptibility to spontaneous combustion determination with fluid oxygen adsorption can not present the essence of coal oxidation process and oxidation reaction. The method is incorrect,paying attention at one aspect and ignoring the rest.The method is not reasonable for coal susceptibility to spontaneous combustion determination.Sus- ceptibility to spontaneous combustion of coal reflects chemical property of coal oxidation with oxygen absorption and heat release at low temperature.Coal's susceptibility to spon- taneous combustion is mainly decided by the number of molecules with reaction activation energy and activation molecule production rate at certain temperature.Therefore,index of susceptibility to spontaneous combustion should adopt accumulative value or trend of heat release or oxygen adsorption during oxidation process.

Deformed coal types and pore characteristics in Hancheng coalmines in Eastern Weibei coalfields

Based on SEM observance,the methods of low-temperature nitrogen and isothermal adsorption were used to test and analyze the coal samples of Hancheng,and pore structure characteristics of tectonic coals were discussed.The results indicate that in the same coal rank,stratification and crack are well developed in cataclastic coal,which is mostly filled by mineral substance in the geohydrologic element abundance,results in pore connectivity variation.Granulated and mylonitic coal being of these characteristics,as develop microstructures and exogenous fractures as well as large quantity of pores resulted from gas generation and strong impermeability,stimulate the recovery of seepage coal,improve coal connectivity and enhance reservoir permeability.Absorption pore(micro-pore) is dominant in coal pore for different coal body structure,the percentage of which pore aperture is from 1 to 100 nm is 71.44% to 88.15%,including large of micro-pore with the 74.56%-94.70%;with the deformation becoming more intense in the same coal rank,mesopore enlarge further,open-end pores become thin-neck-bottle-shaped pores step by step,specific surface area of micro-pore for cataclastic coal is 0.0027 m 2 /g,while mylonitic coal increases to 7.479 m 2 /g,micro-pore gradually play a dominant role in effecting pore structural parameters.

Deformation transition of intact coal induced by gas injection

Gas migration in coal bed is a multiple-physical process, of which not only includes gas desorption/diffusion through coal matrix and gas Darcy flow through the cleat system, but also results in deformation of solid coal. Especially for enhanced coal bed methane(ECBM) and CO2 capture and sequestration(CCS), gas injection is mainly controlled by the gas diffusivity in the coal matrix and coal permeability.Although the relevant coal permeability models have been frequently developed, how the dual-porosity system of coal affects gas adsorption/diffusion is still poorly understood. In this paper, a series of experiments were carried out in order to investigate deformation evolution of intact coal subjected to hydrostatic pressure of different gases(including pure H2, N2 and CO2) under isotherm injection. In the testing process, the coal strain and injected gas pressure were measured simultaneously. The results show that the pressure of non-adsorptive helium remained unchanged throughout the isothermal injection process, in which the volumetric strain of the coal shrinked firstly and maintained unchanged at lower isobaric pressure. With the injected pressure increasing, the coal volume underwent a transition from shrinking to recovery(still less than initial volume of the coal). In contrast, N2 injection caused the coal to shrink firstly and then recover with decreasing gas pressure. The recovery volume was larger than the initial volume due to adsorption-induced swelling. For the case of CO2 injection, although the stronger adsorption effect could result in swelling of the solid coal, the presence of higher gas pressure appears to contribute the swelling coal to shrink. These results indicate that the evolution of coal deformation is time dependent throughout the migration of injected gas. From the mechanical characteristics of poroelastical materials, distribution of pore pressure within the coal is to vary with the gas injection,during which the pore pressure in the cleats will rapidly increase, in contrast, the pore pressure in the matrix will hysteretically elevate. Such a difference on changes of pore pressure between the cleats and the matrix will contribute to the shrinkage of the matrix as a result of initially greater effective stress.Besides, both gas-adsorption-induced swelling and decreasing effective stress also control the coal deformation transition. This work gives us an insight into investigation on influence of effective stress on coal-gas interaction.