Nanopore Structure of Different Rank Coals and Its Quantitative Characterization

Abstract

Based on gas adsorption theory, high-pressure mercury intrusion (HPMI), low-temperature liquid nitrogen gas adsorption (LT-N2GA), CO2 adsorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and small-angle X-ray scattering (SAXS) techniques were used to analyze the pore structures of six coal samples with different metamorphisms in terms of pore volume, specific surface area (SSA), pore size distribution (PSD) and pore shape. Combined with the gas adsorption constant a, the influence and mechanism of the pore structure of different coal ranks on gas adsorption capacity were analyzed. The results show that there are obvious differences in the pore structure of coals with different ranks, which leads to different adsorption capacities. To a large extent, the pore shapes observed by SEM are consistent with the LT-N2GA isotherm analysis. The pore morphology of coal samples with different ranks is very different, indicating the heterogeneity among the coal surfaces. Adsorption analysis revealed that mesopore size distributions are multimodal and that the pore volume is mainly composed of mesopores of 2–15 nm. The adsorption capacity of the coal body micropores depends on the 0.6–0.9 nm and 1.5–2.0 nm aperture sections. The influence of coal rank on gas desorption and diffusion is mainly related to the difference in pore structure. The medium metamorphic coal sample spectra show that the number of peaks in the high-wavenumber segment is small and that it is greater in the high metamorphic coal. The absorption intensity of the C–H stretching vibration peak of naphthenic or aliphatic hydrocarbons varies significantly among the coal samples. Over a small range of angles, as the scattering angle increases, the scattering intensity of each coal sample gradually decreases, and as the degree of metamorphism increases, the scattering intensity gradually increases. That is, the degree of metamorphism of coal samples is directly proportional to the scattering intensity. The influence of coal rank on gas adsorption capacity is mainly related to the difference in pore structure. The gas adsorption capacity shows an asymmetric U-shaped relationship with coal rank. For higher rank coals (Vdaf < 15%), the gas adsorption consistently decreases significantly with increasing Vdaf. In the middle and low rank coal stages (Vdaf > 15%), it increases slowly with the increase of Vdaf. We believe that the results of this study will provide a theoretical basis and practical reference value for effectively evaluating coal-rock gas storage capacity, revealing the law of CBM enrichment and the development and utilization of CBM resources.

Discussion on Technical Defects and Improvement of the Current Prediction Index System for Gas Outburst Risk Areas

“…The analysis showed that the methane adsorption amount showed a "U" shaped arc with the increase of coal rank, and the methane adsorption amount was the lowest in the coking and fertile coal stages. Li (31) studied and analyzed 8 types of coal samples with different metamorphic degrees, and there were signi cant differences in the adsorption capacity of different coal rank coal samples; As the degree of deterioration deepens, the adsorption capacity shows a "U" pattern of rst decreasing and then increasing, which is consistent with the conclusion drawn by Chen(32); Liu (33) selected anthracite, coking coal, gas coal, and long ame coal for mercury intrusion and isothermal adsorption experiments. The experimental results showed that as the degree of coal metamorphism increased, its maximum adsorption capacity showed an overall trend of decreasing increasing.…”

On Gas Desorption-Diffusion Regularity of Bituminous Coal with Different Particle Sizes and Its Influence on Outburst-Coal Breaking

“…The desorption-diffusion tests for coal with different particle sizes were conducted using a self-developed setup at Chongqing University, China, as shown in Figure 2. The structure and function of this setup are similar to the most present gas desorption tests setups [38,39], consisting of an adsorption-desorption unit, a vacuum pumping unit, a high-pressure inflation unit, and a desorption-diffusion measurement unit. In detail, the adsorption-desorption unit comprises a coal sample tank and a reference tank in the material of stainless steel, of which the former provides an airtight environment for coal samples adsorbing gas sufficiently to a dynamic equilibrium state, and the latter functions as a gas-pressure conditioner and compensator.…”

Pore and gas desorption characteristics of primary coal with different degrees of metamorphism

“…1,2 The study and evaluation of the pore structure in coal is the basic premise for accurately mastering the adsorption and desorption characteristics of coal, evaluating the permeability of coal, preventing coal and gas outbursts, and realizing efficient gas extraction. 3,4 Scholars have often examined the pore structure of coal: Jiang et al studied the pore characteristics of coal with different coal structures by low-temperature liquid nitrogen adsorption test. [5][6][7] Qi et al found that the burst coal and block coal had similar properties in terms of pore size distribution and surface area by collecting the burst coal and block coal and using a low-temperature nitrogen adsorption test.…”