Formation mechanism of methane during coal evolution: A density functional theory study

The formation mechanism of methane (CH₄) during coal evolution has been investigated by density functional theory (DFT) of quantum chemistry. Thermogenic gas, which is generated during the thermal evolution of medium rank coal, is the main source of coalbed methane (CBM). Ethylbenzene (A) and 6,7-dimethyl-5,6,7,8-tetrahydro-1-hydroxynaphthalene (B) have been used as model compounds to study the pyrolysis mechanism of highly volatile bituminous coal (R), according to the similarity of bond orders and bond lengths. All possible paths are designed for each model. It can be concluded that the activation energies for H-assisted paths are lower than others in the process of methane formation; an H radical attacking on β-C to yield CH₄ is the dominant path for the formation of CH₄ from highly volatile bituminous coal. In addition, the calculated results also reveal that the positions on which H radical attacks and to which intramolecular H migrates have effects on methyl cleavage.     

应用TG-FTIR联用研究催化剂对煤热解的影响

用TG-FTIR联用技术研究了碱金属、碱土金属和过渡金属对宝日希勒褐煤和包头烟煤热解的催化作用和挥发分析出的影响。结果表明，各种催化剂对褐煤和烟煤热解的催化效果分别为Ni>Fe~Ca>K和Ca~Fe>Ni>K，K2CO3对煤的热解没有明显的催化作用。催化剂使褐煤和烟煤热解转化率增加的最大值分别为10.1％和6.4％。烟煤热解生成的CH4比褐煤的多，不同的催化剂使煤热解挥发产物CO2、H2O、CH4和CO增加的幅度不一样，催化效果与温度和煤的变质程度有关。     

Sorption of methane and CO2 for enhanced coalbed methane recovery and carbon dioxide sequestration

Sequestration of CO2 in deep and unmineable coal seams is one of the attractive alternatives to reduce its atmospheric concentration. Injection of CO2 in coal seams may help in enhancing the recovery of coalbed methane. An experimental study has been carried out using coal samples from three different coal seams, to evaluate the enhanced gas recovery and sequestration potential of these coals. The coals were first saturated with methane and then by depressurization some of the adsorbed methane was desorbed. After partial desorption, CO2 was injected into the coals and subsequently they were depressurized again. Desorption of methane after the injections was studied, to investigate the ability of CO2 to displace and enhance the recovery of methane from the coals. The coals exhibited varying behavior of adsorption of CO2 and release of methane. For one coal, the release of methane was enhanced by injection of CO2, suggesting preferential adsorption of CO2 and desorption of methane. For the other two coals, CO2 injection did not produce incremental methane initially, as there was initial resistance to methane release. However with continued CO2 injection, most of the remaining methane was produced. The study suggested that preferential sorption behavior of coal and enhanced gas recovery pattern could not be generalized for all coals.     

Sorption of methane and CO2 for enhanced coalbed methane recovery and carbon dioxide seauestration

Sequestration of CO2 in deep and unmineable coal seams is one of the attractive alternatives to reduce its atmospheric concentration. Injection of CO2 in coal seams may help in enhancing the recovery of coalbed methane. An experimental study has been carried out using coal samples from three different coal seams, to evaluate the enhanced gas recovery and sequestration potential of these coals. The coals were first saturated with methane and then by depressurization some of the adsorbed methane was desorbed. After partial desorption, CO2 was injected into the coals and subsequently they were depressurized again. Desorption of methane after the injections was studied, to investigate the ability of CO2 to displace and enhance the recovery of methane from the coals. The coals exhibited varying behavior of adsorption of CO2 and release of methane. For one coal, the release of methane was enhanced by injection of CO2, suggesting preferential adsorption of CO2 and desorption of methane. For the other two coals, CO2 injection did not produce incremental methane initially, as there was initial resistance to methane release. However with continued CO2 injection, most of the remaining methane was produced. The study suggested that preferential sorption behavior of coal and enhanced gas recovery pattern could not be generalized for all coals.     

Study on gas adsorption and desorption characteristics on water injection coal

To study the desorption mechanism of methane in coal by H₂O injection and establish the Wiser molecular structure model of bituminous coal, the Grand Canonical Monte Carlo method was used to study the desorption behavior of CH₄ in coal with different amounts of H₂O injection at molecular scale. The results showed that at 293 K, the maximum adsorption capacity of H₂O was about 16 mmol/g, and that of CH₄ was about 8 mmol/g, which was about twice that of CH₄. This indicates that H₂O has a stronger adsorption capacity than CH₄. For methane-bearing coal, when the amount of water injected is 100, the average relative concentration of CH₄ is 0.5446, and the average relative concentration of CH₄ decreases by 33.77% compared to the water content of 20. Under the same time conditions, the root mean square displacement and diffusion coefficient of CH₄ decrease with the increase of H₂O injection quantity. With the increase of H₂O injection, the motion velocity of CH₄ in vacuum layer decreased. When water was injected, methane was trapped in the coal by water. The more H₂O injected, the more methane trapped in the coal, and the less methane desorption. This research lays a theoretical foundation for further research involving coal-water interaction.     

Qualitative and quantitative ATR-FTIR analysis and its application to coal char of different ranks

This paper analyzes the coal to char stages of char formation of six coals of different ranks by using Fourier transform infrared coupled w ith attenuated total reflectance(ATR-FTIR).The chars w ere obtained by coal pyrolysis carried out at temperature range of 450~700℃.The data obtained show s the pragmatic disappearance of the aliphatic hydrogen content w ith increasing char formation temperature.Numerical evaluation of the spectra enabled the determination of aromaticity,fa.The aromaticity w as found to be betw een 0.66~0.79 for lignite,0.75~0.90 for sub-bituminous,0.84~1.00 for low volatile bituminous,0.83~1.00 for high volatile bituminous,0.94~1.00 for semi-anthracite,and 0.97~1.00 for anthracite respectively.With increasing rank of coal samples,spectra exhibit rising aromaticity and enhanced condensation of aromatic rings,w hereas the aliphatic chain lengths decrease.     

Effects of supercritical CO2 exposure on diffusion and adsorption kinetics of CH4, CO2 and water vapor in various rank coals

The physico-chemical effects caused by supercritical CO₂ (ScCO₂) exposure is one of the leading problems for CO₂ storage in deep coal seams as it will significantly alter the flow behaviors of gases. The main objective of this study was to investigate the effects of ScCO₂ injection on diffusion and adsorption kinetics of CH₄, CO₂ and water vapor in various rank coals. The powdered coal samples were immersed in ScCO₂ for 30 days using a high-pressure sealed reactor. Then, the diffusion and adsorption kinetics of CH₄, CO₂ and water vapor in the coals both before and after exposure were examined. Results indicate that the diffusivities of CH₄ and CO₂ are significantly increased due to the combined matrix swelling and solvent effect caused by ScCO₂ exposure, which may induce secondary faults and remove some volatile matters that block the pore throats. On the other hand, the diffusivities of water vapor are reduced due to the elimination of surface functional groups with ScCO₂ exposure. It is concluded that density of the surface function groups is the controlling factor for water vapor diffusion rather than the pore properties. The unipore model and pseudo-first-order equation can simulate the diffusion and adsorption kinetics of CH₄ and CO₂ very well, but the unipore model is not capable of well describing water vapor diffusion. The effective diffusivity (D_e), diffusion coefficient (D) and adsorption rates (k₁) of CH₄ and CO₂ are significantly increased after ScCO₂ exposure, while the values of water vapor are decreased notably. Thus, the injection of ScCO₂ will efficiently improve the transport properties of CH₄ and CO₂ but hinder the movement of water molecules in coal seams.     

FT-IR和SEM用于煤阶对煤催化加氢气化影响的研究

基于加压固定床反应器研究了不同煤阶的煤催化加氢气化效果,对比了煤阶对催化剂添加量、甲烷释放速率以及产品气组成的影响。原煤及气化残渣采用FT-IR和SEM进行表征分析。研究结果表明,不添加催化剂情况下,随着煤阶的升高原煤气化反应性降低,低阶煤的甲烷释放分为两个阶段;加入催化剂之后,3种煤中以神府烟煤的反应性最好,遵义无烟煤和云南褐煤反应性较差。SEM和FT-IR表征结果表明,高阶煤颗粒表面更加光滑、煤结构致密,而煤中的脂肪族结构以及芳香结构振动峰强度随煤阶的降低而升高,催化剂的加入使得脂肪结构吸收峰明显加强。这些差异导致不同煤样催化加氢气化反应活性不同。     

Thermokinetic behavior and microcharacterization of low-rank bitumiteoxidization

Journal of Thermal Analysis and Calorimetry - As to the continuous consumption of limited high-rank coal resources, low-rank bituminous coal is gradually grabbing more attention to the utilization...     

DSC-TG studies of coal structure modification by the inert gas helium

The influence of the inert gases helium and argon on the thermal properties of coal have been studied using DSC and TG. The coals studied were high-volatile bituminous coal and anthracite. It has been stated that the heat capacity of coal is strongly influenced by helium. This phenomenon is especially distinct for high-volatile bituminous coal. The authors attempt to explain this phenomenon.     

Thermal analysis study of the effect of coal-burning additives on the combustion of coals

Thermal analysis is widely used in combustion research for both fundamental and practical investigations. Efficient combustion of coals in cement industry is very important and necessary. In this research, the effects of three coal-burning additives on burning behaviour of bituminous coal and anthracite were studied with the help of thermogravimetry (TG) and differential scanning calorimetric (DSC) analysis. The kinetic study was carried out and the results were presented. The results showed that the coal-burning additives especially LSZ can reduce the ignition temperature, increase the ignition index D _i, combustion ending index D _f and affect the activation energy of the coal samples studied. The coal-burning additives especially LSZ can improve coal combustion effectiveness.     

Competitive adsorption equilibria of CO2 and CH4 on a dry coal

Gases like CO₂ and CH₄ are able to adsorb on the coal surface, but also to dissolve into its structure causing the coal to swell. In this work, the binary adsorption of CO₂ and CH₄ on a dry coal (Sulcis Coal Province, Italy) and its swelling behavior are investigated. The competitive adsorption measurements are performed at 45?°C and up to 190 bar for pure CO₂, CH₄ and four mixtures of molar feed compositions of 20.0, 40.0, 60.0 and 80.0% CO₂ using a gravimetric-chromatographic technique. The results show that carbon dioxide adsorbs more favorably than methane leading to an enrichment of the fluid phase in CH₄. Coal swelling is determined using a high-pressure view cell, by exposing a coal disc to CO₂, CH₄ and He at 45 and 60?°C and up to 140 bar. For CO₂ and CH₄ a maximum swelling of about 4 and 2% is found, whereas He shows negligible swelling. The presented adsorption and swelling data are then discussed in terms of fundamental, thermodynamic aspects of adsorption and properties which are crucial for an ECBM operation, i.e. the CO₂ storage capacity and the dynamics of the replacement of CH₄ by CO₂.     

On the porous structure of coals: Evidence for an interconnected but constricted micropore system and implications for coalbed methane recovery

An experimental and theoretical study of adsorption and diffusion of carbon dioxide and methane in coals of widely varying rank was carried out. Low pressures adsorption isotherms of CO₂ were obtained and analyzed using Dubinin's theory of volume filling of micropores. High-pressure adsorption isotherms of CH₄ were obtained and analyzed using tracer pulse chromatography in conjunction with an appropriate adsorption/diffusion model. A preliminary¹²⁹Xe NMR analysis of chemical shifts experienced by xenon atoms in particles of different sizes is also reported.The heretofore undocumented and/or underestimated effects of activated diffusion of CO₂ at 273–298 K complicate the elucidation of the true microporous structure of coals, especially its dependence on coal rank. Activated diffusion of both CO₂ and methane at room temperature does not allow reliable estimates of coalbed gas content to be made. A model of an interconnected network of pores which includes randomly distributed, numerous and ultramicroporous constrictions (at any size scale) is consistent with all these experimental and theoretical findings.Presented in part at the International Conference on Coal Science, Banff, Alberta, Canada. September 1993, and at the 21st Biennial Conference on Carbon, Buffalo, NY, June 1993.     

烟煤和无烟煤中碘的赋存形态及其环境效应分析

采用逐级化学提取和电感耦合等离子体质谱(ICP-MS)研究烟煤和无烟煤中碘的赋存形态。结果表明,烟煤和无烟煤中各种形态的碘含量由高到低依次都是有机态、铁锰氧化物结合态、水溶态、残留态、离子交换态和碳酸盐结合态。烟煤和无烟煤中有机结合态碘平均含量分别为47.4%±3.28%和43.3%±2.42%,Fe-Mn氧化物结合态碘分别为36.8%±3.70%和34.6%±4.77%,水溶态和离子交换态碘平均含量之和分别为10.3%±3.00%和14.3%±3.37%,碘的平均潜在淋失率分别10.9%和16.0%,平均潜在可淋失浓度分别为0.9μg/g和1.2μg/g。这说明无烟煤中生物有效态碘含量高于烟煤,在表生条件下烟煤和无烟煤中淋出的碘都能导致地表水中碘的浓度升高;无烟煤中碘的有利环境效应大于烟煤,而不良环境效应小于烟煤。     

用电子自旋共振谱研究我国一些煤的自由基

本文用电子自旋共振谱(ESR)研究了我国十种不同变质程度的煤。电子自旋共振最重要的参数是g值、自由基的浓度、线宽和线形。用这些参数与煤中的碳、氧等元素相关联,发现ESR参数与煤的变质程度有密切关系。     

Semiconductor Photocatalysts for Non-oxidative Coupling,Dry Reforming and Steam Reforming of Methane

Methane is one of the promising alternatives of petroleum, which should be used for not only a fuel but also a resource for hydrogen and more useful chemicals as with the petroleum. However, the selective methane conversion to them is still difficult in contrast to the combustion. Three types of photocatalytic reactions for methane conversion, i.e., the photocatalytic non-oxidative coupling of methane (2CH₄ → C₂H₆ + H₂), the photocatalytic dry reforming of methane (CH₄ + CO₂ → 2CO + 2H₂) and the photocatalytic steam reforming of methane (CH₄ + 2H₂O → 4H₂ + CO₂), can take place around room temperature or at a mild condition such as 473 K using photoenergy and semiconductor photocatalyst. In the present short review, the details of each photocatalytic reaction and the design concept of the semiconductor photocatalysts for each photocatalytic methane conversion were summarized and discussed.     

Preparation and characterization of colloidal dispersions of graphene-like structures from different ranks of coals

This paper focuses on preparation of colloidal solution of graphene-like structures from different ranks of coals: brown coal, bituminous coal, low-volatile bituminous coal, anthracite. It was found that brown coal thermo-oxidative destruction leads to formation of small d=32 nm (V=17%) and large d=122 nm (V=11%) fractions of nanoparticles. The thermo-oxidative destruction of bituminous coal leads to formation of nanoparticles d=50 nm (V= 5.2%) and d=164 nm (V=16%). Thermo-oxidative destruction of low-volatile bituminous coal and anthracite leads to formation of nanoparticles, predominantly, d= 122-190 nm. Carbon nanostructures obtained from coal are negatively charged at pH=2-12. Colloidal solution of carbon nanostructures at dispersed phase concentration 0.01 mg/mL is stable for 1 month. Electron diffraction patterns and X-ray analysis of carbon nanostructures showed that nanostructure from brown coal is amorphous and nanostructure from anthracite is crystalline. Results of coal macromolecules modeling and graphene-like structures obtained from them are presented.     

A model for enhanced coal bed methane recovery aimed at carbon dioxide storage

Numerical simulations on the performance of CO₂ storage and enhanced coal bed methane (ECBM) recovery in coal beds are presented. For the calculations, a one-dimensional mathematical model is used consisting of mass balances describing gas flow and sorption, and a geomechanical relationship to account for porosity and permeability changes during injection. Important insights are obtained regarding the gas flow dynamics during displacement and the effects of sorption and swelling on the ECBM operation. In particular, initial faster CH₄ recovery is obtained when N₂ is added to the injected mixture, whereas pure CO₂ allows for a more effective displacement in terms of total CH₄ recovery. Moreover, it is shown that coal swelling dramatically affects the gas injectivity, as the closing of the fractures associated with it strongly reduces coal’s permeability. As a matter of fact, injection of flue gas might represent a useful option to limit this problem.     

中国煤中溴的含量及分布

在测试分析中国27个省、直辖市和自治区的305个煤样中溴含量的基础上,从煤的不同成煤年代、不同变质程度以及不同聚煤区等方面分析和考查中国煤中溴的分布状况。结果表明,中国煤中溴的含量为0.12~69.66μg/g,因其对数值呈正态分布,故可用其几何平均值(7.04μg/g)表征中国煤中溴的含量,该值低于大多数其他国家煤中溴的平均含量,和日本煤中溴的平均值(7.10μg/g)较为接近。按煤中溴的平均值,中国13个省市小于5μg/g;9个省为5~15μg/g;5个省市高于15μg/g。根据煤的变质程度,中国煤中溴含量按照烟煤、无烟煤、褐煤、次烟煤顺序依次下降;根据成煤地质年代,按早石炭纪、新近纪、晚石炭纪、早二叠纪、中侏罗纪、晚三叠纪、早侏罗纪、晚侏罗纪、古近纪、中石炭纪依次下降;根据聚煤区,按西北、华北、华南、滇藏和东北聚煤区依次下降。中国煤中溴的分布受多种因素的影响,任何单一因素和其没有密切的相关性。     

水煤浆制备过程中浆体抗搅拌性能的研究

选用两种不同类型的分散剂，考察了１１种不同变质程度煤在高浓度煤浆制备过程中的浆体抗搅拌性能。发现高浓度煤浆制备对应的最佳搅拌强度不仅与分散剂的种类有关，而且取决于煤的变质程度。浆体抗搅拌性能随制浆用煤的变质程度的提高而增强，从而使较高变质程度煤的高浓度煤浆制备需在较高的搅拌强度下进行，才能最大限度地提高煤的成浆性。相关分析表明，煤表面分布的羧基、酚羟基等亲水性基团对浆体抗搅拌性会产生重要影响，煤表面此类基团的含量较高或吸水性较强时可使高浓度煤浆制备的最佳搅拌强度大幅度降低。