Low-rank Coal Samples Research Articles

Spectral signatures of solvent-extracted macromolecules in Indian coals of different rank: Insights from fluorescence excitation-emission matrix

Solvent-extracted fractions of six Indian coal samples of different ranks were investigated using multiple geochemical, petrological and spectroscopic proxies and an attempt was made to indicate possible fingerprint regions for different polycyclic aromatic hydrocarbons (PAH) with the help of excitation-emission matrix (EEM). In this study, for the very first time, the influence of rank and maturation of organic matter in the characterisation of coal solvent-extracts from Indian coals were perceived from the viewpoint of fluorescence EEM. Vitrinite reflectance (VRo) values were used to determine the general ranks of the original coal samples viz. lignite, subbituminous, bituminous and anthracite. Different fluorescence peak regions corresponding to different fused aromatic ring (FAR) systems were delineated using the EEM and their indicative depositional environments could be inferred. Our observations indicate that solvent-extracted fractions of low rank coals comprise of a larger number of shorter carbon chains compared to the other samples. For the low rank coal samples, the solvent-extracts show a strong humic influence and the presence of smaller PAH rings while for the medium rank coals, the extracted fractions tend to show a more bimodal distribution of PAHs, possibly comprising of different sized PAHs. Higher fluorescence sensitivity and quick response of smaller PAHs impart a singular centralised region in the EEM for the low rank coal samples while interference in the fluorescence of differently sized PAHs indicate a multimodal distribution of the fluorophores in the medium rank coals. The high rank coal used in this study shows a bimodal distribution with very low intensity of the peaks, indicating the low abundance of extractable macromolecules, possibly as a result of deformation.

Low-rank coal upgrading to optimize its utilization as fuel

Abstract Indonesia’s coal resources and reserves are mostly categorized as low-rank coal, lignite and sub-bituminous coals. Most of them have a high content of moisture, a low calorific value, and a high tendency to low-temperature oxidation and self-combustion, making them difficult to transport over long distances. To increase the utilization of the low-rank coals, a drying treatment was carried out by heating the coal at a temperature and pressure of 160°C and 3.5 Bar respectively for about 15 minutes. Two low-rank coal samples, from South Sumatra (A coal) and South Kalimantan (B coal) were used in this research. The result indicates that the degree of dewatering of A coal is 89.00% while B coal is 95.5%. B Coal has a higher inherent moisture content which generally has more capillary water which is relatively easier to remove. The higher the degree of dewatering the higher the calorific value degree of improvement. The calorific value of the A coal increased by 27.16% and the B coal increased by 40.65%. The combustion characteristic obtained from the thermogravimetry-differential thermal analysis shows that the A and B upgraded coals have a lower tendency to spontaneous combustion than that of the raw coals.

Biogenic Degradation of Lignite Coal from Pakistani Coal Mine and Extraction of Humic Acid through Pretreatment Strategies by Fungal Isolate

The utilization of white rot fungal strain for degradation of low rank coal from coal environments is considered to be the best choice for converting raw coal into different kinds of valuable by products. NF-1, a fungal strain was isolated from coal mine of Thar region of Pakistan. For enhancing the process of biodegradation, the indigenous fungal isolate was used in the study. The lignite (low rank) coal sample degradation initiated by NF-1 strain were undergone through optimization process and had shown considerable production of organics at 1.5% Glucose, 1% coal loading ratio and 7 days of incubation period. The FTIR analysis of untreated coal and treated coal after solubilization was performed in order to observe changes in structural pattern by NF-1 fungal isolate. NF1 has the ability to break down the aromatic side linkages and attacking the amine and carboxyl groups. Scanning electron microscopy (SEM) of the coal treated by NF-1 strain was analyzed to investigate the attachment and adherence of the fungal mycelium on the coal surface. At 200µm the NF-1 isolate hyphae has clearly entrapped the coal particles showing erosion and up to 50µm colonization and visible changes on the coal surface can be observed clearly. Moreover, the residual coal after fungal pretreatment was processed for extraction of humic acid as a byproduct for defining the effect of such biological pretreatments. The produced humic acid was analyzed on the basis of E4/E6 ratio and FTIR. The fungal pretreated humic acid has shown more improvement in E4/E6 ratio as well as presence of functional groups related to humic acid through FTIR. This explains that the isolate NF1 have ability to speed up the biodegradation process and shows an aptitude of producing valuable by products i.e. humic acid as an alternative fuel.

Full-Scale Pore Structure Characterization and Its Impact on Methane Adsorption Capacity and Seepage Capability: Differences between Shallow and Deep Coal from the Tiefa Basin in Northeastern China

Deep low-rank coalbed methane (CBM) resources are numerous and widely distributed in China, although their exploration remains in its infancy. In this work, gas adsorption (N2/CO2), mercury intrusion porosimetry, and 3D CT reconstruction were performed on five coal samples of deep and shallow low-rank coal from northeast China to analyze their pore structure. The impact of the features in the pore structure at full scale on the capacity for methane adsorption and seepage is discussed. The results indicate that there are significant differences between deep low-rank coal and shallow low-rank coal in terms of porosity, permeability, composition, and adsorption capacity. The full-scale pore distribution was dispersed over a broad range and exhibited a multi-peak distribution, with the majority of the peak concentrations occurring between 0.45–0.7 nm and 3–4 nm. Mesopores are prevalent in shallow coal rock, whereas micropores are the most numerous in deep coal rock. The primary contributors to the specific surface area of both deep and superficial coal rock are micropores. Three-dimensional CT reconstruction can characterize pores with pore size greater than 1 μm, while the dominating equivalent pore diameters (Deq) range from 1 to 10 μm. More mini-scale pores and fissures are observed in deep coal rock, while shallow coal rock processes greater total and connection porosity. Multifractal features are prevalent in the fractal qualities of all the numbered samples. An enhancement in pore structure heterogeneity occurs with increasing pore size. The pore structure of deep coal rock is more heterogeneous. Furthermore, methane adsorption capacity is favorably connected with D1 (0.4 nm < pore diameter ≤ 2 nm), D2 (2 nm < pore diameter ≤ 5 nm), micropore volume, and specific surface area and negatively correlated with D3 (5 nm < pore diameter ≤ 50 nm), showing that methane adsorption capability is primarily controlled by micropores and mesopores. Methane seepage capacity is favorably connected with the pore volume and connected porosity of macropores and negatively correlated with D4 (pore diameter > 50 nm), indicating that the macropores are the primary factor influencing methane seepage capacity.

Research on the main control factors of gas desorption in middle and low rank coals based on orthogonal testing

To study the factors influencing gas desorption by middle rank and low rank coal, gas adsorption and desorption experiments of middle rank and low rank coal samples were designed with the orthogonal test method by considering the particle sizes, adsorption equilibrium pressures, temperatures and water contents of the coal samples, and the main factors affecting gas desorption from the middle rank and low rank coal samples were investigated. The results showed that there were obvious differences in the desorption capacity of the middle rank and low rank coals in the designed experimental scheme, and the maximum cumulative amounts of gas desorbed were 6.11 ml/g for Xinjiang low rank coal and 6.50 ml/g for Hebi middle rank coal. The relative sensitivities to the factors affecting gas desorption by Xinjiang low rank coal decreased in the order adsorption equilibrium pressure > particle size > water content > temperature, which the effect of adsorption equilibrium pressure on gas desorption from Xinjiang low rank coal was the most obvious and main controlling factor for gas desorption from Xinjiang low rank coal. The relative sensitivities to the factors affecting gas desorption by Hebi middle rank coal decreased in the order particle size > adsorption equilibrium pressure > temperature > water content, which the effect of particle sizes on gas desorption from Hebi middle rank coal was the most obvious and main controlling factor for gas desorption from Hebi middle rank coal. The cumulative amounts of gas desorbed from Xinjiang low rank coal first increased and then decreased with increasing coal grain sizes, and the cumulative amounts of gas desorbed from the Hebi middle rank coal gradually decreased with increasing coal grain size. The cumulative amounts of gas desorbed from both Xinjiang low rank coal and Hebi middle rank coal gradually increased with increasing adsorption equilibrium pressure.

The differences on gas–water relative permeability characteristics of low and high rank coal and its controlling factors

A comprehensive comparison was conducted in terms of the gas–water relative permeability characteristics of high rank coals from south Qinshui Basin and low rank coals from Erlian Basin, China. The results show that: High rank coal samples are featured with a larger percentage of micropores. Compared with high rank coal samples, low rank ones display a smaller irreducible water saturation (S wo) and higher methane (K rg) and water (K rw) permeability and wider spans of two-phase flow region. High rank coal samples with a large percent of micropores are featured with a larger S wo and a narrower two-phase flow span. Gas displacement pressure increases K rg, while decreases K rw and S wo, and the changing rule is more pronounced in low rank coal samples. The effective stress is inversely proportional to the K rg and the span of two-phase flow, whereas proportional to the K rw. Influenced by effective stress, the dynamic changes of the cross points are more obvious in the low rank coal samples, however that of two-phase flow spans and S wo are not so pronounced. The conclusion will be helpful for CBM exploration and development.

Thermodynamic characteristics of methane adsorption on coals from China with selected metamorphism degrees: Considering the influence of temperature, moisture content, and in situ modification

A better understanding of the thermodynamic characteristics of methane (CH4) adsorption on coal surfaces is needed for the high-efficiency coalbed methane (CBM) preextraction or prevention of coal and gas outbursts. In this study, atomistic representations of selected coal samples with different degrees of metamorphism, such as long flame coal, coking coal, and anthracite coal obtained from China were created; CH4 adsorption isotherms considering the temperature, moisture content, and microwave in situ modification for CBM preextraction were simulated and analysed. The coal molecular structural results showed that as the degree of coal metamorphism increased from long flame coal to coking coal and anthracite coal, the functional groups in the molecular structure of the coal reduced, while the degree of graphite increased. In situ modification of coal facilitated this process. The saturation adsorption capacity of CH4 decreased and then increased with increasing coal metamorphism degree and decreased linearly with increasing temperature. When water molecules were present, the adsorbed CH4 showed different degrees of decreasing trends with increasing moisture content. The isosteric heat of CH4 adsorption followed the order of coking coal > anthracite coal > long flame coal. The isosteric heat gradually decreased with increasing methane adsorption, and the presence of water molecules had a limited effect on the isosteric heat. In situ modification disrupted the self-conjugated hydrogen bonds, reduced the oxygen-containing functional groups, and decomposed some associated minerals in the coal samples. After in situ modification, the amount of methane adsorption decreased due to a greater contribution from the functional group change than that from the pore volume change, and the modification effect was more evident for the low-rank coal samples than for the middle and high metamorphism degree coal samples. In situ modification applied in the coal seams could contributed to designs for CBM exploration, and the results of this experiment and simulation could provide a novel theoretical basis for in situ modification technology.

Adsorption processes on a carbonaceous surface: Electron spin resonance study

The effect of pyrolysis at T ∼700 °C of the low-rank coal samples was studied using scanning electron microscopy (SEM), electron paramagnetic resonance (EPR), and Raman scattering of light methods. The kinetics of adsorption-desorption processes on a carbon surface was first investigated based on the dynamics of the spin characteristics of paramagnetic defects (carbon dangling bonds). The EPR signals of these defects in the air are characterized by the anomalously large broadening up to ΔBpp = 32 mT, nearly two orders of magnitude in comparison with the initial samples. It is established that broadening is caused by the interaction of the spins of defects with molecular oxygen. The latter is adsorbed on the surface of the samples in a high concentration because the samples have a high porosity formed with the participation of ash nanoclusters, including FexOy. The observed EPR dynamics are very complex. It depends on the initial conditions of signal observation, the occurrence of other gases, e.g. nitrogen, and drastically on the humidity of the environment. For example, holding samples in the water vapor at T = 100 °C reduces the ΔBpp from ∼17.5 mT to 3.2 mT for a time of nearly t ∼24 s. Using the strong effect of oxygen on the broadening of the studied samples, the kinetics of competitive processes of adsorption/desorption of N2↔O2↔H2O on the carbon surface is studied in detail. The unusual adsorption kinetics for the mixture of O2 and N2 gases, caused by a difference in the diffusion rates, was found.

Adsorption and desorption behavior under coal-water-gas coupling conditions of high- and low-rank coal samples

Adsorption and desorption behavior under coal-water-gas coupling conditions of high- and low-rank coal samples

Pore Structure Characteristics and Adsorption and Desorption Capacity of Coal Rock after Exposure to Clean Fracturing Fluid.

In the hydraulic fracturing process, fracturing fluid contacts coal rock and physical and chemical reactions occur, which inevitably damage the pore structure of the coal rock and affect the adsorption and desorption capacity of the coal rock. In this paper, a low-temperature N2 adsorption method and scanning electron microscopy (SEM) were used to characterize coal samples. Using gas adsorption/desorption tests, high-, medium-, and low-rank coal samples before and after the clean fracturing fluid treatment were systematically studied. According to the relationship between coal pore structure parameters and gas adsorption/desorption characteristics, a correlation between the microscopic pore structure and the macroscopic gas adsorption/desorption characteristics of coal was obtained. The results show that the number of closed pores in high-, medium-, and low-rank coal samples increased after the clean fracturing fluid treatment. The micropore volume increased by 0.0009, 0.00143, and 0.0035 mL/g, respectively, and the specific surface area increased by 4.87, 9.06, and 57.60%. The fractal dimension also increased compared with that of raw coal. SEM analysis indicated that the influence degree of clean fracturing fluid treatment on the pore structure of different-rank coal samples was Gengcun low-rank coal > Pingba middle-rank coal > Jiulishan high-rank coal. The experimental results of methane adsorption and desorption showed that the adsorption capacity of the coal samples after clean fracturing fluid treatment was enhanced, which is related to increases in the micropore proportion, micropore volume, and specific surface area of the coal. The desorption capacity of the coal samples was also enhanced. The desorption rate of medium- and high-rank coal samples increased after the clean fracturing fluid treatment but that of low-rank coal samples decreased. The main reason is the increase in the number of micropores in low-rank coal, which enhances the gas adsorption ability and makes gas desorption difficult. Therefore, clean fracturing fluid is suitable for medium- and high-grade metamorphic coalbed methane mines. These research results provide a theoretical basis for the application of clean fracturing fluid in different coalbed methane wells.

Study of the Microstructural Characteristics of Low-Rank Coal under Different Degassing Pressures

Low-rank coal samples from the Xishanyao Formation in the southern Junggar basin of Xinjiang were processed under different negative pressures in order to examine the microstructural characteristics of low-rank coal reservoirs. The pore structures of low-rank coal under different negative pressures were tested using scanning electron microscopy, low-temperature nitrogen adsorption–desorption, and water saturation and centrifugal low-field NMR experiments. The results showed that for the low-rank coal samples from the study area, a high portion of the porosity and surface area came from micropores and small pores; the fractal dimension of the adsorption pores of the low-rank coal samples was divided into surface fractal dimension D1 and structural fractal dimension D2, which showed that the microstructure of the low-rank coal from the study area was complex. The transverse relaxation times T2 of the low-rank coal samples in the test were approximately 0.1~2.5, approximately 10, and greater than 100 ms; the T2 spectrum had basically three peak types. By combining scanning electron microscopy and nuclear magnetic resonance tests, it was concluded that the pore connectivity of the low-rank coal reservoirs in the study area was poor and the effective porosity was relatively low, which may be unfavorable for the exploration and development of coalbed methane.

A combined experimental and numerical study of coal briquettes pyrolysis using recycled gas in an industrial scale pyrolyser

Low-rank coal pyrolysis plays a significant role in clean coal utilisation in various coal-based industries. Using recycled gas or exhaust gas as heat carrier in the pyrolyser is a promising way for improving its energy efficiency and reducing carbon emission. In this study, an experimental and numerical combined study is conducted to cost-effectively investigate the effect of utilising the recycled gas on coal pyrolysis in an industrial scale gas-heat-carrier pyrolyser. First, a lab-scale two-stage pyrolyser is employed to pyrolyze the low-rank coal samples to obtain the gas products composition. The experimental results are subsequently used in the CFD pyrolyser model as operating conditions to simulate the pyrolysis process of utilising the recycled gas as the heating carrier gas in an industrial scale pyrolyser. The comprehensive in-furnace phenomena evolution during the pyrolysis are illustrated, in terms of flow, temperature, gas composition, and pyrolysis characteristics. The effects of two carrier gases – hydrocarbon-rich recycled gas and CO2-rich exhaust gas are examined and compared on the pyrolysis performance. The results indicate that compared to the exhaust gas, the employment of the pre-heated recycled gas as the heat carrier could significantly reduce the direct carbon emission (58%); improve pyrolysis efficiency (33%) and produce more valuable hydrocarbon gases (71.4%).

Experimental study of the catalytic effect of iron on low-rank coal gasification

Acid-washing low-rank coal samples were loaded with different content of iron catalyst and then pyrolyzed. FT-IR, Raman spectra, and temperature-programmed experiments were used to investigate the influence of iron on the coal char. The FT-IR results revealed that iron catalyst rises the number of –OH, –CH3, and –CH2 functional groups. The Raman spectra results showed that partial large polyaromatic ring structures transform into small polyaromatic ring structures after the addition of iron. The results of temperature-programmed desorption indicated that the number of surface active sites is increased due to the addition of iron. For low-rank coal char with 3% Fe, the number of active sites increased with the increase of adsorption temperature until 800 °C and then start to decrease. At 750 °C, the adsorption capacity of CO2 increased with the increase of time and reached saturation state after 45 min. The results of the char-steam isothermal gasification experiment suggested that the iron catalyst enhances the gasification reactivity of low-rank coal. It is verified that iron catalysts can improve the gasification reactivity of low-rank coal by increasing the number of surface active sites.

Experimental Study on the Pore Structure of Middle- and Low-Rank Coal and Its Influence on Methane Adsorption

The pore characteristics of coal have an important influence on the coal adsorption of methane. The liquid nitrogen adsorption method was used to study the pore structure of low- and middle-rank coal samples from two aspects: pore specific surface area and pore shape. Low-field nuclear magnetic resonance (NMR) technology was used to study the difference in methane adsorption of coal samples under the same adsorption conditions. The influence and mechanism of the pore structure of middle- and low-rank coal samples on gas adsorption are discussed and analyzed. The results show the following: (1) There are differences in the adsorption capacity of medium- and low-rank coal samples. Within 1 h of methane inflation, the methane adsorption capacity of middle-rank coals such as Henan Pingdingshan (PDS) and Shanxi Wangzhuang (WZ) and Xinjiang low-rank coals (XJ) increases by 36.4%, 31.7%, and 35.8%, respectively, and as the inflation time is extended to the cumulative inflation of 10 h, the order of the increase in methane adsorption capacity is still PDS>XJ>WZ. (2) The pore types of the experimental coal samples are mostly wedge-shaped pores and ink bottle-shaped pores. The order of the BET specific surface area of the coal samples is in the order of WZ>PDS> XJ, which means that the WZ samples can provide more adsorption sites for gas molecule adsorption, but under the same adsorption conditions, the WZ sample has the least amount of gas adsorbed. Further analysis shows that the order of the tortuosity of the coal sample is WZ>XJ>PDS, which means that the WZ sample has a large pore tortuosity, a long channel for methane molecules to diffuse into the internal pores of the coal sample, and large diffusion resistance. It is the main reason why the methane adsorption capacity of WZ coal samples is less than that of PDS and XJ coal samples in the same adsorption conditions.

Water adsorption characteristic and its impact on pore structure and methane adsorption of various rank coals.

Coalbed methane not only is a new clean energy source, but also has potential damage to ecological environment. Water and methane coexist in coal reservoir; understanding the adsorption of water on coal and its impact on pore structure and methane adsorption of coal is vital to evaluate the reserves and productivity of coalbed methane. In the paper, water adsorption characteristics of various rank coals are firstly investigated by ten mathematical models. The modified Dent model provides a best fit, followed by GAB and Dent models. For GAB model, the primary site adsorption is more difficult to reach saturation, and the contribution rate of the secondary site adsorption is surprisingly high at P/P0 approaching 0, which can be attributed to the possible overestimation of GAB monolayer adsorption capacity and secondary site adsorption. Besides, the low-rank coal sample YZG2 exhibits more prominent hysteresis than middle- to high-rank coals. The low-pressure hysteresis can be attributed to the water-water interactions over the primary site and the strengthened binding forces of water molecules in the water desorption process. In contrast, the high-pressure hysteresis largely depends on pore structure of coal such as ink-bottle pores, especially for the studied sample YZG2. Besides, pore analyses by low-temperature nitrogen adsorption method show that the pre-adsorbed water has remarkable influence on micropores smaller than 10 nm, and the micropores smaller than 4 nm almost disappear for water-equilibrated coals, which is closely related to the formed water clusters and capillary water in pore throats. This finding reveals that more methane gas can only be adsorbed in the larger pores of moist coal, and provides an explanation for water weakening methane adsorption capacity.

Ash Evaluation of Indonesian Coal Blending for Pulverized Coal-Fired Boilers

Coal calorific value is one of the main considerations for using coal as a power plant fuel. In addition, the requirements for indications of slagging and fouling are also important to maintain combustion efficiency. However, coal power plants often experience problems in boiler operations due to the use of certain types of coal, even though they have a relatively high calorific value. This research investigates the effect of coal blending on ash fouling and slagging in an experimental investigation using a drop tube furnace with or without additives. Five different types of coal from different locations have been used in this study. Pulverized low-rank coal samples are burned in a drop tube furnace at 1,175°C with probe temperatures of 550°C and 600°C, corresponding to the combustion chamber of 600 MW power plants, including superheater and reheater areas. The ash particles’ characteristics and material composition were also analyzed using scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD), respectively. All coal mixture combinations demonstrated potential as a fuel for power plants that use pulverized coal-fired boilers. Because of its capacity to reduce slagging and fouling potentials, combining coal blending with the use of chemical additives yielded the greatest results.

Characteristics of low-rank and medium-rank Indonesian coal using the TG-DSC method

Low-rank and medium-rank coal are dominant coal resources in Indonesia. Considering the decisive role of coal in coal-fired power plants, it is crucial to examine the combustion characteristics before burning coal in the boiler. This paper presents the effect of moisture content, heating value, and volatile matter on ignition temperature and burn out of five samples of low-rank coal and five samples of medium-rank coal using TG-DSC analysis which was carried out using LINSEIS High-Pressure STA at atmospheric pressure with an air rate of 25 ml/min and heating rate of 10 °C/min. The investigation results show that low-rank coal with the higher volatile matter has tremendous reactivity and is more flammable, and favours of burning through itself than medium-rank coal. Medium-rank coal has better combustion with short residence time because it has a lower burnout temperature (Tbo) value than low-rank coal. However, medium-rank coal burns more instantly because it has a lower temperature interval than low-rank coal. Medium-rank coal, which has fixed carbon and higher heating value, but lower moisture content, has a higher Rmax value than low-rank coal. In conjunction with these properties, it is crucial to examine the implementation in boilers.

Particle Size Effect and Temperature Effect on the Pore Structure of Low-Rank Coal.

High moisture content and high volatile content are typical characteristics of low-rank coal. To acquire the pore structure characteristics of low-rank coal accurately, the particle sizes and the pretreatment temperatures are two key parameters that should be considered when the low-pressure liquid-nitrogen adsorption is used. In this study, a low-rank coal sample was collected from Ordos Basin, and it was polished into four different particle sizes, 40–80 mesh, 80–120 mesh, 120–160 mesh, and 160–200 mesh, respectively. Besides, the low-rank coal samples are handled under seven various pretreatment temperatures (ranging from 120 to 300 °C); then, the pore structure characteristics of low-rank coal under various particle sizes and pretreatment temperatures are acquired. The dynamic change of pore volume and pore-specific surface area for low-rank coal is coincident. Under the same pretreatment temperatures, the mesopores’ volume continuously decreases. When the pretreatment temperature reaches 300 °C, a faint increase in their volume is observed. These results mean the mesopores are damaged during the progressive pulverization and heating procedures. When it comes to the same particle sizes, the mesopores’ volume also decreased with the increased pretreatment temperatures. Contrarily, the macropore volume is stable. This is mainly due to the decomposition of volatile matters and collapse of mesopores under the high pretreatment temperatures. However, the enrichment of ash in the mesopores could maintain the coal skeleton. The particle size effect and temperature effect mainly relate to the mesopores in low-rank coal, and the pores with the aperture below 5 nm contribute predominantly, followed by the pores with the aperture ranging from 5 to 10 nm.

The dynamic change of pore structure for low-rank coal under refined upgrading pretreatment temperatures

Pore structure characteristics are significant factor in the evaluation of the physical characteristics of low-rank coal. In this study, three low-rank coal samples were collected from the Xishanyao Formation, Santanghu Basin, and low-temperature liquid-nitrogen adsorption (LP-N2A) measurements were taken under various pretreatment temperatures. Owing to the continuous loss of water and volatile matter in low-rank coal, the total pore volume assumes a three-step profile with knee temperatures of 150 °C and 240 °C. However, the ash in the coal can protect the coal skeleton. Pore collapse mainly occurs for mesopores with aperture smaller than 20 nm. Mesopores with apertures smaller than 5 nm exhibit a continuous decrease in pore volume, whereas the pore volume of mesopores with apertures ranging from 5 to 10 nm increases at lower pretreatment temperatures (<150 °C) followed by a faint decrease. As for mesopores with apertures larger than 10 nm, the pore volume increases significantly when the pretreatment temperature reaches 300 °C. The pore structure of low-rank coal features a significant heating effect, the pretreatment temperature should not exceed 150 °C when the LP-N2A is used to evaluate the pore structure of low-rank coal to effectively evaluate the reservoir characteristics of low-rank coal.

Experimental Research on Dynamic Variation of Permeability and Porosity of Low-Rank Inert-Rich Coal Under Stresses.

The effective stress variation can cause permeability change during coalbed methane (CBM) production. At the same time, macerals have an important influence on the stress sensitivity of coal reservoirs. To investigate the low-rank inert-rich coal permeability dynamic response to effective stress during the production of CBM, eight low-rank coal samples were collected from the Huanglong coalfield, China, and the dynamic changes in helium permeability and porosity were tested under conditions in which the effective stress changed from 0, 15, to 0 MPa. Then, the permeability dynamic change, stress sensitivity control mechanism, and their influence on the development of low-rank CBM are discussed. The results show that with the increase in effective stress, the permeability and porosity of coal samples decrease in the form of negative exponents. During the process of effective stress increase, the permeability variation of low-rank coal includes three stages: rapid loss stage (0–3 MPa), slow loss stage (3–9 MPa), and stable loss stage (>9 MPa). In the process of effective stress reduction, the irreversible damage rate linearly increases. Fracture permeability is easier to recover than pore permeability during pressure relief. Coal macerals have an important influence on the initial permeability and stress sensitivity of low-rank coal. The ratio of vitrinite to inertinite of low-rank coal is positively correlated with initial permeability, but negatively correlated with the permeability loss rate and the fracture compression coefficient (Cf). Cf shows a decreasing trend of the negative index with an increase in effective stress and is highly positively correlated with the average permeability loss rate. The porosity–permeability index model and the permeability–effective stress-sensitive model of low-rank coal under effective stress are established. Research shows that inertinite is helpful to improve the stress sensitivity of low-rank coal. Accordingly, to reduce the damage of coal reservoir permeability, the pressure should steadily change either in the process of fracturing or drainage.