Chinese Bituminous Coal Research Articles

Experimental evaluation of the dynamic water-blocking effect in coalbed methane reservoir

Field observations indicate that almost 59% of coalbed methane (CBM) wells in the southern Qinshui Basin showed insufficient daily gas production (∼600 m3/t); this is caused by the water-blocking effect (WBE) attributed to the invasion of fracturing fluids or the accumulation of formation water. WBE can cause serious damages to the low-permeability reservoir but has not been paid sufficient attention to in CBM studies. To enhance understanding of the dynamic WBE in the multiphase flow process, we conducted core flooding experiments on two typical Chinese bituminous coal and anthracite coal samples. During the flooding experiments, real-time nuclear magnetic resonance (NMR) measurements of the change of water signal in the core were used to calculate the gas-water relative permeability and to evaluate the dynamic WBE. The results highlight that a more pronounced WBE occurred in anthracite coal compared with bituminous coal. The gas breakthrough model can calculate the gas breakthrough time, which is a useful parameter for evaluating water blocking. The water-blocking threshold pressure gradient (TPG) was determined based on experimental data. The results reveal that the water-blocking TPGs for both samples follow a positive power-law relationship with the normalized water saturation. The water-blocking TPG is approximately an order of magnitude higher than that of the conventional gas TPG. A high water-blocking TPG value reflects stronger gas/water interference during multiphase flow, exerting a negligible impact on the gas migration through the coalbed, which should be prioritized in CBM production. The implications of this study are important for a better understanding of the interaction mechanism of gas and water transport in coal reservoirs during the gas production process.

In situ change of fractal structure in coal with coking capability during high-temperature carbonisation

ABSTRACT Coal is an important energy and chemical raw material. Carbonization is an effective way for comprehensive utilization of coal, especially for coal with coking capability. The complex pore structure of raw coal and its carbonization solid products can be well characterized by a fractal description. Owing to a lack of in situ studies, the change in the fractal structure of coal with coking capability during carbonization remains unclear. Here we report briefly on an in situ study on high-temperature (1000°C) carbonization of four kinds of Chinese typical bituminous coal with coking capability, namely gas coal, fat coal, coking coal and lean coal, by synchrotron radiation small-angle X-ray scattering (SAXS). The change of fractal structure of the coal is analyzed in terms of corresponding mechanisms.

Effects of pressure on the chemical looping combustion of coal with CuFe2O4 combined oxygen carrier

Chemical looping combustion (CLC) has fast developed as an attractive technology for clean utilization of coal as well as inherent CO2 separation at the low energy penalty. Increase of the system pressure has been acknowledged as one of the effective measures in CLC for sufficient conversion of coal along with affiliated benefits for CO2 sequestration, but the complex consequences as incurred should be well considered. In this research, reaction of a typical Chinese bituminous coal of high sulfur content (designated as LZ) with CuFe2O4 combined oxygen carrier (OC) within 0.1–3.0 MPa in the CO2 atmosphere was systematically investigated, with focus on the effect of the system pressure to the conversion of LZ coal, transfer of the oxygen involved in CuFe2O4, sulfur evolution and redistribution. Sequential reactions of CuFe2O4 with the volatiles emitted from pyrolysis of LZ coal and further reaction with the released gasification products and residual char were accomplished, which was beneficial to overcome the mass transfer limitation of the reducing gases evolved from LZ coal to OC under the pressurized conditions and alleviate the inhibition effect resulting from accumulation of the transferred reactive gases around the surroundings of OC. And thus, under the pressurized conditions, improved conversion of LZ coal was reached by oxidization of more intractable C-C/C-H groups to oxygen bound carbon groups. Meanwhile, deep reduction of the added CuFe2O4 occurred with deficient Fe3O4 at the lower valence being formed, which would interact with the silicon minerals present in LZ coal to form various iron silicates. Furthermore, sulfur evolution from coal was found to directionally interact with the reduced CuFe2O4 to mainly form solid Cu2S due to their high combination potential. And deeper reduction of Cu2S was instigated especially at 3.0 MPa with Cu1.8S formed, and the total solid sulfur fraction was increased with the elevated system pressure. Overall, comprehensive information on the interaction of coal of high sulfur content with CuFe2O4 under the extended pressurized conditions around 0.1–3.0 MPa was provided, which would be meaningful for the realistic pressurized CLC in the future.

Identification of maceral groups in Chinese bituminous coals based on semantic segmentation models

Automatic identification of coal macerals has been a long-term pursuit for coal petrologists. As the rapid developments of computational processing capacities and algorithms in recent years, deep learning has been experiencing a considerable progress and successfully applied to image analysis. Semantic segmentation algorithms based on deep learning were introduced to make a pixel-level classification of maceral groups in Chinese bituminous coals. A high quality dataset with 739 representative petrographic images was established based on the characteristics of coal macerals in Chinese bituminous coal. The petrographic images were annotated into six classes, i.e., vitrinite, inertinite, liptinite, dark minerals (clay minerals), bright minerals (pyrite), and epoxy (background). The unbalanced proportions of annotated classes were calibrated by mathematical methods. Three classic semantic segmentation models, U-Net, SegNet, and DeepLab V3+, were used to make a pixel-level identification of the coal macerals. Pixel Accuracy (PA), Intersection over Union (IOU) and BFScore were introduced to evaluate the performances of the segmentation. The results have shown that the loss curves and accuracy curves of the three models in the training process rapidly converged after iterations. The training time of U-net, SegNet, and DeepLab V3 + was 172 min, 637 min, and 135 min respectively. The average pixel accuracy of the three models was 73%, 73%, and 92% respectively. The IoU and BFScore of DeepLab V3 + were also higher than those of U-Net and SegNet. The DeepLab V3 + was the preferred method for maceral group identification considering the training efficiency and segmentation accuracy. The maceral group compositions determined by DeepLab V3 + model achieved similar results as the manual point-counting method. The semantic segmentation method based on deep learning meet the accuracy requirements of quantitative analysis of coal maceral groups.

Roles of moisture and cyclic loading in microstructures and their effects on mechanical properties for typical Chinese bituminous coals

This work aimed at studying the roles of moisture and cyclic loading in microstructures and their effects on mechanical properties for typical Chinese bituminous coals. Different relative moisture contents (100%, 75%, 50%, 25%, and 0%) for Shenmu coal (SM), Hongshaquan coal (HSQ), and Wucaiwan coal (WCW) were chosen to study the effects of moisture. The raw SM was then further selected to investigate the effects of cyclic loading. Images of coals surfaces and mechanical properties during simulated crushing process were recorded and combined to be analyzed. The results showed that the moisture possessed significant effects on coal mechanical properties, which strongly depended on their porosities. As for low porosity coal (SM), the adsorption of moisture can soften and lubricate the microstructures, weakening mechanical properties. While the drying process would destroy the microstructures and decease mechanical properties for high porosity coals (HSQ and WCW). Under the cyclic loading process, the cumulative effects of strain showed a step-up state and the first cyclic loading can typically cause the biggest change of microstructures and produce the largest strain under different stress levels. Finally, a normalized quantitative relationship (σr=-0.67Dr2+1.62Dr+0.07) between the relative fractal dimension and relative stress was built.

Proposed Cleaning Potential Gradient Based on a Study of Coals from Three Exploration Areas in Shanxi Province, China.

The cleaning potential of selected Chinese bituminous coals during coal preparation was evaluated in terms of coal quality, cleaning grade, and cleanability. The cleaning potentials were determined for coals sampled from a total of six seams, two each in three different general exploration areas in southern Shanxi, China. Distribution maps of resources with different cleaning potential characteristics were prepared using mapping and geographical information software. In each case, the area and calculated reserves for coals having different classifications of cleaning potentials were determined. The total areal extent of the six coals studied is about 410 million m2, and the total reserves of these coals amount to about 1460 million tonnes, which include coals of poor, fair, good, and proficient cleanabilities. The proportion of high-quality coals is about 28.9%. Coals that can be processed into high-quality coals account for about 69.2%. A cleaning potential gradient is proposed for indicating the cleaning potential level, and an equation for calculating it is established. The necessity of processing raw coal of good quality is also to be considered in terms of economics. It does not make economic sense to process coal with an organic sulfur content more than 1.5%, even though it may have a good cleanability.

Effect of ash removal on structure and pyrolysis/gasification reactivity of a Chinese bituminous coal

In this study, the effect of ash removal on Shenfu bituminous coal was investigated. The coal was pretreated by hydrofluoric acid (HF) pickling, and the raw/pretreated coal chars were prepared at 900 °C in a fixed bed reactor. The structure of coal and char were detected by Fourier transform infrared (FTIR) and Raman spectroscopy. The reactivity was tested in a thermogravimetric analyzer, including coal pyrolysis and char gasification. The reaction kinetics was analyzed through the Coats–Redfern method, master plots, the model-free and model-fitting method. The results show that the HF pickling can remove silicon from coal efficiently, and the macromolecular framework of coal is quite stable according to FTIR. The Raman parameters imply some carbonaceous structure on coal surface changed. For slow pyrolysis of coal, the effect of heating rate is considered. The changes of pyrolysis characteristics and kinetics are insignificant. For char gasification, the reactivity under isothermal and non-isothermal condition are discussed with an emphasis in different residence time of devolatilization process. In kinetic control region (low temperature), the activation energy (Ea) is very close (about 240 kJ/mol) for all chars. With the temperature increases, the reactivity of raw coal char is more easily suffered by diffusion. The random pore model is more suitable for the ash-free coal char, and the char with long residence time has a larger value of structural parameter ψ and smaller value of pre-exponential factor A. The Ea calculated by model-fitting and model-free method were in good agreement.

Effect of Partial Rapid Pyrolysis on Bituminous Properties: From Structure to Reactivity

In this paper, a typical Chinese bituminous coal was pyrolyzed rapidly and repeatedly in a high frequency furnace at 1000 °C, to investigate the changes of coal physicochemical structures, reactivities and their relationship. The physicochemical structure was mainly characterized by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, pore analysis, etc. The results showed that after the rapid pyrolysis, the specific surface area was reduced significantly, the average particle size and the content of amorphous carbon structure increased first and then decreased, the content of oxygen-containing group exposed to the surface decreased and the aromaticity increased gradually. The reactivities were analyzed by thermogravimetric analysis (TGA), including slow pyrolysis (from 105 to 1000 °C) and in-situ gasification (at 850, 900, 950 and 1000 °C). For slow pyrolysis, the primary decomposition peak of pretreated samples was inhibited while the secondary peak was enhanced. For in-situ gasification,...

Cleaning Potential of Selected Coals in Shanxi Province, China: Evaluation of Geological Factors Affecting the Cleaning Potential

The effects of geological factors on the cleaning potential of Chinese bituminous coals from Anping, Xizhuang, and Yitang exploration areas were studied. The cleaning potential was evaluated in terms of coal quality and cleanability. Distribution maps of resources with different cleaning potentials were prepared using mapping and geographical information software. To identify the geological factors responsible for the cleaning potentials of coal resources, the distribution of coal resources with different cleaning potentials was summarized with regard to the geological setting of the three exploration areas. The results are as follows: The depositional environment may have a large role in affecting the cleanabilities of coals, but it may not be the only factor. Coal which formed in an environment prone to producing pyrite can have a better cleanability than coal formed in a depositional environment that is more prone to producing organic sulfur. In contrast to the effects of depositional environment, the local structures (e.g., folds, faults, and collapse columns), where present, are generally considered to have little, if any, effect on the distribution of the coal with different cleaning potential characteristics.

Investigation of Chemical-looping Gasification Characteristics of Chinese Western Coals with Hematite-CuO Oxygen Carrier

In order to realize highly efficient conversion of Chinese western bituminous coals into syngas, a series of chemical-looping gasification experiments was conducted with hematite-CuO oxygen carrier in a laboratory-scale fluidized-bed reactor. The results indicated that the gasification rate of Chinese western bituminous increased by 2-3 times after addition of a hematite-CuO oxygen carrier. Meanwhile, the syngas yields of Chinese western bituminous ranged from 1.84–2.04 m3/kg, three times higher than that of lignite. This shows that the combination of chemical-looping technology and gasification of coal can achieve efficient conversion of Chinese/western bituminous coals. The temperature and the supply oxygen coefficient (O/C) all demonstrated a clear effect on the gasification rates and the gas yields. 10 cycles of redox experiments indicated that the hematite-CuO oxygen carriers have good recycled reaction characteristics. Those results provide theoretical guidance for the efficient conversion of Chinese western bituminous coals into syngas via chemical-looping gasification.

Analysis of distillate product in the direct coal liquefaction of a Chinese bituminous coal

Analysis of distillate product in the direct coal liquefaction of a Chinese bituminous coal

Porosity changes in bituminous and anthracite coal with ultrasonic treatment

Improving pore structures is key for enhancing coalbed methane (CBM) recovery. The variations in pores of Chinese bituminous and anthracite coal were analysed by using low-pressure N2 gas adsorption and CO2 gas adsorption during 20 kHz and 50 kHz ultrasonic treatment. Although the shape of the physisorption isotherm and hysteresis loop, that indicate the pore type of coal, were not changed after ultrasonic treatment, the pore size distribution (PSD) of the treated coal differed from that of the raw coal, especially for bituminous coal with tissue pores, intercrystal pores, and gas pore clusters. An obvious increase in bituminous coal pores was noted, although only a slight change was noted in anthracite pores except for macropores during ultrasonic treatment. The 50 kHz ultrasonic treatment contributed to pore development more than 20 kHz, and created sub-micropores, mesopores, and macropores in bituminous coal fractures, after rapid growth in the treatment process. In the 20 kHz ultrasonic field, these ranges of pores represent continuous and monotonous growth. The range of micropores in bituminous coal increased rapidly before 36 h of ultrasonic treatment, followed by a slow increase. Throughout the ultrasonic treatment process, the largest growth of micropores, sub-micropores, mesopores, and macropores in the bituminous coal were about 148.5%, 418.4%, 228.3%, and 43.6%, respectively, with the higher frequency of ultrasonic treatment; for the lower frequency of the ultrasonic field, the increment values were 115.0%, 179.9%, 103.9%, and 76.4%, respectively. The cumulative pore volume of anthracite showed a 61.9% increase during the 50 kHz ultrasonic treatment after macropore fracturing.

Synergistic effect of CaO and MgO addition on coal ash fusibility in a reducing atmosphere

AbstractDuring coal gasification in entrained‐flow slagging gasifiers, the addition of additives is a promising method to improve coal ash fusibility. In some cases, mixed additives are more effective in decreasing ash fusibility than single additive addition with the same content added. However, mechanisms of mixed additive addition for improving ash melting are scarce. In this study, the effect of CaO, MgO, and their mixtures on the ash flow temperature (FT) of Chinese bituminous coal was investigated. The results show that the single addition of CaO or MgO influenced the ash FT of the test coal sample. The addition of CaO and MgO mixture synergistically decreased the ash FT relative to their single addition. When the ratio of CaO to MgO was 5:5 (mass ratio) in the mixture, the FT decreased to 1297°C, which is 69°C and 110°C lower than that of the single addition of CaO and MgO, respectively. The formation of a low‐temperature eutectic from the interactions between anorthite and olivine caused such synergistic effects.

Impact of pressure on the carbon structure of char during pyrolysis of bituminous coal in pressurized entrained-flow reactor

The impact of pressure on the carbon structure of a Chinese bituminous coal was investigated using a pressurized entrained-flow reactor in the temperature and pressure ranges of 700-900 °C and 0.1-4.0MPa, respectively. Pyrolysis pressure had a significant influence on the physiochemical and carbon structure of chars. The specific surface area and the swelling ratio of chars reached their highest values at 1.0MPa. Fourier transform infrared spectroscopy (FTIR) analysis showed that higher pressures enhanced the decomposition of functional groups in chars. Raman spectroscopy analysis results revealed that at elevated pressures, the organic matrix and functional groups were removed from the char structure, leading to higher ordering of the carbon structure. During X-ray diffraction (XRD) analysis, parameters such as the stacking height (Lc), interlayer spacing (d002) and lateral size of the graphite structures (La) were used to evaluate the graphitic structures in chars. The results showed an increase in Lc, La, and the average number of graphene sheets with pyrolysis pressure, indicating a more ordered carbon structure at elevated pressures. The d-spacing of char was in the range of 3.34-3.37 A, similar to typical graphitic structures.

Mercury release characteristics during pyrolysis of eight bituminous coals

The mercury release during pyrolysis of eight Chinese bituminous coals under N2 at temperatures up to 1200 °C was studied. The pyrolysis experiment was carried out in a quartz reactor. Results show that higher than 93% of mercury release out during pyrolysis. The release of mercury from all the coals begins at about 150 °C with two or three mercury peaks at the similar temperature range. The correlation analysis shows that mercury in the coals has close relation with the mineral and the mercury released at different peaks is mainly correlated with different types of material in the coals. The first peak located at 150–400 °C is mainly correlated with carbonate mineral as well as organic material in the coals. The second peak located at 500–600 °C is mainly correlated with sulfide minerals in the coals. The third peak located at higher than 750 °C is mainly correlated with aluminosilicate mineral in the coals. A large part of mercury in coal can be removed by pyrolysis below 400 °C.

Release of Ca during coal pyrolysis and char gasification in H2O, CO2 and their mixtures

The release characteristics of Ca during a Chinese bituminous coal pyrolysis and char gasification in different gasifying agents was studied. Residual chars with different carbon conversion levels were obtained by gasifying in a quartz fixed-bed reactor under H2O, CO2, and H2O/CO2 atmospheres at 800–1000 °C. Sequential chemical extraction and microwave digestion were used to determine the modes of occurrence of Ca in chars. X-ray diffraction and scanning electron microscopy were used to analyze the ash composition and the morphology of minerals. The contents of Ca in chars were measured by atomic absorption spectroscopy. The results indicate that water-soluble form and HCl-soluble form Ca transforms mainly into gas phase during pyrolysis. The release ratio of Ca at 800 °C under different atmospheres is H2O/CO2 >H2O > CO2. The main reason is that minerals are more prone to sintering in CO2 atmosphere. The interaction of H2O and CO2 can expand pores, which precipitate the minerals to the char surface and release into the gas phase. Increasing temperature induces the collapse of the pore structure which changed the release rule of Ca. The composition of the ash obtained from gasification at 1000 °C in CO2, H2O and H2O/CO2 are nearly same.

Analysis of distillate product in the direct coal liquefaction of a Chinese bituminous coal

To understand the reaction mechanism of coal liquefaction process and further improve the direct coal liquefaction (DCL) technology, a certain kind of DCL oil, which was prepared from a Chinese bituminous Yulin coal, was first separated into different fractions using improved silica-gel column chromatography (SGCC) coupled with the UV-absorbance detector, and then identified by gas chromatography-mass spectrometer (GC-MS), the comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometer (GC × GC-TOFMS), 1H NMR and FTIR. Around 95% of DCL oil was successfully separated into seven fractions by four mobile phases and analysed. It was found that DCL oil consists of 9.06% of alkanes, 36.32% of hydroaromatics, 17.85% of aromatics, 15.39% of oxygen-containing compounds, 16.21% of nitrogen-containing compounds and 2.33% of polyheteroatomic compounds, respectively. [Received: August 22, 2017; Accepted: March 1, 2018]

Coal combustion emission and ash formation characteristics at high oxygen concentration in a 1 MWth pilot-scale oxy-fuel circulating fluidized bed

Combustion experiments for one common Chinese bituminous coal (Datong) were conducted in air and in O2/dry recycle flue gas (RFG) mixtures with input O2 concentrations of 30%, 40% and 50% in a 1MWth pilot-scale oxy-fuel circulating fluidized bed (CFB). The effect of combustion atmosphere and O2 concentration were investigated to determine their impact on coal combustion efficiency, gaseous pollutant emissions, limestone desulfurization efficiency and chemical composition of the fly ash. The experimental results showed that the unburned char content in fly ash for the 30% O2/RFG combustion mode is slightly higher than that obtained for the air combustion mode. However, when the O2 concentration reaches and exceeds 40%, the content of unburned char in fly ash is less than that obtained under the air firing mode. Compared with the air firing mode, the SO2 emissions under O2/RFG modes are notably higher, and the limestone desulfurization efficiency is lower. Under O2/RFG modes, the NO emissions are much lower, but the N2O emissions are higher than that under the air firing mode, with the level of N2O emission being nearly the same as the corresponding NO emission. There are no obvious differences in the chemical composition of fly ash under air and O2/RFG modes, except for a slightly lower percentage of K2O, Na2O and SO3 under O2/RFG modes at higher O2 concentrations.

Trace element concentration and reduction of typical Chinese bituminous coals via dry physical separation techniques

Dry physical separation techniques were used to clean typical Chinese bituminous coals by removing their ash and sulfur contents and major trace elements. Different dry coal separators, such as the conventional dense-medium gas–solid fluidized bed separator, vibrated dense-medium gas–solid fluidized bed separator, and triboelectric separator, were used to clean various-sized coal samples. Results suggested that the coal quality could be significantly improved through coal cleaning, which removed the mineral impurities of coal and therefore decreased its ash and sulfur contents. The majority of the trace elements were efficiently removed from run-of-mine coals and were found concentrated in the tailings after separation, thereby showing the highest removal efficiency for several harmful elements, including As, Co, Cr, Hg, and Mn in particular. Despite the removal percentage of the majority of trace elements, which obviously decreased with a decrease in the particle size of the coal sample, the overall removal efficiency satisfied the requirements of coal cleaning. The proposed dry physical separation flowsheet provides an alternative efficient approach to coal cleaning that can be utilized in the arid and water shortage countries.

Experimental Evaluation of a Novel 20 kWth in Situ Gasification Chemical Looping Combustion Unit with an Iron Ore as the Oxygen Carrier

This study presents the results obtained from the operation of a 20 kWth in situ gasification chemical-looping combustion (iG-CLC) unit with a Chinese bituminous coal as the fuel and a natural iron ore as the oxygen carrier. This unit was built based upon the concept of a novel iG-CLC system evaluated through our previous cold-model tests. It is mainly composed of a high-flux circulating fluidized bed riser as the fuel reactor, a cross-flow moving bed as the air reactor, and a combination of an inertial separator and a cyclone separator as the separation system. The actual thermal power inputs from the coal for the reported tests in this study were about 11 kWth and the total duration reached about 70 h. The flow patterns and reaction characteristics of the whole system were investigated, and the effects of the fuel reactor temperature on the distributions of gas components in the two reactors were elucidated. At the fuel reactor temperature of 950 °C, the CO2 yield, gas conversion, carbon capture efficie...