Coal Moisture Research Articles

Construction of a New Type of Coal Moisture Control Device Based on the Characteristic of Indirect Drying Process of Coking Coal

This paper presents an in-depth study on the indirect drying characteristics and interface renewal. By comparing the drying rate curve, it is found that shortening the surface drying stage, which contains a repeated evaporation-diffusion-condensation process of moisture, is the key to improve the total drying efficiency. By stirring the coal and realizing the interface renewal between the bottom layer and the surface layer, the drying efficiency reached seven times than that of static indirect drying. Based on indirect heat transfer with high heat and mass transfer rate, a new type of indirect heat transfer moving bed coal moisture control device is designed. At the same time, Fluent fluid mechanics software is used for mathematical modeling and simulation experiments. It is proved that the designed moving bed coal moisture control device has a good application prospect in coal pre-drying technology.

Experimental measurements of carbon dioxide, methane and nitrogen high-pressure adsorption properties onto Malaysian coals under various conditions

In this study, the CO2 adsorption capacity was measured on Malaysian sub-bituminous coals in dry and moisture equilibrated state using volumetric technique to understand its carbon sequestration and storage potential. The CO2 adsorption capacity onto dry and wet coal samples was performed at 300–348 K and 6 MPa. Four coal specimens, namely S1, S2, S3 and S4 were analysed by using BET, XRD and FESEM techniques before and after CO2 adsorption. The dry S3 coal had showed the highest CO2 adsorption capacity 1.59 mmol g−1 at 300 K and 6 MPa among all samples. FTIR spectra patterns explain this behaviour as they showed an excessive presence of hydroxyl and carboxyl functional groups in dry coal samples. The functional group analysis of all the wet coal samples exposed to CO2 showed significantly declined adsorption relative to their corresponding dry samples. The mineral phases were identified within all fresh and wet exposed coal samples to CO2 through XRD analysis. The S3 coal had the highest selectivity of CO2 over CH4 and N2 gases at pressures up to 6 MPa. The bituminous Malaysian coal has a high affinity to adsorb CO2 in dry and wet conditions.

Experimental Investigation on the Influence of Water Content on the Mechanical Properties of Coal under Conventional Triaxial Compression

The presence of water is one of the most important factors in coal mining, and it has a dual influence on the mechanical behavior of rock. To study the influence of water content on the mechanical properties of coal under complicated stress conditions, dry coal specimens and wet coal specimens with water contents of 1.8% and 3.6% were conducted by uniaxial and conventional triaxial compression tests. The relations between the uniaxial compressive strength, deformation, and water content were observed. The reductions in the strength and elastic modulus under different confining pressures were obtained. The mechanical properties of coal specimens with different water contents under triaxial compression were studied. The influences of water content on the microstructure, clay minerals, internal friction angle, and cohesive force of coal were discussed. The results show that the strengths and elastic moduli of wet specimens are clearly lower than those of dry specimens under different confining pressures. The water content has a significant influence on the postfailure mechanical behavior of coal. The loss rates of strength and elastic modulus decrease with increasing confining pressure. The water content has almost no effect on the internal friction angle, while the cohesive force of the saturated specimens is 36.5% lower than that of the dry specimens. The results can provide a reference for inhibiting the occurrence of disasters during coal mining and exploiting coal efficiently.

High-temperature pyrolysis behavior of two different rank coals in fixed-bed and drop tube furnace reactors

In order to explore the high-temperature pyrolysis behavior and mechanism, two different rank coals (Shenhua bituminous coal and Baiyinhua lignite coal) were pyrolyzed in fixed-bed and drop tube furnace (DTF) reactors. Pyrolysis gas production was online quantified by a drainage method while the gas composition was detected by the gas chromatography. The physical and chemical characteristics of pyrolysis char were observed by N2 absorption and FTIR. Results show that pyrolysis gas release rate and total production increase with the higher temperature. The gas production of SH coal (618–749 ml/g) is always higher than that of BYH coal (418–510 ml/g) due to its higher volatile content. The main high-temperature pyrolysis gas products are H2 (∼50% vol), CO, CH4 and CO2. Carbon-reduction reaction at high temperatures can further form H2 and CO while the rupture of aromatic heterocyclic ring can generate CH4. High-temperature pyrolysis can greatly remove the coal moisture, develop their pore structures, crack the chemical functional groups (O–H, –CH3, CO, C–O, etc.) and upgrade the coal rank. Different coal structures and pyrolysis heating rates result in various gas evolution and char formation behaviors. Higher heating rate can help to quickly generate large amounts of free radicals and change the pyrolysis behaviors.

A vibration-test-based calculation method of screening material mass of a mining crank-link type flip-flow screen

ABSTRACT Flip-flow screen is a key equipment for dry screening of wet fine coal, and its working load is of great significance to the screening effect and reliability. In order to obtain material mass load characteristics of a crank-link type flip-flow screen in the screening process, a vibration-test-based method is proposed. A complete dynamic model of the crank-link type flip-flow screen system is established by the centralized mass method. The correlation between the material mass and the kinematic parameters is deduced by dynamic theory analysis. The time-frequency analysis, envelope analysis, and Kolmogorov–Smirnov test are used to analyze the material mass statistical distribution characteristics during the screening process. Also, in the vibration tests at different rotation speeds, the relation between the parameters of the material distribution model and the rotational speed is obtained by the least square method. The results of the field experiment show that the maximum deviation of this method is 6.7%, and the material mass distribution model established by this method can well reflect the working state of the flip-flow screen.

Explicit equation derivation for predicting coal moisture content in convective drying process by GMDH-type neural network

ABSTRACT Predicting the instant moisture content of the low-rank coals under the different drying conditions is crucial to construct the optimal system design and operation related to drying processes. Even if the thin-layer drying models are at the center of the field studies, the disadvantage of these type models is that the prediction results are valid only for the conditions of the drying experiment. Conversely, artificial intelligence models can provide accurate prediction results under a wide range of different conditions. Nevertheless, they are not practical because of their implicit forms and require both the specific software and experts. In this study, the GMDH-type neural network is applied for the first time in developing explicit model equations for the prediction of coal moisture at any time during the drying process. 223 experimental instances are used, representing coal moisture contents obtained under the different drying conditions. The considered parameters are bed height (80–150 mm), coal sample size (20–50 mm), drying air velocity (0.4–1.1 m/s), drying air temperature (70–160 °C), and drying time (0–270 minute). The developed equation is nonlinear and provides satisfactory prediction accuracy (R2 is 0.96–0.99) for different drying conditions. Additionally, its usage is quite practical due to the explicit form.

Determination principle of residual Gas and Analysis of influencing factors on the results of Laboratory Measurement

In view of the current situation that the concept of residual gas content in coal seam in our country is relatively chaotic, the definition of residual gas content and the measurement principle of degassing and desorption methods in “The direct method of determining coalbed gas content in the mine” are analyzed. The standard unified method and the explicit concept are proposed. Through the laboratory desorption experiments in some mining areas, it is found that the residual content of gas and the degree of coal metamorphism are positively correlated as a whole. There is a negative correlation between the content of ash and internal moisture in coal and the residual content of gas. The order of the residual gas content of different sampling methods is as follows: core sampler> hydraulic slag sampler ≈ wind slag sampler, coal structure is complete, the change range of measured data is the smallest with core sampler, which can truly reflect the law of coal desorption and status.

Exergy destruction calculation and analysis for a Coal drying system

A coal drying system was modelled and the calculation methodology was developed to calculate the system. Then the system performance was obtained and the exergy destruction and loss in the system is 70.5 kW. The reasons of exergy destruction were tried to be explained from the perspective of energy grade. In DRY-REAC, the wet coal was dried to dry coal, the process of which is irreversible, and the main exergy destructions are used to pay the thermodynamic penalty of the increased energy grade of the wet coal.

Numerical Simulation of a Coke Oven Using Decoupling Techniques

Cokemaking is an important unit operation in an integrated steel plant as coke quality has a strong influence on blast furnace performance. A two-dimensional (2D) model coupling physico-chemical phenomena inside combustion and coking chambers of a coke oven has been presented. It was implemented using the computational fluid dynamics software Ansys Fluent and was solved by a serial decoupling approach to improve convergence and to reduce computational time. The model predictions for gas temperatures inside the combustion chamber and the effect of coal moisture content on evolution of coke bed temperatures are in reasonable agreement with published experimental data. The computational time required for the 2D model is six times lower than that for a corresponding three-dimensional model. This model is useful for model-based optimization of cokemaking and for designing coke ovens with reduced computational burden.

The influence of moisture content on coal deformation and coal permeability during coalbed methane (CBM) production in wet reservoirs

Moisture adsorption in the coal seams affects the gas adsorption capacity and can alter the coal deformation and permeability criteria of the coal seam. The effect of dynamic loss of moisture content, both on moisture and gas sorption-induced coal swelling/shrinkage strains, during the coalbed methane (CBM) production, is crucial. This study investigates the interactions among coal matrix, absorbed gas, and moisture content, based on the coal swelling/shrinkage strains and gas adsorption decay criteria. Consequently, a mathematical model of the coal deformation is developed for the proper evaluation of the moisture effect. For developing the model, this paper considers the standard gas flow and moisture loss equations to assess the volumetric content, equilibrium pressure, and density of the moisture. Finally, it comprehensively analyzes the sensitive factors and effects of elemental parameters of moisture content on coal deformation and coal permeability. The results show that moisture content at adsorbed state significantly changes the coal swelling/shrinkage strain and that distorted swelling and shrinkage characteristics can promote the permeability alternation in wet coal reservoirs. Moreover, the intermolecular attraction between the coal structure and the moisture content has a significant effect on methane adsorption/desorption-induced deformation in coal structure. This study also designs the coal deformation strains as a function of moisture content by the Langmuir type model and evaluates the hysteresis rate between the swelling and shrinkage characteristics. The findings of this paper can characterize a wet coal reserve for CBM production and anticipate future production under different operating conditions.

Using hybridized ANN-GA prediction method for DOE performed drying experiments

Coal is an important component in the energy industry and plays a key role in energy-producing facilities. Moisture is a common condition that has a considerable impact on coal. Coal drying has long been a question of great interest in a wide range of fields. Defining parameters in the coal drying is obtained by experiments. High costs, time constraints, and repetition of an experiment are one of the most frequently stated problems with experimental works. Using qualitative methods with experiments can be more useful for identifying and characterizing the coal drying process. The purpose of this article is finding the effective parameters in the coal drying process by using a hybridized prediction method. Genetic Algorithm (GA) and Artificial Neural Network (ANN) are hybridized with each other to identify and characterize the coal drying process. GA-ANN algorithm is applied to the coal drying process to predict the moisture of coal, but it does not provide a decent result at first. Later, the Design of Experiment (DoE) methodology is performed to determine the main effects of six parameters. Two scenarios are generated because two parameters are not statistically significant. The first scenario excludes the air relative humidity parameter, and the second scenario excludes the air relative humidity and the velocity of air parameters. Following the application of the DoE method, GA-ANN reaches decent results in scenario-2.

Multi-Criteria Efficiency Analysis of Using Waste-Based Fuel Mixtures in the Power Industries of China, Japan, and Russia

This paper presents the results of analyzing the efficiency of the following five fuel types: dry coal, wet coal processing waste, coal–water slurry, and two waste-derived slurries. In the calculations, we employed 16 criteria related to the energy industry, economy, social aspects, safety at plants, and environmental protection. We used the experimental data, obtained from the combustion of the fuels under study at three heating temperatures (700 °C, 800 °C, and 900 °C). Three countries were analyzed, where all of them have a high share of using fossil fuels in the energy industry: Japan, China, and Russia. The total performance indicator was calculated using three multiple-criteria decision analysis techniques (weighted sum method, weighted product method, and analytic hierarchy process). The choice of weight coefficients was confirmed for each method. We found that coal and coal–water slurry had the lowest integral efficiency indicators (0.016–0.535 and 0.045–0.566, respectively). The maximum effect was achieved when using waste-derived slurry with used turbine oil (0.190–0.800) and coal processing waste (0.535–0.907). There were, on average, 3%–60% differences in the integral efficiency indicator for the same fuel in different countries. The difference in the efficiency indicator of the same fuel in different countries was on average 3%–60%; with changes in temperature, the difference in efficiency was 5%–20%; and when changing the calculation procedure, the difference was 10%–90%.

Analysis of Upgrading Process of South Sumatera Low Rank Coal

This research aims to find a method of removing moisture of Indonesian low rank coal by using a combined equipment of evaporator and heater. The heating equipment was controlled at varied temperatures of 343 K, 353 K and 363 K. This method is expected to increase heating value and reduce moisture content of coal. Based on this study, the removal of moisture content of Indonesian low rank coals was affected by the heating temperature and particle size of coals. Ambient air was passed through the evaporator tubes to decrease the temperature until reaching the dew point temperature of the air and then through the heating elements of a heater before entering the fluidised bed of pulverized coal. The results of this research showed that the heating value of coal can be increased up to 35.4% and also the moisture content can be reduced up to 29.8%, both at heater temperature of 363 K and particle size of (-20 +40) mesh.

Decision tree regression model to predict low-rank coal moisture content during convective drying process

ABSTRACT Coal is still a significant energy source for the world. Due to the utilization of low-rank coal, drying is a key issue. There are lots of attempts to develop efficient drying processes. The most prominent method seems as thermal drying. For thermal drying processes, the most important subject is the coal moisture content change with time. In this study, convective drying experiments were utilized to develop a new model based on decision tree regression method to predict coal moisture content. The developed model gives satisfactory results in prediction of instant coal moisture content with changing drying conditions. With the decision tree depth of six, the best test results were achieved as 0.056 and 0.802 for MSE and R 2 analyses, respectively.

Improving permeability of coal seams by freeze-fracturing method: The characterization of pore structure changes under low-field NMR

The method of improving permeability of coal seams by freeze-fracturing is presented to accelerate the diffusion of coalbed gas into exploited wells. The pore structure of coal sample changes caused by freeze-fracturing were characterized by nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), and a permeability-calculation model was employed to estimate the permeability of coal sample. The pore size distribution, porosity, and permeability of coal before and after freezing were analyzed. Results show that freeze-fracturing can complicate the coal pore structure, and its complexity is positively correlated with the freezing temperature and coal moisture content. The main manifestation is the growth and expansion of coal pores, with higher pore connectivity, generation of gas seepage pores with pore diameter > 100 nm, and expansion of original pores into larger ones. By comparing the connectivity and seepage space of the pore structure of coal before and after freezing, it is concluded that freeze-fracturing can promote the coal seam permeability significantly, which is beneficial for the exploitation of coalbed gas.

CO2 adsorption and swelling of coal under constrained conditions and their stage-change relationship

The adsorption of CO2 into coal will cause the volumetric swelling of coal body, which is a highly concerned issue for the CO2 enhanced coalbed methane recovery (CO2-ECBM). However, the research of adsorption and swelling of coal is not all-inclusive and their relationship is ambiguous. This paper conducted the experiments regarding the CO2 adsorption and corresponding swelling of coal influenced by multiple factors using the self-designed device. Results show that the adsorption capacity is promoted by the injection pressure and is restrained by confining pressure, temperature and water content, but the thermal-induced swelling strain and moisture-induced swelling strain can offset the swelling loss to some extent for the wet coal at a high temperature. There has a stage-change relationship between swelling strain and adsorption capacity of coal, and the adsorption swelling can be divided into three parts: the slow swelling stage, the rapid swelling stage and the stable swelling stage. Under the action of the external stress, the adsorption swelling develops towards to the interior of coal body first and then develops towards to the external when the swelling stress exceeds the external stress. Different extended directions of adsorption swelling are the main causes of the stage adsorption swelling. The study contributes to the deep understanding for the gas adsorption swelling mechanism and the attenuation-rebound of the coal reservoir permeability during the CO2-ECBM.

Improvement on gas drainage of soft gassy coal seam with underground hydraulic flushing and fracturing: a case study in Huainan

The low gas drainage efficiency of soft coal seam is a key factor that induces gas disasters, such as gas explosion and outburst, in China. This study aims to improve the gas drainage efficiency of a soft coal seam in Huainan, China, by conducting field trials with a new stimulation technique. The stimulation technique comprises two significant procedures: (1) fracturing the coal seam with a fracturing borehole and (2) flushing wet coal and water out with the adjacent fracturing borehole. Coal seam stress is released by such actions because much coal could be flushed out of the coal seam through the process. Therefore, an artificial fracture network could be created in the coal seam, and the gas drainage efficiency could be improved. Results of field trials show a significant improvement in gas drainage efficiency of up to eight times higher than that of the original part.

Rapid detection of the total moisture content of coal fine by low-field nuclear magnetic resonance

Coal total moisture content has an observable influence on its calorific value. In this investigation, a rapid detection method of total moisture of coal fine through 1H low-field nuclear magnetic resonance (LF-NMR) was proposed. The first peak point value and total signal amplitude analyzed by T2 spectrum were used to fit with the total moisture content of coal. The first peak point value and total signal amplitude increased as the total moisture content of coal increased. The fitting results of both first peak point value and total signal amplitude to the total moisture content of coal agreed with the second-order linear equation best. The first peak point value had higher prediction accuracy than the total signal amplitude and hence it is used preferentially to predict coal moisture content. This method provides a new and effective tool for online detecting the moisture content of coal in the coal preparation plant.

Nondestructive quantification of moisture in powdered low-rank coal by a unilateral nuclear magnetic resonance scanner

ABSTRACT Moisture content in coal powder is a critical index because it controls the difficulty of coal handling and ultimately its combustion efficiency in coal-fired power plants. A unilateral proton nuclear magnetic resonance (NMR) scanner was applied in a laboratory to nondestructively quantify moisture content in powdered samples of brown and subbituminous coals. Due to the open geometry of the sensor unit, the scanner allows the nondestructive moisture quantification of a portion several millimeters below the surface of a large sample. Proton transverse relaxation was measured by the unilateral NMR scanner, and obtained moisture content values were compared with those by a conventional bilateral NMR apparatus. The comparison showed reasonable agreement with a root mean square residual of 4.1 wt.%H2O for a moisture content range of > 12 wt.%H2O (wet basis). This moisture-quantification performance by unilateral NMR is sufficient for the quality control of wet coal having a critical value of approximately 30 wt.%H2O in terms of the handling property (flow-ability). Because unilateral NMR requires no specific sample preparation, such as sample insertion into tubes, the system is potentially applicable to in-situ nondestructive monitoring of the coal moisture in power plants to contribute to improved efficiency of handling and combustion.

Dewatering coal with supercritical Carbon Dioxide

ABSTRACT Supercritical CO2 (sCO2) dewaters coal through two mechanisms. For bituminous coal of 9.5% average moisture content the water is dissolved out of the coal and the concentration of water in the bulk sCO2 approximates its solubility limit. For lignite (54% average moisture content) the water extracted by the sCO2 exceeds its solubility limit in sCO2, which implies that a part of the water is displaced from, rather than dissolved out of the coal. Thus, there is a clear distinction between wet coals where most of the water is displaced, as compared to drier coals where the water is dissolved out. Hence, sCO2 dewatering will be especially efficient for high-moisture coals where there is significant free water available for displacement. Precedent for this process is available from the dewatering of wood flakes.