Coal Recovery Research Articles

Some parameters of coal methane system that cause very slow release of methane from virgin coal beds (CBM)

In some worldwide hard coal basins recovery of methane from virgin coal beds is difficult. In general, mentioned difficulties are related to geo-mechanical, petrographical and physical-chemical properties of coals in question, occurring for example in the Bowen Basin (Australia) or the Upper Silesian Coal Basin (Poland). Among numerous properties and parameters, the following are very essential: susceptibility of coal beds to deformation connected with coal stress state change and contemporary shrinkage of the coal matrix during methane desorption. Those adverse geo-mechanical and physical-chemical effects are accompanied by essential change of the porous coal structure, which under these disadvantageous conditions is very complex. This study aims to show difficulties, which occur in phase of recognition of the methane-reach coal deposit. Volume absorbed methane (not surface adsorbed) in sub-micropores having minimal size comparable with gas molecule diameter must possess energy allowing separation of the nodes and methane release to micropores.

Prediction of Coal Bed Methane Recovery Rate and Its Improvement Measures in Dafosi Mine Field

The recovery rate of coalbed methane (CBM) can reflect the mining situation and the residual gas in coal reservoir. It plays an important role in the calculation of the recoverable resources. This paper mainly uses isothermal adsorption curve method and hydraulic model method to predict recovery rate of CBM. The isothermal adsorption curve method considering desorption lag problem in the prediction process, which is more in line with the actual situation. In the hydraulic model method, the recovery rate of “V” type well is the largest in the early stage. But with the time going on, the recovery rate of multilateral horizontal well is greater than vertical well, “U” type well and “V” type well finally. The factors affecting CBM recovery rate include geological characteristics, development conditions and economic factors. The geological characteristics of coal reservoir are the main factors affecting CBM recovery rate, and corresponding measures can be adopted to improve the recovery rate.

Mesoscale Assessment of CO2 Storage Potential and Geological Suitability for Target Area Selection in the Sichuan Basin

In China, south of the Yangtze River, there are a large number of carbon sources, while the Sichuan Basin is the largest sedimentary basin; it makes sense to select the targets for CO2 geological storage (CGUS) early demonstration. For CO2 enhanced oil and gas, coal bed methane recovery (CO2-EOR, EGR, and ECBM), or storage in these depleted fields, the existing oil, gas fields, or coal seams could be the target areas in the mesoscale. This paper proposed a methodology of GIS superimposed multisource information assessment of geological suitability for CO2 enhanced water recovery (CO2-EWR) or only storage in deep saline aquifers. The potential per unit area of deep saline aquifers CO2 storage in Central Sichuan is generally greater than 50 × 104 t/km2 at P50 probability level, with Xujiahe group being the main reservoir. CO2 storage potential of depleted gas fields is 53.73 × 108 t, while it is 33.85 × 108 t by using CO2-EGR technology. This paper recommended that early implementation of CGUS could be carried out in the deep saline aquifers and depleted gas fields in the Sichuan Basin, especially that of the latter because of excellent traps, rich geological data, and well-run infrastructures.

Factors and challenges affecting coal recovery by opencast pillar mining in the Witbank coalfield

This paper was first presented at the New technology and innovation in the Minerals Industry Colloquium’, 9–10 June 2016, Emperors Palace, Johannesburg, South Africa.

Longwall mining under gateroads and gobs of abandoned small mine

Due to the use of outdated mining technology or room and pillar mining process in small coal mines, the coal recovery ratio is only 10–25%. In many regions of China, the damage area caused by the small coal mines amounted to nearly one hundred square kilometers. Therefore, special mining techniques must be taken to reclaim the wasted resource in disturbed coal areas. This paper focuses on the different mining methods by analyzing the longwall panel layout and abandoned gateroad (AG) distribution in the abandoned area of Cuijiazhai coal mine in northwestern China. On the basis of three-dimensional geological model, FLAC3D numerical simulation was employed. The abutment pressure distribution was simulated when the panel face passed through the disturbed areas. The proper angle of the inclined face was analyzed when the panel face passed through the abandoned gateroads. The results show that the head end of the face should be 13–20m ahead of the tail end. The pillars on both sides of abandoned gateroads had not been damaged at the same time, and no large-area stress concentration occured above the main roof. Therefore, the coal reserves of disturbed areas can be successfully recovered by using underground longwall mining.

Evaluation of Coal Combustion Zone and Gas Energy Recovery for Underground Coal Gasification (UCG) Process

During underground coal gasification (UCG) operations, evaluation of coal gasification cavity evolution and precise control of the underground reactor are important for efficient gasification. It is also essential to estimate the energy recovery of a UCG system and the whole gasification process to ensure an effective combustion and gasification rate. An experimental simulation of UCG using an artificial coal seam comprising a compacted broken coal block was conducted using ex situ UCG models. The main goal of the experiments was the establishment of evaluation methods for the gasification zone and energy recovery during UCG. To investigate the distribution and extent of fracture activity, and to evaluate the propagation of the combustion area in the UCG reactor, we used acoustic emissions (AE) monitoring. This was combined with traditional measurements of temperature variation and product gas content. This paper presents the results of AE analysis of the fracturing activities and damage mechanisms of the...

Use of Correlation Fractal Dimension signatures for understanding the Overlying Strata Dynamics in Longwall Coal Mines

The longwall mining is considered to be the best coal mining practices due to vast recovery of coal over other forms of underground as well as opencast mining methods. But the actual scenario is quite opposite in India where productivity form of the longwall mines lags far behind than the desired level. Irregular caving and sudden rock bursts, which are very hazardous for mine workers and equipment. Usually these are major problem faced by bulk of Indian longwall faces and which are due to the presence of thick sandstone beds as overlying strata. Thus to keep an eye on the rock bursts, it is very necessary to monitor the stressed zones in the hanging overlying strata above and behind the panel. Earlier Correlation Integral ‘C’ and Correlation Fractal Dimension ‘Dc’ has been very helpful in monitoring the stressed zones for several great earthquakes in past. Following the same way, in the present study we have used the mine-induced microseismic data obtained from the retreating longwall panel using various monitoring instruments to calculate the Correlation Fractal Dimension ‘Dc’ for monitoring the stress levels and fractures in the overlying strata and also for spatio-temporal forecasting of roof-falls. The variation of blast charge size with Fractal Dimension is also studied. The use of Fractal Dimension has been very effective in obtaining the precursory signatures for roof-fall, thus ensuring safety in the mines.

Top coal caving mining technique in thick coal seam beneath the earth dam

It is important to study the mining technology under structures for raising the coal resources recovery ratio. Based on the geological and mining conditions, the top coal caving harmonic mining technique in thick coal seam beneath the earth dam was put forward and studied. The 5 factors such as the panel mining direction, panel size, panel location, panel mining sequence and panel advance velocity were taken into account in this technique. The dam movement and deformation were predicted after the thick coal seam mining and the effects of mining on the dam were studied. By setting up the surveying stations on the dam, the movement and deformation of the dam were observed during mining. By taking some protective measures on the dam, the top coal caving mining technique in thick coal seam beneath the earth dam was carried out successfully. The study demonstrates that harmonic mining in thick coal seam is feasible under the dam. The safety of the earth dam after mining was ensured and the coal resources recovery ratio was improved.

An experimental investigation of applicability of CO2 enhanced coal bed methane recovery to low rank coal

Previous studies have shown that carbon dioxide (CO2) injection can enhance CH4 production (CO2-ECBM) compared to traditionally-used methods, mainly due to the higher adsorptive capability of CO2 in coal, which desorbs the CH4 with higher sweep efficiency. Many studies have been conducted to date on the CO2-ECBM technique for high rank coals. However, there have been very few studies on low rank coal. Therefore, this study uses Victorian brown coal samples to investigate the CO2-ECBM potential of low rank coal. A series of CO2 core flooding tests was conducted on CH4 saturated meso-scale brown coal samples for various CO2 injection conditions, phases and pressures.According to the experimental findings, compared to natural recovery, CO2 flooding enhances CH4 production by creating higher production rates, and higher CO2 pressures can drive the CH4 towards the production end with almost 100% sweep efficiency. Furthermore, the rapid CO2 breakthroughs observed under higher CO2 pressures are found to be significant for super-critical CO2. Tests results show that the superior competence of super-critical CO2 in CH4 recovery is independent of coal rank or maturity. However, the greater volumetric strain created by higher CO2 pressures may, have a negative influence on long-term gas productivity with the reduction of flow ability through the seam.

A suitable process for clean coal recovery from tailing pond deposits

ABSTRACTThis study aims to recover fine coal process with efficiency at a very low reasonable investment and operational costs. In the present study, the sample was taken from a tailing pond of an operating coal preparation plant, cleaned using an enhanced gravity separator to produce a clean product of low ash content. The experimental results indicated that a good amount of clean coal recovered from tailing pond with an ash content of 16.24% could be produced from a feed of 33.5% ash with an appreciable yield of 69.15%. The multi-gravity separator (MGS) has an advantage of utilizing the fine- and ultrafine-sized refuse coal with minimum environmental pollution.

Modeling of fine coal flotation separation based on particle characteristics and hydrodynamic conditions

Flotation is a complex multifaceted process that is widely used for the separation of finely ground minerals. The theory of froth flotation is complex and is not completely understood. This fact has been brought many monitoring challenges in a coal processing plant. To solve those challenges, it is important to understand the effect of different parameters on the fine particle separation, and control flotation performance for a particular system. This study is going to indicate the effect of various parameters (particle characteristics and hydrodynamic conditions) on coal flotation responses (flotation rate constant and recovery) by different modeling techniques. A comprehensive coal flotation database was prepared for the statistical and soft computing methods. Statistical factors were used for variable selections. Results were in a good agreement with recent theoretical flotation investigations. Computational models accurately can estimate flotation rate constant and coal recovery (correlation coefficient 0.85, and 0.99, respectively). According to the results, it can be concluded that the soft computing models can overcome the complexity of process and be used as an expert system to control, and optimize parameters of coal flotation process.

Perturbation Solutions for Hagen-Poiseuille Flow and Heat Transfer of Third-Grade Fluid with Temperature-Dependent Viscosities and Internal Heat Generation

Regular perturbation technique is applied to analyze the fluid flow and heat transfer in a pipe containing third-grade fluid with temperature-dependent viscosities and heat generation under slip and no slip conditions. The obtained approximate solutions were used to investigate the effects of slip on the heat transfer characteristics of the laminar flow in a pipe under Reynolds’s and Vogel’s temperature-dependent viscosities. Also, the effects of parameters such as variable viscosity, non-Newtonian parameter, viscous dissipation, and pressure gradient at various values were established. The results of this work were compared with the numerical results found in literature and good agreements were established. The results can be used to advance the analysis and study of the behavior of third-grade fluid flow and steady state heat transfer processes such as those found in coal slurries, polymer solutions, textiles, ceramics, catalytic reactors, and oil recovery applications.

Effect of compound collector and blending frother on froth stability and flotation performance of oxidized coal

The beneficiation of oxidized coal is difficult to achieve by conventional flotation with oily collectors. Oxidized coal samples were prepared by the peroxide solution oxidation and a new type of oxygenated polar compound collector (CC) and a powerful blending frother (BF) were used to intensify oxidized coal flotation in the present study. The effects of the CC and BF on the froth stability and flotation performance (i.e., recovery and selectivity) were investigated. The results show that only 44.51% recovery with 9.71s dynamic froth stability index (DFS) was obtained for oxidized coal flotation by using conventional diesel and octanol as the collector and frother, respectively. Lots of oxygen functional groups (−OH and C-O) were introduced after peroxide oxidation indicated by Fourier Transform Infrared Spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis, leading to poor hydrophobicity and low flotation recovery. An optimal separation result with 76.24% oxidized coal recovery and 19.43s DFS was obtained using the CC combined with octanol, which was attributed to the hydrogen bonding between the hydrophilic sites on coal surface and polar oxygen functional groups in CC. An identical DFS (19.43s) was obtained using conventional diesel combined with the BF, while with only 21.38% recovery. Diesel did not sufficiently render the coal hydrophobicity and as a result only a small amount of coal particles was collected by attaching to bubble surface in pulp phase. The Fuerstenau upgrading curves show that both the maximum recovery and the best flotation selectivity could be obtained using the CC combined with octanol.

Microscopic mechanism for enhanced coal bed methane recovery and outburst elimination by hydraulic slotting: A case study in Yangliu mine, China

AbstractCoal bed methane (CBM) is a clean, efficient, and environmentally friendly energy, and its efficient exploitation can meet the growing demand for energy worldwide. Besides, methane (CH4) is a greenhouse gas (GHG) and is 25–72 times more powerful than carbon dioxide (CO2) in inducing global warming. With regard to the mining industry, CBM is regarded as a major hazard resource, as its inadequate management or control may lead to serious coalmine accidents. In China, gas drainage is the main measure adopted to resolve this problem. However, because the coal seams in China are characterized by low permeability, it is difficult to drain the gas contained in coal mass. Therefore, measures must be taken to reconstruct the coal reservoir to enhance coal seam permeability. In China, hydraulic slotting has been widely accepted and applied in coal mine engineering owing to its permeability‐enhancement effect. In this work, based on our previous research, we systematically elaborate the mechanisms of permeability enhancement and elimination of coal and gas outburst by hydraulic slotting from microscopic perspectives. To examine the rationality and practicability of the research results and the permeability‐enhancement effect of hydraulic slotting, a field experiment was conducted and the monitoring results were analyzed. Our results show that the gas flow rate and concentration in the slotted boreholes are significantly higher than those in the conventional boreholes. In addition, the time required by the slotted borehole to eliminate the outburst is 72.41% less than that needed by the conventional borehole. The practice in Yangliu coalmine could be extended to other coal mines with similar reservoir conditions. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd

Coal mine wastes recycling for coal recovery and eco-friendly bricks production

The mining industry is globally criticized for generating large amount of solid wastes often with a potential environmental impact. This study presents a feasible approach of recovering residual coal from coal mine waste rocks (CMWR) and reusing the derived tailings to produce ecofriendly fired bricks. The first step aims at the production of high grade anthracite coal concentrate using a physical treatment method which consists of consecutive stages of crushing, grinding and froth flotation. Coal (organic matter) was recovered in order to enhance the mechanical strength of bricks and to reduce the greenhouse gas (GHG) emissions during the firing process. The second step of the process consists of the mixture of the decarbonated tailings, called treated coal mine tailings (TCMT), with a small amount of water, pressed and fired at an optimal temperature of 1020°C for the production of ecofriendly bricks at a laboratory scale. Fired bricks containing no other raw materials than CMWR or its derived TCMT have been produced showing in both cases very interesting ornamental looks. The study assessed the effects of removing coal from CMWR on the physical and technical properties of the derived fired bricks. The removal of coal from CMWR enhanced the quality of fired bricks by increasing its flexural strength and decreasing the open porosity and water absorption. In addition, the results show that this integrated process reduces the amount of GHG emissions by more than 70% and allows a sustainable production of fired bricks.

Evaluation of beneficiation options for recovery of ultrafine thermal coal

Recent requirements to reduce thermal coal plant emissions, and ultrafine coal’s tendency to produce these emissions due to its typically high ash and moisture contents have limited the use of ultrafine coal. Ultrafine coal in beneficiation plants are usually either disposed of in tailing ponds at a loss in combustible material or disposed of by blending with coarser higher-grade products, resulting in a reduction in marketed product quality. A study of the options for processing ultrafine coal, consisting of 200 μm fraction and EGS processing of the 10 to 200 μm fraction was found to be the next best option, attaining a combined 90.1 percent CMR and 25.0 percent ash content.

A case study on the effective stimulation techniques practiced in the superposed gas reservoirs of coal-bearing series with multiple thin coal seams in Guizhou, China

The superposed reservoirs of coal-bearing series represent a new type of gas reservoir associated with coal seams, and those with multiple thin coal seams are an important subtype popular in Guizhou, an emerging area for coal-bearing series gas exploitation in China. So it is significance to accommodate hydraulic fracturing, a common stimulation technique for enhanced gas recovery from low permeability reservoirs, to the reservoirs there. Based on a number of engineering and test data from Songhe pilot project for coal seam gas recovery in Guizhou, this paper presents some effective stimulation techniques adapting to the superposed reservoirs of coal-bearing series with multiple thin coal seams, including drilling and well completion selection, gas-bearing intervals targeting, small-layer perforation, staged fracturing, ball sealer diversion and graded proppant injection. The study also focuses on the mechanism on adaptable hydraulic fracturing of the superposed reservoirs associated with multiple thin coal seams, by combining theoretical analyses and actual fracturing curve diagnoses. The results indicate that the fractures opening exhibit a certain temporal order depend on mechanical and other physical properties differences among individual perforated intervals, and the fracture opening sequence can be controlled by adjusting pumping scheme and dropping ball sealers; the fracture height extended into the adjacent rocks is a critical geometric parameter affecting scale of fracture network, and negatively correlated with tensile strength difference between coal seams and adjacent rocks; under a condition of the current maximum pump rate, the fracturing treatment in both multiple thin coal seams and interlayered sandstones have been implemented, and a fracture network has been generated with a uniform fracture geometry and strong flow conductivity by the combined application of the effective stimulation techniques.

Mechanism of a Coal Chemical-Leaching Process and Recovery of Spent Chemicals: A Pilot-Scale Study

ABSTRACTMethods for enriching organic macerals in coal are of great importance, but, due to the yield constraint, physical-beneficiation methods are less effective. Chemical beneficiation using alkali followed by acid has been studied at the pilot stage with a batch of 500 kg of feed coal for three different coal fractions of the physical-beneficiation process using dense media cyclone and froth flotation. The mechanism of silica and alumina reactions during the alkali-leaching process has been derived and found that 150°C is the critical point for precipitation of the unwanted sodalite compound. The composition of sodalite is calculated based on the experimental results. Due to the formation of sodalite, the acid-leaching step is critical for the removal of sodalite and, hence, regeneration of acid is of great importance. A process has been developed for acid regeneration in which the first major impurity, silica, is removed by a polycondensation process and the second major impurity, alumina, is removed by a reaction with sulfuric acid. Optimum operating conditions for the regeneration process have been identified where more than 99% of the silica and 80% of the alumina can be removed from the spent acid. Almost 60% of the spent acid can be regenerated for further use and is recycled.

Residual coal exploitation and its impact on sustainable development of the coal industry in China

Although China owns large coal reserves, it now faces the problem of depletion of its coal resources in advance. The coal-based energy mix in China will not change in the short term, and a means of delaying the coal resources depletion is therefore urgently required. The residual coal was exploited first with a lower recovery percentage and was evaluated as commercially valuable damaged coal. This approach is in comparison to past evaluations when the residual coal was allocated as exploitation losses. Coal recovery rates, the calculation method of residual coal reserves and statistics of its mines in China were given. On this basis, a discussion concerning the impacts on the delay of China's coal depletion, development of coal exploitation and sustainable developments, as well as technologies and relevant policies, were presented. It is considered that the exploitation of residual coal can effectively delay China's coal depletion, inhibit the construction of new mines, redress the imbalance between supply and demand of coal in eastern China, improve the mining area environment and guarantee social stability. The Chinese government supports the exploitation technologies of residual coal. Hence, exploiting residual coal is of considerable importance in sustainable development of the coal industry in China.

Application of central composite design method to coal flotation: Modelling, optimization and verification

In this study, collector dosage, frother dosage, solid ratio and air flow rate that can be effective parameters on the combustible recovery and ash content of clean coal and their interactions in flotation are described with a mathematical model by using statistical design method. The test results have shown that both models obtained for ash content and combustible recovery were found to be statistically significant, and also the effects of collector and frother dosage on the response variables are found to be higher than those of other parameters. Additionally, verification experiments at optimum conditions proposed by the models were done to determine the validity and adequacy of the predicted models.