Coal Grains Research Articles

An Investigation of the Mechanism of Influence of Coal-tar Pitch Additive on Changes in the Ordered Structure of Caking Coals during Pyrolysis

The quantitative X-ray phase analysis and scanning electron microscopy were used to determine the influence of coal tar pitch on structural changes in co-pyrolyzed Polish coals—P, Z, and B—with good caking properties. It was stated that coal tar pitch weakened interactions between fragments of coal macromolecules and favored their higher mobility at the stage of swelling of the coal grains, which contributed to making the removal of plasticizing substances from these grains easier. It reduced the mobility of the macromolecular structure at the viscous-liquid stage. The decrease in the amount of the crystalline phase was influenced by the coal tar pitch at the temperature of re-solidification of the co-pyrolyzates of coals P and B. The intracrystalline ordering was blocked in the plastic layer of the co-pyrolyzates of coal Z and evaluated by an increase in the d002 parameter.

Quantification of the Existence Ratio of Non-Adhesion Grain Boundaries and Factors Governing the Strength of Coke Containing Low-Quality Coal

“Non-adhesion grain boundaries” are formed when low-quality coal grains do not adhere to other grains in the carbonization process because of the low dilation of coke. To better understand the effects of non-adhesion grain boundaries on coke strength, the relationship between the existence ratio of non-adhesion grain boundaries and coke strength was investigated quantitatively. The existence ratio of non-adhesion grain boundaries were measured quantitatively by observing the fracture cross-section of coke using scanning electron microscopy (SEM). Coke strength was measured with a diametral-compression test and an I-shape drum index test. As a result, non-adhesion grain boundaries increased with an increase in the blending ratio of low-quality coal. In particular, non-adhesion grain boundaries increased rapidly when the blending ratio of low-quality coal was over 50%. When the ratio was less than 50%, low-quality coals adhered to other caking coal. However, not many low-quality coals adhered to other caking coals when the ratio was over 50%. The tensile strength of coke was not affected by the porosity of coke. However, the tensile strength and the drum index were affected by the existence ratio of non-adhesion grain boundaries. Tensile strength decreased rapidly even for a few non-adhesion grain boundaries because significant defects caused a fracture in the diametral-compression test. However, the I-shape drum index decreased linearly with the existence ratio of the non-adhesion grain boundaries because many fractures occurred during 600 rotations in the drum. The strength of coke containing low-quality coal is governed by the existence ratio of non-adhesion grain boundaries rather than mean values such as the porosity of coke.

The effect of the fracture distribution on CO2 injection into a coal seam

The effect of the fracture distribution on CO2 injection into coal seams was studied with a heterogeneous model having dual porosity to represent both the primary medium (the coal matrix) and the secondary medium (the fractures) under variable stress conditions. A numerical generation method and a digital image processing method were used to model the heterogeneous fracture distribution in the coal. The model solutions demonstrate that: (1) the fractures are the main channel for gas flow and their distribution has an important impact on the gas injection rate; (2) the fractures only affect the injection rate of CO2 into the coal but not the final storage amount; (3) when gas is injected into coal the fractures will first expand and then close due to the changing effective stresses and the adsorption induced swelling of coal grains. This fully coupled dual-porosity model with a heterogeneous fracture distribution provides a way to predict the CO2 injection into a coal seam.

Experimental Study on Coal Pyrogenation Characteristic and Dynamic Model of Reaction

The study effort was in progress through a thermogravimetric analyzer which export experimental data for construction pyrogenation dynamic model when import varied coal grain diameter in non-isothermal thermogravimetry method. This paper chiefly introduces a study on coal pyrogenation process with different altering parameter like temperature rise rate, grain diameter of coal that has been proved to affect in a big way in the whole work. The experimental research represents that there are four stages of the coal thermal decomposition, taking volatile release feature index as basic characteristic reflection to establish a thermogravimetry dynamic model.

Improving coke strength prediction using automated coal petrography

A range of Australian and overseas coals with a mean maximum vitrinite reflectance (Ro, max) range of 0.68–1.71% were carbonized in a test coke oven. Coal properties were characterized using XRF, XRD and automated imaging of polished sections of discrete coal grains. The Full Maceral Reflectance (FMR) parameter was calculated from the reflectance data and was correlated with coal as well as coke properties. The Ro, max values and the vitrinite estimates from the semi-automated microscopic technique indicated a good correlation with similar data based on manual point count analysis. The FMR parameter is shown to increase with increasing carbon content, structural ordering of carbon and decrease with increasing volatile matter. The FMR parameter of coal was related to cold coke strength DI15150 and coke strength after reaction (CSR). The FMR parameter was modified by diluting the contribution of high reflectance coal grains as well as incorporating the effect of ash contribution to propose a combined coal index (CCI). The new coal index is shown to improve the accuracy of coke strength prediction. The combined coal index provides a promising objective measurement based alternative for predicting coke strength.

Pressure generation in coking furnaces: A review

The pressure field in loading coking chambers with coal batch will be affected by the specific temperature field during the release of volatiles at the stages of softening and swelling of the coal grains, the appearance of a plastic layer, and the formation of semicoke and coke. In monitoring the process, the maximum pressure in the middle of the furnace (coking pressure) and at the charge-wall contact (the swelling pressure) are recorded. These characteristics depend on the properties of the coal’s organic mass (petrographic composition, metamorphic stage, degree of reduction), the batch preparation (reduction in grain size, moisture content, density of the charge), the structural characteristics of the working chambers, the temperature conditions, and the duration of carbonization. The maximum permissible pressure values are established specifically for each coke battery, in the light of the batch and technology employed. Usually, a wall pressure of around 7 kPa (∼1 psi) is adopted as the critical value at which unhindered coke discharge is possible. This value determines the limiting attainable quality of blast-furnace coke (within the framework of the given technology).

Assessment of petrographic model for the prediction of coke strength

The strength of coke produced from Mongolian coal is calculated within the framework of Eremin’s petrographic model, on the basis of literature data from a maceral analysis of such coal. The model produces quantitatively correct values of the strength characteristics M25 and M10. The clinkering microstrength of coal grains for individual ranks and the postreactive strength CSR of the coke may also be estimated.

The Application of the Coal Grain Analysis Method to Coal Liberation Studies

Emerging coal markets such as the use of coal for conversion to liquid fuels and its use in fuels cells and as coal water slurries in diesel engines require coal products with different coal quality specifications than those applicable to traditional coal markets of coke making and conventional power generation. As well as quantifying coals in terms of their chemical and physical properties, detailed knowledge of the mineral inclusions within the coal particles is required to identify coals that are suited to economically produce the low-ash value coals required for these markets. Hence, it is necessary to understand the associations of the minerals and macerals in individual particles as these give a particular coal not only its chemical attributes and utilization performance but also its washability (density distribution) and contribute to its surface chemistry characteristics. These attributes pinpoint the coals that are suited to beneficiation techniques that can be employed to produce economic amounts of low-ash value products for these new markets. After mining and processing, some particles can consist of essentially pure components of a single maceral or mineral phase whilst others are composite particles that are comprised of varying amounts of macerals and minerals. The proportion of particles that are present as pure components or as composites will be a function of the characteristics of the coal and the particle size. In general, it is considered that size reduction will result in liberation and hence increased yield. The amount of liberation that occurs during crushing or grinding a coal is however coal specific. Particle characterization information provided by an optical microscopic-imaging method, Coal Grain Analysis, was used to identify coals that might benefit from additional crushing to improve recovery of clean coal by new density separation techniques and by flotation. As expected, the results of these studies suggest that the degree of liberation that is obtained is coal specific, and, hence, yield improvements are also coal specific. Hence a quantitative method of investigating this issue is required.

Reflotation of Coal Flotation Tailings using Foam Separation / Reflotace Černouhelných Flotačních Hlušin Pomocí Pěnové Separace

Abstract Froth separation is one of the new methods of flotation process in mineral treatment. This method allows a substantial increase in the size upper limit of separated grains. The contribution deals with the verification of the possibility of using the froth separation for retreatment of flotation tailings. For laboratory research samples of flotation tailings from the Paskov Mine treatment complex were used. The results confirmed the ability of froth separation to convert coarse coal grains into a concentrate. Grains with a grain size of 0.5 mm were successfully reflotated.

Stress–strain response of pre-compacted granular coal samples exposed to CO2, CH4, He and Ar

The interaction between supercritical carbon dioxide and coal has been the subject of many studies in recent years. This paper reports two different types of swelling experiments, one on confined and one on unconfined samples of pre-pressed, crushed bituminous coal, performed at gas or fluid pressures up to 10 MPa and temperatures of around 40 °C. These experiments confirm earlier observations of increased swelling of coal due to carbon dioxide adsorption compared with methane and argon. However, the experiments reveal a previously unrecognised swelling phenomenon. The swelling observed in the present samples is up to 3 times larger than that seen in block-shaped coal matrix fragments that have not been pre-compacted and is largely irreversible. We propose that pre-pressing led to permanent compaction of the internal pore structure of the coal matrix grains, decreasing the mean pore aperture and increasing the proportion of micropores. Introduction of CO2 and CH4 resulted in increased micropore adsorption which, through the associated surface forces developed within the micropores, led to a disjoining pressure effect that caused recovery of the pre-pressing-induced permanent strain. The higher affinity of micropores for CO2 than for CH4 adsorption, explains the markedly bigger effect of CO2. After the recovery of the permanent strain was achieved, the samples seem to have exhibited a normal, reversible response to adsorption. We thus attribute the irreversible swelling effect seen in our experiments to the stress-history effect of pre-pressing. The findings of this study may be relevant for coals that have undergone deep burial after formation, or for naturally stressed coals in tectonic zones, which may exhibit more swelling upon gas adsorption than expected from their reversible adsorption capacity.

The Relationship Between the Natural Compaction and the Spontaneous Combustion of Industrial-scale Coal Stockpiles

The companies necessarily store their coal needs in their stock areas in which stock fires occur due to long stay periods of coals and cause serious damages in terms of the economy and environment. In this study, in order to determine the conditions by which spontaneous combustion occurs due to long stays of coals under natural weather conditions, an industrial-scale coal stockpile was formed with 10–18 mm coal grain sizes. Temperature measurements were recorded by using temperature sensors placed at certain locations inside the stockpile. At the same time, the effects of atmospheric conditions on the stockpile were investigated by continuously measuring the values of weather temperature, weather humidity, atmospheric pressure, wind velocity, and direction that are the factors affecting these stockpiles. Stockpiles tend to be faced with compaction due to the gravitational effect under long-term stays and the vibrations resulting from the huge machines working closer to the stockpiles. For the purpose of measuring the magnitude of the compaction, digital photographs were taken at certain periods and then processed photogrammetrically using terrestrial photogrammetry software. Afterward, the relationship between the calculated compaction rate and the spontaneous combustion was determined.

Subaquatic oxidation of coal by water-dissolved oxygen

Subaquatic oxidation of two bituminous coals by water-dissolved oxygen was investigated using batch reactor equipped with membrane oxygen sensor. Effects of time, temperature and coal grain size were studied as basic parameters influencing the oxidation process. Obtained results showed the subaquatic coal oxidation can be considered as interaction of the first reaction order with respect to oxygen. From temperature dependence of oxidation rate, activation energies E = 72 ± 4 kJ mol −1 and/or 50 ± 4 kJ mol −1 were calculated. For the samples, oxygen consumption R O 2 was found to be in the range of 2 × 10 −7 mol O 2 kg −1 s −1 to 6 × 10 −7 mol O 2 kg −1 s −1, such values being quite comparable with R O 2 for aerial oxidation of bituminous coals.

Changes in pore structure of anthracite coal associated with CO 2 sequestration process

Pore structure changing of coal during the CO 2 geo-sequestration is one of the key issues that affect the sequestration process significantly. To address this problem, the CO 2 sequestration process in an anthracite coal was replicated using a supercritical CO 2 (ScCO 2) reactor. Different coal grain sizes were exposed to ScCO 2 and water at around 40 °C and 9.8 MPa for 72 h. Helium pycnometer and mercury porosimetry provide the density, pore size distribution and porosity of the coal before and after the ScCO 2 treatment. The results show that after exposure to the ScCO 2–H 2O reaction, part of the carbonate minerals were dissolved and flushed away by water which made the true density increased as well as total pore volume and porosity most importantly in the micro-pore range. Hysteresis between mercury intrusion and extrusion was observed. Ink bottle shaped pores can be either damaged or created compared with the ScCO 2 treated coal samples. This suggests that the ScCO 2 treatment most likely increase the volumes of pores in anthracite coal, which also contributed to the increase in porosity of the treated samples. Therefore the CO 2 sequestration into coal appears to have the potential to increase significantly the anthracite microporosity which is very advantageous for CO 2 storage.

Determination of Spontaneous Combustion in Industrial-scale Coal Stockpiles

Many companies unavoidably store coals at their stock areas for later usage. Stock fires take place due to long-term stay of these coals at the stock areas and cause serious damages in terms of economy and environment. In order to determine the occurrence conditions of spontaneous combustion of coals due to long-term stay under natural air conditions, an industrial-scale coal stockpile composed of 10 to 18 mm coal grains, having the dimensions of 5 m width, 3 m height, and 10 m length and weighing approximately 120 tons, was formed in a coal stock area of a company using great amounts of coal annually. Recording the temperature values measured at the stockpile, the temperature-time diagrams were plotted. Moreover, the atmospheric effects on the stockpile were investigated by continuously measuring air temperature, air humidity, atmospheric pressure, velocity, and direction of wind, which are some of the parameters affecting the stockpile.

Artificial Neural Network Modeling of the Spontaneous Combustion Occurring in the Industrial-scale Coal Stockpiles with 10–18 mm Coal Grain Sizes

Companies consuming large amounts of coal should work with coal stocks in order to not face problems due to production delays. The industrial-scale stockpiles formed for the aforementioned reasons cause environmental problems and economic losses for the companies. This study was performed in a coal stock area of a large company in Konya, which uses large amounts of coal in its manufacturing units. The coal stockpile with 5 m width, 10 m length, 3 m height, and having 120 tons of weight was formed in the coal stock area of the company. The inner temperature data of the stockpile was recorded by 17 temperature sensors placed inside the stockpile at certain points. In order to achieve this goal, the electrical signal conversion of temperatures sensed by 17 temperature sensors placed in certain points inside the coal stockpile, the transfer of these electrical signals into computer media by using analog-digital conversion unit after applying necessary filtration and upgrading processes, and the record of these information into a database in particular time intervals are provided. Additionally, the data relating to the air temperature, air humidity, atmospheric pressure, wind velocity, and wind direction that are the parameters affecting the coal stockpile were also recorded. Afterwards, these measurement values were used for training and testing of an artificial neural network model. Comparison of the experimental and artificial neural network results, accuracy rates of training and testing were found to be 99.5% and 99.17%, respectively. It is shown that possible coal stockpile behavior with this artificial neural network model is powerfully estimated.

Trace elements in coal: Associations with coal and minerals and their behavior during coal utilization – A review

Trace elements such as mercury, arsenic and selenium present in coal are known to be of concern for public health. Coal-fired power plants have resulted in emission of several tons of TEs in environment. These elements mostly evaporate during combustion and condense either homogeneously as sub-micron ash or heterogeneously onto already existing fine ash. The coal–mineral and mineral–mineral associations play an important role in the formation of fine particles and in subsequent condensation of trace elements in various phases. Any retention of these elements in fly ash particles is strongly influenced by their association with other minerals in individual coal and mineral grains. Clean coal technology development is, therefore, a priority area for research and needs continuous improvements in increased efficiency and decreased pollutant emission. The paper will include trace elements in different coals from around the world. It will consider different modes of occurrences present in coals, the ash formation and evaporation of trace elements and emissions. The typical emissions from typical power stations will be presented. The paper will also review different approaches adopted in estimating the deportment of these elements. The paper at the end would discuss control strategies for reducing emissions and future directions.

Thermal-destruction products of coal in the blast-furnace gas-purification system

Given the shortage and high price of coking coal, efforts are underway to reduce their consumption in their primary uses, including blast-furnace production and the smelting of copper and nickel ore in shaft furnaces. A promising approach is the replacement of some or all of the coke by less expensive coal [1] or by dust‐coal mixture injected in the blast furnace [2]. The lean, poorly clinkering coal and anthracite used to replace coke in blast furnaces has a considerable content of volatile components (low-molecular thermaldestruction products), which enter the water and sludge of the blast-furnace gas-purification system as petroleum products. Therefore, it is important to study the influence of coal on the petroleum-product content in the water and sludge within this system. The liberation of primary thermal-destruction products is investigated for anthracite with around 4 wt % volatiles, using a STA 449C Jupiter thermoanalyzer equipped with a QMC 230 mass spectrometer. The thermoanalyzer determines small changes in mass and thermal effects with high accuracy (weighing accuracy 10 ‐8 g; error in measuring thermal effects 1 mV). This permits experiments with single layers of coal particles, eliminating secondary reactions of its thermal-destruction products. Samples are prepared from natural anthracite in powder form. Particles of size class 0.5‐2 mm (first series of experiments) or <0.5 mm (second series) form a single layer; in the third series, (0.5‐2)-mm particles form several layers. The experiments are conducted in a current of argon. The results are shown in Figs. 1 and 2 in the form of curves of the mass (TG) and the rate of mass change (DTG) and differential scanning-calorimetry (DSC) plots with standard leucosapphire samples, along with mass-spectrometric data. According to experimental data, the evaporation of free water ends at 200‐220 ° C, and the formation of low-molecular thermal-destruction products of coal begins at 250 ° C. The liberation rate of volatiles from a single layer of (0.5‐2)-mm particles, like the evaporation rate of water (Fig. 1), is considerably higher than for the other series of experiments. In the particle layer, more favorable conditions for the reaction of intermediate products released from a single particle with other particles or thermal-destruction products are created. When the coal is ground to powder, the structure of the macromolecules is destroyed, and the resulting fragments of different molecular mass and chemical activity react at once with each other and with the partially disintegrated coal grain. This necessarily affects the decomposition rate. These aspects of the thermal destruction of anthracite particles must be taken into account in selecting the fraction of coal-dust fuel for injection in steel-smelting furnaces. According to mass-spectrometric data (Fig. 2), the basic components of the gas phase in the thermal destruction of anthracite are CH 4 , CO 2 , H 2 , and H 2 O. No carbon monoxide is observed, in contrast to [3]. The content of C n H m and N 2 is no more than 1 vol %, according to [3]; this is probably why it is not detected mass-spectrometrically. The composition of the gas phases varies considerably as a function of the temperature. Intense methane liberation occurs in the range 440‐500 ° C, while the CO 2 content gradually declines. At 580 ° C, hydrogen begins to predominate in the gas. Above 580 ° C, the hydrogen content increases, and the methane content declines; this is associated with displacement of the equilibrium in the reaction CH 4 = C + 2H 2 toward the products.

Type Curves for Dry CBM Reservoirs With Equilibrium Desorption

Abstract The purpose of this work is to model the single-phase radial gas flow in coalbed methane including equilibrium sorption phenomena in the coal matrix and Darcy flow in the natural fracture network. Considering a control volume, the gas desorption rate as a function of time and space is incorporated into the radial continuity equation as a source term. Using a Langmuir-type sorption isotherm, the gas desorption rate is determined at any radius of the reservoir. Introducing the definition of pseudo-pressure and pseudo-time, the resulting continuity equation is converted into the linearized diffusivity equation by modification of total gas compressibility. It is shown how the traditional definition of the material balance pseudo-time is modified for dry CBM reservoirs. With the help of these transformations, the traditional (PTA and RTA) type curves can be employed for analysis of production data of dry CBM reservoirs. The model developed here is validated against Fekete's numerical CBM simulator over a wide range of reservoir parameters. In addition, one set of field data from Horseshoe Canyon coals of the Western Canadian Sedimentary Basin is analyzed using the solution procedure presented in this paper. Introduction Coalbed methane (CBM) is natural gas produced from coal seams. Coal is both the source rock and the reservoir for methane production. The world total CBM resource potential is evaluated at about 143.2 trillion cubic metres(1). CBM reservoirs are naturally fractured reservoirs that are characterized by two distinct porosity systems including:micropores (matrix) with extremely low permeability andmacropores (natural fractures or cleats). Due to the small pore diameter of less than 10 Å, the coal matrix has a large internal surface area of 100 to 300 m(2)/g(2, 3). As a result, substantial quantities of gas can be adsorbed on the surface of the coal grains. Micropores are impermeable to gas and inaccessible to water. However, the desorbed gas can be transported through the primary porosity system by diffusion. The macropores act as a sink to the micropores and provide permeability to fluid flow. In porous media with larger pore size distributions, mass transfer is driven by pressure gradients, whereas in coal, mass transfer is driven by concentration gradients(4, 5). The diffusion through the micropores can be the result of three distinct mechanisms that may act individually or simultaneously(6):bulk diffusion, where molecule/molecule interactions dominate;Knudsen diffusion, where molecule/surface interactions dominate; andtwo-dimensional surface diffusion of the adsorbed gas layer. The steady state diffusion coefficient of methane for most coals is on the order of 10−4 to 10−5 cm(2)/s(6). These experimentally determined diffusion coefficients represent averaged values including the contributions of the bulk, Knudsen and surface diffusion processes. Diffusion effects can be quantified by determining a sorption time. The sorption time is equal to the time required to desorb 63.2% of the initial gas volume. It is determined from whole core canister desorption test. This time is related to fracture spacing and the diffusion coefficient(7). Gas production from the CBM reservoirs is controlled by a four-step process that includes:

Semi-automated petrographic assessment of coal by coal grain analysis

A new classification method, coal grain analysis, which uses optical imaging techniques for the microscopic characterisation of the individual grains present in coal samples is discussed. This differs from other coal petrography imaging methods in that a mask is used to remove the pixels of mounting resin to obtain compositional information of the maceral (vitrinite, inertinite and liptinite) and mineral abundances on each individual grain within each image. Experiments were conducted to establish the density of individual constituents in order to enable the density of each grain to be determined and the results reported on a mass basis. The grains were sorted into eight grain classes of liberated (single component) and composite grains. By analysing all streams (feed, concentrate and tailings) of the flotation circuit at a coal washing plant, the flotation response of the individual grain classes was tracked. This has implications for flotation process diagnostics and optimisation.

Influence of coal tar–water emulsion on carbonisation of caking coals

The influence of coal tar–water emulsion (CTWE) on the dynamics of carbonisation and characteristics of cokes obtained from single coals and their blends was studied. Carbonisation tests were carried out in a Karbotest unit and a laboratory coke oven using X-raying. The qualitative parameters of cokes CRI and CSR, the structure of the coal plastic layer, the thickness of its zones, and the intralayer pressure were determined. The coals and cokes carbonised were investigated using scanning electronic microscopy (SEM). It was found out that the addition of CTWE to coal during pyrolysis affects the structure of the plastic layer, changes the thickness of its zones as well as the parameters characterising the dynamics of the plastic mass movement during carbonisation. CTWE introduced to coal facilitates or hinders the release of decomposition products of the organic matter that is evidenced by SEM micrographs of swollen coal grains. Under the influence of the emulsion the surface of coal grains may become clogged. On examination it was found out that by changing the parameters of the plastic state and phenomena occurring during pyrolysis CTWE plays the role of an external plasticiser of the organic matter and a modifier of the surface of swollen grains.