Coals Of Various Ranks Research Articles

Experimental Study of Dynamic Permeability Changes in Coals of Various Ranks During Hydraulic Fracturing

Experimental Study of Dynamic Permeability Changes in Coals of Various Ranks During Hydraulic Fracturing

CO2 and CH4 sorption on carbon nanomaterials and coals – Comparative characteristics

The aim of this research was to compare the structure, sorption properties and the kinetics of sorption to CO2 and CH4 in nanomaterials with a very orderly structure and in structurally heterogeneous coal. The authors tested synthetic materials such as multiwall carbon nanotubes (MWCNT) and reduced graphene oxide (rGO) as well as natural materials such as coals of various ranks. The value of the surface area of Langmuir and Brunauer, Emmett and Teller, in MWCNTs and rGO was much higher than in the coal. The course of CH4 and CO2 sorption of the materials was investigated and the parameters of sorption isotherms in the pressure of 0–2.0 MPa were determined. The nanomaterials reached the sorption equilibria much faster than the coal. The highest total CH4 sorption capacity was found in low-rank coal. The highest CO2 total sorption capacity was found in MWCNTs and low-rank coal. The highest relative mean drop in CH4 and CO2 sorption capacity resulting from the increasing measurement temperature was found in the MWCNTs and the low-rank coal. The authors analysed the courses of CH4 and CO2 sorption kinetics in the materials of highly different structures. The kinetics of CH4 and CO2 accumulation in MWCNT and rGO nanomaterials progressed almost instantaneously, and their rate was many times higher than in the case of kinetics on hard coals.

Structural analysis of Greek and Bulgarian coals by solid-state <SUP align="right">13</SUP>C nuclear magnetic resonance spectroscopy

In this study, 32 peat, xylite-rich and matrix lignites, and sub-bituminous coal samples from representative lignite and sub-bituminous coal basins of Greece and Bulgaria were used to investigate how their NMR differences reflect the organic matter origin, genesis conditions and coalification process in each one of these basins. The evolution of the major organic structural units in the samples was identified by using high-resolution, solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, using a spectrometer with cross-polarisation and magic angle spinning. Solid-state 13C CP/MAS NMR is considered as a very powerful method for determining the chemical structure of complex organic substances such as coals of various ranks. To investigate a possible influence of paramagnetic species on the CPMAS-NMR spectra of the samples, electron paramagnetic resonance (EPR) spectroscopy was also applied. Similarities were observed between the spectra of coals of similar rank or of similar forming conditions. The peat-lignite and lignite-sub-bituminous coal transition is marked by pronounced differences in the NMR spectra, as do the different lithotypes of lignite, i.e., matrix and xylite-rich. Determination of the coal organic constituents offers valuable knowledge about the chemical and thermal behaviour of coals in order to investigate major issues associated with coal mining, combustion, and post-combustion processes.

Change of parameters in molecular structure of Donbas coals under the influence of external factors

The molecular structure of coal is estimated by 13 parameters of infrared spectra, electron paramagnetic resonance, volatile yield and ash content. A new criterion of hydrophobicity is applied, which has shown its informativeness in analyzing the molecular structure of coals of various ranks. It is more sensitive to changes in coal rank than the standard index of volatile yield. The coalification process leads to a significant increase in this index due to the water release and other hydroxyl-containing compounds from the substance of coal. Two-way analysis of variance showed that the influence of the metamorphism factor is significant for 9 molecular parameters. The strongest effects are manifested in the sorption capacity, the criterion of hydrophobicity and the number of paramagnetic centers. The stratigraphic factor does not have a significant impact on one of the molecular parameters. Factor analysis by the method of principal components of the molecular structure of coal showed that the most significant independent factors are metamorphism and sedimentation conditions, the latter ones include independent processes: accumulation of mineral components, decomposition of biomass and geochemical environment during the sedimentation period.

Thermal decomposition and oxidation of coal processing waste

To expand the database of kinetic parameters used for modeling the ignition of coals and their processing waste, promising coal-water slurry and coal-water slurry containing petrochemicals, studies have been performed on an experimental set-up using thermal gravimetric analysis. The research into coals of various ranks (flame, gas, coking, low-caking, and non-baking) and their processing waste (filter cakes) has yielded the decomposition parameters of the organic matter of coal and the formation of volatile substances as well as the oxidation parameters of the coke residue of all the coals and filter cakes under study. The studies cover the temperature range of the burning processes: 450-1300 K (for low-, medium-, and high-temperature burning modes). We have ascertained the dependence of kinetic parameters (pre-exponential factor and activation energy) describing the thermal decomposition of the organic matter of coal on the rank of coals and filter cakes. The findings show that the kinetic parameters describing the thermal decomposition of the organic matter of coal and its processing waste are practically the same. The thermokinetic parameters of coke residue oxidation are close for all the coals under study but they differ significantly for coke residue of filter cakes. The values of thermokinetic parameters obtained in the research are necessary to devise adequate physical and mathematical models and perform numerical studies (for mathematical modeling) of fuel slurry combustion processes in the combustion chambers of power plants.

Thermovolumetric investigations of steam gasification of coals and their chars

The process of steam gasification of three coals of various rank and three chars obtained from these coals by the ex-situ method at 900 °C was compared. In the coal gasification process, the pyrolysis stage plays a very important part, which is connected with its direct impact on the kinetics of gasification of the resulting char. What is more, taking into consideration the impact of pyrolysis conditions on char properties, it should be anticipated that the gasification kinetics of coal and char, formed from it by the ex situ method, will be different. In order to examine and compare the process of gasification of coals and chars, an isothermal thermovolumetric method, designed by the authors, was applied. For all the examined samples the measurements were performed at three temperatures, i.e. 850, 900, and 950 °C, and at the pressure of 0.1 MPa. An evaluation of the impact of raw material on the steam gasification of the examined samples was made. The carbon conversion degree and the kinetic parameters of CO and H2 formation reaction were calculated. It was observed that the course of gasification is different for coals and chars obtained from them and it can be concluded that coals are more reactive than chars. Values of kinetic parameters of carbon monoxide and hydrogen formation calculated for coals and corresponding chars are also different. Due to the observed differences the process of gasification of coals and of chars with steam should not be equated.

Effect of heating process on the wettability of fine coals of various ranks

AbstractSpontaneous coal combustion can produce a condition of heating process because the atmosphere of coal underground is usually poor in oxygen. Even though the spontaneous combustion of coal is prevented by people, the surface properties of coal are greatly changed. The changes in the wettability of coal influence the flotation of fine coals. This paper investigates the effect of the heating process on the wettability of fine coals of various ranks. Lignite, bituminous, and anthracite coals were used as experimental samples. The heating temperature and time were 500 °C and 2 h. X‐ray photoelectron spectroscopy (XPS) and contact angle measurements were used to indicate the changes in wettability of fine coals before and after the heating process. The results showed that the wettability of bituminous coal was significantly increased whereas that of lignite was greatly decreased after the heating process. However, the wettability of anthracite coal was increased only a little after the heating process.

A CCSEM study on the transformation of included and excluded minerals during coal devolatilization and char combustion

This study reports the transformation pathways of both included and excluded minerals during coal devolatilization and char combustion. Three Chinese coals of various ranks, namely YQ anthracite, DT bituminous coal and CF lignite, were size narrowed to 45−100μm, devolatilized in nitrogen and combusted in air at 1300°C, using a laboratory-scale drop tube furnace, to prepare chars and ashes. The mineral properties of the coals as well as their derived chars and ashes were analyzed using a computer-controlled scanning electron microscopy (CCSEM). The results demonstrate that, during coal devolatilization, whereas quartz and kaolinite are generally stable for all the three coals, included and excluded Fe-rich minerals (i.e., pyrite and siderite) as well as excluded Ca-rich minerals (i.e., calcite and ankerite) are decomposed and some of the intermediate products are incorporated with aluminosilicates. During char combustion, quartz participates in combustion reactions and forms aluminosilicates. Whereas the mullite-like phase in the YQ and DT chars changes little, that in the CF char absorbs Fe/Ca-rich fine particles and is incorporated into Fe–Ca-aluminosilicate. Most of the Ca/Fe-rich intermediate products react with aluminosilicates to form Fe-, Ca-, and Fe–Ca-aluminosilicates. The interactions between Fe/Ca-rich minerals and aluminosilicates are also observed. Calcium in Ca-rich minerals and iron in Fe-rich minerals of the YQ anthracite and the CF lignite interact with aluminosilicates during both coal devolatilization and char combustion processes. However, for the DT bituminous coal, such interactions between Ca-rich minerals and aluminosilicates only take place during devolatilization. The interactions between excluded Fe/Ca-rich minerals and included aluminosilicates during coal devolatilization are also observed.

Emission of inorganic PM10 from included mineral matter during the combustion of pulverized coals of various ranks

Three density-separated (1.4−1.6g/cm3) and size-fractioned (63−90μm) Chinese coals (HLH lignite, PDS bituminous coal and CZ anthracite), containing dominantly included mineral matter, were prepared and then combusted in a drop-tube furnace in air at 1400°C. Under the experimental conditions, the yields of particulate matter (PM) with aerodynamic diameters of <1μm (PM1), 1−10μm (PM1–10) and <10μm (PM10) as well as some key PM10-forming elements (e.g., Si, Fe and Mg) follow a rank sequence of the HLH lignite>the PDS bituminous coal>the CZ anthracite. Such a rank dependence is attributed to the discrepancies in the properties of included mineral matter in these coals and the coal combustion characteristics. The partition of Fe in the PM1 from the HLH lignite appears to result from the fragmentation and disintegration of included siderite during coal combustion.

The influence of air distribution on gas-fired coal preheating method for NO emissions reduction

A gas-fired coal preheating (GFCP) method was investigated on a 35kW drop tube furnace with different air distributions, and three coals of various rank were used in the experiments. The results of this investigation show the GFCP provides a flexible means to control NOx emissions for various coals, which can be used with traditional air staging like close-coupled over fire air (CCOFA), separated over fire air (SOFA) as well as MILD. The principle is that GFCP promotes substantial nitrogen intermediates (HCN and NH3) and soot to be generated in the preheating chamber. With proper air distribution, the former convert into N2 and the later function as reductant to destroy newly formed NO in the subsequent furnace. The global denitrification effect is more remarkable for coals of lower rank. While a primary air ratio at intermediate level may obtain the lowest NO emissions for combustion without GFCP, the NO emissions are essentially independent of variations in primary air ratio when GFCP is adopted. Our experiments also find that GFCP reduces fly-ash level by extending residence time for particles traversing high temperature zones.

Comparative studies on steam gasification of ash-free coals and their original raw coals

Catalytic gasification of raw coals at mild condition is not realized yet mainly due to deactivation of catalysts via their irreversible interaction with mineral matters in coal. As a means to achieve repeated use of catalysts, four different ash-free coals (AFCs) containing less than 0.2 wt% ash are produced in this work. Steam gasification of ash-free coals (AFCs) and their parent raw coals of various ranks ranging from lignite (Eco) to coking coal (Posco) is performed in a fixed bed reactor at 700–900 °C. Regardless of the rank of the parent raw coals, all the AFCs behave like a highly carbonized coal such that their gasification rate are similarly slow and they exhibit relatively low H2/CO ratio. The steam gasification and associated CO to CO2 conversion of the AFCs are, however, significantly enhanced by K2CO3, resulting in the higher H2/CO and CO2/CO molar ratio.

Investigation of the spontaneous combustion susceptibility of coal using the pulse flow calorimetric method: 25 years of experience

The paper summarises 25years of experience with pulse flow calorimetry (PFC) as a method to assess the potential of coals to spontaneously heat up using the values of oxidation heat q30 (Wkg−1). During the period, about 300 coals of various rank and with natural moisture content were investigated, the maximum propensity to oxidation being found in subbituminous coals with moisture content of about 20%. The highest value of oxidation heat q30 was found to be 10.5Wkg−1. Oxidatively altered bituminous coals are presented as coals of evidently increased ability to oxidize, and the reasons for the increased oxyreactivity are reported. Limiting values of q30 heat are then given to rank the coals according to the categories of self-heating risk. Finally, a comparison between oxidation heat q30 and the values of the initial rate of heating (IRH) of adiabatic oxidation tests is presented.

Differentiation of pre-existing trapped methane from thermogenic methane in an igneous-intruded coal by hydrous pyrolysis

So as to better understand how the gas generation potential of coal changes with increasing rank, same-seam samples of bituminous coal from the Illinois Basin that were naturally matured to varying degrees by the intrusion of an igneous dike were subjected to hydrous pyrolysis (HP) conditions of 360°C for 72h. The accumulated methane in the reactor headspace was analyzed for δ13C and δ2H, and mol percent composition. Maximum methane production (9.7mg/g TOC) occurred in the most immature samples (0.5%Ro), waning to minimal methane values at 2.44%Ro (0.67mg/g TOC), and rebounding to 3.6mg/g TOC methane in the most mature sample (6.76%Ro). Methane from coal with the highest initial thermal maturity (6.76%Ro) shows no isotopic dependence on the reactor water and has a microbial δ13C value of −61‰. However, methane from coal of minimal initial thermal maturity (0.5%Ro) shows hydrogen isotopic dependence on the reaction water and has a δ13C value of −37‰. The gas released from coals under hydrous pyrolysis conditions represents a quantifiable mixture of ancient (270Ma) methane (likely microbial) that was generated in situ and trapped within the rock during the rapid heating by the dike, and modern (laboratory) thermogenic methane that was generated from the indigenous organic matter due to thermal maturation induced by hydrous pyrolysis conditions. These findings provide an analytical framework for better assessment of natural gas sources and for differentiating generated gas from pre-existing trapped gas in coals of various ranks.

Physical Properties of Particulate Matter Emitted from Combustion of Coals of Various Ranks in O2/N2 and O2/CO2 Environments

This work examined the particulate emissions from pulverized coals burning under either conventional or oxyfuel combustion conditions. Oxyfuel combustion is a process that takes place in O2/CO2 environments, which are achieved by removing nitrogen from the intake gases and recirculating large amounts of flue gases into the boiler; this is done to moderate the high temperatures caused by the elevated oxygen partial pressure therein. In this study, combustion took place in a laboratory laminar-flow drop-tube furnace (DTF) in environments containing various mole fractions of oxygen in either nitrogen or carbon dioxide background gases. A bituminous coal, a sub-bituminous coal, and a lignite were burned at a DTF temperature of 1400 K. Trimodal ash particle size distributions were observed with peaks in the submicrometer region (∼0.2 μm), as well as in the supermicrometer region (∼5 μm and >10 μm). Both submicrometer and supermicrometer particulate emission yields of all three coals were typically lower in O2/CO2 than in O2/N2 environments. Emission yields typically increased with increasing oxygen concentration in the furnace, with an exception noted at moderate oxygen mole fractions (20%–30%) in CO2, where significant amounts of unburned carbon were detected. Submicrometer particulate yields were found to be comparable in the effluents of all three coals, independently of their ash contents, whereas supermicrometer particulate yields were nearly analogous to the ash contents of the three coals. Scanning electron microscopy (SEM) revealed that submicrometer particles were spherical, whereas supermicrometer particles were often of irregular shapes, fractured spheres, and spheres with small particles attached to their surface.

Mechanism of the dissolution of coals in a mixture of concentrated sulfuric and nitric acids

In this work, coals of various ranks were dissolved in an acid melange. With the use of IR spectroscopy, it was found that the mechanism of coal dissolution consists in the grafting of polar groups to the peripheral benzene rings of graphite-like systems.

Emissions of NOx and SO2 from Coals of Various Ranks, Bagasse, and Coal-Bagasse Blends Burning in O2/N2 and O2/CO2 Environments.

Oxy-coal combustion is a viable technology, for new and existing coal-fired power plants, as it facilitates carbon capture and, thereby, can mitigate climate change. Pulverized coals of various ranks, biomass, and their blends were burned to assess the evolution of combustion effluent gases, such as NOx, SO2, and CO, under a variety of background gas compositions. The fuels were burned in an electrically heated laboratory drop-tube furnace in O2/N2 and O2/CO2 environments with oxygen mole fractions of 20%, 40%, 60%, 80%, and 100%, at a furnace temperature of 1400 K. The fuel mass flow rate was kept constant in most cases, and combustion was fuel-lean. Results showed that in the case of four coals studied, NOx emissions in O2/CO2 environments were lower than those in O2/N2 environments by amounts that ranged from 19 to 43% at the same oxygen concentration. In the case of bagasse and coal/bagasse blends, the corresponding NOx reductions ranged from 22 to 39%. NOx emissions were found to increase with increa...

Supercritical Pure Methane and CO2 Adsorption on Various Rank Coals of China: Experiments and Modeling

Carbon dioxide sequestration in coal seams with enhanced coal-bed methane recovery (CO2-ECBM) is considered as a promising option for permanent carbon dioxide storage. As one of the most important factors to the success of the CO2-ECBM process, adsorption of methane and CO2 on coal helps to assess the amount of recoverable methane as well as the storage capacity of CO2 of the targeted coal seam. In this work, the methane and CO2 adsorption isotherms were measured with a volumetric technique at temperatures of 35, 50, and 65 °C and pressures up to 16 and 12 MPa (CO2 adsorption at 35 °C is limited below 6 MPa), respectively. Four coals of various rank exploited from four main coal seams in China were tested. The isotherms fit well to the Ono-Kondo lattice model, which confirms the applicability of this model in describing the adsorption behaviors of methane and CO2 on coal under the supercritical conditions. In addition, the experimental results show that the excess adsorption of CO2 reaches the maximum lev...

Modification of vitrinite coals in a low-temperature oxygen plasma

Transformations in the organic matter of vitrinites in coals of various ranks as a result of their interaction with oxygen activated in an RF field were characterized using IR spectroscopy and solvent extraction.

Effect of mechanochemical treatment on the solubility of coals of various ranks

The effect of the mechanochemical treatment of coals of various ranks on their dissolution in an acid melange was studied. It was found that the mechanochemical treatment weakly affects the reaction time of coal with the melange because it takes only 1–5 min; however, it determines the melange volume at which the dissolution of coals takes place. Preliminary mechanochemical treatment affects the specific surface area of coals after their treatment with the melange and the increase in the weight of coals.

Moisture-Induced Swelling of Coal

The gas-induced swelling behavior of coal is important when considering CO2 sequestration into coal seams or enhanced coalbed methane applications, but coals may also swell in the presence of moisture, or shrink on drying. In this paper we examine the moisture-swelling properties of coals from Australia and elsewhere. Results on the moisture uptake and corresponding swelling measurements are presented for 15 coals of various ranks (sub-bituminous and bituminous) at 22°C and atmospheric pressure. Measurements were made by exposing sample blocks of coal (nominally 30 × 10 × 10 mm) to relative humidities ranging from 0% to 97%. A selection of coals was also fully saturated in water. Moisture uptake at 97% relative humidity (RH) ranged from about 2.5% to more than 16% db. Maximum linear strain associated with the moisture sorption (measured at 97% RH) varied from about 0.2% to 1.3%, with lower rank coals showing the most swelling. In all cases, swelling was greater in the direction perpendicular to the bedding plane. These results correspond to volumetric swelling of about 0.5% to around 5%. Although exhibiting significant expansion, all of the samples returned to their original dimensions upon drying. Volumetric moisture sorption and the amount of swelling induced were found to be strongly correlated by a single linear expression that held for all of the coals examined. It was further found that the volume of the water adsorbed was linearly related to the pore space within the coal, however, at 97% relative humidity, only about 60% of the available pore space is occupied by water. Exposure to liquid water allowed the pores to completely fill; although for the lowest rank coals, the volume of water absorbed appeared to be slightly more than the corresponding pore volume. Despite the additional water uptake, immersion in water did not produce further swelling beyond that induced at 97% relative humidity.