Lignite Samples Research Articles

Anisole-based assessment for the quantification of methoxyl groups in lignin-rich biomass

The methoxyl group (–OCH3) is abundant in biomass containing lignin. The quantification and biogeochemical evolution features of –OCH3 in these materials are still a research focus. The quantification methods of –OCH3 available up to date overall have a large error range and/or are time consuming through the need for a series of treatments. The present work reports an anisole-based assessment method for –OCH3 quantification in biomass containing lignin, by applying an anisole standard reagent to react with hydroiodic acid to create an external quantification curve used for –OCH3 quantification in samples studied. In practice, 10–200 μL of anisole and 0.2–1 g of lignite samples were reacted with surplus hydroiodic acid under 130 ℃ for 30 min to quantitatively obtain the iodomethane (CH3I) produced in a 20 mL ampere vial. The headspace gas was subjected to GC-FID analysis to create an external calibration curve for –OCH3 quantification in the lignite samples. The results indicated that the external calibration curve had a fairly good correlative relationship (R2 = 0.9523). Quantification of –OCH3 carbon in lignites by this method is well within the scope of previous reports. It seems likely that this simple and precise quantification method will have promising applications for methoxyl group quantification in lignin-rich biomass.

Accurate characterization method of coal pore and pore throat structure of different coal ranks based on two-dimensional NMR liquid nitrogen fracturing cycles

In order to accurately characterize the porosity and pore throat structure of different coal grades under liquid nitrogen fracturing cycles, this paper tests the T2 and T1-T2 sequences of different grades of coal samples (lignite, bituminous coal and anthracite) before and after the fracturing cycles. The results show that the total pore space of A, L and B is arranged in descending order according to the initial state. The internal porous throat distribution of lignite and anthracite coal samples is wider. With the increase of the number of fracturing cycles, the internal pore space of the three coal grade coal samples has been improved, of which lignite has the largest lifting rate; The internal porous throat distribution of L coal samples was reduced by 0.97%. The internal pore space of B and A coal samples increased by 34.62% and 6.33%, respectively, the pore-throat distribution increased by 28.96% and 16.98%, respectively.

Paleo-wildfire signatures revealing co-occurrence of angiosperm-gymnosperm in the early Paleogene: Evidences from woody charcoal and biomarker analysis from the Gurha lignite mine, Rajasthan, India

Charcoal, a common product of wildfire, is abundant in many sedimentary rocks deposited in a wide range of environments, from terrestrial to marine. The analysis of paleofloras and the related paleoecological conditions is of great importance for the understanding of past environmental and paleoclimatic events. Woody charcoal is identified from the early Paleogene sediments of the Gurha lignite mine (Palana Formation), Bikaner, Rajasthan. This evidence is in the form of fragments of tracheids and vessels that show homogenized cell walls and well-preserved anatomical details, like pitting on cell walls, characteristic features of charcoal. These charcoal remains, most of which belong to angiosperms, represent the first verified occurrence of paleo-wildfires in this region during the early Paleogene. Moreover, a gymnospermous taxonomic affinity can be established for some of the charred woods and a relationship with conifers is likely, thus providing additional evidence for the taxonomic composition of early Paleogene floras in this region. Consecutively, the presence of diterpenoid compounds together with the occurrence of (mega-)sporinite in the lignite samples also shows the presence of gymnosperm vegetation (conifers).Furthermore, the petrographic composition reveals that the lignite samples are dominated and sub-dominated by the huminite (av. 43.2 vol%) and inertinite (av. 39 vol%) groups, respectively, followed by liptinite (av. 13.9 vol%) group of macerals. Fusinite, semifusinite (and inertodetrinite) macerals are commonly considered as fossil charcoal. The relatively higher content of these macerals in the studied lignite indicates the regular events of palaeofire during the deposition. Subsequently, the presence of unsubstituted polycyclic aromatic hydrocarbon (PAH) compounds identified in samples further suggests the occurrence of fire. Additionally, the inertinite (fusinite) reflectance indicates that the charcoal/inertinite was formed at a temperature above 300 °C, while the highest reflectance value suggests a burning temperature of ∼800 °C.

Mineral and Chemical Characterization of Magnetic and Non - Magnetic Products Obtained from Dry Magnetic Enrichment of Lignite Coals

The mineral and chemical composition of magnetic (tailing fraction) and non – magnetic (clean coal fraction) products from the dry magnetic separation of coal samples with high ash and sulfur contents obtained from Çanakkale Çan lignite (S1) and Manisa Kula Pabuçlu lignite (S2) were investigated in this study. The feasibility of the dry magnetic separation technique for the separation of the minerals from the coal matrix was identified as the function of coal types and particle sizes. The separation technique depends on coal fragments being weakly diamagnetic, while most of the minerals present in the coal are weakly to moderately paramagnetic. Three particle sizes (coarse-, medium-, and fine-sized) of feed lignite and their magnetic and non-magnetic fractions were separated and characterized. The influence of coal type on the magnetic separation efficiency was determined by the recovery of tailing fractions obtained from S1 and S2 lignites. The particle sizes of S1 lignite were quite lower than S2 lignite. When the fine-sized lignite samples from S1 and S2 were used, the recovery of tailing fractions was the highest compared to other sizes. The mineral compositions of S2 lignite include typically mica, smectite (clay mineral), gypsum, kaolinite (clay mineral), pyrite, quartz, and plagioclase. The results showed that the Permroll High-Intensity Magnetic Separation method for high calorific value and low ash content at the medium-sized particle was more efficient for the S2 lignite sample than the S1 lignite sample.

Structural characteristics of pores and fractures during lignite pyrolysis obtained from X-ray computed tomography

In-situ mining of lignite requires dehydration, pyrolysis, gasification, and other stages. The injected fluid, dehydrated water, and pyrolysis products are transported in the developing pores and fractures. The internal structure and properties of lignite change significantly under the joint action of temperature and fluid pressure. In this study, X-ray computed tomography (X-CT) was used to scan lignite samples in the temperature range of 25°C–450 °C. Grayscale images and three-dimensional reconstruction images of the internal structure were obtained to investigate the evolution of the internal pore structure during lignite pyrolysis. It is found that the porosity of lignite increased as the temperature rose from 25 °C to 250 °C. The porosity was 6.54% at 250 °C. At 350 °C, the porosity decreased to 2.45% due to channel blockage and softening of the coal. At 450 °C, the pyrolysis of the lignite organic matter resulted in numerous large and interconnected honeycomb pore clusters. At this temperature, the porosity was 16.02%. X-CT and nuclear magnetic resonance enabled detailed and quantitative characterization of the internal structure of lignite. The research results provide theoretical and technical information on the evolution of migration channels in lignite for the potential improvement of in-situ pyrolysis and gasification efficiency of in-situ lignite mining.

Application of Humic and Fulvic Acids as an Alternative Method of Cleaning Water from Plant Protection Product Residues

Humic acids (HAs) and fulvic acids (FAs) are naturally occurring compounds that influence the fate and transportation of various compounds in the soil. Although HAs and FAs have multiple uses, the reports about their sorbent potential for environmental pollutants are scanty and sparse. In this study, HA and FA, isolated from lignite samples from two mines in Greece, were studied as sorbent materials for three active compounds of plant protection products, namely glyphosate (herbicide), cypermethrin (pyrethroid insecticide), and azoxystrobin (fungicide). According to the results, both HA and FA are promising sorbent materials for these active compounds, with HA achieving better sorption for cypermethrin and azoxystrobin, while FA was found to be more efficient for glyphosate. Moreover, their performance was not compromised by other components commonly found in commercially available herbicides/insecticides/fungicides. In addition, no significant leaching of the sorbed compounds was recorded. Finally, the two materials achieved similar sorption efficiency of the compounds from lake water.

Quantitative characterization of the pore volume fractal dimensions for three kinds of liquid nitrogen frozen coal and its enlightenment to coalbed methane exploitation

Liquid nitrogen fracturing technology is an important means to improve the permeability of coal seams and realize unconventional natural gas extraction. In order to study the fractal characteristics of pore structure of coal frozen by liquid nitrogen, the pores of both nonfrozen and frozen samples of lignite, bituminous coal, and anthracite were measured with a high-pressure adsorption meter, a mercury porosimeter, and a low-field nuclear magnetic resonance (NMR). The test results showed that after freezing in liquid nitrogen, the pores of coal were effectively expanded. Among them, Nitrogen adsorption of lignite, bituminous coal and anthracite increased by 20.82%, 6.54% and 20.39%. The conversion rates from adsorption pore to seepage pore were 2.16%, 0.78% and 3.57%. The research results of pore fractal dimension showed that the multifractal characteristics of macropores were higher than that of mesopores and micropores. After liquid nitrogen freezing, the pore fractal dimension of lignite, bituminous coal and anthracite decreased by 0.031, 0.009 and 0.003, and the pore structure tends to be smooth. It showed the liquid nitrogen freezing coal enhances the migration and drainage of coalbed methane, which has reference significance for the further research of liquid nitrogen fracturing technology.

Investigation of Combustion Kinetic with Coats–Redfern Method by Using Thermogravimetric Data of Different Lignite Samples

Investigation of Combustion Kinetic with Coats–Redfern Method by Using Thermogravimetric Data of Different Lignite Samples

Thermodynamic variation law and influence mechanism of low-temperature oxidation of lignite samples with different moisture contents

Coal spontaneous combustion (CSC) is one of the main disasters that threaten the safe production in coal mines. Moisture, which is the main factor affecting coal oxidation, has a considerable impact on CSC. In this paper, the variation characteristics of TG-DTG and DSC-DDSC curves of two kinds of lignite during low-temperature oxidation were investigated. The following results were obtained: Oxidation-induced heat production (OHP) starts to play a leading role in the temperature range of 50–75 °C; pyrolysis-induced heat absorption (PHA) grows faster than OHP in the temperature range of 70–130 °C; and OHP is dominant in the temperature range of 130–260 °C. Furthermore, for coal samples from Shengli (SL) Coal Mine and Yuanbaoshan (YB) Coal Mine, the initial heat release temperature and the end temperature both conform to the Lorentz function T=T0+2A(B4(M−Mc)2+B2). Thermodynamic parameters reach the maximum under the moisture content of 12%–17% where the CSC is the most likely to occur. In the oxygen adsorption and weight gain stage, the activation energy decreases and gradually stabilizes with the increase in moisture content. In the accelerated oxidation stage, moisture in coal plays a chemical catalytic role in the processes of oxygen chemical adsorption and intermediate complex formation and decomposition. However, excessive moisture cannot accelerate the coal-oxygen composite reaction.

Thermal desorption of mercury from lignite in a high-temperature furnace and in power plant mills

In this article, the binding forms of two lignite samples are determined by thermal desorption using a high-temperature furnace. Each mercury compound, such as HgCl2, has a specific binding strength whose decomposition requires a certain thermal energy. Hence, the release of mercury from pure substances and lignite samples was analyzed in a high-temperature furnace. The released mercury is determined with a Mercury Vapor Monitor. The obtained characteristic temperature range and peak of the mercury release were compared between lignite samples and mercury pure substances. For the lignite samples investigated, the binding form of mercury was then identified as Humic Acid. These organic compounds vaporize at lower temperatures. About half of the mercury bound in the lignite was already released at 350 °C. Furthermore, the question arises whether mercury is already released during the grinding-drying process in the coal mill of a power plant. At two power plants, lignite samples were taken simultaneously at the feeder before entering the coal mill and at the dust line afterwards. The samples were analyzed for mercury concentration. The results show that up to one third of the mercury was already released in the coal mill. The vaporized mercury enters the combustion chamber detached from the lignite. The stated analysis methods and the results presented in this article contribute to the understanding of the mercury binding forms in lignite. It also shows the potential of thermal coal pretreatment as a favorable alternative mercury separation technology to others such as activated carbon dosing.

Molecular Resolution Imaging of Adsorption of Dodecane on a Lignite Surface by Frequency Modulation AFM

The consumption of dodecane, a critical flotation collector, in the flotation of low-rank coal is particularly high; however, the adsorption mechanism of dodecane on coal still remains unclear. To shed light on this issue, the molecular-scale adsorption structure of dodecane on a lignite surface was investigated in situ by frequency modulation atomic force microscopy (FM-AFM) for the first time. The adsorption structure of dodecane on highly oriented pyrolytic graphite (HOPG) and mica has been detected in advance. Meanwhile, molecular dynamics (MD) simulations are utilized to elucidate and illustrate the adsorption structure on HOPG, mica, and lignite. Experimental results from FM-AFM show that a layered structure of dodecane molecules is formed on HOPG, while there is no stable adsorption structure on mica. Meanwhile, a few local monolayer structures of dodecane molecules are observed on the surface of lignite, and their thickness is approximately 1 nm, which is less than the molecular chain length of dodecane. It is speculated that the dodecane molecules are adsorbed on lignite selectively, with the capacity to form a stable adsorption structure. As indicated by MD simulations, the interaction energy between dodecane and the mentioned surfaces is in the order HOPG > lignite > mica. And the MD simulation results show a desirable match with direct observation by FM-AFM. In accordance with the characterization of the lignite sample surface, large amounts of polar oxygen-containing functional groups may hinder dodecane adsorption behavior. Through in situ molecular resolution imaging of the interface structure between dodecane and a raw lignite surface, we provide a new perspective for exploring the interaction mechanism between nonpolar collectors and a lignite surface.

Study on Structural Evolution of Synthetic Graphite Derived from Lignite Prepared by High Temperature–High Pressure Method

Abundant and cheap lignite is regarded as inferior fuel due to its disadvantages such as low heat and high moisture. In order to realize the high value and clean, non-fuel utilization of lignite, we used lignite as precursor to prepare high-quality synthetic graphite through graphitization treatment using the high temperature–high pressure (HTHP) method, and afterwards characterized the samples by XRD, Raman, FTIR, SEM-EDS, and HRTEM, and systematically analyzed the effect of P-T conditions on the structure evolution of lignite. Our results demonstrated that temperature dominates the graphitization of lignite, and high pressure accelerates the graphitization process of lignite. Under HTHP treatment, the aromatic layer of lignite accelerates directional rearrangement; the ordered domain increases rapidly, and the ductility and stacking degree of the carbon sheets of the lignite sample are greatly enhanced. Compared with the traditional high-temperature graphitization method, the HTHP method greatly reduces the graphitization temperature and shortens the graphitization time. Remarkably, the as-prepared graphite with a graphitization degree of 91.87% superior to graphite fabricated by the conventional high-temperature processing were rapidly prepared from lignite at 6 GPa and 1300 °C in 20 min. This study demonstrates that the HTHP method is a feasible and effective method to realize the green, efficient, and high value utilization of lignite.

Characterization and Removal Potential of Fluorine in Lignite from a Mine in Shaanxi Province, China: A Case Study

Fluorine appears in coal and is released into the atmosphere upon combustion, resulting in harmful impacts on the environment and life, which needs to be removed from coal before utilization. Coal can be processed by flotation and gravity separation to reduce its fluorine content. In this study, a lignite sample from a mine in Shaanxi Province, China, was characterized using the float–sink test, sieving test, X-ray diffraction (XRD), and polarized light microscopy. Mineralogical analysis indicated that the fluorine in coal is mainly contained in Muscovite and polylithionite, and partly in pyrite. The washability and floatability analyses were employed to evaluate the extent of fluorine removal from >0.5 and <0.5 mm size fractions of lignite, respectively. Compared to the raw sample that contained 347.74 μg/g fluorine content, the proposed combination of gravity-flotation separation process decreased the fluorine content to 90.14 μg/g, which meets the requisites of coal standards.

CO2 Gasification Reactivity and Syngas Production of Greek Lignite Coal and Ex-Situ Produced Chars under Non-Isothermal and Isothermal Conditions: Structure-Performance Relationships

The presented work explores the structural properties, gasification reactivity, and syngas production of Greek lignite fuel (LG) and ex-situ produced chars during CO2 gasification. Three different slow pyrolysis protocols were employed for char production involving torrefaction at 300 °C (LG300), mild-carbonization at 500 °C (LG500), and carbonization at 800 °C (LG800). Physicochemical characterization studies, including proximate and ultimate analysis, X-ray Diffraction (XRD), and Raman spectroscopy, revealed that the thermal treatment under inert atmospheres leads to chars with increased fixed carbon content and less ordered surface structures. The CO2 gasification reactivity of pristine LG and as-produced chars was examined by thermogravimetric (TG) analysis and in batch mode gasification tests under both isothermal and non-isothermal conditions. The key parameters affecting the devolatilization and gasification steps in the overall process toward CO-rich gas mixtures were thoroughly explored. The gasification performance of the examined fuels in terms of carbon conversion, instant CO production rate, and syngas generation revealed an opposite reactivity order during each stage. TG analysis demonstrated that raw lignite (LG) was more reactive during the thermal devolatilization phase at low and intermediate temperatures (da/dtmax,devol. = 0.022 min−1). By contrast, LG800 exhibited superior gasification reactivity at high temperatures (da/dtmax,gas. = 0.1 min−1). The latter is additionally corroborated by the enhanced CO formation of LG800 samples under both non-isothermal (5.2 mmol) and isothermal (28 mmol) conditions, compared to 4.1 mmol and 13.8 mmol over the LG sample, respectively. The pronounced CO2 gasification performance of LG800 was attributed to its higher fixed carbon content and disordered surface structure compared to LG, LG300, and LG500 samples.

The effect of plastic type on the yield and quality of lignite and plastic waste pyrolysis products

Lignite is a low rank coal which has great potential in South Kalimantan. However, it has not been used optimally due to its low quality. One of the ways to improve it is pyrolysis. Pyrolysis is the thermal decomposition of organic material in the absence of oxygen which will produce three products (char, tar and gas). Pyrolysis can make lignite into liquid fuel (pyrolysis liquid), but it still requires improvement due to the need for pure hydrogen donors. Plastic waste has a higher hydrogen/carbon ratio than coal. This material can be used as an additive in the pyrolysis process because it is rich in hydrogen. The samples of plastic waste used were ppolyethylene (PE), ppolypropylene (PP), and polystyrene (PS). Samples of lignite and plastic (plastic composition was 25 wt%) were used for every experiment, and pyrolysis was carried out with a holding time of 60 minutes at 500°C. The pyrolysis liquid obtained is then analyzed for its yields and properties (density, kinematic viscosity, heating value). The most feasible result for fuel alternative was obtained with the addition of PE.

Investigation of Combustion Kinetic with Coats-Redfern Method by Using Thermogravimetric Data of Different Lignite Samples

Investigation of Combustion Kinetic with Coats-Redfern Method by Using Thermogravimetric Data of Different Lignite Samples

Gravimetric and instrumental methods comparison for experimental determination of carbonate carbon content in solid mineral fuels

The content of combustible elements in solid mineral fuels (carbon, hydrogen, etc.) are very important, since they most directly affect the heat value. It should be noted that the fuel heat value depends on many other constituents, such as ash and moisture. In this paper, special attention has been paid to carbon content. In solid mineral fuels, carbon is found alone or bound in the form of various compounds. One of them is mineral carbonate compounds bound as carbonate carbon, which originates from absorbed CO2 from atmosphere. Determination of carbonate carbon content of solid mineral fuels was performed by standard gravimetric method (according to ISO 925: 2019), and newly developed instrumental method, using thermogravimetric analyzer LECO TGA 701. Comparison of obtained experimental results was done. Four types of coal, Kolubara lignite, Kostolac lignite, brown coal, and control coal sample were included in experimental analysis. In addition, moisture in the samples was also determined using analytical method and inspected using LECO TGA 701 thermogravimetric analyzer, as well as total carbon content using the LECO CHN 628 elemental analyzer. An analysis and comparison of the obtained results was performed, and comments and conclusions are presented. The experiments were done in the department for fuel characterization, Laboratory for Thermal Engineering and Energy, Institute of Nuclear Sciences Vinca.

Selected Critical Raw Materials in Waste from Coal Gasification in Poland

In an effort to identify new sources of critical raw materials (CRMs) possibility of recovering selected CRMs from Polish coals, chars, and ashes resulting from the combustion of coals and chars was investigated. The samples were collected from pilot fluidized bed gasification systems. The search for CRMs in coal gasification wastes has not been widely reported before. The study used 2 bituminous coal and 1 lignite sample; the concentration of individual critical raw materials (CRMs) was analyzed using the ICP-MS method. The obtained results were compared with Clarke values in coal ash and in the Earth’s crust, and with the adopted cut-off grade. As shown by the analysis, the highest concentrations of CRMs can be found in fly ash, mainly in samples from the eastern part of the Upper Silesian Coal Basin. This applies mostly to Be, Cs, or Sb due to the fact that their concentrations were found to be higher than the Clarke value in the Earth’s crust; the mentioned fly ashes could be used as potential sources of critical elements if appropriate recovery technologies are developed. In addition, the tested materials have elevated Se, Pb, Ni concentrations, but their recovery is currently not economically viable. Compared to the currently adopted cut-off grade levels, there are no critical elements in the analyzed coal gasification waste that could be recovered.

Study on particle radiative properties of lignite, hard coal and biomass fly ashes in the infrared wavelength range

Fly ash, which is a by-product of combustion in furnaces or boilers, is used in certain materials as an additive for waste recycling. The optical properties of fly ash and the effects on the heat transfer phenomena of materials used in engineering applications differ and depend on the kind of solid fuel and the combustion conditions. The morphological and the radiative properties of fly ash samples of Turkish lignite, biomass, and hard coal that are burned in different thermal power plants were studied herein. The samples, which were collected from cyclones, were morphologically examined using scanning electron microscopy (SEM) images, and their chemical compositions were analysed by energy dispersive x-ray spectroscopy (EDX). Absorbance measurements were made in the wavelength range from 2.5 to 25 μm, and the discrete dipole approximation (DDA) was applied for a numerical assessment of the radiative properties of the samples. The measured absorbance values of all samples for particle diameters of 25 and 75 μm displayed significant differences in the related wavelength range. The most prominent change was observed in the biomass sample, and the hard coal fly ash had the lowest absorbance values in the related spectrum range. Although the particle shapes of lignite and biomass fly ashes are not same, the changes in the measured absorbance values were similar. The effects of the k index of the complex refractive index (CRI) on the radiative properties were examined for values of 0.01, 0.1, 0.3, 0.5 and 1.0. According to the measured absorption values and the calculated absorption efficiency results, the k index may be between 0.3 and 1.0 at the related wavelength range. It was concluded that the effects of the particle size and absorption index of fly ash on the heat transfer properties are important in the specified wavelength range.

Evaluation of single particle drying characteristics using drying models and curve fitting

ABSTRACT In this study, Turkish lignite dried with hot air and superheated steam was experimentally investigated. Drying experiments were carried out at 120°C drying temperature and atmospheric pressure. Mass, center and surface temperature changes of lignite having a diameter of 30 mm were continuously measured. Hot air and superheated steam drying methods were compared, and it was found that, because of condensation in the initial stage of superheated steam, the moisture content of the lignite sample temporarily increased. On the contrary, the moisture content of the lignite sample dramatically reduced on drying with hot air. Six different thin-layer models describing the drying kinetics of lignite were fitted to the experimental data, with the Pusat model giving the best predictions with superheated steam drying and the Midilli model giving the best predictions with hot air drying of a single particle.