Raw Lignite Research Articles

Investigation of co-combustion characteristics and kinetics of low-ranked coal and hydrochar derived from cow manure

CO2 emission should be reduced by the effective utilization of low-ranked coal in a coal-based power plant based on co-combustion of coal and biomass. This study investigated the co-combustion behavior of lignite and cow manure using three different kinds of samples: raw lignite coal blended with cow manure (CM-L), raw lignite coal blended with hydrochar derived from cow manure (HCM-L), and co-hydrothermal carbonization (Co-HTC) of lignite coal and cow manure (Co-HCML) using thermogravimetric analysis and a hydrothermal carbonization (HTC) temperature of 200 °C for 60 min. The kinetic parameters were calculated and analyzed using the Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose methods. The combustion characteristics were studied of the various kinds of hydrochar mixed with lignite coal. The results showed that the low-ranked coal improved the fuel properties by increasing carbon content and decreasing sulfur through the HTC processes. The activation energy of Co-HCML was the lowest, which is preferable, and therefore it was recommended as the promising process.

A Novel Microbial Compound Inhibitor and Its Efficacy in Preventing and Controlling Mine Fire Risks: A Case Study on Low-Rank Coal Spontaneous Combustion.

To guarantee the safety and sustainability of coal mining by effectively mitigating the substantial risk associated with coal spontaneous combustion, this study proposes a multifaceted prevention strategy aligned with green environmental principles. A compound flame retardant with a physicochemical control mechanism was prepared using indigenous microorganisms to mineralize residual coal after mining, utilizing Bacillus pasteurelli as a substitute material for inorganic salts. Under laboratory conditions simulating coal self-combustion, biobased flame retardants were employed to investigate the physical and chemical transformations of heat and mass evolution from ambient temperature to combustion in two representative low-rank coals. By quantitatively comparing alterations in microbiome-based groups among raw lignite, bioretarded lignite, and two control samples, the inhibitory mechanism of biobased materials on the oxygen reaction pathway was elucidated. The findings substantiated that biobased modification can consolidate the methyl and methylene groups present in aliphatic hydrocarbon side chains, which are prone to instigating low-temperature oxidation reactions. Additionally, the preventive performance of biobased flame retardants was assessed through temperature-programmed experiments, which involved estimating the critical self-heating temperature, oxygen consumption, and gas production rates of compared coal samples. The results demonstrated significant enhancements in the resistance to spontaneous combustion following bioretarded modification. Notably, the identification grade of long flame coal shifted from easy to moderate susceptibility to spontaneous combustion. Furthermore, biobased flame retardants exhibited remarkable flame retardancy rates of approximately 80% for lignite, thereby validating their efficacy as more environmentally friendly and technologically advanced substitute materials for inhibiting spontaneous combustion in low-rank coals.

Removal of nonylphenol polyethoxylates by raw lignite coal and activated carbon: materials characterization, adsorption studies, and modeling the adsorption isotherms

Removal of nonylphenol polyethoxylates by raw lignite coal and activated carbon: materials characterization, adsorption studies, and modeling the adsorption isotherms

Removal of Heavy Metal Ions from Aqueous Solution Using Biotransformed Lignite

Heavy metal pollution caused by industrial wastewater such as mining and metallurgical wastewater is a major global concern. Therefore, this study used modified lignite as a low-cost adsorbent for heavy metal ions. Pingzhuang lignite was dissolved and modified using Fusarium lignite B3 to prepare a biotransformed-lignite adsorbent (BLA). The O, H, and N contents of the BLA increased after transformation, and the specific surface area increased from 1.81 to 5.66 m2·g−1. Various adsorption properties were investigated using an aqueous solution of Cu(Ⅱ). The kinetic and isothermal data were well-fitted by pseudo-second-order and Langmuir models, respectively. The Langmuir model showed that the theoretical Cu(II) adsorption capacity was 71.47 mg·g−1. Moreover, large particles and a neutral pH were favorable for the adsorption of heavy metal ions. The adsorption capacities of raw lignite and BLA were compared for various ions. Microbial transformation greatly improved the adsorption capacity, and the BLA had good adsorption and passivation effects with Cu(II), Mn(II), Cd(II), and Hg(II). Investigation of the structural properties showed that the porosity and specific surface area increased after biotransformation, and there were more active groups such as –COOH, Ar–OH, and R–OH, which were involved in the adsorption performance.

Re-explosion hazard potential of solid residues and gaseous products of coal dust explosion

In the present work, solid residues and gaseous products after the initial explosion of coal dust were collected by self-made devices, respectively, and their explosiveness was further studied to assess the re-explosion hazard. The results show that the solid residue can explode again, and its explosion pressure (Pex) and pressure rise rate ((dP/dt)ex) both increase gradually as dust concentration and ignition energy increase, but decrease with the larger particle size. Solid residue is characterized by a lower deflagration index (Kst) than raw lignite dust, but it can produce greater severity than some raw high-rank coal dust in some cases. The gaseous products of coal dust explosion are mainly composed of CO, H2 and CH4, and some trace gases. The volume fraction of CO and H2 in the gaseous products rises in proportion to the concentration of coal dust. For coal dust explosion with a concentration >200 g/m3, the gaseous products collected are flammable and have a wider explosion limit and a lower limited oxygen content. This research provides valuable information and reference for future prevention and control of secondary explosion disasters in coal mines.

Hydro-liquefaction of the ashless coal from de-polymerization of Shengli lignite

This work performed de-polymerization of Shengli lignite (SL) to remove minerals, and the resultant product (denoted as SDP) was subjected to hydro-liquefaction to prepare value-added aromatic products, especially n-hexane soluble fractions (HS for abbreviation). Compared with raw lignite, SDP is easily converted to light products (HS) with low H2 consumption due to its small molecular weight and less free radicals generated during hydro-liquefaction. Appropriate temperatures (e.g., 400 °C) are required for efficient conversion of SDP into HS fractions. Ni-Mo-S/Al2O3 catalyst effectively promotes hydrogenation of SDP into HS during the reaction, moreover, the condensation reaction is effectively reduced. The yield of HS (YHS) reaches to 73.7% and the yield of THFI (YTHFI) is only 2% with the use of Ni-Mo-S/Al2O3 at 400 °C. The Aromatic fractions in the solvent effectively reduce YTHFI through suppression of condensation reaction, and the hydrogen donor solvents remarkably increase YHS. The HS fractions primarily consist of aromatic hydrocarbons with 1–3 rings and aromatic derivatives containing alkyl as well as various oxygen-containing functional groups.

Physicochemical properties, combustion kinetics and thermodynamics of oxidized lignite

Lignite oxidizes easily during storage and transportation. Furthermore, the surface characteristics of lignite change during oxidation, affecting its thermal conversion process. Herein, the surface functional groups and pore structures of lignite oxidized at temperatures below 190 °C were investigated. Moreover, the kinetics and thermodynamics of the combustion of lignite were analyzed using thermogravimetric analysis. The relationship between the surface structure and combustion performance of oxidized lignite was established, and the effect of oxidation on the characteristics of the combustion process was evaluated. The results revealed that the value of CH3/CH2 increased considerably upon increasing oxidation temperature from 70 to 150 °C. The number of –OH groups decreased with increasing in oxidation temperature, whereas the number of CO groups decreased gradually, then increased significantly, and lastly decreased with increasing oxidation temperature. The mesoporous structure of lignite was severely damaged during oxidation, the specific surface area and pore volume of oxidized lignite were lower than those of raw coal, and the average pore diameter of oxidized lignite was significantly higher than that of raw lignite. The poor combustion performance of oxidized lignite was attributed to its low mesopore specific surface area and low pore volume. The activation energy (Ea) of lignite during combustion was calculated to be 35.29 kJ/mol using the Ginstling-Broushtein equation, and the Ea of oxidized lignite ranged between 40.53 and 45.93 kJ/mol. These results indicate that a large amount of energy was required to burn oxidized lignite. Therefore, the oxidation of lignite should be avoided during the storage process to damage the pore structure, thus affecting the combustion performance; the pore structure of oxidized lignite can be adjusted to enhance its combustion performance. This will effectively improve the utilization efficiency and economic value of lignite.

Extraction of Humic Acids from Lignite and Its Use as a Biochar Activator.

Current research focuses on extracting humic acid (HA) compounds from low-rank coals to obtain high value-added products. In this study, HAs with high purity and low heavy metal content were obtained from lignite by combining acid pretreatment with hydrothermal treatment. Scanning electron microscopy, elemental analysis (EA), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction, and inductively coupled plasma optical emission spectrometry (ICP-OES) were used to analyze raw lignite and HAs. The effects of acid and hydrothermal treatments on the inorganic elements, functional groups, and yield of HAs were examined. The results showed that acid treatment reduced the ash content of lignite from 20 to 9%, and hydrothermal treatment increased the yield of HAs from 36 to 68%. The chemical properties of HAs exhibited an increase in molecular weight and improved aromaticity after acid and hydrothermal treatments. The results of ICP-OES analysis suggested that the combined method of acid and hydrothermal treatments resulted in a significant reduction of heavy metal elements in HAs. FTIR analysis confirmed the results and demonstrated that the extracted HA from nitric acid pretreated and hydrothermal generation of lignite PHA was rich in carboxyl and phenolic functional groups. PHA was applied to biochar as an activator for the adsorption of heavy metal ions. The experimental results showed that PHA was successfully loaded onto biochar and introduced a large number of functional groups, and the adsorption capacity of the modified biochar for Pb2+ was effectively improved.

Biogasification of methanol extract of lignite and its residue: A case study of Yima coalfield, China.

To investigate the biogas generation characteristics of the organic matter in lignite, methanol extraction was conducted to obtain the soluble fraction and the residual of lignite, which were subsequently taken as the sole carbon source for biogas production by a methanogenic consortium. Afterward, the composition of compounds before and after the fermentation was characterized by UV-Vis, GC-MS, and HPLC-MS analysis. The results indicated that the methanogenic microorganisms could produce H2 and CO2 without accumulating CH4 by utilizing the extract, and the methane production of the residue was 18% larger than that of raw lignite, reaching 1.03 mmol/g. Moreover, the organic compounds in the methanol extract were degraded and their molecular weight was reduced. Compounds such as 1, 6-dimethyl-4-(2-methylethyl) naphthalene, 7-butyl-1-hexylnaphthalene, simonellite, and retene were completely degraded by microorganisms. In addition, both aromatic and non-aromatic metabolites produced in the biodegradation were detected, some of which may have a negative effect on the methanogenesis process. These results revealed the complexity of the interaction between coal and organism from another point of view.

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.

Bio-solubilization of Yunnan lignite by Penicillium ortum MJ51 and characterization of its products

In this study, a newly discovered fungal strain Penicillium ortum MJ51 was isolated from lignite collected from a coal mine in Yunnan Province, China. The strain was then utilized to convert solid Yunnan lignite to liquid value-added chemicals. It solubilized 36.4% of the raw lignite and 82.0% of lignite pretreated with nitric acid within 8 days. The extracellular metabolites and enzymes secreted by MJ51 were detected dynamically, along with an analysis of the organic element composition, Fourier-transform infrared spectroscopy, and gas chromatography-mass spectrometry of the raw lignite, pretreated lignite, and their bio-solubilized residues and soluble fractions. The results suggested that extracellular metabolites including alkaline substances and surfactants, secreted by MJ51 were the primary contributors to the depolymerization of lignite, because the lignite-solubilization ability of MJ51 was correlated with pH and surface tension of the culture. Moreover, extracellular enzymes, such as lignin peroxidase and esterase, secreted by MJ51 participated in the degradation of lignite despite their poor activity. Furthermore, MJ51 could introduce CO into the structure of raw lignite in the same way as HNO3. Under the associated action of extracellular alkaline substances, surfactants, and enzymes, the hydrogen bond in coal was rebuilt, the complex aromatic skeletons were opened, and the attached long aliphatic side chain was cut off. The pretreated lignite could be thoroughly degraded by MJ51 than raw lignite as it had a lower degree of aromatic condensation, shorter aliphatic side chain and more carbonyl groups. The soluble bio-solubilized fractions of lignite contained aliphatic and aromatic compounds, and their derivatives.

Improved Lignite Predrying System Integrated with Heat and Water Recovery from Wet Flue Gas at Lignite-Fired Power Plants: Energy and Water Conservation

To improve energy efficiency and reduce water consumption simultaneously in lignite-fired power plants, an improved lignite predrying system integrated with heat and water recovery from wet flue gas is proposed in this study. In the proposed system, the heat source for the lignite predrying system was the latent heat from the flue gas instead of the extracted steam, so the extracted steam could return to the turbine to generate more power. Moreover, both the water from the raw lignite and wet flue gas desulfurization (WFGD) processes were recovered, which minimized the water consumption of lignite-fired power plants. This study evaluated the energy- and water-saving potential of the proposed system at both air-cooling and wet-cooling power plants firing three types of lignite separately. The results showed that the higher the moisture content of lignite, the larger the net efficiency improvement and the make-up water rate reduction. The net efficiency improvement and make-up water rate reduction in power plants firing Yimin lignite reached 2.33% points and 0.370 t/MWh, respectively, at wet-cooling power plants, and 2.18% points and 0.443 t/MWh at air-cooling power plants. This means that zero water consumption can be achieved, and water and electricity can be output at the same time at air-cooling power plants. The technical and economic analysis showed that the net present value in 10-years of power plants firing different lignite types could reach 49.31, 36.19, and 16.44 million USD depending on the predry degree. The discounted payback period was 2.34–3.67 years, which showed good economic viability in all cases.

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.

Assessment of lignite upgrade and hydrogen evolution via electrolysis

Lignite, considered as the lowest-rank coal, accounts for 45% of the total coal resources worldwide, and its low carbon and high ash contents limit its large-scale application. In order to upgrade lignite effectively, a modified coal electrolytic cell (MCEC) was designed, built, and then evaluated for lignite upgrade and hydrogen production through electrolysis, and the operating conditions were optimized. The results show that the performance of the MCEC outperformed the previous coal electrolytic cell (CEC). The operating temperature and iron concentration significantly impact the average current density per milligram of catalyst (ACDMC). The content of dry fixed carbon and heating value increased by 20.9% (wt%) and 8.15%, respectively, in electrolyzed lignite compared with raw lignite. Besides, 71.5% ash and 39.6% mercury were removed from raw lignite after electrolysis, which is the first report. The Faradaic efficiency (FE) for hydrogen evolution reached 99% using MCEC. In addition, the analysis of spectroscopy methods (SEM, EDX, FTIR, and XRD) revealed that the lignite particles were cracked; the oxygen-containing functional groups increased; the minerals of quartz and hematite formed on the surface of lignite particles after electrolysis. The facts indicate that electrolysis of lignite is a competitive approach for lignite upgrade and hydrogen production.

Removal of Aqueous Nonylphenol Polyethoxylates by Raw Lignite Coal and Activated Carbon: Materials Characterization, Adsorption Studies, and Modelling the Adsorption Isotherms

Removal of Aqueous Nonylphenol Polyethoxylates by Raw Lignite Coal and Activated Carbon: Materials Characterization, Adsorption Studies, and Modelling the Adsorption Isotherms

Removal of Aqueous Nonylphenol Polyethoxylates by Raw Lignite Coal and Activated Carbon: Materials Characterization, Adsorption Studies, and Modelling the Adsorption Isotherms

Removal of Aqueous Nonylphenol Polyethoxylates by Raw Lignite Coal and Activated Carbon: Materials Characterization, Adsorption Studies, and Modelling the Adsorption Isotherms

Investigation of the liquefaction possibility of Ermenek lignite as an alternative clean energy source

Coal liquefaction; can be defined as the transformation of coal into products that have a high energy density, can be easily stored and transported and do not create environmental pollution, to meet both fuel and chemical raw material requirements.
 This study was carried out to determine the yields of the products formed as a result of the liquefaction of Konya-Ermenek lignite in the N2 atmosphere and under non-catalytic conditions. Proximate and ultimate analysis of the obtained solid and liquid products was made. The composition of the oil product was determined by GC-MS. As a result of liquefaction, the char yield was 71.37%, the preasphaltene yield was 12.24%, the asphaltene yield was 2.23% and the oil + gas yield was 14.16%. It was determined that the higher heating value of 4199 kcal/kg in raw lignite is twice as high in liquefaction products. It was determined that the sulfur content of 3.94% in raw lignite decreased to 1.54%, 0.85% and 0.44% in char, preasphaltene and oil, respectively.

Adsorption of Pb(II) cation from aqueous solutions by acid modified low-rank coal: An experimental study and simulation.

An acid modified approach to enhance the adsorption capacity of low-rank coal (lignite) is proposed to reduce the risk of heavy metal ions within the wastewater. Adsorption kinetics, adsorption thermodynamics, adsorption coefficient and density functional theory DFT calculations were studied in this paper, respectively. The results indicate that the adsorption capacity of lignite was enlarged after HNO3 modification, and pseudo-second order kinetics model and Langmuir isothermal adsorption model can be used to describe the adsorption process. The surface chemical properties of lignite play a dominant role rather than the specific surface area and total pore volume in the Pb(II) cation adsorption process, and it is suggested that the adsorption of Pb(II) cation by raw lignite (RL) and modified lignite (ML) is mainly completed by chemical adsorption. The Fourier transform infrared spectroscopy (FTIR) characterization showed that the surface oxygen functional groups of lignite increased after modification. The results of interaction energies between the model molecule and Pb(II) cation show that Pb(II) cation and -C-O-C are most easily combined, followed by -COOH, and -C = O is the weakest.

The effect of torrefaction, slow, and fast pyrolysis on the single particle combustion of agricultural biomass and lignite coal at high heating rates

The present paper studies single particle combustion of olive residue (OR), almond shell (AS), and Tunçbilek lignite (TL) and their thermal pre-treated particles in a vertical wire mesh reactor (WMR) setup. Pre-treatment of the fuels was done via torrefaction, slow, and fast pyrolysis. The particles with different masses were heated to 1000 °C by radiation with a heating rate of 900 °C/s. The combustion process was recorded with a high-speed camera. The time analysis demonstrated that the particle mass and size did not affect ignition delay time; however, slow pyrolysis had a significant effect on the ignition delay time. The ignition delay time was significantly increased for the chars produced from slow pyrolysis compared to raw and other heat-treated particles for all fuels. Burnout times of all fuels increased proportionally with particle mass. Fast and slow pyrolysis generally increased the char combustion and burnout times. The average burnout times for fast and slow pyrolyzed biochars were ∼20 s and ∼29 s, respectively, while those of raw biomass were ∼9 s. The average burnout times for fast and slow pyrolyzed lignite particles was ∼20 s and ∼53 s, respectively, while those of raw lignite was ∼19 s, when 2 mg particles were compared. Fast pyrolyzed biomass particles had similar burnout times compared to those of raw TL particles. This depicts the potential of co-firing TL with biomass chars produced from fast pyrolysis.

Molecular structural characteristics of late Jurassic Shengli lignite submacerals

Different lignite submacerals have complex physical and chemical properties, and exploring their molecular structures are of great significance for enrichment the evolution mechanism of coal microstructure and efficient utilization. In this paper, raw lignite with rich-(eu-ulminite) (FJ) and rich-leviglinite (FW), respectively, were selected on the basis of petrographic identification. The proximate analysis and ultimate analysis, FTIR, XPS, 13C NMR, HRTEM and molecular simulation technique were used to construct the molecular structure of FJ and FW. The experimental data and molecular structural models show that FW has a higher aromaticity, larger size of the aromatic layer, and more pronounced directional arrangement. In a single stable molecular configuration, aromatic rings and the aliphatic ring are arranged in a wrapped arrangement. Furthermore, the type of hydrogen bond mainly depends on the type of oxygen-containing functional group of the molecular structure. In addition, the length and angle of the hydrogen bond tend to be polarized. There is a little difference on the value of experimental and simulated density.