Low-grade Coal Research Articles

Single particle combustion studies of coal/biomass fuel mixtures

Single particle combustion behaviours of coal-biomass fuel mixtures are studied. Different biomass i.e. sawdust, wood and rice husk is blended with locally available low grade coal to carry out the experimental investigation. The combustion of solid fuel consists of flaming combustion and char combustion. Rice husk showed higher flaming time and char glowing time because of higher ash and particle density. The burning time of coal-rice husk mixture fuel sample is higher compared to other samples due to the combine effect of particle density and ash content of the fuel samples. Ignition mass flux of saw dust is higher compared to rice husk and wood fuel particles because of the presence of higher volatiles in sawdust. The diameter index for flaming combustion, char combustion are 1.13 to 0.47 and 0.63 to 1.23 respectively for 0%–40% coal in coal/wood fuel mixtures. It is observed that with the increases in surface area/volume ratio, flaming and char combustion decreases due to reduction in effective surface area of the particle for combustion. This study concludes that particle size or surface area/volume together with particle density are the primary controlling parameter for better conversion of fuel mixtures in any co-combustion system.

Changes in physicochemical properties of brown coal under hydrothermal modification

Hydrothermal drying of low-grade coals was first suggested in Austria as early as in 1920, when it envisaged treatment of a lump coal with 180–240 °C steam in an autoclave. Hydrothermal treatment (HTT) of brown high-ash coals at 200 to 250 °C causes certain changes in their structure and physicochemical properties, similar to those under low-temperature pyrolysis. Moreover, these changes occur at values 50–100 °C below the pyrolysis temperatures, and the HTT product is close to semi-coke in terms of its calorific value. During hydrothermal treatment, the surface of the coal phase becomes more hydrophobic, while the specific surface of the mineral fraction decreases and the swellability is lost, which leads to lower adsorption activity. Taken in combination, these effects significantly modify the processing properties of coal. When only brown coal-water separation is required, the HTT may be performed without adding free water; when changes the processing properties of coals are necessary, it is advisable to carry out hydrothermal treatment in suspensions containing approximately 50 % of free water. This treatment not only causes the removal of moisture, but also leads to certain other changes in the coal structure. The study was supported by a grant from the Russian Science Foundation (project No. 18-17-00169).

Exploring the spatial spillover effects of low-grade coal consumption and influencing factors in China

The residential consumption of low-grade coal (LGC) is a serious threat to the environment and human health in China. In this study, spatial patterns and spatial spillover effects of LGC consumption in China were investigated. The LGC consumption data, economic, household expenditure, income, and energy consumption data at the provincial-level in mainland China were collected. Global/local spatial autocorrelation and spatial regression models were employed. The results presented a significant positive spatial autocorrelation and an obvious spatial clustering feature of LGC consumption in China. Furthermore, the spatial spillover effect of LGC consumption wan detected based on spatial lag modeling. Our result further revealed that LGC consumption is significantly related to energy consumption factors rather than economic and income factors, which is contrary to conventional assumptions about the energy transition but fits well with the fuel stacking model. Additionally, this study provides several implications for residential energy improvement in China, which calls for further policies, e.g., building a specific and strict management system to prohibit of LGC for residential consumption and improving the energy consumption structure, especially in the provinces with a high value of LGC consumption in the middle, southwestern, and northeastern parts of China.

Conceptual kinetic study for the catalytic effect of limestone on mill scale reduction by low grade coal

The reduction output was increased by addition of catalyst, even on utilizing lean grade resources (mill scale and low grade coal). The effect and synthesis for addition of limestone as catalyst was determined. The kinetics of overall reduction process was evaluated with proper explanation. The reduction values of samples containing limestone found to be higher at all times and temperatures. With an increase in the percentage of limestone from 0 to 14%, the degree of reduction increases, reaches to a highest value (0.934, in 10% limestone sample) and then decreases instantaneously. The kinetic data of mill scale reduction for both - with and without the addition of catalyst was found to be chemically controlled, fits on the first order reaction model, with activation energy 29.9 and 51.6 KJ/mol in the reduction process. The SEM micrographs shows the formation of fine grains and porous structure due to the effect of reduction.

General Characterization of Low-rank and Low-grade Coal of East-Kalimantan, Indonesia

General Characterization of Low-rank and Low-grade Coal of East-Kalimantan, Indonesia

Kinetics of Gasification and Co-gasification of Petcoke and Coal

Co-gasification of petroleum coke with other carbonaceous material is making significant inroads in the quest for efficient utilization of petroleum coke. In the present work, gasification and co-gasification of petroleum coke and low-grade Indian coal was studied by thermogravimetric analysis at three heating rates, 10, 20, 30 °C min−1 under oxygen atmosphere. Physicochemical characterization of petroleum coke revealed that it contains significantly high fixed carbon and has high calorific value with low ash content vis-a-vis coal sample. It was observed that with the increase in heating rate, the ignition and the burn out temperature also increases. Kinetic parameters were calculated by employing commonly used two model-free isoconversional methods, i.e., Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) based on TGA data. The activation energies of petcoke and blend (petcoke + coal) calculated by KAS and FWO methods were found to be 126.75 and 109.94 kJ mol−1, and 128.3 and 111.85 kJ mol−1, respectively. Activation energy values obtained by both the methods demonstrate that the blend requires lesser energy than gasification of petcoke alone. Gasification of blend was found to follow complex multi-step nature of kinetics. Overall, the findings demonstrate that co-gasification could be a practical approach for energy recovery from the petcoke–coal mixture than gasification of petcoke alone.

Preliminary Study of Calorific Value Increase on Lignite Coal Using Dialkyl Carbohydrate Biosurfactant

Indonesian coal consumption is dominated by a low-grade coal quality, and supplied from local coals mining which was composed of lignite. A technological breakthrough has to be main objective to convert low grade coal into upper-class coal. This paper reports on improving the quality of lignite coal by washing with biosurfactant. Di-alkyl carbohydrates were tested on coal sizes of 80, 64, 16 and 6 mesh. The optimum biosurfactant applied to increase the calorific value derived from brown algae, namely isopropyl stearate alginate (ISA) 0.5%. This was able to improve a calorific value of lignite coal up to 27722.74% or increased by 11%. The size of the coal also affects the calorific value of lignite coal. Four coal sizes applied in the study (80, 64, 16, and 6 mesh), indicates coal size 80 mesh has given the best calorific value improvement. Conversely, the smaller of the coal size, increase biosurfactant absorption. Thus, improve the washing capability of biosurfactants.

Synthesis and Characterization of Metallic Iron Reduced from Low-grade Coal in Rajasthan

Today’s industrial technology offers to optimize and reutilize the resources available in terms of scum that remains after hot rolling in steel rerolling mills in Rajasthan, India. Vast reserves of low-grade coal exist and tons of mill scale are produced everyday after hot rolling in steel rerolling mills in Rajasthan, India. The present research work aimed to optimize the various parameters of solid-state reduction of mill scale using low-grade coal from Rajasthan, India. This study investigates the effect of reduction temperature (850–950 °C in steps of 50 °C), reduction time (60 to 180 min, in step of 60 min), mill scale size (800–300 μm), and mill scale to coal ratio (1:1, 1:2 and 1:3) on achievable metallic iron percentage and its metallization. A higher percentage of iron content is obtained at a lower particle size of 300 μm. The structural analysis is carried out on 300 μm mill scales using XRD analysis, which shows the percentage of iron is approximately 63%, and metallization above 80% can be obtained under the reduction temperature at 900 °C for 180 min, for 1:3 ratio (mill scale to coal ratio). The chemical composition of the mill scale is obtained using X-ray fluorescence (XRF), and particle sizes as well as distribution of mill scale are determined by scanning electron microscopy (SEM). The elements present in the mill scale are also verified using EDS, which indicates the presence of a major portion of Fe, followed by Fe2O3, Si, and C, respectively.

Beneficiation potential of a low-grade coal from the Emalahleni (Witbank) coalfield

Beneficiation potential of a low-grade coal from the Emalahleni (Witbank) coalfield

Optimization of process parameter of tribo-electrostatic separator for high ash non-coking coal preparation using response surface methodology

ABSTRACT Tribo-electrostatic separation of low-grade non-coking coal with 39–41% ash was optimized using response surface methodology at 50 kg h−1 feed rate. The petrographic and SEM-EDAX analysis reveals that coal has difficult liberation characteristics. The optimum condition to minimize the ash content of the coal was 20% relative humidity, 120 s tribo charging time, and 24.75 kV DC voltage. It is possible to achieve 32.7% clean ash coal with 59% yield from a 41% feed ash coal. The combustible recovered at these conditions was 67.4%. The experiment results corroborated well with the estimated value of 9% ash reduction from the coal.

Synthesis of O-doped coal-based carbon electrode materials by ultrasound-assisted bimetallic activation for application in supercapacitors

Abundantly available low-grade coal is used to prepare activated carbon (AC) for supercapacitor (SCs) electrode materials. Aiming to transform low-grade coal into pore structure, microcrystalline, and surface chemistry multi-scale coordinated AC, a new ultrasound-assisted bimetallic activation strategy is proposed. The introduction of ultrasonic waves in the FeCl3/MgCl2 co-impregnation process synergistically controls the pore and microcrystalline structure of the precursor. Meanwhile, the surface modification is performed in a CO2 atmosphere. Comparative experiments and density functional theory (DFT) calculations show that the AC-20 possesses abundant micropores (0.92 cm3 g−1), appropriate mesopores (0.40 cm3 g−1), high specific surface area (SSA, 2329 cm2 g−1), low impurity content, and is highly O-doped (13.65 at.%), which are essential for enhancing carbon-based electrochemical processes. Meanwhile, the graphitization degree improves (ID/IG is down to 0.89), stability enhances (TGA curves shift right), and electrical resistivity decreases (Re is 0.15 Ω cm−1). Therefore, AC-20 has a specific capacitance (C) of 309 F g−1 at 0.5 A g−1. Furthermore, AC-20//AC-20 can reach an energy density (E) of 9.6 Wh kg−1 when the power density (P) is 250 W kg−1. Combined with low price, abundant reserves, a simple preparation method, and good practicability, the obtained AC-20 has great potential to expand production.

Enhanced reactivity of geopolymers produced from fluidized bed combustion bottom ash

In this study, waste bottom ash obtained from fluidized bed combustion of low-grade South African coal in a bubbling fluidized bed reactor was used to produce geopolymers. The geopolymers were cured using both a conventional oven and a household microwave. An alkaline solution of Na2SiO3/NaOH (1:0.5) was mixed on a 1:1 ratio with a mixture of bottom ash/kaolin (1:1) mixture. Thereafter the resulting mixture was mixed with sand on a 1:0.5 ratio. Characterization of the geopolymers carried out using the following techniques, scanning electron microscope (SEM-EDX) analysis, compressive strength test, Thermogravimetric analysis (TGA), X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis. A household microwave and a conventional oven were used to enhance the geopolymerisation process and strength of the geopolymers. The results showed that the microwave curing enhanced the reactivity and compressive strength of the geopolymers. The microwave and oven cured geopolymer had the highest Si/Al ratio of 4.42 and reached a reasonable high compressive strength test of 31 MPa in 7 days compared to geopolymers cured with a conventional oven only or with a microwave only. The microwave radiation followed by conventional oven curing reduced the heat curing time and energy but improved the reactivity and strength of the geopolymer.

Environmentally friendly thermal power plant based on the principle of a multi-product complex at vortex burning of the Berezovsky coal of the Kansko-Achinsk basin

According to the Energy Program of Russia until 2030, the formation of coal power is associated with large thermal power plants on low-grade coals and supercritical steam parameters. An increase in the proportion of low-quality coal leads to a sharp increase in toxic emissions in flue gases, liquid effluents and ash and slag waste. The article considers the scientific and technical bases and circuit solutions for the creation of environmentally friendly thermal power plants (EFTPP) on the Berezovsky coal of the Kansko-Achinsk Basin (KAB) with vortex combustion technology based on the principle of a multi-complex with the production of not only the main products (electric and thermal energy), but additional waste with consumer properties. Multi-product complex (MPC) is a series of road construction plants and agricultural enterprises, associated with the TPP and efficiently using wastes for construction, metallurgy and various agricultural products. The performed calculations and analysis of technical and economic indicators have shown that with the existing equipment, prices and tariffs for the products, the EFTPP will pay off more than two times during the life cycle. After 8 years from the beginning of construction the MPC starts generating profit.

Effective techniques for bio-coal briquettes production

The production of bio-coal briquettes gives an opportunity to utilize low grade coal as well as the great amount of agricultural residues, to increase the calorific value of obtained briquettes and to improve their chemical and physical properties. The present study is focused on developing the technological scheme which gives an opportunity to reduce the energy consumption in

Immobilization of Atrazine Using Oxidized Lignite Amendments in Agricultural Soils

Leonardite (LND), a low-grade coal, is an inexpensive material used to improve soil fertility in humic-deficient soils. Our objective was to quantify the ability of LND to immobilize atrazine (ATZ). To accomplish this, LND morphological properties, elemental composition, and spectroscopic properties were determined. We found that LND had a high organic matter content (40.53%, w/w) with a mesopore particle structure. Fourier-transform infrared spectroscopy spectral analysis confirmed the presence of aromatic molecules, all of which contributed to ATZ adsorption. The ATZ adsorption characteristics were quantified using batch adsorption kinetic and isotherm experiments. The ATZ adsorption kinetics were best described by a pseudo-second-order kinetic model while the concentrations of ATZ adsorbed (Cads) versus equilibrium concentrations (Ceq) had the best fit with the Freundlich equation compared with five other isotherms tested. The ability of LND to minimize ATZ leaching was tested using one-dimensional transport experiments with packing configurations and materials. The results revealed that up to 95% of ATZ was immobilized in the LND-amended soil. The extractable ATZ residues from both soil and LND corroborated the reduction in ATZ leaching. Overall, the adsorption results indicated that LND had a high affinity for ATZ adsorption and could reduce ATZ transport to the groundwater.

Characterization and optimization of calorific value of low grade coal by statistical experiment and modelling

The effects of environmental factors (temperature, humidity, and airflow) on gross calorific value (GCV) and proximate analysis of low grade coal were systematically investigated. The factorial experiments were conducted according to the statistical experimental design. The results were empirically modeled, statistically tested, and experimentally verified to quantify the change in GCV and proximate analysis of coal directed by environmental factors. The GCV of the coal were most affected by the humidity followed by the temperature. The GCV was increased from 5,365 to 5,986 (kcal/kg) through the decrease in humidity from 80 to 29 (% R.H.) and increase in temperature from 28 to 36 (°C). This increase in GCV could be attributed to the decrease in moisture content of the coal from 16.2 to 7.1 (%). Also, the optimized environmental conditions were identified where GCV ≈ 6,000 kcal/kg of the coal could be obtained. This study can be helpful in (i) managing the variability in GCV of a coal at different places, environment, and weather conditions and, thereby, settling the disputes between buyers and sellers over its variability and (ii) in maintaining the optimum conditions to enhance the GCV of the low grade coal in the power plants.

Thermal performance analysis of low-grade coal pretreated by ionic liquids possessing imidazolium, ammonium and phosphonium cations

This paper aims to investigate the impact of ionic liquid (ILs) pretreatment on low-grade coal for the thermochemical conversion process. Two imidazolium-based ILs, 1-butyl-3-methylimidazolium chloride [Bmim][Cl], and 1-ethyl-3-methylimidazolium chloride [Emim][Cl]; one ammonium-based IL, tributylmethylammonium chloride [N1444] [Cl]; and one phosphonium-based IL trihexyltetradecylphosphonium chloride [P66614] [Cl], were selected for this study. Reactivity, kinetic, and thermodynamic parameters were calculated via thermogravimetric analysis (TGA) conducted under N2 environment at a ramp rate of 20 °C/min. All four ILs increased the higher heating value (HHV) of coal. The IL treatment resulted in greater mass loss and enhanced reactivity of coal. The highest reactivity of 11.45%min−1°C−1 and pyrolysis factor (PF) of 23.28, nearly four and six times, respectively, higher than that of raw coal, were recorded for the [P66614] [Cl] treated sample. This could be attributed to the complete absorption of [P66614] [Cl] in coal and its high hydrogen bond basicity (HBB). Activation energies and pre-exponential factors were calculated using the model-fitting analysis with regression coefficient R2 in the range of 0.979–0.999. All ILs, except [P66614] [Cl], reduced the activation energy of coal, indicating degradation of coal structure. The higher activation energy in the case of [P66614] [Cl] suggests increase in thermal stability of coal. The results indicate that relatively low-cost phosphonium and ammonium-based ILs can be potential pretreatment agents for coal thermochemical conversion processes.

Formation of carbon quantum dots and graphene nanosheets from different abundant carbonaceous materials

Carbon quantum dots (CQDs) and graphene nanosheets (GNs) are one of the most attractive fluorescent carbon nano-sized materials with unique features of optical and physico-chemical properties having various applications in chemical sensing, biomedicine, semiconductor devices, photo-catalyzing, and electrocatalysis. The primary purpose of this preliminary work is the feasibility study for synthesis of nano-structured value-added carbon products i.e., CQDs and GNs from cheap and abundant carbon sources, such as coal, petroleum coke, graphite, and coal-based humic acid by hydrothermal techniques. The physico-chemical properties of the raw precursors were evaluated by proximate and ultimate analysis. High resolution-transmission electron microscopy (HR-TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transforms infrared (FT-IR) spectroscopy, X-ray powder diffractometer (XRD), UV–visible (UV–vis), and fluorescence (FL) spectroscopy were used to achieve physico-chemical parameters of carbon nanostructures, to better comprehend the development of carbon quantum dots and graphene nanosheets. HR-TEM analysis indicates that the dimensions of the synthesized CQDs from different carbon sources exist in the range of 3–6.5 nm. The approximate quantum yields (QY) of the synthesized carbon nanomaterials were found to be about 2–22%. The synthesized carbon based nanomaterials (i.e. CQDs and GNs) have been efficiently used in the photo-degradation of harmful 2-nitrophenols (2-NP) with the efficiency of 80.79% for CQDs and 82.53% for GNs, respectively. The present study provides a simple and efficient technique for large-scale fabrication of typical CQDs and GNs from abundantly available low-grade coal and other carbonaceous materials for photo-catalysis application.

Production of high-strength and low-gasification reactivity coke from low-grade carbonaceous materials by vapor deposition of tar

Carbon/carbon composites were prepared from pyrolyzed-chars of lignite or low-strength coke (drum index of DI1506 = 52) and a coke-oven-gas tar through a gaseous-tar vapor deposition method to investigate the possibility of producing high-strength coke from low-grade coal or coke. The optimum vapor deposition temperature was examined under tar pyrolysis temperature of 700 °C and holding time of 30 min. The highest strength of prepared composite was observed at a vapor deposition temperature of 300 °C. The pore volume of the composite prepared at the vapor deposition temperature of 300 °C was the smallest among other samples prepared at that of 400–700 °C. The strength was improved by filling the carbonaceous materials derived from tar into the pores. The tar utilization coefficient was extremely low at 1.0 wt% or less. When same treatment was carried out for coke with different DI and pyrolyzed-cahr of lignite under the optimum conditions, it became clear that the composite prepared from any coke or pyrolyzed-char of lignite is comparable in strength to high-strength coke (DI1506 = 87). It was thus found that the present method can prepare high-strength coke from low-grade carbonaceous materials. The gasification rate of the composite prepared from low-strength coke was 0.5 times smaller than that of the original sample.

Study on functional group, oil adsorption process, and the effect of chemical wash on lignite and sub-bituminous coal

This study was to determine the characteristics of Indonesia’s low-grade coal, which was sub-bituminous taken from Samarinda coal mine and lignite taken from Palembang coal mine. Experiment was conducted using Fourier Transform Infrared (FTIR) spectroscopy to analyze the functional group of the coal. Inherent moisture was 2. 16 g in 5 g lignite coal, and 1. 42 g in 5 g sub-bituminous coal. Soaking coal with liquid smoke reduced the percentage of residual (volatile matter and ash content) combustion and increased fixed carbon. Adsorption of oil by coal was measured by the change of free fatty acid (FFA) in the oil, in which lignite coal was decreased about 0.1 % and sub-bituminous coal was decreased about 0.07 %. In contrast, by heating adsorption 100 °C, FFA change in the oil was increased about 0.28 % for lignite coal and increased about 0.46 % for sub-bituminous coal, by heating adsorption 125 °C, the increase was about 0.44 %for lignite coal and was about 0.59 % for sub-bituminous coal, and by heating adsorption 150 °C, FFA was increased about 0.41 % for lignite coal and about 0.46 % for sub-bituminous coal.