Ultra Clean Coal Research Articles

Experimental study on metal removal characteristics of Zhundong coal to produce ultra-clean coal using acid-alkali method

The large-scale application of coal-based deash carbon materials has put forward growing requirements for the removal technology of ash within coal. Extensive research has demonstrated that the contents of ash and metal elements have a large influence on the performance of the coal-based electrodes made from ultra-clean coal. However, there is a lack of research on the ash and mental elemental removal characteristics of Zhundong coal. In the present study, one kind of Zhundong coal characterized by a comparatively low ash content, was utilized to examine the characteristics of ash and metals removal during the acid-alkali ash removal process. An inductively coupled plasma optical emission spectrometer (ICP-OES) was utilized to analyze the metal element concentrations in the solution during ash removal and the metal element contents within the coal samples. The removal characteristics of metal elements were further analyzed based on the detection results. The experimental results indicate significant improvements in the removal of ash and metals through acid-alkali multi-stage leaching. Under the specific experimental condition here, a two-stage acid-alkali ash removal process achieves coal with the lowest ash content of 1.6% and the removal of 98.0% of Fe, 95.2% of K, and 94.6% of Na, with the removal of Al reaching up to 40%. In addition, the Fe removed in the acid leaching step accounted for 95% of the total amount removed, and adding an acid pretreatment step based on acid-alkali leaching can remove 90.2% of Fe within coal. Both the acid leaching and alkali leaching steps exhibit a certain removal proportion for Al, K, and Na elements, which are closely related to the existing form of these elements within the coal. Iron-containing minerals are mostly acid soluble and distributed on the surface of coal particles. Potassium-containing minerals are mainly alkaline soluble, resulting in a removal rate of over 60% in the alkali leaching step of acid-alkali leaching combined with acid pretreatment. The low removal rate of Al (∼40%) is due to the wide variety of Al-containing minerals, such as kaolin, illite, and aluminates, which are less removed during the reaction with alkali due to temperature limitations. The present study can offer improved knowledge on mineral removal to obtain ultra-clean coal for utilization as function materials.

Preparation of refined coal oil-water slurry by blending coal tar with ultra-clean coal filter cake

Preparation of refined coal oil-water slurry by blending coal tar with ultra-clean coal filter cake

Study on preparation of ultra-clean coal from bituminous coal using the ionic liquid extraction

ABSTRACT In order to realize clean and efficient utilization of low rank coal, ultra-clean coal preparation is an important method. A clean and efficient method of producing ultra-clean coal is urgently needed, given the high environmental impact and corrosiveness of the chemicals used in the traditional chemical preparation of ultra-clean coal. In this study, ultra-clean coal was prepared by leaching using acidic imidazole ionic liquids, and the effects of temperature, time and concentration on the reaction process were investigated. The ash composition of the sample and the ash removal mechanism under the ionic liquid were revealed, and the evolution law of the microstructure and functional group characteristics was clarified. The research showed that the concentration and temperature had a large influence on the ash content of the product during the leaching process, and the reaction time had a small influence. The minimum ash content of the ultra-clean coal could reach 0.89% (dry basis). In the ionic liquid system, iron, calcium and magnesium in the coal were effectively removed, the iron oxide content was significantly reduced and calcium salts except calcium sulfate were effectively removed, while periclase was almost completely removed. However, the removal of silicon and aluminum was poor. Most of the alumina and silica remained in the ultra-clean coal. Extracted coal had more cracks and rougher surface than raw coal, the BET surface area decreased from 2.8579 to 2.4032. Under the influence of ionic liquid, the Aar/Aal ratio, which evaluates the aromaticity of coal, increased from 0.3702 to 0.8569, indicating that the aromaticity of coal was significantly improved. The CH2/CH3 ratio describing the length and branching degree of aliphatic chains in coal decreased from 4.0849 to 3.1516 which showed that ionic liquids can effectively reduce the length of aliphatic side chains, weakening the coal’s oxidizing activity. The pyrolysis curve of coal shifted to the high temperature zone and the peak temperature increased, indicating the effect of ionic liquids on the chemical stability of coal.

Production of Ultra-Clean Coal by the Combined Method of Grinding and Collector Gasification Flotation

The preparation and application of ultra-clean coal is one of the important aspects of clean energy technology. However, the preparation of ultra-clean coal is mainly chemical methods, which are low in efficiency, high in energy consumption and expensive. It is urgent to find an effective method to prepare ultra-clean coal. In this paper, the combined method of grinding and the collector gasification flotation method was used to obtain ultra-clean coal. The effects of grinding time on the particle size composition, mineral dissociation, surface properties and flotation results of coal samples were studied. The grinding test results show that with the increase in grinding time, the particle size and the pore diameter of coal samples decreased gradually, while the specific surface area and pore volume of coal samples gradually increased. When the grinding time was 20 min, the D90 and D[4,3] of grinding products were 5.20 um and 4.23 um, respectively. The ash content of −1.3 g/cm3 was less than 1% when the grinding time was 20 min. Compared with the traditional flotation method, the collector gasification flotation method can obtain a higher concentrate yield and lower concentrate ash content. When the amount of collector was 2.0 kg/t, the yield of clean coal obtained by the collector gasification flotation method was 4.1% higher than that by the traditional flotation method, while the ash content of clean coal was 0.3% lower.

Influence of grinding methods on the preparation of ultra-clean coal from slime

Influence of grinding methods on the preparation of ultra-clean coal from slime

Effect of Diesel Oil and Mixture of Alcohol-Glycol Ether on Colombian Ultrafine Coal Cleaning Using a Test-Rig Closed-Loop Flotation Column

A test-rig closed-loop flotation column was used to observe the effect of diesel oil (collector) and Flomin F-425 (frother) on mass yield and ash content for two Colombian coals: Caypa (northern zone) and Guachinte (southwestern zone). The coal samples of less than 38 µm (-400 M) were processed in a collector concentration range of 0,32 to 1,60 kg/ton of coal, as well as a frother concentration range of 10 to 50 ppm. The response surface methodology was used for the experimental test runs. The results showed that the maximum mass yield obtained by Caypa coal was 98,39% at 1,28 kg of collector/ton of coal and 40 ppm of frother concentration, whereas Guachinte coal obtained a maximum mass yield of 94,71% at 0,96 kg of collector/ton of coal and 30 ppm of frother concentration. In general, for Caypa coal, the mass yield tends to increase (low ash removal) with the collector and frother concentration increase; while the mass yield tends to decrease (high ash removal) for Guachinte coal when the collector concentration increases (low ash removal) at high frother concentrations. It is worth highlighting that the ash content of 0,65% obtained for Caypa coal is the lowest value reported in the literature while employing a test-rig loop flotation column in a single stage, which is considered to be an ultra-clean coal obtained by a physical cleaning process.

Improving the yield of ultra clean coal by adding acetic acid and propionic acid in selective agglomeration

ABSTRACTIn this study, ash content lower than 1%, the ultraclean coal, using anthracite coal, is prepared by selective agglomeration flotation. ultraclean coal with lower ash is needed as electrode material and solid fuel, aim of obtaining lower ash content of ultraclean coal, dispersant is usually used to reduce ash content, but the yield will decrease. Therefore, clean coal yield can be further increased by regulator. For this purpose, the effects of stirring speed; dispersant (sodium hexametaphosphate) dosage, kerosene dosage, organic regulator (butyric acid; propionic acid; acetic acid; fumaric acid; maleic acid; ethylenediaminetetraacetic acid) on yield and ash content of clean coal were investigated in the experiments. The analyses of zeta potential, contact angle, x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), x-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and Fourier transform infrared spectroscopy (FTIR) were used to illustrate the performance of the process. The results showed that the sodium hexametaphosphate and stirring speed had a significant effect on the reduction of clean coal ash content. Furthermore, adding acetic acid and propionic acid can improve the clean coal yield without increasing the clean coal ash content. Acetic acid was used as regulator, the yield of clean coal increased by 19.42%. At the same time, acetic acid and propionic acid are proposed as a method to produce microbubbles. This important discovery provides an optional method to obtain more target products for fine mineral flotation.

The Effect of Preparation Conditions on Microbubble Flotation Process of Ultra Clean Coal Using Box Behnken Design Model

AbstractWhen coal is widely used as fuel, it also produces lower combustion efficiency and environmental pollution due to direct utilization of coal. The demineralization of coal to prepare ultra clean coal can improve utilization value and reduce pollutant emissions. Thus, Taixi anthracite is chosen to prepare ultra clean coal by microbubble flotation method. Box Behnken design (BBD), a response surface model is introduced as a factorial design of experiments setup and used to find those parameters, which affect the ash content and yield of ultra clean coal and to adjust these parameters in order to achieve the optimized effect. Especially, their interaction effect of different factors in flotation process could be explored by the mathematical model. To explore the mechanism of preparing ultra clean coal, X‐ray fluorescence (XRF), X‐ray diffraction (XRD) and scanning electron microscopy energy dispersive spectroscopy (SEM‐EDS) results are analysed. Combining XRF with SEM‐EDS analysis, the concentrate is polluted by composite particles with particle size of less than 2 μm and organic matter accounting for more than 95 %. Through XRD and SEM‐EDS analysis, it can be shown that a small amount of fine clay minerals including kaolinite and illite agglomerate heterogeneously with coal particles during flotation and pollute concentrate. Further, the existence of composite particles and slime coatings were the main reason of limiting deep demineralization in microbubble flotation process.

Preparation of low silicon ultra clean coal by flotation pretreatment followed by alkali-acid leaching combined process

ABSTRACT The alkali-acid combination leaching has the characteristics of reacting with most minerals from various grades coal and high safety but the removal of silicic minerals as quartz and aluminosilicate is very limited. To further remove quartz and aluminosilicate from coal, we optimized the samples size, alkali leaching concentration, temperature and contract times of leaching process, and designed a new demineralization process of flotation pretreatment followed by alkali-acid combination leaching. On leaching process the percentage demineralization was 86.81, but appending the flotation column pretreatment the percentage demineralization reached 94.51. To investigate the removal mechanism of quartz and aluminosilicate in coal, the mixture coal samples of quartz–coal and aluminosilicate–coal was carried out by combination leaching process. Leach samples characterization of raw coal, flotation pretreatment coal, and mixture coal were done by XRD, ICP-OES, and SEM-EDS analysis. The result of the analysis showed that production of quartz and aluminosilicate in alkali leaching tend to adsorb on the surface of coal, then these products form new impurities in acid leaching. Flotation removes part of the dissociated silicic minerals in advance, which reduces the adhesion of soluble silica-containing sodium salt produced by alkali leaching to the coal surface and thus leads to the reduction of ash content.

Alkali Leaching of Indian Coal for Ash Reduction

Coal is a major source of energy. The available Indian coal quality is very poor having very high ash content and low calorific value. Indian coal generates a large quantity of ash as a by-product of combustion. The objectives of the present study were to reduce the ash content of coal to produce ultra-clean coal. The coal was treated with a dual chemical leaching process consisting of NaOH followed by HCl.During experiments, the concentration of NaOH was varied from 2.5 to 10 M with 1.4 N HCl. Effect of shaking speed and time was analysed at 0, 50, 100, 150 and 200 rpm and 1, 2, 3 and 4 hours respectively. After chemical leaching, the ash content was reduced from 35.33 to 0.98% in the ultra-clean coal. The concentration of alkali, shaking speed and time of duration were found as highly influencing parameters for the reduction of ash content in the coal. Moreover, the current study should result in a better option for the removal of ash content from low-rank Indian coals.

Chemical treatment of low-grade coal using Taguchi approach

Power generation in India is primarily coal-based in the present scenario. About 70% electricity generation produced by combustion of pulverized coal in thermal power plants. The available Indian coal quality is very poor; generate the large quantities of ash produced as by-products of combustion. The objective of the present study was to reduce the ash content and to produce ultra-clean coal. The coal sample was treated with chemical treatment of NaOH followed by HCl. The effect of particle size, temperature, and concentration on ash content was studied and found to be significant. Experimental influence parameters were optimized using Taguchi method. The result indicates that the temperature contributes the most dominant factor followed by acid concentration and particle size. The maximum reduction in ash content was observed at, particle size range of 53–75 µm, temperature 70 °C, and concentration of NaOH 40% (by volume), respectively. After chemical leaching, the ash content was reduced from 35.33% to 0.65% so as to produce ultra-clean coal.

Separative Refining of Coals through Solvolytic Extraction under Milder Conditions: A Review

Not only cleaning but refining of coal has become an area of utmost importance both industrially and academically because of stringent environmental norms being imposed globally for arresting the climate change. However, coal is a polluting fuel, therefore, there is a grave need of deep refining of coal to obtain ashless coal for increasing its efficiency of utilization in order to minimize its impact on environment. The presence of ash in coal also greatly reduces the coal utility because of its deleterious effect on the boiler parts and gasification equipment. Solvent extraction (organo-refining) techniques produce Super Clean Coal (SCC) or Ultra Clean Coal (UCC) with almost zero ash content. The utilization of the SCC in the Integrated Gasification Combined Cycle (IGCC) Power Generation would increase the availability of this otherwise efficient and cleaner coal technology. Research work on different processes of solvent extraction of coal under varying conditions of temperature, pressure, and the natu...

Optimization of leaching conditions for removing sodium from sodium-rich coals by orthogonal experiments

The leaching pretreatment has been adopted to mitigate the ash-related problems encountered during gasification and combustion of sodium-rich coals. However, the wide-spread applications of this technology are limited to produce the ultra clean coal through remove all the ash by some aggressive agents. In the present study, some mild agents, such as deionized water, ammonium acetate solution and diluted hydrochloric acid, were employed to selectively remove troublesome sodium rather than all the ash. The primary leaching conditions such as particle size, temperature, residence time and concentration of agents were optimized by orthogonal experiments for the first time to achieve maximum removal efficiency of sodium (RES). Based on the range analysis, it was found that the significant factor was leaching temperature in the water-leaching (W-leaching) treatment and the RES showed a positive correlation with temperature. However, in both ammonium acetate-leaching (A-leaching) and diluted hydrochloric acid-leaching (H-leaching) treatments, the concentration of agents were found to play the dominant role and RES presented a volcanic relationship with it. Under optimal conditions, the leaching treatments were less time-consuming with improved RES (47.6% for W-leaching, 97.9% for A-leaching and 96.8% for H-leaching). Besides, the properties of coal samples after leaching were determined by a series of analysis methods. The results showed that the leaching treatments could significantly decrease the content of AAEMs and coal ash. Due to the removal of AAEMs, the ash fusion temperatures increased, while both gasification and combustion reactivity decreased obviously. The application to other three sodium-rich coals confirmed that the optimal leaching conditions had a good universality.

Air impact pulverization–precise classification process to support ultraclean coal production

This research demonstrates a novel technique for achieving ultraclean coal production through the coupling of pulverization, classification, and separation of Taixi anthracite. Specifically, ultrafine pulverization techniques used in noncoal fields were newly applied to coal in this study. Taixi anthracite served as the subject material, and an air impact pulverization–precise classification process was designed and demonstrated. The effects of primary variables on process outcomes were investigated, and then, the process was optimized. In order of effect, the primary variables included pulverizing pressure, secondary wind pressure, classifier II rotational speed, and classifier I rotational speed. The classified products with ash content, particle size and classification precision cascade change can be obtained, ultimately, realizing the results of quality-upgrading of coal products. Under optimal process conditions of a 0.5-MPa pulverizing pressure, 0.10-MPa secondary wind pressure, 210-Hz classifier II rotational speed, and 230-Hz classifier I rotational speed, ultraclean coal with an ash content of 1.40% was produced. Ash content gradients for different product classifications were apparent, suggesting that the separation effect can be determined through precise classification. Numerical simulation results indicated that airflow kinetic energy transformation was the primary energy source facilitating pulverization. During impact pulverization, Taixi anthracite experienced selective, differentiated, and surface pulverization effects.

Influence of Chemical Leaching on Rheology and Ash Reduction of Indian Coal

ABSTRACTThe quality and calorific value of Indian coal are very low. The objective of the present study was to reduce the ash content and to produce ultraclean coal (UCC). The low-ash-content coal was then used to prepare a coal–oil slurry. The coal was demineralized with a dual-acid leaching process using hydrofluoric acid (HF) followed by nitric acid (HNO3). The leaching concentration was varied from 10–40% (by volume). After chemical leaching, the ash content was reduced from 38.5 to 0.9% (by weight) in the ultraclean coal. The coal–oil slurry was also prepared from treated coal with different concentrations of chemicals. The rheology of the untreated coal–oil slurry and ultraclean coal–oil slurry was investigated using a RheolabQC rheometer. The solid concentration of the coal–oil slurry was taken in the range of 20–40% (by weight). The coal–oil slurry exhibited shear thinning behavior. The SEM micrograph results revealed that some non-porous materials that were present in the untreated coal were removed after chemical leaching. The rheology data also showed that the apparent viscosity of the coal–oil slurry was greatly influenced by ash content.

The production of ultra-clean coal from Zonguldak bituminous coal by chemical leaching

ABSTRACTFrom run-of-mine coal with an average ash of 50% being produced at Zonguldak coal basin, clean coal with 7% ash can be produced through physical enrichment methods such as jig and heavy media. However, for the production of coal with an ash content of lower than 7%, it is required for the coal to be first ground and then subjected to chemical leaching operation. In this study, chemical leaching tests were performed with aqueous HF, HCL, HNO3, and citric acid (CA) in order to produce ultra-clean coal (UCC) from Zonguldak bituminous coal. The effects of type of chemical, grain size, temperature, and leaching time in the leaching process were studied. It was observed that UCC with 0.82% ash could be produced from sample coal with 8.84% ash as the result of leaching operation with aqueous HF.

Performance evaluation of a dense-medium cyclone using alternative silica-based media

The production of superior metallurgical-grade silicon requires the use of ultra-clean coal containing <1.5% ash with no iron contamination. The magnetite suspension utilized for coal cleaning in a dense-medium cyclone (DMC) is a detrimental impurity in the final coal product that dramatically diminishes the purity of silicon. A prior study has demonstrated that several silica-based materials may be suitable alternatives that eliminate iron contamination. To further improve the separation performance and evaluate the silica-based media, DMC separation tests were conducted based on a statistically designed program to identify the role of various operational parameters (i.e. medium relative density, silica media blend ratio, and cyclone apex diameter) in maximizing organic efficiency and minimizing low-density bypass. An ideal medium composition with a bimodal particle size distribution considerably improves medium stability properties while also eliminating the undesirable effects of apex diameter on the DMC separation efficiency. Low-density cutpoints (<1.35) are hindered by medium instability, but these issues can be rectified by increasing the amount of fine media particles or decreasing the apex diameter.

Assessing the feasibility of silica-based media for coal preparation operations: A novel method of carbon feedstock production for the silicon market

Production of silicon for solar cells and semiconductors requires the use of ultra-clean coal as the main reducing agent in the silicon production process. Most coal preparation plants employ magnetite suspensions as the dense medium in coal cleaning processes. Although downstream operations are designed to remove and recover magnetite, the unavoidable presence of residual magnetite in the final coal product dramatically decreases the purity and suitability of the coal for the silicon market. Given that silicates are the main raw materials in the silicon production, a systematic study through laboratory dense-medium cyclone tests was used to evaluate fine silica-based alternative materials as a substitute for magnetite in coal cleaning operations. These alternative silica materials include byproducts generated during the silicon production process. The results indicate that laboratory tests with silica-based materials were able to achieve similar separation performance when compared to plant operations using magnetite. Following the outcomes of the experimental study, a novel flowsheet arrangement was designed using silica-based media rather than magnetite. If properly implemented, this new separation system can eliminate iron contamination and provide coal that can be marketed as a high-quality silicon feedstock.

Ultra clean coal production for the solar-cell industry

Ultra clean coal production for the solar-cell industry

Comparison of Oxidation Behaviors Between Ash Free Coal and Carbon in a Coin Type Direct Carbon Fuel Cell

Ash free coal extracted from a bituminous coal and carbon made from Oak were served as fuels in a direct carbon fuel cell and their oxidation behaviors were compared in terms of temperature, coal to carbonate ratio, and gas compositions in the cell. The ash free coal had much higher OCV and voltage at polarization states than carbon fuel at 650oC. Dominant product gases from the coal were H2and CO, while carbon had carbon monoxide in the tested range. Therefore difference in the oxidation process would be existed between the two fuels.Coal is relatively abundant energy source compared with other fossil fuels. Regardless of its abundance and long history of usage, inconvenience of its use reduces coal consumption at present. Coal as a solid fuel has a big problem of remaining ash in its use. Ash free coal has been attempted to overcome the problem. So far, ash free coal can be produced by two ways; one is removing ash compounds with acids and alkalines in the coal, calling UCC(Ultra Clean Coal), and another one is extracting organic compounds from the coal with organic solvents, named by ash free coal.In this work, ash free coal was produced by a micro-wave method with a polar solvent of NMP. For the comparison of oxidation behavior, carbon made from Oak was employed in this work. The electrochemical oxidation behaviors of coal and carbon were investigated with respect to temperature, coal to carbonate ratio, and gas compositions in a coin type direct carbon fuel cell (DCFC).The raw coal used in this work was Berau bituminous supplied by KEPRI in Korea. Carbon was made in-house, for 2 hrs at 400oC under N2 environment. A coin type direct carbon fuel cell was fabricated by molten carbonate fuel cell technology. The diameter of electrodes was about 3 cm. The anode was porous Ni-Al alloy, and the cathode was porous Ni. The matrix was made of LiAlO2. The long alumina tube at the upside of the cell was installed for the coal supply to the anode. A mixture of 62 mol% Li2CO3 and 38 mol% K2CO3 was served as electrolyte. The cathode gas was 70 % air and 30 % CO2. More details of the DCFC operation was described in a previous work [1]. The normal H2 fuel for the anode was H2:CO2 = 0.125 L/min: 0.025 L/min with ca. 5 % of H2O. Temperature ranged from 923K to 1123K. Coal to carbonate ratios were 3g to 3g, 3g to 1g, and 1g to 3g. Gas compositions in the cell were analyzed with a gas chromatography (HP 5890II).Figures 1 and 2 show OCV behaviors of fuel change from the normal H2 fuel to carbon fuel and ash free coal, respectively at 650oC, 750oC, and 850oC. The carbon fuel was a mixture of 3 g of carbon and 3 g of Li-K carbonates, while the ash free coal was a mixture of 3 g coal and 3 g of Li-K carbonates. Before 0 s the OCV was determined by the H2 fuel. After 0 s the carbon fuel shows only one minimum in the OCV whereas the coal does two minimums. The difference was attributed to the different gas species that carbon produced mostly CO species, while the coal gave H2 and CO as dominant gas species. In particular, very low OCV is observed at carbon fuel of 650oC, which indicates that very small reaction species in the condition. Very low gasification of carbon is the reason at the condition. On the other hand, coal fuel shows much higher OCV at 650oC. Rather easy gasification of ash free coal can be the reason. The main gas species of coal were CO and H2 because the hydrocarbon species in the coal is readily gasified even at 650oC. Similar amounts of CO and H2 were measured at the coal fuel in the anode chamber. The H2gas is generally more active than CO gas in the oxidation. Then the difference in the oxidation process between coal and carbon can be existed. ACKNOWLEDGEMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0009748).