2015-03-27 ASH-FREE COAL PRODUCTION METHOD USING PRECIPITATION BY SOLUBILITY CHANGE 5 TECHNICAL FIELD One or more embodiments of the present disclosure relate to a method of producing ash-free coal, and more particularly, to a method of producing ash-free coal by using solubility change. 10 BACKGROUND ART With recent explosive rise in crude oil cost, there have been increasing efforts to secure alternative energy sources to crude oil, and vibrant research into novel alternative energy sources to conventional fossil fuel such as solar energy and bio 15 energy. However, such novel alternative energy sources still have limitations in industrial use. For this reason, recently the demand for coal is increasing again, and consequently South Korea imports coal, due to the closure of most mines, from overseas such as Australia and Indonesia. Coal includes ash as an inorganic component that is incombustible even at high 20 temperatures. Accordingly, when coal is used as fuel, ash in the coal may be fused onto a inner wall or flow path during combustion or gasfication to block the flow of materials, thus leading to reduction in heat transfer efficiency or heat efficiency. Meanwhile, ash contained in the coal, not fused, may cause environmental contamination, which needs for an additional ash collection process and additional 25 expenses therefore. Accordingly, it has been actively tried to remove ash from coal in the fuel and various industrial fields for efficient use of coal. Methods of removing ash in coal may include a washing method of selectively dissolving and removing only ash, with organic components in the fuel remaining, by using acid and alkali solutions, and an extraction 30 solvent extraction method of dissolving and extracting only organic components, with ash in the fuel remaining, by using an extraction solvent. 1 2015-03-27 The amount of ash in the final product may be about 0.1% when the washing method is used, and may be about 0.02% when the solution solvent extraction method is used, indicating that a lower amount of ash in the final product may be remain with the extraction solvent extraction method than the washing method. 5 US 2010/0006477 (PCT/JP2007/069833) discloses a method of producing ash-free coal, and in particular, a method of producing ash-free coal by solvent extraction. In this method, ash-free coal in solid state is separated by evaporating a solvent from a extracted solution that is obtained by a solvent extraction process at high temperature. However, when the temperature of the extracted solution is lower than a io boiling point of the solvent, evaporation of the solvent may not be smooth, and the entire extraction solution has to be heated to the boiling point of the solvent. In addition, to prevent the temperature of the extraction solution from being lowered to the boiling point or less of the solvent, external heat needs to be additionally supplied as much as evaporative latent heat of the solvent. Furthermore, the energy consumption 15 may be so high in consideration of heat loss in practical processes. In addition, oxidation or thermal decomposition of some organic components (coal) in the heating at high temperature for evaporating the solvent may occur, thus causing loss in the final product. It is also practically impossible to entirely recover the solvent evaporated during a 20 solvent recovery process through condensation and is thus necessary to supplement expensive solvent, which increases cost. Therefore, there is a need for the development of a simple, effective ash-free coal production method. 25 DETAILED DESCRIPTION OF THE INVENTION TECHNICAL PROBLEM One or more embodiments of the present disclosure an ash-free coal production method in which ash-free coal and a solvent may be effectively separated from an extraction solution by using solubility change. 30 2 2015-03-27 TECHNICAL SOLUTION According to an aspect of the present disclosure, an ash-free coal production method using PRECIPITATION BY solubility change includes: mixing a raw coal and an extraction solvent to make a slurry; heating the slurry to dissolve an organic component 5 of the raw coal; separating the heated slurry into a extracted solution in which the organic component is dissolved and an undissolved inorganic component; adding a precipitation solvent to the extracted solution to precipitate solid coal; and separating the precipitated solid coal by solid-liquid separation. In some embodiments of the ash-free coal production method using solubility 10 change, in the making of the slurry, a mixing ratio of the raw coal to the extraction solvent may be in the range of about 1:2 to about 1:10 by weight. In some embodiments of the ash-free coal production method using solubility change, in the dissolving of the organic component, the slurry may be heated at a temperature of about 2000C to about 4000C and mixed using a stirrer. 15 In some embodiments of the ash-free coal production method using solubility change, the separating of the extracted solution and the undissolved inorganic component comprises separating the undissolved inorganic component by using at least one selected from gravitational settling, filtration, and centrifugation. In some embodiments of the ash-free coal production method using solubility 20 change, in the precipitating of the solid coal, the precipitation solvent may be a diluent that is uniformly miscible with the extraction solvent and does not dissolve the solid coal. In some embodiments of the ash-free coal production method using solubility change, in the precipitating of the solid coal, an ratio of the extracted solution to the 25 precipitation solvent may be in the range of about 1:0.3 to about 1:1 by volume. In some embodiments of the ash-free coal production method using solubility change, the precipitating of the solid coal may include condensing and reusing the precipitation solvent that evaporates when added. In some embodiments of the ash-free coal production method using solubility 30 change, the separating of the precipitated solid coal by solid-liquid separation may be performed by one selected from gravitational settling, filtration, and centrifugation. 3 2015-03-27 In some embodiments of the ash-free coal production method using solubility change, the ash-free coal production method may further include recovering a solvent mixture resulting from the separating of the precipitated solid ash-free coal by solid-liquid separation and separating the extraction solvent and the precipitation 5 solvent from the recoverd solvent mixture to reuse the extraction solvent and the precipitation solvent. In some embodiments of the ash-free coal production method using solubility change, the extraction solvent may be one selected from N-methyl-2-pyrrolidone (NMP), 1 -methylnaphthalene (1-MN), and light cycle oil (LCO) that have a boiling point of about 10 2000C to about 3000C. In some embodiments of the ash-free coal production method using solubility change, the precipitation solvent may be one selected from water or ethanol. [Prior art document] [Patent document] 15 (0001) US 2010/0006477 (PCT/JP07/69833) ADVANTAGEOUS EFFECTS As described above, according to the one or more of the above embodiments of the present disclosure, an ash-free coal production method may include extracting coal 20 with an extraction solvent to obtain a extracted solution and adding a precipitation solvent to the coal extracted solution to precipitate solid coal, thereby efficiently separating ash-free coal from the extraction solvent with reduced loss of the ash-free coal and increasing a recovery rate of the expensive solvent 25 DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a conventional method of producing ash-free coal by a solvent extraction method; FIG. 2 illustrates an ash-free coal production method according to an embodiment of the present disclosure; and 30 FIG. 3 illustrates an ash-free coal production method according to another embodiment of the present disclosure. 4 2015-03-27 BEST MODE According to an embodiment of the present disclosure, an ash-free coal production method of effectively separating ash-free coal from a solvent by using solubility change includes: mixing a raw coal and an extraction solvent to prepare a 5 slurry; heating the slurry to dissolve an organic component of the raw coal; separating the heated slurry into a extracted solution in which the organic component is dissolved and an undissolved inorganic component; adding a precipitation solvent to the coal extracted solution to precipitate solid coal; and separating the precipitated solid coal by solid-liquid separation. 10 The term "coal" used herein refers to a dark brown combustible sedimentary rock that forms when land or water plants that were deposited and buried underwater in the geographical age undergoes alternation by the action of heat or pressure. Coal consists of mainly organic compounds and a small amount of inorganic components. Coal may be classified into peat, lignite (also referred to as brown coal), sub-bituminous 15 coal, bituminous coal, and anthracite. Low ranks of coal (LRC) include from brown coal to sub-bituminous coal, and high ranks of coal (HRC) include bituminous coal or later. The coal used herein is not specifically limited and may be any coals. The term "ash" used herein refers to inorganic components in coal, remaining unburned as an incombustible mineral material after combustion. Such inorganic 20 components in coal include silicon oxide (Si0 2 ), aluminum oxide (A1 2 0 3 ), iron oxide (Fe 2
O
3 ), calcium oxide (CaO), magnesium oxide (MgO), titanium oxide (TiO 2 ), and sodium oxide (Na 2 0). The term " ash-free coal" used herein refers to coal including no ash or coal including, for example, about lwt% or less of ash based on a total amount of coal. 25 The term "solvent extraction" used herein refers to extracting one component (two or more components if required) from solid or liquid solutes by using a solvent. In some embodiments of the present disclosure, only organic components of coal may be extracted by dissolving ash-containing coal in an extraction solvent. The ash in coal as an organic component is indissoluble in the extraction solvent, and only the organic 30 components in coal may be extracted using the extraction solvent. The term "precipitation" used herein refers to, in general, the phenomenon that a specific solid phase is generated in a liquid or solid phase and separated from the 5 2015-03-27 original phase due to, for example, a change in temperature or addition of a third component. In some embodiments, after dissolving coal in an extraction solvent to obtain a extracted solution, a precipitation solvent that is uniformly miscible with the extraction solvent in the extracted solution, but that does not dissolve coal may be 5 added to the extracted solution to precipitate ash-free coal. FIG. 1 illustrates a conventional method of producing ash-free coal by a solvent extraction method. An extraction solvent is mixed with raw coal including ash to make a slurry (slurry making step 10), followed by heating the slurry to a boiling temperature or higher of the extraction solvent to facilitate dissolution of an organic component of io the coal in the extraction solvent (coal extraction step 20), and separating a undissolved including an inorganic component from the heated slurry to obtain a extracted solution in which only the organic component of the coal (inorganic component separation step 30). The extracted solution is heated by applying heat energy, for example, using an 15 evaporation method (solvent evaporation step 40) to remove the extraction solvent, thereby obtaining ash-free coal in solid phase (ash-free coal obtaining step 50). The extraction solvent evaporated using, for example, evaporation is condensed (condensation step 41) for reuse in, for example, the preparing of slurry. When the extracted solution is transferred into an insulating container under 20 atmospheric condition for a next process after obtained at 3500C by using NMP as an extraction solvent, a considerable amount of the extraction solvent may be evaporated, but a portion of the extraction solvent may remain unevaporated in liquid state as the temperature of the extracted solution may be lowered to about 2000C or lower due to loss of evaporative latent heat in the extraction solvent. That is, the lower the 25 extraction temperature becomes, the larger the amount of the extraction solvent remaining unevaporated may become, and the larger amount of heat energy may be required to evaporate the remaining solvent. When the temperature of the extracted solution is higher than 3500C, theoretically nearly all of the extraction solvent may be evaporated without further 30 applying heat energy. However, the resulting coal particles in solid state may be exposed to high-temperature atmospheres above 2000C, and consequentially some components of the solid coal may be lost by thermal decomposition. 6 2015-03-27 It may also be practically impossible to condense all of the evaporated extraction solvent in the condensation step 41 for reuse, so part of the evaporated extraction solvent may be lost by being externally discharged in vapor phase. FIG. 2 is a schematic view illustrating an ash-free coal production method using 5 PRECIPITATION BY solubility change according to an embodiment of the present disclosure. The ash-free coal production method of FIG. 2 may include: mixing a raw coal and an extraction solvent to make a slurry (slurry making step 10); heating the slurry to dissolve an organic component of the raw coal (coal extraction step 20); separating the heated slurry into a extracted solution in which the organic component is io dissolved and an undissolved inorganic component (inorganic component separation step 30); adding a precipitation solvent to the extracted solution to precipitate solid coal (precipitation step 400); and separating the precipitated solid coal by solid-liquid separation (solid-liquid separation step 500). The slurry preparation step 10 may be of mixing ash-containing raw coal with an 15 extraction solvent for extracting coal. The raw coal may be grinded using a grinder. A particle size of the grinded coal may be in the range of about 50 pm to about 300 pm, for example, about 100 pm, but is not limited thereto. When the raw coal has a particle size smaller than 50 pm, the coal particles may agglomerate together and thus may not be dispersed in the extraction solvent. When the raw coal has a particle size 20 larger than 300 pm, longer extraction time may be required. Raw coal in powder form may have a small particle size to increase the reaction area with the extraction solvent and to smoothly transport the slurry prepared by mixing with the extraction solvent. In some embodiments, in the preparing of the slurry, a mixing ratio of the raw coal to the extraction solvent may be in the range of about 1:2 to about 1:10 by weight. When the 25 amount of the extraction solvent is increased so the mixed ratio of the raw coal to the extraction solvent is lower than 1:10, the amount of the raw coal may be too small to produce a final ash-free coal product that makes an economic profit. When the amount of the extraction solvent is reduced so the mixing ratio is lower than 1:2 by weight, the amount of the raw coal may be so high to increase the viscosity of the slurry 30 and cause a problem during processes such as in slurry transport and filtration. The extraction solvent is not specifically limited, and may be varied depending on the type of coal. For example, the extraction solvent may be one selected from 7 2015-03-27 N-methyl-2-pyrrolidone (NMP), 1-methylnaphthalene (1-MN), and light cycle oil (LCO) that have a boiling point of about 2000C to about 3000C. The dissolving of the organic component of the coal in the extraction solvent (coal extraction step 20) may be of extracting the organic component except for ash 5 from the raw coal in the slurry by heating the slurry to a predetermined temperature. The heating may be performed at a temperature of, for example, about 2000C to about 4000C at which intermolecular bonding in the coal may be loosen through thermal softening to allow extraction of the molecules in the coal particles. For example, the heating may be performed with stirring using a stirrer to further facilitate the extraction 10 reaction. While dissolving the organic component of the coal in the extraction solvent for about 30 minutes to about 3 hours, for example, about 50 minutes to about 2 hours, the organic component in the coal may be extracted at the same time with separating inorganic component. The inorganic component separation step 30 may be of separating an inorganic 15 component, for example, ash that is undissolved in the extraction solvent from the extracted solution to remove the inorganic component. In some embodiments, the solid including an inorganic component in particulate form remaining undissolved in the extraction solvent may be separated using, for example, but not limited to, at least one selected from gravitational settling, filtration, and centrifugation. 20 The precipitation step 400 may be of precipitating the solid coal in the extracted solution by adding a precipitation solvent that is uniformly miscible with the extraction solvent but that does not dissolve the coal. In some embodiments, when N-methyl-2-pyrrolidone (NMP) is used as an extraction solvent for extraction (also referred to as extraction solvent), water may be added to the extracted solution that 25 includes NMP, to lower the solubility of the coal in the NMP extraction solvent and consequently precipitate the liquid coal in solid form. This solid coal is ash-free coal free of inorganic components, for example, ash, and may be separated from the mixture of coal with the solvents by using any of various common solid-liquid separation methods, including precipitation, filtration, and centrifugation to remove the solvents. 30 When 1-methylnaphthalene (1-MN) is used as an extraction solvent for extraction, kerosene may be used as the precipitation solvent. The precipitation solvent may be 8 2015-03-27 any precipitation solvents that are uniformly miscible with the extraction solvent in the extracted solution but that do not dissolve the coal. In some embodiments, the precipitation solvent may be water or ethanol, but is not limited thereto. In some embodiments, in the adding of the precipitation solvent, a 5 volume ratio of the extracted solution to the precipitation solvent may be about 1:0.3 to about 1:1. When the amount of the precipitation solvent is reduced so the volume ratio of the extracted solution to the precipitation solvent is lower than 1:0.3, the amount of the precipitated solid may be reduced. When the amount of the precipitation solvent is increased so that the volume ratio of the extracted solution to the precipitation io solvent is higher than 1:1, costs for processing the precipitation solvent may be increased regardless of the amount of the precipitated solid ash-free coal. In some embodiments, the precipitation of the solid coal may occur instantly after the adding of the precipitation solvent to the extracted solution, and thus there is no need for a conventional process of evaporating the extraction solvent in the extracted solution. In 15 addition, the precipitated solid coal may not be exposed to high-temperature atmospheres, so that loss of the produced solid ash-free coal by thermal decomposition may not occur. The solid-liquid separation step 500 may be of separating the solid coal that is precipitated by the addition of the precipitation solvent, by using at least one common 20 solid-liquid separation method, for example, gravitational settling, filtration, or centrifugation, to finally obtain ash-free coal (ash-free coal obtaining step 50). In some embodiments, the ash-free coal production method may further include recovering the solvent resulting from the solid-liquid separation step 500, separating the extraction solvent from the extracted solution (solvent purification step 510), and condensing the 25 extraction solvent to reuse it (condensation step 41). The method of recovering the extraction solvent is not particularly limited, and may be any common methods for separating a liquid mixture, for example, distillation or membrane separation method. The recoverd extraction solvent may be reused in the slurry preparation step 10. The separated precipitation solvent may be reused in the coal precipitation step 400 30 FIG. 3 is a schematic view illustrating an ash-free coal production method according to another embodiment of the present disclosure. According to the method of FIG. 3, the precipitation solvent that evaporates while being added in the coal 9 2015-03-27 precipitation step 400 may be collected and reused via condensation (condensation step 41). In some embodiments, when the temperature of the extracted solution obtained using NMP solvent is about 2000C or higher, the temperature of the extracted solution may be high enough for water added as a precipitation solvent to evaporate 5 immediately after the precipitation of the coal, and consequentially may be further reduced. The evaporated water may be collected and reused via condensation. When the temperature of the extracted solution is reduced, there is a wider choice of materials for equipment that is used in the following solid-liquid separation process. In addition, when the temperature of the extracted solution is lowered due to the addition io of the precipitation solvent, evaporation and consequent loss of the extraction solvent may be reduced. While one or more embodiments of the present disclosure have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from 15 the spirit and scope of the present disclosure as defined by the following claims. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The disclosures of any documents referred to herein are incorporated in their entirety by references. 20 10