AU2012210131B2

AU2012210131B2 - Method for producing low-grade coal slurry, apparatus for producing low-grade coal slurry, and system for gasifying low-grade coal

Info

Publication number: AU2012210131B2
Application number: AU2012210131A
Authority: AU
Inventors: Noriyuki Kimura; Norihiko Kumada; Shoichi Mori; Motohiko Nishimura; Yoshiharu Nonaka; Shoichi Takao
Original assignee: Kawasaki Heavy Industries Ltd; Kawasaki Jukogyo KK
Current assignee: Kawasaki Motors Ltd
Priority date: 2011-01-24
Filing date: 2012-01-11
Publication date: 2016-04-21
Anticipated expiration: 2032-01-11
Also published as: JP2012153761A; CN103298918B; WO2012101964A1; CN103298918A; AU2012210131A1; JP6134088B2

Abstract

The purpose of the present invention is to provide: a production method and production device with which it is easy to produce a low-grade coal slurry having a high slurry concentration and a low viscosity, and which is appropriate for gasification using a pressurized gasification furnace; and a gasification system provided with such a production device. This low-grade coal slurry production method has: a first step for heating and drying coarsely ground low-grade coal, and recovering condensed water emitted from the low-grade coal as steam; and a second step for adding a dispersant and the condensed water obtained during the first step to the heated and dried low-grade coal, and forming a slurry using the wet-grinding method. In addition, this low-grade coal slurry production device has: a drying device for heating and drying the coarsely ground low-grade coal; a condensed water recovery device for recovering condensed water emitted from the low-grade coal as steam through the heating and drying; and a wet-grinding device for adding the condensed water and the dispersant to the heated and dried low-grade coal, and forming a slurry.

Description

[DESCRIPTION] [Title of Invention] METHOD FOR PRODUCING LOW-GRADE COAL SLURRY, APPARATUS FOR PRODUCING LOW-GRADE COAL SLURRY, AND SYSTEM FOR GASIFYING LOW-GRADE COAL [Technical Field] [0001] The present invention relates to a method and an apparatus for producing a slurry suitable as fuel for pressurized gasification furnaces from low-grade coal such as brown coal. The present invention also relates to a low-grade coal gasification system equipped with a pressurized gasification furnace using such a slurry as fuel. [Background Art] [00021 In recent years, the consumption of energy resources has been growing rapidly on a global basis and the price of oil, coal, natural gas, or the like as a main energy source has been increasing. Therefore, how to effectively utilize these energy sources has become a critical issue. [0003] From the viewpoint of not only the efficiency and cost of power generation but also the safety of physical distribution such as transportation or storage, bituminous coal with a high calorific value is usually utilized as fuel coal. However, supply and demand of bituminous coal has become tight on a global basis due to economic growth in developing countries. [00041 On the other hand, it is estimated that the reserves of low-grade coal such as brown coal, lignite, or subbituminous coal are 600 billion tons or more that exceeds the reserves of high-grade coal such as bituminous coal. However, low-grade coal has a high moisture content, and therefore the utilization thereof as fuel for power generation boilers or the like reduces thermal efficiency and increases power generation cost. Further, brown coal has high reactivity and is more likely to spontaneously ignite when dried, and is therefore hardly utilized at present due to difficulty in transportation and storage. [0005] 1 Further, carbon dioxide (C0 2 ) emissions by coal-fired power generation tend to rapidly increase on a global basis, and it is believed that this is a serious factor causing global warming. Coal has higher CO 2 emissions per unit calorific value than oil or natural gas, and therefore the utilization of coal has been reduced in developed countries such as Japan due to an increased demand for reduction in CO 2 emissions. [0006] Low-grade coal such as brown coal is very cheap and is a promising fuel resource with large reserves, but is, in fact, not effectively utilized due to the above problems. Under the circumstances, technical developments are being made to provide a method for gasifying brown coal near a coal mine to utilize obtained syngas. The term "syngas" refers to a crude gas produced using, as a raw material, hydrocarbon such as coal or natural gas and containing hydrogen (H 2 ) , carbon monoxide (CO), and carbon dioxide (C0 2 ) or methane or the like, and is a generic name for a raw-material gas used for ammonia synthesis, methanol synthesis, hydrogen production, or the like. [00071 Particularly, attention is being given to utilization of C0 2 -free syngas obtained by separating and recovering CO 2 from syngas produced by gasifying low-grade coal such as brown coal near a coal mine. For example, it is expected that a method will be established in a future hydrogen society, in which hydrogen is separated from syngas obtained by gasifying brown coal, purified, and liquefied to obtain liquid hydrogen and the liquid hydrogen is transported and used as fuel for gas turbines or gas engines. [0008] A technique for gasifying bituminous coal using a coal gasification furnace has already been applied to a commercial plant in which bituminous coal is wet-ground to prepare a water slurry and the water slurry is supplied together with oxygen to awet-type oxygen-blown gasification furnace operatedunder high temperature and pressure. Further, a method for carrying powdered coal obtained by finely grinding bituminous coal by airflow to supply the powdered coal to a gasification furnace has also already been applied to a commercial plant. [0009] 2 When bituminous coal is gasified using a pressurized gasification furnace, handling and transfer of groundbituminous coal become easy by adding water and an additive such as a dispersing agent thereto so that a slurry is prepared. Bituminous coal is lower in price per unit calorific value than heavy oil or the like, and is therefore attracting attention as a fuel alternative to oil. [0010] When coal is supplied as a slurry to a pressurized gasification furnace, a reduction in the viscosity of the slurry makes it easy to handle and transfer the slurry, but a reduction in the slurry concentration (coal concentration) of the slurry reduces energy efficiency. Particularly, in the case of low-grade coal such as brown coal, a reduction in slurry concentration causes a significant reduction in energy efficiency because of its higher moisture content than bituminous coal. Therefore, there is a demand for a method for producing a high-concentration and low-viscosity low-grade coal slurry. [0011] Patent Document 1 discloses a method in which, for the purpose of obtaining a slurry having a preferred appearance viscosity of about 1000 cP and a coal content as high as possible (about 70 wt%) with the use of low-grade coal as raw-material coal, a dehydrated cake, water, and a dispersing agent are kneaded in two steps: a first step in which a dehydrated cake and water are kneaded with no dispersing agent added or with a dispersing agent added in such a small amount that a kneaded product does not have flowability; and a second step in which the dispersing agent is added when the kneaded product is turned into a paste and the paste is further kneaded. [Citation List] [Patent Literature] [0012] PTL 1: Japanese Laid-open Patent Application Publication No. 11-335680 [Summary of Invention] [Technical Problem] [0013] The moisture content of brown coal reaches as high as 30 3 to 70%, and therefore when a water slurry is prepared by further adding water, the total moisture content of the slurry (the total amount of moisture contained in the brown coal and moisture present outside the brown coal as a dispersion medium of the slurry) becomes extremely high. In this case, a great amount of heat is consumed for moisture evaporation in a pressurized gasification furnace, and therefore energy efficiency becomes extremely low. For this reason, a method has hardly been studied in which a water slurry of low-grade coal, such as brown coal, with a high moisture content is prepared and supplied to a pressurized gasification furnace to gasify the low-grade coal. [00141 Fig. 2 shows a schematic flow chart of a slurry production method disclosed in Patent Literature 1. It is reported that the slurry production method disclosed in Patent Literature 1 is capable of producing a high-concentration slurry even when brown coal or subbituminous coal is used which is highly hygroscopic, has a high moisture content (30 to 70 wt%) , and has a hydrophilic surface. [0015] According to the slurry production method disclosed in Patent Literature 1 (Claim 3) , in a first step (grinding step) water is added to brown coal or subbituminous coal and a low-concentration slurry (10 to 40 wt%) is prepared by wet coarse grinding. However, areas near coal mines are often poor in water infrastructures, and therefore it can be imagined that it is hard to ensure a large amount of water. [0016] Then, the slurry is reformed by hot-water treatment (pressure: 12 to 15 MPa, temperature: 250 to 330 0 C) in a second step (reforming step) , and the reformed slurry is dehydrated in a third step (dehydration step) to obtain a dehydrated cake. However, the dehydrated cake has no flowability and is therefore difficult to handle and transfer. In a subsequent fourth step (first kneading step) , the dehydrated cake is kneaded to obtain a paste to again prepare a slurry, but the paste has no flowability and is therefore difficult to handle and transfer. Further, it cannot be expected that the brown coal or the like will be finely ground by kneading. [00171 4 Further, in a fifth step (second kneading step), a water slurry is prepared by adding a dispersing agent, but it is difficult to uniformly disperse the dispersing agent by kneading. [0018] An object of the present invention is to provide a method for easily producing a high-slurry concentration and low-viscosity low-grade coal slurry suitable for gasification by a pressurized gasification furnace. [Solution to Problem] [0019] The present inventor has intensively studied to solve the above problems associated with the conventional technique for producing a low-grade coal slurry. As a result, the present inventor has found that, when a low-grade coal slurry is prepared, the amount of moisture contained in low-grade coal can be reduced while increasing the amount of moisture present outside the low-grade coal as a dispersion medium not by externally adding water to the ground low-grade coal but by recovering, as condensed water, water vapor generated by heat-drying the low-grade coal to dehydrate the low-grade coal by evaporation and utilizing the condensed water. Further, the present inventor has found that by doing so, it is possible, even when low-grade coal is used as raw material coal, to obtain a slurry that has a total moisture content comparable to that obtained by the conventional technique but has a low viscosity and is easy to handle and transfer. These findings have led to the completion of the present invention. [0020] Specifically, the present invention provides a method for producing a low-grade coal slurry, comprising: a first step of coarsely grinding low-grade coal such that an amount of grains with a grain size of 3 mm or less is 70 wt% or more; a second step of heat-drying coarsely-ground low-grade coal and recovering, as condensed water, water vapor released from the low-grade coal; and a third step of adding the condensed water obtained in the second step and a dispersing agent to the low-grade coal subjected to heat-drying treatment and wet-grinding the 5 low-grade coal to prepare a slurry; wherein the heat drying in the second step is performed by indirect heating treatment at 100 0 C or higher but 200 0 C or lower; and a moisture content of the low-grade coal after the first step is 15 wt% or lower. [00211 In the second step, water contained in the coarsely-ground low-grade coal is released as water vapor so that the low-grade coal is dried. Further, a surface microstructure of the low-grade coal is filled with a tar component and hydrophilic functional groups on the surface of the low-grade coal are changed to hydrophobic functional groups by heating, and therefore it can be expected that the released water is less likely to be reabsorbed by the low-grade coal. [00221 Here, the low-grade coal refers to brown coal, subbituminous coal, lignite, or a mixture of two or more of them, but the present invention is particularly preferably applied to brown coal. [00231 The water vapor (self-release water) released from the low-grade coal, cooled, and recovered as condensed water in the second step is used when the low-grade coal subjected to reforming treatment is wet-ground to prepare a slurry in the third step, which makes it possible to prepare a low-grade coal slurry even in the neighborhood of a coal mine poor in water resources. [00241 This paragraph has been intentionally left blank. [00251 The term "moisture content of the low-grade coal" here refers to the percentage of the weight of moisture contained in the low-grade coal to the total weight of the low-grade coal. [00261 It is preferred that the slurry obtained in the third step has a slurry concentration of 40 wt% or higher but 65 wt% or lower. The term "slurry concentration" here refers to the percentage by weight of the low-grade coal contained in the slurry on a dry coal basis (in terms of the weight of dry coal having a moisture 6 content of 0 wt%, that is, on a dry basis). [00271 It is preferable to further include, after the third step, a fourth step of subjecting the obtained slurry to high-speed stirring treatment. By subjecting the slurry to high-speed stirring treatment, the stability of the slurry can be improved and therefore the slurry can also be stored before supplied to a pressurized gasification furnace. [00281 It is preferred that the dispersing agent is a polystyrene-based dispersing agent, a naphthalenesulfonic acid-based dispersing agent, a methacrylic acid-based dispersing agent, or a lignin sulfonic acid-based dispersing agent, and an amount of the dispersing agent to be added to the slurry is 0.2 wt% or higher but 2 wt% or lower with respect to a dry weight of the low-grade coal. [00291 The present invention also provides an apparatus for producing a low-grade coal slurry, comprising: a drying device that heat-dries coarsely-ground low-grade coal; a condensed water recovery device that recovers, as condensed water, water vapor released from the low-grade coal by heat drying; and a wet-type grinding device that prepares a slurry of the low-grade coal by adding the condensed water and a dispersing agent to the heat-dried low-grade coal. [00301 It is preferred that the drying device is an indirect heat drying device that can be easily upsized. [00311 It is preferred that the apparatus for producing a low-grade coal slurry according to the present invention further includes a dry-type grinding device that coarsely grinds the low-grade coal. [00321 It is preferred that the apparatus for producing a low-grade coal slurry according to the present invention further includes a stirring device that subjects the slurry obtained by 7 the wet-type grinding device to high-speed stirring treatment. [0033] The present invention also provides a system for gasifying low-grade coal, comprising: the apparatus for producing a low-grade coal slurry; a transfer pipe through which a slurry obtained from the wet-type grinding device or the stirring device is transferred; and a pressurized gasification furnace that gasifies the slurry transferred through the transfer pipe. [00341 The foregoing objects, other objects, characteristics, and advantages of the present invention will be clear from detailed descriptions of preferred embodiments givenbelowwith reference to attached drawings. [Advantageous Effects of Invention] [0034a] According to the present invention, it is possible to easily produce a high-slurry concentration and low-viscosity low-grade coal slurry suitable for gasification by a pressurized gasification furnace. Further, it is also possible to efficiently gasify, using a pressurized gasification furnace, low-grade coal, such as brown coal, that has hardly been used effectively. [0034b] A definition of the specific embodiment of the invention claimed herein follows. In a broad format, the invention provides a method for producing a low-grade coal slurry comprising: a first step of coarsely grinding low-grade coal such that an amount of grains with a grain size of 3 mm or less is 70 wt% or more; 8 a second step of heat-drying coarsely-ground low-grade coal and recovering water vapor released from the low-grade coal as condensed water; and a third step of adding the condensed water obtained in the second step and a dispersing agent to the low-grade coal subjected to heat-drying treatment and wet-grinding the low-grade coal to prepare a slurry; wherein the heat drying in the second step is performed by indirect heating treatment at 100 0 C or higher but 200 0 C or lower; and a moisture content of the low-grade coal after the first step is 15 wt% or lower. [Brief Description of Drawings] [00341 Fig. 1 shows a schematic configuration diagram illustrating one example of an apparatus for implementing a method for producing a low-grade coal slurry according to the present invention. Fig. 2 shows a schematic flow chart of a method for producing a low-grade coal slurry disclosed in Patent Literature 1. [Description of Embodiments] [0035] Hereinbelow, the present invention will be described in detail. It should be noted that the present invention is not limited to the descriptions given below. [0036] Fig. 1 is a schematic configuration diagram illustrating one example of an apparatus for implementing a method for producing a low-grade coal slurry according to the present invention. First, low-grade coal is coarsely ground by a dry-type grinding device 1. At this time, the low-grade coal is coarsely ground so that the amount of grains with a grain size of 3 mm or less (grains that pass through a sieve with a mesh [Text continues on page 9]. 8a size of 3 mm) is 70 wt% or more, preferably 90 wt% or more. [00381 Then, the coarsely-ground low-grade coal is supplied to and dried by an indirect heat-drying device 2. As the indirect heat-drying device 2, for example, a steam tube drier, a uniaxial indirect heating drier, or a biaxial indirect heating drier can be used. The heat drying is preferably performed under conditions of a temperature of 100 0 C or higher but 200 0 C or lower. By drying the low-grade coal under such temperature conditions, moisture contained in the low-grade coal is released as water vapor to the outside with little change in grain size (evaporating dehydration). [0039] The low-grade coal is dried by the indirect heat-drying device 2 to achieve a moisture content of 15 wt% or less, preferably 10 wt% or less. Further, the surface microstructure of the low-grade coal is filled with a tar component depending on heat-drying conditions, and therefore it can be expected that the released moisture is less likely to be reabsorbed by the low-grade coal and the surface hydrophobicity of the low-grade coal is enhanced. The low-grade coal dried by the indirect heat-drying device 2 is supplied to a wet-type grinding device 5 through a conveyor 4. [00401 On the other hand, the water vapor released from the coarsely-ground low-grade coal in the indirect heat-drying device 2 is supplied to a condensed water recovery device 3, cooled, and recovered as condensed water. All or part of the recovered condensed water is supplied to the wet-type grinding device 5. Then, the low-grade coal is finely ground by the wet-type grinding device 5 to prepare a low-grade coal slurry. [00411 As the condensed water recovery device 3, for example, an air-cooled condenser or a water-cooled condenser can be used. As the wet-type grinding device 5, for example, a wet-type ball mill or a media agitating mill can be used. It should be noted that a surplus of the condensed water may be disposed of or used for any other purposes. [00421 The amount of water to be supplied to the wet-type grinding 9 device 5 is preferably adjusted so that the concentration of the low-grade coal slurry is in the range of 40 to 65 wt% on a dry coal basis (on a dry basis) , but when only the recovered condensed water is not enough to meet the water requirement of the slurry, the required amount of extra water is added. Further, the low-grade coal slurry is preferably finely ground so that the amount of low-grade coal grains with a grain size of 74 pm or less (low-grade coal grains that pass through 200 mesh) is 30 to 90% of the total amount of low-grade coal grains. [00431 As the dispersing agent, a polystyrene-based dispersing agent, a naphthalenesulfonic acid-based dispersing agent, a methacrylic acid-based dispersing agent, or a lignin sulfonic acid-based dispersing agent can be used. The dispersing agent is preferably added so that its concentration in the low-grade coal slurry is 0.2 wt% or higher but 2 wt% or lower with respect to the dry weight of the low-grade coal contained in the low-grade coal slurry. [00441 In addition to the dispersing agent, a stabilizing agent such as carboxymethyl cellulose and/or a thickening agent such as xanthan gum may be added to the low-grade coal slurry to further improve the stability of the slurry. The concentration of the stabilizing agent or the thickening agent in the low-grade coal slurry is preferably adjusted to be 0.001 to 0.05 wt%. [0045] A low-grade coal slurry obtained by the present invention is smaller in the amount of moisture contained in low-grade coal but larger in the amount of water as a dispersion medium than a conventional low-grade coal slurry even when they have the same level of total moisture content. Therefore, the low-grade coal slurry obtained by the present invention has a higher concentration and a lower viscosity than the conventional low-grade coal slurry and has properties suitable for gasification by a pressurized gasification furnace. [0046] Unlike a method disclosed in Patent Literature 1, in the present invention, a dispersing agent is added at the same time as mixing coarsely-ground low-grade coal with water to prepare a slurry, and therefore the dispersing agent can be uniformly 10 dispersed in the slurry. [00471 As in the case of Patent Literature 1, when a low-concentration slurry is wet-ground, a grain size distribution with a small amount of fine grains is obtained. On the other hand, in the present invention, a very high-concentration mixture of low-grade coal and water is wet-ground, and therefore the wet-ground low-grade coal has an ideal grain size distribution with a large amount of fine grains. [00481 The low-grade coal slurry obtained by the wet-type grinding device 5 is stored in a slurry tank 6. The low-grade coal slurry in the slurry tank 6 is preferably stirred by a stirrer 7 (low-speed stirrer). Then, the low-grade coal slurry is supplied from the slurry tank 6 to a stirring device 10 through a transfer pipe 9 equipped with a transfer pump 8. By subjecting the low-grade coal slurry to high-speed stirring treatment using the stirring device 10 (high-speed stirrer) , the stability of the low-grade coal slurry is improved and therefore the separation of the low-grade coal can be prevented. [0049] As the stirring device 10, a line mixer or a mixing tank equippedwith ahigh speed-stirrer canbe used. When alinemixer is used, the rotation speed of the line mixer is preferably set to 500 to 5000 rpm, and the retention time of the low-grade coal slurry is preferably set to 10 seconds to 2 minutes. The low-grade coal slurry (product slurry) subjected to high-speed stirring treatment is supplied to a pressurized gasification furnace through a transfer pipe 13 equipped with a transfer pump 12. [0050] It should be noted that the stirring device 10 is an optional component, and may be omitted when the low-grade coal slurry contained in the slurry tank 6 is immediately supplied to the pressurized gasification furnace. In this case, the low-grade coal slurry is supplied from the slurry tank 6 to the pressurized gasification furnace through the transfer pipe 9. [00511 The low-grade coal slurry supplied to the pressurized gasification furnace (wet coal slurry feed-type coalpressurized 11 gasification furnace) is gasified therein and converted to syngas. If necessary, the obtained syngas is purified by separating impurities such as CO or C0 2 , to obtain hydrogen gas. The separated CO 2 is preferably fixed. Further, the separated CO is preferably oxidized to CO 2 and then fixed. In this way, a system for gasifying low-grade coal as a raw material in a pressurized gasification furnace is formed. [0051] [Example] (First step) Brown coal (Loy Yang coal from Australia) having properties shown in Table 1 was coarsely ground using a hammer crusher so that the amount of grains having a grain size of 3 mm or less was 90 wt% or higher (so that the amount of grains that pass through a sieve with a mesh size of 3 mm was 90 wt% or higher of the total weight of the brown coal). The coarsely-ground brown coal was fed into an indirect heat drier (a compact steam tube drier manufactured by Kawasaki Heavy Industries, Ltd.) and dried at 0.1 MPa and 180*C for 1 hour to obtain dried brown coal with total moisture content of 10 wt%. Water vapor released from the indirect heat drier was cooled by a water-cooled condenser and recovered as condensed water. [0052] TABLE 1 Total moisture 59.8 wt% Ash 0.8 wt% Volatile matter 20.6 wt% Fixed carbon 18.8 wt% [0053] (Second step) The condensed water and a polystyrene-based dispersing agent were added to 3 kg of the dried brown coal, and then the resulting mixture was fed into a wet-type ball mill (# 300 mm x L 300 mm, capacity: about 21 L). The amount of the condensed water to be added was adjusted to be 49 wt% with respect to the total weight of the brown coal (on a dry coal basis), the condensed water, and the polystyrene. Further, the polystyrene-based dispersing agent was added in an amount of 0. 7 wt% with respect to the weight of the dried brown coal. 12 [0054] The brown coal, the condensed water and the polystyrene-based dispersing agent fed into the wet-type ball mill were subjected to batch grinding to obtain a brown coal slurry containing brown coal grains that pass through 200 mesh in an amount of 85% of the total amount of the brown coal. The brown coal slurry had a concentration of 50 wt% on a dry coal basis, and its appearance viscosity as measured under conditions of a share velocity of 100 sec- 1 and a temperature of 25 0 C was 1.8 Pa-s (1800 cP) . The brown coal slurry was allowed to stand (room temperature) in a plastic container with an internal volume of 500 mL for 7 days, and as a result, the amount of the brown coal settled was about 10 vol%. [0055] (Third step) The brown coal slurry with a concentration of 50 wt% obtained in the second step was further subjected to high-speed stirring treatment using a high-speed stirrer (T.K Homomixer MARK II Type 2.5 manufactured by PRIMIX Corporation) at 1000 rpm for 1 minute. The appearance viscosity of the brown coal slurry after high-speed stirring treatment, as measured under conditions of a shear velocity of 100 sec 1 and a temperature of 25 0 C, was 1.2 Pa-s (1200 cP) . The brown coal slurry subjected to high-speed stirring treatment was allowed to stand (room temperature) in a plastic container with an internal volume of 500 mL for 7 days, and as a result, the amount of the brown coal settled was about 7 vol%. Therefore, it was confirmed that the stability of the brown coal slurry obtained in the second step was improved by further subjecting the brown coal slurry to high-speed stirring treatment. [00581 From the foregoing explanations, many improvements on and other embodiments of the present invention are apparent to a person skilled in the art. Therefore, the explanations above should be construed as illustrative examples provided for the purpose of explaining the best mode for carrying out the present invention to a person skilled in the art. It is possible to substantially alter the details of the structure and/or functions without deviating from the spirit of the present invention. 13 [Industrial Applicability] [0056] The method for producing a low-grade coal slurry, the apparatus for producing a low-grade coal slurry, and the system for gasifying low-grade coal according to the present invention are useful in the field of fuel or energy. [Reference Signs List] [00571 1: Dry-type grinding device 2: Indirect heat-drying device 3: Condensed water recovery device (Condenser) 4: Conveyor 5: Wet-type grinding device 6: Slurry tank 7: Stirrer (Low-speed stirrer) 8,12: Transfer pump 9,13: Transfer pipe 10: Stirring device (High-speed stirrer) 11: Motor [00581 Mere reference to background art herein should not be construed as an admission that such art constitutes common general knowledge in relation to the invention. [0059] Throughout this specification, including the claims, where the context permits, the term "comprise" and variants thereof such as "comprises" or "comprising" are to be interpreted as including the stated integer or integers without necessarily excluding any other integers. 14

Claims

1. A method for producing a low-grade coal slurry comprising: a first step of coarsely grinding low-grade coal such that an amount of grains with a grain size of 3 mm or less, is 70 wt% or more; a second step of heat-drying coarsely-ground low-grade coal and recovering water vapor released from the low-grade coal as condensed water; and a third step of adding the condensed water obtained in the second step and a dispersing agent to the low-grade coal subjected to heat-drying treatment and wet-grinding the low-grade coal to prepare a slurry; wherein the heat drying in the second step is performed by indirect heating treatment at 100 0 C or higher but 200 0 C or lower; and a moisture content of the low-grade coal after the first step is 15 wt% or lower.

2. The method for producing a low-grade coal slurry according to claim 1, wherein the slurry obtained in the third step has a concentration of 40 wt% or higher but 65 wt% or lower.

3. The method for producing a low-grade coal slurry according to claim 1, wherein the dispersing agent is a polystyrene-based dispersing agent, a naphthalenesulfonic acid-based dispersing agent, a methacrylic acid-based dispersing agent, or a lignin sulfonic acid-based dispersing agent, and an amount of the dispersing agent to be added to the slurry is 0.2 wt% or higher but 2 wt% or lower with respect to a dry weight of the low-grade coal.

4. The method for producing a low-grade coal slurry according to any one of claims 1 to 3, comprising, after the third step, a fourth step of subjecting the obtained slurry to high-speed stirring treatment.

5. An apparatus for producing a low-grade coal slurry comprising: a coarse grinding device that coarsely grinds low-grade 15 coal such that an amount of grains with a grain size of 3 mm or less, is 70 wt% or more; a drying device that heat-dries coarsely-ground low-grade coal until a moisture content of the low-grade coal becomes 15 wt% or lower; a condensed water recovery device that recovers, as condensed water, water vapor released from the low-grade coal by heat drying; and a wet-type grinding device that prepares a slurry of the low-grade coal by adding the condensed water and a dispersing agent to the heat-dried low-grade coal; wherein the drying device is an indirect heat-drying device.

6. The apparatus for producing a low-grade coal slurry according to claim 5, wherein the drying device is an indirect heat-drying device.

7. The apparatus for producing a low-grade coal slurry according to claim 5, further comprising a dry-type grinding device that coarsely grinds the low-grade coal.

8. The apparatus for producing a low-grade coal slurry according to claim 5, further comprising a stirring device that subjects the slurry obtained by the wet-type grinding device to high-speed stirring treatment.

9. A system for gasifying a low-grade coal comprising: the apparatus for producing a low-grade coal slurry according to claim 5; a transfer pipe through which a slurry obtained from the wet-type grinding device or the stirring device is transferred; and a pressurized gasification furnace that gasifies the slurry transferred through the transfer pipe. 16