WU 2015/016062 Al I PCT/JP2014/068852 PRODUCTION METHOD OF REFORMED COAL TECHNICAL FIELD [0001] The present invention relates to a production method of reformed coal. BACKGROUND ART [0002] Low-grade coal (low-rank coal) with high moisture content, such as lignite and sub-bituminous coal, has a low calorific value per unit of weight; thus, such coal is heated so as to be dried and pyrolyzed and is also modified in a low-oxygen atmosphere so as to reduce surface activity. As a result, the coal is converted to reformed coal having a high calorific value per unit of weight while preventing spontaneous combustion (see, for example, Patent Document 1). [0003] It is also known that mercury can be removed in coal reformation processes involving such pyrolysis described above through heating operation during pyrolysis (see, for example, Patent Document 2). CITATION LIST Patent Literature [0004] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2011-37937A Patent Document 2: US Patent No. 5403365B Patent Document 3: US Patent No. 8394240B Patent Document 4: US Patent No. 7540384B Non Patent Literature [0005] Non Patent Document 1: R. Weinstein and R. Snoby, "Advances in dry jigging improves coal quality", p.29 to p.34, Mining Engineering, January 2007 Non Patent Document 2: William H. Pollock et al., "Lowering Costs with Dry Coal Cleaning Technology to Meet New Environmental Requirements", p.1 to p, 1 3, presented at the 10th Anniversary CoalGen Conference, Pittsburgh, PA, August 10, 2010 WO 20151/016062 Al 2 PCT/JP2014/068852 SUMMARY OF INVENTION Technical Problem [0006] However, although the coal reformation process described above is capable of obtaining reformed coal by removing mercury from the raw coal through pyrolysis, there is a demand for coal having a lower mercury content. [0007] The present invention has been conceived in order to solve the problem described above, and an object of the present. invention is to provide a production method of reformed coal in which coal having a higher calorific value and lower mercury content can be obtained. Solution to Problem [0008] A production method of reformed coal according to a first invention for solving the problem described above includes: a drying step of drying coal to obtain dried coal; a dry deashing step of removing ash from the dried coal obtained in the drying step to obtain deashed dried coal; and a pyrolyzing step of pyrolyzing the deashed dried coal obtained in the dry deashing step to obtain deashed pyrolyzed coal. [0009] A production method of reformed coal according to a second invention for solving the problem described above is based on the production method of reformed coal according to the first invention of the present invention and further includes: a dividing step of dividing out a portion of the deashed dried coal obtained in the dry deashing step; and a mixing step of mixing the deashed dried coal divided out in the dividing step with the deashed pyrolyzed coal obtained in the pyrolyzing step to obtain mixed coal. [0010] A production method of reformed coal according to a third invention for solving the problem described above is based on the production method of reformed coal according to the first invention or the second invention. In such a production method of reformed coal, the dry deashing step is performed using a pulverizer that pulverizes the dried coal and a magnetic separation apparatus that magnetically separates and removes the ash from the dried coal pulverized by the pulverizer. [0011] WO 2015/016062 Al 3 PCT/JP2014/068852 A production method of reformed coal according to a fourth invention for solving the problem described above is based on the production method of reformed coal according to the first invention or the second invention. In such a production method of reformed coal, the dry deashing step is performed using an air stream separation apparatus that, with an air stream, separates and removes the ash from the dried coal. [0012] A production method of reformed coal according to a fifth invention for solving the problem described above is based on the production method of reformed coal according to any one of the first to fourth inventions, in which the coal is low-grade coal. Advantageous Effects of Invention [0013] In accordance with the production method of reformed coal according to the present invention, it is possible to increase a calorific value of coal by drying and dry deashing the coal, before pyrolyzing the coal. In addition, removing ash from the dried coal through dry deashing before pyrolyzing the dried coal allows for the removal of both the ash and mercury contained within the dried coal, resulting in deashed pyrolyzed coal having a mercury content lower than that of pyrolyzed coal obtained by pyrolyzing dried coal. As a result, coal having a higher calorific value and a lower mercury content can be obtained. Brief Description of Drawings [0014] FIG. 1 is a process flow chart of a first embodiment of a production method of reformed coal according to the present invention. FIG. 2 is a process flow chart of a second embodiment of the production method of reformed coal according to the present invention. Description of Embodiments [0015] Embodiments of a production method of reformed coal according to the present invention will now be described with reference to the drawings. However, the present invention is not limited to the embodiments described hereafter with reference to the drawings. [0016] WO 2015/016062 Al 4 PCT/JP2014/068852 [First Embodiment] A first embodiment of the pro auction method of reformed coal according to the present invention will now be described with reference to FIG. 1. [0017] The production method of reformed coal according to the present embodiment includes, as illustrated in FIG. 1, a drying step SI of drying low grade coal (low-rank coal) I to obtain dried coal 2, a dry deashing step S2 of dry-deashing the dried coal 2 obtained in the drying step SI using a dry deashing apparatus to obtain deashed dried coal 3, and a pyrolyzing step S3 of pyrolyzing the deashed dried coal 3 obtained in the dry deashing step S2 to obtain deashed pyrolyzed coal 4. [0018] The low-rank coal 1 is coal having a high (60 to 70%) moisture content, such as lignite or sub-bituminous coal, which, although vast deposits thereof exist, the coal has a low calorific value per unit of weight and is inefficient to transport. [0019] The drying step S1 is a step of removing water 5 from the low-rank coal 1. In the drying step S], for example, feeding the low-rank coal 1 to a hot-air dryer of belt conveyor type or the like and drying the low-rank coal 1 with hot air (100 to 280'C, preferably 150 to 200'C) produces dried coal 2 having a moisture content of substantially 0%. [0020] The dry deashing step S2 is a step of separating and removing, from the dried coal 2, ash 6 contained in the dried coal 2. In the dry deashing step S2, for example, feeding the dried coal 2 to a dry deashing apparatus provided with a pulverizer that pulverizes the dried coal 2 and a magnetic separation apparatus that magnetically separates and removes the ash 6 from the dried coal 2 pulverized by the pulverizer, and pulverizing the dried coal 2 to, for example, a particle diameter of 200 mesh or less and magnetically separating and removing the ash 6 (especially pyrite and the like having a high mercury content) produces deashed dried coal 3 having, for example, an ash content of about 35% or less. Alternatively, in the dry deashing step S2, for example, feeding the dried coal 2 to the dry deashing apparatus, such as an air stream separation apparatus, allowing the dried coal 2 to flow on a fluidized bed and supplying air thereto to separate and remove the ash 6 (especially substances including high-mercury-content pyrite, which are heavier than the coal) produces the deashed dried coal 3 having an ash content of, for example, about WO 2015/016062 Al 5 PCT/JP2014/068852 35% or less. In other words, the dry deashing step S2 produces deashed dried coal 3 obtained by removing, from the dried coal 2, ash 6 along with the better part of the mercury in the dried coal 2. [0021] The apparatus disclosed, for example, in Non Patent Document I can be used as the dry deashing apparatus provided with the pulverizer and the magnetic separation apparatus. The apparatus disclosed, for example, in Patent Document 4 or Non Patent Document 2 can be used as the dry deashing apparatus such as the air stream separation apparatus. [0022] The pyrolyzing step S3 is a step of removing volatile components 7 such as tar from the deashed dried coal 3. In the pyrolyzing step S3, for example, feeding the deashed dried coal 3 to a continuous pyrolyzer and pyrolyzing the deashed dried coal 3 at an elevated temperature (300 to 500'C, preferably 400 to 450'C) to separate and collect mercury contained in the deashed dried coal 3 along with the volatile components 7 such as tar produces deashed pyrolyzed coal 4. [0023] Thus, in accordance with the production method of reformed coal according to the present embodiment, it is possible to dry, dry deash, and pyrolyze the low-rank coal I to obtain deashed pyrolyzed coal 4, thereby allowing for a higher calorific value than that of the low-rank coal 1. In addition, mercury in the low-rank coal I can be removed in both the dry deashing step S2 and the pyrolyzing step S3, with the mercury being physically removed along with the ash 6 in the dry deashing step S2 before being removed along with the volatile components 7, thereby allowing the deashed pyrolyzed coal 4 to have a lower mercury content than that of pyrolyzed coal obtained by drying and pyrolyzing the low-rank coal 1. As a result, deashed pyrolyzed coal 4 having a higher calorific value and a lower mercury content can be obtained. [0024] [Second Embodiment] A second embodiment of the production method of reformed coal according to the present invention will now be described with reference to FIG. 2. The present embodiment is constituted by the steps of the production method of reformed coal illustrated in FIG. I to which a dividing step and a mixing step are added. The rest of the steps is substantially identical to that illustrated in FIG. I as described above, so the identical steps and substances WO 2015/016062 Al 6 PCT/JP2014/068852 will be given the same reference numerals and duplicated description thereof will be omitted as appropriate. [0025] The production method of reformed coal according to the present embodiment includes, as illustrated in FIG. 2, a drying step Si of drying low grade coal (low-rank coal) I to obtain dried coal 2, a dry deashing step S2 of dry deashing the dried coal 2 obtained in the drying step SI using a dry deashing apparatus to obtain deashed dried coal 3, a dividing step S1I of dividing out a portion of the deashed dried coal 3 obtained in the dry deashing step S2, a pyrolyzing step S12 of pyrolyzing deashed dried coal 3a that has not been divided out in the dividing step S11 to obtain deashed pyrolyzed coal 11, and a mixing step S13 of mixing the deashed pyrolyzed coal 11 obtained in the pyrolyzing step S12 with deashed dried coal 3b that has been divided out in the dividing step S11 to obtain mixed coal 12. [0026] The dividing step SI1 is a step of dividing out a portion of the deashed dried coal 3. In the dividing step S11, for example, feeding the deashed dried coal 3 to a dividing apparatus such as a conveyer belt or a screw feeder divides the deashed dried coal 3 into the deashed dried coal 3a to be sent to the pyrolyzing step S12 and the deashed dried coal 3b to be sent to the mixing step S13. The proportion of how the deashed dried coal 3 is divided can be adjusted according to the target oxygen content or mercury content of the mixed coal 12. This is because the oxygen content or mercury content of the deashed dried coal 3 and the deashed pyrolyzed coal 11 can be adjusted in accordance with respective processing conditions, and can also be determined through analysis. [0027] The pyrolyzing step S12 is a step, identical to the pyrolyzing step S3 described above, of removing volatile components 13 such as tar from the deashed dried coal 3a. In the pyrolyzing step S12, for example, feeding the deashed dry coal 3a to a continuous pyrolyzer, pyrolyzing the deashed dry coal 3a at an elevated temperature (300 to 500'C, preferably 400 to 450'C), and separating and collecting mercury contained in the deashed dried coal 3a along with the volatile components 13 such as tar, produces deashed pyrolyzed coal 11. [0028] The mixing step S13 is a step of mixing the deashed pyrolyzed coal 11 obtained in the pyrolyzing step S12 with the deashed dried coal 3b divided out in the dividing step S11. In the mixing step S13, feeding the deashed dried coal WO 2015/016062 Al 7 PCT/JP2014/068852 3b divided out in the dividing step S 11 into a mixer along with the deashed pyrolyzed coal 11 and stirring until the two are uniformly mixed produces mixed coal 12. [0029] The mixing ratio of the deashed pyrolyzed coal 11 to the deashed dried coal 3b in the mixture are adjusted, as appropriate, according to the respective oxygen content of the deashed pyrolyzed coal 11 and the deashed dried coal 3b, the respective mercury contents of the deashed pyrolyzed coal II and the deashed dried coal 3b, and the like. This is because the oxygen content and mercury content of the deashed dried coal 3 can be obtained through the processing condition of the dry deashing step S2 or analysis, and the oxygen content and mercury content of the deashed pyrolyzed coal 11 can be obtained through the processing condition of the pyrolyzing step S12 or analysis. [0030] Thus, in accordance with the production method of reformed coal according to the present embodiment, it is possible to mix the deashed pyrolyzed coal 11 obtained by drying, dry deashing, and pyrolyzing the low rank coal 1 with divided out deashed dried coal 3b to obtain the mixed coal 12, thereby allowing for a higher calorific value than that of the low-rank coal 1. In addition, because the mixed coal 12 is a mixture of the deashed pyrolyzed coal 11 obtained by removing mercury from the low-rank coal I through dry deashing and pyrolysis and the deashed dried coal 3b obtained by removing mercury from the low-grade coal 1 through dry deashing, with the dry deashing physically removing mercury and the pyrolysis uniformly chemically removing mercury from the entirety of the coal, the mercury content of the deashed pyrolyzed coal 11 can be reduced to less than that of pyrolyzed coal obtained by drying and pyrolyzing the low-rank coal 1. As a result, the deashed pyrolyzed coal 11 having a higher calorific value and a lower mercury content can be obtained. In addition, because the volume of the deashed dried coal 3b to be mixed can be increased according to the respective oxygen content and mercury content of the deashed pyrolyzed coal 11 and the deashed dry coal 3b and the target oxygen content and mercury content of the mixed coal 12, the yield can be increased over the production method of reformed coal according to the first embodiment, which does not include the dividing step S I1 and the mixing step S13, resulting in higher productivity of the mixed coal 12. EXAMPLES [0031] WO 2015/016062 Al 8 PCT/JP2014/068852 Examples performed in order to confirm the effects of the production method of reformed coal according to the present invention will now be described; however, the present invention is not limited to the examples described below with reference to various data. [0032] In the present embodiment, low-rank coal having a mercury content of 69 ppm is used. The low-rank coal is dried with hot air at a temperature of, for example, 100 to 280'C in the drying step to remove water from the low-rank coal and obtain dried coal. As only the water has been removed from the low rank coal, the mercury content of the dried coal is 69 ppm, which is the same as the low-rank coal. [0033] Next, in the dry deashing step, the dried coal is pulverized to 200 mesh or less and the ash is magnetically removed therefrom, for example, by a dry deashing apparatus provided with a pulverizer and a magnetic separation apparatus, and deashed dried coal is obtained. As a result, the ash constituting a portion of the dried coal is removed. The ash is a substance such as pyrite having magnetic properties, which contains more mercury than the other substances in the coal. The mercury content of the deashed dried coal 3 is 24 2 ppm. The size of the finely powdered ash-containing dried coal was set so as to allow 98% of the calorific value of the coal to be retained while reducing mercury content by 65%. In other words, the dry deashing step physically removes the mercury contained in the dried coal. [0034] Next, in the dividing step, a portion of the deashed dried coal is divided out. The remainder of the deashed dried coal is pyrolyzed in the pyrolyzing step at an elevated temperature (300 to 500'C, preferably 400 to 450'C) to obtain deashed pyrolyzed coal. As a result, the mercury contained in the deashed dried coal is volatilized along with the volatile components, and removed by 80%. The mercury content of the deashed pyrolyzed coal is 4.83 ppm. In other words, the pyrolyzing step chemically removes the mercury contained in the deashed dried coal. [0035] Next, the deashed dried coal divided out in the dividing step and the deashed pyrolyzed coal obtained in the pyrolyzing step are mixed to obtain mixed coal. The divided out deashed dried coal and the deashed pyrolyzed coal obtained in the pyrolyzing step are mixed at the ratio of, for example, 54% by WO 2015/0 16062 Al 9 PCT/JP2014/068852 weight to 46% by weight to obtain the mixed coal. The mercury content of the obtained mixed coal is 13.8 ppm. [0036] Thus, it was apparent that oxygen content and mercury content can be adjusted by adjusting the particle size of the dried coal and the amount of ash to be removed in the dry deashing step and the mixing ratio of deashed pyrolyzed coal and deashed dried coal in the mixing step, thereby obtaining mixed coal having a higher calorific value and a lower mercury content. [0037] [Other Embodiments] In the first embodiment, the production method of reformed coal in which the low-rank coal I is dried, dry deashed, and pyrolyzed to obtain the deashed pyrolyzed coal 4 has been described, but a production method of reformed coal in which coal is obtained from the deashed pyrolyzed coal 4 having its surface deactivated through a deactivating process in which the deashed pyrolyzed coal 4 is brought into contact with a specific treatment gas (oxygen-containing gas) is also possible. In addition, a production method of reformed coal is also possible in which, after the coal obtained by the deashed pyrolyzed coal 4 having its surface deactivated through the deactivation process in which the deashed pyrolyzed coal 4 is brought into contact with a specific treatment gas (oxygen-containing gas), the coal is mixed with a binder such as corn starch or asphalt and compressed (at a pressure of 1,200 kg/cm 2 and a temperature of 300 to 450"C, preferably 350 to 450'C) into a solid briquette such as a cylinder or a charcoal briquette to produce formed coal. [0038] In the second embodiment described above, the production method of reformed coal has been described in which the deashed dried coal 3a obtained by drying and dry deashing the low-rank coal 1 is pyrolyzed to obtain the deashed pyrolyzed coal 11, a portion of the deashed dried coal 3 is divided out, and the deashed pyrolyzed coal 11 and the divided out deashed dried coal 3b are mixed to obtain the mixed coal 12, but a production method of reformed coal in which coal is obtained by the mixed coal 12 having its surface deactivated through a deactivation process in which the mixed coal 12 is brought into contact with a specific treatment gas (oxygen-containing gas) is also possible. In addition, a production method of reformed coal is also possible in which, after the coal is obtained by the mixed coal 12 having its surface deactivated through the deactivation process in which the mixed coal 12 is brought into contact with a specific treatment gas (oxygen-containing gas), WO 2015/016062 Al 10 PCT/JP2014/068852 the coal is mixed with a binder such as corn starch or asphalt and compressed (at a pressure of 1,200 kg/cm 2 and a temperature of 300 to 450'C, preferably 350 to 450'C) into a solid briquette such as a cylinder or a charcoal briquette to produce formed coal. Industrial Applicability [0039] The production method of reformed coal according to the present invention can obtain coal having a high calorific value and a lower mercury content, and thus can be used to great advantage in industrial applications. Reference Signs List [0040] 1 Low-grade coal (low-rank coal) 2 Dried coal 3 Deashed dried coal 3a, 3b Deashed dried coal 4 Deashed pyrolyzed coal 5 Water 6 Ash 7 Volatile component 11 Deashed pyrolyzed coal 12 Mixed coal 13 Volatile component SI Drying step S2 Dry deashing step S3 Pyrolyzing step Sll Dividing step S12 Pyrolyzing step S13 Mixing step