DESCRIPTION TITLE OF THE INVENTION: METHOD FOR REFORMING SOLID FUEL TECHNICAL FIELD [0001] The present invention relates to a method for reforming a solid fuel, which is fuel for boilers. BACKGROUND ART [0002] Boilers that use a solid fuel as its fuel are supplied with the solid fuel pulverized in a pulverizer and supplied along with carrier air. Such a boiler includes a furnace for burning the supplied fuel by means of a burner or the like to generate heat, and a set of heat exchanger tubes that are arranged from an upper part to a downstream side of the furnace to cause heat exchange with a combustion gas directed to flow therein. [0003] As fuel for the boilers, the use of low-grade coals, such as a brown coal, having a high moisture content is limited especially in Japan where the rate of utilization of foreign coals is high. The reason is that since the low-grade coals require moisture removal that involves a great heat loss, it is not much beneficial to put efforts into transporting the low-grade coals at a high cost. [0004] Patent Document 1 discloses a solid fuel derived from a porous coal 1 by eliminating its spontaneous combustibility while increasing its calorie as a whole, and a method for producing the solid fuel. The solid fuel is produced as described below. An oil mixture including a heavy oil content and a solvent oil content is mixed with a porous coal to obtain a slurry state, and subsequently heated up to, for example, 100 ~ 250 'C in order to replace moisture in pores with the oil mixture. The thus-produced solid fuel can contribute to a decrease in heat loss even when the solid fuel is used as fuel for the boilers. [0005] For utilization of the low-grade coal in the boilers, however, it is still necessary to reduce the amount of ash deposition on the boilers even though the moisture is removed from the low-grade coal. Compared to a high grade bituminous coal, low-grade coals with a lower degree of coalification have a lower ash content rate, while ash from a majority of the low-grade coals has a lower melting point. When the low-grade coals are used for the boilers, ash adheres to wall surfaces of the furnace or the set of heat exchanger tubes and deposits thereon, resulting in slagging or fouling. This may cause problems in that heat collecting capability of the boiler is deteriorated, or a furnace bottom of the boiler is clogged with deposited ash. [0006] To use the low-grade coals for boilers, the amount of ash deposition onto the boilers should be reduced by mixing the low-grade coals with multiple types of high-grade bituminous coals. Focusing on slag that is a component melted through combustion in the boiler, suspended in combustion air inside the boiler, and carried by a stream of the combustion 2 air while adhering to the furnace wall or the set of heat exchanger tubes, the present inventors have found a method for determining a mixing ratio of low-grade and high grade coals, with which the amount of ash deposition on the boiler can be reduced, and already filed a patent application for the method. In the method, based on a slag fraction (a 5 molten liquid fraction in ash) and the composition of ash components calculated for each solid fuel, the mixing ratio of the multiple types of solid fuels is determined so as to obtain the molten liquid fraction in ash that is lower than or equal to a reference value in the boiler. A range of from 50 to 60 wt% is preferably defined as the reference value for the molten liquid fraction in ash. 10 [0006A] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority 15 date of each claim of this application. [0006B] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of 20 any other element, integer or step, or group of elements, integers or steps. 3 CITATION LIST PATENT DOCUMENT [0007] 5 Patent Document 1: Japan Patent No. 2776278 SUMMARY OF INVENTION TECHNICAL PROBLEM [0008] Meanwhile, in the above-described method, because the mixing ratio is determined 10 in such a manner that the molten liquid fraction in ash (the slag fraction) is maintained at or below the reference value to use a mixture of low-grade and high-grade coals for the boiler, it is preferable that the molten liquid fraction in ash is sufficiently lower than the reference value. Furthermore, a sufficiently low molten liquid fraction in ash of the low-grade coal will increase the possibility of solely using the low-grade coal for the boiler 15 without having to mix the low-grade coal with the high-grade coal. [0009] An aim at least in its preferred form of the present invention is to provide a method for reforming a solid fuel by reducing a molten liquid fraction in ash, with which deposition of ash onto a boiler can be reduced, 20 SOLUTION TO PROBLEM [0010] A method for reforming a solid fuel of this invention comprises the steps of mixing a starting coal with a starting oil and adding at least one of a magnesium compound or an aluminum compound to a mixture of the starting coal and the starting oil for forming 4 a slurry, heating up the starting slurry, separating the heated starting slurry into solid and liquid components, and drying the solid component of the solid-liquid separated starting slurry to form the solid component as a product coal. [0010A] 5 In one aspect of the invention, there is provided a method for reforming a solid fuel comprising the steps of mixing a starting coal and a starting oil, to which an additive containing at least one of a magnesium compound or an aluminum compound is added to form a starting slurry; 10 heating up the starting slurry; separating the heated starting slurry into solid and liquid components, and drying the solid component of the solid-liquid separated starting slurry to thereby obtain a product coal, wherein an additive fraction of the magnesium compound or the aluminum compound is determined in such a manner that a molten 15 liquid fraction in ash is lower than or equal to 60 wt% in the product coal. [0011] A higher shrinkage factor of ash promotes a change of ash from solid to molten liquid (molten slag). According to the above-described structure, the shrinkage factor of ash is lowered by adding at least one of the magnesium compound or the aluminum 20 compound to the mixture of the starting coal and the starting oil, which makes ash resistant to turning into molten liquid. In this way, a molten liquid fraction (a slag fraction) in ash of the product coal is lowered. 5 [0012] Here, the "molten liquid fraction in ash" means a fraction of ash that turns into molten liquid (molten slag) at a certain temperature under a certain atmospheric condition among a fixed amount of ash in a solid state. Further, the "slag" means a 5 component melted through combustion, suspended in combustion air inside the boiler, and carried by a stream of the combustion air while adhering to furnace walls and a set of heat exchanger tubes. [0013] In general, as an additive fraction of an inorganic compound in the product coal 10 increases, inorganic substances contained in the product coal are increased, thereby raising the rate of increase in slag. With regard to the magnesium compound or the aluminum compound, however, as their additive fractions increases, the molten liquid fraction in ash of the product coal decreases, thereby lowering the rate of increase in slag. [0014] 15 Here, the "rate of increase in slag" is a ratio between volumes of formed slag before and after the addition of the inorganic compound. Then, the "volume of formed slag" is a value obtained by multiplying the molten liquid fraction in ash by a weight of ash in a coal to be supplied and a weight of the inorganic compound to be added. [0015] 20 Further, in the method for reforming a solid fuel according to this invention, the additive fraction of at least one of the magnesium compound or the aluminum compound is preferably determined in terms of the molten liquid fraction in ash, which is smaller than or equal to 60 percent by weight (wt%) of the product 6 coal. According to this structure, the magnesium compound or the aluminum compound is added so that the molten liquid fraction in ash of the product coal is maintained at or below 60 wt% even at around 1573 K that is a particular temperature at which ash can easily deposit inside the boiler. In this way, the molten liquid fraction in 5 ash of the product coal comes smaller than or equal to a reference value used for determining a mixing ratio of a plurality of solid fuels, and a deposition fraction of ash is accordingly decreased. When the thus-formed product coal is used for the boiler, an amount of ash deposition onto the boiler can be reduced. [0016] 10 Still further, in the method for reforming a solid fuel of this invention, the additive fraction of at least one of the magnesium compound or the aluminum compound is preferably between or equal to 25 wt% and 50 wt% of coal ash. According to this setting, the molten liquid fraction in ash of the product coal can be preferably lowered by adding at least one of the magnesium compound or the aluminum compound to the mixture of 15 the starting coal and the starting oil at the additive fraction of from 25 wt% to 50 wt%. [0017] Furthermore, in the method for reforming a solid fuel of this invention, it is preferable that at least one of the magnesium compound or the aluminum compound has an average particle diameter of 5pm or smaller. When the average particle 20 diameter of the magnesium compound or the aluminum compound is smaller and finer than that of ash, the effect of reducing deposition of ash is enhanced. Since the average particle diameter of ash is approximately 6.8 gin, at least one of the magnesium 7 compound or the aluminum compound having the average particle diameter of 5 tm or smaller can effect preferable reduction of ash deposition onto the boiler. [0018] Moreover, in the method for reforming a solid fuel of this invention, it is 5 preferable that the additive contains a 70 wt% or more magnesium compound. According to this, the molten liquid fraction in ash of the product coal can be suitably lowered by adding the additive that contains the 70 wt% or more magnesium compound to the mixture of the starting coal and the starting oil. ADVANTAGEOUS EFFECTS OF INVENTION 10 [0019] According to the method for reforming a solid fuel of this invention, the addition of at least one of the magnesium compound or the aluminum compound to the mixture of the starting coal and the starting oil causes the shrinkage factor of ash to decrease, so that ash becomes more resistant to turning into molten liquid, and the molten 15 liquid fraction (slag fraction) in ash of the product coal is accordingly lowered. As the additive fraction of the magnesium compound or the aluminum compound increases, the molten liquid fraction in ash of the product coal is decreased, thereby lowering the rate of increase in slag. Because the decreased molten liquid fraction in ash leads to a decrease in deposition fraction of ash of the product coal, the amount of ash deposition onto the 20 boiler can be reduced by using the thus-formed product coal for the boiler. [0020] Further, when a low-grade coal containing ash with a low melting point is used as a starting coal, at least one of the magnesium compound or the aluminum compound is 8 added to the low-grade coal, to thereby lower the molten liquid fraction in ash, which can provide the product coal whose deposition fraction of ash is lowered. In this way, there is a high possibility that the low-grade coal can be solely used for the boiler without having to mix the low-grade coal with a high-grade coal such as a bituminous coal. 5 [0021] As such, by adding at least one of the magnesium compound or the aluminum compound to the mixture of the starting coal and the starting oil, the molten liquid fraction in ash of the product coal can be lowered, and the deposition fraction of ash in the product coal can be accordingly reduced. When the product coal formed as described above is used for 10 the boiler, deposition of ash onto the boiler can be curbed. BRIEF DESCRIPTION OF DRAWINGS [0022] [FIG. 1] A schematic diagram showing a boiler; [FIG. 2] An explanatory diagram of a method for reforming a starting coal: [FIG. 3] A 15 diagram showing a relationship between a molten liquid fraction in ash and a deposition fraction of ash at 1573 K; 8A [FIG. 4] A diagram showing a relationship between a temperature and the molten liquid fraction in ash; [FIG. 5] A diagram showing a relationship between the temperature and a shrinkage factor of ash; [FIG. 6 A diagram showing a relationship between an additive fraction of an inorganic compound and a rate of increase in slag; [FIG. 7] A diagram showing a relationship between additive fractions of a magnesium compound and an aluminum compound relative to coal ash and the molten liquid fraction in ash; [FIG. 8] A diagram showing a relationship between an MgO content rate in an additive and the molten liquid fraction in ash; [FIG. 9] A diagram showing a relationship among the MgO content rate in the additive, the molten liquid fraction in ash, and an amount of ash deposition; [FIG. 10] A diagram showing a distribution in particle diameter of coal ash; [FIG. 11] A diagram showing a relationship between an additive fraction of an MgO sample relative to coal ash and the amount of ash deposition; [FIG. 12] A diagram showing a relationship between the temperature and the molten liquid fraction in ash, and [FIG. 13] A diagram showing a relationship between the temperature and the shrinkage factor of ash. DESCRIPTION OF EMBODIMENTS [0023] Hereinafter, a preferred embodiment of this invention will be 9 described with reference to the drawings. [0024] <Embodiment 1> (Boiler structure) A product coal formed in a reforming method according to a first embodiment is used as a solid fuel for a boiler. As shown in FIG. 1, a boiler 7 is provided with hoppers 1, 2 for storing solid fuels, a supply amount regulators 3a, 3b for regulating a supply amount of the solid fuels supplied from the hoppers 1, 2, a mixer 4 for mixing the solid fuels supplied from the hoppers 1, 2, a pulverizer 5 for pulverizing the solid fuels mixed by the mixer 4 into a pulverized coal, a burner 6 for burning, as fuel, the pulverized coal supplied together with carrier air from the pulverizer 5, and a calculator 9 for controlling the supply amount regulators 3a, 3b. The boiler 7 collects heat generated through combustion of the pulverized coal. [00251 The hopper I and the hopper 2 respectively store the solid fuels which are different in ash properties from each other. Here, the solid fuels include coals, sludge carbides, biomass fuels, and others. It should be noted that the number of hoppers is not limited to two, and a single hopper or more than two hoppers may be employed. The supply amount of the solid fuel supplied from the hopper 1 to the mixer 4 is regulated by the supply amount regulator 3a, while the supply amount of the solid fuel supplied from the hopper 2 to the mixer 4 is regulated by the supply amount regulator 3b. [0026] 10 Although it is not illustrated in the drawing, the boiler 7 includes a furnace in which the pulverized coal supplied from the pulverizer 5 is burned by means of the burner 6 or the like to generate heat, and a set of heat exchanger tubes, which are arranged from an upper part to a downstream part of the furnace to cause heat exchange with a combustion gas directed to flow therein. The combustion gas generated in the boiler 7 is discharged from a chimney. In addition, the set of heat exchanger tubes is composed of an upper heat transfer unit in which a secondary heater, a tertiary heater, a final heater and a secondary reheater are arranged side by side at predetermined intervals in the upper part of the furnace, and a rear heat transfer unit in which a primary heater, a primary reheater and a coal economizer are arranged in a rear part of the furnace. [0027] The calculator 9 previously collects properties, such as a moisture content rate, a calorific value, an ash content rate, and an ash component composition, of the solid fuels and stores them as data. Using mixing ratios of the solid fuels as parameters, the calculator 9 calculates a composition of ash components in the mixed solid fuels based on the previously measured ash component composition of each solid fuel. Further, based on a relationship between molten liquid fractions (slag fractions) in ash and deposition fractions of ash, which are previously measured, the calculator 9 determines a value of a molten liquid fraction in ash (a reference value) associated with a low deposition fraction of ash of from approximately 5% to 7%. Then, the calculator 9 determines a mixing ratio for each solid fuel by thermodynamic equilibrium calculation so as to obtain an ash composition 11 that leads to the molten liquid fraction in ash of the determined reference value or smaller. Here, the supply amounts of the solid fuels to be used as fuel are determined in terms of a constant amount of heat input into the boiler. [00281 Then, the calculator 9 controls the supply amount regulators 3a, 3b based on the determined mixing ratio for each solid fuel. In this way, the supply amounts of the solid fuels from the hoppers 1, 2 to the boiler 7 are regulated. [0029] Here, the "molten liquid fraction in ash", which is an evaluation index for ash deposition characteristics used in this embodiment, represents a fraction of ash that turns into molten liquid (molten slag) at a certain temperature under a certain atmospheric condition among a fixed amount of ash in a solid state. Further, the "slag" represents the component melted through combustion in the boiler, suspended in combustion air inside the boiler, and carried by the stream of the combustion air while adhering to the furnace wall or the set of heat exchanger tubes. The molten liquid fraction in ash is calculated in accordance with each solid fuel and a mixing condition for each solid fuel. Here, the molten liquid fraction in ash is obtained by finding, using thermodynamic equilibrium calculation, a composition or a phase (a gas phase, a solid phase, or a liquid phase) of previously measured ash in each solid fuel in the most thermodynamically stable state, i.e. the state where Gibbs free energy (AG) is nearly zero under a certain condition (of temperature, atmospheric gas composition). The 12 composition of ash found in the above calculation is a composition of ash obtained from a plurality of coals, which have been mixed at a certain rate. [0030] It should be noted that the thermodynamic equilibrium calculation is performed using an atmospheric temperature and an atmospheric gas composition in a region close to the burner where ash noticeably deposits onto boiler walls. However, the thermodynamic equilibrium calculation is not limited to using the atmospheric temperature and the atmospheric gas composition in the region close to the burner, and may be carried out based on any atmospheric temperature and atmospheric gas composition in a desired region such as the set of heat exchanger tubes where ash can easily deposit. In this way, the molten liquid fraction in ash can be appropriately found for ash in each region inside the boiler, and an appropriate mixing ratio can be calculated for the plurality of solid fuels. Note that the thermodynamic equilibrium calculation is not limited to the above-described form, and may be carried out using the maximum atmospheric gas temperature specified by a design of the boiler and an atmospheric gas composition in the region corresponding to the maximum atmospheric gas temperature. Moreover, an atmospheric gas composition having the greatest reducing ability (including the highest concentration of a reducing gas such as CO or H 2 ) specified by the design of the heating furnace and the temperature in the region corresponding to the atmospheric gas composition may be similarly used. In this case, the mixing ratio can be determined without depending on a combustion temperature inside the furnace of the boiler. 13 [0031] It should be also noted that calculation of the molten liquid fraction in ash is not limited to the above-described way, and may be conducted based on molten liquid fractions in ash measured at each temperature in each atmospheric gas composition by heating ash in each solid fuel in advance. This allows for determination of the molten liquid fraction in ash adapted to actual boiler conditions. Furthermore, the molten liquid fraction in ash may be calculated from the shrinkage factor of actual coal ash using a Thermo Mechanical Analysis (TMA) device. [0032] In addition, the "deposition fraction of ash" is a fraction of a volume of deposited ash on an ash deposition probe inserted into the furnace of the boiler relative to a volume of impinging ash onto the ash deposition probe, and represents a tendency of ash to deposit. The deposition fraction of ash is expressed by the equation described below. Note that the "volume of impinging ash onto the ash deposition probe" is a total volume of ash that impinges onto a projected area of the ash deposition probe, and obtained based on a supply amount of solid fuel, an ash content rate in the solid fuel, and a furnace shape of the boiler. [0033] [Equation 1] Deposition Fraction of Ash [wt%] = (Volume of Deposited Ash [Kg] / Volume of Impinging Ash onto Ash Deposition Probe [Kg]) x 100 [0034] Note that the deposition fraction of ash may be calculated using a 14 combustion test furnace or an in-service boiler rather than the boiler 7. [0035] (Method for reforming starting coal) Next, described is a method for reforming a starting coal used as the solid fuel for the boiler with the above-described structure. [0036] As shown in FIG. 2, a starting coal such as a low-grade coal and a starting oil are firstly supplied to a mixing part 11 and mixed together. Then, an additive containing MgO of a magnesium compound (an inorganic compound) is supplied to the mixing part 11 and added to a mixture contained in the mixing part 11, to thereby form a starting slurry. [0037] The additive contains 70 wt% or more MgO, preferably, 90 wt% or more MgO. The average particle diameter of MgO is 5 gm or smaller, preferably approximately 0.2 pm. The additive fraction of MgO is between or equal to 25 wt% and 50 wt% of an inorganic component in the solid fuel. Note that the magnesium compound is not limited to MgO, which is an oxide, and may be MgCO3 or Mg(OH) 2 . [0038] Then, the starting slurry is supplied to a heating part 12 and preheated to around the boiling point of water at an operating pressure. After the preheating, the starting slurry is dewatered in oil under a condition, such as at 140 *C under a pressure of 4 atmospheres, for example, to remove moisture. [00391 15 Subsequently, the heated starting slurry is supplied to a solid-liquid separating part 13, and separated into solid and liquid components by any desired means such as sedimentation, centrifugal separation, filtration, or pressing. From the separated liquid component, a moisture content is drained out, while an oil content is recycled as the starting oil in the mixing part 11. On the other hand, the separated solid component is conveyed to a molding part 14, dried therein, and extracted as a product coal. The extracted product coal is used as solid fuel in the boiler 7 (refer to FIG. 1). [0040] (Relationship between molten liquid fraction in ash and deposition fraction of ash) Next, a relationship between the molten liquid fraction in ash and the deposition fraction of ash will be described. FIG. 3 shows the relationship between the molten liquid fraction in ash and the deposition fraction of ash obtained from a variety of blended coals at 1573 K that is a particular temperature at which ash can readily deposit inside the boiler. From FIG. 3, it can be seen that when the molten liquid fraction in ash exceeds 60 wt% at the atmospheric temperature and the atmospheric gas composition in the furnace, the deposition fraction of ash sharply increases. To put it another way, the deposition fraction of ash can be lowered by maintaining the molten liquid fraction in ash at or below 60 wt%. In this embodiment, the reference value, which is the value of the molten liquid fraction in ash associated with lower deposition fractions of ash, is 50 to 60 wt%. In FIG. 1, the calculator 9 determines the mixing ratio for each solid fuel through thermodynamic equilibrium calculation so as to obtain an ash 16 composition that provides the molten liquid fraction in ash lower than or equal to the defined reference value. [0041] (Relationship among temperature, molten liquid fraction in ash, and shrinkage factor of ash) Next, a relationship between the temperature and the molten liquid fraction in ash, and a relationship between the temperature and the shrinkage factor of ash will be described. FIG. 4 shows a calculated result of the molten liquid fraction in ash determined by the above-described way. FIG. 5 shows a result of finding the shrinkage fraction of ash while changing the temperature of an ash sample by means of the Thermo Mechanical Analysis (TMA) in which a load is applied to a substance to measure a deformation of the substance. As the ash sample, (a) a low grade coal added with no MgO (herein, ash in an upgraded brown coal), and (b) ash in an upgraded brown coal added with 25 wt% MgO are employed. Here, a higher shrinkage factor of ash means that the ash sample has a stronger tendency to change from solid to molten liquid (molten slag), while a higher temperature leads to increases of both the molten liquid fraction in ash and the shrinkage factor of ash. [0042] It is observed from FIGS. 4 and 5 that both the molten liquid fraction in ash calculated by thermodynamic equilibrium calculation and the measured shrinkage factor of ash are remarkably lowered by adding MgO to the ash sample. That is, the addition of MgO to the ash sample reduces the shrinkage factor of ash, thereby making ash more resistant to turning into 17 molten liquid, and in turn decreases the molten liquid fraction in ash. In particular, at around 1573 K at which ash can readily deposit inside the boiler, the molten liquid fraction in ash is decreased to approximately 40 wt%, and becomes lower than a threshold value (60 wt%) indicated in FIG. 3. In this way, because the deposition fraction of ash is reduced as shown in FIG. 3, there is a great possibility that the low-grade coal can be solely used for the boiler rather than the blended coal in which the low-grade coal containing ash with a low melting point is mixed with a high-grade bituminous coal. [0043] (Relationship between additive fraction of inorganic compound and rate of increase in slag) FIG. 6 shows a calculated result representing a relationship between additive fractions of various inorganic compounds and the rate of increase in slag, obtained by adding the various inorganic compounds to coal ash, and more specifically shows the rate of increase in slag at 1573 K of the temperature at which ash can readily deposit inside the boiler. Here, the "rate of increase in slag" is a ratio between volumes of formed slag before and after the inorganic compound is added, and expressed by the following equation. [0044] [Equation 2] Rate of increase in slag (%) = (Volume of formed slag after addition of inorganic compound [kg/hrl) / (Volume of formed slag before addition of inorganic compound [kg/hr]) x 100 18 [00451 Note that the volume of formed slag is obtained by multiplying the molten liquid fraction in ash by a weight of ash in coal to be supplied and a weight of the inorganic compound to be added. Specifically, the volume of formed slag ([kg/hr]) obtained before the addition of the inorganic compound is expressed by (the molten liquid fraction in ash [wt%] x the supply amount of coal [kg-dry base/hr] x the ash content rate [%]). On the other hand, the volume of formed slag obtained after the addition of the inorganic compound ([kg/hrl) is expressed by (the molten liquid fraction in ash [wt% x (the supply amount of coal [kg-dry base/hr] x the ash content rate [%] + the additive amount of the inorganic compound [kg/hr])). [0046] The rate of increase in slag of 100% shown in FIG. 6 denotes the volume of formed slag (a calculated value) of ash with the low melting point under a condition that no inorganic compound is added. When the rate of increase in slag lies below 100%, formation of slag is curbed. In general, as the additive fraction of the inorganic compound in coal ash increases, the rate of increase in slag is raised because inorganic substances contained in the coal are increased. Referring to MgO and A1 2 03, however, as the additive fractions thereof increase, the molten liquid fraction in ash is decreased, to thereby lower the rate of increase in slag as shown in FIG. 6. Therefore, it can be said that MgO and A1 2 0 3 are inorganic compounds having an effect of reducing deposition of ash with the increasing additive fraction. [0047] 19 (Relationship between additive fraction of magnesium compound relative to coal ash and molten liquid fraction in ash) FIG. 7 shows a relationship between the additive fractions of the magnesium compound and the aluminum compound relative to coal ash and the molten liquid fraction in ash. FIG. 7 shows the molten liquid fractions (calculated values) in ash as functions of additive fractions of MgO and A1 2 0 3 respectively changed at 1573 K of the particular temperature at which ash can readily deposit inside the boiler. As shown in FIG. 3, when the molten liquid fraction in ash reaches or exceeds 60 wt%, the deposition fraction of ash sharply increases. Here, as shown in FIG. 7, the MgO additive fraction related to the molten liquid fraction in ash of 60 wt% or lower is greater than or equal to 15 wt%. In this embodiment, the additive fraction of MgO added to the mixture in the mixing part 11 shown in FIG. 2 is between or equal to 25 wt% and 50 wt% of the inorganic component (coal ash) in the solid fuel. [0048] (Relationship between MgO content rate in additive and molten liquid fraction in ash) FIG. 8 shows a relationship between the MgO content rate in the additive and the molten liquid fraction in ash. The molten liquid fraction in ash decreased to or below 60 wt% corresponds to the MgO content rate at or above 70 wt%. Accordingly, as long as the additive contains 70 wt% or more MgO, preferably 90 wt% or more MgO, the molten liquid fraction in ash can be maintained at or below 60 wt%, to thereby reduce the deposition fraction of ash. In this embodiment, the additive, which contains 70 wt% or 20 more MgO, preferably 90 wt% or more MgO, is added to the mixture in the mixing part 11 of FIG. 2, to form the starting slurry. [0049] (Relationship among MgO content rate in additive, molten liquid fraction in ash, and amount of ash deposition) FIG. 9 shows a relationship between the MgO content rate in the additive to be added at a rate of 25 wt% of coal ash and the molten liquid fraction in ash and between the MgO content rate in the additive and an amount of ash deposition. When the MgO content rate in the additive is 70 wt% or greater, the molten liquid fraction in ash is maintained at or below 60 wt% at 1573 K of the particular temperature at which ash can readily deposit inside the boiler. It is also evident from FIG. 9 that as the MgO content rate in the additive increases, the amount of ash deposition decreases. For this reason, deposition of ash onto the boiler can be curbed by setting the MgO content rate in the additive to or above 70 wt%, preferably 90 wt% or more. In this embodiment, the additive, which contains 70 wt% or more MgO, preferably 90 wt% or more MgO, is added to the mixture in the mixing part 11 of FIG. 2, to form the starting slurry. [0050] (Particle diameter distribution in coal ash) FIG. 10 shows a particle diameter distribution in coal ash used in this embodiment. The coal ash used in this embodiment has an average particle diameter (a particle diameter (a median diameter) measured when an accumulated weight reaches 50%) of 6.8 pin. On the other hand, the average particle diameter of MgO to be added to the mixture in the mixing 21 part 11 of FIG. 2 is 5 pm or smaller, preferably approximately 0.2 pm. [0051] (Ash deposition characteristic test) Then, to demonstrate the effect of MgO for reducing ash deposition, an ash deposition characteristic test was carried out using a coal combustion furnace (furnace inner diameter of 400 mm, in-furnace effective height of 3650 mm) under a condition that the amount of heat input from coal and from city gas for heating is maintained constant at 149 kW. Here, a 25 wt% MgO sample and a 50 wt% MgO sample were respectively added to coal ash. For the MgO sample to be added to coal, three samples having average particle diameters of 10 pm, 5 pm, and 0.2 pm are used. The coal is a pulverized coal, and burned together with combustion air by a burner installed at the top of the furnace. Then, an ash deposition probe is inserted into the furnace below the burner on a location where a gaseous atmospheric temperature is 1573 K, and held for 100 minutes. After that, an amount (a weight) of ash deposition onto the surface of the ash deposition probe is measured. A result of the measurement is shown in FIG. 11. [0052] FIG. 11 shows a relationship between the additive fraction of the MgO sample relative to coal ash and the amount of ash deposition. Since the amount of ash deposition without the addition of MgO sample is 4.4 g ash/100min as shown in FIG. 11, the weight smaller than or equal to 4.4 g ash/100min indicates that reduction in deposition of ash is effected. From FIG. 11, it has been shown that the effect of reducing deposition of ash is 22 obtained by the addition of the MgO sample, which is 5 pm or smaller in average particle diameter. Namely, it can be understood that the effect of reducing deposition of ash is obtained by adding the MgO sample whose average particle diameter is smaller than that of ash. Further, the amount of ash deposition is smaller with respect to the MgO sample having the average particle diameter of 0.2 pjm than the MgO sample having the average particle diameter of 5 pm. From this, it is understood that as the average particle diameter of the added MgO sample becomes finer, a greater effect of reducing deposition of ash is obtained. In this embodiment, the average particle diameter of MgO added to the mixture in the mixing part 11 of FIG. 2 is set to or below 5 pm, preferably approximately 0.2 pm, i.e. smaller than the average particle diameter of coal ash, which is 6.8 pm. [0053] Moreover, it has been shown from a comparison performed between the conditions of adding the MgO sample by 25 wt% and by 50 wt% relative to the coal ash that the effects of reducing deposition of ash are substantially the same under the both conditions. Then, since the amount of ash deposition tends to almost converge on or above the additive fraction of 25 wt% of the MgO sample, the addition of the 25 wt% or more MgO sample can provide the effect of reducing deposition of ash. However, an excessively higher additive fraction of the MgO sample results in a higher rate of increase in slag. For this reason, the additive fraction of the MgO sample is preferably set to or below 50 wt%. In this embodiment, the additive fraction of MgO to be added to the mixture in the mixing part 11 of FIG. 2 is from 25 wt% to 50 wt% of the inorganic component in the solid fuel. 23 [0054] (Effect) As described above, although ash has the stronger tendency to change from solid to molten liquid as the shrinkage factor of ash increases, the magnesium compound added to the mixture of the starting coal and the starting oil causes a decrease in the shrinkage factor of ash, thereby making ash resistant to turning into the molten liquid. As a result, the molten liquid fraction in ash of the product coal is lowered. [00551 Further, the inorganic substances contained in the product coal increases with the increasing additive fraction of the inorganic compound in the product coal, resulting in the higher rate of increase in slag. Nevertheless, as the additive fraction of the magnesium compound increases, the molten liquid fraction in ash of the product coal is decreased to thereby lower the rate of increase in slag. [0056] Because the lowered solution fraction in ash leads to the reduced deposition fraction of ash of the product coal, the use of the product coal for the boiler allows for reduction in the amount of ash deposition onto the boiler. [0057] Still further, in this embodiment, by using the low-grade coal containing ash with the low melting point as the starting coal and adding the magnesium compound to the low-grade coal, the product coal is obtained in which the molten liquid fraction in ash is reduced, and the deposition 24 fraction of ash is accordingly lowered. This raises the possibility that the low-grade coal can be solely used for the boiler without having to mix the low-grade coal with the high-grade coal such as a bituminous coal. [00581 Thus, when the magnesium compound is added to the mixture of the starting coal and the starting oil, the molten liquid fraction in ash of the product coal can be decreased to thereby lower the deposition fraction of ash in the product coal. It is therefore possible to curb deposition of ash onto the boiler through the use of the thus-formed product coal for the boiler. [0059] Furthermore, in this embodiment, the magnesium compound is added in such a manner that the molten liquid fraction in ash of the product coal is decreased to or below 60 wt% at around 1573 K of the particular temperature at which ash can easily deposit inside the boiler. In this way, the molten liquid fraction in ash of the product coal is maintained at or below the reference value used for determining the mixing ratio of a plurality of solid fuels, with a result that the deposition fraction of ash is lowered. Accordingly, the amount of ash deposition onto the boiler can be reduced by using the thus-formed product coal for the boiler. [00601 Moreover, when the magnesium compound is added to the mixture of the starting coal and the starting oil at the additive fraction of from 25 wt% to 50 wt%, the molten liquid fraction in ash of the product coal can be suitably lowered. [0061] 25 Further, as the average particle diameter of the magnesium compound becomes smaller and finer than that of ash, the effect of reducing the deposition of ash is enhanced. Because the average particle diameter of ash is approximately 6.8 pm, the magnesium compound can be defined to be smaller than or equal to 5 pm in average particle diameter, to thereby suitably curb the deposition of ash onto the boiler. [0062] Furthermore, the molten liquid fraction in ash of the product coal can be suitably lowered by adding the additive, which contains 70 wt% or more magnesium compound, to the mixture of the starting coal and the starting oil. [0063] <Embodiment 2> Next, a second embodiment of the present invention will be described. Compared to the first embodiment, a difference in the second embodiment is that the product coal is prepared from the starting coal upgraded by adding an additive containing A1 2 0 3 , which is an aluminum compound (inorganic compound), to the mixture of the starting coal and the starting oil. The average particle diameter of A1 2 0 3 is smaller than or equal to 5 pm, preferably approximately 0.2 pm, while the additive fraction of A1 2 0 3 is between or equal to 25 wt% and 50 wt% of the inorganic component in the solid fuel. It should be noted that the aluminum compound is not limited to an oxide such as A1 2 0 3 , and may be a carbonate oxide or a hydroxide. [0064] 26 (Relationship among temperature, molten liquid fraction in ash, and shrinkage factor of ash) FIG. 12 shows a calculated result of the molten liquid fraction in ash determined by the way as described above other than adding A1 2 03. FIG. 13 shows a result of finding the shrinkage factor of ash while changing the temperature of an ash sample by means of the Thermo Mechanical Analysis (TMA) in which a load is applied to a substance to measure a deformation of the substance. As the ash sample, (c) a low-grade coal added with no A1203 (herein, ash in an upgraded brown coal), (d) ash in an upgraded brown coal added with 25 wt% A1 2 03, and (e) ash in an upgraded brown coal added with 50 wt% A1 2 03 are employed. Here, a higher shrinkage factor of ash promotes the ash sample to change from solid to molten liquid, while a higher temperature leads to increases of both the molten liquid fraction in ash and the shrinkage factor of ash. [0065] It is observed from FIGS. 12 and 13 that the addition of A1 2 0 3 to the ash sample leads to a remarkable decrease in both the molten liquid fraction in ash calculated by the thermodynamic equilibrium calculation and the measured shrinkage factor of ash. In other words, when Al20 3 is added to the ash sample, the shrinkage factor of ash is lowered, thereby making ash more resistant to turning into molten liquid, and the molten liquid fraction in ash is accordingly decreases. In particular, at around 1573 K at which ash can easily deposit inside the boiler, the molten liquid fraction in ash is decreased to approximately 60 wt% when 25 wt% A1 2 0 3 is added to ash in the upgraded brown coal, and decreased to approximately 30 wt% when 50 27 wt% A1 2 0 3 is added to ash in the upgraded brown coal. In both cases, the molten liquid fraction in ash lies below the threshold value (60 wt%) indicated in FIG. 3. Thus, because the deposition fraction of ash is decreased as shown in FIG. 3, there is a great possibility that the low-grade coal can be solely used for the boiler rather than the blended coal in which the low-grade coal containing ash with the low melting point is mixed with the high-grade bituminous coal. [0066] (Relationship between additive fraction of inorganic compound and rate of increase in slag) FIG. 6 shows the calculation result representing the relationship between the additive fractions of various inorganic compounds and the rate of increase in slag obtained by adding the various inorganic compounds to coal ash. FIG. 6 shows the rate of increase in slag at 1573 K of the particular temperature at which ash can easily deposit inside the boiler. In general, as the additive fraction of the inorganic compound in coal ash increases, the rate of increase in slag is raised because the inorganic substances contained in the coal are increased. As shown in FIG. 6, however, as the additive fractions of MgO and A1 2 0 3 increase, the molten liquid fraction in ash is decreased, and the rate of increase in slag is lowered accordingly. From this, it can be said that MgO and A12O 3 are inorganic compounds having the effect of reducing deposition of ash with the increasing additive fraction. [0067] (Relationship between additive fraction of aluminum compound relative to 28 coal ash and molten liquid fraction in ash) FIG. 7 shows the relationship between the additive fractions of the magnesium compound and the aluminum compound relative to coal ash and the molten liquid fraction in ash. FIG. 7 shows the molten liquid fraction (the calculated value) in ash as functions of the additive fractions of MgO and Al2O3 respectively changed at 1573 K of the particular temperature at which ash can readily deposit inside the boiler. Here, although the deposition fraction of ash sharply increases when the molten liquid fraction in ash reaches or exceeds 60 wt% as shown in FIG. 3, the additive fraction of A1 2 0 3 related to the molten liquid fraction in ash of 60 wt% or lower lies at or above 25 wt% as shown in FIG. 7. In this embodiment, the additive fraction of A120 3 added to the mixture in the mixing part 11 illustrated in FIG. 2 is between or equal to 25 wt% and 50 wt% of the inorganic component (coal ash) in the solid fuel. [0068] (Ash deposition characteristic test) In addition, from FIG. 11 showing the test result of the ash deposition characteristic test, it is observed that the addition of the inorganic compound whose average particle diameter is smaller than that of ash has the effect of reducing deposition of ash. It is also found that the finer average particle diameter of the inorganic compound to be added provides the greater effect of reducing deposition of ash. In this embodiment, the average particle diameter of Al20 3 added to the mixture in the mixing part 11 of FIG. 2 is set to or below 5 pam, preferably approximately 0.2 pm, i.e. smaller than the average particle diameter of coal 29 ash, which is 6.8 jm. [0069] The other structures are identical to those of the first embodiment, and descriptions related to the structures will not be repeated. [0070] (Effect) As described above, although ash has the strong tendency to change from solid to molten liquid as the shrinkage factor of ash increases, the aluminum compound added to the mixture of the starting coal and the starting oil causes a decrease in the shrinkage factor of ash, thereby making ash resistant to turning into the molten liquid. As a result, the molten liquid fraction (slag fraction) in ash of the product coal is lowered. [0071] Further, the inorganic substances contained in the product coal increases with the increasing additive fraction of the inorganic compound in the product coal, resulting in the higher rate of increase in slag. Nevertheless, referring to the aluminum compound, as its additive fraction increases, the molten liquid fraction in ash of the product coal is decreased to thereby lower the rate of increase in slag. [0072] The decreased molten liquid fraction in ash leads to the lowered deposition fraction of ash in product coal. Thus, the amount of ash deposition onto the boiler is reduced by using the product coal formed as described above for the boiler. [0073] 30 Still further, in this embodiment, by using the low-grade coal containing ash with the low melting point as the starting coal and adding the aluminum compound to the low-grade coal, the product coal is obtained in which the molten liquid fraction in ash is reduced, and the deposition fraction of ash is accordingly lowered. This raises the possibility that the low-grade coal can be solely used for the boiler without having to mix the low-grade coal with the high-grade coal such as a bituminous coal. [00741 As such, when the aluminum compound is added to the mixture of the starting coal and the starting oil, the molten liquid fraction in ash of the product coal can be decreased, to thereby lower the deposition fraction of ash in the product coal. It is therefore possible to curb deposition of ash onto the boiler through the use of the thus-formed product coal for the boiler. [0075] Furthermore, in this embodiment, the aluminum compound is added in such a manner that the molten liquid fraction in ash of the product coal is decreased to or below 60 wt% at around 1573 K of the particular temperature at which ash can readily deposit inside the boiler. In this way, the molten liquid fraction in ash of the product coal is maintained at or below the reference value used for determining the mixing ratio of multiple solid fuels, with a result that the deposition fraction of ash is lowered. Accordingly, the amount of ash deposition onto the boiler can be reduced by using the thus-formed product coal for the boiler. [00761 Moreover, when the aluminum compound is added to the mixture of 31 the starting coal and the starting oil at the additive fraction of from 25 wt% to 50 wt%, the molten liquid fraction in ash of the product coal can be suitably lowered. [0077] Further, as the average particle diameter of the aluminum compound becomes smaller and finer than that of ash, the effect of reducing the deposition of ash is enhanced. Because the average particle diameter of ash is approximately 6.8 pm, the aluminum compound can be defined to be smaller than or equal to 5 pm in average particle diameter, to thereby suitably curb the deposition of ash onto the boiler. [0078] (Modification of the embodiment) Although the embodiments of this invention have been described above, the embodiments are provided by way of illustration, and in particular, not intended to limit the present invention. Design modifications may be applied as appropriate to specific structures and others without departing from the scope defined in the appended claims. In addition, the functions and effects described in the embodiments for carrying out the invention are merely listing of most preferable functions and effects derived from the present invention, and the functions and effects are not limited to those described in the embodiments for carrying out the invention. The starting slurry may be formed by adding an additive, which contains both of the magnesium compound and the aluminum compound. [0079] This application is based on Japan Patent Application (No. 2010 32 164763) filed on July 22, 2010, which is incorporated herein by reference in its entirety. REFERENCE SIGNS LIST [0080] 1, 2: Hopper 3a, 3b: Supply amount regulator 4: Mixer 5: Pulverizer 6: Burner 7: Boiler 9: Calculator 11: Mixing part 12: Heating part 13: Solid-liquid separating part 14: Molding part 33