Patent Publication Number: US-2022220404-A1

Title: Low-sulfur coal production method

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a low-sulfur coal production method. 
     BACKGROUND ART 
     In an iron manufacturing process, when coal is used as a reducing material for iron ore, a part of sulfur contained in the coal dissolves as a solid in iron obtained by reducing the iron ore. If sulfur remains, toughness and workability of steel deteriorates, so that a great amount of effort has been made to remove sulfur from iron. 
     When coal is used as a heat source, a sulfur oxide is mixed in an exhaust gas, so that a great amount of effort has been required to remove a sulfur content from an exhaust gas from the standpoint of prevention of air pollution. 
     From such background, the industrial value is high if sulfur (sulfur content) in coal can be removed before the coal is used. 
     As a method of producing coal having a reduced sulfur content (low-sulfur coal), the claim of Patent Literature 1 describes “a chemical desulfurization method for coal, characterized in that an aqueous solution of caustic soda or caustic potash alone, or an aqueous solution of a mixture thereof is mixed with pulverized coal, and the resultant mixture is heated and reacted at a high temperature under an atmosphere of an oxygen gas or air or a mixture thereof, thereby removing a sulfur content in the coal.” 
     CITATION LIST 
     Patent Literatures 
     
         
         Patent Literature 1: JP 3-275795 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problems 
     In producing low-sulfur coal by desulfurizing coal (removing sulfur in coal), the conventional method had an insufficient desulfurization effect in some cases. 
     An object of the present invention is therefore to provide a low-sulfur coal production method having an excellent desulfurization effect. 
     Solution to Problems 
     The present inventors have made an intensive study and as a result found that when the configuration described below is employed, the foregoing object is achieved. The invention has been thus completed. 
     Specifically, the present invention provides the following [1] to [11]. 
     [1] A low-sulfur coal production method comprising bringing coal into contact with a chemical agent which is a mixed solution of hydrogen peroxide and acetic anhydride to thereby remove sulfur in the coal. 
     [2] The low-sulfur coal production method according to [1] above, wherein a molar ratio between the acetic anhydride and the hydrogen peroxide (acetic anhydride/hydrogen peroxide) is not less than 0.5 and not more than 12.0. 
     [3] The low-sulfur coal production method according to [1] or [2] above, wherein the acetic anhydride and the hydrogen peroxide are mixed before the chemical agent is brought into contact with the coal, and 
     wherein when 10 minutes or more have elapsed after the acetic anhydride and the hydrogen peroxide are mixed, the chemical agent is brought into contact with the coal. 
     [4] The low-sulfur coal production method according to any one of [1] to [3] above, wherein a mass ratio between the chemical agent and the coal (chemical agent/coal) is not less than 1.0. 
     [5] The low-sulfur coal production method according to any one of [1] to [4] above, wherein a temperature of the chemical agent at a time of being brought into contact with the coal is not less than 5° C. 
     [6] The low-sulfur coal production method according to any one of [1] to [5] above, wherein a temperature of the chemical agent at a time of being brought into contact with the coal is not more than 30° C. 
     [7] The low-sulfur coal production method according to any one of [1] to [6] above, wherein the coal comprises sub-bituminous coal. 
     [8] The low-sulfur coal production method according to any one of [1] to [7] above, wherein the coal that has been brought into contact with the chemical agent is heat-treated at a heat treatment temperature of not less than 150° C. 
     [9] The low-sulfur coal production method according to [8] above, wherein a heating rate at which the coal that has been brought into contact with the chemical agent is heated to the heat treatment temperature is not less than 10° C./min. 
     [10] The low-sulfur coal production method according to any one of [1] to [7] above, wherein the coal that has been brought into contact with the chemical agent is brought into contact with a hydrogen peroxide solution having a temperature of not more than 40° C. 
     [11] The low-sulfur coal production method according to [10] above, 
     wherein a concentration of the hydrogen peroxide solution is not less than 2.0 mass %, and 
     wherein a mass ratio between the hydrogen peroxide solution and the coal (hydrogen peroxide solution/coal) is not less than 1.0. 
     Advantageous Effects of Invention 
     The present invention can provide a low-sulfur coal production method having an excellent desulfurization effect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing a desulfurization rate with respect to a mass ratio between a chemical agent and coal (chemical agent/coal). 
         FIG. 2  is a graph (lower part) showing an amount of peracetic acid generated with respect to a temperature of a chemical agent, and a graph (upper part) showing a desulfurization rate (solid line) and a carbon yield (dashed line) with respect to a temperature of a chemical agent. 
         FIG. 3  is a schematic view showing an example of a facility for producing low-sulfur coal. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     [Low-Sulfur Coal Production Method] 
     The low-sulfur coal production method of the invention (hereinafter, also simply referred to as “the method of the invention”) is a low-sulfur coal production method comprising bringing coal into contact with a chemical agent which is a mixed solution of hydrogen peroxide and acetic anhydride to thereby remove sulfur in the coal. 
     &lt;Primary Treatment (Chemical Treatment)&gt; 
     First, described below is a primary treatment (chemical treatment) in which coal is brought into contact with a chemical agent which is a mixed solution of hydrogen peroxide and acetic anhydride. 
     Sulfur in coal is roughly classified into inorganic sulfur (inorganic sulfur content) and organic sulfur (organic sulfur content). 
     A typical example of inorganic sulfur is FeS 2 . Examples of organic sulfur include: an aromatic sulfur compound in which sulfur is present inside an aromatic ring such as dibenzothiophene; an aliphatic sulfur compound such as mercaptan. Of these, sulfur present inside an aromatic ring constituting coal is known to be particularly difficult to be removed. 
     The present inventors studied various chemical agents (desulfurization agents). As a result, it was found that peracetic acid effectively acts on thiophene form sulfur which is a component particularly difficult to be removed among organic sulfurs in coal, thereby successfully removing sulfur from coal or increasing an efficiency of converting sulfur into an easily removable form. It is assumed that by the action of peracetic acid, thiophene form sulfur is oxidized to be, for example, sulfone form sulfur or sulfide form sulfur, and a bond between carbon and sulfur is relatively weakened to be easily cut off, whereby the sulfur becomes easy to be separated. 
     Meanwhile, peracetic acid is easy to decompose. In the invention, therefore, a mixed solution of hydrogen peroxide and acetic anhydride (hereinafter, also simply referred to as “mixed solution”) is used as a chemical agent. The mixed solution generates peracetic acid which is a reaction product of hydrogen peroxide and acetic anhydride. The mixed solution as above is brought into contact with coal. 
     A reaction of hydrogen peroxide (H 2 O 2 ) and acetic anhydride ((CH 3 CO) 2 O) to obtain peracetic acid (CH 3 COO 2 H) and water (H 2 O) is represented by Formula (I) below. 
       2H 2 O 2 +(CH 3 CO) 2 O⇔2CH 3 COO 2 H+H 2 O  (I)
 
     In Formula (I) above, an equilibrium state changes depending on various conditions such as a temperature and a mixing ratio of a chemical agent. Therefore, the concentration of each component varies depending on the combination of the conditions. Suitable conditions will be described in detail below. 
     When a chemical agent is brought into contact with coal, inorganic sulfur which is easy to be removed dissolves and leaches into the chemical agent in the form of, for example, a sulfate ion. Similarly, a part of organic sulfur is also oxidized and leaches into the chemical agent in the form of, for example, a sulfate ion. Coal is desulfurized (i.e., sulfur in coal is removed) in this manner to thereby obtain coal having a reduced sulfur content (low-sulfur coal). 
     &lt;&lt;Molar Ratio (Acetic Anhydride/Hydrogen Peroxide)&gt;&gt; 
     A molar ratio between acetic anhydride and hydrogen peroxide (acetic anhydride/hydrogen peroxide) in a chemical agent is preferably not less than 0.1 and more preferably not less than 0.5 because peracetic acid which is a reaction product can be formed in a proper amount and the desulfurization effect can become more excellent. 
     Further, when the molar ratio (acetic anhydride/hydrogen peroxide) is within the foregoing range, acetic anhydride can be prevented from becoming excessive with respect to hydrogen peroxide, and residual hydrogen peroxide in the mixed solution can be minimized (as described below, hydrogen peroxide decreases a carbon yield of coal). 
     The molar ratio (acetic anhydride/hydrogen peroxide) is preferably not more than 15.0 and more preferably not more than 12.0. When the molar ratio (acetic anhydride/hydrogen peroxide) is within the foregoing range, as in the above, peracetic acid which is a reaction product can be formed in a proper amount, so that the desulfurization effect can become more excellent. Further, the generated peracetic acid is prevented from being diluted with excessive acetic anhydride. 
     The molar ratio (acetic anhydride/hydrogen peroxide) is calculated as follows. 
     First, a molar amount [mol] of each component (acetic anhydride or hydrogen peroxide) in a chemical agent is represented by Formula (a) below. Therefore, the molar ratio between acetic anhydride and hydrogen peroxide (acetic anhydride/hydrogen peroxide) in the chemical agent is calculated by Formula (b) below. 
       Molar amount=(Li×Ci)/(100×Mi)  (a)
 
       Molar ratio=(L1×C1×M2)/(L2×C2×M1)  (b)
 
     Li: amount of i aqueous solution [g/h] 
     Ci: concentration of i aqueous solution [mass %] 
     Mi: molecular weight of i [g/mol] 
     Here, i is 1 or 2, 1 is acetic anhydride and 2 is hydrogen peroxide. 
     The molecular weight of acetic anhydride is assumed to be 102, and the molecular weight of hydrogen peroxide is assumed to be 34. The amount of an aqueous solution Li is adjusted such that the desired molar ratio (acetic anhydride/hydrogen peroxide) is obtained. 
     &lt;&lt;Elapsed Time after Mixing&gt;&gt; 
     The reaction (forward reaction) of Formula (I) above has a relatively slow rate. Therefore, generation of peracetic acid is insufficient immediately after acetic anhydride and hydrogen peroxide are mixed in some cases. 
     The present inventors determined the quantities of various reaction rates and found out that it takes about 10 minutes for the reaction of Formula (I) above to settle into a steady state. 
     In the invention, therefore, it is preferable that acetic anhydride and hydrogen peroxide are mixed before a chemical agent is brought into contact with coal, and when 10 minutes or more have elapsed after this mixing, the chemical agent is brought into contact with the coal. This allows peracetic acid to be sufficiently generated, whereby the desulfurization effect of removing sulfur in coal can become more excellent. Further, this allows peracetic acid hydrogen to be decreased, whereby decrease in a carbon yield due to a reaction of hydrogen peroxide with coal can be minimized. 
     The elapsed time after mixing is more preferably not less than 20 minutes and even more preferably not less than 30 minutes and, at the same time, preferably not more than 120 minutes, more preferably not more than 90 minutes, and even more preferably not more than 60 minutes. 
     &lt;&lt;Mass Ratio (Chemical Agent/Coal)&gt;&gt; 
     The present inventors studied a mass ratio between a chemical agent and coal (chemical agent/coal). In this study, a chemical agent having a molar ratio between acetic anhydride and hydrogen peroxide (acetic anhydride/hydrogen peroxide) of 5.0 was used. 
       FIG. 1  is a graph showing a desulfurization rate with respect to a mass ratio between a chemical agent and coal (chemical agent/coal). As shown in the graph of  FIG. 1 , as the amount of a chemical agent with respect to coal increases, the desulfurization rate increases, so that the desulfurization effect becomes more excellent. Therefore, the mass ratio (chemical agent/coal) is preferably not less than 0.5, more preferably not less than 1.0 and even more preferably not less than 2.0. 
     As shown in the graph of  FIG. 1 , when the amount of a chemical agent becomes excessive with respect to the amount of coal, the desulfurization rate barely changes. The mass ratio (chemical agent/coal) is preferably not more than 100.0 and more preferably not more than 50.0 for the sake of reducing the amount of a chemical agent used. 
     When a mass of coal (solid content) before desulfurization is W 1  [kg], a sulfur content of coal (solid content) before desulfurization is % S 1  [mass*], a mass of coal (solid content) after desulfurization is W 2  [kg], and a sulfur content of coal (solid content) after desulfurization is % S 2  [mass %], the desulfurization rate [mass %] is defined by Formula (1) below. 
       Desulfurization rate[mass %]=100×{1−(W 2 ×% S 2 )/(W 1 ×% S 1 )}  (1)
 
     &lt;&lt;Temperature of Chemical Agent&gt;&gt; 
     The present inventors also studied a temperature of a chemical agent at the time of being brought into contact with coal (hereinafter, also simply referred to as “a temperature of a chemical agent”). In this study, a chemical agent having a molar ratio between acetic anhydride and hydrogen peroxide (acetic anhydride/hydrogen peroxide) of 5.0 was used. 
       FIG. 2  provides a graph (lower part) showing an amount of peracetic acid generated with respect to a temperature of a chemical agent, and a graph (upper part) showing a desulfurization rate (solid line) and a carbon yield (dashed line) with respect to a temperature of a chemical agent. The amount of peracetic acid generated is an index obtained by setting a calculated value at the time when the reaction contributing substances (hydrogen peroxide and acetic anhydride) completely react to 1.0. 
     As shown in the graphs (lower and upper parts) of  FIG. 2 , when the temperature of a chemical agent at the time of being brought into contact with coal is high, the amount of peracetic acid generated is large, and the desulfurization rate is high, so that the desulfurization effect becomes more excellent. In connection with this, the temperature of a chemical agent is preferably not less than 5° C., more preferably not less than 10° C., even more preferably not less than 20° C. and particularly preferably not less than 25° C. 
     On the other hand, as shown in the graph (upper part) of  FIG. 2 , the temperature of a chemical agent is preferably not too high in order to maintain a high carbon yield. Specifically, the temperature is preferably not more than 40° C., more preferably not more than 35° C. and even more preferably not more than 30° C. because the carbon yield can become more excellent. 
     When a carbon content of coal (solid content) before desulfurization is % C1 [mass %] and a carbon content of coal (solid content) after desulfurization is % C2 [mass %], the carbon yield [mass %] is defined by Formula (2) below. 
       Carbon yield[mass %]=100×(W 2 ×% C 2 )/(W 1 ×% C 1 )   (2)
 
     The presumable reason why the carbon yield decreases is described below. 
     Hydrogen peroxide and peracetic acid may become an oxidizing agent which may destroy a skeleton of coal, and in this case, the carbon yield unintentionally decreases simultaneously with removal of sulfur. The present inventors found, through a study, that peracetic acid first causes cutting off of a bond between sulfur and carbon of thiophene form sulfur, and thereafter destroy of a carbon skeleton (carbon-carbon bond) occurs. The degree of destroy of a carbon skeleton is low with peracetic acid and high with hydrogen peroxide. In particular, it is remarkable with hydrogen peroxide having a high temperature. 
     Therefore, by appropriately controlling a condition when a chemical agent is brought into contact with coal (for example, preventing the temperature of a chemical agent from becoming too high, or appropriately adjusting the mixing ratio of hydrogen peroxide in a mixed solution), the thiophene form sulfur can be effectively removed while the destroy of a carbon skeleton is minimized. 
     &lt;Coal&gt; 
     While the coal used in the invention is not particularly limited and a wide variety of coals can be used, the coal preferably includes coal having a moderate degree of coalification such as sub-bituminous coal, more preferably includes sub-bituminous coal and even more preferably is sub-bituminous coal. 
     When such coal is used, the desulfurization effect tends to be more excellent than that in the case where coal having a high degree of coalification such as anthracite coal is used, and the carbon yield tends to be more excellent than that in the case where coal having a low degree of coalification such as brown coal is used. 
     The grain size (mean grain size) of coal used in the invention is not particularly limited. For example, even when the grain size of coal is on the order of several millimeters, there is no significant change in desulfurization performance. When the grain size of coal is equal to or larger than this, a mild pulverization treatment may be performed as necessary. 
     The primary treatment (chemical treatment) for desulfurizing coal was described above. 
     Next, two types of secondary treatments are described as a treatment for further removing sulfur remaining in coal having been desulfurized by the primary treatment. 
     &lt;Secondary Treatment (Heat Treatment)&gt; 
     By the action of peracetic acid which is a reaction product of hydrogen peroxide and acetic anhydride, thiophene form sulfur which is difficult to be removed is changed into an easily removable form; therefore, the thiophene form sulfur can be removed by a heat treatment at a relatively low temperature (about 150° C.) 
     That is, it is preferable that a heat treatment is further performed on coal which has been brought into contact with a chemical agent because the desulfurization effect can become more excellent. The heat treatment temperature is preferably not less than 150° C., more preferably not less than 250° C., and even more preferably not less than 350° C. 
     Note that a hydrocarbon-containing gas derived from coal and generated by a heat treatment can be recovered and used as a part of a gaseous fuel in an iron manufacturing process. In consideration of performing a heat treatment using, for example, exhaust heat generated at a factory such as ironworks, a heat treatment at a temperature of up to several hundreds Celsius is preferred. 
     One example of a furnace for subjecting coal to a heat treatment in iron manufacturing process is a coke oven. The heat treatment temperature in a coke oven is about 1000 to 1200° C., and the coke oven may be operated at a temperature at or above 1200° C. Coal that has been brought into contact with a chemical agent and desulfurized may be introduced into a coke oven to produce low-sulfur coke. While a hydrocarbon gas and a sulfur-containing gas are generated in this case, the sulfur-containing gas can be separately removed. The generated gas after the sulfur-containing gas is removed can be reused as a fuel gas. 
     Among processes for subjecting coal to a heat treatment, a process having the highest temperature is probably substantially a process of producing coke. As a result of experiments conducted by the present inventors, it was confirmed that a sufficient desulfurization effect was also exhibited even with a heat treatment temperature in a coke oven. 
     Therefore, the heat treatment temperature is, for example, not more than 1300° C. 
     Coal that has been subjected to a heat treatment at about 600° C. is generally called semi-coke. Coal that has been brought into contact with a chemical agent and desulfurized can also be used in producing semi-coke. Since semi-coke is generally inferior in strength to coke, it can hardly be used as coke for a blast furnace, but it can be used for other applications. In particular, semi-coke containing less sulfur is useful as, for example, a heating agent (carburizing material) used for heating in a converter. 
     It is preferable that a heating rate at which coal that has been brought into contact with a chemical agent is heated to the heat treatment temperature (hereinafter, also simply referred to as “heating rate”) is higher. This is because a sulfur compound which has been changed into a form allowing desulfurization by the action of a mixed solution of hydrogen peroxide and acetic anhydride may be resynthesized into thiophene form sulfur which is difficult to desulfurize under heating, and this resynthesis is suppressed. Specifically, the heating rate is preferably not less than 10° C./min and more preferably not less than 20° C./min. 
     While the upper limit of the heating rate is not particularly limited, realization of an excessively high heating rate is difficult for technical and industrial (cost) reasons. Therefore, the heating rate is, for example, not more than 100° C./min. 
     &lt;Secondary Treatment (Hydrogen Peroxide Treatment)&gt; 
     The present inventors found, through the study, that for further desulfurizing coal that has been brought into contact with a chemical agent, a treatment using low-temperature hydrogen peroxide may be performed separately from the above-described heat treatment. 
     When hydrogen peroxide acts on coal that has not been subjected to the primary treatment (chemical treatment), as described above, a carbon skeleton is destroyed, and the carbon yield decreases. However, since a sulfur content remaining in coal that has been subjected to the primary treatment is in an easily removable form, the coal can be easily additionally desulfurized with hydrogen peroxide. 
     That is, it is preferable that the coal that has been brought into contact with the chemical agent is further brought into contact with a hydrogen peroxide solution having a low temperature. 
     The temperature of a hydrogen peroxide solution is preferably not more than 50° C. and more preferably not more than 40° C. The oxidizing ability of hydrogen peroxide becomes increasingly strong as the temperature of the hydrogen peroxide becomes high, and not only the desulfurization effect but also the carbon yield tends to decrease. When the temperature of a hydrogen peroxide solution is within the above range, the desulfurization effect is further excellent, and the carbon yield is also good. 
     The lower limit thereof is not particularly limited, and the temperature of a hydrogen peroxide solution is, for instance, not less than 5° C. 
     The concentration of a hydrogen peroxide solution (the content of hydrogen peroxide in a hydrogen peroxide solution) is preferably not less than 2.0 mass % and more preferably not less than 3.0 mass % because the desulfurization effect can become more excellent. 
     When the concentration of a hydrogen peroxide solution is not less than 3.0 mass %, the effect thus obtained is substantially constant regardless of the concentration of a hydrogen peroxide solution. Therefore, the upper limit thereof is not particularly limited, and the concentration of a hydrogen peroxide solution is preferably not more than 35.0 mass %, for instance. 
     Hydrogen peroxide is often commercially available as an aqueous solution of 30 to 35 mass % because it is easy to decompose on the high concentration side. In the present invention, such a commercially available aqueous solution may be appropriately diluted and used. 
     [Facility for Producing Low-Sulfur Coal] 
     Next, an example in which the present invention is implemented using a specific facility will be described with reference to  FIG. 3 . 
       FIG. 3  is a schematic view showing an example of a facility for producing low-sulfur coal (hereinafter, also simply referred to as “production facility”). 
     The production facility shown in  FIG. 3  has a hydrogen peroxide storage tank  1  for storing hydrogen peroxide and an acetic anhydride storage tank  3  for storing acetic anhydride. 
     The hydrogen peroxide inside the hydrogen peroxide storage tank  1  is supplied to a chemical agent mixing tank  5  via a hydrogen peroxide transport pipe  2 . The acetic anhydride inside the acetic anhydride storage tank  3  is supplied to the chemical agent mixing tank  5  via an acetic anhydride transport pipe  4 . The hydrogen peroxide transport pipe  2  and the acetic anhydride transport pipe  4  are each provided with a suitable flow rate control device (not shown), and the flow rates of the hydrogen peroxide and the acetic anhydride can be controlled. 
     The chemical agent mixing tank  5  is provided with a heating device  6  and a mixing device  7 . The hydrogen peroxide and the acetic anhydride supplied to the chemical agent mixing tank  5  are heated to a predetermined temperature using the heating device  6  as necessary and mixed using the mixing device  7 . 
     A chemical agent which is a mixed solution obtained by mixing in the chemical agent mixing tank  5  is supplied to a desulfurization treatment tank  9  via a chemical agent transport pipe  8 . The chemical agent transport pipe  8  is provided with a suitable flow rate control device (not shown), and the flow rate of the chemical agent can be controlled. 
     The desulfurization treatment tank  9  is further supplied with coal from a coal storage tank  10  for storing coal via a coal transport pipe  11 . The coal transport pipe  11  is provided with a suitable flow rate control device (not shown), and the flow rate of the coal can be controlled. 
     The desulfurization treatment tank  9  is provided with a heating device  12 . The heating device  12  controls the chemical agent supplied from the chemical agent mixing tank  5  and the coal supplied from the coal storage tank  10  to an appropriate temperature as necessary. Further, the desulfurization treatment tank  9  is provided with a mixing device  13 . The mixing device  13  mixes the chemical agent and the coal well as necessary. 
     Thus, in the desulfurization treatment tank  9 , the coal is brought into contact with the chemical agent and desulfurized, thereby obtaining coal with low sulfur content (low-sulfur coal) (hereinafter, also referred to as “chemical-treated coal”) 
     The desulfurization treatment tank  9  is provided with discharge holes at two places. A chemical agent circulation pipe  14  is provided at one discharge hole. Peracetic acid may remain in a part of the chemical agent after use in desulfurization of the coal. In this case, the chemical agent may be flown back from the desulfurization treatment tank  9  to the chemical agent mixing tank  5  and reused. 
     However, sulfur may leach into the chemical agent after desulfurization. Reuse of the chemical agent into which sulfur leaches may adversely affect desulfurization. Therefore, a chemical agent discharge pipe  15  is connected to the chemical agent circulation pipe  14 , and a part or all of the chemical agent after desulfurization can be discharged through the chemical agent discharge pipe  15 . 
     A chemical-treated coal transport pipe  16  is provided at the other discharge hole of the desulfurization treatment tank  9 . The chemical-treated coal transport pipe  16  is further branched into three pipes, i.e., a chemical-treated coal discharge pipe  16   a , a heat treatment device connection pipe  16   b  and a hydrogen peroxide treatment device connection pipe  16   c.    
     The chemical-treated coal discharge pipe  16   a  discharges the chemical-treated coal obtained in the desulfurization treatment tank  9  without performing the secondary treatment. The heat treatment device connection pipe  16   b  transports the chemical-treated coal to a heat treatment device  17 . The hydrogen peroxide treatment device connection pipe  16   c  transports the chemical-treated coal to a hydrogen peroxide treatment device  23 . 
     First, the heat treatment device  17  will be described. 
     When low-sulfur coal (chemical-treated coal) is subjected to a heat treatment in the heat treatment device  17 , sulfur is further volatilized, so that the desulfurization proceeds further. The coal that has been subjected to the heat treatment in the heat treatment device  17  and has been further reduced in sulfur content (hereinafter, also referred to as “heat-treated coal”) is taken out through a heat-treated coal discharge pipe  18  and used for a predetermined use. 
     Further, the heat treatment device  17  is provided with a heat treatment gas exhaust pipe  19 . A gas generated by a heat treatment may include a combustible gas. In this case, the gas can be taken out through the heat treatment gas discharge pipe  19  and used for a predetermined use. 
     Next, the hydrogen peroxide treatment device  23  will be described. 
     The hydrogen peroxide treatment device  23  is supplied with the chemical-treated coal via the hydrogen peroxide treatment device connection pipe  16   c . In the hydrogen peroxide treatment device  23 , the chemical-treated coal is subjected to the above-described secondary treatment (hydrogen peroxide treatment). 
     The hydrogen peroxide treatment device  23  is supplied with the hydrogen peroxide via a hydrogen peroxide supply pipe  20 . The hydrogen peroxide supply pipe  20  is connected to the hydrogen peroxide storage tank  1 . When the hydrogen peroxide is diluted, water may be supplied from a dilution water tank  21  through a dilution water supply pipe  22 . Another hydrogen peroxide storage tank (not shown) may be provided exclusively for the hydrogen peroxide treatment device  23 . 
     The hydrogen peroxide treatment device  23  is provided with a cooling device  24 . The cooling device  24  controls a temperature inside the hydrogen peroxide treatment device  23  to an appropriate temperature as necessary. 
     Further, the hydrogen peroxide treatment device  23  is provided with a mixing device  25 . The mixing device  25  mixes the hydrogen peroxide solution and the chemical-treated coal well as necessary. 
     The hydrogen peroxide treatment device  23  is provided with discharge holes at two places. 
     A hydrogen peroxide circulation pipe  27  is provided at one discharge hole. Hydrogen peroxide may remain in a part of the hydrogen peroxide solution after use in desulfurization of the coal (chemical-treated coal). In this case, the hydrogen peroxide solution may be flown back from the hydrogen peroxide treatment device  23  to the hydrogen peroxide storage tank  1  and reused. A destination of the flowback may be a separately provided hydrogen peroxide storage tank (not shown) or the chemical agent mixing tank  5 . 
     However, sulfur may leach into the hydrogen peroxide solution after desulfurization. Reuse of the hydrogen peroxide solution into which sulfur leaches may adversely affect desulfurization. Therefore, a hydrogen peroxide discharge pipe  28  is connected to the hydrogen peroxide circulation pipe  27 , and a part or all of the hydrogen peroxide solution after desulfurization can be discharged through the hydrogen peroxide discharge pipe  28 . 
     A discharge pipe  26  is connected to the other discharge hole of the hydrogen peroxide treatment device  23 . Coal that has been further desulfurized inside the hydrogen peroxide treatment device  23  (hereinafter, also referred to as “hydrogen peroxide-treated coal”) is taken out through the discharge pipe  26  and used for a predetermined use. 
     Note that since the chemical-treated coal transported to the heat treatment device  17  or the hydrogen peroxide treatment device  23  is already reduced in sulfur content, it may be taken out through the heat-treated coal discharge pipe  18  or the discharge pipe  26  without being subjected to the secondary treatment (heat treatment or hydrogen peroxide treatment). 
     Each part of the production facility described with reference to  FIG. 3  need not have a special specification, and existing devices can be used as appropriate. For example, the heat treatment device  17  may be a heat exchanger using exhaust heat as a heat source, and it may be a furnace such as a semi-coke oven or a coke oven. 
     EXAMPLES 
     The present invention is specifically described below with reference to examples. However, the present invention should not be construed as being limited to the following examples. 
     Examples 1 to 16 and Comparative Example 1 
     By using the production facility described with reference to  FIG. 3 , a test was conducted in which coal was desulfurized to produce low-sulfur coal by the method of the present invention. 
     As the coal, at least one selected from the group consisting of Coal A (sub-bituminous coal), Coal B (sub-bituminous coal) and Coal C (semi-anthracite coal) was used. The details of the coals used are shown in Table 1 below. The granularity of each coal was about 300 μm in a mean grain size. With all coals, permeability of peracetic acid is high, and the desulfurization performance did not vary greatly depending on the granularity. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Industrial analysis value 
                 Industrial analysis value 
               
               
                   
                 [mass % d.a.f.] 
                 [mass % d.b.] 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 C 
                 H 
                 N 
                 S 
                 V.M 
                 Ash 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Coal A 
                 78.5 
                 4.6 
                 0.8 
                 0.2 
                 38.2 
                 6.8 
               
               
                 Coal B 
                 77.1 
                 4.9 
                 1.5 
                 0.5 
                 33.2 
                 6.7 
               
               
                 Coal C 
                 82.1 
                 1.2 
                 1.4 
                 2 
                 9.4 
                 8.1 
               
               
                   
               
            
           
         
       
     
     In Table 1 above, “d.a.f” indicates a dry ash free basis, and means an analytical value of net coal excluding moisture and ash. 
     “d.b.” means an analysis value on a dry basis. 
     “V.M” means a content of volatile matter in industrial analysis. 
     “Ash” means a content of ash in industrial analysis. 
     Test conditions such as supply amounts (flow rates) of coal, hydrogen peroxide and acetic anhydride are shown in Table 2 below. 
     In Examples 1 to 7 and Comparative Example 1, only the above-described primary treatment (chemical treatment) was performed. That is, the coal after being brought into contact with the chemical agent was taken out, and the desulfurization rate and the carbon yield were determined. 
     In Examples 8 to 11, the above-described secondary treatment (heat treatment) was further performed. That is, after the primary treatment (chemical treatment), the coal was further introduced into the heat treatment device capable of raising the temperature to 1200° C. and then subjected to heat treatment under a nitrogen atmosphere, and the desulfurization rate and the carbon yield after the heat treatment were determined. 
     In Examples 12 to 16, the above-described secondary treatment (hydrogen peroxide treatment) was further performed. That is, after the primary treatment (chemical treatment), the coal was further introduced into the hydrogen peroxide treatment device and then subjected to the hydrogen peroxide treatment, and the desulfurization rate and the carbon yield after the hydrogen peroxide treatment were determined. 
     In the primary treatment, an aqueous solution having a concentration of hydrogen peroxide of 35 mass % was used as hydrogen peroxide. As acetic anhydride, acetic anhydride having a purity of 99 mass % was used. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 Example 
                 Example 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Unit 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
               
               
                   
               
               
                 Coal 
                 Coal A 
                 g/h 
                 100 
                 50 
                 0 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                   
                 Coal B 
                 g/h 
                 0 
                 50 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 Coal C 
                 g/h 
                 0 
                 0 
                 100 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 Total amount 
                 g/h 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 Chemical 
                 Hydrogen peroxide 
                 g/h 
                 50 
                 100 
                 50 
                 95 
                 50 
                 20 
                 240 
                 50 
                 50 
               
               
                 agent and 
                 Acetic anhydride 
                 g/h 
                 265 
                 55 
                 265 
                 40 
                 350 
                 70 
                 60 
                 265 
                 265 
               
               
                 primary 
                 Molar ratio 
                 mol/mol 
                 5.0 
                 0.5 
                 5.0 
                 0.4 
                 6.6 
                 3.3 
                 0.2 
                 5.0 
                 5.0 
               
               
                 treatment 
                 (acetic anhydride/ 
               
               
                 (chemical 
                 hydrogen peroxide) 
               
               
                 treatment) 
                 Elapsed time after 
                 min 
                 30 
                 60 
                 30 
                 30 
                 8 
                 30 
                 30 
                 30 
                 30 
               
               
                   
                 mixing 
               
               
                   
                 Mass ratio (chemical 
                 g/g 
                 3.2 
                 1.6 
                 3.2 
                 1.4 
                 4.0 
                 0.9 
                 3.0 
                 3.2 
                 3.2 
               
               
                   
                 agent/coal) 
               
               
                   
                 Chemical agent 
                 ° C. 
                 20 
                 22 
                 20 
                 20 
                 17 
                 20 
                 35 
                 20 
                 20 
               
               
                   
                 temperature 
               
               
                   
                 Desulfurization rate 
                 mass % 
                 52 
                 51 
                 50 
                 48 
                 51 
                 45 
                 58 
                 52 
                 52 
               
               
                   
                 (after primary 
               
               
                   
                 treatment) 
               
               
                   
                 Carbon yield 
                 mass % 
                 96 
                 93 
                 97 
                 96 
                 94 
                 96 
                 90 
                 96 
                 96 
               
               
                   
                 (after primary 
               
               
                   
                 treatment) 
               
               
                 Secondary 
                 Heat treatment 
                 ° C. 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 150 
                 1200 
               
               
                 treatment 
                 temperature 
               
               
                 (heat 
                 Heating rate 
                 ° C./min 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 20 
                 25 
               
               
                 treatment) 
                 Desulfurization rate 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 65 
                 68 
               
               
                   
                 (after secondary 
               
               
                   
                 treatment) 
               
               
                   
                 Carbon yield 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 95 
                 94 
               
               
                   
                 (after secondary 
               
               
                   
                 treatment) 
               
               
                 Secondary 
                 Temperature of 
                 ° C. 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 treatment 
                 hydrogen peroxide 
               
               
                 (hydrogen 
                 solution 
               
               
                 peroxide 
                 Concentration of 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 treatment) 
                 hydrogen peroxide 
               
               
                   
                 solution 
               
               
                   
                 Mass ratio (hydrogen 
                 g/g 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 peroxide solution/coal) 
               
               
                   
                 Desulfurization rate 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 (after secondary 
               
               
                   
                 treatment) 
               
               
                   
                 Carbon yield 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 (after secondary 
               
               
                   
                 treatment) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Comparative 
               
               
                   
                   
                   
                 Example 
                 Example 
                 Example 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Unit 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 1 
               
               
                   
               
               
                 Coal 
                 Coal A 
                 g/h 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                   
                 Coal B 
                 g/h 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 Coal C 
                 g/h 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 Total amount 
                 g/h 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 Chemical 
                 Hydrogen peroxide 
                 g/h 
                 50 
                 50 
                 50 
                 50 
                 50 
                 50 
                 50 
                 300 
               
               
                 agent and 
                 Acetic anhydride 
                 g/h 
                 265 
                 265 
                 265 
                 265 
                 265 
                 265 
                 265 
                 0 
               
               
                 primary 
                 Molar ratio 
                 mol/mol 
                 5.0 
                 5.0 
                 5.0 
                 5.0 
                 5.0 
                 5.0 
                 5.0 
                 0.0 
               
               
                 treatment 
                 (acetic anhydride/ 
               
               
                 (chemical 
                 hydrogen peroxide) 
               
               
                 treatment) 
                 Elapsed time after 
                 min 
                 30 
                 30 
                 30 
                 30 
                 30 
                 30 
                 30 
                 — 
               
               
                   
                 mixing 
               
               
                   
                 Mass ratio (chemical 
                 g/g 
                 3.2 
                 3.2 
                 3.2 
                 3.2 
                 3.2. 
                 3.2 
                 3.2 
                 3.0 
               
               
                   
                 agent/coal) 
               
               
                   
                 Chemical agent 
                 ° C. 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                   
                 temperature 
               
               
                   
                 Desulfurization rate 
                 mass % 
                 52 
                 52 
                 52 
                 52 
                 52 
                 52 
                 52 
                 2.8 
               
               
                   
                 (after primary 
               
               
                   
                 treatment) 
               
               
                   
                 Carbon yield 
                 mass % 
                 96 
                 96 
                 96 
                 96 
                 96 
                 96 
                 96 
                 96 
               
               
                   
                 (after primary 
               
               
                   
                 treatment) 
               
               
                 Secondary 
                 Heat treatment 
                 ° C. 
                 100 
                 150 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 treatment 
                 temperature 
               
               
                 (heat 
                 Heating rate 
                 ° C./min 
                 10 
                 5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 treatment) 
                 Desulfurization rate 
                 mass % 
                 53 
                 61 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 (after secondary 
               
               
                   
                 treatment) 
               
               
                   
                 Carbon yield 
                 mass % 
                 95 
                 95 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 (after secondary 
               
               
                   
                 treatment) 
               
               
                 Secondary 
                 Temperature of 
                 ° C. 
                 — 
                 — 
                 20 
                 40 
                 45 
                 30 
                 30 
                 — 
               
               
                 treatment 
                 hydrogen peroxide 
               
               
                 (hydrogen 
                 solution 
               
               
                 peroxide 
                 Concentration of 
                 mass % 
                 — 
                 — 
                 35.0 
                 35.0 
                 5.0 
                 1.5 
                 3.0 
                 — 
               
               
                 treatment) 
                 hydrogen peroxide 
               
               
                   
                 solution 
               
               
                   
                 Mass ratio (hydrogen 
                 g/g 
                 — 
                 — 
                 2.5 
                 2.5 
                 2.5 
                 2.5 
                 0.9 
                 — 
               
               
                   
                 peroxide solution/coal) 
               
               
                   
                 Desulfurization rate 
                 mass % 
                 — 
                 — 
                 65 
                 66 
                 55 
                 62 
                 63 
                 — 
               
               
                   
                 (after secondary 
               
               
                   
                 treatment) 
               
               
                   
                 Carbon yield 
                 mass % 
                 — 
                 — 
                 95 
                 93 
                 71 
                 95 
                 95 
                 — 
               
               
                   
                 (after secondary 
               
               
                   
                 treatment) 
               
               
                   
               
            
           
         
       
     
     &lt;Summary of Test Results&gt; 
     It was revealed that Examples 1 to 16 using a mixed solution of hydrogen peroxide and acetic anhydride as a chemical agent exhibited a higher desulfurization rate than that of Comparative Example 1 in which such a solution was not used, thus having a sufficient desulfurization effect. The carbon yield was also good. 
     The comparison between Example 1 and Example 4 revealed that Example 1 in which a molar ratio (acetic anhydride/hydrogen peroxide) was 5.0 had a higher desulfurization rate than that of Example 4 in which a molar ratio (acetic anhydride/hydrogen peroxide) was 0.4, thus having a more excellent desulfurization effect. 
     The comparison between Example 1 and Example 5 revealed that Example 1 in which the elapsed time after mixing of acetic anhydride and hydrogen peroxide was 30 minutes had a higher desulfurization rate than that of Example 5 in which the time was 8 minutes, thus having a more excellent desulfurization effect. 
     The comparison between Example 1 and Example 6 revealed that Example 1 in which the mass ratio (chemical agent/coal) was 3.2 had a higher desulfurization rate than that of Example 6 in which the mass ratio (chemical agent/coal) was 0.9, thus having a more excellent desulfurization effect. 
     The comparison between Example 1 and Example 7 revealed that Example 1 in which the temperature of the chemical agent at the time of being brought into contact with coal was 20° C. had a better carbon yield than that of Example 7 in which the temperature was 35° C. 
     The desulfurization rates (after the secondary treatment) of Examples 8 to 11 were equal to or higher than the desulfurization rates (after the primary treatment) of Examples 1 to 7. 
     The comparison between Example 8 and Example 10 revealed that Example 8 in which the heat treatment temperature was 150° C. had a higher desulfurization rate (after the secondary treatment) than that of Example 10 in which the heat treatment temperature was 100° C., thus having a more excellent desulfurization effect. 
     The comparison between Example 8 and Example 11 revealed that Example 8 in which the heating rate at which the temperature was raised to the heat treatment temperature was 20° C./min had a higher desulfurization rate (after the secondary treatment) than that of Example 11 in which the heating rate was 5° C./min, thus having a more excellent desulfurization effect. 
     The desulfurization rates (after the secondary treatment) of Examples 12 to 16 were equal to or higher than the desulfurization rates (after the primary treatment) of Examples 1 to 7. 
     The comparison between Example 12 and Example 14 revealed that Example 12 in which the temperature of the hydrogen peroxide solution was 20° C. had a higher desulfurization rate (after the secondary treatment) than that of Example 14 in which the temperature was 45° C., thus having a more excellent desulfurization effect. 
     The comparison between Example 12 and Example 15 revealed that Example 12 in which the concentration of the hydrogen peroxide solution was 35.0 mass % had a higher desulfurization rate (after the secondary treatment) than that of Example 15 in which the concentration was 1.5 mass %, thus having a more excellent desulfurization effect. 
     The comparison between Example 12 and Example 16 revealed that Example 12 in which a mass ratio (hydrogen peroxide solution/coal) was 2.5 had a higher desulfurization rate (after the secondary treatment) than that of Example 16 in which a mass ratio (hydrogen peroxide solution/coal) was 0.9, thus having a more excellent desulfurization effect. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : Hydrogen peroxide storage tank 
               2 : Hydrogen peroxide transport pipe 
               3 : Acetic anhydride storage tank 
               4 : Acetic anhydride transport pipe 
               5 : Chemical agent mixing tank 
               6 : Heating device 
               7 : Mixing device 
               8 : Chemical agent transport pipe 
               9 : Desulfurization treatment tank 
               10 : Coal storage tank 
               11 : Coal transport pipe 
               12 : Heating device 
               13 : Mixing device 
               14 : Chemical agent circulation pipe 
               15 : Chemical agent discharge pipe 
               16 : Chemical-treated coal transport pipe 
               16   a : Chemical-treated coal discharge pipe 
               16   b : Heat treatment device connection pipe 
               16   c : Hydrogen peroxide treatment device connection pipe 
               17 : Heat treatment device 
               18 : Heat-treated coal discharge pipe 
               19 : Heat treatment gas exhaust pipe 
               20 : Hydrogen peroxide supply pipe 
               21 : Dilution water tank 
               22 : Dilution water supply pipe 
               23 : Hydrogen peroxide treatment device 
               24 : Cooling device 
               25 : Mixing device 
               26 : Discharge pipe 
               27 : Hydrogen peroxide circulation pipe 
               28 : Hydrogen peroxide discharge pipe