EP4660282A1 - Coke production method - Google Patents

Abstract

Provided is a method for producing coke that can inhibit a decrease in the coke strength of coke that is produced, even if the coke is produced with a coal blend in which a carbonaceous material having an anomalous surface tension is blended.

A method for producing coke includes producing coke by carbonizing a coal blend obtained by mixing formed coal with a coal fines mixture, the formed coal including brands of coal, the coal fines mixture including brands of powdered coal. The method includes selecting brands and blending ratios of coal that is included in the formed coal and of coal that is included in the coal fines mixture, such that an interfacial tension γ_inter of the coal blend and an interfacial tension γ_p of the coal fines mixture satisfy γ_inter > γ_p and γ_p < 0.03, where the interfacial tension γ_inter and the interfacial tension γ_p are calculated from surface tensions of heat-treated coal obtained by heat-treating the brands of coal that are included in the coal blend and the blending ratios of the brands of coal.

Description

Technical Field
• The present invention relates to a method for producing coke. In the method for producing coke, a portion of brands of coal used as raw materials that are carbonized is formed coal. The method includes adjusting a composition of the raw materials for the formed coal.
Background Art
• In blast furnaces, blast furnace coke is used as a reduction agent, a heat source, and a supporting material for maintaining gas permeability and liquid permeability. Stable operation of blast furnaces requires ensuring gas permeability and liquid permeability in the blast furnaces and, accordingly, requires coke having excellent properties; the properties include strength, particle size, and strength after reaction. In particular, coke strengths, such as a tumble strength, are particularly important properties for blast furnace coke.
• The blast furnace coke is produced by carbonizing coal in a carbonization furnace, thereby thermally plasticizing the coal to cause coal particles to be bonded to one another. Accordingly, producing coke having high strength requires caking coal having excellent thermoplasticity. Today, however, caking coal suitable for the production of coke is approaching depletion. Hence, there is a need for increased use of materials that have not been used for the production of coke in the past, such as coal having poor thermoplasticity and carbon materials other than coal. Examples of the carbon materials other than coal include biomasses, such as wood pellets and oil palm residues, torrefied biomasses obtained by subjecting a biomass to torrefaction, biomass carbonaceous materials obtained by subjecting a biomass to heat treatment, and plastics. Since the need to reduce the amount of CO₂ emissions is increasing today, there is a need to use a biomass-derived carbon material, waste plastic, and the like for the production of coke.
• The production of blast furnace coke involves using a coal blend containing various types of coal blended together. Accordingly, in the related art, studies have been conducted regarding methods for estimating a coke strength of coke that is produced by using a coal blend as a raw material. It has recently been found that a surface tension of coal in a thermally plasticized state has an influence on a bond strength of the coal. Patent Literature 1 discloses a method for blending coal and a method for producing coke that use surface tensions and interfacial tensions calculated from the surface tensions. Patent Literature 1 discloses that an interfacial tension that occurs between particles of different types of coal is determined based on surface tensions of heat-treated coal (semicoke) obtained by heat-treating coal (hereinafter referred to as "surface tensions of coal"), and that a coal blend is controlled based on a magnitude of the interfacial tension.
• The production of coke involves carbonizing coal, thereby thermally plasticizing the coal to cause coal particles to be fused to one another. Accordingly, it can be presumed that a bond strength between the coal particles affects a coke strength of coke that is produced. In general, the bond strength between coal particles is improved as an interfacial tension at bonding interfaces decreases. The interfacial tension is expressed in units of mN/m and, therefore, can be considered to be free energy present at interfaces. That is, the presence of interfacial tension means that free energy that can act as a force on the interfaces is present. Accordingly, it can be presumed that when there is a high interfacial tension, breakage is likely to occur at the bonding interfaces. Patent Literature 1 states that when the interfacial tension of the coal blend is greater than 0.03 mN/m, the coke strength of coke that is produced significantly decreases, and that, therefore, it is preferable that the interfacial tension of the coal blend be 0.03 mN/m or less.
Citation List Patent Literature
• PTL 1: International Publication No. 2013/054526
Non Patent Literature
• NPL 1: M. C. Williams and D. W. Fuerstenau, International Journal of Mineral Processing, 20 (1987), pp. 153-157
Summary of Invention Technical Problem
• Patent Literature 1 states that, to obtain coke having high strength, it is preferable that the various types of coal be blended together such that the interfacial tension of the coal blend decreases. Accordingly, an effective way to produce coke having high strength is to use various types of coal having similar surface tension values in the coal blend. With this approach, however, it is difficult to use coal having an excessively high surface tension and coal having an excessively low surface tension. Patent Literature 1 does not disclose any approach that prevents a decrease in the coke strength associated with the use of coal or a carbon material having a surface tension significantly different from a weighted average value of the surface tensions of the coal and carbon material included in the coal blend, and, therefore, Patent Literature 1 presents a problem in that the range of coal resources that can be used is limited.
• If the range of coal resources that can be used as raw materials for coke can be expanded, a very significant advantage can be obtained in terms of consistently procuring raw materials and expanding the use of raw materials that are difficult to use. Hence, there is a need for a method for producing coke that can inhibit a decrease in the coke strength of coke that is produced, even if the coke is produced with a coal blend containing, blended therein, coal or a carbon material having an excessively high surface tension or coal or a carbon material having an excessively low surface tension, compared to an average surface tension of the coal blend. In the description below, the coal or carbon material having an excessively high surface tension and the coal or carbon material having an excessively low surface tension, compared to an average surface tension of the coal blend, are referred to as a "carbonaceous material having an anomalous surface tension".
• The present invention has been made in view of these circumstances, and an object of the present invention is to provide a method for producing coke that can inhibit a decrease in the coke strength of coke that is produced, even if the coke is produced with a coal blend in which a carbonaceous material having an anomalous surface tension is blended.
Solution to Problem
• Means for solving the problem are as follows.
1. [1] A method for producing coke, the method including producing coke by carbonizing a coal blend obtained by blending formed coal with a coal fines mixture, the formed coal including brands of coal, the coal fines mixture including brands of powdered coal, wherein the method includes selecting brands and blending ratios of coal that is included in the formed coal and of coal that is included in the coal fines mixture, such that an interfacial tension γ_inter of the coal blend and an interfacial tension γ_p of the coal fines mixture satisfy γ_inter > γ_p and γ_p < 0.03, where the interfacial tension γ_inter and the interfacial tension γ_p are calculated from surface tensions of heat-treated coal obtained by heat-treating the brands of coal that are included in the coal blend and the blending ratios of the brands of coal, by using equation (1) or equation (2), shown below.
   [Math. 1] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}={\mathrm{W\Gamma W}}^{\mathrm{t}}$$ where $$\mathrm{\Gamma }=\left(\begin{array}{llllll}{\gamma }_{11}& {\gamma }_{12}& \cdots & {\gamma }_{1j}& \cdots & {\gamma }_{1n}\\ {\gamma }_{21}& {\gamma }_{22}& & & & ⋮\\ ⋮& & \ddots & & & ⋮\\ {\gamma }_{i1}& & & {\gamma }_{\mathit{ij}}& & {\gamma }_{\mathit{in}}\\ ⋮& & & & \ddots & ⋮\\ {\gamma }_{n1}& \cdots & \cdots & {\gamma }_{\mathit{nj}}& \cdots & {\gamma }_{\mathit{nn}}\end{array}\right)$$ $${\gamma }_{\mathit{ij}}={\gamma }_i+{\gamma }_j-\mathit{exp}\left[-\beta {\left({\gamma }_i-{\gamma }_j\right)}^2\right]\sqrt{{\gamma }_i{\gamma }_j}$$ $${\gamma }_{\mathit{ij}}={\gamma }_{\mathit{ji}}$$ $$W=\left(w_1{w}_2\cdots w_i\cdots w_n\right)$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$
   • γ_i: the surface tension of heat-treated coal i
   • γ_ij: the interfacial tension at an interface between heat-treated coal i and heat-treated coal j
   • β = 0.0001247 [(m²·mJ)²]
   • w_i: the blending ratio of coal i
   • γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
     [Math. 2] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}=0.032{{\mathrm{\sigma }}_{\mathrm{\gamma }}}^2$$ where $${\sigma }_{\gamma }^2=\frac{100}{100{\displaystyle {\sum }_{i=1}^n{w}_i-1}}\left[{\displaystyle \sum _{i=1}^n{\gamma }_i^2{w}_i-\frac{{\left({\displaystyle {\sum }_{i=1}^n{\gamma }_i{w}_i}\right)}^2}{{\displaystyle {\sum }_{i=1}^n{w}_i}}}\right]$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$
     • γ_i: the surface tension of heat-treated coal i
     • γ_ij: the interfacial tension at an interface between heat-treated coal i and heat-treated coal j
     • w_i: the blending ratio of coal i
     • γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
2. [2] The method for producing coke according to [1], in which at least one of the formed coal and the coal fines mixture contains a carbon material, wherein the method includes selecting brands and blending ratios of coal that is included in the formed coal and of coal that is included in the coal fines mixture and selecting a type and a blending ratio of the carbon material that is included in the formed coal and of the carbon material that is included in the coal fines mixture, such that the interfacial tension γ_iter of the coal blend and the interfacial tension γ_p of the coal fines mixture satisfy γ_inter > γ_p and γ_p < 0.03, where the interfacial tension γ_inter and the interfacial tension γ_p are calculated from surface tensions of the heat-treated coal and the carbon material, the heat-treated coal being obtained by heat-treating the brands of coal that are included in the coal blend; and the blending ratios of the brands of coal and the carbon material, by using equation (3) or equation (4), shown below, instead of equation (1) or equation (2).
   [Math. 3] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}={\mathrm{W\Gamma W}}^{\mathrm{t}}$$ where $$\mathrm{\Gamma }=\left(\begin{array}{llllll}{\gamma }_{11}& {\gamma }_{12}& \cdots & {\gamma }_{1j}& \cdots & {\gamma }_{1n}\\ {\gamma }_{21}& {\gamma }_{22}& & & & ⋮\\ ⋮& & \ddots & & & ⋮\\ {\gamma }_{i1}& & & {\gamma }_{\mathit{ij}}& & {\gamma }_{\mathit{in}}\\ ⋮& & & & \ddots & ⋮\\ {\gamma }_{n1}& \cdots & \cdots & {\gamma }_{\mathit{nj}}& \cdots & {\gamma }_{\mathit{nn}}\end{array}\right)$$ $${\gamma }_{\mathit{ij}}={\gamma }_i+{\gamma }_j-\mathit{exp}\left[-\beta {\left({\gamma }_i-{\gamma }_j\right)}^2\right]\sqrt{{\gamma }_i{\gamma }_j}$$ $${\gamma }_{\mathit{ij}}={\gamma }_{\mathit{ji}}$$ $$W=\left(w_1{w}_2\cdots w_i\cdots w_n\right)$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$
   • γ_i: the surface tension of heat-treated coal i or carbon material i
   • γ_ij: the interfacial tension at an interface between heat-treated coal i or carbon material i and heat-treated coal j or carbon material j
   • β = 0.0001247 [(m²·mJ)²]
   • w_i: the blending ratio of coal i or carbon material i
   • γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
     [Math. 4] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}=0.032{{\mathrm{\sigma }}_{\mathrm{\gamma }}}^2$$ where $${\sigma }_{\gamma }^2=\frac{100}{100{\displaystyle {\sum }_{i=1}^n{w}_i-1}}\left[{\displaystyle \sum _{i=1}^n{\gamma }_i^2{w}_i-\frac{{\left({\displaystyle {\sum }_{i=1}^n{\gamma }_i{w}_i}\right)}^2}{{\displaystyle {\sum }_{i=1}^n{w}_i}}}\right]$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$
     • γ_i: the surface tension of heat-treated coal i or carbon material i
     • y_ij: the interfacial tension at an interface between heat-treated coal i or carbon material i and heat-treated coal j or carbon material j
     • w_i: the blending ratio of coal i or carbon material i
     • γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
Advantageous Effects of Invention
• With the present invention, a decrease in a coke strength of coke that is produced can be inhibited even if the coke is produced with a coal blend in which a carbonaceous material having an anomalous surface tension is blended. Hence, a wide range of various types of coal or carbon materials, including carbonaceous materials having an anomalous surface tension that have been unusable in the past, become usable as raw materials for coke, and, consequently, consistent procurement of raw materials and expansion of resources can be achieved.
Brief Description of Drawings
• [Fig. 1] Fig. 1 is a graph illustrating a relationship between an interfacial tension γ_p of a coal fines mixture and a coke strength, regarding Invention Examples 1 to 3 and Comparative Examples 2 and 3.
• [Fig. 2] Fig. 2 is a graph illustrating a relationship between the interfacial tension γ_p of a coal fines mixture and an amount of increase in the coke strength, regarding Invention Examples 4 to 7 and Comparative Examples 5 and 6. Description of Embodiments
• To solve the problem described above, the inventors of the present invention conducted studies, which addressed methods for producing coke that includes using formed coal that is obtained by subjecting coal to a forming process and which constitutes a portion of a coal blend; and carbonizing the coal blend, which contains the formed coal, powdered coal, which is not formed coal, and a carbon material (hereinafter referred to as a "coal fines mixture"). Patent Literature 1 indicates that in the case of a coal blend not containing formed coal, the coal blend has a preferred upper limit of an interfacial tension. However, the preferred upper limit of the interfacial tension may change if formed coal is used, because in formed coals, particles of coal and the like are disposed closer to one another than in coal fines, and, therefore, bonding between particles is facilitated compared to coal fines.
• The inventors diligently conducted studies and, consequently, ascertained that the influence of the interfacial tension of coal included in the formed coal on the coke strength is less significant than the influence of the interfacial tension of coal included in coal fines. Specifically, the following finding was made. In instances where a coal blend containing formed coal and a coal fines mixture is used, and a carbonaceous material having an anomalous surface tension is blended in the formed coal, the interfacial tension of the coal fines mixture has a dominant influence, and as a result, it is possible to inhibit a decrease in the coke strength due to the blending, in the coal blend, of a carbonaceous material having an anomalous surface tension. That is, even in instances where the interfacial tension of the coal blend as a whole is high, if a larger amount of the carbonaceous material having an anomalous surface tension is blended into the formed coal, the interfacial tension of the coal fines mixture is reduced, which makes the interfacial tension of the coal fines mixture dominant, and, consequently, a decrease in the coke strength of coke that is produced by carbonizing the coal blend can be inhibited. With this finding, the present invention was completed. The present invention will be described in detail below with reference to embodiments of the present invention.
• The influence of the surface tension of coal on the coke strength is described in detail in Patent Literature 1. The method for producing coke of the present embodiment firstly determines the surface tension of a heat-treated product of the coal or the carbon material included in the coal blend that serves as a raw material for coke. The measurement of the surface tension may be carried out with the same method as the method described in Patent Literature 1. The method for measuring the surface tension is summarized below.
• Examples of known methods for measuring the surface tension of coal or carbon materials include sessile drop methods, capillary-rise methods, maximum bubble pressure methods, drop weight methods, pendant drop methods, ring methods, Wilhelmy methods, advancing/receding contact angle methods, tilting plate methods, and film flotation methods. Since coal is formed of various molecular structures, it is anticipated that the surface tension of coal is not uniform. Accordingly, it is preferable to use a method that can evaluate a surface tension distribution, for example, a film flotation method as described in Non Patent Literature 1. The film flotation method can be used uniformly for coal, carbon materials, and semicoke obtained by carbonizing any of these, and the method can determine a surface tension distribution by using a finely divided coal sample. An average value of the obtained surface tension distribution can be used as the surface tension of the coal sample.
• Since bonding between coal particles occurs in a state in which the coal is thermally plasticized during carbonization under heat, it is preferable to use heat-treated coal (semicoke) as the coal sample for the measurement of the surface tension. Since no method is known for measuring the surface tension of a coal melt at a high temperature, the present embodiment measures the surface tension of heat-treated coal or carbon material as in the method disclosed in Patent Literature 1.
• Regarding the temperature for the heat treatment of the coal sample, it is preferable that the temperature be within a range of 350°C or greater and 800°C or less, which corresponds to the temperatures at which coal begins to become thermally plasticized and undergoes bonding and solidification until coking is completed. In particular, it is more preferable to use semicoke that has been heated to a temperature within a range of 350°C or greater and 550°C or less, which is a temperature at which coal becomes thermally plasticized and, thus, is a temperature that contributes to bonding. Specifically, it is particularly preferable to use semicoke that has been heated to a temperature within a range of 480°C or greater and 520°C or less.
• The heating of the coal sample is preferably carried out in an inert gas atmosphere that does not react with the coal or carbon material, such as nitrogen, argon, or helium. Preferably, the heating rate is selected in accordance with the heating rate for producing coke in a coke oven. For example, the heating may be carried out at a heating rate of 3°C/min. Furthermore, it is preferable that the sample heated to a target temperature be rapidly cooled at a cooling rate of 10°C/s or greater so that the molecular structure at the target temperature can be maintained. A preferred method for the cooling is cooling in liquid nitrogen.
• Regarding the carbon materials, other than coal, it is also preferable that the sample for the measurement of the surface tension be heat-treated beforehand. Note that if the carbon material is an organic compound that has been carbonized (torrefied) beforehand, and if the temperature for the carbonization is greater than the temperature for the heat treatment of the coal, the measurement of the surface tension may be performed without heat-treating the carbon material beforehand.
• Now, the formed coal will be described. In the method for producing coke of the present embodiment, a portion of the coal blend that is charged into a coke oven is formed coal. The formed coal may be formed by a method such as granulation, compression bonding, molding, bonding, or kneading. Two or more of these methods may be combined to form the formed coal. An example of the formed coal is briquettes, which are formed as follows: various types of coal and a carbon material are mixed together, a binder is added thereto if necessary, the mixture is kneaded in a kneading machine, and subsequently, the resultant is compacted in a double roll briquetting machine. Examples of binders that can be used include water, coal-based binders (e.g., coal tar pitch, solvent refined coal, tar, and tar sludge), petroleum-based binders (e.g., asphalt, asphalt pitch, and propane deasphalted asphalt), and organic binders (e.g., starches, molasses, synthetic polymeric compounds, and resins). Preferably, the coal that is used for the forming has particle sizes adjusted by pulverization such that particles with a size of 3 mm or less are present in an amount of 70 to 100 mass%.
• Preferably, the formed coal has a bulk density of 0.8 g/cm³ or greater, more preferably 0.9 g/cm³ or greater, and even more preferably 1.0 g/cm³ or greater. When the formed coal has an increased bulk density, even a carbonaceous material having an anomalous surface tension can have improved adhesion to the coal and carbon material in the vicinity. Consequently, a decrease in the coke strength of coke that is produced can be inhibited.
• Now, the coal fines mixture will be described. Regarding the method for producing coke of the present embodiment, the portion other than the formed coal in the coal blend that is charged into a coke oven is referred to as the "coal fines mixture". The coal that constitutes the coal fines mixture also preferably has particle sizes adjusted by pulverization such that particles with a size of 3 mm or less are present in an amount of 70 to 100 mass%. Before being charged into a coke oven, the coal may be conditioned and dried to adjust the amount of water present therein. A carbon material may be blended in the coal fines mixture.
• Now, a method for calculating the interfacial tension will be described. The method for producing coke of the present embodiment selects compositions of the formed coal and the coal fines mixture based on the interfacial tension, which is calculated from surface tensions. The interfacial tension can be calculated based on the method described in Patent Literature 1. The method for measuring the interfacial tension is summarized below.
• It is preferable that the interfacial tension between coal particles be directly measured at an interface between different coal particles; however, such measurement is very difficult with existing technique. Accordingly, it is preferable that the method described in Patent Literature 1, which discloses an influence on the coke strength, be used for the measurement. The interfacial tension between coal particles can be calculated from the surface tensions of the coal or the carbon material that constitutes the coal blend and the blending ratio thereof, by using equation (1), shown below.
  [Math. 5] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}={\mathrm{W\Gamma W}}^{\mathrm{t}}$$ where $$\mathrm{\Gamma }=\left(\begin{array}{cccccc}{\gamma }_{11}& {\gamma }_{12}& \cdots & {\gamma }_{1j}& \cdots & {\gamma }_{1n}\\ {\gamma }_{21}& {\gamma }_{22}& & & & ⋮\\ ⋮& & \ddots & & & ⋮\\ {\gamma }_{i1}& & & {\gamma }_{\mathit{ij}}& & {\gamma }_{\mathit{in}}\\ ⋮& & & & \ddots & ⋮\\ {\gamma }_{n1}& \cdots & \cdots & {\gamma }_{\mathit{nj}}& \cdots & {\gamma }_{\mathit{nn}}\end{array}\right)$$ $${\gamma }_{\mathit{ij}}={\gamma }_i+{\gamma }_j-\mathit{exp}\left[-\beta {\left({\gamma }_i-{\gamma }_j\right)}^2\right]\sqrt{{\gamma }_i{\gamma }_j}$$ $${\gamma }_{\mathit{ij}}={\gamma }_{\mathit{ji}}$$ $$W=\left(w_1{w}_2\cdots w_i\cdots w_n\right)$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$
• γ_i: the surface tension of heat-treated coal i
• y_ij: the interfacial tension at an interface between heat-treated coal i and heat-treated coal j $$\mathrm{\beta }=0.0001247\left[{\left({\mathrm{m}}^2\cdot \mathrm{mJ}\right)}^2\right]$$
• w_i: the blending ratio of coal i
• γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
• Regarding the surface tensions of the various types of heat-treated coal, the various types of heat-treated coal, whose surface tensions are used, are those that have been heated to the same temperature. A blending ratio w_i of coal i is a ratio of a mass of each of the brands of coal (coal i) that is included in the coal blend to the total mass of the coal blend. The interfacial tension between two different brands of coal can be calculated by using equation (5), shown below.
  [Math. 6] $${\mathrm{\gamma }}_{\mathrm{ij}}={\mathrm{\gamma }}_{\mathrm{i}}+{\mathrm{\gamma }}_{\mathrm{j}}-\exp \left[-\mathrm{\beta }{\left({\mathrm{\gamma }}_{\mathrm{i}}-{\mathrm{\gamma }}_{\mathrm{j}}\right)}^2\right]\surd {\mathrm{\gamma }}_{\mathrm{i}}{\mathrm{\gamma }}_{\mathrm{j}}$$
• In equation (5), β is a constant, and the value of β is 0.0001247 [(m²·mJ)²], according to Li and Neumann.
• Regarding the blending ratio, differences in yield between the various types of coal associated with the heat treatment may be considered, and ratios associated with the heat treatment may be calculated. However, whether the calculation is based on the blending ratios in the coal blend or on the ratios in the heat-treated product does not make a significant difference in the selection of the composition. An interfacial tension γ_p, which is an interfacial tension of the coal fines mixture (the remaining portion of the coal blend, excluding the formed coal), can be calculated similarly from equation (1), provided that brands of the individual types of coal that are to be present in the coal fines mixture and their blending ratios have been selected.
• The interfacial tension of the coal blend can also be calculated from equation (2), shown below. The calculation result of equation (1) agrees with the calculation result of equation (2), as suggested by Patent Literature 1.
  [Math. 7] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}=0.032{{\mathrm{\sigma }}_{\mathrm{\gamma }}}^2$$ where $${\sigma }_{\gamma }^2=\frac{100}{100{\displaystyle {\sum }_{i=1}^n{w}_i-1}}\left[{\displaystyle \sum _{i=1}^n{\gamma }_i^2{w}_i-\frac{{\left({\displaystyle {\sum }_{i=1}^n{\gamma }_i{w}_i}\right)}^2}{{\displaystyle {\sum }_{i=1}^n{w}_i}}}\right]$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$
• γ_i: the surface tension of heat-treated coal i
• y_ij: the interfacial tension at an interface between heat-treated coal i and heat-treated coal j
• w_i: the blending ratio of coal i
• γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
• Furthermore, equation (3) and equation (4), shown below, are equations that consider the carbon material, which are expanded from equation (1) and equation (2) that determine the interfacial tension of a coal blend. The concept of the calculation by equation (3) and equation (4) is the same as that of equation (1) and equation (2). The interfacial tension of a coal blend containing a carbon material can be calculated by using equation (3) and equation (4).
  [Math. 8] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}={\mathrm{W\Gamma W}}^{\mathrm{t}}$$ where $$\mathrm{\Gamma }=\left(\begin{array}{cccccc}{\gamma }_{11}& {\gamma }_{12}& \cdots & {\gamma }_{1j}& \cdots & {\gamma }_{1n}\\ {\gamma }_{21}& {\gamma }_{22}& & & & ⋮\\ ⋮& & \ddots & & & ⋮\\ {\gamma }_{i1}& & & {\gamma }_{\mathit{ij}}& & {\gamma }_{\mathit{in}}\\ ⋮& & & & \ddots & ⋮\\ {\gamma }_{n1}& \cdots & \cdots & {\gamma }_{\mathit{nj}}& \cdots & {\gamma }_{\mathit{nn}}\end{array}\right)$$ $${\gamma }_{\mathit{ij}}={\gamma }_i+{\gamma }_j-\mathit{exp}\left[-\beta {\left({\gamma }_i-{\gamma }_j\right)}^2\right]\sqrt{{\gamma }_i{\gamma }_j}$$ $${\gamma }_{\mathit{ij}}={\gamma }_{\mathit{ji}}$$ $$W=\left(w_1{w}_2\cdots w_i\cdots w_n\right)$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$
• γ_i: the surface tension of heat-treated coal i or carbon material i
• y_ij: the interfacial tension at an interface between heat-treated coal i or carbon material i and heat-treated coal j or carbon material j
• β = 0.0001247 [(m²·mJ)²]
• w_i: the blending ratio of coal i or carbon material i
• γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
  [Math. 9] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}=0.032{{\mathrm{\sigma }}_{\mathrm{\gamma }}}^2$$ where $${\sigma }_{\gamma }^2=\frac{100}{100{\displaystyle {\sum }_{i=1}^n{w}_i-1}}\left[{\displaystyle \sum _{i=1}^n{\gamma }_i^2{w}_i-\frac{{\left({\displaystyle {\sum }_{i=1}^n{\gamma }_i{w}_i}\right)}^2}{{\displaystyle {\sum }_{i=1}^n{w}_i}}}\right]$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$
  • γ_i: the surface tension of heat-treated coal i or carbon material i
  • y_ij: the interfacial tension at an interface between heat-treated coal i or carbon material i and heat-treated coal j or carbon material j
  • w_i: the blending ratio of coal i or carbon material i
  • γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
• Now, a method for selecting compositions of the formed coal and the coal fines mixture will be described. The method for producing coke of the present embodiment selects compositions of the formed coal and the coal fines mixture such that the interfacial tension γ_p of the coal fines mixture can be less than the interfacial tension γ_inter of the total coal blend, and that the interfacial tension γ_p of the coal fines mixture can be less than 0.03. As used herein, the "composition" refers to the brand and the blending ratio of coal that is included and the type and the blending ratio of a carbon material that is included.
• Note that if the temperature for the heat treatment of the coal is changed, the surface tension of the heat-treated coal changes; however the tendency of the change is the same regardless of the brand of the coal, that is, the value of the interfacial tension between different types of coal does not substantially change, as disclosed in Patent Literature 1. Accordingly, a suitable range of the interfacial tension γ_p of the coal fines mixture can be determined independently of the temperature for the heat treatment of the coal.
• The blending ratio of a brand of coal in the coal blend is the sum of values, where one of the values is the product of the blending ratio of the formed coal in the coal blend multiplied by the blending ratio of the brand of coal in the formed coal, and the other is the product of the blending ratio of the coal fines mixture in the coal blend multiplied by the blending ratio of the brand of coal in the coal fines mixture. Accordingly, for example, selecting a composition of a coal blend and selecting a blending ratio of the formed coal in the coal blend and blending ratios of brands of coal in the formed coal determine the blending ratios of the coal fines mixture.
• The inventors discovered that the influence of the interfacial tension of the formed coal on the coke strength is less significant than the influence of the interfacial tension of the coal fines mixture and that in an instance where a carbonaceous material having an anomalous surface tension is blended in the formed coal, the influence of the interfacial tension of the coal fines mixture is dominant. Thus, a decrease in the coke strength of coke that is produced can be inhibited to a greater degree when the coal blend used is one having a composition in which formed coal and a coal fines mixture have different compositions such that the interfacial tension of the coal fines mixture can be lower than the interfacial tension of the coal blend, than when the coal blend used is one in which the formed coal and the coal fines mixture have the same composition.
• That is, it is preferable to select a composition in which the formed coal and the coal fines mixture have different compositions such that the interfacial tension of the coal fines mixture can be reduced, rather than to select the same composition for the formed coal and the coal fines mixture. This technical concept can be expressed as an inequality: γ_inter > γ_p. "γ_inter > γ_p" indicates that it is appropriate to apply the present invention to a coal blend having a high γ_inter, which tends to cause a decrease in the coke strength.
• Furthermore, according to a suggestion of Patent Literature 1, if the interfacial tension γ_inter of the coal blend calculated by using equation (1) or equation (2) is 0.03 or greater, a decrease in the coke strength may occur. Accordingly, by selecting a composition in which coal having an anomalous surface tension is included in the formed coal in the coal blend so that the interfacial tension γ_p of the coal fines mixture, which has an influence on the coke strength, can be less than 0.03, it is possible to inhibit a decrease in the coke strength and, therefore, to improve the coke strength to a greater degree than when the formed coal and the coal fines mixture have the same blending ratio of the coal having an anomalous surface tension.
• It is preferable that a blending ratio of the formed coal in the total coal blend be within a range of 10 mass% or greater and 50 mass% or less. If the content of the formed coal is less than 10 mass% in the total coal blend, the effect of expanding the range of usable coal resources is reduced, and, therefore, such a content is not preferable. If the content of the formed coal is greater than 50 mass% in the total coal blend, the cost of forming the formed coal increases, and, therefore, such a content is not preferable. It is more preferable that the blending ratio of the formed coal be within a range of 15 mass% or greater and 40 mass% or less in the total coal blend.
• Regarding the composition of the formed coal, if the fluidity of the raw materials of the formed coal is excessively low, many defects may be formed in the formed-coal-derived portion of the coke that is produced, and, therefore, such a composition may not be preferable. Regarding the lower limit of the fluidity of the raw materials of the formed coal, while the lower limit varies depending on the blending amount of the formed coal and the types of coal and carbon material used in the formed coal, it is preferable that an average log MF be 1.4 or greater, where the average log MF is determined based on the common logarithms of Gieseler fluidities MF of the various types of coal and carbon material that are included in the formed coal and on the contents of the various types of coal and carbon material in the formed coal.
• Lastly, the carbonization of the coal blend will be described. The coal blend, which is prepared by mixing the formed coal with the coal fines mixture, is charged into a coke oven and carbonized to produce coke. The carbonization of the coal blend can be carried out by using a common chamber coke oven and performing carbonization at a temperature of approximately 900°C or greater.
EXAMPLES
• Now, Examples will be described in which coke was produced by using the method for producing coke of the present embodiment. Table 1 shows the properties of various types of coal and a carbon material that were used in the Examples. [Table 1]

  Brand	Ro (%)	Log MF (log [ddpm])	γ (mN/m)
  A	0.71	2.05	43.5
  B	1.00	1.63	41.6
  C	1.30	1.58	40.4
  D	1.52	1.55	39.7
  E	1.10	3.15	39.7
  F	1.18	2.89	40.6
  G	1.24	1.71	40.2
  H	1.51	0.48	39.8
  I	-	-	43.7

• In Table 1 above, A to H are coal, and I is a biomass (palm kernel shell) carbonized at 500°C. Among these, coal A and carbon material I have a surface tension γ higher than the average of the surface tensions of the total coal blend, and thus, in this composition, coal A and carbon material I can be regarded as carbonaceous materials having an anomalous surface tension.
• "Ro" is a mean maximum vitrinite reflectance of coal measured in accordance with JIS M 8816:1992. "log MF" is the common logarithm of the Gieseler maximum fluidity of coal measured in accordance with JIS M 8801:2008. The maximum fluidity MF of I was 0 ddpm. "γ" is the surface tension of the coal or the carbon material, which was heat-treated at 500°C, and is an average value of a surface tension distribution determined by the film flotation method described in Patent Literature 1.
• A coal blend containing formed coal and a coal fines mixture was prepared with the brands of coal listed in Table 1. Each of the brands of coal was pulverized such that particles with a size of 3 mm or less were present in an amount of 100 mass%. The formed coal was briquettes with a volume of 34 cm³, which were produced as follows: tar pitch and tar sludge were added as binders in amounts of 4.0 parts and 6.5 parts, respectively, per 100 parts of the raw materials of the formed coal, and these materials were mixed together and subsequently briquetted in a double roll briquetting machine.
• A coal fines mixture, which was not subjected to any forming process, was blended with the briquettes, and, thus, the coal blend was prepared. The coal blend was charged into a carbonization can such that a bulk density thereof could be 870 kg/m³ and was then carbonized for 6 hours in an electric arc furnace with a furnace wall temperature of 1050°C. Subsequently, the resultant was cooled under nitrogen to produce coke. Table 2 below shows the composition of the coal (mass% based on the total mass of the coal blend), the interfacial tension of the formed coal or the coal fines mixture, the average log MF, and the coke strength DI (150/15) of the produced coke. The interfacial tension is a value calculated from the surface tensions and the blending ratios of the brands of coal by using equation (2). The average log MF is a value calculated by weighted averaging the log MFs of the brands of coal with the blending ratios of the brands of coal. The coke strength is a drum strength index DI (150/15) measured by a tumble strength test method specified in JIS K 2151:2004. [Table 2]

  Brand	Comparative Example 1 Same Composition	Invention Example 1		Invention Example 2		Invention Example 3		Comparative Example 2		Comparative Example 3
  		Formed Coal	Coal Fines Mixture	Formed Coal	Coal Fines Mixture	Formed Coal	Coal Fines Mixture	Formed Coal	Coal Fines Mixture	Formed Coal	Coal Fines Mixture
  A	15.0	15.0	0.0	15.0	0.0	15.0	0.0	10.0	5.0	5.0	10.0
  B	15.0	15.0	0.0	0.0	15.0	10.0	5.0	5.0	10.0	15.0	0.0
  C	15.0	0.0	15.0	15.0	0.0	10.0	5.0	15.0	0.0	10.0	5.0
  D	15.0	0.0	15.0	0.0	15.0	0.0	15.0	0.0	15.0	0.0	15.0
  E	4.0	0.0	4.0	0.0	4.0	0.0	4.0	0.0	4.0	0.0	4.0
  F	14.5	0.0	14.5	0.0	14.5	0.0	14.5	0.0	14.5	0.0	14.5
  G	11.5	0.0	11.5	0.0	11.5	0.0	11.5	0.0	11.5	0.0	11.5
  H	10.0	0.0	10.0	0.0	10.0	0.0	10.0	0.0	10.0	0.0	10.0
  Interfacial Tension (mN/m)	0.051	0.030	0.004	0.080	0.016	0.057	0.009	0.064	0.035	0.036	0.050
  Log MF (log [ddpm])	1.81	1.84	1.80	1.82	1.81	1.80	1.82	1.75	1.84	1.68	1.87
  DI (150/15)	82.9	83.3		83.2		83.2		82.6		81.9

• Comparative Example 1 is an example in which the formed coal and the coal fines mixture had the same composition. Accordingly, the interfacial tension of this example was the interfacial tension γ_inter of the coal blend. In Invention Examples 1 to 3 and Comparative Examples 2 and 3, the formed coal and the coal fines mixture had different compositions. The values of the interfacial tension and the average log MF are shown for each of the compositions. The blending ratio of the formed coal in the coal blend, which was the sum of the blending ratios of the brands of coal included in the formed coal, was adjusted to fall within a range of 30 mass% or greater and 35 mass% or less. In the case of the same composition as in Comparative Example 1, the blending ratio of the formed coal was 30 mass%.
• Fig. 1 is a graph illustrating a relationship between the interfacial tension γ_p of the coal fines mixture and the coke strength, regarding Invention Examples 1 to 3 and Comparative Examples 2 and 3. In Fig. 1, the horizontal axis represents the interfacial tension γ_p (mN/m) of the coal fines mixture, and the vertical axis represents the coke strength DI (150/15) (-). "(-)" indicates being dimensionless. The dashed line in Fig. 1 represents the coke strength of Comparative Example 1.
• As shown in Fig. 1 and Table 2, the coke strength of coke that was produced in Invention Examples 1 to 3, which satisfied γ_inter > γ_p and γ_p < 0.03, was higher than the coke strength of coke that was produced in Comparative Example 1, in which the formed coal and the coal fines mixture had the same composition. It was anticipated that in the case where the formed coal and the coal fines mixture had different compositions, problems may arise not only with the interfacial tension of the coal fines mixture but also with bonding within the formed coal and at boundary portions between the formed coal and the coal fines mixture. The results shown in Fig. 1 and Table 2, however, confirm that the strength of coke that is produced is affected dominantly by the interfacial tension γ_p of the coal fines mixture, independently of the interfacial tension of the formed coal in the coal blend. Furthermore, Fig. 1 confirms that under the conditions in which the interfacial tension γ_p of the coal fines mixture is less than 0.03 mN/m, it is possible to achieve a coke strength comparable to or higher than that of the instance (Comparative Example 1) in which the same composition is employed.
• Furthermore, the results indicate that in the case of the conditions in which the same composition (in terms of brands or types of the coal and carbon material that were used and their blending ratios) is used in the total coal blend, it is possible to obtain coke having high strength by selecting compositions of the formed coal and the coal fines mixture such that γ_inter > γ_p and γ_p < 0.03 are satisfied. One way to select compositions of the formed coal and the coal fines mixture is, for example, as follows. A composition of the total coal blend is firstly selected, and then, compositions of the formed coal and the coal fines mixture are selected without changing the composition of the total coal blend. In this instance, by selecting a composition in which a larger amount of a carbonaceous material having an anomalous surface tension is included in the briquettes, the γ_p of the coal fines mixture can be reduced, and, consequently, the composition can satisfy γ_inter > γ_p and γ_p < 0.03.
• Invention Examples 4 to 7 are examples in which coke was produced with a coal blend containing brands of coal and a carbon material. Conditions other than the composition were the same as those of Invention Examples 1 to 3. The carbon material used was a biomass carbonized at 500°C. The properties of the biomass are shown in "Brand I" in Table 1. Regarding Invention Examples 4 to 7, Tables 3 and 4 below show their composition, interfacial tension of the formed coal or the coal fines mixture, average log MF, coke strength DI (150/15) of coke that was produced, and an amount of increase δDI (150/15) in the coke strength. [Table 3]

  Brand	Comparative Example 4 Same Composition	Invention Example 4		Invention Example 5		Comparative Example 5		Comparative Example 6
  		Formed Coal	Coal Fines Mixture	Formed Coal	Coal Fines Mixture	Formed Coal	Coal Fines Mixture	Formed Coal	Coal Fines Mixture
  A	12.0	12.0	0.0	12.0	0.0	7.0	5.0	3.0	9.0
  B	15.0	15.0	0.0	0.0	15.0	5.0	10.0	0.0	15.0
  C	15.0	0.0	15.0	15.0	0.0	15.0	0.0	14.0	1.0
  D	15.0	0.0	15.0	0.0	15.0	5.0	10.0	10.0	5.0
  E	4.0	0.0	4.0	0.0	4.0	0.0	4.0	0.0	4.0
  F	14.5	0.0	14.5	0.0	14.5	0.0	14.5	0.0	14.5
  G	11.5	0.0	11.5	0.0	11.5	0.0	11.5	0.0	11.5
  H	10.0	0.0	10.0	0.0	10.0	0.0	10.0	0.0	10.0
  I	3.0	3.0	0.0	3.0	0.0	3.0	0.0	3.0	0.0
  Interfacial Tension (mN/m)	0.052	0.031	0.004	0.082	0.016	0.072	0.036	0.068	0.047
  Log MF (log [ddpm])	1.75	1.64	1.80	1.61	1.81	1.54	1.86	1.46	1.87
  DI (150/15)	82.7	83.1		83.0		82.5		81.7
  ΔDI (150/15)	-	0.4		0.3		-0.2		-1.0

  [Table 4]

  Brand	Comparative Example 7 Same Composition	Invention Example 6		Invention Example 7
  		Formed Coal	Coal Fines Mixture	Formed Coal	Coal Fines Mixture
  A	10.0	10.0	0.0	10.0	0.0
  B	15.0	15.0	0.0	0.0	15.0
  C	15.0	0.0	15.0	15.0	0.0
  D	15.0	0.0	15.0	0.0	15.0
  E	4.0	0.0	4.0	0.0	4.0
  F	14.5	0.0	14.5	0.0	14.5
  G	11.5	0.0	11.5	0.0	11.5
  H	10.0	0.0	10.0	0.0	10.0
  I	5.0	5.0	0.0	5.0	0.0
  Interfacial Tension (mN/m)	0.053	0.032	0.004	0.083	0.016
  Log MF (log [ddpm])	1.71	1.50	1.80	1.47	1.81
  DI (150/15)	82.6	83.3		83.1
  ΔDI (150/15)	-	0.7		0.5

• Invention Examples 4 and 5 and Comparative Examples 4 to 6 are production examples in which coke was produced with a coal blend containing carbon material I blended in an amount of 3 mass%. Invention Examples 6 and 7 and Comparative Example 7 are production examples in which coke was produced with a coal blend containing carbon material I blended in an amount of 5 mass%. Comparative Examples 4 and 7 are examples in which the formed coal and the coal fines mixture had the same composition. The interfacial tensions of Comparative Examples 4 and 7 were the interfacial tension γ_inter of the coal blend. Comparative Examples 4 and 7 had different coke strengths of coke that was produced, and δDIs of corresponding Invention Examples and Comparative Examples are shown. δDI is the amount of increase in the coke strength. In Table 3, δDI is the value obtained by subtracting the coke strength of Comparative Example 4 from the coke strength of Invention Example 4 or 5 or Comparative Example 5 or 6. In Table 4, δDI is the value obtained by subtracting the coke strength of Comparative Example 7 from the coke strength of Invention Example 6 or 7. The average log MF in Tables 3 and 4 was an average value calculated assuming that the log MF of carbon material I is zero.
• Fig. 2 is a graph illustrating a relationship between the interfacial tension γ_p of the coal fines mixture and the amount of increase in the coke strength, regarding Invention Examples 4 to 7 and Comparative Examples 5 and 6. In the graph of Fig. 2, the horizontal axis represents the interfacial tension γ_p (mN/m) of the coal fines mixture, and the vertical axis represents the amount of increase δDI (150/15) in the coke strength. In Fig. 2, the black circles indicate the results of Table 3, and the white circles indicate the results of Table 4.
• As shown in Fig. 2, it was observed that the coke strength tended to decrease with an increase in γ_p. It was also observed that the coke strength significantly decreased when γ_p was 0.03 or greater. These results confirm that even if the composition is one that may increase the interfacial tension of the coal blend in the case where it includes, in the coal blend, a carbonaceous material having an anomalous surface tension (coal A and carbon material I), which is difficult to use as a typical material for coke, it is possible to inhibit a decrease in the coke strength of coke that is produced and, therefore, to realize the production of coke having high strength, by using formed coal in a portion of the coal blend and selecting a composition such that γ_inter > γ_p and γ_p < 0.03 are satisfied.
• These results confirm that even in instances where coke is produced with a coal blend in which a carbonaceous material having an anomalous surface tension is blended, it is possible to inhibit a decrease in the coke strength of coke that is produced and, therefore, to realize the production of coke having high strength, by selecting a composition such that γ_inter > γ_p and γ_p < 0.03 are satisfied. Hence, a wide range of various types of coal or carbon materials, including carbonaceous materials having an anomalous surface tension that have been unusable in the past, become usable as raw materials for coke, and, consequently, consistent procurement of raw materials and expansion of resources can be achieved.

Claims (2)

1. A method for producing coke, the method comprising producing coke by carbonizing a coal blend obtained by blending formed coal with a coal fines mixture, the formed coal comprising brands of coal, the coal fines mixture comprising brands of powdered coal, wherein

   the method comprises selecting brands and blending ratios of coal that is included in the formed coal and of coal that is included in the coal fines mixture, such that an interfacial tension γ_inter of the coal blend and an interfacial tension γ_p of the coal fines mixture satisfy

   γ_inter > γ_p and γ_p < 0.03

   where the interfacial tension γ_inter and the interfacial tension γ_p are calculated from surface tensions of heat-treated coal obtained by heat-treating the brands of coal that are included in the coal blend and the blending ratios of the brands of coal, by using equation (1) or equation (2), shown below.
   [Math. 1] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}={\mathrm{W\Gamma W}}^{\mathrm{t}}$$ where $$\mathrm{\Gamma }=\left(\begin{array}{cccccc}{\gamma }_{11}& {\gamma }_{12}& \cdots & {\gamma }_{1j}& \cdots & y_{1n}\\ {\gamma }_{21}& {\gamma }_{22}& & & & ⋮\\ ⋮& & \ddots & & & ⋮\\ {\gamma }_{i1}& & & {\gamma }_{\mathit{ij}}& & {\gamma }_{\mathit{in}}\\ ⋮& & & & \ddots & ⋮\\ {\gamma }_{n1}& \cdots & \cdots & {\gamma }_{\mathit{nj}}& \cdots & {\gamma }_{\mathit{nn}}\end{array}\right)$$ $${\gamma }_{\mathit{ij}}={\gamma }_i+{\gamma }_j-\mathit{exp}\left[-\beta {\left({\gamma }_i-{\gamma }_j\right)}^2\right]\sqrt{{\gamma }_i{\gamma }_j}$$ $${\gamma }_{\mathit{ij}}={\gamma }_{\mathit{ji}}$$ $$W=\left(w_1{w}_2\cdots w_i\cdots w_n\right)$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$

   γ_i: the surface tension of heat-treated coal i

   γ_ij: the interfacial tension at an interface between heat-treated coal i and heat-treated coal j $$\mathrm{\beta }=0.0001247\left[{\left({\mathrm{m}}^2\cdot \mathrm{mJ}\right)}^2\right]$$

   w_i: the blending ratio of coal i

   γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
   [Math. 2] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}=0.032{{\mathrm{\sigma }}_{\mathrm{\gamma }}}^2$$ where $${\sigma }_{\gamma }^2=\frac{100}{100{\displaystyle {\sum }_{i=1}^n{w}_i-1}}\left[{\displaystyle \sum _{i=1}^n{\gamma }_i^2{w}_i-\frac{{\left({\displaystyle {\sum }_{i=1}^n{\gamma }_i{w}_i}\right)}^2}{{\displaystyle {\sum }_{i=1}^n{w}_i}}}\right]$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$

   γ_i: the surface tension of heat-treated coal i

   γ_ij: the interfacial tension at an interface between heat-treated coal i and heat-treated coal j

   w_i: the blending ratio of coal i γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
2. The method for producing coke according to Claim 1, in which at least one of the formed coal and the coal fines mixture comprises a carbon material, wherein

   the method comprises selecting brands and blending ratios of coal that is included in the formed coal and of coal that is included in the coal fines mixture and selecting a type and a blending ratio of the carbon material that is included in the formed coal and of the carbon material that is included in the coal fines mixture, such that the interfacial tension γ_iter of the coal blend and the interfacial tension γ_p of the coal fines mixture satisfy

   γ_inter > γ_p and γ_p < 0.03

   where the interfacial tension γ_inter and the interfacial tension γ_p are calculated from surface tensions of the heat-treated coal and the carbon material, the heat-treated coal being obtained by heat-treating the brands of coal that are included in the coal blend; and the blending ratios of the brands of coal and the carbon material, by using equation (3) or equation (4), shown below, instead of equation (1) or equation (2).
   [Math. 3] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}={\mathrm{W\Gamma W}}^{\mathrm{t}}$$ where $$\mathrm{\Gamma }=\left(\begin{array}{cccccc}{\gamma }_{11}& {\gamma }_{12}& \cdots & {\gamma }_{1j}& \cdots & y_{1n}\\ {\gamma }_{21}& {\gamma }_{22}& & & & ⋮\\ ⋮& & \ddots & & & ⋮\\ {\gamma }_{i1}& & & {\gamma }_{\mathit{ij}}& & {\gamma }_{\mathit{in}}\\ ⋮& & & & \ddots & ⋮\\ {\gamma }_{n1}& \cdots & \cdots & {\gamma }_{\mathit{nj}}& \cdots & {\gamma }_{\mathit{nn}}\end{array}\right)$$ $${\gamma }_{\mathit{ij}}={\gamma }_i+{\gamma }_j-\mathit{exp}\left[-\beta {\left({\gamma }_i-{\gamma }_j\right)}^2\right]\sqrt{{\gamma }_i{\gamma }_j}$$ $${\gamma }_{\mathit{ij}}={\gamma }_{\mathit{ji}}$$ $$W=\left(w_1{w}_2\cdots w_i\cdots w_n\right)$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$

   γ_i: the surface tension of heat-treated coal i or carbon material i

   γ_ij: the interfacial tension at an interface between heat-treated coal i or carbon material i and heat-treated coal j or carbon material j

   β = 0.0001247 [(m²·mJ)²]

   w_i: the blending ratio of coal i or carbon material i

   γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture
   [Math. 4] $${\mathrm{\gamma }}_{\mathrm{inter}}\mathrm{or}{\mathrm{\gamma }}_{\mathrm{p}}=0.032{{\mathrm{\sigma }}_{\mathrm{\gamma }}}^2$$ where $${\sigma }_{\gamma }^2=\frac{100}{100{\displaystyle {\sum }_{i=1}^n{w}_i-1}}\left[{\displaystyle \sum _{i=1}^n{\gamma }_i^2{w}_i-\frac{{\left({\displaystyle {\sum }_{i=1}^n{\gamma }_i{w}_i}\right)}^2}{{\displaystyle {\sum }_{i=1}^n{w}_i}}}\right]$$ $${\displaystyle \sum _{i=1}^n{w}_i=1}$$

   γ_i: the surface tension of heat-treated coal i or carbon material i

   γ_ij: the interfacial tension at an interface between heat-treated coal i or carbon material i and heat-treated coal j or carbon material j

   w_i: the blending ratio of coal i or carbon material i

   γ_inter or γ_p: the interfacial tension of the coal blend or the coal fines mixture