JP7827083B2 - Method for placing raw materials in a coke oven and method for producing coke - Google Patents

Description

The present invention relates to a method for placing raw materials in a coke oven and a method for producing coke when recycling synthetic resins, such as waste plastics, as raw materials for steelmaking in the coke oven. In the following description, the unit of mass, "t," represents ¹⁰ kg. In this specification, "synthetic resins" includes not only used plastics that are general waste, commonly known as waste plastics, but also plastics that become industrial waste, such as synthetic resin scraps and defective products generated in the manufacturing process and used plastics.

In recent years, marine pollution caused by waste plastic has become a global issue, with the amount of waste plastic dumped into the ocean estimated to be approximately 8 million tons per year worldwide. Addressing marine pollution is one of the Sustainable Development Goals (SDGs) adopted at the 2015 United Nations Summit. In response, Europe has adopted the EU Plastics Strategy to strengthen plastic recycling and reduce single-use plastics. In Japan, the Containers and Packaging Recycling Law was enacted in 1995, marking an early step in promoting waste plastic recycling. In response to growing momentum for further plastic waste reduction, the Act on Promotion of Resource Recycling Related to Plastics came into effect in 2022, calling for further strengthened waste plastic recycling. In the steel industry, a technology has been put into practical use to recycle waste plastic as raw material for steelmaking by mixing it with coal and charging it into coke ovens. This technology is being implemented as a chemical recycling technology for waste plastics under the Containers and Packaging Recycling Law. However, it is known that mixing waste plastics into coal to produce coke reduces the strength of the coke, and the upper limit of the waste plastic mixing ratio at which coke strength does not decrease is said to be approximately 1% by mass (Non-Patent Document 1). Therefore, various technologies have been developed to prevent the deterioration of coke strength due to the mixing of waste plastics.

For example, Patent Document 1 discloses a method of carbonizing in a coke oven plastic granules that have no holes or cracks extending from the surface to the interior, and have an apparent density of 0.85 to 1.1 g/cm ³ and a volume of 6,000 to 200,000 mm ³ (6 to 200 cm ³ ). To mold such granules, the method discloses a method in which waste plastics containing thermoplastic resins are heated to a temperature of 180 to 260°C in a molding device that extrudes them from a nozzle, and then extruded through the nozzle while gas is being sucked in, followed by water cooling.

Patent Document 2 also discloses a method of pyrolysis recycling in which waste plastic is charged above the raw materials in the coke oven carbonization chamber at least one hour after the raw materials have been charged. It claims that by utilizing the space above the coke oven, large amounts of waste plastic can be recycled without affecting the strength of the coke.

JP 2009-293032 A Japanese Patent Application Laid-Open No. 2019-135281

Nomura, Seiji, Kato, Kenji, Nakagawa, Tomoyuki, Furumaki, Ikuo Journal of the Japan Institute of Energy, Vol. 81, No. 8 (2002) pp. 728-737

However, the conventional technology has the following problems.
The technology disclosed in Patent Document 1 is characterized by the production of plastic granules that do not have holes or cracks that penetrate from the surface to the interior. When molding synthetic resins using a twin-screw extrusion molding machine, the synthetic resin molded product extruded from the nozzle is cut with a cutter to achieve a uniform length, but cutting without cracks or holes is practically difficult. Furthermore, while the heating temperature is set to 180 to 260°C, the specific temperature within the machine is not specified. Furthermore, when the heating temperature is set to 180°C or higher, gas is generated due to thermal decomposition of the synthetic resins, and the accumulated gas forms voids within the molten synthetic resins, resulting in a decrease in density. Furthermore, while it is possible to suppress the decrease in coke strength by modifying the properties of the synthetic resins, the strength decrease due to the generation of voids in the coke itself has not been fundamentally resolved. Furthermore, suppressing the decrease in coke strength is difficult when the synthetic resin addition rate is high.

Furthermore, the technology disclosed in Patent Document 2 can minimize the impact on coke strength by separating coal and waste plastics. However, because the raw material charging lid must be opened after charging coal to charge the waste plastics, strict control is required to ensure that the amount of gas generated does not exceed the amount of gas suction to prevent gas leakage. However, if the amount of gas generated in the coking chamber increases or decreases due to fluctuations in coal quality or other factors, control becomes difficult, posing a risk of the generated gas leaking to the outside and causing abnormal combustion. Furthermore, while a large amount of generated gas is suctioned at the top of the coking chamber, if waste plastics are carbonized at the top of the coking chamber, a large amount of powdered charred waste plastics is suctioned, which may cause blockages in the gas recovery and cleaning line. In addition, this charred waste plastics may be mixed into the recovered tar, potentially reducing the quality of the tar recovered as a product. Furthermore, because plastics are charged after the raw materials are charged into the coal tower, a dedicated hopper for waste plastics must be provided at the top of the coal car. This additional raw material charging procedure reduces the productivity of the coke oven.

The present invention was made in consideration of the above circumstances, and aims to propose a method for arranging raw materials in a coke oven and a method for producing coke that can prevent a decrease in coke strength when synthetic resins are blended.

The coke oven raw material placement method of the present invention, which advantageously solves the above-mentioned problems, is characterized by distributing synthetic resins unevenly near the machine side and the coke side of the coke chamber when they are charged into the coke oven together with raw coal as coke raw materials.

The method for converting synthetic resins into raw materials for iron making according to the present invention includes the steps of:
(a) dividing the coke chamber into four or more equal regions in the longitudinal direction, and allocating more of the synthetic resins to the region in contact with the machine side and the region in contact with the coke side than to the other regions;
(b) the sum of the amounts of the synthetic resins distributed to the area in contact with the machine side and the area in contact with the coke side is 75% or more of the amount of the synthetic resins charged into the entire coke oven;
This may be a more preferable solution.

The coke production method of the present invention, which advantageously solves the above problem, is characterized by carbonizing synthetic resins and raw coal charged using any of the above-mentioned coke oven raw material placement methods.

In this invention, synthetic resins are concentrated at both ends of the coke chamber in the longitudinal direction, which reduces the mixing ratio of synthetic resins in other areas and makes it possible to suppress a decrease in coke strength. This makes it possible to process larger amounts of synthetic resins. Although coke strength decreases at the ends where synthetic resins are concentrated, the severely deteriorated parts are crushed into fine coke by the impacts they receive in the subsequent coke transport process and cooling process (coke dry quenching system, wet quenching system, etc.). The fine coke can be recovered and used as fuel in processes such as iron ore sintering.

Furthermore, because synthetic resins volatilize through thermal decomposition, when they are mixed with coal and carbonized, voids form within the coke, and cracks propagate from these voids within the coke. If synthetic resins are dispersed throughout the coal and charged, cracks propagate from the dispersed voids during coking, making the coke more susceptible to crumbling and making it difficult to push and discharge from the coke oven. However, by concentrating the synthetic resins on the coke side and machine side, cracks are limited to the ends of the coke cake. This prevents the middle part of the coke cake from collapsing, making it easier to push and discharge the coke.

Furthermore, it is known that synthetic resins absorb heat when they undergo thermal decomposition in the absence of oxygen. If they absorb heat while in contact with the refractory bricks of a coke oven, the temperature drops locally, and thermal stress can reduce the durability of the refractory bricks. However, since heat is removed more from the coke side and machine side and the refractory bricks are relatively cooler, they are less susceptible to the temperature drop caused by heat absorption. This has the effect of suppressing brick deterioration.

1A and 1B are schematic diagrams illustrating a method for arranging raw materials in a coke oven according to one embodiment of the present invention, in which FIG. 1A is a plan view and FIG. 1B is a side view. 1A and 1B are schematic diagrams illustrating a method for arranging raw materials in a coke oven according to a comparative example, in which FIG. 1A is a plan view and FIG. 1B is a side view.

The following describes in detail the embodiments of the present invention. The following embodiments are intended to exemplify equipment and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to that described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

(Method of arranging raw materials in a coke oven)
In this embodiment, when coal and molded synthetic resins are supplied as raw materials into the coke oven carbonization chamber, the synthetic resins are unevenly distributed at the ends of the coke oven in the longitudinal direction. That is, the synthetic resins are arranged so that the mixing ratio (= synthetic resin mass/(coal mass+synthetic resin mass)×100, mass-based percentage) of the synthetic resins on the coke discharge side (hereinafter referred to as the coke side (C/S)) and the coke extrusion side (hereinafter referred to as the machine side (M/S)) is as high as possible.

In this embodiment, it is preferable to divide the interior of the coke chamber into four or more equal regions in the longitudinal direction of the coke chamber, and to charge synthetic resins so that the mixing ratio of the synthetic resins in the region adjacent to the coke side and the region adjacent to the machine side exceeds the mixing ratio of the synthetic resins in the other inner regions. Furthermore, it is more preferable that the mixing ratio ratio be three times or more. In this case, it is preferable that the total mass of synthetic resins charged in the region adjacent to the coke side and the region adjacent to the machine side be 75% or more of the total. At this time, the mixing ratio of synthetic resins in the other inner regions is 50% or less of the overall average mixing ratio of synthetic resins. Therefore, the decrease in strength of the coke after carbonization in that region can be suppressed to 50% or less compared to when the synthetic resins are evenly distributed. While there is no upper limit on the number of regions the coke chamber can be divided into, taking into account the installation load of the raw material charging equipment, it is preferable to divide the coke chamber into 10 or fewer regions, and more preferably 6 or fewer.

The carbonization chamber of a typical coke oven is a long, narrow, roughly rectangular parallelepiped (for example, W 0.6m x D 15m x H 7m). To distribute the raw materials evenly throughout the carbonization chamber, coal is charged through multiple coal inlets aligned along the length of the chamber (direction D). By increasing the mixing ratio of synthetic resins charged through the inlet closest to the coke side or machine side, it is possible to concentrate and unevenly distribute the waste plastic near the coke side or machine side.

In this embodiment, the amount of synthetic resins added to the coke oven to be mixed with coal is not particularly limited, but from an environmental perspective, it is preferable to set the amount at 0.5 mass% or more relative to the total raw materials charged to the coke oven. More preferably, it is more than 1.0 mass%, and even more preferably, it is 3.0 mass% or more. There is no particular upper limit, but adding more than 5 mass% may reduce coke strength. In particular, when adding a large amount of synthetic resins, it is preferable to distribute the synthetic resins unevenly in the area near the coke side and machine side so that the amount per raw material charged is 1.0 mass% or less in the inner area excluding the area near the coke side and machine side.

(Molded synthetic resin products)
When the same mass of synthetic resins is added, the more they are added, the lower the coke strength becomes. When synthetic resins are carbonized in coal, they have a high volatile content, which causes voids to form after evaporation, resulting in voids inside the coke after carbonization. Since voids are the starting point for cracks inside the coke, it is thought that the more voids there are, the more cracks will occur, and the lower the coke strength becomes.

Synthetic resins are molded, for example, using a twin-screw extrusion molding machine to produce molded synthetic resin products. The synthetic resins are crushed or pre-granulated before supply. In this case, it is preferable to adjust the moisture content of the synthetic resins to 5% by mass or less. By reducing the moisture content of the synthetic resins, the synthetic resins can be molded stably and the density of the molded products increases. Various types of dryers, including hot air flow dryers, can be used to evaporate the moisture from the synthetic resins.

Any twin-screw extruder suitable for use in this embodiment and capable of molding synthetic resins can be used, with the basic structure remaining the same. The feedstock is mixed using twin screws housed in a casing, and the synthetic resin is extruded through a nozzle attached to a heated plate. The cylindrically extruded synthetic resin is cut with a rotary cutter to a specific length. The volume per synthetic resin is adjusted by the nozzle's inner diameter and the cutter's cutting speed. Nozzles with an inner diameter of 2.0 to 3.0 cm are commonly used. Using a large-diameter nozzle with an inner diameter of 4.0 to 6.0 cm makes it possible to produce large molded products. This increases the mass of synthetic resin per unit, increasing the amount that can be charged for the same number of units.

The molded synthetic resins are cylindrical with a diameter equal to or slightly larger than the inner diameter of the nozzle, and their length can be adjusted by the rotation speed of the cutter. The length of the molded product is affected by the position and condition of the forming nozzle, making it difficult to achieve a uniform length. This results in a distribution of molded products ranging from short to long. While the volume can be increased by increasing the maximum length of the molded product, it is preferable that the maximum length within the molded product length distribution be 20 cm or less. Because the diameter of the charging port at the top of the coke oven chamber is approximately 40 to 50 cm, increasing the maximum length beyond this may cause clogging. The average volume of the molded product is preferably 90 cm ^or more, more preferably 150 ^cm or more, and even more preferably 200 ^cm or more. The upper limit depends on the size of the charging port when charging into the coke oven, but is preferably 1000 cm ^or less, and more preferably 600 cm ^or less.

(Coke manufacturing method)
The molded synthetic resins are loaded into a hopper, fed at a constant speed by a metering feeder, and then fed onto the coal on a belt conveyor that supplies blended coal to the coke oven. In this embodiment, a larger amount of synthetic resins is distributed through the charging ports closest to the coke side and machine side, and a smaller amount of synthetic resins is distributed through the other charging ports. This reduces the mixing ratio of synthetic resins introduced through the other charging ports, thereby suppressing a decrease in coke strength after carbonization in that area. As an index of coke strength, the drum strength index (DI150/15) can be measured according to the drum strength measurement method specified in JIS K2151:2004, a coke test method. After 150 rotations, the coke is charged into a drum testing machine, sieved through a 15 mm mesh sieve, and the mass fraction remaining on the sieve is measured. When the drum strength index (DI150/15) is used, it is preferable that the decrease in DI150/15 be less than 1 point. It is known that the DI150/15 index has a measurement error of about 0.5 points, and a strength drop of 1 point or more clearly indicates a decrease in coke strength. In coke oven operations, a drop in coke strength (DI150/15) of 1 point or more is recognized as a decrease in coke strength, and operations such as improving the quality of the coking coal may be carried out.

The mixing ratio of synthetic resins is basically controlled by measuring the coal and synthetic resin discharge rates in advance and adjusting the respective discharge rates using the quantitative feeder's supply rate or gate opening to maintain a constant ratio. Furthermore, the mixing ratio of synthetic resins in the longitudinal direction (direction D) of a coke oven's carbonization chamber is difficult to measure directly. Therefore, it can be estimated from the mixing ratio of synthetic resins in each hopper of the coal car above the coal inlet of the carbonization chamber. When raw materials are charged into a single furnace through multiple charging ports in a carbonization furnace, they are charged simultaneously through each port, so the mixing ratio of synthetic resins directly below each charging port is considered to be the same as the mixing ratio of synthetic resins in each hopper of the coal car. The mixing ratio of synthetic resins in each hopper of the coal car can be estimated by installing a camera above each hopper, detecting synthetic resins through image analysis, and calculating the mass of synthetic resin charged.

The synthetic resin moldings should preferably be cut out from a location where the conveyor belt is low after passing through the coal drying facility (CMC). This is because the coal is heated and dried in the CMC, and if synthetic resin moldings are added before the CMC, there is a risk that they will melt inside the CMC. The synthetic resin moldings pass through a coal tower and coal transport car together with the blended coal and are supplied to the carbonization chamber. The synthetic resins are thermally decomposed in the carbonization chamber, and some remain as charcoal, but most are recycled as gas or tar.

(Example 1)
A mixture of waste plastics primarily composed of thermoplastic resins was crushed and confirmed to have a moisture content of 5% or less. The crushed mixture was then heated and extruded through a 40 mm diameter nozzle using a twin-screw extruder, cut, and formed into a cylindrical shape. As shown in FIG. 1 , the resulting molded product 1 was loaded into a carbonization can 3 along with approximately 40 kg of a coal blend 2 used as a coke raw material. To arrange the waste plastic molded products 1, the amount of the coal blend 2 was divided into eight portions, and first, 1/8 of the blended coal was laid in the carbonization can 3. A predetermined amount (1/4 of the planned amount to be charged) of waste plastic molded products 1 was then arranged so that they were in contact with one end of the carbonization can 3 in the longitudinal direction. Next, an additional 1/8 of the coal blend was charged and compacted to a charged coal density of 830 kg/ ^m3 . This process was repeated four times, and the waste plastic molded products 1 were arranged so that they were unevenly distributed at one end of the can in the longitudinal direction. FIG. 1( a ) is a plan view illustrating the arrangement of waste plastics in the carbonization can 1. FIG. 1(b) is a side view illustrating the arrangement of waste plastics within the carbonization vessel 1. The blend ratio of waste plastics to the mass of the blended coal was 3% by mass. After carbonization for 20 hours at 700 to 1100°C in a test carbonization furnace, the mixture was quenched by pouring water over it from above and then dried to obtain coke. The obtained coke was charged into a drum testing machine, rotated 150 times, and sieved through a 15 mm mesh sieve. The mass fraction of the remaining particles was measured as the drum strength index DI150/15 according to the drum strength measurement method specified in JIS K2151:2004, a coke testing method. Many cracks were observed on the side of the carbonized coke where the waste plastic molded product 1 was concentrated, suggesting a decrease in coke strength in that area. Meanwhile, the remaining coke was sound.

(Comparative Example 1)
As shown in Figure 2, waste plastic molded products 1, produced using the same method as in the example, were arranged as uniformly as possible in approximately 40 kg of coal blend 2. To arrange the waste plastic molded products 1, the amount of coal blend 2 was divided into eight parts, and 1/8 of the coal blend 1 was first laid in the carbonization tank 3. The waste plastic molded products 1 were then arranged so that their centers were located at 1/4 and 3/4 of the length from one end of the carbonization tank 3 in the longitudinal and transverse directions, respectively. This arrangement was made to correspond to 1/4 of the planned amount of waste plastic to be charged. This process was repeated four times, and when the carbonization tank 3 was divided into four longitudinal sections, the waste plastic mixing ratio was the same in each section. Figure 2(a) is a plan view illustrating the arrangement of waste plastics in the carbonization tank 1. Figure 2(b) is a side view illustrating the arrangement of waste plastics in the carbonization tank 1. The waste plastic mixing ratio relative to the mass of the coal blend was 3% by mass. After carbonization in a test carbonization furnace at 700 to 1100°C for 20 hours, the mixture was quenched by pouring water over it from above and then dried to obtain coke. The obtained coke was charged into a drum testing machine, rotated 150 times, and sieved through a sieve with 15 mm openings, in accordance with the drum strength measurement method specified in JIS K2151:2004, the mass fraction of the remaining coke was measured as the drum strength index DI150/15.

A comparison of the drum strengths of the obtained cokes is shown in Table 1. As a reference example, the strength of coke obtained by carbonizing only blended coal is also shown in Table 1. When waste plastic molded materials 1 are evenly distributed among the coal in the carbonization tank 3, as in the comparative example, the drum strength of the coke is significantly reduced compared to the reference example. It was found that by concentrating the waste plastic molded materials 1 at one end of the carbonization tank 3, as in the invention example, the decrease in drum strength compared to the reference example is suppressed.

(Example 2)
A mixture of waste plastics, primarily composed of thermoplastic resins, was crushed and confirmed to have a moisture content of 5% or less. The crushed material was then heated and extruded through a 40 mm diameter nozzle using a twin-screw extruder, cut into pieces with an axial length of 20 mm or more, and formed into cylindrical shapes. This material was mixed with a coke oven coal blend to a mixing ratio of 3% by mass and then loaded into the hopper of a coal delivery car above the coke oven as coke raw material. The coal delivery car was equipped with four hoppers. Of the hoppers, coal mixed with the waste plastics was loaded into the hoppers closest to the machine side and the coke side, respectively, while only coal was loaded into the inner hopper. Next, coke raw material was simultaneously extruded from the bottom of each hopper of the coal delivery car and loaded into the carbonization chamber. The waste plastic mixing ratio on the machine side and the coke side of the carbonization chamber was 3% by mass, and the waste plastic mixing ratio in the middle section was 0% by mass. The average waste plastic mixing ratio in the carbonization chamber was 1.5% by mass. The strength of the coke obtained after carbonization was measured according to the drum strength measurement method specified in JIS K2151:2004, Testing Methods for Cokes. As a result, the value of the drum strength index DI150/15, which is the mass ratio of the remaining coke when the coke was charged into a drum testing machine, rotated 150 times, and then sieved through a 15 mm mesh sieve, was 85.7. In addition, the coke cake extrusion property after carbonization was better than that of Comparative Example 2 described below.

(Comparative Example 2)
A mixture of waste plastics primarily composed of thermoplastic resins was crushed and confirmed to have a moisture content of 5% or less. The crushed mixture was then heated and extruded through a 40 mm diameter nozzle using a twin-screw extruder, cut into axial lengths of 20 mm or more, and formed into cylindrical shapes. This mixture was mixed with a coke oven coal blend to a mixing ratio of 1.5% by mass, and then evenly charged into four hoppers of a coal car above the coke oven as a coke raw material. Next, coal was simultaneously excavated from the bottom of each hopper of the coal car, so that the mixing ratio of the waste plastics in the coke chamber was evenly 1.5% by mass. The strength of the coke obtained after carbonization was measured according to the drum strength measurement method specified in JIS K2151:2004, a coke testing method. As a result, the drum strength index DI150/15, which is the mass fraction of the sieve remaining after sieving through a 15 mm mesh sieve after 150 rotations in a drum testing machine, was 84.0. Furthermore, the pushability of the coke cake after carbonization was worse than that of Example 2, and a larger power was required to discharge the coke cake.

1 (waste plastic) molding 2 blended coal 3 carbonization vessel (carbonization chamber)

Claims (4)

A method for arranging raw materials in a coke oven, in which synthetic resins are charged into a coke oven together with raw coal as coke raw materials, the carbonization chamber is divided equally into four to six regions in the longitudinal direction, and the mixing ratio of synthetic resins in the region adjacent to the machine side and the region adjacent to the coke side is at least three times the mixing ratio of synthetic resins in other inner regions . 2. A method for arranging raw materials in a coke oven as described in claim 1, wherein the synthetic resins are distributed so as to be 1.0 mass% or less per charged raw material in other inner areas excluding the area in contact with the machine side and the area in contact with the coke side. The raw material placement method for a coke oven according to claim 2, wherein the sum of the amounts of synthetic resins allocated to the area in contact with the machine side and the area in contact with the coke side is 75% or more of the amount of synthetic resins charged to the entire coke oven. A method for producing coke, comprising carbonizing synthetic resins and raw coal charged using the raw material placement method for a coke oven described in any one of claims 1 to 3.