Waste plastic processing method

Abstract

This waste plastic processing method is characterized by: when a carbonization chamber of a coke oven is to be charged by using a charging coal and a chlorine-containing waste plastic and dry distillation is to be performed thereon, charging, by using the chlorine-containing waste plastic, an oven-top space part (5) above the charging coal (11) used for charging the carbonization chamber (2); and, when the temperature of the oven-top space part (12) is 750℃ or higher, charging the carbonization chamber with the waste plastic (12) with which the oven-top space part (5) is charged. As a result, decomposition of a chlorine-containing gas generated from the chlorine-containing waste plastic in the oven-top space part (5) sufficiently progresses, and discharge of an organic chlorine gas to a riser tube (3) can be sufficiently reduced.

Description

How to dispose of waste plastic

[0001] The present invention relates to a method for treating waste plastics by charging coal for coke production (charging coal) and waste plastics into the carbonization chamber of a coke oven and carrying out carbonization.

[0002] Conventionally, the vast amounts of waste plastic generated as industrial plastic waste and general plastic waste have been disposed of by landfilling or partial incineration. When landfilled, waste plastics are not decomposed by bacteria in the soil, and when incinerated, the high heat output has a negative impact on incinerators. Furthermore, in the case of chlorine-containing waste plastics, the disposal of the chlorine in the exhaust gas poses a problem. Given the predicted future shortage of landfill sites and growing environmental concerns, there is a need to promote the recycling of waste plastics. Possible methods for recycling waste plastics include reusing them as plastics, utilizing the heat generated during combustion, and using the gases and oils obtained through thermal decomposition as fuels or chemical raw materials.

[0003] A method for recycling waste plastics by pyrolysis using a coke oven, which is one of the processes for steel production, has long been known. For example, Patent Document 1 discloses a method for reusing waste plastics by mixing plastic granules molded at 100 to 160°C with coal and carbonizing the mixture in a coke oven. On the other hand, Patent Document 2 describes that mixing a large amount of plastic reduces the strength of the coke, and that to limit the decrease in coke strength to a few percent, the mass ratio of plastic to coal needs to be 1% or less, and that the amount that can be reused is limited depending on the target level of coke strength.

[0004] One known method for processing waste plastics by adding them to a coke oven is to charge them into the space above the coal charged in the coke oven. According to Patent Document 3, this method allows most of the waste plastics to be pyrolyzed in the high-temperature coke oven, resulting in the production of high-calorie pyrolysis gases containing hydrogen, methane, ethane, propane, and the like. These pyrolysis gases are recovered as part of the coke oven gas generated in the coke oven chamber by the pyrolysis of coal, and are reused as an energy source.

[0005] Coke ovens can also be used to recycle waste plastics, as waste plastics are also hydrocarbons and can be carbonized together with or instead of coal to produce coke, tar, diesel, and fuel gas.

[0006] For example, Patent Document 4 discloses a method for producing coke by carbonizing coal in a coke oven and then charging waste plastics into the upper space of the coke oven chamber. This document describes a method for charging polymer waste in an atmosphere-shielded environment, in which a hopper is attached to a charging car and a cylindrical chute is attached to the coke oven's coal inlet (see Figure 5 of the document). It states that the preferred charging time is during the coke aging period (approximately 2 hours), which is the usual storage time between the end of carbonization (completion of carbonization) and the coke extrusion. During this time, the amount of coke oven gas generated is low, and the heat dissipated from the coke oven is large, allowing this heat to be utilized as gas decomposition heat. Patent Documents 3, 5, and 6 also disclose inventions in which waste plastics are charged into the top space of a coke oven after the end of carbonization (completion of carbonization).

[0007] Patent Document 7 discloses a method for treating waste plastics, which involves charging a coke oven with either cokemaking coal or cokemaking coal blended with 0.1 to 1.0 wt% of waste plastics, and then charging waste plastics into the top space above the charged coal, and then pyrolyzing the waste plastics during carbonization to recover them as tar, light oil, and gas. The method involves charging the waste plastics into the top space above the charged coal through a charging port after the coal is charged, and supplying the waste plastics from a supply device connected to the top space of the coke oven from after the coal is charged until the end of carbonization. These methods, in the case of the same document in which waste plastics were charged into the top space, are said to enable the treatment of the same amount of waste plastic as when about 5 wt% of waste plastics was blended into the charged coal.

[0008] Chlorine-containing resins such as polyvinyl chloride and polyvinylidene chloride are used in plastic products and then discarded together with other plastic products without sorting, so waste plastic inevitably contains chlorine components carried over from the chlorine-containing resins. In fact, waste plastics collected and separated from households usually contain polyvinyl chloride and polyvinylidene chloride, which contain several mass percent of chlorine when converted to chlorine. When chlorine-containing resins such as polyvinyl chloride are thermally decomposed at high temperatures, chlorine-based gases are generated.

[0009] Patent Documents 8 and 9 disclose methods for safely recycling chlorine-containing resins, chlorine-containing organic compounds, or waste plastics containing them (chlorine-containing waste plastics). These methods involve pyrolyzing chlorine-containing waste plastics in a coke oven, contacting the resulting pyrolysis gas containing chlorine-based gases with an ammonia-containing gas or liquid, incorporating the chlorine in the pyrolysis gas as ammonium chloride into water, and then adding a strong base to convert the chlorine into a strong base salt. When pyrolyzing chlorine-containing waste plastics in a coke oven, the chlorine-containing waste plastics are charged into a carbonization chamber without or after premixing with coal, and then pyrolyzing the chlorine-containing waste plastics together with the coal. When charging chlorine-containing waste plastics into a coke oven together with coal for recycling, the chlorine concentrations in the diesel oil and tar produced as by-products in the coke oven increase. However, it has been confirmed that these concentrations are below the upper limit of each operational tolerance under these conditions, causing no problems.

[0010] JP 2001-49261 A JP 2019-135278 A JP 2002-47494 A JP 4-41588 A JP 2001-323280 A JP 11-263980 A JP 8-157834 A JP 2001-123180 A JP 2000-104075 A

[0011] As described in Patent Documents 8 and 9, when chlorine-containing waste plastics are charged into a coke oven together with coal for recycling, it has been found that the amount of chlorine remaining in the oil (tar and diesel) produced as a by-product and in the gas (COG) produced increases, and further increase in the amount of chlorine-containing waste plastics used leads to problems in quality control.The first object of the present invention is to provide a method for treating waste plastics, which suppresses the increase in chlorine content in the oil and gas produced when chlorine-containing waste plastics are blended and carbonized in a coke oven.

[0012] In the final stage of coal carbonization in a coke oven, the upper part of the carbonization chamber becomes overheated, causing problems such as carbon formation. A second object of the present invention is to provide a method for treating waste plastics that does not cause problems such as carbon formation during coal carbonization in a coke oven.

[0013] That is, the gist of the present invention is as follows: [1] A method for treating waste plastics in which charge coal and chlorine-containing waste plastics are charged into a carbonization chamber of a coke oven and carbonized, wherein the chlorine-containing waste plastics are charged into a top space above the charge coal charged into the carbonization chamber, and the chlorine-containing waste plastics charged into the top space are charged into the carbonization chamber when the temperature of the top space is 750°C or higher. [2] The method for treating waste plastics according to [1], in which the chlorine-containing waste plastics charged into the top space are charged into the carbonization chamber when the temperature of the top space is 800°C or higher.

[0014] The present invention provides a method for treating waste plastics, in which coal for coke production (charging coal) and waste plastics are charged into the carbonization chamber of a coke oven and carbonized, in which chlorine-containing waste plastics are charged into the furnace top space above the charged coal charged into the carbonization chamber, and the waste plastics charged into the furnace top space are charged into the carbonization chamber when the temperature of the furnace top space is 750°C or higher, thereby suppressing an increase in the oil content generated and the chlorine content remaining in the gas.

[0015] 1 is a cross-sectional view showing an example of a state in which waste plastics are charged into a furnace top space portion of a carbonization chamber. FIG. 2 is a diagram showing the relationship between the secondary decomposition temperature of a secondary furnace of a test furnace for dry distilling chlorine-containing waste plastics and the organic chlorine generation rate.

[0016] The coal charged into a coke oven as coal for coke production will be referred to hereinafter as charged coal 11. The present invention relates to a method for treating waste plastics in which the charged coal 11 and waste plastics 12 are charged into a carbonization chamber 2 of a coke oven 1 and carbonized (see FIG. 1 ).

[0017] Chlorine-containing waste plastics contain chlorine as organic matter. The chlorine content in the chlorine-containing waste plastics charged into the carbonization chamber of a coke oven is thermally decomposed in the high-temperature environment inside the furnace and decomposed into organic chlorine-based gases and inorganic chlorine-based gases. The inorganic chlorine-based gas discharged from the carbonization chamber 2 through the riser 3 together with COG comes into contact with an ammonia-containing gas or liquid, as described in Patent Documents 8 and 9, and the chlorine content in the pyrolysis gas is absorbed into water as ammonium chloride. Furthermore, a strong base can be added to convert the chlorine content into a strong base salt.

[0018] However, when chlorine-containing waste plastics are mixed with the charged coal 11 and carbonized in a coke oven, thermal decomposition does not proceed sufficiently in the carbonization chamber 2, and organic chlorine gases are sometimes discharged from the riser 3 without being completely decomposed. In this case, it was found that the organic chlorine remains in the exhaust gas and mixes with the oil (tar and diesel) produced as a by-product, preventing it from being removed with ammonia water, and chlorine remains in the gas and oil of the recycled product. If the chlorine content exceeds the allowable limit, the product cannot be used commercially.

[0019] In the present invention, it was conceived that by charging waste plastic 12 into the furnace top space 5 above the charged coal 11 charged into the carbonization chamber 2 and limiting the conditions for charging the waste plastic, it may be possible to solve the above-mentioned quality control problems.

[0020] Experiments were conducted using an offline test furnace. The test furnace had a carbonization furnace and a secondary furnace directly above it. The chlorine content of the waste plastic used as the sample was 2.4% by mass. 50 g of sample was placed in the carbonization furnace, and the waste plastic was carbonized to 1000°C at a heating rate of 5.1°C / min. A secondary furnace was installed directly above the carbonization furnace. The temperature of the secondary furnace corresponded to the secondary decomposition temperature in the furnace top space. The effect of the secondary decomposition temperature in the furnace top space on the organic chlorine generation rate was evaluated by changing the temperature of the secondary furnace. The generated gas was collected and the total chlorine contained in the gas was measured. Here, inorganic chlorine was removed by passing the gas through water before gas collection, so the measured total chlorine amount was treated as approximately equal to the organic chlorine amount.

[0021] Figure 2 shows the relationship between the temperature of the secondary furnace (secondary decomposition temperature) and the organic chlorine generation rate (%) calculated by dividing the amount of organic chlorine generated by the input chlorine by 100. It can be seen that by setting the temperature of the secondary furnace to 750°C, the organic chlorine generation rate was significantly reduced to about 0.06%. Furthermore, when the temperature of the secondary furnace was increased to 800°C or higher, the organic chlorine generation rate was reduced by another level, to about 0.05%. Furthermore, at temperatures above 800°C, the organic chlorine generation rate did not change significantly.

[0022] After charging coal (charging coal 11) into the carbonization chamber 2, carbonization progresses, and the temperature of the charged coal 11 gradually increases over time until the carbonization is completed and the coal is pushed out. The temperature inside the furnace also increases in the top space 5 above the charged coal 11 as the carbonization progresses. The timing for charging the waste plastics 12 into the top space 5 can be selected from immediately after the coal is charged to immediately before the coal is pushed out after the carbonization is completed. Based on the results of the offline test furnace, it was found that charging the waste plastics 12 into the top space 5 when the temperature of the top space 5 is 750°C or higher can suppress the residual chlorine content in the generated oil as organic chlorine. This is described in detail below.

[0023] When chlorine-containing waste plastics are charged into the furnace top space 5 above the charged coal 11 in the carbonization chamber 2, thermal decomposition of the chlorine-containing waste plastics begins immediately after charging. In the furnace top space 5 within the carbonization chamber, gas generated by the dry distillation of the charged coal 11 is discharged from the riser pipe 3, causing a gas flow from the charged coal 11 toward the riser pipe 3 in the furnace top space 5. The chlorine-containing gas generated by the thermal decomposition of the chlorine-containing waste plastics (waste plastics 12) charged into the furnace top space 5 flows toward the riser pipe 3 together with the gas flow in the furnace and is discharged from the riser pipe 3. The chlorine-containing gas generated by the thermal decomposition of the chlorine-containing waste plastics initially contains organic chlorine. The inventors have discovered that if the temperature of the furnace top space 5 is 750°C or higher, the organic chlorine in the gas is thermally decomposed and converted to inorganic chlorine while moving through the furnace top space 5. On the other hand, if chlorine-containing waste plastics are charged into the furnace top space 5 at the beginning of the dry distillation when the temperature of the furnace top space 5 is low, the progress of thermal decomposition will not be observed due to the low temperature, and the organic chlorine-based gas will reach the riser 3 with remaining residue.

[0024] The present invention has found that by charging the chlorine-containing waste plastics 12 into the carbonization chamber 2 when the temperature of the furnace top space 5 is 750°C or higher, the decomposition of the chlorine-containing gas in the furnace top space 5 proceeds sufficiently quickly, and the possibility of the generated organic chlorine-based gas being discharged into the riser pipe 3 can be sufficiently reduced. It is more preferable that the temperature of the furnace top space 5 is 800°C or higher. On the other hand, the upper limit of the temperature of the furnace top space 5 when the chlorine-containing waste plastics 12 are charged is preferably 1200°C.

[0025] The chlorine content of the chlorine-containing waste plastics is not particularly specified, but on average, 5%, 3%, 1.5%, 1.0%, or 0.5% is used.

[0026] In order to charge the waste plastics 12 when the temperature of the furnace top space 5 is 750°C or higher, it is necessary to avoid the timing when the temperature drops immediately after charging the coal, and to adjust the input heat amount and the amount of coal charged so that the temperature of the furnace top space 5 becomes steady at 750°C or higher. The temperature of the furnace top space 5 can be measured by hanging a thermocouple into the furnace top space from a hole drilled in the charging lid.

[0027] As mentioned above, during coal carbonization in a coke oven, the upper part of the coking chamber becomes overheated toward the end of the coal carbonization process, causing problems such as carbon formation. In the waste plastic processing method of the present invention, even after the coal in a given kiln has completed carbonization, the adjacent kiln continues carbonization and heating from both combustion chambers continues. This excess heat causes the kiln to become overheated, and the temperature in the furnace top space 5 continues to rise until the kiln is unloaded. By charging waste plastic 12 into the furnace top space 5 at this excess heat timing, the waste plastic absorbs the heat required for carbonization, thereby suppressing the temperature rise in the furnace top space 5. This suppresses the rise in oven wall temperature, preventing carbon formation and other problems during coal carbonization in a coke oven.

[0028] The present invention was applied to a coke oven in which the amount of coal charged into the coke chamber was 13 tons / charge. The moisture content of the charged coal was 2%, and the chlorine-containing waste plastic used had a chlorine concentration of 0.8% by mass.

[0029] In Comparative Example 1, 200 kg / charge of chlorine-containing waste plastics was mixed with the charged coal in advance and charged into the carbonization chamber. In Reference Example, no chlorine-containing waste plastics was added, and normal carbonization was carried out using only the charged coal.

[0030] In the present invention, coal was charged into the carbonization chamber, and 15 hours after the coal was charged, 120 kg / charge of chlorine-containing waste plastics 12 was added to the furnace top space 5 above the charged coal 11 (see FIG. 1 ). The temperature of the furnace top space 5 when the waste plastics 12 were charged was measured by inserting a thermocouple through the charging port 4, and the temperature of the furnace top space 5 when the waste plastics were charged was 815°C.

[0031] In Comparative Example 2, coal was charged into the carbonization chamber, and when the temperature of the furnace top space reached 710°C, 120 kg / charge of chlorine-containing waste plastics 12 was added to the furnace top space 5 above the charged coal 11 (see Figure 1). The temperature of the furnace top space 5 when the waste plastics 12 were added was measured by inserting a thermocouple through the charging port 4.

[0032] In Comparative Example 1, Comparative Example 2, Reference Example, and Inventive Example, gas was sampled at the riser bend. Gas sampling was performed over the entire charge, from the charging of the coal to the discharge of the coke, with a fixed amount of gas sampled every 20 to 30 minutes. During sampling, the gas was passed through an aqueous NaOH solution, and then samples were collected using a gas bag. It has been confirmed that inorganic chlorine can be removed by passing the gas through NaOH. The total chlorine concentration in the gas was measured by combustion ion chromatography as an analytical method. The chlorine concentration approximation curve was calculated from the total chlorine concentration in the sampled gas obtained, and the amount of chlorine in the charge (mg / charge) was calculated by multiplying this by the time-varying curve of the amount of gas generated in the charge. The results are shown in Table 1.

[0033]

[0034] The amount of organic chlorine in Comparative Example 1 (when coal was blended) was 4901 mg / charge, the amount of organic chlorine in Comparative Example 2 (when waste plastic was charged into the top space) was 2725 mg / charge, the amount of organic chlorine in the Reference Example (when coal only was used) was 2263 mg / charge, and the amount of organic chlorine in the Example of the present invention (when waste plastic was charged into the top space) was 2396 mg / charge. The chlorine content of the Reference Example was subtracted from the chlorine content of Comparative Example 1, Comparative Example 2, and the Example of the present invention to determine the amount of organic chlorine derived from the chlorine-containing waste plastics. The amount of organic chlorine derived from the chlorine-containing waste plastics in Comparative Example 1 (when coal was blended) was evaluated as 4901 - 2263 = 2638 (mg / charge), and the amount of organic chlorine derived from the chlorine-containing waste plastics in the Example of the present invention (when waste plastic was charged into the top space) was evaluated as 2396 - 2263 = 133 (mg / charge). Comparative Example 2 is expressed in a similar manner. The amount of organic chlorine derived from waste plastics was divided by the amount of waste plastic charged (kg / charge) for each of Comparative Example 1, Comparative Example 2, and the Inventive Example to calculate the increase coefficient of organic chlorine amount per kg of waste plastic (mg / kg). The results were 13.2 mg / kg for Comparative Example 1, 3.85 mg / kg for Comparative Example 2, and 1.1 mg / kg for the Inventive Example, and the Inventive Example had a significantly lower increase coefficient of organic chlorine per kg of added waste plastics than the waste plastic-derived organic chlorine in Comparative Example 1 (from 13.2 mg / kg to 1.1 mg / kg). The Inventive Example also had a significantly lower increase coefficient of organic chlorine than Comparative Example 2.

[0035] The preferred range of the organic chlorine content increase coefficient per kg of waste plastic is 1.1 to 1.7 mg / kg. At levels exceeding 1.7 mg / kg, the furnace top space temperature exceeds 1,000°C, resulting in an overheated state. Therefore, the upper limit was set at 1.7 mg / kg. On the other hand, at levels below 1.1 mg / kg, the furnace top space temperature falls below 750°C, resulting in an underheated state near a non-steady state. Therefore, the lower limit was set at 1.1 mg / kg.

[0036] In the present invention, the amount of organic chlorine in the gas at the bend of the riser pipe was significantly reduced compared with Comparative Examples 1 and 2, and therefore the amount of chlorine remaining in the oil (tar and diesel) produced as a by-product from the coke oven and in the gas (COG) produced was reduced, and it was found that the problem of quality control could be solved. Note that inorganic chlorine in the gas at the bend of the riser pipe was removed from the exhaust gas by the reaction with ammonia water.

[0037] REFERENCE SIGNS LIST 1 Coke oven 2 Carbonization chamber 3 Rising pipe 4 Charging port 5 Furnace top space 11 Charging coal 12 Waste plastic

Claims

1. A method for treating waste plastics in which charged coal and chlorine-containing waste plastics are charged into the carbonization chamber of a coke oven and carbonized, wherein the chlorine-containing waste plastics are charged into the top space above the charged coal charged into the carbonization chamber, and the chlorine-containing waste plastics charged into the top space are charged into the carbonization chamber when the temperature of the top space is 750°C or higher.

2. A method for treating waste plastics as described in claim 1, characterized in that the chlorine-containing waste plastics charged into the furnace top space are charged into the carbonization chamber when the temperature of the furnace top space is 800°C or higher.

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