Brief introduction to the development process of Japan’s technology for coking with waste plastics in coking coal

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After Nippon Steel succeeded in developing a method for coking using 1% waste plastics in coking coal in 2000, its energy utilization rate was as high as 94%, far exceeding the 75% achieved through blast furnace injection and the 65% obtained through gasification and oilification. Additionally, the proportion of chlorinated waste plastics such as PVC that could be used could be increased to 5% (compared to 2% for blast furnace injection). Moreover, it replaced coking coal, which requires large amounts of expensive coal dust. As a result, its usage volume exceeded that of blast furnace injection by 2002. Subsequently, a technique was developed to increase the proportion of recycled plastic to 2%, and this was rolled out across the 5 steel plants under its ownership, with a target of using 380,000 tons by 2010. In contrast, as the world’s largest coking country, a large amount of waste plastic remains unused in our country. To implement the principle of giving priority to energy conservation and environmental protection in the 11th Five-Year Plan, it is urgent to make use of this technology. Based on daily publications, a comparison of this technology overview is provided below for reference. Adding 1% waste plastic to coking coal yields good results, but further research is needed to increase the amount used. According to the \"Containers and Packaging Recycling Law\" enacted in 1995 and fully implemented in 2000, household waste plastics that were previously disposed of through landfilling or incineration must now be recycled; the Waste Plastic Association collects a fee of 20,000 to 40,000 yen per ton and passes it on to other organizations for recycling purposes. Following the successful use of waste plastics to replace coal in the blast furnaces at the NKK Kyobin plant by the Japan Iron and Steel Federation in 1995, the federation outlined in its voluntary corporate plan for energy conservation and environmental protection up to 2016, published in 1996 with a focus on reducing CO2 emissions, a goal of using 1 million tons of waste plastics by 2010 in order to achieve a 1.5% reduction in energy consumption in the steel industry. Based on this, Nippon Steel successfully developed a coking technology that allows 1% waste plastic to be incorporated into coking coal, building upon waste plastic oilification technology. The main details are as follows: (1) Composition of waste plastics: PE 20%, PS 26%, PP 14%, PVC 4%, PVOC 1%, PET 15%, and others 20%. (2) Analysis of major elements in mixed waste plastics: C 72.6%, H 2.9%, N 0.3%, S 0.04%, ash 5%. (3) Pretreatment process: Recycling waste plastic → Manual removal of large impurities → Rough crushing → Mechanical removal of further impurities → Secondary grinding → Reduction in volume and solidification into small particles → Mixing with coking coal. (4) Production and recovery conditions: Coals are loaded into a coke oven for dry distillation, where most of them undergo thermal decomposition and gasification at temperatures of 1100–1200°C. The gaseous portion is recovered at the top of the oven at 900°C; after cooling to 80°C, some crude tar is recovered. The recovery ratio is 20% for coke, and 40% each for coke oven gas and chemical by-products. (5) Coke quality. Through the drum test and thermal reaction intensity test, DI15150 was 82.6% and the CSR value was 59.5%, both of which are the same as those of conventional coke. Based on this, 40,000 t/a pretreatment units were built at the Kunisaki Plant and the Nagoya Plant in the winter of 2000 and put into formal operation ； In 2001, while recycling 80,000 tons of waste plastics, pre-treatment facilities with a capacity of 20,000 tons per year were established at the Yawata and Muroran plants; as a result, a total of 120,000 tons of waste plastics were utilized in 2002, exceeding the 110,000 tons used for blast furnace injection at the NKK Kyobin and Fukuyama plants. In 2003, it maintained a lead of 150,000 t in the bidding process. This is mainly due to its numerous advantages: (a) an energy conversion efficiency of up to 94%, which is better than the 75% achieved by blast furnace injection ； (b) The content of chlorinated waste plastics such as PVC can be relaxed to 5%, which is higher than the 2% used for blast furnace injection ； (c) The price of the substitute coking coal is 50~60% higher than that of coal powder ； This results in better overall economic benefits. However, the drawback is that the incorporation ratio is too low. To increase the incorporation level, it is first necessary to determine the role of different types and particle sizes of major waste plastics during the dry distillation process, in order to ensure that the quality of the coke is not affected. Based on this, the following experiments were conducted. Test results and preliminary analysis (1) The effect of plastic particle size on coke strength is shown in Table 4. Table 4: Effect of plastic particle size on coke strength (pure coal value: 83.3) Parameter: Plastic particle size (particle diameter in mm): 0.29, 0.74, 3.67–3.81, 10–17, 30–35; Corresponding coke strength (DI15150): PE – 84.5, 83.4, 82.5, 82.7, 83.1; PS: 82.4, –, 79.6, 80.2, 80.8; Mixed material: –, –, –, –, 82.0. It can be seen from the above that the lowest values of coke strength were achieved when the PE particle size was between 10–17 mm and the PS particle size was between 3–4 mm ； At the same particle size, PS has the lowest value, followed by the mixture ； This is related to the loss of coal cohesion after adding PS. When 0.74 mm of PE is added, the strength actually increases compared to pure coal, and the same result was observed in tests with movable fireplaces; the reasons will be explained later. (2) Analysis on the improvement in coke strength by adding PE powder. Through a comparative microscopic observation of coke production from pure coal and coke production with the addition of PE powder, it was found that the pore structure changed significantly when plastic was added; the pores became circular and larger in size. According to relevant studies, the addition of PE increases the pressure of the plastic pyrolysis gases generated within the softened and molten layer, which in turn leads to densification of the re-solidified layer and an increase in the strength of the coke. Furthermore, the results of the movable furnace tests show that as the porosity of the coke increases in the direction of the furnace width, the value obtained by adding PE powder is 2–3 percentage points higher than that with pure coal ； The thermal decomposition temperature of PE was also measured to be 400–480°C, which is roughly consistent with the temperature range at which the two types of coal begin to soften and solidify. It is inferred that the pyrolysis gas of PE promotes the growth and merging of bubbles in the softened and molten coal particle layer, thereby enhancing coke strength. (3) Influence of plastic particle size on the coking strength. Based on the research results of coking fines ≤15mm generated by the drum test, those ≤6mm are products of surface destruction, while those ranging from 6 to 15mm are products of volume destruction. Generally, the strength of surface failure is expressed as DI6150 (100 – percentage of powder resulting from surface failure), while the strength of volume failure is expressed as DI6-15150 (percentage of powder resulting from volume failure). The test data for this time are shown in Table 5. Table 5 Relationship between plastic particle size and coke failure strength. Item: Pure coal; PE particle size (mm); PS particle size (mm). For blended plastics: 30.62, 0.74, 3.81, 10.58; 30.62, 87.75; 0.29, 3.81, 16.74, 34.55. DI6150: 85, 86.6, 85.6, 84.8, 84.8, 85.1, 84.3, 81.6, 82.7, 83.0, 84.2. DI6-15150: 1.7, 2.1, 2.2, 2.4, 2.2, 2.0, 1.8, 2.1, 2.3, 2.1, 2.2. As can be seen from the above, in terms of surface failure strength, it varies with particle size, and each type has a minimum value; at the same particle size, the order is PE, blended plastics, and PS. In terms of volume failure strength, the three plastics show little difference, with a thickness of around 10 mm having the least impact on strength. (That is, the powder content is highest at 6–15 mm.) When comparing the above data with that of pure coal, it was found that except for the coarse-grained PE (10.53–87.75), which mainly affects DI6-15150, the others mainly affect DI6150. (4) Analysis of the reasons for the extremely low surface failure strength. When the temperature is increased at a rate of 3°C per minute, the thermal decomposition temperature range of PS in a nitrogen atmosphere is 350–420°C, which is lower than the thermal decomposition range of PE as well as the melting to solidification ranges of the two types of coal; therefore, its DI6150 value is slightly lower than that of PE and pure coal. By comparing the microscopic images of coke formation when 2% of particles were added to both PE and PS, it was found that voids of sizes corresponding to those of the plastic particles were formed in both cases. In particular, around the voids formed after the thermal decomposition of PS, there were fragile layers of coke with poor bonding between the coal particles; this was due to the adverse effect of the gases generated during decomposition on the melting and bonding of the coal particles. In contrast, in PE, where the thermal decomposition temperature is higher, there are fewer areas with poor bonding around the voids. When -w (g) of plastic is added to coal, with a particle size of 2r (cm), a density of ρ (g/cm3), a number of particles of N, and a contact area between the coal and the plastic of A (cm2), it can be seen that A is inversely proportional to 2r·ρ (as shown in the following equation). -w = N × ρ(4πr³/3); A = N × 4πr² = (4π – wr²) / (4πρr³/3) = (3 – w/ρ) / r. It can be seen from the above that, when the ratio of plastic to coal is constant, the larger the particle size of the plastic, the smaller its contact area with the coal; as a result, the surface layer prone to failure due to surface damage also becomes smaller ； Furthermore, the molten layer of coal is generally 5 mm thick; therefore, when the particle size of plastic is larger than this value, the pyrolysis gases will penetrate through the coal layer and the coke layer, thereby eliminating the effect of softening and melting the coal layer to enhance coke strength. However, for plastic powders with a very fine particle size, not only can the aforementioned effects be achieved, but the voids can also be filled by expanded coal, thereby increasing the strength of the coke. It can be summarized as follows: 1. When the particle size of PE is small, its thermally decomposed gases can promote the growth and merging of bubbles within the softened and molten layer of coal, thereby causing changes in the pore structure of coke, which in turn helps to improve the strength of coke. When the particle size of 2 PS is small, the voids formed after thermal decomposition are filled by the expansion of the coal and buried within the pore walls, resulting in very few defects that affect coke strength; thus, the decline in coke strength can also be suppressed. 3 As the particle size of PE and PS exceeds 3–10 mm, the contact area between the plastic and coal decreases; as a result, the formation of fragile layers due to surface damage is reduced, which in turn helps to prevent a decline in coke strength. 4 When the plastic particle size is between 3 and 10 mm, fragile and large coke structures form around the voids created upon the thermal decomposition of the plastic, resulting in extremely poor surface strength of the coke. Furthermore, since PS affects the cohesion of coal, its impact on the particle size of coke is smaller than that of PE. At the same time, coal with high expansibility can be used to fill its voids; in this case, the particle size can be increased appropriately to reduce the impact on the strength of damage to the coke surface. (5) Analysis of plastic particle size factors affecting the volumetric failure strength of coke. Based on the comparative analysis of X-ray CT scans of coke, the following relationship can be observed: 1 When the particle size of plastics is less than 1 mm, the voids formed as a result of their thermal decomposition can be filled in by the expansion of coal, thereby being buried within the pore walls. 2 When the plastic particle size is 3–10 mm, the voids resulting from the thermal decomposition of the plastic within the coke mass are evident, with fragile coke structure surrounding these voids. Especially when the particle size reaches 8–10 mm, cracks several millimeters long form starting from the voids, which significantly undermines the strength of the coke. 3 When the plastic particle size reaches 25 mm, the voids formed upon its thermal decomposition even become visible on the surface of the coke, which is further detrimental to the strength of the coke. 4 The thickness of the fragile carbon layer surrounding the voids formed after plastic pyrolysis is greatest for PS, which hinders coal cohesion. (6) The influence of plastic particle size on the CRI and CSR of coke, etc. The specific test results are shown in Table 6. Table 6: Effect of Plastic Particle Size on CRI and CSR of Coke
Table | Parameter | Pure Coal | PE Particle Size (mm) | PS Particle Size (mm) | Blended Plastic | 30.62 | 0.74 | 3.81 | 10.58 | 30.62 | 87.75 | 0.29 | 3.81 | 16.74 | 34.55 |
CRI | 25.2 | 25.6 | 26.2 | 27.2 | 27.2 | 25.4 | 29.6 | 27.8 | 27.4 | 26.6 | 26.5 |
CSR | 64.5 | 67.5 | 66.0 | 64.4 | 63.5 | 64.8 | 58.0 | 60.0 | 59.5 | 61.2 | 62.6 |
From the above, the following points can be observed: 1. When the plastic particle size increases to over 10 mm, CRI decreases while CSR shows an upward trend; this is likely due to a reduction in fragile structures with high reactivity. The CRI of the 2 PE powder is lower than that of other larger-grained samples and is close to that of pure coal; this may be due to the effect of thermal decomposition, which causes the pore structure to change in a way that reduces reactivity. The CRI of the 3 PS powder is significantly higher than that of the other larger particle sizes; this may be due to the residual dispersion of highly reactive PS thermal decomposition residues (defective carbon), and the increase in microcracks also enhances reactivity. In summary, regarding the effect of adding plastics on reactivity, further evaluation in the future should be conducted from aspects such as the co-carbonization effects of plastics and coal, as well as the analysis of the porous structure of coke. (7) Effect of adding waste plastic on the swelling pressure. Through the measurement and analysis of the changes over time in the expansion pressure and the air pressure within the softened and molten layer during the dry distillation process, it was found that when PE powder and particles were added, the air pressure and expansion pressure within the softened and molten layer were extremely high, remaining at high levels throughout the process; the peak value reached after about 10 hours of dry distillation was 3.5 to 3.8 times that of pure coal. When reduced-volume solidified products containing mixed waste plastics and pure coal were used, the expansion pressure and gas pressure remained essentially unchanged in the initial stage, with only a slight increase observed after 9 hours of carbonization. Preliminary analysis suggests that since the thermal decomposition temperature range of PE coincides with the softening and melting temperature range of coal B, the PE thermal decomposition gases are trapped by the softening and melting layer of the coal with poor permeability, resulting in an increase in pressure. It is predicted that if PE powder is added to other coal types with high air permeability resistance and high expansion pressure in their softened and molten layers, the air pressure and expansion pressure of those softened and molten layers will also increase. In this experiment, it was also observed that the addition of PE led to an increase in both coke strength and expansion pressure; however, there is generally no direct causal relationship between the two, and this may be a special case. Conclusions and brief applications: Based on the above experimental and analytical results, the preliminary conclusions are as follows: (1) The coke strength is lowest when the plastic particle sizes are 10 mm for PE and 3 mm for PS; as these particle sizes increase, the contact area between the plastic and coal decreases, which in turn reduces the formation of fragile structures around the voids generated by thermal decomposition ； When the particle size is reduced further, the voids are filled by the expansion pressure of the coal, which is beneficial for improving coke strength. (2) The addition of PE particles can improve coke strength, mainly because the pyrolysis gases of PE are trapped within the softened and molten layer of coal, which promotes the growth and merging of bubbles, thereby improving the pore structure of the coke. (3) Adding 2% mixed waste plastics to coking coal does not affect the quality of the coke; the size of the compacted material resulting from this addition should be around 20 mm, and plastics such as PS and PET, which can affect the cohesion of the coal, should be used as little as possible. Since 2004, the original 4 plants have expanded their pretreatment capacity and have been using this capacity for production in accordance with the aforementioned methods. Starting in 2005, the Oita plant also installed pretreatment equipment and began to use it as well. In 2005, the total amount used reached 200,000 tons, and efforts are underway to increase this figure to 380,000 tons by 2010. (End) This post was last edited by zslhsy369 on 2009-3-15 18:36.]