Brief introduction of waste plastic injection tech

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According to the policy of giving priority to energy conservation and environmental protection in the eleventh five year plan, the iron and steel industry, as a major energy consumer, should further pay close attention to energy conservation. Blast furnace injection of waste plastics through exhibitions is a mature energy-saving and environmental protection technology in Japan, but its application in China is still blank, and it is urgent to catch up with it as soon as possible in the process of China's steel industry from big to strong. According to the materials in the journal, the process of developing this technology by JFE steel in Japan is briefly introduced as follows for relevant reference

development background

Japan is a large economic country with a narrow territory and poor resources. It attached great importance to energy conservation after the oil crisis in 1973; In the 1990s, we began to solve the shortage of landfill sites, and then paid close attention to waste recycling to expand energy conservation; After the adoption of the "Kyoto Protocol" in 1997, it undertook the task of reducing CO2 emissions by 6% in 2010 compared with 1990. Therefore, the "basic law for building a circular society" and a number of supporting regulations were promulgated in 2000, which became a lasting driving force for the development and continuous development of blast furnace injection waste plastic technology

the volume of waste plastics accounts for a very high proportion of waste, most of which are treated by landfill and incineration. However, after landfilling, it is not easy to degrade, causing pollution of underground water sources. During incineration, PVC and other chlorine containing waste plastics are easy to generate highly toxic dioxins, which is very harmful to people's health. Therefore, it has become a key object requiring classified recycling and expanded reuse in the "container packaging recycling law" promulgated in 1995. JFE steel Jingbin plant has been injecting waste plastics into blast furnaces since 1995

in addition, the iron and steel industry is a high-energy consumption industry. Although the energy efficiency of Japan's iron and steel industry ranks first in the world, its energy consumption still accounts for 11% of the country, and the CO2 emission in 2004 was as high as 185 million tons; In addition, raw fuels are basically imported, with an annual import volume of 1.2 billion tons. Therefore, the samples prepared in the 2010 enterprise voluntary plan for energy conservation and environmental protection with CO2 emission reduction as the center organized and released by the iron and Steel Union in 1996 meet the corresponding standards for mechanical testing of metal materials, which stipulates that the energy saving is 10.5% higher than that in 1990 (including 1million tons of waste plastics consumed, equivalent to 1.5% energy saving), thus becoming a direct and lasting driving force to promote the injection of waste plastics into blast furnaces

the specific background of research and development is as follows:

(1) social problems of waste plastic utilization. Due to the wide range of uses, waste plastics are often mixed together, so it is difficult to use them as raw materials. For example, in 1995, the recycling rate was only 25%, far lower than 66% of the steel tank shell of other container packaging waste. In terms of the development of recycling technology, oilization technology was developed at the initial stage, but due to the strict separation of raw materials, the cost was high, the quality of finished oil was worse than that of oil, and the oil price was low in the late 1990s, so it was not practical. Therefore, great importance was attached to the development of new recycling technology

(2) development process. JFE steel obtained the patent of synthetic plastics as blast furnace reductant in the 1970s, but missed the opportunity of practical technology development because the social system of recycling waste plastics has not been established. After entering the 1990s, the society has improved its understanding of the rational utilization of resources, so it has re studied the technology of using waste plastics in blast furnaces, mainly including: (a) loading ore and coke together from the top of the furnace; (b) The method of injecting hot air into the blast furnace from the tuyere to replace coal

when adding from the upper part of the blast furnace, the waste plastic is decomposed into hydrogen carbide dominated by tar at about 300 ℃ and discharged from the top of the furnace, so that the effect of coke saving is not obvious, and it has a negative effect on improving the permeability of the furnace charge. Therefore, it is decided that the blast furnace should use waste plastic to inject from the lower part, and the technical development is focused on this aspect. In 1993, referring to the experience of countries with low chlorine containing waste plastics such as PVC in Germany, that is, in addition to the small blast furnace blowing from the tuyere, it was also treated by the chemical plant, but the latter's energy utilization rate was low, so it was decided to concentrate on the development of high-efficiency technology for blast furnace blowing waste plastics

(3) the subject of injecting waste plastics into blast furnace. As for the technology of injecting waste plastics into blast furnace, the first is the lack of information about particle size; The second is waste plastics discharged as domestic waste, whose variety composition fluctuates greatly, as shown in Table 1. In particular, the false specific gravity of waste plastics, mainly packaging bags and films, is only 0.1t/m3, which is far from that of pulverized coal; In addition, hydrochloric acid produced by heating chlorine containing waste plastics may cause corrosion to blast furnace equipment. Table 1 content and performance of waste plastics in domestic garbage

in view of the above topics, the following technical development is required:

1 in order to understand the reaction efficiency of waste plastics in blast furnace, it is necessary to analyze its changes in blast furnace, especially to find out the gas characteristics in high temperature area (~2000 ℃) and the best particle size and characteristics of waste plastics

2 develop the classification, crushing and granulation technology of plastics with different characteristics

3 develop efficient separation and removal technology for PVC and other chlorine containing waste plastics that cause equipment failure

4 develop Dechlorination Technology of chlorine containing waste plastics to facilitate rational utilization

pretreatment technology of making waste plastics into blast furnace raw materials

1 evaluation of the combustibility of waste plastics

when waste plastics are used in blast furnaces, (2) according to the use of machines and the service life of oil, achieving high gasification rate in the furnace to play the role of reductant is the most important topic. Generally, when the specific surface area of particles is increased by micronization, the reaction speed will increase, but it is difficult to micronize waste plastics. In this regard, in order to grasp the gasification characteristics of waste plastics injected into blast furnace and select the particle size of waste plastics that can be used efficiently, the following basic research has been carried out

(1) evaluation of single particle combustibility. In order to evaluate the basic combustion characteristics of waste plastic particles, a special laminar flow furnace was used to study. The height of the furnace is 2.4m, and the middle combustion hole is φ 100mm, the height of the combustion section is 500mm. When heated to 1300~1500 ℃ in the furnace, the preheated air and plastic particles are sent into the combustion zone through pipes from the upper part of the furnace. The combustion process of a single particle is directly photographed by a high-speed photography lens through a quartz glass window installed on the side wall of the furnace, and the particle temperature is measured by a two-color thermometer. During the test, the comparative combustion tests of waste plastic particles and pulverized coal of various sizes were carried out. Ready to use pulverized coal (particle size φ 0.044~0.063mm) and waste plastic (particle size φ 0.425~0.85mm) comparing the results of combustion test (the plastic is polypropylene, the volatile content of coal is 33.3%, the air temperature is 1200) ℃, and the feeding speed is 4m/s), it is seen that the pulverized coal will ignite immediately after being added into the furnace, and the volatile combustion will make the particle flame temperature reach 1500 ℃. At the same time, the observed particle flame diameter will also expand rapidly to 20 times, and then as the volatile combustion is completed, the flame diameter will shrink, the temperature will also drop, and it will turn to the surface of fixed carbon for combustion. However, the particle size of waste plastics is larger than that of pulverized coal, resulting in slow ignition. The observed flame temperature is only 1100~1180 ℃, and the expansion of flame diameter is also slower than that of pulverized coal

in addition, the results of the comparative test of the time from falling to ignition of waste plastics of different types and particle sizes are shown in Table 2. Except that it is found that the ignition time increases with the increase of particle size, it has little relationship with the different types of plastics. Table 2 ignition time of different plastics and particle sizes (unit: ms)

and when compared with pulverized coal, the influence of plastic particle size is greater, and it is longer than pulverized coal at 1mm, which is mainly due to the different thermal conductivity between the two particles, that is, it has higher thermal conductivity than pulverized coal, resulting in faster heat transfer

(2) evaluation of particle swarm combustibility

from the combustion test of a single particle, the larger the particle size, the lower the combustibility. However, due to the poor pulverization of waste plastics, it needs to be cooled during micronization, which makes the economy poor. Considering that the actual injection is particle swarm combustion, there is radiation heat transfer between particles, which is different from single particle combustion. Therefore, in order to find out the combustion changes of waste plastics in the coke filling layer, tests and studies were carried out with a 10t/a test blast furnace (waste steel melting furnace). The furnace is a vertical cupola. Waste steel and coke are loaded from the top of the furnace, oxygen (diluted with nitrogen) and pulverized coal are blown from the tuyere, and molten iron and slag are recovered from the lower part of the furnace. Since the lower part of the blast furnace reaches a high temperature of 2000 ℃, the tuyere is cooled with water to protect it. The injection plastic is waste PCP of PE, PP, PS and PE, with particle size

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