Analysis of environmental problems caused by the h

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Analysis of environmental problems caused by PVC (VI)

4.4 other recycling technologies, including co incineration. A newly developed dissolution precipitation process is based on the physical process that does not destroy the polymer chain of the feed mixture. This process is mainly applied to composites containing PVC and other components. PVC in the mixture can be separated by selective solvent, and then the whole mixture can be recovered by precipitation process. In this way, PVC and other components can be reused

now, an experimental factory has been put into operation, and the first formal factory is planned to be put into operation in 2001. This technical plant is a closed circulation system where the solution can be recycled.

this process is mainly used for selective treatment of collected PVC products. The quality of these products is the same as that of mechanical recycling, which means that the cost of making these materials treatable is comparable. Advocates of this process hope that such technology can be used to deal with complex molecular forms of mixtures, such as tarpaulins, cables, bubble packaging for medicine, floor coatings, car mudguards, and hope to compete economically with other recycling methods.

mixed plastic waste is used by German ironmaking plants as a reducing agent for blast furnace ironmaking. Mixed plastic waste can also be used as a substitute for coal, oil and gas in cement kilns to generate heat.

there are some disputes in evaluating the environmental impact of mixed plastic waste used in blast furnaces and cement kilns. According to some biosphere analysis, blast furnaces and cement kilns are more beneficial in energy recovery and prevention of global warming than incineration of municipal solid waste. Considering the potential contribution of PVC to the release of dioxins, it is more difficult to draw accurate conclusions, and further research is needed. Blast furnaces and cement kilns can treat mixed plastic waste with low investment. Therefore, using blast furnace and cement kiln for mixed plastic waste treatment is very competitive compared with other treatment facilities. On the other hand, blast furnaces and cement kilns have strict restrictions on the content of chlorine in mixed plastics, because chlorine can have a negative impact on the produced cement and iron, and the formation of HCI may cause corrosion to the equipment. Theoretically, burning mixed plastic waste in cement kilns is still a very important process in the future.

4.5 incineration

if used for incineration, PVC waste is generally treated in municipal incinerators. Since PVC products are also used in hospitals, PVC waste can also be seen in hospital waste incinerators. About 60000 tons of PVC are burned in a community every year. PVC accounts for 10% of the total incinerated plastics and 0.7% of the total incinerated waste. In the whole incineration waste, PVC accounts for 38% to 66% of the total chlorine. Other major sources of chlorine are perishable substances (about 17 percent) and paper (about 10 percent). On average, 50% of the chlorine put into the incinerator is caused by PVC.

during the incineration process, PVC waste produces hydrochloric acid (hydrogen chloride) and is released with flue gas. If there is no technology for the reuse of hydrochloric acid (hydrogen chloride), these hydrochloric acid (hydrogen chloride) needs to be neutralized. Now in Germany, only five factories have used this special recycling technology, and another three factories are under construction. All acid gases (except hydrogen chloride gas and various oxides of sulfur) produced by incineration of municipal solid waste must be neutralized before being released into the air. Community legislation has long stipulated the minimum value of hydrogen chloride gas release. This minimum value can be modified in more stringent legislation.

in order to meet the minimum release standard of hydrogen chloride gas, it is necessary to add neutralizing substances to the incinerator. Limestone is the most commonly used, so that the acidic components in the incineration gas can be neutralized. The four main neutralization processes are dry neutralization, semi dry neutralization, semi wet neutralization and wet neutralization

through the evaluation of the flue gas washing residue obtained by the impact method of incinerating PVC waste and using the air path switching method to introduce the temperature into the test area, it is concluded that after the flue gas is neutralized by limestone dry method, semi dry method and semi wet method, 1 to 1.4 kg of residue can be produced per 1 kg of PVC on average; In the process of semi dry neutralization with sodium bicarbonate as the neutralizing substance, 1 kg PVC can produce about 0.8 kg residue; The same wet process can produce 0.4 to 0.9 kg of residual liquid objects. During the incineration of rigid PVC and flexible PVC, the demand for neutralizing substances and the amount of residues produced are obviously different. Compared with 270 kg of carbon dioxide emissions that can be reduced in one year, flexible PVC has more demand for neutralizing substances and residues (1 kg of flexible PVC produces 0.5 to 0.78 kg of residues)

flue gas washing residue is defined as hazardous waste. These residues are produced by mixing with fly ash, and may also be produced separately in semi wet and wet systems. These residues include salts produced by neutralization, excessive neutralizing substances, and some pollutants such as heavy metals and undissolved dioxins. With some special exceptions, the only option for Member States is to landfill these residues.

a method has been designed to recover calcium chloride and sodium chloride from dry and wet processes, but these methods are not used for commercial purposes. Except for some special cases, in most cases, it is uncertain whether these technical processes can be used to recover useful substances in a large amount of residues. These technologies can be considered as some "end-of-life" solutions. In contrast, a preventive measure dedicated to reducing the source of residues is more popular.

compared with incineration of municipal solid waste without PVC, the current content of PVC in municipal solid waste has the following effects on flue gas washing residue. In-depth investigation and Research on the first production line of enterprises:

· PVC incineration increases the amount of flue gas washing residue (dry method accounts for 37%, semi dry method accounts for 34%, semi wet method accounts for 42%).

· PVC incineration increases twice the amount of leakable salts in the residue, which are mainly calcium chloride, sodium chloride and potassium chloride.

· PVC incineration increases the amount of leakage in the residue sent to the landfill (dry method accounts for 19%, semi dry method accounts for 18%, semi wet method accounts for 15%, wet method accounts for 4%). The leakage shall be treated before discharge.

· theoretically, it is possible that complex chlorides increase due to PVC incineration, which leads to the increase of leakage substances. However, this possibility needs to be verified by data.

· in the current temperature range of various processes of incineration of municipal solid waste, higher chlorine content will not affect the transfer of heavy metals and some trace elements from bottom ash to gas treatment residue.

as PVC is the largest chlorine supplier in incinerators, the potential impact of PVC waste incineration on dioxin emissions has become the central issue of scientific debate. Between 1993 and 1995, communities accounted for 40% of the contribution of PVC incineration to dioxin emissions. Although the real mechanism is not clear, it can be estimated that reducing the content of chlorine in waste can lead to the formation of dioxin substances. Even though the actual formation mechanism is not very clear. It is hoped that the impact of this reduction in chlorine content on the formation of dioxins can become the second or third element. Of course, the main factors affecting the formation of dioxins are incineration parameters, such as temperature, oxygen concentration and so on.

according to the current level of chlorine content in municipal waste, there seems to be no direct relationship between chlorine content and dioxin formation. There is a possibility that dioxin formation in incinerators will be caused when the chlorine content in waste exceeds a certain threshold. It is suggested that this threshold be determined as containing 1% chlorine, but this threshold cannot be determined. Further experiments are needed to verify the above threshold. This threshold will increase as the amount of chlorine containing waste increases.

at present, not all incinerators in society have been operated according to the latest dioxin air emission standards. The guidance and suggestions on waste incineration predict the emission limit of 0.1ng/m 3. This will reduce dioxin emissions from incinerators. (to be continued)

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