Thermal decomposition of the hottest plastic packa

2022-08-15
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Thermal decomposition of plastic packaging waste

Abstract: plastic packaging waste can be recycled through thermal decomposition. In this paper, the principle, process, equipment and technical parameters of thermal decomposition are studied, and the technology of thermal decomposition to produce fuel oil is preliminarily explored

key words: thermal decomposition of plastic packaging waste

thermal decomposition and recycling of plastic packaging waste can not only make full use of resources, but also protect the environment and human health. The technology of thermal decomposition and recycling of plastic packaging waste can be divided into four aspects: one is the combustion technology for the purpose of generating heat, steam and electricity; The second is the pyrolysis technology for manufacturing medium and low calorific value fuel gas, fuel oil and carbon black; Third, gasification and pyrolysis technology for manufacturing medium and low calorific value fuel gas or NH3, CH3OH and other chemical substances; The fourth is the gasification and pyrolysis technology for manufacturing heavy oil, kerosene and gasoline

1 basic principle of thermal decomposition of plastic packaging waste

thermal decomposition is to use heat energy to break the chemical bond of compounds under the condition of anoxia, and convert large molecular weight organic matter into small molecular weight fuel gas, liquid matter (oil, grease, etc.) and coke. Thermal decomposition is different from incineration. Incineration is to heat organic matter under the condition of sufficient supply of oxygen, so that organic matter can be completely oxidized to produce stable CO2 and H2O

2 thermal decomposition process and equipment of plastic packaging waste

there are many ways of pyrolysis of plastic packaging waste. According to the heating method, there are two categories: external heating and internal heating: according to the operating temperature, there are high-temperature pyrolysis and low walk pyrolysis; According to the products, there are gas production technology and oil production technology; According to the type of pyrolysis furnace, there are rotary furnace, shaft furnace, moving bed and fluidized bed, etc

2.1 external thermal decomposition

external thermal decomposition types mainly include external rotary furnace, etc. The main pyrolysis equipment of external heating rotary furnace is rotary furnace. There is a heating furnace outside the rotary furnace, which is completely separated from the rotary furnace. The rotary furnace tilts slightly downward. The heating furnace is equipped with a fuel nozzle. The fuel sprayed by the nozzle burns and heats the rotary furnace indirectly. Waste is added to the rotary furnace by a screw feeder. Both ends of the rotary furnace are provided with mechanical seals to prevent outside air from entering the furnace. The waste is pyrolyzed in the rotary furnace, the pyrolysis gas and organic matter flow counter current, and the decomposition residue is discharged by the screw discharging machine at the lower end of the converter. Because there is no air flowing into the external heated rotary furnace, it is thermally decomposed in the reducing atmosphere, so carbon black and other products are not oxidized, and the quality is good. The decomposed gas does not produce CO2 and nitrogen, so the gas has high calorific value. By using the indirect heating furnace, the uniform heating can be achieved, and the temperature is reasonably distributed along the axial direction of the converter, so that the raw materials can be pyrolyzed under the most suitable temperature conditions. The input raw materials do not need to be granulated, and the coarse broken fragments can also be decomposed. The disadvantage of this method is that the carbon layer is easy to adhere to the converter, but it can often be solved by setting a scraper device in the rotary furnace

2.2 internal thermal decomposition

internal thermal decomposition furnace mainly includes single tower fluidization furnace, etc. The internally heated single tower fluidized bed reactor is a fluidized bed reactor with better reaction efficiency. The heat required for thermal decomposition is provided by the partial combustion of waste. The waste is continuously added by the screw feeder, mixed with high-temperature sand in the furnace and heated rapidly. Dry thermal decomposition occurs in a short time, and organic matter is decomposed into fuel gas, tar and carbon. These products are partially burned to heat undissolved raw materials and sand, and other parts come out of the upper part of the furnace and enter the cyclone dust collector; Sand and carbon particles are separated; In the fuel gas delivery processing section, the air for fluidization and partial combustion of materials in the furnace is provided by the air nozzle at the lower part of the pyrolysis furnace. The lower part of the pyrolysis furnace is equipped with a dispersion plate and a diffuser pipe, so that the incombustible substances and sand accumulated at the lower part of the furnace can be easily discharged. The single tower fluidizing furnace also has an air distribution plate at the lower part, and the incombustible substances will be discharged by a special discharge method

with this method, the products mainly recovered from pyrolysis can be divided into oil recovery and gas recovery. Oil recovery is carried out in the range of 450-550 ℃, and the reaction temperature of gas recovery is 650-750 ℃

compared with other pyrolysis devices, the single tower partial combustion pyrolysis device has a simple structure and is considered to be particularly suitable for small-scale equipment. In addition, due to the low reaction temperature, compared with the incinerator, the damage to the refractory materials is less, the metal is fixed in the decomposition residue in the reduced state, and the dissolution in the landfill is less. The oil recovery method can recover the oil that can be stored and transported, with low decomposition temperature (450-550 ℃) and no NOx. Compared with incineration, the combustion exhaust is less, but if the moisture of waste raw materials is high, the oiling rate is low. As a pre pyrolysis treatment, it must be dried to form a rail transit industrial cluster; Based on Yuanda Kejian, the drainage treatment is easy, and the combustion exhaust volume is small. However, because the decomposition product is fuel gas, it is difficult to store a large amount, and the calorific value of fuel gas is low, which is not conducive to utilization and heat balance

3 technical parameters and influencing factors of thermal decomposition of plastic packaging waste

3.1 thermal decomposition temperature

different thermal decomposition temperatures lead to different products and yields after thermal decomposition. Even if the same product has different physical properties due to different pyrolysis temperatures, the decomposition temperature is high, the gas output increases, and the oil and carbides decrease correspondingly. In addition, with different decomposition temperatures, the composition of the gas also changes. The higher the temperature is, the lower molecular carbides CH4 and H2 in the gas increase

3.2 thermal decomposition rate

with different thermal decomposition rates, the composition of gas produced by thermal decomposition is also very different

3.3 moisture content

the relationship between the moisture content of plastic packaging waste and the amount of thermal decomposition is negative correlation. The amount of refined fuel produced by 1t waste varies according to the moisture content. For example, the amount of fuel produced by replacing 1t waste with combustible components has almost no effect on the change of moisture content. However, due to the large moisture content of waste, the thermal balance of pyrolysis is difficult, and a large number of auxiliary fuels are required

in the gasification process, if water steam is injected, water gasification reaction will occur due to water, carbon deposition and CO2, which will reduce the carbon content in the residue and increase the proportion of H2 and CO in the gas

3.4 air content

external thermal pyrolysis is carried out under the condition of absolute oxygen, and the calorific value of the gas produced is high; During internal thermal pyrolysis, due to the introduction of air, the pressurized gas products at both ends of the power battery enterprise contain a considerable amount of nitrogen and CO2, and the calorific value of the gas is low. The calorific value of the gas products produced by pure oxygen pyrolysis is higher than that produced by air

4 plastic half bucket packaging waste thermal decomposition to fuel oil

a certain amount of catalyst must be added to the thermal decomposition of plastic packaging waste to fuel oil, and the requirements for cracking raw materials, capacity cracking catalyst and cracking conditions are high

due to different compositions, the cracking behavior of plastics is also different. Polyethylene (PE), polypropylene (PP) and polystyrene (PS) are almost completely decomposed at 300-400 ℃; Polyvinyl chloride (PVC) decomposes in two stages at 200-300 ℃ and 300-400 ℃ and emits hydrogen chloride gas. Since hydrogen chloride gas has serious corrosivity to the reaction equipment, and affects the service life of cracking catalyst and the quality of diesel oil and gasoline, it is generally required that the cracking raw materials do not contain PVC waste plastics

pyrolysis products also vary with different types of plastics, depending on where plastic molecules are cut off under the action of heat energy. Generally, the main chains of polyethylene and polypropylene are irregularly cut off and decomposed into various low molecular weight substances. The higher the decomposition temperature, the lower the molecular weight of the products; Polystyrene and polymethylmethacrylate are mainly decomposed into original polymerized monomers, namely styrene and methyl methacrylate

the pyrolysis reaction of polyethylene and polypropylene is a free radical reaction mechanism. Free radical transfer in molecular bioplastics is the main reaction mechanism during polyethylene cracking, which generates various free radicals, and then bond splitting occurs to obtain various small molecular compounds

the cracking mechanism of polypropylene is random chain breaking to produce primary and secondary free radicals, followed by intramolecular free radical transfer reaction to produce more stable tertiary free radicals, and then proceed β- Various low molecular compounds are formed by fracture

4.1 pyrolysis oiling process

pyrolysis oiling process is to mix waste polyethylene and waste polyethylene with other waste plastics for pyrolysis to produce wax, oil products, carbon black and other products. The experimental results show that the products obtained by pyrolysis of mixed waste plastics are dispersed and of little use value. The diesel oil produced by pyrolysis has the highest wax content, high pour point and low cetane number; The obtained gasoline has a low octane number

4.2 catalytic thermal energy method

catalytic pyrolysis (one-step) oiling process is to add waste polyethylene or the mixture of waste polyethylene and other waste plastics and catalyst into the reactor, and pyrolysis and catalytic pyrolysis are carried out at the same time. According to a patent report (Chinese patent cna, 1993), gasoline and diesel oil can be produced with an overall yield of 85% - 87% using repeatedly modified Y-type molecular sieve and highly active aluminum hydroxide as catalyst

Author: Zhao Yao, Zhao Yanwei

reprinted from: plastic packaging

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