Plastic packaging waste thermal decomposition recovery technology

Abstract: Plastic packaging waste can be recycled through thermal decomposition. In this paper, the thermal decomposition principle, process, equipment and technical parameters are studied, and a preliminary exploration of the thermal decomposition of fuel technology is made.

Keywords: plastic packaging waste thermal decomposition

The thermal decomposition recycling of plastic packaging waste not only makes full use of resources, but also protects the environment and protects human health. The thermal decomposition and recycling technology of plastic packaging waste can be divided into four aspects: First, the combustion technology for the purpose of generating heat, steam, and electricity; The second is the pyrolysis technology for the production of medium and low calorific fuel gas, fuel oil, and carbon black. The third is to produce gasification and pyrolysis technology for medium and low calorific fuel gas or chemical substances such as NH3 and CH3OH; the fourth is to manufacture gasification pyrolysis technology for heavy oil, kerosene and gasoline.

1 Basic principles of thermal decomposition of plastic packaging waste

Thermal decomposition is the use of plastic energy in the plastic packaging waste heat under the conditions of the compound bonds broken, from large molecular weight organic matter into a small molecular weight fuel gas, liquid material (oil, grease, etc.) and coke. Different from thermal decomposition and incineration, incineration heats organics under sufficient supply of oxygen to completely oxidize the organics and produce stable CO2 and H2O.

2 Plastic packaging waste thermal decomposition process and equipment

There are many ways to pyrolyze plastic packaging waste. According to the heating method, there are two types of external heat and internal heat: According to the operating temperature, there are high temperature pyrolysis and low walk pyrolysis; According to the product points, there are gas production technology and oil production technology; According to the pyrolysis furnace Types, there are rotary furnace, shaft furnace, moving bed and fluidized bed.

2.1 External thermal decomposition

External thermal thermal decomposition type mainly includes external thermal rotary furnaces. Outer thermal rotary furnace pyrolysis main equipment is a rotary kiln. Outside the rotary kiln there is a heating furnace. The heating furnace and the rotary kiln are completely separated. The rotary kiln is inclined slightly downwards. There is a fuel nozzle in the heating furnace. The fuel sprayed from the nozzle burns, indirectly heating the rotary kiln, and the waste is fed from the screw feeder. Into the rotary kiln, there are mechanical seals at both ends of the rotary kiln to prevent outside air from entering the furnace. The waste is pyrolyzed in a rotary kiln. The pyrolysis gas flows countercurrently to the organic matter. The decomposition residue is discharged from the screw discharge machine at the lower end of the converter. Since there is no air inflow, the external rotary type rotary furnace performs thermal decomposition in a reducing atmosphere. Therefore, carbon black and other products are not oxidized, and the quality is good. The decomposed gas is not generated by CO 2 and nitrogen, so the gas has a high calorific value. Indirect heating with a heating furnace enables uniform heating and a reasonable distribution of temperature along the axial axis of the converter, allowing the raw material to be pyrolyzed at the most suitable temperature. The input materials do not need to be granulated, and the broken pieces can also be decomposed. The disadvantage of this method is that the carbon layer is easily adhered in the converter. However, the provision of a scraper device in the rotary kiln can often be solved.

2.2 Internal Thermal Decomposition

Internal thermal thermal decomposition furnaces mainly include single tower fluidization furnaces. The internal-heating single-column fluidization furnace is a fluidized-bed reactor with good reaction efficiency. The heat required for thermal decomposition is provided by partial combustion of waste. The waste is continuously added by a screw feeder, mixed with high-temperature sand in the furnace, and rapidly heated. Dry thermal decomposition occurs in a short time, organic matter decomposes into fuel gas, tar and charcoal. These products partially burn and heat undecomposed 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; The fuel gas is sent to the processing section, the fluidization and part of the material in the furnace The air for combustion is provided by the air nozzle at the lower part of the pyrolysis furnace. The lower part of the pyrolysis furnace is provided with a dispersion plate and a diffuser pipe, so that the incombustibles and sand energy accumulated in the lower part of the furnace can be easily discharged. The single tower fluidization furnace also has an air distribution plate at the lower part, and incombustibles will be discharged by a special discharge method.

In this way, the main products recovered by pyrolysis can be divided into oil recovery and gas recovery. The oil recovery is carried out in the temperature range of 450-550°C. The reaction temperature for gas recovery is 650-750°C.

The single-column partial combustion pyrolysis device has a simpler structure than other pyrolysis devices and is considered to be particularly suitable for small-scale equipment. In addition, since the reaction temperature is low, the refractory material is less damaged than the incinerator, the metal is fixed in the decomposed residue in a reduced state, and the elution is small in the buried field. The oil recovery method can recover the oil that can be stored and transported, has a low decomposition temperature (450-550°C), does not generate NOx, and has less combustion and exhaust gas than incineration, but if the moisture content of the waste material is high, the oil is converted into oil. The rate is low. As the pre-pyrolysis treatment, it must be dry, the drainage treatment is easy, and the amount of combustion and exhaust gas is small. However, since the decomposition product is gas, it is difficult to store it in large amounts, and the gas calorific value is low, which is not conducive to utilization and heat balance.

3 Thermal Decomposition Technology Parameters and Influencing Factors of Plastic Packaging Wastes

3.1 Thermal Decomposition Temperature

With different thermal decomposition temperatures, the products and yields obtained after thermal decomposition are also different. Even if the same product has different pyrolysis temperatures, the physical properties are not the same. The decomposition temperature is high, the gas yield is increased, and the oil and carbide are reduced accordingly. In addition, as the decomposition temperature varies, the gas composition also changes, and the higher the temperature, the lower the number of low molecular carbides CH4 and H2 in the gas.

3.2 Thermal decomposition rate

With different thermal decomposition rates, the gas components generated by thermal decomposition are also very different.

3.3 Moisture content

There is a negative correlation between the moisture content of plastic packaging waste and the amount of thermal decomposition generated. The amount of refined fuel gas produced from 1 t waste varies depending on the amount of water contained. For example, the amount of fuel that is replaced with 1 t of combustible waste does not affect the change in water content. However, waste water is large, and the heat balance of pyrolysis is difficult, requiring a large amount of auxiliary fuel.

In the process of gasification, if water vapor is injected, the water coal gasification reaction occurs due to water, coking coal and CO2, the carbon content in the residue is reduced, and the proportion of H2 and CO in the gas increases.

3.4 Air content

External thermal pyrolysis is conducted under anaerobic conditions, resulting in a high calorific value of gas; during internal pyrolysis, due to the introduction of air, the gas product contains a considerable amount of nitrogen and CO2, the calorific value of gas is lower, pure oxygen The pyrolysis gas product has a higher calorific value than the gas generated by air.

4 plastic half bucket packaging waste thermal decomposition fuel

Thermal decomposition of plastic packaging waste requires a certain amount of catalyst to be added to the fuel, and the requirements for cracking raw materials, cracking catalysts and cracking conditions are high.

Because of the different composition of plastics, their cracking behavior is also different. Polyethylene (PE), polypropylene (PP) and polystyrene (PS) are almost completely decomposed between 300-400°C; polyvinyl chloride (PVC) is decomposed in two stages at 200-300°C and 300-400°C. And release hydrogen chloride gas. Since the hydrogen chloride gas has severe corrosiveness to the reaction equipment, and also affects the service life of the cracking catalyst and the quality of the diesel fuel and the gasoline, the cracked raw material generally does not need to contain the polyvinyl chloride waste plastic.

The lysate also varies depending on the type of plastic, depending on where the plastic molecules are cut off under thermal energy. Under normal circumstances, the main chains of polyethylene and polypropylene are cut off irregularly and decomposed to produce various low-molecular-weight substances. The higher the decomposition temperature, the lower the molecular weight of the products. Polystyrene and polymethyl methacrylate are mainly decomposed into primary polymerization orders. Body, namely styrene and methyl methacrylate.

The pyrolysis reactions of polyethylene and polypropylene are all radical reaction mechanisms. The intramolecular free radical transfer is the main reaction mechanism when polyethylene is cracked, various free radicals are generated, and then bond cleavage occurs to obtain various small molecule compounds.

The mechanism of the polypropylene cleavage reaction is that the random chain generates primary and secondary free radicals, followed by an intramolecular free radical transfer reaction to generate more stable tertiary radicals, and β-cleavage to generate various low molecular compounds.

4.1 Thermal decomposition process

The pyrolysis oiling process combines 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 and mixed waste plastics are dispersed and their utilization value is not large. The diesel oil produced by pyrolysis has the highest wax content, high condensation point, and low cetane number; the obtained gasoline has low octane number.

4.2 Catalytic heat energy method

Catalytic pyrolysis (one-step process) The oilification process involves the addition of a mixture of waste polyethylene or waste polyethylene and other waste plastics and a catalyst to a reaction vessel, which is carried out simultaneously with pyrolysis and catalytic pyrolysis. Patented report (Chinese patent CN1077479A, 1993) With multiple modified Y-type molecular sieves and high activity of aluminum hydroxide as a catalyst, it can produce gasoline and diesel with an overall yield of 85%-87%.

Reproduced from: China Plastic Net