CN110229685B

CN110229685B - Method for preparing fuel oil by high-pressure thermal conversion of waste plastics

Info

Publication number: CN110229685B
Application number: CN201910507635.3A
Authority: CN
Inventors: 袁浩然; 程磊磊; 顾菁; 陈勇
Original assignee: Guangzhou Institute of Energy Conversion of CAS
Current assignee: Guangzhou Institute of Energy Conversion of CAS
Priority date: 2019-06-12
Filing date: 2019-06-12
Publication date: 2020-08-25
Anticipated expiration: 2039-06-12
Also published as: CN110229685A; WO2020248534A1

Abstract

The invention discloses a method for preparing fuel oil by high-pressure thermal conversion of waste plastics. The method for preparing fuel oil by high-pressure thermal conversion of waste plastics comprises the following steps: feeding the pretreated waste plastics into a reaction container, introducing nitrogen into the reaction container to replace air in the reaction container, closing a gas outlet on the reaction container after replacement is finished, introducing nitrogen and hydrogen through a gas inlet on the reaction container to enable the pressure in the reaction container to reach 0.6-2.1 MPa, heating the reaction container to the reaction temperature of 340-380 ℃ for reaction, and cooling the air in the reaction container to the normal temperature after the reaction is finished to obtain high-pressure gas and product oil. The invention increases the thermal conversion temperature by about 100 ℃ by means of the temperature runaway effect of the thermal conversion of the waste plastics under the high pressure condition, indirectly reduces the heating initial temperature, further reduces the energy consumption, and avoids the uncontrollable temperature runaway phenomenon due to the small quantity of reactants causing the temperature runaway phenomenon.

Description

Method for preparing fuel oil by high-pressure thermal conversion of waste plastics

The technical field is as follows:

the invention relates to the technical field of resource recycling, in particular to a method for preparing fuel oil by high-pressure thermal conversion of waste plastics.

Background art:

at present, the yield of plastic wastes in China is about 1000 million tons every year, which causes serious 'white pollution' problem and great trouble to the society. A large amount of domestic plastic wastes are discarded, buried and burned. Not only causes the waste of land resources and the pollution of soil, water quality and atmospheric environment, but also loses the opportunity of resource utilization. The plastic is a petroleum product, contains abundant carbon and hydrogen elements, and has a certain energy value. At present, the resource utilization of waste plastics mainly comprises incineration power generation, melting regeneration, thermal cracking and the like. The waste gas generated by burning power generation causes air pollution; the quality of the regenerated plastic is poor, and the regenerated plastic can still be converted into waste in a short time; the plastic waste is converted into industrial raw materials or fuel products through thermal conversion, so that the environmental pollution is eliminated, and the resource utilization of the waste can be realized.

The complete thermal conversion of waste plastics into product oil under normal pressure requires at least a temperature condition of 430 ℃ to 550 ℃. The catalyst is mixed with the raw materials in the catalytic thermal conversion process, and the waste plastic thermal conversion generates carbon slag which is pasted on the surface of the catalyst, so that the catalyst is easy to inactivate and difficult to recycle, and the operation cost is increased.

Therefore, the development of a non-catalytic method for preparing high value-added products by efficiently thermally converting waste plastics is of great significance.

The invention content is as follows:

the invention aims to solve the problems in the prior art and provides a method for preparing fuel oil by high-pressure thermal conversion of waste plastics, which has the advantages of low energy consumption and cost and no secondary pollution and is an environment-friendly treatment technology for resource utilization of waste plastics.

The invention provides a method for preparing fuel oil by high-pressure thermal conversion of waste plastics, which comprises the following steps: feeding the pretreated waste plastics into a reaction container, introducing nitrogen into the reaction container to replace air in the reaction container, closing a gas outlet on the reaction container after replacement is finished, introducing nitrogen and hydrogen through a gas inlet on the reaction container to enable the pressure in the reaction container to reach 0.6-2.1 MPa, heating the reaction container to the reaction temperature of 340-380 ℃ for reaction, and cooling the air in the reaction container to the normal temperature after the reaction is finished to obtain high-pressure gas and product oil. The product oil comprises aromatic hydrocarbon, cyclic hydrocarbon and isomerized alkane, and improves the octane value of fuel oil.

The invention firstly introduces nitrogen and hydrogen into an autoclave to produce high-pressure atmosphere, sets proper initial temperature and pressure conditions, and then carries out thermal conversion on waste plastics under the conditions. When the reaction temperature and pressure reach certain values, the heating of the reaction container is stopped, the long chain of the waste plastic thermal conversion product is continuously broken into shorter chain by means of secondary reaction heat release and pressurization effect generated by the secondary reaction heat release, and experiments prove that the carbon number distribution of the final product oil product is the same as that of gasoline/diesel oil. The exothermic principle of the reaction is as follows: under the conditions of high pressure, 340 ℃ and above, the plastic carbon chains are broken into free radicals, most of the free radicals are kept in a liquid phase region because the reaction system is in a high-pressure atmosphere, and the probability of collision of the carbon chain free radicals in the liquid phase region is higher, so that the temperature of the reaction system is increased by about 100 ℃ by combining with released heat. Meanwhile, small molecular olefins are subjected to diene synthesis and further dehydrogenation reaction to generate aromatic hydrocarbons, carbon chain free radicals are cyclized to form cyclic hydrocarbons, part of the cyclic hydrocarbons are further dehydrogenated to form the aromatic hydrocarbons, the yield of the olefins is reduced, the yield of isoparaffins is also increased, and therefore the octane number of the fuel oil product is increased.

Preferably, the product oil is fractionated to obtain a gasoline component, a diesel oil component and a heavy oil component, and the gasoline component and the diesel oil component reach the industrial oil standard after being further upgraded; the heavy oil component is circularly conveyed to the high-pressure container to be mixed with the waste plastics for the next thermal conversion reaction. The heavy oil component obtained by fractionation is conveyed back to the reaction vessel to be used as a heat conducting agent for recycling, so that the heat conductivity coefficient of waste plastic thermal conversion is increased, and the caking phenomenon during the thermal conversion of the waste plastic is prevented.

Further preferably, the mass ratio of the heavy oil component to the waste plastic is 0-0.4: 1.

Preferably, the volume ratio of the nitrogen to the hydrogen is 9-19: 1.

preferably, the pretreatment of the pretreated waste plastic comprises the steps of: collecting waste plastics, removing impurities and drying to obtain pretreated waste plastics.

Preferably, the mass of the waste plastics is 1/8-1/5 of the mass which can be contained in the reaction vessel.

Preferably, the waste plastic is selected from one of polyethylene, polypropylene and polystyrene.

The invention has the beneficial effects that:

(1) the invention increases the thermal conversion temperature by about 100 ℃ by means of the temperature runaway effect of the thermal conversion of the waste plastics under the high pressure condition, indirectly reduces the heating initial temperature, further reduces the energy consumption, and avoids the uncontrollable temperature runaway phenomenon due to the small quantity of reactants causing the temperature runaway phenomenon.

(2) The invention can realize the quality improvement effect of the product oil without using a catalyst, and reduce the cost.

(3) The invention has high fuel oil yield, and the fuel oil products mainly comprise gasoline, diesel oil fraction and a small amount of heavy oil fraction, and have no secondary pollution.

(4) The invention has the advantages of simple whole process flow, mature equipment manufacture, simple actual operation and easy amplification.

Description of the drawings:

FIG. 1 is a schematic view showing the structure of an apparatus for carrying out the method for producing fuel by high-pressure thermal conversion of waste plastics according to the present invention;

description of reference numerals: 1. a nitrogen tank; 2. an air delivery pump; 3. an intake valve; 4. a temperature controller; 5. a magnetic stirrer; 6. an autoclave reactor; 7. an electric heating jacket; 8. a pressure gauge; 9. an air outlet valve; 10. a gas collection tank; 11. a liquid outlet valve; 12. a fractionating tower.

The specific implementation mode is as follows:

the present invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are only for illustrating the present invention and are not intended to limit the scope of the present invention. In practice, the technical personnel according to the invention make improvements and modifications, which still belong to the protection scope of the invention.

The equipment and reagents used in the present invention are, unless otherwise specified, conventional commercial products in the art.

As shown in fig. 1, the device for realizing the method for preparing fuel oil by high-pressure thermal conversion of waste plastics comprises an autoclave reactor 6, wherein an electric heating jacket 7 is arranged at the bottom outside the autoclave reactor 6, a temperature controller 4 is also arranged outside the autoclave reactor 6, a gas inlet, a gas outlet and a feed inlet are arranged at the top of the autoclave reactor 6, the waste plastics are put into the autoclave reactor 6 through the feed inlet, a liquid outlet is arranged at the bottom of the autoclave reactor 6, and a magnetic stirrer 5 is also arranged in the autoclave reactor 6. The nitrogen gas and the hydrogen gas stored in the nitrogen gas tank 1 are supplied to the autoclave reactor 6 through a gas inlet by a gas supply pump 2, and the amounts of the supplied nitrogen gas and hydrogen gas are adjusted by a gas inlet valve 3. After the reaction is finished, high-pressure gas is conveyed into a gas collecting tank 10 through a gas outlet through a high-pressure gas conveying pipeline, a gas outlet valve 9 and a pressure gauge 8 are arranged on the high-pressure gas conveying pipeline, product oil is conveyed into the fractionating tower through a liquid outlet through a product oil conveying pipeline, and a liquid outlet valve 11 is arranged on the product oil conveying pipeline. The product oil is fractionated in the fractionating tower 12 to obtain a gasoline component, a diesel oil component and a heavy oil component.

Example 1

Polyethylene waste plastics are used as experimental raw materials, and are subjected to pressurized thermal conversion in a device shown in figure 1, and the method comprises the following steps:

adding 1/8 mass polyethylene capable of being contained into the high-pressure autoclave reactor 6, closing the liquid outlet valve 11, opening the air inlet valve 3 and the air outlet valve 9, introducing nitrogen into the high-pressure autoclave reactor 6 for 20min to exhaust the air in the high-pressure autoclave reactor 6, closing the air outlet valve 9 to seal the high-pressure autoclave reactor 6, introducing nitrogen to reach 2.1MPa initial pressure, and closing the air inlet valve 3; setting an initial temperature of 340 ℃ by the temperature controller 4, turning on a switch of the electric heating sleeve 7, cutting off a power supply after the temperature controller 4 reaches the initial temperature, continuously heating to a maximum temperature value of 430 ℃ by means of temperature runaway, dismounting the electric heating sleeve 7, and rapidly cooling the high-pressure autoclave reactor 6 in the air. The polyethylene mainly undergoes chain scission reaction and dehydrogenation reaction to generate alkane and olefin, and the high pressure condition is favorable for generating shorter molecular chain; meanwhile, secondary reaction is carried out to generate cyclic hydrocarbon and aromatic hydrocarbon, and isomerization of straight-chain hydrocarbon occurs, and the series of secondary reaction leads to increase of the octane number of the product fuel oil.

After cooling to normal temperature, firstly opening the gas outlet valve 9 to release high-pressure gas in the high-pressure autoclave reactor 6, and then opening the liquid outlet valve 11 to release liquid product oil; fractionating the product oil in a fractionating tower 12 to obtain a diesel oil component, a gasoline component and a heavy oil component; the heavy oil fractions were collected and mixed with polyethylene for the next thermal conversion. The product yields and family compositions obtained are shown in table 1:

TABLE 1

Mixing the heavy oil component and polyethylene, and repeating the reaction, wherein the mass ratio of the heavy oil component to the polyethylene is 0.4:1, under the conditions of this example, the yield of gaseous product was 4.12%, the yield of liquid was 92.38%, and the yield of coke was 3.5%.

The heavy oil component is mixed with polyethylene to carry out thermal conversion reaction, the thermal conversion heat conductivity coefficient of the polyethylene is increased, and the temperature of reactants rises more quickly under the condition of the same power input. Meanwhile, hydrogen generated during the thermal conversion of the polyethylene is used as a hydrogen source to ensure that a small amount of olefin in the heavy oil is hydrogenated and converted into alkane, the yield of gas products and the yield of liquid products are slightly reduced, and a small amount of coke is generated.

Comparative example 1

And arranging a liquid collecting tank behind the high-pressure autoclave reactor in the cold trap to keep the normal pressure state inside the high-pressure autoclave reactor, and collecting gas by using a gas collecting bag behind the liquid collecting tank. The autoclave reactor was charged with 1/8 mass of polyethylene, the mass of which could be accommodated. Firstly, introducing nitrogen into an autoclave reactor for 20min to exhaust air in the reactor, then carrying out catalytic thermal conversion under the normal pressure state, setting an initial temperature of 340 ℃ by a temperature controller, and avoiding temperature runaway phenomenon in the reaction process. Gas pockets were used to collect the gas phase product, liquid phase product was collected in the liquid collection tank and the anti-autoclave reactor, solid phase product was collected in the autoclave reactor, and the product yields and group compositions are shown in table 2:

TABLE 2

Compared with the high-pressure state of example 1, the thermal conversion performed under the normal-pressure state produces more wax; there is no temperature runaway effect. Thus, the product oil carbon number distribution is lower. In addition, the absence of aromatics and isoparaffins in fuel results in low fuel octane.

Comparative example 2

And arranging a liquid collecting tank behind the high-pressure autoclave reactor in the cold trap to keep the normal pressure state inside the high-pressure autoclave reactor, and collecting gas by using a gas collecting bag behind the liquid collecting tank. The autoclave reactor was charged with 1/8 mass of polyethylene, in an amount to accommodate the mass, using an H-ZSM-5 catalyst (silica to alumina ratio 23) at a polyethylene to catalyst mass ratio of 20: 1. Firstly, introducing nitrogen into an autoclave reactor for 20min to exhaust air in the reactor, then carrying out catalytic thermal conversion under the normal pressure state, setting an initial temperature of 340 ℃ by a temperature controller, and avoiding temperature runaway phenomenon in the reaction process. Gas pockets were used to collect the gas phase product, liquid phase product was collected in the liquid collection tank and autoclave reactor, solid phase product was collected in the autoclave reactor, and the product yields and group compositions are shown in table 3:

TABLE 3

Compared with the high-pressure state of the embodiment 1, the catalytic thermal conversion is carried out under the normal-pressure state to generate more gas products, so that the fuel yield is reduced; the reduction of carbon number of the product oil comes from catalysis, not from temperature runaway effect generated under high pressure.

Example 2

adding 1/8 mass polyethylene which can be contained in the autoclave reactor 6, closing the liquid outlet valve 11, opening the air inlet valve 3 and the air outlet valve 9, introducing nitrogen into the autoclave reactor 6 for 20min to exhaust the air in the autoclave reactor 6, closing the air outlet valve 9 to seal the autoclave reactor 6, introducing mixed gas of 90% by volume of nitrogen and 10% by volume of hydrogen to reach the initial pressure of 2.1MPa, and closing the air inlet valve 3; setting an initial temperature of 340 ℃ by the temperature controller 4, turning on a switch of the electric heating sleeve 7, cutting off a power supply after the temperature controller 4 reaches the initial temperature, continuously heating to a maximum temperature value of 436 ℃ by means of temperature runaway, dismounting the electric heating sleeve 7, and rapidly cooling the high-pressure autoclave reactor 6 in the air. The polyethylene mainly undergoes chain scission reaction and dehydrogenation reaction to generate alkane and olefin, and the high pressure condition is favorable for generating shorter molecular chain; meanwhile, secondary reaction is carried out to generate cyclic hydrocarbon and aromatic hydrocarbon, and isomerization of straight-chain hydrocarbon occurs, and the series of secondary reaction leads to increase of the octane number of the product fuel oil.

After cooling to normal temperature, firstly opening the gas outlet valve 9 to release high-pressure gas in the high-pressure autoclave reactor 6, and then opening the liquid outlet valve 11 to release liquid product oil; fractionating the product oil in a fractionating tower 12 to obtain a diesel oil component, a gasoline component and a heavy oil component; the heavy oil fractions were collected and mixed with polyethylene for the next thermal conversion. The product yields, group compositions and average carbon numbers obtained are shown in table 4:

TABLE 4

Mixing the heavy oil component and polyethylene, and repeating the reaction, wherein the mass ratio of the heavy oil component to the polyethylene is 0.4:1, under the conditions of this example, the yield of gaseous product was 4.65%, the yield of liquid was 91.16%, and the yield of coke was 4.19%.

The heavy oil component is mixed with polyethylene to carry out thermal conversion reaction, the thermal conversion heat conductivity coefficient of the polyethylene is increased, and the temperature of reactants rises more quickly under the condition of the same power input. Meanwhile, hydrogen and hydrogen generated during polyethylene thermal conversion are jointly used as hydrogen sources to hydrogenate and convert part of olefins in heavy oil into alkane, so that the yield of gas products and the yield of liquid products are slightly reduced, and a small amount of coke is generated.

Example 3

adding 1/5 mass polyethylene which can be contained in the autoclave reactor 6, closing the liquid outlet valve 11, opening the air inlet valve 3 and the air outlet valve 9, introducing nitrogen into the autoclave reactor 6 for 20min to exhaust the air in the autoclave reactor 6, closing the air outlet valve 9 to seal the autoclave reactor 6, introducing mixed gas of 90% by volume of nitrogen and 10% by volume of hydrogen to reach 1.6MPa initial pressure, and closing the air inlet valve 3; the temperature controller 4 sets the initial temperature of 360 ℃, the switch of the electric heating sleeve 7 is turned on, the power supply is cut off after the temperature controller 4 reaches the initial temperature, the temperature is continuously heated to the maximum temperature value of 465 ℃ by means of temperature runaway, the electric heating sleeve 7 is dismounted, and the high-pressure autoclave reactor 6 is rapidly cooled in the air. The polyethylene mainly undergoes chain scission reaction and dehydrogenation reaction to generate alkane and olefin, and the high pressure condition is favorable for generating shorter molecular chain; meanwhile, secondary reaction is carried out to generate cyclic hydrocarbon and aromatic hydrocarbon, and isomerization of straight-chain hydrocarbon occurs, and the series of secondary reaction leads to increase of the octane number of the product fuel oil.

After cooling to normal temperature, firstly opening the gas outlet valve 9 to release high-pressure gas in the high-pressure autoclave reactor 6, and then opening the liquid outlet valve 11 to release liquid product oil; fractionating the product oil in a fractionating tower 12 to obtain a diesel oil component, a gasoline component and a heavy oil component; the heavy oil fractions were collected and mixed with polyethylene for the next thermal conversion. The product yields, group compositions and average carbon numbers obtained are shown in table 5:

TABLE 5

Comparative example 3

A mixed gas cylinder with the volume fraction of 90% of nitrogen and the volume fraction of 10% of hydrogen and a gas flowmeter are arranged in front of the high-pressure autoclave reactor, a liquid collecting tank is arranged behind the high-pressure autoclave reactor and placed in a cold trap to keep the normal pressure state inside the high-pressure autoclave reactor, and a gas collecting bag is adopted behind the liquid collecting tank to collect gas. The autoclave reactor was charged with 1/5 mass of polyethylene, in an amount to accommodate the mass, using an H-ZSM-5 catalyst (silica to alumina ratio 23) at a polyethylene to catalyst mass ratio of 20: 1. Firstly, introducing nitrogen into an autoclave reactor for 20min to exhaust air in the autoclave, then carrying out catalytic thermal conversion under a normal pressure state, introducing nitrogen and hydrogen mixed gas, setting an initial temperature of 360 ℃ by a temperature controller, and avoiding temperature runaway phenomenon in the reaction process. Gas pockets were used to collect the gas phase product, liquid phase product was collected in the liquid collection tank and autoclave reactor, solid phase product was collected in the autoclave, and the product yields and group compositions are shown in table 6:

TABLE 6

Compared with the high-pressure state of the embodiment 3, the catalytic thermal conversion is carried out under the normal-pressure state to generate more gas products and coke, and the fuel yield is reduced; the reduction of carbon number of the product oil comes from catalysis, not from temperature runaway effect generated under high pressure.

Example 4

adding 1/5 mass polyethylene which can be contained in the autoclave reactor 6, closing the liquid outlet valve 11, opening the air inlet valve 3 and the air outlet valve 9, introducing nitrogen into the autoclave reactor 6 for 20min to exhaust the air in the autoclave reactor 6, closing the air outlet valve 9 to seal the autoclave reactor 6, introducing mixed gas of 90% by volume of nitrogen and 10% by volume of hydrogen to reach the initial pressure of 2.1MPa, and closing the air inlet valve 3; the temperature controller 4 sets the initial temperature of 380 ℃, the switch of the electric heating sleeve 7 is turned on, the power supply is cut off after the temperature controller 4 reaches the initial temperature, the temperature is continuously heated to the maximum temperature value of 482 ℃ by means of temperature runaway, the electric heating sleeve 7 is dismounted, and the high-pressure autoclave reactor 6 is rapidly cooled in the air. The polyethylene mainly undergoes chain scission reaction and dehydrogenation reaction to generate alkane and olefin, and the high pressure condition is favorable for generating shorter molecular chain; meanwhile, secondary reaction is carried out to generate cyclic hydrocarbon and aromatic hydrocarbon, and isomerization of straight-chain hydrocarbon occurs, and the series of secondary reaction leads to increase of the octane number of the product fuel oil.

After cooling to normal temperature, firstly opening the gas outlet valve 9 to release high-pressure gas in the high-pressure autoclave reactor 6, and then opening the liquid outlet valve 11 to release liquid product oil; fractionating the product oil in a fractionating tower 12 to obtain a diesel oil component, a gasoline component and a heavy oil component; the heavy oil fractions were collected and mixed with polyethylene for the next thermal conversion. The product yields, group compositions and average carbon numbers obtained are shown in table 7:

TABLE 7

Example 5

Adding 1/8 mass polyethylene which can be contained in the autoclave reactor 6, closing the liquid outlet valve 11, opening the air inlet valve 3 and the air outlet valve 9, introducing nitrogen into the autoclave reactor 6 for 20min to exhaust the air in the autoclave reactor 6, closing the air outlet valve 9 to seal the autoclave reactor 6, introducing mixed gas of 90% by volume of nitrogen and 10% by volume of hydrogen to reach the initial pressure of 1.1MPa, and closing the air inlet valve 3; setting an initial temperature of 380 ℃ by the temperature controller 4, turning on a switch of the electric heating sleeve 7, switching off a power supply after the temperature controller 4 reaches the initial temperature, continuously heating to a maximum temperature value of 474 ℃ by means of temperature runaway, dismounting the electric heating sleeve 7, and rapidly cooling the high-pressure autoclave reactor 6 in the air. After cooling to normal temperature, firstly opening the gas outlet valve 9 to release high-pressure gas in the high-pressure autoclave reactor 6, and then opening the liquid outlet valve 11 to release liquid product oil; fractionating the product oil in a fractionating tower 12 to obtain a diesel oil component, a gasoline component and a heavy oil component; the heavy oil fractions were collected and mixed with polyethylene for the next thermal conversion. The product yields, group compositions and average carbon numbers obtained are shown in table 8:

TABLE 8

Example 6

Adding 1/5 mass polyethylene capable of containing water into the high-pressure autoclave reactor 6, closing the liquid outlet valve 11, opening the air inlet valve 3 and the air outlet valve 9, introducing nitrogen into the high-pressure autoclave reactor 6 for 20min to exhaust air in the autoclave, closing the air outlet valve 9 to seal the high-pressure autoclave reactor 6, introducing a mixed gas of 95% by volume of nitrogen and 5% by volume of hydrogen to reach 0.6MPa initial pressure, and closing the air inlet valve 3; the temperature controller 4 sets the initial temperature of 380 ℃, the switch of the electric heating sleeve 7 is turned on, the heating power supply is cut off after the initial temperature is reached, the temperature is continuously heated to the maximum temperature value of 463 ℃ by means of temperature runaway, the electric heating sleeve 7 is dismounted, and the high-pressure autoclave reactor 6 is rapidly cooled in the air. After cooling to normal temperature, firstly opening the gas outlet valve 9 to release high-pressure gas in the high-pressure autoclave reactor 6, and then opening the liquid outlet valve 11 to release liquid product oil; fractionating the product oil in a fractionating tower 12 to obtain a diesel oil component, a gasoline component and a heavy oil component; the heavy oil fractions were collected and mixed with polyethylene for the next thermal conversion. The product yields and group compositions obtained are shown in table 9:

TABLE 9

The above detailed description is specific to one possible embodiment of the present invention, and the embodiment is not intended to limit the scope of the present invention, and all equivalent implementations or modifications that do not depart from the scope of the present invention are intended to be included within the scope of the present invention.

Claims

1. The method for preparing fuel oil by high-pressure thermal conversion of waste plastics is characterized by comprising the following steps of: feeding the pretreated waste plastics into a reaction container, introducing nitrogen into the reaction container to replace air in the reaction container, closing a gas outlet on the reaction container after replacement is finished, and introducing nitrogen and hydrogen through a gas inlet on the reaction container to enable the pressure in the reaction container to reach 0.6-2.1 MPa, wherein the volume ratio of the nitrogen to the hydrogen is 9-19: 1, heating a reaction container to a reaction temperature of 340-380 ℃, then disconnecting a power supply, continuing heating by virtue of temperature runaway for reaction, and cooling air in the reaction container to normal temperature after the reaction is finished to obtain high-pressure gas and product oil, wherein the waste plastic is polyethylene.

2. The method for preparing fuel oil by high-pressure thermal conversion of waste plastics according to claim 1, wherein the product oil is fractionated to obtain a gasoline component, a diesel oil component and a heavy oil component, and the gasoline component and the diesel oil component are further upgraded to meet industrial oil standards; the heavy oil component is circularly conveyed to the high-pressure container to be mixed with the waste plastics for the next thermal conversion reaction.

3. The method for preparing fuel oil by high-pressure thermal conversion of waste plastics according to claim 2, wherein the mass ratio of the heavy oil component to the waste plastics is 0-0.4: 1.

4. The method for preparing fuel oil by high-pressure thermal conversion of waste plastics according to claim 1 or 2, wherein the pretreatment of the pretreated waste plastics comprises the steps of: collecting waste plastics, removing impurities and drying to obtain pretreated waste plastics.

5. The method for preparing fuel oil by high-pressure thermal conversion of waste plastics according to claim 1 or 2, wherein the mass of the waste plastics is 1/8-1/5 of the mass which can be accommodated in a reaction vessel.