Co-pyrolysis characteristics of typical components of waste plastics in a fallin

Zechen Jin ,Lijie Yin *,Dezhen Chen Yuanjie Jia Jun Yuan Yuyan Hu

1 Thermal and Environmental Engineering Institute,Tongji University,Shanghai 200092,China

2 Shanghai Municipal Engineering Design Institute(Group)Co.,Ltd.,Shanghai 200092,China

Keywords:Co-pyrolysis Falling film reactor Waste plastics

ABSTRACT Waste plastics mainly come from MSW and usually exist in the form of mixed plastics.During the co-pyrolysis process of mixed plastics,various plastic components have different physicochemical properties and reaction mechanisms.Considering the high viscosity and low thermal conductivity of molten plastics,a falling film pyrolysis reactor was selected to explore the rapid co-pyrolysis process of typical plastic components(PP,PE and PS).The oil and gas yields and the compositions of pyrolysis products of the three components under different ratios at pyrolysis temperatures were analyzed to explore the co-pyrolysis characteristics ofPP,PE,and PS.The study is of great significance to the recycling of waste plastics.

1.Introduction

Municipal solid wastes(MSWs)are increasing and the proportion of waste plastics is continuously increasing.Domestic wastes generally contain many kinds of waste plastics,including polyethylene(PE),polypropylene(PP),polystyrene(PS)and polyvinyl chloride(PVC),which account for 70%of total waste plastics[1].Since plastics are polymers and the calorific values of these components(45 kJ·kg-1for PE,46 kJ·kg-1for PP,41 kJ·kg-1for PS and 22 kJ·kg-1for PVC)are close to those of crude oil(48 kJ·kg-1)[2].The bonds in the polymers of waste plastics can be broken through pyrolysis to obtain low-molecular-weight and high-calorific-value fuels or industrial raw materials.In this way,waste plastics are disposed and the supply of non-renewable fossil fuels may be indirectly increased.

The pyrolysis reactions of plastics involve the radical chain mechanism and the reactions can be divided into chain initiation reactions,chain transfer reactions and chain termination reactions.Waste plastics are mostly from MSW and many kinds of waste plastics are usually mixed together.Therefore,it is necessary to study the co-pyrolysis process of various plastics.Liu et al.[3]found that PP accelerated the pyrolysis of PE,whereas PVC decelerated the pyrolysis of PE and PP due to the intermolecular transfer of free radicals.Faravelli et al.[4]studied the co-pyrolysis process of PE and PS and found that the reaction between PE and PS did not occur when they were not completely mixed,but the radicals generated from the depolymerization process of PS tended to accelerate the radical chain mechanism of PE after they were completely mixed.Nilgun et al.[5]studied the pyrolysis process of PP/PS and PP/PE and found the decreased yield of pyrolysis oil,the slightly increased aliphatic pyrolysis oil of C7-C11and the increased non-condensable gas of C3and C4when the content of PP in the mixed waste plastics increased.Pinto et al.[6]explored the pyrolysis process of the mixed waste plastics of PP/PS/PE in a batch reactor at 430°C,realized the conversion rate of above 90%and the pyrolysis oil yield of above 80%,and found that the content of alkanes in pyrolysis oil increased when the content of PE in the mixed waste plastics increased.Jude et al.[7]investigated the co-pyrolysis process of LDPE/PS and found that the co-pyrolysis of mixed plastics increased the yield of pyrolysis oil and decrease the yield of coke compared to the pyrolysis of single plastic.

In the pyrolysis process of plastics,solid plastic particles firstly absorb heat,melt into liquid and then absorb heat for pyrolysis and the heat energy ratio in the two stages is about 1:4[8].Molten plastic is characterized by the high viscosity and low thermal conductivity.In the temperature range of the pyrolysis,the viscosity is from several Pa·s to several thousand Pa·s and the thermal conductivity is only 1/4 of that of water.The fluidity of molten plastic is low and its heat cannot be easily transferred from the inner wall of the reactor to raw materials.The internal pyrolysis temperature of the material may be lower than the designed pyrolysis temperature,thus leading to the poor product consistency,which is not conducive to the subsequent use or processing.In addition,the mixing degree of the materials also affects the yield and composition of pyrolysis products[4].Yin et al.[9]studied the pyrolysis process of molten PP in a vertical falling film pyrolysis reactor and found that the oil yield from the vertical falling film pyrolysis reactor was slightly higher than from the tubular reactor,equal to that from the rotary kiln reactor,and slightly lower than that in the fluidized-bed reactor.In this paper,the co-pyrolysis experiment of typical components of waste plastics in a falling film pyrolysis reactor was carried out to explore the interaction of different plastic components in the pyrolysis process and the effect of different mixing ratios on the yield and quality of pyrolysis oil and gas.

2.Experimental

2.1.Materials

The experimental materials were the particles of PE,PP and PS(PetroChina Daqing Re fining&Chemical Co.,Ltd.)with an average particle diameter of 2 to 3 mm.The mixing proportion of PE/PP/PS was set as 24:9:8 according to the content of PE/PP/PS in fresh MSW[10,11].The ultimate and proximate analyses are shown in Table 1.

Table 1 Ultimate and proximate analyses of the feedstock

2.2.Methods

The pyrolysis plant is shown in Fig.1.It consists of the pyrolysis system,the heating and control system,condensation and collection systems and non-condensable gas processing system.The pyrolysis system consists of the upper melting tank and the falling film plate.The melting tank is mainly used to melt solid plastic particles.A feeding inlet is set on the top of the melting tank and the bottom is connected with a falling plate.The size of the vertical falling plate is 0.05 m×1 m.When the molten plastic flowed down from the melting tank,it was pyrolyzed along the falling plate.Then the pyrolysis products were discharged from the top.Quartz sand was laid at the bottom to collect residues.Three sets of 9 K-type thermocouples were installed according to the same interval along the falling film plate to monitor the temperatures of the heating wall,the liquid film and the volatile in real time and the measured temperature was recorded by the control system.The pyrolysis products were collected by segment condensation.Cooling water at 85 °C and 0 °C was respectively used to collect highpour-point(HPO)oil and low-pour-point(LPO)oil.The remaining non-condensable gases were processed with the exhaust gas processing system.A nitrogen pore was reserved in the upper part of the melting tank for pressing the molten plastic onto the falling plate through the U-shaped pipe.Another nitrogen pore was reserved at the bottom of the falling plate for purging observation glass.

In the experiment,200 g of quartz sand was firstly laid on the bottom.Then 300 g of mixed-plastic particles was added from the top of the melting tank and the melting tank was sealed.The entire system was purged with nitrogen for about 10 min.Then the power was supplied to start the heating process.According to the melting temperatures of the three kinds of plastics,the temperature of the melting tank was set to ensure the complete melting process.Moreover,the temperature of the falling film plate was raised to the pyrolysis temperature and maintained until the reaction was completed.After plastic particles were completely melted in the melting tank,nitrogen was added into the upper melting tank so that the molten plastics flowed down to the falling film plate under the action of the pressure and pyrolyzed while flowing.

The devices used in the experiments are listed in Table 2,where the GC analytical program of GC-MS can be referred to Yin et al.[9].

3.Results and Discussion

3.1.Co-pyrolysis kinetics analysis of typical components of plastics

Fig.1.Afalling film reactor forplastic pyrolysis.1-Controlpanel;2-Nitrogen inlet;3-Feedstock inlet;4-Melting tank;5-Non-condensable gas collector;6-Thermocouple;7-Glass window;8-Falling film plate;9-Heating tube;10-Nitrogen purge inlet;11-Votalite outlet;12-Condenser tube;13-Digitaldisplay thermostatic bath;14-Non-condensable gas treatment;15-Ice salt water.

Table 2 Experiment parameters and used devices

Fig.2.TG/DTG curves for the co-pyrolysis process of typical components of plastics.

In the falling- film pyrolysis reactor,after the molten plastic entered the falling film plate,the pyrolysis process was completed quickly.In order to explore its kinetic characteristics,the heating rate in the thermogravimetric experiment was set as 100 °C·min-1and the pyrolysis temperature ranged from 50 °C to 650 °C.Fig.2 shows the TG/DTG curves for the pyrolysis process of the typical components of plastics and their mixture,of which the principle of mixing ratio is according to their relative contents in the fresh MSW[10,11].Table 3 shows the corresponding chemical kinetic parameters[12].

The temperature range of the main mass loss of PE/PP co-pyrolysis was 490-575°C.The mass loss rate was about 96%and the maximum mass loss rate occurred at 530°C.After the temperature reaches 625°C,the pyrolysis is almost complete.The main mass loss temperature of PE/PP co-pyrolysis was between those of PE pyrolysis and PP pyrolysis and the maximum mass loss rate of PE/PP copyrolysis was between those of PE pyrolysis and PP pyrolysis.The initial pyrolysis temperature of PE/PP co-pyrolysis was lower than that of PE pyrolysis or PP pyrolysis.This indicates that when PE/PP was copyrolyzed,the energy required to the transformation from the normal state to the active state was reduced.

The temperature range of the main mass loss of PP/PS was 425°C-575°C.The mass loss rate was about 97%and the maximum mass loss rate occurred at 516 °C.When the temperature rose to 570 °C,the pyrolysis process was almost completed and the total mass loss rate reached 99%.The initial pyrolysis temperature of PP/PS co-pyrolysis was lower than thatofPP pyrolysis and PS pyrolysis,butthe main mass loss temperature range of PP/PS co-pyrolysis was wider.This indicated that the mass loss process was slowerand the corresponding pyrolysis cycle was longer.

The temperature range of the main mass loss of PE/PP/PS was 480-590°C.The weight loss rate was about 96%and the maximum mass loss rate occurred at538 °C.When the temperature rose to 590 °C,the pyrolysis process was almostcompleted and the totalmass loss rate reached 99%.The maximum mass loss rate of PE/PP/PS was slower than those of PP and PE and faster than that of PS.

In this paper,the temperature ranges of the main mass loss were firstly determined and then the reaction kinetics equations were selected for each temperature range based on the TG/DTG curves for the copyrolysis process of mixed plastics[3].The optimal model was determined based on the correlation coefficients,the main reaction mechanisms of PE,PP,PS and mixed plastics were consistent with the diffusion theory,indicating that the pyrolysis process from the molten state to the volatile state was the diffusion process of small molecules generated from macromolecular polymers.Besides,the activation energy of PE and PP was higher than that of PS.The activation energy of the copyrolysis process of mixed plastics was among the activation energy of the individual plastic component and the reaction mechanism of the co-pyrolysis process was not changed compared to that of the pyrolysis process of individual plastic component.

Compared with the conventional heating rates for plastic pyrolysis,the heating rate of 100 °C·min-1increased the characteristic temperatures of TG/DTG curves[3,13]because the heat transfer process was delayed at a higher heating rate,especially when thethermal conductivity of molten plastics was low.Therefore,the time required for heat transfer at a higher heating rate was longer.

Table 3 Chemical kinetic parameters of the co-pyrolysis process of plastics

3.2.Co-pyrolysis characteristics of typical components of plastics

3.2.1.Co-pyrolysis characteristics of PP/PE

Fig.3 shows the conversion rate and yields of pyrolysis oil and noncondensable gas of PP/PE mixtures with different mixing ratios at the pyrolysis temperature of 550°C in the vertical falling film reactor.The histogram shows the experimentally results of the conversion rate and yields,and the line graphs show the weighted values calculated with the mixing ratio as follows:

whereαiis the mass fraction ofcomponent i in the mixture,and Yiis the yield of component i when pyrolysis alone.

Fig.3.Conversion rates and yields ofpyrolysis oiland non-condensable gas ofPP/PEunder different mixing ratios.

The conversion rate of PP/PE co-pyrolysis was lower than the average conversion rate of PP pyrolysis and PE pyrolysis.Compared with the pyrolysis of PP or PE,PP/PE co-pyrolysis reduced the activation energy required for the chain initiation reaction(Fig.2),but in the chain transfer reaction,PE inhibited the autocatalytic activity of the free radical,slowed the pyrolysis endotherm of PP,and improved the thermal stability of free radicals[14,15].The higher proportion of PE in PP/PE corresponded to the lower conversion rate and the more significant difference from the average value,indicating that PE inhibited the pyrolysis of PP.The yield of pyrolysis oil increased with the increase in the proportion of PE.That is to say,increasing the proportion of PE in PP/PE could increase the yield of pyrolysis oil,whereas increasing the proportion of PP in PP/PE could increase the yield of non-condensable gases.The result was in accordance with that of Nilgun[5].Meanwhile,in the co-pyrolysis process of PP/PE,with the increase in the proportion of PE,the yield of HPO obtained in the high-temperature condensation section increased.

The main components of plastic pyrolysis oil were alkanes,alkenes,naphthenes,and aromatics.The intermolecular hydrogen transfer between two radicals was the main reason for the formation of alkanes,whereas β-scission of the radicals was the main reason for the formation of alkenes[16].Fig.4 shows the composition of pyrolysis oil of PP/PE under different mixing ratios.As PP and PE are both fatty hydrocarbon polymers,the contentofaromatics in the pyrolysis oilwas small.With the increase in the proportion of PE in PP/PE,the content of alkanes increased from 5%to 39.2%;the contentof naphthenes decreased from 31%to 7.7%;the content of alkenes decreased slightly.The content of alkanes obtained in the co-pyrolysis process of PP/PE was lower than the average contentof alkanes obtained in PP pyrolysis and PE pyrolysis and the content of alkenes was higher than the content of alkanes obtained in PP pyrolysis and PE pyrolysis.The results indicated that PP/PE co-pyrolysis inhibited the intermolecular hydrogen transfer between two radicals and promoted the β-scission of the radicals.This is because of the fact that PP contains more methyl branches and is less stable,and PP is pyrolyzed prior to PE.The free radicals generated by PP pyrolysis plunder H radicals on the PE molecular chain,which makes it easier for β-scission of the radicals to generate alkenes when PE is decomposed.The higher the content of PP was,the more signi ficant the promotion on the β-scission was.

Fig.4.Composition of pyrolysis oil of PP/PE under different mixing ratios.

Table 4 Components of HPO and LPO of PP/PE co-pyrolysis under different mixing ratios

Table 4 shows the components ofHPOand LPO ofPP/PE co-pyrolysis under differentmixing ratios.Due to the chain structure of PP and PE,in spite of the interaction of free radicals in the chain transfer reaction,the products are also fatty hydrocarbon polymers[17].The carbon atom numbers of HPO were C9-C26and mainly concentrated on the carbon atom number of C12and above.The contents of C12and C18increased with the increase in the proportion of PP in PP/PE.The components of the carbon atom number of above C18increased with the increase in the proportion of PE in PP/PE,especially 1-Nonadecen,1-heneicosene and 1-Tricosene,which were mainly produced in PE pyrolysis.The carbon atom numbers of LPO were C6-C23,and the light component(C6-C12)increased with the increase in the proportion of PP in PP/PE.In the later experimental stage,some heavy components were mixed with LPO due to the condensation effect.

3.2.2.Co-pyrolysis characteristics of PP/PS

Fig.5.Conversion rates and yields of pyrolysis oil and non-condensable gas of PP/PS copyrolysis under different mixing ratios.

Fig.5 shows the conversion rate and yields of pyrolysis oil and noncondensable gas of PP/PS co-pyrolysis in the vertical falling film reactor under differentmixing ratios at550°C.The conversion rate of PP/PS copyrolysis was above 90%,but it was lower than the average conversion rate of PP pyrolysis and PS pyrolysis.The higher content of PS corresponded to the more significant difference compared to the average conversion rate,indicating that PS inhibited the pyrolysis of PP during the co-pyrolysis of PP/PS.The difference mightbe interpreted as follows:The primary radicals generated by the chain reaction of PS pyrolysis were relatively stable radicals containing benzene ring.The apparent activation energy required for the chain reaction of PS was lower than that of PP chain reaction.PS was more easily pyrolyzed than PP and the reaction process of PS was not affected by PP chain initiation reaction[18].Therefore,the time required to reach the temperature of PP pyrolysis was prolonged.The result was in agreement with that of Wong et al.[19].The intermediate radicals produced by PS in the pyrolysis reaction were mainly free radicals containing benzene ring[6].The yield of pyrolysis oil increased from 74.4%to 84.7%with the increase in the proportion of PS in PP/PS.

Fig.6 shows the composition of pyrolysis oil of PP/PS under different mixing ratios.PS is a polymer of styrene.During PS pyrolysis,besides the irregular degradation reactions,a large number of styrene monomers are generated in the depolymerization reaction.Therefore,the content of aromatics in pyrolysis oil increased with the increase in the proportion ofPS in PP/PS.Alkanes,alkenes and naphthenes were mainly generated in the pyrolysis of PP and their contents decreased with the decrease in the proportion of PP in PP/PS.In the pyrolysis oil of PP/PS,the content of aromatics was higher than the average value of PP pyrolysis and PS pyrolysis and the contentsofalkanes,alkenes,naphthenesof PP/PS co-pyrolysis were lower than the average values.The difference might be interpreted as follows:PS generated a large number of free radicals containing benzene ring in the initial chain reaction.These radicals participated in the initialchain reaction of PP,thus affecting the intermolecular hydrogen transfer and β-scission of free radicals and increasing the free radicals containing benzene ring[18].

Table 5 shows the composition ofHPOand LPO ofPP/PS co-pyrolysis under different mixing ratios.In addition to the pyrolysis oil generated in PP pyrolysis and PS pyrolysis,alkylbenzene compounds such as cumene were also produced by the combination of the free radicals of aromatics generated by the PS chain reaction and the free radicals of fatty hydrocarbons generated by the PP chain reaction.The carbon atom numbers of HPO were C7-C24and mainly concentrated in the carbon atom numberofC8and C12-C18.With the increase in the proportion of PS in PP/PS,the contents of C8and C13-C17increased due to the formation of monocyclic and bicyclic aromatics such as Styrene,1,3-Diphenylpropane,and 1,3-Diphenylbutene and the contents of C12and C18decreased.The carbon atom numbers of LPO were C6-C21and the content of light components(C6-C12)in C6-C21was higher than 80%.

Fig.6.Composition of pyrolysis oil of PP/PS co-pyrolysis under different mixing ratios.

Table 5 Components of HPO and LPO of PP/PS co-pyrolysis under different mixing ratios

3.2.3.Co-pyrolysis characteristics of PE/PP/PS

Fig.7 shows the conversion rates and yields of pyrolysis oil and non-condensable gas of PE/PP/PS co-pyrolysis in the vertical film reactor at different pyrolysis temperatures.When the pyrolysis temperature increased from 550 °C to 625 °C,the conversion rate of PE/PP/PS co-pyrolysis increased from 90.3%to 97%,but it was lower than the average conversion rate of PE pyrolysis,PP pyrolysis,and PS pyrolysis.This indicates that the co-pyrolysis of the three components of plastics inhibited the conversion rate compared to the pyrolysis of single component.The content of non-condensable gases increased from 19.6%to 39.2%due to the secondary pyrolysis of long chain compounds.The content of the pyrolysis oil decreased with the increase in the pyrolysis temperature.As the pyrolysis temperature increased,the heat absorbed by the C--C bond increased,thus leading to the complete radical reaction in the chain initiation stage and more products with a lower pour point.Therefore,the yields of HPO decreased.With the increase in the pyrolysis temperature,the yields of LPO increased and then decreased.The changing tendency might be interpreted as follows:With the increase in the pyrolysis temperature,more intermediates with a low pour point were condensed at low temperature.In addition,high temperature promoted the secondary pyrolysis to generate more non-condensable gas.In addition,with the increase in the pyrolysis temperature,the temperature of oil and gas mixtures entering the condensing tube increased,thus resulting in the worse condensation effect.

Fig.7.Effects of temperature on the yield of pyrolysis products of PE/PP/PS.

Fig.8 shows the components of pyrolysis oil in the vertical falling film reactor at different pyrolysis temperatures.Alkenes and aromatics are the main components of pyrolysis oil and the total content of alkenes and aromatics was about 80%.When the temperature was higher than 575°C,the content of alkene increased slightly,whereas the contents of alkanes and naphthenes decreased slightly.This difference might be interpreted as follows:The high temperature promoted the break of the C--H bonds of the alkanes so that the individual electrons on C were combined with each other to form carbon-carbon double bonds.Moreover,the high temperature also promoted the ring-opening reaction of naphthenes to form alkenes.At high temperatures,diole fins and monoole fins were prone to Diels-Alder reaction and dehydrogenation reaction to generate aromatics[20]and the chemical properties of benzene-containing substances were relatively stable.Therefore,the aromatic content increased with the temperature increase.Compared with the average contents of PE pyrolysis,PP pyrolysis,and PS pyrolysis,the alkanes and naphthenes contents of PE/PP/PS co-pyrolysis are lower and the aromatic content of PE/PP/PS co-pyrolysis is much higher.This is because that,on the one hand,the initial pyrolysis temperature of PS was low and the generated radicals containing benzene ring were involved in the chain initiation reaction of PP and PE[18];On the other hand,the aromatic hydrocarbons produced by PS pyrolysis are mainly styrene monomers,the styrene is unstable and is easy to combine small-volume free radicals by means of intermolecular free radical transfer at high temperatures[21].

3.3.Properties of pyrolysis oil

At present,the common quality evaluation criteria of the pyrolysis oil of waste plastics,have not been reported[22,23].Pinto et al.[6]investigated the density,research octane number(RON),and heatofcombustion of pyrolysis oil of mixed plastics,Sarker et al.[22]measured the sulfur contentin PP pyrolysis oil.This paper focuses on the in fluences of different proportions of typical plastic components on physical properties of HPO and LPO,such as RON,DV and high calorific value(HHV).The RON of LPO was calculated by the chemical analysis method[24].

Fig.9 shows the HHV of HPO and LPO obtained in PE/PP/PS copyrolysis under different mixing ratios.The HHV of HPO obtained in PE pyrolysis was higher than that of PP and PS.The HHV of HPO obtained in PE/PP co-pyrolysis was 40.52 MJ∙kg-1-44.31 MJ∙kg-1and the calorific value varied slightly with the mixing ratio of PE/PP.The HHV of HPO obtained in PP/PS co-pyrolysis was 43.23 MJ∙kg-1-43.81 MJ∙kg-1and decreased with the increase in the proportion of PS in the PP/PS mixture.The HHV of HPO obtained in PE/PP/PS copyrolysis was 44.11 MJ∙kg-1.The HHV of LPO in PE/PP co-pyrolysis was 40.52 MJ∙kg-1-42.45 MJ∙kg-1and increased with the increase in the proportion of PE in the PP/PE mixture.The HHV of LPO obtained in PP/PS co-pyrolysis was 38.45 MJ∙kg-1-40.98 MJ∙kg-1and decreased with the increase in the proportion of PS in the PP/PS mixture.The HHV of LPO obtained in PE/PP/PS co-pyrolysis was 42.53 MJ∙kg-1.

Fig.8.Effects of temperature on the components of pyrolysis oil of PE/PP/PS.

Fig.9.HHV of HPO and LPO obtained in PE/PP/PS co-pyrolysis under different mixing ratios.

Fig.10.Dynamic viscosity of HPO obtained in PE/PP/PS co-pyrolysis under different mixing ratios.

Fig.10 shows the DVofHPOobtained in PE/PP/PS co-pyrolysis under different mixing ratios.The DV of HPO decreased with the increase in the shear rate.The DV of HPO obtained in PE/PP/PS co-pyrolysis was lower than that obtained in PE/PP co-pyrolysis.With the increase in the proportion of PP in the PP/PE mixture,the DV of HPO obtained in PE/PP co-pyrolysis decreased.The result was in accordance with the results by Jing et al.[25].When PS was added,the DV of HPO obtained in PE/PP/PS co-pyrolysis decreased with the increase in the PS content.According to the previous research,the activation energy of PE is largest,result in the carbon number distribution of HPO concentrating on high molecular components,making the DV of PE is the largest.

Table 6 shows the properties of LPO obtained in PE/PP/PS copyrolysis under different mixing ratios.The RON of LPO obtained in PS pyrolysis was the largest,and that of PE and PP was lower for the higher content of alkenes.With the increase in the proportion of PS in the PE/PP/PS mixture,the RON of LPO increased to the RON standard of gasoline(GB17930-2013).The higher the proportion of PS was,the greater the content of aromatic hydrocarbons in the LPO was.The higher the proportions of PE and PP were,the higher the content of alkenes in the LPO was.

Table 6 Properties of LPO obtained in PE/PP/PS co-pyrolysis under different mixing ratios

4.Conclusions

In this paper,the co-pyrolysis experimentof3 typicalcomponents of waste plastics in a falling film pyrolysis reactor was carried out to explore the interaction of different components in the pyrolysis process and the effects ofdifferentmixing ratio on the yield and quality ofpyrolysis oil.The main conclusions are drawn below.

During the co-pyrolysis process of PE/PP/PS mixture at the heating rate of100°C·min-1,the maximumweightloss rate ofmixture was determined by that of the pyrolysis process of single component and the initial pyrolysis temperature of PE/PP/PS mixture was lower than that of PE pyrolysis,PP pyrolysis,and PS pyrolysis.The main reaction mechanism of PE/PP/PS was consistent with the 3D diffusion theory.

The conversion rate of PP/PE co-pyrolysis was lower than that of PE pyrolysis and PP pyrolysis.PP/PE co-pyrolysis inhibited the intermolecular hydrogen transfer between two radicals and promoted the βscission of the radicals.The content of alkanes obtained in PP/PE copyrolysis was lower than the average value and the content of alkenes obtained in PP/PE co-pyrolysis was higher.

The conversion rate of PP/PS co-pyrolysis was above 90%,but it was lower than the average value.Alarge number offree radicals containing benzene rings generated by PS in the chain initiation reaction were involved in the initial chain reaction of PP.The content of aromatics of PP/PS co-pyrolysis was higher than the average value and the contents of alkanes,alkenes,and naphthenes were lower.

When the pyrolysis temperature increased from 550 °C to 625 °C,the conversion rate of PE/PP/PS co-pyrolysis increased from 90.3%to 97%,but it was lower than the average conversion rate.Alkenes and aromatics were the main components ofpyrolysis oiland the totalcontent of alkenes and aromatics was about 80%.The aromatic content in the productsofPE/PP/PS co-pyrolysiswas much higherthan the average aromatic value.

The HHV of HPO and LPO obtained in PE/PP/PS co-pyrolysis under different mixing ratios was more than 40 MJ·kg-1.The DV of HPO obtained in PE/PP/PS co-pyrolysis was lower than that obtained in PE/PP co-pyrolysis.The RONof LPO obtained in PE/PP/PS co-pyrolysis reached the RON standard of gasoline(GB17930-2013).