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Accepted Manuscript Title: Study of the thermal behavior, kinetics, and product characterization of biomass and low-density polyethylene co-pyrolysis by thermogravimetric analysis and pyrolysis-GC/MS Authors: Yunwu Zheng, Lei Tao, Xiaoqing Yang, Yuanbo Huang, Can Liu, Zhifeng Zheng PII: DOI: Reference:

S0165-2370(17)31086-0 https://doi.org/10.1016/j.jaap.2018.04.001 JAAP 4300

To appear in:

J. Anal. Appl. Pyrolysis

Received date: Revised date: Accepted date:

11-12-2017 9-3-2018 1-4-2018

Please cite this article as: Yunwu Zheng, Lei Tao, Xiaoqing Yang, Yuanbo Huang, Can Liu, Zhifeng Zheng, Study of the thermal behavior, kinetics, and product characterization of biomass and low-density polyethylene co-pyrolysis by thermogravimetric analysis and pyrolysis-GC/MS, Journal of Analytical and Applied Pyrolysis https://doi.org/10.1016/j.jaap.2018.04.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Study of the thermal behavior, kinetics, and product characterization of biomass and low-density polyethylene co-pyrolysis by thermogravimetric

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analysis and pyrolysis-GC/MS

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Yunwu Zheng 1, 2, Lei Tao 2, Xiaoqing Yang 1, Yuanbo Huang1, Can Liu 1, Zhifeng Zheng1

1 University Key Laboratory for Biomass Chemical Refinery & Synthesis, Yunnan Province; Engineering

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Laboratory for Highly-Efficient Utilization of Biomass, Yunnan Province; College of Materials Science and

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Engineering, Southwest Forestry University, Kunming 650224, China.;

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2 Key Laboratory of Bio-based Material Science & Technology, Ministry of Education; College of Materials

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Science and Engineering, Northeast Forestry University, Harbin 150040, China;

*Corresponding Author: Zhifeng Zheng, Yunwu Zheng

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Email: [email protected], [email protected], Tel.: +86-13708707976

Highlights

Mechanism and kinetic study of synergy effect during catalytic co-pyrolysis of biomass and LDPE was

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

studied by TGA and Py-GC/MS.



Secondary reactions of volatiles in the char bed were mainly responsible for synergy effect.



Synergy effect promoted the aromatic yield (BTXE) during catalytic co-pyrolysis and decreased the content of aromatic hydrocarbons larger than C10.



Catalytic co-pyrolysis had a lower Ea than non-catalytic co-pyrolysis.



Catalyst, plastic, pyrolysis temperature and catalyst ratio have great effects on pyrolysis aromatic hydrocarbon product fractional yields and chemical composition.

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ABSTRACT The present study aims to improve the yields and selectivity of aromatic hydrocarbons in the catalytic

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pyrolysis of biomass by the addition of low-density polyethylene (LDPE), which has a higher hydrogen-carbon ratio than biomass. We have investigated the thermal decomposition behavior and kinetics, as well as the product distribution, of the co-pyrolysis of biomass (cellulose and pine sawdust) and plastic (LDPE) both with and without

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a catalyst (HZSM-5) using thermogravimetric analysis (TGA) and analytical pyrolysis-gas chromatography/mass

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spectrometry (Py-GC/MS). Our results, based on the weight loss difference (△W), show that there is a positive

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and synergistic interaction between the biomass and LDPE. After the addition of LDPE, the synergistic reactions inhibited catalyst coking effectively and decreased the formation of solid residues. In addition, the ZSM-5 catalyst

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improved the reaction activity and reduced the activation energy, although the reaction mechanism is not changed.

and

catalytic

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At the same time, the Flynn-Wall-Ozawa (FWO) method was used to fit the kinetic data for both non-catalytic co-pyrolysis

of

biomass

and

LDPE,

and

the

activation

energies

(Ea)

of

the

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cellulose + LDPE + catalyst and pine + LDPE + catalyst systems were found to be 168.81 and 185.87 kJ/mol, respectively. The co-pyrolysis of biomass and LDPE effectively improved the yield and selectivity of aromatics

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and increased the selectivity for benzene, toluene, xylene, and ethylbenzene (BTXE). The addition of LDPE can effectively improve the selectivity for naphthalene family products (methylnaphthalene and 2-methylnaphthalene) in the catalytic pyrolysis of biomass and decrease the content of aromatic hydrocarbons larger than C10.

Keywords：Biomass, LDPE, Co-pyrolysis, Kinetic, Aromatic Hydrocarbon

1. Introduction With the increasing fossil fuel crisis and serious haze fog and other environmental pollution, the need for the

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development and utilization of renewable energy is increasing. Biomass is a source of organic carbon that could replace fossil resources by using catalytic pyrolysis to prepare chemical feedstocks such as olefins and aromatic

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hydrocarbons, whose supply is a widespread importance in society [1]. However, because the oxygen content of bio-oil is much higher than those of fossil fuels and the content of polycyclic aromatic hydrocarbons is also

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higher, pyrolysis catalysts suffer from coking and deactivation more easily, which has hindered the use and

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development of bio-oil. Many studies have been carried on the catalytic thermal decomposition of different

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biomass sources and have shown that an appropriate hydrogen/carbon ratio (H/Ceff) is the essential factor

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affecting the yield and selectivity of biomass catalytic pyrolysis products [2–3]. However, because of the lower

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H/Ceff of biomass (usually less than 0.3), the yield of petrochemicals is reduced, and the production of coke is increased when biomass undergoes catalytic pyrolysis over an HZSM-5 zeolite catalyst. Thus, to increase the bio-

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oil yield, improve the aromatic selectivity, and minimize coking, the co-feeding of biomass and hydrogen-rich

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feedstocks (such as plastics [4–5], vegetable oil [6], and alcohol [7]) for catalytic pyrolysis has become popular, and this novel and feasible route could solve these problems [8–9]. Waste plastics have an appropriate hydrogen/carbon ratio and show synergistic effects in co-pyrolysis. In addition, they are abundant sources of

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hydrogen [10–11], making them an ideal co-reactant for the catalytic pyrolysis of biomass [12–15] to improve the aromatic carbon efficiency and reduce coke formation. Among waste plastics, polyethylene formed by the polymerization of olefins with a H/Ceff ratio of 2 accounts for up to 40% of gross waste plastics [16]. Hence, the co-feeding of lignocellulose with waste plastics in catalytic pyrolysis is beneficial for the environment and energy

recapture [17]. At the same time, the pyrolysis kinetics is also important for biomass conversion, and understanding the kinetics of non-catalytic co-pyrolysis and catalytic co-pyrolysis is vital to the design, optimization, and scaling up of industrial biomass conversion applications [18–19]. Currently, the pyrolysis kinetics of different types of has

been

widely

studied

using

thermogravimetric

analysis

[20–23]

and

pyrolysis-gas-

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biomass

chromatography/mass spectrometry (Py-GC/MS) [24–25]; however, the use of these two methods together to

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study the pyrolysis characteristics has not been reported.

Thus, this work aims to study the catalytic thermal decomposition of synthetic polymers, biomass, and

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commercial cellulose mixtures using TG and Py-GC/MS, as well as laboratory scale batch pyrolysis, to clarify the

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thermal decomposition processes based on the complementary information obtained by the above methods. The

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effects of the HZSM-5 catalyst on the decomposition kinetics, synergistic effects of biomass and plastic, pyrolysis

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mechanism, and the composition of pyrolysis products of plastics, biomass, and biomass/plastics model waste

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mixtures were studied. The results help us understand the possible changes in the catalytic activity during the catalytic thermal conversion of waste mixtures into valuable products and to enhance the carbon yield of

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aromatics, reduce the formation of coke, and increase the aromatic selectivity.

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2. Materials and methods 2.1. Materials

Particles of Yunnan pine were used in this study and collected from Pu’er city in Yunnan province. Before the

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experiments, the pine particles were sieved to a particle size in the range of 0.250–0.420 mm. The feedstock was dried at 105 ± 2°C until it reached a constant weight; then, the feedstock was sealed in bags. The main characteristics of Yunnan pine are listed in Table 1. The cellulose (CAS number: 9004-34-6) used in the experiments is a commercial product produced by

Aladdin Industrial Corporation, Shanghai, China. The low-density polyethylene pellets (LDPE) were purchased from BASF-YPC Co., Ltd., China. The particles were sieved to a particle size in the range of 0.250–0.420 mm. Tetrahydrofuran (THF, AR) was provided by the Tianjin Zhiyuan Chemical Reagent Company. High-purity nitrogen was provided by the Kunming Messer Company. The HZSM-5 molecular sieve catalyst in the form of

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white pellets was obtained from the catalyst plant of Nankai University. The Si/Al ratio of ZSM-5 is approximately 25. The chemical composition of the catalysts is as follows: 5-5.5 wt.% Al2O3 and 80-85 wt.%

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SiO2. The absorption capacities for hexane, cyclohexane, and water are 9.5-10.5, 2.0-2.5, and 11.0-12.0 wt.%, respectively. Before the experiment, the bare HZSM-5 was ground manually in a mortar and sieved to a size of

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0.185-0.250 mm.

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The catalysts were dried in a high-temperature carbonization furnace at a temperature of 500 C and

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activated for 4 h, followed by cooling using 0.5 mol/mL NH4NO3 by solution impregnation (quality ratio: 1 g/100

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mL) at 80 C for 2 h. The product was then filtered, washed, dried at 105 C for 12 h, and then calcined at 500 C

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for 4 h. The obtained product was cooled using the above-mentioned cooling steps and sealed for the experiment.

2.2 Thermogravimetric analysis

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Pyrolysis experiments were carried out with a thermogravimetric analyzer (TGA 209 F3, NETZSCH Co.,

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Bavaria, Germany). Al2O3 crucibles were used for each run, and the experiments were carried out under nonisothermal conditions. A 5-10-mg sample was pyrolyzed from room temperature to 600 C at heating rates of 5, 10, 20, 40, and 60 K/min and an argon gas flow rate of 20 mL/min. The biomass/plastic samples were blended in a

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1:1 weight ratio in an agate mortar to achieve homogeneity. The biomass/plastic/catalyst samples were blended in a 1:1:2 weight ratio.

2.3 Kinetics study Among the mathematical approaches that could be used to calculate the kinetic parameters, the Flynn-Wall-

Ozawa (FWO) method has been widely demonstrated to be a good model-free approach, and it was referred in this study to evaluate the activation energies of the biomass and LDPE during the thermal decomposition process. The general kinetic equation of heterogeneous solid-state thermal transformation in a linear temperature heating rate has been traditionally described as

d

K T

 LDPE

 K  T f 



   Char  Volatiles

,

(1)

,

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Biomass

(2)

dt

mi -m

.

(3)

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mi - m

 

f

Here, m (mg) is the sample mass, where m0, mt, and mf are the initial, actual, and final masses of the samples,

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respectively. Because this is a solid-state reaction, first-order decomposition is assumed. The mathematical

  1 -  

n

For n = 1.

(4)

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f 

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function, f(α), for first-order decomposition is given by the following expression:

 -Ea

/ RT



.

(5)

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K T   Ae

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The activation energy, Ea (kJ/mol), can be calculated using the Arrhenius equation:

Here, A is the pre-exponential factor (min−1), and R is the universal gas constant (8.314 J K-1 mol-1).

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Then, substituting the obtained expressions for k and f(α) from (Eqs. 4 and 5) into Eq. 2, the rate expression

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yields: d

-Ea

 Ae

1    .

(6)

n

RT

dt

When the heating rate β is constant and   

A

d dt

A



d

, for non-isothermal TGA experiments,

dt

-Ea

e RT 1  

 . n

(7)

Eq. 7 can be rewritten as d dt



A



-Ea

e RT 1  

.

(8)

In this work, the activation energy was obtained from non-isothermal TGA measurements, and the kinetic

parameters of the co-pyrolysis of biomass and LDPE were determined using the FWO method (Eq. 9) [26]. The α ranges of the biomass, LDPE, and the blend were 0.05–0.7, 0.05–0.9, and 0.05–0.3, respectively, which were based on the TG experimental data. '

ln   ln i

A E a

R (a)

E RT

a

(9)

a

ai

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g

 5 . 331 - 1 . 052

a

In Eq. 9, β, Tp, and Ea represent the heating rate, the peak temperature of the TG curve at different α, and the

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activation energy, respectively. R is the gas constant (8.314 J K-1 mol-1), and A is the pre-exponential factor (min-1).

This method can be used to obtain the value of the activation energy from a plot of ln(β) against 1/Tp for a series of

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experiments at different heating rates (β), and the activation energy can be calculated from the slope of the plot, which

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is equal to –1.052 Ea/R.

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To further illuminate the synergetic effects between the LDPE and biomass samples, we defined the difference of

 W  W blend -  x 1 W 1  x 2 W 2

,

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mass loss as W [27]:

(11)

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 W  W blend -  x 1 W 1  x 2 W 2  50  .

(10)

Here, Wblend is the mass loss of blend, xi is the mass fraction of each material in the blend, and Wi is the mass loss

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of each material under the same operating conditions. Obviously, △W1 describes the "extent" of the synergetic effect during the co-pyrolysis process between biomass and LDPE, and △W2 describes the "extent" of the

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synergetic effect during the co-pyrolysis of biomass, LDPE, and the catalyst.

2.4 Elemental analysis The C, H, N, and O contents of Yunnan pine and its products were quantified by elemental analysis (EA1108 Elemental Analyzer). Those of C, N, and H were determined first, and the mass fraction of O was calculated by subtracting the ash contents of C, N, and H from the total mass of the sample. Next, the water content of bio-oil

was analyzed by the Karl-Fischer titration. A mixture of methanol and chloroform with a mass ratio of 3:1 was used as the titration solvent. The higher heating values (HHV) of biochar and bio-oil were calculated using the following equation [28]: HHV(MJ/kg)

= 0.3491

× C  1.1783

× H  0.1005

× S - 0.1034

× O - 0.0151

× N - 0.0211

× A

,

(12)

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where C, H, S, O, and N represent the weight percentages of carbon, hydrogen, sulfur, oxygen, and nitrogen, respectively, and A represents the weight percent of ash.

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2.5 Fixed-bed pyrolysis experiments

A fixed-bed reactor for the pyrolysis of Yunnan pine was specially designed for this investigation. The furnace

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was heated electrically, and the temperature was measured using an internal thermocouple. A schematic diagram

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of the fixed-bed is shown in Fig. 1. The pyrolysis reaction was conducted in the reactor, and a steel pipe was

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placed through a hole in the furnace. The top of the reactor was connected to a straight condenser. The condenser

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was maintained at approximately 5 C using ice-water. The distance between the top of the furnace and condenser

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was approximately 30 cm. First, a dry glass fiber was inserted into the vertically installed reactor, and it was subsequently used for loading the feedstock. Then, 1.20 g of Yunnan pine was placed into the reactor. The bottom

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of the reactor was filled with high-purity nitrogen, while the top of the reactor was connected to the condenser.

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The reactor was purged with high-purity nitrogen for 5 min before the experiment began. Then, heat was applied rapidly to attain the set temperatures at a heating rate of 250 °C/min, followed by a holding time of 30 min. The flow rate of nitrogen was 150 mL/min throughout the experiment, the catalytic temperature was 500 C, the

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pyrolysis temperature was 450 C, and the biomass to catalyst ratio was 1:2. Then, the obtained bio-oil was dissolved using THF and analyzed. THF was found to be an appropriate solvent to dissolve and dilute the bio-oil. The yield of biochar was obtained by weighing the material. The yield of the gaseous products was calculated from the difference in the solid and liquid yields.

YL 

M

1

M

0

Y S - ex - situ 

 100 %

M

2

M

0

(13)

 100 %

(14)

YG  1 - Y L - YS

(15)

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Here, YL is the yield of bio-oil, YS-ex-situ is the yield of solid residue with ex situ catalytic pyrolysis upgrading, YG is the yield of non-condensable gas, M0 is the weight of the biomass feedstock, M1 is the weight of the liquid, M2

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is the weight of the solid residue, and M3 is the weight of the initial catalyst.

2.6 Gas chromatography-mass spectrometry (GC-MS) analysis

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Gas chromatography-mass spectrometry analysis of bio-oil was performed on an ITQ 900 instrument

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(Thermo Fisher Scientific) using an HP-5MS (30 m × 0.25 mm × 0.25 μm) capillary chromatographic column.

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The temperature of the injector was 280 °C, and the split ratio of the carrier gas was 1:10 using high-purity

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helium. At first, the oven temperature was held at 50 °C for 5 min; then, the temperature was ramped from 50 to

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280 °C at 5 °C/min and held for 5 min. For the MS measurements, electron ionization (EI) was used with an

of 230 °C.

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ionization energy of 70 eV, a scan per second over the m/z range of 30 to 500 amu, and an ion source temperature

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3. Results and discussion

3.1. Thermogravimetric analysis

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Thermogravimetric analysis was used to show the relationship between the weight change of the sample and

the temperature, allowing an understanding of the thermal decomposition and reaction mechanism during pyrolysis to be obtained. The TG curves indicate the mass loss of the sample versus temperature change during thermal degradation, and the DTG curves show the corresponding rates of mass loss of the TG curves. Fig. 2(a) shows the TG curves of isolated materials (cellulose and LDPE) and that of their mixture. The TG curves of

cellulose with LDPE are shown in Fig. 1(a). The thermal degradation of cellulose took place between 300 and 400 C, and, when the temperature reached 341.5 C, the degree of thermal degradation reached the maximum, resulting in the activation of cellulose, depolymerization, and the breakdown of compounds. With increasing temperature, a large amount of volatile gas was produced and the degree of decomposition was significant,

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accompanied by an equally significant weight loss. The solid residue was 19.82%. In contrast, the thermal decomposition of LDPE occurred at 400–500 C. It was found that LDPE was nearly completely degraded

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without the formation of a solid residue (the solid residues was only 2.97%). The decomposition range was narrow, and the decomposition domain was much more intense. When 50 wt.% LDPE was mixed with cellulose,

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the degradation curve could be divided into two stages. In the first stage (300–360C), pyrolysis occurred by

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devolatilization, and cellulose was degraded in this stage, whereas LDPE was not degraded. The solid residue was

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36.29%. In the second stage (360–500 C), continuous devolatilization occurred with the formation of small

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partial lignin compounds and charring. At the same time, the weight loss accounted for 50.57%, and the solid

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residual quality was only 12.97%. These results indicate that the presence of both LDPE and catalyst had a strong effect on the degradation temperature. The presence of LDPE inhibited the coking of cellulose, and the solid

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residue of the mixed product was reduced. In contrast, in the presence of the ZSM-5 catalyst, the high-temperature

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peak shifted to a lower temperature. The high-temperature peak shifted to around 380–450 C, which is lower than that of cellulose and LDPE mixture. The initial temperature was reduced to 435.5 C, but the quantity of the residue increased in the following order: LDPE < LDPE + cellulose < cellulose < cellulose + LDPE + catalyst.

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The order of thermal decomposition temperature is cellulose < cellulose + LDPE < LDPE < cellulose + LDPE + catalyst. As such, the TG curve of the cellulose and LDPE mixture is comparable with that of the cellulose and LDPE mixture. Additionally, the curve of cellulose + LDPE + catalyst was identical to that of cellulose + LDPE + catalyst. Fig. 2(b) shows the DTG curves of the different samples. As shown in Fig. 2(b), the decomposition of

cellulose and LDPE alone resulted in single peaks at 341.5 and 469.7 C, respectively. After mixing, the curves contained two peaks, and, in the presence of the ZSM-5 catalyst, the high-temperature peak shifted to a lower temperature, around 435.5 C, which is lower than that of the cellulose and LDPE mixture. This result confirms that the catalyst can significantly reduce the decomposition temperature.

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The TG and DTG curves shown in Fig. 3 are extremely similar to those of Fig. 2, as shown from Fig. 2 and Table 3. Similarly, the Yunnan pine power mainly decomposed at 200–400 C. The early part of decomposition

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corresponds to the decomposition of hemicelluloses and lignin, which occurred when the temperature reached 280 C and can be mainly attributed to the pyrolysis of xylan. A small peak in the DTG curves of the biomass

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appeared at about 360 C, as shown in Fig. 3(b), which corresponds to the partial decomposition of cellulose and

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lignin. This is in good agreement with previous results that the temperature range of hemicellulose decomposition

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is from 190 to 380 C, and the temperature range for cellulose decomposition is from 250 to 380 C with the

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maximum mass weight loss at about 350 C [29]. After the addition of LDPE and the ZSM-5 catalyst, the

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temperature of thermal decomposition decreased, and the residual mass of solid was clearly affected, indicating an obvious synergistic effect. The order of solid residues was LDPE < LDPE + biomass < biomass <

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LDPE + biomass + catalyst. The results indicated that, to some extent, the addition of LDPE inhibited the

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formation of residual carbon during biomass pyrolysis. This is due to the thermal decomposition of biomass and LDPE via free radical reactions. After mixing, with increasing temperature, the poor thermal stability of the biomass resulted in its decomposition, producing primary radicals, and, as the temperature continued to increase,

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the LDPE decomposed. Hydrocarbon compounds are rich hydrogen sources for the radicals produced by the decomposition of the biomass, resulting in the formation of volatile substances, which prevents LDPE from coating the biomass surface by melting at high temperatures and the precipitation of volatile substances that hinder the process of free radical polymerization to form further coke [30]. The process is shown in Scheme 1.

To determine the synergistic effect of the biomass (cellulose/pine powder) and LDPE, we defined the difference in the weight loss (∆W). Taking ∆w as a synergistic index parameter for biomass and LDPE copyrolysis. Fig. 4 shows the variation of ∆W with temperature for the different biomass/plastics blends. For the two blends, ∆W is less than ±1% below 250 C. This is because, at these temperatures, LDPE was still decomposing,

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and, obviously, there were no interactions between the cellulose/wood and the LDPE. However, ∆W is not equal to zero in the two cases, which may be caused by experimental errors such as the different initial weights of the

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samples and different thermal conduction conditions. For both the cellulose/LDPE and wood/LDPE blends, ∆W is greater than 1% at pyrolysis temperatures greater than 500 C, which indicates that the synergistic effect during

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co-pyrolysis occurred mainly at high temperatures. At the same time, the ∆W of the two samples first declined

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and, then, increased sharply at 250–500 C. This special pyrolysis behavior of the blends can be explained by the

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fact that LDPE first softens at about 300 C, reaching a plastic state that inhibits the evolution of volatile matter.

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With further increasing temperature, the LDPE began to decompose quickly, forming volatile hydrocarbons.

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Therefore, ∆W increased considerably. However, the biomass was much more stable, and more free radicals were produced from cellulose during the pyrolysis process. These free radicals could react with LDPE to accelerate its

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decomposition, so greater volatilization occurred and the change in ∆W was larger, indicating that the biomass has

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an important effect on the softening of LDPE and the conversion of volatiles. After the addition of the ZSM-5 catalyst, similar results were obtained, but two peaks at 250–400 and 400–500 C were observed. The former peak results from the high viscosity of LDPE and the low thermal conductivity because the pore size of ZSM-5 is

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relatively small and its thermal conductivity is relatively low. Thus, when they were mixed, the catalyst attached to the surface of LDPE, preventing LDPE softening and volatile precipitation, thus prolonging the softening time and resulting in the response lag [31]. Thus, the peaks appeared over a wide temperature range of about 400– 500 C. At the same time, above 500 C, the devolatilization processes of the blends had essentially completed,

and ∆W is stable at this stage. On the other hand, woody biomass is more stable than cellulose during pyrolysis, and this is consistent with effect on the evolution of volatile matter of the softening of LDPE at 300–500 C.

3.2. Thermal degradation kinetics The kinetic parameters, including the apparent activation energy (Ea) and pre-exponential factor for LDPE,

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biomass, biomass + LDPE, and biomass + LDPE + catalyst, for the pyrolysis reaction were determined by TGA, as shown in Tables 4 and 5 and Fig. 5. Because the R2 of the linear models in Figs. S1 and S2 are all larger than

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0.95, the first-order reaction mechanism fits well for the pyrolysis of LDPE, biomass + LDPE, and biomass + LDPE + catalyst, respectively. We determined that the apparent average Ea of LDPE and cellulose are

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249.87 and 169.29 kJ/mol, respectively. However, the apparent Ea of the cellulose and LDPE mixture is only

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201.57 kJ/mol, which is in an intermediate value. This result indicates that there was a positive synergistic effect

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between cellulose and LDPE on lowering the activation energy. In addition, in the presence of the catalyst, Ea

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decreased (168.81 kJ/mol) compared with Ea of the mixture without the addition of catalyst. This confirmed that

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the catalyst had a vital influence on the decrease in Ea. The reactivity of the material was increased, and the Ea was decreased. Moreover, the catalyst had an obvious effect on decreasing Ea but did not change the mechanism of the

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pyrolysis reaction; these results are similar to those of cellulose and LDPE mixtures under the same conditions.

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The biomass was composed of cellulose, hemicellulose, and lignin The phenylpropanoid structure in lignin has a low reaction activity, so the decomposition was difficult, and the Ea was higher, about 185.87 kJ/mol. The

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presence of LDPE and the catalyst obviously increased the reaction activity, and the Ea was significantly reduced. The Ea of LDPE and biomass were 249.87 and 185.87 kJ/mol, however, the apparent Ea of the biomass and LDPE mixture is only 224.34 kJ/mol, which is an intermediate value. This result indicates that there was a positive synergistic effect between biomass and LDPE on lowering the activation energy. In addition, in the presence of the catalyst, the Ea decreased to 173.23 kJ/mol compared with the Ea of the mixture without the catalyst. This result

confirms that catalyst had a strong effect in reducing the Ea.

3.3. Non-catalytic and catalytic pyrolysis upgrading Fig. 6 presents the product yields obtained in larger scale pyrolysis experiments for the total decomposition of the samples. As the results clearly show, the solid residue yields were 16.80%, 7.12%, and 5.57% without the

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catalyst in the cases of cellulose, LDPE, and the mixed samples, respectively, which changed to 32.46%, 3.81%, and 13.54%, respectively, with the catalyst. The solid residue of the mixed samples of biomass and LDPE

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decreased from 22.14% to 17.58%. The pyrolysis process produces coke on the surface of the catalyst, resulting in coking and the deactivation of the catalyst, hindering cellulose pyrolysis. As a result, the pyrolysis of cellulose and

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LDPE was incomplete, and the solid residual amount was higher. However, wood powder and LDPE had a

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synergistic effect on the pyrolysis process, and the existence of hemicellulose, which can decompose at a lower

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temperature to form free radicals, triggered LDPE pyrolysis, resulting in more complete pyrolysis of the raw

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materials and the formation of more small molecule hydrocarbons. Thus, the solid content decreased.

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It is also clear that a significantly higher yield of gaseous products was measured from each sample with the HZSM-5 catalyst than without the catalyst. The yields of gas were 31.46%, 10.20%, and 19.84% without the

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catalyst in the cases of biomass, LDPE, and the mixture, which increased to 52.99%, 27.24%, and 39.55% after

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applying the HZSM-5 catalyst, respectively. However, after catalytic pyrolysis, the yield of gas increased by 15.21%, 17.04%, and 3.24% in the cases of cellulose, LDPE, and the mixture, which is lower than the pine catalytic pyrolysis yield. These results show that cellulose was more easily decomposed, and HZSM-5 was highly

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active for the cracking of the pyrolysis gas to smaller molecules on the catalyst surface and inside the catalyst micropores, resulting in further cracking to yield more gas. At the same time, the bio-oil yields were 36.82%, 82.68%, and 58.02% without the catalyst in the cases of biomass, LDPE, and the mixture, respectively, which decreased to 18.06%, 68.95%, and 42.87% after applying

the HZSM-5 catalyst, respectively. However, for cellulose, after catalytic pyrolysis, the yield of gas decreased by 30.87%, 17.04%, and 11.05% in the cases of cellulose, LDPE, and the mixture, which produced more bio-oil compared to biomass. The HZSM-5 catalyst induced severe degradation that led to increased gas yields and decreased pyrolysis oil

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yields. The polymer pyrolysis leads to more oil than biomass pyrolysis, and the thermo-catalytic pyrolysis of each mixture resulted in much less oil than the catalyst-free pyrolysis. Furthermore, it also can be seen that cellulose

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was the source of bio-oil, whereas lignin in the biomass was the origin of the biochar.

Fig. S3 presents the total ion chromatograms obtained from the pyrolysis of biomass, LDPE, and the blend,

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and Fig. 7 illustrates the relative contents of different groups of chemicals in the pyrolysis volatiles from biomass,

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LDPE, and the blend. As shown by Fig. 7, the main pyrolysis products of cellulose were acetoin, furfural, 5-

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hydroxymethylfurfural, levoglucosan (1,6-dehydratase--D-pyran (type) glucose, 1,4:3,6-2-dehydro--D-pyran

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glucose) and other small oxygen-containing compounds such as ketones, aldehyde sugars, furans, and carboxylic

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acids, which will be further categorized. The contents were 4.89%, 8.26%, 15.44%, 7.83%, and 5.39%, respectively. It is well known that, for cellulose depolymerization, the 1,4-glycosidic linkage can be cleaved, as

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well as the acetalization reaction of C1 and C6. OH was formed and released, which can, together with C4, form

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the 1,6-dehydratase--D-pyran (type) glucose. Although the hemiacetal bond of cellulose was quite reactive, the stability of the C2–C3 bond was relatively poor, resulting in cracking, and four-carbon fragments can be formed. C5 and C6 in the four-carbon fragment lose H2O, forming hydroxybutanone. 5-Hydroxymethyl furfural (5-HMF)

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is mainly derived from cellulose units, and the glycosidic bond will cleave to form the aldehyde structure on C1. C4 and C5 will rearrange to form double bonds, and, then, another double bond between C2 and C3 forms through the dehydration of the corresponding hydroxyl groups. Finally, the acetal reaction of the hydroxyl groups on C2 and C5 is considered to be the essential step in the formation of 5-HMF. However, because of the instability of 5-

HMF, it continues to degrade, and furfural formic acid is formed. The main reaction path is shown in Scheme 2. The main carbon number distribution of paraffin, olefins, and dienes from the pyrolysis of LDPE are shown in Fig. 7(b). The products produced by thermal cracking reaction contained approximately 57.87% C7–C9, 27.31% C10–C16, and 14.81% C17–C22, and the ratio of these compounds was 1.0:0.47:0.26, respectively. After

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fast pyrolysis at 550 C, most of the LDPE had been decomposed into small molecule hydrocarbons; moreover, the same number of carbon atoms of the olefin content is higher than the corresponding alkane content and dienes.

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This thermal polyolefin plastic cracking occurs by three major free radical reactions: depolymerization (the polymer chain fracture into the polymer’s parent monomer or an oligomer), stochastic decomposition (the

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polymer chain randomly fractures into low-molecular-weight compounds with a wide molecular weight

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distribution), and elimination reactions (eliminating polymer functional groups when present or hydrogen to

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increase the degree of unsaturation) [32].

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Throughout the analysis of pyrolysis products, the separate pyrolysis products are defined as

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theoretical values, and the results of the mixed pyrolysis are found by experimental values.Through comparative study, it is found that no new products are produced after mixed pyrolysis, and the

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pyrolysis products are basically superimposed on the composition of independence pyrolysis

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products, but the content is changed.The theoretical value of the oxygen containing compounds from the individual pyrolysis products is higher however, the experimental values are lower. At the same time, it is known from the carbon number that the theoretical value is less than the

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experimental value under the less than C15, while the carbon number is higher than C15+, and the result is the opposite. It shows that the presence of cellulose/pine changes the reaction path of LDPE. The above phenomena fully indicate that there is a synergistic effect between cellulose and LDPE during the pyrolysis process, which makes the content of oxygen compounds decreased

obviously.The reaction pathways are shown in Scheme 3. Wood powder pyrolysis produces a few hydrocarbons, esters, and furans and a large number of aldehydes, ketones, acids, and guaiacol, 3,4-dimethylphenol, 4-methylguaiacol, and other phenolic substances. The phenolic compounds mainly derive from the decomposition of the phenylpropanoid lignin structure. In the pyrolysis of

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lignin, the main chain alkyl ether bond can undergo dehydration with loss of a hydroxyl group, which is the main source of the formation of phenols, and aldehydes, ketones, acids, esters, and other substances that are mainly

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derived from the pyrolysis products of cellulose and hemicelluloses. Of the products obtained by the thermal degradation of the mixture of biomass and LDPE, only the hydrocarbon content increased; that is, the C5–C9

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content decreased, but the C10–C16 content increased. The ratios of LDPE, cellulose + LDPE, and wood + LDPE

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were 3.90:1.84:1, 0.11:3.11:1, and 0.13:4.01:1, respectively. This is due to the synergistic effect of the co-

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pyrolysis of biomass and LDPE. Because the initial pyrolysis temperature was relatively low, biomass first

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decomposes and produces primary free radicals; with increasing temperature, the production of primary free

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radical occurs, promoting chain scission and hydrogen transfer reactions in the plastic, resulting in the transfer of hydrogen to the biomass. Thus, radicals are produced from pyrolysis, inhibiting free radical polymerization and

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resulting in the formation of more light hydrocarbons and producing more volatile components [33].

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To understand the interactions between biomass and plastic during pyrolysis with HZSM-5 (25) better, poplar wood powder, cellulose, LDPE, and mixtures were pyrolyzed separately. Fig. 8 shows the product distribution during the catalytic co-pyrolysis of biomass and plastic. As shown in Fig. 8(a), the monocyclic aromatics derived

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from cellulose were much lower, only 47.67%, but the contents of polycyclic aromatic hydrocarbons (PAHs) such as naphthalene and anthracene were higher, up to 52.33%. The LDPE pyrolysis products are monocyclic aromatics, comprising 85.60%. A few naphthalenes and indene, formed by the alkylation/cyclization of aliphatic fragments from plastics, were produced when the LDPE pyrolysis vapors were exposed to HZSM-5 (25).

Pyrolyzing pine wood powder alone produced a large number of monocyclic and polycyclic aromatic hydrocarbons (PAHs) when the vapors passed through HZSM-5 (25), and the contents were 69% and 31%, respectively. This corresponds to the complex poplar wood composition and many oxygen-containing substances that contribute to catalyst modification and coking. Branched-chain alkylated benzene derivatives, produced

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during lignin pyrolysis, may be converted to PAHs over the ZSM-5 catalyst. Compared with LDPE, pine wood powder generates fewer monocyclic aromatic hydrocarbons and more PAHs [32]. After mixing, the contents of

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monocyclic aromatics from cellulose/LDPE and pine/LDPE were 51.58% and 62.85%, increasing by 11.27% compared to cellulose/LDPE and increasing by 22.07% and 6.19%, respectively, compared to cellulose and pine.

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It is agreed that aromatization reaction pathways share a series of common steps. For the catalytic pyrolysis

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of the feedstock, first, cellulose undergoes cracking and deoxidization to form small molecule olefins such as C2–

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C5 [34]. The small olefins are polymerized to form C6–C10 olefins, which then undergo hydrogen transfer

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reactions to form dienes and, finally, form aromatics through cyclization and aromatization reactions. Compared

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with the catalytic fast pyrolysis (CFP) of cellulose, aromatics can be formed more quickly via cyclization and aromatization of LDPE-derived C6–C10 dienes during the CFP of LDPE because of the presence of the ZSM-5

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catalyst, which results in the formation of C1–C5 and C10–C26 species produced by LDPE pyrolysis. These

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species are then cracked and reformed to C3–C5 olefins. Finally, these products undergo further oligomerization, cyclization, and aromatization to form aromatic hydrocarbons [11, 35]. The cracking of large polymer molecules into C3–C5 olefins in the catalytically active acidic sites, followed by oligomerization, cyclization, and hydrogen

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transfer reactions, results in the formation of aromatics, light paraffin, and olefins. The high proportion of aromatics generated has been attributed to both the strong acidity and the shape selectivity properties of the HZSM-5 zeolite because of its three-dimensional microporous structure (0.55 nm ×0.51 nm, 0.54 nm ×0.56 nm), which reduces the occurrence of side reactions.

When LDPE was added to biomass, a synergistic effect between the wood and the plastic occurs during pyrolysis. This supports the hypothesis that the plastic-derived pyrolysis vapors may prevent or reduce the number of unwanted side reactions (such as coking). Because these plastic-derived vapors have a high H/Ceff-ratio and contain no oxygen, they may partially quench the repolymerization reactions of wood pyrolysis vapors. It also

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possible that Diels–Alder reactions between the biomass-derived oxygenated compounds and plastic-derived olefins increase the yield of aromatics. This was proposed by both Dorado and Li et al. [36–37]. In other words,

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LDPE provides an effective hydrogen source for biomass and promotes biomass catalytic conversion. In addition, oxygen-containing compounds further generated by the catalytic pyrolysis of biomass promote the

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depolymerization of polyethylene. However, compared to cellulose, on the basis of existing synergistic effects, the

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pine wood had a remarkable catalytic effect due to the presence of alkali metals, which promote LDPE cracking,

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and more low-molecular-weight organic compounds are generated [38]. Figs. 8 (b–c) shows the effect of aromatic

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selectivity on the pyrolysis of the mixture. Different bio-oil products, each with different selectivity for different

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components, were produced by cellulose pyrolysis. The content of benzene was the lowest, whereas naphthalene and its derivatives were the highest. This is because in the process of pyrolysis, furan can react with benzene to

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produce naphthalene and its derivatives [39], but, for the wood pyrolysis products, the content of benzene and

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toluene was higher, 28.60% and 23.13%, respectively. After the addition of LDPE, which improved the selectivity of toluene and xylene, these values reached 24.94% and 20.83%, respectively, an increase of 1.81% and 16.19%, although the benzene content decreased significantly. This may be due to the increase in benzene alkylation

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reactions because monocyclic aromatic polysubstituted compounds (such as trimethylbenzene and dimethylbenzene) undergo disproportionation reactions to form toluene and xylene [40]. At the same time, the addition of LDPE improved the selectivity of naphthalene group products (methyl and dimethyl naphthalene); this is mainly because LDPE is a high-molecular-weight polymer, and pyrolysis and chain breakage are not complete,

thus producing larger molecules [4]. Fig. 8(d) shows the carbon number distribution of aromatic hydrocarbon; the pyrolysis products were mainly composed of C6, C7, and C8 chemical compounds. After mixing, due to the synergistic effects, the C10+ polycyclic aromatic hydrocarbon content decreased obviously, and the contents in cellulose/LDPE and pine wood powder/LDPE decreased from 22.57% to 23.22% and from 19.13% to 26.46%,

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respectively.

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3.4. Optimizing catalytic pyrolysis of biomass and plastic for light aromatics 3.4.1 Effect of pyrolysis temperature

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The pine and LDPE were also co-pyrolysis and the effect of pyrolysis temperature was evaluated by

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changing the pyrolysis temperature of the first reactor from 450℃ to 650℃ while fixing the catalytic temperature

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of the second reactor at 550℃ and the ratio of biomass to LDPE was 2. The yields and distribution of co-pyrolysis

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products were shown in Fig.S4. when the pyrolysis temperature was increased during co-pyrolysis, the yields of aromatic increased significantly, when the temperature reached to 500℃, the relative content of aromatic

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increased to 93.82%, when the pyrolysis temperature continued increased, the aromatic yields began to decreased,

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while, the content of alkanes and olefins increased with the increase of temperature. This is due to the effect of temperature on the product yield of biomass and LDPE mainly in two parts: the degradation of plastics was an

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exothermic reaction, higher temperature favored thermal decomposition, which resulted in producing small molecules of olefin, on the other hand, there is a competitive reaction in the catalytic pyrolysis of biomass, and the

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higher temperature will help to promote the conversion of biomass into small molecules of oxygen- containing compounds. Then these oxygen-contained compounds were more easily accessible to the catalyst pores and converted to small molecular substances such as aromatics, olefin and so on. However, if the temperature was much too high, the oxygen compounds of small molecules will generate more small molecules such as non condensable gases, which will impede the generation of aromatics, so the content will be reduced. Moreover, for

selectivity, the content of benzene and its derivatives first increased and then decreased with the increase of temperature. The content of naphthalene and its derivatives continued to increase, while the content of polycyclic aromatic hydrocarbons such as indene, phenanthracene and anthracene remained at a relatively stable level. For the selectivity of mono aromatic hydrocarbons, the selectivity of toluene and xylene increases first and then

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decreases with the increase of temperature. This is mainly due to the product of benzylation of toluene and xylene, and the methylation reaction is exothermic reaction. With the increase of temperature, the methylation reaction

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was inhibited and the selectivity of toluene and xylene decreased, while the selectivity of benzene and ethyl benzene changed little. Therefore, when the pyrolysis temperature was 500 ℃, benzene, toluene and xylene had

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higher selectivity, and the yields of aldehydes, ketones, ethers, esters and undetected substances were at the lowest

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level. The content of aromatics was 93.82%, and the content of polycyclic aromatic hydrocarbons was the lowest.

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3.4.2 Effect of catalytic temperature

The product yield and distribution of catalytic co-pyrolysis of biomass and LDPE with different catalytic

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temperature were shown in Fig.S5. The results showed that the relative content of aromatic hydrocarbon gradually

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increased from 70.27% at 400℃ to 87.39% at 450℃ and then decreased to 55.41% at 650℃, whereas that of oxygen-contained increased, the maximum yield of aromatic hydrocarbon was 87.39%, which occurred at 450℃.

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The selectivity of main aromatic hydrocarbon compounds included benzene and its derivative, naphthalene and phenanthrene were changed a little. The selectivity of mono cyclic aromatic hydrocarbons was different with

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different catalytic temperatures. The alkylation reaction belongs to exothermic reaction. Benzene and toluene have the highest selectivity at 600℃, the selectivity was 4.32% and 27.58% respectively, and the maximum selectivity of xylene was 29.41% at 550℃. Under low temperature conditions, there were no enough thermal to make biomass complete decomposition due to the insufficient heat of the pyrolysis material. When the temperature was higher than 600℃, although pyrolysis feedstock can be fully completely pyrolyzed, but it was easy to cause bio-

oil cracking for two times and generate more coke and small molecular components, thus reducing the yield of target products. Therefore, under the condition of catalytic temperature of 550℃, the selectivity of mono ring aromatics was highest.

3.4.3 Effect of the amount of catalyst

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The product yields and distribution as a function of the amount of catalyst in catalytic co-pyrolysis of

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biomass and LDPE were shown in Fig.S6. The catalytic conversion efficiency of biomass and LDPE co-pyrolysis could be improved by increasing the amount of catalyst. Under the condition of constant carrier gas flow rate, the contact time between cracking product and catalyst was limited, increasing the amount of catalyst that was

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improving the conversion efficiency of cracking products, thus making more cracking products catalyzed into

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target substances. As can be seen in Fig.S6, with the increase of catalyst dosage, the total yield of aromatics

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increased, the highest reached 97.42%, the content of olefin and paraffin decreased, and the content of polycyclic aromatic hydrocarbons increased significantly, and the content of mono cyclic aromatic hydrocarbons decreased.

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This is because inside a limited furnace chamber, with the increase of the amount of catalyst, which increases the

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material placed length, therefore, the resistance of mass transfer and heat transfer in biomass pyrolysis is increased, a large amount of pyrolysis steam had condensation reaction, which was attached to the surface of the

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catalyst, thus causing the coking deactivation of the catalyst, thus reducing the content of the catalyst. For the selectivity of mono cyclic aromatic hydrocarbons, the selectivity of toluene and xylene first increased and then

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decreased, and the content of ethyl benzene increased significantly. Among them, the highest selectivity of xylene can reach 39.54%, and the highest selectivity of toluene was 25.23%.

3.4.4 Effect of the type of catalyst

The relative content of aromatic yield and distribution under different catalysts were shown in Fig.S7. The present of catalyst greatly increased the yield of aromatic compounds. β , H-ZSM-5 and aromatization catalyst favored aromatic hydrocarbon producing, the content of those are similar, but the USY catalyst has a strong

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selectivity to olefin, however, the content of aromatics was lower.USY catalyst, which had the strong acidity and abilities of deep catalytic deoxygenating, so, there were so much incondensable gas such as CO, CO2, CH4 and so

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on. Moreover, its own pore size was larger, which reduces the contact probability of the thermal gas and the acid site, so the aromatics content was lower. At the same time, the selectivity of USY catalyst for benzene (5.79%)

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and olefin (37.91%) was much stronger. The selectivity of aromatization catalyst to xylene is better than that of

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other catalysts. And the selectivity of HZSM-5 catalyst for naphthalene group (13.62%) was stronger, so the yield

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of aromatic group gradually increased with the following order: USY < < HZSM-5 < Aromatization catalyst.

3.4.5 Effect of the mass ratio of biomass to LDPE

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Fig.S8 showed the effect of mass ratio of biomass to LDPE on the yield and aromatic selectivity. Obviously,

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the aromatic hydrocarbon yield increased nonlinearly with the increase of LDPE proportion, this curvature indicated that there was a synergistic effect between the two feeds; the maximum yield of aromatic hydrocarbon

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was obtained at the point of 50% (93.28%), the aromatic yield gradually increased and then decreased with the increasing LDPE proportion. Because of the addition of LDPE, on the one hand, the effective hydrogen to carbon

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ratio (H/Ceff ratio) of the system was increased, so that more biomass can be transformed into aromatics. On the other hand, the oxygenated compounds produced by biomass pyrolysis will further promote the degradation of plastics, so the content of polycyclic aromatic hydrocarbons will decrease. However, because the pyrolysis conditions of LDPE and biomass are different, the pyrolysis temperature of LDPE was higher than that of biomass, so there is a certain lag difference between the two. Resulting in the occurrence of olefins is lagging

behind, so the maximum value occurs at the ratio of biomass to LDPE of 0:1. Moreover, with the addition of LDPE, the content of benzene and its derivative increased first and then decreased, and the content of naphthalene and other polycyclic aromatic hydrocarbons decreased. This is due to the synergistic effect between the biomass and plastic, which promotes the transformation of biomass and plastics and advances to the target products. At the

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same time, the addition of LDPE made the selectivity of benzene down, and the selectivity of toluene and xylene first increases and then decreases. When the mixing ratio was 1:1, the selectivity of toluene was higher, while the

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selectivity of xylene produced by mixing ratio is 1:3(39.23%), which creates the condition for later xylene preparation.

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3.4.6 Effect of the kinds of plastic

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Fig.S9 showed the effects of different plastic (Polylactic acid (PLA), Polyhexyl ester (PCL), Polypropylene

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(PP) and low density polyethylene (LDPE)) on the aromatic yield and distribution in catalytic co-pyrolysis of biomass and plastic with HZSM-5 catalyst. Catalytic co-pyrolysis of LDPE and pine produced the highest yield of

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aromatic hydrocarbon (82.17%), followed by PP, and the lowest content of aromatics produced by PLA and pine

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co-pyrolysis, which was 63.96%. This is because PLA is an oxygenated polymer with high oxygen content and lower effective hydrogen carbon ratio. Therefore, the lower hydrogen carbon ratio makes the aromatics content

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lower and the content of oxygenated compounds higher. While, the content of olefins and alkanes produced by PP was relatively higher, this depends on the special monomer structure of PP. It has high selectivity to propylene, so

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the selectivity of olefin was quiet higher. As shown in Fig.S9 (b-c), the selectivity of xylene was the highest selectivity, followed by naphthalene and its derivative. Catalytic co-pyrolysis of PLA and pine presented the highest selectivity of benzene and its derivative (90.45%) and PCL favored the selectivity of naphthalene, because of the six membered ring furan structures which could react with olefin by Diels-Alder reaction to produce naphthalene and its derivative. For mono cyclic aromatic hydrocarbons, the selectivity of xylene is relatively high,

followed by toluene. The selectivity of toluene and xylene produced by PP was the highest, which were 15.30% and 37.09%, respectively. So, based on the statistical analysis above, the overall proposed reaction mechanism for biomass and plastics

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catalytic co-pyrolysis with HZSM-5 catalyst to form aromatics was shown in scheme 4 as follows:

4. Conclusions

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(1) The thermal stability of cellulose was high, and, thus, the solid residue was higher after pyrolysis. LDPE, which has a higher thermal decomposition temperature, showed a faster thermal weight loss rate but the pyrolysis

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temperature range was relatively narrow. There was a positive synergistic interaction between the biomass and

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LDPE, which decreased the formation of solid residue at the end of experiment. The addition of LDPE inhibited

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the coking reaction of biomass effectively, resulting in an increase in the volatile content;

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(2) The ZSM-5 catalyst can improve the reaction activity effectively, and the reaction activation energy

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decreased significantly. However, because of the surface coking deactivation of catalyst, the activation energy loss was not obvious. Thus, the activation energies of cellulose-LDPE-catalyst and pine-LDPE-catalyst are only

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mechanism.

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168.81 and 185.87 kJ/mol, respectively. Moreover, the addition of catalyst did not change the decomposition

(3) The co-pyrolysis of biomass and plastic can improve the yield and selectivity of aromatics effectively.

The selectivity for benzene, toluene, xylene, and ethylbenzene (BTXE) increased. Furthermore, the addition of

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LDPE efficiently improved the selectivity for naphthalene group compounds (methyl naphthalene and dimethyl naphthalene), resulting in a decrease in the C10+ aromatic hydrocarbons.

Acknowledge This work was supported by the National Natural Science Foundation of China (31670599), the 948 project (Grant No. 2013-4-08) in the State Forestry Administration, the Special Fund for Renewable Energy Development in Yunnan Province (Yunnan Finance Industry No. [2015]86), the Yunnan Provincial Department of Education

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Major Project of Scientific Research Foundation (Grant No. ZD2014012), and the Key Laboratory of Bio-based Material Science & Technology (Northeast Forestry University), Ministry of Education. We also thank the

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support from Key Discipline of Forestry Engineering and Yunnan Provincial Scholarship/ Fellowship Project

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A

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(Award No. 2016060).

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CC E

1051-1063.

PT

[28] Channiwala S A, Parikh P P. A unified correlation for estimating HHV of solid, liquid and gaseous fuels[J]. Fuel, 81(2002):

[29] Bu Q, Lei H, Wang L, et al. Bio-based phenols and fuel production from catalytic microwave pyrolysis of lignin by activated

carbons[J]. Bioresource technology, 162(2014): 142-147.

A

[30] Marcilla A, Gomez-Siurana A, Valdés F J. Influence of the temperature on the composition of the coke obtained in the catalytic

cracking of low density polyethylene in the presence of USY and HZSM-5 zeolites[J]. Microporous and Mesoporous Materials,

109(2008): 420-428.

[31] Liu M, Zhuo J K, Xiong S J, et al. Catalytic degradation of high-density polyethylene over a clay catalyst compared with other

catalysts[J]. Energy & Fuels, 28(2014): 6038-6045.

[32] Lin X, Zhang Z, Tan S, et al. In line wood plastic composite pyrolyses and HZSM-5 conversion of the pyrolysis vapors[J].

Energy Conversion and Management, 141(2017): 206-215.

IP T

[33] Levine S E, Broadbelt L J. Detailed mechanistic modeling of high-density polyethylene pyrolysis: Low molecular weight

product evolution[J]. Polymer Degradation and Stability, 94(2009): 810-822.

SC R

[34] Gayubo A G, Aguayo A T, Atutxa A, et al. Transformation of oxygenate components of biomass pyrolysis oil on a HZSM-5

zeolite. II. Aldehydes, ketones, and acids[J]. Industrial & engineering chemistry research, 43(2004): 2619-2626.

U

[35] Serrano D P, Aguado J, Escola J M, et al. An investigation into the catalytic cracking of LDPE using Py–GC/MS[J]. Journal of

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analytical and applied pyrolysis, 74(2005): 370-378.

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[36] Dorado C, Mullen C A, Boateng A A. Coprocessing of agricultural plastic waste and switchgrass via tail gas reactive

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pyrolysis[J]. Industrial & Engineering Chemistry Research, 54(2015): 9887-9893.

ED

[37] Li X, Li J, Zhou G, et al. Enhancing the production of renewable petrochemicals by co-feeding of biomass with plastics in

catalytic fast pyrolysis with ZSM-5 zeolites[J]. Applied Catalysis A: General, 481(2014): 173-182.

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[38] Long J, Song H, Jun X, et al. Release characteristics of alkali and alkaline earth metallic species during biomass pyrolysis and

CC E

steam gasification process[J]. Bioresource technology, 116(2012): 278-284.

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A

[40] Tsai T C, Liu S B, Wang I. Disproportionation and transalkylation of alkylbenzenes over zeolite catalysts[J]. Applied Catalysis

A: General, 181(1999): 355-398.

List of Figures Fig.1 The schematic diagram of fixed-bed Fig.2 TG and DTG curves of cellulose with LDPE Fig.3 TG and DTG curves of biomass with LDPE Fig.4 Variation of △W for Biomass(Pine and Cellulose) LDPE and catalyst blends

Cellulose, b Pine)

IP T

Fig.5 Activation energy values of LDPE, biomass, and their blends during pyrolysis (a

Fig.6 Carbon yields of the pyrolysis products in non-catalytic and catalytic pyrolysis of

SC R

biomass, LDPE and their mixture (1:1:2) with ZSM-5 zeolite

Fig.7 Relative contents of different groups of chemicals in pyrolysis volatiles from biomass,

U

LDPE and blend（a: production content, b: carbon number）

N

Fig.8 Production yields and aromatic selectivity for the biomass catalytic pyrolysis using H-

A

ZSM-5 catalyst（a:production yields, b:single-ring aromatic selectivity, c:polycyclic

M

aromatic selectivity, d:carbon number）Key—Ben. Benzene and its derivatives; Ind.: Indene and its derivatives; Nap. : Naphthalene and its derivatives; Phen and an.:

ED

Phenanthrenes and anthraceens; SBTXE: The total yields of benzene, toluene, xylene and

PT

ethyl benzene

Scheme 1. The schematic diagram of co-pyrolysis process of biomass and LDPE

CC E

Scheme.2 The reaction pathway of cellulose Scheme.3 The reaction pathway of LDPE

A

Scheme.4 Reaction pathway of catalytic co-pyrolysis of biomass and LDPE

IP T SC R

A

CC E

PT

ED

M

A

N

U

Fig.1 The schematic diagram of fixed-bed

100

-35 -30

DTG (%/min)

60

40

Cellulose LDPE Cellulose+LDPE Cellulose+LDPE+Catalyst

20

-25 -20 -15 -10 -5

0

0 100

200

300

400

500

600

100

200

300

IP T

TG (%)

80

Cellulose LDPE Cellulose + LDPE Cellulose + LDPE +Catalyst

400

Temperature (°C)

SC R

Temperature (°C)

A

CC E

PT

ED

M

A

N

U

Fig.2 TG and DTG curves of cellulose with LDPE

500

600

100

-35 -30

DTG (%/min)

60

40

-20 -15 -10

Biomass LDPE Biomass + LDPE Biomass + LDPE + Catalyst

20

-25

-5

0

0 100

200

300

400

500

600

100

200

300

IP T

TG (%)

80

Biomass LDPE Biomass + LDPE Biomass + LDPE + Catalyst

400

Temperature (°C)

Temperature (°C)

A

CC E

PT

ED

M

A

N

U

SC R

Fig.3 TG and DTG curves of biomass with LDPE

500

600

5

10 5

0

W (%)

-10

△

-5

-10 -15

-15

Biomass + LDPE Cellulose + LDPE

-20

-20

Biomass + LDPE + Catalyst Cellulose+ LDPE + Catalyst

-25 100

200

300

400

500

600

100

200

Temperature (°C)

300

IP T

△

W (%)

0 -5

400

500

Temperature (°C)

A

CC E

PT

ED

M

A

N

U

SC R

Fig.4 Variation of △W for Biomass(Pine and Cellulose) LDPE and catalyst blends

600

260

280

a

b

240

Cellulose LDPE Cellulose + LDPE Cellulose + LDPE +Catalyst

180

200

Biomass LDPE Biomass + LDPE Biomass + LDPE +Catalyst

160 160

140 0.0

0.2

0.4

α

0.6

120 0.0

0.8

0.2

0.4

α

IP T

Ea (kJ/mol)

200

Ea (kJ/mol)

240 220

0.6

0.8

A

CC E

PT

ED

M

A

N

U

SC R

Fig. 5 Activation energy values of LDPE, biomass, and their blends during pyrolysis (a Cellulose, b Pine)

Liquid yield

100

Gaseous yield

Solid yield

Product yield (%)

80

60

20

SC R

0 E Pine DPE DPE alyst alyst alyst alyst alyst se lulo LDP + L + L Cat Cat Cat Cat Cat Cel lose Pine ulose + DPE + Pine + DPE + DPE + u l l l L L L Ce Cel se + Pine + lulo l e C

IP T

40

A

CC E

PT

ED

M

A

N

U

Fig.6 Carbon yields of the pyrolysis products in non-catalytic and catalytic pyrolysis of biomass, LDPE and their mixture (1:1:2) with ZSM-5 zeolite

67

Cellulose Pine LDPE Cellulose+LDPE Pine+LDPE

34.5 34.0

Relative area(%)

66

Peak area(%)

35.0

Cellulose Pine Cellulose+LDPE Pine+LDPE

65

64 30

33.5 33.0 15

20

10

C23

C22

C21

C20

C19

C18

C15

C14

C13

C12

SC R

C11

C9

C10

C8

C7

C6

C5

0

C17

l s des etons Acids sters ohols urans rbons heno omps gar E Su ldehy F P K oca Alc ty c r n A i d ta Hy cer n U

C16

5 0

IP T

10

A

CC E

PT

ED

M

A

N

U

Fig. 7 Relative contents of different groups of chemicals in pyrolysis volatiles from biomass, LDPE and blend（a: production content, b: carbon number）

80

a

Cellulose+Catalyst LDPE+Catalyst Pine+Catalyst Cellulose+LDPE+Catalyst Pine+LDPE+Catalyst

60

Cellulose+Catalyst LDPE+Catalyst Pine+Catalyst Cellulose+LDPE+Catalyst Pine+LDPE+Catalyst

70

Aromatic selectivity (%)

Production relative yield (%)

80

40

20

60

b

50 40 30 20

0 Ben

Ind

Nap

Phen and an

0

SBTXE

Benzene

Toluene

Production species

6 3

M

ne ne ne ne Naphthale MethylNaphthale imethylNaphthalerimethylNaphthale D T

SC R

30 25 20

U

9

Cellulose+Catalyst d LDPE+Catalyst Pine+Catalyst Cellulose+LDPE+Catalyst Pine+LDPE+Catalyst

35

15 10

N

12

Polycyclic aromatic compounds

SBTXE

A

Aromatic selectivity (%)

15

Aromatic production relative yield (%)

Cellulose+Catalyst LDPE+Catalyst Pine+Catalyst Cellulose+LDPE+Catalyst Pine+LDPE+Catalyst

c

Ethyl benzene

Aromatic species 40

18

Xylene

IP T

10

5 0

C6

C7

C8

C9

C10 C11 C12 C13 C14 C15 C10+

Carbon numbers

A

CC E

PT

ED

Fig.8. Production yields and aromatic selectivity for the biomass catalytic pyrolysis using H-ZSM-5 catalyst （a:production yields, b:single-ring aromatic selectivity, c:polycyclic aromatic selectivity, d:carbon number ）Key—Ben. Benzene and its derivatives; Ind.: Indene and its derivatives; Nap. : Naphthalene and its derivatives; Phen and an.: Phenanthrenes and anthraceens; SBTXE: The total yields of benzene, toluene, xylene and ethyl benzene

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Scheme 1. The schematic diagram of co-pyrolysis process of biomass and LDPE

O

O

Other anhydrous hexose .

OH

OH

+ OH.

1

OH

6 5 *

OH

OH

OH

OH

O

4

3 1

3

OH

O

CHO

*

+

O.

CH2OH .

2

O

CH2 OH

CH2 OH

C=O

C=O

CH2

CH2

CHO

CH3

OH

CHOH

COH

OHCH

CH

CH

CH

COH

COH

CH2OH

CH2 OH

SC R

CHO

CH2 OH

U

4

CHO

CHOH +

A

M ED PT CC E

CHO

O

Scheme.2 The reaction pathway of cellulose

A

O

N

Cellulose unit

+ H2O

IP T

2

CH2O

R-CH2-CH2-CH2-CH2-CH2.

k1

R-CH2-CH2-CH2-CH2-CH2 . + R.

R-CH2-CH2-CH2 -CH2 -CH3.

k2

R-CH2-CH2-CH2. + CH2=CH2

RCH2-CH2-CH2 -CH2 -CH2. +

k3

CH3-CH2-CH2-CH2=CH2 (alkene/dienes) + R.

R-CH2-CH2 -CH2 -R

k4

IP T

R-CH2-CH2-CH2-CH2-CH2..

R-CH2 -CH2-CH2-CH2-CH3 + R-CH2-CH2-CH2-R. k5

R-R

SC R

R. + R.

A

CC E

PT

ED

M

A

N

U

Scheme.3 The reaction pathway of LDPE

O depolymerization HO

Cellulose

HO

O

O

OH OH OH

ring-opening reaction

xH2O yHCOH zCOx

O xH2O yCOx

small molecular productions

dehydration OH OH OH

OH anhydro monosaccharides

N

OH O

A

O

U

O Lignin

Diels-Alder reaction dehydration reaction

Aromatics

M

R

ED

n

catalytic conversion

n

PE

HZSM-5

PT

n

CC E

Scheme.4 Reaction pathway of catalytic co-pyrolysis of biomass and LDPE

A

O

furan compounds

OH deoxygenation O HZSM-5 catalyst

O

SC R

acid catalyzed oligomerization decarboxylation decarbonylation

Hemicellulose depolymerization

IP T

O

OHO

OH

OH

O

O

Biomass

O

List of Tables Table 1 Main characteristics of the Biomass and LDPE Table 2. Kinetic parameters of Cellulose and LDPE co-pyrolysis process Table 3. Kinetic parameters of Biomass and LDPE co-pyrolysis process Table 4 The results of Ea using FWO Method

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Table 5 The results of Ea using FWO Method

Table 1 Main characteristics of the Biomass and LDPE Cellulose

LDPE

a Moisture

2.12

2.17

0

a Volatile

83.22

95.36

99.08

13.45

2.15

0

a Ash

1.21

0.32

0.92

C

47.59

41.91

86.35

H

6.179

6.557

13.58

N

0.09

0.16

0

S

0.024

0.073

0.074

Ob

46.117

51.3

0

H/C molar ratio

1.56

1.88

1.89

a Fixed

Ultimate analysis (wt.%)

Carbon

SC R

Proximate analysis (wt.%)

Raw

O/C molar ratio Cellulose Component analysis (wt.%)

Hemi cellulose Lignin

Air-dry basis; b:by difference, O(wt%)=100-C-H-N-S

A

CC E

PT

ED

M

A

N

a

0.73

0.92

0

44.39

100

0

24.16

0

0

31.45

0

0

19.13

17.06

46.15

U

Calorific value ( MJ/kg)

IP T

Samples

Table 2. Kinetic parameters of Cellulose and LDPE co-pyrolysis process Temp. (℃)

Cellulose

（dw/dt） max（ wt%/min）

Ea (KJ/mol)

Char percentage(%)

3-97

19.24

168.29

19.82

469.7

2-100

34.74

249.87

2.97

298.1

468.3

3-98

6.75

14.25

201.57

12.96

303.8

435.5

5-100

2.82

4.81

168.81

65.89

Tp1 (℃)

Tp2 (℃)

Conv. (%)

250-500

341.5

LDPE

350-500

Cellulose+LDPE Cellulose+LDPE+ Catalyst

250-500 250-500

dw/dt）max （wt%/min）

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Samples

Table 3. Kinetic parameters of Biomass and LDPE co-pyrolysis process Temp. (℃)

Biomass

（dw/dt） max（ wt%/min）

Ea (KJ/mol)

Char percentage(%)

5-95

9.18

185.87

28.44

469.7

2-100

34.74

249.87

2.97

356.9

468.8

3-100

5.63

13.82

224.34

14.5

350.5

443.5

3-99

5.54

6.54

173.23

47.95

Tp1 (℃)

Tp2 (℃)

Conv. (%)

250-500

359.3

LDPE

350-500

Biomass+LDPE Biomass+LDPE+ Catalyst

250-500 250-500

dw/dt）max （wt%/min）

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Samples

Table 4 The results of Ea using FWO Method Cellulose

LDPE

Cellulose+LDPE

Cellulose+LDPE+Catalyst

Ea

R2

Ea

R2

Ea

R2

Ea

R2

0.05

155.81

0.9967

255.94

0.9777

154.98

0.9997

183.75

0.9835

0.1

169.78

0.9977

246.78

0.9934

152.05

0.9991

180.82

0.9827

0.2

172.19

0.9986

247.31

0.9986

157.68

0.9979

150.48

0.9884

0.3

170.84

0.9991

250.09

0.9994

161.29

0.988

160.19

0.9996

0.4

169.86

0.9994

250.16

0.9993

196.60

0.9249

0.5

170.23

0.9998

250.62

0.9988

238.29

1.0000

0.6

174.59

0.9998

250.47

0.9991

244.45

0.9989

0.7

163.02

0.9728

246.48

0.9988

249.41

0.9985

0.8

249.79

0.9991

259.33

0.9985

0.9

250.62

0.9982

SC R

168.29

249.87

201.57

A

CC E

PT

ED

M

A

N

U

Average

IP T

α

168.81

Table 5 The results of Ea using FWO Method Biomass

LDPE

Biomass+LDPE

Biomass+LDPE+Catalyst

α

Ea

R2

0.05

165.28

0.9428

255.94

0.9777

154.38

0.9976

141.68

0.9981

0.1

183.60

0.9911

246.78

0.9934

164.74

0.9993

146.86

0.9924

0.2

184.73

0.9984

247.31

0.9986

171.06

0.9994

136.05

0.9856

0.3

190.29

0.9992

250.09

0.9994

172.56

0.9988

185.56

0.9928

0.4

190.44

0.9994

250.16

0.9993

277.36

0.9966

203.81

0.9904

0.5

187.21

0.9986

250.62

0.9988

261.21

0.9992

225.29

0.9497

0.6

199.52

0.8621

250.47

0.9991

265.71

0.9972

0.7

0.00

246.48

0.9988

269.77

0.9982

0.8

0.00

249.79

0.9991

282.32

0.9964

0.9

0.00

250.62

0.9982

Average

185.87

249.87

Ea

R2

Ea

R2

173.23

A

CC

EP

TE D

M

A

N

U

SC R

224.34

IP T

Ea

R2