Elsevier

Renewable Energy

Volume 164, February 2021, Pages 87-95
Renewable Energy

Catalytic co-pyrolysis of torrefied poplar wood and high-density polyethylene over hierarchical HZSM-5 for mono-aromatics production

https://doi.org/10.1016/j.renene.2020.09.071Get rights and content

Highlights

  • The catalytic co-pyrolysis of TPW/HDPE over hierarchical HZSM-5 was studied.

  • Torrefaction promoted the hydrogen transfer reaction between lignin radicals and HDPE.

  • Mild torrefaction temperature and meso-HZSM-5 enhanced mono-aromatic production.

  • Torrefaction favored BTX production, meso-HZSM-5 favored alkylbenzenes production.

Abstract

The catalytic co-pyrolysis of torrefied poplar wood sawdust (TPW) and high-density polyethylene (HDPE) was investigated over hierarchical HZSM-5. Compared with raw PW/HDPE, the bio-oil yield from co-pyrolysis of TPW/HDPE decreased gradually while the quality of bio-oil was upgraded. With increasing torrefaction temperature from 220 to 280 °C, the amounts of acids, furans, and anhydrosugars in bio-oil were significantly reduced due to the removal of hemicellulose, whereas the production of phenols and alkenes were improved due to the enhanced hydrogen transfer reaction. In the catalytic co-pyrolysis, increasing torrefaction temperature caused an enhanced production of mono-aromatics as well as the selectivity of BTX (benzene, toluene, and xylene). Nevertheless, severe torrefaction (280 °C) lead to a rapid reduction of aromatic yield and selectivity due to the loss of cellulose. Compared to parent HZSM-5, hierarchical HZSM-5 treated with alkaline concentration (0.2–0.3 mol/L) favored the formation of mono-aromatics at the expense of polyaromatics. The maximum mono-aromatics yield of 71.75% was obtained during catalytic co-pyrolysis of 260-TPW/HDPE over 0.3-HZSM-5. The present work suggests that torrefaction pretreatment followed by the catalysis of hierarchical HZSM-5 is an efficient way to promote the production of valuable mono-aromatic hydrocarbons from biomass and plastic wastes.

Introduction

Renewable resources have gained increasing attention worldwide due to the shortage of fossil fuels and environment pollution. Biomass is an abundant, low-cost, and eco-friendly energy source and can be directly converted into liquid fuels through fast pyrolysis, which is considered as a promising alternative to fossil fuels [1,2]. However, the pyrolysis oil obtained from biomass, known as bio-oil, has some undesirable properties such as corrosive, unstable, low heating value, and incompatible with fossil fuel, limiting its use in conventional engines and boilers [3]. Incorporation of hydrogen-rich materials such as thermoplastics into biomass pyrolysis process can enhance the quality of the end-product bio-oil [4,5]. It is reported that more than 300 million tons of plastics are produced annually worldwide [6]. The continuous increase of waste plastics needs to be disposed harmlessly. Thereby, co-pyrolysis of plastic waste with biomass is an efficient method to mitigate plastic pollution and simultaneously obtain value-added liquid fuels [7].

Torrefaction, as one of the main pretreatment ways of biomass, is an effective technique to gain an improved quality of bio-oil by reducing oxygen content. Torrefaction is a thermal pretreatment to increase biomass energy density and remove water from biomass at the temperature ranging from 200 to 300 °C [8,9]. During the torrefaction of biomass, part of the hemicellulose and the branches of cellulose and lignin are eliminated from biomass as volatile vapors, enhancing the pyrolysis characteristics of biomass [10]. Torrefaction pretreatment has been demonstrated to cause the deoxygenation of biomass feedstock and facilitate aromatic production [[11], [12], [13], [14]]. Aromatics, especially mono-aromatics such as benzene, toluene, and xylenes (BTXs), are important chemical materials [15,16]. Thus, it is crucial to increase the content of aromatic compounds in bio-oil for its use as fuel additives or value-added chemical production. Several researchers have reported that torrefaction pretreatment can increase the yields of aromatics in the bio-oil from the co-pyrolysis of biomass and plastics [17,18], which confirmed that the combination of torrefaction and co-pyrolysis is a promising strategy. Recently, the effects of factors such as co-pyrolysis temperature and catalyst type/amount on the production of aromatic hydrocarbons through catalytic fast pyrolysis of torrefied biomass and plastics have been studied [6,19]. For example, Park et al. [19] investigated the effect of catalyst properties on the aromatic production during catalytic co-pyrolysis of torrefied yellow poplar and high-density polyethylene (HDPE). They summarized that both the large pore size and strong acidity of the catalyst were important for aromatics production. However, little attention has been focused on the effect of torrefaction pretreatment conditions (e.g. torrefaction temperature and time) on the aromatic production during catalytic co-pyrolysis of biomass and plastics.

Over the past decades, a lot of catalysts have been tested in the catalytic co-pyrolysis process ranging from zeolites [20,21], metal oxides [22], acidic mesoporous materials [23], carbon-based materials [24], etc. Among these, HZSM-5 zeolite is found to be the most frequently used and the most effective catalyst in aromatic production due to its unique pore structure and relatively strong acidity. However, the micropores of HZSM-5 zeolite severely hindered the diffusion of bulky compounds causing coke deposition on the surface and reducing catalyst efficiency, which is a well-known issue in the HZSM-5 upgrading process. Moreover, the undesirable polymerization of aromatic hydrocarbon precursors at the acid sites of HZSM-5 zeolite resulted in the high polyaromatic hydrocarbons (PAHs) yield [25,26]. Fortunately, this hindering diffusion effect could be reduced by introducing mesopores connected to micropores in the HZSM-5 zeolite. Hierarchical HZSM-5 zeolite containing both micropores and mesopores can be prepared by alkaline treatment that involves partial desilication of the HZSM-5 framework to generate larger pore openings and higher external surface area. The incorporation of mesopores in the HZSM-5 zeolite was found to improve the catalytic activity by decreasing the diffusion and accessibility limitation of bulky molecules in petrochemical and biomass upgrading process [27]. For example, Li et al. [28] prepared a series of mesoporous HZSM-5 zeolites with varying alkaline concentrations, finding that hierarchical HZSM-5 improved the diffusion of bulky oxygenates and produced more aromatic hydrocarbons and less coke compared to the parent HZSM-5 in catalytic pyrolysis of beech wood. Chen et al. [29] also reported that suitable mesopores of HZSM-5 zeolite created by alkaline treatment enhanced mass transfer efficiency, increasing the yield of aromatics in catalytic pyrolysis of rice straw. Thus, the hierarchical HZSM-5 zeolite is hypothesized to improve the aromatic production during co-pyrolysis of torrefied biomass and plastics.

Poplar wood (PW) is one of the widespread and abundant species in China, which can be used for timbers and furniture. Consequently, large amounts of PW sawdust and residues are generated every year. In this study, to obtain a high content of valuable mono-aromatics in the bio-oil, PW was first pretreated by torrefaction and then was catalytically co-pyrolyzed with HDPE in the presence of hierarchical HZSM-5 zeolite by using a fixed bed reactor. The torrefaction temperatures of PW (220, 240, 260, and 280 °C) and concentrations of alkaline treated HZSM-5 zeolite (0.2, 0.3, and 0.4 mol/L) were studied to obtain their effects on mono-aromatic hydrocarbons production. The influence of combining the torrefaction pretreatment with hierarchical HZSM-5 catalysis on the mono-aromatic formation mechanism was proposed.

Section snippets

Materials

PW, obtained from Heilongjiang, China, was smashed in a high-speed rotary cutting mill and screened to the particle size of 200–400 μm, then dried at 105 °C for 48 h. HDPE (particle size 250–500 μm) was purchased from Tianjin Petrochemical Company, China. HZSM-5 zeolite (powder, Si/Al = 35) was purchased from Nankai University Catalyst Co., Ltd., China. The zeolite was calcined at 550 °C for 5 h before use. NaOH and NH4NO3 were purchased from Tianjin Hengxing Chemical Co., Ltd., China.

Torrefaction of PW

The

Properties of TPW

The ultimate and proximate analyses of raw and torrefied PW are shown in Table 1. When the torrefaction temperature increased, the carbon content of TPW was increased at the expense of oxygen, resulting in lower O/C ratios. It is evident that torrefaction pretreatment caused deoxygenation of PW as the main reactions. Oxygen was released in the form of H2O, CO2, CO, and some organic volatiles via dehydration, decarboxylation, and decarbonylation reactions [30]. The volatile content of PW was not

Conclusions

The catalytic co-pyrolysis of TPW and HDPE over hierarchical HZSM-5 was performed to produce value-added mono-aromatic hydrocarbons in a fixed bed reactor. Although torrefaction reduced bio-oil yield, it was demonstrated to be effective in deoxygenation of bio-oil compounds, favoring the formation of aromatics. Moreover, torrefaction promoted the hydrogen transfer reaction between lignin and HDPE, resulting in the increase of phenols and alkenes with torrefaction temperature. In the

CRediT authorship contribution statement

Xiaona Lin: Conceptualization, Methodology, Writing - original draft. Lingshuai Kong: Investigation, Resources, Data curation. Xiajin Ren: Investigation, Data curation. Donghong Zhang: Investigation, Resources, Data curation. Hongzhen Cai: Writing - review & editing. Hanwu Lei: Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors are grateful for financial support from the National Natural Science Foundation of China (51806129), and the Natural Science Foundation of Shandong Province of China (ZR2017BEE062), the Agriculture and Food Research Initiative Competitive Grant no. 2016-67021-24533 and 2014-38502-22598 from the National Institute of Food and Agriculture, United States Department of Agriculture.

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