Jet fuel and hydrogen produced from waste plastics catalytic pyrolysis with activated carbon and MgO

https://doi.org/10.1016/j.scitotenv.2020.138411Get rights and content

Highlights

  • Catalytic pyrolysis of waste plastics with activated carbons and MgO was studied.

  • The placement of catalysts had a crucial effect on the catalytic pyrolysis of LDPE.

  • 100% jet fuel range liquid products were obtained in the catalytic pyrolysis of LDPE.

  • More than 90% of hydrogen was achieved in gaseous products.

Abstract

Catalytic pyrolysis of waste plastics to produce jet fuel and hydrogen using activated carbon and MgO as catalysts was studied. The effects of catalyst to waste plastics ratio experimental temperature, catalyst placement and activated carbon to MgO ratio on the yields and distributions of pyrolysis products were studied. The placement of catalysts played an important role on the catalytic pyrolysis of LDPE, and the pyrolytic volatiles first flowing through MgO and then biomass-derived activated carbon (BAC) could obtain an excellent result to produce H2 and jet fuel-rich products. The higher pyrolysis temperature converted diesel range alkanes into jet fuel range alkanes and promoted the aromatization of alkanes to generate aromatic hydrocarbons. BAC and MgO as catalysts had excellent performance in catalytic conversion of LDPE to produce hydrogen and jet fuel. 100 area.% jet fuel range products can be obtained in LDPE catalytic pyrolysis under desired experimental conditions. The combination of BAC and MgO as catalysts had a synergy effect on the gaseous product distribution and promoted the production of hydrogen, and up to 94.8 vol% of the obtained gaseous components belonged to hydrogen. This work provided an effective, convenient and economical pathway to produce jet fuel and hydrogen from waste plastics.

Introduction

Plastics are used as raw materials in many aspects of our life such as electronics, healthcare, agriculture, transportation, construction, and packaging, which play an indispensable role in industries because of their properties of low cost and versatility (Kunwar et al., 2016; Serrano et al., 2005; Cho et al., 2010). The consumption of plastics is increasing with global economic development in recent decades, and the usage amount of plastics was 405 million tons in 2015 (Geyer et al., 2017) and was forecasted to progressively increase at a rate of 4% over the next few years (Burange et al., 2014). Solid waste caused by the use of plastics has become one of the main environmental pollutions (Fan et al., 2017). Thus, the effective treatment of plastics is an urgent need to solve the environmental problem. Incineration and burial are the two most common methods for plastics disposals (Cho et al., 2010; López et al., 2011). However, these methods create numerous problems for the economy and ecosystem (Jambeck et al., 2015). In recent years, the recovery of energy from plastics to meet growing energy consumption has increasingly attracted interest (Sørum et al., 2001; Stelmachowski, 2010; Ryu et al., 2019). Thus, it is necessary to develop a simple, efficient, and low-cost method to solve plastic pollution and convert them into valuable energy products.

Pyrolysis, an effective and simple method, was widely used to convert waste plastics into valuable energy sources, such as the production of aromatics, olefins, and paraffin (Sharuddin et al., 2016). However, the products from waste plastics pyrolysis are difficult to use as conventional liquid fuels due to the wide variety of pyrolysis products, which lead to required subsequent refining processes in order to be used (Miandad et al., 2016). According to the previous studies (Sharuddin et al., 2016; Miandad et al., 2017; Demirbas, 2004; Miskolczi et al., 2006), catalytic pyrolysis can be a promising method to convert plastics into high-value biofuels. The use of catalysts for the catalytic upgrade of volatiles produced in the pyrolysis process has significant advantages because it facilitates the production of the desired target products. This means that the target diesel, gasoline, or jet fuel could be selectively produced by selecting a suitable catalyst. The catalytic pyrolysis can reduce the reaction temperature, thereby reducing the energy consumption of the entire process (López et al., 2011; Vasile et al., 2001). Therefore, the use of suitable catalysts is essential for the catalytic pyrolysis of plastics, which not only maximize the production of target products, but also reduce energy consumption and play a role in environmental protection. Recently years, many catalysts were used and studied in the catalytic conversion of waste plastics such as solid acid catalysts (zeolites, activated carbons) and base catalysts. Zeolites are considered to be effective catalyst for waste plastics thermal conversion due to their high acidity and the ability to obtain higher yields of aromatic hydrocarbon products (Liu et al., 2010; Zhang et al., 2015a). The carbocationic cleavage and subsequent reaction of the pyrolysis volatiles were accelerated by these catalysts (Lopez et al., 2017; Boronat and Corma, 2008). However, base catalysts have received less attention for catalytic conversion of waste plastics compared to zeolites. MgO is an effective, cost-efficient, and sustainable base catalyst for catalytic upgrading of waste plastic pyrolysis products (Fan et al., 2017), and the catalytic pyrolysis of diesel into gasoline fractions and the hydrogenation of alkenes to generate alkanes can be promoted by MgO. In recent years, biomass-derived activated carbons with acidity, high porosity and surface area to convert biomass and waste plastics into high-valued chemicals and fuels have received more and more attentions (Fraga et al., 2016; Yang et al., 2018; Zhang et al., 2019). The molecular weight of the products was positively correlated with the pore sizes of activated carbons, that is to say, higher molecular weight products can be obtained via biomass catalytic pyrolysis with large pore size catalysts. Not only that, activated carbons used in waste plastics pyrolysis also have good catalytic effect. According to the research results of Sun et al. (2018), aromatics, alkenes and alkanes could be produced from waste polyethylene pyrolysis over activated carbon catalysts, and the target products obtained from the catalytic conversion of long-chain hydrocarbons was affected by the phosphorus-containing functional groups (P-OH, Pdouble bondO and C-O-PO3). Zhang et al. (2019) investigated the catalytic conversion of plastics with activated carbons. The results showed that the aviation fuel hydrocarbons in the liquid products obtained from LDPE can be up to 100 area.%, of which the selectivity of aromatics and alkanes were 28.7 area.% and 71.3 area.%, respectively. This study showed that activated carbons have a good prospect to catalytic convert the LDPE into high purity biofuels.

In recent years, many researchers have focused their attention on the combination of solid acid catalysts and base catalysts to improve the yield and quality of pyrolysis liquid products (Zhang et al., 2014; Wang et al., 2017; Ding et al., 2018; Iftikhar et al., 2019). Combining the base catalyst (MgO) and the solid acid catalyst (HZSM-5) as catalysts in a dual-stage catalytic bed could provide a wide pore size range, and includes both acidic and basic active sites, which can have a synergistic effect to improve bio-oil quality (Kabir and Hameed, 2017). It has been proven that the combination of solid acid and base catalysts can improve the yield of aromatic hydrocarbons. Thus, combining MgO with BAC as catalysts can be used as an attempt to convert waste plastics into high-purity hydrocarbon fuels. The aim of this work was to study the combined impact of BAC and MgO on the quality and yield of waste plastics derived fuels. Operating conditions were optimized by studying the placement of catalysts, experimental temperature, the ratio of catalyst to waste plastic and the ratio of BAC to MgO to attain the aim of converting waste plastics into high purity biofuels.

Section snippets

Material

LDPE (low-density polyethylene) from Sigma-Aldrich Corporation (St. Louis, MO, USA) was used as a reactant, and it was ground into powder for catalytic pyrolysis experiments. Phosphoric acid (H3PO4, 85 wt%) was purchased from Alfa-Aeser (USA). MgO, as part of the catalyst, was purchased from Alfa Aesar (USA). Corncob, as raw material for preparing activated carbon catalyst, was purchased from Home Depot.

The preparation process of activated carbons

Activated carbons as part of the catalyst from this work, obtained by microwave

The characterization of activated carbon catalyst

The porous structure of activated carbon catalyst can increase the catalytic residence time of pyrolytic volatiles, so the porous structure is known as a critical factor of activated carbon catalysts. Table 1 lists the structural properties of BAC. The mesopore surface area and BET surface area of BAC were 125.84 and 1167.06 m2·g−1, respectively. This confirms that activated carbon had a developed mesopore structure. More reaction surfaces were provided by large BAC surface areas to catalyze

Conclusion

The catalytic pyrolysis of waste plastics with biomass-derived activated carbons and MgO as a catalyst was studied to generate H2-rich gaseous products and jet fuel in this work. The results showed that the liquid products produced by LDPE catalytic pyrolysis can be used as jet fuel. The placement of catalysts had a crucial impact on the LDPE catalytic pyrolysis, and the effect of the pyrolytic volatiles first flowing through MgO and then BAC had a more pronounced effect on the catalytic

CRediT authorship contribution statement

Erguang Huo: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. Hanwu Lei: Conceptualization, Funding acquisition, Writing - review & editing, Supervision. Chao Liu: Conceptualization, Funding acquisition, Writing - review & editing, Supervision. Yayun Zhang: Methodology, Writing - review & editing. Liying Xin: Methodology, Writing - review & editing. Yunfeng Zhao: Methodology, Writing - review & editing. Moriko Qian: Validation, Writing - review &

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

This study was supported by the Agriculture and Food Research Initiative Competitive Grant no. 2016-67021-24533, 2018-67009-27904, and 2014-38502-22598 from the National Institute of Food and Agriculture, United States Department of Agriculture, the National Natural Science Foundation of China (No. 51576019) and China Scholarship Council (No.201806050178).

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