Microwave-assisted catalytic upgrading of co-pyrolysis vapor using HZSM-5 and MCM-41 for bio-oil production: Co-feeding of soapstock and straw in a downdraft reactor

https://doi.org/10.1016/j.biortech.2019.122611Get rights and content

Highlights

  • The addition of MCM-41 was beneficial in prolonging the life of HZSM-5.

  • The optimal operating conditions were determined.

  • Co-pyrolysis of soapstock with straw advanced the deoxygenation especially phenol.

  • Co-pyrolysis with a dual-catalyst system is a promising technology.

Abstract

Microwave-assisted co-pyrolysis of low hydrogen-to-carbon and high hydrogen-to-carbon effective ratio materials with the aid of HZSM-5 and MCM-41 is a promising technique to improve the bio-oil quality. The low content of hydrocarbons and short life cycle of catalyst limit the application of pyrolysis technology in biomass energy conversion. The effects of catalytic temperature, and HZSM-5-to-MCM-41, feedstock-to-catalyst, and straw-to-soapstock ratios on the yield and composition of bio-oil were studied in this work. The quality of bio-oil during biomass pyrolysis can be improved by adjusting the operating conditions. The optimal catalytic temperature, and ratios of HZSM-5-to-MCM-41, feedstock-to-catalyst, and straw-to-soapstock were 400 °C, 1:1, 2:1, and 1:2, respectively. The addition of MCM-41 was beneficial in prolonging the life of HZSM-5 since the macromolecular compounds cracked when MCM-41 was added which restrain the generation of coke. The co-pyrolysis of soapstock with straw advanced the deoxygenation of oxygen-containing compounds especially phenol from straw during pyrolysis.

Introduction

With the over-consumption of fossil fuels and increasing world energy demand, the development of renewable energy has attracted the attention of many researchers. Bio-oil, a renewable energy resource, is one of the most promising alternatives to fossil fuels. Bio-oil is most commonly produced via the pyrolysis of lignocellulosic biomass. Straw is one of the most common sources of biomass. Pyrolysis is a thermochemical conversion technique for converting biomass into bio-oil, which is eco-friendly and efficient (Uzoejinwa et al., 2018). But some hurdles, such as high oxygen, low hydrocarbon content, corrosiveness, and low calorific value, should be addressed (Dai et al., 2019, Lu et al., 2009). These challenges suggest the need to improve raw material pretreatment or selection, catalyst selection, and process to increase calorific value and hydrocarbon content and reduce the oxygen content of bio-oil.

In many studies conducted for the improvement of bio-oil quality (lower yield of oxygen-containing compounds and higher yield of hydrocarbons in bio-oil), catalytic cracking is one of the most common and best techniques used (Dai et al., 2017). HZSM-5 is a typical zeolite catalyst with a unique pore structure and acidity and performs well in deoxygenation (Jae et al., 2011). However, the quality of bio-oil from catalytic cracking using only HZSM-5 is unsatisfactory, and the lifecycle is limited due to coke formation (Zhang et al., 2007). HZSM-5, a microporous material, has a pore size of about 0.5 nm (Mohabeer et al., 2019). Thus, large molecules are prevented from entering the pores, causing the deactivation of the catalyst (Li et al., 2019). The deactivation of zeolite catalysts is related to the pore structure and acidic properties. Li et al. (Li et al., 2017) studied the mechanism of coke formation in Ni/HZSM-5 and Ni-Cu/HZSM-5 and showed that Ni/HZSM-5 is coked easier than Ni-Cu/HZSM-5 due to the formation of carbon plates. The Cu site in the Ni-Cu/HZSM-5 catalyst can reduce the particle of Ni and the formation of carbon plates. The results showed that Ni/HZSM-5 has a smaller total specific pore volume than Ni-Cu/HZSM-5. Hence, it is necessary to choose a suitable pore structure and avoiding coke deposition for developing an effective catalyst for bio-oil deoxygenation. The mesoporous zeolite material has a bigger pore size than the microporous one. The application of MCM-41 (mesoporous material) catalytic bed, which is located in the upstream position from the HZSM-5 catalytic bed, converts large molecules into small molecules. The pore size of MCM-41 can vary from 1.5 nm to 10 nm (Li et al., 2019). Yu et al. (Yu et al., 2012) showed that the mesoporous zeolite catalyst can promote the cracking of high-carbon chain compounds, which is beneficial for the catalysis of HZSM-5 due to reduced molecule size.

Co-pyrolysis with a high hydrogen-to-carbon effective (H/Ceff) ratio raw material is another method to upgrade the bio-oil quality. Soapstock is one of the by-products of the food industry and one of the best feedstocks for co-pyrolysis. Soapstock is suitable for bio-oil production due to its eco-friendly and chemical composition property (Wang et al., 2016). Elemental analysis indicated that the H/Ceff of soapstock is 1.39 (Table 1), whereas that of straw is − 0.38. Pyrolyzing raw materials with low H/Ceff ratio (less than 1) to obtain bio-oil of HZSM-5 is economically unreasonable due to the limited catalytic life cycle, which is mainly caused by coke formation (Chen et al., 1986). One of the main reasons for heavy hydrocarbon and coke formation is the low H/Ceff ratio of raw materials, which is due to the hydrogen-deficient hydrocarbon pool inside the zeolite catalyst (Cheng and Huber, 2011, Foster et al., 2012). However, the co-pyrolysis of lignocellulosic biomass (low H/Ceff ratio) and soapstock (high H/Ceff ratio) can effectively solve this problem. Zhang et al. (Zhang et al., 2015) found that the yield of total organic pyrolysis products and the relative content of aromatics increase nonlinearly as the H/Ceff ratio of feedstock increases. The co-pyrolysis coupling downdraft reactor and the microwave-absorbent bed may improve bio-oil quality. With the application of the downdraft reactor, the chance of contact between the pyrolysis vapors increases, which is helpful in enhancing the synergistic effect. Compared with electric heating, microwave heating has the advantage of fast heating rate, no temperature gradient, no hysteretic effect, volumetric heating. Wang et al (Wang et al., 2016, Wang et al., 2018a, Wang et al., 2018b, Wang et al., 2019) have successfully applied microwave heating technology to biomass pyrolysis.

The content of aromatics in bio-oil increased with the application of downdraft reactor and SiC (microwave absorber) as shown in our previous study (Wang et al., 2019). However, to the best of our knowledge, existing research on the effects of applying downdraft reactor and microwave-absorbent bed on co-pyrolysis is few. The present study examines the microwave-assisted catalytic upgrading of co-pyrolysis vapor using HZSM-5 and MCM-41 for bio-oil with the application of downdraft reactor and SiC (microwave absorber). Also, the effects of catalytic temperature (HZSM-5 catalytic bed) and feedstock-to-catalyst, straw-to-soapstock, and HZSM-5-to-MCM-41 (H-to-M) ratios on the yield and quality of bio-oil are reported. The raw materials used in the study are straw and soapstock.

Section snippets

Materials

Soapstock was purchased from Yihai Kerry Oils and Foodstuffs Co. Ltd. (China). Soapstock was saponified using NaOH and absolute alcohol, dried under a vacuum environment at 105 °C for 48 h, and crushed into granules. The elemental analysis showed that the concentrations of C, H, and O were 64.51 wt%, 9.93 wt%, and 19.82 wt%, respectively (Table 1). The chemical composition of soapstock also shown in Table 1. Raw straw, which was obtained from the surrounding countryside in Nanchang Jiangxi

Fractional yield analysis of the pyrolysis product

This work reported the H-to-M ratio (no MCM-41, 3:1, 1:1, 1:3, and no HZSM-5) effects on the yield and chemical composition of bio-oil. The catalytic temperature was maintained at 400 ℃, and the ratios of feedstock-to-catalyst and straw-to-soapstock were fixed at 2:1 and 1:2, respectively. The fractional yields (gas, liquid, and solid phases) of the pyrolysis product at different H-to-M ratios are shown in Fig. 2A. The bio-oil yield initially increased slightly with the increased amount of

Further discussion

This work studied several factors that affect the yield and chemical composition of bio-oil, and the optimum experimental conditions were discussed in this paper. The specific pyrolysis process was summarized in this section in accordance with the results of this paper, from the microwave-assisted catalytic pyrolysis of soapstock (Wang et al., 2019), the co-catalytic pyrolysis of biomass to bio-oil (Li et al., 2019), and the thermochemical conversion of waste oil over basic composite catalysts (

Conclusion

The optimal catalytic temperature and ratios of H-to-M, feedstock-to-catalyst, and straw-to-soapstock were 400 °C, 1:1, 2:1, and 1:2, respectively. MCM-41 loading was beneficial in extending the life of HZSM-5, and a high catalytic temperature promoted the deoxygenation of oxygen-containing compounds. Also, co-pyrolysis of soapstock with straw can promote the deoxygenation of oxygen-containing compound especially phenol from straw during pyrolysis. These findings promoted the application of

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.

Acknowledgements

This project is financially supported by the National Natural Science Foundation of China (Nos. 21766019, 21878137), Key Research and Development Program of Jiangxi Province (20171BBF60023), China Scholarship Council (201806820035), Natural Science Foundation of Jiangxi Province (20181BAB206030), Innovation and Entrepreneurship Development Fund of “Thousand talents program” Talent (1001-02102082).

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