Enhancing production of hydrocarbon-rich bio-oil from biomass via catalytic fast pyrolysis coupled with advanced oxidation process pretreatment
Graphical abstract
Introduction
With the development of industrialization and urbanization, people are more dependent on energy. However, due to the depletion of fossil fuels and the improvement of global environmental protection requirements, people have to open up new ways to find renewable and clean energy. As a renewable resource, biomass resources can be used to produce liquid fuels and high value-added chemicals. The annual yield of biomass resources in the world is huge, and only the yield of corn stalk reaches about 75 million tons of dry matter a year (Menardo et al., 2015). Its rational use can effectively solve the problem of energy shortage. Fast pyrolysis is a thermochemical technique to convert biomass into bio-oil, char and gas, having been developing for decades, which has many advantages, such as high bio-oil yield, low consumption, and short reaction time. However, bio-oil which contains many unstable oxygen-containing functionalities is a complex compound with different characteristics from traditional liquid fuels. There are some factors for the complex components of pyrolytic liquid: (1) biomass includes three major components, cellulose, hemicellulose and lignin. For disparate biomass feedstocks, there are differences in the contents of the three components which makes the pyrolysis products different and even can not effectively enrich some specific products (Gholizadeh et al., 2019, Liu et al., 2020); (2) among the three components, lignin is an important component in the formation of biomass xylem, which endows biomass with rigidity and allows biomass to keep a shape, but its existence makes the process of biomass pyrolysis difficult to decompose (Figueirêdo et al., 2020); (3) lignin is a three-dimensional amorphous polymer composed of phenylpropane structural units connected by β-O-4 ether bonds and C–C bonds (Melro et al., 2018). It is a complex organic polymer that is prone to coking in the pyrolysis. These problems greatly affect the quality of bio-oil and the yield of high value-added chemicals.
Pretreatment and catalytic pyrolysis can effectively deal with the above problems. Catalytic pyrolysis is a technology to achieve selective separation during pyrolysis, which can transform and upgrade pyrolysis products. However, due to the coking problem of macromolecular substances in the pyrolysis products, the catalyst pores will be blocked and deactivated, resulting in low efficiency, high cost, and difficulty in recycling. Pretreatment is a method of treating biomass feedstocks to improve the quality of bio-oil and the yield of high-value chemicals (Akhlisah et al., 2021, Chen et al., 2018), which can directionally adjust biomass composition or structure and achieve biomass components separation before pyrolysis. Currently, several pretreatment technologies including torrefaction, ionic-liquid and advanced oxidation have been widely applied. Compared with other pretreatment technologies, advanced oxidation processes (AOPs) are one of the best pretreatment technologies that can effectively degrade lignin in biomass (M'Arimi et al., 2020). AOPs, techniques with easy operation, low energy consumption, and high reaction efficiency, can be defined as methods in which radicals are produced in sufficient quantities to act as the main oxidizing agent.
The radicals are commonly used in research including ·OH, SO4-· and oxygen free radicals. SO4-· (2.5ev-3.1 eV) has a similar oxidation potential to ·OH (2.8 eV), and its ability to destroy aromatic structures is efficient, especially for lignin (Li et al., 2021, Ran and Li, 2020). Therefore, this paper used the AOPs method based on SO4-· to pretreat biomass in order to achieve the effect of destroying and converting lignin. SO4-· can be obtained from persulfate by different activation methods, including heating, UV light, and transition metal ions (Mn+) (Gao et al., 2016, Ishak et al., 2021, Wang et al., 2019b). As shown in Eq. (1), (2), (3):S2O82-+heat → 2SO4-·S2O82-+hv → 2SO4-·Mn++S2O82-→M(n+1) + SO4-·+ SO42-
The current researches are mostly focused on using AOPs technology to depolymerize lignin promoting the conversion of residues into sugars, which can reduce its inhibitory effect on enzymatic hydrolysis and ethanol fermentation in the subsequent biomass fermentation process (Wang et al., 2022a, Wang et al., 2022b). It also can be used to depolymerize biomass improving biomass components, so as to prepare biomass products efficiently such as biochar and methane (Shen et al., 2021, Ban et al., 2022). However, the utilization methods of AOPs ignore that lignin is the only renewable aromatic resource that can be catalytically fast pyrolyzed to produce aromatic fuels (e.g., benzene, toluene, and xylene) (Wu et al., 2022).
Therefore, this study innovatively used sodium persulfate pretreatment which SO4-· was activated by heat/UV combined with catalytic pyrolysis technology to improve the quality of biomass pyrolysis products. The pretreatment effect of SO4-· on corn stalk was investigated by changing the two reaction conditions of different reaction time and sodium persulfate concentration. Pyrolysis and catalytic pyrolysis were used to discuss the pyrolysis products of corn stalk under different pretreatment conditions, especially the overall changes of hydrocarbons and aromatics in bio-oil after catalytic pyrolysis. The possible formative pathways of chemicals and liquid fuels from SO4-· pretreatment coupled with fast pyrolysis were proposed.
Section snippets
Materials
Corn stalk was bought at a farm in Lianyungang city, Jiangsu Province, China. Before sodium persulfate pretreatment, corn stalk was ground into powders that can pass through 40 mesh sieves (≤0.425 mm). The elemental analyzer (Euro Vector EA3000, Italy) was used to analyze the ultimate analysis of corn stalk, and proximate analysis was carried out based on Chinese National Standards (GB/T 28731, 2012). The results were shown in Table. 1. Before sodium persulfate pretreatment, the corn stalk
Productivity and chemical compositions of raw and pretreated feedstocks
Fig. 1 showed the changes of yield and compositions of corn stalk pretreated with sodium persulfate under different reaction conditions. As shown in the Fig. 1(a), in the concentration group, the yields of corn stalk were all above 69.00%. In the time group, it was found that with the increase of time, the yield of corn stalk decreased which gradually decreased from 72.40% in 2 h to 65.26% in 8 h. The results showed that after sodium persulfate pretreatment, some components of the biomass were
Conclusion
This work confirmed that sodium persulfate pretreated coupled with fast pyrolysis was an effective biorefining process to promote the quality of bio-oil. After pretreatment, AAEMs were removed and the phenols produce of fast pyrolysis was significantly increased. Additionally, the highest relative content of aromatic hydrocarbons and hydrocarbons yield from catalytic pyrolysis can be remarkably reached 28.66% and 33.72% respectively, which was mainly ascribed to the effective depolymerization
CRediT authorship contribution statement
Jiapeng Wang: Writing – original draft. Bo Zhang: Supervision, Conceptualization, Funding acquisition, Project administration. Awsan Shujaa Aldeen: Writing – review & editing, Validation. Stephen Mwenya: Methodology. Haoqiang Cheng: Visualization, Investigation. Zhixiang Xu: Investigation, Conceptualization. Huiyan Zhang: Data curation, Project administration.
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 work was supported by the National Key Research and Development Program of China (No. 2018YFC1901200), the National Natural Science Fund Program of China (No.51706043), and the ‘Zhishan Young Scholar’ Program of Southeast University and the Fundamental Research Funds for the Central Universities (No. 2242021R41076).
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