Insight into liquefaction process of sawdust with hydrogen donor solvents

https://doi.org/10.1016/j.biombioe.2022.106444Get rights and content

Highlights

  • Cyclohexanol is selected as a hydrogen donor solvent for biomass liquefaction.

  • Hydrogen donor solvent can improve the conversion and liquefaction yield of biomass.

  • Different kinds of solvents can affect the distribution of bio-oil products.

  • Possible cracking mechanism of biomass liquefaction is proposed.

Abstract

As a zero-carbon energy source, biomass will play an important role in achieving the goal of carbon neutrality. The efficient conversion and utilization of biomass energy remains a great challenge. Thermochemical liquefaction is one of the most efficient ways to convert biomass into liquid fuels. In this study, five different solvents were used to liquefy sawdust. The results showed that cyclohexanol had the highest conversion of 90.5% with a liquid yield of 79.1%, followed by tetralin (81.7% and 72.0%), ethanol (70.2% and 57.0%), isopropanol (64.5% and 47.7%), and cyclohexane (57.0% and 44.5%). The better performance of cyclohexanol and tetrahydrofuran is due to their greater mass and heat transfer capacity, which is determined by the comparative solvent properties. The liquefied gas, liquid and solid products were characterized by GC, GC-MS and FT-IR, respectively. The results demonstrated that the composition of hydrogen and hydrocarbons in the gas was influenced by the hydrogen donor of the solvent; the distribution of bio-oil was strongly influenced by the hydrogen donor and side reactions of the solvent; the solid residue was mainly from lignin in the wood chip composition. In addition, microcrystalline cellulose and lignin were used as models to further investigate solvent effects and liquefaction mechanisms. The formation pathways of some special chemicals, including acid intermediates, ketones, and phenols, were proposed based on the structural units of cellulose and lignin.

Introduction

The extensive consumption of traditional resources (e.g. coal and petroleum) has aroused severe environmental issues such as greenhouse gas (e.g. CO2) and detrimental gases (e.g. SOx and NOx) emissions [[1], [2], [3]]. The search for a renewable alternative energy source has become a universal focus of the world [4]. Therefore, biomass-related energy sources are attracting more and more attention, largely due to the abundance of reserves, cheapness and renewability [[5], [6]]. As an environmentally friendly energy source [7]. Biomass absorbs CO2 as it grows, capturing carbon from the environment, which offsets the CO2 emitted when it is burned. The net CO2 emissions from biomass to the atmosphere is close to zero, which can effectively reduce the greenhouse effect [8,9]. Thus, the efficient use of biomass can help achieve carbon neutrality goals [10]. However, because of its low calorific value, biomass is not suitable for direct incineration and needs to be processed into biofuels through different conversion processes.

Biomass conversion and utilization technologies mainly include hydrolysis, biochemical conversion and thermochemical conversion. Thermochemical conversion has a higher degree of conversion and a shorter conversion period than hydrolysis and biochemical conversion. Thermochemical conversion includes gasification, thermal cracking and liquefaction technology, among which liquefaction technology is more conducive to industrial operation, especially the subsequent purification and upgrading of bio-oil. Biomass liquefaction is commonly performed in a solvent at 200–400 °C, 2–20 Mpa in the presence or absence of a catalyst, which is considered as one of the most potential technologies to transform lignocellulose into bio-oil [11]. However, biomass liquefaction often suffers from two distinguished challenges, the low conversion and/or the low products selectivity [12]. In the past work, researchers have found that these two aspects were considerably affected by the solvents employed [[13], [14], [15], [16], [17]]. When a reaction medium interacted with the raw material, the difference in the properties of the solvent itself has a great influence on the mass transfer and heat transfer. The reactivity of solvent is referred as the solubility to the raw material, the ability to disperse the fragments decomposed, and the ability to react with the intermediates, which will affect the liquefaction progress [12,[11], [18]]. On the other hand, the heat transfer rate of solvent may affect the decomposition rate of feedstock, thereby affecting the conversion behavior and product composition [19].

Fan et al. [13] investigated a comparative solvolysis of fruit fibers and the order of conversion rate was ethylene glycol > water > ethanol > acetone > toluene. They believed that it might be related to the combination of the polarity and ability to donate hydrogen of solvents. Moreover, the bio-oil produced by ethanol, water and toluene treatment was mainly composed of phenolic, while ethylene glycol tended to form alcohols and acetone favored the production of ketones and aldehydes. Wang et al. [15] observed a phenomenon that a large amount of phenolic appeared in phenol-liquefied bio-oil from bamboo liquefaction. Recently, ionic liquids are widely used to dissolve cellulose and lignin due to their strong anions and cations, which can destroy hydrogen bonding presented in the nature bulk [20,21].

Additionally, the solvents can also react with intermediates to form secondary products in liquefaction. For example, alcohol solvents can act as a reactant to esterify with acids derived from biomass. Yuan et al. [14] found that the liquefaction of Spirulina in methanol and ethanol mainly produced methyl esters and ethyl esters. Therefore, solvents such as alcohols or polycyclic aromatics can be considered as a potential source of hydrogen and radical terminator to promote bio-oil formation [18].

In our previous study, it was demonstrated that aromatics-enriched slurry exhibited an excellent hydrogen donation capacity to promote sawdust and oil co-refinery [22]. Therefore, in this paper, cyclohexanol, a common cyclic alcohol with a certain polarity and hydrogen donating capacity, was selected to liquefy sawdust to verify its feasibility. In addition, another four solvents with different properties (polarity, hydrogen bond strength, hydrogen donating capacity, heat transfer capacity, etc.) were chosen for comparison, focusing on investigating solvent effects on product yields and distributions from a comprehensive perspective. To the best of our knowledge, no study has been done on biomass liquefaction in cyclohexanol. Moreover, microcrystalline cellulose and lignin were employed as the models to further explore liquefaction mechanism. This work can be recongnized with reference value for research on mechanism of solvent effect and choices of solvent.

Section snippets

Materials

The sawdust was obtained from a lumber mill adjacent of Qingdao, China. Before test, the sawdust was washed with deionized water for three times, dried at 105 °C in an oven for 15 h, milled into 60–100 meshes in a High-speed Pulverizer (LFP-800T) and then preserved in a desiccator for further use. The microcrystalline cellulose (AR, Tianjin Guangfu FCRI) and lignin (AR, XiYa reagent) are used directly without any pre-treatment. The chemical and elemental compositions of the three types of

Thermal stability of biomass feedstocks

The TG and DTG curves of three feedstocks are presented in Fig. 2. The main weight loss temperature range of sawdust, microcrystalline cellulose and lignin are 240–395 °C, 300–380 °C and 150–500 °C, respectively. Correspondingly, the thermal stability of the biomass decreases in the order of microcrystalline cellulose > sawdust > lignin. The cellulose presents superior thermal stability probably because cellulose networks are bound by extensive intramolecular and intermolecular hydrogen bonding

Conclusion

The liquefaction of sawdust and models in different solvents was conducted at 320 °C and 30 min. The results suggested that the types of solvent have a great effect on conversion and products distributions. The order of both conversion rate and liquid yields were cyclohexanol > tetralin > ethanol > isopropanol > cyclohexane. This was mainly due to the nature of solvent itself affecting the mass transfer and heat transfer of the reaction system. Moderate polarity and higher heating rates make

Acknowledgements

We thank China University of Petroleum (East China) for their support of the materials testing approach. This project was supported by the Natural Science Foundation of Shandong Province (ZR2016BM29).

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