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Article

Research on Deoxygenation Pyrolysis of Larch Based on Microwave Heating

by
Shuang Xue
*,
Xin Wang
,
Biao Zhang
,
Bin Xiao
and
Yongyi Song
*
SINOPEC (Dalian) Research Institute of Petroleum and Petrochemicals Co., Ltd., Dalian 116045, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(11), 808; https://doi.org/10.3390/catal14110808
Submission received: 30 September 2024 / Revised: 2 November 2024 / Accepted: 8 November 2024 / Published: 10 November 2024
(This article belongs to the Special Issue Sustainable Catalytic Routes for the Production of Green Synthetic Fuels and Other Value-Added Products)
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Versions Notes

Abstract

:
Aiming at problems such as low energy utilization efficiency and the high oxygen content of liquid products in the process of conventional biomass conversion to prepare liquid fuels, the deoxygenation pyrolysis technology route of larch based on microwave heating was proposed in this paper. Two kinds of calcium–iron composite oxygen carriers, including Ca2Fe2O5 with iron ore structure and CaFe2O4 with spinel structure, were successfully synthesized. The results showed that the selectivity of ideal products was improved under the action of single iron-based oxygen carriers; however, the deoxygenation effect was undesirable. Under the action of CaFe2O4, the selectivity of aromatics was increased to 27.17% and the selectivity of phenols was decreased to 36.46%, which mainly existed in the form of O1P with low oxygen content. The oxygen content of bio-oil was reduced to 27.70% and the calorific value was increased to 29.05 MJ/kg, thus leading to a great improvement in the quality of liquid products. After the pyrolysis reaction, the Fe2P3/2 XPS peak of CaFe2O4 shifted to a higher binding energy and was characterized as higher valence of iron oxide, which proved its “oxygen grabbing” capacity in microwave pyrolysis. The deoxygenation conversion of larch without an external hydrogen supply was achieved.

1. Introduction

Biomass is the only renewable carbon source that can be directly converted into liquid fuels [1], with near-zero carbon emissions occurring during the whole life cycle utilization process. Under the goal of “carbon peak and carbon neutrality”, the use of biomass conversion to prepare liquid fuels has significant ecological benefits.
The conversion of biomass into liquid fuels involves a series of technical approaches, including gasification-coupled Fischer–Tropsch synthesis, pyrolysis, hydrolytic fermentation, etc. [2,3]; the approach of pyrolysis can process biomass raw materials quickly in large quantities and has a high oil yield (70–95%) [4,5]. However, the pyrolysis oil obtained from the fast pyrolysis of biomass has a high oxygen content (30–40%), high corrosiveness, low energy density, and poor thermal stability [6,7,8]. Although the pyrolysis oil can be further converted into alkane biofuel via hydrogenation [9,10], a series of problems still exists [11,12,13]. First, the route of pyrolysis coupled with hydrogenation requires two reactors, and the two-step reaction results in the high energy consumption and low energy utilization efficiency of the system [14]. Second, the hydrogen source is expensive and the production cost is too high. Third, during the processes of condensation, storage, and heating of pyrolysis oil, unstable components are prone to secondary polymerization, resulting in serious deactivation of hydrogenation catalysts in the subsequent upgrading process [13,15,16,17]. Therefore, it is urgent to choose an economical and feasible method to improve the energy utilization efficiency of biomass conversion to liquid fuels, reduce the oxygen content of pyrolysis oil, and improve the quality of liquid products.
Aiming at the bottleneck problems existing in conventional biomass pyrolysis technology, this paper intends to solve them by focusing on two aspects.
First, chemical looping pyrolysis technology was adopted to effectively solve the problems of high oxygen content in biomass pyrolysis liquid products and the high cost of external hydrogen supply, using the oxidation/reduction capacity of oxygen carriers to adjust the hydrogen and oxygen in biomass in space and time [18] so as to achieve the fractional high-value transformation of biomass. Specifically, in the pyrolysis reactor, the reductive oxygen carrier was used to “capture” oxygen from biomass to obtain low-oxygen and hydrogen-rich liquid products of a high quality while generating H2 in situ to upgrade bio-oil. After the pyrolysis reaction, the reductive oxygen carrier experienced oxidization to become an oxidized oxygen carrier, which can be reduced through a regeneration reaction to achieve recycling. The generalized reaction occurring in the pyrolysis reactor is shown as (1), where biomass, bio-oil, semi-char, reductive oxygen carrier (fresh oxygen carrier before pyrolysis reaction), and oxidized oxygen carrier (oxygen carrier after pyrolysis reaction) are simplified as CxHyOz, Cx-bHy-cOz-a, C, MeOx-1, and MeOx, separately. This paper mainly studied the effect of reductive oxygen carriers on the oxygen content of biomass pyrolysis liquid products.
CxHyOz + aMeOx-1 = Cx-bHy-cOz-a + aMeOx + bC + 0.5cH2   △H < 0
Second, microwave heating was employed as a heating method. Traditional biomass pyrolysis technology used external heating from the surface to the inside, which took a long time to reach thermal balance and had low thermal efficiency. Microwave heating had the characteristics of a fast heating rate, uniform heating, and the “homotropy” of heat and mass transfer, which could separate the reactants and target products during the reaction process and reduce the probability of secondary reactions [19,20,21]. Thus, the efficiency of pyrolysis reaction and the yield of target products can be improved.
Iron-based oxygen carriers had numerous advantages, such as low price and high catalytic efficiency [22,23]. Common iron-based oxygen carriers included Fe2O3, cobalt iron oxide, iron ore, etc. However, in the reaction process, single iron-based oxygen carriers had a single catalytic effect, and it was necessary to add a second active metal component to modify the iron-based oxygen carriers to improve the oxygen carrying and catalytic capacity. A calcium–iron composite oxygen carrier can promote the generation of aromatics and ketones in chemical looping gasification and pyrolysis [24,25,26,27,28], possessing a better oxygen carrying and catalytic effect. Meanwhile, CaO can enhance the decarbonylation reaction and capture CO2 generated by the pyrolysis reaction in situ to achieve a “negative carbon” emission in the whole life cycle of biomass pyrolysis products [29,30,31].
In summary, this paper proposed conducting deoxygenation pyrolysis of forest residues (larch) based on the microwave heating method without an external hydrogen supply. Four reductive oxygen carriers, including reduced iron powder (Fe), Fe3O4, and two calcium–iron composite oxygen carriers (Ca2Fe2O5 and CaFe2O4), were adopted. The oxidation/reduction capacity of the oxygen carriers was utilized so as to prepare bio-oil with a low oxygen content and high calorific value, further achieving the deoxygenation conversion of larch without an external hydrogen supply.

2. Results

2.1. Characterization Results of Calcium–Iron Composite Oxygen Carrier

The XRD results of calcium–iron composite oxygen carriers prepared with different molar ratios of calcium/iron precursors are shown in Figure 1. By comparing with standard PDF cards, it can be concluded that, when the molar ratio of Fe/Ca was 2, a CaFe2O4 diffraction peak with a stable crystal form (JCPDS 72-1199) was obtained; when the molar ratio of Fe/Ca was 1, a Ca2Fe2O5 diffraction peak with a stable crystal form (JCPDS 71-2264) was formed. No miscellaneous peaks, such as Fe2O3, Fe and CaO, were found in the XRD pattern. In addition, according to the semi-quantitative result of elements from the XRF in Table 1, the actual stoichiometric values of each element of the calcium–iron composite oxygen carriers prepared with different molar ratios of calcium/iron precursors were calculated based on the Fe element, and it was verified that the stoichiometric values were close to the theoretical stoichiometric values. According to the results of the XRD and XRF, the precise preparation of different phases (CaFe2O4 and Ca2Fe2O5) was achieved in this study by adjusting the molar ratio of calcium/iron precursors, adding the appropriate amount of citric acid and adopting a moderate drying temperature and calcination temperature.
To determine the presence of metal elements in calcium–iron composite oxygen carriers, XPS analysis was performed. According to the satellite characteristics of Ca2p shown in Figure 2a, it can be observed that, compared with Ca2Fe2O5, the peak intensity of Ca2p in CaFe2O4 was significantly reduced. The XPS spectra of Ca2p were divided into four sub peaks, representing Ca2p3/2-Ca2Fe2O5/CaFe2O4, Ca2p3/2-CaCO3, Ca2p1/2-Ca2Fe2O5/CaFe2O4, and Ca2p1/2-CaCO3 in descending order of binding energy. The peak positions of Ca2p-CaCO3 were about 347 eV and 351 eV, respectively, while the peak positions of Ca2p3/2- Ca2Fe2O5/CaFe2O4 were 345.85 eV and 345.45 eV. The peak area of Ca2p-CaCO3 in CaFe2O4 was lower than that in Ca2Fe2O5, corresponding to the lower amount of CaCO3 in the prepared oxygen carrier. Compared with Ca2p3/2 in Ca2Fe2O5, although the valence of calcium was the same, the introduction of a larger amount of iron in CaFe2O4 affected its electron arrangement outside the nucleus, causing its binding energy to shift towards a decreasing direction by about 0.4 eV. The presence of iron was judged by the satellite characteristics of Fe2p shown in Figure 2b, which had two 2p peaks because of spin–orbit coupling. The binding energy was at approximately 710 eV and 725 eV, corresponding to Fe2p3/2 and Fe2p1/2. Two small peaks appeared at about 718.8 eV and 732.5 eV, corresponding to the satellite peaks for Fe2p3/2 and Fe2p1/2. The two satellite peaks were distinguishable due to there being no overlap with both the Fe2p3/2 and Fe2p1/2 peaks. The presence of the satellite peaks demonstrated that iron existed in the form of Fe(III) [32,33]. The XPS spectra of C1s in Figure 2c further confirmed the lower intensity of the O-C=O and π-π* satellite in CaFe2O4 corresponding to fewer carbonates or hydrocarbonates on the surface of CaFe2O4.
To analyze the surface oxygen types and chemical states of two oxygen carriers, the O1s-XPS spectra were divided into three peaks shown in Figure 2d [34,35,36,37]. OI was characterized by lattice oxygen in Ca2Fe2O5 and CaFe2O4, OII as oxygen vacancies, and OIII as hydroxyl groups or C-O bonds adsorbed on the surface of the oxygen carrier. The proportion of different oxygen types calculated from the peak division results is shown in Table 2. The percentage of lattice oxygen atoms on the surface of Ca2Fe2O5 and CaFe2O4 was calculated. The oxygen atom percentage of Ca2Fe2O5 was 37.90% and the percentage of OI species in all surface oxygen was 51.12%, so the distribution concentration of O species on the surface was roughly calculated to be about 19.37%. The oxygen atom percentage of CaFe2O4 was 41.10%, and the percentage of OI species in all surface oxygen was 61.99%, resulting in a lattice oxygen concentration of 25.48% that can participate in surface reactions. In contrast, CaFe2O4 had a faster initial reaction rate based on the analysis of surface oxygen atom concentration distribution.

2.2. Analysis of Liquid Products from Microwave Pyrolysis of Biomass

In the absence of an oxygen carrier, the liquid product from larch pyrolysis were mainly phenols (39.69%), ketones (26.95%), aldehydes (24.07%), etc. Phenols accounted for the highest proportion of oxygenates, which mainly came from the fracture of the β-O-4 bond of lignin during the thermal pyrolysis process. The selectivity of catechol (C8H10O2) containing two hydroxyl groups was as high as 11.03%, which was the product from secondary reaction of pyrolysis vapor, and not conducive to the improvement of the quality of liquid products.
Two single iron-based oxygen carriers (Fe and Fe3O4) were introduced in the pyrolysis stage to upgrade the liquid products. Under the action of reduced Fe powder, the pyrolysis liquid products were mainly phenols (47.82%), ketones (15.07%), aldehydes (11.68%), and a small amount of aromatics (2.26%). The selectivity of catechol decreased to 10.16%, indicating that the reduced Fe powder inhibited the secondary reaction, but its catalytic performance was weak. Under the action of Fe3O4, the selectivity of aromatics increased to 8.91%, the content of aldehydes decreased significantly (4.67%), and catechol further decreased to 6.87%, indicating that Fe3O4 had better ability in terms of aromatization and inhibition of the secondary reaction.
Compared with pyrolysis liquid products without oxygen carriers, the addition of reduced Fe powder and Fe3O4 increased the selectivity of aromatics, alcohols, and esters and decreased the selectivity of aldehydes. Among the two single iron-based oxygen carriers, Fe3O4 greatly promoted the formation of aromatics and Fe significantly promoted the formation of alcohols. The addition of the two reductive oxygen carriers promoted the efficient conversion of lignin, which showed the improvement of the phenols’ selectivity. Fe increased the selectivity of phenols to 47.82% and Fe3O4 to 52.76%. In this paper, phenols are subdivided according to oxygen content, including O1P (phenols containing only one oxygen element, such as phenol, 2-methylphenol, 2,4-dimethylphenol, 3-ethylphenol, etc.), O2P (phenols containing two oxygen elements, such as guaiacol, 2-methoxy-4-methylphenol, engenol, catechol, etc.), and O3P (phenols containing three oxygen elements, such as 2,6-dimethoxyphenol, vanillin, 4-ethenyl-2,6-dimethoxyphenol, etc.). Under the action of Fe, the selectivity of O2P was as high as 69.43%, while O1P was only 14.95%. Fe3O4 significantly increased the O1P selectivity to 33.72%, reducing the proportion of O2P and O3P. In comparison, Fe3O4 possessed a better deoxygenation performance for phenols.

2.3. Effect of Calcium–Iron Composite Oxygen Carrier on Liquid Product

A single iron-based oxygen carrier promoted the generation of ideal products to some extent, but the catalytic capacity was relatively weak. Under the condition of physical mixing of Fe and CaO, the selectivity of aromatics increased to 9.64%. Phenols mainly existed in the form of O2P (64.09%), and the selectivity of O1P increased to 20.54%, with an increase of 37.39% compared with the condition without CaO, so the deoxygenation performance was improved to a certain extent.
The specific liquid product distribution under different calcium–iron composite oxygen carriers is listed in Table 3. Under the action of calcium–iron composite oxygen carriers (Ca2Fe2O5 and CaFe2O4), the selectivity of aromatics undertook an obvious increase. Ca2Fe2O5 improved the selectivity of aromatics to 15.99%. The selectivity of O1P increased to 68.92%, and O2P and O3P decreased to 22.88% and 8.20%. Phenols mainly existed in the form of O1P, so the deoxygenation effect of Ca2Fe2O5 was significant. CaFe2O4 further improved aromatics to 27.17% and also inhibited the formation of phenols with a selectivity of 36.46%. Phenols still mainly existed in the form of O1P (68.60%), and the selectivity of O3P decreased to 7.37%. In general, CaFe2O4 possessed a better capacity in regard to aromatization and deoxygenation. Both calcium–iron composite oxygen carriers inhibited the production of secondary product catechol, promoted the formation of phenol and alkyl-substituted phenols, and the selectivity of aromatic hydrocarbons with high carbon numbers was significantly improved.

3. Discussion

Figure 3a more intuitively compared the distribution of liquid products under the conditions of different oxygen carriers. The proportion of phenols was extremely high. Single iron-based oxygen carriers (Fe and Fe3O4) promoted the conversion of lignin and reduced the oxygen content of the liquid product by seizing “oxygen” from biomass, thus improving the quality of bio-oil. Under the condition of single iron-based oxygen carriers, the oxygen content of phenols was high and the deoxygenation capacity was weak. It can be seen from the curve of O1P in Figure 3a that calcium–iron composite oxygen carriers promoted deoxygenation reactions and aromatization reactions, to some extent. The proportion of O1P in phenols increased greatly, resulting from large removals of the methoxy group on the aromatic ring. The selectivity of aromatics witnessed an obvious increase, demonstrating that part of the phenolic hydroxyl group was removed. CaFe2O4 had little effect on the deoxygenation of phenols, but it reduced the proportion of phenols in liquid product and promoted the formation of aromatics, thus optimizing the distribution of liquid products.
Figure 3b shows the distribution of gas products under the different conditions of oxygen carriers. A calcium–iron composite oxygen carrier reduced the volume concentration of CO2 and promoted the generation of H2 and CO, thus optimizing the distribution of gas products. In contrast, under the physical mixing of Fe and CaO, the ratio of CO2 was reduced to some extent, but the H2/CO ratio was the lowest, demonstrating that the H2 production capacity was the weakest. Meanwhile, the selectivity of O1P in phenols was relatively low, corresponding to the poor ability regarding the deoxygenation of phenols compared with the calcium–iron composite oxygen carrier.
Table 4 shows the elemental analysis and calorific value of pyrolysis oil under different oxygen carrier conditions. Under the action of CaFe2O4, the oxygen content of pyrolysis oil was reduced to 27.70%, 3.7% lower than the action of Ca2Fe2O5, 5.8% lower than Fe3O4, 29.2% lower than the reduced Fe powder, and 40.8% lower than the condition without an oxygen carrier. The oxygen content witnessed a significant decrease. For aromatic oxygenates such as phenols, benzaldehydes, and methoxy aromatics, they experienced deoxygenation reactions, predominantly demethoxylation, decarbonylation, and dehydration, so aromatics and O1P with low oxygen contents were more frequently produced, according to Figure 3a. Other oxygen-containing compounds, such as ketones, aldehydes and carboxylic acids, can be converted through decarbonylation due to the increase in CO in gas products from Figure 3b.
In addition, the calcium–iron composite oxygen carrier improved the H/C ratio of the pyrolysis oil. Compared with Ca2Fe2O5, CaFe2O4 had a higher content of C element content, thus increasing the carbon chain of pyrolysis oil. The proportion of oxygen in the biomass raw material removed in the form of CO or CO2 was reduced relatively. After the pyrolysis reaction, the Fe2p3/2 peak of both Ca2Fe2O5 and CaFe2O4 was significantly shifted to a higher binding energy according to the XPS results (Fe2p3/2 of Ca2Fe2O5: from 710.19 eV to 712.15 eV; Fe2p3/2 of CaFe2O4: from 710.60 eV to 711.69 eV), characterizing as the higher valence of iron oxide. Combined with the results in Table 4, the hydrogen content of pyrolysis oil under the action of Ca2Fe2O5 and CaFe2O4 was maintained and the oxygen content was reduced, proving the “oxygen capture” ability of the calcium–iron composite oxygen carrier in the microwave pyrolysis of larch, thus realizing the deoxygenation conversion of biomass without an external hydrogen supply.
The analysis of the 1H−NMR spectroscopy of pyrolysis oil under the action of CaFe2O4 is shown in Figure 4. The observed high-intensity signal peak was methylene hydrogen on the side chain of the benzene ring, indicating that there was a large number of substances with long side chains in the benzene ring structure of pyrolysis oil, which favored the retention of the carbon chain. This result corresponded to the high selectivity of 3−ethylphenol (C8H10O) and 2−ethyl−5−methylphenol (C9H12O) in the GC/MS results shown in Table 3.
In this study, both the single iron-based oxygen carrier and composite oxygen carrier promoted the rupture of C−O−C and C=O bonds, thus forming a more stable C−C bond structure. Under the action of spinel structure oxygen carrier CaFe2O4, the calorific value of the pyrolysis oil was increased from 19.04 MJ/kg to 29.05 MJ/kg and there were a large number of substances with long side chains in the benzene ring structure. When compared to the work producing low-oxygen bio-oil via in situ deoxygenation using CO2 as co-reaction gas (the oxygen content was reduced by 12.68% and the HHV value was increased by 26.43%) [38], the conversion performance here was better due to the higher rate of oxygen content reduction (39.95%) and calorific value improvement (52.57%). In the whole reaction, only oxygen was removed and the utilization of carbon and hydrogen was high. Therefore, the deoxygenation conversion of biomass into high-quality bio-oil was achieved without an external hydrogen supply.

4. Materials and Methods

4.1. Materials

In this study, larch was selected as the biomass raw material, being presented as forest residue from the Yichun area of the Heilongjiang province. The industrial, elemental, and fiber composition analysis of larch are shown in Table 5. From elemental analysis, it can be seen that larch has a high hydrocarbon content and low sulfur and nitrogen content, which has obvious advantages in terms of obtaining high-quality liquid fuels. However, its oxygen content is high. Although the high oxygen content can reduce the amount of carbon residue in the pyrolysis process, it has a disadvantageous factor in terms of obtaining bio-oil with a high calorific value. From the analysis of fiber composition, the total fiber composition content of larch is very high and the total of the three elements is more than 90%, indicating that more than 90% of organic components in larch can be converted, theoretically. From the perspective of the ratio of three elements, lignin accounted for 31.67%. Due to the high-oxygen polar groups, the dielectric constant and dielectric loss of lignin are higher than that of cellulose and hemicellulose, so the microwave heating characteristics of larch will be better. Before each pyrolysis experiment, the biomass raw material was ground by a ball mill and sieved through a 60–80 mesh; it was then dried in an oven at 110 °C for 24 h.
Silicon carbide (SiC) was used as the microwave absorbent. Reduced iron powder (Fe), ferric oxide (Fe3O4), calcium oxide (CaO) and two self-prepared calcium–iron complex oxygen carriers with different structures (Ca2Fe2O5 with an iron ore structure and CaFe2O4 with a spinel structure) were used as reductive oxygen carriers in the pyrolysis experiment. Before the pyrolysis experiment, the biomass raw material larch was screened through a ball mill, with 60–80 mesh powder being used as the pyrolysis raw material, and then dried at 110 °C for 24 h.

4.2. Methods of Pyrolysis Experiment and Product Detection

The pyrolysis experiment based on microwave heating was carried out in the microwave fixed-bed system shown in Figure 5. The microwave output power can be adjusted to between 0.6 and 2.3 kW, with a frequency of 2450 MHz. The reaction tube was a quartz tube that can withstand a high temperature of 1600 °C. Also, a three-stage condensing unit and a gas collection system were equipped.
Before each experiment, a certain amount of biomass raw materials, microwave absorbents, and reduced oxygen carriers were mixed evenly and installed in the reaction tube. In the pyrolysis experiment, N2 was adopted as the carrier gas, with a flow rate of 200 mL/min. The pyrolysis temperature was 550 °C and the reaction time was 30 min.
The pyrolysis vapor and tar generated by the microwave pyrolysis reaction were gaseous at high temperatures and then experienced condensation. The condensable product was a liquid product, which was washed by dichloromethane (CH2Cl2), and then the collected liquid was filtered by a PTFE filter to remove impurities before being dehydrated with anhydrous sodium sulfate. The components of the liquid product were analyzed by a gas chromatography-mass spectrometer (GC/MS). The non-condensable product was a gas product, which was analyzed by a gas chromatograph (GC).

4.3. Preparation and Characterization of Composite Oxygen Carrier

Two kinds of calcium–iron composite oxygen carriers, with an iron ore structure (Ca2Fe2O5) and a spinel structure (CaFe2O4), respectively, were prepared by the sol–gel combustion method [39,40,41]. Fe(NO3)3·9H2O and Ca(CH3COO)2 were adopted as the precursor in two ratios of Fe/Ca molar ratio of 1 and 2, and deionized water was added to prepare a certain concentration solution (metal cation concentration of 0.5 mol/L). The solution was stirred at 40 °C until the precursor salt was completely dissolved. Then, citric acid with 1.5 times of the molar amount of cations was added to the solution, heated, and then stirred in a water bath at 80 °C until a wet gel was formed; the wet gel was foamed in an oven at 180 °C for 12 h to obtain a dry gel. At a high temperature of 180 °C, the citric acid rapidly decomposed and provided functional groups of hydroxyl and carboxyl groups, prompting the cross-linking foaming of the metal salt solution to form a loose and highly dispersed calcium–iron precursor. Then, the formed dry gel particles were ground evenly and put into the muffle furnace, pre-burned at 300 °C for 1 h, and then heated at 750 °C for 4 h. Finally, Ca2Fe2O5 and CaFe2O4 were obtained.
The prepared calcium–iron composite oxygen carrier was characterized to analyze the structure and composition. The crystal phase structure of the calcium–iron composite oxygen carriers was tested using an X-ray diffractometer (XRD) with a diffraction angle of 5°–70° and a scanning frequency of 5°/min. An X-ray fluorescence spectrometer (XRF) was used for qualitative and quantitative analysis of all elements of the calcium–iron composite oxygen carriers. The existence form and valence state of the metal elements were analyzed by X-ray photoelectron spectroscopy (XPS). In the data analysis, a C1s (284.8 eV) signal was used as the standard for binding energy correction.

5. Conclusions

This paper proposed a new type of biomass deoxygenation method, specifically a deoxygenation pyrolysis technology based on microwave heating. Two calcium–iron composite oxygen carriers with stable crystal structures (an iron ore structure, Ca2Fe2O5, and a spinel structure, CaFe2O4) were successfully synthesized. CaFe2O4 possessed a higher concentration of lattice oxygen (25.48%) that can participate in surface reactions. By introducing a reductive oxygen carrier into the pyrolysis stage of larch, the oxygen content of bio-oil was reduced and the calorific value was increased, achieving the preparation of high-quality bio-oil. A single iron-based oxygen carrier improved the selectivity of the ideal products, including aromatics, alcohols, and esters, and the formation of aldehydes was inhibited. Fe3O4 enhanced the formation of aromatics and Fe promoted the formation of alcohols. However, the selectivity of O2P and O3P with higher oxygen contents in phenols was high and the deoxygenation ability was weak. The calcium–iron composite oxygen carrier made phenols mainly exist in the form of O1P, the selectivity of which was increased to 68.92%, while the volume concentration of H2 and CO in the gas products was increased and the formation of CO2 was inhibited. CaFe2O4 further improved the selectivity of aromatics to 27.17%, reduced phenols to 36.46%, and increased the calorific value from 19.04 MJ/kg to 29.05 MJ/kg. The calcium–iron composite oxygen carrier increased the H/C ratio of pyrolysis oil, increased the carbon chain of pyrolysis oil, and reduced the oxygen content to 27.70%. The quality of pyrolysis oil was greatly improved.
In this investigation, the oxygen capture, in situ hydrogen supply, and catalysis capacity of reductive oxygen carriers were utilized to achieve the upgrading of biomass pyrolysis products in situ. High-quality bio-oil and clean syngas were obtained simultaneously. In the future, the design of efficient oxygen carriers with the capacity for catalysis, oxygen carrying, and microwave absorption should still be the focus of biomass deoxygenation pyrolysis based on microwave heating.

Author Contributions

Conceptualization, S.X. and X.W.; methodology, S.X. and X.W.; software, S.X.; validation, S.X. and B.X.; formal analysis, S.X.; investigation, S.X. and B.Z.; resources, S.X.; data curation, S.X.; writing—original draft, S.X.; writing—review and editing, S.X., X.W., B.Z., B.X., and Y.S.; visualization, S.X.; supervision, Y.S.; project administration, S.X.; funding acquisition, S.X. and Y.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Youth Doctoral Support Program Project of the SINOPEC (Dalian) Research Institute of Petroleum and Petrochemicals Co., Ltd., DLYZL2024052304.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors Shuang Xue, Xin Wang, Biao Zhang, Bin Xiao, and Yongyi Song declare that this study received funding from the Youth Doctoral Support Program Project of the SINOPEC (Dalian) Research Institute of Petroleum and Petrochemicals Co., Ltd. The funder had the following involvement with the study: study design, collection, analysis, interpretation of data, and the writing of the report.

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Catalysts 14 00808 g001
Figure 1. X-ray diffraction result of calcium–iron composite oxygen carriers.
Figure 1. X-ray diffraction result of calcium–iron composite oxygen carriers.
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Catalysts 14 00808 g002
Figure 2. XPS spectrum diagram of calcium and iron composite oxygen carrier: (a) Ca2p; (b) Fe2p; (c) C1s; (d) O1s.
Figure 2. XPS spectrum diagram of calcium and iron composite oxygen carrier: (a) Ca2p; (b) Fe2p; (c) C1s; (d) O1s.
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Catalysts 14 00808 g003
Figure 3. Effect of oxygen carrier on the distribution of pyrolysis products: (a) liquid products; (b) gas products.
Figure 3. Effect of oxygen carrier on the distribution of pyrolysis products: (a) liquid products; (b) gas products.
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Catalysts 14 00808 g004
Figure 4. The analysis of 1H−NMR spectroscopy of pyrolysis oil under the action of CaFe2O4.
Figure 4. The analysis of 1H−NMR spectroscopy of pyrolysis oil under the action of CaFe2O4.
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Catalysts 14 00808 g005
Figure 5. Schematic diagram of pyrolysis based on microwave heating.
Figure 5. Schematic diagram of pyrolysis based on microwave heating.
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Table 1. X-ray fluorescence analysis of calcium–iron composite oxygen carriers.
Table 1. X-ray fluorescence analysis of calcium–iron composite oxygen carriers.
Oxygen CarrierElemental Semi-Quantitative Results (Mass%)Stoichiometry of Each Element
CaOFe2O3CaFeO
CaFe2O425.3974.610.9723.97
Ca2Fe2O542.7157.292.1225.12
Table 2. Results of peak division statistics for different oxygen types of XPS-O1s.
Table 2. Results of peak division statistics for different oxygen types of XPS-O1s.
Oxygen CarrierOxygen Type (%)OII/OI
OIOIIOIII
CaFe2O461.9928.979.040.47
Ca2Fe2O551.1241.627.260.81
Table 3. The distribution of liquid products from GC/MS results under different calcium–iron composite oxygen carriers.
Table 3. The distribution of liquid products from GC/MS results under different calcium–iron composite oxygen carriers.
Chemical FormulaProduct NameSelectivity
Fe + CaOCa2Fe2O5CaFe2O4
C6H10O32-Butanone,1-(acetyloxy)-1.32%--
C4H8O31,2-Ethanediol,monoacetate-1.38%1.76%
C4H6O3Propanoic acid,2-oxo-,methyl ester1.12%1.17%1.46%
C5H8OCyclopentanone1.46%0.63%0.64%
C5H4O23-Furaldehyde5.32%6.78%9.89%
C6H10OCyclopentanone,2-methyl0.80%--
C5H6O22-Furanmethanol2.43%0.66%0.78%
C6H8O2-Cyclopenten-1-one,2-methyl2.70%1.40%1.67%
C5H8O32-Propanone,1-(acetyloxy)-0.71%-0.73%
C4H4O22(5H)-Furanone1.38%1.76%2.05%
C5H6O21,2-Cyclopentanedione0.59%1.17%0.89%
C7H10O2-Cyclopenten-1-one,3,4-dimethyl0.80%--
C6H6O22-Furancarboxaldehyde, 5-methyl0.95%2.03%2.19%
C6H8O2-Cyclopenten-1-one,3-methyl1.99%0.87%1.03%
C6H6OPhenol2.24%8.82%8.51%
C8H17NOOxazolidine,2,2-diethyl-3-methyl-2.25%2.29%
C7H10O2-Cyclopenten-1-one,2,3-dimethyl4.48%-0.56%
C6H8O22-Cyclopenten-1-one,2-hydroxy-3-methyl1.79%1.92%1.80%
C9H8Indene1.21%1.24%1.45%
C7H8OPhenol,2-methyl1.99%3.65%2.81%
C8H12O2-Cyclopenten-1-one,2,3,4-trimethyl1.23%--
C7H8OPhenol,3-methyl2.93%11.59%9.11%
C7H8O2Phenol,2-methoxy4.12%2.31%2.59%
C9H8O(Z)-3-Phenylacrylaldehyde-0.72%-
C9H8OBenzofuran,2-methyl-0.75%0.65%
C5H10O2-Penten-1-ol,(Z)-0.76%--
C8H12O2-Ethyl-3-methylcyclopent-2-en-1-one0.64%--
C8H10OPhenol,2,4-dimethyl1.71%1.61%0.99%
C10H101H-Indene,3-methyl1.11%0.74%0.65%
C8H10OPhenol,2,5-dimethyl0.64%0.89%0.68%
C8H10OPhenol,3-ethyl--1.67%1.13%
C8H10OBenzene,1-methoxy-4-methyl-1.47%-
C8H10OPhenol,2,3-dimethyl-0.95%0.84%
C12H14N4O42-Vinyl-9-[3-deoxy-á-dribofuranosyl] hypoxanthine-0.99%0.89%
C13H20O2(3-Methoxyphenyl) methanol,2-methylbutyl ether--1.14%
C10H8Azulene-8.22%10.02%
C8H8OBenzofuran,2,3-dihydro-1.43%0.56%
C6H6O35-Hydroxymethyl furfural-0.95%0.68%
C8H16OCyclohexanol,2,4-dimethyl0.89%--
C8H10O2Creosol10.11%--
C9H12OPhenol,2-ethyl-5-methyl-1.82%0.94%
C10H16O2Ascaridole0.82%--
C9H12O23,4-Dimethoxytoluene1.05%--
C12H18O5,9,9-Trimethylspiro[3.5]non5-en-1-one0.99%--
C9H12O2Phenol,4-ethyl-2-methoxy1.67%1.02%0.99%
C9H8O1H-Inden-1-one,2,3-dihydro0.67%--
C7H8O21,2-Benzenediol,4-methyl-0.76%-
C11H10Naphthalene,1-methyl1.41%1.27%2.21%
C9H10O22-Methoxy-4-vinylphenol4.05%2.16%1.75%
C11H10Naphthalene,2-methyl1.06%1.01%1.65%
C8H10O3Phenol,2,6-dimethoxy1.94%1.34%1.02%
C10H12O2Eugenol1.39%1.02%0.84%
C10H14O2Phenol,2-methoxy-4-propyl0.64%--
C8H8O3Benzaldehyde,3-hydroxy-4-methoxy0.74%0.63%0.56%
C9H8OBenzofuran,2-methyl-1.05%0.64%
C12H10Biphenyl--1.48%
C10H12O2trans-Isoeugenol0.83%0.63%2.59%
C9H12O33,5-Dimethoxy4-hydroxytoluene1.85%1.36%1.07%
C10H12O2Phenol,2-methoxy-4-(1-propenyl)-5.49%3.14%-
C12H8Acenaphthylene1.41%1.25%1.13%
C9H10O3Apocynin1.21%0.58%-
C13H12Diphenylmethane-0.50%0.64%
C10H8O1-Naphthalenol-2.13%-
C19H24O5Bicyclo[3.2.2] non-6-ene-6-carboxylic acid,8-(3,4-dimethoxyphenyl)-9-methyl-3-oxa-,methyl ester0.62%--
C10H14O35-tert-Butylpyrogallol0.77%-0.99%
C9H10O4Gallacetophenone-4′-methylether-0.55%-
C12H8ODibenzofuran--1.45%
C11H16O2Guaiacol,4-butyl1.35%--
C10H12O3Phenol,4-ethenyl-2,6-dimethoxy2.08%0.82%0.56%
C14H10O2-Fluorenecarboxaldehyde1.85%1.34%-
C13H10Fluorene--2.27%
C10H14O3Benzenepropanol,4-hydroxy-3-methoxy1.55%1.23%-
C11H14O3(E)-2,6-Dimethoxy-4-(prop-1-en-1-yl)phenol3.09%1.53%1.11%
C11H10O1-Naphthalenol,2-methyl-0.53%-
C14H10Anthracene2.85%1.20%3.04%
C18H18Phenanthrene,3,4,5,6-tetramethyl0.59%--
C21H28O37-Oxodehydroabietic acid,methyl ester0.61%--
C14H14Dehydrochamazulene-0.56%-
C10H10O3Coniferylaldehyde-0.52%-
C14H129H-Fluorene,2-methyl--0.78%
C15H12Phenanthrene,1-methyl--0.56%
C16H10Fluoranthene--1.30%
Table 4. Elemental analysis and calorific value of pyrolysis oil under different oxygen carrier conditions.
Table 4. Elemental analysis and calorific value of pyrolysis oil under different oxygen carrier conditions.
Oxygen CarrierElemental Analysis of Pyrolysis Oil, wt%Calorific Value
MJ/kg
wC,adwH,adwO,adwN,adwS,ad
Without oxygen carrier47.096.6546.130.100.0319.04
Fe53.367.3639.150.100.0322.53
Fe3O462.487.9929.410.090.0327.19
Ca2Fe2O561.989.1428.760.090.0328.13
CaFe2O467.178.0327.700.080.0229.05
Table 5. The industrial, elemental, and fiber composition analysis of larch.
Table 5. The industrial, elemental, and fiber composition analysis of larch.
Industrial Analysis/%
MadAadVadFC, ad
3.40.4280.7115.47
Elemental Analysis/%
wC,adwH,adwO,adwN,adwS,ad
37.955.1752.900.110.05
Fiber Composition Analysis/%
CelluloseHemicelluloseLigninTotal
33.0226.7331.6791.41
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Xue, S.; Wang, X.; Zhang, B.; Xiao, B.; Song, Y. Research on Deoxygenation Pyrolysis of Larch Based on Microwave Heating. Catalysts 2024, 14, 808. https://doi.org/10.3390/catal14110808

AMA Style

Xue S, Wang X, Zhang B, Xiao B, Song Y. Research on Deoxygenation Pyrolysis of Larch Based on Microwave Heating. Catalysts. 2024; 14(11):808. https://doi.org/10.3390/catal14110808

Chicago/Turabian Style

Xue, Shuang, Xin Wang, Biao Zhang, Bin Xiao, and Yongyi Song. 2024. "Research on Deoxygenation Pyrolysis of Larch Based on Microwave Heating" Catalysts 14, no. 11: 808. https://doi.org/10.3390/catal14110808

APA Style

Xue, S., Wang, X., Zhang, B., Xiao, B., & Song, Y. (2024). Research on Deoxygenation Pyrolysis of Larch Based on Microwave Heating. Catalysts, 14(11), 808. https://doi.org/10.3390/catal14110808

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