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Article

Thermal and Oxidative Stability of Biocrude Oil Derived from the Continuous Hydrothermal Liquefaction of Spirulina

by
Yingxian Wang
1,*,
Maojiong Cao
2,
Weijuan Lan
1 and
Dongxue Yin
1
1
College of Agricultural Equipment Engineering, Henan University of Science and Technology, Luoyang 471000, China
2
Laboratory of Environment-Enhancing Energy (E2E), College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(12), 4884; https://doi.org/10.3390/su16124884
Submission received: 1 April 2024 / Revised: 15 May 2024 / Accepted: 21 May 2024 / Published: 7 June 2024
(This article belongs to the Special Issue Green Chemistry and Sustainable Biomass Conversion)
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Abstract

:
The stability of biocrude oil is a significant challenge for its storage, transportation, and refining. In this investigation, the thermal and oxidative stability of Spirulina-biocrude oil derived from a plug-flow continuous hydrothermal reactor was systematically studied. The biocrude oil was stored at three temperatures to simulate the winter (4 °C), spring and autumn (15 °C), and summer (35 °C) seasons, and in two atmospheres (air and N2) to simulate the conditions of a storage tank being sealed or kept open. Results demonstrated that the physicochemical properties of biocrude oil were highly influenced by the storage environment. The viscosity of biocrude oil increased with increasing storage temperature and time. The maximum viscosity (17,577 mPa·s) was observed when biocrude oil was stored at 35 °C and in an air condition over 84 days, 145% higher than fresh biocrude oil (7164.2 mPa·s). The viscosity increased by 10.9% when biocrude oil was sorted at 4 °C in an N2 atmosphere after being stored for 28 days. After long-term storage, biocrude oil still exhibited comparable characteristics to petroleum, with a slight decrease in HHV (31.36–33.97 MJ·kg−1) and the nitrogen-to-carbon ratio (0.087–0.092). This study indicated that the viscosity and HHV of the biocrude oil derived from a continuous reactor stored at 4 °C in an N2 atmosphere condition remained relatively unchanged, which enables the scheduling of oil refining production.

1. Introduction

Microalgae, as a renewable biomass, have been identified as a promising feedstock for biofuel production due to their high photosynthetic efficiency, ability to grow without occupying arable land, rapid growth rate, abundant organic matter content, and strong capability to absorb CO2 [1,2,3]. Hydrothermal liquefaction (HTL) is a thermochemical conversion technology that converts biomass with low value and high moisture into biocrude oil with high energy density under the conditions of subcritical water temperatures of 250–375 °C and pressures of 4–22 MPa [4,5]. Biocrude derived from the HTL of microalgae is considered to have the potential to become a sustainable alternative to fossil fuels due to its high heating value [6], while also decreasing the overall emissions of CO2. Previous studies have demonstrated a high energy recovery rate of 79% for HTL of Spirulina and a heating value of 37.2 MJ·kg−1 for Spirulina biocrude oil [7]. Li et al. [8] conducted research on biocrude oil production from Spirulina and Chlorella using a plug-flow continuous hydrothermal reactor. The result indicated that the biocrude yield and heating value for Spirulina were 42.3% and 32.6 MJ·kg−1, respectively, while for Chlorella they were 25.4% and 35.6 MJ·kg−1 [8]. Despite the advantages of biocrude oil derived from microalgae, a notable drawback is the presence of high heteroatomic compounds, such as ketones, acids, phenols, and nitrogenous compounds [7,8]. These compounds can negatively impact the storage stability of biocrude oil.
The storage stability of fuel oil refers to its ability to maintain its original physical and chemical properties at a certain temperature for a certain storage time. Biofuel with poor storage stability will lead to an increase in viscosity, density, average molecular weight, total acid value or pH value, and phase separation over time [9,10]. These changes in the physical and chemical properties of biofuel are called “aging”, which is influenced by factors such as temperature (thermal stability), headspace ambient gas (oxidation stability), optical density, and storage time [11,12,13]. The poor stability of HTL biocrude oil can significantly impact its transportation and refining processes, potentially causing issues like low ignition efficiency and fuel injection nozzle blockages when burned in engines. Therefore, ensuring the sufficient stability of HTL biocrude oil throughout its production, transportation, storage, and final utilization stages is essential. Liu et al. [13] utilized a high-pressure batch reactor with a volume capacity of 2.0 L to produce Nannochloropsis biocrude oil, which was then stored at room temperature (25 °C) for 12 months. This research resulted in an increased viscosity (48.4%) and average molecular weight (41.9%) compared with raw biocrude oil. Palomino et al. [14] determined that the HTL (in a 50mL batch reactor) reaction temperature (300 °C and 350 °C) and biomass type (Spirulina and Chlorella vulgaris) were significant factors influencing the stability of biocrude oil. Previous studies indicated that biocrude oil derived from feedstock with a high protein content tends to form high-molecular-weight and complex nitrogenous compounds through condensation and alkylation processes [9,15]. Conversely, biocrude oil from feedstock with high lignocellulose was more prone to forming polymerized phenol compounds, leading to poor oil quality.
To date, efforts have been made to determine the impact of HTL reaction temperature, storage temperature, and feedstock on the storage stability of biocrude derived from the batch reactor. However, there is a research gap related to the stability of biocrude derived from continuous hydrothermal reactors under various long-term external storage conditions. The physiochemical properties were the difference between the biocrude oil from the batch reactor and the continuous hydrothermal reactor. Therefore, this study aims to (1) expose the biocrude oil produced from Spirulina through a continuous hydrothermal reactor to different thermal (4 °C, 15 °C, and 35 °C) and oxidation (air and N2) storage environments over a period of three months and (2) assess the changes in the viscosity, HHV, boiling point distribution, and chemical distribution to determine how the thermal and oxidation environments influence the properties of biocrude produced from a continuous hydrothermal reactor. The findings of this study will contribute to a better understanding of the potential of biocrude as a sustainable alternative to fossil crude oil, thereby promoting its industrial applications as a biofuel.

2. Materials and Methods

2.1. Materials

The Spirulina was obtained in dry powder form from Xindaze Spirulina Co. Ltd. Fuqing, China. The water content (3.79 ± 0.31%) was measured at 105 °C for 12 h using an oven (KST101, Shandong Instruments Co., Ltd., Jinan, China.) based on NREL/TP-510-42621 [16]. The ash content (6.32 ± 0.06%) was measured at 575 °C using a muffle furnace based on ASTM E1755-01 [17]. The crude lipid content (2.89 ± 0.11%) was determined through a Soxhlet extractor (XA-06, Qingdao Environmental Protection Technology Co., Ltd., Qingdao, China.) based on NB/T 34057.9-2020 [18], and the crude protein content (71.3 ± 0.04%) was determined using a Kjeldahl nitrogen analyzer (K1305, Shengsheng Automated Analytical Instruments Co., Ltd., Shanghai, China.) based on GB/T 6432-2018 [19]. The total carbohydrate contents (20.24 ± 0.15%) were analyzed using a fiber analyzer (A220, ANKOM Technology Co., New York, NY, USA) based on NY/T 3494-2019 [20]. The experiments were repeated three times, and the average results were reported.

2.2. Continuous Biocrude Oil Production

The HTL process was carried out using a plug-flow continuous hydrothermal reactor at 300 °C with a flow rate of 45 mL·min−1 and a solid content of 18%. The HTL reaction condition was selected based on the maximum oil yield from a previous study [8]. The solid phase and aqueous phase were separated by the gravity separation method. Then, the solid phase was dissolved in acetone, and a filter paper was used to separate the solid residue from the acetone-soluble portion. The acetone in the acetone-soluble portion was separated using a vacuum evaporator. The remaining oil was defined as “biocrude oil”.

2.3. Storage Environment of Biocrude Oil

The biocrude oil was divided into six groups, each group containing about 30 g of sample, which were put into 40 mL glass containers. The biocrude oil was exposed to three thermal environments and two atmospheric gas environments over three months in the dark. The thermal environment involved three temperatures: cold (4 °C), middle (15 °C), and hot (35 °C), simulating the average temperature of the winter, spring/autumn, and summer seasons, respectively. The two atmospheres involved exposure to two gases: air and N2. The storage conditions for biocrude oil are detailed in Table 1.

2.4. Analysis

The viscosity was primarily used to determine the stability of biocrude oil, which was measured after 0-, 14-, 28-, 42-, 63-, and 84-days’ storage using the NDJ-8S rotational viscometer (Shanghai Changji Geological Instrument Co., Ltd., Shanghai, China.) at 40 °C based on ASTM D445 [21]. At each time of viscosity testing, the total weight of the sample and container was measured using electronic scales (METTLER TOLEDO). Other physiochemical compositions of biocrude oil were measured at the start (0 days) and end of the storage time (84 days).
The ultimate analysis of biocrude oil was conducted using an elemental analyzer (Vario MICRO Cube, Element analysis system, Langenselbold, Germany). The oxygen content was calculated by the difference. The higher heating value (HHV) of biocrude oil was calculated according to the Dulong formula:
HHV(MJ/kg) = 0.3414C(wt%) + 1.445H(wt%) − [N(wt%) + O(wt%) − 1]/8
The effective hydrogen-to-carbon ratio H/Ceff for the biocrude oil was calculated according to the previous study:
H/Ceff = (H − 2O − 3N − 2S)/C
where H, O, N, S, and C represent the atomic concentrations of hydrogen, oxygen, nitrogen, sulfur, and carbon, respectively.
The hydrogen-to-carbon ratio (H/C), oxygen-to-carbon ratio (O/C), and nitrogen-to-carbon ratio (N/C) were calculated by taking the ratio of the element weight percent and the element molecular weight.
The chemical characterization of the biocrude oil samples was conducted via gas chromatography–mass spectrometry (GC-MS, QP2010, Shimadzu Co., Tokyo, Japan). Using helium as the carrier gas at a flow rate of 1.78 mL/min. The injection temperature, inlet temperature, and source temperature were set at 280 °C, 250 °C, and 230 °C. The oven temperature was initially set at 40 °C, hold 2 min, and then ramped at 10 °C/min to 200 °C, hold 3 min, and then ramped at 4 °C/min to 300 °C, hold 6 min. The electron ionization voltage was set at 70 eV, the spectra were scanned from 50 to 550 m/z, and the characterization of individual peaks was determined by comparing the mass fragmentation patterns of the peak to the NIST database (NIST11).
The organic functional groups present in the biocrude oil were determined via FT-IR (Thermo Fisher Nicolet ls 10, Waltham, MA, USA). The boiling point of the biocrude oil was estimated through thermal gravimetric analysis (METTLER TOLEDO TGA/DSC 3+). Both testing methods were based on a previous study [15].

3. Results and Discussion

3.1. Physical Properties

The physical properties of the biocrude oil were highly impacted by the storage temperature and gas environment. Figure 1A illustrates the quality changes in the biocrude oils over time in storage. It was observed that all biocrude oils experienced varying degrees of mass loss, indicating volatilization during storage. Although the sample bottle was tightly sealed and protected by N2, small molecular compounds such as water and hydrocarbons with less than 16 carbon atoms were still found to volatilize [9,15]. This is due to the release of these compounds into the external environment upon opening the bottle cap, even if stored in a sealed headspace. It can be clearly seen that under the same storage ambient gas conditions, the rate of mass change in biocrude was found to increase with the increase in temperature, with the minimum change observed at 4 °C. This is attributed to the faster movement of molecules at higher temperatures (15 °C and 35 °C), leading to increased volatilization. At the same storage temperature, the mass change rate was higher in air compared to nitrogen protection. This is because small molecules of biocrude under air conditions can easily be released into the atmosphere, while those protected by N2 remain within the system, either being adsorbed on the bottle cap or returning to the biocrude during storage. It is worth noting that the rate of mass change in the biocrude oil stored in air at 35 °C exhibited an initial rapid increase within the first 42 days, followed by a subsequent decrease from day 42 to day 84. This phenomenon can be attributed to the formation of an “oxide layer” (OL) during biocrude oil storage, which serves as a protection barrier for its internal biocrude oil content.
The OL is formed within a certain period, exhibiting a defined thickness and hardness akin to that of a solid substance with limited fluidity. This OL was thickening over time and with increased storage temperature. Compared with the biocrude oil stored in the N2 environment, the OL becomes harder when the biocrude oil is stored in the air environment. The phase change in biocrude oil from a liquid to a solid phase was due to oxidation and solidification [22]. The oxidation and solidification of the biocrude oil were reduced when the biocrude was stored at a low temperature and in a nitrogen-protected condition. It was noted that, despite the formation of an OL when stored in air, the biocrude beneath it remains fluid. The characteristic of the OL may protect the internal oil from air exposure, thereby mitigating volatilization and minimizing quality deterioration. Therefore, to comprehensively analyze the aging properties of the biocrude oil, the physicochemical properties of the OL and its internal biocrude oil were analyzed, respectively. Since the internal biocrude oil is the subject part of the sample, the biocrude oil discussed below is the internal biocrude oil.
Figure 1B shows the viscosity change in the biocrude oil. During storage, the viscosity of the Spirulina biocrude oil increased from 7164.2 mPa·s to 17,577 mPa·s after 84 days. Under the two kinds of storage gas environments, the viscosity change in all biocrudes increases with the increase in temperature. At the same temperature, the viscosity change in the sample in the N2 environment is basically lower than that of the sample under air conditions, except for the biocrude in 15 °C and 35 °C conditions after 14 days of storage. The lower viscosity changes in Air-15 and Air-35 after 14 days may be due to the protection of the OL, which insulates the sample from air. However, the viscosity changes in Air-15 and Air-35 were 15–20% higher than N2-15 and N2-35 after 84 days, indicating that the N2 environment can reduce the aging rate of the biocrude oil during long-term storage. When the storage temperature is 4 °C, the effect of nitrogen protection is the most obvious. The viscosity change in N2-4 was 5.6%, 10.9%, 18.5%, 20.9%, and 25.6% after 14, 28, 42, 63, and 84 days of storage. The viscosity change trends indicate that the biocrude protected by nitrogen and placed at 4 °C is conducive to maintaining its stability, reducing the change in oil viscosity, and maintaining its fluidity.
The maximum viscosity changes (145.3%) in the biocrude oil derived from a plug-flow continuous hydrothermal reactor were lower compared to those derived from a batch reactor, up to 453.8% after 84 days [9]. However, the initial viscosity of the biocrude obtained from the continuous hydrothermal reactor (7164.2 mPa·s) was considerably higher than that obtained from the batch reactor (650 mPa·s). The difference in viscosity of biocrude oil may partially result from the different mass and heat transfer dynamics between the two reactors [8]. Viscosity is an important factor that affects the transportation of biocrude oil. The high viscosity of biocrude oil can have detrimental effects on burner operation, while its use in engines may lead to increased viscosity of moving parts and an elevated risk of fuel filter blockage [9]. Research has indicated that the rise in biocrude viscosity primarily stems from olefin condensation, polymerization, coke formation, evaporation, and oxidation of volatile matter during storage [23,24,25]. Furthermore, acids, aldehydes, and nitrogen-containing compounds in biocrude will accelerate these reactions [24,25,26]. Therefore, it is necessary to subject these continuous biocrudes to distillation or catalytic refining before use as transportation fuels. Additionally, to mitigate oil aging, storing the biocrude oil at low temperatures under a nitrogen atmosphere before refining is recommended.

3.2. Element Distribution and Calorific Value

The organic element distribution and HHV of both fresh biocrude oil and aged biocrude oil stored after 84 days are presented in Table 2. Compared to fresh biocrude, the elements and HHV of the OL and its internal oil have changed in varying degrees after long-term storage. The content of C, H, and N in the OL decreased due to the volatilization of micromolecular compounds, while the content of O increased due to oxidation. Notably, the C content decreases with the decrease in storage temperature. At the same storage temperature, the C content of the OL in the N2 environment decreased more than that in the air environment. Conversely, the changing trend of the C content of the biocrude oil showed an opposite pattern. However, the overall analysis found that the C content of the OL was higher than the biocrude oil, but the H content of biocrude was higher, indicating that the aging reactions of the OL and its IB during storage were different. What is more, the OL and its internal biocrude also reacted with each other during storage, which caused the migration of elements. Both C and H are closely related to the energy potential of biocrude. During storage, the HHV of all OLs and IBs was reduced due to the changes in C and H. The HHV decreased by 1.7–7.3% for the OL and by 5.2–7.7% for biocrude oil. Under the same storage conditions, the HHV was higher for the OL compared to the biocrude oil, suggesting that although the biocrude has become solid, it still has high energy and can be used as a solid fuel source. Furthermore, despite a decrease in HHV during storage, the HHV of the Spirulina biocrude oil after 84 days exceeded 31 MJ·kg−1, which was still higher than that of the biocrude oil prepared by partial sludge and feces [27,28]. In this way, the Spirulina biocrude oil has the potential to become a power fuel even after being stored for the long term.
After 84 days of storage, significant changes were observed in the O content and O/C value (Figure 2) of both the OL and biocrude oil. The O content of the OL showed the greatest change (34.3%) under the conditions of 4 °C and being in a N2 environment, while the IB showed the greatest change (39.9%) under the conditions of 15 °C and being in a N2 environment. This phenomenon can be explained by the oxidation and volatilization of the biocrude oil during storage [15]. The increase in O content in biocrude oil is likely due to the formation of new oxygenated compounds when biocrude is exposed to air [9,15,29]. The decrease in O content in biocrude oil may be attributed to the release of small molecularly oxygenated compounds into the environment by evaporation when opening the sample bottle [30]. These small molecularly oxygenated compounds may be water and ketones with carbon numbers below 20 [9].
After storage, the N content and N/C value in all samples showed a decreasing trend, indicating that some nitrogen compounds volatilized during long-term storage [29]. Those nitrogen compounds may be indolizine based on GC-MS analysis. The presence of nitrogen compounds can contribute to environmental pollution upon combustion. The N/C value of Spirulina biocrude is comparable to petroleum (<0.1), promoting the clean combustion of the biocrude oil [30].

3.3. Boiling Point Distribution

Thermogravimetric analysis (TGA) is a process used to measure the continuous mass loss of a sample as the temperature increases. TGA, when used with N2 as the purge gas, can be considered a micro “distillation device”, which can estimate the boiling point range of the biocrude oil [27]. Although there may be some thermal degradation during the heating process, it is very small and can be ignored. Currently, TGA is widely used to determine the boiling point of biomass fuel oils [31,32,33]. The boiling point (bp) of the Spirulina biocrude, which was analyzed by TGA, was divided into naphtha fraction (≤193 °C), kerosene fraction (193–271 °C), diesel fraction (271–343 °C), vacuum diesel fraction (343–538 °C), and residue fraction (>538 °C) [15]. The naphtha fraction, kerosene fraction, and diesel fraction are low-boiling-point compounds (bp < 343 °C). The vacuum diesel fraction and residue fraction are mid-boiling-point compounds (343 °C < bp < 538 °C) and high-boiling-point compounds (bp > 538 °C), respectively [27].
The boiling point distribution of all fresh biocrude oil, the OL, and aged biocrude is shown in Figure 3. Each distillation section has changed to varying degrees after storage, with the most significant changes observed in the naphtha and residual distillation sections. Under the conditions of two kinds of storage gases, the changing rate of all biocrude oil fractions increased with increasing storage temperatures. The naphtha fraction content of the OL decreased from 23.2% to 8.3%, while the residue content increased from 17.2% to 19.9%. Under the air environment, the kerosene fraction content increased by 0.41%, 2.16%, and 0.69% for the OL stored at 4 °C, 15 °C, and 35 °C, respectively. The vacuum diesel fraction content showed the opposite trend to that of the kerosene fraction. The diesel fraction content in the OL increased with the increase in temperature—a 79.5% increase in the OL at 35 °C. The increase in diesel and residue fractions was due to the polymerization of naphtha and kerosene fractions [9]. Under the N2 environment, the vacuum diesel fraction content (22.3–25.6%) increased with increasing temperature, while the kerosene fraction content (23.6–25.8%) first decreased and then increased, and the diesel fraction content (12.2–15.1%) changed slightly with temperature. Although the variation law of biocrude fractions is similar to that of the OL, the degree of variation is not as large as that of the OL. Under all storage conditions, the variation in all biocrude fractions under 15 °C and N2 protected storage conditions was small (0.1–3.6%), indicating that these storage conditions are conducive to maintaining the boiling point of the biocrude oil. The comprehensive analysis shows that the boiling point distribution of the biocrude oil is affected not only by the storage temperature but also by the storage atmosphere.
According to the mass change analysis, the decrease in the low-boiling-point fraction was not only due to evaporation and volatilization but also converted into mid- and high-boiling-point compounds, which led to an increase in heavier oil and then further converted into residue, finally forming the “oxide layer”. James et al., found that the order of boiling point volume content of the biocrude oil prepared from different raw materials corresponds to the order of average molecular weight and long-branched aliphatic molecular content ratio identified by 13C nuclear magnetic resonance (NMR) spectroscopy [34]. Previous studies have also found that the molecular weight of the biocrude oil or bio-oil will increase during long-term storage [13,15,35]. The increase in high-molecular-weight and long-branched aliphatic groups reflects the polymerization and condensation reactions during storage, which will cause an increase in viscosity [36]. High storage temperatures and oxygen environments will aggravate these polymerization reactions [37,38,39]. Therefore, the boiling point distribution data showed that some small molecular compounds of the biocrude oil were converted into high-molecular-weight compounds after 84 days. The high content of heavy oil and residue will not only reduce the flow performance of biocrude oil but also lead to incomplete combustion of biocrude and affect the service life of internal combustion engines [11,12,13]. Therefore, the aging rate of biocrude oil should be controlled during production and storage.

3.4. GC-MS Analysis

The HTL biocrude oil is a complex mixture composed of various compounds, with their composition determined by GC-MS. As shown in Figure 4, those compounds are divided into six categories: hydrocarbons, acids, esters, alcohols, ketones, and nitrogenous compounds, according to their functional group characteristics. The changes in the content of these compounds in each biocrude sample can be roughly evaluated based on the chromatographic peak areas. It should be noted that the maximum column temperature for GC-MS was 300 °C. So, the compounds in the biocrude oil samples with boiling points above 300 °C could not be determined using GC-MS. In this study, only approximately 50% of the organic compounds in the biocrude oil samples could be detected by GC-MS. After 84 days of storage, the OL and its internal biocrude underwent a varying degree of change as compared to the fresh biocrude oil, indicating that the compounds of Spirulina biocrude are unstable and susceptible to chemical reactions during storage. The nitrogenous content of all samples increased, while ketones and alcohols decreased. The acids were basically in a decreasing trend except for the OL, which was stored at 35 °C. The content of nitrogenous compounds in the OL was lower than that of the aged biocrude oil when stored at 15 °C and 35 °C. This could be attributed to an increase in the molecular weight of nitrogenous compounds, which is not detected by GC-MS.
During storage, with an increase in temperature, the content of nitrogenous compounds in biocrude increases, while ketones, alcohols, and acids continue to decrease, indicating esterification and polymerization occurred under the air atmosphere [9,10,14]. The same trend is observed under the nitrogen atmosphere. However, the rate of change is relatively lower, suggesting that high temperatures and the presence of air or oxygen accelerate the aging reactions in the biocrude oil. Compared to the aged biocrude oil, the nitrogenous content in the OL at low temperatures is higher than at high temperatures. This can be attributed to the lower solubility of the OL in acetone formed at high temperatures, which cannot be detected by GC-MS. The formation of the OL in the biocrude oil was caused by the aging reactions between compounds in the biocrude and oxygen in the atmosphere. The components of the oxide layer may be gum, sediment, asphaltene, or their mixture [40,41]. Researchers agree that the oxygen-containing compounds and nitrogen-containing components, especially nitrogen heterocycles in the biocrude oil, contribute most to the oil instability and sediment formation [39,41]. Therefore, it is essential to control the increase in oxygen and nitrogen-containing compounds during biocrude oil combustion to reduce the possibility of odor and nitrogen oxide emissions.

3.5. FT-IR Analysis

The FT-IR analysis could comprehensively determine the “overall” functional group characteristics of the biocrude oil [27]. Figure 5 shows the peak appearance of the biocrude oil in the wave number range of 4000–500 cm−1. The change in the FT-IR spectrum curve of the biocrude oil increased with the storage temperature. The biocrude oil stored at 4 °C in N2 condition had the smallest change after 84 days, suggesting that it is a suitable storage condition for the biocrude oil.
The peak at 3600–3000 cm−1 indicates that the biocrude oil contains alcohols, phenols, and acids, which contain -OH groups, or nitrogenous compounds, which contain N-H groups [42,43,44]. In the air environment, this peak began to become obvious at 35 °C, indicating that chemical reactions related to alcohols, phenols, acids, and nitrogen-containing compounds occurred during storage. The peak at 3000–2800 cm−1 (C-H) indicates the presence of aliphatic compounds such as -CH2 and -CH3 in biocrude [42,43]. In addition, the -CH2 at 1456 cm−1 indicates that it is an olefin compound [15,43]. After storage, these compounds increased at storage temperatures of 15 °C and 35 °C, which was consistent with the results of GC-MS. The peak at 1667 cm−1 (C=O) comes from ketones and acids [42], which increased significantly in high-temperature and oxide layer samples. The peak at 900–650 cm−1 (C-H) comes from aromatic compounds and their derivatives in the biocrude oil [9,44]. These compounds may participate in aging reactions, resulting in a reduction after storage. The peak at 966 cm−1 originated from C-O vibration, indicating the changes in alcohols, phenols, and acids in the biocrude oil [9,15,42,43,44]. After 84 days, this peak of Air-35OL disappeared, and a new peak of C-N stretching appeared at 1045 cm−1, indicating that alcohols, phenols, and acids reacted with nitrogenous compounds to form new and higher molecular weights [44]. This reaction only occurred in the OL, which is stored in an air environment. The biocrude oil with N2 protection did not occur, indicating that N2 protection can slow down the aging process to a certain extent.

3.6. Relationship between Storage Conditions and Characteristics of Biocrude Oil

To further reveal the relationship between storage conditions (atmosphere, temperature, time) and the characteristics of biocrude from a statistical perspective, a correlation analysis was conducted in Figure 6. The positive correlation means that when one variable increases, the other variable also increases, and vice versa [45]. Compared with storage atmosphere and temperature, storage time has the greatest impact on the properties of Spirulina biocrude. Storage time was significantly positively related to viscosity, H content, O content, H/C, O/C, and kerosene fraction. While it was negatively related to C content, N content, N/C, HHV, and vas gas oil fraction. Both storage temperature and time have impacts on the boiling point distribution of the biocrude oil. The temperature was negatively related to the naphtha fraction while being positively related to the kerosene and diesel fractions. The storage time was significantly positively related to kerosene and diesel fractions while being negatively related to vas gas oil fractions. The boiling point distribution of the OL was also highly impacted by the storage time, temperature, and atmosphere. This may suggest that the storage conditions significantly affect the boiling point distributions of the biocrude oil.
Both the storage temperature and the atmosphere were positive for the viscosity of the biocrude oil. But compared with ambient gas storage, the temperature has a greater impact on the viscosity of biocrude. The viscosity of the biocrude oil was positively related to O content, O/C, kerosene, and diesel fraction. This suggests the increase in viscosity was significantly due to the oxygen and heavy molecular weight compounds. The negative relationship between viscosity and N content of the biocrude oil may be owing to the N migration from biocrude to the OL during storage. Si et al., reported that the viscosity of the biocrude distillates demonstrated a decrease or slightly elevated viscosity during the first eight weeks and then an increase after sixteen weeks [45]. The little changes in the viscosity of distillates during the first eight weeks were due to the removal of solid particles, ash, and nitrogen in biocrude during distillation. Previous studies suggested that inorganic elements and metal species in biocrude could act as catalysts to speed aging reactions such as polymerization and condensation during storage [10]. In that case, distillation is a better way to increase the storage stability of biocrude and is a potential refining technology for the application of Spirulina biocrude derived from a continuous reactor.
During about three months of storage, the thermal and oxidative stability of biocrude oil derived from the continuous hydrothermal reactor was poor. However, previous studies have suggested that a 30-day storage period is sufficient since the realistic application of bio-oil is usually within this time frame. Based on this, before the biocrude oil refining, it may be possible to store the biocrude oil without cooling treatment during the spring, autumn, and winter seasons, but it needs to be stored in cold conditions during the summer season.

4. Conclusions

The thermal and oxidative stability of Spirulina biocrude derived from a plug-flow continuous hydrothermal reactor at various storage conditions was investigated based on changes in viscosity, organic elemental, HHV, boiling point, and chemical compound analysis. An “oxide layer” was formed above the sample bottle during long-term storage. After 84 days, the maximum viscosity change was 145% when biocrude oil was stored at 35 °C and in the air condition, and the minimum viscosity change was 25.6% when biocrude oil was stored at 4 °C in an N2 atmosphere. Compared with ambient gas storage, the storage temperature has a greater impact on the viscosity of biocrude oil. The thickness and hardness of the oxide layer and the viscosity of the biocrude oil were more aggravated at higher storage temperatures and air environments. The HHV decreased by 1.7–7.3% for the oxide layer and by 5.2–7.7% for the biocrude oil. The aging process of the biocrude oil involves volatilization, oxidation, esterification, and polymerization. According to the physicochemical properties’ analysis, to mitigate oil aging, storing the biocrude oil at low temperatures under a nitrogen atmosphere before refining is recommended, especially during the summer season. To comprehensively understand the storage stability of biocrude oil, further investigations are needed to elucidate the aging mechanism of biocrude oil and compare the characteristics of biocrude oil produced by different reactors. Overall, biocrude oil has the potential for industrial applications in the future. In the future, distillation could be used to improve the storage stability of biocrude oil.

Author Contributions

Y.W.: conceptualization, methodology, formal analysis, investigation, experiments, data curation, and writing—original draft; M.C.: experiments and writing—review and editing; W.L.: writing—review and editing; D.Y.: writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Key Scientific and Technological Project of Henan Province (232102110291), the Key R&D Projects in Henan Province (241111321700), and the Doctoral Research Foundation of Henan University of Science and Technology (13480041), China.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Informed consent was obtained from all subjects involved in this study.

Data Availability Statement

The data belongs to Henan University of Science and Technology.

Acknowledgments

We thank Hugang Li and Deliang Kong for their assistance with oil production. We also acknowledge Yongdong Xu for his suggestions on the writing of this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The rate of mass change (A) and viscosity change (B) of biocrude oil.
Figure 1. The rate of mass change (A) and viscosity change (B) of biocrude oil.
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Figure 2. Van Krevelen diagram of biocrude oil: (A) and (B) represent H/C and N/C, respectively.
Figure 2. Van Krevelen diagram of biocrude oil: (A) and (B) represent H/C and N/C, respectively.
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Figure 3. Boiling point distributions of the fresh biocrude oil (A), oxide layer (B–F, scatter diagram), and aged biocrude oil (B–F, bar chart), respectively.
Figure 3. Boiling point distributions of the fresh biocrude oil (A), oxide layer (B–F, scatter diagram), and aged biocrude oil (B–F, bar chart), respectively.
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Figure 4. Organic compounds’ distribution of the fresh biocrude oil, oxide layer, and aged biocrude oil measured by GC-MS.
Figure 4. Organic compounds’ distribution of the fresh biocrude oil, oxide layer, and aged biocrude oil measured by GC-MS.
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Figure 5. FT-IR spectra of the fresh biocrude oil, oxide layer, and aged biocrude oil.
Figure 5. FT-IR spectra of the fresh biocrude oil, oxide layer, and aged biocrude oil.
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Figure 6. Correlation between storage conditions (atmosphere, temperature, and time) and characteristics of the biocrude oil (A) and oxide layer (B).
Figure 6. Correlation between storage conditions (atmosphere, temperature, and time) and characteristics of the biocrude oil (A) and oxide layer (B).
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Table 1. Storage condition of the biocrude oil.
Table 1. Storage condition of the biocrude oil.
SampleStorage Temperature/°CAtmosphere
41535AirN2
Air-4√ √
N2-4√ √
Air-15 √ √
N2-15 √ √
Air-35 √√
N2-35 √ √
Table 2. Elemental distribution of fresh biocrude oil, oxide layer, and aged biocrude oil at different storage conditions.
Table 2. Elemental distribution of fresh biocrude oil, oxide layer, and aged biocrude oil at different storage conditions.
IDC (%)H (%)N (%)O 1 (%)HHV (MJ·kg−1)
Fresh biocrude69.26 ± 0.099.175 ± 0.157.475 ± 0.0114.0933.97
Oxide layer
Air-4UO67.41 ± 0.038.886 ± 0.047.047 ± 0.0516.6632.48
Air-15UO68.93 ± 0.068.897 ± 0.017.202 ± 0.0314.9733.31
Air-35UO69.88 ± 0.098.641 ± 0.017.177 ± 0.0214.3033.39
N2-4UO65.26 ± 0.168.994 ± 0.076.817 ± 0.0518.9331.50
N2-15UO66.54 ± 0.118.878 ± 0.046.936 ± 0.0017.6532.00
N2-35UO67.11 ± 0.039.11 ± 0.017.230 ± 0.0216.55 32.72
Aged biocrude
Air-465.63 ± 0.039.311 ± 0.066.855 ± 0.0118.2032.21
Air-1564.21 ± 0.019.522 ± 0.246.691 ± 0.0219.5831.78
Air-3564.49 ± 0.059.491 ± 0.056.718 ± 0.0019.3031.88
N2-466.04 ± 0.019.364 ± 0.046.919 ± 0.0217.6832.51
N2-1564.41 ± 0.079.192 ± 0.106.688 ± 0.0519.7131.36
N2-3564.82 ± 0.029.226 ± 0.566.961 ± 0.0218.9931.67
1 O(%) = 100—C(%)—H(%)—N(%).
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Wang, Y.; Cao, M.; Lan, W.; Yin, D. Thermal and Oxidative Stability of Biocrude Oil Derived from the Continuous Hydrothermal Liquefaction of Spirulina. Sustainability 2024, 16, 4884. https://doi.org/10.3390/su16124884

AMA Style

Wang Y, Cao M, Lan W, Yin D. Thermal and Oxidative Stability of Biocrude Oil Derived from the Continuous Hydrothermal Liquefaction of Spirulina. Sustainability. 2024; 16(12):4884. https://doi.org/10.3390/su16124884

Chicago/Turabian Style

Wang, Yingxian, Maojiong Cao, Weijuan Lan, and Dongxue Yin. 2024. "Thermal and Oxidative Stability of Biocrude Oil Derived from the Continuous Hydrothermal Liquefaction of Spirulina" Sustainability 16, no. 12: 4884. https://doi.org/10.3390/su16124884

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