Unprecedented climate shift and brisk depletion of conventional resources have raised socioeconomic and environmental concerns; thus the pursuit of clean, independent and alternative renewable and sustainable commercialized solutions for fuels has been expedited [1
]. Fuels produced from so-called 2nd and 3rd generation biomass feedstocks, such as sewage sludge and microalgae, offer the advantage of a potentially favorable carbon footprint and a lower risk of negative indirect land use change (ILUC) [2
Microalgae have been identified as an attractive energy source for the production of biofuels due to their ability to accumulate relatively large amount of lipids along with carbohydrates, and proteins [5
]. Microalgae show higher photosynthetic efficiencies compared to terrestrial plants which could be beneficial in terms of environmental impact [7
]. High growth rates and non-competition with arable land and fresh water resources, triggered escalating interests to convert microalgae into liquid fuels, which engage researchers to develop environment-friendly methods with remarkable potential roots for mass production [8
Out of all available energy valorization techniques at our disposal, hydrothermal liquefaction (HTL) is considered as one of the most promising technologies for the production of biocrude oil, due to its advantages in rapid reaction, using all types of wet or dry feedstocks with no lipid-content restriction [10
]. Recent techno-economic analyses and life cycle assessments on the conversion of microalgae into liquid fuels show that HTL has lower GHG emissions and higher economic potential, with a better energy return on investment as compared to conventional lipid extraction and transesterification technologies [13
At temperatures around 300–420 °C and pressure above the saturation pressure of water the HTL process produces a viscous and energetically dense black liquid usually referred as biocrude. This biocrude contains ~10–15 wt. % of heteroatoms (primarily O and N) with a high heating value (HHV) of ~30–38 MJ/kg [17
]. Biocrude as such cannot be directly utilized in the transportation sector. To produce fungible on-specification transportation fuel, further upgrading processes such as deoxygenation, denitrogenation, etc. are required. Over the years, researchers have investigated both heterogeneous and homogeneous in-situ catalysts to achieve improved bio-diesel [19
]. Catalytic upgrading of biocrude through hydroprocessing is the most promising pathway for the conversion of polar compounds into hydrocarbon-rich mixtures [21
]. During hydroprocessing, undesirable components such as metals, oxygen, sulfur, nitrogen, olefins, and aromatics are removed from biocrude by selectively reacting them in the presence of a heterogeneous catalyst, at relatively high pressure (30–170 bar H2
) and at high temperature 300–450 °C [22
Several studies on the hydrotreating of algal biocrudes are documented in the open literature. Biller et al. [21
] investigated the hydroprocessing of biocrude obtained from Chlorella
microalgae, by using sulfided CoMo and NiMo catalysts at 350 °C and 405 °C, with an initial H2
pressure between 60–66 bar and a residence time of 2 h. Relatively low yields (41–69%) were observed at high temperature, and 86 and 91% reduction of O, along with 60 and 55% of reduction of N, were obtained with NiMo and CoMo catalyst, respectively. Both catalysts showed no difference in their catalytic activity at batch scale; furthermore, after pentane extraction the oil analysis indicated that the majority of remaining O is contained in high molecular weight compounds, while no adequate reduction of nitrogen was achieved. Bai et al. [23
] carried out the catalytic hydrothermal processing of biocrude in the presence of 15 different catalysts in a batch reactor. By using a combination of Ru/C and Raney Ni catalyst they were able to obtain 2 wt. % O and 2 wt. % N with a yield of 77%. Duan et al. [24
] observed, 90% of O and 59% of N removal after the catalytic hydrothermal upgrading of microalgae biocrude by using a mixture of Ru/C and Mo2
/C as catalyst at 400 °C for 4 h. Li and Savage [25
] explored the degree of deoxygenation and denitrogenation by using an acidic cracking catalyst (HZSM-5) in the presence of H2
at 400, 450 and 500 °C. They found that the upgraded oil yield was reduced from 75 to 42% as the temperature increases. Elliott et al. [26
] reported the continuous hydrotreatment of biocrudes from four different microalgae in a bench-scale trickle bed reactor in the presence of sulfided Co-Mo/γ-Al2
catalyst at 405 °C and operating pressure of 136 bar. They obtained oil yields of 80–85% with final O and N levels of ~1 and ~0.1 wt. %, respectively. However, the authors also highlighted the difficulty to compare results from continuous and batch system, because the products from batch system are equilibrium limited.
Although these studies featured hydrotreating tests in different process conditions, the overall effect of temperature, pressure and residence time on heteroatom removal has not been systematically evaluated. A thorough investigation could be of great importance to understand which parameters, or combinations thereof, influence the performance of the process. In the current work, a two-level factorial experimental design was developed and the HTL hydrotreatment of microalgae biocrude, obtained from Spirulina
was evaluated. The effect of temperature, H2
pressure, and residence time on the hydrodeoxygenation (HDO), hydrodenitrogenation (HDN), and hydrogen consumption was discussed. In this evaluation, a maximum temperature of initially 350 °C, then 400 °C was considered, which differs from the previous literature (e.g., [21
]). Following this analysis, three confirmatory experiments were performed at high temperatures to understand the significance of heteroatom removal. Moreover, a small distillation unit (following ASTM D-1160) was used to fractionate the hydrotreated oils showing complete deoxygenation, in order to investigate the concentration of nitrogen and its distribution in lower (<340 °C) and higher (>340 °C) fractional cuts. Comprehensive analysis on the upgraded fuel and distilled fractional cuts was carried out.
3. Results and Discussion
Hereby, the hydrotreating of Spirulina
biocrude is conducted as described in Section 2
. The results of the single experiment are presented in terms of degree of deoxygenation (de-O), degree of denitrogenation (de-N), and hydrogen consumption as response variables. After the identification of influencing factors, the characterization of upgraded biocrude is discussed in terms of elemental composition, chemical structure and boiling point distribution.
3.1. The Effect of the Process Parameters on Heteroatoms Removal
In order to identify the most influencing parameters during hydrotreating, a two-level factorial design of experiments with varying reaction temperature, initial H2
pressure, and residence time was performed until experiment 8 as described in Table 3
. To check the influence of these varying parameters on bio-oil properties, the effect and the interaction between each parameter have been calculated based on the elemental composition of the upgraded oil as shown in Table 4
. Furthermore, the half-normal probability plot is used to assess which factors are important and which are of no significance.
Potential coke and product gases during hydrotreatment were considered negligible as compared to the losses during liquid (oil plus water) recovery in a micro-batch reactor. Thus, the overall yield (wt. %) of upgraded oil is not reported in the present study in order not to present an inaccurate picture. Though, the liquid recovered was around ~90 wt. % ± 5 during all experiments which is similar to other works in the literature, for example [21
]. Current study focuses on the removal of heteroatoms (O and N) by varying the initial H2
pressure and reaction time at considerably low operating temperatures (250 °C and 350 °C).
shows intriguing results in case of heteroatom removal. Within the detection limit of CHN/O, 100% O content is removed from algal biocrude at 350 °C, 80 bar and 4 h of reaction time. At these severe conditions of present experimental matrix N content is reduced up to 47 %. Till now, the most prominent studies on heteroatom removal by Elliott et al. [26
], Biller et al. [21
] and Bai et al. [23
] showed the removal of O and N up to ~90% and ~99% respectively at ~400 °C.
Biller et al. [21
] and Duan et al. [24
] reported 91 and 90% reduction of O along with 60 and 59% reduction of N at 405 °C and 400 °C. Furthermore, Elliott et al. [26
] was able to reduce final O and N levels down to ~1 and ~0.05 wt. % (99 %) at 405 °C in a continuous flow catalytic fixed bed reactor. Lower denitrogenation rates until experiment 8 from the above study are evidence that high temperatures are vital for the removal of nitrogen. It also seems obvious, that mild conditions relating with each variable factor just slightly affect the O content, whereas the N content almost remain intact.
Based on this knowledge, a set of three confirmatory experiments (Exp. 9, 10 and 11) were performed as shown in Table 4
. They are based on the anticipation that N content will be further reduced with the 100 % removal of O at high temperatures. Subsequently, the results presented in Table 4
show complete deoxygenation for experiments 9, 10 and 11, while at best the denitrogenation was only achieved up to 60% at 375 °C and 70 bar initial H2
for three hours. However, 50% and 58% of denitrogenation is achieved at 400 °C by changing H2
pressure and reaction time.
The absolute values from Table 5
indicate the most relevant factors affecting the responses (R1
). Moreover, half-normal probability graph as shown in Figure 1
is used to assess the significant and insignificant factors for de-O (R1
) and de-N (R2
Assuming that the estimated effects of each factor and each interaction term on the de-O follow a normal distribution, the absolute estimated effects will then follow a half-normal distribution. By plotting the ranked estimated effects against the cumulative percent contribution (the contribution to the total sum of squares) of each factor, significant (and insignificant) factors can be identified. Insignificant factors are those having small estimated effects and tend to fall on a straight line starting at the origin (0:0).
From Figure 1
a, it is clear that the only significant factor influencing the de-O is the temperature. Obviously, the half-normal plot show only positive effects (absolute estimated effects). By examining Table 4
it is evident that temperature indeed has a positive effect on the de-O. At the lower level temperature, the higher degree of deoxygenation obtained was 28% (Exp. 3), whereas the de-O range from 72–100% at the higher level. Likewise, a similar identification of the significant factors influencing the de-N can be observed in in Figure 1
b. It is apparent that temperature is also significantly influencing the de-N along with the hydrogen pressure. Thus, the highest de-N was obtained in Exp. 8 (all factors set at high level) yielding a de-N of 47%, which is only marginally higher than Exp. 6 (de-N = 43%) for which the reaction time was only 2 h. As for the de-O, any interaction effects seem insignificant.
The hydrotreated oil after all 11 experiments has a higher C and a lower O and N content. However, H/C molar ratio remains more or less stable with the algal biocrude at lower temperatures and lower pressures. While, at higher temperatures (350–400 °C) and higher pressures (80 bar) overall H/C molar ratio increases and thus the aromatics in the oil decreases. In addition, a negligible effect of reaction time is observed on the molar H/C ratio. However, at 400 °C lower H/C is observed as compared to 375 °C, which is anticipated as a result of cracking reaction. Subsequently, after the complete deoxygenation and 60% of N reduction the HHV increases from 37.6 MJ/kg of the algal biocrude to 44.4 MJ/kg. Higher temperatures are needed for complete denitrogenation, but both H/C and HHV decrease at higher temperature of 400 °C in relation with 375 °C, which means that we are getting higher degree of denitrogenation with a loss of fuel quality. This may be caused by the formation of coke, as HTL biocrude due to high polarity are unstable and prone toward the polymerization and deactivation of NiMo catalyst. This complete deoxygenated oil with reduced N content is considered more stable and thus reduces the possibility of catalyst deactivation during further upgrading for on-specification fuel.
3.2. Analysis of the Up-Graded Oil
Simulated distillation (Sim-Dis) was used to evaluate the boiling point distribution of the catalytically upgraded oil samples and then compared with the untreated biocrude. Five different boiling point ranges are defined and described as gasoline (>190 °C), jet fuel (190–290 °C), diesel (290–340 °C), vacuum gas oil (340–538 °C) and vacuum residue (>538 °C). Table 6
along with Figure 2
shows yield of different fractions and boiling point distribution of all the experiments with complete degree of deoxygenation along with the biocrude and experiment 1 with the lowest possible operating range (250 °C, 40 bar H2
for 2 h). The biocrude shows large fractions in high molecular weight compounds and therefore, it associates with the higher residual cuts (72.7% above 340 °C). In addition, the overall recovery at 720 °C increases from 72% to 92%, thus the amount of potential distillable products also increases. Furthermore, after 100% HDO more than 32% and 14% oil is in jet fuel and diesel range and around 15% is confined in residue. High temperatures and high initial H2
pressures are vital for the cracking of biocrude into lower fractional cuts. Present study shows that at 350 °C and 80 bar initial H2
pressure highest boiling point distribution above diesel is obtained with minimum residual fraction cut of 12.5%. This looks very promising as by just tuning operating conditions at mild temperature (350 °C) high production of diesel drop-in fuel could be achieved (Table 6
and Figure 2
a,b show the GC-MS chromatograms of the HTL biocrude and the complete deoxygenated oils at 350 °C (47% HDN).
The untreated biocrude (Figure 3
a) shows a complex mixture of numerous different hydrocarbons and heteroatoms (mainly O and N), as expected from the elemental analysis. The oxygen containing compounds are mainly saturated fatty acids (hexadecanoic acid) and phenol derivatives such as p- and m-cresol. Moreover, fatty alcohols such as dodecanol are also observed in small amounts. Whereas, nitrogen-containing compounds are mostly found in the form of non-heterocyclic high molecular weight compounds such as various amides. However, N compounds are also present in the form of ring type structures with either one N atom, such as indoles and pyrroles, or two N atoms, such as pyrazines. The sole purpose of hydrotreating is to convert heteroatoms into their respective hydrocarbons along with the saturation of double bonds and cracking of high molecular weight compounds. Figure 3
b shows a chromatogram of upgraded oil (Exp. 8) after the complete removal of O atoms at 350 °C and reveals the presence of dominating aliphatic hydrocarbons ranging from C14
. Aromatic hydrocarbons such as ethylbenzene and toluene are also present in the upgraded oil. At the same time, N is observed in the GC-MS spectrum, such as the presence of amides in Figure 3
b at the retention time of 25.51 min and 27.09 min. However, the GC-MS spectrum also indicates minor presence of amine, indole, nitrile, pyrrole, pyrazine, pyridine and quinoline in the hydrotreated oil.
3.3. Distribution of Nitrogen-Containing Compounds
GC-MS analysis is only able to investigate the volatile fraction of the sample, hence it does not account for all molecules in the oil. Therefore, in order to assess the distribution of N content in the light and heavy fractional cuts of the upgraded oil, a more relevant industrial approach following ASTM D-1160 is carried out. Biller et al. [21
] reported inappreciable amounts of nitrogen-containing compounds through GC-MS; while, through elemental analysis they observed 1–1.5% of O and 2.4–2.7% of N. They performed C5
(pentane) extraction of hydrotreated oil to investigate the distribution of O and N between C5
soluble (flowable) and C5
insoluble (asphaltene) fractions. They were able to completely remove O content in C5
soluble fraction, but the removal of N content was not satisfactory and it seems that it was almost homogenously distributed in C5
soluble and insoluble fractions.
In order to determine the distribution of N content in lower and higher molecular weight compounds, the hydrotreated oil from experiments 8 (47% HDN) and 9 (60% HDN) were subjected to further investigation. Consequently, we performed true boiling point distillation following ASTM D-1160 to assess the concentration and the distribution of heteroatom content in low and high molecular weight fuel fractional cuts which correspond up to diesel (<340 °C) and vacuum residue (>340 °C). Results are shown in Table 7
shows, that 32% and 37% of N content is concentrated in the light fraction while vacuum residue contains 68% and 63% of N content for experiments 8 and 9, respectively. These results reveal that most of the nitrogen-containing compounds are distributed in the higher fractional cuts. Additionally, no O content is associated with the light fraction for both experiments while, 0.24 wt. % of O and 0.08 wt. % of O is incorporated in the vacuum residue obtained after the fractional distillation of experiments 8 and 9 respectively. The results indicate that oxygen is in fact concentrated in the heavy fraction, although in small amount. This means that when analyzing the total sample it is diluted to below the error associated with the CHN analyzer and therefore reported as zero. The presence of 0.08 wt. % of O in vacuum residue of hydrotreated oil (375 °C) as compared to the 0.24 wt. % of O (350 °C) also validate the statistical analysis, which showed that temperature is a main driver for O removal. Moreover, overall higher H/C molar ratio is observed for diesel range (>1.82) and very low H/C molar ratio (<1.48) was observed for vacuum residue in case of both experiment.
illustrates a modified Van Krevelen plot as a function of H/C and N/C molar ratio for biocrude, hydrotreating experiments and distillation cut of upgraded oil from experiments 8 and 9. Starting from the same biomass composition, it is clear that both temperature and pressure have a significant effect on the removal of N heteroatoms. Thus, the N/C molar ratio decreases along with the increase of hydrocarbon content. This plot also elucidates the distribution of nitrogen-containing compounds in lower and higher fractional cuts for experiments 8 and 9; and shows that the N/C molar ratio decreases in the hydrocarbon range (higher H/C molar ratio) but on the other hand N/C molar ratio increases as the aromatic content of the fractional cut increases.
Furthermore, the yields of the small fractional distillation unit are also stated in Table 7
. 2.76 g and 1.09 g of sample is used for the fractional distillation of experiments 8 and 9, respectively. Higher recovery of 87 wt. % is obtained after the fractional distillation of experiment 8 as compared to 76 wt. % for experiment 9. Vacuum residue cut for both experiments is in line with the Sim-Dis results. However, some losses in the light fractional cut (<340 °C) were observed after comparing it with Sim-Dis results, which may be due to the loss of lighter fraction in the distillation column and column head. From experiment 8 it is clear that the losses are less as compared to experiment 9 due to the use of higher amount of sample. Losses for lighter fractional cuts could be minimized by subjecting higher amount of sample to the fractional distillation unit.
Even though nitrogen-containing compounds are mostly found in the higher fractional cuts (>340 °C), still some nitrogen is found in the light fraction. Therefore, this could not be used directly as a fuel, because of the indirect regulations applied on the fuel specification due to the presence of N compounds. The reason for the presence of remaining N compounds could be found in thermodynamics and the reduction of partial H2 pressure during hydrotreating. Thus, we suggest that hydrotreating of algal biocrude for the removal of heteroatom N either possibly requires higher temperatures (which will result in reduced yields due to the formation of volatiles) or multi-stage catalytic treatment, to partially stabilize the oil before further upgrading at higher temperatures in order to completely remove nitrogen-containing compounds.