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Hydrothermal liquefaction of Chlorella vulgaris: Effect of reaction temperature and time on energy recovery and nutrient recovery
G Model JIEC 4127 No. of Pages 7
Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx
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Hydrothermal liquefaction of Chlorella vulgaris: Effect of reaction temperature and time on energy recovery and nutrient recovery Ji-Hyun Yang1, Hee-Yong Shin1, Young-Jin Ryu, Choul-Gyun Lee* National Marine Bioenergy R&D Consortium & Department of Biological Engineering, Inha University, 100 Inha-Ro, Michuhol-Gu, Incheon 22212, South Korea
A R T I C L E I N F O
Article history: Received 30 May 2018 Received in revised form 27 July 2018 Accepted 28 July 2018 Available online xxx Keywords: Hydrothermal liquefaction Microalgae Chlorella vulgaris Energy recovery Nutrient recovery
A B S T R A C T
Hydrothermal liquefaction of Chlorella vulgaris feedstock containing 80% (w/w) water was conducted in a batch reactor as a function of temperature (300, 325 and 350 C) and reaction times (5, 10 and 30 min). The biocrude yield, elemental composition and higher heating value obtained for various reaction conditions helped to predict the optimum conditions for maximizing energy recovery. To optimize the recovery of inorganic nutrients, we further investigated the effect of reaction conditions on the ammonium (NH4+), phosphate (PO43), nitrate (NO3) and nitrite (NO2) concentrations in the aqueous phase. A maximum energy recovery of 78% was obtained at 350 C and 5 min, with a high energy density of 34.3 MJ/kg and lower contents of oxygen. For the recovery of inorganic nutrients, shorter reaction times achieved higher phosphorus recovery, with maximum recovery being 53% at 350 C and 5 min. Our results indicate that the reaction condition of 350 C for 5 min was optimal for maximizing energy recovery with improved quality, at the same time achieving a high phosphorus recovery. © 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Introduction Fossil fuels are a finite resource with limited reserves [1]. Although fossil fuels are continually formed via natural processes, they are considered to be non-renewable resources since they take millions of years to form and the known viable reserves are being depleted faster than new ones being generated [2]. Environmental controversy surrounds the use of fossil fuels as energy sources. The burning of fossil fuels produces approximately 21.3 billion tons of carbon dioxide (a greenhouse gas) per year [3], thereby increasing radiative forcing and contributing to global warming. A global movement towards the generation of renewable energy is underway to help replace fossil fuels and reduce global greenhouse gas emissions. Biofuels are derived from biomass such as plants, photosynthetic bacteria and microalgae which can be regenerated every year. The biofuels are carbon neutral due to the ability of biomass to fix carbon dioxide during growth. Biofuels derived from sugar crop and vegetable oils are being produced on a commercial scale. However, the first generation biofuels use land-based-feedstock
* Corresponding author. E-mail address: [email protected] (C.-G. Lee). The first two authors contributed equally to this work.
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which compete with agricultural crops. Such feedstock may also be harmful to natural ecosystems due to the requirement of large cultivation areas. In order to minimize these problem, there is a proposal to derive biofuels from microalgae. Microalgae are a diverse group of photosynthetic eukaryotes with simple cell structures ranging from single cell to multicellular forms. They have the ability to capture carbon dioxide and convert the energy of sunlight to chemical energy. Furthermore, microalgae have higher productivity than terrestrial crops [4] and a very short harvesting cycle [5,6]. Recent attention has focused on microalgae for the production of liquid biofuels for power generation and transportation [7–10]. Algae conversion to bio-fuels is typically performed via bio- and thermo-chemical methods such as fermentation [11,12], transesterification of lipids [13–15], fast pyrolysis [16,17], and hydrothermal liquefaction [18–22] (HTL). Of these techniques, HTL is considered one of the most promising for wet algal biomass conversion. Since HTL applies subcritical water (200–350 C, 5–20 MPa) to promote biomass decomposition and reformation [23], the reaction avoids the heat of vaporization at elevated temperatures and converts both proteins and carbohydrates into a bio-oil with a higher energy density, in comparison to the raw biomass [24]. Therefore, this thermo-chemical conversion process needs no energy consuming drying step [25] and has a higher bio-oil yield [26]. In addition,
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suitable dilution enables the recycling of the nutrient rich HTL aqueous phase to the growth media for cultivation of microalgae [27]. Compared to a conventional catalytic process, the reaction parameters of a non-catalytic HTL significantly affect the reaction rate and reaction pathway due to the relatively severe reaction conditions, including higher temperature, pressure, and molar ratio of water. Thus, to determine the optimal reaction conditions, biocrude production from microalgae using subcritical water requires a specific study on the effects of varying reaction parameters. Additionally, reaction conditions for biocrude production via the HTL process is also dependent on the type of feedstock. Chlorella vulgaris has a fast growth rate and high biomass productivity and can easily be cultured using an inexpensive nutrient regime. In this study, the hydrothermal liquefaction of C. vulgaris was performed at different reaction temperatures and times. This was followed by a detailed investigation on the effect of reaction temperature and holding time on the HTL for biocrude production and recovery of inorganic nutrients. Numerous studies on HTL processes of microalgae have been investigated optimum reaction conditions for maximizing energy recovery and nutrient recovery separately. However, few researches have been reported to determine the reaction conditions that maximize the recovery of energy and nutrients (especially phosphorus) simultaneously. This study optimized the operating conditions of HTL for the maximum recovery of energy and nutrients, and also assessed the resultant fuel quality. Experimental C. vulgaris (Daesang Co., Ltd.) was the feedstock for hydrothermal liquefaction. The elemental and biochemical analyses of microalgae biomass was evaluated prior to hydrothermal liquefaction. The CHON composition and phosphorous content were determined using an elemental analyzer (Thermo EA1112, Thermo Electron Corp) and inductively coupled plasma optical emission spectroscopy (OPTIMA 7300DV, PerkinElmer), respectively. The
Table 1 Elemental and biochemical composition of Chlorella vulgaris and its HHV (wt% dry basis). Elemental composition (%) C 48.5 Biochemical composition (%) Lipid 18.7 Higher heating value (MJ/kg) 20.2 a,b
H 7.0 Protein 54.0
N 8.5
S 0.2
Oa 35.0
Carbohydrateb 24.3
Ash 3.0
P 0.8
Calculated by difference.
oxygen content was calculated by difference from the sum of carbon, hydrogen, nitrogen and ash contents. To estimate the ash content, 100 mg of sample was incinerated at 575 C for 12 h using a muffle furnace. Lipid content was determined by the Folch method [28] and protein content was calculated by multiplying the nitrogen content of biomass and nitrogen-to-protein conversion factor of 6.35 [29,30]. Carbohydrate content was calculated by difference. The higher heating value (HHV) was also estimated using data from the elemental composition in the following equation proposed by Channiwala and Parikh [31]. MJ O ¼ 0:3383 C þ 1:422 H þ 0:0942 S HHV kg 8 where C, H, O and S are the mass percentage of carbon, hydrogen, oxygen and sulfur, respectively. The element and biochemical composition of C. vulgaris and its higher heating value are presented in Table 1. Hydrothermal liquefaction of C. vulgaris was performed in a stainless-steel batch-type tubular reactor of 10 ml volume. Heating the reactor was achieved by using a molten salt comprising KNO3 and NaNO3. The reactor was loaded with 5 grams of wet algae containing 80% (w/w) water. The reactor was then immersed in the molten salt bath at varying temperatures of 300, 325 and 350 C [19]. After reaching the prescribed temperature, the reaction continued for a set period of pre-set duration (5, 8, 10 and 30 min) [21,22]. On conclusion, the reactor was cooled in a water bath to
Fig. 1. Flow diagram for separation and recovery of HTL products.
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terminate the reaction. After cooling, the reactor was opened and the reduced weight was measured to quantify the yield of gas phase. Addition of 5 ml chloroform facilitated removal of the other
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products (biocrude, aqueous phase, and solid residue) from the reactor, and filtration achieved separation of the solid residue. Layer separation was then performed to separate the biocrude
Fig. 2. Influence of the reaction temperature and time on the HTL product yields: (a) 300 C, (b) 325 C, (c) 350 C.
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(chloroform phase) and aqueous phase. The yields of recovered products were calculated using the following equation: Weight of each productðgÞ Weight of dry microalgae f eedstock ðgÞ 100
Yield of productsð%Þ ¼
A flow diagram of separation and the recovery procedure of HTL products is presented in Fig. 1. The biocrude was dissolved in chloroform and flushed for 24 h with nitrogen to remove the solvent and residual water. Dark colored biocrude samples with
high viscosity were obtained. Each biocrude sample was analyzed for elemental composition and HHV. The energy recovery (ER) of biocrude was calculated using the following equation: Energy recoveryð%Þ ¼
HHV biocrude Y biocrude HHV f eedstock
where HHVfeedstock is the higher heating value of feedstock, HHVbiocrude is the higher heating value of biocrude, and Ybiocrude is mass fraction of biocrude.
Fig. 3. Elemental balance of the experiments performed at various temperature and time in this study: (a) phosphorus balance, (b) nitrogen balance.
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Fig. 4. Effect of reaction temperature and time on the energy recovery of biocrude.
The aqueous phase concentrations of ammonium (NH4+), phosphate (PO43), nitrate (NO3) and nitrite (NO2) were measured using a water quality analyzer (QuikChem 8500 Series 2, Lachat Instruments, USA). Nitrogen recovery (NR) and phosphorus recovery (PR) were calculated using the following equations: Nitrogen recoveryð%Þ ¼
amount of nitrogen in aqueous phase amount of nitrogen in f eedstock
Phosphorus recoveryð%Þ ¼
amont of phosphorus in aqueous phase amount of phosphorus in f eedstock
Results and discussion The effects of reaction temperature and reaction time on the HTL of C. vulgaris using subcritical water were studied by varying the temperature (300–350 C) and time (5–30 min) at a fixed ratio (w/w) of water:microalgae of 4. The mass fraction of products acquired from each variable reaction temperature and time is presented in Fig. 2. The biocrude yields significantly exceeded the lipid content of the microalgae feedstock under all reaction conditions. At 300 C, the biocrude
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yield was augmented as the reaction proceeded, and the highest yield of 49% was obtained at a reaction time of 10 min, after which a slight decrease was observed. At 325 C, the biocrude yield increased up to 47% at 8 min and then decreased. At 350 C, the highest biocrude yield of 46% was obtained at 5 min. We observed that biocrude yields increased up to maximum levels and gradually decreased with increasing reaction time at each reaction temperature. With increasing reaction temperature, the maximum yield was obtained in a shorter time due to the enhanced reaction at a higher temperature. Unlike biocrude, the yield of gas phase increases with increasing temperature and time. As the reaction time and temperature increases, some of the biocrude may be thermally converted to smaller gaseous molecules, resulting in the increase in the gas phase yields as well as the decrease in the biocrude yield. Fig. 3 shows the phase distribution of nitrogen and phosphorus depending on the reaction temperature and time. Nitrogen was partitioned into the aqueous phase and biocrude, and phosphorus into the aqueous phase, biocrude, and solid phase. A higher reaction temperature and a longer reaction time generate a less amount of phosphorus in the aqueous phase. A higher temperature and a longer time conditions turn phosphate into precipitates by combining PO43 with divalent cations [18,19]. On the other hand, more nitrogen tends to move to the aqueous phase as the temperature and reaction time increase, because nitrogen in both the biocrude and the solid phase dissolve into the aqueous phase over time. It can also be seen that this phenomenon occurs faster under high temperature conditions. These results can be attributed to the decomposition of the amine compounds in the biocrude and solids into ammonia through the deamination [20,32]. The primary goal of the hydrothermal liquefaction process is to produce the biocrude; hence, energy recovery and the quality of biocrude are important. Considering energy recovery, each temperature condition has an optimal reaction time for energy recovery. As shown in Fig. 4, maximum energy recovery was about 78% at the reaction time of 10 min at 300 C, 8 min at 325 C, and 5 min at 350 C. The quality of biocrude is strongly associated with HHV, and the oxygen and nitrogen contents. Presence of oxygenated and nitrogenized compounds in the biocrude adversely affect the quality by interfering with the storage stability and catalytic upgrading [33]. In addition, combustion of high N content biocrude emits high NOx [34]. The elemental composition and HHV of biocrudes obtained at different reaction conditions are presented in Table 2. As shown in Table 2, the N and O contents of biocrude decreased with increasing reaction temperature and time, and higher HHV was observed at higher reaction temperature. The HHV of biocrude obtained at various reaction conditions were 32.6–36.2 MJ/kg, which is substantially higher than that of the original feedstock (20.2 MJ/kg). It is generally believed that during HTL reactions, oxygen is recovered as H2O and CO2 through a series of dehydration and decarboxylation [34] and nitrogen is
Table 2 Elemental composition and HHV of biocrude obtained at different reaction conditions. Temp. ( C)
300
325
350
Time (min)
5 10 30 5 8 10 30 5 10 30
Elemental composition (%)
HHV (MJ/kg)
C
H
N
S
68.73 (1.17) 68.18 (0.11) 68.28 (0.74) 68.80 (2.97) 69.31 (1.71) 69.62 (1.16) 71.02 (0.39) 70.70 (2.12) 71.84 (1.33) 73.22 (1.02)
8.37 (0.15) 8.55 (0.23) 8.68 (0.06) 8.60 (0.23) 8.75 (0.36) 8.83 (0.39) 9.00 (0.21) 8.88 (0.51) 9.06 (0.40) 9.28 (0.36)
7.84 (0.07) 7.51 (0.07) 7.26 (0.31) 7.68 (0.07) 7.61 (0.14) 7.48 (0.23) 7.20 (0.28) 7.44 (0.04) 7.45 (0.30) 7.03 (0.22)
0.39 0.33 0.27 0.26 0.27 0.31 0.32 0.34 0.37 0.33
O (0.55) (0.18) (0.13) (0.21) (0.18) (0.13) (0.17) (0.18) (0.18) (0.16)
14.67 (0.53) 15.43 (0.11) 15.51 (1.12) 14.66 (2.93) 14.05 (1.75) 13.77 (1.19) 12.47 (0.64) 12.64 (2.40) 11.28 (1.24) 10.14 (0.99)
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32.6 32.5 32.7 32.9 33.4 33.7 34.6 34.3 35.2 36.2
(0.2) (0.4) (0.4) (1.8) (1.4) (1.1) (0.0) (1.8) (1.2) (1.0)
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Fig. 5. Van Krevelen diagram (a) and H/C and N/C ratios (b) for different microalgae feedstock and corresponding biocrude.
removed as NH3 by deamination reactions [35], leading to decrease in oxygen and nitrogen content within the biocrude. Considering Table 2, it is observed that longer reaction temperatures and times are desirable for dehydration, decarboxylation, and deamination reactions, and are thus more favorable for improved quality of biocrude. The highest HHV of 36.2 MJ/kg and the lowest contents of nitrogen (7.03%) and oxygen (10.14%) were obtained at 350 C, 30 min. However, long holding times may be economically unfavorable for biocrude production in terms of energy consumption. Among the reaction conditions for maximum energy recovery, the highest HHV of 34.3 MJ/kg with low nitrogen and oxygen contents was observed at 350 C, 5 min. Hence, 350 C and 5 min was selected as the appropriate reaction condition in terms of energy recovery and quality of biocrude. Fig. 5 demonstrates the H/C ratios according to N/C and O/C ratios for different microalgae feedstock and corresponding biocrude. N/C and O/C are significantly reduced during the conversion of C. vulgaris into biocrude through HTL. As shown in Fig. 5(a), the H/C and N/C ratios obtained in the current study was similar to those of previous studies [33,36,37] at optimal reaction
conditions. Fig. 5(b) classifies the various microalgae feedstock and biocrude obtained from different reaction conditions based on the Van Krevelen diagram. The Van Krevelen diagram indicates that the biocrude produced in this study also exhibits close similarities for the H/C and O/C ratios to other microalgal biocrudes [33,36,37], suggesting that C. vulgaris is a potential feedstock for the production of biocrude. In the hydrothermal liquefaction process, the nutrient-rich and carbon-containing aqueous phase (the second largest fraction of HTL products) can be recovered and reused for microalgal cultivation [23]. The aqueous byproducts from microalgae HTL were often found to be high in nitrogen, phosphorus and potassium, along with other minerals and essential micronutrients required for microalgae growth [27,38]. Hence, recovery of these nutrients could be beneficial from a techno-economic perspective. The effect of the reaction temperature and time on HTL was investigated to determine optimum conditions for the recovery of inorganic nutrients. The nutrient recovery was calculated by measuring ammonium (NH4+), phosphate (PO43), nitrate (NO3) and nitrite (NO2) concentrations in the aqueous phase. Nitrate
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times are economically favorable for biocrude production in terms of energy consumption, and for high phosphorus recovery. The reaction conditions of 350 C for 5 min was found to be most suitable for maximizing energy recovery (78%) with improved quality, and at the same time achieving a high phosphorus recovery (53%). This study further confirms that C. vulgaris is a promising feedstock for biocrude production. Acknowledgements This research was supported by a grant from the Marine Biotechnology Program (PJT200255, Development of Marine Microalgal Biofuel Production Technology) funded by the Ministry of Oceans and Fisheries, Korea. References
Fig. 6. Nutrient recovery in aqueous phase: (a) nitrogen recovery and (b) phosphorous recovery.
and nitrite were not detected in all conditions. As presented in Fig. 6, the nitrogen recovery was augmented with increasing temperature and reaction time. The reaction conditions that showed lowest nitrogen recovery of 12% was 300 C, 5 min, and the highest nitrogen recovery of 27% was obtained at 350 C for 30 min. These observations are in accordance with the results of previous studies [36], where both longer reaction times and higher temperatures resulted in increased nitrogen content in the aqueous phase. There is a trade-off relation between energy recovery and nitrogen recovery, which may be due to the fact that amino acids converted from algae proteins are further hydrolyzed to form ammonia [36]. On the other hand, phosphorus recovery was higher at lower temperature and shorter reaction time. The lowest phosphorus recovery was 27% at 350 C, 30 min and the highest phosphorus recovery was 54% at 300 C, 5 min. Since phosphorus is obtained from finite mineral resources [38], we focused more on the recovery of phosphorus than nitrogen. Thus, we selected 300 C and 5 min as the optimal condition for nutrient recovery. Based on these results, we consider the reaction conditions of 350 C for 5 min to be the most suitable for maximizing energy recovery with improved quality, while at the same time achieving high phosphorus recovery. Conclusions This study performed the hydrothermal liquefaction of C. vulgaris at varying reaction temperatures and times. We investigated the effect of reaction temperature and time on the HTL to determine optimum conditions for the recovery of energy and inorganic nutrients. Our results indicate that shorter reaction
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Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx
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Hydrothermal liquefaction of Chlorella vulgaris: Effect of reaction temperature and time on energy recovery and nutrient recovery Ji-Hyun Yang1, Hee-Yong Shin1, Young-Jin Ryu, Choul-Gyun Lee* National Marine Bioenergy R&D Consortium & Department of Biological Engineering, Inha University, 100 Inha-Ro, Michuhol-Gu, Incheon 22212, South Korea
A R T I C L E I N F O
Article history: Received 30 May 2018 Received in revised form 27 July 2018 Accepted 28 July 2018 Available online xxx Keywords: Hydrothermal liquefaction Microalgae Chlorella vulgaris Energy recovery Nutrient recovery
A B S T R A C T
Hydrothermal liquefaction of Chlorella vulgaris feedstock containing 80% (w/w) water was conducted in a batch reactor as a function of temperature (300, 325 and 350 C) and reaction times (5, 10 and 30 min). The biocrude yield, elemental composition and higher heating value obtained for various reaction conditions helped to predict the optimum conditions for maximizing energy recovery. To optimize the recovery of inorganic nutrients, we further investigated the effect of reaction conditions on the ammonium (NH4+), phosphate (PO43), nitrate (NO3) and nitrite (NO2) concentrations in the aqueous phase. A maximum energy recovery of 78% was obtained at 350 C and 5 min, with a high energy density of 34.3 MJ/kg and lower contents of oxygen. For the recovery of inorganic nutrients, shorter reaction times achieved higher phosphorus recovery, with maximum recovery being 53% at 350 C and 5 min. Our results indicate that the reaction condition of 350 C for 5 min was optimal for maximizing energy recovery with improved quality, at the same time achieving a high phosphorus recovery. © 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Introduction Fossil fuels are a finite resource with limited reserves [1]. Although fossil fuels are continually formed via natural processes, they are considered to be non-renewable resources since they take millions of years to form and the known viable reserves are being depleted faster than new ones being generated [2]. Environmental controversy surrounds the use of fossil fuels as energy sources. The burning of fossil fuels produces approximately 21.3 billion tons of carbon dioxide (a greenhouse gas) per year [3], thereby increasing radiative forcing and contributing to global warming. A global movement towards the generation of renewable energy is underway to help replace fossil fuels and reduce global greenhouse gas emissions. Biofuels are derived from biomass such as plants, photosynthetic bacteria and microalgae which can be regenerated every year. The biofuels are carbon neutral due to the ability of biomass to fix carbon dioxide during growth. Biofuels derived from sugar crop and vegetable oils are being produced on a commercial scale. However, the first generation biofuels use land-based-feedstock
* Corresponding author. E-mail address: [email protected] (C.-G. Lee). The first two authors contributed equally to this work.
1
which compete with agricultural crops. Such feedstock may also be harmful to natural ecosystems due to the requirement of large cultivation areas. In order to minimize these problem, there is a proposal to derive biofuels from microalgae. Microalgae are a diverse group of photosynthetic eukaryotes with simple cell structures ranging from single cell to multicellular forms. They have the ability to capture carbon dioxide and convert the energy of sunlight to chemical energy. Furthermore, microalgae have higher productivity than terrestrial crops [4] and a very short harvesting cycle [5,6]. Recent attention has focused on microalgae for the production of liquid biofuels for power generation and transportation [7–10]. Algae conversion to bio-fuels is typically performed via bio- and thermo-chemical methods such as fermentation [11,12], transesterification of lipids [13–15], fast pyrolysis [16,17], and hydrothermal liquefaction [18–22] (HTL). Of these techniques, HTL is considered one of the most promising for wet algal biomass conversion. Since HTL applies subcritical water (200–350 C, 5–20 MPa) to promote biomass decomposition and reformation [23], the reaction avoids the heat of vaporization at elevated temperatures and converts both proteins and carbohydrates into a bio-oil with a higher energy density, in comparison to the raw biomass [24]. Therefore, this thermo-chemical conversion process needs no energy consuming drying step [25] and has a higher bio-oil yield [26]. In addition,
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suitable dilution enables the recycling of the nutrient rich HTL aqueous phase to the growth media for cultivation of microalgae [27]. Compared to a conventional catalytic process, the reaction parameters of a non-catalytic HTL significantly affect the reaction rate and reaction pathway due to the relatively severe reaction conditions, including higher temperature, pressure, and molar ratio of water. Thus, to determine the optimal reaction conditions, biocrude production from microalgae using subcritical water requires a specific study on the effects of varying reaction parameters. Additionally, reaction conditions for biocrude production via the HTL process is also dependent on the type of feedstock. Chlorella vulgaris has a fast growth rate and high biomass productivity and can easily be cultured using an inexpensive nutrient regime. In this study, the hydrothermal liquefaction of C. vulgaris was performed at different reaction temperatures and times. This was followed by a detailed investigation on the effect of reaction temperature and holding time on the HTL for biocrude production and recovery of inorganic nutrients. Numerous studies on HTL processes of microalgae have been investigated optimum reaction conditions for maximizing energy recovery and nutrient recovery separately. However, few researches have been reported to determine the reaction conditions that maximize the recovery of energy and nutrients (especially phosphorus) simultaneously. This study optimized the operating conditions of HTL for the maximum recovery of energy and nutrients, and also assessed the resultant fuel quality. Experimental C. vulgaris (Daesang Co., Ltd.) was the feedstock for hydrothermal liquefaction. The elemental and biochemical analyses of microalgae biomass was evaluated prior to hydrothermal liquefaction. The CHON composition and phosphorous content were determined using an elemental analyzer (Thermo EA1112, Thermo Electron Corp) and inductively coupled plasma optical emission spectroscopy (OPTIMA 7300DV, PerkinElmer), respectively. The
Table 1 Elemental and biochemical composition of Chlorella vulgaris and its HHV (wt% dry basis). Elemental composition (%) C 48.5 Biochemical composition (%) Lipid 18.7 Higher heating value (MJ/kg) 20.2 a,b
H 7.0 Protein 54.0
N 8.5
S 0.2
Oa 35.0
Carbohydrateb 24.3
Ash 3.0
P 0.8
Calculated by difference.
oxygen content was calculated by difference from the sum of carbon, hydrogen, nitrogen and ash contents. To estimate the ash content, 100 mg of sample was incinerated at 575 C for 12 h using a muffle furnace. Lipid content was determined by the Folch method [28] and protein content was calculated by multiplying the nitrogen content of biomass and nitrogen-to-protein conversion factor of 6.35 [29,30]. Carbohydrate content was calculated by difference. The higher heating value (HHV) was also estimated using data from the elemental composition in the following equation proposed by Channiwala and Parikh [31]. MJ O ¼ 0:3383 C þ 1:422 H þ 0:0942 S HHV kg 8 where C, H, O and S are the mass percentage of carbon, hydrogen, oxygen and sulfur, respectively. The element and biochemical composition of C. vulgaris and its higher heating value are presented in Table 1. Hydrothermal liquefaction of C. vulgaris was performed in a stainless-steel batch-type tubular reactor of 10 ml volume. Heating the reactor was achieved by using a molten salt comprising KNO3 and NaNO3. The reactor was loaded with 5 grams of wet algae containing 80% (w/w) water. The reactor was then immersed in the molten salt bath at varying temperatures of 300, 325 and 350 C [19]. After reaching the prescribed temperature, the reaction continued for a set period of pre-set duration (5, 8, 10 and 30 min) [21,22]. On conclusion, the reactor was cooled in a water bath to
Fig. 1. Flow diagram for separation and recovery of HTL products.
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terminate the reaction. After cooling, the reactor was opened and the reduced weight was measured to quantify the yield of gas phase. Addition of 5 ml chloroform facilitated removal of the other
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products (biocrude, aqueous phase, and solid residue) from the reactor, and filtration achieved separation of the solid residue. Layer separation was then performed to separate the biocrude
Fig. 2. Influence of the reaction temperature and time on the HTL product yields: (a) 300 C, (b) 325 C, (c) 350 C.
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(chloroform phase) and aqueous phase. The yields of recovered products were calculated using the following equation: Weight of each productðgÞ Weight of dry microalgae f eedstock ðgÞ 100
Yield of productsð%Þ ¼
A flow diagram of separation and the recovery procedure of HTL products is presented in Fig. 1. The biocrude was dissolved in chloroform and flushed for 24 h with nitrogen to remove the solvent and residual water. Dark colored biocrude samples with
high viscosity were obtained. Each biocrude sample was analyzed for elemental composition and HHV. The energy recovery (ER) of biocrude was calculated using the following equation: Energy recoveryð%Þ ¼
HHV biocrude Y biocrude HHV f eedstock
where HHVfeedstock is the higher heating value of feedstock, HHVbiocrude is the higher heating value of biocrude, and Ybiocrude is mass fraction of biocrude.
Fig. 3. Elemental balance of the experiments performed at various temperature and time in this study: (a) phosphorus balance, (b) nitrogen balance.
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Fig. 4. Effect of reaction temperature and time on the energy recovery of biocrude.
The aqueous phase concentrations of ammonium (NH4+), phosphate (PO43), nitrate (NO3) and nitrite (NO2) were measured using a water quality analyzer (QuikChem 8500 Series 2, Lachat Instruments, USA). Nitrogen recovery (NR) and phosphorus recovery (PR) were calculated using the following equations: Nitrogen recoveryð%Þ ¼
amount of nitrogen in aqueous phase amount of nitrogen in f eedstock
Phosphorus recoveryð%Þ ¼
amont of phosphorus in aqueous phase amount of phosphorus in f eedstock
Results and discussion The effects of reaction temperature and reaction time on the HTL of C. vulgaris using subcritical water were studied by varying the temperature (300–350 C) and time (5–30 min) at a fixed ratio (w/w) of water:microalgae of 4. The mass fraction of products acquired from each variable reaction temperature and time is presented in Fig. 2. The biocrude yields significantly exceeded the lipid content of the microalgae feedstock under all reaction conditions. At 300 C, the biocrude
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yield was augmented as the reaction proceeded, and the highest yield of 49% was obtained at a reaction time of 10 min, after which a slight decrease was observed. At 325 C, the biocrude yield increased up to 47% at 8 min and then decreased. At 350 C, the highest biocrude yield of 46% was obtained at 5 min. We observed that biocrude yields increased up to maximum levels and gradually decreased with increasing reaction time at each reaction temperature. With increasing reaction temperature, the maximum yield was obtained in a shorter time due to the enhanced reaction at a higher temperature. Unlike biocrude, the yield of gas phase increases with increasing temperature and time. As the reaction time and temperature increases, some of the biocrude may be thermally converted to smaller gaseous molecules, resulting in the increase in the gas phase yields as well as the decrease in the biocrude yield. Fig. 3 shows the phase distribution of nitrogen and phosphorus depending on the reaction temperature and time. Nitrogen was partitioned into the aqueous phase and biocrude, and phosphorus into the aqueous phase, biocrude, and solid phase. A higher reaction temperature and a longer reaction time generate a less amount of phosphorus in the aqueous phase. A higher temperature and a longer time conditions turn phosphate into precipitates by combining PO43 with divalent cations [18,19]. On the other hand, more nitrogen tends to move to the aqueous phase as the temperature and reaction time increase, because nitrogen in both the biocrude and the solid phase dissolve into the aqueous phase over time. It can also be seen that this phenomenon occurs faster under high temperature conditions. These results can be attributed to the decomposition of the amine compounds in the biocrude and solids into ammonia through the deamination [20,32]. The primary goal of the hydrothermal liquefaction process is to produce the biocrude; hence, energy recovery and the quality of biocrude are important. Considering energy recovery, each temperature condition has an optimal reaction time for energy recovery. As shown in Fig. 4, maximum energy recovery was about 78% at the reaction time of 10 min at 300 C, 8 min at 325 C, and 5 min at 350 C. The quality of biocrude is strongly associated with HHV, and the oxygen and nitrogen contents. Presence of oxygenated and nitrogenized compounds in the biocrude adversely affect the quality by interfering with the storage stability and catalytic upgrading [33]. In addition, combustion of high N content biocrude emits high NOx [34]. The elemental composition and HHV of biocrudes obtained at different reaction conditions are presented in Table 2. As shown in Table 2, the N and O contents of biocrude decreased with increasing reaction temperature and time, and higher HHV was observed at higher reaction temperature. The HHV of biocrude obtained at various reaction conditions were 32.6–36.2 MJ/kg, which is substantially higher than that of the original feedstock (20.2 MJ/kg). It is generally believed that during HTL reactions, oxygen is recovered as H2O and CO2 through a series of dehydration and decarboxylation [34] and nitrogen is
Table 2 Elemental composition and HHV of biocrude obtained at different reaction conditions. Temp. ( C)
300
325
350
Time (min)
5 10 30 5 8 10 30 5 10 30
Elemental composition (%)
HHV (MJ/kg)
C
H
N
S
68.73 (1.17) 68.18 (0.11) 68.28 (0.74) 68.80 (2.97) 69.31 (1.71) 69.62 (1.16) 71.02 (0.39) 70.70 (2.12) 71.84 (1.33) 73.22 (1.02)
8.37 (0.15) 8.55 (0.23) 8.68 (0.06) 8.60 (0.23) 8.75 (0.36) 8.83 (0.39) 9.00 (0.21) 8.88 (0.51) 9.06 (0.40) 9.28 (0.36)
7.84 (0.07) 7.51 (0.07) 7.26 (0.31) 7.68 (0.07) 7.61 (0.14) 7.48 (0.23) 7.20 (0.28) 7.44 (0.04) 7.45 (0.30) 7.03 (0.22)
0.39 0.33 0.27 0.26 0.27 0.31 0.32 0.34 0.37 0.33
O (0.55) (0.18) (0.13) (0.21) (0.18) (0.13) (0.17) (0.18) (0.18) (0.16)
14.67 (0.53) 15.43 (0.11) 15.51 (1.12) 14.66 (2.93) 14.05 (1.75) 13.77 (1.19) 12.47 (0.64) 12.64 (2.40) 11.28 (1.24) 10.14 (0.99)
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32.6 32.5 32.7 32.9 33.4 33.7 34.6 34.3 35.2 36.2
(0.2) (0.4) (0.4) (1.8) (1.4) (1.1) (0.0) (1.8) (1.2) (1.0)
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Fig. 5. Van Krevelen diagram (a) and H/C and N/C ratios (b) for different microalgae feedstock and corresponding biocrude.
removed as NH3 by deamination reactions [35], leading to decrease in oxygen and nitrogen content within the biocrude. Considering Table 2, it is observed that longer reaction temperatures and times are desirable for dehydration, decarboxylation, and deamination reactions, and are thus more favorable for improved quality of biocrude. The highest HHV of 36.2 MJ/kg and the lowest contents of nitrogen (7.03%) and oxygen (10.14%) were obtained at 350 C, 30 min. However, long holding times may be economically unfavorable for biocrude production in terms of energy consumption. Among the reaction conditions for maximum energy recovery, the highest HHV of 34.3 MJ/kg with low nitrogen and oxygen contents was observed at 350 C, 5 min. Hence, 350 C and 5 min was selected as the appropriate reaction condition in terms of energy recovery and quality of biocrude. Fig. 5 demonstrates the H/C ratios according to N/C and O/C ratios for different microalgae feedstock and corresponding biocrude. N/C and O/C are significantly reduced during the conversion of C. vulgaris into biocrude through HTL. As shown in Fig. 5(a), the H/C and N/C ratios obtained in the current study was similar to those of previous studies [33,36,37] at optimal reaction
conditions. Fig. 5(b) classifies the various microalgae feedstock and biocrude obtained from different reaction conditions based on the Van Krevelen diagram. The Van Krevelen diagram indicates that the biocrude produced in this study also exhibits close similarities for the H/C and O/C ratios to other microalgal biocrudes [33,36,37], suggesting that C. vulgaris is a potential feedstock for the production of biocrude. In the hydrothermal liquefaction process, the nutrient-rich and carbon-containing aqueous phase (the second largest fraction of HTL products) can be recovered and reused for microalgal cultivation [23]. The aqueous byproducts from microalgae HTL were often found to be high in nitrogen, phosphorus and potassium, along with other minerals and essential micronutrients required for microalgae growth [27,38]. Hence, recovery of these nutrients could be beneficial from a techno-economic perspective. The effect of the reaction temperature and time on HTL was investigated to determine optimum conditions for the recovery of inorganic nutrients. The nutrient recovery was calculated by measuring ammonium (NH4+), phosphate (PO43), nitrate (NO3) and nitrite (NO2) concentrations in the aqueous phase. Nitrate
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times are economically favorable for biocrude production in terms of energy consumption, and for high phosphorus recovery. The reaction conditions of 350 C for 5 min was found to be most suitable for maximizing energy recovery (78%) with improved quality, and at the same time achieving a high phosphorus recovery (53%). This study further confirms that C. vulgaris is a promising feedstock for biocrude production. Acknowledgements This research was supported by a grant from the Marine Biotechnology Program (PJT200255, Development of Marine Microalgal Biofuel Production Technology) funded by the Ministry of Oceans and Fisheries, Korea. References
Fig. 6. Nutrient recovery in aqueous phase: (a) nitrogen recovery and (b) phosphorous recovery.
and nitrite were not detected in all conditions. As presented in Fig. 6, the nitrogen recovery was augmented with increasing temperature and reaction time. The reaction conditions that showed lowest nitrogen recovery of 12% was 300 C, 5 min, and the highest nitrogen recovery of 27% was obtained at 350 C for 30 min. These observations are in accordance with the results of previous studies [36], where both longer reaction times and higher temperatures resulted in increased nitrogen content in the aqueous phase. There is a trade-off relation between energy recovery and nitrogen recovery, which may be due to the fact that amino acids converted from algae proteins are further hydrolyzed to form ammonia [36]. On the other hand, phosphorus recovery was higher at lower temperature and shorter reaction time. The lowest phosphorus recovery was 27% at 350 C, 30 min and the highest phosphorus recovery was 54% at 300 C, 5 min. Since phosphorus is obtained from finite mineral resources [38], we focused more on the recovery of phosphorus than nitrogen. Thus, we selected 300 C and 5 min as the optimal condition for nutrient recovery. Based on these results, we consider the reaction conditions of 350 C for 5 min to be the most suitable for maximizing energy recovery with improved quality, while at the same time achieving high phosphorus recovery. Conclusions This study performed the hydrothermal liquefaction of C. vulgaris at varying reaction temperatures and times. We investigated the effect of reaction temperature and time on the HTL to determine optimum conditions for the recovery of energy and inorganic nutrients. Our results indicate that shorter reaction
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