Research Article pubs.acs.org/journal/ascecg
Optimum Utilization of Biochemical Components in Chlorella sp. KR1 via Subcritical Hydrothermal Liquefaction Mingyu Jin,† You-Kwan Oh,‡ Yong Keun Chang,*,†,§ and Minkee Choi*,† †
Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡ Department of Clean Fuel, Korea Institute of Energy Research, Daejeon 34101, Republic of Korea § Advanced Biomass R&D Center, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *
ABSTRACT: Product distributions in bio-crude, aqueous phase, and solid residue were rigorously analyzed during the hydrothermal liquefaction (HTL) of Chlorella sp. KR1 in order to optimize utilization of energy and chemicals. A nonasphaltene (paraffinic) fraction in the bio-crude, which can be readily upgraded to high-quality fuels via a subsequent catalytic process, was mainly produced due to lipid extraction. Above 170 °C, lipid extraction was almost complete, and hence, the non-asphaltene content did not increase further with increasing temperature. Carbohydrates could be extracted, mainly as polysaccharides, in the aqueous phase at mild temperatures (200 °C), they decompose and react with proteins via the Maillard reaction to form asphaltene (polycyclic aromatics), which contains large amounts of heteroatoms such as N and S. Although high-temperature carbohydrate conversion could yield more bio-crude with high energy values, it dominantly contributed to formation of the asphaltene fraction, which is difficult to upgrade catalytically. As high-temperature HTL requires a large energy input, the recovery and utilization of intact carbohydrates and proteins at mild temperatures (300 °C) increases the overall bio-crude yield but mainly contributes to the increase of the asphaltene fraction (polycyclic aromatic compounds), which contains a high concentration of heteroatoms such as N and S.35 The asphaltene fraction is a low quality fuel and difficult to convert via subsequent catalytic processes. Due to the increased energy input required for elevating the reaction temperature, the selective increase in asphaltene content may not be beneficial in terms of energy balance and overall economics. Several studies also proposed recycling the aqueous phase that contains various carbon and N/P sources as biomass cultivation media36 or separating value-added polysaccharides.37,38 If the HTL temperature is unnecessarily high, however, the valuable carbohydrates in the aqueous phase can be converted to asphaltene or other toxic compounds, deteriorating the overall economic viability of HTL process. With these regards, the optimum HTL temperature should be chosen for maximizing the overall energy balance and the entire value of products in all different phases including the biocrude, aqueous phase, and solid residue. To the best of our knowledge, however, there have been limited studies that carefully considered the product distributions in all phases and the overall energy balance as a function of HTL temperature. In the present work, we analyzed product distributions in the biocrude (e.g., non-asphaltene and asphaltene), aqueous phase (carbohydrates and various water-soluble organics), and solid residue during the HTL of Chlorella sp. KR1 at different temperatures. Energy Return on Investment (EROI) analysis was also carried out as a function of the reaction temperature. The present study demonstrates that the mild HTL temperature of 170 °C is the most desired in terms of EROI and due to the possibilities of recycling carbohydrates, proteins, and N/ P sources.
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Scheme 1. Product Separation Processes after Hydrothermal Liquefaction (HTL) of Chlorella sp. KR1
mixed with 40 mL of dichloromethane (DCM) and 17 mL of distilled water and stirred for 12 h. At this step, bio-crude is dissolved in a DCM phase, while water-soluble organic species (e.g., carbohydrates) are dissolved in an aqueous phase. Solid residue was first collected by filtration and dried at 80 °C overnight. The filtrate liquid mixture was phase-separated by centrifugation at 6000 rpm. The DCM phase was evaporated under a vacuum at room temperature overnight to collect
EXPERIMENTAL SECTION
Biochemical Composition Analysis of Chlorella sp. KR1. Chlorella sp. KR1 was supplied by KIER (Daejeon, Korea). A modified B
DOI: 10.1021/acssuschemeng.7b01473 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Table 1. Formulas for the Calculation of EROIa
the bio-crude. To separate non-asphaltene components (hexanesoluble paraffinic species) from the bio-crude,35 40 mL of n-hexane was mixed with the separated bio-crude and sonicated for 2 h. The hexane soluble species (non-asphaltene) were collected by removing undissolved solid residue (asphaltene) via filtration (CHMLAB F2040 filter paper, pore size 7−9 μm) and evaporating n-hexane via vacuum evaporation at room temperature. The yields of bio-crude, nonasphaltene, and asphaltene were calculated with respect to the dry biomass. Bio-Crude Analysis. The carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents in non-asphaltene and asphaltene fractions were analyzed using an elemental analyzer (Thermo scientific FLASH 2000), and the oxygen (O) content was calculated by the difference. Saponifiable lipid contents in asphaltene and nonasphaltene fractions were analyzed via transesterification, following the same method that was used for quantification of saponifiable lipid in the initial cell. Higher heating value (HHV) of the bio-crude was calculated following Boie’s formula.42
formula energy input (Ein) energy for heating microalgae (qm) energy for heating water (qw) internal energy of water at T (UT) mass of liquid water at T (ml,T) energy output (Eout) energy of bio-crude (EBC) energy of carbohydrate (EC) energy return on investment
qw = UTf − UTi UT = ml,T × Ul,T + (mw − ml,T) × Uv,T ml,T = (mw × vv,T − Vr)/(vv,T − vl,T) Eout = EBC + EC EBC = HHVNA × YNA × mm + HHVA × YA × mm EC = HHVC × YC × mm EROI = Eout/Ein
a
Cm and mm are the specific heat (1.25 J/g·°C)46 and the mass of microalgae, respectively. Tf is the HTL temperature, and Ti is the initial temperature (25 °C). mw is the mass of water added for HTL. vv,T and vl,T are the specific volumes of water in vapor and liquid phase at the temperature T, respectively. Vr is the volume of a reactor. HHVNA, HHVA, and HHVC are the higher heating values (HHVs) of non-asphaltene (Table 3), asphaltene (Table 3), and carbohydrate (17.32 MJ/kg), respectively. YNA, YA, and YC are the yields of nonasphaltene, asphaltene, and carbohydrate, respectively. vv,T, vl,T, Ul,T, and Uv,T can be obtained from steam tables.
HHV (MJ/kg) = 0.3517 × C + 1.1625 × H + 0.105 × S − 0.111 × O + 0.0628 × N
Ein = qm + qw qm = Cm × mm × (Tf − Ti)
(1)
Aqueous Phase Analysis. The total amount of carbohydrates in the aqueous phase was analyzed using the phenol-sulfuric acid method41 described before. Carbohydrate recovery was calculated with respect to the weight of carbohydrate in the initial dried biomass. Concentration of monosaccharides, organic acids, glycerol, and acetone were determined using high performance liquid chromatography (HPLC, Ultimate 3000, Dionex, USA) with an Aminex HPX87P column (300 × 7.8 mm, Bio-Rad, USA) and RefractoMax521 refractive index detector. The total phenol content was determined using Folin-Denis methods.43 20 μL of aqueous phase was mixed with 1.58 mL of distilled water and 100 μL of 2 N Folin-Ciocalteu’s phenol reagent. Then, 300 μL of saturated Na2CO3 solution was added to the mixture, and the resultant mixture was kept at 20 °C for 2 h. UV absorbance at 765 nm was used to determine the total phenol concentration. Gallic acid was used to obtain a standard curve. Phosphorus content was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES, SPECTRO BLUE). Total nitrogen content in the aqueous phase was measured using a Humas kit (KIT, Humas, Korea).44 Solid Residue Analysis. The elemental composition of solid residue was analyzed using an elemental analyzer (Thermo scientific FLASH 2000). Fourier transform infrared spectra (FT-IR) were collected using an ALPHA FT-IR (Bruker Optics) spectrometer in an attenuated total reflection (ATR) mode. Prior to the measurement, all samples were dried at 80 °C for 12 h. The particle morphology of solid residues was also investigated using a field emission scanning electron microscope (SEM, Magellan400) operating at 2 kV without metal coating. Calculation of Energy Return on Investment (EROI). EROI in the HTL of Chlorella sp. KR1 was calculated as a function of the reaction temperature (calculation methods are summarized in Table 1). EROI of the process can be defined as the total energy output (Eout) divided by the energy input (Ein).35 The energy input (Ein) was calculated as the sum of heat required to elevate the temperature of microalgae (qm) and water (qw) from 25 °C to the specified temperature. Heat loss and heat of reaction during HTL were ignored for simplicity. Although the heat of reaction during HTL is difficult to measure or calculate, we could obtain valuable information from earlier studies on the biomass pyrolysis because they share many common reaction pathways. Previous studies reported that the heat of reaction for pyrolysis of microalgae is 23.2−82.0 kJ/kg (dry cell basis).45 Considering that the energy consumption for raising the temperature of water in HTL is 787−1660 kJ/kg, the heat of reaction is indeed negligible. To calculate the energy required for heating microalgae (qm), the specific heat of microalgae (Cm) was assumed to be 1.25 J/ g·°C, as reported by Minowa and colleagues.46 The energy required for heating water (qw) was calculated by subtracting the specific internal energy of water at 25 °C (Ui) from that at the specified HTL
temperature (UT). For the calculation, saturation properties of steam were applied for the 42 mL (Vr) batch reactor.47 Mass of water in the liquid state (ml) was calculated using specific volumes of water in the liquid (Vl) and vapor (Vv) states. Total internal energy of water at the specified temperature (UT) was calculated by summing the specific internal energy of water in the liquid (Ul) and vapor (Uv) states. Energy output (Eout) was calculated as the sum of calorific values of bio-crude (EBC) and carbohydrate (EC) in the aqueous phase. EBC was calculated using the higher heating values (HHVs) and yields of nonasphaltene and asphaltene fractions. EC was calculated assuming that the HHV of carbohydrate is 17.32 MJ/kg.
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RESULTS AND DISCUSSION Biochemical and Elemental Analysis of Chlorella sp. KR1. Table 2 shows the biochemical and elemental compositions of Chlorella sp. KR1 used for the HTL process. The microalgae contained 36.58 wt % lipids: approximately 80% was saponifiable lipid (fatty acid derivatives) and 20% was unsaponifiable lipid (e.g., cholesterol). The microalgae also contained substantial amounts of carbohydrates (36.12 wt %)
Table 2. Biochemical and Elemental Compositions of Chlorella sp. KR1 biochemical composition lipid
36.58
saponifiable lipid unsaponifiable lipid carbohydrate protein and others ash elemental composition C H O N S HHV (MJ/kg) C
contents (wt %) 29.21 7.37 36.12 22.17 5.13 contents (wt %) 53.62 8.10 35.08 2.59 0.61 24.61 DOI: 10.1021/acssuschemeng.7b01473 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
lipids (or fatty acid derivatives), similar to the lipid composition in the microalgae (Table 2). As the HTL temperature increased above 200 °C, the non-asphaltene yield did not increase appreciably, but the yield of asphaltene gradually increased. These results validate our earlier results obtained with other microalgae that the reactions of carbohydrates and proteins at high temperatures mainly contribute to the formation of asphaltene fraction.35 The asphaltene fraction contained negligible amounts of saponifiable lipids (200 °C) can be attributed to the Maillard reaction between carbohydrates and amino acids.12,48 The reactive carbonyl group of the carbohydrate is known to react with the nucleophilic amino groups of amino acids, producing N-containing cyclic compounds such as pyridines and pyrroles via complex reaction pathways.49 The presence of S atoms in the asphaltene fraction can be attributed to the Maillard reaction between S-containing amino acids (e.g., cysteine) and carbohydrates.35 It is notable that the N contents in asphaltene fractions (3.31−4.69 wt %) are very high, as compared with those in low-quality heavy
and proteins (22.17 wt %). The HHV of dried biomass was calculated as 24.61 MJ/kg. Analysis of Bio-Crudes Obtained at Different HTL Temperatures. In Figure 1, the yields of non-asphaltene and
Figure 1. Non-asphaltene and asphaltene yields as a function of HTL temperature.
asphaltene fractions in the bio-crude are plotted as a function of the HTL temperature. The result shows that the total bio-crude yield gradually increased with the HTL temperature. At mild temperatures (
