Hydrothermal Liquefaction of Microalgae in an Ethanol–Water Co

Jun 30, 2014 - In this paper, hydrothermal liquefaction (HTL) of microalgae Chlorella pyrenoidosa was conducted for biocrude oil production...
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Hydrothermal Liquefaction of Microalgae in an Ethanol−Water CoSolvent To Produce Biocrude Oil Jixiang Zhang*,†,‡ and Yuanhui Zhang‡ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China Department of Agricultural and Biological Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States

‡

ABSTRACT: In this paper, hydrothermal liquefaction (HTL) of microalgae Chlorella pyrenoidosa was conducted for biocrude oil production. The ethanol−water co-solvent was introduced to take advantage of the special properties of supercritical ethanol and hot compressed water. The effects of the ethanol/water mass ratio (R) on the product distribution and characterization were discussed. The co-solvent showed better performance over any constituent mono-solvent, indicating synergistic effects of ethanol and water. The highest biocrude oil yield of 57.3% and the lowest solid residue yield of 9.4% were achieved at R = 5:2. The loading factor was considered as another influencing factor under supercritical conditions. Furthermore, potential reaction pathways for major biomacromolecule monomers in the ethanol−water co-solvent were also proposed on the basis of the gas chromatography−mass spectrometry (GC−MS) analysis of the liquid products.

1. INTRODUCTION Microalgae are considered as a more favorable feedstock for next-generation biofuels. Because nutrients for microalgae cultivation can be obtained from wastewater and CO2 can be fed from external sources, such as power plants, the synergistic coupling of microalgae propagation with carbon sequestration and wastewater treatment potential for mitigation of environmental impacts associated with energy conversion and use can be achieved.1 Hydrothermal liquefaction (HTL) converts biomass feedstock into biocrude oil in hot compressed water.2,3 In comparison to a traditional biodiesel production process, HTL dramatically reduces the energy input for dewatering and drying.4,5 Furthermore, it uses the whole alga cell rather than the lipid content only and, therefore, is ideal for processing low-lipid-content microalgae with a tremendous amount of water. To enhance the liquefaction reaction, water is sometimes replaced by reactive organic solvents, among which ethanol has attracted much attention, and the process is so-called solvolysis.6 Ethanol enjoys several advantages over water: it could (1) react with acidic components to produce diesel-like products, (2) dissolve high-molecular-weight liquid products to prevent coke formation, (3) stabilize the free radicals to reduce the repolymerization reaction because of its hydrogen donor capability, and (4) provide milder reaction conditions because its critical temperature and pressure are much lower.7 It has been reported that ethanol is one of the most suitable solvents to convert high-protein-content (Spirulina8,9 and Chlorella pyrenoidosa10,11), high-carbohydrate-content (Epiactis prolifera12), and low-lipid-content (Nannochloropsis sp.13) microalgae into biocrude oil with and without catalysts. However, processing with pure ethanol and dry alga increases the energy input and makes it difficult to recycle the nutrients. If the biomass feedstock is not completely dried, then the conversion will be actually processed in ethanol−water solutions. Moreover, in one of our previous work,14 a rare © 2014 American Chemical Society

phenomenon was reported that using pure ethanol as the solvent caused a significant increase in solid residue yields under high loading factor conditions (>0.6 g/mL; over 60 g of feedstocks with 30% of dry biomass were loaded in a 100 mL batch reactor). As a subsequent research, this study aims to apply an ethanol−water co-solvent to the HTL of microalgae, to explore the influences of the ethanol/water mass ratio (R) on the product distribution and characterization, to solve the pre-described problem and to gain further insights into the presented process.

2. EXPERIMENTAL SECTION 2.1. Materials. Microalgae C. pyrenoidosa with the physical form of dry and fine powder was obtained as commercial-food-grade material. Its characteristics are shown in Table 1. The macromolecular and chemical compositions were analyzed according to the standard

Table 1. Characteristics of C. pyrenoidosa Chemical Composition (Dry Basis, %) crude fat

crude protein

0.1 hemicellulose

71.3

0.5

lignin

0.3 non-fibrous carbohydratea

0.2 22.0 Elemental Composition (Dry Basis, %)

C

H

51.4

a

cellulose

Oa

N

6.6 11.1 Properties (Dry Basis, %)

30.9

volatile solida

ash

94.4

5.6

Calculated by difference.

Received: May 6, 2014 Revised: June 21, 2014 Published: June 30, 2014 5178

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Figure 1. Schematic diagram of the HTL system. 0.1109O. The energy yields of biocrude oil were calculated with and without the consideration of ethanol.

methods of the AOAC International. The ash content was measure as the combustion residue at 550 °C. Reagent-grade ethanol and acetone were purchased from Fisher Scientific; high-purity nitrogen was purchased from S.J. Smith Co.; and all chemicals were used as received. 2.2. HTL Experiments. HTL experiments were conducted in three 100 mL batch reactors according to the previously reported method14 at 280 °C for 120 min with 0.69 MPa of initial nitrogen pressure. The schematic diagram of the HTL system is presented in Figure 1. Ethanol−water co-solvents of different R values (0:7, 2:5, 1:1, 5:2, and 7:2) were mixed with C. pyrenoidosa to make a 60 g slurry feedstock containing 30% dry biomass for each experiment. Duplicate or triplicate runs were performed for every experimental condition. Average values and standard deviations were reported. 2.3. Product Separation and Analysis. The product separation was carried out according to the previously reported method,14 as shown in Figure 2. HTL products were separated into gas, biocrude

EY‐1 =

EY‐2 =

0.3HHVbiocrude oil × yield biocrude oil R

0.3HHValga + 0.7 R + 1 HHVethanol

HHVbiocrude oil × yield biocrude oil HHValga

Energy return on investment (EROI) of the presented process was defined as

EROI = =

energy recovered from biocrude oil energy required to heat the feedstock HHV × yield biocrude oil

( R +1 1 Δhw + R R+ 1 Δhe)

0.3ΔTc p,a + 0.7

where the specific enthalpy changes of water and ethanol from 20 °C/ 0.1 MPa to 280 °C/10.0 MPa (Δhw and Δhe) were taken to be 1151 kJ kg−116 and 1093 kJ kg−1,17 respectively, and the average specific heats of dry alga at a constant pressure (cp,a) were taken to be 1.25 kJ kg−1 K−1.18

3. RESULTS AND DISCUSSION 3.1. Effects of the Ethanol−Water Co-solvent. The effects of the ethanol−water co-solvent on the product yields are illustrated in Figure 3a. The gas yields decreased with increasing R. The solid residue yields first fell from 14.9% to a minimum of 9.42% at R = 5:2 and then rose significantly to 17.4% at R = 7:0. The biocrude oil yields showed an opposite trend, first increasing to a maximum of 57.3% and then declining to 50.0%. The effects of the ethanol−water co-solvent on the elemental composition of biocrude oil are presented in Table 2. Increasing R was more favorable in terms of deoxygenation, while reducing R facilitated denitrification and resulted in products with higher H/C ratios. The HHV of biocrude oil were in the range of 31.4−32.2 MJ/kg. The EROI and the energy yields of biocrude oil of the presented process are shown in Figure 3b. All experiments using co-solvent had higher biocrude yields and energy yields (EY-2) than any constituent mono-solvent, indicating synergistic effects of ethanol and water. EROI was in the range of 5.1−8.9 and greatly improved with the increase of R. It means that the

Figure 2. Product separation procedure. oil, solid residue, and volatile components. Volatile components evaporated along with the solvent during the drying process and were not collected. The chemical composition of the gas and filtrate were analyzed by gas chromatography (GC) and gas chromatography−mass spectrometry (GC−MS), respectively. The elemental composition of the biocrude oil was determined using a CE 440 elemental analyzer. For detailed operation parameters, see ref 14. The yields of gas, solid residue, and biocrude oil, all expressed in mass percentage, were calculated on the dry basis. The higher heating values (HHV, MJ/kg) of alga and biocrude oil were calculated using the Boie formula:15 HHV = 0.3516C + 1.16225H + 0.0628N − 5179

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Figure 3. Effects of the ethanol−water co-solvent on (a) product distribution and (b) EROI and energy yields of biocrude oil with (EY-1) and without (EY-2) consideration of ethanol.

Table 2. Effects of the Ethanol−Water Co-solvent on the Elemental Composition of Biocrude Oil R 0:7 2:5 1:1 5:2 7:0 a

C 67.20 67.54 67.40 67.80 68.65

± ± ± ± ±

H 0.63 0.40 0.18 1.48 0.59

7.91 7.65 7.96 7.65 7.63

± ± ± ± ±

Oa

N 0.11 0.14 0.03 0.33 0.21

8.54 8.94 9.40 10.24 10.35

± ± ± ± ±

0.04 0.12 0.11 0.09 0.06

16.34 15.87 15.24 14.31 13.38

± ± ± ± ±

0.68 0.37 0.07 1.85 0.74

HHV

H/C

O/C

N/C

31.5 31.4 31.8 31.8 32.2

1.41 1.36 1.42 1.35 1.33

0.18 0.18 0.17 0.16 0.15

0.11 0.11 0.12 0.13 0.13

Calculated by difference.

presented process is a net energy producer. For the HTL process only, pure ethanol is recommended from the standpoint of energy consumption, while the ethanol−water co-solvent of R = 5:2 is favorable for maximizing biocrude oil yields. The energy yields (EY-1) decreased with increasing R mainly because the unreacted ethanol and the volatile components produced were not collected for further application in this study. It is reasonable to anticipate a higher energy yield of the presented process with integration of solvent recovery and hydrogen production from volatile components via aqueous-phase reforming. Some other works on HTL of various feedstocks using ethanol−water reported similar synergistic effects of the cosolvent and proposed respective optimized concentrations.19−25 Wu et al.19 found that the conversion and biocrude oil yield of Dunaliella tertiolecta peaked with the 40% (v/v) ethanol−water solution (at 320 °C) and were greatly reduced as the ethanol content further increased. Yuan et al.20 indicated that the maximum biocrude oil yield of rice straw was achieved with 50% (v/v) ethanol−water solution (at 300 °C), and the HHV of biocrude oil increased with the ethanol concentration. Xu et al.21 showed that 50% (w/w) ethanol−water solution was a much more effective solvent than any constituent mono-solvent for liquefaction of sawdust (at 300 °C). As discussed in the literature, in addition to the advantages brought by ethanol, the special properties of hot compressed (subcritical) water also contribute to the presented synergistic effects. Hot compressed water has stronger acidity (kw ≈ 10−11) over ethanol at 200− 350 °C,26 and the addition of water would thus enhance the hydrolysis/solvolysis reaction.25 Another possible explanation was proposed in this paper. When processing in the subcritical state (with the system pressure higher than the saturated pressure to avoid boiling), the diffusion and reaction of the free radicals were, hence,

limited in the liquid phase; the loading factor could not be a major influencing factor because the volume of the liquid phase proportionally changed with it. However, in the case of the supercritical state, a single-phase reaction environment was formed in the constant volume of the reactor. The concentration of free radicals increased at higher loading factors; therefore, the polymerization was promoted, and the solid residue yields remarkably increased. The critical values of the ethanol−water co-solvent calculated by the CHEMCAD program20 and the reaction conditions are shown in Figure 4. Only HTL with pure ethanol (R = 7:0) was conducted at supercritical conditions, and the solid residue yields were significantly higher compared to sub- or near-critical conditions (R = 5:2). This explanation is also supported by our previous study,14 in which the HTL product distribution showed

Figure 4. Critical values (Tc and Pc) of the ethanol−water co-solvent and the reaction conditions (T and P). 5180

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Figure 5. Potential reaction pathways for (a) amino acids, (b) glucose, and (c) fatty acids in the ethanol−water co-solvent.

Table 3. Major Components of the Liquid Products peak area (%)a component group/name ketones 3-penten-2-one, 4-methyl2-pentanone, 4-hydroxy-4-methyl2-pentanone, 4-amino-4-methyl2-cyclopenten-1-one, 2,3-dimethylesters hexadecanoic acid, ethyl ester octadecanoic acid, ethyl ester linoleic acid, ethyl ester nitrogen-containing heterocyclic compounds 1H-pyrrole, 3-ethyl-2,4,5-trimethylpyrazine, methyl4-piperidinone, 2,2,6,6-tetramethylpyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro-3-(phenylmethyl)hydrocarbons cyclopentene, 1-hexyl(2E)-3,7,11,15-tetramethyl-2-hexadecene a

R = 5:2

R = 1:1

R = 2:5

41.95 18.08 21.24 1.44 1.19 28.45 13.64 1.76 7.86 24.66 5.68 1.15 12.83

61.77 32.24 28.55

58.69 27.56 29.73

0.98 17.18 8.07 1.15 6.03 19.68 5.47 2.43 5.41 1.15 0.88 0.27 0.44

1.40 7.27 3.62 0.42 2.27 30.14 7.02 1.89 16.35

3.61 2.47 1.03

2.97 1.90 0.91

Other components were not presented but included in the group total area.

factors. The impacts of the loading factor (or solvent filling factor) were rarely discussed because they were usually fixed or investigated in the name of the biomass/solvent ratio in the literature. 3.2. Reaction Pathways for Major Biomacromolecule Monomers in the Ethanol−Water Co-solvent. The potential liquefaction mechanisms for alga in hot compressed

insensitiveness to the loading factor under subcritical water conditions, but the solid residue yields increased at high loading factors when using ethanol as the solvent. In the co-liquefaction of microalgae and synthetic polymer mixture, Yuan et al.27 reported that supercritical ethanol conditions were more favorable at lower solvent filling factors but the solid residue yields exhibited an analogous increase at higher solvent filling 5181

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water28 and supercritical ethanol11 were discussed in our previous works: biomacromolecules first broke down into the corresponding monomers under hydrolysis/solvolysis conditions; then further decomposition of the monomers produced various types of intermediates; and finally, the monomers and intermediates went through a series of reactions to form gas, liquid, and solid products. Therefore, the reaction pathways of biomacromolecules are important to provide further insights into the liquefaction mechanism. Peterson et al.26 reviewed the reactions of biological molecules in hydrothermal systems mainly focused on the chemistry behind individual biochemicals. In this study, potential reaction pathways for major biomacromolecule monomers related to the ethanol−water system were proposed on the basis of the GC−MS analysis of the filtrate, as illustrated in Figure 5. The filtrate was a complex mixture composed of various oxygenated and nitrogenous compounds, including ketones, esters, hydrocarbons, and nitrogen-containing heterocyclic/aliphatic compounds. The major components (>1% peak area) of the liquid products are listed in Table 3. It is obvious that increasing R promoted the esterification reaction and resulted in a higher ester content. The peptide bonds between amino acids, the building blocks of proteins, rapidly hydrolyzed in hydrothermal systems. Amino acids subsequently degraded, as shown in Figure 5a, underwent decarboxylation and deamination reactions to form corresponding acids and amines/ammonia. The acids reacted with ethanol through esterification to form C4−C11 esters, which were found abundant in the liquid products. Meanwhile, short-chain ketones (e.g., 2-pentanone, 4-hydroxy-4-methyl-, from which 2-pentanone, 4-amino-4-methyl-, and 3-penten-2-one, 4-methyl-, could be formed through substitution and dehydration, respectively) could be derived from the cleavage of the benzene derivatives,29 such as the aromatic side chain of amino acids. Starches were easily hydrolyzed in hydrothermal conditions to produce glucose monomers. As illustrated in Figure 5b, glucose further decomposed under elevated temperatures, forming furfural derivatives and C1−C2 carboxylic acids.26 The acids finally formed esters through esterification. Furfural derivatives (e.g., hydroxymethylfurfural) could be converted to cyclic ketones (e.g., cyclopentanone, 2-methyl-, and 2-cyclopenten-1-one, 2,3-dimethyl-).30 Furfural derivatives also underwent condensation/polymerization with phenols or themselves, forming polymers and finally coke. Another pathway of aldose was to react with amino acids through the Maillard reaction, forming nitrogen-containing heterocyclic compounds, including pyrazines, pyridines, pyrroles, and oxazoles. Ketones, esters, and nitrogen-containing heterocyclic compounds became dominant in the liquid products because C. pyrenoidosa was abundant in crude protein and non-fibrous carbohydrates. Amines (e.g., nheptylamine) also reacted with aldose, forming nitrogencontaining aliphatic compounds [e.g., D-glucitol, 1-deoxy-1(heptylamino)-]. Another part of the nitrogen-containing aliphatic compounds was derived from the reaction of fatty acids and amines/ ammonia. In hydrothermal conditions, triacylglycerides (TAGs), the most common form of lipid in biological systems, were split to form glycerol and free fatty acids. Fatty acids also contributed to the formation of hydrocarbons and fatty acid esters (C18−C20), as presented in Figure 5c.

Article

CONCLUSION In this work, the ethanol−water co-solvent was introduced to the HTL process of microalgae to produce biocrude oil. Synergistic effects of ethanol and water were confirmed because all experiments using the co-solvent had higher biocrude yields and lower solid residue yields than any constituent monosolvent. The optimal ethanol/water mass ratio was R = 5:2 from the standpoint of product distribution. Besides the explanation in the existing literature, the high loading factor was considered as another influencing factor under supercritical ethanol conditions. The potential reaction pathways for amino acids, glucose, and fatty acids in the ethanol−water co-solvent were proposed to gain further insights into the liquefaction mechanism. It revealed the transformation and separation of oxygen and nitrogen during the HTL process and provided useful information for developing efficient deoxygenation and denitrification methods.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-89733939. E-mail: zhangjixiang.zju@ gmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project is financially supported by the Science Foundation of China University of Petroleum, Beijing, China (2462013YJRC022). The authors appreciate the University of Illinois for providing the experiment facilities and supplies for the research. Acknowledgment also goes to fellow graduate students and researchers in the Bioenvironmental Engineering Division at the University of Illinois at Urbana−Champaign for their assistance during the project.

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