Characterization of biocrudes recovered with and without solvent after hydrothermal liquefaction of algae

Algal Research 6 (2014) 1–7

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Characterization of biocrudes recovered with and without solvent after hydrothermal liquefaction of algae Donghai Xu a,b, Phillip E. Savage b,⁎ a b

Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi Province 710049, China Department of Chemical Engineering, University of Michigan, Ann Arbor, MI, United States

a r t i c l e

i n f o

Article history: Received 6 June 2014 Received in revised form 28 July 2014 Accepted 16 August 2014 Available online xxxx Keywords: Algae Hydrothermal liquefaction Biocrude Dichloromethane

a b s t r a c t Bench-scale, batch experiments exploring algae hydrothermal liquefaction typically use organic solvents such as dichloromethane (DCM) to recover biocrude from the reactor contents. Commercial-scale, continuous processes, however, may separate biocrude and the aqueous phase co-product without solvent use. Herein, we provide the first thorough examination of the influence of DCM extraction on biocrude yield and composition from microalgae liquefaction in hot compressed water (350 °C, 20 min). We provide gravimetric biocrude yields and biocrude characterization via elemental analysis, GC-MS, NMR, and IR spectroscopies. The application of DCM increases the biocrude yield because it extracts water-soluble compounds from the aqueous phase co-product. Indeed, the biocrude recovered from the aqueous phase accounted for about 8 wt% of the total amount of biocrude. The addition of these molecules diminishes the biocrude quality, however, by decreasing the C and H content, increasing the O and N content, and thereby decreasing the higher heating value of the biocrude. The biocrude extracted from the aqueous phase had a remarkably different elemental composition than did the biocrude recovered without solvent. The O and N contents were more than twice as high, and the higher heating value was 20% lower. The biocrudes recovered directly from hydrothermal liquefaction (e.g., without water and DCM coming into contact) were 87%–88% C and H, and possessed a higher heating value of N39 MJ/kg. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Microalgae have a much higher photosynthetic efficiency than terrestrial biomass, which leads to higher growth rates and a greater potential for CO2 fixation [1,2]. Hydrothermal liquefaction (HTL) converts wet algae paste to biocrude, thus avoiding the cost and energy consumption associated with drying the biomass [3]. In general, algae HTL occurs at 280 °C–370 °C and 10–25 MPa for 5–120 min. The process generates a biocrude as well as gaseous, aqueous, and solid phase byproducts [4]. The biocrude yield may reach 50–60 wt% d.a.f. (dry ash-free) in the presence of catalysts [5,6], but yields are more typically around 40 wt%. In a typical algae HTL experiment, after the reaction products have cooled, an organic solvent is added to dissolve the biocrude and separate it from the aqueous and solid phases [7,8]. Almost all of the algae HTL experiments in the literature apply this approach to collect the biocrude [4], and dichloromethane (DCM) is the most frequently used solvent [3,6,7,9]. Valdez et al. [8] used several different organic solvents to separate the algae HTL products and found that the biocrude yield and compositions differed from solvent to solvent. Additionally, the literature shows that biocrude yields may differ even when using the same alga and ⁎ Corresponding author. Tel.: +1 734 764 3386; fax: +1 734 763 0459. E-mail address: [email protected] (P.E. Savage).

http://dx.doi.org/10.1016/j.algal.2014.08.007 2211-9264/© 2014 Elsevier B.V. All rights reserved.

solvent, if different amounts of solvent are used in the product workup procedure [8]. Thus the solvent type and amount may play a significant role in biocrude collection. Although an organic solvent is typically used in the laboratory for experimental convenience during HTL experiments, such use is undesired in a large-scale biorefinery for both economic and environmental reasons. Indeed, there have been recent reports [10,11] of continuous HTL processes that avoid solvent use in biocrude collection. Given the recent progress made toward solvent-free biocrude recovery, that the vast majority of literature data on biocrude yield and composition were obtained from studies that used a solvent, and that the amount and identity of the solvent can affect biocrude yields and composition, there is a pressing need to clarify what role the solvent (DCM) plays in influencing the biocrude yield and properties. Therefore, we undertook the present study to determine the differences between solvent-free biocrude and DCM-recovered biocrude from algae HTL. 2. Experimental section 2.1. Materials A preservative-free Nannochloropsis sp. slurry was purchased from Reed Mariculture, Inc. We determined its biomass content to be 32.5 ± 2.5 wt% by drying three separate samples at 70 °C in an oven

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for 48 h. Its biochemical composition (dry basis) was 59 wt% crude proteins, 20 wt% carbohydrates, and 14 wt% lipid [12]. Table 1 provides its elemental composition. The P, O, and ash contents were previously reported to be approximately 0.6, 28, and 3 wt%, respectively [12]. Solvents were purchased from Fisher Scientific and deionized water was produced in-house. Gases (He, N2) were from commercial sources. We assembled several 4.1 ml mini-batch reactors for the HTL experiments using 1/2-in. 316 stainless steel Swagelok port connectors and caps.

A B A S

2.2. Experimental procedures We loaded each reactor with 1.237 g algae slurry and 1.403 ml deionized water, leading to an algal biomass content of about 14.1 wt%, and then sealed the reactor using a torque wrench to 45 ft-lb. Experiments and stoichiometric calculations have clarified that oxygen from room air in the initial reactor headspace does not measurably affect product yields [8]. We then placed the reactor in a Techne Fluidized Sand Bath (Model SBL-2), and its temperature was maintained at 350 °C by a Techne TC-8D temperature controller. The reactor temperature increased to the sand bath temperature in less than 3 min [6,12, 13], and as it did, the liquid water expanded to occupy about 95% of the reactor volume. The pressure within the reactor would be essentially the vapor pressure of water at the reaction temperature (~16.5 MPa). We removed the reactor from the sand bath after 20 min, quenched it in ambient-temperature water for 20 min, and allowed it to equilibrate at room temperature for one hour before collecting products. The products were classified as gases, solids, aqueous phase, or biocrude. We calculated the yield of gaseous products by weighing the reactor before and after release of the gases. To explore the influence of DCM on biocrude, we collected the remaining algae HTL products by three different methods, which we refer to as M1, M2, and M3. Fig. 1 illustrates the first product-collection method (M1), which is the general method used in most previous algae HTL work. This method involves the use of DCM to extract biocrude (DCM-solubles) simultaneously from all phases present in the reactor. The rectangle on the left shows that the opened reactor contained biocrude (B), an aqueous phase (A), and some solids (S). We added a total of 7 ml DCM (in smaller sequential aliquots) to the reactor to recover all of its contents. This material was collected in a conical tube, which was then vortexed at 3000 rpm for 10 s and then centrifuged at 500 rcf for 1 min. The rectangle on the right in Fig. 1 shows the resulting phases. The DCM phase, which contained the biocrude, was then transferred to a pre-weighed glass tube. Next, we vortexed again the residual aqueous and solid phases at 3000 rpm for 10 s and centrifuged them at 1500 rcf for 3 min. After this treatment, we transferred the aqueous phase (denoted M1-A) into a pre-weighed glass tube and determined its mass. We dried the DCM phase and the solid phase remaining in the tube by flowing N2 over them for 6 h in a SUPELCO VISIDRYTM system. The dried material was the biocrude (denoted M1-B) and solids (denoted M1-S). We added 4 ml of hexane to the conical tube containing the biocrude (M1-B) and treated it for about 15 min in an ultrasonic cleaner (Fisher Scientific FS20) to extract the hexane-soluble portion, which we define as light biocrude (BL). We then vortexed the tube at 3000 rpm (10 s), centrifuged it at 500 rcf (3 min) and drew off the hexane phase. We dried the separated hexane phase using N2 to recover the light biocrude (M1-BL) and the hexane-insoluble heavy biocrude (M1-BH).

Table 1 Elemental composition (wt % dry basis) of algae. Element

Content

C H N S

52.56 7.35 8.58 0.57

Add DCM Centrifuge

S

M1-A Dry

DCM Remove Phase phase DCM

M1-S

M1-B

Fig. 1. Schematic diagram of conventional HTL product-collection method (M1). A = aqueous phase, B = biocrude, S = solids, DCM = dichloromethane.

The second method, M2, differs from M1, the conventional product collection method, in that we first removed the aqueous phase from the reactor. This aqueous phase may contain some solids (SA) as noted in Fig. 2, which we separated after centrifuging at 1500 rcf for 3 min. We refer to these dried solids as M2-SA. We then added 4 ml of DCM to the aqueous phase, mixed the two phases, and then centrifuged to separate the DCM phase from the water layer. Removing the solvent from the DCM phase provided the water-soluble biocrude, termed M2-BA. We note that DCM does enjoy some limited solubility in water and that this solubility could influence the extraction of biocrude compounds from the aqueous phase. The majority of the biocrude remained in the reactor, so we added a total of 7 ml DCM in smaller increments to extract and recover biocrude. Removing the solvent provided the water-insoluble biocrude components (M2-B) and residual solids (M2-SB). The total mass of the dichloromethane-soluble biocrude in M2 is the sum of M2-B (waterinsoluble portion) and M2-BA (water-soluble portion). Likewise, the total mass of the solids is the sum of M2-SA and M2-SB. We also separated the light biocrude (M2-BL) and the heavy biocrude (M2-BH) from M2-B through the same method used in M1. The third method, M3, was identical to M2 with the exception that we recovered much of the biocrude from the reactor before adding DCM. This modification provided a biocrude sample that had never been in contact with the extraction solvent. To recover this biocrude directly, we sealed the reactor and placed it vertically into a water bath at 50 °C for 20 min and then in an ultrasonic cleaner (Fisher Scientific FS20) in 50 °C water for an additional 5 min. The purpose of this treatment was to facilitate biocrude recovery by having it flow toward one end of the reactor. We opened the warmed reactor and physically removed the biocrude from the bottom cap using a spatula. We refer to this directly recovered, solvent-free biocrude as M3-B, and it is shown in Fig. 3b. We then added 7 ml of DCM to recover residual biocrude in the reactor, the bottom cap, the spatula, and the pipette used to remove the aqueous phase. This DCM-biocrude mixture is shown in Fig. 3c. The total mass of biocrude in M3 is the sum of M3-B (material recovered directly from the reactor with no solvent), the additional material recovered via addition of DCM, and M3-BA. Prior to analyzing or weighing M3-B, we removed the cap from the conical tube containing M3-B (Fig. 3b) and placed the tube in a fume hood for N48 h to allow any moisture or volatiles to evaporate. The water/volatile content of this directly recovered biocrude was 15.9 ± 5.4 wt%. It is this dried material that was then weighed and characterized using the different analytical methods. These three product work-up procedures provided samples of conventional DCM-recovered biocrude that contains contributions from all phases in the reactor effluent (M1-B), samples of the watersoluble portion of the biocrude (M2-BA and M3-BA), samples of the water-insoluble portion of the biocrude (M2-B and M3-B), and a sample of biocrude that was never in contact with DCM (M3-B).

D. Xu, P.E. Savage / Algal Research 6 (2014) 1–7

3

A A A SA B

Add DCM Centrifuge

M2-A

DCM Remove phase DCM M2-BA

Centrifuge separate

Separate

A

SA

Dry

M2-SA

S B SB

Add DCM Centrifuge

DCM Remove phase DCM M2-B Dry SB M2-SB

Fig. 2. Schematic diagram of second HTL product-collection method (M2). A = aqueous phase, B = biocrude, S = solids, DCM = dichloromethane.

2.3. Analysis methods Atlantic Microlab, Inc. performed elemental (C, H, N, S) analysis. We analyzed biocrudes (redissolved in DCM) with a GC-MS (Agilent Technologies 6890 N) equipped with an Agilent HP-5 capillary column (50 m × 200 μm × 0.33 μm). The column was initially held at 40 °C for 3 min and then heated to 300 °C at 4 °C/min and held isothermally for 10 min. The inlet temperature was 310 °C, the sample volume injected was 2 μl, and the split ratio was 14.7:1. Helium served as the carrier gas (23.6 ml/min). A mass spectral library was used for compound identification. Nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopic analyses were conducted to characterize the biocrudes. 1H NMR and 13C NMR spectra were recorded separately at 400.0 MHz and 100.6 MHz (45° pulse width, broadband proton decoupling), respectively, on a Varian, Inova 400 NMR spectrometer at around 25 °C. Approximately 256 scans were accumulated for the 13C spectrum. FT-IR analysis of a thin film of sample was carried out at ambient temperature on a Perkin Elmer Spectrum BX instrument (resolution, 4 cm−1; scan, 254; range, 4000–600 cm−1).

We estimated the higher heating value (HHV) of the biocrude according to the Dulong formula, HHVðMJ=kgÞ ¼ 0:338C þ 1:428ðH  O=8Þ þ 0:095S

ð1Þ

where C, H, O, and S are the wt% composition of each element in the biocrude. Results reported herein are the average values from three independent runs at each experimental condition. The uncertainties are the standard deviations. The yield of a given product fraction was calculated as its mass relative to the mass of algae (dry basis) loaded into the reactor. 3. Results and discussion This section provides information about the yields and compositions of the biocrudes recovered using the different methods outlined above. The mass balance exceeded 90% for all of the product recovery methods. 3.1. Product yields

Fig. 3. Products in M3. (a) Aqueous phase with solids (M3-SA) at the bottom, (b) biocrude untouched by DCM (M3-B) before drying, and (c) DCM phase containing biocrude and solids (SB). DCM = dichloromethane. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

In accord with previous algae HTL studies, the yields of gases and solids were low. The mean values were 4.9 wt% and 2.9 wt%, respectively. Table 2 shows that the total biocrude yield was 44% for each of the three product collection methods. The conventional method, M1, achieved this yield by extracting DCM-soluble material from all phases in the reactor simultaneously. M2 and M3, on the other hand, involved separate extractions of the organic and aqueous phases (with accompanying solids). The biocrude yield for M3 includes the biocrude recovered directly from the reactor (biocrude that never experienced contact with DCM) along with the biocrude recovered from the reactor components via addition of DCM. In the analyses that follow, only the solvent-free biocrude is in view. Table 2 shows that of the 44% biocrude yield, the aqueous phase delivered a biocrude yield of 3.7 wt%. Thus, one would lose about 8.4% of the total biocrude available if the water-soluble portion is not recovered. M2 and M3 involved direct removal of the aqueous phase and its associated solids (SA) from the reactor. Additional solids (SB) were recovered from the biocrude phase, and these represented approximately 14% of the total mass of solids. We estimate that the residual solids in DCM-free M3-B added no more than 0.12 wt% to the reported biocrude

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Table 2 Biocrude yields (wt%) from algae HTL at 350 °C and 20 min. Method

Method Description

Biocrude (B) yield

% of light biocrude

BA yield

Total biocrude yield

M1 M2 M3

DCM extraction of all phases simultaneously Separate DCM extraction of aqueous and non-aqueous phases Biocrude sampled w/o extraction, separate DCM extraction of aqueous phase

43.7 ± 2.5 40.1 ± 3.4 40.5 ± 2.7

65 ± 4 66 ± 4 66 ± 3

– 3.7 ± 1.9 3.7 ± 1.6

43.7 ± 2.5 43.8 ± 3.2 44.2 ± 3.2

yield. The large majority of solids in the HTL reactor effluent are associated with the aqueous phase. Elliott et al. [11] also reported that most solid matter was recovered in the aqueous phase. 3.2. Elemental analysis Table 3 lists the elemental compositions of the different biocrude samples. We did not analyze the O content but rather calculated this amount by difference. The composition of M1-B was similar to that reported previously for DCM-extracted biocrude from HTL of this alga under similar experimental conditions. For instance, the C, H, and N contents of biocrudes were reported as 76.0, 10.3, and 3.9 wt% [13], 75.3, 10.2, and 4.18 wt% [6], and 75.8, 10.6, and 4.5 wt% [8], respectively. The biocrudes recovered without DCM and the aqueous phase HTL co-product coming into contact (M2-B and M3-B) had a higher carbon and hydrogen content (statistically significant at a 0.10 level for C), a lower nitrogen and oxygen content (statistically significant at a 0.001 level for N), and a larger estimated heating value compared with the biocrude obtained by simultaneous extraction of the aqueous and non-aqueous phases (M1). Thus, these materials are likely to be higher quality biocrudes. This result shows that extracting DCM-solubles from the aqueous phase enriches the resulting biocrude in carbon-deficient but oxygen- and nitrogen-rich compounds. Indeed, Table 3 shows that the biocrude extracted from the aqueous phase (M2-BA) has more than twice the oxygen and nitrogen content of the biocrude that naturally separated from the aqueous phase after HTL. As a result, the heating value of this biocrude component is about 25% lower. Moving these O- and Ncontaining molecules from the aqueous phase to the biocrude increases the biocrude yield, but at the expense of biocrude quality. To the best of our knowledge, this is the first report on the amount and elemental composition of biocrude (i.e., DCM-solubles) in the HTL aqueous phase effluent. Table 4 shows the elemental compositions of the various light and heavy biocrudes. One can reasonably expect the uncertainty in these data to be similar to those in Table 3. In all cases, and consistent with previous work, the light biocrude was richer in C and H and had a lower N content than the corresponding heavy biocrude. Additionally, the light biocrudes recovered without DCM contacting the aqueous phase co-product from HTL (M2-BL and M3-BL) were richer in C and H compared with the corresponding light biocrude from M1, which included compounds extracted by DCM from the aqueous phase. This outcome is likely due to the absence of the more N- and O-rich, water-soluble compounds in these materials. The C and H contents in the heavy fraction of the biocrude that never came in contact with DCM (M3-BH) being lower than those in M2-BH, which was obtained by DCM extraction of the reactor vessel after removing the aqueous phase, is probably due to the presence of the

solids in M3-BH that are absent in M2-BH. Since no DCM touched M3-BH, we could not recover or analyze these solids. However, Table 4 shows that M2-SA, the solids recovered from the aqueous phase in M2, which would be the same types of material to be retained by M3-BH, had a carbon content of just 34%. Thus, it seems reasonable that the presence of solids in M3-BH could have been responsible for the lower carbon content of this biocrude fraction. The data in this section shows that the water-insoluble biocrude produced directly from algae HTL has a higher C and H content and HHV compared with biocrude recovered using DCM to extract compounds in the aqueous phase. The use of the solvent increases the biocrude yield, but decreases its quality. 3.3. GC-MS analysis Fig. 4 displays total ion chromatograms for M1-B, M2-B, and M2-BA. Panel a, for M1-B, gives the tentative identities of many of the peaks in the chromatogram. The identities are necessarily tentative since they were obtained solely from computer matches of product mass spectra with spectra stored in the GC-MS database. M2-B differed from M1-B in that it either did not contain or contained only in very small amounts compounds appearing in the retention time range of 8–34 min. The absence of these compounds in M2-B suggests that they appear in M1-B because the DCM extracts them from the aqueous phase co-product that emerges from the HTL reactor. Fig. 4c, which displays the total ion chromatogram for the biocrude extracted from the aqueous phase alone in M2, confirms this expectation. This sample of M2-BA contained many of the compounds appearing in M1-B that were absent in M2-B. Many of the compounds in Fig. 4c contain nitrogen, which is consistent with the high N-content of this material shown in Table 3. Reactions such as the Maillard reaction might be responsible for the formation of N-containing cyclic organic compounds, which are common components in biocrude produced by biomass HTL [14]. 3.4. NMR analysis We performed 1H and 13C NMR analysis of the biocrude to identify the types and amounts of functional groups in the molecules in the biocrude. Fig. 5 shows 1H NMR spectra of the biocrude components that are water-insoluble (M2-B) and water-soluble (M2-BA). The spectrum for M2-B, biocrude that spontaneously separated from the aqueous phase after HTL, shows large peaks at 0.83, 1.21, and 2.1 ppm, which are characteristic of protons in terminal methyl groups, methylene groups in alkyl chains, and carbon atoms α to an acyl group in fatty acids, respectively [6,15]. The two smaller peaks at around 1.4–1.6 ppm and 1.8–2.0 ppm matched resonances expected from protons on carbon atoms β to an acyl group and the C = C bond together Table 4 Elemental composition (wt%) of light (BL) and heavy (BH) biocrude and solids.

Table 3 Elemental composition (wt%) and HHV of biocrudes. Sample C M1-B M2-B M2-BA M3-B

75.37 ± 77.22 ± 63.59 ± 77.59 ±

H 0.54 9.77 1.50 9.91 0.33 8.43 1.10 10.05

N ± 0.30 5.36 ± 0.06 ± 0.17 4.70 ± 0.03 ± 0.25 11.20 ± 0.07 ± 0.06 4.51 ± 0.05

S 0.53 ± 0.46 ± 0.40 ± 0.56 ±

O 0.01 9.50 0.04 8.17 0.02 16.78 0.03 7.85

HHV (MJ/kg) 38.0 39.0 30.8 39.4

Sample

C

H

N

M1-BL M2-BL M3-BL M1-BH M2-BH M3-BH M2-SA

76.58 78.16 79.11 73.90 76.11 71.04 33.56

10.42 10.68 10.72 8.61 8.81 8.53 4.19

4.51 4.03 4.02 6.93 5.90 5.66 3.15

D. Xu, P.E. Savage / Algal Research 6 (2014) 1–7

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Fig. 4. Total ion chromatograms of biocrudes: (a) M1-B, (b) M2-B, and (c) M2-BA. M1, M2 = first and second product collection method, A = aqueous phase, B = biocrude.

with the methylene group, respectively. The peaks at 5.2–5.4 ppm were likely alkenyl protons. Overall, the 1H NMR results show that most of the hydrogen content in M2-B was due to the presence of fatty acids, alkanes, and other compounds with aliphatic methylene and methyl groups. Fig. 5b shows that the 1H NMR spectrum for the biocrude extracted from the aqueous phase also contains peaks at b1.5 ppm, indicative of methylene and terminal methyl groups. Distinct from the waterinsoluble biocrude, however, the water-soluble components show several peaks from 1.5 to 3.5 ppm. It is precisely in this region that protons in cyclic N- and O-containing compounds such as piperidine and

2-pyrrolidinone appear. Moreover, the smaller peaks around 7.0 ppm could arise from amine protons in molecules such as 2-pyrrolidinone [6]. Thus, the 1H NMR spectrum is consistent with the GC-MS analysis, which showed the abundance of cyclic N- and O-containing products. Fig. 6 displays 13C NMR spectra of M2-B and M2-BA. In both samples, there are numerous peaks in the 10–50 ppm region, where aliphatic methyl and methylene carbon (such as alkane carbons) atoms appear [16]. One also observes a larger number of peaks in the 10–50 ppm region for M2-BA, however, than for M2-B. These additional peaks are consistent with the resonances expected from protons in molecules such as piperidines and pyrrolidinones. Apart from the solvent peak

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a)

b)

12

10

8

6 ppm

4

2

0

12

10

8

6 ppm

4

2

0

Fig. 5. 1H NMR spectra of biocrudes: (a) M2-B and (b) M2-BA. M2 = second product collection method, A = aqueous phase, B = biocrude.

~1700 cm−1 than at ~3000 cm−1, whereas the water-insoluble fraction (M2-B) showed the opposite behavior. This difference suggests a larger carbonyl content and lower methylene content for the water-soluble biocrude components. The relative intensities of the two major absorbances for M2-BA are entirely consistent with the IR spectra for various alkyl-substituted pyrrolidinones. Moreover, the IR spectra for these compounds also show strong absorption at ~ 1300 cm− 1 and ~1400 cm−1, as is evident in Fig. 7 for M2-BA. To summarize this section, which provided the first comprehensive analysis of solvent-free biocrude and biocrude compounds extracted from the aqueous HTL effluent, M2-B and M3-B had similar elemental compositions and, although not shown, similar total ion chromatograms and IR and NMR spectra. M1-B, on the other hand, had a visibly different total ion chromatogram and C and N contents that differed from the solvent-free biocrudes by statistically significant amounts. The differences between M1-B and M2-B or M3-B arise from the M1-B biocrude including compounds obtained from the aqueous phase via DCM extraction. Hence, the application of a solvent to recover biocrude from HTL affects not only the total biocrude yield but also the types of compounds and functional groups present in the biocrude.

(76–78 ppm), there was almost no signal in the 70–100 ppm region where carbohydrate carbons appear. This reveals that carbohydrate carbons (~20 wt% in the algal biomass) were converted to other products appearing in the biocrudes. The weaker peaks appearing at about 130 ppm suggest the presence of alkenyl and aromatic carbon in the biocrudes. Each of the biocrude samples showed a peak at about 208 ppm, which is about where the carbonyl carbon in ketones and aldehydes appears. Furthermore, the peak at 170–180 ppm in M2-BA is in the region where the carbonyl carbon atom in 2-pyrrolidinones [6] appears. 3.5. FT-IR analysis GC-MS and NMR analysis showed that the biocrudes primarily contained methylene groups, probably either in alkanes or in fatty acids, and the FT-IR results in Fig. 7 further confirm this feature. The asymmetrical and symmetrical C—H stretching vibrations in aliphatic methylene groups appear in the range of 2800–3000 cm−1 [6], where two large bands are present in Fig. 7. The presence of carbonyl carbons (e.g., carboxylic acids and esters) is consistent with the strong bands at 1650–1760 cm−1. The high intensity in these two regions is consistent with a significant amount of the hydrogen in the biocrudes being in aliphatic structures [6]. The spectra also exhibited strong absorbance at approximately 1456 cm−1, where the scissoring band in methylene groups emerges [15]. These peaks might also be due to some mononuclear aromatic compounds that are present in these biocrudes. Although the IR spectra show similarities regarding the locations of the strongest absorption peaks, there are also some differences. The water-soluble biocrude (M2-BA) shows a much stronger absorbance at

4. Conclusions Biocrude recovered directly from algae HTL without the addition of a solvent had a combined C and H content of 88% and a HHV of 39.4 MJ/kg. Using a solvent (i.e., DCM) to recover biocrude from the entire reactor effluent from algae HTL extracts water-soluble compounds from the aqueous phase and transfers them to the biocrude product. For algae HTL at the conditions examined herein, the aqueous

a)

b) CDCl3

200

150

100 ppm

CDCl3

50

0

200

150

100 ppm

50

0

Fig. 6. 13C NMR analysis of biocrudes: (a) M2-B and (b) M2-BA. M2 = second product collection method, A = aqueous phase, B = biocrude.

D. Xu, P.E. Savage / Algal Research 6 (2014) 1–7

100

(no. 21206132), the Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120201120069), the Fundamental Research Funds for the Central Universities (no. xjj2012032), the National Science Foundation for Post-doctoral Scientists of China (no. 2013 M540748), and the State Scholarship Fund for University Key Teachers from the Ministry of Education of China to Study Abroad as a Visiting Scholar (grant no. 201206285008).

90

Transmittance

7

80

70 M2-B M2-BA

60

50

3600

3000

References

2400 1800 Wavenumbers (cm-1)

1200

600

Fig. 7. FT-IR spectra of the different biocrudes. M2 = second product collection method, A = aqueous phase, B = biocrude. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

phase contributed 8.4 wt% of the total amount of biocrude. The watersoluble biocrude component has a lower C and H content (72 wt%), a higher N and O content, and a lower heating value (30.8 MJ/kg) compared with the biocrude that spontaneously separates from the aqueous phase after cooling the liquefaction products. Moreover, it contains an abundance of alkyl-substituted, cyclic N- and O-containing compounds. Thus, using a solvent to recover biocrude from the entire reactor effluent increases the biocrude yield but at the expense of biocrude quality. A solvent-free algae HTL process offers the potential advantages of producing a higher quality biocrude that is probably easier to upgrade to hydrocarbons, eliminating environmental, health, and sustainability issues associated with the use of petroleum-derived solvents, reducing cost through process simplification and solvent avoidance, and producing an aqueous phase co-product richer in organic compounds, which can be gasified hydrothermally to produce methane for combined heat and power generation in a biorefinery. Potential disadvantages of a solvent-free approach are a reduced biocrude yield, which could reduce biorefinery revenue, and perhaps a more difficult separation of biocrude from the aqueous phase co-product as emulsification is possible. Acknowledgements We gratefully acknowledge the financial support from the University of Michigan College of Engineering, the National Science Foundation (EFRI-0937992), the National Natural Science Foundation of China

[1] L. Brennan, P. Owende, Biofuels from microalgae—a review of technologies for production, processing, and extractions of biofuels and co-products, Renew. Sust. Energ. Rev. 14 (2010) 557–577. [2] K. Anastasakis, A.B. Ross, Hydrothermal liquefaction of the brown macro-alga Laminaria saccharina: effect of reaction conditions on product distribution and composition, Bioresour. Technol. 102 (2011) 4876–4883. [3] P. Biller, A.B. Ross, Potential yields and properties of oil from the hydrothermal liquefaction of microalgae with different biochemical content, Bioresour. Technol. 102 (2011) 215–225. [4] D. López Barreiro, W. Prins, F. Ronsse, W. Brilman, Hydrothermal liquefaction (HTL) of microalgae for biofuel production: state of the art review and future prospects, Biomass Bioenergy 53 (2013) 113–127. [5] P. Biller, R. Riley, A.B. Ross, Catalytic hydrothermal processing of microalgae: decomposition and upgrading of lipids, Bioresour. Technol. 102 (2011) 4841–4848. [6] P. Duan, P.E. Savage, Hydrothermal liquefaction of a microalgae with heterogeneous catalysts, Ind. Eng. Chem. Res. 50 (2011) 52–61. [7] S. Amin, Review on biofuel oil and gas production processes from microalgae, Energy Convers. Manag. 50 (2009) 1834–1840. [8] P.J. Valdez, J.G. Dickinson, P.E. Savage, Characterization of product fractions from hydrothermal liquefaction of Nannochloropsis sp. and the influence of solvents, Energy Fuels 25 (2011) 3235–3243. [9] N. Neveux, A.K.L. Yuen, C. Jazrawi, M. Magnusson, B.S. Haynes, A.F. Masters, A. Montoya, N.A. Paul, T. Maschmeyer, R. de Nys, Biocrude yield and productivity from the hydrothermal liquefaction of marine and freshwater green macroalgae, Bioresour. Technol. 155 (2014) 334–341. [10] D.C. Elliott, T.R. Hart, G.G. Neuenschwander, L.J. Rotness, G. Roesijadi, A.H. Zacher, J.K. Magnuson, Hydrothermal processing of macroalgal feedstocks in continuous-flow reactors, ACS Sust. Chem. Eng. 2 (2013) 207–215. [11] D.C. Elliott, T.R. Hart, A.J. Schmidt, G.G. Neuenschwander, L.J. Rotness, M.V. Olarte, A.H. Zacher, K.O. Albrecht, R.T. Hallen, J.E. Holladay, Process development for hydrothermal liquefaction of algae feedstocks in a continuous-flow reactor, Algal Res. 2 (2013) 445–454. [12] P.J. Valdez, M.C. Nelson, H.Y. Wang, X.N. Lin, P.E. Savage, Hydrothermal liquefaction of Nannochloropsis sp.: systematic study of process variables and analysis of the product fractions, Biomass Bioenergy 46 (2012) 317–331. [13] T.M. Brown, P.G. Duan, P.E. Savage, Hydrothermal liquefaction and gasification of microalga Nannochloropsis sp. Energy Fuels 24 (2010) 3639–3646. [14] A. Kruse, P. Maniam, F. Spieler, Influence of proteins on the hydrothermal gasification and liquefaction of biomass. 2. Model compounds, Ind. Eng. Chem. Res. 46 (2007) 87–96. [15] R.M. Silverstein, F.X. Webster, D.J. Kiemle, Spectrometric Identification of Organic Compounds, 7th ed. Wiley, New York, 2005. [16] P.G. Duan, P.E. Savage, Upgrading of crude algal bio-oil in supercritical water, Bioresour. Technol. 102 (2011) 1899–1906.