Hydrothermal Liquefaction of Microalgae in a Continuous Stirred-Tank

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Hydrothermal Liquefaction of Microalgae in a Continuous StirredTank Reactor Diego López Barreiro,*,† Blanca Ríos Gómez,‡ Ursel Hornung,‡ Andrea Kruse,‡,§ and Wolter Prins† †

Department of Biosystems Engineering, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium ‡ Institute for Catalysis Research and Technology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmoltz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § Conversion Technology and Life Cycle Assessment of Renewable Resources (440f), Institute of Agricultural Engineering, University Hohenheim, Garbenstrasse 9, 70599 Stuttgart, Germany S Supporting Information *

ABSTRACT: The microalgae species Nannochloropsis gaditana (N. gaditana, marine) and Scenedesmus almeriensis (S. almeriensis, freshwater) were subjected to hydrothermal liquefaction (HTL; 350 °C; residence time of 15 min) in a continuous stirred-tank reactor (190 mL) at microalgae loadings of 9.1 and 18.2 wt % in the feed. The results indicate that the high loading of biomass in the feed promotes the formation of biocrude oil, with a maximum yield of 54.8 ± 3.4 wt % for N. gaditana. A similar type of biocrude was obtained with both species. Its nitrogen and carbon contents increased at the high biomass loading, as well as the higher heating value. The HTL product yields varied from those reported for batch experiments carried out at the same reaction conditions. Most of the nutrients initially present in the feedstock were recovered to some extent in bioavailable forms in the aqueous phase, especially in the case of N. gaditana. slurries at 350 °C and 20 MPa in a later publication, with microalgae loadings in the feed between 17.0 and 34.4 wt %. The biocrude yields varied between 38.0 to 63.8 wt %. Jones et al.9 carried out continuous HTL for Nannochloropsis and Chlorella at 350 °C. They achieved biocrude oil yields of 59 and 41 wt %, respectively, and concluded that the biocrude oil yield was enhanced at high biomass loadings in the feed. Recently, Patel and Hellgardt10 investigated continuous HTL in a novel quartz lined chamber with a volume of 2 cm3 at temperatures of 300−380 °C and residence times of 0.5−5 min, using a low loading of microalgae in the feed (1.5 wt %). The maximum biocrude oil yield was 38 wt % at 380 °C and 0.5 min. However, higher biomass loadings will be needed for industrial scales: at least 15 wt % is required for an energy and economically efficient operation.11 The studies on batch HTL significantly outnumber those on continuous HTL. It has been reported in some of these studies that instant heating rates enhance the formation of biocrude oil during HTL.12,13 Moreover, most of the experiments reported in the literature have been done in tubular reactors. These issues motivated the present study, in which a CSTR of 190 mL is used, viz., 350 °C and 20 MPa, in order to achieve instant heating rates. A residence time of 15 min was applied, at two different biomass loadings in the feed (9.1 and 18.2 wt %). Two microalgae species from marine and freshwater environments were used (Nannochloropsis gaditana (N. gaditana) and Scenedesmus almeriensis (S. almeriensis), respectively). To the best of our knowledge, this work is the first reporting on the

1. INTRODUCTION The research about microalgae has experienced a notable boost during the past decade due to the increasing interest in renewable feedstock for biofuels. Quite some work has been done regarding microalgal hydrothermal liquefaction (HTL) because HTL can convert a significant fraction of the whole microalgae biomass into biofuel, instead of just a single fraction, such as in biodiesel production from the lipids. HTL uses the properties of hot compressed water (5−25 MPa, 280−375 °C) to degrade microalgae and form a liquid biofuel, the so-called biocrude oil,1 thus avoiding the energy penalty of drying the feedstock. However, this biocrude needs further upgrading steps2 to decrease its content of heteroatoms and make it usable as transportation fuel. So far, most of the experiments about microalgae HTL have been carried out in batch microautoclaves using an organic solvent (usually dichloromethane) to separate the biocrude oil and aqueous phases.3,4 Only a few publications are available on HTL in continuous reactors.5 Most of them are referring to tubular reactors or combinations that use a continuous stirredtank reactor (CSTR) as preheater. Elliott et al.6 provided the first report on continuous microalgae processing in hot compressed water, using a supported Ru catalyst at 350 °C. Nevertheless, this study focused mainly on gas production, with very little information about the biocrude oil produced. Jazrawi et al.7 studied the continuous HTL of Chlorella and Spirulina at varying biomass loadings (1−10 wt %), temperatures (250− 350 °C), residence times (3−5 min), and pressures (15−20 MPa). They reported high biomass loadings, high temperatures, and long residence times to enhance the production of biocrude oil, attaining a maximum yield of 41.7 wt %. Elliott et al.8 reported on continuous HTL of Nannochloropsis sp. © XXXX American Chemical Society

Received: May 10, 2015 Revised: September 22, 2015

A

DOI: 10.1021/acs.energyfuels.5b02099 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Scheme of the setup used for continuous HTL experiments of microalgae. All of the experiments were carried out at 350 °C. The reactor was heated electrically by heating cartridges inserted in the wall of the reactor. The heating to the desired temperature was done while running distilled water through the reactor. This startup period took usually 3 h. Once the reaction temperature was attained inside the reactor, the water supply was closed and the feeding of the microalgae slurry was started. Calibration experiments (data not shown) revealed that the time required for achieving the stationary state was ca. 80 min (approximately 5 times the residence time) after starting to feed the microalgae slurry. The collection of samples started at 60 min, and was performed each 20 min until 180 min of run time. The data windows for the HTL products at each sampling time were 10 min in the experiments with low microalgae loading (9.1 wt %) and 7.5 min in the experiments with high loading (18.2 wt %). 2.4. Product Separation and Analyses. The pressure in the reactor was kept at the set-point (20 MPa) by a back-pressure regulating valve controlled with a PID controller. This was done by letting out regularly part of the products inside the reactor to avoid exceeding the pressure set-point. Once the products exited the reactor, they passed through a cooling system and were collected in a glass container. This container had an exit connected to a gas meter to measure the volume of gas produced. Gas samples were collected with a gas sampling tube for further analysis. Each collected sample (containing solids, aqueous products, and biocrude oil) was filtered (Whatman 589/1 black ribbon) to recover the aqueous product. The oil and solid products remained on the top of the filter and on the walls of the collection system. Once the aqueous product was taken apart, the mixture of oil and solids was dissolved in dichloromethane (DCM) (VWR, ≥99.7% purity) to reduce the viscosity of the biocrude oil and make it filterable. The mixture was filtered to remove the solids over the same filter used for the aqueous product. The DCM was used also to maximize the collection of any possible products that could remain on the walls of the collection system. Enough DCM was used so that no biocrude oil remained in the collection system or in the filter cake. With this procedure, an aqueous product, a solid residue, and biocrude oil dissolved in DCM were obtained. It should be noted that the HTL products were separated avoiding the contact between DCM and the aqueous product, which is undesired because DCM would extract some water-soluble organic compounds, thus reducing the yield of organics in the aqueous phase.16 The yields of the different product phases were calculated on an organic basis as the ratio between the recovered organic mass (mi) of

evolution of the yields throughout the run time for continuous HTL of microalgae, while also providing an extensive characterization of the biocrude oil and aqueous phase produced.

2. MATERIALS AND METHODS 2.1. Chemicals. All of the solvents and chemicals used in this study were obtained from Sigma-Aldrich and Merck (purities ≥ 98%) and used without any further purification. 2.2. Feedstock. N. gaditana (CCAP 849/5) and S. almeriensis (CCAP 276/24) were obtained in a freeze-dried state from Estación Experimental Las Palmerillas (University of Almerı ́a, Spain). The same feedstock was already used in previous studies,14 where a detailed explanation about the characterization and composition of the algal feedstock is provided. The freeze-dried state of the feedstock resulted in the presence of big aggregates that hindered the formation of a homogeneous microalgae slurry upon mixture with water. A rotor mill Fritsch Pulverisette 14 device was used to reduce the size of these particulates below 0.2 mm and improve the miscibility of the feedstock with water. Samples of the feedstock were analyzed with a field emission scanning electron microscope (FE-SEM, type DSM 982 Gemini, Carl Zeiss Ltd.) to assess the effect of the milling step on the structure of the feed. Microalgae were then mixed with deionized water to produce slurries with the desired biomass loading (either 9.1 or 18.2 wt %). Xanthan gum from Sigma-Aldrich was used as thickener at a loading of 0.1 wt % to prevent the biomass from settling during pumping. The biomass settling velocity is expected to be lower when using fresh microalgae suspensions, and therefore the use of thickeners is not envisaged at industrial scales. The microalgae slurry was placed in a feed tank that was continuously stirred to prevent the microalgae from settling. 2.3. Hydrothermal Liquefaction and Product Collection. The HTL experiments were done in an in-house constructed continuous stirred-tank reactor with an internal volume of 190 mL made of Inconel 625, a nickel-based alloy. A scheme of the setup is shown in Figure 1. The reactor was stirred with a magnetically coupled impeller made of stainless steel EN 1.45471 and was fed continuously by a double screw press that ran asynchronously to ensure a continuous flow rate. The flow corresponded with an average residence time of 15 min. This value was calculated considering the change in density of water at the reaction conditions.15 B


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Table 1. Feedstock Characterization: Elemental Composition, Ash Content, Biochemical Composition, Mineral Elements (wt %) and HHV (MJ·kg−1)

a

species

C

H

N

S

Oa

ash

lipids

proteins

HHV

Ca

Fe

K

Mg

Na

P

N. gaditana S. almeriensis

47.6 38.0

7.5 5.6

6.9 5.5

0.5 0.5

25.1 30.4

12.4 20.0

13.4 13.1

32.2 30.0

23.1 16.8

0.50 6.96

0.05 0.27

1.30 0.90

0.27 0.83

3.02 0.45

1.43 3.67

By difference (100 − C − H − N − S − ash).

each HTL product and the mass of microalgae (dry, ash free (daf)) initially loaded to the reactor, following

yield/(wt %) =

mi × 100 mmicroalgae(daf)

was calculated by a differential method, subtracting TIC from TC. Ion chromatography was used to determine the content of organic acids, phenols, and anions. Ammonium was measured photometrically using the standard test LCK 303 from Hach Lange in a Hach spectrophotometer DR5000. A detailed explanation of the analytical equipment and methods used can be found in the Supporting Information.

(1)

The mass of biocrude oil was determined after flushing the mixture biocrude−DCM with a constant flow of nitrogen to evaporate the DCM, until the weight remained invariable. This process might cause the loss of some organic molecules present in the biocrude oil, thus reducing the mass balance closure. The mass of aqueous product collected was weighed, and the organic matter present in it was determined by drying two aliquots of 2 mL at 60 °C overnight. The dry residue was then treated in air at 550 °C for 5 h in a muffle furnace to determine its ash content. The HTL solid residue was quantified by drying it at 105 °C overnight to remove any residual water and DCM that could remain after the filtration. The ash content was also determined at 550 °C for 5 h under oxidative conditions. The ideal gas law was used to calculate the mass of gas produced, taking into account the gas volume (measured with the gas meter) and its composition. The composition was measured in an Agilent 7890A gas chromatograph equipped with a front flame ionization detector (FID) and a back thermal conductivity detector (TCD). The mass of gas was then used in eq 1 to calculate the gas yield. For each experiment, all of the biocrude oil samples collected from 60 to 180 min were then mixed together for further analyses, after determining the yields at each sampling time. Preliminary experiments with the same feedstock showed that the variability over the run time of the results from the several analytical techniques applied was minimal, which allowed mixing all the biocrude oil samples. The CHNS elemental composition (in weight percentage) was measured by a Vario EL cube analyzer, and the oxygen content (among other elements) was determined by difference (100 − C − H − N − S). The resulting values were used in Boie’s formula17 eq 2 to calculate the higher heating value (HHV) of the biocrude oil. HHV/(MJ·kg −1) = 0.3516C + 1.16225H − 0.1109O + 0.0628N

3. RESULTS AND DISCUSSION 3.1. Feedstock. Both species contained a similar quantity of lipids and proteins (Table 1), though the lipids had a different nature in each species.14 Most of the total lipids were in the form of neutral lipids for N. gaditana, but not for S. almeriensis. These neutral lipids are typically in the form of triacylglycerides (TAGs), which consist of a backbone of glycerol with three fatty acids bound to it by ester bonds.11 The ashes also differred between species. A relevant fraction of the ash from S. almeriensis was formed by calcium and phosphorus, while in the case of N. gaditana the main inorganic compound was sodium, most likely because of the marine environment. A rotor mill was used to reduce the size of the aggregates in the feedstock to a maximum diameter of 0.2 mm. This size is much larger than the typical size of microalgae cells (80 wt % of ash for S. almeriensis; 50−80 wt % for N. gaditana). The low yield of solids indicated a very good conversion of the microalgal feedstock. Probably the high concentration of alkalis in the feedstock (Table 1) played a role here, as alkalis are said to reduce the formation of solid residue.29 In this sense, the lower content of alkalis in S. almeriensis could be the reason for its higher solid residue yields. The low gas yield was probably caused by the temperature used (350 °C), beneath the gasification region.30 The gas formed was mainly CO2 (more than 95 vol %), likely coming from decarboxylation reactions.7 Trace amounts of methane and C2−C4 hydrocarbons were formed as well. Table 3 presents the elemental balances of carbon and nitrogen in the biocrude oil, aqueous and gas phases. The results are referred only to the period assumed as the stationary state (80−140 min for S. almeriensis; 80−180 min for N. gaditana). No data are provided for the solid residue because its Table 3. Elemental Balances for Carbon and Nitrogen (wt %) for Continuous Microalgae HTL at 350 °C and 15 min of Residence Time S. almeriensis

N. gaditana

9.1 wt %

18.2 wt %

9.1 wt % 18.2 wt %

44.5 2.7 44.6

54.0 2.0 28.4

53.1 1.6 43.4

57.5 1.5 35.3

91.8

84.5

98.1

94.3

21.4 66.9

26.6 63.4

18.1 86.2

25.6 74.7

88.3

90.0

104.4

100.3

carbon biocrude oil gas water-soluble organics total nitrogen biocrude oil water-soluble organics total

E


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Figure 3. Molecular weight distribution of the biocrude oil collected from 60 to 180 min of processing time from Scenedesmus almeriensis (left) and Nannochloropsis gaditana (right) at 350 °C and 15 min of residence time.

Figure 4. FT-IR spectra of biocrude oil collected from 60 to 180 min of processing time from Scenedesmus almeriensis (left) and Nannochloropsis gaditana (right) at 350 °C and 15 min of residence time.

probably originating from the reaction of side chains of proteins.31 In the case of N. gaditana, cholesterol or cholesterol derivatives, such as cholestane or cholestadiene, were also detected, which agreed with the reported presence of cholesterol in this sort of microalgae.32 Molecules coming from the degradation of pigments (such as chlorophyll)33 were identified in both species (such as 2-hexadecene, 3,7,11,15 tetramethyl-). The absence of oxygenated aromatic or cyclic molecules (such as cyclopentanones or furanones) indicated a low content of carbohydrates in the microalgae tested. These products are typically present in the biocrude oils obtained from HTL of feedstocks rich in saccharides, such as seaweeds.34 The molecular composition herewith reported is similar to that of HTL biocrude oils from other microalgae species.3 This indicates that HTL of different species leads to a similar type of biocrude oil. It is however important to notice that GC-MS analyses only give a limited image of the biocrude oil composition, because most of the molecules cannot be analyzed by GC due to their high molecular weight.31 The heavier, nonGC-amenable fraction of the oil, could hypothetically exhibit larger differences between species. Further research is needed to unveil the composition of the heavy fraction of the biocrude oil produced by HTL. 3.3.4. FT-IR Analyses. The FT-IR analyses revealed similar spectra for all of the biocrude oils produced. The types of functional groups present in the oils were identical for both species. No significant effects were detected by FT-IR when the microalgae loading was doubled from 9.1 to 18.2 wt %. The chemical functionalities found by FT-IR analyses agreed with the molecules detected by GC-MS analyses. Figure 4 shows the data for each experiment. The main vibrations (2920, 2850, and 1450 cm−1) were related to the presence of methylene groups from alkanes or fatty acids35 (e.g., in n-hexadecanoic acid and docosene). The carbonyl signals at 1700 and 1660 cm−1 were indicative of the

these results, the increase in the biocrude oil yield at the high biomass loading does not result in any large alteration in the molecular weight distribution. 3.3.3. GC-MS Analyses. The molecular composition of the biocrude oil, as analyzed by GC-MS, indicated the presence of a broad range of molecules for both species. The chromatograms can be found in the Supporting Information (Figures S3 and S4). In general, the same types of molecules were found, regardless of the microalgae loading in the feed. The chemical compounds present in the biocrude oil were in both cases typically derived from the lipid and protein fractions. Apparently, doubling the microalgae loading did not modify the type of molecules present in the GC-amenable fraction of the biocrude oil. The most abundant molecule detected for both species was n-hexadecanoic acid, a product from the hydrolysis of lipids. Its concentration was much higher for N. gaditana, as demonstrated by the height of its peak compared to the internal standard (pentadecane). This constituted the main difference between both oils. Other types of compounds derived from the lipids were detected, mostly in the form of unsaturated fatty acids (e.g., oleic acid). Two abundant molecules in the biocrude oil from N. gaditana were tetradecanoic acid and (11Z)hexadecenoic acid, which were absent from the biocrude oil from S. almeriensis. This was consistent with the high content of TAGs in N. gaditana, as TAGs typically lead to the formation of fatty acids upon HTL1. Other long-chain unsaturated aliphatic molecules were also detected, such as docosene. Molecules arising from the thermochemical conversion of the proteinic fraction were found as well. Long-chain saturated amides, such as hexadecanamide, or unsaturated, like (9Z)octadecenamide, were detected. Other degradation products from proteins were present in the biocrude oil as well, e.g., indole derivatives and pyrrole derivatives (for both species) or pyrrolidine derivatives (only for N. gaditana). Substituted (methyl- or ethyl-) phenols were also found for both species, F


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Table 4. pH Values and Organic Compounds (mg·gdaf−1) Detected by Ion Chromatography in the Aqueous Product from Continuous Microalgae HTL at 350 °C and 15 min of Residence Time S. almeriensis 9.1 wt %

18.2 wt %

N. gaditana 9.1 wt %

18.2 wt %

a

time (min)

pH

glycolic acid

formic acid

acetic acid

formaldehyde

phenol

p-cresola

m-cresola

o-cresola

80 120 160 80 120 160

8.0 8.0 7.9 8.2 8.2 8.1

40.3 39.2 28.6 24.0 18.4 19.8

1.4 2.0 0.9 0.8 0.5 0.2

22.8 25.0 20.8 18.9 12.5 10.4

25.9 27.0 21.6 24.9 18.3 16.3

1.3 1.6 1.3 1.2 0.7 0.8

0.9 0.6 0.5 0.5 0.3 0.3

bdl bdl bdl bdl bdl bdl

bdl bdl bdl 0.1 0.2 0.2

80 120 160 80 120 160

7.9 8.1 8.0 8.1 8.1 8.1

8.9 13.4 11.6 5.5 5.8 5.7

0.5 0.9 1.1 0.7 1.1 1.2

7.1 11.3 10.6 9.8 9.6 9.2

21.1 33.6 29.5 bdl bdl bdl

0.9 1.1 1.1 0.7 0.7 0.5

0.4 0.5 0.5 0.3 0.3 0.2

bdl bdl bdl 0.5 0.5 0.5

bdl bdl bdl 0.1 0.2 0.2

bdl, below detection limits.

Table 5. Recovery of Nutrients (wt %) in the Aqueous Product from Continuous Microalgae HTL at 350 °C and 15 min of Residence Time recovery S. almeriensis 9.1 wt %

18.2 wt %

N. gaditana 9.1 wt %

18.2 wt %

time (min)

PPO43−

SSO42−

C

org C

N

N−NH4+

Ca

K

Mg

Na

80 120 160 80 120 160

7.1 7.1 4.7 5.8 3.7 4.4

22.7 23.9 18.1 19.7 15.6 14.4

42.8 46.3 35.6 32.3 24.5 21.8

39.1 41.8 31.3 28.6 21.8 19.3

64.8 69.0 55.5 73.7 53.1 47.6

43.9 47.2 38.9 41.6 36.4 29.0

3.0 3.0 2.9 0.1 0.1 0.1

118.0 126.6 98.6 113.0 87.5 78.1

0.5 0.3 0.3 0.1 0.1 0.1

79.7 82.4 64.8 56.8 49.4 48.2

80 120 160 80 120 160

59.4 61.1 60.4 51.9 63.8 61.9

33.3 30.9 30.1 31.5 31.0 31.1

46.6 43.1 40.4 33.8 35.0 37.1

42.0 38.7 36.3 30.1 31.3 33.4

91.8 84.4 82.5 73.3 73.9 76.8

50.3 46.2 46.5 36.8 41.3 35.5

1.7 1.5 1.1 0.9 0.6 0.8

106.4 105.1 99.3 101.7 102.0 103.7

0.6 0.9 1.4 0.1 0.1 0.1

96.1 94.1 88.9 88.8 88.9 90.1

presence of fatty acids arising from the degradation of lipids.36 Nitrogen-containing molecules led to vibrations in the regions of 1274 and 1170 cm−1, which was consistent with the detection of nitrogen heterocycles by GC-MS (i.e., amides, indoles, and pyrrolidines) produced by Maillard reactions.37 The vibrations at 740 cm−1 were attributed to the presence of aromatic rings (phenols) that can be formed from proteins.31 3.4. Aqueous Phase. Samples of the aqueous products obtained at 80, 120, and 160 min were analyzed to check the stability of the composition of this phase throughout the run time. Besides, the effect of changing the species and its loading on the composition of the aqueous phase was examined. Tables 4 and 5 show the data collected. Table 4 presents the quantity of some organic compounds detected by ion chromatography, in relation to the organic mass (daf) loaded to the reactor (wt % of recovery). The pH value is also shown. The organic molecules detected represented less than one-third of the organic matter in the aqueous product, as determined by the mass balances. Therefore, the results presented in this section cannot be taken as a complete image of the composition of this phase. Other methods and techniques are needed to obtain more information about its composition. For a further character-

ization of the HTL aqueous phase the reader is referred to Pham et al.38 and Garcı ́a Alba et al.39 The molecules with a higher concentration detected were glycol acid, acetic acid, formic acid, formaldehyde, and various types of phenols. Glycerol was detected as well (but not quantified), probably originating from the degradation of TAGs under HTL conditions. Despite the presence of several organic acids in the aqueous phase, it was slightly basic (pH ∼ 8) because of the high content in ammonium (see Table 5). In general, the concentration of all of the chemical species detected was quite constant during the period from 80 to 160 min. The exception was a decrease taking place for the compounds detected at 160 min for S. almeriensis, likely related to the problems in pumping that occurred at that sampling time. This effect was not observed for N. gaditana, where the concentration of the various chemical compounds remained stable in all cases. The influence of the species becomes clear when looking at the data from Table 4. S. almeriensis produced much more glycolic acid than N. gaditana. The amount of formic acid produced was low for both species, while acetic acid was significantly higher for S. almeriensis. The content of formaldehyde was higher in the case of S. almeriensis for the two G


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Figure 5. Comparison of the product yields from HTL in batch42 and CSTR experiments (wt % of the dry, ash free microalgae) for Scenedesmus almeriensis (left) and Nannochloropsis gaditana (right). Experiments at 9.1 wt % microalgae loading, 350 °C, and 15 min of residence time.

microalgae loadings tested, whereas in the case of N. gaditana this molecule was only detected when a loading of 9.1 wt % was used. With respect to the presence of phenolic compounds, the most abundant ones were phenol and p-cresol, although the content of phenolic compounds was low in all cases. The effect of increasing the microalgae loading in the feed was not linear: doubling the biomass loading did not double the content of the various molecules present in the aqueous phase. At a biomass loading of 18.2 wt % the concentration of glycolic acid increased in the case of S. almeriensis, whereas it decreased for N. gaditana. Formaldehyde decreased below detection limits in the case of N. gaditana but remained quite constant for S. almeriensis. The amounts of phenol and p-cresol stayed quite constant for both species, while m-cresol increased significantly for N. gaditana, and o-cresol increased for both species. The broad range of molecules present in microalgae and the complexity of the reactions governing microalgae HTL make it difficult to explain all of the observed effects. In any case, the fact that doubling the microalgae loading in the feed does not double the content of the various organic molecules detected in the aqueous phase is expected, according to the mass balances. The results from Table 4 indicate that the loading of microalgae in the reactor feed plays a relevant role in the reactivity of the process, affecting not only the yields but also the composition of the aqueous product. The recovery of several elements in the aqueous phase, compared to their concentration in the feedstock, was also investigated. Table 5 shows that the recovery was strongly dependent on the type of element and its concentration in the feed. As a general trend, it can be said that on the one hand the monovalent ions K+ and Na+ led to very high recoveries for both species, and on the other hand the divalent ions Ca2+ and Mg2+ could not be recovered in the aqueous product and were most likely found in the solid residue from the process. This calls for studies with regard to the possible applications of the solid byproduct to make use of the nutrients that are recovered in it. The recovery of phosphorus, a very relevant nutrient for microalgae cultivation which might face a shortage in the coming years,40 is strongly dependent on the species used (between 3.7 and 7.1 wt % for S. almeriensis; up to ca. 60 wt % for N. gaditana). The recovery of sulfur was higher also for N. gaditana (ca. 30 wt %). The recovery of carbon in the aqueous product was in the same range for both species, being reduced when the biomass loading was doubled. Most of the carbon was in organic form. Nitrogen was more species-dependent, being remarkably high for N. gaditana at a loading of 9.1 wt %. Doubling the biomass loading reduced the recovery of nitrogen in the aqueous

product for both species and increased its content in the biocrude oil (as shown in Table 2). This again confirms that the proteinic material is more prone to repolymerization and formation of biocrude oil at higher microalgae loadings in the feed, as hinted at by the elemental analyses (see Section 3.3.1). An important part of the nitrogen was recovered in the form of ammonium (30−50 wt %). The presence of other nitrogencontaining molecules in the aqueous product was clearly indicated by the gap between total nitrogen and nitrogen in the form of ammonium. No nitrates or nitrites were found, showing that the unidentified nitrogen was in organic form. This is consistent with the literature,38 where thorough analyses of the aqueous product from microalgae HTL reported the presence of numerous nitrogen-containing molecules, such as pyridine or piperidone. The differences in the recovery of elements dissolved in the aqueous product are related to the variations in the thermochemical equilibria of salt mixtures when subjecting them to hydrothermal conditions. This behavior is strongly influenced by the type and concentration of salts present in the feedstock,41 which is in line with the significant differences in the salt content of the two species used in this work (Table 1). The recoveries herewith reported are similar to those from batch experiments under similar reaction conditions.42 The capability of the aqueous product to recover nutrients and recycle them to microalgae cultivation has been already highlighted in several studies.39,43 This work shows that the recovery of some nutrients is not only species-dependent but also dependent on the biomass loading in the reactor feed. Results are especially promising for N. gaditana, which presents several advantages over S. almeriensis; i.e., its cultivation would not require freshwater, the biocrude yields are higher, and a higher recovery of phosphorus and sulfur in the aqueous phase is obtained. However, as a disadvantage, the high chloride content of marine species might cause corrosion in the hydrothermal equipment on a long-term operation.7 3.5. Differences between CSTR and Batch Experiments. This section provides a comparison between the performances of batch and CSTR experiments for the same microalgal species. The results for batch experiments are taken from our own study42 with microautoclaves. HTL was applied to the same feedstock and according to the same product separation procedure, using microalgae loadings of 9.1 wt %. The time at the reaction temperature (350 °C) was also 15 min. Figure 5 shows the product yields for batch and CSTR experiments. The mass balance closures were higher in the CSTR, probably because the higher volumes processed in a CSTR reduced the relative importance of the mass losses H


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Energy & Fuels occurring during the product collection and separation. The yield of solid residue was low for both types of reactor, indicating that microalgae were degraded to a high extent under the conditions applied. Organics in the aqueous product increased significantly in a CSTR, while the gas product underwent minimal decreases. The biocrude oil yield decreased for both strains when using a CSTR. The decrease was much higher for S. almeriensis (from 51.5 ± 2.1 wt % in batch to 42.6 ± 5.5 wt % in CSTR) than for N. gaditana (52.6 ± 2.3 wt % for batch and 50.8 ± 1.1 wt % for CSTR). This species-dependency of the variation in the biocrude oil yields between reactors was probably caused by differences in the biochemical composition of the species. N. gaditana is richer in TAGs, and TAGs typically lead to a straightforward formation of biocrude oil molecules (fatty acids) under HTL conditions. Other biochemical fractions form biocrude oil through more complex repolymerization reactions, which are weakened in CSTR reactors.26 This could hypothetically explain the larger differences in the formation of biocrude oil between batch and CSTR experiments for S. almeriensis. Primary intermediates can react with final products (i.e., hydrogenation reactions) in a CSTR due to the intense mixing, which enables reaction pathways different from those in batch reactors. Kruse and Faquir26 reported that the mixing in a CSTR at supercritical conditions was providing “active hydrogen” formed in situ to all the reaction steps. The active hydrogen would saturate the intermediate products formed, thus suppressing repolymerization reactions and reducing the yield of biocrude oil. This could explain the lower biocrude oil yields found in the CSTR experiments: a higher saturation of the reaction intermediates and a weaker repolymerization would retain more organic matter in the aqueous phase. However, the formation of hydrogen under the conditions applied in the present study would be lower than in the study from Kruse and Faquir26 because of the lower temperature applied in the present work, and therefore the impact of this effect on the yields is expected to be limited in our experiments. Many molecules are involved in the HTL process, each of them having a different reactivity. The conversion of microalgae to water-soluble compounds takes place likely through splitting reactions that have a reaction order typically below one. The repolymerization of water-soluble compounds to form biocrude oil or char exhibits normally reaction orders higher than one,44 and the degradation to form gas compounds generally presents reaction orders below one.26 In this regard, the choice of a batch reactor or a CSTR will influence the concentration of the various molecules in the reaction medium, thus affecting the kinetics and the product yields of the HTL process. The higher dilution in a CSTR would reduce the rate of the pathways with reaction orders higher than one. Batch and CSTR modes presented differences also in the elemental composition of the biocrude oil. The carbon and nitrogen contents were higher for batch operation, while the oxygen content was higher for the CSTR (Table 6). The higher nitrogen content in batch reactions was probably caused by the higher relevance of repolymerization reactions occurring in batch systems. The pathways toward the formation of biocrude oil from proteins typically imply the occurrence of repolymerization reactions.34 Since such reactions are reduced in a CSTR (see Section 3.2), the formation of biocrude oil from the proteinic fraction is weakened, hence explaining the lower nitrogen content of biocrude oils produced in a CSTR. The

Table 6. Elemental Composition of the Biocrude Oil from Batch and CSTR Operation Modes: Experiments at 9.1 wt % Microalgae Loading, 350 °C, and 15 min of Residence Time operation mode S. almeriensis N. gaditana a

b

batch CSTR batchb CSTR

C

H

N

S

Oa

74.9 73.2 74.8 74.2

9.1 9.3 9.9 9.3

5.9 5.1 5.3 4.0

0.7 0.8 0.5 0.6

9.6 11.7 9.5 11.8

By difference. bData from López Barreiro et al.42

cause for the higher oxygen content remains obscure at this moment and demands more research to elucidate the mechanisms and rates governing the formation of biocrude oil in batch and continuous reactors, which was out of the scope of the present study. It could be hypothesized that the lower gas formation (which is almost entirely composed of CO2) in CSTR experiments is a sign of a lower relevance of decarboxylation reactions, leading to a reduced deoxygenation of the biocrude oil. In terms of molecular weight distribution, batch experiments led to a biocrude oil containing molecules with higher molecular weight, as shown by the tailing of the chromatograms from Figure 6. The higher molecular weight can be again attributed to the preponderance of repolymerization reactions in batch reactors.26 The heating rate probably affects as well the molecular weight distribution. Instant heating up to the reaction temperature occurred in a CSTR, while for batch experiments a heating rate of ca. 18 °C·min−1 was applied. At the slow heating rates applied in batch reactors, the feedstock undergoes stepwise hydrolytic and pyrolytic reactions as the temperature rises.45 In CSTR, these two reactions take place simultaneously due to the instant heating, thus resulting in a different reactivity. The shorter heating and cooling periods in CSTR, together with the continuous removal of products from the reaction medium and the previously explained effect of active hydrogen, could weaken the repolymerization reactions, therefore producing a biocrude oil poorer in heavy molecules. No differences were found in the type of molecules detected by GC-MS in the biocrude oils from batch and CSTR reactions, although this might be related to the reduced fraction of the oil that can be identified by GC-MS. The FT-IR spectra (Supporting Information Figure S5) was identical as well for batch and CSTR operations, indicating that they lead to the production of biocrude oils with the same chemical functionalities. In any case, comparing batch and continuous HTL is a complex issue because of the difficulty in accounting for the effect of the heating and cooling periods. The information provided in this study indicates that the results available in the literature for microalgae HTL (carried out in the vast majority in batch experiments) may vary when continuous processing is applied. In this sense, our results show different trends than in Faeth et al.12 or Bach et al.13 In those studies it was reported that remarkable increases in the biocrude oil yields occurred when high heating rates were applied in batch experiments. This did not apply to the data presented in the current study. The biocrude oil decreased for both species in these CSTR experiments. Therefore, no positive influence was found in terms of biocrude oil formation from stirring the medium while applying instant heating rates to achieve the reaction temperature. I


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Figure 6. Molecular weight distribution of the biocrude oil collected from batch42 and continuous operation modes from Scenedesmus almeriensis (left) and Nannochloropsis gaditana (right). Experiments at 9.1 wt % microalgae loading, 350 °C and 15 min of residence time.

4. CONCLUSIONS HTL experiments in a CSTR were successfully conducted for two microalgae species at 350 °C and a residence time of 15 min, using two different microalgae loadings in the feed. The operation appeared to be stable, though some problems were observed when pumping microalgae slurries from S. almeriensis. Biocrude oils produced in a CSTR presented lower yields and less heavy molecules than when obtained in batch experiments at the same reaction conditions. The biocrude oil was the main product phase in all cases, achieving a maximum yield of 54.8 wt % for N. gaditana at a microalgae loading in the feed of 18.2 wt %. The high microalgae loading led to the formation of a biocrude oil with a reduced oxygen content and an increased nitrogen content. The biocrude oil yield was also increased, probably by the enhancement of repolymerization reactions (typically with a reaction order higher than one). This was not affecting dramatically either the molecular composition (as analyzed by GC-MS) or the molecular weight distribution. The recovery of inorganic nutrients in the aqueous product was species-dependent, but essentially independent of the microalgae loading in the feed.

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Habicht, H. Habicht, A. Lautenbach, H. Köhler, G. Zwick, A. Böhm, and C. Altesleben (KIT) are acknowledged for their assistance in the analyses.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02099. Analytical methods and equipment, scanning electron microscope pictures of the feedstock before and after milling, chromatograms of the biocrude oils, and FT-IR for batch and CSTR HTL biocrude oils (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +32 9 264 6190. E-mail: Diego.LopezBarreiro@UGent. be. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the Institute for the Promotion of Innovation by Science and Technology (IWT) from Belgium is acknowledged (Grant 121018). Fundación Cajamar is also acknowledged for providing the microalgae biomass. B. Rolli, S. J


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