Applied Energy 154 (2015) 815–828

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Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

Process application of Subcritical Water Extraction (SWE) for algalbio-products and biofuels production

http://dx.doi.org/10.1016/j.apenergy.2015.05.0760306-2619/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +60 3 8946 6289; fax: +60 3 8656 7120.E-mail address: mh_razif@upm.edu.my (R. Harun).

Selvakumar Thiruvenkadam a, Shamsul Izhar a, Hiroyuki Yoshida a, Michael K. Danquah b, Razif Harun a,⇑a Department of Chemical and Environmental Engineering, Universiti Putra Malaysia, 43400 Serdang, Malaysiab Department of Chemical and Petroleum Engineering, Curtin University of Technology, 98009 Sarawak, Malaysia

h i g h l i g h t s

�We summarize the studies on subcritical water extraction of algae.� Factors affecting biocrude yield, such as temperature and time, were reviewed.� The produced biocrude evidence the future prospects of algal biofuels.� Biocrude upgrading and scale-up strategies were too included in this paper.

a r t i c l e i n f o

Article history:Received 20 November 2014Received in revised form 19 May 2015Accepted 21 May 2015Available online 7 June 2015

Keywords:Subcritical water extractionAlgaeBiocrudeBio-oilBiofuelsThermochemical

a b s t r a c t

Algal biomass is appreciated as an essential bioenergy feedstock owing to the rapid growth rate of algalcells and the capacity to harbor substantial quantities of biochemicals via CO2 biosequestration forbiofuel production. Amongst the various thermochemical technologies for converting algal biomass tobiofuels, Subcritical Water Extraction (SWE) demonstrates significant capacity for generating liquidtransportation fuels from algae with minimal environmental impacts. The SWE process expends pressur-ized water to produce biocrude or bio-oil as well as aqueous, solid, or gaseous by-products. However, theexistence of high levels of heteroatoms in biocrude hinders its application in internal combustionengines, and this has triggered studies into parametric characterization of biocrude production from algalbiomass. This article comprehensively reviews the process principles, optimal conditions, engineeringscale-up and products development to ascertain the viability of an industrial-scale SWE process forbiofuel production from algae.

� 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8162. The theory of SWE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8163. Biocrude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817

3.1. Factors affecting biocrude yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817

3.1.1. Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8173.1.2. Effect of residence time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8193.1.3. Effect of water density and biomass loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8203.1.4. Effect of other solvent/co-solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8203.1.5. Effect of catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8203.1.6. Effect of heating rate and stirring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8213.1.7. Effect of algae strain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822

3.2. Energy recovery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8223.3. Modeling and LCA analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8243.4. Biocrude upgrading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824

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3.5. Scale-up implications and recent developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825

4. Other products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825

4.1. Aqueous product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8254.2. Solid residue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8264.3. Gas product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 826

5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 826Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 826References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 826

Fig. 1. The phase diagram of water representing subcritical region [23].

1. Introduction

Climate change, increasing energy consumption, growinghuman population and the development of new economies callfor the creation of new and sustainable energy technologies suchas biofuels [1]. The development of first and second generation bio-fuels from proposed candidates, such as energy crops, lignocellu-losic biomass, and agricultural wastes carry many demerits froma biodiversity and economic perspective [2]. In recent years, therehas been an increasing interest in algal biofuels, so-called thirdgeneration biofuels. Algae are a unique biomass feedstock for sus-tainable production of biofuels. Algae, one of the fastest growingphotosynthetic organisms on earth, have biomass productivityrates higher than terrestrial plants [3]. They have several advan-tages including tolerance to extreme environmental conditions,eco-friendly cultivation process, simple life cycle and resourceavailability for large-scale production [2–5]. Additional benefitsof algae over food crops include fast growth rates, less waterintake, adaptation to various water sources (fresh, seawater,saline/brackish and wastewater), high photosynthetic efficiency,carbon dioxide (CO2) biosequestration, phytoremediation, inex-pensive cultivation techniques using non-arable land and shortharvesting periods. Notwithstanding these benefits, algal biofueldevelopment faces a few drawbacks which include low biomassdensities and high operating costs for biomass generation and con-version [6]. Although algal based biofuels generate approximately13% CO2 lower emissions from combustion relative to CO2 emis-sions from petroleum diesel [7], in terms of absolute emissionlevels, algal biofuels can be significantly high for full-scale applica-tions. The development of biofuels from algal biomass has beensignificantly successful under lab-scale conditions. However,opportunities for commercial-scale applications should focus onaddressing related environmental, technological and economicdrawbacks [2]. A wide variety of bioactive materials for pharma-ceutical, nutraceutical, and biomedical applications can beextracted from algae. Apart from bio-oil, other products, such ascarbohydrates, polyunsaturated fatty acids (PUFAs), vitamins,minerals, and dietary fibers, have been realized from algae.

Most existing technologies for production of algal biofuels pri-marily aim on extracting lipids from algae and these technologiesinvolve downstream harvesting or dewatering processes such as,flocculation and centrifugation [8]. However these processes areexpensive, energy intensive, unsustainable, involve lengthyprocessing steps and result in lower product yields. Furthermore,thermochemical conversion technologies, including gasification,pyrolysis and Fischer–Tropsch process, require dry biomass andoperational temperatures higher than 600 �C [9]. Cost-effectiveand energy-effective strategies play a key role in commercializa-tion of algal biofuels. One of the newer technologies for generatingalgal biofuels is Subcritical Water Extraction (SWE). The techniqueinvolves liquefaction of high moisture content biomass containinglow solids (5–30%), with no requirement of energy-intensive cul-ture dewatering and drying operations prior to extraction [10].Unlike pyrolysis, SWE does not involve water volatilization, andthe technique is more preferable to pyrolysis in terms of energy

consumption [11]. Moreover, SWE process uses water which is acleaner solvent than chemical solvents, making it more favorablein terms of environmental concerns and the lack of solvent recov-ery operations [12]. Water plays a dual role as solvent and catalystduring SWE and after the reaction, water is recovered from the pro-duct separator and recycled back into SWE process. A simple heatintegration system would economically help to scale-up this tech-nology by reducing energy penalty [13]. The main products of SWE,biocrude or bio-oil, can be used as transportation fuel alone orblended with petroleum diesel [14]. A body of work have beenreported on the application of SWE for the extraction of essentialoils from coriander seeds [15], citrus fruit Yuzu [16], and herb zaa-tar [17]; proteins and amino acids from deoiled rice bran [18];antioxidant compounds from canola meal [19], and winery waste[20]; phenolic compounds from bitter melon [21], and pomegra-nate [22]. The high moisture content of algal feedstock has madeSWE a promising technology to produce biocrude with minimaldewatering requirements. This article presents an overview ofSWE process with bioprocess insights into the application of SWEfor the production of biofuels and bioproducts from algal biomass.

2. The theory of SWE

In SWE, water is pressurized to a temperature and pressureunder its critical level conditions, described as the subcritical regionaccording to Fig. 1. During SWE process, the feed slurry is processedin a hot pressurized water system at a temperature between 100and 374 �C and a pressure of up to 22.1 MPa with or without thepresence of a reducing gas and/or catalyst [14,24]. The operatingpressure is kept constant above the vapor pressure to hold thewater in the liquid state at subcritical conditions. When operatingbelow the vapor pressure, water boils to generate water vaporand this vaporization process increases the pressure above the

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vapor pressure level, resulting in the water being present in theliquid state [25]. Besides the liquid state of water at subcriticalconditions, the increase in dielectric constant and the decrease indensity (1 g/cm3 at 25 �C to 0.75 g/cm3 at 300 �C) compared toambient conditions, result in hydrocarbons becoming morewater-soluble [26,27]. The decrease in density increases the dielec-tric constant (relative permittivity; er), dissociation constant (Kw)and diffusion, resulting from the breakdown of hydrogen-bondingin the water molecule [27]. The polarity of water changes fromcomplete polarity to moderately non-polar in the subcriticalregions and this tends to increase the affinity of water towardnon-polar organic hydrocarbons [9]. Many acid- or base- catalyzedreactions are accelerated at subcritical conditions due to highconcentrations of hydronium (H+) and hydroxide (OH�) ions,derived from auto-ionization of water [12,28]. Close to criticalpoint, auto-ionization of water triples in order of magnitudecompared to ambient conditions; Kw of water increases from10�14 at 25 �C to 10�11 at 250 �C and er of water decreases from78.85 at 25 �C to 19.66 at 300 �C [9,27]. At these conditions, acascade of degradation and repolymerization reactions takes placewith the biomass, resulting in the establishment of a viscid liquid,referred to as biocrude or bio-oil, which normally consists oforganic acids, various ketones and phenols. Other co-products fromSWE include aqueous products of dissolved organics, gas and solidresidue. Biocrude from SWE of algal biomass does not only containthe lipid fraction, but also protein and carbohydrate fractions. Thedegradation reactions transform the organic biomass into manyfragments which are converted to hydrocarbons by the additionof hydronium ions to the existing open carbon bonds [9]. Theorder of biocrude conversion efficiency during SWE process is:lipids > protein > carbohydrates [29]. Biocrude, a precursor ofbiodiesel, is the main product generally targeted from SWE of algalbiomass. After refining, the biocrude is used directly orco-processed with petroleum diesel as a transportation fuel[30,31]. Biocrude can also be used as the feedstock to produce otherbio-products such as resins and polyurethane foam materials [30].After the SWE process, the products are cooled down, and thisrestores water to its ambient ion concentrations, acting as aself-neutralizing medium. This demonstrates the simplicity ofSWE process with the elimination of solvent recovery steps andreducing waste production [27]. Recycling of aqueous products tothe algal cultivation system can be developed to provide a closedloop bioprocess for the algal biorefinery paradigm. The proposedscheme for the biorefinery model employing SWE process forproduction of biofuel from algae is illustrated in Fig. 2 and reactionmechanisms are also presented.

The schematic diagram of batch SWE process is shown in Fig. 3,as depicted in Li et al. [14]. The setup consists of three main parts:batch reactor with reactant(s) sampling and product(s) drainingports, gas cylinder(s) and data recorder(s). The reactor is rinsedwith ethanol prior to the run. The feedstock(s) and extractingsolvent (water, ethanol or acetone) are loaded into the reactorand closed tightly. The reactor headspace air is evacuated andrefilled with pure N2 [14] or He [5] gas for three cycles. The vesselcontaining the reaction mixture is fitted with an electric heatedjacket for a defined residence time. The reactor is removed fromthe bath and quenched with cold water [5]. Gaseous products canbe analyzed by connecting to a gas chromatographic unit beforedepressurizing the system. The liquid product mixture is collectedfrom the reactor and subsequently cooled to yield three differentproducts: biocrude/bio-oil, aqueous product and solid residue.

3. Biocrude

Biocrude is an energy dense product derived from SWE of wetbiomass [32]. The algal biocrude has less dissolved water with

low density and provides a lower acid content (Total AcidNumber, TAN), in comparison to biocrude from liquefied lignocel-lulosic biomass [33]. This makes algal biocrude advantageous forbiofuel development. However, algal biocrude is more viscous atroom temperature with high levels of nitrogen and sulfur. Bio-oilfrom lignocellulosic biomasses can directly be used as heavy fueloil but the presence of high concentrations of oxygen and nitrogenrestricts its direct utilization as a transportation fuel. The conven-tional petroleum hydroprocessing technologies can process algalbiocrude to hydrocarbon fuels similar to existing fossil petroleumproducts [25]. The effects of physical and chemical parameterson biocrude produced from SWE of algal biomass are discussedin the following section.

3.1. Factors affecting biocrude yield

3.1.1. Effect of temperatureThe most dominating operational parameter in SWE process is

temperature. High thermal energy released during elevated tem-peratures increases the solvation power of the diluent or solventthereby overcoming cohesive and adhesive forces. High tempera-tures contribute to decreasing water polarity [34]. This decreasein polarity of water molecules at high temperatures favorscomplete miscibility between lipids and water molecules, thusincreasing aqueous lipid extraction yield from algal biomass [35].Close to the critical point (Tc = 374 �C and Pc = 22.1 MPa), the ionicproduct of water increases, leading to hydrothermal breakage ofbiomass into many fragments, favoring the increase in biocrudeyield with increasing temperature. However, with further temper-ature increase, biocrude yield decreases as a result of the formationof new compounds from degraded products. The formation of newcompounds is induced during condensation, polymerization and/orcyclization reactions. Shuping et al. [24] observed the same resultswhen evaluating the effect of temperatures on biocrude yield ofDunaliella tertiolecta. The optimum liquefaction temperature wasfound to be 360 �C. Chen et al. [36] reported an increase in bio-crude yield with increasing temperature up to 320 �C; with a fur-ther increase in temperature resulting in the decomposition ofbiocrude to gaseous products. During SWE of Chlorella pyrenoidosa[37] and planktonic algae [38], oil yield notably increased withincreasing temperature from 260 �C to 280 �C, and a furtherincrease in temperature decreased the oil yield. Similar outcomeswere reported by Anastasakis and Ross [39], where biocrude pro-duction increased with increasing temperature during SWE ofLaminaria saccharina. A threshold temperature of 340 �C was foundto maximize biocrude yield during hydrothermal co-liquefaction ofSpirulina platensis (microalgae) and Entermorpha prolifera (macroal-gae) [40]. It was understood that the synergistic effect on biocrudeproductivity during liquefaction of both algal strains is influencedby temperature.

Plaza et al. [41] reported the influence of temperature onextraction yield while characterizing bioactive compounds fromthe green alga Chlorella vulgaris by SWE and Ultrasound-AssistedExtraction (UAE) techniques. It was shown that the extractionyields were higher in SWE than UAE and this was attributed tothe increased mass transfer in SWE. Moreover, the highest yieldand the maximum antioxidant activity were recorded at the high-est temperature examined (200 �C). SWE with different solventsshowed that extraction with water resulted in high yields ofantioxidant compounds compared to ethanol and acetone. Theantioxidant activity of the water extracts increased with increasingtemperature, but this trend was not observed with the acetone andethanol extracts. However, generally, the diffusion coefficients ofthe compounds increase with increasing extraction temperatureand this favors desorption kinetics from the cellular matrix [42].

Fig. 2. Algae biorefinery model featuring SWE.

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High aqueous phase and solid residue yields accompanying lowyields of biocrude and gas are usually noted at low temperaturesdue to hydrolysis reactions [43]. However, increasing temperatureshows increasing biocrude and gas yield with decreasing organic

phase and solid residue yields [44]. Eboibi et al. [45] examinedthe effect of temperature on SWE of Tetraselmis species.Increasing the reaction temperature from 330 �C to 350 �C for5 min increased the biocrude yield. However, increasing the

Fig. 3. A schematic drawing of a typical batch scale SWE process. Source: Li et al. [14]; reprinted with permission.

S. Thiruvenkadam et al. / Applied Energy 154 (2015) 815–828 819

temperature further favored gasification reactions to increase gasyield. High temperatures (330–340 �C) were found to be optimalfor rapid degradation of proteins and carbohydrates, and biocrudeformation at these conditions are favored with rapid heating andcooling rates [46]. It has also been demonstrated that differentoperating conditions can promote the conversion of individual bio-chemical components. On comparing SWE of freshwaterOedogonium sp. and marine Derbesia tenuissima, Neveux et al.[46] concluded that the biocrude production is dependent on theorganic fraction of the algae regardless of its origin, freshwater ormarine environment. The extent of depolymerization, crackingand repolymerization during SWE is influenced by the reactionconditions [43]. It is unclear if the three main reactions (hydrolysis,cracking and polymerization) happen sequentially or simultane-ously to produce biocrude or bio-oil. To understand the cell break-age point during SWE, Garcia-Alba et al. [44] visually monitoredDesmodesmus sp. cells after SWE using scanning electron micro-scopy (SEM). The changes in algal cell morphology were correlatedwith the differences in biocrude yield. At 225 �C, the cells werefound to be distorted and closely filled with some solid molecules,which are probably from adhesions of intracellular denatured pro-teins and/or maillard reactions producing melanoidins. The pres-ence of algaenan in green algae such as Desmodesmus sp.provides thermal resistance to the cell wall at higher temperatures,thus denaturation of intracellular components can occur withoutcell wall breakage. After increasing the temperature to 250 �C,the visually compact cells at 225 �C ruptured to release intracellu-lar components.

Valdez et al. [47] investigated the effects of temperature, reac-tion time, water densities, and biomass loadings on the yieldsand elemental analyses of the product fractions from SWE ofNannochloropsis sp. in mini-batch reactors under sub- andsuper-critical conditions. The biocrude yields at subcritical temper-atures (300 �C and 350 �C) were higher compared to the yields at250 �C and 400 �C. A steady increase in the yield of light biocrudewas observed with increasing temperature above 300 �C. In con-trast, the yield of heavy biocrude decreased above 300 �C, resultingfrom the polymerization reactions and/or the conversion of heavybiocrude to light biocrude. Extract viscosity is used to distinguishbetween heavy and light biocrude. The formation of water solubleproducts decreased with increasing process time at 300 �C and350 �C, and this decrease was validated by the increase of volatilecompounds, indicating the decomposition of aqueous phase com-pounds to volatile compounds. Increasing yields of gases (H2, CO,

CO2, CH4, C2H4 and C2H6) were observed with increasing processtime and temperature. Jin et al. [48] studied the SWE behavior ofC. pyrenoidosa in an acetone medium under sub- andsuper-critical conditions (Tc = 235 �C and Pc = 4.8 MPa). A decreas-ing trend in biocrude yield was observed with increasing temper-ature. The decrease in biocrude yield was accompanied by aincrease in solid yield. Compared to water, liquefaction of algaein acetone occurs at milder conditions with a trade-off in biocrudequality including high viscosity and high nitrogen content.

3.1.2. Effect of residence timeIn a study to investigate the effect of residence time on SWE of

L. saccharina, 15 min was found to be the optimum holding time at350 �C and further increase in residence time decreased biocrudeyield due to subsequent condensation and/or polymerization ofbiocrude intermediates to form new high molecular weight prod-ucts [39]. This was supported by Xu et al. [38], Jin et al. [40] andShuping et al. [24], who observed decreasing in biocrude yieldbeyond the threshold points 30 min, 40 min and 50 min respec-tively. Also, a short residence time during SWE increases biocrudeyield due to the rapid release of the intracellular components of thebiomass [45]. A short residence time is beneficial to reduce thecosts for commercial scale applications involving small reactors.However, Eboibi et al. [45] and Jazrawi et al. [26] reported thatshorter residence times, though produce high biocrude yield, gen-erates low quality biocrude due to high oxygen content.Notwithstanding, Shuping et al. [24] discovered a consistentincrease in oil yield up to 60 min residence time during SWE ofD. tertiolecta. With increasing residence time, the aqueous phaseand solid residue showed a gradual decrease while the gaseousyield gradually increased. Tsigie et al. [31] showed that the fattyacid methyl ester (FAME) yield increased with increasing reactiontime during SWE for biodiesel production from C. vulgaris. It hasalso been suggested that a longer residence time is essential toincrease FAME yield, and the series of process steps takes placein the order: cell wall breakage, release of lipids out of the cell,and reaction of lipids with methanol [31]. Longer retention timesare recommended for repolymerization of small organic materialsformed from protein hydrolysis [49]. On the contrary, longer reten-tion times do not affect the lipid extraction recovery from algal bio-mass [50]. Garcia-Alba et al. [44] demonstrated that at amoderately low temperature (200 �C), a longer residence timecauses thermal degradation of intracellular constituents withoutrupturing the cell wall, resulting in oil yield increase as well as

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the composition of N and C elements. It was also found that shorterresidence times increase biocrude yield at temperatures above300 �C. In another set of experiments carried out at higher heatingrates 585 �C/min [10], the residence time was varied within therange 5–20 min to find the optimum retention time. It was notedthat neither a shorter nor longer residence time outside the rangeinvestigated increased the biocrude yield. The maximum biocrudeyield (79%) was obtained at 15 min residence time. While studyingthe bio-oil distribution after SWE of Desmodesmus sp. [44], thebio-oil yield doubled with increasing reaction time from 5 to60 min at a low temperature (200 �C). However, a slight increasein bio-oil yield was observed for the same variation in reactiontime at a higher temperature (300 �C).

3.1.3. Effect of water density and biomass loadingThe quantity of water in the extraction system is another crucial

parameter that influences biocrude yield. The presence of watercould act as a green solvent, hydrolyzing agent, catalyst, andhydrogen donor [39,40]. Maximum biocrude yield was achievedat an optimum reactor loading of 1:10 ratio (biomass: water,g/mL) from SWE of the brown alga L. saccharina [39], and furtherincrease in water ratio did not affect the biocrude yield. This find-ing has been supported by Jin et al. [40] after studying the influ-ence of algal biomass/water ratio on co-liquefaction of S.platensis/E. prolifera. The maximum biocrude yield was obtainedat 1:5.8 ratio (biomass:water, g/g), and a further increase in thisratio decreased biocrude yield. The dilution of biocrude precursorsat higher water loadings does not support further polymerizationreactions. According to the experimental results from SWE of C.pyrenoidosa [37], the yield of bio-oil decreased with increasingtotal solid ratio from 15% to 25%. However, the contrary wasobserved with further increase in total solids from 25% to 35%. Itwas established that total solid ratio had no effect on the higherheating value (HHV) of the biocrude or bio-oil produced.However, it had a positive effect on carbon and nitrogen contentsof the biocrude. Co-liquefaction of microalgae/cyanobacteria andmacroalgae was demonstrated in a 20 mL stainless steel reactorby Jin et al. [40]. The effect of both algae mass ratios on biocrudeyield was monitored by varying the mass ratio (S. platensis and E.prolifera) from 0% to 100%. The highest biocrude yield was achievedat a mass ratio of 50%, and any further increase in the mass ratioreduced the synergetic effect. The authors also reported that fattyacids produced from S. platensis promoted the hydrolysis of pro-teins and carbohydrates in E. prolifera. Garcia-Alba et al. [44] inves-tigated the effect of algal concentration on biocrude yield and theresult showed that the increase in algae loading rate did not havea significant effect on biocrude yield. Pham et al. [51] presented theliquefaction behavior of Spirulina on the removal of bioactive com-pounds. The results indicated that the removal of bioactive com-pounds (ceftiofur and florfenicol) decreased with increasing solidconcentration of Spirulina.

3.1.4. Effect of other solvent/co-solventsThe introduction of ethanol as the extracting solvent instead of

subcritical water in SWE resulted in a slight increase in biocrudeyield and quality under H2 as the processing gas [52]. Chen et al.[50] attempted subcritical co-solvents extraction (SCE) ofNannochloropsis sp. at 90 �C, 1.4 MPa and 50 min with a mixtureof hexane and ethanol. They also optimized the extraction condi-tions such as the hexane to ethanol (3:1) ratio and theco-solvents to microalgae (10:1) ratio. Hexane enhanced theextraction efficiency by displacing of the largely non-polar lipidsinto the hexane phase from the lipid carrier, ethanol. In the pres-ence of organic solvents, the extraction efficiency increases withincreasing temperature and pressure, and this is due to decreasingviscosity of the solvent, increasing lipid solubility and decreasing

ethanol polarity. Alcohol is a reactant for in situ transesterificationprocess to produce biodiesel [31]. It is used to convert lipids tofatty acid alkyl esters, and also considered as an extracting solventfor lipids extraction from biomass. The effect of methanol concen-tration on extraction efficiency has been reported by Tsigie et al.[31]. Increasing methanol to algal biomass ratio increased the con-version rate of biocrude lipids to biodiesel until a maximum yieldis achieved, and a further increase in the ratio decreased FAMEyield.

3.1.5. Effect of catalystCatalyst addition during SWE has generated a steady interest

since they are expected to enhance biocrude yields. Comparativestudies on the effect of organic acid (CH3COOH and HCOOH) andalkali (KOH and Na2CO3) catalysts on SWE of C. vulgaris andSpirulina displayed higher yields of bio-oil at higher temperatures,and the yield was higher for lipid-rich C. vulgaris compared toSpirulina [53]. It is generally assumed that lipid-rich algae producemore biocrude, and this was verified by Toor et al. [54] during SWEof Nannochloropsis salina, producing the highest biocrude yield of46% at 350 �C. In their study, Shuping et al. [24] evaluated theeffect of Na2CO3 catalyst on the liquefaction of D. tertiolecta andobtained the optimum biocrude yield in the presence of 5%Na2CO3. High-carbohydrate algae are probably the most efficientlyliquefied with Na2CO3 catalyst. These carbohydrates break downprecursors for biocrude formation. However, in the case ofhigh-lipid algal species, Na2CO3 has a negative effect on oil yielddue to the formation of soap between algal lipids and alkaline,causing serious biochemical and process challenges to biodieselproduction [31,49]. Zeolite catalyst types, raney-Ni and HZSM-5,produced no significant results during SWE of C. pyrenoidosa withethanol as the solvent [52]. The inexpensive catalyst zeoliteHZSM-5 was tested for treating crude bio-oil from SWE ofNannochloropsis sp. [55]. Three parameters namely reaction tem-perature, reaction time and catalyst loading were examined andit was found that the reaction temperature had a significant effectin the treated bio-oil composition compared to the otherparameters.

Biller et al. [56] studied the effect of three catalysts (Pt/Al2O3,Ni/Al2O3 and Co/Mo/Al2O3) during SWE of C. vulgaris andNannochloropsis occulta. Among the catalysts tested, Co/Mo/Al2O3

increased biocrude yield from C. vulgaris by 3 wt.% while the bio-crude yield from N. occulta decreased by 10 wt.%. A similar trendwas obtained for the Pt/Al2O3 catalyst. Ni/Al2O3 catalyst decreasedbiocrude yield from both algal species due to hydrogen formationpromoted by increased decarboxylation and gasification reactions.Algal biocrudes derived from these heterogeneous catalysts con-tain no alkanes, indicating the deoxygenation of protein and carbo-hydrate fractions of the algal species. Anastasakis and Ross [39]tested KOH catalyst and found that the biocrude yield decreased,with a significant increase in the solubility of organic compoundsin the aqueous phase. The addition of organic acid and alkaline cat-alysts to SWE of C. vulgaris and Spirulina was shown to improve theheat content of bio-oils [53]. The alkaline catalyst increased theheat content slightly higher than the organic acid. Furthermore,it was found that the organic acid catalyst reduced the boilingpoint and increased the flow properties of the bio-oil.

Yu et al. [57] evaluated the effect of five heterogeneous (Pd/C,Pd/Al2O3, Pt/C, Pt/Al2O3 and Raney Nickel) and two alkaline(NaOH and Na2CO3) catalysts on SWE of C. pyrenoidosa. The cat-alyzed SWE analyses at 240 �C produced no significant increasein biocrude yield except NaOH. However, all the catalysts producedmore significant results at 280 �C. The overall effect of catalystpresence on biocrude yield was not overwhelmingly high, and thislikely due to instability of the catalysts under SWE operating con-ditions. To study the effect of catalyst dosage, the authors utilized

S. Thiruvenkadam et al. / Applied Energy 154 (2015) 815–828 821

5% and 30% Pt/Al2O3 at the same SWE conditions. They observedthat the catalyst dosage did not affect the biocrude yield. The studyalso established that pre-reduction of metal catalysts is unneces-sary as oxygen is removed from fatty acids by decarboxylation.The catalysts were found to be less effective on the boiling pointdistribution of biocrude. Also, the elemental composition of bio-crude did not considerably change after catalyst application. Theheterogeneous catalysts increased the hydrocarbon fractions inbiocrude (except for Pd/Al2O3) and decreased the organic acidsfraction. The compositions of amides, nitrogen and oxygen hetero-cyclic compounds were not affected by the heterogeneous cata-lysts. The alkaline catalysts increased the N and O heterocycliccompounds, probably due to polymerization of oil intermediatespromoted by hydrogen ions which are dissociated from fatty acids.

The effect of various catalysts on biocrude yield is presented inTable 1. The possibility of carbon deposition on the surface of thecatalysts were investigated using environmental scanning electronmicroscopy (ESEM) [57]. This is essential for repeated use of thecatalysts. To minimize catalytic poisoning by algal feedstock,Yang et al. [5] proposed a two-chamber reactor, where the algalfeedstock and the catalyst (Pt/C) are loaded into each chamber.The chambers were separated by a porous metal frit to allow freeflow of molecules between the chambers. This arrangement was

Table 1Summary of elemental compositions and calorific values of fresh algae and biocrudes pro

Algae Feedstock Biocrud

Elemental Compositiona HHVb Element

C H N O S C

Catalyst: Na2CO3

Chlorella vulgaris 52.6 7.1 8.2 32.2g 0.5 23.2 73.6Dunaliella tertiolecta 39f 5.37f 1.99f 53.02f,g ND 20.08 63.55f

Nannochloropsis oculata 57.8 8 8.6 25.7g NA 17.9 69.6Porphyridium cruentum 51.3 7.6 8 33.1g NA 14.7 46.1Spirulina sp. 55.7 6.8 11.2 26.4g 0.8 21.2 75.4

Catalyst: HCOOHChlorella vulgaris 53.6 7.3 9.2 29.4g 0.5 23.2 72.1Chlorella vulgaris 52.6 7.1 8.2 32.2g 0.5 23.2 70.8Nannochloropsis oculata 57.8 8 8.6 25.7g NA 17.9 74.7Porphyridium cruentum 51.3 7.6 8 33.1g NA 14.7 72.5Spirulina sp. 55.7 6.8 11.2 26.4g 0.8 21.2 72.7Spirulina sp. 54.4 7.6 10.9 26.3g 0.83 21.2 72.7

Catalyst: CH3COOHChlorella vulgaris 53.6 7.3 9.2 29.4g 0.5 23.2 70.8Spirulina sp. 54.4 7.6 10.9 26.3g 0.83 21.2 71.7

Catalyst: KOHChlorella vulgaris 53.6 7.3 9.2 29.4g 0.5 23.2 74Spirulina sp. 54.4 7.6 10.9 26.3g 0.83 21.2 74.6

Catalyst: Pt/Al2O3

Chlorella vulgaris 52.6 7.1 8.2 32.3g 0.5 23.3 74.8Nannochloropsis occulta 57.8 8 8.6 25.7g ND 17.9 74

Catalyst: Ni/Al2O3

Chlorella vulgaris 52.6 7.1 8.2 32.3g 0.5 23.3 75.4Nannochloropsis occulta 57.8 8 8.6 25.7g ND 17.9 76.8

Catalyst: Co/Al2O3

Chlorella vulgaris 52.6 7.1 8.2 32.3g 0.5 23.3 75.2Nannochloropsis occulta 57.8 8 8.6 25.7g ND 17.9 77

Catalyst: Ni–Mo/Al2O3

Nannochloropsis salina 56.52 8.28 8.51 26.69g 24.17 73.46

a wt.%, ash-free dry weight.b Higher heating value, MJ/Kg.c wt.%, dry weight.d temperature, �C.e time, min.f wt.%, dry weight.g Calculated by difference.h wt.%, ash-free dry weight.

compared with the conventional method of loading algal feedstockwith catalyst presence in a single-chamber reactor. SWE ofNannochloropsis sp. from the two-chamber reactor produced aslightly better biocrude yield than the conventionalsingle-chamber reactor setup. This slight increase in biocrude yieldis due to the reduction of catalyst deactivation from algal biochem-icals. SWE of Nannochloropsis sp. at 350 �C for 60 min under H2

(30 bar) atmosphere produced 80% of light biocrude resulting fromhydrogen-promoted catalytic reactions such as hydrogenation andhydrodeoxygenation. Li et al. [14] studied the effect of hydrogena-tion on SWE of N. salina. For hydrogenated SWE under Ni–Mo/Al2O3 catalyst, a biocrude yield of 78.5% was reported, and thiswas higher than those reported for non-hydrogenated SWE(55.6%). In addition, the hydrogenation process produced less gas-eous products. Bach et al. [10] reported a different effect of catalyst(KOH) addition on biocrude yield of seaweed L. saccharina. Theyobtained a slight increase in biocrude yield �2%. However, thisstudy was carried out at higher heating rates (585 �C/min) usingreactors with relatively small volumes.

3.1.6. Effect of heating rate and stirringBach et al. [10] carried out the optimization of SWE of L. saccha-

rina by using small quartz capillary reactors at different heating

duced from catalyzed SWE processes.

e SWEconditions

Reference

al Compositiona HHVb Yieldc Tempd Timee

H N O S

10.7 4.9 10.7g 0 37.1 30 350 60 [49]7.66f 3.71f 25.08f,g ND 30.74 25.8 360 5 [24]9.2 3.8 17.3g 0 35.5 28 350 60 [49]5.6 3.2 13.3g 0.2 22.8 30 350 60 [49]10.8 4.6 8.7 0.5 34.8 18 350 60 [49]

9.5 6.4 11.5g 0.5 35 22h 300 60 [53]9.4 5.3 13.9g 0.6 33.2 30 350 60 [49]10.6 4.3 10.4g 0 39 28 350 60 [49]9.1 5.7 13.3g 0.4 36.3 20 350 60 [49]9.8 5.7 10.9g 1 35.1 22 350 60 [49]9.8 5.7 10.9g 1 35.6 14.2h 350 60 [53]

9.4 5.3 14.1g 0.4 34.2 23h 300 60 [53]9.7 6.1 11.6g 0.9 35.1 16.6h 350 60 [53]

12.9 4.3 8.9g 0.2 39.9 22.4h 350 60 [53]11.4 5.1 8.5g 0.5 33.4 15.2h 350 60 [53]

9.7 5.6 9.3g 0.6 37.9 38.9h 350 60 [49]10.2 3.6 12g 0.1 38.2 30.2h 350 60 [49]

6.7 5.4 12.6g 0 34.5 30.0h 350 60 [49]9.4 3.6 10.2g 0 38.2 18.1h 350 60 [49]

8.3 5.7 10.7g 0 36.2 38.7h 350 60 [49]8.9 4.6 9.4g 0 37.6 25.5h 350 60 [49]

10.31 4.88 11.35g 37.53 78.5h 340 30 [14]

822 S. Thiruvenkadam et al. / Applied Energy 154 (2015) 815–828

rates (146 �C/min, 321 �C/min and 585 �C/min). The study wasaimed to minimize the heat and mass transfer limitations. At thehighest heating rate of 585 �C/min, the maximum biocrude yieldof 79% was achieved while the biocrude yield of 53% and 65% wereobtained at heating rates of 146 �C/min and 321 �C/min respec-tively. They also concluded that higher heating rates are morefavorable in enhancing biocrude yield than the utilization of cata-lysts. The beneficial effects of rapid heating [10] are: (i) accelerat-ing degradation reactions, (ii) limiting unwanted reactions, (iii)facilitating cell disruption, and (iv) preventing biochar formation.In the case of lignocellulosic compounds, heating and cooling rateare characterized as crucial parameters during liquefaction [58].The effect of stirring has been demonstrated to have a negligibleeffect on biocrude yield [31]. Stirring is mostly considered duringSWE to avoid clustering of biomass, and also to make biomassaccessible to methanol during in situ transesterification [31]. Theauthors also confirmed that biomass stirring at lower temperaturesprovided better results with simultaneous extraction and transes-terification of wet microalgal biomass.

3.1.7. Effect of algae strainThe production of biofuels from algal biomass is influenced by

both biological (e.g. algal cell morphology, growth conditions suchas, temperature, nutrients and photo period) and technological(cultivation, harvesting, extraction and/or conversion) factors[59]. As presented in Fig. 4, biochemical composition variesbetween algae species. Neveux et al. [46] reported a low biocrudeyield from the marine alga Chaetomorpha linum and this is partlydue to its high inorganic content (36.6 wt.% ash and 5.1 wt.% mois-ture). Algae species with high inorganic content are recognized asundesirable for SWE as they result in low biocrude yields. Ash con-tent in algal biomass could be reduced by either selected culture orpost-harvest processing [46]. Chen et al. [64] proposed a two-steppretreatment method, centrifugation followed by ultrasonication,to reduce ash content and increase biocrude yield for amixed-culture wastewater algae (WA). Centrifugation segregatesthe volatile matter from ash while the ultrasonication process

0 20 40

Ulva ohnoiUlva fasciata

Tetraselmis sp.Spirulina platensis

Spirulina sp.

Porphyridium cruentumOedogonium sp.

Nnannochloropsis occultaNannochloropsis salina

Nannochloropsis oceaniaGelidium amansii

Enteromorpha proliferaDunaliella ter�olectaDerbesia tenuissima

Cladophora vagabundaCladophora coelothrix

Chlorogloeopsis fritschiiChlorella vulgaris

Chlorella sorokinianaChaetomorpha linum

Bio

lip

Scenedesmus dimorphous

Fig. 4. Biochemical composition (%) of different s

breaks down volatile mater into individual components bycavitation.

The biochemical composition and the structural characteristicsof the cell wall affect the SWE process and the corresponding bio-crude yield. Algal strains with no or reduced cell wall (e.g.Phaeodactylum tricornutum and D. tertiolecta) produce more bio-crude yield at mild SWE conditions compared to species withstrong cell walls (e.g. Scenedesmus obliquus) [65]. Under harshand severe SWE conditions, strain-specific parameters are lessinfluential on biocrude yield. Reddy et al. [35] carried out SWE oftwo algal species, Chlorella sorokiniana and D. tertiolecta, cultivatedin tap water and geothermal water media. They studied SWE ofbiocrude oil by comparing biocrude oil yield and lipid content.The alga D. tertiolecta cultivated in tap water produced the maxi-mum biocrude oil of 30%. Comparison between biocrude oil yieldand lipid content showed that the biocrude oil yield was higher,probably due to protein hydrolysis. The highest heating value(38 ± 1.8 MJ/kg) of biocrude oil was obtained from D. tertiolectacultivated in geothermal water. The elemental analyses and HHVof some selected algae and their biocrude compositions are tabu-lated in Table 2. Zhu et al. [67] reported a high efficiency of SWEin converting lipid-extracted algae (LEA) to biofuels using a benchscale continuous SWE reactor, and the resulting bio-oil wasupgraded by hydrocracking and hydrotreating. Li et al. [66] carriedout SWE on two algal strains, Nannochloropsis sp. and Chlorella sp.,with varying lipid/protein compositions. They investigated theinfluence of temperature, extraction time and total solid contentof the feedstock. Algal feedstock compositions greatly influencedthe SWE performance. The results also imply that the hydrother-mal conversion process is not only affected by lipid content. Theyalso concluded that algal biomacromolecules greatly influencebio-oil yield and quality.

3.2. Energy recovery

The energy recoveries reported for the biocrudes of Oedogoniumsp., U. ohnoi, D. tenuissima, Cladophora vagabunda, C. coelothrix and

60 80 100 120

chemical composi�on (%)

id protein carbohydrate

pecies of algae [35,40,45,46,49,54,56,60–63].

Table 2Ultimate analysis and higher heating values of different algae strains and their corresponding biocrudes.

Algae Feedstock Biocrude SWEconditions

Reference

Elemental Compositiona HHVb Elemental Compositiona HHVb Yieldc Tempd Timee

C H N O S C H N O S

Chaetomorpha linum 26.5 4.1 3.4 31 2.1 10.3 70.9 7.7 6.8 11.4 0.1 32.5 9.7 330 5 [46]Chlorella sp. 53.5f 7.4f 11f 27.6f,g 0.5f 24.3 70.7f 8.8f 7.7f 12.0f,g 0.8f 33.8 41.7 350 3 [26]Chlorella sp. 60.5 9.1 1.9 21.8 32.3 75.6 12 0.3 11.5 34.2 82.9 220 90 [66]Chlorella vulgaris 42.3 5.1 6.2 0.4 70.6 9.2 5.5 12.3 0.4 34.4 33 250 5 [65]Chlorella vulgaris 52.6f 7.1f 8.2f 32.2f,g 0.5f 23.2 70.7f 8.6f 5.9f 14.8f,g 0 35.1 40 350 60 [49]Chlorella vulgaris 52.6f 7.1f 8.2f 32.3f,g 0.5f 23.2 70.7f 8.6f 5.9f 14.8f,g 0 35.1 35.8h 350 60 [56]Cladophora coelothrix 30.9 5 5.2 34.9 2.3 12.7 71.6 8 7.1 10.6 0.9 33.3 13.5 330 5 [46]Cladophora vagabunda 37.5 5.9 6.5 32.9 1.8 16.4 71.1 8.3 6.8 10.6 1.3 33.5 19.7 330 5 [46]Derbesia tenuissima 29.2 4.8 4.5 27.4 2.8 12.4 73 7.5 6.5 10.6 0.7 33.2 19.7 330 5 [46]Desmodesmus sp. 51.96f 7.31f 6.86f 33.87f,g 23.44 75.8f 9.1f 6f 9.1f 36.6 46.5 350 60 [44]Dunaliella tertiolecta 51.9 7.5 8.6 0.5 71.3 9.1 5.3 12.2 0.4 34.6 44.8 250 5 [65]Gelidium amansii 28.04 4.85 2.62 64.48g 9.8 78.46 10.24 4 7.3g 40 11.98 350 20 [61]Laminaria saccharina 39.44f 5.14f 2.99f 52.03f 0.6f 14.46 75.54f 9.16f 3.65f 11.66f 0.62f 35.97 79h 350 15 [10]Laminaria saccharina 31.3 3.7 2.4 26.3 0.7 12 82 7.1 4.9 5.4g 36.5 19.3 350 15 [39]Mixed culture algae 27.9 3.01 3.9 65.2g 0.4 12.9 59.4 7.79 2.50 30.3 g 25.8 300 60 [32]Nannochloropsis gaditana 51 6.6 6.9 0.4 71.5 9.7 3.7 11.5 0.2 35.4 34.4 250 5 [65]Nannochloropsis occulta 57.8f 8f 8.6f 25.7f,g ND 17.9 68.1f 8.8f 4.1f 19f,g 0 34.5 34.3h 350 60 [56]Nannochloropsis oceanica 50.06 7.46 7.54 34.47g 0.47 21.46 72.58 9.75 5.09 12.06g 0.52 36.35 40.08 300 30 [60]Nannochloropsis oculata 57.8f 8f 8.6f 25.7f,g NA 17.9 68.1f 8.8f 4.1f 18.9f,g 0 34.5 38 350 60 [49]Nannochloropsis salina 55.16 6.87 2.73 33.97g 1.27 25.4 77.2 9.01 2.75 8.71 1 38.1 350 30 [54]Nannochloropsis salina 56.52 8.28 8.51 26.69g 24.17 72.59 9.79 5.2 12.42g 36.3 55.6 340 30 [14]Nannochloropsis sp. 43.7 7.7 7.5 29.1 22.4 74 10.2 5.4 9.5 31.5 55.0 260 60 [66]Oedogonium sp. 36.6 5.7 4.8 30.9 0.4 15.8 72.1 8.1 6.3 10.4 0.8 33.7 26.2 330 5 [46]Phaeodactylum tricornutum 38 4.8 5.2 0.7 62.9 8 4.7 12 0.3 30.3 40.8 250 5 [65]Porphyridium cruentum 51.3f 7.6f 8f 33.1f,g NA 14.7 72.8f 8.5f 5.4f 13.3f,g 0.4f 35.7 22 350 60 [49]Porphyridium purpureum 45.6 6.1 6 1.1 69.1 8.4 5 15.2 0.5 32.7 24.7 250 5 [65]Scenedesmus almeriensis 50.6 6.4 6.8 0.4 72.6 9.4 4.1 12.5 0.3 35.3 35.7 250 5 [65]Scenedesmus obliquus 44.4 5.4 5.8 0.3 69.3 9.1 5.1 12.9 0.2 33.8 17.6 250 5 [65]Spirulina sp. 53.7f 7.7f 12.1f 25.9f,g 0.6 24.9 68.3f 8.3f 6.9f 15.4f,g 1.1f 32 300 5 [26]Spirulina sp. 55.7f 6.8f 11.2f 26.4f,g 0.8 21.2 73.3f 9.2f 7f 10.4f,g 1f 36.8 34 350 60 [49]Spirulina platensis 42.26 5.86 3.47 47.26g 1.15 20.4 70.69 8.05 7.22 10.06 0.77 34.3 350 30 [54]Tetraselmis sp. 42 6.8 8 40.2g 3 19.2 71 9.5 5 14g 0.6 35 65 350 5 [45]Tetraselmis suecica 45 5.9 6.3 1.1 62.6 7.4 4.8 14 0.4 29.3 29.4 250 5 [65]Ulva ohnoi 27.7 5.5 3.5 41.4 5 11.7 72.6 8.2 5.8 11 0.4 33.8 18.7 330 5 [46]

a wt.%, dry weight.b Higher Heating value, MJ/Kg.c wt.%, dry weight.d Temperature, �C.e Time, min.f wt.%, ash-free dry weight.g Calculated by difference.h wt.%, ash-free dry weight.

S. Thiruvenkadam et al. / Applied Energy 154 (2015) 815–828 823

C. linum were 55.7%, 54%, 52.5%, 40.1%, 35.3% and 30.6% respec-tively [46]. Energy recoveries of 38–87% and 52.5% were obtainedfrom biocrudes of Tetraselmis sp. [45] and mixed-culture algal bio-mass [36] respectively. Another similar study on a mixed-culturealgal biomass and swine manure mixture produced an energyrecovery of 49.9% and also promoted net energy gain [32]. The bio-crude from S. platensis co-liquefied with E. prolifera containedhigher nitrogen and lower oxygen contents [40]. Moreover, thisco-liquefaction enhanced the conversion of proteins in the algalbiomass.

Eboibi et al. [45] obtained up to 63–88% and 24–53% for carbonand nitrogen recovery respectively in the biocrude from SWE ofTetraselmis species. Low nitrogen recovery (8.41–16.8%) wasobtained from the biocrude of mixed-culture algal biomass, andthis is probably due to the presence of calcium carbonate in thefeedstock [36]. While comparing the effect of various heteroge-neous and alkaline catalysts on recovery, Yu et al. [57] reportedthe highest carbon recovery from the biocrude of C. pyrenoidosain the presence of alkaline catalyst NaOH at 240 �C. However, thecarbon content in biocrude from NaOH-catalyzed SWE (68.3%)was lower than that obtained from non-catalyzed SWE (75.7%) at240 �C. At 280 �C, the carbon recovery of biocrude from catalyzed

SWE was higher than that obtained from non-catalyzed SWE.They also noted that the alkaline catalyst reduced the carbonrecovery of solid residue to less than 2%, revealing the benefits ofalkaline catalysts for SWE. Catalysts, especially alkaline catalyst,increased the nitrogen recovery of biocrude at 240 �C and 280 �C.This increase was accompanied by a decrease in nitrogen recoveryof solid residue at both temperatures. Gai et al. [37] carried out theSWE of C. pyrenoidosa and reported that high temperatures, hightotal solid ratio, and moderate retention time contribute to highercarbon recovery of the biocrude. On the contrary, a low nitrogenrecovery could be obtained at low temperatures, low total solidratios, and long residence times. The authors also stated an impor-tant foremost step in selecting the residence time to achieve bio-crude with higher carbon and lower nitrogen recoveries. Chenet al. [32] investigated mixed-culture algal biomass (25%)co-liquefied with swine manure (75%). The crude fat in swine man-ure increased the nitrogen recovery due to its reactions with fattyacid derivatives, but the carbon recovery did not increase. Theseauthors also inferred that the recoveries may differ with varyingcombinations of feedstock. Nitrogen recovery in the biocrude canbe reduced by promoting deamination of proteins prior to SWE[60]. SWE in large-scale continuous flow reactors resulted in

824 S. Thiruvenkadam et al. / Applied Energy 154 (2015) 815–828

fouling and slagging due to the presence of alkali metals [45].Aluminum, copper, iron, calcium, magnesium, potassium, sodium,manganese, nickel, sodium, and zinc were detected during metallicanalysis of biocrude from Tetraselmis sp. [45]. The presence of met-als such as Cu, Fe, Mn, Cr, Zn and Mo have been reported as catalystpoisons during biocrude upgrading for denitrogenation and desul-furization [45].

3.3. Modeling and LCA analysis

Johnson and Tester [43] developed a model for triglyceridehydrolysis of two microalgae strains, Isochrysis sp. andThalassiosira weissflogii, operated at 250–350 �C in a batch reactor.They tested the model on saturated fatty acids and then to unsat-urated fatty acids. The total hydrolysis process comprised of threesequential reversible reactions converting triglycerides, diglyc-erides and monoglycerides, respectively, to diglycerides, mono-glycerides and glycerol. A free fatty acid is released in eachreaction step on hydrolysis of water molecule with one ester bond.Valdez et al. [68] conducted a useful kinetic modeling, includingthe biochemical content of the microalgae for SWE. In the model,the decomposition of algal biomacromolecules such as proteins,carbohydrates and lipids were described by six rate constants.Analysis of the rate constants showed that lipid decomposition ratewas exceeding that of protein decomposition. This explains thatlipids decomposition conditions are adequate for the decomposi-tion of proteins. The authors also stated that SWE conditions forlipid conversion will also enhance protein conversion to biocrudewith high N/C ratio, which would reduce the biocrude quality.This model also suggested that protein- and lipid-rich algal speciesare better feedstock than carbohydrate-rich algae for biocrude for-mation. Previously, the same group developed a kinetic modelspecific for Nannochloropsis sp. [69]. This kinetic model was devel-oped from a reaction network that interconnected all product frac-tions from SWE, and the model suggested that a reaction time offew minutes is enough to accomplish SWE.

Zhu et al. [67] devised a process simulation model of LEA bio-mass subjected to SWE and upgrading technologies such ashydrotreating and hydrocracking. This model predicted the eco-nomic benefits of large scale SWE plant. A techno-economic modelon SWE for algal biofuel production was developed by Delrue et al.[70]. This model included elements such as continuous SWE, sepa-ration and recovery of biocrude/bio-oil technologies, integratingheat exchanger with SWE system, and a hydrotreating process toupgrade biocrude. The sensitivity analysis of this model predictedfour important factors; biocrude yield, lower heating value (LHV)of the produced biodiesel, CO2 aeration rate and heat recycling por-tion. The authors reported that SWE yielded more biodiesel thanconventional lipid extraction pathway, regrettably with higherGHG emissions. Moreover, the authors also preferred SWE overanaerobic digestion for recovering energy from defatted biomassresidue. Venteris et al. [71] compared the biomass and nutrientdemands of lipid extraction (LE) and SWE under five different sce-narios: Fuel and co-products yields from LE (Scenario 1) or SWE(Scenario 2), LE with post algal extract recycled via anaerobicdigestion (Scenario 3) or Catalytic Hydrothermal Gasification(CHG) (Scenario 4), and SWE followed by post algal residue treat-ment with CHG (Scenario 5). On the basis of production levels,the most efficient pathway was scenario 5 (SWE + CHG), thoughwith a trade-off in increased N consumption.

Life cycle assessment (LCA) studies reflect the possible environ-mental concerns of this technique. LCA and techno-economic stud-ies on algal biofuel systems have been published by Rickman et al.[72]. The group developed a Utility-Connected Algae System(UCAS) with a modular approach consisting of parameters suchas total pond area, flue gas pumping rate, and flue gas CO2

concentration and uptake efficiency. In this study, a significantmitigation of CO2 emissions was difficult due to the complexityof the parameters. LCA analysis by Fortier et al. [73] illustratedthe benefits of combining nutrient recycling and SWE, where thealgae could be cultivated at associated wastewater treatmentplants (WWTPs). Operating SWE reactors at a WWTP site is feasibleas the methane generated from anaerobic digestion can be used asthe heat source for SWE process. SWE at WWTPs results in lowerlife-cycle greenhouse gas (LC-GHG) emissions than SWE takingplace at a petroleum refineries and this lower LC-GHG emissionsfrom WWTP/SWE pathway are the consequence of cutting logisticsspending on transporting algae from WWTP to petroleum refinery.

3.4. Biocrude upgrading

Algal oil generated directly after extraction contains impurities,such as metals and heteroatom-containing substituents, whichmight adversely affect the direct usage of the oil for biofuel pro-duction. Bio-oils from SWE are of low quality and can be upgradedby various strategies namely: (i) hydrotreating; (ii) hydrocracking;(iii) supercritical fluids; (iv) solvent addition/esterification; (v)emulsification; and (vi) steam reforming [74]. Prior to SWE, thealgae should be washed with distilled water to remove any watersoluble inorganic matter present with the algae species. AlthoughSWE is applied on wet biomass, the requirement for washing withdistilled water is dependent on the intended use of the extract.Washing with distilled water only enhances the purity of theextracts and does not affect the efficiency of SWE. For a genericSWE, intensive culture dewatering is unnecessary compared toother extraction approaches such as solvent extraction. The pri-mary product of SWE for biodiesel production is biocrude orbio-oil. The biocrude is then transesterified to biodiesel by theaddition of alcohol catalyzed by an acid or base catalyst. The estab-lished method for biodiesel production from algae is a two-stepprocess: (i) extraction of bio-oil from algae and (ii) conversion ofbio-oil to biodiesel [75]. However, it is still a challenge to maximizebiodiesel production from algae as algal lipids contain highamounts of unsaturated fatty acids. FAME resulting from transes-terification of unsaturated fatty acids has an increased inclinationtoward oxidation reaction, consequently deteriorating the biodie-sel production process. This problem can be eliminated by hydro-genating the double bonds of unsaturated fatty acids with thehelp of hydrogenating catalysts. A high concentration of carbohy-drates in algae is undesirable as it primarily results in biochar for-mation [46]. Garcia-Alba et al. [44] reported that high-proteinalgae species such as Spirulina contain high nitrogen content inthe biocrude and thus denitrogenation is necessary in terms ofoil upgrading steps. Direct infusion Fourier Transform IonCyclotron Resonance Mass Spectrometry (FT-ICR MS) analysis ofbio-oil fractions of N. salina showed the presence of numerousnitrogen heterocyclic and oxygenated compounds [4]. High nitro-gen content has a detrimental effect on biocrude quality as itresults in NOX emissions during combustion [44]. Similarly, bio-crude with high oxygen and sulfur contents are also prohibitive[11,65]. The maximum permitted levels of sulfur and phosphorousin biodiesel are respectively 15 ppm and 10 mg/kg as per US ASTMD6751 standards [76]. The presence of polar compounds in bio-crudes leads to corrosion and chemical instability after storage[4]. Studies have been conducted on oil upgrading techniques suchas hydrodenitrogenation, hydrodeoxygenation and hydrocracking.During nitrogen removal, harsh treatment conditions are recom-mended since basic nitrogen compounds tend to deactivate thecatalysts by adsorbing to their active sites [29].

As mentioned previously, fresh bio-oil from algal biomass con-tains high nitrogen and oxygen contents, deteriorating its quality.With the intention of decreasing those contents, Cheng et al. [60]

S. Thiruvenkadam et al. / Applied Energy 154 (2015) 815–828 825

demonstrated a biodiesel and biocrude co-generation process. Inthis work, high-grade biodiesel was obtained from transesterifica-tion of algal lipids via microwave irradiation. The algal residue wasprocessed for SWE to produce low-grade biocrude from the algalcarbohydrates and proteins. Comparison of the co-generation pro-cess with direct SWE of raw microalgae delivered the followingresults: (1) The conversion of lipids were effective in both pro-cesses, (2) The partially hydrolyzed proteins in the transesterifica-tion step of the co-generation process inhibited the reaction offatty acids with nitrogen-rich organics to yield long-chain nitrogencompounds and thus reducing nitrogen content in the final bio-crude, (3) Initial deamination of proteins in the cogeneration pro-cess enhanced the formation of liquid fuels from carbohydrates,preventing gas product which was observed in direct SWE of algalbiomass. However, compared to direct SWE, the co-generation pro-cess increased oxygen content slightly from 14.02% to 15.86%,while the nitrogen content decreased from 27.06% to 16.02% [60].An interesting alternative proposed by Miao et al. [77] was aimedat the removal of proteins and polysaccharides from the feedstockbefore SWE treatment. The process is named as sequentialhydrothermal fractionation, where proteins and polysaccharidesare isolated from the feedstock at low temperature followed byhigh temperature SWE for biocrude formation from the extractedbiomass. Complete in-depth information on individual compo-nents present in the biocrude is particularly necessary for optimiz-ing the SWE and its upgrading process [4].

Researchers have explored ways to tackle these downstreamprocesses. Roussis et al. [78] demonstrated a thermal treatmentprocess to remove oxygen and metals from fresh algal oils. Theyfound that the thermal treatment improves oil fluidity characteris-tics by increasing volatility and decreasing viscosity thus reducingtheir boiling point. This is advantageous to further downstreamprocessing steps, minimal poisoning effects and improved flowproperties. Bai et al. [79] examined the catalytic treatment of pre-treated algal oil from SWE of C. pyrenoidosa. Different catalystswere used: Activated carbon, Alumina, CoMo/c-Al2O3 (sulfided),HZSM-5, Mo2C, MoS2, Ni/SiO2–Al2O3, Pd/C, Pt/C, Pt/C (sulfided),Raney-Ni and Ru/C. In these experiments, the algal biocrude afterSWE was pretreated at 350 �C for 4 h, repeatedly for four times,before the upgrading process. Amongst the catalysts investigated,Ru/C catalyst produced the highest upgraded bio-oil yield (68.5%)followed by Ni/SiO2–Al2O3 (68.2%). All the catalysts tested showeddenitrogenation activity, while only Pt/C, Ru/C, alumina, HZSM-5,and Raney-Ni promoted deoxygenation activity. Higher C contentand HHV were observed for the catalyzed-upgraded bio-oil com-pared to the uncatalyzed bio-oil. Deoxygenation and denitrogena-tion processes were executed best by Ru/C and Raney-Nirespectively. The combination of Ru/C and Raney-Ni showed a pos-itive effect in terms of yield, C and H contents, and HHV of theupgraded bio-oil. A two-step upgrading process, described byZhang et al. [52], involved SWE at a low temperature of 260 �Cfor 60 min in H2 followed by upgrading SWE filtrates in supercrit-ical ethanol at 300 �C for 60 min in H2. Unfortunately, this two-stepprocess did not provide significant results when compared withSWE at 300 �C for 30 min in H2.

3.5. Scale-up implications and recent developments

Batch experiments of SWE have provided insights into biofuelproduction from algal feedstock [37,45,60,61]. In spite of thisincreased interest, commercial and continuous SWE are still underinvestigation and development. A research group from Australia[26] focused on the development of continuous pilot scale SWEreactors and achieved a maximum biocrude yield of 41.7% fromChlorella sp. processed at 350 �C and 3 min residence time.Though higher yields were achieved at higher SWE reaction

temperatures with reduced oxygen content, an increase in nitrogencontent was observed at high temperatures; hence the quality ofthe biocrude was compromised. Low-lipid algal species are consid-ered to be favorable feedstock for SWE process [32]. With difficul-ties pertaining to culturing pure-monocultures, some researchershave proposed integrating algal cultivation with wastewater sys-tems, resulting in a positive net energy balance [29,32]. Recently,co-liquefaction of swine manure (SW) and mixed-culture wastew-ater algae (AW) was explored by Chen et al. [32]. Co-liquefaction of25% AW and 75% SW gave the highest biocrude yield (35.7%) underthe experimental conditions employed. High ash content of about47.5% in AW harvested from full-scale mixed algal wastewatertreatment plant was found to block biocrude formation. The dilu-tion of total ash contents in the combined feedstock with 75%SW improved biocrude yield. For other combinations, high ashcontents of the feedstock resulted in the formation of aqueous orsolid products. The saponification of ash contents result in aqueousproducts. Direct deoxy-liquefaction, a modified SWE process, wasfirst explored by Li et al. [61] and this liquefaction aims to producebiocrude with lower oxygen content. Macroalga Gelidium amansiiwas tested for deoxy-liquefaction at 350 �C and it produced11.98% oil with 7.30% oxygen, which was very much lower thanthose found in the raw material (64.48% oxygen). The presence ofnitrogen compounds in the bio-oil leads to serious environmentalconcerns, and these nitrogen compounds are derived from proteinand polysaccharide fractions. Thus, a two-step SWE process namedas Sequential SWE (SEQSWE) has been developed with the aim ofreducing obnoxious nitrogen compounds in bio-oil by removingproteins and polysaccharides at low temperature as a first step ofSEQSWE, followed by SWE of extracted biomass to producebio-oil [77].

4. Other products

4.1. Aqueous product

The aqueous phase product from SWE contains carbon andnitrogen with negligible amounts of hydrogen and carbon monox-ide [33]. Alkaline catalyst (Na2CO3) increased the carbon recoveryof the aqueous phase from SWE of C. pyrenoidosa to 52.1%, whichis higher than the value obtained from non-catalyzed SWE testsat 240 �C (32.0%) [57]. This study also implied that the aqueousproduct gets promoted with the help of alkaline catalysts duringSWE. High-carbon aqueous phase can be recycled as a substratefor the growth of mixotrophic organisms [26]. Biller et al. [62]tested the feasibility of using the aqueous phase for microalgaecultivation. After filtering the aqueous product, the filtrate wasdiluted prior to microalgae cultivation in order to reduce extremeconcentrations of nutrients present in the aqueous phase.Microalgal growth profile can be optimized by selecting the rightdilution of SWE aqueous product. A report by Johnson et al. [80]studied the effect of nutrient production on algae LCA, consideringall energy and materials inputs. This study reported the signifi-cance of nutrient contributions to algae biofuel production.Fertilizer production and dewatering process are among the mostendoergic steps during algal-to-biofuel pathway and a potentialsolution is the recycling of aqueous water for biomass synthesiscombined with the SWE process [73]. Orfield et al. [81] havereported that the recovery and recycling of aqueous productimprove the life cycle performance of algal biorefinery. The authorsexplored two different pathways in recycling the aqueous product:(i) CHG of the aqueous products and (ii) cultivation of the bac-terium Escherichia coli on the aqueous products. The flue gas pro-duced from CHG is directed to a hydrogen production plantwhich produces hydrogen gas for hydrotreating. The bacterial

826 S. Thiruvenkadam et al. / Applied Energy 154 (2015) 815–828

growth on the aqueous products increased the carbon footprintand reduced the energy return on investment (EROI). EROI is animportant metric that facilitates the quantification of environmen-tal performance of a biofuel together with economic viability [82].

4.2. Solid residue

Another product from SWE is solid residue, which includes ahigh concentration of ash and low concentrations of hydrogen,nitrogen and sulfur [1]. The distribution of products from SWE ofalgae can be seen in Table 3. The solid yields vary according tothe feedstock characterization. For example, algal species withhigh inorganic contents yield more solid residue. It has beenreported that the solid yield decreases with increasing tempera-ture [38]. The solid residues can be used as asphalt [36] and/or fer-tilizer [11] if N partitions into the solid phase during the SWEprocess. FTIR spectra of solid residue from SWE of Ulva fasciatawere presented by Singh et al. [63]. The macroalga carbohydratesand proteins were indicated by O–H and N–H bonds observed frombroad bands around 3406 cm�1. Peaks found at 1451 cm�1 and1103 cm�1 represented C–O stretching vibrations belonging to sec-ondary and primary alcohols of the carbohydrates respectively. Thedecomposition of proteins and carbohydrates during SWE wereconfirmed by the absence of relevant peaks in SWE-treated solidresidue compared to that of the raw macroalga U. fasciata. It wasalso observed that the solid residue yields were lower for alkalinecatalyzed SWE than heterogeneous catalyzed SWE at 240 �C [57].The carbon recovery of the solid residue increased when amixed-culture algal biomass (25%) was co-liquefied with swinemanure (75%) in a SWE process. This was due to the presence ofhemicellulose [32].

4.3. Gas product

Gaseous products are released from SWE reactors significantly,and in many cases, the yield of gases are calculated on differencebasis [10,61]. The most abundant gas product in most SWE studies

Table 3SWE yields from specific product phases for different algae species.

Feedstock Biocrudea Solidsa Aqueousa Gasa Reference

Freshwater algaeChlorella vulgaris 40 2 54 4 [49]Cladophora vagabunda 19.7 18.7 61.7b [46]Desmodesmus sp. 46.5 9.1 17.8 22.3 [44]Nannochloropsis gaditana 34.4 24.7 36.2 6.7 [65]Oedogonium sp. 26.2 10.2 63.6b [46]Phaeodactylum tricornutum 40.8 17.8 22.9 12.5 [65]Scenedesmus obliquus 17.6 41.6 27.1 6.1 [65]Spirulina sp. 34 2 14 50 [49]

Marine algaeChaetomorpha linum 9.7 8.4 82.0b [46]Chlorella vulgaris 33 27 29.9 10.8 [65]Cladophora coelothrix 13.5 10.4 76.1b [46]Derbesia tenuissima 19.7 8.1 72.2b [46]Dunaliella tertiolecta 44.8 11.9 38.3 8.7 [65]Gelidium amansii 11.98 40.23 – 26.55 [61]Laminaria saccharina 79f 7f 5f 3f [10]Nannochloropsis oculata 38 2 58 2 [49]Porphyridium cruentum 22 4 74 2 [49]Porphyridium purpureum 24.7 27.6 34 10.9 [65]Scenedesmus almeriensis 35.7 36.6 17.5 9.1 [65]Tetraselmis sp. 65 13 14 8 [45]Tetraselmis suecica 20.4 21.1 28 12.6 [65]Ulva fasciata 11 24 – 9 [63]Ulva ohnoi 18.7 12.1 69.2b [46]

a wt.%, dry basis.b Total yields of aqueous and gaseous products.f wt.%, ash-free dry weight.

is CO2, formed via steam reforming or water-gas shift reactions.Other gases include H2, CO, CH4 and C2H6 [5,48]. High tempera-tures [45] and longer residence times result in gas formation[36,38]. At elevated temperatures, partial decomposition of bio-crude leads to gas production [61] and also increases the carbonrecovery of gas products [57]. Interestingly, a denser feedstockmedium with a low biomass to water ratio decreases gas yieldsdue to enhanced extraction of biocrude oils [37,38]. A similar trendwas observed for increasing acetone/biomass ratio from 10:2.5 to16:2.5 (mL/g) [48]. Bai et al. [79] reported that most catalyzedreactions, except those involving Ru/C catalyst, have no effect ongas formation, explaining that gas formation is a thermal process.High gas yields from Ru/C could be suppressed by combining itwith Raney-Ni catalyst because the combination of these two cat-alysts showed good deoxygenation and denitrogenation of the bio-crude oils. Pt/C catalyst was also seen to promote the formation ofmethane and ethane gases, generated from cracking reactions [5].Gas products rich in CO2 are potentially beneficial and these prod-ucts can be recycled back to algae ponds or could be utilized forheat generation and/or power.

5. Conclusion

Advances in SWE have led to massive interests in the conver-sion of algae to transportation fuels via SWE. This low-cost andeco-friendly technology could in principle be used for productionof bio-products and biofuels from algae and other biomasses.This is appreciated as a substantive effort to the contribution ofbiofuels in the future bioenergy sector. In a closed loop system,algae production costs could be reduced by few approaches, suchas recycling nutrient-rich aqueous products into the cultivationsystem, utilizing co-products, and implementing heat integrationsystem. The use of heterogeneous catalysts has shown to producehigh quality bio-oil but requires some improvements to minimizethe effect of fouling and catalyst deactivation. The road to commer-cialization will mainly be associated with energy demands to avoidthe production of biofuel with higher greenhouse gas emissionsthan conventional fuel. For full-scale application, pilot studiesshould be focused on addressing the present technological prob-lems associated with product yield and composition. Beyond tech-nological problems, algae cultivation systems also play a key rolein improving this technology. Additional research and develop-ment studies are necessary to thoroughly design and identify therealistic potential of this technology for production of biofuels.

Acknowledgements

This work has been supported by Fundamental Research GrantScheme (Project Code: 03-02-13-1297FR) and the Department ofChemical and Environmental Engineering, Universiti PutraMalaysia.

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