Chemical Engineering Journal 263 (2015) 194–199

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Chemical Engineering Journal

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Pyrolysis of microalgae residual biomass derived from Dunaliellatertiolecta after lipid extraction and carbohydrate saccharification

http://dx.doi.org/10.1016/j.cej.2014.11.0451385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding authors. Tel.: +82 31 201 2492; fax: +82 31 204 8114 (J. Kim).Tel.: +82 31 201 3839; fax: +82 31 204 8114 (E.Y. Lee).

E-mail addresses: jkim21@khu.ac.kr (J. Kim), eunylee@khu.ac.kr (E.Y. Lee).

Seung-Soo Kim a, Hoang Vu Ly b, Jinsoo Kim b,⇑, Eun Yeol Lee b,⇑, Hee Chul Woo c

a Department of Chemical Engineering, Kangwon National University, 346 Joongang-ro, Samcheok, Gangwon-do 363-883, Republic of Koreab Department of Chemical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Koreac Department of Chemical Engineering, Pukyong National University, 365 Sinseon-ro, Nam-gu, Busan 608-739, Republic of Korea

h i g h l i g h t s

� Dunaliella tertiolecta residual biomass was further utilized by pyrolysis as a biorefinery concept.� Effects of pyrolysis temperature and time on product distributions were investigated.� The product bio-oil from pyrolysis of the biomass residue was analyzed by GC–MS.� The pyrolysis reaction mechanism was studied using the lumped kinetic model.

a r t i c l e i n f o

Article history:Received 22 August 2014Received in revised form 3 November 2014Accepted 6 November 2014Available online 14 November 2014

Keywords:MicroalgaeDunaliella tertiolectaResidual biomassLumped kinetic modelThermogravimetric analysis

a b s t r a c t

Microalgae (Dunaliella tertiolecta) are considered potential feedstock for production of biodiesel andbioethanol due to their high lipid and carbohydrate contents. To achieve complete utilization ofmicroalgae in a microalgae biorefinery, residual biomass after conversion of lipids and carbohydrates intobiodiesel and bioethanol can be converted into bio-oils by pyrolysis. D. tertiolecta residual biomassdecomposed mainly between 200 �C and 550 �C at heating rates of 5–20 �C/min. The apparent activationenergy increased from 163.12 kJ mol�1 to 670.24 kJ mol�1 with increasing pyrolysis conversion.Experimental results were consistent with the proposed lumped kinetic model, and the kinetic rate con-stant for D. tertiolecta residual ? bio-oil (k2) was the highest. This result indicates that the predominantreaction pathway of D. tertiolecta residual was A (D. tertiolecta residual) to B (bio-oil), rather than A(D. tertiolecta residual) to C (gas; C1–C4, CO, CO2, H2) or B (bio-oil) to C (gas; C1–C4, CO, CO2, H2).

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Biomass is considered to be a renewable feedstock for biofuelproduction and therefore an alternative to fossil fuels [1–5].Among various biomasses, microalgae are a promising feedstockbecause they have several advantages compared to sugarcane,corn, and lignocellulosic biomass [6]. Microalgae can grow fasterthan plant crops, and are free of the controversy surrounding theuse of food for fuel production. Furthermore, large-scale cultivationof microalgae can be implemented on non-arable land, such asdeserts, or the ocean.

Microalgae are good feedstocks for biodiesel productionbecause of their high oil content (up to 80% biomass) [7]. Manystudies have been conducted to commercialize microalgae-based

biodiesel production [8–10]. However, biodiesel production frommicroalgae is still too expensive to be commercialized at present[11]. To develop economically feasible processes for microalgaebiofuels, whole components need to be used-up completely basedon the microalgae biorefinery concept [12]. The main componentsof microalgae biomass are proteins, carbohydrates, and lipids.Residual biomass after oil extraction for biodiesel production isrich in proteins and carbohydrates. This residual biomass can beused as animal feed [6]. Carbohydrates in residual biomass couldalso potentially be exploited as feedstock for bioethanol produc-tion [13].

Previously, we developed a microalgae biorefinery for thesequential production of biodiesel and bioethanol. Dunaliellatertiolecta LB999 was cultivated in a photobioreactor using asemi-permeable membrane at a large-scale at a Korean coastalarea [14]. Biodiesel was produced from the cultured D. tertiolectabiomass, and the fuel properties satisfied the fuel quality require-ments of transportation fuel of the Korean Institute of Petroleum

S.-S. Kim et al. / Chemical Engineering Journal 263 (2015) 194–199 195

Management. Residual biomass after oil extraction was subjectedto saccharification. More than 81% of the total carbohydrate con-tent was saccharified by enzymes such as AMG 300L without anypretreatment. The saccharification cocktail was directly used forbioethanol fermentation using Saccharomyces cerevisiae with 82%yield [15].

To achieve complete utilization of whole cellular components,the materials that remain after biodiesel and bioethanol produc-tion can be subjected to pyrolysis to convert them into bio-oils.Ross et al. [16] performed pyrolysis of Macrocystis pyrifera at500 �C using pyrolysis–GC–MS, and confirmed that degradationproducts were carbohydrates, proteins, or polyphenolics. Than-galazhy-Gopakumar et al. [17] reported catalytic pyrolysis of greenalgae for hydrocarbon production using HZSM-5 catalyst in a fixedbed reactor. They confirmed that certain negative attributes ofalgae bio-oil, such as its high nitrogen and oxygen content, werereduced by using HZSM-5 as a catalyst.

In the present study, we investigated the pyrolysis characteris-tics and kinetics of D. tertiolecta residual biomass obtained afterlipid and carbohydrate utilization to obtain information regardingthe types of fuel (gas, bio-oil, or bio-char) that can be generated.Thermogravimetric analysis was used to study the pyrolysis char-acteristics of D. tertiolecta residual and to obtain global kineticparameters, including activation energy. Kinetic parameters froma lumped kinetic model were also calculated.

Table 1Analysis of composition of D. tertiolecta LB999 after lipid extraction followed bycarbohydrate saccharification.

Chemical composition (%)

Lipid Carbohydrate Protein Ash

After culture 22.0 40.5 27.2 10.3After lipid extraction – 51.9 35.0 13.1After saccharification – 15.5 67.7 16.8

2. Experimental

2.1. Microlagae biomass sample preparation

D. tertiolecta LB999 was cultured in a 70 L plate-type photobior-eactor with fluorescent lighting (60 lE/m2 s) at 20–25 �C [14].Residual microalgae biomass was obtained after lipid extractionand saccharification as follow: total lipids were extracted twicedirectly from dried cells using a 2:1 (v/v) methanol/chloroformmixture for 2 h at 65 �C [18]. For enzymatic saccharification, 5%(w/v) lipid-extracted microalgae biomass was treated using AMG300L at pH 4.5 and 55 �C for 16 h [15].

2.2. Chemicals and analyses

Commercial cellulase (Celluclast 1.5L, Novoprime B957),amyloglucosidase (AMG 300L), and Viscozyme L were used for sac-charification [15]. All other chemicals were of analytical or reagentgrade and were used with no pretreatment. Lipid content of thedried biomass was determined using the Soxhlet method (method920.39) [19]. Carbohydrate content was determined based on themethods of the National Renewable Energy Laboratory (NREL)[20]. Protein content was determined using a micro-Kjedahlmethod (method 976.05) [19].

2.3. Thermogravimetric analysis and tubing reactor

Moisture and ash contents of D. tertiolecta residual biomasswere determined using ASTM E 1756 and ASTM E 1755, respec-tively [21]. Thermogravimetric analysis of D. tertiolecta residualbiomass samples (25.0 ± 1.0 mg) was carried out using thermo-gravimetric analysis (TGA; TA Instrument Q50). Nitrogen was usedas the carrier gas at a flow rate of 25 mL/min. Heating rates werecontrolled at 5, 10, 15, and 20 �C/min from 30 �C to 900 �C.

A tubing reactor was used to pyrolyze the D. tertiolecta residualbiomass sample. The reactor was used to test the effect ofresidence time on the pyrolysis of D. tertiolecta residual biomassat a constant temperature. A sample mass of 2 g was used in eachexperimental run. Descriptions of the experimental apparatus and

procedure are provided in our earlier publications [22,23]. Basedon data from the differential thermogravimetric (DTG) curves, weselected pyrolysis temperatures of 410 �C, 420 �C, and 430 �C inthe tubing reactor. Reaction time was varied from 1 to 5 min ateach reaction temperature. After reaction, the reactor was removedfrom the molten salt and cooled to room temperature. Reactionproducts were analyzed by weighing gas, oil, and char products.Gas yield, defined as (gas weight) � 100/(feed weight), wasobtained by weighing the tubing reactor before and after gasrelease. Other pyrolyzed products were separated into oil (acetonesoluble) and char (acetone insoluble) using a solvent extractiontechnique [24]. Solid yield was defined as (weight of acetoneinsoluble) � 100/(weight of feed), while oil yield was defined as(100 � gas yield � char yield).

3. Results and discussion

3.1. Analysis of composition of D. tertiolecta LB999 after lipidextraction and carbohydrate saccharification

Cellular composition of D. tertiolecta LB999 after photoautotro-phic culture was analyzed; results are shown in Table 1. RawD. tertiolecta biomass sample consisted of lipid (22.0 wt.%), carbo-hydrate (40.5 wt.%), protein (27.2 wt.%), and ash (10.3 wt.%). Forbiodiesel production, lipids in the raw D. tertiolecta biomass samplewere completely extracted using a solvent mixture of dimethylcarbonate and methanol. After lipid extraction, the residual bio-mass contained 51.9 wt.% carbohydrate, 35.0 wt.% protein, and13.1 wt.% ash, without any trace of lipids (Table 1). This residualbiomass was further utilized for saccharification using enzymes(AMG 300L) for bioethanol fermentation. After saccharification, asecond residual biomass was obtained by separating supernatantcontaining various monosaccharides from bioethanol fermenta-tion. As shown in Table 1, the second residual biomass obtainedafter lipid extraction followed by saccharification was composedmainly of protein (67.7 wt.%). To achieve complete utilization ofthe D. tertiolecta biomass sample, the second residual biomasswas pyrolyzed to obtain valuable bio-oil.

The characteristics of the raw D. tertiolecta biomass and the sec-ond D. tertiolecta residual biomass samples are presented in Table 2.The ash content of the D. tertiolecta residual biomass was higherthan that of the raw biomass, as inorganic materials were concen-trated in the residual biomass during lipid extraction and sacchar-ification. In general, the ash content of algal biomass is higher thanthat of lignocellulosic biomass. Bird et al. [25] produced algal bio-char from macroalgae (seaweed) by pyrolysis of eight species ofgreen tide algae. They reported that the ash content of raw algaewas 10.5–33.8 wt.%, which is consistent with our findings.

The carbon (C) content of D. tertiolecta residual biomass washigher than that of other green tide algae such as Cladophora linumand Cladophora coelothrix that had C contents ranging from 20.5 to32.1 wt.% [25]. The oxygen and nitrogen contents of D. tertiolectaresidual biomass were 40.04 wt.% and 8.40 wt.%, respectively. Thehigh content of nitrogen was likely due to the high protein contentof 67.7 wt.% (Table 1). The higher heating values (HHVs) of the raw

Table 2Characteristics of D. tertiolecta residual biomass.

Sample V.M.a Fixed carbona Moisture (%)b Ash (%)c Element (%) HHV (MJ/kg)

C H N Od

D. tertiolecta – – – 10.3 38.23 6.19 11.12 44.46 11.66D. tertiolecta residue 56.52 17.34 9.38 16.76 44.78 6.78 8.40 40.04 14.83

a Determined by thermal gravimetric analysis.b ASTM E1756, standard test method for the determination of total solids of biomass.c ASTM E1755, standard test method for determination of the ash content of biomass.d By difference.

196 S.-S. Kim et al. / Chemical Engineering Journal 263 (2015) 194–199

D. tertiolecta biomass and the D. tertiolecta residual biomass werecalculated as 11.66 and 14.83 MJ/kg, respectively, based on theequation proposed by Channiwala and Parikh [26]. HHV is stronglydependent on biomass composition, mainly the oxygen to carbon(O/C) and the hydrogen to carbon (H/C) ratios. HHV increases withdecreasing O/C ratio or increasing H/C ratio.

Concentrations of calcium (Ca), magnesium (Mg), phosphorous(P), lithium (Li), potassium (K), and sodium (Na) are shown inTable 3. It is noteworthy that the D. tertiolecta residual biomasssample contained higher concentrations of Ca, Mg, P, and Na, andlower concentration of K than the raw D. tertiolecta sample. Thedifference in concentration of inorganic matter between the rawbiomass and the residual biomass was quite large depending onthe species. This can be attributed to the nature of individualspecies, such as strong adherence to the biomass or removal fromthe biomass during solvent extraction and washing.

Fig. 1. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves forraw D. tertiolecta biomass at heating rates of 5, 10, 15, and 20 �C/min.

3.2. Thermogravimetric analysis and kinetic parameters of the

pyrolysis of D. tertiolecta residual biomass

The results of the thermogravimetric experiments areexpressed as a function of the conversion X, which is defined asshown below in Eq. (1):

X ¼ W0 �WW0 �W1

ð1Þ

where W0 (g) is the initial mass of the sample, W (g) is the mass ofthe pyrolyzed sample, and W1 (g) is the final residual mass.

The degrees of conversion versus temperature at heating ratesof 5, 10, 15, and 20 �C/min for the raw D. tertiolecta biomassobtained via TGA are shown in Fig. 1. At temperatures lower than190 �C, we attributed the small change in conversion of thesamples to vaporization of the moisture attached to the surfacesof the samples. The TGA graphs of the raw D. tertiolecta biomassshowed similar patterns at heating rates of 5, 10, 15, and 20 �C/min, and decomposition started at 200 �C. TGA curves shiftedtowards the right with increasing heating rate.

The differential rate of conversion, dX/dt, was obtained from dif-ferential thermogravimetric analysis (DTG) at heating rates of 5,10, 15, and 20 �C/min (Fig. 1). The DTG curve at each heating ratehas four extensive peaks between 20 and 450 �C. The first peak,at temperatures lower than 190 �C, corresponds to dehydration.The second peak, between 200 and 250 �C, corresponds to

Table 3Calcium, magnesium, phosphorous, lithium, potassium, and sodium contents of raw D. ter

Sample Element (ppm)

Ca Mg

D. tertiolecta 2388.10 6121.38D. tertiolecta residue 17083.02 6422.78

decomposition of carbohydrates, while the third peak between250 and 350 �C is attributed to decomposition of proteins. Thefourth peak below 450 �C denotes decomposition of lipids[16,27]. The maximum rate in the DTG curves increased withincreasing heating rate as shown in Fig. 1. The maximum pointrates of decomposition observed in the DTG curves occurred at303, 310, 316 and 318 �C at heating rates of 5, 10, 15, and 20 �C/min, respectively. Maximum peaks corresponded to decompositionof proteins.

Differential rate of conversion, dX/dt, for the D. tertiolectaresidual biomass was obtained from DTG curves at heating ratesof 5, 10, 15, and 20 �C/min (Fig. 2). DTG curves of the D. tertiolectaresidual biomass were clearly different from those of the raw D.tertiolecta biomass (Fig. 2). This was due to the absence of a lipidcomponent in the D. tertiolecta residual biomass, as shown inTable 1. Therefore, the forth peak in the DTG curves below450 �C, indicating the decomposition of lipids, disappeared(Fig. 2). This result confirms that all lipids were extracted by thesolvent mixture. We attributed the major peaks between 250 and350 �C to decomposition of protein. The small left shoulders atca. 270 �C corresponds to decomposition of carbohydrate. Themaximum point rates of decomposition for the D. tertiolectaresidual biomass were 309, 320, 325, and 326 �C at heating rates

tiolecta biomass and residual biomass.

P Li K Na

14239.42 – 7557.35 15141.3324214.94 – 2231.26 18585.76

Fig. 2. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves forD. tertiolecta residual biomass at heating rates of 5, 10, 15, and 20 �C/min.

S.-S. Kim et al. / Chemical Engineering Journal 263 (2015) 194–199 197

of 5, 10, 15, and 20 �C/min, respectively. These values are similar tothose obtained for the raw D. tertiolecta biomass.

The TGA graph was analyzed to determine the kineticparameters of D. tertiolecta residual biomass pyrolysis, includingthe activation energy and Arrhenius frequency. The differentialmethod was used to extract the pyrolysis kinetic parameters fromthe thermogravimetric data [28–30].

lndXdt

� �¼ lnðAXnÞ � E

R1T

ð2Þ

The activation energy, E, based on Eq. (2), can be determinedfrom the slope of �E/R based on a plot of ln(dX/dt) vs. 1/T. Theintercept (lnA�Xn) was obtained from Eq. (2) for each conversion.When the apparent order of reaction (n) is assumed to be 0, 1, or2, the pre-exponential factor (A) can be obtained from Eq. (3):

lnðA � XnÞ ¼ ln Aþ n lnðXÞ ð3Þ

The moisture attached to the surfaces of the D. tertiolecta resid-ual biomass was vaporized at temperatures less than 190 �C. Thus,the activation energy at the 5% conversion level was not forpyrolysis of D. tertiolecta residual biomass, but vaporization ofmoisture. The activation energy for the pyrolysis of the D. tertiolec-ta residual biomass ranged from 102.52 to 269.68 kJ/mol, depend-ing on the conversion. Up to a conversion of 40%, activationenergies were similar, and reflected carbohydrate decomposition.Activation energies increased slowly with increasing conversionfrom 50% to 95%; we attributed these increases to protein degrada-tion. These results are in good agreement with those reported byKim and Agblevor [28].

Table 4 shows the apparent activation energies and pre-expo-nential factors calculated from Eqs. (2) and (3) assuming zero-,first-, or second-order reactions. The pre-exponential factors werebetween 107 and 1012 min�1 for conversion efficiencies of 10%and 95%, respectively.

3.3. Bio-oil composition

The bio-oil obtained from pyrolysis of D. tertiolecta residual bio-mass at 430 �C for 5 min in a tubing reactor was analyzed by GC–MS (Agilent 7890A) using an Agilent 19091S-433 capillary column(30 m � 0.25 mm � 0.25 lm). Table 5 shows the typical composi-tion of bio-oil analyzed by GC–MS. A total of 78.32% of compoundsin the liquid products were identified using this method.

Maddi et al. [31] performed comparative pyrolysis of algal bio-mass from natural lake blooms and lignocellulosic biomass. Algal

bio-oils were compositionally different from those obtained fromlignocellulosic biomass, and contained several N-compounds,which were attributed to protein degradation.

Kim et al. [32] investigated the characteristics of bio-oils frompyrolysis of microalgae, and reported that microalgae-derivedbio-oil had a high proportion of fatty acid alkyl esters. The majorcomponents of the fatty oxygenate in the bio-oil were palmitic acid(ca. 7.4%) and oleic acid (ca. 5.4%). Wang et al. [33] pyrolyzedlipid-extracted microalgae remnants in a fluidized bed reactor.They categorized the bio-oil products into carbohydrate-derived,protein-derived, and lipid-derived compounds. The microalgalremnant bio-oil comprised a wide variety of compounds includingaromatics, amides, amines, carboxylic acids, and phenols.

In the current study, hexamethyl-cyclotrisiloxame, ketole, phe-nol, and 4-methyl-phenol were produced in relatively high propor-tions, while the remaining bio-oil products produced by pyrolysisof D. tertiolecta residual biomass were mostly aromatic compoundssuch as pyrrole, benzyle nitrile, 4-ethyl-phenol, benzene propanenitrile, 5,5-dimethyl-2,4-imidazolidinedione, 3-methyl indole,and hexahydro-pyrrole [1,2-a] pyrazine-1,4-dione. All identifiedcompounds contained oxygen and nitrogen. Wang et al. [33] alsoidentified similar compounds from the bio-oil obtained by pyroly-sis of lipid-extracted microalgae remnants, and categorized theseas protein-derived compounds. D. tertiolecta residual biomass con-tained 67.70 wt.% protein, including 8.40 wt.% nitrogen and40.04 wt.% oxygen elements. Therefore, N-compounds in bio-oilare likely produced by the decomposition of proteins.

3.4. Pyrolysis mechanism

Figs. 3–5 show the effects of the pyrolysis residence time onproduct yields at 410 �C, 420 �C, and 430 �C, respectively. Yieldsof oil and gas were in the range of 8.21–28.78 wt.% and 2.01–12.09 wt.%, respectively, at pyrolysis temperatures of 410 �C,420 �C, and 430 �C with a reaction time of 1–5 min. D. tertiolectaresidual biomass yield was greater than 59.0 wt.% under the sameconditions. An increase in the pyrolysis temperature from 410 �C to430 �C resulted in an increase in the yield of oil, whereas the D. ter-tiolecta residual biomass yield decreased. However, the yield of gasremained essentially constant at pyrolysis temperatures between410 �C and 430 �C.

The fractions of gas, oil, and solid (D. tertiolecta residualbiomass) were considered to simplify the kinetic model of theexperimental data. A pyrolysis kinetic model was assumed fortwo groups of series and parallel reactions. The pyrolysis mecha-nisms assumed for the kinetic model developed previously[22,29] is shown below:

A (D. tertiolecta residual)

B (bio-oil)

C (gas; C1-C4, CO, CO2, H2)k1

k2

k3

Based on the assumed mechanism, the kinetic equations for thereaction in terms of yields (CA, CB, and CC) are as follows:

CA ¼ CA0 exp½�ðk1 þ k2Þt� ð4Þ

CB ¼ CB0 þ k2CA0exp½�ðk1 þ k2Þt�

k3 � ðk1 þ k2Þ� expð�k3tÞ

k3 � ðk1 þ k2Þ

� �ð5Þ

Table 4Calculated kinetic parameters for the pyrolysis of D. tertiolecta residual biomass.

Conversion (%)

5 10 20 30 40 50 60 70 80 90 95

Ea (kJ/mol) 35.25 102.52 118.21 121.83 123.99 157.12 186.06 284.03 208.40 244.78 269.68n 0th 1.17 � 10 5.11 � 107 2.26 � 109 7.42 � 108 8.42 � 108 7.35 � 1011 1.66 � 1014 2.38 � 1022 1.83 � 1013 2.38 � 1013 3.07 � 1011

1st 1.23 � 10 5.65 � 107 2.76 � 109 1.00 � 109 1.25 � 109 1.21 � 1012 3.03 � 1014 4.79 � 1022 4.07 � 1013 5.86 � 1013 7.94 � 1011

2nd 1.30 � 10 6.24 � 107 3.38 � 109 1.35 � 109 1.87 � 109 1.99 � 1012 5.53 � 1014 9.65 � 1022 9.07 � 1013 1.44 � 1014 2.05 � 1012

Table 5Compounds identified by GC–MS in D. tertiolecta residue-derived bio-oil produced bypyrolysis at 380 �C for 5 min.

RT Composition Area% Structure

4.422 Pyrrole 2.8

4.809 Phenol 9.16

6.936 4-Methyl-phenol 8.32

8.142 Benzyl nitrile 3.67

8.607 4-Ethyl-phenol 0.12

9.839 Benzene propane nitrile 5.73

20.64 Ketole 15.68

20.708 5,5-dimethyl-2,4-imidazolidinedione 5.71

22.949 3-Methyl indole 5.98

26.393 Hexahydro-pyrrole [1,2-a] pyrazine-1,4-dione

2.31

28.709 Hexamethyl-cyclotrisiloxane 15.84

Time [ min ]0 1 2 3 4 5 6

Yiel

d [ w

t% ]

0

20

40

60

80

100

D. tertiolecta residueOilGasD. tertiolecta residue, ModelOil, ModelGas, Model

Fig. 3. Effect of reaction time (1–5 min) on product distributions at 410 �C.

Time [ min ]0 1 2 3 4 5 6

Yiel

d [ w

t% ]

0

20

40

60

80

100

D. tertiolecta residueOilGas D. tertiolecta residue, ModelOil, ModelGas, Model

Fig. 4. Effect of reaction time (1–5 min) on product distributions at 420 �C.

198 S.-S. Kim et al. / Chemical Engineering Journal 263 (2015) 194–199

CC ¼ CA0 � CA � CB ð6Þ

where CA (wt.%) is the yield of reactant (D. tertiolecta residual), CB

(wt.%) is the yield of liquid product, and CC (wt.%) is the yield ofgas product.

The reaction rate constants k1, k2, and k3 were obtained byapplying an optimization procedure to the values of each reactantand product at the three different pyrolysis temperatures.Parameter measurements can be formulated using the followingconstrained nonlinear optimization program:

minimize S ¼XN

i¼1

ðYcalculated;i � Yexperimental;iÞ2 ð7Þ

where Ycalculated;i and Yexperimental;i refer to the values calculated byEqs. (4)–(6) and the experimental values at a given pyrolysistemperature and residence time, respectively.

Reaction rate constants were estimated by applying Eq. (7) tothe results shown in Figs. 3–5. The calculated reaction rateconstants for the pyrolysis of D. tertiolecta residual biomass areshown in Table 6. As the pyrolysis temperature increased from410 �C to 430 �C, the reaction rate constants k1 and k2 increased,while k3 decreased. Based on the proposed mechanism, thereaction pathway from bio-oil to gas comprises a series of reactionsin a micro tubing reactor; k3 might therefore be affected by reactorconditions such as increased pressure at 410–430 �C.

Reaction rate constants were higher for k2 (D. tertiolecta resid-ual ? bio-oil) than for k1 (D. tertiolecta residual ? gas) and k3

(bio-oil ? gas) at each temperature. These results indicate that thepredominant pyrolysis reaction pathway of D. tertiolecta residualbiomass was from A to B rather than from A to C and/or B to C.

Time [ min ]0 1 2 3 4 5 6

Yiel

ds [

wt%

]

0

20

40

60

80

100

D. tertiolecta residueOilGasD. tertiolecta residue, ModelOil, ModelGas, Model

Fig. 5. Effect of reaction time (1–5 min) on product distributions at 430 �C.

Table 6Reaction rate constants (min�1) for the pyrolysis of D. tertiolecta residual biomass.

Temperature (�C) Rate constant (min�1)

k1 k2 k3

410 0.0024 0.1467 0.0537420 0.0069 0.1500 0.0451430 0.0127 0.1647 0.0315

S.-S. Kim et al. / Chemical Engineering Journal 263 (2015) 194–199 199

Kinetic parameters for pyrolysis of D. tertiolecta residual bio-mass were different from those reported for the macroalgal speciesSaccharina japonica. In the case of S. japonica, k3 was higher than k1

and k2 [22]. First-order lumped kinetics provided an excellent fit tothe products obtained from the D. tertiolecta residual biomass at410 �C, 420 �C, and 430 �C. The lumped reaction scheme proposedpreviously [22,29] is supported by the pyrolysis mechanism of theD. tertiolecta residual biomass as represented in Eqs. (4)–(6). Thismechanism is consistent with the experimental results andaccounts for the formation of bio-oil and gas.

4. Conclusions

Residual biomass of microalgae (D. tertiolecta) obtained afterlipid extraction and saccharification was pyrolyzed for completeutilization of biomass based on the microalgae bio-refinery con-cept. The apparent activation energy for pyrolysis of D. tertiolectaresidual biomass ranged from 102.5 to 269.7 kJ/mol dependingon the conversion efficiency. Based on the proposed lumped kineticmodel, the predominant pyrolysis reaction pathway was fromD. tertiolecta residual biomass to bio-oil rather than D. tertiolectaresidual biomass to gas and/or bio-oil to gas. We demonstratedpyrolysis conversion of microalgae residual biomass to bio-fuel;this could be integrated into microalgae bio-refinery systems tomake microalgae-to-liquid fuels a reality.

Acknowledgements

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Science, ICT and Future Planning (NRF-2013R1A2A2A01068863), and Ministry of Oceans and Fisheries ofKorea (Contract No. 20131039449).

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