Bioresource Technology 139 (2013) 242–248

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Bioresource Technology

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Thermogravimetric characteristics and pyrolysis kinetics of AlgaSagarssum sp. biomass

0960-8524/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.biortech.2013.03.192

⇑ Corresponding authors. Tel.: +82 33 570 6544; fax: +82 33 570 6535 (S.-S. Kim),tel.: +82 31 201 2492; fax: +82 31 202 1946 (J. Kim).

E-mail addresses: sskim2008@kangwon.ac.kr (S.-S. Kim), jkim21@khu.ac.kr(J. Kim).

Seung-Soo Kim a,⇑, Hoang Vu Ly b, Jinsoo Kim b,⇑, Jae Hyung Choi c, Hee Chul Woo c

a Department of Chemical Engineering, Kangwon National University, 1 Joongang-ro, Samcheok, Gangwon-do 363-883, Republic of Koreab Department of Chemical Engineering, Kyung Hee University, 1 Seocheon-dong, Yongin, 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

� Systematically study and the first attempt of pyrolysis for Macroalge Sagarssum sp.� Determination of global kinetic parameters including activation energy from TG analysis.� Influence of pyrolysis temperature and residence time on product yields and compositions.� Calculation of kinetic parameters from a lumped kinetic model.

a r t i c l e i n f o

Article history:Received 15 February 2013Received in revised form 28 March 2013Accepted 31 March 2013Available online 6 April 2013

Keywords:BiomassMacroalgaeSagarssum sp.Lumped kinetic modelThermogravimetric analysis

a b s t r a c t

Alga Sagarssum sp. can be converted to bio-oil, gas, and char through pyrolysis. In this study, the pyrolysischaracteristics and kinetics of Sagarssum sp. were investigated using a thermogravimetric analyzer andtubing reactor, respectively. Sagarssum sp. decomposed below 550 �C, but the majority of materialsdecomposed between 200 and 350 �C at heating rates of 5–20 �C/min. The apparent activation energyincreased from 183.53 to 505.57 kJ mol�1 with increasing pyrolysis conversion. The kinetic parametersof Sagarssum sp. pyrolysis were determined using nonlinear least-squares regression of the experimentaldata, assuming second-order kinetics.

The proposed lumped kinetic model represented the experimental results well and the kinetic rate con-stants suggested a predominant pyrolysis reaction pathway from Sagarssum sp. to bio-oil, rather thanfrom Sagarssum sp. to gas. The kinetic rate constants indicated that the predominant reaction pathwaywas A (Sagarssum sp.) to B (bio-oil), rather than A (Sagarssum sp.) to C (gas; C1–C4).

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

With the global economy increasing, the demand for fossil fuelshas also increased. Fossil fuels are limited resources, and the deple-tion of oil is expected by the middle of this century (Kraan, 2013).Thus, there has been increasing interest in renewable energysources such as solar, wind, fuel cells, hydrogen, geothermal,ocean, and biomass. Biomass is one of the most promising optionsas an alternative fuel because it is clean, renewable and fastgrowing.

Marine biomass from sources such as micro-algae and macroal-gae is being considered as a potential source of third-generationbiofuel feedstocks (Um and Kim, 2009; Demirbas, 2010; Leeet al., 2010; Adams et al., 2011). Macroalgae especially are fast-

growing marine and freshwater plants that can grow to a consider-able size, up to tens of meters in length in the case of Pacific kelpspecies (Lüning, 1990). Growth rates of marine macroalgae are fas-ter than those of terrestrial biomass. Many countries are trying todevelop marine biomass (Kraan, 2013), and Korea has producedmacroalgae and converted it into chemicals and bio-energy since2009 through the financial support of the Ministry for Food, Agri-culture, Forestry and Fisheries (http://www.abrc.re.kr).

Marine biomass such as macroalgae, which are primarily com-posed of polysaccharides, can be converted into fuels such asbio-alcohol (Adams et al., 2009; Borines et al., 2011) or bio-oil(Ross et al., 2008, 2009; Bae et al., 2011) by fermentation or pyro-lysis, respectively. Cancela et al. (2012) researched microwave-as-sisted transesterification of macroalgae and compared the resultswith the traditional transesterification. Hu et al. (2013) carriedout pyrolysis experiments of blue–green algae blooms (BGAB) ina fixed-bed reactor, and the bio-oil from BGAB was characterizedwith high heating value of 31.9 MJ/kg and an O/C molar ratio of0.16 at optimum conditions. Wang et al. (2013) researched the

Nomenclature

A pre-exponential factor (s�1)E activation energy (kJ mol�1)k pyrolysis rate constant (min�1)n reaction orderR gas constant (8.314 J g mol�1 K�1)t pyrolysis time (min)T absolute temperature (K)

W weight of sample at time t (g)W0 initial weight of sample (g)Wt final weight of sample (g)X conversion of samplesk1, k2, k3 reaction rate constants (min�1)CA, CB, CC yields of reactant and product (wt.%)

S.-S. Kim et al. / Bioresource Technology 139 (2013) 242–248 243

pyrolysis of two seaweed Enteromorpha clathrata and Sargassumnatans. The bio-oil compositions were greatly different from thoseof terrestrial biomass, especially in the presence of many nitrogen-containing compounds. Anastasakis et al. (2011) investigated thepyrolysis characteristics of alginic acid, mannitol, laminarin andfucoidan using TGA and py-GC/MS. Their decompositions werefound to start at lower temperatures than terrestrial biomass con-stituents such as cellulose, hemi-cellulose and lignin, which re-vealed that seaweed constituents were easier to break.

To understand the pyrolysis characteristics of macroalgae, ther-mogravimetric analysis (TGA) has been performed (Ross et al.,2009; Li et al., 2010, 2011; Maddi et al., 2011). Anastasakis et al.(2011) performed a pyrolysis behavior study of the main carbohy-drates of brown macroalgae, and Py-GC/MS results showed thateach of the model compounds produced a characteristic fingerprint.

In this study, the characteristics and kinetics of Sagarssum sp.pyrolysis were investigated to obtain information regarding thetypes of fuel (gas, bio-oil, or bio-char) that can be generated. Globalkinetic parameters including apparent activation energy weredetermined during pyrolysis of Sagarssum sp. using TGA. The kineticparameters from a lumped kinetic model were also calculated.

2. Experimental

2.1. Thermogravimetric analysis

Alga Sagarssum sp. was obtained from the Institute of TropicalBiology of the Vietnamese Academy of Science and Technologyand used after drying. The alga material was ground with a knifemill and sieved to obtain fragments 150 and 355 lm in lengthfor thermogravimetric analysis.

The moisture and ash contents were determined using ASTM E1756 and ASTM E 1755, respectively (Annual Book of ASTM Stan-dards, 1997). Thermogravimetric analysis of the Sagarssum sp.samples (25.0 ± 1.0 mg) was carried out using TGA (Q50, TA Instru-ment, New Castle, USA). Nitrogen was used as the carrier gas at aflow rate of 25 mL/min. The heating rates were controlled at 5,10, 15, and 20 �C/min from 30 to 800 �C.

2.2. Tubing reactor

A tubing reactor was used to pyrolyze Sagarssum sp. samples.The experimental apparatus consisted of a salt bath, temperature

Table 1Characteristics of the Sagarssum sp. biomass.

Sample V.M. (%)a Fixedcarbon (%)a Moisture (%)b Ash (%

Sagarsum sp. 44.50 9.34 9.34 36.82

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

controller, mechanical stirrer, and a micro-tubing reactor. Thereactor was used to test the effect of residence time on the pyroly-sis of Sagarssum sp. at a constant temperature. A sample mass of3 g was used in each experimental run. The molten salt bath wascomposed of a eutectic salt of KNO3 (59 wt.%) and Ca(NO3)2

(41 wt.%) (Kim and Kim, 2000; Park et al., 2009; Kim et al., 2010,2012). The temperatures of the molten salt bath were kept con-stant at 360, 370 �C, and 380 �C ± 1 �C. The experimental apparatuswas described in our previous work (Kim et al., 2012).

Based on data from the differential thermogravimetric (DTG)curves, pyrolysis temperatures of 360, 370, and 380 �C were se-lected with heating rates of 5, 10, 15, and 20 �C/min. The tubingreactor containing the Sagarssum sp. sample was placed into themolten salt bath. The reaction times ranged from 1 to 4 min at eachreaction temperature. After the reaction, the reactor was removedfrom the molten salt and cooled to room temperature. The reactionproducts were analyzed by weighing the gas, oil, and solid prod-ucts. The gas yield, defined as (gas weight) � 100/(feed weight),was obtained by weighing the tubing reactor before and after gasrelease. The other pyrolyzed products were separated into oil (ace-tone soluble) and solid (acetone insoluble) using a solvent extrac-tion technique (Ding et al., 1997). The solid yield was defined as(weight of acetone insoluble) � 100/(weight of feed), while theoil yield was defined as (100 – gas yield – solid yield). The solidwas separated by micro filter paper of 0.45 lm.

3. Results and discussion

3.1. Material characterizations

The characteristics of the Sagarssum sp. samples are presentedin Table 1. In our previous work, the ash content of Saccharinajaponica, a type of macroalgae that grows near the southern coastof Korea, was found to be 20.21 wt.% (Kim et al., 2012). However,the ash content of Sagarssum sp. was much higher, at 36.82 wt.%.Bird et al. (2011) produced algal biochar from macroalgae (sea-weed) through the pyrolysis of seven species of green tide algae.The ash content of the raw algae was 10.5–33.8 wt.%, and Cladno-phoropsis sp. and Cladophora vagabunda were 33.3 and 33.8 wt.%,respectively. Thus, the ash content of Sagarsum sp. was higher thanthat of Cladophoropsis sp. and C. vagabunda.

The carbon (C) content of Sagarssum sp. was lower than that ofS. japonica , and S. japonica had a C content of 32.89 wt.% (Kim et al.,

)c Element (%) HHV (MJ/kg)

C H N S Od

26.70 4.23 1.35 0.19 67.53 10.10

ass.

Table 2Calcium, magnesium, phosphorous, lithium, potassium, and sodium contents of ofSagarssum sp. biomass.

Sample Element (ppm)

Ca Mg P Li K Na

Sagarsum sp. 37260.87 14201.97 808.17 N.D. 104535.01 53998.44

Temperature [ oC ]0 100 200 300 400 500 600 700 800

Wei

ght %

0

20

40

60

80

100

1: 5 oC/min2: 10 oC/min3: 15 oC/min4: 20 oC/min

Char : 46.17 wt%

Fig. 1. Weight loss curves for Sagarssum sp. at heating rates of 5, 10, 15, and 20 �C/min.

Temperature [ oC ]

0 100 200 300 400 500 600 700 800

Con

vers

ion,

X

0.0

0.2

0.4

0.6

0.8

1.0

DTG

, dX/

dt X

105 [

wt%

/min

]

0

50

100

150

200

250

3001. 5 oC/min2. 10 oC/min3. 15 oC/min4. 20 oC/min

4

3

2

1

Fig. 2. Thermogravimetric and differential thermogravimetric curves for Sagarssumsp. at heating rates of 5, 10, 15, and 20 �C/min.

244 S.-S. Kim et al. / Bioresource Technology 139 (2013) 242–248

2012). The carbon contents of green tide algae such as Cladophor-opsis sp. and Cladophora coelothrix were between 20.5 and32.1 wt.% (Bird et al., 2011). The oxygen content of Sagarssum sp.was higher than that of most woody biomasses (Park et al., 2009;Kim et al., 2010), and was even higher than that of S. japonica,which had an oxygen content of 60.01 wt.% (Kim et al., 2012).The H/C molar ratio of Sagarsum sp. was 1.90. The higher heatingvalue (HHV) of Sagarssum sp. was 10.06 MJ/kg, which was analyzedusing a bomb calorimeter (IKA 2000).

The concentrations of calcium (Ca), magnesium (Mg), phos-phorous (P), lithium (Li), potassium (K), and sodium (Na) areshown in Table 2. Lithium was not detected in the preparedsamples. Sagarssum sp. contained much more inorganic matterthan S. japonica, (Kim et al., 2012). Specifically, the calcium, mag-nesium and potassium contents were higher in Sagarssum sp.These higher levels of inorganic compounds are the reasonwhy the ash content of Sagarssum sp. was higher than that ofS. japonica, as shown in Table 2.

3.2. Thermogravimetric analysis and kinetic parameters of thepyrolysis of Sagarssum sp.

Fig. 1 shows thermogravimetric analysis results of Sagarssumsp. in nitrogen atmosphere at the heating rate of 5–20 �C/min.When the thermogravimetric analysis was carried out to 800 �C,the final weights were 45.94–46.49 wt.% for each heating rate.Therefore, the content of char is estimated to be 46.17 wt.% in aver-age, suggesting the existence of a large amount of inorganic com-pounds as shown in Table 2.

The results of the thermogravimetric experiments for Sagarssumsp. were expressed as a function of the conversion X, which is de-fined as in Eq. (1).

X ¼ W0 �WW0 �W1

ð1Þ

where W0 is the initial mass of the sample, W is the mass of thepyrolyzed Sagarssum sp. sample, and W1 is the final residual massafter thermogravimetric analysis.

The degrees of conversion versus temperature at heating ratesof 5, 10, 15, and 20 �C/min for Sagarssum sp. obtained via TGAare shown in Fig. 2. At temperatures lower than 180 �C, the changein the conversion of the samples can be attributed to vaporizationof the moisture attached on the surfaces of the samples. Pyrolysisof Sagarssum sp. samples started at 200 �C. The TGA graphs at dif-ferent heating rates of 5, 10, 15, and 20 �C/min showed similar re-sults, and the Sagarssum sp. samples showed two weight loss steps,with major decomposition occurring between 200 and 350 �C. Sim-ilar thermal degradation patterns were noted for three red macro-algae samples (Pophyra yezoensis, Polcarnium telfairiae Harv, andCorallina pilulifera) by Li et al. (2011). S. japonica showed similardegradation patterns as well (Kim et al., 2012).

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. 2). The DTG curve at each heating ratehad three extensive peaks between 50 and 350 �C. The first step,at temperatures lower than 180 �C, corresponds to evaporation ofwater attached on sample’s surface, and the next steps, between200 and 350 �C, correspond to decomposition of the differentbio-polymer fractions. The second peak between 200 and 270 �Ccan be attributed to the decomposition of carbohydrates, and thethird peak below 350 �C indicates decomposition of proteins (Rosset al., 2008; Bae et al., 2011). In the case of S. japonica, the algashowed a higher second peak than third peak (Kim et al., 2012),however, for Sagarssum sp., the second peak was highest and theother peaks were reversed. This means that Sagarssum sp. has morecarbohydrate in its structure than S. japonica.

Decomposition of Sagarssum sp. at 350 �C or higher likely pro-gressed more slowly because of the remaining protein and char(Ross et al., 2008; Bae et al., 2011; Sanchez-Silva et al., 2012;Kim and Agblevor, 2007; Yang et al., 2007). The maximum rate ofdX/dt in the DTG curves increased with increasing heating rate(Fig. 2). The maximum rate of decomposition tended to increaseat higher heating rates because there was more thermal energy,which facilitated the transfer of heat between the surroundingsand the interiors of the samples (Caballero et al., 1997). The max-imum point rates of decomposition observed in the DTG curves oc-curred at 256.3, 250.0, 257.0 and 259.0 �C at heating rates of 5, 10,15, and 20 �C/min, respectively.

The TGA graph was analyzed to determine the kinetic parame-ters of Sagarsum sp. pyrolysis, including the apparent activation

1/T [ K-1 ]0.0010 0.0015 0.0020 0.0025 0.0030

ln(d

X/dt

)

-10.5

-10.0

-9.5

-9.0

-8.5

-8.0

-7.5

-7.0

-6.5

-6.0

-5.55% 10% 20% 30% 40% 50% 60% 70% 80% 90% 95%

Fig. 3. Activation energies calculated from Eq. (5) at heating rates of 5, 10, 15, and20 �C/min at conversion values ranging from 5–95%.

S.-S. Kim et al. / Bioresource Technology 139 (2013) 242–248 245

energy and the Arrhenius frequency. The differential method in Eq.(2) was used to determine the pyrolysis kinetic parameters fromthe thermogravimetric data (Kim and Agblevor, 2007; Maitiet al., 2007; Kim et al., 2010, 2012).

lndXdt

� �¼ lnðAXnÞ � E

R1T

ð2Þ

The activation energy, E, based on Eq. (2), can be determinedfrom a plot of ln(dX/dt) vs. 1/T. Fig. 3 shows plots of ln(dX/dt) vs.1/T at various conversions ranging from 5% to 95%, resulting in afamily of parallel straight lines with slopes of �E/R. For example,when the conversion of Sagarssum sp. was 20%, for example, thecorresponding temperatures were 207.92, 210.67, 213.75, and221.67 �C for heating rates of 5, 10, 15, and 20 �C/min, respectively.At these temperatures, the values of ln(dX/dt) were �9.32, �8.28,�7.88, and �7.52 for heating rates of 5, 10, 15, and 20 �C/min,respectively. From the slope of ln(dX/dt) vs. 1/T at 20% conversion,the activation energy was calculated to be 183.53 kJ/mol. Theintercept (lnA�Xn) for each conversion was obtained from Fig. 3.When the apparent order of reaction (n) from Eq. (2) is assumedto be 0, 1, or 2, the pre-exponential factor (A) can be obtained fromEq. (3).

lnðA � XnÞ ¼ ln Aþ n lnðXÞ ð3Þ

The moisture on the surfaces of Sagarssum sp. vaporized at tem-peratures less than 180 �C, as seen in Fig. 2. Thus, the activation en-ergy at the 5–10% conversion level was not the pyrolysis ofSagarssum sp., but the vaporization of moisture. The activation en-ergy for the pyrolysis of Sagarssum sp. ranged from 183.53 to525.57 kJ/mol, depending on the conversion. Up to a conversionof 40%, the activation energies increased, which corresponds tocarbohydrate decomposition. The activation energies increased

Table 3Calculated kinetic parameters for the pyrolysis of Sagarssum sp. biomass.

Conversion (%)

5 10 20 30 40 50

Ea (kJ/mol)

48.46 56.06 183.53 308.61 402.03 32

n 0th 1.88 � 103 7.26 � 103 7.96 � 1015 1.07 � 1028 7.09 � 1037 1.1st 3.74 � 104 7.24 � 104 3.98 � 1016 3.54 � 1028 1.78 � 1038 3.2nd 7.43 � 105 7.22 � 105 1.99 � 1017 1.18 � 1029 4.46 � 1038 7.

from 320.74 to 525.57 kJ/mol with increasing conversion from50% to 60%, after which the activation energy showed higher valuescompared to carbohydrate decomposition. Those values canmainly be attributed to protein degradation, and are in good agree-ment with those reported by Kim and Agblevor (2007). The overallactivation energies of Sagarssum sp. were higher than those of S.japonica.

Table 3 shows the apparent activation energies and pre-expo-nential factors calculated using Eq. (2) and (3) based on Fig. 3,assuming a zero-, first-, or second-order reaction. The pre-expo-nential factors were between 1015 and 1042 s�1 when the conver-sion was between 10% and 95%, and was the highest at aconversion of 60%.

3.3. Bio-oil composition

The bio-oil obtained from the pyrolysis of Sagarssum sp. at380 �C for 4 min in a tubing reactor was analyzed by GC–MS(Agilent Technologies 7890A) with an Agilent 19091S-433 capil-lary column (30 m � 0.25 mm � 0.25 lm). Helium was used asthe carrier gas at a flow rate of 1 mL/min. The GC oven temper-ature was initiated at 40 �C for 5 min, then gradually increasedto 280 at 10 �C/min, and finally maintained at 280 �C for10 min. The injector and detector temperatures were set at 250and 280 �C, respectively. In each trial, 1 lL of the sample was in-jected. Table 4 shows the composition of bio-oil by GC–MS. A to-tal of 78.54% of the compounds in the liquid products wereidentified using this method.

Ross et al. (2008) studied macroalgae pyrolysis in Macrocystispyrifera using pyrolysis-GC–MS at 500 �C. In their research, thedegradation products were classified as carbohydrates, proteins,or polyphenolics. Unlike woody biomass, macroalgae does notcontain as many phenolic compounds because in contains no lig-nin-type materials. Maddi et al. (2011) performed a comparativepyrolysis study of algal biomass from natural lake blooms andlignocellulosic biomass. The algal bio-oils were compositionallydifferent and contained several N-compounds, which was attrib-uted to protein degradation. Bae et al. (2011) carried out pyroly-sis on three marine macroalgae in a pecked tube reactorbetween 300 and 600 �C. Bio-oils of laminaria showed higherselectivity for ethanone and dianhydro-mannitol. Dianhydro-mannitol was highest (27.28%) at 300 �C and decreased to2.17% at 600 �C. In our previous work, S. japonica was pyrolyzedin a tubing reactor with the same reaction conditions as Sagar-sum sp., with dianhydro-mannitol showing the highest selectiv-ity among the produced compounds, and the remaining bio-oilproducts produced by pyrolysis of S. japonica were mostly aro-matic compounds such as ethanone, furanone, phenol, cyclopen-tene, cyclopentenolone, pyridine, and isosorbide (Kim et al.,2012).

In this study, 5-methyl furfuran showed the highest selectivityamong the produced compounds, and the remaining bio-oil prod-ucts produced by the pyrolysis of Sagarssum sp. were phenol,4(1H)-pyridine, and hexadecanoic acid. Pyrolysis products derived

60 70 80 90 95

0.74 525.57 397.73 461.89 260.03 397.95

86 � 1026 2.97 � 1042 3.65 � 1026 6.41 � 1026 2.03 � 1010 1.00 � 1017

73 � 1026 4.95 � 1042 5.23 � 1026 7.99 � 1026 2.26 � 1010 1.06 � 1017

43 � 1026 8.24 � 1042 7.50 � 1026 9.95 � 1026 2.53 � 1010 1.11 � 1017

Table 4Compounds identified by GC–MS in Sagarssum sp. bio-oil produced by pyrolysis at 380 �C for 4 min.

RT (min) Compositions Area (%) Structure

13.991 5-Methyl furfuran 24.31

14.568 Phenol 10.62

17.410 4(1H)-pyridinone 17.42

28.193 Hexadecanoic acid 20.19

Table 5Oil, gas, and solid yields (wt.%) from pyrolysis Sagarssum sp.biomass pyrolyzed underdifferent conditions in a micro-tubing reactor.

Reaction time (min)

1 2 3 4

Pyrolysis at 360 �CGas (wt.%) 4.62 ± 0.22 9.86 ± 0.22 13.74 ± 0.29 17.09 ± 0.16Oil (wt.%) 20.52 ± 0.9 26.65 ± 1.05 32.73 ± 0.67 33.85 ± 0.45Solid (wt.%) 74.86 ± 0.79 63.49 ± 1.05 53.53 ± 0.41 49.06 ± 0.29

Pyrolysis at 370 �CGas (wt.%) 5.21 ± 0.1 11.09 ± 0.09 14.80 ± 0.11 17.61 ± 0.12Oil (wt.%) 23.55 ± 0.26 29.27 ± 0.53 34.11 ± 0.44 35.23 ± 0.62Solid (wt.%) 71.24 ± 0.16 59.64 ± 0.44 51.09 ± 0.33 47.16 ± 0.59

Pyrolysis at 380 �CGas (wt.%) 7.59 ± 0.27 12.47 ± 0.42 18.98 ± 0.24 18.71 ± 0.29Oil (wt.%) 21.71 ± 0.72 29.93 ± 0.78 31.28 ± 0.3 36.54 ± 0.08Solid (wt.%) 71.07 ± 0.82 57.60 ± 0.62 49.74 ± 0.13 44.75 ± 0.35

Reaction time [ min ]

0 1 2 3 4 5

ln(C

A0/

CA)

0.0

0.2

0.4

0.6

0.8

1.0

360 oC

370 oC

380 oC

Fig. 4. First-order kinetic fitting of Sagarssum sp. decomposition at 360 �Cforreaction times of 1–4 min.

246 S.-S. Kim et al. / Bioresource Technology 139 (2013) 242–248

from Sagarssum sp. are a potential source of transport fuels as asubstitute and/or additive to gasoline and diesel.

3.4. Pyrolysis mechanism

The results shown in Table 5, illustrate the effects of pyrolysistemperature and residence time on product yields. The yields ofoil and gas were in the range of 20.52–37.26 and 4.62–18.71 wt.%, respectively, at pyrolysis temperatures of 360, 370,and 380 �C with a reaction time of 1–4 min. The solid yield wasgreater than 44 wt.% under the same conditions, which containschar and unreacted Sagassum sp. The content of unreacted Sagas-sum sp. might be decreased with increasing reaction temperatureand/or time. The high yields of Sagarssum sp. are attributed to highconcentrations of inorganic compounds as shown in Table 2. Tosimplify the kinetic model of the experimental data, the fractionsof gas, oil, and solid were considered to the kinetic model, andthe pyrolysis kinetic model was assumed for two groups of seriesand parallel reactions. The initial fraction of solid (Sagarssum sp.)was considered to be 100 wt.% and decreased with increasing reac-tion time at a given temperature.

The pyrolysis mechanisms assumed for the kinetic model,which was developed previously (Park et al., 2009; Kim et al.,2010, 2012) was:

ð4Þ

Overall conversion of Sagarssum sp. decomposition was consid-ered to either a first-order reaction or a second-order reaction. Theapparent rate equation can be applied as a power law expression todetermine the reaction order, and is in the form.

dCA

dt¼ �kCn

A ð5Þ

where CA is the wt.% of Sagarssum sp., k is the apparent rate con-stant, and n is the apparent reaction order.For first-order kinetics,where n = 1, the integrated form of Eq. (5) would be Eq. (6).

lnCA0

CA¼ kt ð6Þ

Reaction time [ min ]0 1 2 3 4 5

1/C

A - 1

/CA0

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

360 oC370 oC380 oC

Fig. 5. Second-order kinetic fitting of Sagarssum sp. decomposition at 360 �Cforreaction times of 1–4 min.

CA2/CB

0 50 100 150 200 250 300

-[(d

CA/d

t + d

CB/d

t)/C

B]

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

360 oC

370 oC

380 oC

Fig. 6. Kinetic plot of Eq. (15) for Sagarssum sp. decomposition at 360 �C, 370 �C, and380 �C for reaction times of 1–4 min.

Table 6Reaction rate constants (min�1) for the pyrolysis of Sagarssum sp. biomass.

Temperature(�C)

R2 Reaction rate constant (min�1)

1storder

2ndorder

k k1 k2 k3

360 0.96 0.99 0.0026 0.0017 0.0254 0.0009370 0.94 0.98 0.0028 0.0018 0.0080 0.0011380 0.95 0.99 0.0031 0.0018 0.0035 0.0013

Note:R2: determined in Fig. 4 and Fig. 5.k: determined from the slopes of the fitting with Eq. (7) in Fig. 4.k3: determined from the slopes of the fittings with Eq. (15) in Fig. 6.k2: determined from the intercept of the fittings with Eq. (15) in Fig. 6.k1: evaluated by k–k3.

S.-S. Kim et al. / Bioresource Technology 139 (2013) 242–248 247

For second-order kinetics, where n = 2, the integrated form ofEq. (5) could be solve from Eq. (5) to give Eq. (7).

1CA� 1

CA0¼ kt ð7Þ

Figs. 4 and 5 show Eq. (6) and Eq. (7) fit to the experimentaldata, respectively. The fit of the second-order kinetics had a higherR2 than the first-order kinetics (Table 6). Therefore, it was assumedthat the decomposition of Sagarssum sp. followed a second-orderreaction, and was adopted as follows to solve Eq. (4).

1. A combination of series and parallel reactions is assumedfor the pyrolysis mechanism of Sagarssum sp.

2. All the reactions are irreversible.3. Sagarssum sp. are decomposes by second-order kinetic.4. Bio-oil to gas formation is through first-order kinetics.

Based on Eq. (4) and the adopted assumptions, the rate of for-mation/depletion of various components can be written as follows:

dCA

dt¼ �ðk1 þ k3ÞC2

A ð8Þ

dCB

dt¼ k1C2

A � k2CB ð9Þ

dCC

dt¼ k3C2

A þ k2CB ð10Þ

The kinetic equation of Eq. (8) for the reaction in terms of yield(CA) can be solved by Eq. (11).

1CA� 1

CA0¼ ðk1 þ k3Þt ð11Þ

Substituting Eq. (11) into Eq. (9) gives Eq. (12).

dCB

dtþ k2CB ¼

k1

ðk1 þ k3Þt þ 1CA0

h i2 ð12Þ

Solving Eq. (12), produces a very complicated polynominalfunction, and thus the variations with time in the experimentalyield data (Sagarssum sp., bio-oil and gas) fit using the followingpolynomial function (Zhao and Yu, 2011).

Ci ¼Xn

j¼0

ai;jtj ð13Þ

For each lumped component in Sagarssum sp. pyrolysis, the bestfit was found with a correlation coefficient very close to unity(R2 > 0.95). The time derivatives of the components were obtainedas shown in Eq. (14) as well:

dCi

dt i¼Xn

j¼0

ai;zjtj�1 ð14Þ

Combining Eqs. (8) and (9) produces the following expressions:

�dCAdt þ

dCBdt

CB

!¼ k2 þ k3

C2A

CB

!ð15Þ

The left-side of Eq. (15) can be evaluated from Eqs. (14) and(15), which were determined from the experimental data.

The reaction rate constants k1, k2, and k3 were obtained for eachcomponent product at the three different reaction temperatures.The calculated reaction rate constants for the pyrolysis of Sagars-sum sp. are shown in Table 6. The overall reaction rate constant kslightly increased with increasing temperature from 360 to380 �C. The reaction rate constants k1 and k3 did not vary muchdepending on temperature, whereas k2 sharply decreased withincreasing temperature. The reaction rate constant k2 was muchhigher than k1 and k3, especially at the temperatures of 360 and370 �C. This indicates that bio-oil can be easily converted to a gas-eous product when Sagarssum sp. decomposes to bio-oil. Whencompared in terms of a parallel reaction, the rate constant k1

(Sagarssum sp. ? bio-oil) was higher than k3 (Sagarssum sp. ? gas).

248 S.-S. Kim et al. / Bioresource Technology 139 (2013) 242–248

These results indicate that the predominant pyrolysis reactionpathway of Sagarssum sp. was from A to B rather than from A toC with the proposed mechanism.

The results from Sagarssum sp. were similar to the results ob-tained from the pyrolysis of S. japonica, even though the decompo-sition of S. japonica was a first-order reaction, where k2 was higherthan k3. Second-order lumped kinetics provide an excellent fit forthe products obtained from Sagarssum sp. at 360, 370, and380 �C. The lumped reaction scheme previously proposed (Parket al., 2009; Kim et al., 2012; Zhao and Yu, 2011) is supported bythe pyrolysis mechanism of Sagarssum sp. shown in Eqs. (8)–(10).This mechanism is generally in agreement with the experimentalresults and accounts for the formation of the products.

4. Conclusions

The global kinetic parameters of Sagarssum sp. including theapparent activation energies, were determined by differentialmethods. The activation energies of the pyrolysis of Sagarssumsp. were between 183.53 and 525.57 kJ/mol. Sagarssum sp. bio-oilshowed the highest selectivity for dianhydro-mannitol when pyro-lysis was carried out at 380 �C for 4 min in a micro-tubing reactor.The proposed lumped kinetic model represents the experimentalresults well, and the kinetic rate constants suggest a predominantpyrolysis reaction pathway from Sagarssum sp. to bio-oil ratherthan from Sagarsum sp. to gas.

Acknowledgement

This work was financially supported by the Ministry for Food,Agriculture, Forestry and Fisheries (contract no.12128295500).

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