Research on Pyrolysis Characteristics of Seaweed - American

a DTG-60H thermal analyzer, and the pyrolysis characteristics obtained at different heating rates (20, 30, 40, and 50 °C min-1) are analyzed. The res...
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Energy & Fuels 2007, 21, 3723–3729

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Research on Pyrolysis Characteristics of Seaweed S. Wang,† X. M. Jiang,*,† N. Wang,† L. J. Yu,† Z. Li,‡ and P. M. He‡ Institute of Thermal Energy Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, P. R. China, and College of Aqua-life Science and Technology, Shanghai Fisheries UniVersity, Shanghai 200090, P. R. China ReceiVed April 25, 2007. ReVised Manuscript ReceiVed August 2, 2007

Pyrolysis experiments of Enteromorpha clathrata (ENT) (a species of seaweed) have been conducted using a DTG-60H thermal analyzer, and the pyrolysis characteristics obtained at different heating rates (20, 30, 40, and 50 °C min-1) are analyzed. The results indicate that the nonisothermal mass loss process of samples is composed of dehydration, rapid mass loss, slow mass loss, and solid residue decomposition. The devolatilization stage of ENT starts earlier than that of woody biomass because the basic components in seaweed are preferable for pyrolysis compared to lignocellulosic materials. The FTIR analysis is employed to investigate the changes in the main components of sample, while the TG-MS analysis is used for the gaseous products analysis during the pyrolysis. And because of the difference between the compositions of seaweed and woody biomass, gas formation here is not the same as the one from woody biomass. The characteristic parameters of pyrolysis at different heating rates show that the maximum rate of pyrolysis mass loss, the peak temperature, the initial and final temperature for devolatilization, and the heat release will increase with increasing heating rate. The kinetic parameters are calculated by using the Coats–Redfern method, which indicates that the kinetic function of pyrolysis mechanism is different from the woody biomass. The kinetic compensation effect exists between the activation energy and the frequency factor.

1. Introduction As a limited energy source, fossil fuel could hardly afford long-lasting consumption, and environmental contamination becomes an apparent and serious issue. All facts hasten a growing concern to the exploitation and application of biomass in energy researches and industries. Biomass can be defined as any organic material mainly consisting of carbon, hydrogen, oxygen, nitrogen, and some other components in small proportions.1 Biomass mainly includes woody and herbaceous species, organic wastes, animal wastes, agricultural and industrial residues, and so on. Comparing with wood and crop, marine biomass as another sort of energy plants is applied to energy consumption in a relatively small area all over the world. Seaweed is an important constituent part of marine biomass. Most of seaweeds are the green (1200 species), brown (2000 species), or red (6000 species) kinds, which can be found throughout the world’s oceans and seas. Many countries are surrounded by wide coastal areas and territorial seas. China is one of them. In its 14200 km coastal area lives a great variety of seaweeds, with the number of species ranging from 3000 to 4000.2 With short life cycle and fast breeding, seaweeds are easy to breed, and the growth seasons of seaweeds are not single. A variety of seaweeds can be cultivated alternately in any time of the year, which ensures the biomass source is abundance. Moreover, seaweed lives in sea areas, not occupying land areas. In tradition, biomass resources are difficult to put into wide * Corresponding author: e-mail [email protected]; Tel +86-2134205681. † Shanghai Jiao Tong University. ‡ Shanghai Fisheries University. (1) Yaman, S. Energy ConVers. Manage. 2004, 45, 651–671. (2) Xia, B. M. Flora algarum marinarum sinicarum, Tomus 2 Rhodophyta, No 3 Gelidiales Cryptonemiales Hildenbrandiales; Science Press: Beijing, China, 2000. (in Chinese).

application because they are scattered and influenced by seasons. But seaweeds do not have these matters. Seaweeds are also able to absorb large quantities of nitrogen, phosphorus, and carbon dioxide and produce a lot of oxygen. In eutrophic sea area, the eutrophication is the factor for the occurrence of massive blooms of harmful microalgae and the decrease of the biodiversity, which finally causes red tides. In these sea areas, seaweed cultivation has an excellent effect on decreasing eutrophication. The nutrient control could be an effective way to reduce the risk of red tide occurrence. Algicidal activity of some seaweeds also has positive effects on the decreasing of red tide microalgae.3 A lot of seaweeds can be planted to help ocean remediation. However, another problem comes out, that is, how to deal with so many seaweeds. Seaweeds can be used in some fields like medication and food industry. Besides, industry chain of seaweed biomass energy and ocean bioremediation can be built up along the coastal lines where seaweed concentrates. If those rich seaweeds sources are explored efficiently and put into clean and proper use, they may contribute a lot to the worldwide energy usage, both theoretically and industrially. Many experts have done broad and deep research works on biomass like lignocellulosic biomass.4–6 Some specialists and experts have made related reports on how to make use of marine biomass as energy sources nowadays.7 For example, a great amount of oil in B. braunii was obtained by Dote et al., with a (3) Jeong, J. H.; Jin, H. J.; Sohn, C. H.; Suh, K. H.; Hong, Y. K. J. Appl. Phycol. 2000, 12, 37–43. (4) Matsuoka, K.; Shinbori, T.; Kuramoto, K.; Nanba, T.; Morita, A.; Hatano, H.; Suzuki, Y. Energy Fuels 2006, 20, 1315–1320. (5) Lv, P. M.; Chang, J.; Wang, T. J.; Wu, C. Z.; Tsubaki, N. Energy Fuels 2004, 18, 1865–1869. (6) Dai, X. W.; Wu, C. Z.; Li, H. B.; Chen, Y. Energy Fuels 2000, 14, 552–557. (7) Miao, X. L.; Wu, Q.Y.; Yang, C. Y. J. Anal. Appl. Pyrolysis 2004, 71, 855–863.

10.1021/ef700214w CCC: $37.00  2007 American Chemical Society Published on Web 09/19/2007

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yield of 57–64 wt % at 300 °C. The oil was equivalent in quality to petroleum oil.8 In biomass energy technologies, pyrolysis is of importance as this thermal degradation of fuels occurs in both combustion and gasification. So the research on the pyrolysis characteristics is of great significance. A large number of researches on biomass pyrolysis have been performed. Experiments on fast pyrolysis of rice straw, sugar cane bagasse, and coconut shell in an externally heated fixed-bed reactor were carried out by Tsai et al.9 Fixed-bed fast pyrolysis experiments were also conducted on a sample of linseed.10 The pyrolysis of large beechwood particles in fixed beds and the composition of the products were analyzed.11 Enthalpy for pyrolysis for several types of biomass was also studied.12 There are also some researches on the pyrolysis of microalgae. For example, Peng has studied the pyrolytic characteristics of heterotrophic Chlorella protothecoides.13 Huber et al.14 have mentioned both microalgae and seaweed are fast-growing plants. Both plants can convert more of their solar energy into cellular structure. However, there are only a few investigations on seaweeds.15 In fact, in his paper on biomass studies, Yaman1 showed about 200 articles in biomass research studies, and none of them were on seaweeds. Enteromorpha clathrata (ENT) belongs to green algae. It grows fast and can absorb plenty of nutrition. Researchers also find that it can inhibit the growth of the red tide microalgae. During 5 h, the concentration of ammonium nitrogen in seawater declined form 213.84 µmol L-1 to zero by cultivated ENT.16 In this way, ENT can effectively prevent seawater from any eutrophication resulted by excessive N and P. Therefore it is considered as an ideal seaweed for ocean bioremediation. Meanwhile, it is a potential biomass.17 Regretfully, the application of ENT as an energy source is not given close attention yet. To study the thermal degradation of biomass, many techniques have been used. For example, TG-MS and FTIR in the study of the pyrolysis of chicken litter by Whitely et al.18 and DSC-TG in the study of wood ceramics by Ozao.19 The thermal behavior of biomass during pyrolysis is very important for the effective design and operation of the thermochemical conversion units. The devolatilization process is always a fundamental step.20 TG analysis can obtain the information on real-time mass loss. However, during the process of thermochemical conversion of biomass, the gas emission composition should be determined before industrial application. So mass spectrometry coupled with TG is a useful technique for the pyrolysis and gasification. (8) Dote, Y.; Sawayama, S.; Inoue, S.; Minowa, T.; Yokoyama, S. Y. Fuel 1994, 73, 1855–1857. (9) Tsai, W. T.; Lee, M. K.; Cheng, Y. M. J. Anal. Appl. Pyrolysis 2006, 76, 230–237. (10) Acıkgoz, C.; Onay, O.; Kockar, O. M. J. Anal. Appl. Pyrolysis 2004, 71, 417–429. (11) Schröder, E. J. Anal. Appl. Pyrolysis 2004, 71, 669–694. (12) Daugaard, D. E.; Brown, R. C. Energy Fuels 2003, 17, 934–939. (13) Peng, W. M.; Wu, Q. Y.; Tu, P. G. J. Appl. Phycol. 2001, 13, 5–12. (14) Huber, G. W.; Iborra, S.; Corma, A. Chem. ReV. 2006, 106, 4044– 4098. (15) Wang, J.; Wang, G. C.; Zhang, M. X.; Chen, M. Q.; Li, D. M.; Min, F. F.; Chen, M. G.; Zhang, S. P.; Ren, Z. W.; Yan, Y. J. Process. Biochem. 2006, 41, 1883–1886. (16) Xu, S. N.; He, P. M. J. Fish. China 2006, 30, 554–561 (in Chinese). (17) Moll, B.; Deikman, J. Bioresour. Technol. 1995, 52, 225–260. (18) Whitely, N.; Ozao, R.; Cao, Y.; Pan, W. P. Energy Fuels 2006, 20, 2666–2671. (19) Ozao, R.; Pan, W. P.; Whitely, N.; Okabe, T. Energy Fuels 2004, 18, 638–643. (20) Vamvuka, D.; Kakaras, E.; Kastanaki, E.; Grammelis, P. Fuel 2003, 82, 1949–1960.

Wang et al.

Figure 1. Mass loss process of ENT at the heating rate of 20 °C min-1. Table 1. Proximate and Ultimate Analysis of ENT Samples proximate analysisa moisture (%) volatile (%) ash (%) fixed carbon (%) net calorific value (MJ kg-1) a

13.30 41.82 37.09 7.79 7.89

ultimate analysisa (%) C H O N S

22.74 6.27 16.19 3.14 1.27

Air-dry base.

In this study, the simultaneous technology of TG-DTG-DTA is used to analyze thermal decomposition process of seaweed, especially pyrolysis profiles and kinetics. FTIR is used to analyze the changes in the main components during the pyrolysis. TG-MS helps to obtain some knowledge about the gaseous products during the pyrolysis of ENT. 2. Experimental Section 2.1. Materials. The seaweed sample used in this work is Enteromorpha clathrata (ENT), which was collected in Rudong, Jiangsu province of China, in April 2006. The analytical data are shown in Table 1. Before studied, the air-dried samples were ground to a small particle size of less than 0.18 mm by miniature type coal grinding machine. 2.2. Pyrolysis Experiments. This study was performed in a DTG-60H thermal analyzer from Shimadzu Co. The thermal analyzer was controlled by a computer to collect data and get the TG, DTG, and DTA curves. An ambient gas of 100 mL min-1 was used, which was N2 (99.99%) for pyrolysis experiment. The N2 gas could bring away the gaseous products as soon as possible; otherwise, any secondary reaction in high temperature might cause minor errors on the instant mass loss of samples. Approximately 17 mg of samples was heated from room temperature to 1200 °C at the heating rates of 20, 30, 40, and 50 °C min-1. 2.3. FTIR Analysis of the Changes in the Main Components during the Pyrolysis. The FTIR spectra of the samples were determined with the PARAGON1000 Fourier transform infrared analyzer made by PerkinElmer Co. Samples were the raw ENT and the ENT pyrolyzed at 210, 320, 620, 860, and 1000 °C. Each of these samples was mixed alone with IR grade KBr with the ratio 1:99 between sample and KBr. Then each mixture was milled fully to shape the potassium bromide disk for IR analysis. FTIR scans were made in the frequency range of 4400–450 cm-1 to obtain the FTIR spectra. 2.4. TG-MS Analysis of the Gaseous Products during the Pyrolysis. The TG-MS experiments were performed using a Netzsch STA 409 PC thermobalance connected to a quadrupole mass spectrometer (QMS403C Aëolos). Instead of nitrogen, helium

Pyrolysis Characteristics of Seaweed

Figure 2. Comparison of FTIR spectra of samples dealt at six temperatures.

was used for the inert atmosphere because N2 has the same molecular mass as CO. Approximately 15 mg samples were heated from room temperature to 1200 °C under the helium flow rate of 50 mL min-1. The mass spectrometer was operated in electron impact ionization mode with 100 eV electron energy. The gaseous products were led from TGA to MS through a heated capillary transfer line. The signals for mass numbers of 2, 16, 18, 27, 28, 30, 34, 44, 46, and 64 (H2, CH4, H2O, HCN, CO, NO, H2S, CO2, NO2, and SO2) were continuously detected. Then the ion current intensities were measured by the mass spectrometer.

3. Results and Discussion 3.1. Pyrolysis Process. In Figure 1 are the TG, DTG, and DTA curves of ENT being heated at a rate of 20 °C min-1. It shows that the pyrolysis process of ENT is divided into four regions. The first one is from 30 to 180 °C, in which ENT begins to release moisture and the DTG curve appears fluctuant with the first peak at 85 °C. The second region is 180–540 °C, where

Figure 3. MS curves during the pyrolysis of ENT.

Energy & Fuels, Vol. 21, No. 6, 2007 3725

most of the volatiles are released. Much mass loss happens in this temperature interval. There exist two peak temperatures in DTG curve, which shows that the rate of mass loss changes quickly. It represents that the initial temperature of pyrolysis mass loss of ENT is lower than that of woody biomass because of the difference between the components of seaweed and woody biomass. The components of seaweeds are reported by some researchers. Zhang studied five species of seaweeds and found soluble sugar, the lipid, and the raw protein could be 25.66–52.00%, 0.18–1.28%, and 2.99–29.05%.21 Another chemical analysis indicated that Enteromorpha spp. has 9–14% protein, 2–3.6% ether extract, and 32–36% ash.22 The major components “hemicellulose, cellulose, and lignin” in woody biomass are more difficult to pyrolyze than the components “protein, lipid, and soluble polysaccharide” in seaweed.23,24 The DTA curve shows that there is a complex exothermic chemical reaction when volatile is released. However, the release of volatile in woody biomass always absorbs heat. This phenomenon was also found in other seaweeds by Wang.15 The lowtemperature peak represents the degradation of protein and soluble polysaccharide, and the high-temperature peak corresponds to the degradation of crude cellulose in the cell wall, other insoluble polysaccharides, and crude lipid. The third temperature stage is 540–790 °C, in which the organic residues decompose slowly. The last region is 790–1200 °C where inorganic residues decompose and the rate of mass loss changes slowly. In Figure 4b, the pyrolysis process on all work conditions can be divided into the above four stages. 3.2. FTIR Spectra of Samples. Figure 2 shows the comparison of FTIR spectra of the six samples pyrolyzed at six temperatures. In the infrared spectrogram of the raw sample at room temperature, the peak at 2926 cm-1 corresponds to C–H stretching vibration, which is present in saccharide. The band at 3450 cm-1 is ascribed to O–H stretching vibration, which is present in H2O and saccharide, and the band at 1250 cm-1

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Figure 4. Pyrolysis TG, DTG, and DTA curves of ENT at different heating rates.

attributed to total sulfate radical is relevant to sulfated polysaccharides contents. The peak at 1658 cm-1 is assigned to C)O stretching vibration of protein peak. By comparing it with the infrared spectrogram of the samples at 210 °C constant temperature, it can be found that there are few changes in the band at 3442 cm-1 which corresponds to saccharide contents.

Wang et al.

But the peak at 1250 cm-1 attributed to sulfate radical25 becomes a bit different, which indicates that the sulfated polysaccharides begin to react. When comparing the infrared spectrogram between 320 and 210 °C, the peak at 3442 cm-1 and the peak at 1658 cm-1 vary within wide limits. The change indicates that the pyrolysis reaction happens, which is consistent with the second stage stated in section 3.1. In the infrared spectrogram at 620 °C, the bands due to saccharide and protein almost disappear, which represents the pyrolysis of polysaccharides and protein ends. The end indicates that most of the volatiles have been released. The band due to inorganic residue becomes apparent in the infrared spectrogram at 860 °C. In the infrared spectrogram at 1000 °C, the band due to the inorganic ash and the organic bond of C)C are left. The changes stated above are in agreement with the pyrolysis process. The main pyrolytic polysaccharides of woody biomass are cellulose and hemicellulose, but the ones of seaweed are galactan, etc., belonging to soluble polysaccharides. The compounds of seaweed are probably pyrolyzed more easily than that of woody biomass. The main pyrolytic saccharides of ENT are soluble sulfated polysaccharides which are composed of rhamnose, xylose, and glucuronic acid.25 Meanwhile, the proteins in seaweeds are pyrolyzed at a lower temperature. 3.3. MS Analysis of the Gaseous Products during the Pyrolysis. Figure 3 reports the MS curves obtained under inert condition. Results of analysis in terms of TG and DTG curves have been presented in section 3.1. The formation behaviors of SO2, CO2 are well accompanied by the trend of the DTG curve. The CO production is overestimated in the TG-MS technique, because not all the signals for mass number of 28 are from CO release. In fact, some of them are from C2H4. For mass number of 30, both NO and C2H6 might be assigned. Figure 3a shows two peaks at 90 and 240 °C for the H2O intensity. The first is associated with moisture, and the second is with the polycondensation of hydroxyl groups in seaweed. The slow H2 release is observed at around 400–600 °C, which is likely caused from the polycondensation of free radicals generated during the pyrolysis. In Figure 3b, the low-temperature region (180–500 °C) of CO2 evolution is quite consistent with the second pyrolysis region of ENT (as previously described in section 3.1) because the CO2 evolution is from the decomposition of carboxy groups in protein and saccharides. The evolution profile also displays a peak around 720 °C. It is caused by the decomposition of organic residues and also by the pyrolysis of carbonates minerals possibly. Three intense CH4 peaks in the temperature region of 180–600 °C probably suggest the possibility of the secondary reactions of volatile. In Figure 3c, the SO2 evolution occurring at such a low temperature is associated with the sulfate radical in polysaccharides mentioned in the FTIR results. The SO2 evolution observed above 550 °C might be related to the degradation of sulfides in organic residues, corresponding to the third pyrolysis region of ENT (as previously described in section 3.1). At 200 and 570 °C is the H2S evolution section, and its peak is at 270 °C, with a shoulder on the right-hand side and also a stem from the decomposition of some organic sulfur materials. It is significant in Figure 3d that (21) Zhang, G. H. Nat. Sci. J. Hainan UniV. 2002, 20, 324–327 (in Chinese). (22) Aguilera-Morales, M.; Casas-Valdez, M.; Carrillo-Domínguez, S.; González-Acosta, B.; Pérez-Gil, F. J. Food Compos. Anal. 2005, 18, 79– 88. (23) Ginzburg, B. Z. Renewable Energy 1993, 3, 249–252. (24) Wu, Q. Y.; Dai, J. B.; Shiraiwa, Y.; Sheng, G. Y.; Fu, J. M. J. Appl. Phycol. 1999, 11, 137–142. (25) Ji, M. H. Seaweed Chemistry; Science Press: Beijing, China, 1997; pp 358–366,1250 (in Chinese).

Pyrolysis Characteristics of Seaweed

Energy & Fuels, Vol. 21, No. 6, 2007 3727

Table 2. Pyrolysis Characteristics of ENT temp at initial temp at the end of max rate decomposition devolatilization (°C) temp (°C) (°C)

heating rate (°C min-1)

max rate (mg s-1)

20 30 40 50

-0.01605 -0.0247 -0.0356 -0.0424

255.8 263.4 269.6 274.9

176.5 183.5 193.5 202.8

600 608 618 625

Table 3. Pyrolysis Characteristics of Biomass15,27,28 sample

heating rate (°C min-1)

temp at max rate (°C)

initial decomposition temp (°C)

Meranti Meranti Metasequoia Metasequoia Subabul wood fir wood U. lactuca D. divaricata G. filicina

20 30 20 30 50 10 10 10 10

389.2 397.8 390.9 414.9 410.0 364.00 234.38 262.62 223.78

285.4 290.2 280.1 275.7 225.0

NO and C2H6 evolve with peaks at 243 and 473 °C and HCN evolves with the peak at 460 °C. The N-containing compounds are mainly associated with the protein in ENT. These results are in accordance with the high sulfur and nitrogen contents in the ultimate analysis shown in Table 1. The existence of S in ENT is complicated. The little evolution of SO2 may result from interactions between the sulfur radical and the oxygen functional groups.26 The N-containing compounds in ENT are complicated, and the oxygen content in ENT is high, so the element N is also very likely to be released in form of NOx during the pyrolysis, not only HCN and NH3. Compared with ENT, most woody biomass contains lower sulfur and nitrogen content and thus has less SO2 and NOx released during the pyrolysis. In the second pyrolysis region of ENT, the components (saccharide, protein analyzed from FTIR) are degraded, and at the same time the gases emitted are complicated, including H2, CH4, H2O, HCN, CO, H2S, etc. The analysis of gaseous products can give fundamental information for the thermochemical convertion of seaweed. 3.4. Effects of Heating Rate on Pyrolysis. The effects of heating rate on pyrolysis can be generally stated from two aspects. First, the higher the heating rate is, the sooner the sample reaches the temperature needed.27 Second, the higher rate may influence the pyrolysis inside each sample grain. The pyrolysis gas outside grains is hard to diffuse in time when temperature difference between inside and outside of each sample grain becomes larger. All these factors result in the unsatisfied pyrolysis. That means both these two effects should be taken into consideration when choosing heating rate. Pyrolysis TG, DTG, and DTA curves at the heating rates of 20, 30, 40, and 50 °C min-1 are given in Figure 4. These curves show pyrolysis characteristics at different heating rates. The second region, the focus of the present study, corresponds to the release of volatiles. For every DTG curve, the maximum mass loss rate in the low-temperature stage is always greater than that in the high-temperature stage. For the DTA curve, in the exothermic zone, the heat release increases with increasing heating rate. Table 2 gives the maximum rate of mass loss, the initial and final temperature (26) Xu, L.; Yang, J. L.; Li, Y. M.; Liu, Z. Y. Fuel Process. Technol. 2004, 85, 1013–1024. (27) Yu, J.; Zhang, M. C.; Shen, Y.; Fan, W. D. J. Shanghai Jiaotong UniV. Sci. 2002, 36, 1475–1478 (in Chinese).

Table 4. f(x) Functions Applied to Thermal Degradation of Solids model

f(x)

reaction order: n ) 0, 0.5, 1, 1.5, 2 (1 - x)n geometric functions: cylindrical symmetry 2 (1 - x)0.5 spherical symmetry 3 (1 - x)2/3 diffusion functions: one dimension 1/2x two dimensions ( - ln(1 - x))-1 three dimensions: Jander equation 1.5 (1 - x )2/3(1 - (1 - x)1/3)-1 Ginstling–Brounshtein 1.5 ((1 - x)-1/3 - 1)-1 Anti–Jander 1.5 (1 + x)2/3((1 + x)1/3 - 1)-1 Zhuralev, Lesokin, and Tempelman 1.5 (1 - x)4/3((1/(1 - x)1/3 - 1))-1 crystal growth Avrami–Erofeev for n ) 1, 1.5, 2, 3,4 n(1 - x)(- ln(1 - x))(n-1)/n

for devolatilization, and the peak temperature which increase with heating rate. According to the previous studies in woody biomass, the degradation of cellulose and lignin occurs in rough temperature regions between 300 and 430 °C and 250 and 550 °C, respectively.28 The pyrolysis characteristics of four kinds of woody biomass and three kinds of seaweeds are shown in Table 3.15,27,28 It is found that volatiles of ENT and other seaweeds release earlier than that of woody biomass by comparing them. So ENT probably pyrolyzes more easily than woody biomass. The reason is the difference between their pyrolysis composition.23,24 Wang also found that the thermal stability of seaweed is lower than that of fir wood.15 3.5. Kinetic Analysis of Pyrolysis. The kinetics of pyrolysis could be described as dR ) kf(R) dt

(1)

The mass loss ratio (R ) can be calculated as follows: R)

w0 - w w0 - w∞

(2)

k is determined by the Arrhenius equation: k ) A exp(-E ⁄ RT)

(3)

where w∞, w0, and w are the final, initial, and actual weight of the sample being decomposed, respectively (mg); E is the activation energy (kJ mol-1); t is the time of pyrolysis process (s); T is the pyrolysis temperature at time t (K); k is the chemical reaction kinetics Arsenics constant; A is the Arrhenius preexponential factor (s-1); and R is the universal gas constant (kJ mol-1 K-1). The function f(R)is determined in the presumed reaction mechanism (Table 4).29 Equations 1 and 3 can get eq 4: E dR ) Af(R)e- RT dt

(4)

Introducing the heating rate given in the equation β ) dT⁄dt , eq 4 turns to eq 5. A E dR ) e- RT dT f(R) β

(5)

(28) Raveendran, K.; Anuradda, G.; Kartic, C. K. Fuel 1996, 75, 987– 998. (29) Li, Y. Z. Thermal Analysis; Tsinghua University Press: Beijing, China, 1987 (in Chinese).

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Figure 5. Comparison between experimental data and calculation results. Table 5. Kinetic Analysis of ENT in Low-Temperature Stage at Different Heating Ratesa heating rate (°C min-1)

temp section (°C)

20 30 40 50 a

177–298 187–315 195–320 204–330

fitting equation Y Y Y Y

) ) ) )

36.7 – 27512.4 X 36.55 – 28130.3 X 37.12 – 28755.9 X 37.68 – 29504.7 X

A (s-1)

E (kJ mol-1)

r

7.61×1019

228.74 233.87 239.08 245.30

-0.9850 -0.9885 -0.9901 -0.9904

1.05×1020 2.54×1020 5.69×1020

X ) 1/T, Y ) ln[g(R)/T2].

Table 6. Kinetic Analysis of ENT in High-Temperature Stage at Different Heating Ratesa heating rate (°C

min-1)

temp section (°C)

20 30 40 50 a

298–540 315–559 320–574 330–580

fitting equation Y Y Y Y

) ) ) )

2.90 3.27 3.47 3.95

– 10331.4 X –10843.3 X –11119.5 X – 11604.6 X

A (s-1)

E (kJ mol-1)

r

6.24×104 1.43×105 2.39×105 5.04×105

85.90 90.15 92.45 96.48

-0.9757 -0.9781 -0.9802 -0.9788

X ) 1/T, Y ) ln[g(R)/T2].

The Coats and Redfern30 integral is used in eq 5, and then the equation that describes the pyrolysis of biomass is ln

{ [

g(R) AR 2RT ) ln 12 βE E T g(R) )

∫

R

0

]} - RTE

(6)

dR f(R)

As the term of 2RT⁄E can be neglected because it is much less than 1, eq 6 could be simplified as ln

E AR g(R) ) ln βE RT T2

(7)

Equation 7 is transformed into linear function, as follows: F(x) ) C + DX where F(x) ) C ) ln(AR/βE), D ) –E/R, and X ) 1/T. So pre-exponential (A) and activation energy (E) can be determined. On the basis of the proximate analysis and FTIR spectra of sample, the authors refer that the region above 800 °C is the ash decomposition region and it is unimportant to kinetic analysis. So the kinetic analysis is only carried out for the interval where the pyrolysis takes place. The interval at each heating rate is divided into the low and high temperature stage and the kinetic mechanism functions in Table 4 are used to fit them. Tables 5 and 6 give the calculation results. The correlation coefficient (r) provides a good quantitative criterion for the validity of the kinetic model. Tables 5 and 6 show that Zhuralev, Lesokin and Tempelman equation and second-order equation are the most probable kinetic mechanism functions for describln[g(R)/T2],

(30) Coats, A. W.; Redfern, J. P. Nature (London) 1964, 201, 68–69.

ing the low and high temperature stage respectively. Its counterpart, the kinetic models of woody biomass31,32 and cellulose33 have been studied and their kinetic parameters31,32 have been calculated. Many researchers consider first-order equation suits the pyrolysis process of biomass.34–36 The difference is caused by the fact that the main components in seaweed and woody biomass is not the same. In the low temperature stage, the pyrolysis components in ENT are complex and the skeleton structure can be formed from the high ash when devolatilizing, which makes it reasonable that the thermolysis mechanism of ENT is the three-dimensional diffusion. In the high temperature stage, the second-order equation is the most probable mechanism function for describing the pyrolysis of the major reaction component, insoluble saccharide, which is similar to the pyrolysis of the cellulose.37 The theoretical results are calculated by using the kinetic parameters. In Figure 5, the experimental data and calculation results are plotted. The good agreements of the calculated curves with the experimental ones prove the accuracy of the kinetic mechanism models. Tables 5 and 6 show that the activation energy increases with increasing heating rate and the kinetic (31) Reina, J.; Velo, E.; Puigjaner, L. Ind. Eng. Chem. Res. 1998, 37, 4290–4295. (32) Várhegyi, G.; Antal, M. J., Jr.; Jakab, E.; Szabó, P. J. Anal. Appl. Pyrolysis 1997, 42, 73–87. (33) Yamaguchi, Y.; Fushimi, C.; Tasaka, K.; Furusawa, T.; Tsutsumi, A. Energy Fuels 2006, 20, 2681–2685. (34) Fang, M. X.; Shen, D. K.; Li, Y. X.; Yu, C. J.; Luo, Z. Y.; Cen, K. F. J. Anal. Appl. Pyrolysis 2006, 77, 22–27. (35) Kastanaki, E.; Vamvuka, D.; Grammelis, P.; Kakaras, E. Fuel Process. Technol. 2002, 77–78, 159–166. (36) Tsamba, A. J.; Yang, W. H.; Blasiak, W. Fuel Process. Technol. 2006, 87, 523–530. (37) Rao, T. R.; Sharma, A. Energy 1998, 23, 973–978.

Pyrolysis Characteristics of Seaweed

compensation effect (KCE)29 exists between frequency factor and activation energy in the low- and high-temperature stage respectively. E and A depend on many factors such as experimental condition, sample size, and heating rate. The KCE can provide a way to predict the kinetic parameters at different heating rates when limited data are available.38 The two linear relations are presented as follows: in the lowtemperature stage ln A ) 16.728 + 0.126 E; correlation coefficient ) 0.9881. In the high-temperature stage ln A ) -6.019 + 0.199E; correlation coefficient ) 0.9996. 4. Conclusions ENT can not only be applied in ocean remediation but also be explored as a potential biomass energy resource. It has advantages such as fast growth and good character for pyrolysis. The experimental study on the pyrolysis of ENT has been presented, using a TG-DTG-DTA thermal analysis technology, TG-MS, and IR spectrometric analysis. On this basis, the following conclusions are drawn: 1. The pyrolysis reactions take place mainly between 180 and 580 °C, during which heat energy is released. ENT can be pyrolyzed together with other fuels. This requires less external heat input15 because heat is released during the pyrolysis of (38) Liu, N. A.; Wang, B. H.; Fan, W. C. Fire Safety Sci. 2002, 11, 63–69.

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ENT. ENT is pyrolyzed at lower temperatures than woody biomass because the components of ENT mainly contain sulfated polysaccharides and protein. 2. The composition of ENT determines that its gaseous products during the pyrolysis are complicated, including H2, CH4, CO, CO2, etc. Among the products, the SO2 release is associated with the sulfate radical in polysaccharides and NOx is related to the protein in ENT. Gasification can be a utilization path of ENT. Considering that ENT has less O content than that of woody biomass4,5,36 in ultimate analysis, ENT could also be used to produce high quality bio-oil. 3. The maximum rate of pyrolysis mass loss, the initial and final temperature for devolatilization, the peak temperature, and heat release will increase with increasing heating rate. 4. The kinetic equations are given as follows: the Zhuralev, Lesokin, and Tempelman equation in the low-temperature stage; the second-order equation in the high-temperature stage. The kinetic parameters at each heating rate are calculated by using the Coats–Redfern method. It is indicated that the activation energy increases with increasing heating rate, and the kinetic compensation effect exists between the activation energy and the frequency factor. Acknowledgment. This work was supported by Shanghai Pujiang Program of China (Grant 05PJ14086). EF700214W