Energy Fuels 2009, 23, 5173–5178 Published on Web 09/01/2009
: DOI:10.1021/ef900414x
Combustion Characteristics of Seaweed Biomass. 1. Combustion Characteristics of Enteromorpha clathrata and Sargassum natans S. Wang, X. M. Jiang,* X. X. Han, and J. G. Liu Institute of Thermal Energy Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Received May 6, 2009. Revised Manuscript Received August 18, 2009
The fuel characteristics of two typical seaweeds (Enteromorpha clathrata and Sargassum natans) were studied. It was found that both the contents of volatile and ash are high, and the ash melting characteristics were different. High contents of alkali metals especially K and Na were also found in seaweed ash. The ignitions of seaweed particles were observed using a thermal microscope, which demonstrated that particle ignition proceeded in the homogeneous mode. Combustion experiments of seaweed have been conducted using a DTA-60H thermal analyzer, and the combustion processes were studied. Furthermore, the thermogravimetric mass spectrum (TG-MS) analysis was used for the gaseous products analysis during the combustion. It provided the fundamental data for the development and utilization of seaweed biomass.
In fact, many experts have done extensive research on biomass like lignocellulosic biomass.4-7 Some specialists and experts also have made related reports on how to make use of microalgae and seaweed biomass as energy sources nowadays.8-10 In the world, the techniques of using biomass energy forms are various such as combustion, gasification, pyrolysis, hydrogen production, and so on. The combustion method is relatively mature. However, little has been reported about seaweed combustion. In fact, there are considerable differences between combustion for seaweed and traditional biomass due to their physical and chemical characteristics imposed by their different living environments. Therefore, the study on the combustion of seaweed is very necessary. The ignition and combustion characteristics of biomass are very important for the effective design and operation of the combustion units. To study the information on real-time combustion processes of biomass, thermal analysis techniques have been used. Some examples include the thermogravimetry mass spectrum (TG-MS) analysis in the study of the cocombustion of sewage sludge and coal by Otero et al.11 and thermogravimetric analysis (TG) in the study on combustion characteristics of fine and micropulverized coal in the mixture of O2/CO2 by Huang.12 The combustion characteristics and kinetic model of municipal solid wastes are also studied by Guo.13
1. Introduction Biomass has the potential of being a more important energy source in the future, because biomass is a potentially renewable and CO2 neutral fuel. Now, the utilization of biomass focuses on lingocellulosic biomass. Comparing with wood and crops, marine biomass as another sort of energy plant is applied for energy consumption in a relatively small area all over the world. Seaweed is an important constituent part of marine biomass. Most seaweeds are the green (1200 species), brown (2000 species), or red (6000 species) kinds, which live in sea areas, not occupying land areas. The whole seaweed front can be used as energy resource. With a short life cycle and fast growing, seaweeds are easy to breed. For example, in the summer of 2008, in the offshore Qingdao Olympic sailing competition area, more than 60 million tons of two species of green seaweed (Enteromorpha clathrata, and Enteromorpha compressa) grew very rapidly. Moreover, the growth seasons of seaweeds are not single. A variety of seaweeds can be cultivated alternately in any time of the year, which ensures that the biomass source is abundant. Seaweeds can convert more solar energy into their cellular structure than terrestrial biomass.1 Seaweeds are also able to absorb large quantities of nitrogen, phosphorus, and carbon dioxide and produce a lot of oxygen. They also have an excellent effect on ocean remediation.2,3 Therefore, the appropriate development of seaweed biomass energy is of great potential. Currently, seaweed as an energy source has begun to attract attention in many countries.
(6) Munir, S.; Daood, S. S.; Nimmo, W.; Cunliffe, A. M.; Gibbs, B. M. Bioresour. Technol. 2009, 100, 1413–1418. (7) Skrifvars, B. J.; Yrjas, P.; Kinni, J.; Hupa, M. Energy Fuels 2005, 19, 1503–1511. (8) Miao, X. L.; Wu, Q. Y.; Yang, C. Y. J. Anal. Appl. Pyrolysis 2004, 71, 855–863. (9) 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. (10) Wang, S.; Jiang, X. M.; Wang, N.; Yu, L. J.; Li, Z.; He, P. M. Energy Fuels 2007, 21, 3723–3729. (11) Otero, M.; Dı´ ez, C.; Calvo, L. F.; Garcı´ a, A. I.; Moran, A. Biomass Bioenergy 2002, 22, 319–329. (12) Huang, X. Y.; Jiang, X. M.; Han, X. X.; Wang, H. Energy Fuels 2008, 22, 3756–3762. (13) Guo, X. F.; Wang, Z. Q.; Li, H. B.; Huang, H. T.; Wu, C. Z.; Chen, Y. Energy Fuels 2001, 15, 1441–1446.
*To whom correspondence should be addressed. E-mail address:
[email protected]. Tel.: þ86-21-34205681. (1) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044– 4098. (2) Anderson, D. M. Nature 1997, 388, 513–514. (3) Senthilkumar, R.; Vijayaraghavan, K.; Thilakavathi, M.; Iyer, P. V. R.; Velan, M. J. Hazardous Mater. 2006, 136, 791–799. (4) Chau, J.; Sowlati, T.; Sokhansanj, S.; Preto, F.; Melin, S.; Bi, X. Appl. Energy 2009, 86, 364–371. (5) Lima, A. T; Ottosen, L. M.; Ribeiro, A. B. J. Hazardous Mater. 2009, 161, 1003–1009. r 2009 American Chemical Society
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: DOI:10.1021/ef900414x
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The aim of the present study was to investigate the combustion characteristics of seaweed by thermal analysis. The ignition mechanism was studied through the heated stage microscope. TG-MS also helps to obtain some knowledge about the gaseous products during the combustion of seaweeds. The experimental results can provide some fundamental information for combustion experiments of seaweeds. It may also be useful when evaluating the combustion properties of seaweeds to be used in different appliances as an opportunity feedstock for the bioenergy sector of the future.
Table 1. Properties of Seaweed Biomass EN
moisture volatile fixed carbon ash carbon hydrogen nitrogen sulfur oxygen LHV (MJ kg-1)
2. Experimental Section 2.1. Materials. The samples used in this work are Enteromorpha clathrata (EN) and Sargassum natans (SA), which were obtained from Rudong, Jiangsu province of China, and Zhanjiang, Guangdong province of China, respectively. Their shapes are shown in Figure 1. The air-dried samples were ground to a small particle size of less than 0.18 mm by a miniature grinding machine. The two seaweeds belong to the green and brown seaweed families, respectively. 2.2. Ignition Experiments. Ignition phenomenon was observed and photographed using a Leitz II-A heated stage microscope with a high definition video camera. Using a figuring machine, cube samples were made from the seaweed samples (d < 0.18 mm), and the experimental conditions are the following: (1) a cube seaweed sample size of 3 mm 3 mm 3 mm; (2) 100% O2 concentration with a flow rate of 350 mL min-1; (3) heating rate of 10 °C min-1. 2.3. Thermal Analysis on Combustion. This work was performed in a DTG-60H thermal analyzer made by the Shimadzu Company. The thermal analyzer was controlled by a computer to collect data and get the thermogravimetric (TG), derivative thermogravimetric (DTG), and differential thermal analysis (DTA) curves, respectively. The gas mixture with 20% O2 and 80% N2 was used as the reactive gas for the combustion study, and the gas flux was 100 mL min-1. Approximately 17 mg of samples (d < 0.18 mm) were heated from room temperature to 1200 °C at the heating rate of 20 °C/min. 2.4. TG-MS Analysis of the Gaseous Products during the Combustion. The TG-MS experiments were performed using a Netzsch STA 409 PC thermobalance connected to a quadrupole mass spectrometer (QMS403C Aeolos). The gas mixture with 20% O2 and 80% He was used as the reactive gas with flow rate of 50 mL min-1 for the combustion study. Approximately 15 mg samples (d < 0.18 mm) 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 18, 27, 44, 46, and 64 (H2O, NO, CO2, NO2, and SO2) were continuously detected. Then, the ion current intensities were measured by the mass spectrometer.
proximate analysis (air-dry basis; %a) 13.30 41.82 7.79 37.09
10.46 48.85 11.60 29.09
ultimate analysis (air-dry basis; %) 22.74 6.27 3.14 1.27 16.19 7.89
25.9 5.57 3.58 1.22 24.18 8.68
ash composition (%) 11.4 12.08 5.72 4.8 3.6 4.48 0.28 1.56 47.79
17.2 11.78 0.83 3.17 10.84 4.45 0.53 0.71 47.96
fusibility temperatures (°C) 1120 1183 1202 1231
721 786 1379 1402
K2O Na2O Al2O3 Fe2O3 CaO MgO TiO2 P2O5 SiO2 initial deformation soften hemisphere fluid a
SA
(%) Weight percent of mass.
3. Results and Discussion 3.1. Fuel Properties. The analytical data of seaweed samples are shown in Table 1. The heating values of the seaweed samples are around 8 MJ kg-1, which are lower than that of terrestrial biomass. In relation to the elementary analysis, seaweed has a lower oxygen content (16.19 and 24.18 mass %, respectively) than terrestrial biomass (around 40 mass %). The moisture and ash contents of seaweed are high. As combustion proceeds, carbon is consumed from the biomass/char particles leaving noncombustible ash-rich particles which causes the mean temperature to decrease.14 There are also many metal elements such as potassium (K), sodium (Na), and calcium (Ca) in seaweed ash. Combustion is also accompanied by preferential loss of catalytic elements such as K and Ca. These transformations would have a large impact on the reactivity of biomass chars at later times. Nevertheless, biomass chars are quite reactive in the early stages of char conversion and burn almost under diffusion control.14 The potassium content is likely to affect not only the char reactivity but also the ignition temperature.15 Table 2 shows ash compositions of terrestrial biomass. The comparison of ash composition analysis shows seaweed biomass ashes contain more potassium and sodium than terrestrial biomass due to the differences of living environment. The fusion temperature of seaweed is low because of the high content alkali metal (particularly sodium and potassium). Especially initial deformation temperature (14) Sami, M.; Annamalai, K.; Wooldridge, M. Prog. Energy Combust. Sci. 2001, 27, 171–214. (15) Grotkjær, T.; Dam-Johansen, K.; Jensen, A. D.; Glarborg, P. Fuel 2003, 82, 825–833.
Figure 1. Seaweed biomass samples.
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Table 2. Ash Compositions of Terrestrial Biomass (%) sample
K2O Na2O Al2O3 Fe2O3 CaO MgO TiO2 P2O5 SiO2 ref
pine chips 4.49 corn straw 16.1 rape straw 12.1 forest residue 6.9 reed canary grass 3.4 poplar 16 brassica 21
1.19 0.14 0.4 0.3 0.9 0.14 0.69
7 4.4 4.9 4.3 1.3 0.34 0.61
5.42 2.2 1.8 3.6 0.7 0.36 0.28
7.85 12.8 27.6 25.3 5.6 29 23
2.42 3.9 1.8 2.8 1.2 3.0 3.2
0.55 0.25 0.26 n.a n.a n.a n.a
1.55 2.1 2 3 2.5 3.0 9.1
67.84 16 43.4 16 36.7 16 24.6 17 72.5 17 4.2 18 7.6 18
Figure 2. Ignition phenomenon of seaweed under a heated stage microscope.
(IDT) of SA ash is only 721 °C, which is much lower than that of woody biomass.19 The slagging problem of ash in seaweed biomass should be considered during the combustion utilization of seaweed. Therefore the low temperature combustion technique in fluidized bed could be adopted. Meanwhile one of the major advantages of circulating fluidized bed combustors is their efficiency for the combustion of fuels with low calorific value and high ash content, such as lignites20 and oil shales.21 A large amount of ashes can serve as bed materials22,23 and reduce the SO2 emission due to the desulphurization effect of CaO in seaweed ash. Therefore fluidized bed combustion technology may be one of the utilization methods of seaweed. Further study is necessary. Part 2 will present the fluidized combustion test with seaweed. Seaweed contains a high K, Na, and Mg content which may cause fouling problems during combustion. Pretreatment similar to that of the algae in water can be considered it reduces around 30-40% of the Mg, K, and Na.24 3.2. Ignition Mechanism. The ignition process is observed through the heatable stage microscope. Initially, the ash particle begins to expand because of the release of volatile matters. Meanwhile the pore is enlarged, which also promotes the release of volatile from the inner of particle. Then, the fog appears, followed by the ignition. The ignition phenomenon observed at ignition moment is shown in Figure 2. Clear flame appears outside seaweed particles and solid phases are dark, which means it is pyrolyzed volatile that is burning. It is judged by the appearance25 that the ignition behavior of seaweed is homogeneous. The shape and size of EN particle change little when it is burning, but SA particles shrink rapidly. When the flame disappears, EN particles still keeps their original size and tight shape. However, SA particles not only reduce considerably, but also present a loose state. Many pores inside particles can be also seen. The difference is related to the volatile and ash contents of the two kinds of seaweeds. 3.3. Thermal Analysis of Combustion. 3.3.1. Combustion Process. The combustion of seaweed is a kind of severe
chemical reaction releasing plenty of heat, whose process is divided into five stages; presented in Figure 3. The first one is dehydration and desiccation, in which fluctuations appear in the DTG curve and an inapparent endothermic peak is found in the DTA curve. The second stage is the release and combustion of volatiles. It is found that mass loss of seaweed starts earlier than that of woody biomass.9 The reason is that seaweed mainly contains crude lipid, protein, and solubility polysaccharide. EN has 23.99% protein; 1.28% lipid; 40% carbohydrate. SA has 9.6% protein; 1.39% lipid; and 63.97% carbohydrate. 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.26,27 The second stage involves the decomposition of proteins and carbohydrates.28 And, the decomposition temperature of this protein is in range of 300-400 °C.29 With the mass decreasing, the endothermic peak turns into an exothermic peak in the DTA curve. When the temperature reaches the ignition temperature of volatiles, gas components start to combust. According to the previous analysis on the ignition mechanism of seaweed, the stage is called homogeneous combustion. At this point, plenty of volatiles are quickly released from the seaweed particles in little time, which weakens the diffusion transmission of oxygen to the surface of the seaweed particles. The activation energy of fixed carbon is higher than that of volatile. Therefore char can not burst into flame in such a low temperature. The third stage is a transition stage. The combustion rate of volatiles begins to decrease. Surrounded by volatiles, char starts to ignite when oxygen diffuses to the surface of char. The combustion of residual volatiles and char happens at the same time. The fourth stage is the combustion of char. While the combustion of volatiles approaches to its end, the surface of char is surrounded sufficiently by the oxygen. Then char can burn quickly and give off lots of heat, which shows an apparent peak on the DTA curve. At this stage, the mass loss rate is lower than that of the second stage, which could be caused by the fact that fixed carbon is surrounded by ash after releasing volatiles. The oxygen diffusion is inhibited from the fixed carbon, which decreases the combustion rate of this stage. When the temperature exceeds about 600 °C, the combustion of fixed carbon is close to terminal. The fifth stage is the reaction at high temperature, a little loss in mass which may be attributed to volatile metal loss and carbonate decomposition. After the combustion of fixed carbon
(16) Tortosa Masi a, A. A.; Buhre, B. J. P.; Gupta, R. P.; Wall, T. F. Fuel Process. Technol. 2007, 88, 1071–1081. € (17) Skrifvars, B. J.; Ohman, M.; Nordin, A.; Hupa, M. Energy Fuels 1999, 13, 359–363. (18) Fern andez Llorente, M. J.; Murillo Laplaza, J. M.; Escalada Cuadrado, R.; Carrasco Garcı´ a, J. E. Fuel 2006, 85, 1157–1165. (19) Heinzel, T.; Siegle, V.; Spliethoff, H. Fuel Process. Technol. 1998, 54, 109–125. € (20) Ozkan, G.; Do gu, G. Chem. Eng. Process. 2002, 41, 11–15. (21) Jiang, X. M.; Han, X. X.; Cui, Z. G. Prog. Energy Combust. Sci. 2007, 33, 552–575. (22) Han, X. X.; Jiang, X. M.; Wang, H.; Cui, Z. G. Fuel Process. Technol. 2006, 87, 289–295. (23) Redemann, K.; Hartge, E.-U.; Werther, Joachim Powder Technology 2009, 191, 78–90. (24) Ross, A. B.; Anastasakis, K.; Kubacki, M.; Jones, J. M. J. Anal. Appl. Pyrol. 2009, 85, 3–10. (25) Han, X. X.; Jiang, X. M.; Cui, Z. G. J. Therm. Anal. Calorim. 2006, 84, 631–636.
(26) Ginzburg, B. Z. Renewable Energy 1993, 3, 249–252. (27) Wu, Q. Y.; Dai, J. B.; Shiraiwa, Y.; Sheng, G. Y.; Fu, J. M. J. Appl. Phycol. 1999, 11, 137–142. (28) Marcilla, A.; G omez-Siurana, A.; Gomis, C.; Chapuli, E.; Catala, M. C.; Valdes, F. J. Thermochim. Acta 2009, 484, 41–47. (29) Thipkhunthod, P.; Meeyoo, V.; Rangsunvigit, P.; Rirksomboon, T. J. Anal. Appl. Pyrol. 2007, 79, 78–85.
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Figure 3. Combustion TG, DTG, and DTA curves of seaweeds. Table 3. Combustibility Parameters of Seaweeds sample EN SA
T1 (°C)
T2 (°C)
T3 (°C)
T4 (°C)
ΔT1 (μV)
ΔT2 (μV)
TΔT1 (°C)
TΔT2 (°C)
238 242
255 277
548 470
674 685
82 91
159 196
345 335
556 490
According to column T1, the ignition temperatures of the seaweeds are both lower than that of most terrestrial biomass. For example, the measured ignition temperatures of straw are mainly in the range 242-292 °C.32 Strawdust and wheat straw ignite at 279 and 277 °C, respectively.33 And EN is slightly easier to ignite than SA. The DTG data for seaweed shows a two stage combustion process with peaks at T2 and T3. These parameters are used mainly in the assessment of combustibility, which represents the place where the rate of weight loss is at a maximum due to rapid combustion. Compared with T2 and T3 of other biomass showed in Table 4, it indicates that the seaweed can reach the maximum combustion rate more quickly after ignition. The burnout temperature indicates the temperature where the sample oxidation is completed. According to column T4, the burnout temperature of char of EN is close to that of SA. According to columns ΔT1 and ΔT2, SA has more severe combustion of volatile and char than EN. Columns ΔT1 and ΔT2 show that the maximum exothermic peaks are consistent with the combustion of char. In order to analyze comprehensively the combustion characteristic of seaweeds, the combustibility index S is defined as ðdW=dtÞmax ðdW=dtÞmean Where, (dW/dt)max is the follows:12,36 S ¼ Ti 2 Th maximum combustion rate, % min-1; (dW/dt)mean is the average combustion rate, % min-1; Ti is the ignition temperature, °C; and Th is the burnout temperature of char, °C. The product of the above items reflects the combustion characteristics of seaweed. It is deduced that the higher the S, the better the combustion property of the seaweed. The inclusion of several important parameters such as ignition temperature, burnout temperature, or mean burning rate in the index gives a comparatively comprehensive evaluation and can be regarded as a reference for practical operation.
Figure 4. Ignition temperature definition sketch.
reaches a terminal level, there is a clear mass loss in the TG curve of SA and the exothermic peak occurs in the DTA curve. Most probably, the cause is the dissociation of CaCO3. The peaks in EN and SA DTG curves above 1000 °C are probably due to the decomposition of ash. 3.3.2. Combustion Characteristics. Several combustion characteristic parameters (T1, T2, T3, T4, ΔT1, ΔT2, TΔT1, and TΔT2) are defined: T1 is the ignition temperature; as shown in Figure 4, the ignition temperature (T1) was defined as the corresponding temperature at the intersecting point C between TG baseline and the tangent line of TG descending point B which corresponds to the peak point A at the DTG curve.30,31 On the basis of Figure 3, it is calculated that the ignition temperature of EN and SA are 238 and 242 °C, respectively. T2 and T3 are temperatures at the maximum combustion rate of volatiles and char in DTG curves respectively. T4 is the burnout temperature that is identified as the corresponding temperature of no weight loss in TG-DTG curves.31 In this work, the burnout temperature (T4) is defined as the temperature at which the rate of combustion at the end of stage 4 diminishes to 0.5% min-1. ΔT1 represents the maximum temperature difference between reference sample and volatile combustion of the seaweed; and ΔT2 represents the maximum temperature difference between reference sample and char combustion of the seaweed. TΔT1 and TΔT2 are the temperatures at the maximum temperature difference in DTA curves, respectively. These parameters are shown in Table 3.
(32) Blasi, C. D.; Portoricco, G.; Borrelli, M.; Branca, C. Fuel 1999, 78, 1591–1598. (33) Wang, C. P.; Wang, F. Y.; Yang, Q. R.; Liang, R. G. Biomass Bioenergy 2009, 33, 50–56. (34) Yorulmaz, S. Y.; Atimtay, A. T. Fuel Process. Technol. 2009, 90, 939–946. (35) Munir, S; Daood, S. S.; Nimmo, W.; Cunliffe, A. M.; Gibbs, B. M. Bioresour. Technol. 2009, 100, 1413–1418. (36) Jiang, X. M.; Zheng, C. G.; Qiu, J. R.; Li, J. B.; Liu, D. C. Energy Fuels 2001, 15, 1100–1102.
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Table 4. Peak Temperatures (T2 and T3) of Other Biomass T2 (°C) T3 (°C) ref
pine
MDF
particleboard
plywood
cotton stalk
sugar cane bagasse 1
sugar cane bagasse 2
shea meal
366 523 34
346 514 34
324 448 34
332 483 34
283 na 35
304 na 35
312 na 35
287 na 35
Figure 5. MS curves during the combustion of seaweed.
In Figure 5a, the low temperature region (200-400 °C) of CO2 evolution is weak and the CO2 release is obvious at around 500-700 °C, which indicates that the release of CO2 occurs mainly in the stage of the combustion of fixed carbon. The CO2 evolution profile in Figure 5c displays two peaks in temperature regions of 200-400 and 400-600 °C. The release of CO2 is also observed at around 700-800 °C, which is likely caused from the decomposition of carbonate in ash. CO2 release patterns are very similar to the corresponding DTA and DTG curves; this is not so for SO2 release. The SO2 release of EN is observed at around 200-400 and 500700 °C, which are likely caused from the decomposition and oxidization of S-containing compounds (such as sulfated polysaccharide). The existence of S in EN is complicated. The little evolution of SO2 at around 200-400 °C may result from interactions between the sulfur radical and the oxygen-functional groups. The SO2 release in the region of 500-700 °C is quite consistent with the fourth combustion region of EN (as previously described in section 3.3.1). Above 1100 °C, the SO2 evolution may be associated with the inorganic matters in the ash. In Figure 5d, the SO2 evolve with peaks at 265 °C, with a shoulder at right-hand side, also stem from the decomposition and oxidization of some organic sulfur materials of SA.
Indexes S of two seaweeds (EN, SA) are calculated, respectively: 3.082 10-7, 4.628 10-7. When the index S increases, the seaweed ignites much earlier and the combustion characteristics of seaweeds become better. Therefore, S of SA is greater, indicating its better combustion performance than EN. 3.3.3. MS Analysis of the Gaseous Products during the Combustion. Figure 5 reports the MS curves obtained under oxidative condition. Results of analysis in terms of TG and DTG curves have been presented in section 3.3.1. The formation behaviors of CO2 and NO2 are well accompanied by the trend of the DTG curve. Figure 5a shows three peaks at 90, 250, and 515 °C for the H2O intensity. There are also three H2O peaks at 100, 265, and 485 °C in Figure 5c. Both the first are associated with moisture, and the others are with the oxidization during the combustion of volatiles and char. Combustion is a chemical process, an exothermic reaction between a substance and an oxidizer, to release heat.37 Seaweed contains the organic carbon, and the oxidizer is the O2; CO2 is emitted during heat releasing so the profiles of CO2 curves resemble DTA curves. (37) Otero, M.; S anchez, M. E.; Garcı´ a, A. I.; Moran, A. J. Therm. Anal. Calorim. 2006, 86, 489–495.
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components are degraded and the volatile matters are ignited. At this time, SO2 and NOx are released. The analysis of gaseous products above can give fundamental information for the combustion of seaweed. 4. Conclusions To study the combustion characteristics of seaweed (a kind of marine biomass), the ignition phenomenon was observed by a thermal microscope, the thermal analysis was carried out using a TG-DTG-DTA thermal analysis technology, and the gaseous products during the combustion were measured by TG-MS. On the basis of the experiments, the following conclusions are drawn: 1. From the proximate analysis of seaweed, it is clear that they have a large quantity of ash. In relation to the elementary analysis, seaweed has a lower oxygen content than terrestrial biomass. The fusion temperature is low because of the many alkali metal elements in ash. The fouling problems caused by K, Na, and Mg should be considered in the practical application of seaweed biomass. 2. The ignition behaviors of the seaweed are homogeneous. Their combustion characteristics are different from the woody biomass. The ignition temperatures of seaweed are all low, and the biomasses easily burst into flame. The ignition temperature of EN is lower than that of SA. 3. The combustion process is composed of dehydration, the pyrolysis and combustion of volatiles, transition stage, the combustion of char, and reaction at high temperature. The DTA curve of EN shows two exothermic peaks in accordance with the combustion of volatile and fixed carbon. Beside the two exothermic peaks, the DTA curve of SA shows another narrow exothermic peak, which probably is caused from the oxidation of uncompleted burned matters. 4. During the combustion of seaweeds, NO2 release curves follow the CO2 release curves. They are both according to the mass loss peaks in DTG curve and exothermic peaks in the DTA curve. The obvious SO2 release of EN is observed at around 200-400 and 500-700 °C, which is likely caused from the decomposition and oxidization of S-containing compounds. However, the SO2 release of SA can be found at around 200-400 °C, which is the region of the release and combustion of volatile matters. There is also a great difference of the NO evolution between two seaweeds. EN gives off NO at around 200-300 °C, but SA shows several NO release peaks in the temperature region of 200-800 °C, especially the peaks at 270 and 790 °C are obvious. The cause is mainly associated with the protein in seaweed.
Figure 6. Cumulative release of products during combustion of seaweed.
Above 1100 °C, the SO2 release is also found because of the decomposition of S-containing matters in ash. There is a great difference of the NO evolution between two seaweeds. It is significant that EN gives off NO at around 200-300 °C and SA shows several NO release peaks in the temperature region of 200-800 °C, especially the peaks at 270 and 790 °C are obvious. The N-containing compounds are mainly associated with the protein in seaweed. NO release may be associated with the degradation of protein in the devolatilization stage and the oxidization in the ignition-and-burning stage. NO2 releases are similar with CO2 releases. NOx is a primary component of photochemical smog, being partially responsible for corroding metals and causing acid rain. In order to make the quantitative analysis of the production of different chemical species, the results of the MS curves have been further analyzed according to the following procedure:38 1. Integrals subtended by the MS curves have been calculated. 2. Integral values have been normalized by the initial sample mass of seaweed sample and the value of the total ion intensity. These are proportional to the cumulative amounts released throughout an experiment. Figure 6 reports the cumulative amounts released throughout combustion experiments. Results obtained for the two seaweed samples are compared. The CO2 and NO release are both more than NO2 and SO2. And all the gases released from EN are more than SA. Compared with most woody biomass, seaweed contains higher sulfur and nitrogen content and, thus, has more SO2 and fuel-NOx released during the combustion. The sulfur and nitrogen mainly come from the components such as saccharide and protein. In combustion stage 2, these
Acknowledgment. This work was supported by Shanghai Pujiang Program of China (Grant 05PJ14086).
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