Energy & Fuels 2008, 22, 2229–2235
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Fusion Characteristic Study on Seaweed Biomass Ash S. Wang, X. M. Jiang,* X. X. Han, and H. Wang Institute of Thermal Energy Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, P. R. China ReceiVed February 20, 2008. ReVised Manuscript ReceiVed April 10, 2008
The ash fusing characteristics on three sorts of seaweed (a kind of marine biomass) were studied by using thermal microscope, X-ray diffractometer (XRD), ash composition analysis, and simultaneous thermogravimetry/ differential thermal analysis. It was presented that there were lots of alkali metals especially K and Na in all seaweed ash samples. XRD analysis shows that the crystalline phase intensities of alkali chlorides reduce with increasing the ashing temperature, due to the evaporization of alkali chlorides. Therefore, the evaporization of alkali chlorides in seaweed biomass should be considered during the thermal conversion. The low ash fusion temperatures of seaweed make the ashing temperatures recommended by both GB and ASTM norms to exceed the limit of temperature to prepare seaweed ash. At a high ashing temperature, Gracilaria cacalia and Sargassum natans will generate some high-melting matters which influence the identification of the ash fusion points. So, it is more exact and referenced that the seaweed biomasses were ashed at a lower temperature (such as 530 °C). Besides, the slagging characteristics and fusing characteristics of each sample are quite different. Gracilaria cacalia is the easiest to slag, followed by Sargassum natans. Enteromorpha clathrata is the hardest to slag among them. Gracilaria cacalia and Sargassum natans show a great range from the ash deformation temperature to hemispherical temperature, and Enteromorpha clathrata reveals a small temperature difference between deformation temperature and hemispherical temperature.
1. Introduction Bioenergy is regarded as a green renewable energy and has the potential of being more important in the future. Biomass energy offers remarkable worldwide potential to reduce net CO2 emission. Many countries are putting great emphasis on the exploration of biomass energy, and the techniques used are various such as combustion, gasification, pyrolysis, hydrogen production, and so on. Interest in the combustion of biomass fuels for power generation has increased in recent years, due to its CO2 neutrality.1 The compositions of different types of biomass are commonly sophisticated. Biomass has high organic content which contains cellulose, hemicellulose, lignin, lipid, protein, saccharide, and other compounds. Beside those organisms composed by C, H, and O, it also contains inorganic mineral substances like K, Na, Ca, P, etc. The inorganic mineral substances left after the thermochemical conversion of biomass are ashes. The ashes in biomass gradually form accompanying the growth of the plant, which is determined by many factors like the species and the surroundings of the biomass. The ashes have great passive influence on the chemical conversion of biomass. This influence would not only reduce the utilization efficiency of equipment but also shorten their service life. Inorganic species existing in biomass such as alkali oxides and salts can aggravate agglomeration, deposition, and corrosion problems on boiler’s heat transfer surfaces.1 The alkali metal in biomass may cause high temperature corrosion on gas turbine blades, and sedimentation in the form of slag or fly ash may occur on tail-heating surfaces. Alkali compounds emitted during thermochemical biomass conversion have been paid more and more attention recently because they are proved to play a * Corresponding author. E-mail address:
[email protected]. Tel.: +86-21-34205681. (1) Tortosa Masia´, A. A.; Buhre, B. J. P.; Gupta, R. P.; Wall, T. F. Fuel Process. Technol. 2007, 88 (11-12), 1071–1081.
critical role in the most common problems in biomass thermochemical utilization, such as agglomeration, slagging, and fouling. Many studies have been focused on the characteristics of biomass ash,2–4 which are found to have direct effects on slagging, ash fouling, and bed agglomeration5–7 during biomass combustion. Marine biomass as another sort of plant which is applied to energy consumption in a relatively small area all over the world. Seaweed is an important constituent of marine biomass. Most 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 14 200 km coastal area lives a great variety of seaweeds, with the number of species ranging from 3000 to 4000.8 Seaweeds are easy to breed because of their short life cycle and fast breeding, and the growth seasons of seaweeds are not single, which ensures that the biomass source is abundance. Moreover, seaweed lives in sea areas, not occupying land areas. Traditional biomass resources are difficult to apply for wide application because they are scattered and influenced by seasons. But, seaweeds do not have these limitation. Seaweeds are also able to absorb large quantities of carbon (2) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13 (2), 379–389. (3) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13 (2), 390–395. (4) Skrifvars, B. J.; Yrjas, P.; Kinni, J.; Siefen, P.; Hupa, M. Energy Fuels 2005, 19 (4), 1503–1511. ¨ hman, M.; Nordin, A.; Skrifvars, B.-J.; Backman, R.; Hupa, M. (5) O Energy Fuels 2000, 14 (1), 169–178. ¨ hman, M.; Pommer, L.; Nordin, A. Energy Fuels 2005, 19 (4), (6) O 1742–1748. ¨ hman, M.; Nordin, A. Energy Fuels 2005, 19 (3), 825– (7) Brus, E.; O 832. (8) Xia, B. M. Flora algarum marinarum sinicarum, Tomus 2 Rhodophyta, No 3 Gelidiales Cryptonemiales Hildenbrandiales; Science Press: Beijing, China, 2000 (in Chinese).
10.1021/ef800128k CCC: $40.75 2008 American Chemical Society Published on Web 06/06/2008
2230 Energy & Fuels, Vol. 22, No. 4, 2008
Wang et al.
Table 1. Content of Main Composition of Seaweed Samples proteina lipida carbohydratea (%) (%) (%)
sample Gracilaria cacalia (GR) Enteromorpha clathrata (EN) Sargassum natans (SA) a
29.41 23.99 9.6
1.53 1.28 1.39
46.03 40.0 63.97
ref this study this study 15
Dry weight.
dioxide and produce a lot of oxygen. In eutrophic sea areas, seaweed cultivation has the excellent effect of decreasing eutrophication. 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 decreasing red tide microalgae.9,10 A large number of seaweeds can be planted to help ocean remediation. If those rich seaweed sources are explored efficiently and put into clean and proper use, they may make a great contribution to worldwide energy usage, both theoretically and industrially.11 So far, some specialists and experts have made related reports on how to make use of marine biomass as energy sources. For example, a great amount of oil in B. braunii was obtained by Dote et al.,12 with a yield of 57-64 wt % at 300 °C. The oil was equivalent in quality to petroleum oil.13 Experiments on pyrolysis of Enteromorpha clathrata were carried out by Wang et al.11 A comparison of thermolysis characteristics and kinetics between seaweeds and fir wood was also analyzed.14 However, little has been reported about seaweed ash characteristics. In fact, there are considerable differences in ash characteristics between seaweed and traditional biomass. This paper studies the compositions and characteristics of seaweed ashes, which provides not only the foundation of exploitation and utilization of seaweed biomass, but also the reference to comprehensive utilization of seaweed ashes. 2. Experimental Section 2.1. Materials. Three species of seaweeds (Gracilaria cacalia (GR), Enteromorpha clathrata (EN), and Sargassum natans (SA)) are used, respectively collected from Xiangshan Port (Zhejiang), Rudong (Jiangsu), and Zhanjiang (Guangdong) in China. They are red, green, and brown seaweeds individually. The component analyses of three species of seaweeds are shown in Table 1. However, the major components in woody biomass are hemicellulose, cellulose, and lignin. Therefore, the differences in characteristics between seaweed and woody biomass will be great when they are used as fuels. Also, inorganic constituents in seaweed should be also different from that in woody biomass due to the difference in living environment. 2.2. Seaweed Ash Preparation. Before they were studied, the air-dried samples were ground to a small particle size of less than 0.18 mm by a grinding machine. 2.2.1. Ash Preparation Following Norms. Until now, there were no relevant norms established for biomass prepared ash in China. In the present experiments, the ashing temperature was set at 815 and 600 °C, according to GB/T 212-2001 (Chinese norm) for coal and ASTM E870-82 for biomass. (9) Nagayama, K.; Shibata, T.; Fujimoto, K.; Honjo, T.; Nakamura, T. Aquaculture 2003, 218, 601–611. (10) Jeong, J. H.; Jin, H. J.; Sohn, C. H.; Suh, K. H.; Hong, Y. K. J. App. Phycol. 2000, 12, 37–43. (11) Wang, S.; Jiang, X. M.; Wang, N.; Yu, L. J.; Li, Z.; He, P. M. Energy Fuels 2007, 21 (6), 3723–3729. (12) Dote, Y.; Sawayama, S.; Inoue, S.; Minowa, T.; Yokoyama, S. Fuel 1994, 73 (12), 1855–1857. (13) Miao, X. L.; Wu, Q. Y.; Yang, C. Y. J. Anal. Appl. Pyrolysis 2004, 71, 855–863. (14) 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.
Figure 1. Schematic diagram of the thermal microscope.
Experiments were carried out in the muffle furnace. Three types of grains (
