Energy Fuels 2009, 23, 5144–5150 Published on Web 09/08/2009
: DOI:10.1021/ef900496b
Physico-Chemical Characteristics and Mineral Transformation Behavior of Ashes from Crop Straw Youqing Wu, Shiyong Wu, Ye Li, and Jinsheng Gao* Department of Chemical Engineering for Energy Resources and Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, 200237Shanghai, China Received March 6, 2009. Revised Manuscript Received July 19, 2009
In this paper, the physicochemical characteristics and mineral transformation behavior of three crop straw ashes (rice straw, wheat straw, and cotton straw) were investigated using ICP-AES, XRF, SEM-EDS, XRD, and ash fusion point analyzer. It was found that these ashes had quite high content of alkali metals (especially K) and quite low fusion point (especially cotton straw ashes) and caused easily the corrosion, slagging, and fouling of equipments. These straw ash particles were uniformly distributed in the small, medium, and large size. As the ashing temperature increased, some amounts of K (in the form of KCl) were released into the gas phase, quartz transformed to cristobalite and then keatite, and some mineral components also occurred to transform. Up to 1088 K, K2O and Na2O separately reacted with Al2O3 and SiO2 to produce albite (K-Al-Si) and sanidine (Na-Al-Si). Above 1273 K, other alkaline earth oxides also participated in the above reactions to form amorphous matters like augite. Meanwhile, comprehensive utilization of crop straw ashes is also worth our attention, due to their special compositions.
equipments but also shortening their life.6-9 In the thermochemical conversion process of biomass, these problems are significantly pertinent with ash compositions, ash fusion characteristics, and mineral transformation behaviors.10-14 Thereby, understanding the physicochemical characteristics and mineral transformation behavior of ashes is very critical to the effective thermo-chemical utilization of biomasses. Presently, there have been some correlative reports on mineral behaviors of biomass ashes. For examples, Wang et al.11 reported that with the increasing temperature, the volatilization of alkali chlorides in biomass ashes became stronger and stronger; the studies of Womat et al.15 suggested that the partial volatilization of alkali metals took place after the fuel devolatilization, which was characterized by the release of oxygen- and hydrogen-rich gases; French et al.16 considered that in the thermo-chemical utilization process of biomass, potassium chloride was the dominant potassium compound released to the vapor phase, with potassium hydroxide presented as a secondary volatile species; Bryers et al.17 found that in the biomass ashes, potassium compounds might also react with silicates and aluminates to produce a low-melting eutectic. The physicochemical characteristics and
Introduction Due to the sustainable development and increase in the demand of fuels, biomass as a renewable energy source is considered to have the potential of being increasingly important in the future. Biomass sources include agricultural wastes, straws, firewood, seaweed, agro residues, and so on, which occupy a major role in this scenario. Many countries have put or are putting great emphasis on the exploration and utilization of various biomasses. At present, some biomass utilization technologies such as combustion, gasification, and pyrolysis1-5 have been used or are being explored. The compositions of different species of biomasses are commonly complicated. Generally, biomass consists of organic components that mainly contain C, H, and O, and inorganic components, which contain K, Na, Ca, Mg, Al, Si, P, S, and Cl. After combustion, the inorganic minerals are left as ashes. It has been observed that ashes can take some great negative effects on the thermo-chemical conversion process of biomass. Inorganic species such as alkali oxides and salts can aggravate agglomeration, deposition, and corrosion on heat transfer surfaces of equipment, not only reducing the utilization efficiency of *Corresponding author. Telephone or Fax: þ86-21-64252058. E-mail:
[email protected]. (1) Ni, M.; Leung, D. Y. C.; Leung, M. K. H.; Sumathy, K. Fuel Process. Technol. 2006, 87, 461–472. (2) D’Jesus, P.; Boukis, N.; Kraushaar-Czarnetzki, B.; Dinjus, E. Fuel 2006, 85, 1032–1038. (3) Erlich, C.; Bjornbom, E.; Bolado, D.; Giner, M.; Fransson, T. H. Fuel 2006, 85, 1535–1540. (4) Albertazzi, S.; Basile, F.; Brandin, J.; Einvall, J.; Hulteberg, C.; Fornasari, G; Rosetti, V.; Sanati, M.; Trifiro, F.; Vaccari, A. Catal. Today 2005, 106, 297–300. (5) Senneca, O. Fuel Process. Technol. 2007, 88, 87–97. (6) Tortosa Masia, A. A.; Buhre, B. J. P.; Gupta, R. P.; Wall, T. F. Fuel Process. Technol. 2007, 88, 1071–1081. (7) Ohman, M.; Pommer, L.; Nordin, A. Energy Fuels 2005, 19, 1742– 1748. (8) Ohman, M.; Nordin, A.; Backman, R.; Hupa, M. Energy Fuels 2000, 14, 169–178. r 2009 American Chemical Society
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Energy Fuels 2009, 23, 5144–5150
: DOI:10.1021/ef900496b
Wu et al.
Table 1. Proximate and Ultimate Analysis of Three Crop Strawsa
samples RS WS CS
proxiamte analysis (%), db
ultimate analysis (%), daf
A
V
FC
C
H
N
S
Ob
17.12 10.01 7.44
72.95 76.90 71.05
9.93 13.09 21.51
49.40 47.03 48.93
6.88 10.80 6.26
1.39 0.58 0.59
0.26 0.24 0.29
42.07 41.35 43.93
Table 2. Elemental Compositions of Three Straw Ashes at the Ashing Temperature of 1088 K
a db: dry basis; daf: dry-ash free basis; A: ash; V: volatile matter; FC: fixed carbon; C: carbon; H: hydrogen; N: nitrogen; S: sulphur; O: oxygen; b By difference.
mineral transformation behaviors of some biomass ashes have been reported by many literatures,11,15-22 but the studies appear quite dispersive. Obviously, these reports are not enough to deeply understand mineral transformation behaviors of biomass ashes, especially for crop straw ashes. Moreover, the different biomass ashes probably present significant differences in the physicochemical characteristics and mineral behaviors, due to considerable differences in their compositions. Therefore, using the rice straw ash, the wheat straw ash, and the cotton straw ash as the three representatives of crop straw ashes, this paper investigates their compositions, mineral forms, fusion characteristics, and mineral transformation behaviors, aiming to provide some useful references for the exploration and utilization of crop straw biomasses.
oxides
RSA
WSA
CSA
SiO2 (wt %) Al2O3 (wt %) TiO2 (wt %) Fe2O3 (wt %) K2O (wt %) Na2O (wt %) CaO (wt %) MgO (wt %) P2O5 (wt %) SO3 (wt %) Cl (wt %) LOI (loss of ignition at 1273 K, wt %) base/acid ratio (B/A)
61.63 2.79 0.10 3.14 9.10 3.77 4.47 2.15 4.16 3.66 3.39 1.03 0.35
36.90 8.00 0.43 6.29 17.38 9.16 8.07 2.98 2.01 4.47 5.20 0.89 0.97
17.67 10.8 0.18 2.43 30.20 6.78 8.98 7.29 4.28 5.81 4.93 1.57 1.94
Table 3. Alkali Index (A Measure of Fouling/Slagging Potential) of Three Straws samples RSA WSA CSA
high calorific value (MJ/kg)
alkali index (kg alkali oxide/GJ)
origin of data
15.35 14.75 15.38 16.15 17.15 15.18 15.86
1.44 1.22 2.05 1.65 1.66 2.15 1.73
This study Miles et al.14 Dayton et al.25 This study Miles et al.14 Dayton et al.25 This study
content in ashes is measured by a sequential X-ray fluorescence spectrometer (XRF, XRF-1800). The fusion characteristic temperatures of ash samples are determined by an ash fusion point autoanalyzer (ZRC 2000) according to GB/T 219-1974 (Chinese standard for coals). The analysis of the morphology and elemental composition of individual ash particle is performed using a scanning electron microscope coupled with an electron detection scanning (SEM-EDS, JEOL JSM-6360LV). An X-ray diffraction analyzer (XRD, JSM-6360LV, D/max-rA12 kW, 12 kW, 40 kV, 100 mA, Cu KR radiation) is employed to determine the mineral components of ash samples, and major components are identified using the standard cards of JCPDSICDD (PDF-2 database Sets).
Experimental Section Materials. Three species of crop straws (rice straw (RS), wheat straw (WS), and cotton straw (CS)) from the suburb of Shanghai in China are used as raw materials. Their proximate and ultimate analysis data are given in Table 1. From the table, it is found that the three crop straws are different from other biomasses such as woody biomass,23 due to their relatively high content of ashes. Preparation of Ash Samples. Before the preparation of ash samples, the air-dried straw samples are ground to less than 76 μm. The straw ashes are prepared in a muffle furnace according to the procedures proposed by ASTM E870-82. First, the straw samples are heated up to 1088 K at a heating rate of 30 K/min and held at this temperature for 2 h (the mass of samples is almost unchanged after 2 h) in the atmosphere. In succession, the ashes are taken out and quenched immediately by immersing them into cool water. Finally, the quenched ashes are dried at 423 K for 3 h and stored as the experimental samples. The ash samples from rice straw, wheat straw, and cotton straw are, respectively, referred to as RSA, WSA, and CSA. For a purpose of comparison, some ash samples are also prepared separately at the ashing temperatures of 873 K (a holding time for 3 h) and 1273 K (a holding time for 1 h) according to the above same procedures. Analysis and Test Methods for Ash Samples. Prior to all analysis (excluding the SEM-EDS nanlysis), all ash samples are ground into less than 73 μm. All elemental contents (excluding chlorine) in ashes are measured using the chemical methodology coupled with an inductively coupled plasma-atomic emission spectrophotometer (ICP-AES, IRIS 1000). Only the chlorine
Results and Discusssion Elemental Compositions of Straw Ashes. The elemental compositions of RSA, WSA, and CSA at the ashing temperature of 1088 K are presented in Table 2. The table shows that the straw ashes contain the following main elements: Si, Al, Fe, K, Na, Ca, Mg, P, S, Cl and only a little titanium. This result suggests that the elemental compositions of the three straw ashes are commonly similar to those of other crop straw ashes such as corn straw ashes, bean straw ashes, and kaoliang straw ashes.24 Making a comparison among the three straw ashes, significant differences in the two main elemental contents (Si and K) are found. The Si content (in the form of SiO2, 61.63%) in RSA is the largest, and the differences between RSA and the other two ashes (WSA and CSA) are 24.73 and 43.96%, respectively. The content of K (in the form of K2O) in CSA is up to 30.20% and occupies a larger proportion than those in the other two (WSA and RSA). Furthermore, the differences among them in the content of other elements also are presented more or less. Obviously, all above differences in elemental contents are ascribed to their different living conditions (or surroundings) and the difference of species. The
(18) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13, 379–389. (19) Skrifvars, B. J.; Yrjas, P.; Kinni, J.; Siefen, P.; Hupa, M. Energy Fuels 2005, 19, 1503–1151. (20) Umamaheswaran, K.; Batra, V. S. Fuel 2008, 87, 628–638. (21) Wang, S.; Jiang, X. M.; Han, X. X.; Wang, H. Energy Fuels 2008, 22, 2229–2235. (22) Lan, F.; Ma, X. Q.; Wang, J. J. Renew. Energy Res.(Chinese) 2007, 25, 25–28. (23) Srivastava, V. C.; Mall, I. D.; Mishra, I. M. J. Hazard. Mater. B 2006, 134, 257–267.
(24) 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 biochemistry 2006, 41, 1883–1886.
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: DOI:10.1021/ef900496b
Wu et al.
Figure 1. XRD analysis of RSA, WSA, and CSA at the ashing temperature of 1088 K (A: RSA; B: WSA; C: CSA). 1: quartz [SiO2]; 2: cristobalite [SiO2]; 3: calcite [CaCO3]; 4: dolomite [CaMg(CO3)2]; 5: potassium chloride [KCl]; 6: kaolimite [Al4Si4O10]; 7: aphthitalite [K3Na(SO4)2]; 8: albite [NaAlSi3O8]; 9: sanidine [KAlSi3O8]; 10: arcanite [K2SO4]; 11: sillimanite [Al2SiO5]; 12: mullite [Al6Si2O13]; 13: rankinite [Ca3Si2O7]; 14: potassium silicate [K2Si4O9]; 15: calcium magnesium silicate [Ca7Mg9(SiO4)4].
above results suggest that RSA with high silica content is suitable for ceramic products, and the high content of potassium in CSA can make it suitable for soil amendment. At the same time, it is also found that the contents of alkali metals (K and Na), especially the content of K, are very high in the three straw ashes, which suggests that the corrosion of alkali metals on heat transfer surfaces and main reactor bodies is probably easy to occur in the thermo-chemical conversion process of the three straws. Some literatures have reported that the relatively high content of alkali metals (in the forms of chloride, sulfate, carbonate, and hydroxide) in biomass ashes is considered as one of main reasons for increased corrosion in boilers.6-9,25,26 Meanwhile, the high contents of alkali and alkaline-earth metals in the straw ashes make it feasible for the gasification (or combustion) catalysts of other fuels with low activity (such as petroleum coke and oil shale). According to Table 2, the alkali index (a measure of fouling and slagging potential expressed as the mass of alkali oxides per unit fuel energy) of the three straws are calculated as Table 3. From the table, it can be found that these alkali index obtained in the study are within those reported by literatures.14,25 Besides, two significant results can be obtained as follows: (I) Alkali index of the three straws are in the range of 1.44-1.73 kg/GJ and quite above 0.34 kg/GJ, which suggests that the three straws have a highly severe fouling and slagging potential in the thermo-chemical conversion process (It is thought that fouling is of low severity for fuels with alkali index below 0.17 kg/GJ and of high severity above 0.34 kg/GJ14); (II) The ordering of alkali
index of the three straws is: CS > WS > RS, which suggests that CS has the severest fouling and slagging potential in the thermo-chemical conversion process. In a word, these problems on the corrosion of alkali metals and the fouling and slagging of ashes should be attached more importance in the thermo-chemical utilization of the three straws, especially cotton straw. However, comprehensive utilization of the straw ashes is also worth our attention. Mineral Forms of Straw Ashes. Figure 1 shows the XRD patterns of the three straw ashes at the ashing temperature of 1088 K. From the figure, it is observed that in these ashes, the compounds containing Si, K, Al, Ca, Na, and Mg are presented predominantly and that these elements exist in various different forms. Si is mainly in the form of quartz and cristobalite. In literatures,27,28 the presence of cristobalite along with quartz has been also reported for other biomass ashes. K is in the main form of KCl (this has been reported in ref 10) and a large amount of K also exists in the forms of sulfate and silicate. Other elements (Al, Na, Ca, and Mg) are also distinctly presented in these forms of carbonate, sulfate, or silicate. Nevertheless, no characteristic peaks of compounds containing Fe are observed in all straw ashes. Olanders et al.29 found the same phenomenon and considered that Fe probably existed in the quite small crystallite, which could not be detected by XRD due to a quite small granularity of below 0.4 μm. Analyzing XRD patterns of the three straw ashes, their mineral forms can be separately obtained as follows. In RSA, (27) Rachakornkij, M.; Ruangchuay, S.; Teachakulwiroj, S. Songklanakarin J. Sci. Technol. 2004, 26, 13–24. (28) Olsson, J. G.; Jaglid, U.; Pettersson, J. B. C.; Hald, P. Energy Fuels 1997, 11, 779–784. (29) Olanders, B.; Steenari, B. M. Biomass Bioenergy 1995, 8, 105–115.
(25) Dayton, D. C.; Belle-Oudry, D.; Nordin, A. Energy Fuels 1999, 13, 1203–1211. (26) Davidsson, K. O.; Engvall, K.; Hagstrom, M.; Korsgren, J. G.; Lonn, B.; Pettersson, J. B. C. Energy Fuels 2002, 16, 1369–1377.
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Energy Fuels 2009, 23, 5144–5150
: DOI:10.1021/ef900496b
Wu et al.
Figure 2. SEM/EDS analysis of three straw ash particles at the ashing temperature of 1088 K. Table 4. Fusion Temperatures of Three Straw Ashesa samples
DT (K)
ST (K)
HT (K)
FT (K)
origin of data
RSA WSA CSA Shenhua coal Yanzhou coal Huainan coal Guizhou coal
1198 1188
1258 1243
1393 1353
1398 1364 1523 1488
1483 1442 1783 1518
1493
1508 1478
