Energy & Fuels 2005, 19, 1503-1511
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The Fouling Behavior of Rice Husk Ash in Fluidized-Bed Combustion. 1. Fuel Characteristics Bengt-Johan Skrifvars,*,† Patrik Yrjas,† Jouni Kinni,‡ Peter Siefen,‡ and Mikko Hupa† Åbo Akademi University, Process Chemistry Centre, Turku/Åbo, Finland, and Kvaerner Power Oy, Tampere, Finland Received November 8, 2004. Revised Manuscript Received March 18, 2005
Rice husk can be considered as an “opportunity fuel” for energy production. However, although rice husk has long been identified as a source for energy production, only limited experience exists from rice husk firing in larger-scale combustors. Only a few units worldwide are reported to be using rice husk as their main fuel. One concern in rice husk firing is the behavior of the ash, i.e., its slagging and fouling tendency, as well as its abrasiveness. This paper presents the very characteristic properties of the rice husk ash, as measured by a variety of laboratory tests and analyses, and compares these characteristics with eucalyptus bark and rice straw, as well as with some other biomass fuels. The paper is the first in a series of two, where we report from a recently finished study on the slagging and fouling behavior of rice husk when fired alone or in combination with other fuels in a fluidized-bed boiler. In the second part of the series, we will report the results of fireside fouling measurements in a pilot-scale burning test facility and in a 157 MWth full-scale fluidized-bed boiler burning rice husk and bark in different ratios. Rice husk has been shown to produce large, almost millimeter-sized ash particles with a characteristic shape. The particles consisted almost purely of silica, with just a few percent of potassium. The melting properties for this type of ash is characterized by a very high initial melting temperature, and, consequently, no molten fly ash is expected under any fluidized-bed combustion conditions when husks are burned alone.
1. Introduction The co-firing of fossil fuels with CO2-neutral fuels or of several CO2-neutral fuels simultaneously is an attractive alternative to decrease CO2 emissions from energy production. Fluidized-bed combustion (FBC) technology has been shown to meet the technical needs of such co-firing well and also has recently been successfully demonstrated at a large scale.1 Because all waste and biomass-based materials are considered CO2neutral, several new “opportunity fuels” are being introduced in the marketplace. These fuels include husk and hull residues of various types. Rice husk is an important byproduct in the rice production industry. Worldwide, there is a production of some 700 million tons/yr of rice-related waste, such as straw, shells, and stalk.2 The fuel potential of rice husk alone is some 80 million tons/yr worldwide, corresponding to an energy potential of some 1.2 × 109 GJ.3 * Author to whom correspondence should be addressed. Telephone: 358 2 215 4648. Fax: 358 2 215 4962. E-mail address:
[email protected]. † Åbo Akademi University. ‡ Kvaerner Power Oy. (1) Hupa, M. Challenges in Multi-Fuel Firing in Fluidized Bed CombustionsInvited Plenary Lecture. In Proceedings of the 17th ASME International Conference on Fluidized Bed Combustion, May 18-21, 2003, Jacksonville, FL; American Society of Mechanical Engineers: New York, 2003. (CD-ROM, ISBN 0791836754.) (2) Werther, J.; Saenger, M.; Hartge, E.-U.; Ogada, T.; Siagi, Z. Combustion of Agricultural Residues. Prog. Energy Combust. Sci. 2000, 26, 1-26.
Although rice husk has long been identified as a source for energy production, experiences from large-scale rice husk firing is quite limited.4,5 Only a few boilers worldwide have reported rice husk to be their main fuel. One major concern in rice husk firing is the carbon conversion efficiency. The recent overview by Natarajan et al.3 gives a summary of four previous studies of the combustion of rice husk and seven previous studies on the gasification of rice husk in various types of test facilities and fluidized-bed reactors.6-16 Another concern in rice husk firing is the behavior of the ash, specifically its slagging and fouling tendency as well as its abra(3) Natarajan, E.; Nordin, A.; Rao, A. N. Overview of Combustion and Gasification of Rice Husk in Fluidized Bed Reactors. Biomass Bioenergy 1998, 14 (5/6), 533-546. (4) Advance Agro Public Co., Ltd., & Thai Power Supply Co., Ltd., Thailand, http://www.advanceagro.com, 1998: 157 MWth (480 °C, 84 bar). (5) Agrielectric Power Partners Ltd, Lake Charles, LA, http:// www.agrielectric.com, August 9, 2000: 10 MWel (750 °F, 650 psig). (6) Peel, R. B.; Santos, F. J. Fluidized Bed Combustion of Vegetable Fuels. In Proceedings of the International Conference Fluidised Combustion: Systems and Applications; Institute of Energy: London, 1980. (7) Bhattacharya, S. C.; Shah, N.; Alikhani, Z. Some Aspects of Fluidized Bed Combustion of Paddy Husk. Appl. Energy 1984, 16 (4), 307-316. (8) Preto, F.; Anthony, E. J.; Desai, D. L.; Friedrich, F. D. Combustion Trials of Rice Hulls in a Pilot-Scale Fluidized Bed. In Proceedings of the 9th International Conference on Fluidized Bed Combustion; American Society of Mechanical Engineers: New York, 1987; Vol. 2, pp 1123-1127. (9) Bhattacharya, S. C.; Wu, W. Fluidized Bed Combustion of Rice Husk for Disposal and Energy Recovery. In Energy from Biomass and Wastes XII; Elsevier Applied Science: Amsterdam, 1989; pp 591-601.
10.1021/ef049714b CCC: $30.25 © 2005 American Chemical Society Published on Web 05/05/2005
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siveness. Very few published reports are available on this subject. One pulverized-fuel-fired unit in the United States has reported erosion problems in the flue gas duct.5 The issue of slagging or fouling caused by the rice husk has not been addressed. The recent study by Armesto et al.17 briefly mentioned that no slagging or fouling was observed when testing rice husk firing in a pilot FBC facility. There are no previous studies on the behavior of the rice husk ash when the husk is burned together with other biomass fuels. The interaction of ashes from different fuels may be very complex, and there is a great need to study these interactions further. The purpose of this study was to learn to understand the chemical and physical properties of rice husk ash and to determine the slagging and fouling tendency of this ash at conditions relevant to bubbling FBC. Of particular interest was the fouling behavior of rice husk ash when rice husk was to be burned together with other fuels, such as eucalyptus bark. The work is to be reported in two papers. This first paper presents the special characteristics of the rice husk ash as measured by a variety of laboratory tests. It includes detailed analyses of the elemental composition of the ash-forming matter in the original fuel as well as scanning electron microscopy (SEM) and elemental analysis (energy-dispersive X-ray (EDX) analysis) of ash and partially burned fuel particles. The paper compares these detailed characteristics with two other biomass fuelsseucalyptus bark and rice strawsstudied with the same techniques as the rice husk. The results are also compared with typical characteristics of other biomass fuels. In the second paper of the series, we will report the results of fireside fouling measurements in a pilot-scale burning test facility and in a 157 MWth fullscale fluidized-bed boiler burning rice husk and bark in different ratios.18 (10) Van den Aarsen, F. G.; Beenackers, A. A. C. M.; Swaij, W. P. M. Performance of a Rice Husk Fluidized Bed Pilot Plant Gasifier. In Producer Gass1982: 1st International Conference, Sri Lanka, 1982; pp 381-391. (11) Hiler, E. A. On-Site Energy Production from Agricultural Residues, Report TENRAC/EDF-074, Order No. DE83900814, Center of Energy and Mineral Resources, University of Texas, Austin, TX, 1982. (12) Xu, B. Y.; Hang, W. C.; Flanigan, V. J.; Sitton, O. C. Design and Operation of a 6 in. Fluidized Bed Gasifier for Rice Hulls. In Symposium on Energy from Biomass and Waste IX; Institute of Gas Technology: Chicago, IL, 1985; pp 595-613. (13) Bingyan, X.; Zongnan, L. A Study of Gasification of Rice Hulls. In Proceedings of the Biennial Congress of the International Solar Energy Society, Hamburg, Federal Republic of Germany, 13-18 September 1987; Bloss, W. H., Pfisterer, F., Eds.; Advances in Solar Energy Technology; Elsevier: Amsterdam, 1988; pp 2312-2316. (14) (a) Flanigan, V. J.; Xu, B. Y.; Huang, E. Fluidized Bed Gasification of Rice Hulls. In Proceedings of the 10th Annual EnergySource Technology Conference and Exhibition, Dallas, TX, 1987; pp 19-34. (b) Hartiniati, A.; Soemardjo, A.; Youvial, M. Performance of a Fluidized Bed Gasifier, Fuelled by Rice Husks. In Proceedings of the International Conference on Pyrolysis and Gasification, 1989; pp 257263. (15) Panaka, P. Operating Experiences of Biomass Gasifiers in Indonesia. In Advanced Thermochemical Biomass Conversions; Bridgewater, A. V., Ed.; Blackie: Glasgow, U.K., 1994; pp 392-402. (16) Sanches, C. G.; Lora, E. S. Biomass Fluidized Bed Gasification Research in the State University of Campinas. Energy Sustainable Dev. 1994, 1 (4), 31-33. (17) Armesto, L.; Bahillo, A.; Veijonen, K.; Cabanillas, A.; Otero, J. Combustion Behaviour of Rice Husk in a Bubbling Fluidized Bed. Biomass Bioenergy 2002, 23, 1871-1879. (18) Skrifvars, B.-J.; Yrjas, P.; Laure´n, T.; Kinni, J.; Tran, H.; Hupa, M. The Behavior of Rice Husk Ash during Fluidized Bed Combustion. 2. Laboratory and Full-Scale Measurements. Energy Fuels, 2005, 19, 1512-1519.
Skrifvars et al.
2. Experimental Section The main focus in the study was on rice husk; however, two other fuels were also chosen for comparison: rice straw and eucalyptus bark. We further compared the analyses results with other similar data for average values of husks and hulls, straw, and Scandinavian bark. These data were taken from our own unpublished work.19 All three fuels, i.e., rice husk, rice straw, and eucalyptus bark, were subjected to conventional fuel analyses. The presence of the ash-forming elements in the fuels was also analyzed by the stepwise leaching test, also referred to as “chemical fractionation”.20 We further performed scanning electron microscopy and X-ray-based elemental analysis (SEM/EDXA) on the fuels, as well as on partially ashed fuel samples. In the stepwise leaching test, the fuels were treated with increasingly aggressive solvents, starting with water, continuing with ammonium acetate, and finally ending with hydrochloric acid. After each leaching step, the ash-forming elements of interest were analyzed from the solvent, giving the amount of each element being leached out by that solvent. Also, the remaining fraction, as well as the untreated original fuel sample, was analyzed. Based on the elements that were leached out in the various solvents, conclusions can be made on how the element in question was associated in the fuel:21-23 (1) Water-soluble parts of the ash forming elements are most likely present as simple soluble salts, such as chlorides and alkali sulfates. (2) The acetate-soluble fraction of the elements could be directly bound to the organic groups in the fuel. Ammonium acetate can act as an ion exchanger in addition to being a buffer solution. Examples of elements that are partially present in organically bound form are calcium, sodium, and potassium. (3) The acid-soluble elements may be carbonate minerals in the fuel (for instance, calcium and iron carbonates). Also, crystalline calcium oxalate, which has been found to be present in certain bark biomass fuels, will be mostly dissolved in the acid leaching.22,24 (4) The insoluble remainder is, most typically, various types of silicate minerals. The SEM analyses were performed on both the original fuels and the partially ashed samples of the fuels. The ashing was conducted in a muffle furnace for 15-30 min at three different temperaturess500, 600, and 900 °Csin ambient air. The SEM/ EDX analysis technique permitted study of the presence of the ash-forming matter in the fuels and, furthermore, the changes in the ash-forming matter as they were released from the fuel matrix in the ashing process. (19) Zevenhoven, M.; Yrjas, P.; Skrifvars, B.-J.; Backman, R.; Hupa, M. Characterization of Ash Forming Matter in Biomass Fuels for Power Production. Unpublished work. (20) Benson, S.; Holm, P. L. Comparison of Inorganic Constituents in Three Low-Rank Coals. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 145-149. (21) Skrifvars, B.-J.; Blomquist, J.-P.; Hupa, M.; Backman, R. Predicting the Ash Behavior during Biomass Combustion in FBC Conditions by Combining Advanced Fuel Analyses with Thermodynamic Multicomponent Equilibrium Calculations. Presented at the 15th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 1998. (22) Zevenhoven, M. Ash Forming Matter in Biomass Fuels, Report 01-03, Dr. Thesis, Åbo Akademi University, Åbo/Turku, Finland, 2001. (23) Zevenhoven, M.; Skrifvars, B.-J.; Yrjas, P.; Hupa, M.; Nuutinen, L.; Laitinen, R. Searching for Improved Characterization of Ash Forming Matter in Biomass. Proceedings of the 16th International Conference on Fluidized Bed Combustion, May 2001, Reno, NV. (CDROM, ISBN 0791835235.) (24) Fink, S. The Micro Morphological Distribution of Bound Calcium in Needles of Norway Spruce [Picea abies (L.) Karst.]. New Phytol. 1991, 119, 33-40.
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Table 1. Analyses of Rice Husk, Rice Straw, and Eucalyptus Bark fuel
moisture (wt %, ar)
ash (wt %, db)
C
H
rice husk rice straw eucalyptus bark
9.4 10.0 53.9
19.7 15.0 13.0
40.1 40.3 44.4
4.7 5.1 4.8
Composition (wt %) N Cl 0.5 0.5 0.3
0.06 0.61 0.66
S
O
lower heating value, LHV (MJ/kg, db)
0.05 0.09 0.04
54.5 45.1 43.0
15.5 14.7 14.8
Table 2. Elemental Analyses of Laboratory Ash of Rice Husk, Rice Straw, and Eucalyptus Barka Composition (wt % Oxide in Ash) fuel
SiO2
Al2O3
Fe2O3
TiO2
MnO
CaO
MgO
P2O5
Na2O
K2O
sum
rice husk 1 rice husk 2 rice straw eucalyptus bark
95.9 95.4 69.9 0.1
0.1 0.1 0.3 0.2
0.1 0.05 0.2 0.3
0.005 0.002 0.01 1600
1370 1575 >1600
>1600 >1600 >1600
1074 1369 1417
1200 1250 1275
Table 5. Alkali Index and MFF Index for Rice Husk, Rice Straw, and Eucalyptus Bark Samples, as Well as for Some Other Husks and Hulls (Two Samples), Straws (Four Samples), and Scandinavian Barks (Seven Samples) Alkali Index
MFF Index
fuel
value (kg/GJ)
risk of fouling/slagging
value (%)
risk of fouling/slagging
rice husk, sample 1 rice husk, sample 2 husks and hulls, average rice straw straw, average eucalyptus bark, sample 1 eucalyptus bark, sample 2 Scandanavian bark, average
3.70 3.54 7.43 1.48 8.71 1.32 0.53 2.06
certain certain certain certain certain certain certain certain
2 2 52 17 34 41 42 30
low low high medium medium high high medium
not directly correlate with the ash behavior in a fullscale combustor. This is due to many reasons, for example, the fact that inorganic mixtures such as fuel ashes do not melt at a single temperature. An ash may have a temperature range of several hundreds of degrees Celsius in which it is both in a molten state and a solid state. Another way to estimate ash behavior is to calculate indices based on various analysis data of the fuel. One example is the alkali index, where the total amount of sodium and potassium in the fuel, expressed as their corresponding oxides, is divided by the heat value of the fuel. This alkali index was originally intended for prediction of the fouling tendencies of different coals,27 but has also been suggested for biomass fuels and biomass mixtures.28 Based on experience and field tests, Miles et al.28 suggested that the fouling and slagging risk increases if this index reaches values between 0.17 kg/GJ and 0.34 kg/GJ. Above 0.34 kg/GJ, a fuel or fuel mixture is categorized to be virtually certain to slag or foul to an unmanageable degree. Table 5 (column 2 and 3) presents the calculated alkali index values for rice husk and the other fuels studied here, as well as for some other fuels. As indicated, all alkali index values exceed the critical value of 0.34 kg/GJ, indicating that all the fuels presented would be seriously fouling or slagging. Table 5 also shows another index (columns 4 and 5). This index, which is called the “MFF index” (Multi Fuel Fouling index), was developed in the beginning of the 1980s for the co-firing of bark with oil, gas, and coal or peat.29 The index focuses on sulfate deposits and total ash amount and is calculated as the percentage of water-soluble alkali and alkaline-earth metals (given as oxides) in the total ash of the fuels. Values of 40% high fouling intensity.29 As can be seen, according to the MFF index, rice husk would be a low fouling fuel, whereas all straws would be medium fouling fuels. Also, Scandinavian bark was predicted to be a medium fouling fuel, whereas eucalyptus bark and the other hulls would be high fouling fuels. Indexes of this type clearly can only give rough indications of the fouling or slagging potentials. Rice husk seems to be a very unusual fuel also from the standpoint of the indices. The two indices resulted in conflicting predictions of the fouling or slagging potential. The alkali index only focuses on the total amount of alkalis, which are known to form low melting compounds of various types, which may cause fouling problems. However, ash components other than the alkalis also may influence on the fouling behavior of an ash, which is partially taken into account in the MFF index, because the soluble alkali and alkaline-earth metal contents are related to the total ash content. In rice husk, the alkali content is high, which results in a high fouling potential, according to the alkali index. However, the extremely high silica content of the rice husk dilutes the alkali and alkaline-earth metal content, which accordingly resulted in a low value for the fouling potential, when calculated with the MFF index. 5. Conclusions Rice husk is a very different fuel, compared to rice straw and eucalyptus bark. The rice husk is characterized by its edged shell-like outer cover, which remains almost intact during combustion. The silica completely dominates the ash composition, and very little alkali silicates seem to be formed at temperatures up to 900 °C. Rice straw showed a different behavior than rice husk when heated. Rice straw ash was found to contain potassium- and silicon-rich ash particles when heated to 900 °C. At 600 °C, chlorine also was detected, most likely as potassium chloride (KCl). Eucalyptus bark ash contained mainly calcium. In the untreated fuel, the calcium was found in the form of
Fouling of Rice Husk Ash in FBC. 1
10-20 µm cubic crystals, most likely calcium oxalate. As the bark was heat-treated, the crystals were released from the organic matrix and formed particles 10-20 µm in size, most likely calcium carbonate. As the temperature was further raised to 900 °C, the particles became porous, most likely because of the decomposition of calcium carbonate to calcium oxide. Another ash fraction also was found: a small fraction with particles on the order of a few micrometers in size. These particles consisted of potassium, calcium, phosphorus, and chlorine. The high value of the melting temperatures indicated that none of the tested fuels would have a problematic ash, from the standpoint of fouling or slagging. On the other hand, because of the high contents of alkali metal, the alkali index indicated that all fuels would have a
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seriously fouling ash. However, because of the high total content of ash, the MFF index graded rice husk as low fouling, despite its high content of alkalis. For the other fuels, the MFF index predicted moderate or severe fouling potentials. In the second part of this work, we will report pilotscale and full-scale measurement results, which clearly show that rice husk indeed is a nonfouling fuel, despite its high potassium content. Acknowledgment. The final publishing of this work was made possible by the support of the Academy of Finland to the Åbo Akademi Process Chemistry Centre in the National Centers of Excellency Program. EF049714B
