Energy & Fuels 2006, 20, 2737-2742
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Fluidized-Bed Combustion Characteristics of Cedar Pellets by Using an Alternative Bed Material Tadaaki Shimizu,† Jun Han,‡ Sunyong Choi,‡ Laehyun Kim,§ and Heejoon Kim*,‡,§ Department of Chemistry and Chemical Engineering, Niigata UniVersity, Ikarashi, Niigata, 950-218, Japan, Department of Ecological Engineering, Toyohashi UniVersity of Technology, Tempaku cho, Toyohashi, 441-8580, Japan, and Graduate School of Energy & EnVironment, Seoul National UniVersity of Technology, Nowon-Gu, Seoul, 139-743, Korea ReceiVed April 23, 2006. ReVised Manuscript ReceiVed September 20, 2006
The agglomeration of bed material is a serious problem during biomass combustion. In this paper, an alternative bed material (porous alumina) was used to prevent defluidization and to improve efficiency during cedar pellet combustion. During the tests, we observed that porous alumina was more difficult to agglomerate than silica sand, because of its specific properties. The volatile matter from cedar pellets mostly burned in the dense bed when the porous alumina was employed, as porous alumina captured part of the volatile matter released from the fuel pellets. Therefore, the temperature of the alumina bed was higher than that of silica sand under the same experimental conditions (fuel feed rate and air feed rate), and the temperature gradient in the freeboard region using the former was larger than that using the latter. In addition, the temperature distribution in the horizontal direction was notably uniform. When silica sand was used as a bed material, the CO concentration sharply decreased with the height in the freeboard region. In the horizontal direction, the maximum CO concentration was found in the middle of the bed. By contrast, the height of the combustor had a minor influence on the CO concentration when porous alumina was used as the bed material. Our experimental results also showed that NOx emission was not affected by the type of bed material.
Introduction Biomass was one of humanity’s earliest energy sources and now ranks fourth in the world, providing approximately 14% of our global energy needs.1 As the coal and oil resources are depleted, the utilization of biomass has gained importance because it is renewable. In particular, biomass consumes the same amount of CO2 from the atmosphere during growth as that released from combustion. Among the combustion technologies, fluidized-bed combustion (FBC) seems to be the most suitable for converting biomass into energy, because of its inherent advantages of fuel flexibility, low operating temperature, and low pollution emission. Many studies of biomass combustion in fluidized-bed combustors have been reported.2-8
Although fluidized-bed technology has many merits, there are still some shortcomings, such as sintering, deposition, and bed material agglomeration during biomass conversion. Previous bench-scale studies9-11 have shown that the combustion of several typical biomass fuels results in critical agglomeration at the normal FBC temperature. Because the interaction between silica sand (QS) and alkaline materials such as potassium in the ash is known to cause the agglomeration, frequent bed material changes are often used to avoid the accumulation of ash in the bed as a precautionary measure to achieve problemfree operation, although this is not economically sustainable on a long-term basis. The use of silica-free alternative bed materials or the addition of sorbents such as porous alumina, dolomite, limestone, and kaolin has also been proposed.12-19
* To whom correspondence should be addressed. Phone: 81-532-446908. Fax: 81-532-44-6929. E-mail:
[email protected]. † Niigata University. ‡ Toyohashi University of Technology. § Seoul National University of Technology. (1) Li, X. Biomass Gasification in a Circulating Fluidized Bed. The Dissertation of the University of British Columbia, 2002. (2) Werther, J.; Saenger, M.; Hartge, E-U.; Ogada, T.; Siagi, Z. Combustion of Agricultural Residues. Prog. Energy Combust. Sci. 2000, 26, 1-27. (3) Broek, V. d.; Faaij, A.; Van Wijk, A. Biomass Combustion for Power Generation. Biomass Bioenergy 1996, 11, 271-281. (4) Bhattacharya, S. C. State-of-the-Art of Utilizing Residues and Other Types of Biomass as an Energy Source. RERIC Int. Energy J. 1993, 15, 1-21. (5) Kuprianov, V. I. Co-Firing of Sugar Cane Bagasse with Rice Husk in a Conical Fluidized-Bed Combustor. Fuel 2006, 85, 434-442. (6) Armesto, L. Combustion Behavior of Rice Husk in a Bubbling Fluidized Bed. Biomass Bioenergy 2002, 23, 171-179. (7) Fang, M. Experimental Study on Rice Husk Combustion in a Circulating Fluidized Bed. Fuel Process. Technol. 2004, 85, 1273-1282.
(8) Permchart, W. Emission Performance and Combustion Efficiency of a Conical Fluidized-Bed Combustor Firing Various Biomass Fuels. Biores. Technol. 2004, 92, 83-91. (9) Ohman, M.; Nordin, A. The Role of Kaolin in Prevention of Bed Agglomeration during Fluidized Bed Combustion of Biomass Fuels. Energy Fuels 2000, 14, 737-737. (10) Ohman, M.; Nordin, A.; Skrifvars, B.-J.; Backman, R.; Hupa, M. Bed Agglomeration Characteristics during Fluidized Bed Combustion of Biomass Fuels. Energy Fuels 2000, 14, 169-178. (11) Skrifvars, B.-J.; Ohman, M.; Nordin, A.; Hupa, M. Predicting Bed Agglomeration Tendencies for Biomass Fuels Fired in FBC Boilers: A Comparison of Three Different Prediction Methods. Energy Fuels 1999, 13, 359-363. (12) Lin, W.; Jensen, A. D.; Johnsson, J. E.; Dam-Johansen, K. Combustion of Biomass in Fluidized Beds - Problems and Some Solutions Based on Danish Experiences. Proc. 17th Int. Conf. on Fluidized Bed Combustion, New York, 2003. (13) O ¨ hman, M.; Nordin, A.; Lundholm, K.; Bostro¨m, D.; Hedman, H. Ash Trans-Formations during Combustion of Meat-, Bonemeal, and RDF in a (Bench-Scale) Fluidized Bed Combustor. Energy Fuels 2003, 17, 11531159.
10.1021/ef0601723 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/25/2006
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In addition to mitigation of the agglomeration problem, porous alumina is known to have positive effects to improve the burnout of volatile matter. Shimizu and colleagues19-22 successfully employed porous alumina as a bed material instead of conventional silica sand, in order to suppress the emission of unburned gases (CO and hydrocarbons). However, they conducted experiments in laboratory-scale reactors that required external heating to compensate for the heat loss. Combustion behavior in the porous alumina bed in a large reactor that can be operated under autothermal conditions has not yet been investigated. In the current paper, a semi-pilot-scale fluidized-bed combustor without additional heat was employed as a reactor. Porous alumina is used as an alternative bed material to prevent agglomeration and to improve the combustion efficiency in a fluidized-bed combustor during cedar pellet combustion. Silica sand was employed as conventional bed material. The temperature and CO, O2, CO2, and NOx concentration profiles of the two bed materials are compared. Experimental Apparatus A schematic diagram of the fluidized-bed combustor is shown in Figure 1; it was made of stainless steel; its cross section was rectangular, and its length and width were 0.52 and 0.12 m, respectively. The total height is 1.7 m, allowing an air preheater of up to 0.35 m. The air distributor was located at the bottom of the fluidized-bed combustor, which consisted of four wind boxes (0.12 × 0.12 m), and each part had 36 holes (1 mm in diameter). A variable-speed screw feeder was connected to the combustor by a pipe. The pipe was located at a height of 40 cm above the air distributor. The angle between the pipe and the horizontal direction was about 60°. An external cyclone at the exit of the combustor was used to separate particulate matter from the flue gas. The fluidized air was provided by a compressor, and the flow rate was controlled by four mass-flow controllers.
Experimental Procedures Before the experiment, the bed material was fed into the chamber. The chamber was then heated to the ignition temperature of the cedar pellets by two electrical heaters. After the predetermined (14) Bhattacharya, S. P.; Harttig, M. Control of Agglomeration and Defluidization Burning High-Alkali, High-Sulfur Lignites in a Small Fluidized Bed Combustors - Effect of Additive Size and Type, and the Role of Calcium. Energy Fuels 2003, 17, 1014-1021. (15) Man, M. D. Agglomeration Mechanisms and Mitigating Measures in Fluid-Bed Combustion. Circulating Fluidized Bed Technology VII; Canadian Society of Chemical Engineering: Ottawa, Canada, 2002. (16) Tran, K. Q.; Iisa, M. K.; Steenari, B.-M.; Lindqvist, O.; Hagstro¨m, M.; Pettersson, J. B. C. Capture of Alkali Metals by Kaolin. Proc. 17th Int. Conf. on Fluidized Bed Combustion, Jacksonville, FL, 2003. (17) Daavitsainen, J. H. A.; Nuutinen, L. H.; Ollila, H. J.; Tiainen, M. S.; Laitinen, R. S. FB Combustion of Bark and Sawdust in Silica Sand Bed with Dolomite Addition. A Case Study. Proc. 16th Int. Conf. on Fluidized Bed Combustion, Reno, NV, 2001. (18) Daavitsainen, J. H. A.; Nuutinen, L. H.; Ollila, H. J.; Tiainen, M. S.; Laitinen, R. S. The Coating Layers on Silica-Free Bed Particles during FB-Combustion. Proc. 16th Int. Conf. on Fluidized Bed Combustion, Reno, NV, 2001. (19) Shimizu, T.; Nemoto, T.; Tsuboi, H.; Shimoda, T.; Ueno, S. Rice Husk Combustion in a BFBC using Porous Bed Material. Proc. 18th International Conference on Fluidized Bed Combustion, Toronto, Canada, 2005. (20) Tadaaki, S.; Hans-Ju¨rgen, F.; Satoko, H.; Yasuo, T.; Kazuaki, Y.; Masato, T. Reduction of Dioxins Emission from a Bubbling Fluidized Bed Waste Incinerator by Use of Porous Bed Material. J. Jpn. Inst. Energy 2001, 80, 1060-1063. (21) Hans-Ju¨rgen, F.; Tadaaki, S.; Yasuo, T.; Satoko, H.; Marko, S.; Makoto, I.; Masato, T. Reduction of Devolatilization Rate of Fuel during Bubbling Fluidized Bed Combustion by Use of Porous Bed Material. Chem. Eng. Technol. 2001, 24, 725-733. (22) Tadaaki, S.; Hans-Ju¨rgen, F.; Satoko, H.; Yasuo, T.; Kazuaki, Y. Porous Bed Material - An Approach To Reduce Both Unburnt Gas Emission and NOx Emission from a Bubbling Fluidized Bed Waste Incinerator. J. Jpn. Inst. Energy 2001, 80, 333-342.
Figure 1. Schematic diagram of the fluidized-bed combustor. Table 1. Properties of the Cedar Pellets Used in the Tests Ultimate Analysis (wt %, daf) carbon hydrogen oxygen nitrogen sulfur
48.58 6.11 45.04 0.2 0.07 Proximate Analysis (wt %)
moisture ash volatile
10.37 0.74 84.32
heat value (kJ/kg)
19 700
temperature was reached, the heaters were turned off and the fuel was fed into the fluidized-bed combustor. When the temperature in the sand bed and the concentration of CO became stable, the temperature and the O2, CO, CO2, and NOx concentration profiles were recorded. The temperatures within the chamber were recorded by five thermocouples located along the height of the combustor. There were 12 holes at the upper surface of the combustor and five holes at the side, which were used as sampling ports. Four stainlesssteel sampling tubes (1.4 m in length) on the upper surface were used to sample the flue gas from the freeboard. The flue gas compositions (O2, CO, CO2, and NOx) were measured by an analyzer (Horiba-5000) after removing particles and water vapor. Before the experiment, the gas analyzer was calibrated using standard gases. The range of concentrations for CO monitoring was 0-5000 ppm. For O2, CO2, and NOx, the monitoring ranges were 0-25%, 0-20%, and 0-200 ppm, respectively. The pressure drop was also measured with a U-tube manometer. In the tests, cedar pellets were used as fuel, and their properties are summarized in Table 1. The diameter and length of the pellets were about 6 mm and 20 mm, respectively. The properties of the alumina and silica sand are listed in Table 2. The static bed height was previously reported to have a relatively weak influence on the major combustion characteristics (the temperature and gas concentrations) of a fluidized-bed combustor fired with different biomass fuels.5 For this reason, only a 15 cm bed height was investigated in the current experimental study. Table 3 shows the detailed operation parameters of the experiments.
Combustion Characteristics of Cedar Pellets
Energy & Fuels, Vol. 20, No. 6, 2006 2739
Figure 2. Temperature profile along the combustor for different fuels’ combustion. Table 2. Properties of the Sands Used in the Tests
name
specific surface area (m2/g)
average diameter (µm)
bulk density (kg/m3)
true density (kg/m3)
alumina sand silica sand
214 9.2
500 150
780 1223
3200 2600
Table 3. Operational Conditions
case 1 case 2 case 3 case 4
bed material
air rate (l/min)
fuel rate (g/min)
O2 at outlet (%)
silica sand alumina sand alumina sand alumina sand
280 280 240 200
44 44 38 32
6-10 6-10 6-10 6-10
Experimental Results and Discussions The experimental results of Ergudenler and Ghaly23 showed that, when silica sand was used as the bed material, it was not possible to operate the fluidized-bed reactor at (or above) 800 °C because of severe agglomeration. In case 1 of the present work, silica sand was used as a bed material; the operation of cedar pellet combustion had to be stopped after 1 h because of defluidization of the bed material, which was consistent with the conclusions of Ergudenler and Ghaly. This phenomenon can be explained by the interaction between silica sand and fuel ash that forms low-melting-point material. In cases 2-4, porous alumina was used as a substitute for silica. The operation conditions of cases 2-4 are similar except for the fuel and air feed rate; the details of parameters can be found in Table 3. The combustion tests were successfully carried out for several hours at bed temperatures as high as 950 °C without any agglomeration. A comparison of the vertical temperature profiles along the combustor for different fuels is shown in Figure 2. With the exception of cases 2-4, all other cases used silica sand as the bed material. It was found that most temperature profiles slightly increased or decreased along the height of the combustor in the freeboard region. For example, the temperature only decreased from 809 to 805 °C when the height shifted from 0.05 to 0.55 m in case 1 of the present study. Generally, biomass combustion undergoes the following processes:
biomass f devolatilization
{
f gas combustion f char combustion
(23) Ergudenler, A. Ghaly, A. E. Agglomeration of Alumina Sand in a Fluidized Bed Straw Gasifier at Elevated Temperatures. Biores. Technol. 1993, 43, 259-268.
According to Table 1, the volatile matter of cedar pellets is relatively high, about 84%, and is released after the fuel is fed into the chamber. The volatile matter is carried by the fluidizing gas and burned along the combustor. Thus, the temperature in the freeboard is higher than that of the bed, or the variation of temperature along the combustor is small. When using porous alumina, the highest temperature was observed in the dense bed and the temperature declined in the freeboard. The temperature was 900 °C at a height of 0.05 m but decreased to 780 °C at a height of 0.55 m. The reason for this phenomenon is that the porous alumina can capture part of the volatile matter, and most of the combustion takes place in the dense zone. Volatile matter combustion that occurred in the dense zone caused the reduced combustion in the freeboard; thus, the operation temperature decreased sharply. Visual observation revealed that there was an obvious flame in the freeboard region when the bed material was silica sand. In the case of porous alumina, flame was weaker in the freeboard region. Moreover, the temperature of the porous alumina was about 900 °C, while that of the silica sand was only 809 °C, which also confirmed that there was more volatile matter burned in the dense bed in the case of porous alumina than that of silica sand. As seen in Figure 3, the temperature was uniformly distributed in the horizontal direction at different heights, which implied good fluidization during cedar pellet combustion. The local small temperature difference is also one of the reasons that there is not serious agglomeration in the case of porous alumina used as bed material. The variation of temperature profiles from the start to the end of the experiments can be found in Figure 4, which shows that the temperature fluctuation is very limited and the combustion is considerably stable.
Comparison of Flue Gas Emission In order to further understand the difference between QS and porous alumina sand (AS), the CO, O2, CO2, and NOx concentration profiles were also compared (Figures 5-8). In the two runs (cases 1 and 2), the operation parameters were the same, except for the bed material. When we used silica sand as a bed material, the CO concentration at 0.4 m was notably high, at about 5000 ppm, but sharply decreased to 620 ppm when the height was increased to 0.7 m. In this region, CO was rapidly oxidized by OH radicals or O2. The CO concentration then slightly decreased with an increase in the height. The concentra-
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Figure 3. Horizontal temperature profile under the two bed materials.
Figure 4. Temperature changes at different heights.
Figure 5. Comparing horizontal CO concentration profile under the two bed materials.
Figure 6. Comparing horizontal O2 concentration profile under the two bed materials.
tion at 0.7 m was nearly the same as that in 1.1 m. Moreover, an obvious CO concentration gradient was detected in the horizontal direction at 0.4 m. The maximum CO occurred in the middle of the chamber, which is considered to be a result of nonuniform fuel distribution. In the combustor system, the fuel fell into the middle of the bed and then dissipated slowly.
Thus, the cedar pellet concentration near the falling point was higher than that elsewhere. For alumina sand, as mentioned above, most devolatilization and combustion occurred in the dense bed. The CO concentration at 0.4 m was notably low, at about 361 ppm, and had decreased to 72 ppm at the combustor’s exit. The concentrations of CO in the freeboard for the porous
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Energy & Fuels, Vol. 20, No. 6, 2006 2741
Figure 7. Comparing horizontal CO2 concentration profile under the two bed materials.
did not show such a decrease in NOx by the porous alumina bed. An explanation is the difference in the bed temperature between this work and the literature;19 the bed temperature for the porous alumina was higher than that of the silica sand in this work. It is widely accepted that the emissions of NOx tend to increase with increasing temperature. Thus, the reduced NOx emission caused by the present porous alumina is considered to be canceled out by the increase in temperature. Combustion Efficiency
Figure 8. Comparing NOx emission under the two bed materials.
alumina bed were 1 order of magnitude lower than those for the silica sand bed. In the horizontal direction, the peak of CO concentration also appeared in the middle of the chamber. Figure 6 shows the horizontal concentration profile of oxygen at different heights under the two bed materials. An O2 concentration difference was observed between 0.4 and 0.7 m in case 1 (silica sand bed) due to the combustion in the freeboard region, while that in case 2 (porous alumina bed) was not significant. At the same time, a large amount of CO formed in the middle of the bed, where the O2 concentration was also at a minimum. Figure 8 shows that NOx emission had no relationship to the bed materials used. According to the conclusion of Quaak et al.,24 it is impossible to form thermal NOx when the temperature is lower than 950 °C; thus, the emissions of NOx shown in Figure 8 are from fuel-bound nitrogen. The results of rice husk combustion in a laboratory-scale reactor showed that the emissions of NOx from the porous alumina bed were slightly lower than those from the silica sand bed when the emissions were compared at the same temperature.19 The present results
When estimating combustion efficiency, the heat loss owing to unburned carbon contained in the particulate matter is neglected, because carbon in the fly ash is limited. The combustion efficiency for each test run is commonly calculated as follows:25
combustion efficiency )
CO2 [%] CO2 [%] + CO [%]
× 100
Table 4 shows the combustion efficiencies for different fuels in the fluidized-bed combustor. The combustion efficiency for cedar pellets ranged from 99.92% to 99.93% in the case using porous alumina, which was significantly higher than the range for silica sand. The reason for this was the low CO emission, as discussed previously. Conclusions The results of the current study show that porous alumina can capture part of the volatile matter released from biomass and that more volatile matter combustion takes place in the dense
Table 4. Comparison of CO and Combustion Efficiency for Different Fuels fuel
sand
CO (ppm)
combustion efficiency (%)
source
sawdust rice husk bagasse rice husk sawdust olive cake rice husk peach stone apricot stone cedar pellet cedar pellet cedar pellet cedar pellet
silica silica silica silica silica silica silica silica silica silica alumina alumina alumina
350 650 830 372-788 1700-2300 197-929 1085-1808 1374-3092 3207-16036 170-350 30-95 200-300 500-1000
99.82 81.31 99.41 97.46-98.05 99.03-99.7 98.0-98.66 97.0-98.9 96.0-97.5 93.4-96.3 99.50-99.88 99.92-99.93 99.12-99.88 97.22-98.43
Permchart Permchart Permchart Fang Kuprianov Topal Armesto Kaynak et al. Kaynak et al. our experiment (case1) our experiment (case2) our experiment (case3) our experiment (case4)
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zone. Thus, the temperature of the porous alumina bed is higher than that of the silica bed. In the freeboard region, the temperature along the height of the combustor decreases sharply with the height, while the CO concentration profile is only slightly decreased. Moreover, alumina is difficult to react with alkali (24) Quaak, P.; Knoef, H.; Stassen, H. E. Energy From Biomass: A Review of Combustion and Gasification Technologies. World Bank Technical Paper, Energy Series; World Bank: Washington, DC, 1999; pp 82-83. (25) Chiou-Liang, L.; Ming, Y. W.; Shr, D. Y. Effect of Concentration of Bed Materials on Combustion Efficiency during Incineration. Energy 2004, 29, 125-136. (26) Kuprianov, V. I.; Permachart, W.; Janvijitsakul. Fluidized Bed Combustion of Pre-Dried Thai Bagasse. Fuel Process. Technol. 2005, 86, 849-860.
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metals from biomass ash, and the agglomeration of bed material was not found at 950 °C in this work. However, when using silica sand as a bed material, the operation of the fluidized-bed combustor for cedar pellet combustion had to be stopped after 1 h, because of the serious agglomeration and defluidization of bed materials. On the basis of the combustion efficiencies and stabilization, we conclude that alumina sand is more favorable than silica sand for use in a fluidized-bed combustor during biomass combustion. EF0601723 (27) Kaynak, B.; Huseyin, T.; Aysel, T. A. Peach and Apricot Stone Combustion in a Bubbling Fluidized Bed. Fuel Process. Technol. 2005, 86, 1175-1193.
