Characterization of Waste Tire Incineration in a Prototype Vortexing Fluidized Bed Combustor

Transcription

1 TECHNICAL PAPER ISSN J. Air Teng, & Waste Chyang, Manage. Assoc. Shang, 47: and Ho Copyright 1997 Air & Waste Management Association Characterization of Waste Tire Incineration in a Prototype Vortexing Fluidized Bed Combustor Hsisheng Teng, Chien-Song Chyang, Sheng-Hui Shang, and Jui-An Ho Department of Chemical Engineering, Chung Yuan Christian University, Chung-Li, Taiwan, R.O.C. ABSTRACT To investigate the characteristics of incinerating waste tires in a prototype vortexing fluidized bed combustor, performance tests were conducted with two sizes of waste tire fragments. The results from the combustion experiments showed that increasing the tire particle size caused less of the volatiles to be burned in the freeboard and thus lowered freeboard temperature. Uniform bed temperature could also be achieved by increasing the size of the tire particles. Variations in the secondary and tertiary air rates simultaneously affected the swirling intensity and the axial gas velocity in the freeboard, and thus resulted in the variations in ash elutriation, combustion efficiency, and pollutant emissions from the combustion system. INTRODUCTION Scrap tires with a high sulfur content present a solid waste disposal problem. If not disposed of properly, waste tires provide a refuge for disease-carrying creatures such as rats and mosquitoes, create fire hazards, and limit the uses of closed landfill sites. Several technologies have been proposed to solve this problem. 1,2 Due to their high heating value, waste tires have become an increasingly attractive fuel option. Incineration and heat recovery not only solve the disposal problem, but also produce additional energy. Fluidized bed combustion shows promise as a cheaper, more compact, and cleaner mode of coal combustion. 3-8 The combustion systems offer the advantages of low NO x emissions, an in situ SO x reaction with lime, and a high combustion efficiency. 3-8 All of these are also expected in burning waste material. Combustion of waste tires in an 18-inch, pilot-scale fluidized bed combustor (FBC) has been tested by Chyang and Chen. 9 The results showed that the fluidized bed combustor was an efficient system for waste tire treatment. The Johnson Boiler Company has designed and manufactured a 3-MW vertical shell boiler prototype that has successfully burned shredded rubber. 10 The Ebara Corporation has developed a new internally-circulating, fluidized-bed boiler to handle fuels containing noncombustible materials, such as waste tires containing steel belts (tire reinforcement). The steel belts are rolled into balls by internal circulation in the bed before being discharged. 11 A novel combustion technology using a vortexing fluidized bed combustor (VFBC), an integrated fluidized bed with a cyclone, was developed and demonstrated early in the 1970s. 12,13 A vortex-generating system was developed by injecting a secondary air stream tangentially into the freeboard. The first VFBC pilot plant was constructed and operated by Korenberg. 14 Korenberg used a series of secondary air injection nozzles vertically along the freeboard. In this combustor, most of the air was introduced tangentially and evenly distributed in the freeboard. The VFBC, named and modified by Nieh and Yang, 15 resulted in an increase in the combustion intensity, the calcium utilization, and the turndown capability. Recently, a two-stage, swirl-flow fluidized bed combustor was also developed by Lee et al. 16 The purpose of this study was to determine the characteristics of a prototype VFBC system for waste tire incineration. The effects of tire particle size and secondary air flow rate on temperature distribution, combustion efficiency, and pollutant emissions of a VFBC are extensively discussed. IMPLICATIONS Landfilling used tires generated each year worldwide is increasingly becoming an unacceptable solution. A better solution from an environmental standpoint is incineration, which is a simple and convenient method to convert tires into steam energy. Characterization of the process of burning tires in a vortexing fluidized bed combustor (VFBC) reveals that through operational adjustments, a high combustion efficiency and low pollutant emissions can be achieved. This research demonstrates the environmental viability of a VFBC to dispose of waste tires. EXPERIMENTAL APPROACH Description of the Combustor Figure 1 shows the configuration of the FBC tested in this study. This system was designed to incinerate waste tires at 120 kg/h. The combustor assembly was fabricated with refractory-lined steel to limit heat loss. In this study the total heat loss to the surroundings was negligible (<5%). The combustor was divided into four sections: the windbox, the distributor, the combustion chamber, and the freeboard. The windbox was built with two independent parts. The volume of one part was two times that of the other. The primary air Volume 47 January 1997 Journal of the Air & Waste Management Association 49

2 a waste-heat boiler via an economizer, and then passed into an air preheater. Finally, the flue gas passed to a venturi scrubber and then to the stack. The flue gases were sampled before they passed through the venturi scrubber. The samples were dehumidified before passing through gas analyzers to determine the concentrations of O 2, CO, SO 2, and NO x. The values of the pollutant concentrations reported in this study were all corrected to 10% residual oxygen on a dry basis. Since the fluidized bed system used in this study was large and complicated, even under the same reported operating conditions, the discrepancy in the absolute values of the data from different sequences of experiments was hardly avoidable. However, this study focused on the trends in the data under operating conditions, rather than the absolute values. The reproducibility of the trends has proven to be good. The temperature in the FBC was measured with thermocouple probes. The thermocouple locations are indicated in Figure 1 and marked with a plus (+) sign. All of the thermocouple probes were located near the wall of the combustor. Figure 1. The configuration of the fluidized bed combustor. Thermocouple locations are indicated by +. supply to each part was individually controlled. The distributor plate was water-cooled and had a slanted design. The design objective of the windbox and distributor was to cause a rotational flow of the bed material within the fluidized bed. The combustion chamber was 0.7 by 1.4 m at its base and 2 m in height. The chamber could be divided into high- and low-velocity zones. Above the smaller part of the windbox was the high-velocity zone. The freeboard section was approximately 1 m in diameter and 4 m in height. The tangential secondary air injection nozzles were placed at two levels on the freeboard. Each level had four equally spaced air nozzles. To distinguish between these two levels, the lower one is the secondary and the upper one is the tertiary, as shown in Figure 1. Both of the air streams were preheated in the freeboard before injection. The feeding system in this study employed over-bed feed of both shredded tires and limestone. The combined streams of tire particles and limestone fell directly through an airlock and a cooled chute into the fluidized bed chamber. The feeding point was located on the side of the low-velocity zone. Flue gas leaving the combustor entered a cyclone for primary cleaning. Ash, unburned char, and limestone particles dropped from the cyclone into a sealed vessel for removal and analysis. After it exited the cyclone, the flue gas entered Fuel and Bed Material Waste tires were the fuel used in the combustor. Two sizes of scrap tire fragments were used in this study: mm and mm. The proximate and ultimate analyses of the waste tires are shown in Table 1. Silica sand was employed as bed material. The mean size of the sand particles was 520 µm in diameter. The high sulfur content of waste tires can cause high sulfur oxide emissions during combustion. Limestone was employed to reduce the sulfur emissions in this study. The mean size of the limestone particles used was 710 µm in diameter. The ratio of calcium to sulfur was fixed at three for each experiment. RESULTS AND DISCUSSION Combustion Phenomena Bed Temperature. In considering bed temperature, it is important to differentiate between combustion in the bed and in the freeboard. The characteristics of the bed temperature for burning mm tire fragments are shown in Figure 2. Figure 2 reveals that the temperature profile in the freeboard showed heavy fluctuation. It was also at a higher temperature than in the bed. Some reasons for this could be that a large amount of volatiles was burned in the freeboard, since the volatile content of tires is as high as 65% by weight; or, Table 1. The compositions of the waste tires. Proximate analysis (%) Ultimate analysis (%) Fixed carbon 29.6 Carbon 86.1 Volatile 65.4 Hydrogen 7.1 Moisture 1.45 Sulfur 1.6 Ash 3.55 Oxygen 3.3 Other Journal of the Air & Waste Management Association Volume 47 January 1997

3 due to vortexing resulting from the secondary and tertiary air stream injections, there was an increase in the residence time of volatile and unburned carbon in the freeboard for burning; or, also due to vortexing, vigorous turbulence caused temperature fluctuation in the freeboard. The maximum bed temperature was just below the feeding point (C-1). This may be because the combustion rate was higher than the mixing rate. The minimum bed temperature was found at the C-5 location. Reasons for this could be that the axial mixing rate was faster than the lateral mixing rate, and there was a heat recovery tube immersed on the C-5 side of the chamber. Figure 3 shows the characteristics of the bed temperature for the combustion of mm tire particles. The Figure 4. Effect of secondary air flow rate on freeboard temperature for mm tire particles. Feed rate: 66.0 kg/h; primary air: 12.0 Nm 3 /min; tertiary air: 1.3 Nm 3 /min. ( ) Excess air ratio = 0.53; ( ) excess air ratio = 0.58; ( ) excess air ratio = 0.62; ( ) excess air ratio = 0.67; ( ) excess air ratio = Figure 2. Temperature of the bed during steady state combustion for mm tire particles. Feed rate: 66.0 kg/h; primary air: 12.0 Nm 3 /min; excess air ratio: 67%. Figure 5. Effect of secondary air flow rate on freeboard temperature for mm tire particles. Feed rate: 63.8 kg/h; primary air: 12.0 Nm 3 /min; tertiary air: 1.3 Nm 3 /min. ( ) Excess air ratio = 0.58; ( ) excess air ratio = 0.63; ( ) excess air ratio = 0.68; ( ) excess air ratio = 0.73; ( ) excess air ratio = Figure 3. Temperature of the bed during steady state combustion for mm tire particles. Feed rate: 63.8 kg/h; primary air: 12.0 Nm 3 /min; excess air ratio: 73%. temperature profile in the combustion chamber was similar to that in Figure 2, but the temperature in the freeboard was lower than that in the bed. This may have occurred because a longer devolatilization time was required for larger tire particles used in fluidized bed combustion. Thus, it caused less volatiles to be burned in the freeboard. Also, because of the lower combustion rate for the larger tire particles, the lateral temperature differences (180 C between C-1 and C-5 and 30 C between C-2 and C-6) were less than those for the smaller tire particles (280 C between C-1 and C-5 and 80 C between C-2 and C-6). Figures 2 and 3 indicate that a variation in fuel size caused a different volatile-release rate and a different volatile-combustion region, and influenced the distribution of the bed Volume 47 January 1997 Journal of the Air & Waste Management Association 51

4 temperature. The results showed that uniform bed temperature could be achieved by increasing the size of the particles used as fuel. However, the figures also demonstrate that the increase in fuel particle size may have resulted in increased bed temperature fluctuation over time. This was probably caused by increased burning of volatiles in the combustion chamber for the larger tire particles, and because the flame generated from volatile combustion could easily be disturbed by the turbulence of the air flow in the chamber. Figures 4 and 5 show the effect of the secondary air flow rate on the temperature profile in the freeboard for burning mm and mm tire fragments, respectively. In this experiment, primary and tertiary air flow rates remained constant, and thus the excess air ratio increased with the secondary air flow rate. One can see that the greater the distance from the distributor, the lower the temperature became. This was because of cooling by the secondary and tertiary air stream preheaters along the freeboard. Also, in most cases the temperature in the freeboard increased with the excess air ratio. The reason for this is that as the excess air increased, the intensity of the vortexing flow became more forceful, causing longer residence times for volatiles and unburned carbon. Since the residence time was extended, the unburned carbon and volatiles were burned more completely. In Figure 4, the lower temperature for the excess air ratio of 0.58, rather than 0.53, was one exception; this was probably due to the cooling effect of the increased excess air. Figures 6 and 7 display the effect of the tertiary air flow rate on the temperature profile in the freeboard for burning mm and mm tire particles, respectively. In this experiment, primary and secondary air streams remained constant, and thus the excess air ratio increased with the tertiary air flow rate. A perusal of Figure 6 reveals that when the excess air ratio was lower (0.64 and 0.67), the temperature between the tertiary air injection and the top of the combustor tended to decrease with the distance from the expansion crosssection. As the excess air further increased to intensify the vortexing above the tertiary air injection zone, more volatiles and unburned carbon were burned to raise the temperature at the higher portion of the freeboard. The tendency for mm tire fragments, shown in Figure 7, is similar to that shown in Figure 6. Figure 6. Effect of tertiary air flow rate on freeboard temperature for mm tire particles. Feed rate: 66.0 kg/h; primary air: 12.0 Nm 3 / min; secondary air: 4.0 Nm 3 /min. ( ) Excess air ratio = 0.64; ( ) excess air ratio = 0.67; ( ) excess air ratio = 0.71; ( ) excess air ratio = 0.74; ( ) excess air ratio = Figure 7. Effect of tertiary air flow rate on freeboard temperature for mm tire particles. Feed rate: 63.8 kg/h; primary air: 12.0 Nm 3 / min; secondary air: 4.0 Nm 3 /min. ( ) Excess air ratio = 0.70; ( ) excess air ratio = 0.73; ( ) excess air ratio = 0.75; ( ) excess air ratio = 0.80; ( ) excess air ratio = Figure 8. Effect of secondary air flow rate on particle elutriation for mm tire particles. Feed rate: 63.8 kg/h; primary air: 12.0 Nm 3 / min; tertiary air: 1.3 Nm 3 /min. 52 Journal of the Air & Waste Management Association Volume 47 January 1997

5 The temperature between the secondary and tertiary air injections was also affected by the tertiary air flow rate. As shown in Figure 6, for the mm tire particles, the temperature decreased with the increase in the tertiary air flow rate, when the excess air ratio increased from 0.64 to This was probably due to cooling from the increased tertiary air injection. However, the temperature became an increasing function of the tertiary air flow rate when the excess air ratio was greater than This means that the tertiary air stream was able to flow downwards to burn out more of the unburned char and volatiles. The results in Figure 7 for mm tire particles show similar influences of the tertiary air flow rate. Ash Elutriation. The influences of the secondary air flow rate on ash elutriation are discussed in this section. Although the collection efficiency of the cyclone could be affected by the air flow, the information concerning this aspect was not available in this study. Therefore, in the following discussion, the influence of air flow on the cyclone efficiency is not considered a major factor in determining the results of the ash elutriation experiments. Figure 8 shows the effect of the secondary air flow rate on the weight of fly ash collected by the cyclone. As shown, the weight of fly ash decreased with the excess air to a minimum at a secondary air flow rate of 4.0 Nm 3 /min (equivalent to 0.73 excess air ratio) and then began to increase. The combustion efficiency, which will be discussed in the next section, also varied with the change in the secondary air rate. By assuming that the weight of unburned fuel equaled that of the unburned carbon, the weight of the unburned carbon could be estimated from the combustion efficiency of the operation. This analysis was done, and it was found that the variation in the ash collection with the secondary air flow Figure 9. Effect of secondary air flow rate on particle size distribution for mm tire particles. Feed rate: 63.8 kg/h; primary air: 12.0 Nm 3 /min; tertiary air: 1.3 Nm 3 /min. rate could be explained, to a large extent, by the change in the amount of unburned carbon. The cumulative analysis of fly ash in the cyclone collector is shown in Figure 9. The cyclone collection characteristics with air flow might possibly have affected the particle size distribution of the ash collected by the cyclone. As stated previously, however, such information is not available at present, and this aspect is not taken into account in the following discussion. The fly ash collected includes limestone, CaSO 4 particles from sulfation of the limestone, ash from tire oxidation, bed material, and unburned carbon. The color of the fly ash was a function of the unburned carbon content; the higher the carbon content, the darker the color of the ash. After seiving, it was found that particles of the largest size (>149 µm) were gray, and the color became darker as the particle size decreased. When the excess air ratio increased, the weight fraction of particles larger than 149 µm decreased. The weight fractions of the smaller sizes (<125 µm) generally increased with the excess air ratio to a certain extent, and then began to decrease; the maximum occured at an excess ratio of The change in the weight fractions of the different size particles revealed that the combustion intensity increased with the excess air ratio up to a value of 0.73, and that the unburned carbon particles became smaller, due to the increased combustion intensity. When the excess air ratio was above 0.73, the vortexing flow was overwhelmed by the axial flow, and the weight fraction of smaller particles decreased. Combustion Efficiency. Since the rates of the secondary and tertiary air flow injections influenced the combustion behavior in the freeboard, it is necessary to understand how the combustion efficiency of the fluidized bed is affected by the air injections. For different fluidized bed combustion systems, different formulas have been adopted for calculating the combustion efficiency. In this system the combustion efficiency is defined as the ratio of the consumed oxygen to the theoretical oxygen during the combustion. According to the previous discussion, the more secondary or tertiary air injected tangentially to the freeboard, the longer the residence time of unburned carbon in the freeboard. However, this also increased the axial gas velocity and thus enhanced the elutriation rate. Therefore, the characteristics of combustion efficiency can be explained by competing effects between carbon burning and elutriation. The effects of the secondary and tertiary air flow rates on combustion efficiency are shown in Figures 10 and 11, respectively. Similar tendencies appear concerning the effect of the secondary air flow on both sizes of tire particles and that of the tertiary air flow on the mm tire particles, showing that the combustion efficiency increased with the air rates to a maximum and then began to decrease. The results can be explained by the competing effects described Volume 47 January 1997 Journal of the Air & Waste Management Association 53

6 above. The tertiary air flow rate had little influence on the combustion efficiency for the mm tire particles. This was because the combustion of the large tire particles was almost completed before they entered the tertiary air injection zone, since less of the fuel was burned in the freeboard for larger tire particle combustion. Figure 10. Effect of secondary air flow rate on combustion efficiency. Primary air: 12 Nm 3 /min; tertiary air: 1.3 Nm 3 /min. ( ) mm tire particles, feeding rate: 66.0 kg/h; ( ) mm tire particles, feeding rate: 63.8 kg/hr. Pollutant Emissions SO 2 Emissions. Sulfur is used to cross-link the polymer chains in the process of rubber manufacture. Due to environmental concerns, the emission of sulfur compounds has to be taken into account in assessing waste-tire conversion processes. Burning the waste in contact with limestone at appropriate temperatures causes reactions to occur between the limestone and the SO 2 that is formed, producing CaSO This is important for the widespread commercialization of fluidized bed combustion technology. Figure 12 shows the change in SO 2 emissions and in the secondary air flow rate for two sizes of waste tire particles. For both sizes of particles, the SO 2 emissions increased with the secondary air rate to a maximum and then began to decrease. This was because at lower secondary air rates, an increase in the secondary air did not cause a noticeable swirling effect but a dilution of the SO 2, which retarded SO 2 contact with limestone. As the secondary air velocity further increased Figure 11. Effect of tertiary air flow rate on combustion efficiency. Primary air: 12 Nm 3 /min; secondary air: 4.0 Nm 3 /min. ( ) mm tire particles, feeding rate: 66.0 kg/h; ( ) mm tire particles, feeding Figure 12. Effect of secondary air flow rate on SO 2 emission. Primary air: 12 Nm 3 /min; tertiary air: 1.3 Nm 3 /min. ( ) mm tire particles, feeding rate: 66.0 kg/h; ( ) mm tire particles, feeding 54 Journal of the Air & Waste Management Association Volume 47 January 1997

7 Figure 13. Effect of tertiary air flow rate on SO 2 emissions. Primary air: 12 Nm 3 /min; secondary air: 4.0 Nm 3 /min. ( ) mm tire particles, feeding rate: 66.0 kg/h; ( ) mm tire particles, feeding Figure 14. Effect of secondary air flow rate on NO x emissions. Primary air: 12 Nm 3 /min; tertiary air: 1.3 Nm 3 /min. ( ) mm tire particles, feeding rate: 66.0 kg/h; ( ) mm tire particles; feeding to improve the swirling in the freeboard, the capture of SO 2 could thus be increased. Moreover, a higher velocity promoted a more vigorous grinding effect in the freeboard, leading to smaller limestone particles. Also, one can observe from Figure 12 that at lower secondary air rates, the SO 2 emission for the smaller tire particle combustion was higher than that for the larger particles. This was due to larger amounts of volatiles being burned in the freeboard for the smaller tire particles; the SO 2 formed in the freeboard did not have enough time to be captured by the limestone when the secondary air rate was low. The variation in the SO 2 emission with respect to the tertiary air flow rate is shown in Figure 13. For the mm particles it is likely that a large fraction of SO 2 was released and reacted with limestone within the bed and in the secondary air stream injection zone. Therefore, there was no noticeable effect of tertiary air flow rate on SO 2 emissions for the mm particles. However, since a large amount of SO 2 remained unreacted in the freeboard for the smaller particles, an increase in the tertiary air rate was expected to increase the swirling effect and therefore to enhance the reduction of SO 2. Results in Figure 13 verify this expectation. NO x Emission. Another significant advantage of fluidized bed combustion is a low NO x emission level. The operating Figure 15. Effect of tertiary air flow rate on NO x emissions. Primary air: 12 Nm 3 /min; secondary air: 4.0 Nm 3 /min. ( ) mm tire particles, feeding rate: 66.0 kg/h; ( ) mm tire particles; feeding Volume 47 January 1997 Journal of the Air & Waste Management Association 55

8 temperature in the fluidized bed (~800 C) was too low for thermal NO x formation. 3,4 Therefore, the fuel-no x mechanism accounted for most of the NO x formation in fluidized bed combustion. Figures 14 and 15, respectively, show the effects of the secondary and tertiary air rates on NO x emission for both sizes of waste tire particles. Both figures show that the NO x emissions decreased with the excess air ratio to a minimum and then began to increase. The trends differ from those for SO 2 emission. This difference may be attributed to the different reducing agents for NO x and SO x. The species responsible for NO x reduction in this fluidized bed combustion are CO, 18,19 hydrocarbon free radicals, 20 and unburned carbon The increase in the secondary or tertiary air rates increased mixing between NO x and the reducing agents and lowered the concentrations of the reducing agents, due to the higher extent of oxidation. As a result, the NO x emissions decreased with the excess air ratio to a minimum, and then began to increase as the excess air was further increased. It should be understood 3,4 that in a fluidized bed combustion system, NO x concentration in the freeboard is generally lower than in the combustion chamber, due to the fact that the NO x initially formed in the chamber can later be converted to N 2 by reacting with the reducing species. Generally speaking, the freeboard is the zone responsible for NO x reduction rather than for NO x formation. Since a higher fraction of the volatiles burn in the combustion chamber for the larger particles, it was expected that a higher fraction of fuel-n converted to NO x in the chamber, and therefore the final NO x emission from the larger particle combustion, would be more sensitive to the secondary air rate, which determined the NO x reduction efficiency in the freeboard. Results in Figure 14 reflect this fact. Data in Figure 15 show that the NO x minimums occurred at different tertiary air rates for different particle sizes. The results demonstrate that a higher intensity of vortexing is required for the smaller particles to reach the NO x minimum, although the reason for this is not clear at present. CO Emission. Figures 16 and 17 display the correlation between CO emission and the rates of flow of the secondary and tertiary air streams, respectively. During combustion in the fluidized bed, there was more burning of volatiles and carbon within the bed for the larger tire particles, and the required oxygen supply and residence time for complete combustion in the freeboard was reduced. The CO emission should have become lower as the combustion became more complete. Therefore, Figures 16 and 17 reveal that the CO emissions for the mm tire particles were low and there was no clear effect from the secondary and tertiary air flow rates. However, the results show that the influence of the secondary and tertiary air flow rates on CO emission was significant Figure 16. Effect of secondary air flow rate on CO emissions. Primary air: 12 Nm 3 /min; tertiary air: 1.3 Nm 3 /min. ( ) mm tire particles, feeding rate: 66.0 kg/h; ( ) mm tire particles; feeding Figure 17. Effect of tertiary air flow rate on CO emissions. Primary air: 12 Nm 3 /min; secondary air: 4.0 Nm 3 /min. ( ) mm tire particles, feeding rate: 66.0 kg/h; ( ) mm tire particles; feeding 56 Journal of the Air & Waste Management Association Volume 47 January 1997

9 for the mm tire particles. The increase in the air rates resulted in more oxygen supply and stronger intensity of the swirling flow, and thus increased the combustion rate in the freeboard to reduce CO emissions. CONCLUSIONS Incineration of different sizes of tire particles in a prototype vortexing fluidized bed combustor showed that using larger tire particles as fuel caused less volatiles to burned in the freeboard and thus lowered freeboard temperatures. The results also reveal that uniform bed temperature could be achieved by using larger particles as fuel. The combustion characteristics were significantly influenced by the secondary and tertiary air flow rates. The increases in the flow rates of the secondary and/or tertiary air injections simultaneously enhanced the swirling intensity and the axial gas velocity in the freeboard. Variations in ash elutriation, combustion efficiency, and pollutant emissions from the combustion process, due to the change in the air flow rates, can be attributed to the competition between the changes in the swirling intensity and the axial gas velocity. In general, the secondary air flow rate had a stronger impact on the combustion results than the tertiary air flow rate. From the viewpoints of saving energy and controlling pollution, this study suggests that it is essential to understand the combustion characteristics in a vortexing fluidized bed combustor in order to achieve optimal operation for tire incineration. ACKNOWLEDGMENTS Financial support from the School of Engineering at Chung Yuan Christian University is gratefully acknowledged. 6. Johnsson, J.E. Formation and reduction of nitrogen oxides in fluidized bed combustion, Fuel 1994, 73, Chi, Y.; Basu, P.; Cen, K. A simplified technique for measurement of sorbent reactivity for use in circulating fluidized bed combustors, Fuel 1994, 73, Lyngfelt, A.; Leckner, B. SO 2 capture and N 2 O reduction in a circulating fluidized-bed boiler: Influence of temperature and air staging, Fuel 1993, 72, Chyang, C.S.; Chen, W.H. Combustion of waste-tires in a pilot scale fluidized bed combustor, J. Chung Yuan (Taiwan, R.O.C.) 1989, XVIII, Highley, J. Development of fluidized-bed combustion for industry, 1st International Symposium of Fluid Combustion and Applied Technology; McGraw-Hill: New York, 1984; pp. I/21-I/ Hirose, T.; Kosugi, S.; Hirota, T.; Makiyama, Y.; Shshia, T. Characteristics of the internally circulating fluidized bed boiler, In Proceedings of the 11th Int. Conf. Fluid. Bed Combust.; American Society of Mechanical Engineers: Montreal, Canada, 1991, p Sowards, N.K. Low pollution incineration of solid waste, U.S. Patent assigned to Environmental Products, Inc., Idaho Falls, ID, Sowards, N.K. Low pollution incineration of solid waste, U.S. Patent assigned to Energy Products of Idaho, Coeur d Alene, ID, Korenberg, J. Integrated fluidized bed/cyclone combustion development, In Proceedings of the 4th Int. Conf. on Fluidization; Kunii, D.; Tori, R. Eds.; United Engineering Trustees, Inc.: Kashikojima, Japan, 1983; p Neih, S.; Yang, G. Modeling of solid flow in a fluidized bed with secondary tangential air injectory in the freeboard, American Institute of Chemical Engineers Annual Meeting; American Institute of Chemical Engineers: Miami Beach, FL, 1986; paper 41K. 16. Lee, J.K.; Hu, C.G.; Chun, H.S. Characteristics of a two-stage swirl flow fluidized bed combustor-part II. Combustion characteristics and combustion efficiency, In Proceedings of the1st Asian Conference on Fluidized-Bed & Three-Phase Reactors; Yoshida, K.; Morooka, S., Eds.; Tokyo, Japan, 1988; p Anderson, D.C.; Anderson, P.; Galwey, A.K. Surface textural changes during reaction of CaCO 3 crystals with SO 2 and O 2, Fuel 1995, 74, Chan, L.K.; Sarofim, A.F.; Beer, J.M. Kinetics of the NO-carbon reaction at fluidized bed combustor conditions, Combustion and Flame 1983, 52, Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Nitric oxide reduction by char and carbon monoxide, Fuel 1985, 64, Miller, J.A.; Bowman, C.T. Mechanism and modeling of nitrogen chemistry in combustion, Prog. Energy Comb. Sci. 1989, 15, Teng, H.; Suuberg, E.M.; Calo, J.M. Studies on the reduction of nitric oxide by carbon: The NO-carbon gasification reaction, Energy & Fuels 1992, 6, 398. REFERENCES 1. Teng, H.; Serio, M.A.; Wójtowicz, M.A.; Bassilakis, R; Solomon, P.R. Reprocessing of used tires into activated carbon and other products, Ind. Eng. Chem. Res. 1995, 34, Merchant, A.A.; Petrich, M.A. Pyrolysis of scrap tires and conversion of chars to activated carbon, AIChE J. 1993, 39, Pereira, F.J.; Beér, J.M.; Gibbs, B.; Hedley, A.B. NO x emissions from fluidized-bed coal combustors, 15th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1974; p Sarofim, A.F.; Beér, J.M. Modeling of fluidized bed combustion, 17th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1979; p Kunii, D.; Levenspiel, O. Fluidization Engineering; John Wiley & Sons: New York, 1969; Chapter 1, p. 1. About the Authors Hsisheng Teng, Ph.D., (corresponding author) is an associate professor of Chemical Engineering and Chien-Song Chyang, Ph.D., is a professor of Chemical Engineering at Chung Yuan Christian University, Chung-Li 32023, Taiwan. At the time of this research, Sheng-Hui Shang and Jui-An Ho were graduate research assistants at Chung Yuan Christian University. Dr. Teng can be reached by at hsisheng@cchp01.cc.cycu.edu.tw; and by fax at Volume 47 January 1997 Journal of the Air & Waste Management Association 57