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Energy & Fuels 2006, 20, 114-119
An Experimental Study of Temperature of Burning Coal Particle in Fluidized Bed Mirko Komatina,† Vasilije Manovic,*,†,‡ and Dragoljub Dakic§ Faculty of Mechanical Engineering, UniVersity of Belgrade, 27 marta 80, 11000 Belgrade, Serbia and Montenegro, UniVersity of Belgrade, Djusina 7, 11000 Belgrade, Serbia and Montenegro, and Institute for Nuclear Sciences, Vinca, P.O. Box 522, 11001 Belgrade, Serbia and Montenegro ReceiVed July 20, 2005. ReVised Manuscript ReceiVed September 28, 2005
The purpose of this study was to investigate the temperature of coal particle during combustion in fluidized bed (FB). It is necessary to know the coal particle temperature in order to predict kinetics of chemical reactions within and at the surface of coal particle, accurate NOx and SO2 emission, fragmentation, attrition, the possibility of ash melting, etc. The experimental investigations were conducted in order to obtain the reliable data on the temperature of particle burning in the FB. A method using thermocouple was developed and applied for measurements. Thermocouple was inserted in the center of the particle shaped into spherical form with various diameters: 5, 7, 8, and 10 mm. Two characteristic types of low-rank Serbian coals were investigated. Experiments were done at the FB temperature in the range of 590-710 °C. Two types of experiments were performed: (i) combustion using air as fluidization gas and (ii) devolatilization with N2 followed by combustion of obtained char in air. The temperature histories of particles during all stages after introducing in the FB were analyzed. Temperature difference between the burning particle and the FB was defined as a criterion, for comparison. It was shown that the temperature profile depends on the type of the coal and the particle size. The higher temperature difference between the burning particle and the FB was obtained for smaller particles and for lignite (130-180 °C) in comparison to the brown coal (70-130 °C). The obtained results indicated that a primary role in the temperature history of coal particle have the mass and heat transfer through combusting particle.
Introduction Fluidized bed (FB) combustion is one of the advanced technologies for coal combustion. It has been developed as a promising technology, which can ensure in situ emission control of gaseous pollutants (NOx and SO2) and fuel flexibility. FB has an inert fluidizing medium, which contains only a few percent by weight of coal enabling each coal particle to burn surrounded by the moving inert particles. Understanding the behavior of the coal particle in the hot FB is the basis for investigation of the processes occurring in the FB combustor.1,2 Temperature history of single coal particle in the hot FB determines the processes and phenomena like devolatilization and char combustion, fragmentation and attrition, sulfur selfretention by coal ash, and ash agglomeration. Combustion of coal particle in the FB is a complex process, which, from the aspect of coal particle temperature, may be studied in three main stages: drying, devolatilization, and char combustion. After entering the coal particle into the combustion device, the rapid release of moisture (drying) and volatile * To whom correspodence should be addressed. Fax: +381-11-3235 539. E-mail:
[email protected]. † Faculty of Mechanical Engineering, University of Belgrade. ‡ University of Belgrade. § Institute for Nuclear Sciences. (1) Grubor, B.; Manovic, V.; Oka, S. Chem. Eng. J. 2003, 96, 157169. (2) Oka, S.; Ilic, M.; Dakic, D.; Grubor, B.; Komatina, M.; Barisic, V.; Arsic, B.; Manovic, V. Single Coal Particle Behavior in Fluidized Bed Combustion: Recent Results Achieved in Vinca Institute. Proceedings of the ASME-ZSITS International Thermal Science Seminar, June 11-14 (Bled, Slovenia) 2000, 83-92.
substances (devolatilization) into the surroundings occurs as a result of high temperature of the surrounding medium. Temperature of the coal particle as well as heat and mass transfer during these processes are different from those during combustion of the char particle. Heat transferred from the FB media to a coal particle is consumed by heating and drying of the particle and endothermic reactions of forming volatiles. Exothermic reaction of char combustion causes increase of the particle temperature and heat transfer from particle to surrounding medium.3,4 Measurement of coal particle temperature in the hot FB can be conducted by different methods: photographic method,5 fusible wire ring,6 thermocouple technique,3,7-10 and optical probe.11,12 The experimental investigations were performed with coal or char particles. It was noticed that photographic and (3) Komatina, M. Temperature of Coal Particle during Combustion in Fluidized Bed. Ph.D. Thesis, University of Belgrade, Faculty of Mechanical Engineering, Belgrade, Serbia and Montenegro, 1997. (4) Komatina, M.; Oka, S.; Grubor, B.; Voronjec, D. Experimental Investigation of Heat Transfer a Bubbling Fluidized Bed and Large Particle, Proceedings of the 10th International Heat Transfer Conference. (Brighton, England) 1994, 215-220. (5) Ross, I. B.; Patel, M. S.; Davidson. J. F. Trans. Inst.Chem. Eng. 1981, 59, 83-88. (6) Yates, G.; Walker, R. Partical Temperature in Fluidized Bed Combustor, Fluidization; Cambridge University Press: 1978; pp 241-245. (7) Stubington, J. F. Chem. Eng. Res. Des. 1985, 63, 241-249. (8) Prins, W. Fluidized Bed Combustion of a Single Carbon Particle. Ph.D. Thesis, University of Twente, Enschede, The Netherlands, 1987. (9) Winter, F.Single Fuel Particle and NOx/N2O Emission Characteristics under Circulating Fluidized Bed Combustor Conditions. Ph.D. Thesis, Vienna University of Technology, Vienna, Austria, 1995. (10) Winter, F.; Prah, M. E.; Hofbauer, H. Combust. Flame 1997, 108, 302-314.
10.1021/ef050222o CCC: $33.50 © 2006 American Chemical Society Published on Web 11/25/2005
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Table 1. Proximate and Ultimate Analysis of the Investigated Coals proximate analysis (%)
ultimate analysis (%, daf)
coal
moisture
ash
VM
Cfix
calcd value (MJ/kg)
C
H
N
S
O
porosity (%)
lignite (Kosovo) brown coal (Bogovina)
25.16 13.41
21.73 8.61
30.71 32.31
21.69 41.49
11.17 19.81
66.56 70.39
5.35 4.64
1.01 1.02
1.47 5.17
25.61 18.78
48.2 31.6
optical probe methods were more suitable for temperature measurements of small coal particles. Unfortunately, the measurement of temperature history of individual large coal particle was not possible with these methods. A major obstacle in measurement by optical probe method is how to determine the fraction of receiving radiation from the burning particle to optical probe. By photographic method, it is possible to photograph particles on the bed surface, where the heat and mass properties are different from those within bed. Fusible wire ring methods are not suitable for the coal particle temperature measurements, since properties of shaped coal particles are different from those of parent coal and it is uncertain whether the particle temperature is above or below the point of fusible rings. The thermocouple techniques are the most commonly used methods. Temperature of coal particle can be measured continuously by these techniques during almost a whole period after introducing into a hot FB. The thermocouple method imposes some restrictions in relation to particle motion in the FB, and the temperature of small coal particles (dc < 3 mm) cannot be measured. The obtained results were different, depending on the kind of experimental conditions and applied measurement methods. The maximal temperature difference between the burning particles and the FB was in the range of 20-400 °C. The size of coal particles was not exactly known at the moment when a temperature was measured, and usually it was assumed that coal particles retained its initial size. The temperature difference between burning particle and surrounding media in the first place depends on coal and char characteristics, which determine diffusion transport and thermal conductivity through combusting particle, and concentration of oxygen in the surrounding gas.13-15 The aim of this paper was to present the results of investigation of single coal particle temperature during combustion of two Serbian coals in the FB at low to medium temperatures. In this work, the embedded thermocouple in the center of the coal particle was applied as measurement method. This method is widely accepted for coarse particles temperature measurement. Experimental Section In the experiments, two Serbian coals were used, Kosovo lignite rank coal and Bogovina brown rank coal. The samples were characterized by proximate and ultimate analyses (Table 1), as well as chemical analyses of ash (Table 2). The experiments of combustion were performed in an experimental FB reactor. The schematic presentation of the experimental apparatus is shown in Figure 1. The FB was heated by a 2.0-kW electrical heater, regulated by the variable transformer. The ceramic FB furnace had an inside diameter of 80 mm, and the static bed height 90 mm was used in this experiments. The electrical heater and a spiral preheater were insulated by glass fibers. The bed (11) Macek, A.; Bulik, C. Direct Measurement of Char-Particle Temperatures in Fluidized-Bed Combustors, Proceedings of the 20th Symposium (International) on Combustion 1984, 1223-1230. (12) Linjewile, T.; Hull, A.; Agarwal, P. Fuel 1994, 73, 1880-1888. (13) Chan, C.; Kojima, T. Fuel Process. Technol. 1996, 47, 215-232. (14) Zajdlik, R.; Jelemensky, L.; Remairova, B.; Markos, J. Chem. Eng. Sci. 2001, 56, 1355-1361. (15) Remairova, B.; Markos, J.; Zajdlik, R.; Jelemensky, L. Fuel Process. Technol. 2004, 85, 303-321.
Table 2. Chemical Analyses of Coal Ash coal ash
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O
lignite 27.34 (Kosovo) brown coal 8.56 (Bogovina)
SO3
8.51
8.20
32.89 6.42
1.40
0.79 11.02
5.78
3.37
39.95 8.43
1.23
0.65 27.40
temperature was measured by a thermocouple (Cr-Ni), connected to a digital thermometer and an acquisition system. The fluidizing gas (air or nitrogen) was supplied into the reactor using an axial fan through four nozzles symmetrically placed in the ceramic tube. The gas flow rate was measured by a previously calibrated standard measuring orifice. Before entering the FB, gas was preheated. Inert material was silica sand with the mean particle diameter of dp ) 0.25 mm. Fluidization velocity was in the range Vf ) 0.18-0.22 m/s (Vmf ) 0.063-0.078 m/s). Fluidization velocities were chosen so that maximum particle heat transfer coefficient was obtained at specific temperature.4,16 During the experiments, the coal particle temperature, the FB temperature, and concentrations of gases CO, CO2, SO2, and O2 were continuously measured in all stages of the combustion process and collected by data acquisition system.
Figure 1. Experimental FB reactor: 1, fan; 2, N2; 3, tubes; 4, coal particle; 5, FB; 6, thermal insulation; 7, electrical heater; 8, thermocouple in coal particle center; 9, FB reactor; 10, thermocouple in FB; 11, data acquisition system; 12, gas analyzer; 13, digital thermometer.
The coal particles were shaped into spherical form in order to eliminate the uncertainties associated with the difference in shape, such as the area of heat and mass transfer. Investigations were performed with bed temperatures between Tbed ) 590 and 710 °C. The experiments at higher, usual FB temperatures were difficult because the coal showed a high degree of fragmentation, attrition, and cracking. This dependency was noticed in previously published literature data17-19 and in our previous investigations.20 The diameters of shaped coal particles were: dc ) 5, 7, 8, and 10 mm. Experimental conditions were designed in such a manner that coal particle diameter, coal type, and type and size of inert material were in the range usually used in FB combustion. (16) Komatina, M. Heat and Mass Transfer in Fluidized Bed Combustion of a Single Coal Particle. M. Sc. Thesis, University of Belgrade, Faculty of Mechanical Engineering, Belgrade, Serbia and Montenegro, 1992.
116 Energy & Fuels, Vol. 20, No. 1, 2006
Figure 2. Temperatures of FB during both types of experiments with lignite (dc ) 8 mm) at Tbed ) 650 °C.
The temperature was measured in the coal particle center with a Cr-Ni thermocouple embedded at either 0.5 or 1.0 mm. The coal particle was drilled before the thermocouple was set in. The thermocouple was fastened to coal particle with ceramic glue. The coal particles with thermocouples were simultaneously introduced into the hot FB with several spherical coal particles without thermocouples, at the moment when the desired FB temperature was reached. Total mass of particles introduced in the FB in all experiments was 2.0 g, enabling similar experimental conditions, in the first place gas concentrations and temperatures. It should be mentioned that in this type of experiments (batch) conditions around single particle vary with time and significantly depend on the mass of the sample. The two groups of experimental investigations with coal were done. In the first group of measurements combustion of coal particles in oxidizing (air) atmosphere was conducted. The second group of the investigations was performed in order to find the influence of inert atmosphere at the beginning of process. In the hot bed, fluidized by nitrogen, after entering the coal particle was intensively heated and approached the bed temperature. It was assumed that devolatilization was completed when the particle temperature became equal the bed temperature.21 When the devolatilization process in inert atmosphere was completed, the nitrogen as fluidizing gas was replaced with the air and the char particle had started to burn (N2 and air). In this manner, the drying and devolatilization processes while the char was obtained were performed in inert atmosphere. This approach enabled to gather data on the similarity and differences in the coal particles behavior as a function of the type of atmosphere at the beginning of the combustion process.
Results and Discussion The temperature of coal particle after entering in the hot FB was investigated. The batch combustion tests were carried out, and the temperature in the center of coal particles, bed temperature, and gas concentrations (O2, CO, CO2, SO2) were measured, controlled, and stored using data acquisition system. The aim was to illustrate the real surrounding conditions of coal particle after its introduction into the hot FB. As an example of the obtained results the change of measured FB temperature during experiment with lignite (dc ) 8 mm) is given in Figure 2. The drop of temperature after introducing the batch of coal particles in bed preheated at 650 °C can be (17) Chern, J.-S.; Hayhurst, A. N. Combust. Flame 2004, 139, 208221. (18) Zhang, H.; Cen, K.; Yan, J.; Ni, M. Fuel 2002, 81, 1835-1840. (19) Lee, J. M.; Kim, J. S.; Kim, J. J. Fuel 2003, 82, 1349-1357. (20) Dakic, D.; van den Honing, G.; Valk, M. Fuel 1989, 68, 911-916. (21) Ross, D. P.; Heidenreich, C. A.; Zhang, D. K. Fuel 2000, 79, 873883.
Komatina et al.
observed. Except for heating coal particles, the additional heat was consumed for thermal decomposition of the coal matrix as well as for heating volatiles that were liberated through and from the coal particles. The initial decrease of temperature is about 5 °C, which is a consequence of small mass of coal particles in comparison with the mass of bed, i.e., high heat capacity of bed. After initial decrease, the FB temperature quickly achieves initial value, more rapidly in the case of the air atmosphere as a result of volatile combustion. For the lignite coal particle (dc ) 8 mm) the typical changes of O2, CO2, and CO concentrations for both investigated atmospheres: combustion in air and devolatilization in N2 and subsequent combustion in air are presented in Figure 3. Similarly to the changes of temperature, significant changes in gas concentrations were noticed at the beginning of the experiments. The later changes were generally steady, with occasional changes as a result of fragmentation and cracking, which generate additional external surface of the char particle and therefore easier penetration of oxygen. In all experiments temperatures of several coal particles were measured, with aim to confirm reproductively and repeatability of the obtained results. It was shown that the obtained timetemperature profiles are significantly different (Figure 4). These differences were a result of variation in coal particle properties as well as attrition, fragmentation, cracking and drop off particle from thermocouple, and the degree of discrepancy increased with the increase of the bed temperature. As a consequence the original time-temperature profile was not appropriate for comparison of temperature histories of different particle types and under different FB conditions. To be able to perform the suitable comparison at least three curves were selected from all experimental data based on criteria that no attrition and fragmentation occurred during the experiment and that thermocouple stayed attached to the particle until the end of combustion. The average time-temperature profiles were defined using selected curves having additional criteria that temperature registrated for given curve principally did not vary more then (20 °C in comparison with temperature in average timetemperature profile. The examples of calculated average timetemperature profiles obtained at 650 °C for lignite (dc ) 8 mm) are shown in Figure 4. The further discussions given in this paper are based on the average time-temperature profiles. The processes that occur during drying, devolatilization, and char combustion in the FB reactor are analyzed on the basis of the temperature changes. The corresponding differential curves (dTc/dt) are shown in Figure 4. There was no change in the measured temperature in the center of the coal particle that could be regarded as a result of drying of the coal particle. The thermocouple was settled in the center of the coal particle, and when the measured temperature in the center of the coal particle reached 100 °C, the rest of the coal particle was already totally dried.22 Besides that, the heat-transfer rate in the FB was very high, therefore the drying process was very intensive and the applied measurement technique could not be detected the drying process of the coal particle. The effect of moisture content on temperature during devolatilization and char combustion was not significant, as was moreover known from literature.10 The characteristic peaks in the differential curves (that show change of temperature gradient) and corresponding temperatures can be noticed. The maximum heating rate was measured when temperature of the coal particle center had reached about 150 (22) Komatina, M.; Manovic, V.; Saljnikov A. Energy Sources, accepted for publication.
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Energy & Fuels, Vol. 20, No. 1, 2006 117
Figure 3. Concentrations of gases during experiments with lignite (dc ) 8 mm) at Tbed ) 650 °C: (a) combustion in air; (b) devolatilization in N2 and combustion in air.
Figure 4. Analyses of temperatures during experiments with lignite (dc ) 8 mm) at Tbed ) 650 °C: (a) combustion in air; (b) devolatilization in N2 and combustion in air.
°C. The first peak corresponds to completed drying process, while devolatilization had not yet started. After this, the endothermic reactions of devolatilization of light volatile components had started, which resulted in a decrease of the temperature gradient. Furthermore, the volatiles created additional heat and mass transfer resistance within and around the coal particle. The corresponding temperatures in the center Tc,1 was about 150 °C, regardless of surrounding atmosphere. It is in an agreement with fact that during devolatilization mass flow is outward of particle, reducing the effect of surrounding atmosphere. The temperature is about 150 °C in the center, assigned as the temperature of starting of devolatilization is realistic. Corresponding temperatures on the places near the particle surface are significantly higher, enabling endothermic reactions of devolatilization that obstruct the heat transfer to the center of the particle.23 The maximal rate of devolatilization reactions is obtained when the temperature in center of particle (Tc,2) was about 300 °C. The peak, which corresponds to the temperature Tc,3 is a result of decrease of devolatilization rate. The intensity of peak, i.e., increase of temperature gradient, is more noticeable in the case of air atmosphere as a consequence of ignition and combustion of volatiles and char particle. Thus, this moment was assumed as a start of char combustion in the air atmosphere. In the last
period of char combustion the temperature gradient was mostly constant. During this period the change of temperature in the center of coal particle was similar as in experiments with char formed in the inert atmosphere. The temperature (Tc,1/2) at half time (τ1/2) of char combustion was chosen as a characteristic temperature. This temperature was used for comparison of temperature difference between FB and center of burning particle during combustion of different type of coals and various particle diameters for both types of experiments. The influence of coal particle size, coal type, and experimental conditions on the temperature history in the center of the particle can be observed in Figure 5. The differences in temperature history of coal particle center were influenced by coal type and particle size. The higher temperature gradient during combustion of smaller particles and combustion of lignite can be noticed. It indicates that a leading role in the temperature history of coal particle is played by the mass and heat transfer through combusting particle, i.e., regime of combustion is controlled by diffusion. This justifies the reason measurements were performed at lower temperature then usually used for FB combustion. Regime controlled by diffusion may be expected with increase of bed temperature. The important characteristic of coal is porosity, which determines rate of combustion in this regime.24 The porosity of lignite is significantly higher than the
(23) Komatina, M.; Manovic, V.; Saljnikov, A. Energy Sources, accepted for publication.
(24) Ilic, M.; Grubor, B.; Manovic, V. J. Serb. Chem. Soc. 2003, 68, 137-145.
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Figure 5. Influence of coal type and particle diameter on temperature history of particle center during combustion in air (a, c) and devolatilization in N2 and combustion in air (b, d) at Tbed ) 650 °C.
porosity of brown coal (48.2 and 31.6%, retrospectively, Table 1). It affects the porosity of produced char and ash and determines diffusion of oxygen through particle, rate of combustion, and generated heat. As a consequence of greater porosity, the burning rate of lignite particle was higher than that of brown coal particle. Apart from coal porosity, an important role in change of temperature during the char combustion period may have the ash layer resistance to heat transfer.13 Higher ash content in the lignite (21.73%) than ash content in the brown coal (8.61%) implicates higher resistance to heat transfer through formed ash layer, toward surface of the burning particle and surroundings. The coal particle size in the moment when the temperature difference between bed and burning char particle was defined is not exactly estimated. As a characteristic value the maximum temperature difference authors usually use and usually assigned to the parent coal particle size. For analyses of temperature differences between the burning particle and the FB that should be assigned to the initial size of coal and char particle, a new reliable criterion was defined (Figure 4) in this study. According to our findings, the temperature difference in the first period was not reliable, since in this period beside combustion of char the combustion of volatiles also occurs and the maximum temperature difference was measured when the burning particle surely changed its initial size, and hence that moment also was not suitable. The temperature (Tc,1/2) at the half time (τ1/2) of char combustion was chosen as a characteristic temperature, and the reliable temperature difference was taken at this moment. Different experimental investigations were performed, with aim to find the effect of the size of coal particles on the moment τ1/2. The change of the coal particle size during combustion period was measured. For the different combustion time, the
Figure 6. The measured temperature differences between the burning particle and the FB temperature considered for half time combustion of char.
process was broken up using N2 instead air and particles were removed from fluidizing bed. It was concluded that average change of the initial particle size for τ1/2 was approximately 10% and maximum measured change was 12.5%. Although the experiments were carried at different temperatures, between 590 and 710 °C, and different starting atmospheres (air or N2 and air) the obtained temperature-time profiles and temperature differences were similar for particles of the same size. Therefore, the obtained temperature differences are shown in the same diagram (Figure 6), regardless of the bed temperature. The obtained temperature differences with corresponding amplitude obtained by comparison of experiments
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Table 3. Temperature Differences between Burning Particle and FB temperature difference, ∆T, (°C) lignite N2 and air
particle diameter (mm)
Air
5.0 7.0 8.0 10.0
175 ( 25 155 ( 26 149 ( 18 128 ( 20
182 ( 24 157 ( 25 142 ( 21 125 ( 22
air
brown coal N2 and air
128 ( 22 85 ( 10 79 ( 11 70 ( 12
127 ( 18 82 ( 10 78 ( 10 72 ( 12
at different temperatures are given in Table 3. It was found that the temperature differences between the burning particle and the FB decrease with the increase of initial size of the coal particle. The temperature differences were higher in the case of lignite combustion. Summary and Conclusion The temperature of coal particles during combustion in FB at low-medium temperatures was investigated. The experimental investigations were conducted in an experimental FB reactor in order to obtain data on the temperature of burning particle. A method using thermocouple was developed and applied for the measurements. A thermocouple was inserted in the center of particle shaped into spherical forms having different diameters: 5, 7, 8, and 10 mm. Two characteristic types of lowrank Serbian coals were investigated. Experiments were done at the FB temperature in the range of 590-710 °C. Air and N2 were used as fluidization gases. The two groups of experimental investigations with coal were performed, combustion of coal particles and combustion of char particles previously obtained by coal devolatilization in N2. The obtained time-temperature profiles were significantly different, as a result of variation in coal particle properties as well as attrition, fragmentation, cracking, and drop-off particle from thermocouple. To solve this problem, the average timetemperature profiles were calculated. The temperature (Tc,1/2)
in half time (τ1/2) of char combustion was chosen as a characteristic temperature. This temperature was used for comparison of temperature difference between FB and center of burning particle during combustion of different types of coals, particle diameter in both types of experiments. The analysis of obtained results has shown that differences in temperature history of coal particle center are mostly influenced by coal type and particle size. It indicated that the leading role in the temperature history of coal particle have the mass and heat transfer through combusting particle. The higher temperature differences between the burning particles and the FB was obtained for smaller particles, and for lignite (130-180 °C) in comparison with the brown coal (70-130 °C). The obtained temperature data may be useful for creating reliable mathematical models and predicting rate of chemical reactions within and at the surface of coal particle, NOx and SO2 emission, fragmentation, attrition, ash melting, and other phenomena occurring in burning coal particle. Glossary dc Tc Tbed Tc,1/2 ∆T t τ τ1/2
initial diameter of coal particle, m temperature in the center of burning coal particle, °C temperature of FB, °C temperature in the center of burning coal particle for half time of char combustion, °C difference between temperature in burning coal particle and FB, ∆T ) Tc,1/2 - Tbed, °C time, s time of char combustion, s half time of char combustion, s
Acknowledgment. This work was supported by the Ministry of Science and Environmental Protection, Republic of Serbia. The authors thank Dr. Mile Djurdjevic for helpful suggestions and assistance during preparation of the paper. EF050222O
