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Combustion and heat transfer characteristics in a square internally circulating fluidized bed combustor with draft tube
Fuel 87 (2008) 3710–3713
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Combustion and heat transfer characteristics in a square internally circulating fluidized bed combustor with draft tube Jin Hee Jeon a, Sang Done Kim a,*, Seung Jae Kim b, Yong Kang c a Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Energy and Environment Research Center, Daejeon 305-701, Republic of Korea b Department of Environmental Engineering, Chonnam National University, Gwangju 505-757, Republic of Korea c School of Chemical Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 July 2007 Received in revised form 20 May 2008 Accepted 14 July 2008 Available online 8 August 2008 Keywords: Internally circulating fluidized bed Heat transfer coefficient Overall combustion efficiency
a b s t r a c t The effects of bed temperature (800–950 °C) and gas velocity to the draft tube (0.26–0.37 m/s) on the heat transfer coefficient and the overall combustion efficiency have been determined in a square internally circulating fluidized bed combustor (0.28 m width 2.6 m height) with an orifice-type square draft tube (0.1 m width 0.9 m height). The overall combustion efficiency increases with increasing the excess air and the bed temperature. At a given aeration rate to the moving bed, the heat transfer coefficient goes through a maximum value with increasing gas velocity in the fluidized bed. The heat transfer coefficients in the moving bed and in the freeboard increase with increasing gas velocity to the fluidized bed and bed temperature. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction To reduce the height of conventional circulating fluidized bed and its construction cost, internally circulating fluidized beds using a central draft tube [1,2] or a flat plate [3] to divide the bed for internal solids circulation in a single vessel have been developed. Internally circulating fluidized beds (ICFBs) with a separate aeration in the annulus section may provide more flexible operation of gas and solid reacting systems. Therefore, ICFB reactors have been utilized to coal combustion [4], coal gasification [5] and incineration of solid wastes [6]. Normally, water-walls are used to contain and extract heat from large-scale fluidized bed combustors (FBCs). The wall-to-bed heat transfer coefficients are needed to design and predict performance of the combustors. Numerous research works have been carried out to determine the heat transfer characteristics in conventional bubbling fluidized beds [7] and circulating fluidized beds [8]. However, no information is available in the literature on the combustion efficiency and heat transfer characteristics in the ICFB with a draft tube. Needless to say, temperature in the ICFB combustor should be controlled and maintained within a certain level to maintain stable operation. Thus, information on the combustion efficiency and heat transfer coefficient in this system is indispens-
* Corresponding author. Tel.: +82 42 869 3913; fax: +82 42 869 3910. E-mail address: [email protected] (S.D. Kim). 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.07.013
able for the effective industrial applications of ICFB with draft tube. Therefore, the heat transfer coefficient and the overall combustion efficiency have been determined to provide prerequisite knowledge for optimum design and operation of the ICFB combustor. In the present study, a square shaped internally circulating fluidized bed was designed for easy modulation for scale-up purposes with easy repairs and maintenance. The effects of bed temperature, the excess air and gas velocity to the fluidized bed (draft tube) on the heat transfer coefficient and the overall combustion efficiency were determined in a square ICFB combustor with an orifice-type square draft tube. 2. Experimental The co-combustion of coal and sludge was carried out in an ICFB combustor (0.28 m square 2.6 m high) with a centrally located draft tube (0.1 m square 0.9 m high) as shown in Fig. 1. The bed was loaded with a known weight of sand particles [particle diameter (dp) = 0.3 mm, particle density (qs) = 2620 kg/m3, minimum fluidizing velocity (Umf) = 0.074 m/s] were fluidized by compressed air through a pressure regulator, a filter and a gas flow meter. Based on the hydrodynamic properties in the cold bed test [9], the bed height in the annulus section was maintained at 0.9 m above the distributor and the gas velocity to the annulus section was maintained at 1.5 Umf for all the experimental conditions. Gas sampling probes were installed at the outlet of a condenser
3711
J.H. Jeon et al. / Fuel 87 (2008) 3710–3713
9
13
8
15 water outlet
10
14
1
11
12
water inlet
15
7
6
4
5
2
3
1
Fig. 1. Schematic diagram of ICFB reactor. (1. flowmeter; 2. air plenum; 3. distributor; 4. bed drain; 5. overflow drain; 6. draft tube; 7. main body; 8. screw feeder; 9. feed hopper; 10. cyclone; 11. condenser; 12. collector; 13. ID fan; 14. water-wall; 15. thermistor).
and flue gas compositions (CO2, CO, SO2 and NO) were measured by gas analyzer (ZKJ, Fuji electronics). Fly ash was captured by a cyclone and the bed materials from the reactor through an overflow pipe were sampled to determine the overall combustion efficiency (qov) [4]. Properties of coal and sludge used in this study are shown in Table 1. Water-walls were mounted at walls of the annulus section (8 mm i.d. 0.1 m length at 0.405 m from the distributor), draft tube (15 mm i.d. 0.472 m length at 0.505 m from the distributor) and freeboard section (8 mm i.d. 0.1 m length at 2.070 m from the distributor) to determine the heat transfer coefficients. When the combustor reaches steady state condition, cooling water temperatures were measured at the inlet and outlet of the water-walls by J-type thermocouples. From the energy balance between the
bed and the cooling water in the water walls, the heat transfer coefficients in the bed can be determined [4].
3. Results and discussion The effect of excess air percentage on the overall combustion efficiency (qov) is shown in Fig. 2 where qov increases with an increase in excess air percentage. This can be attributed to the increase in reactivity between carbon and oxygen due to an increase in O2 supply with increasing contacting between gas and solid phases caused by vigorous solid mixing by bubbles [10]. In the present combustor, gov lie between 87% and 92% that corresponds well with those in the ICFB of previous studies
J.H. Jeon et al. / Fuel 87 (2008) 3710–3713
Coal
Sludge
Proximate analysis (%), dry base Moisture Ash Fixed carbon Volatile
3.1 11.4 50.9 34.6
7.4 47.7 3.9 41
Ultimate analysis (%), dry base Carbon Hydrogen Oxygen Nitrogen Combustible sulfur Calorific value (kcal/kg), dry base
74.6 4.7 7.6 1.1 0.3 6670
9.8 2.8 25.8 0.7 9.4 1085
Uf /Umf [-] 3.5
4.0
4.5
550
0.4
500 0.3 450
400 0.2 350 800 °C 850 °C 900 °C 950 °C
300
Solid symbol :hf Open symbol: εsf 0 0.0
5.0
0.1
Solid holdup of fluidized bed, εsf [-]
Table 1 Chemical properties of Tinto coal from Indonesia and wastewater sludge from S electronic company
Heat transfer coefficient of fluidized bed, hf [Wm-2K-1]
3712
0.0 3.5
4.0
4.5
5.0
5.5
6.0
Uf /Umf [-] Fig. 3. Effect of Uf/Umf on the heat transfer coefficient and solid holdup (esf) in the fluidized bed.
90
88
800 °C 850 °C 900 °C 950 °C
86
0 0
10
20
30
Excess air [%] Fig. 2. Effect of excess air percentage on the overall combustion efficiency.
[11,12] because the entrained particles in the cyclone were not fed back into the fluidized bed and consequent reduction in gov in the present study. If the unburned carbon particles are recycled to the fluidized bed, gov in the ICFB combustors might increase accordingly [4,11]. The combustion efficiency in a fluidized bed combustor [12] would be the major objective parameter for designing and operating the combustion system. Naude and Dutkiewicz [13] proposed correlations between combustion efficiency and the operating variables, such as bed temperature, superficial gas velocity and static bed height in a fluidized bed combustor using three types of coals. Choi [14] proposed a correlation of gov as a function of bed temperature (Tb), and fluidizing gas velocity (Uf) in an ICFB with a partition plate. In the present study, gov in the ICFB with a draft tube is correlated with Tb (°C) and Uf (m/s) as
gov ¼ 39:048T 0:135 U 0:061 b f
ð1Þ
in the range of 800 < Tb < 950 °C and 0.26 < Uf < 0.37 m/s, with a correlation coefficient of 0.97. The effect of gas velocity (Uf) to the draft tube (fluidized bed) on the heat transfer coefficients (hf) in the fluidized bed and solid holdup (esf) are shown in Fig. 3 where hf increases with increasing Tb since thermal conductivity of the fluidizing gas, solids heat
capacity and radiative component of the heat transfer coefficient increase with increasing Tb [15] and the contact frequency of solid with the water-wall increases with increasing Uf. A maximum hf exhibits with increasing Uf and the maximum hf lies at 4–4.5 Umf and esf = 0.34–0.37 in the present study. The contact frequency of solids is the controlling factor to increase hf and particle density is the controlling factor to decrease hf from the maximum value [16,17]. The effect of Uf on the transfer coefficient (hm) in the moving bed (annulus section) is shown in Fig. 4 where hm increases with increasing Uf and Tb. At a given gas velocity to the moving bed, hm increases since the surface renewal rate on the heat transfer surface increases with increasing solid circulation rate by Uf.. Therefore, thermal conductivity of solids is the controlling factor to govern hm with relatively slow particle downward movement in the moving bed.
Heat transfer coefficient at moving bed, hm [ Wm-2K-1 ]
Overall combustion efficiency, ηov [%]
92
480
450
420
390
800 850 900 950
360
0 0.0
3.5
4.0
4.5
5.0
°C °C °C °C
5.5
U f /Umf [-] Fig. 4. Effect of Uf/Umf on the heat transfer coefficient in the moving bed.
J.H. Jeon et al. / Fuel 87 (2008) 3710–3713
The ht is lower than that of Biyikli et al. [20] since particle entrainment in the ICFB is less than that of the bubbling fluidized bed.
180
Heat transfer coefficient at moving bed, hm [W m-2 K-1]
3713
800 °C 850 °C 900 °C
160
950
4. Conclusion
°C
The effects of the bed temperature and the excess air percentage on the heat transfer coefficient and the overall combustion efficiency have been determined in a square internally circulating fluidized bed combustor with a square draft tube. The overall combustion efficiency increases with increasing the excess air and the bed temperature. At a given gas velocity to the moving bed, the heat transfer coefficient in the fluidized bed exhibits a maximum value with increasing gas velocity to the fluidized bed. The heat transfer coefficients in the moving bed and the freeboard zone increase with increasing gas velocity to the fluidized bed and bed temperature at a given gas velocity to the moving bed.
140
120
100
Acknowledgment 0 0.0
3.5
4.0
4.5
5.0
5.5
Uf /Umf [-] Fig. 5. Effect of Uf/Umf on the heat transfer coefficient at freeboard region of the bed.
The authors acknowledge a Grant-in-aid for research to S.D. Kim from the Korea Science and Engineering Foundation (R012002-000-00337-0, KOSEF FO1-2004-000-10207-0). References
700
heat transfer coefficients, h [Wm-2K-1]
This study, ICFB, orifice type draft tube, Um/Umf = 1.5, 800 °C
600
ht
hm
hf
hf, Park et al. (1991) ICFB with partition plate Um/Umf = 1, 800 °C
500
hm, Lee et al. (1992), ICFB gap height type draft tube Um/Umf = 1.3, 800 °C ht, Biyikli et al. (1987) BFBC, 750 °C
400
300
200
100
0 0
2
3
4
5
6
7
Uf /Umf [-] Fig. 6. Comparison of the heat transfer coefficients in fluidized bed, moving bed and freeboard region between the present and previous studies.
The effect of Uf on the heat transfer coefficient in the freeboard (ht) is shown in Fig. 5 where ht increases with increasing Uf and Tb. The ht is ranged from 105 to 160 W/m2 K that is much lower than hf since solid holdup in the freeboard is very small. According to Bak et al. [18], ht is higher with smaller coal size since the amount of entrained coal particles increases at a given gas velocity. Comparison of the heat transfer coefficients of the present and previous studies is shown in Fig. 6. As can be seen, the maximum hf in this study lies at 4–4.5 Umf that is corresponds well with those lies at 2–5 Umf of previous studies [11,19]. According to Kharchenko et al. [19], hf is mainly governed by heat conduction and convection but hm may be controlled by heat conduction of solids.
[1] Yang WC, Keairns DL. Design of recirculating fluidized beds for commercial applications. AIChE Symp 1978;74(176):218–28. [2] LaNauze RD. A circulating fluidized bed. Powder Technol 1976;15:117–27. [3] Kuramoto M, Kunii D, Furusawa T. Flow of dense fluidized particles through an opening in a circulation system. Powder Technol 1986;47:141–9. [4] Lee WJ, Cho YJ, Kim JR, Kim SD. Coal combustion characteristics in a fluidized bed combustor with a draft tube. Kor J Chem Eng 1992;9:206–11. [5] Kim YJ, Lee JM, Kim SD. Coal gasification characteristics in an internally circulating fluidized bed with draft tube. Fuel 1997;76:1067–73. [6] Roh SA, Jeong DS, Kim SD, Guy C. Combustion characteristics of spent catalyst and paper sludge in an internally circulating fluidized bed combustor. J Air Waste Manage Assoc 2005;55:1269–76. [7] Highley J, Kaye WG. Fluidized bed industrial boilers and furnaces. London: Applied Science; 1983. [8] Oakes EJ, Engstrom F. Fluidized bed combustion provides for multifuel, economic cogeneration systems. Power Eng 1982;86(3):56–9. [9] Jeon JH, Kim SD, Kim SJ, Kang Y. Hydrodynamic characteristics of a square internally circulating fluidized bed for sludge combustion. J Kor Ind Eng Chem 2004;15:618–24. [10] Yerushalmi J, Cankurt NT. Further studies of the regimes of fluidization. Chem Tech 1978;8:564–72. [11] Park SS, Choi YT, Lee GS, Kim SD. Coal combustion characteristics in an internal circulating fluidized bed combustor. In: Basu P, Horio M, Hasatani M, editors. Circulating fluidized bed technology III. Oxford: Pergamon Press; 1991. p. 497–503. [12] Poersch WW. Combustion efficiency of fluidized bed combustor: some thoughts about the combustion of small carbonic particles in the fly ash from fluidized bed combustor. Powder Technol 1984;40:331–41. [13] Naude DP, Dutkiewicz RK. Fluidized bed combustion of poor quality coal. In: Davidson JF, Keairns DL, editors. Fluidization. London, New York: Cambridge University Press; 1978. p. 280–5. [14] Choi YT. Hydrodynamic properties and combustion characteristics in an internal circulating fluidized bed. PhD thesis, Korea Advanced Institute of Science and Technology, Korea; 1991. [15] Grewal NS, Menart J, Hajicek DR, Zobeck BJ. Heat transfer to horizontal tubes immersed in fluidized bed combustor. Powder Technol 1987;52:149–59. [16] Kim SW, Ahn JY, Kim SD, Lee DH. Heat transfer and bubble characteristics in a fluidized bed with immersed horizontal tube bundle. Int J Heat Mass Trans 2003;46:399–409. [17] Grewal NS, Saxena SC. Heat transfer between a horizontal tube and a gas–solid fluidized bed. Int J Heat Mass Trans 1980;23:1505–19. [18] Bak YC, Son JE, Kim SD. Heat transfer characteristics of a vertical tube in a fluidized bed combustor. Int Chem Eng 1989;29:166–76. [19] Kharchenko NV, Makhorin KE. The rate of heat transfer between a fluidized bed and an immersed body at high temperature. Int Chem Eng 1964;4:650–4. [20] Biyikli S, Tuzla K, Chen JC. Freeboard heat transfer in high temperature fluidized beds. Powder Technol 1987;53:187–94.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Combustion and heat transfer characteristics in a square internally circulating fluidized bed combustor with draft tube Jin Hee Jeon a, Sang Done Kim a,*, Seung Jae Kim b, Yong Kang c a Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Energy and Environment Research Center, Daejeon 305-701, Republic of Korea b Department of Environmental Engineering, Chonnam National University, Gwangju 505-757, Republic of Korea c School of Chemical Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 July 2007 Received in revised form 20 May 2008 Accepted 14 July 2008 Available online 8 August 2008 Keywords: Internally circulating fluidized bed Heat transfer coefficient Overall combustion efficiency
a b s t r a c t The effects of bed temperature (800–950 °C) and gas velocity to the draft tube (0.26–0.37 m/s) on the heat transfer coefficient and the overall combustion efficiency have been determined in a square internally circulating fluidized bed combustor (0.28 m width 2.6 m height) with an orifice-type square draft tube (0.1 m width 0.9 m height). The overall combustion efficiency increases with increasing the excess air and the bed temperature. At a given aeration rate to the moving bed, the heat transfer coefficient goes through a maximum value with increasing gas velocity in the fluidized bed. The heat transfer coefficients in the moving bed and in the freeboard increase with increasing gas velocity to the fluidized bed and bed temperature. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction To reduce the height of conventional circulating fluidized bed and its construction cost, internally circulating fluidized beds using a central draft tube [1,2] or a flat plate [3] to divide the bed for internal solids circulation in a single vessel have been developed. Internally circulating fluidized beds (ICFBs) with a separate aeration in the annulus section may provide more flexible operation of gas and solid reacting systems. Therefore, ICFB reactors have been utilized to coal combustion [4], coal gasification [5] and incineration of solid wastes [6]. Normally, water-walls are used to contain and extract heat from large-scale fluidized bed combustors (FBCs). The wall-to-bed heat transfer coefficients are needed to design and predict performance of the combustors. Numerous research works have been carried out to determine the heat transfer characteristics in conventional bubbling fluidized beds [7] and circulating fluidized beds [8]. However, no information is available in the literature on the combustion efficiency and heat transfer characteristics in the ICFB with a draft tube. Needless to say, temperature in the ICFB combustor should be controlled and maintained within a certain level to maintain stable operation. Thus, information on the combustion efficiency and heat transfer coefficient in this system is indispens-
* Corresponding author. Tel.: +82 42 869 3913; fax: +82 42 869 3910. E-mail address: [email protected] (S.D. Kim). 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.07.013
able for the effective industrial applications of ICFB with draft tube. Therefore, the heat transfer coefficient and the overall combustion efficiency have been determined to provide prerequisite knowledge for optimum design and operation of the ICFB combustor. In the present study, a square shaped internally circulating fluidized bed was designed for easy modulation for scale-up purposes with easy repairs and maintenance. The effects of bed temperature, the excess air and gas velocity to the fluidized bed (draft tube) on the heat transfer coefficient and the overall combustion efficiency were determined in a square ICFB combustor with an orifice-type square draft tube. 2. Experimental The co-combustion of coal and sludge was carried out in an ICFB combustor (0.28 m square 2.6 m high) with a centrally located draft tube (0.1 m square 0.9 m high) as shown in Fig. 1. The bed was loaded with a known weight of sand particles [particle diameter (dp) = 0.3 mm, particle density (qs) = 2620 kg/m3, minimum fluidizing velocity (Umf) = 0.074 m/s] were fluidized by compressed air through a pressure regulator, a filter and a gas flow meter. Based on the hydrodynamic properties in the cold bed test [9], the bed height in the annulus section was maintained at 0.9 m above the distributor and the gas velocity to the annulus section was maintained at 1.5 Umf for all the experimental conditions. Gas sampling probes were installed at the outlet of a condenser
3711
J.H. Jeon et al. / Fuel 87 (2008) 3710–3713
9
13
8
15 water outlet
10
14
1
11
12
water inlet
15
7
6
4
5
2
3
1
Fig. 1. Schematic diagram of ICFB reactor. (1. flowmeter; 2. air plenum; 3. distributor; 4. bed drain; 5. overflow drain; 6. draft tube; 7. main body; 8. screw feeder; 9. feed hopper; 10. cyclone; 11. condenser; 12. collector; 13. ID fan; 14. water-wall; 15. thermistor).
and flue gas compositions (CO2, CO, SO2 and NO) were measured by gas analyzer (ZKJ, Fuji electronics). Fly ash was captured by a cyclone and the bed materials from the reactor through an overflow pipe were sampled to determine the overall combustion efficiency (qov) [4]. Properties of coal and sludge used in this study are shown in Table 1. Water-walls were mounted at walls of the annulus section (8 mm i.d. 0.1 m length at 0.405 m from the distributor), draft tube (15 mm i.d. 0.472 m length at 0.505 m from the distributor) and freeboard section (8 mm i.d. 0.1 m length at 2.070 m from the distributor) to determine the heat transfer coefficients. When the combustor reaches steady state condition, cooling water temperatures were measured at the inlet and outlet of the water-walls by J-type thermocouples. From the energy balance between the
bed and the cooling water in the water walls, the heat transfer coefficients in the bed can be determined [4].
3. Results and discussion The effect of excess air percentage on the overall combustion efficiency (qov) is shown in Fig. 2 where qov increases with an increase in excess air percentage. This can be attributed to the increase in reactivity between carbon and oxygen due to an increase in O2 supply with increasing contacting between gas and solid phases caused by vigorous solid mixing by bubbles [10]. In the present combustor, gov lie between 87% and 92% that corresponds well with those in the ICFB of previous studies
J.H. Jeon et al. / Fuel 87 (2008) 3710–3713
Coal
Sludge
Proximate analysis (%), dry base Moisture Ash Fixed carbon Volatile
3.1 11.4 50.9 34.6
7.4 47.7 3.9 41
Ultimate analysis (%), dry base Carbon Hydrogen Oxygen Nitrogen Combustible sulfur Calorific value (kcal/kg), dry base
74.6 4.7 7.6 1.1 0.3 6670
9.8 2.8 25.8 0.7 9.4 1085
Uf /Umf [-] 3.5
4.0
4.5
550
0.4
500 0.3 450
400 0.2 350 800 °C 850 °C 900 °C 950 °C
300
Solid symbol :hf Open symbol: εsf 0 0.0
5.0
0.1
Solid holdup of fluidized bed, εsf [-]
Table 1 Chemical properties of Tinto coal from Indonesia and wastewater sludge from S electronic company
Heat transfer coefficient of fluidized bed, hf [Wm-2K-1]
3712
0.0 3.5
4.0
4.5
5.0
5.5
6.0
Uf /Umf [-] Fig. 3. Effect of Uf/Umf on the heat transfer coefficient and solid holdup (esf) in the fluidized bed.
90
88
800 °C 850 °C 900 °C 950 °C
86
0 0
10
20
30
Excess air [%] Fig. 2. Effect of excess air percentage on the overall combustion efficiency.
[11,12] because the entrained particles in the cyclone were not fed back into the fluidized bed and consequent reduction in gov in the present study. If the unburned carbon particles are recycled to the fluidized bed, gov in the ICFB combustors might increase accordingly [4,11]. The combustion efficiency in a fluidized bed combustor [12] would be the major objective parameter for designing and operating the combustion system. Naude and Dutkiewicz [13] proposed correlations between combustion efficiency and the operating variables, such as bed temperature, superficial gas velocity and static bed height in a fluidized bed combustor using three types of coals. Choi [14] proposed a correlation of gov as a function of bed temperature (Tb), and fluidizing gas velocity (Uf) in an ICFB with a partition plate. In the present study, gov in the ICFB with a draft tube is correlated with Tb (°C) and Uf (m/s) as
gov ¼ 39:048T 0:135 U 0:061 b f
ð1Þ
in the range of 800 < Tb < 950 °C and 0.26 < Uf < 0.37 m/s, with a correlation coefficient of 0.97. The effect of gas velocity (Uf) to the draft tube (fluidized bed) on the heat transfer coefficients (hf) in the fluidized bed and solid holdup (esf) are shown in Fig. 3 where hf increases with increasing Tb since thermal conductivity of the fluidizing gas, solids heat
capacity and radiative component of the heat transfer coefficient increase with increasing Tb [15] and the contact frequency of solid with the water-wall increases with increasing Uf. A maximum hf exhibits with increasing Uf and the maximum hf lies at 4–4.5 Umf and esf = 0.34–0.37 in the present study. The contact frequency of solids is the controlling factor to increase hf and particle density is the controlling factor to decrease hf from the maximum value [16,17]. The effect of Uf on the transfer coefficient (hm) in the moving bed (annulus section) is shown in Fig. 4 where hm increases with increasing Uf and Tb. At a given gas velocity to the moving bed, hm increases since the surface renewal rate on the heat transfer surface increases with increasing solid circulation rate by Uf.. Therefore, thermal conductivity of solids is the controlling factor to govern hm with relatively slow particle downward movement in the moving bed.
Heat transfer coefficient at moving bed, hm [ Wm-2K-1 ]
Overall combustion efficiency, ηov [%]
92
480
450
420
390
800 850 900 950
360
0 0.0
3.5
4.0
4.5
5.0
°C °C °C °C
5.5
U f /Umf [-] Fig. 4. Effect of Uf/Umf on the heat transfer coefficient in the moving bed.
J.H. Jeon et al. / Fuel 87 (2008) 3710–3713
The ht is lower than that of Biyikli et al. [20] since particle entrainment in the ICFB is less than that of the bubbling fluidized bed.
180
Heat transfer coefficient at moving bed, hm [W m-2 K-1]
3713
800 °C 850 °C 900 °C
160
950
4. Conclusion
°C
The effects of the bed temperature and the excess air percentage on the heat transfer coefficient and the overall combustion efficiency have been determined in a square internally circulating fluidized bed combustor with a square draft tube. The overall combustion efficiency increases with increasing the excess air and the bed temperature. At a given gas velocity to the moving bed, the heat transfer coefficient in the fluidized bed exhibits a maximum value with increasing gas velocity to the fluidized bed. The heat transfer coefficients in the moving bed and the freeboard zone increase with increasing gas velocity to the fluidized bed and bed temperature at a given gas velocity to the moving bed.
140
120
100
Acknowledgment 0 0.0
3.5
4.0
4.5
5.0
5.5
Uf /Umf [-] Fig. 5. Effect of Uf/Umf on the heat transfer coefficient at freeboard region of the bed.
The authors acknowledge a Grant-in-aid for research to S.D. Kim from the Korea Science and Engineering Foundation (R012002-000-00337-0, KOSEF FO1-2004-000-10207-0). References
700
heat transfer coefficients, h [Wm-2K-1]
This study, ICFB, orifice type draft tube, Um/Umf = 1.5, 800 °C
600
ht
hm
hf
hf, Park et al. (1991) ICFB with partition plate Um/Umf = 1, 800 °C
500
hm, Lee et al. (1992), ICFB gap height type draft tube Um/Umf = 1.3, 800 °C ht, Biyikli et al. (1987) BFBC, 750 °C
400
300
200
100
0 0
2
3
4
5
6
7
Uf /Umf [-] Fig. 6. Comparison of the heat transfer coefficients in fluidized bed, moving bed and freeboard region between the present and previous studies.
The effect of Uf on the heat transfer coefficient in the freeboard (ht) is shown in Fig. 5 where ht increases with increasing Uf and Tb. The ht is ranged from 105 to 160 W/m2 K that is much lower than hf since solid holdup in the freeboard is very small. According to Bak et al. [18], ht is higher with smaller coal size since the amount of entrained coal particles increases at a given gas velocity. Comparison of the heat transfer coefficients of the present and previous studies is shown in Fig. 6. As can be seen, the maximum hf in this study lies at 4–4.5 Umf that is corresponds well with those lies at 2–5 Umf of previous studies [11,19]. According to Kharchenko et al. [19], hf is mainly governed by heat conduction and convection but hm may be controlled by heat conduction of solids.
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