- Email: [email protected]
Clean and ecological coal combustion in the binary circulating fluidized bed
Energy 26 (2001) 1109–1120 www.elsevier.com/locate/energy
Clean and ecological coal combustion in the binary circulating fluidized bed W. Nowak *, W. Muskała Technical University of Czestochowa, Dabrowskiego 69, 42-200 Czestocowa, Poland
Abstract The aim of this paper is to increase the understanding of the role of the binary circulating fluidized bed in the process of clean and ecological coal combustion. The operating range of a stable fluidized bed, as a function of gas velocity changes and the flow rate of fine particles, is determined for all possible conditions. Experiments concerning the combustion and desulfurization processes in multi-solid fluidized bed (MSFB) and circulating fluidized bed (CFB) systems give evidence that the residence time of burnt particles in the combustion chamber of MSFB is much extended. This is directly reflected in better combustion conditions, especially those for fine particles, as well as in the process of desulfurization. The advantages of the binary circulating fluidized bed over typical circulating systems make it one of the most efficient methods of clean and ecological coal combustion. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Circulating fluidized bed (CFB) technology has many advantages compared with typical systems containing a bubbling fluidized bed. Good sulfur capture efficiency, low emission of NOx and good fuel flexibility, as well as satisfactory turn-down characteristics, are the main advantages of CFB. Significant progress in exploitation of the so-called clean coal technologies is possible due to the development of fluidized bed combustors. Clean coal technologies include atmospheric and pressurized fluidized bed combustion, circulating fluidized bed combustion, advanced fluidized bed combustion and a number of others. Multi-solid fluidized bed (MSFB) technology is particularly attractive in areas where environmental regulations are stringent and a very wide range of fuels is utilized. Fluidized bubbling bed boilers and circulating fluidized bed boilers meet the requirements concerning low gas and dust emission. However, the required flexibility in boiler
* Corresponding author. E-mail address: [email protected] (W. Nowak).
0360-5442/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 1 ) 0 0 0 7 3 - 1
1110
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
loads as well as the strict requirements for admissible size of burnt fuel particles in those boilers call for improvements. A new solution called MSFB was introduced in the late 1980s. MSFB consists of two parts: 앫 a dense, stationary phase of coarse particles, which is situated in the lower part of a reaction chamber; and 앫 a circulating dilute phase bed. In the lower part of the MSFB there is a bubbling bed consisting mainly of spherical ceramic particles ranging from 10 to 15 mm. These spheres are fluidized by the stream of air installed under the combustion chamber (primary air), which contains fine particles of circulating material such as ash, sorbent and fuel. Thus, in the lower part of combustion chamber, there are good operating conditions for the process of disintegration (possibility of utilizing a fuel whose particles are not larger than 50 mm). Although MSFBs are used in boilers [1–3], there is only limited information available concerning their performance. In this paper an attempt is made to analyze the processes concerning MSFB. Fig. 1 presents the boiler model used. 2. Some chosen results of flow experiments Analysis of the flow of fine and coarse particles creating a bubbling bed in the lower part of the combustion chamber gives a precise view of the circulating fluidized bed for various gas velocities in the MSFB. Flexibility is another advantage of MSFB systems. Typical gas velocities for fast fluidized beds are 3.7–6.1 m/s, while for the fluidized beds of MSFB the value is 3.7–
Fig. 1. Model of a boiler with a circulating fluidized bed (MSFB).
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
1111
12.2 m/s. Thus the range of velocities for which MSFB can be realized is almost twice as wide as that for a circulating fluidized bed. The typical range of velocities used in boilers with a binary circulating fluidized bed, resulting from the velocity values at the beginning of fluidization and the transportation for the fine and coarse particles, is shown in Fig. 2 [4]. In order to determine the operating range of the binary bed it is important to find out what the minimum velocities and maximum bed concentrations are. These data are necessary for determining the bottom line describing the stable state of the circulating fluidized bed, as well as for determining the top line in Fig. 2, describing the impact of coarse particle transport velocities on the circulating rate of the fine solids. In order to determine the velocities of coarse particles in a flowing suspension consisting of fine particles, it is necessary to find out the level of interaction between the fine and coarse particles. A simplified model of fine and coarse particles flow is shown in Fig. 3(A) and 3(B). Velocities of the fine particles are close to the gas velocities, while the velocities of coarse particles are typically below the gas velocities. Often coarse particles move in the opposite direction to the gas flow [5]. Theoretical analysis of the problem shows the influence of the mixture of fine particles and gas on the coarse particles. It can be assumed that, in a circulating fluidized bed, all fine particles carried up by gas and following with a velocity of U⬎Utf will move from direction y with an average velocity and mass of (wf)i=wf and mfi=mf, respectively. Thus, angle a=b=q and the unit force due to collision of fine particles on the surface element of the sphere with diameter D, according to Fig. 3(B), is: dF⫽p dA [N],
(1)
where p⫽⌬PN˙ [kg/m s2].
Fig. 2. Typical range of occurrence of the binary bed in MSFB boilers [4].
(2)
1112
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 3. Schematic diagram of a collision between a single fine particle and a coarse particle.
In Eq. (2), p is defined as the rate of momentum change of a single fine particle: ⌬P⫽P1⫺P2⫽2mfwf cos q [kg m/s],
(3)
and the fine particle flow rate, representing the number of fine particles flowing through the crosssectional area of 1 m2 within 1 s, is: N˙ ⫽nwf [1/m2 s],
(4)
where n is the number of fine particles in a volume of 1 m3 [1/m3]. Since in CFB and MSFB flows of gas and solids are organized in opposite directions to the gravitational force (along the y direction), only the component dF is to be considered, i.e.: dFy⫽p dA cos q,
(5)
where dA⫽2pR2c sin q cos2 q dq;
Rc+f⫽dc+f/2.
(6)
Substituting Eqs. (2)–(4) into Eq. (5) and integrating over the range 0ⱕqⱕp/2 results in:
冕
p/2
Fy⫽具F典⫽4pR2c+f nmfw2 cos2 q sin q dq⫽p/3(dc⫹df)2nmfw2f. 0
Since
(7)
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
冕
1113
p/2
nmfw2 cos2 q sin q dq⫽1/3,
0
˙ f [kg/m2 s] and wf=U⫺Utf, we obtain: and using known dependencies that nmfwf=G p˙ Fy⫽ G (d ⫹d )2(U⫺Utf). 3 f c f
(8)
Assuming that there is no interaction among coarse particles, their velocities may be described by the following equation: dwc mc ⫽Fg⫹Fy⫺G, dt
(9)
where Fg⫽
pd 2c 兩U−wc兩(U−wc) CDcrg ; G⫽mcg. 4 2
Assuming that coarse particles are spheres of diameter dc, for conditions of their convection dwc/dt=0; wc=0; Uwc, then U⫺wc⬵U⬵Utc, ˙ f[1+(df/dc)]2(Utc−Utf) 3 rg(Utc)2 3G CDc ⫹ ⫺g⫽0. (10) 4 rc dc 2 rc dc ˙ f, the physical The transport velocity of large particles Utc depends on the solids circulating rate G properties of the particles (both coarse and fine) and the fluidizing gas. Since the dependence is ˙ f, Utc, …]=0), the solution of the dependence can be obtained by using the the implicit one (F[G method of successive approximations. Comparison of the results obtained for the dependence [Eq. (10)] and the experiments (results of experiments concerning various loose materials and various temperatures in a binary fluidized bed) are shown in Fig. 4(A) and 4(B). Theoretical analysis and the experiments confirm that increasing the solids circulating rate, especially for values under 20 kg/m2 s, significantly reduces the velocities of coarse particles in a circulating fluidized bed. This significantly reduces the range of possible changes of flow parameters in the binary system. This fact should be taken into account while designing such systems. Eq. (10) can also be presented in dimensionless form:
冋
Retc⫽
册
˙ ∗f dcRetf 4 Arc ˙ ∗f )2 G (G ⫹2 ⫹ 2 CD c CDcdf 3CDc
where: Utcdc Retc⫽ , vg
0.5
⫺
˙ ∗f G , CDc
(11)
1114
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 4. Results of experiments and velocity calculations of coarse particles with convection changes in the binary fluidized bed: (A) for different bed temperatures; (B) for different parameters of the coarse particles.
冉 冊
2 ˙ ˙ ∗f ⫽Gsdc 1⫹df , G rgvg dc
冪3 C
Re∗f ⫽ and
4 Arf Df
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
1115
Arc⫽d 3crg(rc⫺rg)g/m2g.
3. Results and discussion Experiments concerning the processes of combustion, gas emission and desulfurization were carried out with the experimental work-stand shown in Fig. 5. The range of flow parameters was varied in the experiments. They are shown in Fig. 6(A). The changes of gas exhaust components were analyzed in order to investigate the combustion processes in MSFB and CFB. The fuel sample was introduced into the system. The time of combustion of fuel samples for different conditions ranged within 60–120 s. As a result of integration of temporary courses of oxygen in gas exhausts, the level of gas conversion in given samples and conditions of combustion is described in the following way:
冕 t
冘 t
n˙ gRT n˙ gRT X⫽ (c0⫺ct) dt⬇ (2c ⫺c ⫺c )⌬t, mpp mpp t⫽0 0 t−1 t
(12)
0
Fig. 5. Schematic diagram of the experimental work-stand used in the studies of combustion and desulfurization processes in the binary circulating fluidized bed (MSFB).
1116
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 6. (A) Ranges of flow parameter changes of the circulating fluidized bed in MSFB and CFB systems; (B) effect of circulating material stream on the size of convected coarse particles in MSFB.
where X is the level of conversion [–], t is time [s], ng is flow of the oxygen stream [mol/s], R is the universal gas constant [8.314 J/mol K], T is the temperature in the reaction zone [K], mp is the amount of the constant regulating substance [g], p is the pressure of the regulating system [Pa], c0, ct⫺1, ct are initial and instantaneous concentrations of the reacting gas [g/m3]. Fig. 7(A) and 7(B) shows some results of conversion for given fuels. Because the residence time of burnt particles is much longer in MSFB systems, the operating conditions of combustion in this system are much better than those of CFB. This is especially the case for such fuels as petroleum coke and brown coal. The MSFB system, in comparison to a similar CFB system, is less susceptible to the influence of flow and temperature changes (on the result of combustion of different fuels) because of the much longer residence time of the particles. Besides, the temperature distribution in a column is more uniform than in a CFB system. This is shown in Fig. 8(B). Studies of the combustion process of various fuels providing a constant fuel supply under different temperature and flow conditions in binary circulating MSFB and typical CFB are shown in the paper. In Fig. 9(A) and 9(B), CO and NOx emissions in the process of hard and brown coal combustion within average bed temperature changes ranging from 1023 K to 1223 K are shown. The average gas velocity in the lower part of the combustion chamber is 6 m/s and the solids circulating rate is 10 kg/m2 s. Studies of the desulfurization process of fuels in the binary system of MSFB and typical CFB have been conducted. Two sorbents are used in the desulfurization process of fuels: disintegrated calcium carbonate (CaCO3) and calcium hydroxide [Ca(OH)2]. Fig. 10(A) and 10(B) presents the results of dry desulfurization of fuels for different temperature conditions and Ca/S ratios using
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
1117
Fig. 7. (A) Examples of changes in [O2] and [CO2] during the process of combustion of a fuel sample; (B) conversion of different fuels in MSFB and CFB systems.
these two sorbents for MSFB and CFB systems. The studies proved that the optimal temperatures in the desulfurization process of fuels range from 1073 K to 1123 K. Moreover, the studies gave evidence of a significant effect of Ca/S ratio on the efficiency of desulfurization of fuels providing that the ratio is lower than 2. Finally, there is also a slight effect of flow parameters on the efficiency of desulfurization of fuels in MSFB. 4. Conclusion 앫 The MSFB is a modification of circulating systems. There is an organized bubbling bed, consisting of a mixture of coarse and fine particles in the lower part of the main fluidized column. 앫 Parameter changes of the operating conditions of the system, such as fluidized gas velocity and solids circulating rate, should be adjusted to the operating conditions of the system and the physical properties of the bed material.
1118
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 8. (A) Average flow period of fuel particles through MSFB and CFB under different flow conditions of these systems; (B) examples of temperature distribution in the combustion chamber (phase temperature, 1123 K; gas velocity, 6 m/s; solids circulating rate, 10 kg/m2 s).
앫 The residence time of fine particles in the fluidized system, especially in the combustion chamber, is significantly extended due to a dense phase occurring in the fluidized column. Positive conditions for the processes of combustion and dry desulfurization are created as a result of a dense phase. Thus, one of the advantages of an MSFB system is the fact that it helps to organize the combustion and desulfurization processes in the cases of fuels for which combustion is rather difficult. 앫 In the process of fuel combustion under conditions similar to those of a CFB, the system can operate with lower values of the solids circulating rate because of the extended residence time of particles in the lower part of the combustion chamber. Thus, values of the multification factor of circulation are lower in comparison with CFB systems.
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
1119
Fig. 9. Examples of emission changes, [CO] and [NOx], in the process of combustion in MSFB: (A) hard coal; (B) brown coal.
앫 Another advantage of MSFB systems is the flexibility of the boiler to load changes. This is obtained by adjusting the velocity of fluidization as well as the solids circulating rate to the operating data of the system.
References [1] Nowak W, Matsuda H, Win KK, Hasatani M. Studies of the hydrodynamics of a multi-solid fluidized bed. In: Proceedings of the 5th SCEJ Symposium on Circulating Fluidized Beds, Tokyo (Japan). Japan: The Society of Chemical Engineers, 1993:14–24.
1120
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 10. Results of desulfurization experiments in MSFB and CFB systems: (A) as a function of bed temperature changes; (B) as a function of molar ratio. [2] Nowak W, Win KK, Matsuda H, Hasatani M. Diagnosis of a multisolid fluidized bed. In: Proceedings of the International Conference on Energy Systems and Ecology, Cracow (Poland), 1993:37–54. [3] Nowak W, Muskała W. Spalanie we gla w binarnej cyrkulacyjnej warstwie fluidalnej typu MSFB. Gospodarka Paliwami i Energia 1995;7:2–8. [4] Kojima Y, Mii T, Kuno M, Takebayashi T, Murata A, Tomoyasu T et al. Design and operating experience of the first comercial MSFB boiler in Japan. In: Basu P, Large JF, editors. Circulating fluidized beds technology. Oxford: Pergamon Press, 1988:369–76. [5] Win KK, Nowak W, Matsuda H, Hasatani M, Bis Z, Krzywan´ ski J et al. Transport velocity of coarse particles in multi-solid fluidized bed. J Chem Eng Jpn 1995;28(5):535–40.
Clean and ecological coal combustion in the binary circulating fluidized bed W. Nowak *, W. Muskała Technical University of Czestochowa, Dabrowskiego 69, 42-200 Czestocowa, Poland
Abstract The aim of this paper is to increase the understanding of the role of the binary circulating fluidized bed in the process of clean and ecological coal combustion. The operating range of a stable fluidized bed, as a function of gas velocity changes and the flow rate of fine particles, is determined for all possible conditions. Experiments concerning the combustion and desulfurization processes in multi-solid fluidized bed (MSFB) and circulating fluidized bed (CFB) systems give evidence that the residence time of burnt particles in the combustion chamber of MSFB is much extended. This is directly reflected in better combustion conditions, especially those for fine particles, as well as in the process of desulfurization. The advantages of the binary circulating fluidized bed over typical circulating systems make it one of the most efficient methods of clean and ecological coal combustion. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Circulating fluidized bed (CFB) technology has many advantages compared with typical systems containing a bubbling fluidized bed. Good sulfur capture efficiency, low emission of NOx and good fuel flexibility, as well as satisfactory turn-down characteristics, are the main advantages of CFB. Significant progress in exploitation of the so-called clean coal technologies is possible due to the development of fluidized bed combustors. Clean coal technologies include atmospheric and pressurized fluidized bed combustion, circulating fluidized bed combustion, advanced fluidized bed combustion and a number of others. Multi-solid fluidized bed (MSFB) technology is particularly attractive in areas where environmental regulations are stringent and a very wide range of fuels is utilized. Fluidized bubbling bed boilers and circulating fluidized bed boilers meet the requirements concerning low gas and dust emission. However, the required flexibility in boiler
* Corresponding author. E-mail address: [email protected] (W. Nowak).
0360-5442/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 1 ) 0 0 0 7 3 - 1
1110
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
loads as well as the strict requirements for admissible size of burnt fuel particles in those boilers call for improvements. A new solution called MSFB was introduced in the late 1980s. MSFB consists of two parts: 앫 a dense, stationary phase of coarse particles, which is situated in the lower part of a reaction chamber; and 앫 a circulating dilute phase bed. In the lower part of the MSFB there is a bubbling bed consisting mainly of spherical ceramic particles ranging from 10 to 15 mm. These spheres are fluidized by the stream of air installed under the combustion chamber (primary air), which contains fine particles of circulating material such as ash, sorbent and fuel. Thus, in the lower part of combustion chamber, there are good operating conditions for the process of disintegration (possibility of utilizing a fuel whose particles are not larger than 50 mm). Although MSFBs are used in boilers [1–3], there is only limited information available concerning their performance. In this paper an attempt is made to analyze the processes concerning MSFB. Fig. 1 presents the boiler model used. 2. Some chosen results of flow experiments Analysis of the flow of fine and coarse particles creating a bubbling bed in the lower part of the combustion chamber gives a precise view of the circulating fluidized bed for various gas velocities in the MSFB. Flexibility is another advantage of MSFB systems. Typical gas velocities for fast fluidized beds are 3.7–6.1 m/s, while for the fluidized beds of MSFB the value is 3.7–
Fig. 1. Model of a boiler with a circulating fluidized bed (MSFB).
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
1111
12.2 m/s. Thus the range of velocities for which MSFB can be realized is almost twice as wide as that for a circulating fluidized bed. The typical range of velocities used in boilers with a binary circulating fluidized bed, resulting from the velocity values at the beginning of fluidization and the transportation for the fine and coarse particles, is shown in Fig. 2 [4]. In order to determine the operating range of the binary bed it is important to find out what the minimum velocities and maximum bed concentrations are. These data are necessary for determining the bottom line describing the stable state of the circulating fluidized bed, as well as for determining the top line in Fig. 2, describing the impact of coarse particle transport velocities on the circulating rate of the fine solids. In order to determine the velocities of coarse particles in a flowing suspension consisting of fine particles, it is necessary to find out the level of interaction between the fine and coarse particles. A simplified model of fine and coarse particles flow is shown in Fig. 3(A) and 3(B). Velocities of the fine particles are close to the gas velocities, while the velocities of coarse particles are typically below the gas velocities. Often coarse particles move in the opposite direction to the gas flow [5]. Theoretical analysis of the problem shows the influence of the mixture of fine particles and gas on the coarse particles. It can be assumed that, in a circulating fluidized bed, all fine particles carried up by gas and following with a velocity of U⬎Utf will move from direction y with an average velocity and mass of (wf)i=wf and mfi=mf, respectively. Thus, angle a=b=q and the unit force due to collision of fine particles on the surface element of the sphere with diameter D, according to Fig. 3(B), is: dF⫽p dA [N],
(1)
where p⫽⌬PN˙ [kg/m s2].
Fig. 2. Typical range of occurrence of the binary bed in MSFB boilers [4].
(2)
1112
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 3. Schematic diagram of a collision between a single fine particle and a coarse particle.
In Eq. (2), p is defined as the rate of momentum change of a single fine particle: ⌬P⫽P1⫺P2⫽2mfwf cos q [kg m/s],
(3)
and the fine particle flow rate, representing the number of fine particles flowing through the crosssectional area of 1 m2 within 1 s, is: N˙ ⫽nwf [1/m2 s],
(4)
where n is the number of fine particles in a volume of 1 m3 [1/m3]. Since in CFB and MSFB flows of gas and solids are organized in opposite directions to the gravitational force (along the y direction), only the component dF is to be considered, i.e.: dFy⫽p dA cos q,
(5)
where dA⫽2pR2c sin q cos2 q dq;
Rc+f⫽dc+f/2.
(6)
Substituting Eqs. (2)–(4) into Eq. (5) and integrating over the range 0ⱕqⱕp/2 results in:
冕
p/2
Fy⫽具F典⫽4pR2c+f nmfw2 cos2 q sin q dq⫽p/3(dc⫹df)2nmfw2f. 0
Since
(7)
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
冕
1113
p/2
nmfw2 cos2 q sin q dq⫽1/3,
0
˙ f [kg/m2 s] and wf=U⫺Utf, we obtain: and using known dependencies that nmfwf=G p˙ Fy⫽ G (d ⫹d )2(U⫺Utf). 3 f c f
(8)
Assuming that there is no interaction among coarse particles, their velocities may be described by the following equation: dwc mc ⫽Fg⫹Fy⫺G, dt
(9)
where Fg⫽
pd 2c 兩U−wc兩(U−wc) CDcrg ; G⫽mcg. 4 2
Assuming that coarse particles are spheres of diameter dc, for conditions of their convection dwc/dt=0; wc=0; Uwc, then U⫺wc⬵U⬵Utc, ˙ f[1+(df/dc)]2(Utc−Utf) 3 rg(Utc)2 3G CDc ⫹ ⫺g⫽0. (10) 4 rc dc 2 rc dc ˙ f, the physical The transport velocity of large particles Utc depends on the solids circulating rate G properties of the particles (both coarse and fine) and the fluidizing gas. Since the dependence is ˙ f, Utc, …]=0), the solution of the dependence can be obtained by using the the implicit one (F[G method of successive approximations. Comparison of the results obtained for the dependence [Eq. (10)] and the experiments (results of experiments concerning various loose materials and various temperatures in a binary fluidized bed) are shown in Fig. 4(A) and 4(B). Theoretical analysis and the experiments confirm that increasing the solids circulating rate, especially for values under 20 kg/m2 s, significantly reduces the velocities of coarse particles in a circulating fluidized bed. This significantly reduces the range of possible changes of flow parameters in the binary system. This fact should be taken into account while designing such systems. Eq. (10) can also be presented in dimensionless form:
冋
Retc⫽
册
˙ ∗f dcRetf 4 Arc ˙ ∗f )2 G (G ⫹2 ⫹ 2 CD c CDcdf 3CDc
where: Utcdc Retc⫽ , vg
0.5
⫺
˙ ∗f G , CDc
(11)
1114
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 4. Results of experiments and velocity calculations of coarse particles with convection changes in the binary fluidized bed: (A) for different bed temperatures; (B) for different parameters of the coarse particles.
冉 冊
2 ˙ ˙ ∗f ⫽Gsdc 1⫹df , G rgvg dc
冪3 C
Re∗f ⫽ and
4 Arf Df
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
1115
Arc⫽d 3crg(rc⫺rg)g/m2g.
3. Results and discussion Experiments concerning the processes of combustion, gas emission and desulfurization were carried out with the experimental work-stand shown in Fig. 5. The range of flow parameters was varied in the experiments. They are shown in Fig. 6(A). The changes of gas exhaust components were analyzed in order to investigate the combustion processes in MSFB and CFB. The fuel sample was introduced into the system. The time of combustion of fuel samples for different conditions ranged within 60–120 s. As a result of integration of temporary courses of oxygen in gas exhausts, the level of gas conversion in given samples and conditions of combustion is described in the following way:
冕 t
冘 t
n˙ gRT n˙ gRT X⫽ (c0⫺ct) dt⬇ (2c ⫺c ⫺c )⌬t, mpp mpp t⫽0 0 t−1 t
(12)
0
Fig. 5. Schematic diagram of the experimental work-stand used in the studies of combustion and desulfurization processes in the binary circulating fluidized bed (MSFB).
1116
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 6. (A) Ranges of flow parameter changes of the circulating fluidized bed in MSFB and CFB systems; (B) effect of circulating material stream on the size of convected coarse particles in MSFB.
where X is the level of conversion [–], t is time [s], ng is flow of the oxygen stream [mol/s], R is the universal gas constant [8.314 J/mol K], T is the temperature in the reaction zone [K], mp is the amount of the constant regulating substance [g], p is the pressure of the regulating system [Pa], c0, ct⫺1, ct are initial and instantaneous concentrations of the reacting gas [g/m3]. Fig. 7(A) and 7(B) shows some results of conversion for given fuels. Because the residence time of burnt particles is much longer in MSFB systems, the operating conditions of combustion in this system are much better than those of CFB. This is especially the case for such fuels as petroleum coke and brown coal. The MSFB system, in comparison to a similar CFB system, is less susceptible to the influence of flow and temperature changes (on the result of combustion of different fuels) because of the much longer residence time of the particles. Besides, the temperature distribution in a column is more uniform than in a CFB system. This is shown in Fig. 8(B). Studies of the combustion process of various fuels providing a constant fuel supply under different temperature and flow conditions in binary circulating MSFB and typical CFB are shown in the paper. In Fig. 9(A) and 9(B), CO and NOx emissions in the process of hard and brown coal combustion within average bed temperature changes ranging from 1023 K to 1223 K are shown. The average gas velocity in the lower part of the combustion chamber is 6 m/s and the solids circulating rate is 10 kg/m2 s. Studies of the desulfurization process of fuels in the binary system of MSFB and typical CFB have been conducted. Two sorbents are used in the desulfurization process of fuels: disintegrated calcium carbonate (CaCO3) and calcium hydroxide [Ca(OH)2]. Fig. 10(A) and 10(B) presents the results of dry desulfurization of fuels for different temperature conditions and Ca/S ratios using
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
1117
Fig. 7. (A) Examples of changes in [O2] and [CO2] during the process of combustion of a fuel sample; (B) conversion of different fuels in MSFB and CFB systems.
these two sorbents for MSFB and CFB systems. The studies proved that the optimal temperatures in the desulfurization process of fuels range from 1073 K to 1123 K. Moreover, the studies gave evidence of a significant effect of Ca/S ratio on the efficiency of desulfurization of fuels providing that the ratio is lower than 2. Finally, there is also a slight effect of flow parameters on the efficiency of desulfurization of fuels in MSFB. 4. Conclusion 앫 The MSFB is a modification of circulating systems. There is an organized bubbling bed, consisting of a mixture of coarse and fine particles in the lower part of the main fluidized column. 앫 Parameter changes of the operating conditions of the system, such as fluidized gas velocity and solids circulating rate, should be adjusted to the operating conditions of the system and the physical properties of the bed material.
1118
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 8. (A) Average flow period of fuel particles through MSFB and CFB under different flow conditions of these systems; (B) examples of temperature distribution in the combustion chamber (phase temperature, 1123 K; gas velocity, 6 m/s; solids circulating rate, 10 kg/m2 s).
앫 The residence time of fine particles in the fluidized system, especially in the combustion chamber, is significantly extended due to a dense phase occurring in the fluidized column. Positive conditions for the processes of combustion and dry desulfurization are created as a result of a dense phase. Thus, one of the advantages of an MSFB system is the fact that it helps to organize the combustion and desulfurization processes in the cases of fuels for which combustion is rather difficult. 앫 In the process of fuel combustion under conditions similar to those of a CFB, the system can operate with lower values of the solids circulating rate because of the extended residence time of particles in the lower part of the combustion chamber. Thus, values of the multification factor of circulation are lower in comparison with CFB systems.
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
1119
Fig. 9. Examples of emission changes, [CO] and [NOx], in the process of combustion in MSFB: (A) hard coal; (B) brown coal.
앫 Another advantage of MSFB systems is the flexibility of the boiler to load changes. This is obtained by adjusting the velocity of fluidization as well as the solids circulating rate to the operating data of the system.
References [1] Nowak W, Matsuda H, Win KK, Hasatani M. Studies of the hydrodynamics of a multi-solid fluidized bed. In: Proceedings of the 5th SCEJ Symposium on Circulating Fluidized Beds, Tokyo (Japan). Japan: The Society of Chemical Engineers, 1993:14–24.
1120
W. Nowak, W. Muskała / Energy 26 (2001) 1109–1120
Fig. 10. Results of desulfurization experiments in MSFB and CFB systems: (A) as a function of bed temperature changes; (B) as a function of molar ratio. [2] Nowak W, Win KK, Matsuda H, Hasatani M. Diagnosis of a multisolid fluidized bed. In: Proceedings of the International Conference on Energy Systems and Ecology, Cracow (Poland), 1993:37–54. [3] Nowak W, Muskała W. Spalanie we gla w binarnej cyrkulacyjnej warstwie fluidalnej typu MSFB. Gospodarka Paliwami i Energia 1995;7:2–8. [4] Kojima Y, Mii T, Kuno M, Takebayashi T, Murata A, Tomoyasu T et al. Design and operating experience of the first comercial MSFB boiler in Japan. In: Basu P, Large JF, editors. Circulating fluidized beds technology. Oxford: Pergamon Press, 1988:369–76. [5] Win KK, Nowak W, Matsuda H, Hasatani M, Bis Z, Krzywan´ ski J et al. Transport velocity of coarse particles in multi-solid fluidized bed. J Chem Eng Jpn 1995;28(5):535–40.