2326
Energy & Fuels 2008, 22, 2326–2331
Limestone Calcination with CO2 Capture (II): Decomposition in CO2/Steam and CO2/N2 Atmospheres Yin Wang,*,† Shiying Lin,‡ and Yoshizo Suzuki† National Institute of AdVanced Industrial Science and Technology, 16-1 Onogawa Tsukuba, Ibaraki 305-8569, Japan, and Japan Coal Energy Center, 3-14-10 Mita, Minato-ku, Tokyo 108-0073, Japan ReceiVed January 16, 2008. ReVised Manuscript ReceiVed April 5, 2008
Decomposition of limestone particles (0.25-0.5 mm) in a steam dilution atmosphere (20-100% steam in CO2) was investigated by using a continuously operating fluidized bed reactor for CO2 capture. The decomposition conversion of limestone increased as the steam dilution percentage in the CO2 supply gas increased. At a bed temperature of 1193 K, the conversions were 72% without dilution (100% CO2) and 98% with 60% steam dilution. The decomposition conversions obtained with steam dilution and N2 dilution differed significantly, and this result is explained in terms of the difference between the heat transfer to particle in steam and N2 dilution atmosphere. The reactivities of the CaO produced from limestone decomposition with steam dilution and without dilution (100% CO2) were tested by means of hydration and carbonation reactions. In the hydration test, the time required for complete conversion [CaO f Ca(OH)2] of the CaO produced by steam dilution was approximately half that required for the CaO produced without dilution. In the carbonation test, carbonation conversion (CaO f CaCO3) of the CaO produced by steam dilution was approximately 70%, whereas the conversion was approximately 40% for the CaO produced without dilution.
we investigated the decomposition of In a previous limestone (main component, CaCO3) in a CO2 atmosphere with a continuously operating fluidized bed reactor for obtaining nearly pure CO2 from CaCO3 decomposition. Our results
indicated that the influence of the bed temperature on the limestone decomposition conversion is more substantial than the influence of the residence time of the particles in the bed. The bed temperature must be raised above 1293 K for decomposition of the limestone in a nearly pure CO2 atmosphere. We also observed that the reactivity (hydration and carbonation) of the CaO produced at higher temperature (>1293 K) is lower, owing to the sintering of CaO. Previous studies5,6 have indicated that the CaO sintering that occurs during limestone decomposition at high temperatures is the main cause of the reduction in CaO reactivity. Therefore, reducing the temperature required for limestone decomposition and enhances the reactivity of the CaO and obtains nearly pure CO2, decreasing the CO2 partial pressure by means of steam dilution is desirable. The decomposition of limestone (main component, CaCO3) in a CO2/N2 atmosphere7,8 and in an atmosphere containing a small amount of steam9–11 has been studied. Garcia-Labiano et al.7 reported that decreasing the CO2 partial pressure (from 80% to 0%) increases the decomposition rate and lowers the decomposition temperature of CaCO3. Khinast et al.8 found that at a constant temperature the decomposition rate increases exponentially with decreasing CO2 partial pressure. Khraisha and Dugwell9 and Wang and Thomson10 studied limestone decomposition in the presence of a small amount of
* Author to whom correspondence should be addressed. Phone: +8129-861-8223. Fax: +81-29-861-8209. E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Japan Coal Energy Center. (1) Lyngfelt, A.; Leckner, B.; Mattisson, T. Chem. Eng. Sci. 2001, 56, 3101–3113. (2) Gupta, H.; Fan, L. S. Ind. Eng. Chem. Res. 2002, 41, 4035–4042. (3) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada M. Proceedings of 10th International Conference on Coal Sciences, Taiyuan, China, 1999; B-24. (4) Wang, Y.; Lin, S.; Suzuki, Y. Study of limestone calcination with CO2 capture (I): Decomposition behavior in a CO2 atmosphere. Energy Fuels 2007, 21, 3317–3321.
(5) Abanades, J. C.; Alvarez, D. Energy Fuels 2003, 17, 308–315. (6) Chrissafis, K.; Dagounaki, C.; Paraskevopoulos, K. M. Thermochim. Acta 2005, 428, 193–198. (7) Garcia-Labiano, F.; Abad, A.; de Diego, L. F.; Gayan, P.; Adanez, J. Chem. Eng. Sci. 2002, 57, 2381–2393. (8) Khinast, J.; Krammer, G. F.; Brunner, Ch.; Staudinger, G. Chem. Eng. Sci. 1996, 51, 623–634. (9) Khraisha, Y. H.; Dugwell, D. R. Chem. Eng. Res. Des. 1991, 69, 76–78. (10) Wang, Y.; Thomson, W. J. Chem. Eng. Sci. 1995, 50, 1371–1382. (11) Burnham, A. K.; Stubblefield, C. T.; Campbell, J. H. Fuel 1980, 59, 871–877.
1. Introduction Because of the threat of global warming, the need to reduce emissions of the greenhouse gas CO2 from fossil fuels is urgent. One method for reducing CO2 emissions involves capturing and sequestering CO2 before it is released into the atmosphere. However, the cost of CO2 capture, in the three steps including capture, transport, and sequestration processes, is reportedly much higher than the costs of others.1 Using calcined lime (main componet, CaO) to capture CO2 in the exhaust gas2 or in the reactor3 during the combustion or gasification of fossil fuels is effective; the lime absorbs CO2 in the fuel gas to form the calcium carbonate (CaCO3; eq 1), and then, the CaCO3 is thermally decomposed to CaO with the release of nearly pure CO2 (eq 2) for sequestration. Increasing the temperature enhances the thermal decomposition of CaCO3, and increasing the CO2 partial pressure inhibits the decomposition of CaCO3. CaO + CO2 f CaCO3; exothermic
(1)
CaCO3 f CaO + CO2; endothermic
(2)
study,4
10.1021/ef800039k CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
Limestone Calcination with CO2 Capture
Energy & Fuels, Vol. 22, No. 4, 2008 2327 Table 2. Experimental Conditions
Table 1. Chemical Analysis of Limestone composition [wt %] Kuzuu (Tochigi, Japan)
FCaO
FMgO
FCO2
Fothers
50.60
3.98
44.32
1.10
steam, and the results of the two groups were not consistent with regard to the influence of steam concentration on limestone decomposition. Burnham et al.11 calcined oil shale in N2/CO2 in the presence of steam and observed that the presence of steam lowers the calcite decomposition temperature and enhances the decomposition. However, none of these previous studies provide information about the effect of high concentrations of steam on the decomposition of limestone. In this work, we studied the decomposition of limestone (main component, CaCO3) in the presence of high steam dilution (∼100%) by using a continuously operating fluidized bed reactor. Limestone decomposition under these conditions was also compared with limestone decomposition in various N2 dilution atmospheres. The effects of temperature, average residence time of particles and gas heat transfer on CaCO3 decomposition conversion were investigated. The reactivities (hydration and carbonation) of the CaO produced were also examined. 2. Experimental Section 2.1. Sample. Limestone (Kuzuu, Japan; Table 1) was ground and sieved to 0.25-0.50 mm for the experiments. 2.2. Experimental Apparatus. A continuously operating fluidized bed reactor was used to measure the decomposition conversion of the limestone. Detailed information about the experimental apparatus has already been reported.4 For the present study, a steam generator and a condenser were added to the apparatus (Figure 1). The steam generator was made of a stainless coil held in an electric furnace and a water pump supplied water to the heated zone (stainless coil, ∼1073 K) of the generator. CO2 diluted by steam was supplied from the bottom of the preheater; the gas mixture passed through the fluidized bed reactor and then flowed out the top of the reactor. For experiments of the limestone decomposition with steam dilution, all the lines from the outlet of the steam
CO2/steam or CO2/N2 ratio limestone fluidized bed [10-3 (N m3 s-1)/ feed rate temperature gas flow rate [g s-1] run no. [K] [10-3 (N m3 s-1)] 10-3 (N m3 s-1)] 1 2 3 4 5 6 7 8
1123 1153 1193 1193 1193 1193 1193 1223
steam dilution 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.10/0.15 0.10/0.15 0.10/0.15 0.20/0.05 0.15/0.10 0.10/0.15 0.10/0.15 0.10/0.15
0.18 0.18 0.18 0.18 0.18 0.23 0.13 0.18
9 10 11
1193 1193 1193
N2 dilution 0.25 0.25 0.25
0.20/0.05 0.15/0.10 0.10/0.15
0.18 0.18 0.18
12
1293
without dilution (100% CO2) 0.25
generator to the inlet of the gas preheater and from the reactor outlets to the condenser and to the overflow holder, as well as the overflow holder, were heated up to about 573 K with heater. A condenser was placed downstream of the cyclone to separate the steam from the exhaust gas. 2.3. Limestone Decomposition with Steam Dilution. Temperatures of the fluidized bed reactor and the gas preheater were raised by means of electric furnaces to the target values while the CO2 stream was injected at a flow rate determined by the desired CO2/ steam ratio. The steam generator was heated to 973 K and supplied a flow rate of steam determined by the desired CO2/steam ratio. Table 2 shows the experimental conditions. A purge gas (N2, 0.83 × 10-6 N m3 s-1) was injected into the overflow holder to prevent steam in the reactor from flowing into the overflow holder. When the temperatures of the reactor and steam generator reached the target values, the limestone particles were supplied by a screw feeder to start limestone decomposition in the fluidized bed. The differential pressure, ∆P, between the bottom and top of the fluidized bed was continuously measured and increased with particle supply until overflow began (Figure 2). After ∆P and the bed temperatures stabilized, the experiment was then continued about 70 min. The time between the start of particle supply and the beginning of overflow was defined as the average residence time (to) of the particles in the bed; the bed temperature used was the average value measured by K-type thermocouples located at three points T1, T2, and T3. 2.4. Solid Analysis. The solid sampled from the overflow holder after limestone decomposition was analyzed by a thermogravimetric analysis (TGA; Rigaku ThermoPlus 8120) technique under a N2 atmosphere (heating rate, 20 K min-1 up to 1123 K). The decomposition conversion of limestone (CaCO3 to CaO), X, in the sample was defined as
[
X) 1-
Figure 1. Continuously operating thermal decomposition apparatus.
0.18
(100 - FCO2)Wchange56 FCaO(W0 - Wchange)44
]
× 100%
(3)
Figure 2. Plots of reactor temperature and differential pressure between the bottom and the top of the bed versus operation time.
2328 Energy & Fuels, Vol. 22, No. 4, 2008
Wang et al.
Figure 3. Effect of dilution gas percentage on the decomposition conversion of limestone.
Figure 4. Effect of temperature on the decomposition conversion of limestone in various atmospheres.
The reactivity of CaO in the sample after limestone decomposition was tested by subjecting the decomposed sample to steam hydration and CO2 carbonation with the TGA apparatus under a total pressure of 3.0 MPa.4 Hydration reactivity was measured at 923 K under 2.0 MPa of steam partial pressure, and carbonation reactivity was measured at 923 K under 0.04 MPa of CO2 partial pressure.
3. Results and Discussion 3.1. Limestone Decomposition with Steam Dilution. The decomposition conversions of limestone (CaCO3 f CaO) were measured at various steam dilution percentages (Table 2), and the results are shown in Figure 3. It can be seen that the decomposition conversion of limestone increased with increasing steam percentage at a bed temperature of 1193 K and an average residence time of 40 min. For example, in the absence of steam (100% CO2), the decomposition conversion of limestone was 72%; at 20% steam dilution, the decomposition conversion was 96%, and at 60% steam dilution, the conversion was 98%. These results indicate that the rate of limestone decomposition increased with increasing steam dilution. In a previous study,4 the limestone decomposition rate, Rr, was described to strongly depend on the term (P* - PCO2) in the following equation:
[
P* - P ( -E RT )](
Rr ) KD(P* - PCO2) ) A exp
CO2
)
(4)
where P* is the equilibrium CO2 pressure for CaCO3 decomposition and PCO2 is the partial pressure of CO2 in the reactor. Steam dilution reduced PCO2 in the reactor, which increased the limestone decomposition rate, Rr, and consequently increased the decomposition conversion of limestone. The results for N2 dilution are also shown in Figure 3. The decomposition conversion increased with increasing N2 dilution percentage. For example, the conversion increased from 89% to 97% when the percentage of N2 was increased from 20% to 60%. However, the conversion increase obtained with N2 dilution was slower than that obtained with steam dilution and the difference was largest at 20% dilution (at 20% steam, the conversion was 96%, whereas at 20% N2, the conversion was 89%). This result was due to the different thermal properties of steam and N2, which will be discussed below. The decomposition conversions of limestone with or without steam dilution were examined at various fluidized bed temperatures. The average residence time of particles in the bed was 40 min. Figure 4 shows that at a constant steam dilution of 60%, the conversions of CaCO3 to CaO were 60% at 1123 K, 82% at 1153 K, and 98% (nearly complete decomposition) at 1193 K, significantly increased with bed temperature. At 1123 K and 100% steam, the conversion was 99%. Whereas, the conversions for limestone decomposition without dilution (100% CO2) were 72% at 1193 K, 93% at 1273 K, and 95% at 1293
Figure 5. Effect of the average residence time of particles in the bed on the decomposition conversion of limestone.
K. Comparison of these results clearly indicates that reducing the CO2 partial pressure by steam dilution led to higher conversions of CaCO3 to CaO at comparatively lower bed temperatures. Figure 5shows the variation of decomposition conversion of limestone with residence time. The increase in conversion was slowly by increasing residence time: with steam dilution at 1193 K, the conversion increased by only 2% from 30 min (97%) to 50 min (99%), and without dilution at 1223 K, the conversions were only 83% and 90% at 60 and 90 min, respectively. However, comparing the result of steam dilution with that without dilution, such as with steam dilution 60%, 97% conversion was obtained even at a residence time as short as 30 min, whereas the conversion was still 90% at a residence time of 90 min without dilution (100% CO2), which indicates that steam dilution decreased not only the decomposition temperature of limestone but also the residence time required for nearly complete decomposition of CaCO3. 3.2. Effect of Gas Heat Transfer on the Decomposition Conversion of Limestone. As stated above, the decomposition conversions observed differed for steam dilution and N2 dilution, especially at 20% dilution (Figure 3). This difference was due to the different thermal conductivities of steam and N2: at 1193 K, the thermal conductivity of steam (0.127 W m-1 K-1) is much higher than that of N2 (0.071 W m-1 K-1). The effect of this difference on the heat transfer between gas and particles in fluidized bed reactor in the CO2/steam and CO2/N2 systems can be evaluated as follows. In the reactor, the rate of heat transfer, QT, between the carrier gas and limestone particles is given by12 QT ) hAp(Tb - Tp) + σεAp(Tb4 - Tp4)
(5)
where h is the heat transfer coefficient, Ap is the particle surface area, Tb and Tp are the bed temperature and particle temperature, respectively. σ is the Stefan-Boltzmann constant, and ε is the (12) Ross, I. B.; Patel, M. S.; Davidson, J. F. Trans. Inst. Chem. Eng. 1981, 59, 83–88.
Limestone Calcination with CO2 Capture
Energy & Fuels, Vol. 22, No. 4, 2008 2329
Table 3. Parameters for QT and QR Calculations (1193 K, 0.1 MPa) terms
Fg [kg m-3]
µg [10-5 (Pa s)]
kg (W m-1 K-1)
CO2 steam N2
0.438 0.181 0.286
5.00 4.00 4.70
0.086 0.127 0.071
dp [m]
u [m s-1]
∆H1193K [kJ mol-1]
0.000380
0.870
166.2
(6)
(7)
Because the radiation term in eq 7 is much smaller than the conduction term in the fluidized bed temperature range, the radiation term can be ignored. Nu for heat transfer in a fluidized bed was examined by Kunii and Levenspiel13 on the basis of experimental data reported by many investigators. Kunii and Levenspiel13 found that Nu can be correlated with the Reynolds number, Rep by means of the following equations: 0.5 < Rep < 50
Nu ) 0.03Rep1.3 ; Rep )
dpuFg µg
(8) (9)
where u, Fg, and µg are the superficial velocity, density, and viscosity of the gas, respectively. Accordingly, the rate of heat transfer between gas and limestone particles is given by
( )
QT ) 0.03
dpuFg µg
1.3
πdpkg(Tb - Tp)
CO2 [%] steam [%] N2 [%]
Fg [kg m-3] µg [10-5 (Pa s)] kg [10-2 (W m-1 K-1)]
where dp is the mean diameter of the particle. Equation 5 can be rewritten as QT ) Nuπdpkg(Tb - Tp) + σεAp(Tb4 - Tp4)
atmosphere
terms
gas emissivity. The first term on the right-hand side of eq 5 represents the rate of heat transfer from the carrier gas by conduction or convection, and the second term represents the rate of heat transfer by radiation. The unknown in eq 5 is h, which depends on the thermal conductivity, kg, of the gas. The relevant Nusselt number, Nu, is given by Nu ) hdp/kg
Table 4. Density, Viscosity, and Thermal Conductivity of Gas Mixture (1193 K, 0.1 MPa)
(10)
All of the parameters in eq 10 can be obtained from experimental data (shown in Table 3), except for Tp. Therefore QT is a function of the unknown parameter Tp.
Figure 6. Heat transfer, QT, and heat, QR, needed for decomposition reaction calculated from eqs 10 and 14.
80 20
80
60 40
60
20
40
40 60
40 60
0.387 0.408 0.330 0.377 0.284 0.347 4.90 4.96 4.77 4.92 4.58 4.86 9.28 8.35 1.00 8.09 10.8 7.40
Figure 6shows the relationship between QT and Tp at Tb ) 1193 K. It can be seen that QT decreased rapidly with increasing Tp. When Tp approached Tb, QT approached zero. Figure 6 also shows that the QT value for steam dilution was larger than the value for N2 dilution because the thermal conductivity of steam is larger than that of N2. For example, at Tp ) 1160 K, QT was 3.81 × 10-4 J s-1 in 20% steam and 3.61 × 10-4 J s-1 in 20% N2. However, increasing the percentage of steam or N2, that is, decreasing the CO2 fraction in the supply gas, led to a reduced QT. Because CO2 is much denser than steam or N2, decreasing the CO2 fraction reduced the density of the gas mixture, Fg, and, consequently, reduced QT as shown by eq 10. The rate of limestone decomposition for a single particle can be obtained from eq 3 as follows. Silcox et al.14 reported the following values for A, E, and P*: A ) 0.012
(11)
E ) 33470
(12)
(
P* ) 4.192 × 109 exp
-20474 Tp
)
(13)
The rate of heat, QR, needed for initial reaction of a single particle is given by
[ ( ) ](
QR ) πdp∆H A exp
-E RTP
P* - PCO2)
(14)
QR values calculated for various Tp values at three CO2 partial pressures, PCO2, which correspond to the steam or N2 dilution percentages, are also shown in Figure 6. It can be seen that QR increased with Tp, and the QR curves intersected the QT curve at only one point for each CO2 partial pressure. Because the decomposition of limestone particles (CaCO3 f CaO) must occur close in the capacity of heat transfer for the particle, we suggest the points where the curves intersect represent the particle temperatures suitable for heat transfer and the heat required for decomposition. It can see that the QR line for PCO2 ) 0.08 MPa intersects the QT line for 20% steam at point A (Tp ) 1161.5 K and a heat transfer of 3.65 × 10-4 J s-1) and intersects the QT line for 20% N2 at point B (Tp ) 1161.1 K and a heat transfer of 3.49 × 10-4 J s-1). The heat transfer for steam dilution was 1.05 times that for N2 dilution. This faster heat transfer may explain why the decomposition conversion (rate of decomposition) of limestone with steam dilution was higher than that with N2 dilution (Figure 3). Figure 6 also shows that an increase in the percentage of dilution gas, steam or N2, caused the QT values to decrease and the difference between the QT for steam dilution and the QT for N2 dilution to increase. The decrease in QT was caused by the decrease in the density of the gas mixture as the fraction of the dilution gas was increased (Table 4). The increase in the difference between the QT values for steam and N2 was due to the fact that the thermal conductivity of steam is higher than that of N2, as mentioned above.
2330 Energy & Fuels, Vol. 22, No. 4, 2008
Wang et al.
Table 5. Particle Temperatures at Intersections of QT and QR steam dilution
N2 dilution
dilution percentage [%]
intersection point
Tp [K]
QT ) QR [10-4 J s-1]
intersection point
Tp [K]
QT ) QR [10-4 J s-1]
20 40 60
A C E
1161.5 1149.7 1135.7
3.65 4.56 5.63
B D F
1161.1 1148.9 1132.9
3.49 4.28 4.88
However, QR also increased with increasing dilution gas percentage, because the reaction rate increased when PCO2 decreased (Table 5). As shown in Figure 6, for steam dilution, the curves for QR and QT intersect at point C with 4.56 × 10-4 J s-1 for PCO2 ) 0.06 MPa (40% steam) and at point E with 5.63 × 10-4 J s-1 for PCO2 ) 0.04 MPa (60% steam). For N2 dilution, the QR and the QT curves intersect at point D with 4.28 × 10-4 J s-1 for PCO2 ) 0.06 MPa (40% N2) and at point F with 4.88 × 10-4 J s-1 for PCO2 ) 0.04 MPa (60% N2). The fact that the QR values at the intersections for N2 percentages of 40% and 60% were higher than the QR value for 20% steam explains the phenomenon observed in Figure 3, wherein the rates of limestone decomposition in 40% and 60% N2 were faster than the rate of limestone decomposition in 20% steam. The decomposition conversions in 40% and 60% N2 were close to the final conversion value, whereas the difference between the conversions for steam dilution and N2 dilution was much smaller (Figure 3). 3.3. Comparison of the Reactivities of the CaO Produced. After limestone decomposition, we compared the hydration and carbonation reactivities of the CaO products with nearly the same decomposition conversions, 98% obtained with 60% steam and 95% obtained without dilution (100% CO2) by means of TGA. In the hydration test at 923 K and a steam partial pressure of 2.0 MPa (total pressure, 3.0 MPa), the time required for complete conversion [CaO f Ca(OH)2] of the CaO produced with steam dilution was 6 min, nearly half that required for the CaO produced in 100% CO2 (Figure 7). For the carbonation test at 923 K and a CO2 partial pressure of 0.04 MPa (total pressure 3.0 MPa), close to 70% carbonation conversion (CaO f CaCO3) was obtained with the CaO produced by steam dilution, whereas the conversion was about 40% for the CaO produced in 100% CO2 (Figure 8), which indicates that the reactivity of the CaO was greatly improved by limestone decomposition in steam dilution. These results can be explained in terms of the decreased calcination temperature (due to the fact the steam dilution lowered the CO2 partial pressure) and the shortening of the average residence time of the particles in the bed, which effectively prevented the sintering of particles. Prevention of sintering enhanced the reactivity of the CaO produced by increasing the specific surface area and pore volume.15 For example, in this case, the specific surface area
Figure 8. Comparison of carbonation reactivities of the CaO produced by limestone decomposition with steam dilution (60%) and without dilution (100% CO2).
and pore volume were 8 m2 g-1 and 0.030 cm3 g-1 with 60% steam dilution, whereas they were 3 m2 g-1 and 0.010 cm3 g-1 without steam dilution, respectively. 4. Conclusions The decomposition of limestone particles in various CO2/ steam (steam dilution) and CO2/N2 (N2 dilution) atmospheres (about 0.1 MPa) was investigated using a continuously operating fluidized bed reactor. The hydration and carbonation reactivities of the CaO produced by limestone decomposition were also tested. The following results were obtained: (1) The decomposition conversion of limestone increased with increasing steam or N2 dilution percentage. At a bed temperature of 1193 K and an average residence time of 40 min, the conversions were 96% for 20% steam dilution and 98% for 60% steam dilution and were 89% for 20% N2 dilution and 97% for 60% N2 dilution, whereas the conversion was 72% without dilution. (2) The difference between the decomposition conversions obtained with steam dilution and N2 dilution could be explained in terms of the difference between the heat transfer from gas to limestone particle in steam and N2 dilution atmospheres. (3) In the hydration test, the time required for complete conversion [CaO f Ca(OH)2] of the CaO produced with steam dilution was nearly half that for CaO produced without dilution (100% CO2). In the carbonation test, the carbonation conversion (CaOfCaCO3) of the CaO produced was nearly 70% for the CaO produced with steam dilution, whereas the conversion was about 40% for the CaO produced in 100% CO2. Appendix 1. General equation for the viscosity of a binary gas mixture:16 µg )
Figure 7. Comparison of hydration reactivities of the CaO produced by limestone decomposition with steam dilution (60%) and without dilution (100% CO2).
µ1 µ2 + φ12 ) y2 y1 1 + φ12 1 + φ21 y1 y2 µ1 0.5 M2 0.25 2 µ2 0.5 M1 0.25 1+ 1+ µ2 M1 µ1 M2 φ21 ) 0.5 0.5 M M 4 1 4 2 1+ 1+ 0.5 0.5 M2 M1 2 2
[ ()( ) ] [ ]
[ ()( ) ] [ ]
2
Limestone Calcination with CO2 Capture
Energy & Fuels, Vol. 22, No. 4, 2008 2331
2. General equation for the thermal conductivity of a binary gas mixture:17 kc1
kg )
+ 1 + (y2/y1)(M12/M1)0.125φ12 kd1 kd2 kc2 + + 0.125 1 + (y2/y1)φ12 1 + (y1/y2)φ21 1 + (y /y )(M /M ) φ 1
2
kc1 )
21
2
21
9.5k1 ; 1.32Cp1 + 0.26 kd1 ) k1 - kc1 ;
kc2 )
9.5k2 1.32Cp2 + 0.26
kd2 ) k2 - kc2
M12 ) M21 ) (M1 + M2)/2 Cp,CO2 ) 5.316 + 1.4285 × 10-2Tb - 0.8362 × 10-5Tb2 + 1.784 × 10-9Tb3 Cp,steam ) 7.7 + 0.04594 × 10-2Tb + 0.2521 × 10-5Tb2 0.8587 × 10-9Tb3 Cp,N2 ) 6.903 - 0.03753 × 10-2Tb2 + 0.193 × 10-5Tb2 0.6861 × 10-9Tb3 Notation A ) frequency factor (mol m-2 s-1 kPa-1) Ap ) particle surface area (m2) dp ) mean diameter of limestone particle (m) E ) activation energy (J mol-1) (13) Kunii, D.; Levenspiel, O. Fluidization Engineering: ButterworthHeinemann: Woburn, MA, 1991. (14) Silcox, G. D.; Kramlich, J.; Pershing, D. W. Ind. Eng. Chem. Res. 1989, 28, 155–160. (15) Borgwardt, R. H. Chem. Eng. Sci. 1989, 44, 53–60. (16) Bromley, L. A.; Wilke, C. R. Ind. Eng. Chem. 1951, 43, 1641– 1648. (17) Cheung, H.; Bromley, L. A.; Wilke, C. R. AIChE J. 1962, 8, 221– 228.
FCaO, FCO2 ) CaO and CO2 contents in raw limestone (wt %) h ) heat transfer coefficient (W m-2 K-1) ∆H ) reaction heat for the decomposition of CaCO3 at 1193 K (J mol-1) KD) reaction coefficient, KD ) 0.012 exp(-4026/T) (mol m-2 s-1kPa-1) kg ) thermal conductivity of single gas or gas mixture (W m-1 K-1) Nu ) Nusselt number PCO2 ) partial pressure of CO2 in reactor (kPa) P* ) equilibrium CO2 pressure for CaCO3 decomposition (kPa) QR ) rate of heat needed for initial reaction of a particle (J s-1) QT ) rate of heat transfer (J s-1) R ) gas constant (J K-1 mol-1) Rr ) decomposition rate of limestone (mol m-2 s-1) Rep ) Reynolds number Tb, Tp ) bed temperature and limestone particle temperature (K) u ) superficial velocity of gas (m s-1) W0, Wchange ) initial weight and weight change of the sample (sampled from overflow holder) analyzed by means of TGA measurement (g) X ) decomposition conversion of limestone (%) ε ) gas emissivity σ ) Stefan-Boltzmann constant for radiation (W m-2 K-4) Fg ) density of single gas or mixture gas (kg m-3) µg ) viscosity of single gas or mixture gas (Pa s) µ1, µ2 ) viscosity of pure components 1 and 2 at the temperature and pressure of the mixture (Pa s) y1, y2 ) mole fractions of components 1 and 2 φ21, φ12 ) functions of molecular weight and viscosity of the components M1, M2 ) molecular weight of components 1 and 2 (g) kc1, kc2 ) collisional conductivity of components 1 and 2 kd1, kd2 ) diffusional conductivity of components 1 and 2 Cp ) heat capacity at constant pressure (J mol-1 K-1) EF800039K
