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Quantitative X-ray Diffraction Analysis for Sulfation of Limestone in Flue Gas Desulfurization Soon Kook Kang, Sang Mun Jeong, and Sang Done Kim* Department of Chemical Engineering and Energy & Environmental Research Center, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea
Sulfation and calcination characteristics of pure CaO (lime) and limestone under a CO2 gas atmosphere (0-15%) were determined by using both X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The decomposition temperature of limestone in a calcination reaction has been correlated with CO2 partial pressure. The sulfation conversions of pure CaO and limestone are predicted by quantitative XRD analysis, and the sulfation conversion by an XRD method is good according to that measured by TGA. The sulfation reactivity and sulfur capture capacity of limestone increase with increasing temperature and decreasing CO2 partial pressure in the ambient gas stream. The full width at half-maximum (fwhm) of the main peak (CaO) in the XRD spectrum on the sintered lime decreases with increasing sintering temperature and time. The CaO grain size of sintered lime is determined by a qualitative XRD analysis and correlated with the specific surface area of lime and the true density of CaO. Introduction The emission of sulfur oxides (SOx) from combustion of high sulfur coals in industrial boilers is generally viewed as a major contributor for acid rain. Various sorption processes are under operation to remove SOx from flue gas.1-3 Though the process using limestone to remove SO2 is old, the process is widely utilized in coal-fired boilers.4,5 Although the detailed mechanism of desulfurization is not well understood, it is generally accepted that the reaction involves two steps (calcination and sulfation reactions):
CaCO3 f CaO + CO2
(1)
CaO + SO2 + 1/2O2 f CaSO4
(2)
Nonporous limestone decomposes to porous lime (CaO) of the same volume with a porosity of 54% in a combustor. When CaO is exposed to SO2 and O2 at higher temperature, CaSO4 is formed on the internal and external surfaces of calcined lime. It is generally agreed that the optimum temperature of sulfur retention in a fluidized-bed combustor is around 800-900 °C. The exact optimum temperature is a function of sorbent properties and operating conditions in the bed. Sulfur capture decreases because of the lower rate of the sulfation reaction below the optimum temperature. Above the optimum temperature, the reverse sulfation reaction becomes increasingly important. The structure of calcined lime changes primarily because of sintering at higher temperatures (>900 °C) with decreasing active site and consequent reduction in the reactivity toward SO2.6,7 Conventional techniques for measuring solid-state kinetic data are to measure mass or enthalpy changes * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: 82-42-869-3913. Fax: 82-42869-3910.
of the reacting sample. Thermogravimetric analysis (TGA) or differential thermal analysis (DTA) techniques can be applied easily to simple gas-solid reacting systems (i.e., lime-SO2-O2 reaction). However, in the case of competitive reactions (i.e., a direct limestone sulfation reaction), they may have difficulties explaining the product composition exactly during the reaction because of the production of various species during the competitive reaction.8 X-ray diffraction (XRD) may offer the possibility of obtaining kinetic data in a complex gas-solid reacting system. Because XRD can provide phase-specific solidstate concentration data, it has a distinct advantage over conventional techniques (TGA and DTA). Modern X-ray diffractometers are capable of obtaining the concentration data with a higher degree of quantitative accuracy, precision, and repeatability. Recently, a quantitative XRD analysis has been used to determine the extent of reaction to produce La2CuO4 as a function of time.9 Also the mole fraction of sulfated calcium carbonate was measured by quantitative XRD analysis.10 In the present study, the effects of calcination temperature (750-1000 °C), CO2 partial pressure (0-1 atm) on the calcination reactivity, the sulfation reactivity of lime-SO2, and the sulfur capture capacity of limestone have been determined by using TGA and quantitative XRD. The CaO grain size of sintered lime at a given sintering temperature (800-1000 °C) was determined using the qualitative XRD analysis. The calculated grain size of CaO based on the XRD lines has been correlated with the surface area of calcined lime and the true density of CaO. Theoretical Background. Calcination and sulfation reaction kinetics of limestone. Calcination and subsequent sulfation of limestone proceed in a coal combustor. The unreacted shrinking core model for spherical particles in the chemical control regime can represent the calcination of CaCO3 as11,12
()
r t )1) 1 - (1 - Xc)1/3 τrxn r0
10.1021/ie990749m CCC: $19.00 © 2000 American Chemical Society Published on Web 06/16/2000
(3)
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where Xc and τrxn are calcination conversion and reaction time constants, respectively. The time constant (τrxn) is defined as
τrxn )
Fsr0 ks(Ce - Cg)n
(4)
where Fs, r0, ks, Ce, and Cg are the solid density, initial particle radius, reaction constant, and equilibrium and bulk concentrations of CO2, respectively. The model of the sulfation reaction can be applied to the chemical reaction and diffusion mechanisms simultaneously as
t ) g(Xs) + δ2p(Xs) τpid
(5)
where
Dividing eqs 10 and 11 by eq 9 yields
Iβ R′βCβ ) IR R′RCR
(12)
Iω R′ωCω ) IR R′RCR
(13)
where CR, Cβ, and Cω are the volume fractions of the R, β, and ω phases in the mixture, respectively. The values of Cβ/CR and Cω/CR can therefore be obtained from the measurements of Iβ/IR and Iω/IR and the calculation of R′R, R′β, and R′ω. Once volume fractions Cβ/CR and Cω/CR of the phases are calculated, CR, Cβ, and Cω can be obtained from the additional eq 14 as
C R + Cβ + Cω ) 1
(14)
Experimental Section
g(Xs) ) 1 - (1 - Xs)1/3 δ2 )
ksdp0 12Deff
[
p(Xs) ) 3
(6) (7)
]
Z - {Z + (1 - Z)(1 - Xs)}2/3 - (1 - Xs)2/3 Z-1 (8)
In eqs 5 and 6, g(Xs) and p(Xs) are the conversion functions, which may indicate the sulfation is under the complete control of the chemical reaction or diffusion, respectively. Also Z ()3.09) is the molar volume ratio of the product (CaSO4) to the reactant solid (CaO), and δ2 is the ratio of diffusion to chemical reaction resistance. When δ2 < 0.1, the process is considered to be under chemical reaction control; when δ2 > 10, the system is considered to be under diffusion control.13 Quantitative Analysis in Sulfated Limestone by XRD. A quantitative XRD analysis is based on the diffraction intensity of a particular phase in a mixture that depends on the phase concentration.14 The direct comparison method for a quantitative XRD analysis does not require a sample of the pure phase whose composition can be determined from the reference line of another phase in the mixture. The strength of this method is that it is completely general, requiring no external knowledge of how the concentration of the internal standard changes as the sample reacts.8 Considering that the sulfated sorbent is composed of three phases (R ) CaCO3, β ) CaO, ω ) CaSO4), the particular diffraction intensity of each phase can be written as
IR )
KR′RCR 2µ
(9)
Iβ )
KR′βCβ 2µ
(10)
Iω )
KR′ωCω 2µ
(11)
The chemical and physical properties of natural limestone used in the present study are listed in Table 1. Calcination and sulfation reactions of limestone were carried out in a thermobalance reactor as shown in Figure 1. The main reactor was made of a stainless steel pipe (0.055 m i.d. × 1.0 m height), which was heated by a 5 kV Kanthal electric heater around the reactor. At the bottom of the reactor, a gas inlet port was installed for reactant gases. At the upper port of the reactor, a hatch-type opening was made to insert or remove the sampling basket. The sample basket suspended from an electronic balance was lowered into the reaction zone of the TGA system.15 The reactor was heated to the reaction temperature under a stream of dry air. When the system reached steady state at an isothermal condition, a stream of the desired reactant gas (air, CO2, or SO2) was injected into the reactor at 0.5 m/s (17.7-21.7 std. L/min). A known weight of limestone (0.41 ( 0.005 g) in the sampling basket was lowered into the reaction zone of the thermobalance using a winch assembly. The reactivity in terms of weight gain with reaction time was obtained from an electronic balance, and a computer recorded its weight signals. For a Brunauer-Emmett-Teller (BET) surface area and a quantitative XRD analysis, a sample was taken from the basket through the hatch opening and then it was sealed in a vial tube. The specific surface areas of calcined lime were determined by the BET method. The volume fraction of CaCO3, CaO, and CaSO4 in sulfated limestone was determined from a quantitative XRD analysis, and the CaO grain size in sintered lime was determined at room temperature using a diffractometer (Rigaku goniometer PMG-S2) employing a Ni-filtered Cu KR radiation target source at 50 kV and 40 mA. Results and Discussion Calcination and Sulfation Kinetics under a CO2 Gas Mixture Atmosphere. In a coal combustor, CO2 is always present and its partial pressure may play a significant role in determining the optimum reaction temperature in sulfation of limestone because the thermal decomposition of limestone is a reversible reaction. Limestone (CaCO3) is decomposed to produce lime (CaO) if the ambient partial pressure of CO2 is below the equilibrium value at a given temperature, and
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Table 1. Physical Properties and Chemical Analysis (wt % on a Dry Basis) of Limestonea surface area (m2/g)
pore volume (cm3/g)
1.606 SiO2 2.79
mean pore diameter (µm)
0.0416 Fe2O3 1.11
Al2O3 0.80
bulk density (g/cm3)
1.346 CaO 92.0
MgO 2.46
apparent density (g/cm3)
1.359 SO3 0.04
Na2O 0.02
2.913 K2O 0.47
ZnO 0.04
MnO 0.01
a The surface area is obtained by BET. The pore volume, mean pore diameter, bulk density, and apparent density are obtained by mercury porosimetry.
Figure 1. Schematic diagram of the experimental apparatus: (1) preheater, (2) thermocouple, (3) temperature controller, (4) electric heater, (5) sample basket, (6) cold trap, (7) vacuum pump, (8) hatch, (9) N2 purge line, (10) electronic balance, (11) winch assembly, (12) flowmeter, (13) reactant gas.
Figure 3. Calcination conversion of limestone as a function of the reaction temperature under different CO2 gas concentrations in the ambient gas stream at 900 °C.
The decomposition temperatures of limestone in the present and previous16 studies have been correlated with CO2 partial pressure as
Tdoc (K) ) 37.9 ln(yCO2) + 1020
(15)
with a correlation coefficient of 0.97. Calcination conversion as a function of the reaction time with various CO2 concentrations (0-15%) in the bulk gas stream at 850-1000 °C is shown in Figure 3. The equilibrium partial pressure of CO2 over the limestone is calculated from the equation of Hill and Winter17 as
log Pe ) -8792.3/T + 10.4
(16)
The calcination rate for spherical particles of unchanging size under a CO2 gas atmosphere can be expressed as Figure 2. Effect of CO2 partial pressure on the limestone decomposition temperature.
-
N0 dXc ) kc(Ce - Cg)n 4πr 2 dt
(17)
c
conversely the calcined lime will be transformed back to calcium carbonate if the partial pressure of CO2 exceeds the equilibrium value. The effect of CO2 partial pressure on the decomposition temperature at which limestone (dp ) 320 µm and W0 ) 0.4 g) decomposed into CaO and CO2 is shown in Figure 2. As can be seen, limestone decomposes at 675 °C under an air atmosphere; however, the decomposition takes place around 820 °C under a CO2 partial pressure of about 0.1 atm in the bulk gas stream. On the other hand, when the CO2 partial pressure is closed to 1 atm, limestone is decomposed around 917 °C.
The calcination rate of limestone decreases with decreasing temperature and increasing CO2 concentration because the driving force (Ce - Cg) for thermal decomposition of limestone increases with increasing temperature and decreasing CO2 concentration. If the reaction kinetics of calcination is represented by eq 17, the apparent reaction order can be found from a plot of ln(dXc/dt)ini vs ln(Ce - Cg) for a given temperature. The obtained apparent reaction order for limestone is found to be unity with respect to Ce - Cg as found in previous studies (Table 2). The different values of the apparent activation energies in previous studies may reflect the
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2499 Table 2. Experimental Values of the Calcination and Sulfation of Limestone author
operating temp (°C)
sample type (size, µm)
reaction order
Beruto and Searcy23 Dennis and Hayhurst24 Khraisha and Dugwell12 this work
660-839 824-874 802-999 720-1000
Calcination of Limestone pellet (500) 1.0 particle (500) powder (90) 1.0 particle (320) 1.0
Hajaligol et al.16 Snow et al.18 this work
500-940 850-1250 800-1000
Direct Sulfation of Limestone powder (2-45) powder (2-12) powder (320)
activation energy (kcal/kmol)
equipment
49 000 40 200 46 000 31 500
TGA fluidized bed suspension reactor TGA
16 400 16 400 19 500
TGA TGA TGA
Figure 5. Direct sulfation conversion of limestone with CO2 concentration in an ambient gas stream at 120 min.
Figure 4. Weight variations of limestone as a function of the reaction time with different temperatures (A) and CO2 concentrations (B) for direct sulfation.
varying dependency of the decomposition rate of CaCO3 because the apparent activation energies are sensitive to the composition, size, and weight of powdered samples.23 The weight variation during the direct sulfation with time as a function of the reaction temperature and CO2 concentration is shown in Figure 4. Initially, the weight of the limestone sample decreases with time because calcination is more predominant than sulfation in the direct sulfation of limestone. Thereafter, the reaction proceeds in the sulfation reaction of calcined lime, where the sample weight increases with time. The calcination reaction proceeds longer (=30 min) at 850 °C under a 10% CO2 atmosphere than that at higher temperatures because of lower driving forces (Ce - Cg). The sulfation reactivity and sulfur capture capacity of limestone increase with increasing temperature (Figure 4A). The effect of CO2 concentration on the sulfation capacity at 900 °C is shown in Figure 4B, where the SO2 removal capacity of limestone increases with decreasing CO2 concentration because the calcination reaction is slowly terminated by increasing the CO2 concentration. The apparent activation energy for the direct sulfation of limestone is found to be 19 500 kcal/kmol from the Arrhenius plot for the direct sulfation of limestone under the CO2 concentration (Table 2). This value is well according to the results of previous studies.16,18
Sulfation conversion of limestone during direct sulfation after a reaction time of 120 min under different CO2 concentrations (0-15%) in an ambient gas stream is shown in Figure 5. The sulfur capture capacity of the sorbent increases with decreasing CO2 concentration and with increasing sulfation temperature because the calcination of limestone is rapidly terminated by decreasing the CO2 concentration and increasing the temperature. This result may indicate that the sulfation conversion of limestone in flue gas desulfurization has a relation to the calcination condition such as the CO2 concentration and reaction temperature. When the shrinking-core model is fitted to the experimental data of the initial sulfation reaction rate, a straight line with respect to the ratio of diffusion to reaction resistance (δ2) can be obtained.16 For the direct sulfation without CO2 gas as can be seen in Figure 6, the ratio (δ2) increases with a reaction temperature up to 850 °C and then decreases with a further increase in the reaction temperature. This may indicate that the direct sulfation becomes less diffusion-controlled as the temperature increases above 850 °C but never completely controlled by chemical reaction as reported previously.16 It may be ascribed that the pore size (mean pore diameter 0.40 µm at 850 °C, 0.59 µm at 900 °C, and 1.11 µm at 1000 °C) of calcined lime rapidly increases with increasing calcination temperature because of thermal sintering under the absence of CO2 in an ambient gas stream. Therefore, the rate of increase in Deff of SO2 gas is higher than that in ks. This result can be confirmed by the fact that the apparent activation energy (35 000 kcal/kmol) for diffusivity of SO2 gas19
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Figure 6. Effect of reaction temperature on the ratio of product layer diffusion to chemical reaction resistance at CO2 concentrations of 0 and 5%. Figure 8. XRD spectra of sulfated lime at 850 °C with variation of the reaction time.
Figure 7. XRD spectra of reagent-grade samples.
is higher than that (19 500 kcal/kmol) of the direct sulfation reaction without a CO2 atmosphere. However, δ2 under a 5% CO2 atmosphere increases up to 950 °C and then slowly decreases with a further increase in temperature (Figure 6). This result may indicate that the sulfation reaction under a 5% CO2 atmosphere is less controlled by the chemical reaction because the pore size of calcined lime is nearly constant for the reaction temperature because of sintering by CO2 gas that may accelerate the sintering rate of oxide materials. Quantitative Composition Analysis of Sulfated Limestone by XRD. The XRD spectra of reagent-grade samples (CaSO4, CaO, and CaCO3) are shown in Figure 7. As can be seen, the main peak of the XRD spectrum for CaCO3 appears at 2θ ) 29.4° and the minor peaks appear at 2θ ) 23.0, 39.4, 43.1, and 48.51°, respectively. The main peak of the XRD spectrum for CaO appears at 2θ ) 37.1°, and the minor peaks appear at 2θ ) 30.5,
53.1, 64.0, and 69.4°, respectively. The main peak of the XRD spectrum for CaSO4 appears at 2θ ) 25.5°, and the minor peaks appear at 2θ ) 30.3, 36.0, 39.6, and 51.1°, respectively. The peaks of the XRD spectra for CaCO3, CaO, and CaSO4 are well according to the JCPDS file. The quantitative analytical method in the present study has several important advantages over the other analytical methods such as internal or external methods. The present XRD calculation method does not require standard samples for calibration because the model structure can simulate specific reflections of phases. Therefore, XRD peak intensities of specific reflection are strongly dependent on the composition that can be obtained more accurately than that determined by using inappropriate standard specimens. This analytical method provides routine, accuracy, and rapid quantitative analysis as reported by Anderson and Thomson8 for calcite decomposition kinetics. The XRD spectra of sulfated lime at a sulfation temperature of 850 °C and a SO2 concentration of 3000 ppm with variation of sulfation time are shown in Figure 8. Only CaSO4 and CaO exist in the sulfated lime because the main peaks of the XRD spectra appear at 2θ ) 25.5 and 37.42° in Figure 8. CaSO3, which is one of the potential products in sulfation of CaO, is not found by XRD because CaSO3 is thermally unstable and immediately transformed into CaSO4 and CaS above 1015 K.20 It is ascribed that CaCO3 in limestone is perfectly thermally decomposed to CaO. The intensity of the CaSO4 main peak (2θ ) 25.5°) in the XRD spectra increases with the reaction time because of produced CaSO4 from sulfation of CaO, whereas the intensity of the CaO main peak (2θ ) 37.42°) decreases with increasing reaction time. Sulfation conversion (open symbol) of lime as a function of the reaction time under SO2 concentration of 3000 ppm in an ambient gas stream is shown in Figure 9. Sulfation conversion (Xs) of lime (CaO) can be obtained from weight variations of sorbent in TGA.
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2501
Figure 9. Sulfation conversion measured by TGA and XRD as a function of the reaction time with variation of temperature.
Figure 10. XRD spectra of sulfated limestone under a 10% CO2 concentration at 850 °C.
Sulfation conversion (Xs) based on the CaO molecule can be expressed as
Xs )
∆wMCaO w0ycMSO3
(18)
The volume fractions of the CaO and CaSO4 phases in the sulfated lime can be determined from the main peak intensity (I) of CaO and CaSO4 XRD spectra and eqs 12-14. Calculated sulfation conversion (closed symbol) obtained from volume fractions (CR and Cω) and the Z value ()3.09) is also shown in Figure 9. As can be seen, the sulfation conversion of lime obtained from TGA is good according to the sulfation conversion calculated from XRD. The small discrepancy between the conversions determined from XRD and TGA may be attributed to the lack of truly uniform particle size.25 The sulfation reactivity and sulfur capture capacity (t ) 50 min f Xs) of lime increase with increasing temperature in the present study. The XRD spectra of the direct sulfated sorbent with SO2 under a CO2 concentration of 10% in an ambient gas stream at 850 °C are shown in Figure 10. CaSO4, CaCO3, and CaO still exist in the direct sulfated limestone because the main peaks at 10 min appear at 25.42, 29.45, and 37.42° 2θ in the XRD spectra. However, the main peaks of the XRD spectra at 60 min appear at 25.42 and 37.42° 2θ. It may suggest that only CaSO4 and CaO remain in the sulfated limestone. Also, the main peak ratio of CaSO4 to CaO in the XRD spectra increases with increasing reaction time because CaO in limestone reacts with SO2 to produce CaSO4 with increasing reaction time. Assuming that a mixture in the sulfated limestone contains three phases (CaCO3, CaO, and CaSO4), intensities IR, Iβ, and Iω of the main peak are then given from Figure 10. At 850 °C under the CO2 concentrations of 5 and 10%, the calculated mole fractions of the particular phase in the sulfated limestone using Z values and volume fractions (CR, Cβ, and Cω) from the quantitative XRD analysis are shown in Figure 11. As can be seen, the mole fraction of the CaCO3 phase in the sulfated limestone rapidly decreases with reaction
Figure 11. Calculated CaCO3, CaO, and CaSO4 mole fraction in sulfated limestone with sulfation time at 850 °C.
time because the calcination rate is much higher than the sulfation rate. The mole fraction of the CaO phase in the sulfated limestone increases up to 10 min (CO2 ) 5%) and 20 min (CO2 ) 10%) because of CaCO3 decomposition and thereafter decreases with time because of the sulfation reaction. The conversion of CaO to CaSO4 in the sulfated limestone increases with reaction time during the calcination and sulfation reactions. At 850 °C under different CO2 concentration (0-10%) atmospheres in an ambient gas stream, sulfation conversions of limestone from both TGA (line) and XRD quantitative analyses (symbol) are shown in Figure 12. As can be seen, sulfation conversion of limestone from the XRD quantitative analysis is good accord to the sulfation conversion obtained from TGA. Competitive reactions in a limestone sample (calcination and sulfation) under CO2 concentrations of 0.0, 5.0, and 10.0% in an ambient gas stream have been continuously progressed during the time period of 9, 12, and 17 min,
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Figure 12. Sulfation conversion measured by TGA and XRD as a function of the reaction time with variation of the CO2 concentration.
Figure 13. Specific surface area variation of calcined lime with sintering temperature.
Figure 14. XRD spectra of calcined lime with sintering temperature at 10 h.
calcined lime samples obtained at 800-1000 °C under an air atmosphere were determined by using the BET method. The specific surface area of calcined lime under an air atmosphere with different reaction temperatures (800-1000 °C) is shown in Figure 13. As can be seen, the surface area of calcined lime decreases with increasing sintering time. Also, the reduction rate of the surface area increases with increasing sintering temperature. The CaO grain size of calcined lime can be determined from the main peak broadening in the XRD spectra. The XRD spectra of sintered lime during 10 h at sintering temperatures of 1000 °C (A) and 900 °C (B) are shown in Figure 14. As can be seen, the main peak of calcined lime in the XRD spectra appears at 37.42° 2θ. The main peak of calcined lime at 900 °C is broader than that of calcined lime at 1000 °C. It can be expected that the grain size of calcined lime at 900 °C is smaller than that at 1000 °C. The mean grain size (dg) based on the XRD line broadening from the Scherrer equation22 has been determined.
dg ) respectively. The limestone sample loses its weight by calcination and gains its weight by sulfation at the same time as the competitive reaction proceeds. Therefore, the calcination conversion (Xc) of CaCO3 and sulfation conversion (Xs) of CaO in a limestone sample during the competitive reaction cannot be obtained from the weight variation in TGA, whereas the conversions of calcination and sulfation in a limestone sample can be calculated by using the XRD quantitative analysis for each phase (CaCO3, CaO, and CaSO4) of the XRD spectra. Therefore, it can be claimed that the XRD quantitative analysis for the direct sulfation of limestone as the competition reaction may be an effective analysis method. Grain Size of Sintered Lime by XRD. In a coal combustor at higher temperature, lime is attended with the sintering process of a CaO grain during the sulfation reaction. Sintering of a CaO grain reduces the surface area of lime, which reduces the sulfation reactivity of lime.20 To determine the effect of sintering on the surface area reduction of lime, the surface areas of
kλ β cos θ
(19)
The calculated grain size of CaO with different calcination temperatures (900 and 1000 °C) and time are listed in Table 3. In general, the grain size of calcined lime increases with increasing sintering temperature and reaction time. To quantify the surface area and the grain size of sintered lime under various temperatures and reaction times, the grain size can be expressed as a function of the surface area and the true density of CaO (3400 kg/ m3) as shown in Figure 15. The resulting correlation of the grain size can be expressed as
( )
dg ) 104.1 ln
1 + 890.3 Sm
(20)
with a correlation coefficient of 0.985. In eq 20, dg and Sm are the calculated values of the grain size and the specific surface area of calcined lime.
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2503 Table 3. Surface Area, fwhm of the XRD Spectra, and Grain Size of Calcined Lime with Sintering Temperature and Time calcination temperature (°C) 900 (m2/g)
surface area fwhm (rad × 103) grain size (Å)
1000
0.2 h
0.5 h
1.0 h
2.0 h
6.0 h
10.0 h
0.1 h
0.5 h
1.0 h
2.0 h
10.0 h
14.0 2.95 551
9.4 2.50 650
7.1 2.41 675
5.7 2.15 723
4.0 2.14 760
3.4 2.11 774
10.2 2.56 635
3.2 2.14 760
2.3 2.05 793
1.1 1.85 880
0.6 1.73 940
Nomenclature
Figure 15. Calculated grain size of calcined lime using the line broadening of the main peak in XRD spectra with the specific surface area and the true density of CaO.
Conclusions Calcination and sulfation of pure CaO and limestone under a CO2 gas atmosphere (0-15%) were carried out in TGA. Also, the mole ratio of CaCO3, CaO, and CaSO4 in the sulfated limestone was measured quantitatively by XRD. The sulfation conversions of pure CaO and limestone calculated by the XRD method are well according to the sulfation conversion from TGA. The sulfation reactivity and sulfur capture capacity of limestone increase with increasing temperature and decreasing CO2 partial pressure in an ambient gas stream. The direct sulfation without CO2 gas becomes less controlled by diffusion at a temperature above 850 °C but never completely controlled by chemical reaction, whereas the initial direct sulfation of limestone under a 5% CO2 atmosphere becomes less controlled by chemical reaction with increasing temperature. The decomposition temperature of limestone can be correlated with CO2 partial pressure as Tdoc ) 35 ln(Pg) + 915. The fwhm of the main peak (CaO) in the XRD spectrum decreases with increasing sintering temperature. The CaO grain size of lime is determined by the qualitative XRD analysis and is correlated with the specific surface area of lime and CaO true density.
C ) volume fraction Ce ) equilibrium decomposition the CO2 concentration of limestone (gmol‚m-3) Cg ) CO2 concentration in the bulk gas stream (gmol‚m-3) C0 ) SO2 concentration in the bulk gas stream (ppm) Deff ) effective product layer diffusivity (m2‚s-1) dg ) grain size of the calcined lime (Å) dp ) average particle diameter (m) dp0 ) initial particle diameter (µm) hkl ) Miller indices I ) diffracted intensity of the XRD spectra (CPS) K ) constant independent of the kind and amount of the diffracting substances k ) Shearer constant kc ) intrinsic reaction rate constant (m‚s-1) kv ) sulfation reaction rate constant per unit volume of solid (gmol‚cm-3s-1) Mi ) molecular weight of species of i m ) reaction order of sulfation N0 ) initial mole of CaCO3 in a spherical particle (gmol) n ) reaction order of calcination under a CO2 atmosphere Pe ) equilibrium partial pressure of CO2 (mmHg) Pg ) CO2 partial pressure in gas stream (atm) R′ ) constant depending on hkl and the kind of substance rc ) radius of the unreacted shrinking core in the particle (m) r0 ) initial radius of limestone particle (m) Sm ) specific surface area (m2) t ) reaction time (s) T ) absolute temperature (K) Tdoc ) decomposition temperature (°C) w0 ) initial weight of the calcined sample (kg) ∆w ) weight gain of the sample (kg) Xc ) calcination conversion of limestone Xs ) sulfation conversion of lime or limestone yCO2 ) CO2 mole fraction in an ambient gas stream yc ) CaO weight fraction of a limestone sample Z ) ratio of molar volumes of product and reactant solid Greek Symbols R ) CaCO3 phase β ) CaSO4 phase δ2 ) ratio of diffusion to chemical resistance λ ) X-ray wavelength ) 1.5406 Å µ ) linear absorption coefficient (m‚kg‚C-2) θ ) refracted angle linear absorption coefficient (m‚kg‚C-1) Fp ) true density of CaO (kg‚m-3) Fs ) calcite molar density ) 2.71 × 10-1 gmol‚cm-3 τpid ) characteristic time for product layer diffusion (s) τrxn ) characteristic time for chemical reaction (s) ω ) CaO phase
Literature Cited Acknowledgment The authors gratefully acknowledge the grant-in-aid for research from the Korea Science and Engineering Foundation.
(1) Armor, J. N. Environmental catalysis. Appl. Catal. B 1992, 1, 221-256. (2) Jeong, S. M.; Kim, S. D. Enhancement of the SO2 sorption capacity of CuO/γ-Al2O3 sorbent by and alkali-salt promoter. Ind. Eng. Chem. Res. 1997, 36, 5425-5431.
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Received for review October 15, 1999 Revised manuscript received April 13, 2000 Accepted April 19, 2000 IE990749M
