ARTICLE pubs.acs.org/EF
Experimental and Modeling Study of Sulfur Capture by Limestone in Selected Conditions of Air-Fired and Oxy-fuel Circulating Fluidized-Bed Boilers Sirpa Takkinen,*,† Timo Hypp€anen,† Jaakko Saastamoinen,‡ and Toni Pikkarainen‡ † ‡
Lappeenranta University of Technology, Post Office Box 20, FI-53851 Lappeenranta, Finland VTT Technical Research Centre of Finland, Post Office Box 1603, 40101 Jyv€askyl€a, Finland ABSTRACT: The influence of different conditions on the sulfur-capture efficiency during fluidized-bed desulfurization was studied using both experimental and modeling methods. The effects of the temperature (∼1120 or ∼1200 K) and gas atmosphere (90% N2 or 90% CO2) were studied using one limestone type. The CO2 atmosphere increased the degree of conversion compared to traditional air combustion conditions using both calcinationsulfation and direct sulfation methods. The scanning electron microscopyenergy-dispersive spectrometry analysis of spent sorbent particles revealed different sulfation patterns in different conditions. The N2 atmosphere produced a network sulfation or coreshell sulfation structure depending upon the temperature. Direct sulfation produced a coreshell structure with a thicker sulfate layer. A uniform pattern was observed for many particles in the CO2 atmosphere using indirect sulfation. The experimental results were analyzed using a time-dependent one-dimensional particle model that can accommodate simultaneous reactions. The model was used to interpret the test results and to determine the magnitude of reactions and diffusion rates as a function of the radius and time. The development of a Thiele number, conversion curve, and conversion profile during the reactions was used to explain the observed results.
1. INTRODUCTION In fluidized-bed boilers, sulfur capture is usually carried out with in situ injection of calcium-containing sorbents, such as limestone or dolomite. In air-fired atmospheric units, sulfur capture occurs via relatively rapid calcination and much slower sulfation reactions. CaCO3 ðsÞ f CaOðsÞ + CO2 ðgÞ
ð1Þ
CaOðsÞ + SO2 ðgÞ + 1=2O2 ðgÞ f CaSO4 ðsÞ
ð2Þ
In pressurized combustion and oxy-fuel combustion, the partial pressure of CO2 in the system can be higher than the equilibrium CO2 pressure over the limestone and sulfation can occur directly without the calcination step. CaCO3 ðsÞ + SO2 ðgÞ + 1=2O2 ðgÞ f CaSO4 ðsÞ + CO2 ðgÞ ð3Þ In air-fired atmospheric units, the calcium conversion of limestone seldom exceeds 3040%. The low conversion is due to the development of a sulfated shell that leads to high diffusion resistance and drastically inhibits the use of the sorbent material in the particle core. The incomplete use of the sorbent particles results in an increase in both the cost and the environmental impact of the process.1 However, the development of oxy-fuel circulating fluidized-bed (CFB) boilers can also offer an enhancement in the sulfur-capture efficiency because of the differences in the gas composition and the temperatures of these systems. In oxy-fuel combustion, the gas is composed mainly of CO2, while the H2O content can be up to 30% depending upon the recycling of the wet or dry combustion gas. Literature on sulfur capture in O2/CO2 atmospheres is r 2011 American Chemical Society
limited, especially for CFB conditions. However, differences in the calcium conversion and SO2 emissions have been observed in oxy-fuel combustion tests,28 but no clear explanation for these observations has been provided. Furthermore, the optimal sulfurcapture mechanism for oxy-fuel CFB combustion conditions is not clear. There are many possible factors that could account for the different sulfation efficiencies observed. Liu et al.5 noticed the influence of the system itself, noting that SO2 is enriched in the furnace of oxy-fuel combustion systems because the flue gas is recycled. This enrichment leads to good sulfation conditions, and the total SO2 emission can be much lower. Moreover, the mechanisms of sulfur capture can differ in oxyfuel combustion and conventional coal combustion. In conventional coal combustion, the sulfation efficiency is usually low because of the pore blockage of the sorbent. Direct sulfation can lead to higher sulfation when compared to the calcination sulfation process,5 but conflicting results have also been reported.9 It has been explained that the counter-diffusion of CO2 generated during the direct sulfation reaction forms a porous product layer, offering a lower diffusion resistance than the essentially nonporous layer formed during the CaOSO2 sulfation reaction.10 This report has since been cited repeatedly by several authors to explain the porosity in the product layer and the high rate of the direct sulfation reaction compared to the indirect sulfation reaction at higher conversions. Nevertheless, some authors refute this argument and doubt that the diffusion flow of CO2 could be responsible for the Received: March 17, 2011 Revised: June 14, 2011 Published: June 30, 2011 2968
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Energy & Fuels porosity.5,11 Hu et al.11 claimed that the porosity in the product layer resulted from the formation of crystal grains in the solid product and that the formation of CO2 was irrelevant in the development of porosity in the product layer. They explained the porosity by the size of the grains and pores formed during the reactions. Calcination leads to micrograins and micropores, leading easily to pore blockage during sulfation. Recently, it has been reported that calcination and sulfation can occur simultaneously, especially under CFB conditions, and that the overlap of these reactions can affect sulfur capture.1214 The results by Olas et al.12 indicate that the CaSO4 product layer can prevent both the diffusion of SO2 toward the particle interior and the movement of released CO2 outward, thus increasing the partial pressure of CO2 inside the particle. High CO2 concentrations prevent or hinder calcination reactions, which has been studied in several papers.15,16 According to the study by Olas et al.,12 a simultaneous movement of both calcination and sulfation fronts from the surface to the interior is highly presumable under CFB conditions. Additionally, Saastamoinen et al.14 showed that, under their sulfation conditions (gas atmosphere, air; temperature, 1123 K), the maximum rate of sulfur capture took place during calcination. The effect of simultaneous calcination and sulfation in oxy-fuel conditions was highlighted by Chen and Zhao.13 They studied the sulfur-capture mechanism in an O2/CO2 coal combustion atmosphere at different temperatures and obtained the highest desulfurization efficiency (62.6%) in a high CO2 atmosphere (80%) and high temperature (1323 K). On the basis of these results, Chen and Zhao13 suggested that simultaneous calcination and sulfation would lead to high sulfation conversion in O2/CO2 atmospheres. They explained the high conversion to be a result of the low calcination rate, noting that fresh CaO is readily available over a long time period and that different porous structures are obtained from those in the air. Additionally, they suggested that released CO2 keeps the CaSO4 product layer porous. These mechanisms allow for the sulfur-capture process to maintain a high conversion rate for long periods of time. In a separate experiment, Chen et al.17 also studied the influence of an O2/CO2 atmosphere on calcination. They found that, in an O2/CO2 environment, the specific pore volume and specific surface area were smaller than in air at the same temperature (within the temperature range from 1223 to 1373 K). Additionally, the pore diameter and grain size in O2/CO2 were larger than those in air. They suggested that the pore diameters (40300 Å, peak of 85 Å) found in CaO produced in O2/CO2 were more suitable for sulfation because the size of the pores prevents rapid pore filling and pore-mouth plugging. The existence of different particle sulfation patterns has been demonstrated by Laursen et al.18 These authors showed that, depending upon the sorbent properties, the sulfation patterns that can be observed are coreshell, network, and uniform sulfur distribution. They indicated that the main affecting properties were the pores, fractures, and grain size in the calcined limestone and the type of limestone particle formed after sulfation. Limestones having a uniform sulfation pattern as their main pattern enabled the highest calcium use (∼70%), while limestones that reacted mainly with the unreacted core pattern lead to the lowest values of use (∼40%). Several limiting processes of the limestone particle have been observed during sulfur capture. Sulfation is usually considered to be limited by chemical kinetics or pore diffusion in the beginning of the reaction and by product layer diffusion later in the reaction.
ARTICLE
Table 1. Characteristics of the Limestone moisture (wt %, ar)
0.03
CaCO3 (calculated) (wt %, dry)
96.0
MgCO3 (calculated) (wt %, dry)
2.8
inert (calculated) (wt %, dry)
1.2
element analysis (dry basis) Ca (wt %)
38.5
Mg (wt %)
0.80
S (wt %)
0.03
C (wt %)
N/A
In direct sulfation, chemical kinetics,10 diffusion,19 or both20 are considered to have an influence during the reaction. Hu et al.11 concluded that the direct sulfation of limestone involves five general steps, including gasfilm diffusion, pore diffusion in the particle and product layer, chemical reaction, solid-state diffusion, and the nucleationgrowth process, and that the process may be controlled by each of these steps or by a combination of them. In their conditions and conversion range, the process was controlled by the chemical reaction and solid-state diffusion. In this work, the influence of different conditions on the sulfation, degree of sulfur capture, and sulfation pattern of one limestone type is investigated. Limestones vary in chemical and physical properties. Similar phenomena occur in other types of limestones but at different temperatures or with other particle sizes. The Thiele modulus, which will be discussed later, is a way to characterize different limestones. A characterization of the sulfur distribution pattern was performed with the aid of scanning electron microscopyenergy-dispersive spectrometry (SEMEDS) analyses. An unsteady multi-layer particle model based on the control volume method was used to simulate the limestone reactions in different environments. The relative effects of chemical kinetics and diffusion in the experimental conditions were analyzed using the particle model and characterized by the Thiele modulus. The results show different limitations in different conditions, leading to different sulfation patterns and degrees of conversion.
2. EXPERIMENTAL SECTION 2.1. Description of the Limestone and Test Matrix. The limestone used in the tests was Nordkalk’s Parfill P6. This particular limestone has a low comminution tendency, and its particle size remains practically constant during experiments. The characteristics of the limestone are presented in Table 1. The inerts in the studied limestone include components of the following elements: Na, Al, Si, P, K, Ti, Mn, Fe, and Sr. The test matrix is presented in Table 2. In each test, 30 g of limestone was fed into the reactor. The limestone was sieved to 125180 μm fractions. After the test, the sample was blown out from the reactor and separated from the gas flow by cyclone. 2.2. Apparatus. The bench-scale bubbling fluidized-bed (BFB) reactor with the limestone batch and continuous feed used in the tests is illustrated in Figure 1. In these experiments, the apparatus was operated batchwise with respect to the limestone. The height of the riser tube (heated part) was 669 mm, and the inner diameter of the lower part (height of 225 mm) was 36.0 mm. The inner diameter of the upper part of the riser tube was 53.1 mm. Preheated primary gas was fed into the reactor through a perforated grid. Different types of gas mixtures, including O2, CO2, SO2, CO, air, and nitrogen, can be used. SO2 is fed just under the grid to prevent the possible SO2 reaction, with O2 2969
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Table 2. Test Matrix gas atmosphere description
calcination
sulfation in N2, medium temperature
sulfation in N2, high temperature
direct sulfation in CO2, medium temperature
sulfation in CO2, high temperature
N2 (vol %) CO2 (vol %) O2 (vol %) SO2 (ppm) time (min) temperature (K) SEM/EDS, OM, and XRD 100
0
0
0
20
1123
100
0
0
0
20
1223
0
90
10
0
30
1201
90
0
10
2300
5
1123
90
0
10
2300
30
1123
90
0
10
2300
180
1123
90
0
10
2299
360
1115
90 90
0 0
10 10
2300 2300
5 30
1223 1223
90
0
10
2300
180
1223
90
0
10
2154
360
1187
0
90
10
2300
5
1123
0
90
10
2300
30
1123
0
90
10
2300
180
1123
0
90
10
2222
360
1115
0 0
90 90
10 10
2300 2300
5 30
1223 1223
0
90
10
2300
180
1223
0
90
10
2222
360
1187
different calcium species (CaO, CaCO3, and CaSO4) were calculated for each sample. The amount of CaCO3 was analyzed by heating the sample from 823 to 1223 K and weighting the sample before and after the heating. The mass loss is assumed to be the amount of CO2 from the calcination of CaCO3. The amount of sulfur and calcium was analyzed by XRF. All of the sulfur was assumed to be in the form of CaSO4. The remainder of calcium (not in the form of CaCO3 or CaSO4) was assumed to be in the form of CaO. The size, shape, and porosity of the particles were analyzed from thin cross-cutting sections of the samples by optical microscopy (OM). Furthermore, cross-cutting sections of samples were analyzed by SEMEDS. From the SEMEDS pictures, the distributions of different elements (carbon and heavier elements) were analyzed. The X-ray diffraction (XRD) method was used to identify inorganic crystalline compounds. XRF and CaCO3 analyses (by heating from 823 to 1223 K) were applied for all of the samples. The samples analyzed by SEMEDS, OM, and XRD are shown in Table 2.
Figure 1. Test device.
forming SO3 inside the gas heater. The temperature inside the reactor was measured at six levels, and the temperature was controlled by two surrounding electric heaters. Flue gas composition, including O2, CO2, CO, SO2, and NO, was measured with online gas analyzers. 2.3. Chemical and Physical Analyses. The chemical composition of the samples was analyzed using an X-ray fluorescence (XRF) spectrometer. Fluorine and heavier elements were analyzed (except rare gases). A total of 79 elements was measured. The contents of the
3. NUMERICAL PARTICLE MODEL The influence of different atmospheres on sulfur uptake was modeled using a single particle model of limestone. The model includes a description of mass and energy transfer inside the particle and is based on a control-volume method. The model describes the transient behavior of a particle, including the spatial and temporal changes in both the physical properties of the particle (density, porosity, thermal conductivity, and heat capacity) and the process parameters (temperature, gas concentration, effective diffusivity, and conversion degree). The assumptions and simplifications required for the model are the following: (1) The particle is assumed to be spherical. (2) The boundary and initial conditions are symmetrical, and thus, 1-dimensional analysis is applied. (3) Changes in the shape and size (diameter) of the particle are not considered (fragmentation and attrition do not occur). (4) Each gas species and the gas mixture follow the equation of state for an ideal gas. (5) A common value of effective diffusivity has been assumed for all of the gaseous species. 2970
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CaCO3
CaO
CaSO4
(mass %)
(mass %)
(mass %)
a
3.5
61.7
28.7
b
6.7
60.9
28.4
a
2.4
63.1
28.7
surface were determined by a ghost cell and a numerical grid. The conservation equations were discretized for each cell. The diffusion term was approximated using a first-order scheme. The time dependence of the equation of the gas species was modeled with an implicit method to allow for stable calculations with larger time steps, which were needed for faster simulations. The tridiagonal matrix algorithm (TDMA) method (Thomas algorithm) was used to solve a tridiagonal system of linear algebraic equations.
b
10.6 53.7
63.3 2.3
22.1 40.3
4. RESULTS AND DISCUSSION
b
69.4
0.0
26.6
a
7.3
36.6
49.8
b
41.2
17.0
37.8
Table 3. Calcium Compounds Based on Gas Integration Measurement and Chemical Analyses after 360 min of Test Time test
measurement
1115 K, 90% N2 1187 K, 90% N2
a
1115 K, 90% CO2 1187 K, 90% CO2 a
b
Chemical analyses. Gas integration measurement.
Figure 2. Development of the CaSO4 fraction in different atmospheres as a function of the time.
The conservation equation of chemical species is written as Z I Z d εg Fg Yi dV ¼ ½εg Fg vYi εg Fg Deff ∇Yi ndA + υi Si dV dt V A V ð4Þ where the first term on the right accounts for the advection and the second term on the right accounts for the diffusion. Fluxes of solid species are assumed to be equal to zero. The four gas species considered are SO2, O2, CO2, and N2, and the three solids considered are CaO, CaCO3, and CaSO4. The conservation equation of energy is written as Z I Z d F cp TdV ¼ ½keff ∇TndA + ΔHSm dV ð5Þ dt V tot A V Only the reaction enthalpy and the thermal conduction are considered as significant terms in the energy equation. The values of the source terms for mass and heat balance are obtained using the relations for chemical reaction kinetics. In this study, calcination, sulfation, and direct sulfation (reactions 1, 2 and 3) were considered in the model. The control-volume numerical method was used for solving the system of the conservation equations for obtaining the values of temperatures and gas concentrations along the limestone particle radius and their change in time. For numerical purposes, the spherical particle was divided into computational cells of equal radial thickness. Boundary conditions for the center and
4.1. Measured Conversion Curves, Observed Conversions, and Calcium Compounds. The calcium use was calculated by
integrating the gas responses and from the chemical analysis of the samples. The differences found are shown in Table 3. The chemical analysis always gave higher CaSO4 fractions than the gas analysis. The definition of components based on the gas analysis is sensitive to the errors at the base levels of CO2 and SO2. The calculation of calcination is especially difficult at a high base level CO2. In these cases, a very small error in the base level CO2 leads to a large error in the fractions of CaCO3 and CaO. The fractions of calcium compounds defined by chemical analysis are more reliable. Gas analysis results are used when defining reaction rates (as a function of the time), and the total extent of reactions (conversion) during a certain time are taken from the chemical analysis results. The gas integration results are aligned, so that the fractions of calcium compounds equal the shares of chemical analysis results at the end of each test. Two sets of tests were performed. The total duration of the first test was 180 min, and the second test lasted 360 min. The results were in agreement; however, the temperature levels of the tests differed slightly, as seen in Table 2. The sulfur-capture conversion curves at 1115 and 1187 K in N2 and CO2 atmospheres are shown in Figure 2. Interestingly, the highest sulfur-capture capacity within the time range examined was obtained at a higher temperature in the CO2 atmosphere, which enables indirect sulfation. The sulfur-capture capacity attained is considerably higher compared to the result in a traditional air combustion atmosphere. A lower temperature in a CO2 atmosphere provides the conditions for direct sulfation and for the second highest conversion measured. The calcium compounds of samples from the chemical analysis at three different times (5, 30, and 180 min) under different conditions, which can confirm the assumed reaction routes, are reported in Table 4. In the N2 atmosphere, where the reaction follows the traditional calcinationsulfation, the higher final conversion is obtained at a temperature of 1123 K rather than at a temperature of 1223 K after 180 min, as shown in Table 4. This result corresponds to the sulfur-capture maximum at approximately 1143 K in air-fired atmospheric units. However, the final conversion was the same after longer test runs (360 min), where the temperature levels were 1115 and 1187 K, as seen in Table 3. Although the conversions are higher in a CO2 atmosphere at the end of the test period, the sulfur-capture rate was slightly higher in the N2 atmosphere during the initial stage of the test. Especially, a low reaction rate in the initial stage was observed with direct sulfation. This suggests that the calcined limestone has a higher global reaction rate than the uncalcined limestone. After the 180 min test run, the conversion curves in CO2 exceed the N2 conversion curves, also in noncalcining conditions, and 2971
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Table 4. Measured Calcium Compounds after 5, 30, and 180 min Duration of Test test
1123 K, 90% N2
1223 K, 90% N2
1123K, 90% CO2
1223 K, 90% CO2
test time
CaCO3
CaO
CaSO4
(min)
(mass %)
(mass %)
(mass %)
5 30
66.6 5.9
25.9 78.0
2.7 13.2
180
5.5
62.3
28.0
5
8.9
84.7
2.9
30
4.2
75.8
14.0
180
11.0
63.1
18.7
5
88.7
5.3
2.3
30
83.6
2.6
8.9
180 5
62.0 55.9
0.05 36.1
34.4 2.7
30
7.5
72.9
13.2
180
10.3
44.6
40.8
Figure 3. Calcination curves in different atmospheres as a function of the time.
Table 5. Measured Particle Size after Each Test test
1123 K, 90% N2
1223 K, 90% N2
1123K, 90% CO2
1223 K, 90% CO2
test time
largest particle
mean particle
(min)
size (μm)
size (μm)
5 30
160 170
120 120
180
130
100
5
140
100
30
170
110
180
140
100
5
150
120
30
180
130
180 5
150 180
100 110
30
170
120
180
140
110
the sulfation rate in the N2 atmosphere decelerates substantially after 90 min of test run. It is well-known that, under oxidizing conditions, the maximum conversion attained occurs at approximately 90 min in atmospheric conditions.1 Such a deceleration phenomenon was not observed under CO2 conditions in the reaction time range examined. On the contrary, the sulfation rate
Figure 4. Sulfur distribution at 1123 K and 90% N2 in (a) 5 min, (b) 30 min, and (c) 180 min.
seems to stay approximately constant in CO2, and the conversion curve reveals that the sulfation maximum is not attained. Therefore, even higher conversion degrees could be obtained if the reaction time were extended. Separate calcination studies were performed to evaluate the calcination rate under different conditions without the influence of sulfation. Figure 3 shows that calcination is relatively fast in all calcining atmospheres; 80% conversion was achieved in 12 min in the N2 atmosphere and in 16 min in the CO2 atmosphere. The higher temperature in N2 leads to a faster calcination rate. These results are consistent with the results in the literature, although divergent results can be found. Indirect sulfation in a 2972
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Figure 5. Sulfur distribution at 1223 K and 90% N2 in (a) 5 min, (b) 30 min, and (c) 180 min.
CO2 atmosphere has led to higher conversion than indirect sulfation in a N2 atmosphere.13 Also, direct sulfation has led to higher conversion compared to sulfation (in a N2 atmosphere).5 In a CO2 atmosphere, higher conversion has been attained in calcining conditions compared to noncalcining conditions.9 4.2. Particle Size Measurement. The sorbent material was originally sieved to 125180 μm particle size range. The limestone examined was chosen to be hard to avoid the proneness to fragmentation or attrition, and the particle size was measured after 5, 30, and 180 min, as seen in Table 5. After the experiments, the largest and mean particle sizes were determined by OM. Particle sizes (largest and mean) were determined as an average
Figure 6. Sulfur distribution at 1123 K and 90% CO2 in (a) 5 min, (b) 30 min, and (c) 180 min.
of minimum diameters measured from the representative sample. There were no detectable differences in the N2 and CO2 atmospheres, and the effect of primary fragmentation and attrition could be excluded. 4.3. SEMEDS Micrographs. Figures 47 report the SEMEDS micrographs of the sulfur distribution of the multiparticle reacted samples under different conditions at different 2973
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Table 6. Input Parameters
Figure 7. Sulfur distribution at 1223 K and 90% CO2 in (a) 5 min, (b) 30 min, and (c) 180 min.
times. At 5 and 30 min, there is no remarkable difference between the results, and the amount of sulfur is relatively low. The SEMEDS micrographs at 180 min reveal the establishment of network sulfation structures as the prevailing sulfation pattern in the N2 atmosphere at 1123 K, while the dominant pattern at 1223 K in the N2 atmosphere is the coreshell structure, as seen from Figures 4c and 5c. Laursen et al.18 states that the network
particle diameter (μm)
110
sorbent particle voidage initially, εg
0.3
pressure (kPa)
101
temperature (K)
1115/1187
gas atmosphere
90% N2/90% CO2
particles are typically highly sulfated around the periphery and in the proximity of fractures, while being slightly or totally unsulfated in the core of the “sub-grains” separated by the fractures. Furthermore, in the coreshell sulfation, most of the particles show a layered structure with an outer shell, where sulfur mainly concentrates, and a core consisting almost entirely of uniformly distributed calcium oxides.18 The coreshell sulfation pattern is also the dominant structure in the direct sulfation test (1123 K, 90% CO2), as seen from Figure 6c. A thicker sulfation layer was noted in most of the particles after the 180 min test. Figure 7 shows that, in the CO2 atmosphere with calcinationsulfation, there were two kinds of sulfation patterns. Some of the particles were uniformly sulfated, while others had a coreshell structure. However, it can be assumed that the coreshell pattern can lead to a uniform pattern when the reaction time is extended. After 30 min of test time, only small particles are uniformly sulfated, while at 180 min of test time, some of the larger particles have a uniform pattern. Thus, the uniform pattern cannot be explained solely by the particle size or cross-sectioning through the rim of the unreactedcore type, as Laursen et al.18 also noted. 4.4. Evaluation of Calculation Parameters. The influence of different conditions on sulfur uptake was modeled using the single particle model of limestone. The calculations were performed assuming the set of input parameters reported in Table 6. Table 7 shows the values of the kinetic parameters and the diffusion coefficients obtained in each case. The evaluation of the calcination rate was based on the separate calcination tests accomplished. The diffusion and kinetic parameters during the reactions were chosen to fit the measured and calculated conversion curves and the conversion profiles inside the particle under different conditions. 4.5. Modeled Conversion Curves and Development of Diffusion Coefficients. The modeled and measured fractions of calcium compounds are presented in Figures 811. The model is able to predict the curves with sufficient accuracy. There is a large difference between the modeled and measured CaO fractions in the high CO2 concentration, as seen from Figure 11. This difference is due to the inaccuracy of the gas measurement at high CO2 concentrations, which is discussed in section 4.1. The figures also show how the calcination and sulfation occur simultaneously in both a N2 and a CO2 atmosphere in calcining conditions. A closer look at the model calculations reveals the simultaneous development of CaSO4 and CaO mass fraction profiles inside the particle, as shown in Figures 12 and 13, showing the mass fractions at 5 and 15 min. In a N2 atmosphere, calcination occurs uniformly throughout the particle, whereas in a CO2 atmosphere, the CaO is formed faster in the exterior of the particle than in the interior. The fractions of CaSO4 attained are relatively small during the first 15 min under every condition. It seems that the product layer of CaSO4 is not formed before the calcination has ended and does not affect the calcination. Figures 1417 present the development of the effective diffusion coefficient in each case. The coefficients in a N2 2974
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Table 7. Calculation Parameters description
calcination (mol/m3s)
sulfation (mol/m3s)
diffusion (m2/s) YCaSO4 < 0.2
sulfation in N2, 1115 K
Sc = 2(2 106T + 0.002808)(Peq PCO2)(YCaCO3 0.037)
Ss = 31.2xSO2YCaO
Deff = 1 107 YCaSO4 g 0.2
Deff = 1 1014YCaSO410 YCaSO4 < 0.2
sulfation in N2, 1187 K
Sc = 2(2 106T + 0.002808)(Peq PCO2)(YCaCO3 0.025)
direct sulfation, 1115 K sulfation in CO2, 1187 K
Sc = 2(2 106T + 0.002808)(Peq PCO2)(YCaCO3 0.078)
Ss = 35.1xSO2YCaO Ss = 37xSO2YCaCO3 Ss = 45xSO2YCaO
Deff = 1 107 YCaSO4 g 0.2
Deff = 5 1015YCaSO410 Deff = 1.2 109YCaSO42 Deff = 4.5 108εg2
Figure 8. Measured (—) and modeled (- - -) calcium compounds at 1115 K and 90% N2.
Figure 10. Measured (—) and modeled (- - -) calcium compounds at 1115 K and 90% CO2.
Figure 9. Measured (—) and modeled (- - -) calcium compounds at 1187 K and 90% N2.
Figure 11. Measured (—) and modeled (- - -) calcium compounds at 1187 K and 90% CO2.
atmosphere develop in a similar way at both temperatures, as seen from Figures 14 and 15. The diffusion coefficients decrease strongly after the initial period where the diffusion is assumed to be constant, and the decrease is greatest in the outer layer of the particle. The decrease is slightly faster at the higher temperature (at the outer layer of the particle) than in the lower temperature. Figures 16 and 17 show that, in CO2 atmospheres, the decrease of diffusion coefficients is moderate, especially at higher temperatures, and the values of the average diffusion coefficients are at the same level at the end of the examination time. The rapid increase in the diffusion coefficient in Figure 17 during the first
30 min is due to the calcination phase when the porosity of the particle increases. Nonetheless, the rate of change differs between the direct and indirect sulfation in CO2. During indirect sulfation, the decrease exists almost concurrently through the particle, but in direct sulfation, the decrease is faster in the outermost layers. 4.6. Modeled Profiles of CaSO4 Inside the Particle. Figures 1821 show the CaSO4 mass fraction as a function of the radius at different times in different atmospheres calculated with the numerical particle model. There are only modest differences between the different atmospheres evaluated at 5 and 30 min. In CO2 atmospheres, the mass fractions of CaSO4 at 2975
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Figure 12. CaO mass fractions inside the particle at different times (2, 5 min; 9, 15 min) in different atmospheres (1115 K, 90% N2, 3 3 3 ; 1187 K, 90% N2, —; and 1187 K, 90% CO2, - - -).
Figure 13. CaSO4 mass fractions inside the particle at different times (2, 5 min; 9, 15 min) in different atmospheres (1115 K, 90% N2, 3 3 3 ; 1187 K, 90% N2, —; and 1187 K, 90% CO2, - - -).
Figure 14. Development of the local effective diffusion coefficient inside a particle at 1115 K and 90% N2.
30 min are slightly lower at the temperature of 1115 K and especially lower at the temperature of 1187 K, which were also noted in the conversion curves as having a lower reaction rate in the initial stage. In all of the cases, CaSO4 was formed at some level throughout the particle. This result is consistent with the micrographs, because the difference is negligible in the figures of different atmospheres at 5 and 30 min. Also, the values of calcium compounds in Table 4 reveal that the difference in the total CaSO4 content at 5 and 30 min is small between any test conditions.
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Figure 15. Development of the local effective diffusion coefficient inside a particle at 1187 K and 90% N2.
Figure 16. Development of the local effective diffusion coefficient inside a particle at 1115 K and 90% CO2.
Figure 17. Development of the local effective diffusion coefficient inside a particle in 1187 K and 90% CO2.
On the contrary, there is a clear difference between the modeled cases at 180 and 360 min reaction time. Figures 18 and 19 show that, in the N2 atmosphere, a similar structure with a distinct sulfate layer at the particle surface is formed at both 1115 and 1187 K, corresponding respectively to the network and core shell structure in the SEM figures (Figures 4c and 5c). The conversion profiles produced with the model correspond more with the coreshell than with the network structure. The unsuccessful prediction of the network structure at lower temperatures by the model is due to the description of the reaction parameters and the model of the porous structure. These 2976
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Figure 18. CaSO4 mass fraction inside a particle at different times at 1115 K and 90% N2.
Figure 19. CaSO4 mass fraction inside a particle at different times at 1187 K and 90% N2.
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Figure 21. CaSO4 mass fraction inside a particle at different times at 1187 K and 90% CO2.
Figure 22. Development of the Thiele parameter in different atmospheres.
The micrographs give only a qualitative amount of sulfur, complicating the estimation of the minor amounts of sulfur inside the particle. Therefore, an accurate comparison of the modeling and micrographs is difficult. 4.7. Evaluation of the Limiting Mechanism of Sulfur Capture under Different Conditions. The limiting process during the reaction can be evaluated with the aid of the Thiele parameter, which is a nondimensional ratio of the time scales involved in the heterogeneous system, the time scale for diffusion Lc2/D to the reaction time scale. Φm ¼ L c Figure 20. CaSO4 mass fraction inside a particle at different times at 1115 K and 90% CO2.
parameters do not depend upon the pore or grain sizes, thus not taking into account the differences and development in size distribution, surface area, or pore volume inside the particle. Instead, the porous structure is approximated with radially changing porosity values based on the local relation between the molar volumes of the reaction products and reactants. Direct sulfation led to a thick sulfate layer, whereas indirect sulfation in CO2 led to a more uniform sulfate formation within the particle, as can be seen from Figures 20 and 21. These results are consistent with the micrographs presented in Figures 6c and 7c.
k Deff
!1=2
R k ¼ 3 Deff
!1=2 ð61Þ
It is generally accepted that, when Thiele . 1, the intraparticle diffusion is the controlling mechanism and, when Thiele , 1, kinetics is the controlling mechanism. The reaction kinetics and diffusion both have an effect, when 0.1 < Thiele < 10. Figure 22 shows the Thiele parameter in each case. The reported sulfation patterns and Thiele parameters can explain the sulfation attained as a function of the time in each case, which is discussed below. It can be seen from Figure 22 that, in a N2 atmosphere, the chemical kinetics and diffusion are the limiting processes in the beginning of the reaction and the effect of diffusion limitation increases after the initial reaction period at both temperatures. The diffusion resistance increases faster at the higher temperature than in the lower temperature, and the resistance at the end 2977
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Energy & Fuels is higher. This phenomenon is due to a higher reactivity at the higher temperature, which also produces a sulfate layer faster. This layer effectively prevents the SO2 inflow. Therefore, it has inhibited the extension of reactions and led to a coreshell sulfation pattern. The diffusion limitation can also occur at the surface of the grains via cracks and fissures in the structure. In that case, the reactions occur in the inner parts of the particle to a greater extent as in the lower temperature case. The sulfation pattern is then network sulfation, which can result in higher or similar conversion than the diffusion limitation at the particle surface. Figure 22 shows that the growth of the Thiele number is minor during direct sulfation (1115 K, 90% CO2) and that it remains in the same region during the examined reaction time, limited by both the chemical kinetics and diffusion. In direct sulfation, the calcination will not occur and the particle porosity is low when the reaction begins. The particle porosity decreases as the reaction proceeds. Thus, SO2 is not able to penetrate the interiors of the particle. As mentioned in section 4.1, the initial rate of reaction is lower than the rate of sulfation in a N2 atmosphere, which has also been observed.20 The lower reaction rate may be caused by the slower kinetics of lower diffusion because of lower porosity. The lower reaction rate also hinders the formation of the sulfate layer and forms a coreshell structure with a thicker sulfate layer. The formation of a thicker sulfate layer during direct sulfation enables higher conversion than sulfation in a N2 atmosphere where coreshell structures also exist. As in the lower temperature in a CO2 atmosphere, the change in the Thiele number is minor at the higher temperature and the Thiele number remains in the same region (chemical kinetics and diffusion limitation) during the period investigated, as shown in Figure 22. The uniform sulfation pattern has enabled the highest conversion because calcium is used throughout the particle. The calcination increases the limestone porosity, and SO2 can penetrate into the particle. Even though the rate of calcination is slower than in a N2 atmosphere, the reaction is finished relatively fast. Thus, it is questionable that the lower calcination rate could affect the sulfation rate by the production of CO2 (and fresh CaO), because the difference in the reaction times is small between the N2 and CO2 atmospheres. However, calcination in a CO2 atmosphere can produce a different microstructure as Chen et al.13,17 observed. Laursen et al.18 also observed that calcined limestones, which consist of certain types of grains and pores, gain a uniform sulfation pattern. This microstructure will prevent the development of a sulfate layer because it is not prone to blocking, and it enables the reaction to occur throughout the particle. Therefore, the formation of a thick or dense sulfate layer can be avoided during calcinationsulfation in a CO2 atmosphere.
5. CONCLUSION The influence of different atmospheres on the sulfation and particle sulfation pattern during fluidized-bed desulfurization was assessed using experimental and modeling approaches. The prevailing sulfur distribution patterns were characterized qualitatively by the visual inspection of scanning electron micrographs of the cross-sections of spent sorbent particles. The ultimate calcium conversion and calcium compounds at different test times were also determined. Experimental results show that the limestone reaches the maximum final sulfation at high temperatures and in high CO2
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concentrations, which enables calcinationsulfation chemistry. The second best sulfur-capture capacity is achieved at low temperatures and high CO2 concentrations via direct sulfation. The results of the SEMEDS analyses of spent sorbent particles reveal different sulfation patterns in different atmospheres. In 1223 K and 90% CO2, a uniform sulfation pattern is established for a part of the particles, and in 1123 K and 90% CO2, a coreshell pattern is valid. A N2 atmosphere at a lower temperature (1123 K) leads to a network sulfation type, while a higher temperature (1223 K) leads to a coreshell structure. The experimental results were analyzed using a single particle model of limestone to be able to interpret the effect of different conditions on sulfur capture. The model describes the spatial and temporal changes of reactions and diffusion, and it is able to predict the observed conversion curves and sulfation patterns. The Thiele parameter revealed that, in the N2 atmosphere, sulfation diffusion limitation was increased more than in the CO2 atmosphere, where the increase of the Thiele number was negligible. The equation describing the porous structure within the model could be developed for better validity. A noticeable feature of the model used is its ability to consider simultaneous reactions. The overlap of calcination and sulfation is especially important in oxy-fuel conditions, where the high CO2 concentration can hinder calcination, although in this study, the effect of CO2 was minor. This study reveals the limestone-capture behavior under different conditions, which is important when oxy-fuel CFB is developed. It will also be important to test how the changing conditions during the CFB loop affect the sulfur-capture capacity at higher CO2 concentrations, where carbonation can also take place. It was important to accomplish the sulfur-capture capacity comparison tests for a long time period. In high CO2 contents, the maximum conversion is attained much slower than in a N2 atmosphere. Thus, false conclusions could have been made from shorter test times. The observed effect of H2O on the limestone reactions21,22 should be addressed in future studies because of the importance of H2O in the oxy-fuel CFB.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: sirpa.takkinen@lut.fi.
’ ACKNOWLEDGMENT Sirpa Takkinen is grateful for the scholarship from the Graduate School of Energy Technology. This work has also been supported by the Academy of Finland under Grant 124368 and Foster Wheeler Energia Oy. ’ NOMENCLATURE A = area (m2) cp = specific heat capacity (J kg1 K1) D = diffusion coefficient (m2/s) ΔH = reaction enthalpy (J/mol) k = thermal conductivity (W m1 K1) and reaction rate (s1) Lc = characteristic length (m) = V/A = r/3 for a sphere P = pressure (Pa) r = radial coordinate within the particle (m) R = radius of the particle (m) S = source/sink (kg m3s1) t = time (s) 2978
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Energy & Fuels T = temperature (K) v = velocity (m/s) V = volume (m3) x = molar amount of species (mol/m3) Y = mass fraction of species Greek Symbols
ε = volume fraction F = density (kg/m3) υ = stoichiometric coefficient Φ = Thiele modulus
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(19) Iisa, K.; Hupa, M. Sulphur absorption by limestone at pressurized fluidized bed conditions. Proceedings of the 23rd International Symposium on Combustion; Orleans, France, July 2227, 1990. (20) Hajaligol, M. R.; Longwell, J. P.; Sarofim, A. F. Ind. Eng. Chem. Res. 1988, 27, 2203–2210. (21) Wang, C.; Jia, L.; Tan, Y.; Anthony, E. J. Fuel 2010, 89, 2628–2632. (22) Wang, C.; Jia, L.; Tan, Y.; Anthony, E. J. Energy Fuels 2011, 25, 617–623.
Subscripts
c = calcination eff = effective eq = equilibrium g = gas i = species i m = mass tot = total s = sulfation
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