Bench-Scale and Modeling Study of Sulfur Capture by Limestone in

Article pubs.acs.org/EF

Bench-Scale and Modeling Study of Sulfur Capture by Limestone in Typical CO2 Concentrations and Temperatures of Fluidized-Bed Air and Oxy-fuel Combustion Sirpa Rahiala,*,† Timo Hyppan̈ en,† 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, FI-40101 Jyväskylä, Finland

‡

ABSTRACT: In this study, the influence of CO2 on sulfur capture efficiency was studied during fluidized-bed desulfurization by experiments and modeling. During calcination−sulfation and direct sulfation, the effect was examined with one limestone type. A time-dependent multilayer particle model was used for analyzing the experimental results. The model determines the magnitude of the reactions and the diffusion as a function of the radius and time. In high temperatures (∼1200 K), CO2 increased the conversion degree during calcination−sulfation. In direct sulfation, the effect of CO2 was opposite; lower conversion was obtained when the CO2 concentration was increased. When the CO2 concentration was increased in low temperatures (∼1100 K) (close to the calcination curve), CO2 retarded the conversion strongly. The detected differences between the results are explained with the development of the Thiele number, conversion curve, and conversion profile during the reactions.

1. INTRODUCTION The evidence for the warming of the climate system have raised interest toward the concept of carbon dioxide (CO2) capture and storage (CCS). For the energy sector, there are three main approaches for capturing CO2: post-combustion, pre-combustion, and oxy-fuel combustion systems. Oxy-fuel combustion can be implemented in pulverized fuel firing in fluidized-bed (FB) boilers. Fluidized-bed boilers have many inherent advantages, such as the possibility to use cheap sulfur removal. The sulfur capture is carried out with calcium injection into the furnace using limestone or dolomite. If the furnace is operated in atmospheric pressure, sulfur capture is obtained via calcination and sulfation. CaCO3(s) → CaO(s) + CO2 (g)

(1)

CaO(s) + SO2 (g) + 1/2O2 (g) → CaSO4 (s)

(2)

In oxy-fuel combustion, the gas composition and temperature level can differ from the traditional combustion application. The CO2 concentration will be much higher in oxy-fuel solutions, and the H2O concentration can also be higher if the recycling of wet combustion gas is used. These different conditions can offer an enhancement for sulfur capture. Also, the system itself can lead to lower SO2 emissions, as noticed by Liu et al.,2 because SO2 can be enriched in the furnace of oxy-fuel combustion systems. The higher SO2 concentration leads to good sulfation conditions and, thus, much lower total SO2 emissions. In fluidized-bed conditions, there is only a limited amount of research studies in the literature about the capturing of sulfur in O2/CO2 atmospheres. However, differences have been detected in the calcium conversion and SO2 emissions in oxy-fuel combustion tests.2−7 The observed differences can be caused by several reasons. Although the change of indirect sulfation to direct sulfation can result in a higher sulfation degree,2 opposite results have been reported.8 Recently, de Diego et al.9 and ́ Garcia-Labiano et al.10 found that indirect sulfation gave better conversion degrees compared to direct sulfation and suggested that 1198−1223 K would be an optimal temperature for sulfur retention in oxy-fuel conditions (for gaining calcination). However, the increase of the average CO2 in oxy-fuel combustion (in comparison to air-fired combustion) has not gained much interest. The main focus in the studying of the CO2 effect in oxy-fuel combustion has been in the comparison of indirect and direct sulfation.2,3 Also, the effect of CO2 on direct sulfation11−13 and the effect of CO2 on limestone calcination14,15 have been studied more extensively. Only few studies have focused on the CO2 effect in indirect sulfation

If the furnace is operated in pressurized or oxy-fuel conditions (the partial pressure of CO2 in the system can be higher than the equilibrium CO2 pressure over the limestone), the sulfur capture can occur via direct sulfation. CaCO3(s) + SO2 (g) + 1/2O2 (g) → CaSO4 (s) + CO2 (g) (3)

Traditional atmospheric and pressurized fluidized combustion applications are well-established technologies, and limestone has been successfully used for sulfur capture in these applications for 30−40 years. Although the subject has been deeply studied during the last few decades, there are still some unclarities and the calcium conversion in these units is usually relatively low (30−40%).1 The main reason for this low conversion is the gradual growth of a sulfated shell, which leads to high diffusion resistance and prevents the reaction from proceeding through the particle. This leads to higher costs and higher environmental impact.1 © 2013 American Chemical Society

Received: September 30, 2013 Revised: November 29, 2013 Published: December 9, 2013 7664

dx.doi.org/10.1021/ef401971m | Energy Fuels 2013, 27, 7664−7672

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Table 1. Test Matrix gas atmosphere description

N2 (vol %)

CO2 (vol %)

O2 (vol %)

SO2 (ppm)

time (min)

temperature (K)

calcination−sulfation high temperature

90 49 0 90 70 28 0

0 41 90 0 20 63 90

10 10 10 10 10 9 10

2154 1943 2222 2299 1882 1936 2222

360 180 360 360 180 180 360

1187 1182 1187 1115 1122 1122 1115

calcination−sulfation low temperature direct sulfation

conditions and especially in oxy-fuel circulating fluidized-bed (CFB) combustion conditions.9,10 Snow et al.11 did not notice any effect of CO2 on direct sulfation, but on the calcination side, CO2 did enhance the sulfation, although this is not mentioned in the results. Borgwardt16 studied the effects of water vapor and CO2 on the sintering rate of CaO. The results of Borgwardt16 showed that the reduction of porosity and surface area are accelerated by either H2O or CO2 in the sintering atmosphere and the combination of H2O and CO2 has a greater effect than either gas individually. Tullin et al.12 found that CO2 inhibited the direct sulfation and the gas diffusion coefficient decreases with the increasing CO2 concentration. More recently, Liu et al.2 varied CO2 at the temperature of 1123 K and found that an increase of CO2 caused a higher conversion because of the change from indirect sulfation to direct sulfation. Chen et al.17 studied the effect of CO2 on a large scale (temperature of 1073−1473 K and 20−80% CO2). They found that, when the CO2 concentration was increased, the optimal desulfurization temperature and the corresponding desulfurization efficiency increased. Chen et al.17 explained the observations with delayed calcination and pore structure development. Later, Chen et al.18 made an study in which they examined the influence of an O2/CO2 atmosphere on calcination. Their findings were in agreement with the results of Borgwardt;16 in an O2/CO2 environment, the specific surface area and specific pore volume were less than in air at the same temperature. In addition, the pore diameter and grain size were larger in O2/CO2 than in air. The pore diameters (40−300 Å, with a peak at 85 Å) in CaO produced in O2/CO2 were more suitable for sulfation because the pore sizes do not lead to rapid pore filling and pore-mouth plugging. Hu et al.13 found that the conversion rate decreases consistently with the increasing CO2 concentration in direct sulfation. They explained the effect with solid-state diffusion; the higher CO2 concentration causes a decrease in solid-state diffusivity. However, the temperature level and particle size used in the test are much smaller compared to oxy-CFB ́ et al.10 studied conditions. de Diego et al.9 and Garcia-Labiano the effect of CO2 in oxy-CFB conditions, but they concluded that, once calcining or non-calcining operating conditions were defined, the influence of the CO2 concentration on the sulfation reaction rate was negligible. Lately, Stewart et al.19 found that, in indirect sulfation, the increased CO2 concentration enhanced the conversion in a diffusion-controlled regime. They explained the observation with accelerated sintering and improved solidstate diffusion. Thus, in general, in direct sulfation, the effect of CO2 is found to be negative to sulfur capture, whereas the effect in indirect sulfation is positive, if any effect is detected. The reasons given for these phenomena are the changes in gas- and

solid-state diffusion and sintering, which are usually in connection. Stewart20 brought out the time window used in the sulfur capture tests, because the effect of CO2 can be opposite in diffusion and chemical kinetics-limited regions. Stewart20 also mentioned the combined effect of the temperature, CO2, and H2O, which should be included in the modeling work of sulfation. However, this study focuses only on the effect of CO2 on sulfur capture, and the effect of water will be investigated in future studies. The limestone reactions in fluidized beds have been studied extensively in past decades and reviews,1,21,22 and different models23−27 have been presented. However, most of the models do not take the effects of CO2 into consideration. The process conditions for new fluidized-bed process applications using limestone reactions enable several parallel reactions existing in changing conditions. Thus, in general, the particle model for limestone reactions should include the unsteady solution of mass and energy transport in the time and space domains for a number of species involved in the reactions. This kind of a particle model was developed by Takkinen et al.28 The calcination reaction and model accuracy were studied first,28 and it was followed by the study of the sulfur capture phenomenon in air-fired and oxy-fuel conditions. 7 Then, the different modeling methods were combined.29 A steady-state three-dimensional (3D) model produced the inhomogeneous process conditions inside the furnace, and a stochastic Lagrangian particle flow model determined the particle trajectories of the sorbent inside the furnace and produced a realistic dynamic process environment, which the particles experience. The particle model simulated the effects of the transient environment on limestone reactions. To develop the accuracy of the particle model and description of limestone behavior in the comprehensive 3D process model, sub-phenomena, such as the effect of CO2, should be studied and included in the model. Therefore, in this work, the influence of CO2 on sulfur capture during fluidized-bed desulfurization is investigated by experiments and modeling. During calcination−sulfation and direct sulfation, the effect was examined with one limestone type in conditions (temperature and CO2 concentration), which are similar to fluidized-bed air and oxy-fuel combustion. The unsteady multilayer particle model, which is based on the control volume method, was used for simulating the CO2 effect on sulfur capture. Now, with the experiments and modeling, the effect of CO2 can be evaluated in detail. With this model, the development of the kinetics and diffusion in experiments is evaluated and the Thiele parameter is used for estimating the relation of these magnitudes during reactions. The results show different limitations as a function of time in different conditions, resulting in different sulfation patterns and conversion degrees. 7665


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2. EXPERIMENTAL SECTION

Table 2. Calcium Compounds Based on Chemical Analyses at 180 min

2.1. Description of the Limestone and Test Matrix. The limestone used in the tests was Parfill P6 of Nordkalk Corporation. More details of the limestone are found in the previous study.7 Table 1 presents the test matrix. The test method is described in our previous study.7 The mean particle sizes after different test times (5, 30, and 180 min) were determined by optical microscopy (OM), and the arithmetic mean particle sizes (D[1,0]) were determined on the basis of the minimum particle diameters. In the N2 and CO2 atmospheres, there were no detectable differences and the influences of primary fragmentation and attrition could be omitted. The mean particle size of all tests was 110 μm. After the test, the test material was blown out from the reactor (with N2 or CO2, depending upon the main gas in the test) and all of the particles were separated from the gas flow using a cyclone and used as a sample for chemical analyses. The duration of the new tests was 180 min, and the duration of the previous tests7 was 360 min. 2.2. Apparatus. The bench-scale bubbling fluidized-bed (BFB) reactor was used in the tests. In this study, the apparatus was used batch-wise with respect to the limestone. The limestone was fed through a drop tube by opening a valve above the test device, and the limestone was then fed by gravitation. The details of the reactor can be found in the previous study.7 Preheated primary gas was supplied to the reactor through a perforated grid. Various kinds of gas mixtures can be used, including O2, CO2, SO2, CO, H2O, air, and nitrogen. The gas velocity was 0.40 m/s. Inside the reactor, the temperature was measured at six levels and controlled by two surrounding electric heaters. Flue gas composition, including O2, CO2, and SO2, was determined with online gas analyzers. The test device was operated in atmospheric pressure, and the value of 101 kPa was used in the calculations. 2.3. Chemical Analyses. The detailed information of chemical analyses can be found in our previous study.7 Now, the contents of the different calcium species, CaO, CaCO3, and CaSO4, were analyzed from the samples after each test. The amount of CaCO3 was determined by heating the sample from 550 to 950 °C and weighting it before and after the heating. The amount of sulfur and calcium was determined using the X-ray fluorescence (XRF) for ground finegrained material.

description calcination−sulfation high temperature

calcination−sulfation low temperature

direct sulfation

a

test 0% CO2, 1223 Ka 41% CO2, 1182 K 90% CO2, 1223 Ka 0% CO2, 1123 Ka 20% CO2, 1122 K 63% CO2, 1122 K 90% CO2, 1123 Ka

CaCO3 (mass %)

CaO (mass %)

CaSO4 (mass %)

11.0

63.1

18.7

55

31.4

10.3

44.6

40.8

5.5

62.3

28.0

60.8

19.1

15.9

59.2

3.5

33.5

62.0

0.05

34.4

7.9

Obtained from Takkinen et al.7

The sulfur capture conversion curves obtained as a function of the reaction time at different atmospheres are shown in Figure 1. As seen, the highest sulfur capture capacity was reached in the operating conditions, in which the limestone calcined at the higher temperature in the high CO 2 concentration (90%). Two of the new conversion curves obtained come between the earlier curves. The second highest conversion at 180 min is measured in the case of 63% CO2, which provides the conditions for direct sulfation. Almost the same conversion is attained in the case of 41% CO2, which enables the calcination−sulfation mechanism. On the contrary, in the case of 20% CO2, the operating conditions are near the CaCO3 calcination curve and the sulfation curve departs from the other results. The attained conversion is considerably lower than what is gained in traditional air-combustion conditions (30−40%).1 Tests also revealed that the sulfur capture rate was slightly higher during the initial stage of the test (approximately in 30 min) in the operating conditions with zero CO2 concentration than in high CO2 concentrations (40−90% CO2). Especially, in the initial stage, a low reaction rate was observed with the direct sulfation test (63% CO2). Thus, the uncalcined or slow calcined limestone has a lower global reaction rate than the calcined limestone. It is recognized that, under oxidizing conditions, the maximum conversion obtained exists at approximately 90 min in atmospheric conditions and the sulfation process has two stages: a fast stage (∼90 min), which is controlled by chemical reaction and/or diffusion through the porous system of the particle, and a slow stage (which can continue for days30), which is controlled by diffusion through the product layer. The sulfation rates in the 0% CO2 atmospheres decelerate substantially after 90 min of the test run. The oxy-fuel conditions seem to change the sulfur capture process in such a way that the deceleration is minor and similar clear stages cannot be obtained at least within similar time periods. In high CO2 concentrations, it seems that the sulfation rate remains approximately constant and the conversion curve shows that the maximum sulfation is not achieved in the examined time. Therefore, even higher conversion degrees could be attained if the reaction times were extended and direct sulfation (63% CO2) could exceed the indirect curve (90% CO2). 4.2. Evaluation of Calculation Parameters. The particle model described in section 3 was used for modeling the effect of various conditions on sulfur uptake. In this work, the results

3. NUMERICAL PARTICLE MODEL A detailed description of the one-dimensional dynamic particle model of limestone is presented in the study of Takkinen et al.,28 in which the accuracy and validity of the particle model were examined. The model is based on a control-volume method, and it contains a description of the mass and energy transfer inside the particle. The model interprets the transient behavior of the particle, including the temporal and spatial 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 five gas species considered are SO2, O2, CO2, H2O, and N2, and the three solids considered are CaO, CaCO3, and CaSO4. Calcination, sulfation, and direct sulfation (reactions 1, 2, and 3) were considered in the model. 4. RESULTS AND DISCUSSION 4.1. Experimental Results. The calcium use was calculated from both the gas responses by integrating them and the chemical analysis of the samples, and the differences between these results were explained in our previous study.7 The gas integration results are adjusted in such a way that, at the end of each test, the fractions of calcium compounds equal the shares of chemical analysis results, which are presented in Table 2. 7666


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Figure 1. Conversion curves in different atmospheres in (a) 360 min and (b) 90 min.

of 20, 41, and 63% CO2 are presented in more detail; the results of the previous modeling calculation (0/90% CO2) can be found in the work of Takkinen et al.7 The input parameters for the calculations are reported in Table 3. The values of the Table 3. Input Parameters particle diameter (μm) sorbent particle voidage initially pressure (kPa) temperature (K) CO2 concentration (%)

110 0.3 101 1122/1182 20, 41, and 63

kinetic parameters and the diffusion coefficients obtained in each case are shown in Table 4. The diffusion and kinetic parameters during the reactions were fitted until the measured and calculated conversion curves matched. The kinetic parameters were fitted in the early period when diffusion had no effect on the reaction, and the diffusion development is evaluated in the later period. Figures 2, 5, and 8 show that the model can predict the curves with adequate accuracy. 4.3. Model Calculations of 41% CO2, 1182 K. The modeled and measured fractions of calcium compounds from the test 41% CO2, 1182 K are presented in Figure 2. The calcination takes approximately 30−40 min, whereas in 0% CO2 (1223 K) with the same limestone, the calcination was accomplished in less than 5 min.7 Thus, the calcination is clearly slowed to the increased CO2 concentration. Figure 3 shows the CaSO4 mass fraction as a function of the radius at different times calculated with the numerical particle model. The CaSO4 profiles are similar at 5 and 30 min, but at 180 min, there is a clear difference. First, the profiles are flat, but at 180

Figure 2. Measured (solid line) and modeled (dashed line) calcium compounds at 41% CO2, 1182 K.

Figure 3. CaSO4 mass fraction inside a particle at different times at 41% CO2, 1182 K.

Table 4. Calculation Parameters calcination (mol m−3 s−1)

case 41% CO2, 1182 K

−6

Sc = 2(−2 × 10 T + 0.002808)(Peq − PCO2)

0.88

sulfation (mol m−3 s−1) (YCaCO3 − 0.084)

Ss = 33.75xSO2

diffusion (m2 s−1) YCaSO4 < 0.2 Deff = 1 × 10−6 YCaSO4 ≥ 0.2 Deff = 5 × 10−14YCaSO4−9 YCaSO4 ≥ 0.336

20% CO2, 1122 K

−6

Sc = 2(−2 × 10 T + 0.002808)(Peq − PCO2)

0.88

(YCaCO3 − 0.635)

Ss = 43.68xSO2

Deff = 1 × 10−9 YCaSO4 < 0.04 Deff = 1 × 10−6 YCaSO4 ≥ 0.04 Deff = 7 × 10−11YCaSO4−2

Ss = 33.3xSO2YCaCO3

63% CO2, 1122 K 7667

Deff = 5 × 10−9YCaSO4−2


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min, the distinct layer is developed in the outermost of the particle because of the decreasing diffusion, as seen from Figure 4, which shows the local effective diffusion development.

Figure 6. CaSO4 mass fraction inside a particle at different times at 20% CO2, 1122 K. Figure 4. Development of the local effective diffusion coefficient inside a particle at 41% CO2, 1182 K.

Initially, the diffusion is high and not limiting the reactions. Consequently, its role in reactions is insignificantly small, and the model fitting cannot define an accurate value for the diffusion and has been omitted in the chemical kinetics-limited regime. In Figure 4, this area is presented with a dashed line. The diffusion at the shell of the particle at 180 min is approximately an order higher than in the case of 0% CO2, 1187 K (∼1 × 10−10 m2/s)7 and an order smaller than in the case of 90% CO2, 1187 K (∼1 × 10−8 m2/s).7 4.4. Model Calculations of 20% CO2, 1122 K. The progress of calcination is extremely slow, and only a small share is calcined during the test, as seen from Figure 5, which

Figure 7. Development of the local effective diffusion coefficient inside a particle at 20% CO2, 1122 K.



presents the modeled and measured fractions of calcium compounds from the test 20% CO2, 1122 K. Figure 6 shows that the profile of CaSO4 is initially flat in the 20% CO2 case, but after 30 min, sulfation continues only in the shell of the particle. Figure 7 shows the local effective diffusion development. The diffusion decreases greatly during the first 10 min, but after that, the decrease is minor. On the contrary, in the case of 0% CO2, the decrease of diffusion occurs later but the amount of decrease was greater, so that the diffusion at 180 min was close to 1 × 10−10 m2/s at the outermost of the particle.7 4.5. Model Calculations of 63% CO2, 1122 K. The modeled and measured fractions of calcium compounds from the test 63% CO2, 1122 K are presented in Figure 8. Figure 9 shows the CaSO4 mass fraction as a function of the radius at

different times. The CaSO4 profiles are similar at every time. At 180 min, only a modest increase is detected in the shell of the particle; otherwise, the profiles are flat. The local effective diffusion in Figure 10 shows that the change of diffusion in the test 63% CO2 is minor. The diffusion in the particle shell at 180 min is almost an order higher in the test 63% CO2 than in the 90% CO2, 1115 K case (1 × 10−8 m2/s).7 4.6. Effect of CO2: Comparison. When the conversion curves of calcination−sulfation cases (0, 41, and 90% CO2) are compared in the higher temperature in Figure 11, it can be observed that the reaction rate of 0% CO2 is higher, but after ∼90 min, the 0% CO2 decelerates and both the 41 and 90% CO2 curves exceed it. Thus, it seems that the effect of CO2 is 7668


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Figure 9. CaSO4 mass fraction inside a particle at different times at 63% CO2, 1122 K.

Figure 12. CaSO4 mass fraction inside a particle at 180 min in various CO2 concentrations in the higher temperature.

dimensional ratio of the time scales involved in a heterogeneous system: the time scale for diffusion and the reaction time scale. 1/2 ⎛ k ⎞1/2 R⎛ k ⎞ Φ = Lc⎜ ⎟ = ⎜ ⎟ 3 ⎝ Deff ⎠ ⎝ Deff ⎠

(4)

It is generally acknowledged that when Thiele ≫ 1, intraparticle diffusion is the controlling mechanism, while when Thiele ≪ 1, the controlling mechanism is kinetics. Both the reaction kinetics and diffusion are affected when 0.1 < Thiele < 10. The Thiele curve of 41% CO2 stays between the other two curves, as seen in Figure 13. First, chemical kinetics is Figure 10. Development of the local effective diffusion coefficient inside a particle at 63% CO2, 1122 K.

Figure 13. Development of the Thiele parameter as a function of time in different CO2 concentrations in the higher temperature. Figure 11. Modeled CaSO4 fractions as a function of time in various CO2 concentrations in the high temperature.

the limiting phenomenon, and the diffusion limitation increases, so that, at the end, both are limiting. Thus, it seems that, in calcination−sulfation in the higher temperature, the effect of CO2 is positive because of the enhanced diffusion and delayed calcination. This has also been detected in the literature.11,17,19 Figure 14 compares the conversion curves of calcination− sulfation cases (0 and 20% CO2) at the lower temperature. The initial reactivity is similar in both cases, but the 20% CO2 curve retards quickly. Because the operating conditions are near the CaCO3 calcination curve and the measured calcination curve is slow, it could be assumed that the direct sulfation is dominating. However, the behavior of the calcium sorbent is not similar to the behavior of the sorbent in non-calcining conditions (Figure 1), and the total conversion is much smaller. Figure 15 compares the profiles between the 0 and 20% CO2 cases. In the 0% case, a shell is also developed but the reactions

first negative and then positive in indirect sulfation in the higher temperature. Figure 12 compares the profiles at 180 min in different cases (0, 41, and 90% CO2); the profiles of 0 and 41% CO2 are similar, whereas in the case of 90% CO2, the sulfate layer is thicker. The 41% CO2 case resembles the core−shell sulfation pattern, as was the case in the case of 0% CO2 in the previous study,7 in which the scanning electron microscopy−energydispersive spectrometry (SEM−EDS) micrographs confirmed the sulfation patterns. On the contrary, two kinds of sulfation patterns were found in the case of 90% CO2: a uniform and a core−shell structure.7 The Thiele parameter can be used to evaluate the limiting process during the reaction. The Thiele parameter is a non7669


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in the 0% case than in the 20% case at 180 min. These findings fit for the conversion curves: in the 20% case, the conversion can still increase, whereas in the 0% case, the conversion does not increase after 90 min. In this case, it seems that the delayed calcination does not result in high sulfur capture efficiency like in the higher temperature cases (41 or 90% CO2). There has been discussion in the literature about excessive sintering in certain conditions, because sintering is dependent upon the temperature and H2O and CO2 concentrations.19 Now, it seems that the delayed calcination because of the closeness of the calcination curve (increased CO2 concentration in the lower temperature) can result in an unfavorable microstructure, which will speed up the deceleration of the sulfation process. For example, the increased CO2 concentration can prevent or affect the calcination stage, so that small grains, which are filled quickly, are developed or the external pores of the particle are blocked before the calcination is finished and the cover of the particle is closed up. When the conversion curves of the direct sulfation cases (63 and 90% CO2) are compared in Figure 17, it can be observed

Figure 14. Modeled CaSO4 fractions as a function of time in different CO2 concentrations in the lower temperature.

Figure 15. CaSO4 mass fraction inside a particle at different times in different CO2 concentrations in the lower temperature.

also continue in the inner part of the particle. Thus, the particles have a sulfated surface with a gradual change from the sulfated external coating to the less sulfated core, and the sulfation structure is more uniform. The sulfation structure in the 0% CO2 case was detected to have a network sulfation pattern.7 The 20% CO2 case strongly resembles the core−shell sulfation pattern, and the change from the sulfated covering to the non-sulfated core is much sharper. The Thiele curves of these two cases are presented in Figure 16, which shows how the 0% case changes in the initial period from the chemical kinetics limitation to the chemical kinetics and diffusion limitation. However, the Thiele number is higher

Figure 17. Modeled CaSO4 fractions as a function of time at different CO2 concentrations in direct sulfation.

that the reaction rate of the high CO2 concentration is higher but, after approximately 120 min, the 63% curve exceeds the 90% curve. Thus, it seems that the effect of CO2 is positive first and then negative in direct sulfation. However, the difference between curves is minor, and thus, the accuracy of the measurement can include the difference between curves in the initial period. The shape of the curves is nevertheless different; in lower CO2, the curve is rather straight, whereas the 90% CO2 begins to slow. Figure 18 shows that, in 63% CO2, the profile is flat, whereas in 90% CO2, the sulfate layer is developing and the gradient is greater at the surface of the particle. The SEM−EDS micrographs in the previous work7 confirmed that the core− shell sulfation pattern with a thicker layer is the main pattern in direct sulfation (90% CO2, 1115 K). The uniform sulfation pattern in the 63% CO2 case enables the higher conversion because calcium is used throughout the particle. The change in the Thiele number is minor in the case of 63% CO2 compared to the change in 90% CO2, as shown in Figure 19. Thus, it seems that, in direct sulfation, the effect of CO2 is negative and the development of diffusion explains the behavior. This has also been detected in the literature; Tullin et al.12 found that the gas diffusion decreased with an increasing CO2 concentration.

Figure 16. Development of the Thiele parameter in different CO2 concentrations in the lower temperature. 7670


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sulfation.2 In a CO2 atmosphere, the calcination−sulfation has resulted in higher conversion than the calcination−sulfation in air.17 Also, in a CO2 atmosphere, higher conversion has been achieved in calcining conditions compared to non-calcining conditions.8 It appears that the optimal sulfur capture in fluidized-bed oxy-fuel conditions is not only dependent upon the sulfur capture mechanism (indirect/direct) but also the combination of several parameters, such as temperature, gas concentrations, and limestone type, determines in which conditions the highest capture capacity is obtained and in which conditions the “optimal” sulfation pattern is gained inside the particle.

5. CONCLUSION The effects of CO2 on sulfur capture mechanisms were studied with the bench-scale experiments and modeling in conditions (temperature and CO2 concentration) that are similar to fluidized-bed air and oxy-fuel combustion. Experimental results show different effects of CO2 in different conditions. Sulfur capture was improved during calcination−sulfation in the higher temperature when CO2 was increased. During direct sulfation, the conversion was decreased with higher CO2 concentrations. Close to the calcination curve, CO2 retarded the conversion strongly. A single particle model of limestone was used to explicate the effect of different conditions on sulfur capture, and the model was able to predict the detected conversion curves. The model produced the sulfation patterns inside the particle and the Thiele number development; the highest sulfur capture was accomplished with a uniform sulfation pattern and a low Thiele number. This study showed how the analyses of the experimental results with this model reveal the temporal and spatial changes of reactions and diffusion during the reactions in different conditions. This study reveals the limestone capture behavior under different conditions similar to fluidized-bed air and oxy-fuel combustion, which is important when oxy-fuel CFBs are developed. The results are valid for the used particle size of 110 μm and one limestone type; with different particle sizes and limestone types, the results could be varied because of different diffusion and chemical kinetics limitations, and this could be examined in future studies. The combined effect of the temperature and gas concentrations is more essential than the sulfur capture mechanism (indirect/direct) when the optimal sulfur capture in fluidized-bed oxy-fuel conditions is determined. In high CO2 contents, the maximum conversion is obtained much more slowly than in a N2 atmosphere and the effect of CO2 can be opposite in different limitation regions. Thus, shorter test times could have led to incorrect conclusions, and it is important to notice that, in a CO2 atmosphere, the sulfur capture capacity comparisons are made for long enough periods of time. The combined effect of CO2 and H2O should be included in future studies developing the understanding of the sulfur capture in oxy-fuel conditions because they both exist in large amounts in oxy-CFBs and the combined effects are unclear.

Figure 18. CaSO4 mass fraction inside a particle at different times in different CO2 concentrations in direct sulfation.

Figure 19. Development of the Thiele parameter in different CO2 concentrations in direct sulfation.

4.7. Summary. In the previous study,7 the SEM−EDS micrographs confirmed the obtained sulfation patterns in different cases. Figures 12, 15, and 18 show that the core− shell structure was the sulfation pattern in most of the studied cases. Additionally, in most of the studied cases, at the end of the examination period, Thiele is in the region where both the reaction kinetics and diffusion are limiting, as seen from Figures 13, 16, and 19. The highest capture capacity was obtained in the case where the conditions lead to a uniform and core−shell structure (higher temperature, 90% CO2). Also, the second highest conversion (63% CO2) was obtained with a rather uniform sulfation pattern. The results also show that the faster the core−shell is developed, the smaller the obtained conversion and the Thiele number increases faster in the region where diffusion and kinetics limit the reaction (20% CO2). If the developed shell is thicker and the profile is not stiff (there is rather a gradual change from the high sulfated external shell to the less sulfated core), then the conversion is also higher (like in the lower temperature, 90% CO2). It seems that the sulfur capture capacity will depend upon the CO2 concentration when determining which is the “optimal” condition for sulfur capture and which mechanism (indirect/ direct) will gain higher capture capacity. For example, at intermediate concentrations (40−60%), direct sulfation can gain higher conversions, whereas at higher CO2 concentrations (∼90%), the indirect sulfation can result in good capture efficiency. These results are conflicting with the results of de ́ Diego et al.9 and Garcia-Labiano et al.,10 who found that the indirect sulfation gave higher conversions. However, direct sulfation can lead to higher conversion compared to indirect

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AUTHOR INFORMATION

Corresponding Author

*E-mail: sirpa.rahiala@lut.fi. Notes

The authors declare no competing financial interest. 7671


Energy & Fuels

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Article

(20) Stewart, M. C. Sulfation phenomena under oxy-fuel circulating fluidized bed conditions. Master’s Thesis, University of Ottawa, Ottawa, Ontario, Canada, March 2011. (21) Stanmore, B. R.; Gilot, P. Fuel Process. Technol. 2005, 86, 1707− 1743. (22) Hu, G.; Dam-Johansen, K.; Wedel, S.; Hansen, J. P. Prog. Energy Combust. Sci. 2006, 32, 386−407. (23) Sahimi, M.; Gavalas, G. R.; Tsotsis, T. T. Chem. Eng. Sci. 1990, 45, 1443−1502. (24) Ramachandran, P. A.; Doraiswamy, L. K. AIChE J. 1982, 28, 881−900. (25) Adánez, J.; Gayan, P.; García-Labiano, F. Ind. Eng. Chem. Res. 1996, 35, 2190−2197. (26) Mattisson, T.; Lyngfelt, A. Chem. Eng. Sci. 1998, 53, 1163−1173. (27) Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K. Chem. Eng. Res. Des. 1999, 77, 62−8. (28) Takkinen, S.; Saastamoinen, J.; Hyppänen, T. AIChE J. 2012, 58, 2563−2572. (29) Rahiala, S.; Myö hänen, K.; Hyppänen, T. Fuel 2013, DOI: 10.1016/j.fuel.2013.08.019. (30) Abanades, J. C.; de Diego, L. F.; García-Labiano, F.; Adánez, J. AIChE J. 2000, 46, 1888−1893.

ACKNOWLEDGMENTS This work was carried out in the Carbon Capture and Storage Program (CCSP) research program coordinated by CLEEN, Ltd., with funding from the Finnish Funding Agency for Technology and Innovation, Tekes.

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NOMENCLATURE D = diffusion coefficient (m2/s) k = reaction rate (s−1) 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 m−3 s−1) T = temperature (K) x = molar amount of species (mol/m3) Y = mass fraction of species

Greek Symbols

Φ = Thiele modulus Subscripts

c = calcination eff = effective eq = equilibrium s = sulfation

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