Flue Gas Desulfurization by Calcined Phosphate Rock and Reaction

Article pubs.acs.org/EF

Flue Gas Desulfurization by Calcined Phosphate Rock and Reaction Kinetics Jale Naktiyok,* Hatice Bayrakçeken, A. Kadir Ö zer, and M. Şahin Gülaboğlu Department of Chemical Engineering, Atatürk University, 25240 Erzurum, Turkey ABSTRACT: The phosphate rock was calcined at temperatures ranging between 775 and 850 °C in a fluidized bed using air. The changes in the pore structure and surface area of the calcined phosphate samples were determined by a mercury porosimeter and Brunauer−Emmett−Teller (BET) analysis. It was determined that the calcined phosphate samples at 850 °C for 8 min have the highest surface area, particle volume, and porosity. Then, the reaction kinetics of SO2 with calcined phosphate sample (at 850 °C for 8 min) was investigated in a differential fluidized bed by changing the sulfation temperature and SO2 and CO2 concentrations. It was observed that the sulfation ratio increased with an increasing temperature and SO2 concentration but decreased with an increasing CO2 concentration. The sulfation reaction was well-represented by a shrinking core model, which can be divided into two regions with different rate controlling steps. For low conversions, the controlling step was found to be the chemical reaction at the interface but the diffusion through the product layer for high conversion. The activation energies for the chemical reaction at the interface and the diffusion through the product layer were calculated as 92 and 197 kJ mol−1, respectively.

1. INTRODUCTION Fluidized-bed combustion technology continues to be a focus of coal-fired electric utilities because of its ability to control emissions within acceptable limits. In addition, the fluidized bed is an excellent gas−solid contactor and is therefore not only an ideal combustion system but also provides attractive possibilities on removing sulfur oxides from combustion gases. Using a suitable sorbent as the bed material, highsulfur-containing coal can be burned while maintaining low SO2 emissions without a substantial decrease in combustion efficiency.1−5 Limestone or dolomite addition to the bed minimizes SO2 emissions from high-sulfur fuels by capturing SO2 in situ.6 The principal reactions involving the sorbent are as follows: CaCO3(s)⇔CaO(s) + CO2 (g)

(1)

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

(2)

In our previous studies, the possibility of using the Mazıdagı (Turkey) raw phosphate rock as an alternative to basic materials13 and the effect of carbon dioxide on simultaneous calcination/sulfation during the desulfurization of flue gas with phosphate rock14 have been investigated. The results showed that the conversion of sulfate obtained using phosphate ore was higher than that obtained with limestone and dolomite. Sulfate conversion, in actual fluidized-bed combustion units, is generally in between 30 and 40%,15 but using raw phosphate rock, the conversion is approximately 60% at 850 °C for 30 min.13 An increase in the operating temperature from 750 to 850 °C was accompanied by an increase in the extent of calcination and sulfation. It was observed that calcination was dominant during the early stages of the reaction to produce CaO. However, once a significant concentration of CaO was produced, the sulfation occurred at an appreciable rate. An increase in the CO2 concentrations always decreases the calcination rate. The objective of the present study is to evaluate the effect of the temperature and SO2 and CO2 concentrations on the sulfation reaction during the desulfurization of flue gas with a calcined phosphate sample in a differential fluidized bed and to investigate the reaction kinetics of SO2 with a calcined phosphate sample.

Sorbent calcination generates a porous solid (lime), which reacts with SO2, producing CaSO4. Because CaSO4 has a larger molar volume than CaO, pore blockage takes place during the reaction and complete conversion of the sorbent cannot be achieved.7−10 Using a natural limestone or dolomite as bed material has some environmental drawbacks, including quarrying of large amounts of limestone and disposing of large quantities of solid waste. The inefficient use of calcium can introduce an economic burden to a power plant, especially when transporting the sorbent over long distances.11 On the other hand, the carbonates in the phosphate rocks consume an additional acidulent (sulfuric acid) during the production of phosphoric acid and super phosphates. During the reaction, carbon dioxide produced causes more foaming and also results in the production of smaller size gypsum crystals that may blind the downstream phosphorus gypsum filters and a low-quality phosphoric acid may be produced.12 © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Material. Phosphate ore used in this investigation was provided from the phosphate deposits in Mazıdagı (Turkey). The sample was crushed, ground, and then sieved to give the different size fractions using ASTM standard sieves. In this study, the particles having a 500−710 μm particle size were used. The chemical analysis of the phosphate rock was carried out by standard gravimetric, Received: June 7, 2012 Revised: February 4, 2013 Published: February 4, 2013 1466

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volumetric, and spectrometric methods, and the results of chemical analysis of the sample are given in Table 1.

WCaO = WSO4

500−710 μm

CaO P2O5 MgO Fe2O3 Al2O3 SiO2 F2 loss on ignition (CO2)

50.27 23.27 1.60 0.03 1.13 4.05 3.60 14.85 (11.23)

(3)

Percent sulfate conversion was calculated as follows (eq 4):

Table 1. Analysis of the Phosphate Rock Samples (wt %) components

MCaO MSO4

sulfation conversion (%) =

WCaO × 100 Wt,CaO

(4)

Each experiment was repeated 3 times, and the average of the results of these experiments was used in the calculation of the sulfation conversions. The specific surface areas of the raw and calcined samples were measured by nitrogen adsorption at −195 °C using the Brunauer− Emmett−Teller (BET) method. The BET surface areas of the samples were determined in a Quantachrome QS-17 model apparatus. A Quantachrome Pore Master 60 model mercury porosimeter was used to investigate the pore-size distribution, pore volume, and porosity of the calcined samples in the pressure range from 1.5 to 60 000 psia.

2.2. Apparatus. A schematic diagram of the experimental setup is shown in Figure 1. Experiments were performed in a fluidized-bed reactor, which consisted of quartz (3 cm inner diameter) with a gas preheating section below the distributor. It was heated by positioning the tubular electric furnace centrally. A chromel−alumel thermocouple and a temperature controller were used to control the temperature of the bed. The reacting gas was prepared by mixing 4% O2, 5−20% CO2, 1000−6000 vppm SO2, and the rest being N2. To minimize variations in the bed temperature resulting from the heat absorbed during sulfation and to achieve the differential conditions, a minimum practical quantity of phosphate rock (approximately 1 g) was used. The minimum fluidization velocities, Umf, for the calcined phosphate samples were determined as 18.7, 17.8, 15.7, and 13.8 cm/s for the temperatures of 775, 800, 825, and 850 °C, respectively. The velocity of the fluidizing gas was maintained higher than the minimum fluidization velocity to ensure fluidizing conditions of the phosphate ore particles. The temperature of the bed was raised and kept for 10 min at the desired level, before charging the phosphate rock sample from the top of the bed. At the end of the reaction, the sample was discharged by applying vacuum and was collected in the sample hold up. Samples of dust and fine particles were recovered using two cyclones connected to this sample hold up. After calcination and sulfation, the samples were stored in an airtight bottle. Silica gel crystals were added to the bottle to absorb any moisture during the storage. Then, the sulfate sample was dissolved in diluted HCl. The amount of sulfate in the sample was analyzed by a spectrometric method, and the amount of CaO converted to CaSO4 was calculated using eq 3.

3. RESULTS AND DISCUSSION The reaction between SO2 and calcareous phosphate rock under oxidizing conditions during calcination and sulfation can be described as at least two consecutive reaction steps: Ca5(PO4 )F(s) + CaCO3(s) ⇔ Ca5(PO4 )F(s) + CaO(s) + CO2 (g)

(5)

Ca5(PO4 )F(s) + CaO(s) + SO2 (g) + 1/2O2 (g) ⇔ Ca5(PO4 )F(s) + CaSO4 (s)

(6)

In our previous works by our research group, physical structure, chemical and mineralogical composition,16 and calcination in the fluidized and fixed beds of the Mazidagı (Turkey) phosphate rock have been investigated.17 The results of X-ray powder diffraction analysis showed that the main minerals of the rock are calcite, flourapatite, and carbonate−fluorapatite at small amounts. It was determined that the carbonate− fluorapatite changed to fluorapatite during calcination. It was found by microscopical examination that the rock consists of two different phases, namely, a calcite-rich phase and a phosphorus-rich phase, which were dispersed heterogeneously.

Figure 1. Schematic diagram of experimental setup: (A) air compressor, (B) dryer, (C) control valve, (D) flowmeter, (E) manometer, (F) SO2 analyzer, (G) temperature controller, (H) preheating column, (I) reactor, (J) thermocouple, (K) sample hold up, (L) cyclone, (M) vacuum pump, and (N) gas out. 1467

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was observed that, after calcination at 850 °C for 8 min, the fraction of pores in between 20 and 150 nm is increased and the pores seen in the raw phosphate sample in between 4 and 16 nm disappeared because of the coalescing into macropores, which causes a decrease in the surface area. In addition, the specific surface areas of the raw and calcined samples were measured. BET surface area, pore volume, and porosity of the raw and calcined samples are given in Table 2. The surface area of the raw phosphate rock was decreased with calcination, whereas the pore volume and porosity were increased with the calcination. The pore volume, porosity, and surface areas of calcined samples were increased with an increasing calcination temperature. Freeman et al.20 also found that calcination above 800 °C of the phosphate ore was accompanied by a decrease in the fine pores and an increase in the coarse pores of the original rock. It was determined that phosphate samples calcined at 850 °C for 8 min have the highest surface area, pore volume, and porosity when compared to other calcined samples (Table 2). These samples were more reactive than the others. Therefore, the calcined sample at 850 °C for 8 min was chosen for carrying out the sulfation experiments. 3.2. Effect of the Temperature on the Sulfation Conversion. The effect of the temperature on sulfation conversion was investigated by performing experiments at atmospheric pressure with the phosphate rock calcined at 850 °C for 8 min and having particle sizes of 500−710 μm. A gas flow rate of Uf/Umf = 2 was applied within a temperature range of 775−850 °C. The reactions were carried out using a synthetic flue gas composition containing 4% O2, 15% CO2, 3000 vppm SO2, and the rest being N2. The period of exposure was varied from 1 to 30 min. The results are given in Figure 4. The results of the sulfation reactions indicate that, as the temperature increased from 775 to 850 °C, the activity of the calcined samples for SO2 removal also increased. The maximum conversion was obtained at approximately 85% at 850 °C for 15 min. It is well-known that sulfur capture with calcium-based sorbents is a process highly dependent upon the temperature and CO2 concentration according to the equilibrium curve of CaCO3 calcination.21 According to the thermodynamic data,22 the CO2 equilibrium concentrations are about 12.5 and 20 vol % at 775 and 800 °C, respectively. The sulfated samples were studied at 15 vol % CO2 concentration at 775 and 800 °C, where the sulfation reaction was markedly affected by the concentration of CO2. This result is likely due to the recarbonation of CaO and the lower reactivity of CaCO3 toward SO2, as a result of being quite close to the line corresponding to the calcination of CaCO3 at 775 and 800 °C. Although CaCO3 formed in the early stage would react with SO2 in a later stage, the overall sulfation rate decreased with an increasing fraction of CaCO3, owing to the lower reactivity of CaCO3 than CaO. Calcined samples gave the higher conversions of the sulfation when compared to uncalcined samples because of the increase in pore volume and particle porosity with calcination.15 Therefore, it was deduced that the calcination behavior played an important role in the observed variation in the sulfur uptake with temperature. These results are in agreement with those reported by other researchers on the influence of the temperature on the sulfation behavior of calcined sorbents.23−25 The structural properties strongly influence the pore diffusion and product-layer diffusion, which are the rate-limiting processes in the high-temperature sulfation of calcined

Calcite having particle sizes smaller than approximately 2 μm was agglomerated among compact phosphorus-rich phases having particle sizes in between 100 and 300 μm. Porous parts in the rock have generally been accumulated in the calcite-rich phase. Big holes occur in the parts with carbonate of the calcined samples resulting from the change to coarse pores, with calcination of fine pores seen in the parts with carbonate of the phosphate rock. It was also determined that the shrinkage and cracks at the surrounding parts with phosphate occur because of sintering. In addition, at another study, the results of X-ray powder diffraction analysis and scanning electron microscopy (SEM) photographs confirmed that, while the calcite-containing parts in the phosphate rock were reacting with SO2, the parts containing phosphorus gave no reaction. As seen from the reactions, Ca5(PO4)F existing in phosphate rock is inert and does not react. 3.1. Effect of the Temperature on the Calcination Conversion. The calcination of the phosphate samples was investigated in a fluidized bed using air as the reacting gas to determine the fully calcination times at four different temperatures. A gas flow rate of Uf/Umf = 2 was applied within a temperature range of 775−850 °C. As seen from Figure 2, the

Figure 2. Effect of the temperature on calcination of phosphate rock.

calcination conversion rate increased and fully calcination time decreased with an increasing temperature. Fully calcination times were determined as 60, 30, 15, and 8 min for the temperatures of 775, 800, 825, and 850 °C, respectively. Our previous results showed that the samples calcined at 900 °C in a fluidized bed are becoming more sintered;17 therefore, in this study, the optimum temperature for flue gas desulfurization is selected as 850 °C. The pore structure of sorbent plays a crucial role in determining the overall sorbent use and the rate of sulfation. Initial pore-size distribution of the solid reactant affects both the diffusional resistance to gaseous reactant (SO2) and the surface area of the solid.18,19 It is essential for SO2 to diffuse into the particle and react with the available CaO to achieve high calcium use. Furthermore, adequate porosity must exist to accommodate the product of sulfation, which has a higher molar volume than the reactant. As a result, the physical structure of a sorbent is critically important for its performance in the sulfation reaction. The change in pore structure during calcination, the poresize distribution, pore volume, and porosity of the raw and calcined samples were investigated by a mercury porosimeter. The results for pore-size distribution are shown in Figure 3. It 1468

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Figure 3. Pore-size distributions of the raw and calcined samples at the different temperatures.

mouths of the micropores begin to plug faster than the diffusion of SO2 inside the pores. Therefore, the lower sulfate conversions were obtained. However, the behavior in phosphate rock is in contrast. Raw phosphate rock has a high surface area, and especially, the CaO-containing parts in the phosphate rock have micropores. During the calcination, micropores are converted to macropores and shrinkage and cracks at the surrounding parts with Ca5(PO4)F because of sintering resulted in the increase in the pore volume and porosity but the surface area decreased. The SO2 diffusion to the CaO phases in internal parts of the calcined sample is provided by the shrinkage and cracks at the surrounding parts with Ca5(PO4)F because of sintering. In addition to this, SO2 is diffused easily to the macropores in CaO-containing parts because of the late plugging of the pore mouths and higher sulfate conversions are obtained with respect to limestone. At high temperatures, sulfation of calcined limestone is dominated by diffusion processes that are strongly dependent upon the particle structure. A large volume of data compiled by

Table 2. Pore Volume, Porosity, and BET Surface Area of the Raw and Calcined Phosphate Samples

raw phosphate 775 °C for 60 min 800 °C for 30 min 825 °C for 15 min 850 °C for 8 min

pore volume (mL/g)

porosity (%)

BET surface area (m2/g)

0.0584 0.0668 0.0919 0.0978 0.1016

4.26 15.29 18.63 25.54 36.84

9.40 2.25 2.28 2.62 2.98

limestone. The behavior of the reacting limestone particles generally depends upon not only the bulk properties of the porous calcine (such as the specific surface area) but also the pore-size distribution, especially at high temperatures. Natural limestones have compact structure, and they have lower surface areas. Their surface areas are becoming higher after calcination because of the formation of micropores. Although there is enough internal surface area, because of the higher molar volume of CaSO4 formed during the sulfation, the 1469

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mixture has accelerated the sintering of CaO, which resulted in the decrease in sulfation conversion. 3.4. Effect of the SO2 Concentration on the Sulfation. The effects of the SO2 concentration on the sulfation conversion at 850 °C were investigated. The compositions of synthetic flue gases were 4% O2, 15% CO2, 1000−6000 vppm SO2, and the rest being N2. The times of exposure, gas flow rate, and particle size were set to the same conditions as temperature studies. Effects of SO2 concentrations on the sulfation conversion of the calcined samples are depicted in Figure 6. As expected, the sulfation conversion increased with

Figure 4. Effect of the temperature on sulfation of the calcined phosphate sample.

Hartman et al.26 on the saturation capacity of calcined limestone for SO2 absorption showed that the actual capacity had no clear relation to the total pore volume, although the maximum capacity predicted by the stoichiometry of the reaction provided a satisfactory upper bound. 3.3. Effect of the CO2 Concentration on Sulfation. Experiments at atmospheric conditions were carried out to determine the effect of CO2 concentrations on the sulfation conversion at 850 °C. The compositions of synthetic flue gases were 4% O2, 5−20% CO2, 3000 vppm SO2, and the rest being N2. The times of exposure, gas flow rate, and particle size were set to the same conditions as temperature studies. Figure 5

Figure 6. Effect of the SO2 concentration on sulfation of the calcined phosphate sample at 850 °C.

an increasing SO2 concentration. It can be seen that the sulfation conversion is rapid in the initial period at all concentrations of SO2 but slows after 4 min. It could be possibly explained by the decrease in the slope of curves for SO2 concentrations of 3000 and 6000 vppm, which indicates a pore plugging process; at higher SO2 concentrations, the mouths of the pores will begin to plug faster than the SO2 diffusion inside the pores; thus, the sulfation conversion slows after 4 min at higher SO2 concentrations. 3.5. Modeling. In reaction studies, models are often used for the evaluation of kinetic parameters as well as for process simulations. The unreacted shrinking core model is the most frequently used one for the description of the kinetics of the sulfation reaction.27−29 At the beginning, reaction takes place on the surfaces of grains in the solid, where the rate is controlled by the chemical reaction. As the reaction proceeds, a layer of product is formed on the surface of each grain, which separates the reaction surface of the solid from reacting gas. In this case, the overall reaction rate is controlled by internal pore diffusion of the gas in combination with ionic diffusion of the solid reactant. However, the pore mouth is slowly plugged, and then the reaction shifts to the product layer diffusion-controlled stage. The experimental results were analyzed using the relationship between time and conversion for the unreacted shrinking core model with spherical particles of unchanging size, which Szekely et al.30 have modeled as follows: (a) chemical reaction at the interface

Figure 5. Effect of the CO2 concentration on sulfation of the calcined phosphate sample at 850 °C.

shows the conversions of sulfation versus time curves obtained for different CO2 concentrations. The CO2 equilibrium concentration is about 48 vol % at 850 °C. CO2 cannot react with CaO if the CO2 concentration is below 48% when the reaction temperature is 850 °C. In this study, in which the CO2 concentration is 5−20%, the sulfation reaction was affected by the concentration of CO2; the initial rate and sulfate conversion decreased with an increasing CO2 concentration, as shown in Figure 5. The increase in the CO2 concentration in the gas

k1t = 1 − (1 − X )1/3

(7)

(b) diffusion through the product layer 1470

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k 2t = 1 − 3(1 − X )2/3 + 2(1 − X )

(8)

where k1 and k2 are the apparent reaction rate constants, t is the reaction time, and X is the fraction of sulfate conversion. To determine the effect of the reaction parameters on the rate constant of the reaction, a semi-empirical model can be given as follows: k = koCCO2 aCSO2 be−Ea / RT t

(9)

Thus, eq 9 can be written as follows for chemical reaction at the interface and diffusion through the product layer, respectively: 1 − (1 − X )1/3 = koCCO2 aCSO2 be−Ea / RT t

(10)

1 − 3(1 − X )2/3 + 2(1 − X ) = koCCO2 aCSO2 be−Ea / RT t (11)

The results of statistical calculations obtained by simultaneous multiple regression of eqs 10 and 11 are given in Table 3. In the Table 3. Kinetic Parameters for Two Regions with Different Rate-Controlling Steps at the Sulfation of the Calcined Phosphate Sample ko chemical reaction diffusion through the product layer

6.1 × 10 1.6 × 108 3

a

b

Ea

R2

−0.23 −0.23

0.35 0.37

92 197

0.95 0.98

Figure 7. Agreement between experimental and theoretical conversion values.

conversions are reached with the calcined phosphate sample than with calcined limestone and dolomite. Dependent upon initial porosity of the calcined material, the difference in molar volumes between the product and reactant results in an idealized maximum of conversion between 55 and 65%.36,37 In this study, the maximum conversion was obtained approximately 85% at 850 °C for 15 min. Higher sulfation conversions achieved with calcined samples can be attributed to the pores kept open with the help of inert material, such as Ca5(PO4)F, which enhances the diffusion of sulfur dioxide through the porous structure of the reacting particle. The sulfation rate of CaO in the phosphate rock is fast at the initial stages of reaction, but at later stages, diffusion through the impervious CaSO4 product layer surrounding CaO becomes ratecontrolling. A shrinking core model showed that the reaction rate and diffusion control are both important. The activation energies for the chemical reaction at the interface and diffusion through the product layer were calculated as 92 and 197 kJ mol−1, respectively. Low activation energies were obtained for the calcined sample when compared to the raw sample.15 It was determined that the size of the pores developed by calcination has a critical effect on the rate and ultimate capacity for SO2 sorption.

studies involving the sulfation of calcined limestones, apparent activation energies of reactions can vary over a very wide range (103−238 kJ mol−1) caused by (i) sintering of the limestone particles, (ii) influence of the temperature on the structure of the product layer, and (iii) differences in the chemical and physical properties of the limestone and differences in experimental conditions, techniques, and particle sizes.31−35 In our previous work,15 it was found that the activation energies for the chemical reaction at the interface and diffusion through the product layer at the direct sulfation of phosphate particles were calculated as 100 and 296 kJ mol−1, respectively. Low activation energies were obtained for the calcined sample when compared to the raw sample. It was concluded that the size of the pores developed by calcination has a critical effect on the rate and ultimate capacity for SO2 sorption. To test the agreement between the experimental conversion values and the values calculated from eqs 7 and 8 for chemical reaction at the interface and diffusion through the product layer, a plot of Xtheoretical versus Xexperimental was constructed, as given in Figure 7, which shows that the experimental and calculated values were in good agreement.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: (90) 442-2314544. Fax: (90) 442-2314562. Email: [email protected].

4. CONCLUSION In this research, the pore-size distribution, surface area, particle volume, and porosity of the raw and calcined samples were determined by a mercury porosimeter and BET analysis. It was determined that phosphate samples calcined at 850 °C for 8 min have the highest surface area, pore volume, and porosity when compared to other calcined samples. The effects of the temperature and CO2 and SO2 concentrations on sulfation of calcined phosphate samples (at 850 °C for 8 min) in a differential fluidized-bed reactor were also studied. The sulfation ratio increased with an increasing temperature and SO2 concentration but decreased with the increase in the CO2 concentration. The results have indicated that higher sulfation

Notes

The authors declare no competing financial interest.

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NOMENCLATURE MCaO = molar weight of CaO MSO4 = molar weight of SO4 Umf = velocity of the fluidizing gas at minimum fluidization (cm/s) Uf = velocity of the fluidizing gas (cm/s) WCaO = mass of CaO converted to CaSO4 WSO4 = mass of SO4 dx.doi.org/10.1021/ef301806x | Energy Fuels 2013, 27, 1466−1472

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(22) Manovic, V.; Charland, J. P.; Blamey, J.; Fennell, P. S.; Lu, D. Y.; Anthony, E. J. Influence of calcination conditions on carrying capacity of CaO-based sorbent in CO2 looping cycles. Fuel 2009, 88, 1893− 1900. (23) Wen, C. Y.; Ishida, M. Reaction rate of sulfur dioxide with particles containing calcium oxide. Environ. Sci. Technol. 1973, 7, 703− 708. (24) Doğu, T. The importance of pore structure and diffusion in the kinetics of gas−solid noncatalytic reactions: Reaction of calcined limestone with SO2. Chem. Eng. J. 1981, 21, 213−222. (25) Hansen, P. F. B.; Dam-Johansen, K.; Ostergaard, K. Hightemperature reaction between sulphur dioxide and limestoneV. The effect of periodically changing oxidizing and reducing conditions. Chem. Eng. Sci. 1993, 48, 1325−1341. (26) Hartman, M.; Coughlin, R. W. Reaction of sulfur dioxide with limestone and the influence of pore structure. Ind. Eng. Chem. Process Des. Dev. 1974, 3, 248−253. (27) Snow, M. J. H.; Longwell, P. J.; Sarofim, A. F. Direct sulfation of calcium carbonate. Ind. Eng. Chem. Res. 1988, 27, 268−273. (28) Tullin, C.; Nyman, G.; Ghardashkhani, S. Direct sulfation of calcium carbonate: The influence of carbon dioxide partial pressure. Energy Fuels 1993, 7, 512−519. (29) Hajaligol, M. R.; Longwell, J. P.; Sarofim, A. F. Analysis and modeling of the direct sulfation of CaCO3. Ind. Eng. Chem. Res. 1988, 27, 2203−2210. (30) Szekely, J.; Evans, J. W.; Sohn, H. Y. Gas−Solid Reactions; Academic Press: New York, 1976. (31) Hu, G.; Dam-Johansen, K.; Wedel, S.; Hansen, J. P. Review of the direct sulfation reaction of limestone. Prog. Energy Combust. Sci. 2006, 32, 386−407. (32) Suyadal, Y.; Oğuz, H. Dry desulphurization of simulated flue gas in a fluidized bed reactor for a broad range of SO2 concentration and temperature: A comparison of models. Ind. Eng. Chem. Res. 1999, 38 (8), 2932−2939. (33) Stouffer, M. R.; Yoon, H. An investigation of CaO sulfation mechanisms in boiler sorbent injection. AIChE 1989, 35 (8), 1253− 1262. (34) Borgwardt, R. H.; Bruce, K. R.; Blake, J. An investigation of product-layer diffusivity for CaO sulfation. Ind. Eng. Chem. Res. 1987, 26, 1993−1998. (35) Arias, B.; Cordero, J. M.; Alonso, M.; Abanades, J. C. Sulfation rates of cycled CaO particles in the carbonator of a Ca-looping cycle for post combustion CO2 capture. AIChE J. 2012, 58 (7), 2262−2269. (36) Dam-Johansen, K.; Hansen, P.; Ostergaard, K. High-temperature reaction between sulphur dioxide and limestoneIII. A grain− micrograin model and its verification. Chem. Eng. Sci. 1991, 46, 847− 853. (37) Manovic, V.; Anthony, E. J.; Loncarevic, D. SO2 retention by CaO-based sorbent spent in CO2 looping cycles. Ind. Eng. Chem. Res. 2009, 48, 6627−6632.

Wt,CaO = mass of total CaO formed from calcination of CaCO3 in phosphate rock

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dx.doi.org/10.1021/ef301806x | Energy Fuels 2013, 27, 1466−1472