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Sulfation reaction between SO2 and limestone: application of deactivation model
Chemical Engineering and Processing 41 (2002) 179– 188 www.elsevier.com/locate/cep
Sulfation reaction between SO2 and limestone: application of deactivation model I: rfan Ar, Suna Balci * Department of Chemical Engineering, Faculty of Engineering and Architecture, Gazi Uni6ersity, 06570, Maltepe, Ankara, Turkey Received 23 November 2000; received in revised form 2 March 2001; accepted 5 March 2001
Abstract Among the several efficient methods for in combustion sulfur dioxide removal methods, sulfation with carbonate based systems are the most widely used one due to its low cost. In this study sulfation reaction kinetics was investigated using limestones of different origin. Limestones pre-calcinated at 900°C in inert atmosphere were subjected to 0.35% by volume SO2. It was observed that sulfation reaction rate was high in the early stages. The conversion for the sulfation of Kinik and Goynuk limestones increased with increasing reaction temperature. For temperatures of 850°C and lower the sulfation reaction was in kinetic control regime. The occurred pore plugging caused a considerable decrease in the rate of sulfation with the extent of the reaction under the all studied conditions. For the sulfation reactions carried out at high temperatures, the model estimating the increase of activation energy with conversion were found to be in agreement with the experimental data. Activation energies of the sulfation reactions for Kinik and Goynuk limestones were estimated around 12 179 and 11 543 cal/mol, respectively. Deactivation model gave good estimates of the conversion especially at high sulfation reaction temperatures. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sulfation; Limestone; Deactivation; Apparent reaction rate constant
1. Introduction Sulfur dioxide released from the stacks as a result of combustion of fossil fuels, forms a very serious danger for the human and environmental health. Depending on the concentration it causes some diseases that may result even death. Therefore besides the investigation of clean, renewable energy sources the studies to develop a low-cost, retrofit technology for control of SO2 emissions are continuing. Although there are large number of methods for removal of SO2, calcium based processes are mostly preferred due to abundance and economical availability of limestone. In these studies various materials such as lime, limestone, dolomite, sodium carbonate, and bicarbonate are used as sorbents for SO2 removal. Among these materials limestone is the most widely used sorbent due to its low cost * Corresponding author. Tel.: +90-312-2317400, ext. 2506; fax: +90-312-2308434. E-mail addresses: [email protected] (I: . Ar), [email protected] (S. Balci).
and easy availability. The reaction between sulfur dioxide and limestone under oxidizing conditions can be described as taking place at least two consecutive reaction steps; calcination and sulfation: CaCO3(s) CaO(s)+CO2(g),
"
(1)
CaO(s)+ SO2(g) CaSO3(s) CaO(s)+ SO2(g) CaSO3 + 1/2O2(g) CaSO4(s) + 1/2O2 CaSO4(s).
(2)
Second reaction that once may have seemed a simple reaction is now recognized as a complex, high temperature, short-time heterogeneous reaction limited by gasphase and solid-state diffusion with simultaneous physical transformations of solid. Pore diffusion and diffusion through the product layer (CaSO4) which are the controlling processes in the sulfation of CaO are highly dependent on the structural properties of solid reactant CaO. Therefore, it is a very important property for a comprehensive sulfation model to incorporate these physical aspects of the heterogeneous reaction to
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the pore plugging of the reactant by solid product CaSO3 and/or CaSO4 [3]. Because the molar volume of CaSO4 is 2.72 times larger that of CaO, CaO grains swell from sulfation. (Often molar volumes of CaSO4 are reported to be 3.09 times that of CaO. This value is correct for temperatures less than 473 K where the low density — 2.61 g/cm3 — CaSO4 structure is stable). The role of the CaSO4 deposits on the sulfation process therefore has been the subject of several investigations. Pore plugging and the loss of porosity at the outer edge of the limestone particles is the main factor for the
accurately simulate the complex process of limestone– SO2 reaction [1]. Pore and product layer diffusion resistances, changes in pore structure and variation in surface area during the reaction are some important factors to be considered in modelling of SO2 – CaO reaction. It was reported by Dogu and Dogu that there were over 30 models for this gas solid reaction in the literature [2]. The main drawback of the use of limestone in a desulfurization process is the low conversion of the solid reactant. These phenomena may be explained by
Table 1 Chemical compositions and physical properties of limestones used in this study Limestone type
CaO (%)
MgO (%)
A12O3+Fe2O3 SiO2 (%) (%)
Ignition loss (%)
Maximum conversion
Porosity
rpore (nm)
Vpore (cm3·g−1)
Kinik Goynuk Cesme Baglum Koselerik Dikmen Alacaatli Kurucasile Ayas
47.9 53.1 51.8 42.7 44.0 52.8 47.8 51.8 51.8
1.5 1.1 1.5 2.5 1.2 2.0 1.2 1.4 1.1
1.0 0.2 0.2 4.1 1.3 0.5 0.6 0.1 0.3
41.2 43.6 43.3 33.9 34.2 43.3 39.5 43.6 42.3
0.67 0.53 0.53 0.35 0.34 0.31 0.25 0.23 0.22
0.49 0.37 0.30 0.42 0.35 0.31 0.33 0.35 0.33
125.30 61.91 45.31 102.53 131.08 23.21 56.73 43.27 37.63
34.97 25.14 19.32 26.58 19.56 20.45 19.29 22.18 21.34
8.4 2.2 3.2 16.8 19.3 1.7 10.9 3.1 4.5
Fig. 1. Variation of conversion of sulfation of different limestones at 900°C.
Fig. 2. Variation of conversion of sulfation of Kinik limestone at different temperatures.
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Fig. 3. Variation of conversion of sulfation of Goynuk limestone at different temperatures.
Fig. 4. Variation of reaction rate constant for sulfation of different limestones at 900°C.
deactivation of large particles in desulfurization processes. Models describing this process are semi-emprical in nature [4,5]. More fundamental mechanistic models have been also proposed previously [6– 9]. These models are very similar in that they treat the diffusion of SO2 through the porous structure, the development of product layer and the ultimate plugging of the porous structure. They also include an intermediate rate-limiting step; the activated diffusion of the SO2 through the product layer [10,11]. In most of the recent studies some additives like fly ash and/or gypsium and hydrated lime [Ca(OH)2] were used in order to increase the conversion of the solid reactant. Authors stated that
CaO derived from Ca(OH)2 was more reactive than that derived from limestone. They explained this difference by the needle like structure and hence low resistance to the diffusion of SO2 within the Ca(OH)2 derived CaO [11–16]. There are a number of factors, which play a critical role in determining the reaction rate and the overall solid conversion. In addition to that limestones from different origin have very widely changing properties. Due to these facts, the development of a model having general applicability is a very hard task to overcome. A more recent study on this subject is the development of a mathematical model by Mahuli et al. [17]. In that study a mathematical model
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Fig. 5. Variation of reaction rate constant for sulfation of Kinik limestone at different temperatures.
Fig. 6. Variation of reaction rate constant for sulfation of Goynuk limestone at different temperatures.
Table 2 Model constants of apparent reaction rate constant for different kinds of limestone Region/name
Kinik
Goynuk
Cesme
Baglum
Koselerik
Dikmen
Alacaaltı
Kurucasile
Ayas
A0, s−1 Ea 0, cal.mol−1 kd, s−1 (kapp/x = 0), s−1
2.7407 4148.73 13.4984 0.46215
2.2139 3053.28 16.4887 0.59667
3.6394 3075.19 15.7810 0.97279
0.8716 4052.48 24.9090 0.15318
0.8530 6795.53 19.0077 0.04621
6.6450 2904.12 33.6259 1.91145
4.8987 6137.10 37.5963 0.35199
7.7098 3142.78 53.4756 2.00167
1.5886 9608.05 41.2121 0.02574
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Table 3 Model constants of apparent reaction rate constant for Kinik limestone Sıcaklık, °C
700
750
800
850
900
A0, s−1 Ea 0, cal.mol−1 kd, s−1 (kapp/x = 0), s−1
1.0717 11 670.67 21.5072 2.5621e-3
1.1447 11 164.20 18.9999 4.7143e-3
1.1718 12 121.99 12.4275 3.9776e-3
2.1154 11 462.03 10.7809 0.01243
2.7407 4148.73 13.4984 0.46218
Table 4 Model constants of apparent reaction rate constant for Goynuk limestone Sıcaklık, °C
700
750
800
850
900
A0, s−1 Ea 0, cal.mol−1 kd, s−1 (kapp/x = 0), s−1
0.7496 10 991.49 35.3096 2.5455e-3
0.8831 11 224.57 40.3540 3.5305e-3
0.9826 12 728.98 13.6447 2.5090e-3
1.2178 12 852.87 11.1084 3.8374e-3
2.2139 3053.28 16.4887 0.59735
Fig. 7. Arrhenius plots of reaction rate constant and deactivation rate constant for sulfation of Kinik limestone.
in which multiple rate processes (calcination, sintering and sulfation reactions) were taken into account, based on the grain– subgrain concept was proposed. For gas– solid reactions, the chemical and physical structure of the solid undergoes considerable changes with the reaction extent resulting in a decrease in solid activity [18]. The deactivation models were commonly used for the reactions in which an appreciable difference between the molar volumes of the solid reactant(s) and product(s) occurs. Lee et al. [7], Dogu [8], and Lee and Georgakis [9] used deactivation models for the SO2 –CaO reaction. Balci et al. [19], proposed several models for the explanation of the change of activity of solid for gas–solid reactions and observed a good agreement of the models with the experimental data for coal gasification. These proposed models considered an exponential decrease of solid reactivity with time. Gul-
dur et al. [20] used the deactivation model of Balci et al. [19] for the trona-SO2 reaction system. They observed a good agreement for the reaction with the deactivation model compared with the commonly used unreacted core model. In the removal of SO2 using limestone, which is a gas–solid reaction, the solid product CaSO4 causes some variation in the physical and the chemical structure of the solid reactant. High molar volume of CaSO4 causes pore plugging in addition to the deactivation of the solid reactant. All these variations lead to change in reaction rate during the SO2 removal process. In this study SO2 –CaO reaction kinetics was investigated by using various limestone samples taken from different regions of Turkey and deactivation model proposed by Balci et al. [19] was applied to predict the behaviour of the limestone samples during sulfation reactions. This
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Fig. 8. Arrhenius plots of reaction rate constant and deactivation rate constant for sulfation of Goynuk limestone.
model which is the modified form of the deactivation model developed for the SO2 – CaO system by Dogu [8], was preferred because of its advantage that a more general variable, conversion was used instead of sulfation time. 2. Experimental work In this study SO2 – CaO reaction system is investigated using limestones from different regions of Turkey. Chemical analysis of the limestones used in this study were performed using classical gravimetric and volumetric methods [21]. Pore size analysis (including the pores having the diameters greater than 2 nm) was done using Quantochromo Autoscan 60 mercury porosimeter. Results of the chemical analysis and some physical properties of the limestones are given in Table 1. Sulfation experiments were performed by using Linseis L81 Thermogravimetric Analyser (TGA). The experiments were carried out using 50 mg oven dried samples having the average particle diameter of around 1.02 mm. In sulfation experiments limestone samples were first heated to 900°C in 12 l/h flow of inert atmosphere at a rate of 20 K/min, after reaching this temperature 10 more min waited in order to ensure temperature stabilization and completion of the calcination. Then SO2 was given to the system so that its concentration was 0.35% by volume, and the calcined samples were subjected to the sulfation for an hour at that temperature. The sulfation conversion values calculated from the thermogravimetric data are presented in Fig. 1. Maximum conversion values are also tabulated in Table 1. In order to see the effect of the reaction temperature on the reaction rate and to obtain the kinetic parame-
ters, sulfation experiments repeated under the identical conditions except reaction temperatures for the Kinik and Goynuk limestones. In these group of experiments the calcination temperature was again kept as constant at 900°C for all reactions. Sulfation reaction temperatures were varied between 700 and 900°C. Results of these experiments carried out at different temperatures are given in Figs. 2 and 3 for the sulfation of Kinik and Goynuk limestones, respectively.
3. Results and discussion
3.1. Reaction mechanism of the sulfation reaction In the present study, sulfation reactions between SO2 and limestones calcined at 900°C were investigated. The active material in limestone for the sulfation reaction is CaO, the solid product of the calcination reaction of the limestone. Apart from that, knowledge on the chemical composition of the limestone is necessary for the conversion calculations. The used limestones had CaO content over 42% by weight and MgO contents were between 1.1 and 2.5% by weight. All the sulfation reactions were carried out above 700°C. Although MgO which arises as a result of thermal decomposition of the Table 5 Arrhenius results for sulfation reaction and deactivation rate constant Sulfation reaction
Deactivation of solid
Sample
A0, s−1
Ea 0, cal mol−1
Ad, s−1
−Ed, cal mol−1
Kinik Goynuk
2.1099 0.7400
12 179.75 11 543.19
0.10799 0.01899
10 298.26 14 795.50
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MgCO3 content of the limestone is much more reactive to SO2 than CaO, due to the decomposition of the MgSO4 at temperatures higher than 650°C, it can be assumed that the all weight gain during the sulfation is due to the conversion of CaO only. Therefore conversion values of limestones were calculated by using weight vs time data obtained from TGA experiments performed by Ar [21]. On the basis of the assumption that all of the CaO content of the limestone to be convertable to CaSO3 or CaSO4, conversion of CaO content of the limestones to product in sulfation reaction was defined as follows: x=
(Wt −W0)/MWSO3 . (wCaO W0)/MWCaO
(3)
Here W0 is the weight of the oven dried sample before calcination, Wt is the weight of sample at any sulfation time (conversion) and wCaO is the weight fraction of CaO in limestone. Results of sulfation experiments (carried out at 900°C) of different limestones are given as the variation of conversion with time in Fig. 1. The sulfation reaction results reflect the fact that the general behaviour of the limestones in sulfation experiments was the same. The reaction rate of all limestone samples was high in the early stages of the sulfation, and then it started to decrease slowly showing that the reactions had reached to the completion. For the sulfation times of above 200–250 s a more rapid decrease in the reaction rate occurred. A close examination of conversion vs time curves for different limestones together with chemical analysis results presented in Table 1, reveals the fact that although chemical compositions of the limestones are close to each other, there are significant differences between their final conversions. Especially the limestones from Ayas, Kurucasile and Dikmen showed low conversion values compared with ones obtained from the limestones having nearly the same CaO content. Because these limestones had low pore volumes and average pore diameters so the pore plugging and the mass transfer limitations might become significant with the extent of the sulfation. Kinik limestone which achieved the highest conversion value (0.67 in 3000 s) reached a stable form nearly in 1000 s. Goynuk and Cesme limestones showed a similar behaviour and they reached a conversion of 0.53 in 500 s. On the other hand Baglum limestone, although its initial rate was slower than Koselerik and Dikmen (their maximum conversions were 0.34 and 0.31, respectively) reached a higher conversion (0.35) than the others. Alacaatli, Kurucasile and Ayas limestone samples did not show appreciable conversion values and the values in 1000 s were 0.24, 0.21 and 0.17, respectively. Kinik and Goynuk limestones had porosity values as 0.49 and 0.37 and pore volumes of around 34.97 and 25.14 cm3/g, re-
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spectively. So during the sulfation pore plugging might be small and mass transfer limitations could be low, so they yielded high conversion values. Cesme limestone also had high maximum conversion. These observed high conversion for Kinik, Goynuk and Cesme limestones could also be the result of fracture of the particles of these samples during the formation of large volume solid product and hence exposure of the new active surfaces to the gaseous reactant. The decreasing of particle size due to the fracture could also decrease the limitations to the gaseous diffusion. In the further kinetic investigations Kinik and Goynuk limestone should be preferred due to their highest conversion in shortest reaction time. Conversion values of Kinik limestone calculated from the experimental data at different reaction temperatures were plotted as a function of reaction time (Fig. 2). It was observed that the reaction rate at 900°C decreased much after 1000 s. Reaction rate increased with the increasing reaction temperature and also maximum conversion values achieved at high reaction temperatures were higher than those of the low reaction temperatures. For example, the maximum conversion value achieved at 2000 s was around 0.20 at 850°C while the conversion value of 0.64 was achieved much shorter time, around 1000 s at 900°C. The time necessary to achieve the maximum conversion, decreased with increasing sulfation reaction temperature. The maximum conversion values achieved at reaction temperatures of 700, 800, and 900°C were 0.27 (1500 s), 0.36 (1340 s), 0.64 (1140 s), respectively. An appreciable increase in the reaction rate between 850 and 900°C was occurred. In a similar manner, conversion-time results for Goynuk limestone that was obtained from TGA experiments are presented in Fig. 3. Lower conversion values were obtained compared with those of Kinik limestone. As in the case of the Kinik limestone, increase in reaction rate with the increasing reaction temperature was observed. The maximum conversion values achieved at high reaction temperatures were higher than the corresponding values at low reaction temperatures. For example, the conversion value of 0.52 was achieved in 700 s at the reaction temperature of 900°C. The maximum conversion values achieved for Kinik limestone are higher than the corresponding values for the Goynuk limestone at all reaction temperatures. It was also observed that time required to reach maximum conversion values are shorter for the Kinik limestone than for the Goynuk limestone. Although conversion value of Kinik limestone is 21.8% higher than the Goynuk limestone, conversion time is 53.33% shorter relative to the Goynuk limestone. This is very important for the industrial application point of view.
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3.2. Experimental 6erification of the deacti6ation model When there is an appreciable difference between the molar volumes of the solid reactant(s) and product(s), physical and pore structure of the solid undergo some variations. These variations lead to variations in the activity of the solid reactant. Several models which, define the change of activity as a function of conversion are given in the literature. The selected model must explain the reaction mechanism and it has few adjustable model parameter for the ease of application to experimental data. Pore plugging is inevitable due to the formation of denser solid product. Literature studies also confirmed that pore structure of the solid reactant changed significantly during the sulfation reaction [8,9,11]. Among the several structural models, in the explanation of such reactions during which the formation of highly denser solid product is occurring, the most commonly used one is the unreacted core model. As mentioned before, the diffusional resistances are very important in the sulfation reactions. The formation of the exhausted and denser product layer near the outer surface is possible for bigger particle sizes causing diffusional resistances. However, the used limestones had macro-meso porosities over 0.30 and it was obvious that these values increased upon calcination. From kinetic point of view, the presence of large size pore volumes helps the transportation of the gaseous reactant to the reaction sites. So, it was thought that the sulfation reactions and as a result pore plugging take place at every point of the solid. From the conversion vs time data it was concluded that the sulfation reaction mechanism just differed at 900°C reaction temperature. For the sulfation temperature of 850°C and lower the reactions were in the kinetic control region (Figs. 1–3). In this regime, the conversion values were small so diffusional limitations caused by pore plugging were also small. So the sulfation reactions could take place at every point of the particle although the rate was higher near the outer surface. Unreacted core model might not explain the reaction mechanism well in kinetic control regime. In such a case volume reaction model could fit the experimental data well. In the study of Guldur et al. [20] it was also seen that the unreacted core model weakly explained the reaction between trona–SO2. As seen in Figs. 1– 3, conversions (weight gains) were very high in the early stages of the sulfation, and then they showed a decreasing trend generally after 200 s conversion. This might be caused by the deactivation of the solid reactant. So for the explanation of the sulfation mechanism the deactivation model of Balci et al. [19] was used. The rate expression based on first-order decomposition of the solid reactant is written in terms of fractional conversion as,
dx = kapp(1−x). dt
(4)
The deactivation of apparent reaction rate constant from the Arrhenius constant is due to the decrease of reactivity (activity) of the solid. The apparent reaction rate constant was given in the following form [19]. kapp = akArr = aA0 e − Ea 0/RT,
(5)
where ‘a’ is the activity of the solid which is only a function of solid reactant properties under uniform gas reactant concentration. They related the rate of change of activity of solid with respect to conversion as a function of deactivation rate constant, kd, and activity. −
da =kda n. dx
(6)
For the sake of simplicity of the model (few adjustable parameters), the rate of change of activity is expressed to be proportional to activity itself (n= 1). Activity of solid reactant is unity at zero conversion, and it decreases with conversion and it approaches zero when conversion approaches its maximum value. Including these limitations, variation of apparent reaction rate constant with conversion is expressed as follows:
kapp = akArr = A0 exp −
n
Ea 0 RT 1+ kd x RT Ea 0
.
(7)
The implication of this model is an increase of activation energy with conversion. The deactivation rate constant ‘kd’ might also be temperature dependent. The calculated apparent reactipn rate constant values for high temperature sulfation reactions of different origin and the ones for the sulfation of Kinik limestone and Goynuk limestones at different temperatures are plotted in Figs. 4–6, respectively. Apparent reaction rate constant for high temperature sulfation showed nearly linear relation with conversion in semi-log plots. At lower temperatures, two different linear segments were observed. In the first segment in which apparent reaction rate constant decreased very rapidly, conversion of the solid reactant was very fast. As conversion increased, the plugging of pores due to the formation of CaSO4 caused considerable mass transfer limitations in the particles. As a result of this increased mass transfer limitations and the decrease in the composition of the solid reactant CaO, reaction mechanism differed so positive deviations were observed. The validity of the model on the change of activity of solid and apparent reaction rate constant has been investigated using non linear regression analysis. The predicted model parameters are given in Tables 2–4 for the sulfation reaction of different limestones at 900°C and the sulfation reactions of Kinik and Goynuk limestones at different temperatures respectively. The results of the modelling study are also reported together with the experimental values in Figs. 4–6 for several kinds
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of limestones, Kinik and Goynuk limestones, respectively. For the sulfation of different limestones at 900°C, the activation energy values ranged from 2904.12 cal/mol (for Dikmen limestone) and 9608.05 cal/mol (for Ayas limestone). It was observed that, as expected, the deactivation rate constant values showed an increasing trend in the order of the limestone kinds having the decreasing maximum conversion values (Fig. 4 and Table 2). When model equation applied to the experimental results obtained at different sulfation reaction temperatures, especially at temperature of 850°C and lower, positive deviations were observed for Kinik and Goynuk limestones (Figs. 5 and 6). As previously discussed, with increasing conversion the occurred pore plugging caused the change of reaction mechanism. Experimental data showed nearly two different linear segments in semi-plot analysis. Therefore, in the first zone in which sulfation rate was very rapid, high conversion values achieved in a shorter time with the increasing temperature. The plots of kapp-conversion obtained from the experimental values and values predicted by model were agree very well especially in the low conversion values region for low sulfation temperatures. After a certain conversion values, apparent reaction rate constants predicted by the model were not confirm with the experimental results. For example, confirmation between the values predicted by the model and the experimental ones were very poor for the sulfation at 700°C in the higher conversion region. Confirmity between the model values and the experimental ones became better and better with the increasing of temperature to 750 and 800°C, and finally at 850°C model predicted values and experimental ones confirmed exactly (Tables 3 and 4; Figs. 5 and 6). As the sulfation reaction temperature increased from 700 to 850°C, a decrease in deactivation rate constant values were observed. Nearly the same activation energy values were predicted in that temperature range. Since sulfation reaction mechanism just changed to mass transfer control regime at 900°C, an appreciable decrease in the activation energies were observed. The predicted values were less than the one third of the ones obtained sulfation temperatures of 850°C and lower for both Kinik and Goynuk limestones. Best fit between the model and experimental values were obtained for the reaction at 850°C. It was said that, the agreement between the model prediction and experimental values improved by sulfation temperature increase. For the sulfation temperature of 900°C, reaction mechanism differed. Due to the high rate of sulfation in the early stages, the pore plugging and as a result mass transfer limitations might become dominant and these had much more effects on the deactivation rate constant than the effects of the decreasing of the solid reactant with conversion.
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The values of reaction rate constant at zero conversion predicted from the model were reported in Tables 2–4 for each sample. Activity of solid reactant is unity at zero conversion, so initial value of the apparent reaction rate constant must equal to the Arrhenius reaction rate constant. Arrhenius plots of sulfation reaction of Kinik and Goynuk limestones at different temperatures are given in Figs. 7 and 8, respectively. Considering the data points between 850 and 700°C, activation energy values were estimated around 12 179.75 and 11 543.19 cal/mol for the sulfation of Kinik and Goynuk limestones, respectively (Table 5). The values matched with the values found from regression analysis of the apparent reaction rate constant vs conversion data (Tables 3 and 4). The deactivation rate constant is temperature dependent term. The temperature dependencies are also given in Figs. 7 and 8, respectively. It was seen that for temperatures 850°C and below, the dependencies were in Arrhenius form, yielding the negative deactivation energies around 10 298.26 and 14 795.50 cal/mol for Kinik and Goynuk limestones sulfation reactions, respectively (Table 5). Conversion values estimated (using numerical integration) from the model are also presented in Figs. 1–3 with solid and several shapes of the lines for different kinds of limestones, Kinik and Goynuk limestones respectively together with the experimental values. Good agreement of model was observed for the limestones, which gave high sulfation conversion. For the ones showing lower sulfation conversions, small deviations with the model predictions and the experimental data were observed (Fig. 1). For the sulfation of Kinik and Goynuk limestones the model were in agreement with the data especially at higher reaction temperatures. At lower temperatures deviations were observed with increasing sulfation time. In apparent reaction rate constant plots the starting point of the second regime (diffusion controlling region) were also occurred around the same conversion levels. Approximately the same behaviour was also seen for other samples. 4. Conlusions Sulfation reaction rate was very high in the early stages, after 200 s it showed a very rapid decrease. For temperatures of 850°C and below, reaction took place in two proceeding mechanism due to the pore plugging and the changes in the chemical composition of the solid. It was observed that, the agreement between the model prediction and experimental values improved by temperature increase. The SO2 capture capacities of different limestones at 900°C were investigated together by taking into account this temperature effect. Activation energies of the sulfation reactions carried with Kinik and Goynuk limestones were found to be 12 179.75 and 11 543.19 cal/mol, respectively.
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Appendix A Symbols a activity of solid Ad pre-exponential factor of the deactivation rate constant (s−1) A0 pre-exponential factor for the sulfation reaction (s−1) Ea0 initial activation energy (cal.mol−1) Ed deactivation energy (cal.mol−1) kapp apparent reaction rate constant (s−1) kArr Arrhenius type reaction rate constant (s−1) deactivation rate constant of solid (s−1) kd MW molecular weight (g/g mol) R universal gas constant rpore average pore diameter (nm) T sulfation temperature (K) t sulfation time (s) Vpore pore volume (for pores having diameter greater than 2 nm) (cm3.g−1) W0 initial weight of sample before the calcination (g) Wt weight of sample at any sulfation time (g) wCaO CaO weight fraction of limestone samples, g CaO/g limestone x conversion of CaO–CaSO4
References [1] C.R. Milne, G.D. Silcox, D.W. Pershing, D.A. Kirchgessner, High temperature, short time sulfation of calcium-based sorbents 1. theoretical sulfation model, Ind. Eng. Chem. Res. 29 (1990) 2192 – 2201. [2] G. Dogu, T. Dogu, Kinetics of Capture of Sulfur Dioxide and Applications to Flue Gas Desulfurization, NATO ASI on Chemical Reactor Technology For Environmentally Safe Reactors and Products, Kluwer Academic Publisher, 1992, pp. 467 –498. [3] E. Efthimiadis, S.V. Sotirchos, A partially overlapping grain model for gas – solid reactions, Chem. Eng. Sci. 48 (1993) 1201 – 1212.
[4] S.K. Bhatia, D.D. Perlmutter, A random pore model for fluid– solid reactions-II: diffusion and transport effects, AIChE J. 27 (1981) 247 – 258. [5] S.V. Krishnan, S.V. Sotirchos, A variable diffusivity shrinking core model and its application to the direct sulfation of limestone, Can. J. Chem. Eng. 71 (1993) 734 – 745. [6] M. Hartman, R.W. Cuoghling, Reaction of sulfur dioxide with limestone and the influence on pore structure, Ind. Eng. Chem. Process Des. Dev. 13 (1974) 248. [7] D.C. Lee, J.L. Hodges, C. Georgakis, Modelling SO2 emissions from fluidized bed coal combustors, Chem. Eng. Sci. 35 (1980) 302 – 306. [8] T. Dogu, Importance of pore structure and diffusion in the kinetics of gas – solid noncatalytic reactions: reaction of calcined limestone with SO2, Chem. Eng. J. 21 (1981) 213 – 222. [9] D.C. Lee, C. Georgakis, A single-particle-size model for sulfur retention in fluidized bed combustors, AIChE J. 27 (1981) 472 – 481. [10] P.G. Christman, T.F. Edgar, Distributed pore-size model for sulfation of limestone, AIChE J. 29 (1983) 388 – 396. [11] T. Bardakc¸ ı, Diffusional study of the reaction of sulfur dioxide with reactive porous matrices, Thermochim. Acta 76 (1984) 287 – 300. [12] N. Karatepe, E. Meric¸ boyu, S. Ku¨ c¸ u¨ kbayrak, Preparation of fly ash Ca(OH)2 sorbents by pressure hydration for SO2 removal, Energy Sources 20 (1998) 945 – 953. [13] M.J. Renedo, J. Fernandez, A. Garea, A. Ayerbe, J.A. Irabien, Microstructural changes in the desulfurization reaction at low temperature, Ind. Eng. Chem. Res. 38 (1999) 1384 – 1390. [14] Y. Li, M. Nishioka, M. Sadakata, High calcium utilization and gypsum formation for dry desulfurization process, Energy and Fuels 13 (1999) 1015 – 1020. [15] T. Ishizuka, H. Tsuchai, T. Murayama, H. Hattori, Preparation of active absorbent for dry-type flue gas desulfurization from calcium oxide, fly ash, and gypsum, Ind. Eng. Chem. Res. 39 (2000) 1390 – 1396. [16] S. Sasaoka, N. Sada, M.A. Uddin, Preparation of macroporous lime from natural lime by swelling method with acetic acid for high temperature desulfurization, Ind. Eng. Chem. Res. 37 (1998) 3943 – 3949. [17] S.K. Mahuli, R. Agnihotri, R. Jadhav, S. Chank, L.S. Fan, Combined calcination, sintering and sulfation model for CaCO3 – SO2 reaction, AIChE J. 45 (1999) 367 – 382. [18] P.A. Ramachandran, J.M. Smith, A single pore model for gas – solid noncatalytic reactions, AIChE J. 23 (1977) 353 –360. [19] S. Balci, T. Dogu, H. Yu¨ ce1, Pyrolysis kinetics of lignocellulosic materials, Ind. Eng. Chem. Res. 32 (1993) 2573 – 2579. [20] C. Guldur, G. Dogu, T. Dogu, Kinetics of trona sulfur dioxide reaction, Chem. Eng. Proc. 40 (2001) 13 – 18. [21] I: . Ar, Kirec¸ tas¸ı-SO2 reaksiyonunun kinetig˘ i c¸ alıs¸maları ve baca gazındaki SO2 ‘in tutulması amacıyla sobalara uygulanması’, PhD Thesis, Gazi University, Ankara, (1992).
Sulfation reaction between SO2 and limestone: application of deactivation model I: rfan Ar, Suna Balci * Department of Chemical Engineering, Faculty of Engineering and Architecture, Gazi Uni6ersity, 06570, Maltepe, Ankara, Turkey Received 23 November 2000; received in revised form 2 March 2001; accepted 5 March 2001
Abstract Among the several efficient methods for in combustion sulfur dioxide removal methods, sulfation with carbonate based systems are the most widely used one due to its low cost. In this study sulfation reaction kinetics was investigated using limestones of different origin. Limestones pre-calcinated at 900°C in inert atmosphere were subjected to 0.35% by volume SO2. It was observed that sulfation reaction rate was high in the early stages. The conversion for the sulfation of Kinik and Goynuk limestones increased with increasing reaction temperature. For temperatures of 850°C and lower the sulfation reaction was in kinetic control regime. The occurred pore plugging caused a considerable decrease in the rate of sulfation with the extent of the reaction under the all studied conditions. For the sulfation reactions carried out at high temperatures, the model estimating the increase of activation energy with conversion were found to be in agreement with the experimental data. Activation energies of the sulfation reactions for Kinik and Goynuk limestones were estimated around 12 179 and 11 543 cal/mol, respectively. Deactivation model gave good estimates of the conversion especially at high sulfation reaction temperatures. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sulfation; Limestone; Deactivation; Apparent reaction rate constant
1. Introduction Sulfur dioxide released from the stacks as a result of combustion of fossil fuels, forms a very serious danger for the human and environmental health. Depending on the concentration it causes some diseases that may result even death. Therefore besides the investigation of clean, renewable energy sources the studies to develop a low-cost, retrofit technology for control of SO2 emissions are continuing. Although there are large number of methods for removal of SO2, calcium based processes are mostly preferred due to abundance and economical availability of limestone. In these studies various materials such as lime, limestone, dolomite, sodium carbonate, and bicarbonate are used as sorbents for SO2 removal. Among these materials limestone is the most widely used sorbent due to its low cost * Corresponding author. Tel.: +90-312-2317400, ext. 2506; fax: +90-312-2308434. E-mail addresses: [email protected] (I: . Ar), [email protected] (S. Balci).
and easy availability. The reaction between sulfur dioxide and limestone under oxidizing conditions can be described as taking place at least two consecutive reaction steps; calcination and sulfation: CaCO3(s) CaO(s)+CO2(g),
"
(1)
CaO(s)+ SO2(g) CaSO3(s) CaO(s)+ SO2(g) CaSO3 + 1/2O2(g) CaSO4(s) + 1/2O2 CaSO4(s).
(2)
Second reaction that once may have seemed a simple reaction is now recognized as a complex, high temperature, short-time heterogeneous reaction limited by gasphase and solid-state diffusion with simultaneous physical transformations of solid. Pore diffusion and diffusion through the product layer (CaSO4) which are the controlling processes in the sulfation of CaO are highly dependent on the structural properties of solid reactant CaO. Therefore, it is a very important property for a comprehensive sulfation model to incorporate these physical aspects of the heterogeneous reaction to
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the pore plugging of the reactant by solid product CaSO3 and/or CaSO4 [3]. Because the molar volume of CaSO4 is 2.72 times larger that of CaO, CaO grains swell from sulfation. (Often molar volumes of CaSO4 are reported to be 3.09 times that of CaO. This value is correct for temperatures less than 473 K where the low density — 2.61 g/cm3 — CaSO4 structure is stable). The role of the CaSO4 deposits on the sulfation process therefore has been the subject of several investigations. Pore plugging and the loss of porosity at the outer edge of the limestone particles is the main factor for the
accurately simulate the complex process of limestone– SO2 reaction [1]. Pore and product layer diffusion resistances, changes in pore structure and variation in surface area during the reaction are some important factors to be considered in modelling of SO2 – CaO reaction. It was reported by Dogu and Dogu that there were over 30 models for this gas solid reaction in the literature [2]. The main drawback of the use of limestone in a desulfurization process is the low conversion of the solid reactant. These phenomena may be explained by
Table 1 Chemical compositions and physical properties of limestones used in this study Limestone type
CaO (%)
MgO (%)
A12O3+Fe2O3 SiO2 (%) (%)
Ignition loss (%)
Maximum conversion
Porosity
rpore (nm)
Vpore (cm3·g−1)
Kinik Goynuk Cesme Baglum Koselerik Dikmen Alacaatli Kurucasile Ayas
47.9 53.1 51.8 42.7 44.0 52.8 47.8 51.8 51.8
1.5 1.1 1.5 2.5 1.2 2.0 1.2 1.4 1.1
1.0 0.2 0.2 4.1 1.3 0.5 0.6 0.1 0.3
41.2 43.6 43.3 33.9 34.2 43.3 39.5 43.6 42.3
0.67 0.53 0.53 0.35 0.34 0.31 0.25 0.23 0.22
0.49 0.37 0.30 0.42 0.35 0.31 0.33 0.35 0.33
125.30 61.91 45.31 102.53 131.08 23.21 56.73 43.27 37.63
34.97 25.14 19.32 26.58 19.56 20.45 19.29 22.18 21.34
8.4 2.2 3.2 16.8 19.3 1.7 10.9 3.1 4.5
Fig. 1. Variation of conversion of sulfation of different limestones at 900°C.
Fig. 2. Variation of conversion of sulfation of Kinik limestone at different temperatures.
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Fig. 3. Variation of conversion of sulfation of Goynuk limestone at different temperatures.
Fig. 4. Variation of reaction rate constant for sulfation of different limestones at 900°C.
deactivation of large particles in desulfurization processes. Models describing this process are semi-emprical in nature [4,5]. More fundamental mechanistic models have been also proposed previously [6– 9]. These models are very similar in that they treat the diffusion of SO2 through the porous structure, the development of product layer and the ultimate plugging of the porous structure. They also include an intermediate rate-limiting step; the activated diffusion of the SO2 through the product layer [10,11]. In most of the recent studies some additives like fly ash and/or gypsium and hydrated lime [Ca(OH)2] were used in order to increase the conversion of the solid reactant. Authors stated that
CaO derived from Ca(OH)2 was more reactive than that derived from limestone. They explained this difference by the needle like structure and hence low resistance to the diffusion of SO2 within the Ca(OH)2 derived CaO [11–16]. There are a number of factors, which play a critical role in determining the reaction rate and the overall solid conversion. In addition to that limestones from different origin have very widely changing properties. Due to these facts, the development of a model having general applicability is a very hard task to overcome. A more recent study on this subject is the development of a mathematical model by Mahuli et al. [17]. In that study a mathematical model
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Fig. 5. Variation of reaction rate constant for sulfation of Kinik limestone at different temperatures.
Fig. 6. Variation of reaction rate constant for sulfation of Goynuk limestone at different temperatures.
Table 2 Model constants of apparent reaction rate constant for different kinds of limestone Region/name
Kinik
Goynuk
Cesme
Baglum
Koselerik
Dikmen
Alacaaltı
Kurucasile
Ayas
A0, s−1 Ea 0, cal.mol−1 kd, s−1 (kapp/x = 0), s−1
2.7407 4148.73 13.4984 0.46215
2.2139 3053.28 16.4887 0.59667
3.6394 3075.19 15.7810 0.97279
0.8716 4052.48 24.9090 0.15318
0.8530 6795.53 19.0077 0.04621
6.6450 2904.12 33.6259 1.91145
4.8987 6137.10 37.5963 0.35199
7.7098 3142.78 53.4756 2.00167
1.5886 9608.05 41.2121 0.02574
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Table 3 Model constants of apparent reaction rate constant for Kinik limestone Sıcaklık, °C
700
750
800
850
900
A0, s−1 Ea 0, cal.mol−1 kd, s−1 (kapp/x = 0), s−1
1.0717 11 670.67 21.5072 2.5621e-3
1.1447 11 164.20 18.9999 4.7143e-3
1.1718 12 121.99 12.4275 3.9776e-3
2.1154 11 462.03 10.7809 0.01243
2.7407 4148.73 13.4984 0.46218
Table 4 Model constants of apparent reaction rate constant for Goynuk limestone Sıcaklık, °C
700
750
800
850
900
A0, s−1 Ea 0, cal.mol−1 kd, s−1 (kapp/x = 0), s−1
0.7496 10 991.49 35.3096 2.5455e-3
0.8831 11 224.57 40.3540 3.5305e-3
0.9826 12 728.98 13.6447 2.5090e-3
1.2178 12 852.87 11.1084 3.8374e-3
2.2139 3053.28 16.4887 0.59735
Fig. 7. Arrhenius plots of reaction rate constant and deactivation rate constant for sulfation of Kinik limestone.
in which multiple rate processes (calcination, sintering and sulfation reactions) were taken into account, based on the grain– subgrain concept was proposed. For gas– solid reactions, the chemical and physical structure of the solid undergoes considerable changes with the reaction extent resulting in a decrease in solid activity [18]. The deactivation models were commonly used for the reactions in which an appreciable difference between the molar volumes of the solid reactant(s) and product(s) occurs. Lee et al. [7], Dogu [8], and Lee and Georgakis [9] used deactivation models for the SO2 –CaO reaction. Balci et al. [19], proposed several models for the explanation of the change of activity of solid for gas–solid reactions and observed a good agreement of the models with the experimental data for coal gasification. These proposed models considered an exponential decrease of solid reactivity with time. Gul-
dur et al. [20] used the deactivation model of Balci et al. [19] for the trona-SO2 reaction system. They observed a good agreement for the reaction with the deactivation model compared with the commonly used unreacted core model. In the removal of SO2 using limestone, which is a gas–solid reaction, the solid product CaSO4 causes some variation in the physical and the chemical structure of the solid reactant. High molar volume of CaSO4 causes pore plugging in addition to the deactivation of the solid reactant. All these variations lead to change in reaction rate during the SO2 removal process. In this study SO2 –CaO reaction kinetics was investigated by using various limestone samples taken from different regions of Turkey and deactivation model proposed by Balci et al. [19] was applied to predict the behaviour of the limestone samples during sulfation reactions. This
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Fig. 8. Arrhenius plots of reaction rate constant and deactivation rate constant for sulfation of Goynuk limestone.
model which is the modified form of the deactivation model developed for the SO2 – CaO system by Dogu [8], was preferred because of its advantage that a more general variable, conversion was used instead of sulfation time. 2. Experimental work In this study SO2 – CaO reaction system is investigated using limestones from different regions of Turkey. Chemical analysis of the limestones used in this study were performed using classical gravimetric and volumetric methods [21]. Pore size analysis (including the pores having the diameters greater than 2 nm) was done using Quantochromo Autoscan 60 mercury porosimeter. Results of the chemical analysis and some physical properties of the limestones are given in Table 1. Sulfation experiments were performed by using Linseis L81 Thermogravimetric Analyser (TGA). The experiments were carried out using 50 mg oven dried samples having the average particle diameter of around 1.02 mm. In sulfation experiments limestone samples were first heated to 900°C in 12 l/h flow of inert atmosphere at a rate of 20 K/min, after reaching this temperature 10 more min waited in order to ensure temperature stabilization and completion of the calcination. Then SO2 was given to the system so that its concentration was 0.35% by volume, and the calcined samples were subjected to the sulfation for an hour at that temperature. The sulfation conversion values calculated from the thermogravimetric data are presented in Fig. 1. Maximum conversion values are also tabulated in Table 1. In order to see the effect of the reaction temperature on the reaction rate and to obtain the kinetic parame-
ters, sulfation experiments repeated under the identical conditions except reaction temperatures for the Kinik and Goynuk limestones. In these group of experiments the calcination temperature was again kept as constant at 900°C for all reactions. Sulfation reaction temperatures were varied between 700 and 900°C. Results of these experiments carried out at different temperatures are given in Figs. 2 and 3 for the sulfation of Kinik and Goynuk limestones, respectively.
3. Results and discussion
3.1. Reaction mechanism of the sulfation reaction In the present study, sulfation reactions between SO2 and limestones calcined at 900°C were investigated. The active material in limestone for the sulfation reaction is CaO, the solid product of the calcination reaction of the limestone. Apart from that, knowledge on the chemical composition of the limestone is necessary for the conversion calculations. The used limestones had CaO content over 42% by weight and MgO contents were between 1.1 and 2.5% by weight. All the sulfation reactions were carried out above 700°C. Although MgO which arises as a result of thermal decomposition of the Table 5 Arrhenius results for sulfation reaction and deactivation rate constant Sulfation reaction
Deactivation of solid
Sample
A0, s−1
Ea 0, cal mol−1
Ad, s−1
−Ed, cal mol−1
Kinik Goynuk
2.1099 0.7400
12 179.75 11 543.19
0.10799 0.01899
10 298.26 14 795.50
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MgCO3 content of the limestone is much more reactive to SO2 than CaO, due to the decomposition of the MgSO4 at temperatures higher than 650°C, it can be assumed that the all weight gain during the sulfation is due to the conversion of CaO only. Therefore conversion values of limestones were calculated by using weight vs time data obtained from TGA experiments performed by Ar [21]. On the basis of the assumption that all of the CaO content of the limestone to be convertable to CaSO3 or CaSO4, conversion of CaO content of the limestones to product in sulfation reaction was defined as follows: x=
(Wt −W0)/MWSO3 . (wCaO W0)/MWCaO
(3)
Here W0 is the weight of the oven dried sample before calcination, Wt is the weight of sample at any sulfation time (conversion) and wCaO is the weight fraction of CaO in limestone. Results of sulfation experiments (carried out at 900°C) of different limestones are given as the variation of conversion with time in Fig. 1. The sulfation reaction results reflect the fact that the general behaviour of the limestones in sulfation experiments was the same. The reaction rate of all limestone samples was high in the early stages of the sulfation, and then it started to decrease slowly showing that the reactions had reached to the completion. For the sulfation times of above 200–250 s a more rapid decrease in the reaction rate occurred. A close examination of conversion vs time curves for different limestones together with chemical analysis results presented in Table 1, reveals the fact that although chemical compositions of the limestones are close to each other, there are significant differences between their final conversions. Especially the limestones from Ayas, Kurucasile and Dikmen showed low conversion values compared with ones obtained from the limestones having nearly the same CaO content. Because these limestones had low pore volumes and average pore diameters so the pore plugging and the mass transfer limitations might become significant with the extent of the sulfation. Kinik limestone which achieved the highest conversion value (0.67 in 3000 s) reached a stable form nearly in 1000 s. Goynuk and Cesme limestones showed a similar behaviour and they reached a conversion of 0.53 in 500 s. On the other hand Baglum limestone, although its initial rate was slower than Koselerik and Dikmen (their maximum conversions were 0.34 and 0.31, respectively) reached a higher conversion (0.35) than the others. Alacaatli, Kurucasile and Ayas limestone samples did not show appreciable conversion values and the values in 1000 s were 0.24, 0.21 and 0.17, respectively. Kinik and Goynuk limestones had porosity values as 0.49 and 0.37 and pore volumes of around 34.97 and 25.14 cm3/g, re-
185
spectively. So during the sulfation pore plugging might be small and mass transfer limitations could be low, so they yielded high conversion values. Cesme limestone also had high maximum conversion. These observed high conversion for Kinik, Goynuk and Cesme limestones could also be the result of fracture of the particles of these samples during the formation of large volume solid product and hence exposure of the new active surfaces to the gaseous reactant. The decreasing of particle size due to the fracture could also decrease the limitations to the gaseous diffusion. In the further kinetic investigations Kinik and Goynuk limestone should be preferred due to their highest conversion in shortest reaction time. Conversion values of Kinik limestone calculated from the experimental data at different reaction temperatures were plotted as a function of reaction time (Fig. 2). It was observed that the reaction rate at 900°C decreased much after 1000 s. Reaction rate increased with the increasing reaction temperature and also maximum conversion values achieved at high reaction temperatures were higher than those of the low reaction temperatures. For example, the maximum conversion value achieved at 2000 s was around 0.20 at 850°C while the conversion value of 0.64 was achieved much shorter time, around 1000 s at 900°C. The time necessary to achieve the maximum conversion, decreased with increasing sulfation reaction temperature. The maximum conversion values achieved at reaction temperatures of 700, 800, and 900°C were 0.27 (1500 s), 0.36 (1340 s), 0.64 (1140 s), respectively. An appreciable increase in the reaction rate between 850 and 900°C was occurred. In a similar manner, conversion-time results for Goynuk limestone that was obtained from TGA experiments are presented in Fig. 3. Lower conversion values were obtained compared with those of Kinik limestone. As in the case of the Kinik limestone, increase in reaction rate with the increasing reaction temperature was observed. The maximum conversion values achieved at high reaction temperatures were higher than the corresponding values at low reaction temperatures. For example, the conversion value of 0.52 was achieved in 700 s at the reaction temperature of 900°C. The maximum conversion values achieved for Kinik limestone are higher than the corresponding values for the Goynuk limestone at all reaction temperatures. It was also observed that time required to reach maximum conversion values are shorter for the Kinik limestone than for the Goynuk limestone. Although conversion value of Kinik limestone is 21.8% higher than the Goynuk limestone, conversion time is 53.33% shorter relative to the Goynuk limestone. This is very important for the industrial application point of view.
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3.2. Experimental 6erification of the deacti6ation model When there is an appreciable difference between the molar volumes of the solid reactant(s) and product(s), physical and pore structure of the solid undergo some variations. These variations lead to variations in the activity of the solid reactant. Several models which, define the change of activity as a function of conversion are given in the literature. The selected model must explain the reaction mechanism and it has few adjustable model parameter for the ease of application to experimental data. Pore plugging is inevitable due to the formation of denser solid product. Literature studies also confirmed that pore structure of the solid reactant changed significantly during the sulfation reaction [8,9,11]. Among the several structural models, in the explanation of such reactions during which the formation of highly denser solid product is occurring, the most commonly used one is the unreacted core model. As mentioned before, the diffusional resistances are very important in the sulfation reactions. The formation of the exhausted and denser product layer near the outer surface is possible for bigger particle sizes causing diffusional resistances. However, the used limestones had macro-meso porosities over 0.30 and it was obvious that these values increased upon calcination. From kinetic point of view, the presence of large size pore volumes helps the transportation of the gaseous reactant to the reaction sites. So, it was thought that the sulfation reactions and as a result pore plugging take place at every point of the solid. From the conversion vs time data it was concluded that the sulfation reaction mechanism just differed at 900°C reaction temperature. For the sulfation temperature of 850°C and lower the reactions were in the kinetic control region (Figs. 1–3). In this regime, the conversion values were small so diffusional limitations caused by pore plugging were also small. So the sulfation reactions could take place at every point of the particle although the rate was higher near the outer surface. Unreacted core model might not explain the reaction mechanism well in kinetic control regime. In such a case volume reaction model could fit the experimental data well. In the study of Guldur et al. [20] it was also seen that the unreacted core model weakly explained the reaction between trona–SO2. As seen in Figs. 1– 3, conversions (weight gains) were very high in the early stages of the sulfation, and then they showed a decreasing trend generally after 200 s conversion. This might be caused by the deactivation of the solid reactant. So for the explanation of the sulfation mechanism the deactivation model of Balci et al. [19] was used. The rate expression based on first-order decomposition of the solid reactant is written in terms of fractional conversion as,
dx = kapp(1−x). dt
(4)
The deactivation of apparent reaction rate constant from the Arrhenius constant is due to the decrease of reactivity (activity) of the solid. The apparent reaction rate constant was given in the following form [19]. kapp = akArr = aA0 e − Ea 0/RT,
(5)
where ‘a’ is the activity of the solid which is only a function of solid reactant properties under uniform gas reactant concentration. They related the rate of change of activity of solid with respect to conversion as a function of deactivation rate constant, kd, and activity. −
da =kda n. dx
(6)
For the sake of simplicity of the model (few adjustable parameters), the rate of change of activity is expressed to be proportional to activity itself (n= 1). Activity of solid reactant is unity at zero conversion, and it decreases with conversion and it approaches zero when conversion approaches its maximum value. Including these limitations, variation of apparent reaction rate constant with conversion is expressed as follows:
kapp = akArr = A0 exp −
n
Ea 0 RT 1+ kd x RT Ea 0
.
(7)
The implication of this model is an increase of activation energy with conversion. The deactivation rate constant ‘kd’ might also be temperature dependent. The calculated apparent reactipn rate constant values for high temperature sulfation reactions of different origin and the ones for the sulfation of Kinik limestone and Goynuk limestones at different temperatures are plotted in Figs. 4–6, respectively. Apparent reaction rate constant for high temperature sulfation showed nearly linear relation with conversion in semi-log plots. At lower temperatures, two different linear segments were observed. In the first segment in which apparent reaction rate constant decreased very rapidly, conversion of the solid reactant was very fast. As conversion increased, the plugging of pores due to the formation of CaSO4 caused considerable mass transfer limitations in the particles. As a result of this increased mass transfer limitations and the decrease in the composition of the solid reactant CaO, reaction mechanism differed so positive deviations were observed. The validity of the model on the change of activity of solid and apparent reaction rate constant has been investigated using non linear regression analysis. The predicted model parameters are given in Tables 2–4 for the sulfation reaction of different limestones at 900°C and the sulfation reactions of Kinik and Goynuk limestones at different temperatures respectively. The results of the modelling study are also reported together with the experimental values in Figs. 4–6 for several kinds
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of limestones, Kinik and Goynuk limestones, respectively. For the sulfation of different limestones at 900°C, the activation energy values ranged from 2904.12 cal/mol (for Dikmen limestone) and 9608.05 cal/mol (for Ayas limestone). It was observed that, as expected, the deactivation rate constant values showed an increasing trend in the order of the limestone kinds having the decreasing maximum conversion values (Fig. 4 and Table 2). When model equation applied to the experimental results obtained at different sulfation reaction temperatures, especially at temperature of 850°C and lower, positive deviations were observed for Kinik and Goynuk limestones (Figs. 5 and 6). As previously discussed, with increasing conversion the occurred pore plugging caused the change of reaction mechanism. Experimental data showed nearly two different linear segments in semi-plot analysis. Therefore, in the first zone in which sulfation rate was very rapid, high conversion values achieved in a shorter time with the increasing temperature. The plots of kapp-conversion obtained from the experimental values and values predicted by model were agree very well especially in the low conversion values region for low sulfation temperatures. After a certain conversion values, apparent reaction rate constants predicted by the model were not confirm with the experimental results. For example, confirmation between the values predicted by the model and the experimental ones were very poor for the sulfation at 700°C in the higher conversion region. Confirmity between the model values and the experimental ones became better and better with the increasing of temperature to 750 and 800°C, and finally at 850°C model predicted values and experimental ones confirmed exactly (Tables 3 and 4; Figs. 5 and 6). As the sulfation reaction temperature increased from 700 to 850°C, a decrease in deactivation rate constant values were observed. Nearly the same activation energy values were predicted in that temperature range. Since sulfation reaction mechanism just changed to mass transfer control regime at 900°C, an appreciable decrease in the activation energies were observed. The predicted values were less than the one third of the ones obtained sulfation temperatures of 850°C and lower for both Kinik and Goynuk limestones. Best fit between the model and experimental values were obtained for the reaction at 850°C. It was said that, the agreement between the model prediction and experimental values improved by sulfation temperature increase. For the sulfation temperature of 900°C, reaction mechanism differed. Due to the high rate of sulfation in the early stages, the pore plugging and as a result mass transfer limitations might become dominant and these had much more effects on the deactivation rate constant than the effects of the decreasing of the solid reactant with conversion.
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The values of reaction rate constant at zero conversion predicted from the model were reported in Tables 2–4 for each sample. Activity of solid reactant is unity at zero conversion, so initial value of the apparent reaction rate constant must equal to the Arrhenius reaction rate constant. Arrhenius plots of sulfation reaction of Kinik and Goynuk limestones at different temperatures are given in Figs. 7 and 8, respectively. Considering the data points between 850 and 700°C, activation energy values were estimated around 12 179.75 and 11 543.19 cal/mol for the sulfation of Kinik and Goynuk limestones, respectively (Table 5). The values matched with the values found from regression analysis of the apparent reaction rate constant vs conversion data (Tables 3 and 4). The deactivation rate constant is temperature dependent term. The temperature dependencies are also given in Figs. 7 and 8, respectively. It was seen that for temperatures 850°C and below, the dependencies were in Arrhenius form, yielding the negative deactivation energies around 10 298.26 and 14 795.50 cal/mol for Kinik and Goynuk limestones sulfation reactions, respectively (Table 5). Conversion values estimated (using numerical integration) from the model are also presented in Figs. 1–3 with solid and several shapes of the lines for different kinds of limestones, Kinik and Goynuk limestones respectively together with the experimental values. Good agreement of model was observed for the limestones, which gave high sulfation conversion. For the ones showing lower sulfation conversions, small deviations with the model predictions and the experimental data were observed (Fig. 1). For the sulfation of Kinik and Goynuk limestones the model were in agreement with the data especially at higher reaction temperatures. At lower temperatures deviations were observed with increasing sulfation time. In apparent reaction rate constant plots the starting point of the second regime (diffusion controlling region) were also occurred around the same conversion levels. Approximately the same behaviour was also seen for other samples. 4. Conlusions Sulfation reaction rate was very high in the early stages, after 200 s it showed a very rapid decrease. For temperatures of 850°C and below, reaction took place in two proceeding mechanism due to the pore plugging and the changes in the chemical composition of the solid. It was observed that, the agreement between the model prediction and experimental values improved by temperature increase. The SO2 capture capacities of different limestones at 900°C were investigated together by taking into account this temperature effect. Activation energies of the sulfation reactions carried with Kinik and Goynuk limestones were found to be 12 179.75 and 11 543.19 cal/mol, respectively.
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Appendix A Symbols a activity of solid Ad pre-exponential factor of the deactivation rate constant (s−1) A0 pre-exponential factor for the sulfation reaction (s−1) Ea0 initial activation energy (cal.mol−1) Ed deactivation energy (cal.mol−1) kapp apparent reaction rate constant (s−1) kArr Arrhenius type reaction rate constant (s−1) deactivation rate constant of solid (s−1) kd MW molecular weight (g/g mol) R universal gas constant rpore average pore diameter (nm) T sulfation temperature (K) t sulfation time (s) Vpore pore volume (for pores having diameter greater than 2 nm) (cm3.g−1) W0 initial weight of sample before the calcination (g) Wt weight of sample at any sulfation time (g) wCaO CaO weight fraction of limestone samples, g CaO/g limestone x conversion of CaO–CaSO4
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