Review of the direct sulfation reaction of limestone

ARTICLE IN PRESS

Progress in Energy and Combustion Science 32 (2006) 386–407 www.elsevier.com/locate/pecs

Review of the direct sulfation reaction of limestone Guilin Hua, Kim Dam-Johansena,, Stig Wedela, Jens Peter Hansenb a

CHEC, Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark b FLSmidth A/S, Denmark Received 20 December 2005; accepted 22 March 2006 Available online 8 May 2006

Abstract The direct sulfation reaction is defined as the sulfation reaction between SO2 and limestone in an uncalcined state, and is typically relevant for flue gas desulfurization by direct sorbent injection during pressurized fluid-bed combustion (PFBC) and SO2 absorption on limestone in the cyclone preheater used in cement production. In the past decades, this reaction has been extensively studied due to its potential for providing an economical control of SO2 emissions during PFBC and other similar processes. In this paper, a literature review of the direct sulfation reaction is presented. Various subjects, such as the influence of the reaction conditions (gas concentrations, temperature and system pressure), limestone properties and additives to the reaction kinetics, the reaction mechanism and modeling, are discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Limestone; Sulfation; Sulfur dioxide; Mechanism; Additive; Eutectic; Sintering

Contents 1. 2.

3. 4. 5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . Influence of reaction conditions . . . . 2.1. Gas concentrations . . . . . . . . . 2.2. System pressure . . . . . . . . . . . 2.3. Temperature . . . . . . . . . . . . . Reactivity of limestones . . . . . . . . . . Influence of additives . . . . . . . . . . . . Porosity of the product layer . . . . . . Reaction mechanism . . . . . . . . . . . . Kinetics . . . . . . . . . . . . . . . . . . . . . 7.1. Intrinsic reaction rate . . . . . . . 7.2. Diffusion in the product layer . 7.3. Modeling. . . . . . . . . . . . . . . .

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Corresponding author. Tel./fax: +45 45 25 28 45.

E-mail address: [email protected] (K. Dam-Johansen). 0360-1285/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pecs.2006.03.001

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8. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405

1. Introduction

indirect sulfation reaction and is expressed by the following overall reactions:

Emissions of SO2 from different industrial activities such as power production, the metallurgical industry and cement production are undesired due to its harmful effects. The world has long been aware of the destructive effects of SO2 emissions. In the past decades, great effort has been made to abate SO2 emissions. Many different processes have been developed for the purpose of cleaning flue gases for sulfur, including wet scrubbing, dry scrubbing, direct dry sorbent injection and regenerable processes. Of these processes, direct dry sorbent injection is a relatively simple and low-cost process. With this method, the sorbent, often limestone, is injected directly into the process at the place where the absorption of sulfur dioxide on the sorbent can readily take place, such as the combustion chamber in power plants. With limestone as the sorbent, the sulfation reaction can proceed via two different routes depending on whether calcination of the limestone takes place under the given reaction conditions. The dissociation of limestone is normally determined by the CO2 partial pressure in the system. Limestone decomposes to form CaO and CO2 when the partial pressure of CO2 in the system is lower than the equilibrium CO2 pressure over limestone at the same temperature. The equilibrium CO2 pressure over limestone has been investigated by a number of authors (Johnston [1], Mitchell [2], Smyth et al. [3], Hill et al. [4] and Baker [5]). Fig. 1 shows the dependence of the equilibrium CO2 pressure over limestone on temperature, as measured by Hill et al. [4] and Baker [5]. The curve in the figure can be described by the following equation [5]:

CaCO3 ðsÞ ! CaOðsÞ þ CO2 ðgÞ,

(2)

CaOðsÞ þ SO2 ðgÞ þ 0:5O2 ðgÞ ! CaSO4 ðsÞ.

(3)

8308 þ 7:079. ¼
T

peCO2

(1)

Here, is given in atmospheres (1 atm. ¼ 0.101 MPa), and T is given in Kelvin. If calcination of the limestone takes place (the CO2 partial pressure in the system is lower than the equilibrium CO2 pressure over limestone), the limestone first decomposes to form CaO. The CaO then reacts with SO2. This process is often called the

CaCO3 ðsÞ þ SO2 ðgÞ þ 0:5O2 ðgÞ ! CaSO4 ðsÞ þ CO2 ðgÞ.

ð4Þ

This reaction is typically relevant in the application of direct dry sorbent injection for the reduction of SO2 emission during pressurized fluid-bed combustion (PFBC) and SO2 absorption in the cyclone preheater used in cement production. In PFBC, due to the high operation pressure, the partial pressure of CO2 in the combustor is normally sufficiently high to prevent the calcination of the limestone. The sulfation reaction in the combustor is thus the direct sulfation reaction. In cement production, the so-called ‘‘dry-process’’ is today the dominant process. In this process, a multistage cyclone preheater is used for preheating of the raw meal—powder mixture of the raw materials—by direct countercurrent heat exchange with the hot flue

12 11 Equilibrium CO2 pressure, atm.

log10 peCO2

If calcination of the limestone does not take place (the CO2 partial pressure in the system is higher than the equilibrium CO2 pressure over limestone), the limestone may react directly with SO2. This process is often called the direct sulfation reaction and is expressed by the following overall reaction:

Data from Hill et al. 1956

10 9

Data from Baker 1962

8

Equation by Baker 1962

7 6 5 4 3 2 1 0 700

800

900

1000

1100

1200

1300

Temperature, K

Fig. 1. Equilibrium CO2 pressure over limestone.

1400

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Nomenclature C gas concentration, mol/m3 x CO3CO regular carbonate (CO2
3 ) site with no net 3 charge Ea activation energy, J/mol ks rate constant n reaction order of SO2, O2 and CO2, respectively

gas from the rotary kiln and the calciner. During the heating process, SO2 is formed mainly by the oxidation of pyrite that is contained in the raw meal. The formed SO2 partly is absorbed by the limestone particles—the major constituent in the raw meal— through sulfation of the limestone. The CO2 partial pressure in the hot flue gas is normally around 30 vol%, which is higher than the equilibrium CO2 pressure over limestone at the highest temperature of about 1073 K in the cyclone preheater. The sulfation reaction in the cyclone preheater is thus the direct sulfation reaction as well. This review is actually part of the project work which aims to get a better understanding of the mechanism and kinetics of the sulfation reaction of limestone in the cyclone preheater for the purpose of reducing the SO2 emission from cement production. This paper discusses the direct sulfation reaction based on studies of the relevant literature. Subjects such as the influence of reaction conditions, the influence of additives, reaction mechanism, kinetics and modeling are discussed. Besides references on the direct sulfation reaction, a number of references related to the indirect sulfation reaction are also included; to the extent they are relevant to the subjects and beneficial for the discussions. 2. Influence of reaction conditions The direct sulfation reaction can be significantly influenced by various parameters, such as temperature, system pressure and gas concentrations. The degree of influence of each of these parameters on the direct sulfation reaction varies with the reaction conditions and is often difficult to describe by using a simple rate law. The following sections provide an overview of literature findings concerning the influence of the above-mentioned parameters on the direct sulfation reaction, which illustrates the complexity of this reaction.

O00i peCO2 rs T V xi V  CO3

interstitial O2
with two negative charges equilibrium CO2 pressure over CaCO3, atm. surface reaction rate, mol/(m2 s) temperature, K neutral and vacant interstitial site vacant carbonate site with two positive charges

2.1. Gas concentrations The direct sulfation reaction is observed to be affected by the concentrations of the two gaseous reactants (SO2 and O2) and the gaseous product (CO2) in varying degree depending on reaction conditions. Furthermore, the reaction can also be significantly influenced by water. The degree of influence of these gases is normally measured in terms of their apparent reaction orders. Table 1 shows the apparent reaction orders of these gases that were evaluated by various authors. The observed apparent reaction order of SO2 varies from 0.4 to greater than 1. Some of the reaction orders were evaluated by initial reaction rates (supposed to represent intrinsic kinetics), while others were evaluated at high conversions by assuming diffusion control. No clear trend is evident for the variation of the reaction order with the reaction conditions. Few authors tried to explain the reaction order(s) that they observed. Iisa et al. [8] suggested that the low reaction order of SO2 that they observed at high conversions is related to solid-state diffusion control. Spatinos et al. [7] believed that the high reaction order they observed was due to a possible increase of the micro-porosity of the product layer with the increase of the SO2 concentration caused by the faster evolution of the CO2 gas at higher SO2 concentrations. For O2, the general trend is that the reaction order becomes zero at high O2 concentrations. The reason for this zero-order behavior at high concentrations is not clear. No explanations were given by those authors who observed this phenomenon. The rate of the direct sulfation reaction can be significantly reduced by higher CO2 concentrations under certain conditions, as observed by Ulerrich et al. [16], Dam-Johansen [13] and Tullin et al. [19]. Ulerrich et al. [16] believed that the lower sulfation

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rates caused by higher CO2 concentrations were related to slower diffusion of the formed CO2 away from the limestone particles. However, no explanation of how the sulfation reaction is actually affected by the slow diffusion of the CO2 is given. Tullin et al. [19] suggested that the negative effect of higher CO2 concentrations was related to the reverse reaction of the dissociation of the limestone. Another plausible explanation on the effect of the CO2 partial pressure on the direct sulfation reaction is its influence on solid-state mobility as shown by Tetard et al. [21] and Beruto et al. [22]. Tetard et al. [21] observed in their studies of sintering of limestone that the degree of densification of the limestone samples and the growth of the limestone particles (grains) decreased with increase of the CO2 partial pressure. The effect of the CO2 partial pressure is suggested to be related to its influence on the CO2
3 vacancies in the crystal lattice of limestone (calcite). An increase of the CO2 partial pressure reduces the number of CO2
3 vacancies and thus also solid-state mobility of the limestone. (Vacancies, a kind of crystal lattice point defects, and their influence on solid-state mobility will be discussed later in the text.) The following equilibrium between CO2 in the gas phase, CO2
and 3 anion vacancies in limestone is supposed to exist: COx3CO þ V xi Ð O00i þ V  CO3 þ CO2 ðgÞ.

(5)

3

This mechanism also implies a dynamic exchange of CO2 in the gas phase and CO2 contained within carbonate ions. The existence of this dynamic exchange is in fact demonstrated by Haul et al. [23] via isotopic exchange with carbon-13 dioxide. The influence of the CO2 partial pressure on the solid-state mobility of carbonate salts was also observed earlier by Beruto et al. [22]. Beruto et al. [22] conducted sintering experiments with CaCO3, Li2CO3 and a mixture of CaCO3 and Li2CO3 at 800 K. It was observed that sintering of Li2CO3 and the mixture of CaCO3 and Li2CO3 was more significant in nitrogen than in CO2. With CaCO3, it was concluded in the paper that the sintering was more significant in CO2. By looking at the scanning electron microscope (SEM) pictures shown in their paper, the opposite conclusion can in fact be made. The direct sulfation reaction can be significantly influenced by the presence of water in the gas phase. Water may affect the apparent reaction order of SO2 [6], or directly promote the sulfation reaction [20]. Water is clearly not inert for the direct sulfation

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reaction. However, no explanations of the observed phenomena were given in these two papers. The observed influences of all these gases on the direct sulfation reaction indicate that this reaction has a more complicated mechanism than that represented by the overall reaction (Reaction (4)). The unawareness of details of this mechanism is one of the main reasons for the lack of satisfactory explanations of the many experimental observations. 2.2. System pressure The direct sulfation reaction can be significantly hindered by higher system pressures under certain conditions, probably due to its influence on the gas phase diffusivity. Qiu et al. [12] investigated the effect of the system pressure in a PTGA by maintaining constant partial pressures of SO2 and CO2. The oxygen content in the gas was 5%. It was observed that the rate of the sulfation reaction at 1123 K was significantly lower at higher system pressures despite the increase of oxygen concentrations. The effective diffusivity in the product layer was also evaluated to be lower at higher system pressures. The authors suggested that the effect of the higher system pressures was caused by possible structure variation of the product layer or increased resistance of the outward diffusion of the formed CO2. The latter seems, however, to be a more reasonable explanation, as at constant concentrations the diffusion of both SO2 and CO2 is slower at higher system pressures. This is because the gas phase diffusivity is inversely proportional to the system pressure [24]. The slower diffusion of both SO2 and CO2 can have an adverse influence on the sulfation reaction. Bulewicz et al. [25] investigated the effect of the system pressure at constant gas composition (volume percentage). In this case, the gas concentrations were increased with the increase of the system pressure. The higher SO2 concentration is supposed to increase the sulfation rate. The overall decrease in the sulfation rate that they observed is then most likely caused by the negative influence of the higher CO2 concentrations at higher system pressures or a combination of the increased CO2 concentrations and the decreased gas phase diffusivity. 2.3. Temperature The influence of temperature on the direct sulfation reaction gets complicated due to its influence on

Experimental equipment

Gravimetric method

Fixed-bed

PTGAa

TGA

TGA

Fixed-bed

PTGAa

Gravimetric method

Fixed-bed

PTGA

Author

SO2: Yang et al. [6]

Spatinos et al. [7]

Iisa et al. [8]

Krishnan [9]

Zhong [10]

Liu et al. [11]

Qiu et al. [12]

O2: Yang et al. [6]

Dam-Johansen [13]

Iisa et al. [14]

T: 1023 K; P: 0.1 MPa Gas composition: NA Particle size: 1.1 mm T: 873 K; P: 0.1 MPa Gas composition: SO2: 0.15%; O2: 0–4%; CO2: 1.8% Particle size: 0.327–2.0 mm T: 1123 K; P: 1.5 MPa Gas composition: SO2: 0.3%; O2: 1–6%; CO2: 15% Particle size: 125–180 mm

T: 1023 K; P: 0.1 MPa Gas composition: SO2: 0.1–3.1%; O2: 5%; CO2: 15%; H2O: 0–2.9% Particle size: 1.1 mm T: 573–873 K; P: 0.1 MPa Gas composition: SO2: 0.5–3%; O2: 10%; CO2: NA Particle size: 2–4 mm T: 1073 K; P: 1.5 Mpa Gas composition: SO2: 0.1–0.5%; O2: 4%; CO2: 15% Particle size: 125–180 mm T: 1023 K; P: 0.1 MPa Gas composition: SO2: 0.15–0.6%; O2: 6%; CO2: 70% Particle size: 53–350 mm T: 1073 K; P: 0.1 MPa Gas composition: SO2: 0.1–0.5%; O2: 10%; CO2: 70% Particle size: 4–5.4 mm T: 883–1123 K; P: 0.1 MPa Gas composition: SO2: 0–0.24%; O2: 10%; CO2: 20–80% Particle size: 8.4–54 mm T: 1123 K; P: 1.3 MPa Gas composition: SO2: 0.16–0.45%; O2: 5%; CO2: 14% Particle size: 125–180 mm

Experimental conditions

Table 1 Apparent reaction orders of SO2, O2, CO2 and H2O obtained by different authors

0

40 and o1

0.22 (with water)

0.58 (evaluated by initial reaction rate)

1

1

0.4 (evaluated by initial reaction rate)

0.49

41

1 (with water) 0.76 (without water)

Observed apparent reaction order

390 G. Hu et al. / Progress in Energy and Combustion Science 32 (2006) 386–407

ARTICLE IN PRESS

PTGA

Fixed-bed

TGA

PTGA

Fixed-bed

TGA

Gravimetric method

TGA

CO2: Ulerrich et al. [16]

Dam-Johansen [13]

Snow et al. [17]

Iisa et al. [14]

Illerup et al. [18]

Tullin et al. [19]

H2O: Yang et al. [6]

Hajaligol et al. [20]

T: 1023 K; P: 0.1 MPa Gas composition: SO2: 0.1–3.1%; O2: 5%; CO2:15%; H2O: 1–40% Particle size: 1.1 mm T: 1173 K; P: 0.1 MPa Gas composition: SO2: 0.3%; O2: 5%; CO2:95%, H2O: 6–12% Particle size: 10–12 mm

T: 1088 K; P: 1 MPa Gas composition: SO2: 0.5%; O2: 10.5–14%; CO2: 5.8–8.7% Particle size: 125–180 mm T: 873 K; P: 0.1 MPa Gas composition: SO2: 0.15%; O2: 0–4%; CO2: 0–10% Particle size: 0.327–2.0 mm T: 298–1373 K; P: 0.1 MPa Gas composition: SO2: 0.3%; O2: 5%; CO2: 2–95% Particle size: 2–106 mm T: 1123 K; P: 1.5 MPa Gas composition: SO2: 0.3%; O2: 4%; CO2: 15–90% Particle size: 125–180 mm T: 1123 K; P: 1 MPa Gas composition: SO2: 0.15%; O2: 4%; CO2: 10% Particle size: 0.85–1 mm T: 1023–1048 K; P: 0.1 MPa Gas composition: SO2: 0.3%; O2: 4%; CO2: 30–80% Particle size: 9–37 mm (consisting of 1–5 mm primary particles)

T: 1123 K; P: 1.2 MPa Gas composition: SO2: 0.5%; O2: 3–7%; CO2: 12–15% Particle size: 100–595 mm T: 883–1123 K; P: 0.1 MPa Gas composition: O2:4ca. 5%; others: NA Particle size: 8.4–54 mm

TGA ¼ thermal gravimetric analysis; PTGA ¼ pressurized thermal gravimetric analysis.

Fixed-bed

Liu et al. [11]

a

PTGA

Alvarez et al. [15]

40

0

o0

No influence on the final conversion

0

0

o0

o0

0

0

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the physical properties of the solid phases, both the solid reactant (limestone) and the solid product (calcium sulfate). The influence of the temperature is normally reflected in the apparent activation energy of the sulfation process. Table 2 shows the apparent activation energies obtained by different authors at both low and high conversions. The apparent activation energies measured at very low conversions were often assumed to represent the activation energies of the intrinsic kinetics of the sulfation reaction, and those measured at higher conversions were often assumed to represent the activation energies of the diffusion process in the product layer. As shown in Table 2, the observed apparent activation energies vary over a very wide range. There can be various causes for this variation. One is sintering of the limestone particles. At sufficiently high temperatures, for example, higher than the Tammann temperature [30] of limestone, sintering of limestone particles can occur. The Tammann temperature is the temperature at which sintering of crystalline materials starts to take place. The Tammann temperature of a crystalline material is usually around 0.4–0.5 times its melting point (in Kelvin). The sintering may significantly reduce the surface area necessary for the reaction due to the loss of micro-porosity. The experimental observations of Illerup et al. [18], Hanson et al. [31] and Zevenhoven et al. [29] clearly demonstrate the

sintering of limestone at high temperatures and its consequence for the sulfation reaction. Hanson et al. [31] studied the sintering of calcium carbonate in a tube furnace in 100% CO2 gas at atmospheric pressure. It was observed by SEM that sintering of the particles of CaCO3 started at about 973 K. This observation is in good accordance with the Tammann temperature of calcite. The melting point of calcite is 1612 K. The Tammann temperature is thus 645–806 K. At 1123 K, about 500 K lower than the melting point of CaCO3, the sintering was observed to be so severe that the particles lost their characteristic trigonal form and became round. The negative influence of the sintering of limestone particles on the sulfation reaction is well demonstrated by the observations of Illerup et al. [18] and Zevenhoven et al. [29]. Illerup et al. [18] studied the direct sulfation reaction in a pressurized fixed-bed reactor at 1023 K with Stevns Chalk: a porous limestone. The particle size was 0.85–1 mm. It was observed that the limestone particles lost partly their reactivity after a heat treatment at 1123 K. The loss of the reactivity was dependent on the duration of heat treatment: a longer heat treatment caused greater loss of reactivity. Sintering of the limestone particles is proposed to be the reason. In a later study, Zevenhoven et al. [29] studied the direct sulfation reaction in a PTGA at 1.5 MPa over

Table 2 Apparent activation energies obtained by different authors Author

Hajaligol et al. [20] Snow et al. [17] Iisa et al. [8] Iisa et al. [26] Fuertes et al. [27] Krishnan [9] Tullin [19] Fuertes et al. [28] Zhong [10] Zevenhoven et al. [29] Alvarez et al. [15] Qiu et al. [12] Liu et al. [11] a

Experimental method

Temperature (K)

Pressure (MPa)

Particle size (mm)

Apparent activation energy (kJ/mol) At low or zero conversions

At high conversions

TGA

773–1213

0.1

2–45

68.7

146.5

TGA PTGA PTGA TGA TGA TGA TGA TGA PTGA

298–1373 923–1123 923–1223 1023–1173 1023–1123 773–1123 1023–1173 773–1073 1123–1223

0.1 1.5 1.5 0.1 0.1 0.1 0.1 0.1 1.5

2–106 125–180 125–180 26–780 53–350 9–37 20–780 4–5.4 250–300

64 NA 77 96 110–138 70–160 NA 35.9 9.14–82.2

NAa 92–130 133 NA NA 170–390 148 66.5 (
70.7)
338

PTGA PTGA Fixed-bed

1073–1198 1023–1173 883–1123

1.2–2.5 0.6–1.3 1

105–595 200–250 8.4–54

87.2 96.8 80–90

NA 144 83.1

NA ¼ not available.

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a temperature interval of 1123–1223 K. The limestone particle size was 250–300 mm. It was observed that the apparent activation energies obtained from the porous limestones were lower than the apparent activation energies obtained from the denser limestones. Even a negative apparent activation energy was observed. Pore closing by sintering is again proposed to be the reason. The influence of temperature on the structure of the product layer is another cause for the variation of the measured apparent activation energies. The product layer that is formed by the direct sulfation reaction is porous [17,20,11]. The pore size increases with the increase of the temperature [20,11]. This pore structure variation with the temperature can significantly influence the sulfation reaction due to its influence on diffusion in the product layer (there will be more discussions about the product layer later in the text). At sufficiently high temperatures, sintering of the product layer can also occur, which will then adversely affect the sulfation reaction. CaSO4 has a melting point of 1723 K. Its Tammann temperature is around 690–860 K. Glasson [32] observed significant sintering of CaSO4 at a temperature of around 973 K. Hajaligol et al. [20] observed in their studies of the direct sulfation reaction that the conversions after 2 h reaction time were almost constant in a narrow temperature interval close to 1173 K. This may be an example of the occurrence of significant sintering of the product layer that adversely affected the sulfation reaction. 3. Reactivity of limestones Different limestones often show different reactivities as shown by the studies of Zevenhoven et al. [29] and Alvarez et al. [15]. Zevenhoven et al. [29] studied the kinetics of the direct sulfation reaction of five different limestones at 1123 and 1223 K. The measured rate constants of these limestones varied from 0.00071 to 0.0013 m/s, an approximately 2-fold variation. Alvarez et al. [15] performed similar studies on five different limestones at 1123 K. The measured sulfation rates of these limestones varied from 0.00038 to 0.0012 g/(m2 s), an approximately 3-fold variation. There are no clear explanations on the variation in the reactivity of the different limestones. As natural materials, limestones differ normally in their chemical compositions due to the presence of various impurities. The percentage of impurities in

393

different limestones can very from less than 1 to higher than 10 [33,34,15]. The variation in chemical composition is a possible reason for the variation in the reactivity of different limestones. There is, however, no specific experimental evidence that can show how and to what degree the sulfation reaction is influenced by the impurities. Nevertheless, MgCO3 appears to be an impurity that can significantly promote the sulfation reaction if it is present in relatively high concentrations. This promotion effect is mainly due to the surface area increase resulting from the decomposition of MgCO3 at a temperature lower than the actual reaction temperature, since MgCO3 has a much high equilibrium decomposition pressure than CaCO3. The effect of MgCO3 is evident in the results obtained by Alvarez et al. [15] (Table 3) and Zevenhoven et al. [29]. As shown in Table 3, Focino, which contains 7.6 wt% MgCO3, is the most reactive of the five tested limestones. Zevenhoven et al. [29] measured the rate constants of five different limestones at 1123 K. Among these five limestones, Gotland, which contains 2.9 wt% MgCO3 is the most reactive one. (According to the authors, the particles of Gotland limestone disintegrated severely after heating to temperatures above 1073 K during the experiments. The high reactivity of this limestone may partly be explained by a smaller particle size.) Porous limestones often show higher sulfation rates than dense limestones [20,34,29,15]. The high surface areas of porous limestones are usually the main reason for this trend. The difference in sulfation rates between porous limestones and dense limestones is often significantly reduced when the sulfation rates are evaluated per unit internal surface area. This is demonstrated by the results obtained by Alvarez et al. [15] shown in Table 3. The ratio of the initial conversion rates of Maria and Macael (approximately 10) is roughly equal to the ratio of their internal surface areas. 4. Influence of additives Both the direct and indirect sulfation reactions can be enhanced by various additives. Most of the results published in the literature are related to the indirect sulfation reaction. Considering that both the direct and indirect sulfation reactions may be enhanced by the additives by the same mechanisms, the results related to the indirect sulfation are presented and discussed here together

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Table 3 Initial reaction rate of different limestones (Alvarez et al. [15]) Limestone name

Content of CaCO3 and MgCO3 (w%)

Surface area (m2/g)

Initial sulfation rate on mass basis ( ¼ conversion rate) (s
1)

Initial sulfation rate per unit internal surface area (g/(m2 s))

Maria

CaCO3: 99.1 MgCO3: 0.1

0.424

0.00019

0.00045

Focino

CaCO3: 92.1 MgCO3: 7.6

0.188

0.00022

0.0012

Andreu

CaCO3: 99.1 MgCO3: 0.83

0.366

0.00019

0.00050

Macael

CaCO3: 97.6 MgCO3: 1.25

0.043

0.000017

0.00038

Horcallana

CaCO3: 95.6 MgCO3: 1.25

0.241

0.00015

0.00063

with the results related to the direct sulfation reaction. Table 4 lists the additives that are found to have an enhancing effect by different authors. The additives observed to have enhancing effects on the sulfation reaction are mainly different kinds of alkali metal (Li, Na and K) salts, chlorides (including HCl) and hematite (Fe2O3), as shown in the table. There are mainly three explanations that are proposed by different authors to explain the enhancements by alkali metal salts and chlorides. They are formation of eutectics, pore enlargement and formation of more crystal lattice point defects (‘‘crystal lattice point defects’’ are referred as ‘‘point defects’’ in the following text). Formation of eutectics is suggested by Matsukata et al. [35,36], Xie et al. [38] and Zhao et al. [50] to be the reason for the enhancement on the sulfation reaction by the additives (HCl, CaCl2) that they used in their studies. The possibilities for the formation of eutectics between the additives and the solid reactants and products during the sulfation reaction are numerous. The following tables (Table 5–7) show some of the reported eutectic systems that contain CaO, CaCO3 and CaSO4 and have melting points that are lower than 1200 K. The eutectics are listed in alphabetic sequence of the second and third components in the eutectic system. The compositions are listed in the same order as the system (for example, for the system of CaO–CaCl2, 28.7–71.3 means that the mole fractions are 28.7% and 71.3% for CaO and CaCl2, respectively). As shown in Table 5–7, CaCl2 is able to form eutectics with CaO, CaCO3 or CaSO4 having

melting points of 866, 895 and 981 K, respectively. The enhancement effects of CaCl2 and HCl at temperatures higher than about 866 K are most likely due to the formation of eutectics as in Van Houte et al. [48], Matsukata et al. [35,36], Xie et al. [38] and Zhao et al. [50]. The observation by Matsukata et al. [35,36] is an especially good example. Matsukata et al. [35,36] studied the sulfation of calcined limestone in the presence of hydrogen chloride at 1023 K in a TGA under atmospheric pressure. The limestone particle size was 32–1000 mm. The gas consisted of 100–1000 ppm HCl, 1000 ppm SO2 and 5% O2. It was observed that the sulfation of larger particles was enhanced by the presence of HCl. With small particles, no noticeable difference was observed. It was also observed that the addition of HCl after the particles were first sulfated for a period without HCl was able to open the plugged pores and resulted in a high sulfation rate. Energy dispersive X-ray microanalysis analysis showed that the sulfation reaction progressively advanced from the surface into the entire volume of the limestone particles when HCl was added. SEM examinations of the reacted particles also showed that the large pores (cracks) in the particles after certain reaction time were still open when HCl was added, but plugged when HCl was absent. A melting layer was visible on the surface of these large pores (cracks). The liquid phase was believed to be eutectic between CaCl2 and CaSO4. CaCl2 was formed by the reaction between the added HCl and the limestone. Another

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Table 4 Additives observed having enhancing effect on both the direct and indirect sulfation reactions Additive

Author

Temperature (K)

Doped material

Dosage or optimal dosage

Mixing method

Sulfation type (direct or indirect)

HCl (gas)

Matsukata et al. [35,36] Lawrence et al. [37] Xie et al. [38]

1023

CaO

100–1000 ppm

In gas phase

Indirect

953–1123 1140–1160

CaO Limestone

2000 ppm PVC combustion

In gas phase

Indirect Indirect

Yang [39] Shearer et al. [40] Stouffer et al. [41]

1173 1123 975–1275

CaO Limestone Limestone

1.4 wt% 0.5 wt% 0.25–5 wt %

Indirect Indirect Indirect

Wieczorek-Ciurowa et al. [42] Davini et al. [43] Fuertes et al. [44] Adanez et al. [45]

1100

CaO

2 wt%

Dry mixing Impregnation Impregnation, dry mixing Impregnation

1073–1223 1123 1223–1523

Limestone Limestone Ca(OH)2

0–2.7 wt% about 3.5 mol% 0–3 wt%

Indirect Direct Indirect

Fan et al. [46] Liu et al. [47]

1173–1373 1273

CaO Coal

0–8 wt% 6 wt%

Impregnation Impregnation Impregnation, dry mixing Impregnation Dry mixing

Indirect Indirect

Davini et al. [43] Fan et al. [46]

1073–1223 1173–1373

Limestone CaO

0–8 wt%

Impregnation Impregnation

Indirect Indirect

Van Houte et al. [48] Van Houte et al. [49] Stouffer et al. [41]

873–1173

Limestone

2 mol%

Impregnation

Indirect

573–908

Limestone

2 mol%

Impregnation

Indirect/direct

975–1275

Limestone

0.25–5 wt%

Indirect

Liu et al. [47] Zhao et al. [50]

1273 973

Coal CaO

6 wt% 2 mol%

Impregnation, dry mixing Dry mixing Impregnation

Indirect Indirect

FeCl2

Liu et al. [47]

1273

Coal

6 wt%

Dry mixing

Indirect

FeCl3

Stouffer et al. [41]

975–1275

Limestone

0.25–5 wt%

Indirect

Liu et al. [47]

1273

Coal

6 wt%

Impregnation, dry mixing Dry mixing

Indirect

NaF

Fuertes et al. [44]

1123

Limestone

about 7 mol%

Impregnation

Direct

Li2CO3

Fuertes et al. [44] Wang et al. [51]

1123 1173

Limestone Limestone

about 3.5 mol% 2 wt%

Impregnation Impregnation

Direct Indirect

Na2CO3

Stouffer et al. [41]

975–1275

Limestone

0.25–5 wt%

Indirect

Fuertes et al. [44] Fan et al. [46] Wang et al. [51] Laursen et al. [52]

973–1148 1173–1373 1173 1098–1173

Limestone CaO Limestone Limestone

ca. 3.5 mol% 0–8 wt% 2 wt% 3 wt%

Impregnation, dry mixing Impregnation Impregnation Impregnation Impregnation

Direct Indirect Indirect Indirect

K2CO3

Fuertes et al. [44] Fan et al. [46]

1123 1173–1373

Limestone CaO

about 3.5 mol% 0–8 wt%

Impregnation Impregnation

Direct Indirect

Li2SO4

Borgwardt et al. [53] Wang et al. [51]

1073 1173

CaO Limestone

9 wt% 2 wt%

Dry mixing Impregnation

Indirect Indirect

Na2SO4

Borgwardt et al. [53] Fuertes et al. [44] Wang et al. [51]

1073 1123 1173

CaO Limestone Limestone

9 wt% about 3.5 mol% 2 wt%

Dry mixing Impregnation Impregnation

Indirect Direct Indirect

K2SO4

Borgwardt et al. [53] Wang et al. [51]

1073 1173

CaO Limestone

9 wt% 2 wt%

Dry mixing Impregnation

Indirect Indirect

NaCl

KCl CaCl2

Indirect

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396 Table 4 (continued ) Additive

Author

Temperature (K)

Doped material

Dosage or optimal dosage

Mixing method

Sulfation type (direct or indirect)

Na2SO3

Wang et al. [51]

1173

Limestone

2 wt%

Impregnation

Indirect

NaO

Desal et al. [54]

1173

Limestone dolomite

about 1–7 wt%

Impregnation

Indirect

Fe2O3

Yang [39]

1173

1–1.4 wt%

1123 1173

Impregnation, dry mixing Dry mixing Impregnation

Indirect

Yang et al. [55] Desal et al. [54]

Limestone, CaO CaO Limestone dolomite

4 wt% About 1–7 wt%

Indirect Indirect

Table 5 Eutectics containing CaO Eutectic system

Eutectic composition (mol%)

Eutectic melting point (K)

Reference

CaO–CaCl2 CaO–CaCl2 CaO–CaCl2 CaO–Na2O–SiO2 CaO–NaF CaO–P2O5 CaO–P2O5 CaO–TiO2–V2O5 CaO–TiO2–V2O5 CaO–TiO2–V2O5 CaO–V2O5

28.7–71.3 6–94 (30–22.5)–(70–77.5) 5.6–20.6–73.8 48–52 8–92 37–63 7.5–7.5–85 21–17–62 29–30–41 13.5–86.5

866 1030 1073 998 923 753 1013 863 1003 1213 886

Treadgilla [56] Neumann et al.a [57] Neumann et al.a [57] Eitel a [58] Sychev a [59] Kreidler et al.a [60] Kreidler et al.a [60] Solacolu et al.a [61] Solacolu et al.a [61] Solacolu et al.a [61] Fedorov et al.a [62]

a

References from Janz et al. [63].

Table 6 Eutectics containing CaCO3 Eutectic system

Eutectic composition (mol%)

Eutectic melting point (K)

Reference

CaCO3
Ca(OH)2 CaCO3–CaCl2 CaCO3–CaCl2 CaCO3–CaF2 CaCO3–CaF2–Ca(OH)2 CaCO3–Li2CO3 CaCO3–Li3PO4 CaCO3–LiF CaCO3–Na2CO3–Na2SO4 CaCO3–Na2CO3–Na2SO4

35.8–64.2 30–70 30–70 58.1–41.9 29.7–20.1–50.2 37–63 98–2 (wt%) 29.7–70.3 37.5–46.5–16 38–38–24

926 895 908 1153 848 935 1038 853 1043 1068

Gittins et al.a [64] Nigglia [65] Freidina et al. [66] Gittins et al.a [64] Gittins et al.a [64] Eitel et al.a [67] Tetard et al.a [68] Sycheva [59] Polaetaev et al.a [69] Polaetaev et al.a [69]

a

References from Janz et al. [63].

possibility that is not mentioned by the authors is the formation of a eutectic between CaO and CaCl2. According to Treadgill [56] (as listed in Table 5), CaO and CaCl2 can form a eutectic with a melting point of 866 K.

It appears that the enhancement by the formation of eutectics is related to its capability of keeping the pores open and thus reducing the intraparticle diffusion resistance. However, the reality may be more complicated than this. The formation of a

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397

Table 7 Eutectics containing CaSO4 Eutectic system

Eutectic composition (mol%)

Eutectic melting point (K)

Reference

CaSO4–BaCl2–CaCl2 CaSO4–BaCl2–CaCl2–NaCl CaSO4–BaSO4–K2SO4 CaSO4–BaSO4–KCl CaSO4–BaSO4–Na2SO4 CaSO4–BaSO4–NaCl–Na2SO4 CaSO4–CaCl2 CaSO4–CaCl2 CaSO4–CaCl2 CaSO4–CaCl2 CaSO4–CaCl2–KCl CaSO4–CaCl2–KCl CaSO4–CaCl2–LiCl CaSO4–CaCl2–NaCl CaSO4–K2SO4

6–33–61 1.65–14.8–48.15–35.4 32–9.5–58.5 15.6–4.2–80.2 43–11–46 15.5–4.6–45–34.9 12.5–87.5 14–86 (30–80) – (70–20) (90–50) – (10–50) 2.5–24–73.4 5.7–69.7–24.5 3.4–32.2–64.4 2.7–51.7–45.5 42–58

842 708 1140 937 1140 893 981 985 996–999 997 853 877 723 758 1140

CaSO4–K2SO4 CaSO4–K2SO4 CaSO4–K2SO4 CaSO4–K2SO4–MgSO4 CaSO4–K2SO4–MgSO4 CaSO4–KCl CaSO4–KCl–K2SO4 CaSO4–KCl–K2SO4 CaSO4–KCl–LiCl CaSO4–KCl–NaCl CaSO4–Li2SO4 CaSO4–Li2SO4–Rb2SO4 CaSO4–Li2SO4–Rb2SO4 CaSO4–Li2SO4–Rb2SO4 CaSO4–LiCl CaSO4–LiCl CaSO4–LiCl CaSO4–LiCl–Li2SO4 CaSO4–Na2SO4 CaSO4–Na2SO4 CaSO4–Na4P2O7–Na2SO4 CaSO4–NaCl CaSO4–NaCl CaSO4–NaCl–Na2SO4 CaSO4–NaCl–Na2SO4 CaSO4–Rb2SO4

40–60 39.7–60.3 40.3–59.7 4.9–20.8–74.3 19.1–25.1–55.8 18–82 13.5–58.1–28.4 18–79–3 4.9–38.1–57 10.5–42.6–47 18.5–81.5 NA NA NA 14–86 14.4–85.6 14.3–85.7 6.8–64–29.2 40–60 50–50 49.2–2.5–48.2 17.6–82.4 17.7–82.3 2.7–45.5–51.7 18.8–46.1–35 39–61

Zimina et al.a [70] Zimina et al.a [71] Finkel’shtein et al.a [72] Finkel’shtein et al.a [73] Ingraham et al.a [74]; Nagornyi et al.a [75] Zimina et al.a [71] Golubeva et al.a [76]; Golubeva et al.a [77] Bergman et al.a [78] Matsukata et al. [35,36] Zhao et al. [50] Golubeva et al.a [77] Golubeva et al.a [77] Tkachenko et al.a [79] Zimina et al.a [70] Palkinaa [80]; Plyushchev a [81] Grahmanna [82] Finkel’shtein et al.a [72] Rowe et al.a [83] Rowe et al.a [84] Rowe et al.a [84] Rowe et al.a [84] Golubeva et al.a [77]; Rubleva et al.a [85] Golubeva et al.a [77] Golubeva et al.a [77] Liteanu et al.a [86] Rubleva et al.a [85] Plyushchev a [81] Finkel’shtein et al.a [87] Finkel’shtein et al.a [87] Finkel’shtein et al.a [87] Tkachenko et al.a [79] Liteanu et al.a [86] Golubeva et al.a [76] Tkachenko et al.a [79] Plyushcheva [81] Bergman et al.a [78] Bergman et al.a [88] Bergman et al.a [78] Rubleva et al.a [85] Bergman et al.a [78]; Zimina et al.a [70] Bergman et al.a [78]; Zimina et al.a [70] Plyushcheva [81]

a

1143 1148 1148 1153 1155 960 917 948 601 878 968 817 825 927 785 787 806 739 1163 1183 1191 992 999 758 907 1118

References from Janz et al. [63].

liquid phase often causes pore size enlargement and surface area reduction. The sulfation reaction may also take place in the liquid phase rather than at the interface of the solid reactant. Van Houte et al. [48] observed high initial sulfation rates with limestone particles doped with CaCl2 at 973–1173 K even though pore plugging is not supposed to be a limiting factor. This observation indicates that the formation of eutectics causes an increase in the

reactivity of the limestone, probably due to the higher sulfation rate in a liquid phase. Based on observations of pore enlargement following addition of the additives, some authors (Shearer et al. [40], Stouffer et al. [41], WieczorekCiurowa et al. [42], Davini et al. [43], Fan et al. [46] and Laursen et al. [52]) suggested that the improved intraparticle diffusion by the enlarged pore size is the reason for the enhancement on the sulfation

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reaction. The above suggested mechanism may be right if intraparticle diffusion resistance is the controlling factor. Otherwise, the benefit from pore enlargement may be very limited as the internal surface area of the particles is usually significantly reduced as a consequence of pore enlargement by the additives [40,41,43,45]. The reduction in internal surface area may significantly reduce the sulfation rate as discussed earlier. There may be other reasons for the observed enhancements as will be discussed in the following paragraphs. There are also authors (Borgwardt et al. [53], Fuertes et al. [44], Adanez et al. [45] and Wang et al. [51]) who believe that formation of more point defects in the solid phases and thus improvement in solid-state diffusion by the additives is the reason for the enhancement of the additives. Crystalline materials contain defects [89]. One of the defect types is point defects. Point defects can be, for example, vacant lattice sites that are normally occupied or occupied interstitial sites that are normally vacant. The existence of point defects is an intrinsic property of crystalline materials. The thermodynamic basis for the existence of point defects is the increase of entropy that is caused by the disorder. The number of point defects increases exponentially with the temperature [89]. For ionic crystal materials, point defects can also be caused by aliovalent impurities in the crystal. The existence of other cations of lower valence can, for example, create more anion vacancies due to the requirement of electrical neutrality. The point defects caused by aliovalent impurities are normally referred to as extrinsic. At high temperatures, the intrinsic point defects will be dominant due to the exponential increase in number with the temperature, whereas the effect of the aliovalent ions will be very limited. The existence of point defects is necessary for solid-state diffusion to proceed. An increase of the number of point defects leads to a higher solid-state diffusivity. Thus, factors that can increase the number of point defects in limestone and/or the product layer will then increase solid-state diffusivity. Alkali metal ions are of single valence. The incorporation of these ions in the crystal lattice of limestone and/or the solid product will thus increase the vacancies of the anions and thus also the solid-state diffusivity. The formation of point defects in limestone is also influenced by the CO2 partial pressure in the gas phase, as mentioned earlier. The formation of point defects is reduced by higher CO2 partial pressures.

The atomic radius of alkali metal ions is an important parameter for their incorporation into the crystal lattice of limestone. Effective incorporation requires that the alkali metal ion has a similar atomic radius to that of Ca2+. Na+ has the closest atomic radius to Ca2+. This explains why enhancement is almost always observed with Na+. Cs+ has a too large radius. Investigations by Davini et al. [43] and Fuertes et al. [44] also showed that Cs+ has no enhancing effects. Formation of point defects, pore enlargement and formation of eutectics are often interrelated. The pore enlargement by enhanced sintering is usually the result of the increased solid-state mobility that is associated with the formation of more point defects. With the addition of alkali metal salts, the solidstate mobility is often increased before the eutectics are formed due to the formation of more point defects. The investigations by Tetard et al. [21,68] are good illustrations of this interrelationship. Tetard et al. [68] studied the sintering of CaCO3 particles of a size of 2 mm at atmospheric pressure with Li3PO4 as an additive. The sintering was measured in CO2 gas using the dilatometric method. The temperature was 973 K, about 65 K lower than the melting point of the eutectic between CaCO3 and Li3PO4. With the addition of 0.5 wt% Li3PO4, the particles were observed to grow up to 23 times their original size after the defined thermal treatment. The bulk density increased significantly at the same time. The grain size remained unchanged after the same thermal treatment without addition of the additive. In another investigation, Tetard et al. [21] performed a similar study with Li2CO3 as the additive. Fig. 2 shows the relative shrinkage of the calcite samples with and without the additive with the temperature (the relative shrinkage here is a measurement of the disappearing of spaces between the particles and is also an indication of sintering). Fig. 2 shows that the shrinkage of the doped calcite was much higher than the undoped sample starting from a temperature that was about 60 K lower than the eutectic melting point. Sintering experiments performed at 893 K, about 42 K lower than the melting point of the eutectic between CaCO3 and Li2CO3 showed almost identical phenomenon as with Li3PO4. The addition of 0.5 wt% of Li2CO3 caused a 40 times grain growth and significant increase in the bulk density as well. These observations were explained by the formation of more point defects that increased solid-state mobility of the calcite. A solid solution between Li2CO3

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Fig. 2. Sintering of CaCO3 particles with and without addition of Li2CO3 (Tetard et al. [21]).

and CaCO3 may be formed during the thermal treatment. This was confirmed by X-ray diffraction measurements, which showed that the unit cell dimension of the calcite crystal is expanded due to the incorporation of lithium ions into the crystal lattice of calcite after the thermal treatment. The incorporation of lithium ions causes the formation of more point defects, as Li+ is of single valence. The different behaviors of the enhancement by the additives are usually a combined result of the different effects caused by the additives, such as formation of eutectics, pore enlargement, surface area reduction and increase of solid-state mobility. Enlargement of the pores can positively influence the sulfation process if the process is limited by diffusion in the pores. Reduction in the surface area usually reduces the conversion rate due to less surface area available for the reaction. The increase in solid-state mobility usually promotes the sulfation reaction when solid-state diffusion is a limiting factor. In practice, the surface area of limestone particles is usually reduced as a consequence of pore enlargement by the additives. Shearer et al. [40], Stouffer et al. [41], Davini et al. [43] and Adanez et al. [45] observed higher conversion rates despite reduction in surface area. This clearly demonstrates the net positive effect of the addition of additives due to the significant increase in the reactivity (or solid-state mobility) of the limestone and probably also improved pore diffusion. The net positive effect from the formation of eutectics is related to the increased reactivity, probably due to the higher sulfation rate in a liquid phase, and its capability of keeping pores open at higher conversions.

399

The catalytic effect of hematite (Fe2O3) on the conversion of SO2 to SO3 is often suggested [55,39,54,47] to be the reason for its enhancement on the sulfation reaction. Hematite is a well-known commercial catalyst for the conversion of SO2 to SO3 in the manufacturing of sulfuric acid. The good catalytic property of hematite is well illustrated by the investigation performed by Schu¨th et al. [90]. Schu¨th et al. [90] studied the catalytic properties of hematite coated on Mesoporous Molecular Sieves MCM-41. The gas phase consisted of 20 vol% SO2, 22 vol% O2 and the rest N2. Hematite is shown to have good catalytic property at temperatures higher than around 800 K. As the sulfation of limestone involves the oxidation of SO2, there may be good reason to believe that the enhancement on the sulfation reaction by hematite is related to its catalytic effect. 5. Porosity of the product layer The direct sulfation reaction (Reaction (4)) is a gas–solid reaction with the formation of a solid product. Calcium sulfate (CaSO4) is normally the final product in an oxygen-containing atmosphere [91,92]. The formed CaSO4 is of the type anhydrate II [93] and has a molar volume of 46 cm3, which is 24.7% higher than the molar volume of limestone (calculated as calcite with a molar volume of 36.9 cm3). The percentage of the volume increase is much higher than the normal porosity of a natural limestone. Despite the high molar volume of the product, the product layer formed by the direct sulfation reaction is in fact porous [17,20,11]. The porosity and pore size increase with the temperature and decrease with the conversion [20,11]. The pore size can vary from a few nm to over 100 nm depending on the reaction conditions and the conversion [20,11]. The direct sulfation reaction can often proceed at a fairly high rate even at high conversions [17,20,14,94,9,34,11] mainly due to the porosity in the product layer. Snow et al. [17] suggested that the porosity was caused by the flow of CO2 gas formed by the direct sulfation reaction. This explanation has since then been cited repeatedly by other authors to explain the observation of porosity in the product layer and/or the higher rate of the direct sulfation reaction than the indirect sulfation reaction at higher conversions. However, it is questionable if this explanation is correct. The direct sulfation reaction consumes 1.5 moles of gaseous reactants for each mole of

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released CO2. There is a net inward flow of gas to the reaction front. The outward flow of the CO2 gas is thus diffusional. It is doubtful that this diffusional flow of CO2 can be responsible for the formation of the porosity. 6. Reaction mechanism The detailed mechanism of the direct sulfation reaction is presently not well known. There are only few suggestions presented in the literature. Van Houte et al. [49,95] suggested that the direct sulfation reaction takes place according to the following reaction steps at low temperatures (in the range of 573–900 K): CaCO3 ðsÞ þ SO2 ðgÞ ! CaSO3 ðsÞ þ CO2 ðgÞ,

(6)

2CaSO3 ðsÞ þ O2 ðgÞ ! 2CaSO4 ðsÞ,

(7)

2CaSO3 ðsÞ þ SO2 ðgÞ ! 2CaSO4 ðsÞ þ SðgÞ,

(8)

SðgÞ þ O2 ðgÞ ! SO2 ðgÞ.

(9)

In this mechanism, Reactions (6) and (7) were assumed to be the main reactions. The sulfation process was suggested to be controlled by Reaction (6) or Reaction (7) depending on the reaction conditions. (It needs to be mentioned here that Van Houte et al. [49,95] performed the experiments with gas mixtures containing no CO2. The conditions for the direct sulfation reaction were therefore not ensured. The sulfation experiments on which their suggestion was based might possibly be the indirect sulfation reaction rather than the direct sulfation reaction.) Tullin et al. [19] suggested the following reaction mechanism to explain their experimental observations, in particular, the depressing effect of higher CO2 partial pressures on the direct sulfation reaction: Step 1 : dissociation of CaCO3 : CaCO3 ðsÞ Ð CaOðsÞ þ CO2 ðgÞ.

ð10Þ

Step 2 : Formation of sulfite : CaOðsÞ þ SO2 ðgÞ Ð CaSO3 ðsÞ.

ð11Þ

Step 3 : Oxidation of sulfite : 2CaSO3 ðsÞ þ O2 ðgÞ ! 2CaSO4 ðsÞ.

ð12Þ

In this suggested mechanism, Step 2, the sulfation of CaO, was considered to be the rate-limiting step under the reaction conditions used in the study. The

depressing effect of higher CO2 partial pressures was assumed to be caused by its influence on Step 1, the dissociation of the limestone. The above two different mechanisms are in common represented by a set of overall reactions. However, this kind of method is often not sufficient for the description of gas–solid reaction mechanisms. Gas–solid reactions usually involve gas–solid interactions at the gas–solid interface, phase changes (consumption of the solid reactants and formation of new solid phases) and interactions between the solid phases, which are often important steps in the reaction mechanisms. Special intermediates that are different from the known pure materials are often formed. For example, the escape of a CO2 molecule from the limestone into the gas phase may mean formation of an extra point defect in the crystal lattice of the limestone and does not necessarily mean formation of CaO. Furthermore, it is not always suitable to consider a gas–solid reaction in the same way as for a gas phase reaction. For a gas phase reaction with a multi-step mechanism, the rate of one step may affect the other steps or even control the whole reaction due to the influence of gas concentrations on the forward/reverse reaction or the equilibrium of other steps. The situation becomes different for a gas–solid reaction that also has a multi-step mechanism in light of the fact that pure solids have activities close to 1 independent of their amounts and thus do not have the same effect as gaseous components do when their amounts are changed. A slow step may thus not affect or control its preceding step(s). For example, in the mechanism suggested by Van Houte et al. [49,95], if the solid reactants and products are understood as pure materials in separate phases, the sulfation reaction can only be controlled by Reaction (6). Reaction (7) is only a step that determines the final product. Solid-state diffusion is suggested by a number of authors (Hepola et al. [96], Iisa et al. [8,26] Tullin et al. [19], Fuertes et al. [28], Alvarez et al. [15]) to be the limiting step for the direct sulfation reaction at higher conversions. Fuertes et al. [28] proposed the following reaction mechanism to illustrate the process: Interchange of sulfate and carbonate ions at the interface between the product (CaSO4) layer and the solid reactant (CaCO3): 2
SO2
4 þ CaCO3 ! CaSO4 þ CO3 .

(13)

Diffusion of carbonate ions through the product layer to the surface of the product layer and

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successive dissociation at the surface: 2
CO2
3 ! ðCO2 Þads þ O .

(14)

Formation of sulfate at the surface: O2
þ SO3 ! SO2
4 .

(15)

Desorption of CO2 from the surface: ðCO2 Þabs ! CO2 ðgÞ.

(16)

The mechanism proposed by Fuertes et al. [28] considers both these processes taking place at the gas–solid interface and in the solid phases. This is clearly an improved way to express the mechanism of the direct sulfation reaction. The suggested mechanism itself is, however, incomplete. The step for the formation of SO3 is, for example, missing. There are no descriptions of how diffusion in the solid phases and the adsorption/desorption processes take place. In general, the knowledge of the mechanism of the direct sulfation reaction is presently very limited and highly speculative. Except that the final product is CaSO4, nothing else is confirmed. None of the above mechanisms is able to give satisfactory explanations of the many experimental observations such as the varying reaction order of SO2 under different reaction conditions, the zero-order behavior of O2 at high O2 concentration and the effect of water. 7. Kinetics The studies of the kinetics of the direct sulfation reaction are generally in a quite empirical stage, probably due to the lack of detailed knowledge of the reaction mechanism. In the following sections, the results of the intrinsic reaction rate of the direct sulfation reaction and the different opinions on diffusion in the product layer published in the literature are presented and discussed. 7.1. Intrinsic reaction rate The rate expressions used by different authors are heavily empirical and include often only the influence of SO2 as shown below: rs ¼ ks C nSO2 ðmol=ðm2 sÞÞ.

(17)

The influence of the concentrations of other gases are either incorporated in the rate constant or assumed to be zero order. Due to this empirical method of evaluation, application of the obtained

401

kinetic parameters is often limited to the specific conditions under which they are evaluated. Table 8 shows the rate expressions and the corresponding rate constants obtained by different authors. Due to the different reaction conditions used by the above authors and the different reaction orders of the SO2, it is difficult to make a reasonable comparison of the given rate constants. For the purpose of a rough comparison, the initial sulfation rates at 1123 K are calculated by using the rate expressions and rate constants listed in Table 8 and by assuming a system pressure of 0.1 MPa and an SO2 concentration of 3000 ppm. The results of the calculations are listed in Table 8. As shown in the table, the initial reaction rates evaluated at 1123 K by the different authors are not consistent but lie in the same order of magnitude. The inconsistence may be caused by a number of factors, such as differences in the chemical/physical properties of the used limestones, differences in the concentrations of CO2 and O2 and the surface areas used for the calculation of the rates. Among these factors, the surface area is a factor that can often cause relatively large errors, if it is not treated properly. The surface area used for the evaluation of the reaction rate should be equal to the surface area at which the sulfation reaction takes place. With porous particles, there is a risk of overestimation of the reaction rate by using the outer surface area for the evaluation, as the reaction may actually also take place in the pores, which is often the case, especially at lower temperatures. Krishnan [9], Zhong [10] and Qiu et al. [12] used the outer surface area for the evaluation of the reaction rate, which most likely is the reason for the much high reaction rates that they obtained. 7.2. Diffusion in the product layer At higher conversions, the sulfation process can be controlled by diffusion in the product layer. There are two different views concerning the type of diffusion in the product layer that controls the sulfation process. One is gas phase diffusion in the pores, as was suggested by Hajaligol et al. [20], Krishnan et al. [9] and Liu et al. [11], mainly based on the fact that the product layer is porous. The other one is solid-state diffusion, as suggested by Hepola et al. [96], Iisa et al. [8,26] Tullin et al. [19], Fuertes et al. [28] and Alvarez et al. [15], mainly based on the observed large apparent activation

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Table 8 Rate expressions and rate constants of the direct sulfation reaction obtained by different authors Author

Experimental condition

Rate expression (mol/ (m2 s))

Rate constant, ks

Estimated sulfation rate at 1123 K and 3000 ppm SO2 (mol/(m2 s))

Snow et al. [17]

T: 773–1373 K; P: 0.1 MPa

ks C SO2

0.72e(
64046/(8.314T)), m/s

2.5  10
5

ks C SO2

1.5e(
68650/(8.314T)), m/s

3.2  10
5

Gas composition: SO2: 0.3%; O2: 5%; CO2: 2–95% Particle size: 2–106 mm Hajaligol et al. [20]

T: 773–1213 K; P: 0.1 MPa Gas composition: SO2: 0.3%; O2: 5%; CO2: 95% Particle size: 2–106 mm

Fuertes et al. [27]

T: 923–1173 K; P: 0.1 MPa Gas composition: SO2: 0.25%; O2: 3.6%; CO2: 96.4% Particle size: 2–106 mm

ks pSO2

104e(
95700/(8.314T)), mol/(m2 s atm)

1.1  10
5

Krishnan [9]

T: 1123 K; P: 0.1 MPa

ks C 0:4 SO2

0.00031–0.0015, mol0.6/(m0.8 s)

7.9–39  10
5

ks C SO2

0.0049, m/s (Ea ¼ 35.9 kJ/mol)

19  10
5

ks C SO2

0.0007–0.0014, m/s

2.3–4.6  10
5

Gas composition: SO2: 0.15–0.6%; O2: 6%; CO2: 96.4% Particle size: 53–350 mm Zhong [10]

T: 1073 K; P: 0.1 MPa Gas composition: SO2: 0.1–0.5%; O2: 10%; CO2:70% Particle size: 4–5.4 mm

Zevenhoven et al. [29]

T: 1123 K; P: 1.5 MPa Gas composition: SO2: 0.3%; O2: 4%; CO2: 20% Particle size: 250–300 mm

Alvarez et al. [15]

T: 1073–1198 K; P: 1.2–2.5 MPa Gas composition: SO2: 0.5%; O2: 3–7%; CO2:12%–15% Particle size: 100–595 mm

ks C SO2

0.00011 m/s at 1073 K 0.0003 m/s at 1198 K (Ea ¼ 87.2 kJ/mol)

5.6  10
5

Liu et al. [11]

T: 883–1123 K; P: 1.5 MPa

ks C SO2

19e(
90000/(8.314T)), m/s

4.1  10
5

ks C 0:58 SO2

0.00015, kmol0.42/ (m0.26 s)

38  10
5

Gas composition: SO2: 0–0.24%; O2:10%; CO2:20–80% Particle size: 8.4–54 mm Qiu et al. [12]

T: 1123 K; P: 0.6 MPa Gas composition: SO2: 0.35%; O2: 5%; CO2: 30% Particle size: 125–180 mm

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403

Table 9 Effective diffusivity obtained by different authors Author

Experimental condition

Model used for evaluation of the effective diffusivity

Effective diffusivity at 1123 K (m2/s)

Hajaligol et al. [20]

T: 773–1213 K; P: 0.1 MPa Gas composition: SO2: 0.3%; O2: 5%; CO2: 95% Particle size: 2–106 mm

Shrinking unreacted core model

1.5  10
6 (De ¼ 9.96e(
146510/(8.314 T)))

Iisa et al. [14]

T: 1123 K; P: 1.5 MPa Gas composition: SO2: 0.3%; O2: 4%; CO2: 15% Particle size: 125–180 mm

Shrinking unreacted core model

0.6–4  10
10

Iisa et al. [94]

T: 1133 K; P: 0.8–2 MPa Gas composition: SO2: 0.3%; O2: 4%; CO2: 15% Particle size: 150 mm

Shrinking unreacted core model

2–4  10
10 (at 1133 K)

Fuertes et al. [28]

T: 923–1173 K; P: 0.1 MPa Gas composition: SO2: 0.25%; O2: 3.6%; CO2: 96.4% Particle size: 2–106 mm

Shrinking unreacted core model

1.3  10
9 (De ¼ 0.0086e(
146500/(8.314T)))

Zhong [10]

T: 1073K; P: 0.1 MPa Gas composition: SO2: 0.1–0.5%; O2:10%; CO2:70% Particle size: 4–5.4%

Shrinking unreacted core model

2.1  10
9a

Zevenhoven et al. [97]

T: 1123–1223 K; P: 1.5 MPa Gas composition: SO2: 0.3%; O2: 4%; CO2: 20% Particle size: 250–300 mm

Shrinking unreacted core model Changing internal surface model

6.6
102  10
10 1.8
16.8  10
15

Alvarez et al. [15]

T: 1073–1198 K; P: 1.2–2.5 MPa Gas composition: SO2: 0.5%; O2: 3–7%; CO2:12–15% Particle size: 100–595 mm

Shrinking unreacted core model

6  10
13

Liu et al. [11]

T: 883–1123 K; P: 1.5 MPa Gas composition: SO2: 0–0.24%; O2:10%; CO2:20–80% Particle size: 8.4–54 mm

Shrinking unreacted core model

9.1  10
10 (De ¼ 6.71  10
6e(
10000/T))

Qiu et al. [12]

T: 1023–1123K; P: 1.3 MPa Gas composition: SO2: 0.16%; O2: 5%; CO2: 14% Particle size: 125–180 mm

Shrinking unreacted core model

0.1–1.0  10
9

a

Calculated by using the activation energies obtained by the respective authors.

energies (listed in Table 2) and the magnitude of the effective diffusivities, which are of the same order of magnitude as those of solid-state diffusion. Table 9 shows the effective diffusivities obtained via model simulations by different authors. As shown in the above table, except for the values obtained by Alvarez et al. [15], Hajaligol et al. [20] and Zevenhoven et al. [97] with the changing internal surface (CIS) model (a model considering

the internal surface area as well), the effective diffusivities obtained by the different authors are generally of the same order of magnitude, in the range of 10
910. It indicates strongly that it is solid-state diffusion in play rather than gas phase diffusion. In order to clarify whether the diffusion is gas phase diffusion or solid-state diffusion, Iisa et al. [8] investigated the influence of temperature on the

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further sulfation of presulfated limestone particles. They observed an apparent activation energy of approximately 120 kJ/mol, which is too high to justify gas phase diffusion control. There are apparently more arguments for solid-state diffusion control. Although the high apparent activation energies obtained by Iisa et al. [8] and other authors provide good support for solid-state diffusion control, the fact that the product layer is porous raises still questions, like: how can the reaction be controlled by solid-state diffusion in a porous layer? Diffusion in the product layer appears to be more complicated than imagined. Based on model simulations, it was observed by a number of authors (Hajaligol et al. [20], Krishnan et al. [9], Alvarez et al. [15] and Qiu et al. [12]) that the effective diffusivity decreases with the increase of the conversion and the SO2 concentration. Tullin et al. [19] and Qiu et al. [12] also observed that the activation energy of the effective diffusivity increases significantly with the increase of the conversion. It is clear that there are much more things about diffusion in the product layer that need to be known. 7.3. Modeling The shrinking unreacted core model [30,98] is the most frequently used model for the description of the kinetics of the direct sulfation reaction [8–12,17,20, 26,19,28,29,15]. Few authors tried other approaches, like ‘‘parallel pore model’’ [30] by Spartinos et al. [7] and CIS model by Zevenhoven et al. [7]. The shrinking unreacted core model is a simplified model for gas–solid reactions, which assumes a sharp boundary between the unreacted core and the formed product layer. This assumption is often valid for poreless particles, but not always for porous particles [30,33,93,99,100]. For a porous limestone particle, it depends on the actual reaction conditions as to whether the sulfation reaction proceeds with a sharp boundary between the product layer and the unreacted core. This is well demonstrated by the observations of Illerup et al. [18]. Illerup et al. [18] studied the direct sulfation reaction of Stevns Chalk. The size of the limestone particles was 0.85–1 mm. It was observed that the sulfation reaction took place in a shrinking unreacted core pattern at 1123 K, but not at 1023 K. At 1023 K, the sulfation reaction took place throughout the particles.

In studies of reaction kinetics, models are often used for evaluation of kinetic parameters, as well as for process simulations. The proper choice of model is therefore critical for both the extraction of the right kinetic parameters from experimental data and optimal process simulations. The strong influence of the models on the extracted kinetic parameters is well demonstrated by Zevenhoven et al. [97]. Zevenhoven et al. [97] used a CIS model—a model based on similar principles to the random pore model of Bhatia et al. [101,102]—to compare the results obtained by the shrinking unreacted core model. The model simulation, performed by assuming negligible diffusion resistance in the pores of the particles, shows that the rate constants and the effective diffusivities obtained from the CIS model are approximately 3 and 5 orders of magnitude lower, respectively, than the results obtained from the shrinking unreacted core model. These results indicate that to obtain the right kinetic parameters, it is important to ensure that the applied model is in accordance with the actual sulfation pattern in the full range of the applied reaction conditions. The scattering of the sulfation rates and the apparent activation energies that were obtained by the different authors may be partly caused by the mismatch between the applied models and the actual sulfation patterns, as a number of authors (Tullin et al. [19], Fuertes et al. [28], Zhong [10], Alvarez et al. [15], Liu et al. [11] and Qiu et al. [12]), for example, chose the shrinking unreacted core model without having a clear indication for it. 8. Conclusions The direct sulfation reaction can be significantly affected by all the gaseous reactants and products (SO2, O2 and CO2) as well as water. The degree of influence of each of these gases varies with reaction conditions. The apparent reaction order of oxygen usually goes to zero at high concentrations. Higher CO2 concentrations can significantly hinder the direct sulfation reaction, most likely via its influence on the solid-state mobility. Higher temperatures and higher system pressures may adversely affect the direct sulfation reaction. The adverse influence of higher temperatures is due to sintering of the solid reactant (the limestone) and the product layer (CaSO4), which usually becomes significant at temperatures that are a couple of hundreds of degree higher than their Tammann temperatures. Higher system pressures may adversely

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influence the direct sulfation reaction by its negative influence on both gas phase diffusion and solid-state diffusion (through the influence of CO2). The direct sulfation reaction can be enhanced by additives such as alkali metal salts. The enhancements by the additives are related to their capabilities in improving ionic movement in the solid phases via formation of more point defects and eutectics. The conversion rates of different limestones often vary over a wide range. This is mainly because of the dependence of the conversion rate on the total surface area of the limestone particles, which often vary tremendously with different limestones. The conversion rate of a porous limestone is therefore normally higher than that of a dense limestone. The solid product from the direct sulfation reaction is calcium sulfate (CaSO4). This product layer of calcium sulfate is porous. The porosity is the main reason for the relatively high conversion rate (compared to the indirect sulfation reaction) obtainable at high conversions. The porosity of the product layer increases with the temperature but decreases with the conversion. The direct sulfation reaction may proceed in a shrinking unreacted core pattern, within the entire particle volume or in a way in-between the above two extreme situations depending on the reaction conditions, such as particle size, particle morphology and temperature. The shrinking unreacted core pattern often occurs at high temperatures or with dense particles. Although large amount of knowledge has been obtained concerning the direct sulfation reaction, the detailed mechanism of this reaction is, however, not well known. Accordingly, many experimental observations are not explained or are not explained satisfactorily, such as:



  

The variations in the apparent reaction orders of SO2 and O2 with the reaction conditions and the zero-order behavior of O2 at higher concentrations. The influence of water. The reasons for the porosity in the product layer and its variation with the temperature and the conversion. The variations in the effective diffusivity and its apparent activation energy with the conversion.

The intrinsic rate expressions available in the literature are generally empirical. The reaction is

405

often assumed to be first order with respect to the concentration of SO2. The influences of O2 and CO2 are either assumed to be zero order or incorporated into the kinetic parameters. The heavily empiric nature of the kinetic studies means that the applicability of the published kinetic data is greatly limited. To improve this situation, more research work is necessary, particularly concerning the detailed mechanism of the direct sulfation reaction. A good understanding of the reaction mechanism will be greatly beneficial for reducing the empirical nature in the future kinetic studies. Acknowledgements This work was carried out in the Combustion and Harmful Emission Control (CHEC) research centre at the Department of Chemical Engineering, Technical University of Denmark, and financially supported by the Technical University of Denmark and FLSmidth A/S. References [1] [2] [3] [4] [5] [6]

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