Investigation on water vapor effect on direct sulfation during wet-recycle oxy-coal combustion

Applied Energy 108 (2013) 121–127

Contents lists available at SciVerse ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Investigation on water vapor effect on direct sulfation during wet-recycle oxy-coal combustion Lunbo Duan ⇑, Zhongxiao Jiang, Xiaoping Chen, Changsui Zhao Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China

h i g h l i g h t s  Effect of water vapor on direct sulfation during wet-recycle oxy-coal combustion was investigated.  The presence of water vapor promotes the calcium conversion of limestone during direct sulfation process.  Pore structure of limestone is improved due to the presence of water vapor.  No intermediate product is generated.

a r t i c l e

i n f o

Article history: Received 30 October 2012 Received in revised form 7 March 2013 Accepted 10 March 2013 Available online 2 April 2013 Keywords: O2/CO2 atmosphere Water vapor Direct sulfation Pore structure character X-ray diffraction

a b s t r a c t During oxy-coal combustion process, limestone desulfurization may change from indirect into direct sulfation due to the high partial pressure of CO2. When the wet flue gas is recycled, the water vapor will also be enriched in the furnace and affect the desulfurization reactions. In the paper, two limestone sorbents were used to study the effect of water vapor on direct sulfation. Parameters including water vapor concentration, temperature and SO2 concentration were analyzed. Results show that the presence of water vapor has a negligible effect on the direct sulfation during the kinetically-controlled regime, while enhances the calcium conversion during the diffusion-controlled stage. The presence of water vapor promotes the solid-state diffusion of the sulfated product, and sintering of the product layer is intensified. The vacancies in the particles migrate along crystal grain boundaries and through the crystal lattice, resulting in a reverse flow of mass into the pores. The pore structure character of the products sulfated in the presence of water vapor is improved. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction

CaOðsÞ þ SO2 ðgÞ þ 1=2O2 ðgÞ ! CaSO4 ðsÞ

The utilization of coal in power plants generates a large amount of CO2, which is the chief contributor to global warming [1,2]. Oxy-fuel combustion of fossil fuel provides a viable means to reduce anthropogenic greenhouse gas emission from coal-fired power plants [3–8]. At the same time, SO2 emissions may also be simultaneously reduced by injecting the sorbent into the oxy-fuel combustion furnace [9–12]. In conventional combustion, the sulfation of limestone can be divided into two global reaction steps [13]. The limestone initially undergoes calcination to form lime (CaO) before reacting with SO2 as shown in Formulas (1) and (2). The overall process is known as indirect sulfation. The calcium conversion of indirect sulfation is originally very small due to sintering of CaO and blocking of pores on the surface of sorbents.

An oxy-fuel combustion boiler necessitates high recirculation ratio of flue gas to reduce the furnace temperature. The concentration of CO2 is enhanced in the furnace and can come up to more than 90 vol.% (dry basis). At the combustion temperature, limestone is in dynamic equilibrium with its calcined products, lime and CO2 [14]. For elevated CO2 concentration, the limestone calcination will not occur at lower temperature, and the sulfation of sorbents can occur between CaCO3 and SO2 directly. The reaction step can be represented by the following overall formula:

CaCO3 ðsÞ ! CaOðsÞ þ CO2 ðgÞ ⇑ Corresponding author. Tel./fax: +86 25 83790147. E-mail address: [email protected] (L. Duan). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.03.022

ð1Þ

CaCO3 ðsÞ þ SO2 ðgÞ þ 1=2O2 ðgÞ ! CaSO4 ðsÞ þ CO2 ðgÞ

ð2Þ

ð3Þ

Sulfation is characterized by two distinct reaction regimes: a first regime in which the sulfation rate is mainly controlled by kinetics of chemical reaction, and a second one where the rate drops substantially as the control switches to a diffusion-limited process once a product layer (CaSO4) forms and covers the inner surface of larger pores, and plugs smaller pores, due to the higher molar volume of CaSO4 [15].

122

L. Duan et al. / Applied Energy 108 (2013) 121–127

Table 1 XRF analysis of limestone samples (mass fraction (%)). Samples

CaO

SiO2

Al2O3

MgO

Fe2O3

K2O

Na2O

TiO2

Others

LOI

American limestone Chinese limestone

55.990 57.410

0.392 1.920

0.100 0.795

0.538 0.744

0.050 0.388

0.015 0.083

0.041 0.059

0.004 0.040

0.100 0.091

42.770 38.470

There are two different modes of flue gas recycle according to whether condensing the water vapor in the flue gas or not, namely ‘‘dry-recycle’’ and ‘‘wet-recycle’’, and different gas composition in the furnace can be expected. Under the wet-recycle condition, the overall moisture content inside the boiler can reach 45 vol.%. Despite many comprehensive studies of limestone sulfation, there are still uncertainties and disagreements on this subject. The effect of water vapor on sulfation is relatively little investigated, especially for direct sulfation. Wang et al. [16,17] studied the effect of water vapor on direct and indirect sulfation using a TGA method. The results showed that the conversion of calcium was enhanced remarkably when the concentration of water vapor rose to 10%. The author considered that the transient Ca(OH)2 was the cause of the enhancement of calcium conversion. The activation energy of forming Ca(OH)2 was much lower than that of the reaction between CaO and SO2. The author did not investigate the characteristics of the sulfated products, and the mechanism was obscure. Therefore we have made an additional study of the mechanism of sulfation. Stewart et al. [13] employed the TGA and a tube furnace to investigate the effect of water vapor on indirect sulfation. The author found a significant difference between different regimes. There was no effect during the kinetically-controlled stage. Nevertheless, the enhancement was very remarkable during the diffusion-controlled regime. Suyadal et al. [18] had different viewpoints. They investigated the effect of water vapor on indirect sulfation in a bubbling fluidized bed and found that the calcium conversion degraded as the concentration of water vapor was elevated from 0% to 5%. The presence of water vapor had a negative effect on the sulfation of limestone sorbents. However, the mechanism was not discussed. In summary, the mechanism of limestone desulfurization in the presence of water vapor is still not clear now, especially for the direct sulfation process. In this paper, the effect of water vapor on the direct sulfation process of limestone is studied and parameters like water vapor concentration, temperature and SO2 concentration are analyzed.

analysis results of the two limestone sorbents are given in Table 1. The samples were ground and sieved to a particle size less than 100 lm.

2.2. Experiments Sulfation experiment of these two limestone sorbents using a synthetic flue gas with a varying concentration of water vapor was conducted in a tube furnace during 180 min. A diagram of the tube furnace and the ancillary equipment is shown in Fig. 1. The ceramic column with an electrical heating jacket, which is the inner layer of the tube furnace, is 80 mm ID with 1.2 m length. Compressed gas cylinders, a water vapor generator and a syringe pump were employed to produce the synthetic flue gas. The water vapor pipeline was heated with a heating jacket capable of heating to more than 150 °C to ensure that no condensation of water vapor occurred. The temperature in the furnace was monitored by an automatic controller with a range of 20–1200 °C. The water flux rate and the temperature were calibrated before each experiment. Two ceramic pans with limestone samples spread in them were crammed into the central section of the tube after the temperature and atmosphere had been constant for 1 h. During the experiment, the samples were taken out to measure the weight increase using an electronic balance. At the end of each experiment, the samples were cooled to atmospheric temperature in a hermetic drying basin for nitrogen adsorption and X-ray diffraction (XRD) analysis. The total gas flow rate during the sulfation was 1.5 mL/min. The experimental conditions were given in Table 2. Formula (4) is employed to calculate the calcium conversion of samples, where X is conversion of calcium; mt means the sample

2. Experimental section 2.1. Limestone samples Two limestone sorbents were used to investigate the effect of water vapor on direct sulfation. The X-ray fluorescence (XRF)

Fig. 1. The system of tube furnace.

Table 2 Operating conditions for experiments (gas concentration in vol. (%)). Temperature (°C)

750/800/850

O2 (%) SO2 (%) H2O(g) (%) CO2 (%)

7 3/2/1/0.5 0/10/20/30/40 Balance

123

L. Duan et al. / Applied Energy 108 (2013) 121–127

mass at the time t; m0 represents the original sample mass; A is the mass fraction of CaCO3 in the original samples; W CaCO3 and W CaSO4 represent the mole masses of CaCO3 and CaSO4.

W CaCO3 mt  m0 m0 A W CaSO4  W CaCO3

ð4Þ

3. Results and discussion 3.1. Effect of water vapor on direct sulfation

Calcium conversion / %

Fig. 2 shows how the calcium conversion of American limestone and Chinese limestone varies with H2O(g) concentration. The calcium conversion is defined as the amount of calcium carbonate in the sample that is converted to CaSO4. It is observed that the rate of calcium conversion of the two limestone sorbents declines gradually with reaction time. This phenomenon indicates that the coverage of CaSO4 on the surface and the blocking of pores degrades the reaction rate. From the profiles, it is seen that the American limestone achieves higher calcium conversion during a short time. However, when the reaction time reaches 180 min, the degree of sulfation of the Chinese limestone has become greater than for the American one. The larger calcium carbonate fraction of the American limestone is the main reason for higher conversion at the initial stage. Another reason for lower conversion of the Chinese limestone is the lower initial surface areas caused by larger iron oxides fraction. The final higher conversion of the Chinese limestone may be attributed to the catalytic effect of alkali metal salts [19]. The experimental results have a slight difference from the TGA curves obtained by Wang et al. [17]. The calcium conversion of the two sorbents does not change perceptibly with elevated concentration of water vapor when the sulfation is carried out for 30 min. It appears that water vapor does not have any effect during the kinetically-controlled regime. The presence of H2O(g) obviously enhances the calcium conversion of the two limestone sorbents when the reaction is conducted for 180 min due to the negligible effect of the CO2 concentration in direct sulfation [20]. Addition of 10% H2O(g) results in an enhancement of 6.4% in the calcium conversion comparing to that of 0% H2O(g) for American limestone. The calcium conversion of Chinese limestone reaches 89.0%, equivalent to an increase in conversion of 18.0%. However,

100

100

80

80

60

0 % H2O 10 % H2O

40

20 % H2O 30 % H2O

20

0

40 % H2O

0

30

60

90

120

Time / min

(a) American limestone

150

180

Calcium conversion / %

X¼

the effect is not so strong when the concentration is increased to a higher level. The conversion of American and Chinese limestones with 40% H2O(g) is 85.6% and 95.8%, respectively, equivalent to an enhancement of 8.0% and 6.8% compared to those with 10% H2O(g). The results indicate that the influence of water vapor on the direct sulfation occurs in the diffusion-controlled regime, the later period of reaction. As the particle surface is covered by a compact product layer, gas–solid diffusion is suppressed to a large extent, and the action of solid state diffusion may become obvious. According to the hypothesis suggested by Fuertes et al. [21], the exchange of SO2 and CO2 will occur on the interface of the 4 3 product layer and the unreacted inner core. The presence of H2O(g) has a positive effect on solid-state diffusion on the product surface and promotes the ion exchange. Recrystallization and sintering of the sulfated product are usually strengthened along with solid state diffusion. The Tammann temperature is always used to determine the degree of sintering of crystal solids, and it equals half the absolute melting point of the solid compound. Tammann temperature corresponds to the temperature at which vacancies start to migrate at an appreciable rate in the crystal lattice. The melting point of CaSO4 given by a number of researchers is about 1450 °C, which can give a Tammann temperature of 588 °C [22]. It is observed that the sintering phenomenon has already occurred during the progress of sulfation. The presence of water vapor aggravates the sintering of CaSO4 crystals on the particle surface. The vacancies in the particle migrate along crystal grain boundaries and through the crystal lattice at a drastic rate, resulting in a reverse flow of mass into the pores. Stewart et al. discovered that, for a given conversion, the sulfated product in the presence of H2O(g) had a higher skeletal density than that sulfated without H2O(g) using gas pycnometry [13]. The phenomenon obtained by Stewart is consistent with our results, because a process of recrystallization and sintering can increase the density of the sulfated product [23]. On the other hand, the average CaSO4 crystal grain-size generated in the presence of water vapor is larger in diameter than that formed without water vapor, both on the particle outer surface and well within the bulk of the particles. The cracks between larger crystals can lower the resistance of diffusion, and will be more plentiful for the reaction of a gaseous reactant with the sorbent. The reaction rate is promoted and the calcium conversion is enhanced.

60

0 % H2O

40

10 % H2O 20 % H2O

20

30 % H2O 40 % H2O

0

0

30

60

90

120

Time / min

(b) Chinese limestone

Fig. 2. The conversion of calcium in the limestone with varying water vapor concentration.

150

180

124

L. Duan et al. / Applied Energy 108 (2013) 121–127 2

-1

SBET/ (m ·g )

3

-1

VBJH/ (cm ·g )

-1

3

-1

VBJH/ (cm ·g )

dPore / nm

73

0.007

63

0.8 63

0.6

0.005 53

0.005

53

0.6 43

43

0.4

33

0.003

0.4

33

0.003

23

23

0.2

0.2 13

0.001 0.0

2

SBET/ (m ·g )

dPore / nm

3 BET Surface Area

BJH Pore Volume

13

0.001 0.0 Average Pore Diameter

3 BET Surface Area

(a) American limestone

BJH Pore Volume

Average Pore Diameter

(b) Chinese limestone

Fig. 3. Characteristic of pore structure of direct sulfation products.

3.2. Nitrogen adsorption analysis

3.3. X-ray diffraction analysis

The surface area, pore volume and average pore diameter in 40% and 0% H2O(g) atmosphere are shown in Fig. 3. The reaction temperature and time are 850 °C and 180 min. Two kinds of sulfated product present similar pore structure characteristics. The surface area, the pore volume and the average diameter of the products sulfated in the presence of water vapor are improved in comparison with that in a dry atmosphere. As the sulfation reaction proceeds, the surface of CaCO3 crystals will be covered with CaSO4 fleetly. As Stewart suggested, all the visible surfaces of the CaCO3 particles can be seen covered by a layer of crystals packed tightly even when the conversion is 10% [13]. The presence of water vapor promotes the solid-state diffusion and enhances the calcium conversion. The escape of CO2 during the direct sulfation results in a greater porosity of the product in the presence of water vapor. Due to the larger mole volume of CaSO4, the crystal grain size formed in the presence of water vapor is larger, and there are more cracks between crystals. Larger cracks promote gaseous diffusion and provide more reaction sites for the reactant with the sorbents. The results indicate that water vapor can improve the pore structure characteristics of product layer for the direct sulfation reaction.

Fig. 4 shows the maps of XRD analysis of the products of the sulfation of American and Chinese limestones after the reaction time of 30 min. It is observed from the results that only peaks corresponding to CaCO3 and CaSO4 can be detected. No CaO peak is present prove that the calcium carbonate reacts with SO2 directly without calcination. All the sulfation products were cooled to lower temperature than the decomposition temperature of Ca(OH)2 before taken out from the furnace. The absence of a peak corresponding to Ca(OH)2 illustrates that no intermediates such as Ca(OH)2 are generated during the sulfation. From the map, diffracted intensity of the principal peak of CaSO4 (2h = 25.36) is pretty much the same with the two atmospheres. The results indicate that the calcium conversion is not enhanced in the presence of water vapor and the effect of water vapor is negligible in the initial stage. Fig. 5 is the XRD results of the products after 180 min. The main component in the products is CaSO4 for two sorbents with H2O(g). The diffracted intensity is much higher than that without water vapor. CaCO3 still occupies a large share of products in the dry atmosphere after the reaction. It indicates that the presence of water vapor promotes the direct sulfation of the two limestone sorbents.

8000

8000 2

6000

2

6000

1 2

1

4000

CaSO4 CaCO3

4000

1 2

CaSO4 CaCO3

1 2

CaSO4 CaCO3

70

80

1

2000

1 11

1

1

CPS

CPS

2000 80000 6000

2

1 2

1

4000

1

1

1

2

6000

CaSO4 CaCO3

1 1

80000

4000 1

2000

2000

1 11

0 10

20

30

40

1

1

50

60

70

80

90

0 10

20

1

11

1

30

40

50

1

60

2 THETA

2 THETA

(a) American limestone

(b) Chinese limestone

Fig. 4. XRD analysis of products at 30 min.

90

125

L. Duan et al. / Applied Energy 108 (2013) 121–127

8000

8000 1

1

6000

6000

1 2

4000 2

1 11

1

4000

1

80000

1 2

1 2

4000

0 10

20

30

1

40

60

70

80

1 2

CaSO4 CaCO3

70

80

1

1 2

4000

1

1

50

CaSO4 CaCO3

0 8000

2000

11

1

6000

CaSO4 CaCO3

1

2000

11 2

6000

1 2 1

2000

CPS

CPS

2000

CaSO4 CaCO3

0 10

90

20

30

11

1

40

1

50

60

2 THETA

2 THETA

(a) American limestone

(b) Chinese limestone

90

100

100

80

80

Calcium conversion / %

Calcium conversion / %

Fig. 5. XRD analysis of products at 180 min.

60

40

40

20

20

0

60

0

30

60

90

120

150

180

0

0

30

60

90

120

Time / min

Time / min

(a) American limestone

(b) Chinese limestone

150

180

Fig. 6. The conversion of calcium in the limestone at various temperatures.

The results of the XRD analysis validate the experiment results further.

3.4. Effect of temperature on direct sulfation in the presence of water vapor Fig. 6 shows the two sorbents at various temperatures in 40% H2O(g) + 3% SO2 atmosphere. Higher temperature has a positive effect on the direct sulfation. In comparison with the calcium conversion at 850 °C, the calcium conversions of the two sorbents are 64.1% and 55.8% when the temperature reduces to 750 °C, equivalent to a decrease in conversion of 21.5% and 39.9%, respectively. The sulfation becomes more intensive at 850 °C. It is observed that at 850 °C with 0% H2O(g) the calcium conversion of the American limestone is higher than that at 800 °C with 40% H2O(g) when the reaction time is 30 min. Nevertheless, at 180 min, the former is lower than the latter. The Chinese limestone behaves similar to the other sorbent. The sulfation reaction proceeds more fully at 850 °C with 0% H2O(g) at 30 min compared to 40% H2O(g)

atmosphere at 800 °C, and after 180 min the conversions of two atmospheres become nearly the same.

3.5. Effect of SO2 concentration during direct sulfation in the presence of water vapor SO2 concentration is bound to affect the reaction rate due to its direct involvement in the limestone’s sulfation reaction. Possible variations in fuel sulfur between 0.3% and 6% lead to a wide range of SO2 concentrations in the power plant, so it is necessary to investigate the effect of SO2 concentration. Fig. 7 shows the conversion of the two kinds of limestone with varying SO2 concentration. The atmosphere consists of 40% H2O(g) with 40% CO2, and O2 is employed as balance. O2 concentration has a negligible effect on direct sulfation when it passes beyond 5% as Chen et al. [24] suggested. Results indicate that the reaction rate is enhanced as the concentration of SO2 increases. The calcium conversion of the two sorbents increases from 57.3% to 85.6% and 56.5% to 95.8%, respectively, with the enhancement of SO2 concentration from 0.5% to 3% after 180 min. At higher concentration sulfation leads

L. Duan et al. / Applied Energy 108 (2013) 121–127

100

100

80

80

60

40

0.5% SO2,40% H2O 1% SO2,40% H2O 2% SO2,40% H2O

20

3% SO2,40% H2O

Calcium conversion / %

Calcium conversion / %

126

60

40 0.5% SO2,40% H2O 1% SO2,40% H2O 2% SO2,40% H2O

20

3% SO2,40% H2O 3% SO2,0% H2O

3% SO2,0% H2O

0

0

30

60

90

120

Time / min

150

180

0

0

(a) American limestone

30

60

90

120

Time / min

150

180

(b) Chinese limestone

Fig. 7. The conversion of calcium in the limestone with varying SO2 concentration.

to the enrichment of SO2 on the surface of the limestone sorbents and promotes the diffusion of gaseous reactant in the gas film and the product layer. Calcium conversions of these two sorbents in the 0% H2O(g) + 3% SO2 atmosphere are significantly higher than that of 40% H2O(g) + 2% SO2 for 30 min reaction time. However, when the reaction proceeds to 180 min an interesting phenomenon is observed. Due to the promoting action of water vapor, the conversions in the 0% H2O(g) + 3% SO2 atmosphere are lower than that of 2% SO2 with 40%H2O(g). This interesting phenomenon also illustrates that the effect of water vapor is mainly seen in the later diffusion-controlled stage. Due to the higher reaction rate in the early period, the calcium conversion in the 40% H2O(g) + 2% SO2 atmosphere should be higher than that in dry atmosphere at 30 min if water vapor participates in the chemical reaction. 4. Conclusion (1) The conversion of the calcium in the limestone increases at higher water vapor concentration at the same temperature and SO2 concentration. The presence of water vapor has a negligible effect on the direct sulfation during the kinetically-controlled regime and enhances the calcium conversion during the diffusion-controlled stage. (2) The pore structure of the sulfation products is different in the presence of water vapor or not. The surface area, pore volume and average diameter of the product sulfated in the presence of H2O(g) increase. (3) Results of XRD further validate the experiment results. When the reaction proceeds for 30 min, the peak corresponding to CaSO4 is pretty the same, however, the peak is enhanced heavily in the presence of water vapor at 180 min. (4) Temperature has a remarkable influence on direct sulfation of limestone sorbents in the presence of water vapor. The calcium conversion increases as the temperature is elevated. SO2 concentration also affects the reaction, as it does in the enhancement of calcium conversion with elevated SO2 concentration.

Acknowledgements This work was financially supported by Project supported by the National Natural Science Foundation of China (51206023), by

the Special Funds for Major State Basic Research Projects of China (2011CB707301-3). References [1] Hu YK, Li HL, Yan JY. Techno-economic evaluation of the evaporative gas turbine cycle with different CO2 capture options. Appl Energ 2012;89:303–14. [2] Bolea I, Romeo LM, Pallarés D. The role of external heat exchangers in oxy-fuel circulating fluidized bed. Appl Energ 2012;94:215–23. [3] Tan Y, Jia L, Wu Y, Anthony EJ. Experiences and results on a 0.8MWth oxy-fuel operation pilot-scale circulating fluidized bed. Appl Energ 2012;92:343–7. [4] Wang CA, Zhang XM, Liu YH, Che DF. Pyrolysis and combustion characteristics of coals in oxyfuel combustion. Appl Energ 2012;97:264–73. [5] Buhre BJP, Elliott LK, Sheng CD, Gupta RP, Wall TF. Oxy-fuel combustion technology for coal-fired power generation. Prog Energ Combust Sci 2005;31:283–307. [6] Gil MV, Riaza J, Álvarez L, Pevida C, Pis JJ, Rubiera F. Oxy-fuel combustion kinetics and morphology of coal chars obtained in N2 and CO2 atmospheres in an entrained flow reactor. Appl Energ 2012;91:67–74. [7] Wu YH, Wang CB, Tan YW, Jia LF, Anthony EJ. Characterization of ashes from a 100 kWth pilot-scale circulating fluidized bed with oxy-fuel combustion. Appl Energ 2011;88:2940–8. [8] Molina A, Shaddix CRP. Ignition and devolatilization of pulverized bituminous coal particles during oxygen/carbon dioxide coal combustion. Combust Inst 2007;31:1905–12. [9] Shimazaki Y, Akisawa A, Kashiwagi T. A model analysis of clean development mechanisms to reduce both CO2 and SO2 emissions between Japan and China. Appl Energ 2000;66:311–24. [10] Kim H, Park D. Reactivity of calcined limestone and dolomite with sulfur dioxides. Korean J Chem Eng 1987;4:143–8. [11] Li QZ. Investigation on pulverized coal combustion and SO2/NO emission characteristic under O2/CO2 environment. Nanjing: Southeast University; 2009. [12] Li YR, Yang LZ, You CF, Qi HY. Sulfation diffusion model for SO2 capture on the T–T sorbent at moderate temperatures. Korean J Chem Eng 2009;26:1155–7759. [13] Stewart MC, Manovic V, Anthony EJ, Macchi A. Enhancement of indirect sulfation of limestone by steam addition. Environ Sci Technol 2011;44:8781–6. [14] Baker EH. The calcium oxide–carbon dioxide system in the pressure range 1– 300 atm. J Chem Soc 1962;87:464–70. [15] Hu GL, Johansen KD, Wedel S. Direct sulfation of limestone. AIChE J 2007;53:949–60. [16] Wang CB, Jia LF, Tan Y, Anthony EJ. The effect of water on the sulfation of limestone. Fuel 2010;89:2628–32. [17] Wang CB, Jia LF, Tan Y, Anthony EJ. Influence of water vapor on the direct sulfation of limestone under simulated oxy-fuel fluidized-bed combustion conditions. Energy Fuels 2011;25:617–23. [18] Suyadal Y, Erol M, Og’uz H. Deactivation model for dry desulfurization of simulated flue gas with calcined limestone in a fluidized-bed reactor. Fuel 2005;84:1705–12. [19] Hu GL, Johansen KD, Wedel S, Hansen JP. Review of the direct sulfation reaction of limestone. Prog Energ Combust Sci 2006;32:386–407. [20] Chen CM, Zhao CS, Liang C, Pang KL. Experimental study on the sulfation characteristics of limestone under high CO2 atmosphere. J Combus Sci Technol 2006;12:453–6.

L. Duan et al. / Applied Energy 108 (2013) 121–127 [21] Fuertes AB, Velasco G. Study of the direct sulfation of limestone particles at high CO2 partial pressures. Fuel Process Technol 1994;38:181–92. [22] Gregg SJ. The surface chemistry of solids. 2nd ed. New York: Reinhold; 1961. p. 158–161.

127

[23] Iribarne AP, Iribarne JV, Anthony EJ. Reactivity of calcium sulfate from FBC systems. Fuel 1997;76:321–7. [24] Chen CM, Zhao CS, Liu ST. Direct sulfation of limestone based on oxy-fuel combustion technology. Environ Eng Sci 2009;26:1481–8.