H2O combustion conditions in circulating fluidized bed

H2O combustion conditions in circulating fluidized bed

International Journal of Greenhouse Gas Control 95 (2020) 102979 Contents lists available at ScienceDirect International Journal of Greenhouse Gas C...

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International Journal of Greenhouse Gas Control 95 (2020) 102979

Contents lists available at ScienceDirect

International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc

Sulfation of limestone under O2/H2O combustion conditions in circulating fluidized bed

T

Liang Chena, Zhongrui Wanga, Chunbo Wanga,*, Huijie Wanga, Edward J. Anthonyb a b

School of Energy, Power and Mechanical Engineering, North China Electric Power University, Baoding, 071000, China School of Power Engineering, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK

A R T I C LE I N FO

A B S T R A C T

Keywords: O2/H2O combustion Sulfation Limestone Circulating fluidized bed H2O

O2/H2O combustion is a new generation of oxy-fuel technology for CO2 capture, offering better thermodynamic and economic performance than O2/CO2 combustion. The main feature of O2/H2O combustion is that an O2steam mixture is used as the oxidant and, as a result, H2O in the furnace can be as high as 80 %. To determine whether limestone can still be used to capture SO2 in-situ in circulating fluidized bed (CFB) boilers with such a high H2O concentration, its sulfation behavior was studied. Using a constant-temperature thermogravimetric analyzer, the influences of high H2O concentration, SO2 concentration, temperature and particle size on sulfation of limestone under O2/H2O combustion were examined. The fast sulfation stage was hardly influenced by H2O over the entire range of 0–80 %, but the sulfation rate in the slow sulfation stage improved with the increase of H2O concentration. Therefore, limestone can still be used to capture SO2 in O2/H2O combustion in CFB boilers, in spite of the high in-furnace H2O concentration. H2O is more likely to enhance the solid-state ion diffusion in the CaSO4 product layer, rather than the SO2 gas diffusion in the pores of the particles. Under conditions with 80 % H2O, when the SO2 concentration in the flue gas increased from 0.15 % to 0.45 %, the sulfation rate in the fast sulfation stage increased remarkably, while in the slow sulfation stage it barely changed. The sulfation rates in both the fast and slow sulfation stages were significantly increased with the decrease of limestone particle size from 0.45 mm to 0.075 mm. In the tested range of 840 °C–930 °C, the optimum temperature for SO2 capture under O2/H2O combustion is about 900 °C.

1. Introduction To mitigate global warming, various CO2 capture and storage (CCS) technologies have been developed, such as calcium looping (Blamey et al., 2015; Fan et al., 2018; Li et al., 2012), oxy-fuel combustion (Tan et al., 2012; Leckner and Gómez-Barea, 2014) and amine scrubbing with materials like monoethanolamine (MEA) (Mores et al., 2012; Cousins et al., 2011). Of these technologies, oxy-fuel combustion is considered to be one of the most promising for CO2 capture from fossil fuel power plants. Oxy-fuel technology employs O2/CO2 combustion, and produces a high concentration of CO2 (≥90 % by volume, dry basis) which can easily be separated. However, the massive recycled flue gas in O2/CO2 combustion decreases its economic/energetic performance when compared with air-fired combustion. O2/H2O fuel combustion is a new generation oxy-fuel combustion technology for CO2 capture and was proposed by Salvador et al. (Salvador et al. (2009)) in 2009. In O2/H2O combustion, an O2-steam mixture is used as the oxidant instead of air. Seepana and Jayanti



(Seepana and Jayanti (2010)) put forward a power generation system with CO2 capture based on O2/H2O combustion, and demonstrated its thermodynamic feasibility. A simplified O2/H2O combustion system for CO2 capture is shown in Fig. 1. In this system, water condensed from flue gas is recycled to the boiler. Thus the H2O concentration in flue gas can reach levels of up to 80 %. When steam in flue gas is condensed, a high concentration (90 %) of CO2 is obtained (Salvador et al., 2009), which can be easily separated in the CCS unit. Compared with O2/CO2 combustion, O2/H2O combustion has several key advantages: (i) higher thermodynamic and economic performance (Jin et al., 2015); (ii) the avoidance of recycled flue gas (Seepana and Jayanti, 2010); (iii) the specific heat of steam is larger than that of CO2, and the volume of steam to achieve the same combustion temperature in the O2/H2O system is much less than that in the O2/CO2 system, thus, the O2/H2O system is more compact (Seepana and Jayanti, 2010); (iv) easy to start up and shut down; and (v) has lower NOx emissions (Li et al., 2018). In conclusion, O2/H2O combustion is a

Corresponding author. E-mail address: [email protected] (C. Wang).

https://doi.org/10.1016/j.ijggc.2020.102979 Received 10 October 2019; Received in revised form 1 January 2020; Accepted 28 January 2020 1750-5836/ © 2020 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic diagram of O2/H2O combustion system for CO2 capture.

Table 1 Composition of limestones. Compound (wt%)

SiO2

Al2O3

Fe2O3

TiO2

P2O5

CaO

MgO

SO3

Na2O

K2O

Loss on Fusion

Baoding Xinxiang

0.67 0.45

0.78 0.56

< 0.10 0.15

< 0.05 0.05

< 0.03 < 0.03

54.93 55.02

< 0.10 0.48

< 0.10 < 0.10

< 0.10 < 0.20

< 0.10 0.24

42.90 42.78

In this system, the samples start to react instantaneously once arriving in the furnace, so they do not undergo the long heating-up stage as in a commercial TGA. Using this system, the effects of high steam concentration, temperature, limestone particle size and SO2 concentration on the sulfation of limestone under O2/H2O combustion conditions in a CFB were tested.

promising new generation technology for CO2 capture. However, only a few researchers have looked at the O2/H2O combustion technology. Jin et al. (Jin et al. (2015)) evaluated the performance of a 600 MW power generation system based on O2/H2O combustion, and found that it can achieve 0.90 percentage points higher net efficiency than for O2/CO2 combustion. Zou et al. (Zou et al. (2015), 2014) and Cai et al. (Cai et al. (2016), 2015) experimentally investigated the ignition and combustion of pulverized coal in O2/H2O atmospheres, and showed that coal has better ignition and combustion performance in O2/H2O than in O2/CO2 atmospheres. Li et al. (Li et al. (2018)) studied the NO emission of coal and biomass in O2/H2O combustion, and found that it was reduced by H2O. In recent years, circulating fluidized bed (CFB) boilers have undergone remarkable development (Cai et al., 2018). In particular, largescale (300−600 MWe) supercritical CFB boilers have come into service (Yue et al., 2017). Compared with pulverized coal (PC) boilers, CFB boilers have higher combustion stability and are more flexible for a wider range of fuels, especially for poor quality fuels like low-rank coals, petroleum coke, and refuse-derived fuels. Moreover, CFB boilers are also suitable for operation in the O2/H2O combustion mode. However, the investigation of O2/H2O combustion in CFBs is essentially lacking, with only one recent study in the available literature, namely Li et al. (Li et al. (2019)), who studied the ignition and volatile combustion behaviors of lignite particles in a fluidized bed under O2/H2O conditions. Limestone is widely used to capture SO2 in-situ in CFB boilers. However, because of the high H2O concentration in the flue gas, it is uncertain whether in-situ sulfur capture by limestone is still feasible in O2/H2O combustion in CFB boilers. It has been proved that H2O in flue gas can improve the sulfation performance of limestone (Wang and Chen, 2016; Wang et al., 2016a; Jiang et al., 2013; Stewart et al., 2012), but all such tests were performed under conditions with H2O lower than 40 %, corresponding to O2/CO2 or air combustion conditions. However, in O2/H2O combustion the H2O concentration in flue gas can reach levels of 80 %. The influence of such a high percentage of H2O on the sulfation of limestone has never been previously explored. H2O level as high as 80 % in flue gas has a possibility to negatively affect the sulfation of limestone, since it has been proven that H2O can seriously increase the sintering of CaO (Borgwardt, 1989). A high concentration of H2O may, therefore, cause the collapse of the pore structure of CaO and, as a result, decrease its sulfur capture capacity. If the sulfation reaction is significantly affected by the high concentration of H2O, in-situ sulfur capture by limestone could potentially have to be abandoned. Therefore, the sulfation of limestone under high-concentration H2O conditions is a key issue to O2/H2O combustion in CFB boilers. In this work, the sulfation of limestone under O2/H2O combustion conditions in a CFB was investigated. The tests were performed using a self-designed constant-temperature thermogravimetric analyzer (TGA).

2. Materials and experimental Two types of limestones, Baoding and Xinxiang, were used in the tests. Prior to the tests, the limestones were milled and sieved to narrow size ranges (0.075-0.125 mm, 0.25-0.3 mm and 0.4-0.45 mm). The main components of these sorbents are given in Table 1. The pore structures of the two limestones after calcination (under 850 °C in N2) are shown in Table 2. It can be seen that the pore structures of CaO formed by these two limestones are quite different. The experimental system is shown in Fig. 2. The main reactor consists of an electrical tube furnace (40 mm internal diameter by 800 mm length). It is placed on a guide rail and, thus, can be moved horizontally. The sample pan is placed on one end of a high-temperature stent, and the other end of the stent is connected to a weight monitor. Thus the sample mass can be measured continuously. Prior to a test, the furnace is heated to a set temperature. It takes less than three seconds to move the sample pan to the center of the tube furnace. Once arriving at the furnace center, the materials in the sample pan reach the set temperature and start to react, at a high temperature with a rapid heating rate as it would in a real CFB boiler. The elimination of the long heating-up stage is the key advantage for this system compared to a commercial TGA. Synthetic flue gas is composed of pure gases (CO2, SO2, O2 and N2) from gas cylinders and their flow rates are controlled by flowmeters. Water vapor is generated by evaporation of water in a tube heated by an electrical heating strip. The flow rate of water is controlled by a peristaltic pump. A total gas flow of 1200 mL/min (at 25 °C) was used in all the tests. It was determined that this flow rate can eliminate external gas diffusion resistance for the reactions studied in this work. In each test, a given amount of limestone sample (80 mg) was spread as a single layer on the sample pan. The mass of the sample during reaction was measured continuously by the weight monitor (accuracy ± 0.1 mg) and recorded by computer. Table 2 Pore structures of calcined limestones.

2

Parameters

Baoding

Xinxiang

Pore surface area, m2/g Pore volume, cm3/g Porosity Average pore width, nm

23.7 0.261 0.46 44.0

17.1 0.225 0.43 52.6

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Fig. 2. The experimental system (constant-temperature TGA).

two processes under air and O2/CO2 combustion conditions. Results show that these two processes are distinctly different, and that only the simultaneous reaction can represent the real reaction process of limestone in CFB boilers. However, under O2/H2O combustion conditions, the differences between the two reaction processes are still unknown. Therefore, in this study the two processes were tested and compared first, as shown in Fig. 4. The tests were under 900 °C, 80 % H2O and 0.3 % SO2, with 0.25-0.3 mm Baoding limestone. As shown in Fig. 4(a), each of the two reaction processes had two stages, a mass-loss stage and a mass-growth stage. The mass-loss stages are shown in Fig. 4(b) in more detail. For the sequential reaction, only the calcination reaction occurs in the mass-loss stage, while for the

Table 3 Experimental conditions. Conditions

Value

H2O concentration, % SO2 concentration, % O2 concentration, % CO2 concentration, % Temperature, °C Particle size, mm

0, 20, 50, 80 0.15, 0.3, 0.45 5 As balance 840, 870, 900, 930 0.075–0.125, 0.25–0.3, 0.4–0.45

Table 3 summarizes the experimental conditions in this work. 3. Results and discussion 3.1. Simultaneous calcination/sulfation under O2/H2O combustion conditions In most investigations on limestone sulfation (Anthony and Granatstein, 2001; Anthony et al., 2007), limestone is first calcined to form CaO and then the sulfation of CaO is studied, as shown in Fig. 3(a). We call this the sequential calcination-then-sulfation reaction process (or the sequential reaction, in short). However, in a real CFB boiler, raw limestone particles enter the furnace, and the particles are calcined and sulfated simultaneously in hot flue gases. The calcination and sulfation reactions occur simultaneously in each limestone particle, as shown in Fig. 3(b). We call this process the simultaneous calcination/sulfation reaction process (the simultaneous reaction, in short). We have performed a series of tests (Wang et al., 2015; Chen et al., 2019a, b; Chen et al., 2017) to demonstrate the differences between the

Fig. 4. Differences between the calcination-then-sulfation and the simultaneous calcination/sulfation reactions under O2/H2O conditions.

Fig. 3. Different sulfation reaction processes of limestone. 3

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Fig. 5. XRD analysis of sample sulfated under 80 % H2O condition.

simultaneous reaction, the calcination and sulfation reactions occur at the same time in the mass-loss stage. As shown Fig. 4(b), the sample mass in the sequential reaction decreased faster than that in the simultaneous reaction. According to our previous investigation (Chen et al., 2017), there are two reasons why the sample mass in the massloss stage of the simultaneous reaction decreases more slowly than in the sequential reaction: (i) the sample in the simultaneous reaction absorbed SO2; and (ii) the calcination of CaCO3 was impeded by the formed CaSO4 in the sulfation reaction. It should be noted that, although the H2O reaches levels as high as 80 %, no Ca(OH)2 is formed in the samples, since Ca(OH)2 decomposes over 500 °C with H2O partial pressures of 0.8 atm. (Ghosh-Dastidar et al., 1995). This is validated by the x-ray diffraction (XRD) analysis of the final sample undergoing the simultaneous reaction, as shown in Fig. 5. Fig. 5 shows that only CaO and CaSO4 were detected in the final sample. Thus, it can be concluded that only the calcination of CaCO3 (Reaction (1)) and sulfation of CaO (Reaction (2)) occurred in the reaction processes. CaCO3 → CaO + CO2

(1)

CaO + SO2+1/2O2 → CaSO4

(2)

Fig. 6. Effect of H2O concentration on sulfation of limestone. (a) normalized sample mass; (b) sulfation rate.

recycle ratio of steam, and can reach levels as high as 80 %. To determine the influence of H2O concentration on the sulfation performance of limestone, the simultaneous reactions under 20 %, 50 %, and 80 % H2O were tested. For comparison, the simultaneous reaction without H2O was also tested. All the tests were under 900 °C and 0.3 % SO2, with 0.25-0.3 mm Baoding limestone. The sample mass and sulfation rate are shown in Fig. 6. As shown in Fig. 6(a), each of the mass-growth stages can be roughly divided into a fast sulfation stage (before 20 min) and a slow sulfation stage (after 20 min) according to the sulfation rate. Compared with the no-H2O condition, 20 % H2O in the flue gas improved the sulfation ratio considerably. The 90-min sulfation ratio increased from 25.7 % under no-H2O condition to 31.2 % under 20 % H2O. Further increase of H2O concentration (from 20 % to 80 %) showed a much smaller effect on the 90-min sulfation levels. Under each test condition in this work, the sample at 1 min after the minimum mass point was removed from the furnace, cooled in N2, broken and re-calcined in 900 °C N2 atmosphere. However, no further mass loss was found in the re-calcination process for all samples, which indicated that the samples were completely calcined. Thus, in the test process in Fig. 6(a), only the sulfation reaction occurred over 1 min after the minimum mass point. Therefore, the sulfation ratio X of the samples can be calculated by the sample mass, according to:

As shown in Fig. 4(a), in the mass-growth stage, the sample mass increased faster for the simultaneous reaction than for the sequential reaction. The mass-growth stage is dominated by the sulfation stage, so Fig. 4(a) indicates that the simultaneous reaction has a faster sulfation rate than the sequential reaction. As a result, the final sulfation ratio (32.2 %) for the simultaneous reaction is much higher than that (26.8 %) for the sequential reaction. The different calcination processes may be the source for the differences in the sulfation rate. In the simultaneous reaction process, the calcination and sulfation reaction occurred simultaneously, thus the pore structure after the calcination stage is different from the CaO formed without SO2. The different pore structures formed in the calcination stage may cause the different sulfation performance between the two reaction processes. From Fig. 4, it can be seen that the simultaneous reaction is very different from the sequential reaction. Since in CFB boilers the simultaneous reaction is the real reaction process, the investigation on the sulfation of limestone should be based on the simultaneous reaction. Therefore, in this study all the tests followed the simultaneous calcination/sulfation process.

X=

3.2. Effect of H2O concentration

mt m0

+

λMCO2 MCaCO3

−1

λ (MCaSO4 − MCaO) MCaCO3

(3)

in which mt is the sample mass; m0 is the initial sample mass; λ is the

Under O2/H2O combustion, the H2O concentration changes with the 4

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CaCO3 content of the initial sample; MCaSO4, MCaCO3, MCaO, and MCO2 are the molar mass of CaSO4, CaCO3, CaO, CO2, respectively. To compare the sulfation performance of limestone under different conditions, the average sulfation rate in the fast sulfation stage and in the slow sulfation stage were calculated by:

v=

Xt2 − Xt1 t2 − t1

(4)

in which v is the average sulfation rate (1/s); t1 and t2 are reaction times (s), and Xt1 and Xt2 are sulfation ratios corresponding to t1 and t2, respectively. When calculating the average sulfation rate in the fast sulfation stage, t1 = 300 s and t2 = 900 s; while for calculation of the average sulfation rate in the slow sulfation stage, t1 = 2400s and t2 = 5400 s. As shown in Fig. 6(b), the sulfation rate in the fast sulfation stage barely changes with H2O concentration. The reaction rate in the slow sulfation stage always increased with H2O concentration, from 1.7 × 10−5/s under the no-H2O condition to 2.8 × 10−5/s under 80 % H2O. The H2O level at a concentration of 80 % still has a positive effect on the sulfation of limestone, which is the key finding here. This indicates that the sulfation of limestone is not negatively affected by the high H2O concentration, and limestone can still be used for in-situ capture of SO2 in O2/H2O combustion in CFB boilers. To verify that the positive effect of H2O on the sulfation rate in Fig. 6 is a common phenomenon rather than limited only to the Baoding limestone, the sulfation of another limestone (Xinxiang limestone) was also tested. The tests were under 900 °C and 0.3 % SO2 with 0.250.3 mm particles. Results are shown in Fig. 7. As shown in Fig. 7(a), the 90-min sulfation ratio for Xinxiang limestone clearly increased by H2O, which is similar in behavior to the Baoding limestone. However, the sulfation rates in the fast sulfation stage, as shown in Fig. 7(b), are almost the same under different H2O concentrations, and the sulfation rates in the slow sulfation stage are enhanced by H2O over the entire range of 0–80 %. The positive effect of H2O on the sulfation rate of Xinxiang limestone is, therefore, similar to that on Baoding limestone. Therefore, it can be expected that this is a common phenomenon rather than limited to a particular type of limestone. The effect of low H2O concentration (0–40 %) on the sulfation of CaO has been studied frequently in recent years (Wang et al., 2010, 2016b; Duan et al., 2013; Wang et al., 2017). A common finding is that the slow sulfation stage can be significantly improved by H2O, while the fast sulfation stage is not affected much, similar to the findings in this work. However, researchers have not reached a consensus on the effect mechanism of H2O. Wang et al. (Wang et al., 2010) put forward the hypothesis that H2O may react with CaO and form a transient intermediate Ca(OH)2, and the sulfation of Ca(OH)2 occurs more easily and, thus, faster than that of CaO. Jiang et al. (Jiang et al. (2013)) and Stewart et al. (Stewart et al. (2012)) speculated that since H2O has negligible effect on the fast sulfation stage but strongly improved the sulfation rate in the slow sulfation stage, H2O is more likely to enhance solid-state ion diffusion in the CaSO4 layer. However, more work is needed to unambiguously clarify this issue. In the sulfation of limestone, there are three possible rate-controlling steps, the intrinsic sulfation reaction rate, the SO2 gas diffusion in the pores of particles, and the solid-state ion diffusion in the CaSO4 product layer. A common consensus (Anthony and Granatstein, 2001) is that the fast sulfation stage is controlled by the intrinsic sulfation rate, and the slow sulfation stage is controlled by SO2 gas diffusion in the pores, or the solid-state ion diffusion in the compact CaSO4 layer. From Fig. 6 or 7, it can be seen that the fast sulfation stage is barely affected by H2O; thus, H2O should have negligible effect on the intrinsic sulfation reaction. H2O strongly increased the sulfation rate in the slow sulfation stage and, thus, H2O may accelerate the SO2 gas diffusion in the pores or

Fig. 7. Effect of H2O concentration on Xinxiang limestone. (a) the normalized sample mass; (b) sulfation rate.

enhance the solid-state ion diffusion in the CaSO4 layer. To determine whether H2O improved both of these two steps, the pore structures of samples undergoing 60 min of reaction (just in the slow sulfation stage) were measured by the N2 adsorption method. The pore surface area, pore volume and pore width are given in Table 4. From Table 4, the pore structure parameters for the samples under different H2O concentrations showed only small differences, which means that high concentrations of H2O did not significantly affect the pore structure of the sorbent. Since the pore widths of the samples in Table 4 are only about 10 nm, SO2 gas diffusion in these pores follows the Knudsen diffusion (Milne et al., 1990). The Knudsen diffusion coefficient Dk (m2/s) can be calculated by:

Dk =

97 × d p 2

T MSO2

(5)

in which dp is the pore width, m; T is gas temperature, K; and MSO2 is the molar mass of SO2, g/mol. The gas diffusion in a porous particle is Table 4 Pore structures for 60-min sulfated samples under different H2O concentrations. conditions no 20 50 80

5

H2O % H2O % H2O % H2O

surface area S, m2/g

pore volume Vp, cm3/g

pore width dp, nm

1.46 1.39 1.40 1.42

0.0049 0.0037 0.0035 0.0041

13.4 10.6 10.0 11.5

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Fig. 8. Effective diffusion coefficients of SO2 in particles undergoing 60-min reaction.

also affected by the porosity and the tortuosity of the pores, and should be described by the effective diffusion coefficient De (m2/s), which can be calculated by:

De = D k

ε τ

(6)

in which ε is porosity of the particle; and τ is the tortuosity of the pore. The porosity ε can be calculated by:

ε=

Vp Vp + (1 ρt )

(7)

in which ρt is the true density of the particle, g/cm ; Vp is the pore volume of the particle, cm3/g. The tortuosity is difficult to determine, but is usually correlated to porosity by (García-Labiano et al., 2002): 3

τ=

1 ε

(8)

Fig. 9. Effect of SO2 concentration on sulfation of limestone. (a) normalized sample mass; (b) sulfation rate.

Based on the parameters in Table 4, the effective diffusion coefficients of SO2 were calculated, as shown in Fig. 8. From Fig. 8, the effective diffusion coefficient of SO2 in particles under the no-H2O condition is much larger than those with H2O, which is contrary to the variation in the trend of the sulfation rate as shown in Fig. 6. Thus, it can be concluded that the slow sulfation stage of samples in this work was not controlled by SO2 gas diffusion in pores, and should be controlled by solid-state diffusion in the CaSO4 layer. Therefore, the faster sulfation rate in the slow sulfation stage under conditions with H2O should be attributed to the fact that H2O improved solid-state ion diffusion in the CaSO4 product layer. However, it should be noted that the reason why H2O enhances ion diffusion in the CaSO4 layer is still not clear and more work is necessary to clarify this issue.

40 min (the turning point is defined as a point of time that the average sulfation rate in the whole duration before this point is double of that after this point). From Fig. 9(b), the sulfation rate in the fast sulfation stage obviously increased with the SO2 concentration. In the fast sulfation stage, the sulfation rate under the 0.45 % SO2 condition was about double that under the 0.15 % SO2 condition. However, the sulfation rates in the slow sulfation stage remained almost unchanged under the three different SO2 concentrations. Under 0.3 % and 0.45 % SO2 conditions, the sulfation rates in the fast sulfation stage were more than 4 times those in the slow sulfation stage. While under the 0.15 % SO2 condition, the sulfation rate in the fast sulfation stage was only double that in the slow sulfation stage.

3.3. Effect of SO2 concentration In O2/H2O combustion, the SO2 concentration in the flue gas changes with the sulfur content of fuels and the recycle ratio of steam. Different SO2 concentrations lead to different sulfation performance of the limestone. To understand the effect of SO2 concentration on the sulfation of limestone under O2/H2O combustion conditions, the simultaneous reactions under three concentrations of SO2 were tested. The tests were carried out at 900 °C and 80 % H2O, with 0.25-0.3 mm Baoding limestone. The results are shown in Fig. 9. As shown in Fig. 9(a), the sample mass improved remarkably with the increase of SO2 concentration. The 90-min sulfation ratio increased from 28.5 % under 0.15 % SO2 to 36.1 % under 0.45 % SO2. For the 0.3 % SO2 and 0.45 % SO2 conditions, the turning points from the fast sulfation stage to the slow sulfation stage were at about 20 min, while for the 0.15 % SO2 conditions the turning point was postponed to about

3.4. Effect of particle size Particle size is another key parameter that influences the sulfation rate of limestone. To examine the effect of particle size on the sulfation of limestone under O2/H2O combustion, the simultaneous reactions of three sizes of limestone particles were tested. All tests were under 900 °C, 80 % H2O and 0.3 % SO2, with Baoding limestone. The results are shown in Fig. 10. From Fig. 10(a), the final sulfation ratio decreased significantly with the increase of particle size, from 38.3 % for 0.075–0.125 mm particles to 23.3 % for 0.4–0.45 mm particles. The fast sulfation stage was about 2–4 times faster than the slow sulfation stage for all particle sizes, as shown in Fig. 10(b). The sulfation rate in both the fast sulfation stage and the slow sulfation stage decreased significantly with the increase of 6

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Fig. 10. Effect of particle size on sulfation of limestone. (a) normalized sample mass; (b) sulfation rate.

Fig. 11. Effect of temperature on sulfation of limestone: (a) normalized sample mass; (b) sulfation rate.

particle size. In the fast sulfation stage, the sulfation rate of 0.0750.125 mm particles was 40 % higher than that of 0.4-0.45 mm particles. While in the slow sulfation stage, the sulfation of 0.075-0.125 mm particles was about 3 times as fast as that of the 0.4-0.45 mm particles. This means that the slow sulfation stage is more sensitive to particle size than the fast sulfation stage under the conditions examined here.

Temperature may affect the sulfation of limestone in two ways: directly increasing the intrinsic sulfation rate of limestone and changing the pore structure by sintering. To better understand the effect mechanism of temperature on the reaction rate in the slow sulfation stage, the pore structures of samples reacted for 60 min were measured, and the results are shown in Table 5. From Table 5, both the pore surface area and pore volume decreased with temperature, but the pore width changed little. The sulfation rate in the slow sulfation stage was not monotonic with the pore surface area. An explanation is that the sulfation rate was affected by both the solid-state ion diffusion rate and the pore surface area. When temperature increased, the ion diffusion rate increased but the pore surface area decreased. Therefore, the sulfation rate is a complex function of temperature. When temperature increased in the range of 840 °C–900 °C, the enhanced ion diffusion dominated the sulfation rate. When temperature increased further, the decrease of the pore surface area controlled the variation tendency of the sulfation rate.

3.5. Effect of temperature Under air-fired CFB conditions, the optimum temperature for SO2 capture is around 850 °C. But under the high H2O concentration in O2/ H2O combustion, the optimum temperature may be different. To determine the effect of temperature on the sulfation reaction under O2/ H2O combustion, the simultaneous reactions under 840 °C, 870 °C, 900 °C and 930 °C were tested. The tests were under 80 % H2O and 0.3 % SO2, with 0.25-0.3 mm Baoding limestone. The results are shown in Fig. 11. From Fig. 11(a), the 90-min sulfation ratio always increased with temperature, from 23.3 % at 840 °C to 32.6 % at 930 °C. In the fast sulfation stage, as shown in Fig.11(b), with temperature increased from 840 °C to 870 °C the sulfation rate increased obviously, but with further temperature increase (from 870 °C to 930 °C) the sulfation rate showed negligible variation. In the slow sulfation stage, the sulfation rate increased with temperature in the range of 840 °C–900 °C, while further temperature increase (from 900 °C to 930 °C) decreased the sulfation rate. It can be seen from Fig. 11(a) that the optimum temperature for sulfation under 80 % H2O should be around 900 °C.

Table 5 Pore structures for 60-min sulfated samples under different temperatures.

7

temperature

surface area, m2/g

pore volume, cm3/g

pore width, nm

840 °C 870 °C 900 °C 930 °C

2.03 1.94 1.42 1.35

0.006 0.0048 0.0041 0.0035

11.8 9.9 11.5 10.4

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4. Conclusions The effect of high H2O concentration, temperature, particle size and SO2 concentration on the sulfation of limestone under O2/H2O combustion conditions in a CFB were tested. Critically, this work shows that limestone capture of SO2 is possible even at extremely high H2O levels of 80 %. Here we also note that the simultaneous calcination/sulfation reaction is different from the sequential calcination-then-sulfation reaction under O2/H2O combustion conditions, thus all tests should be based on the simultaneous reaction process, that which actually occurs in a CFB. The fast sulfation stage was barely affected by H2O in the range of 0–80 %, but the sulfation rate in the slow sulfation stage was improved with the increase of H2O concentration. Since the sulfation of limestone is improved rather than reduced by the 80 % H2O level in flue gas, it is clear that limestone can still be used to capture SO2 in-situ in O2/H2O combustion in CFB boilers. The mechanism for the increased sulfation rate by H2O in the slow sulfation stage seems to be that H2O can enhance solid-state ion diffusion in the CaSO4 layer. When SO2 concentration in flue gas was raised from 0.15 % to 0.45 %, the sulfation rate in the fast sulfation stage increased markedly, while the sulfation rate in the slow sulfation stage was largely unaffected. The sulfation rates in both the fast and slow sulfation stages were increased significantly with the decrease of limestone particle size. In the tested range of 840 °C–930 °C, the optimum sulfation temperature was about 900 °C. CRediT authorship contribution statement Liang Chen: Conceptualization, Methodology, Investigation, Writing - original draft. Zhongrui Wang: Data curation. Chunbo Wang: Supervision, Funding acquisition. Huijie Wang: Visualization, Data curation. Edward J. Anthony: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China [51976059], and the Fundamental Research Funds for the Central Universities [2018ZD03]. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.ijggc.2020.102979. References Anthony, E.J., Granatstein, D.L., 2001. Sulfation phenomena in fluidized bed combustion systems. Prog. Energy Combust. 27 (2), 215–236. Anthony, E.J., Bulewicz, E.M., Jia, L., 2007. Reactivation of limestone sorbents in FBC for SO2 capture. Prog. Energy Combust. 33 (2), 171–210. Blamey, J., Manovic, V., Anthony, E.J., Dugwell, D.R., Fennell, P.S., 2015. On steam hydration of CaO-based sorbent cycled for CO2 capture. Fuel 150, 269–277. Borgwardt, R.H., 1989. Calcium oxide sintering in atmospheres containing water and carbon dioxide. Ind. Eng. Chem. Res. 28 (4), 493–500. Cai, L., Zou, C., Liu, Y., Zhou, K., Han, Q., Zheng, C., 2015. Numerical and experimental studies on the ignition of pulverized coal in O2 /H2O atmospheres. Fuel 139, 198–205. Cai, L., Zou, C., Guan, Y., Jia, H., Zhang, L., Zheng, C., 2016. Effect of steam on ignition of pulverized coal particles in oxy-fuel combustion in a drop tube furnace. Fuel 182, 958–966. Cai, R., Zhang, H., Zhang, M., Yang, H., Lyu, J., Yue, G., 2018. Development and

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