SO3 formation under oxy-CFB combustion conditions

International Journal of Greenhouse Gas Control 43 (2015) 172–178

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SO3 formation under oxy-CFB combustion conditions Lunbo Duan a,b,∗ , Yuanqiang Duan a , Yerbol Sarbassov b , Yanmin Li c , Edward J. Anthony b a Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China b Centre for Combustion and CCS, School of Energy, Environment and Agrifood, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK c Jiangsu Wisdom Engineering and Technology Co., Ltd, Nanjing 210009, China

a r t i c l e

i n f o

Article history: Received 13 August 2015 Received in revised form 30 October 2015 Accepted 30 October 2015 Available online 15 November 2015 Keywords: Oxy-fuel CFB combustion SO3 formation Homogeneous and heterogeneous reactions Pilot-scale testing

a b s t r a c t Due to the enrichment of SO2 and H2 O, SO3 formation during oxy-fuel circulating fluidized bed (CFB) combustion may significantly increase over the air fired case and this requires special attention in terms of safety consideration. In an attempt to better elucidate the formation mechanism of SO3 under oxy-fuel CFB conditions, homogenous and heterogeneous experiments were performed using a small vertical tube reactor to model the SO3 formation condition in the back pass channels and then the mechanism deduced was further validated by tests and measurements using a pilot-scale 50 kWth oxy-fuel CFB combustor with wet flue gas recycle. Results show that replacing N2 by CO2 does not change the SO3 formation levels while the addition of water enhances SO3 formation. The increased O2 , SO2 , H2 O concentrations along with increasing temperature are favorable for enhancing SO3 formation over the range of tested parameters. Fe2 O3 , CuO and V2 O5 are shown to be able to catalyze SO2 conversion to SO3 under oxyfuel atmosphere; of these V2 O5 s catalyzing ability is the strongest. Fly ash can either catalyze the SO3 formation or absorb SO3 , depending on the temperature and the alkalinity of the ash. The results from the pilot plant burning bituminous coal demonstrate that SO3 concentration in the flue gas is about 4.5 times higher during oxy-fuel combustion than that under air combustion. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Oxy-fuel combustion (Buhre et al., 2005; Scheffknecht et al., 2011; Toftegaard et al., 2010; Yi et al., 2015) is one of the most promising carbon capture technologies which is currently under demonstration and proceeding to commercialization. However, there are still some important issues requiring further investigation, one of which is related to SO3 formation. Due to the enrichment of O2 , SO2 and moisture under the oxy-fuel atmosphere, more SO3 tends to be formed than that under air combustion. Higher SO3 concentration together with high moisture concentration in the flue gas will substantially elevate the acid dew point of the flue gas, increasing the risk of corrosion on the equipment and ducts in the heat recovery zone as well as the flue gas recycle line and the CO2 purification line. Much effort has been made to clarify the SO3 formation mechanism (Alzueta et al., 2001; Fleig et al., 2013; Hindiyarti and Glarborg,

∗ Corresponding author at: Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China. E-mail address: [email protected] (L. Duan). http://dx.doi.org/10.1016/j.ijggc.2015.10.028 1750-5836/© 2015 Elsevier Ltd. All rights reserved.

2007; Jørgensen et al., 2007; Srivastava et al., 2004) during combustion by both experimental measurement and modeling methods. Theoretically, SO3 is formed either via homogeneous gas phase reaction or via heterogeneous reactions catalyzed by solid materials. Two reaction routines are normally recognized as the dominant pathways in the homogeneous SO2 oxidation to SO3 process. One is the primary oxidation by the oxygen radicals occurring in the post-flame area where the temperature is higher than 900 ◦ C: SO2 + O(+M)  SO3 (+M)

(1)

The other is the secondary formation of SO3 via HOSO2 , which is oxidized by the molecular oxygen in the downstream flue gas: SO2 + OH(+M)  HOSO2 (+M)

(2)

HOSO2 + O2  SO3 + HO2

(3)

It has been confirmed that reactions (2) and (3) are insignificant above 700 ◦ C because of the low thermal stability of HOSO2 (Alzueta et al., 2001). In principal, as the gas cools, the formation of SO3 is thermodynamically favored (Fleig et al., 2011). However, simultaneously, this formation becomes kinetically controlled, and only a small amount of SO3 is typically formed in the limited time. Experimental work from Fleig et al. (2013) on a quartz glass reactor

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from 573 ◦ C to 1373 ◦ C showed that reactive gases like NO, CO and CH4 contributed to the SO3 formation by increasing the concentration of radicals in the gas phase. Also it has been found that several metal oxides such as Fe2 O3 , CuO and V2 O5 can catalyze SO3 formation (Belo et al., 2014; Jørgensen et al., 2007) and this phenomenon is well known from the selective catalytic reduction (SCR) process for NOx reduction (Srivastava et al., 2004). A few studies have also been done on SO3 formation during oxy-fuel combustion. Fleig et al. (2012a,b) carried out oxy-fuel combustion testing on their top-fired burner using propane as fuel and found homogeneous reaction is an important contributor to SO3 formation. They also concluded that wet flue gas recycle contributes to greater SO3 emission because of H2 O-enhanced SO3 formation. Their modeling work (Fleig et al., 2011) on gas phase reactions also confirmed there was a significant increase in SO3 concentration during oxy-fuel combustion. Belo et al. (2014) carried out SO3 formation experiments on a horizontal tube furnace and investigated both the homogenous and heterogeneous reactions under air and oxy-fuel atmosphere. They found H2 O has negligible effects on SO3 formation at 900 ◦ C, which is different from the results of Fleig et al. (2012a,b). They also observed the strong catalytic influence of fly ash on SO2 conversion to SO3 . Ahn et al. (2011) carried out the SO3 formation study on oxy-fuel pulverized coal (PC) combustion as well as oxy-fuel CFB combustion burning both high sulfur and low sulfur coals. Very interesting results from this study showed that during PC combustion, SO3 concentration was much higher under oxy-fuel atmosphere than that under air combustion when burning high sulfur coal, while it was at similar level when burning low sulfur coal. However, during CFB combustion, the SO3 formation was notably higher under oxy-fuel atmosphere than that under air atmosphere using the same low sulfur coal, which indicated that SO3 formation was quite dependent on the type of combustion. Mitsui et al. (2011) measured the SO3 concentration at the inlet and outlet of the SCR and Dry Electrostatic Precipitator (DESP) on a 1.5 MWth pilot oxy-fuel combustor and found that SCR had little effect on the SO3 concentration while DESP removed nearly all the SO3 during oxy-fuel combustion. It is important to mention that careful analytical SO3 measurements are crucial to detect most formed SO3 , since all available methods are very sensitive in terms of operation. By nature, SO3 is found to be exceptionally reactive, where it can react easily with tube surfaces or fly ash particles during the sampling process. Also, it can be easily condensed to form sulfuric acid as the temperature falls below the dew point. Previous measured values vary from several ppm to above 100 ppm in different studies (Ahn et al., 2011; Barrett et al., 1966; Belo et al., 2014). Five SO3 measurement methods including the controlled condensation method (CCM), isopropanol absorption bottle method, pentol SO3 monitor, salt method and continuous indirect measurements with FTIR are usually proposed. Usually, the CCM is recognized as the most reliable one by many researchers (Cao et al., 2010; Fleig et al., 2012a,b). Overall, even though there have already been some studies on the SO3 formation during oxy-fuel combustion, there are still some discrepancies and uncertainties among the previous studies. Also, the previous studies focused mainly on the high-temperature zone, while the low-temperature reaction has been studied to a lesser degree. Furthermore, studies on SO3 formation during oxy-fuel CFB combustion are rare. Regarding the fact that oxy-fuel combustion in CFB has attracted a lot of attention due to a wide variety of potential advantages (Czakiert et al., 2006, 2010; Duan et al., 2011a, 2014; Jia et al., 2007, ˜ 2010; Lupiánez et al., 2013; Tan et al., 2012) such as low capital investment, low operation and maintenance cost and good pollutant control, it is essential to take a deep look at the SO3 formation mechanism during oxy-fuel CFB combustion.

173

Fig. 1. Vertical tube reactor system. 1. Gas cylinders, 2. Gas flowmeters, 3. Gas mixer, 4. Steam generator, 5. Water metering pump, 6. Vertical tube furnace, 7. Element heater, 8. Quartz tube, 9. Temperature control panel, 10. SO3 measurement module, and 11. Vacuum pump.

During the oxy-fuel CFB combustion, because the temperature is comparatively low, the primary oxidation of SO2 via reaction (1) is considered as less important than that in oxy-PC furnace. In this study, homogeneous and heterogeneous experiments were designed in a vertical tube flow reactor to model the SO3 formation environment in low temperature zone (400–700 ◦ C) under oxy-fuel conditions to focus the SO3 formation in the back pass channel. Then SO3 formation results obtained from a 50 kWth pilot oxy-fuel CFB combustor with wet flue gas recycle are reported to validate the proposed mechanism. 2. Experimental 2.1. Homogeneous and heterogeneous tests in a vertical tube reactor The experimental rig used for the homogeneous and heterogeneous reactions in this study is a vertical tube reactor, as shown in Fig. 1. The system consists of a 15.5 mm inner diameter quartz tube flow reactor placed in an electrically heated furnace. The total length of the quartz tube is 1115 mm. A porous quartz plate was placed at the center of the quartz tube, which can support the solids on it. The furnace below the plate is used to preheat the gas entering the reaction zone. The isothermal length above the plate is calibrated as 420 mm. Another porous quartz plate was placed at the exit of the quartz tube to prevent the solids from being carried out of the reactor. High purity (>99.99%) reactant gases (O2 , SO2 , and N2 /CO2 ) were supplied by gas cylinders and were metered by the corresponding flow meters before entering the gas mixer. Deionized water was metered and pumped into a steam generator by a metering pump (BT 100-2J, LongerPump, China). The steam generator was controlled strictly at 200 ◦ C. The steam and other gases were mixed in the gas mixer which is also heated and then flowed into the reactor. The whole gas line was electrically heated to avoid condensation. The total gas flow rate into the reactor was controlled at 3.17 L/min (@0.1 MPa, 0 ◦ C) to fix the residence time of gas at 1.5 s under the actual condition. Four temperature levels

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(400 ◦ C, 500 ◦ C, 600 ◦ C, and 700 ◦ C), four oxygen concentration levels (3%, 6%, 11%, and 16%), four SO2 concentration levels (2000 ppm, 3000 ppm, 4000 ppm, and 5000 ppm) and three steam concentration levels (5%, 10%, and 15%) were tested to investigate the effects of different atmospheres on the SO3 formation. During the homogenous tests, the quartz tube was empty. During the heterogeneous tests, three different kinds of metal oxides (0–10 ␮m) including ␣-Fe2 O3 , V2 O5 and CuO with purity higher than 99% and two different kinds of fly ash (0–45 ␮m) were used as the catalysts. 1.6 g Fe2 O3 , 0.795 g CuO and 1.82 g V2 O5 were used, respectively, in the heterogeneous tests (same molar amount). 1.0 g fly ash was used in the fly ash tests. The two fly ash used was collected from our 50 kWth pilot oxy-CFB system burning Chinese bituminous coal and anthracite under 30% O2 /70% CO2 atmosphere. The information of the parent coals and the operation parameters can be found in reference (Duan et al., 2011a). The composition of the two fly ashes was shown in Table 1. The controlled condensation method (CCM) (ASTM 3226-73T) was used to measure the outlet concentration of sulfur compounds, including SO2 and SO3 . The sampling line consisted of the quartz glass liner in the heated probe, a quartz filter to remove the particles, a cooling coil glass tube in a condenser, four impingers in an ice bath and flow control and metering system. The gas temperature in the condenser was kept above the dew point of moisture (typically 75–85 ◦ C) to prevent the condensation of water and associated SO2 capture, but was also kept below the acid dew point at which the sulfuric acid started to condense. This approach allowed for selective condensation of SO3 . The first two impingers contained a hydrogen peroxide solution that captured sulfur dioxide. The last two impingers were filled with silica gel, which removed any moisture left in the gas before the sample passed into the dry gas meter. The amount of sulfur trioxide and sulfur dioxide present in the condensed acid and hydrogen peroxide solutions was quantified through a titration method using barium perchlorate with a thorin indicator, according to the EPA method 8A. All samples were titrated at twice to get accurate results. An Ion Chromatograph Analyzer (DX-120, DIONEX, USA) was also used to measure the SO4 2− concentration in the solutions for comparison. The detailed description of the sampling method can be found in reference (Ahn et al., 2011). 2.2. 50 kWth oxy-fuel CFB test with wet flue gas recycle SO3 sampling was also done in a 50 kWth oxy-fuel CFB system with wet flue gas recycle. The detailed description of the system can be found elsewhere (Duan et al., 2014). One Chinese bituminous coal was used as the feeding fuel, and its analysis is shown in Table 2. The firing rate of this system is 50 kWth, equaling to 7.18 kg bituminous coal used. The overall oxygen concentration in the atmosphere is 22.2% under the oxy-fuel combustion condition. The bed temperature is 881 ◦ C for air combustion, while it is 891 ◦ C for oxy-fuel combustion. All other details of the operating parameters of the test were described in reference (Duan et al., 2014). The oxygen concentration at the inlet and outlet of the furnace were online measured by ZrO2 oxygen sensors (Yokogawa, Japan). SO2 was online measured by a gas analyzer (NGA2000, Emerson, USA). The moisture concentration in the flue gas was online measured by a moisture analyzer (MAC125, MAC INSTRUMENTS, USA). All the analyzers were calibrated before the tests. CCM was used for SO3 sampling at the flue gas duct between the gas cooler and the bag filter, where the temperature is about 180 ◦ C. A sintering metal filter was used to remove the particles before the flue gas entering the sampling train, and then the effect of the fly ash on the SO3 measurement can be avoided. Each sampling run lasted for 30 min under stable

6.0

0.3% SO2+6% O2+ balance N2 0.3% SO2+6% O2+ balance CO2

5.5

SO3 concentration / ppm

174

0.3% SO2+6% O2+10% H2O + balance CO2

5.0 4.5 4.0 3.5 3.0

400

500

600

700

o

Temperature / C Fig. 2. Effect of temperature on SO3 formation under different atmospheres.

condition. Four sampling runs were achieved to ensure the accuracy and repetition of the data. The determination of the SO3 concentration was the same as stated in Section 2.1. 3. Results and discussion 3.1. Homogeneous SO3 formation under different atmospheres The SO3 concentration at different temperature under different atmospheres is shown in Fig. 2. As temperature increases from 400 ◦ C to 700 ◦ C, SO3 concentration increases for all the “different atmospheres” tested and a constant inlet concentration of SO2 at 3000 ppm. It is known that for the temperature range in this study, SO3 is formed by the oxidation of the molecular oxygen via reaction (2) and (3). The formation of SO3 is thermodynamically favored as the temperature falls, but the reaction rate is very slow and it cannot reach equilibrium during the short residence time in the test. The results are consistent with previous studies (Belo et al., 2014; Fleig et al., 2013). Replacing N2 by CO2 has very slight effect on the SO3 formation at low temperature, which indicates CO2 serve essentially as an inert in the SO3 formation. This behavior is different from the results of Fleig et al. (2013) where CO2 favors SO3 formation at higher temperature range. At lower temperature, CO2 is thermally stable and the third body efficiency of CO2 in reaction (2) under this condition is also expected to be low. As can be seen in Fig. 2, steam addition over the temperature range can clearly increase the SO3 concentration, which matches the trend that Fleig et al. (2013) got at temperature from 573 ◦ C to 1173 ◦ C. They suggested that the increase in H2 O concentration promotes SO3 formation by increasing secondary SO3 formation via HOSO2 and to some extent by increasing direct SO2 oxidation. However, Belo et al. (2014) found H2 O concentration from 3 vol.% to 9 vol.% at 900 ◦ C did not have much effect on the conversion of SO2 to SO3 . Results of this study confirm that the presence of steam contributes significantly to SO2 oxidation, presumably by means of OH radical formation. Further results of the steam effect can be found in Fig. 3. As the steam concentration increases from 5% to 15%, SO3 concentration increases from 3.7 ppm to 7.1 ppm, while the SO2 conversion ratio increases from 0.24% to 0.46%. Fig. 4 shows the effect of O2 concentration on SO3 formation. As anticipated, the oxygen enrichment elevates the SO3 formation because there is more molecular oxygen available. At the low temperature used in this study, the excess oxygen serves as molecular oxygen which shifts the reaction (3) to the right side. As O2 concentration increases from 3% to 6%, SO3 concentration increases

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175

Table 1 The composition of fly ash (wt.%). Composition

Na2 O

MgO

Al2 O3

SiO2

K2 O

CaO

Fe2 O3

TiO2

SO3

P2 O5

Fly ash 1 Fly ash 2

0.462 2.55

1.36 4.51

34.14 18.52

45.12 37.84

0.99 0.37

8.66 19.99

4.16 10.37

1.53 1.09

3.06 3.07

0.09 1.09

Table 2 Ultimate and proximate analysis of coal (air dried basis). Ultimate analysis (wt.%)

Coal

Heating value (MJ/kg)

C

H

O

N

S

65.00

3.85

9.95

0.76

0.50

Proximate analysis (wt.%)

25.05

FC

V

A

M

51.68

28.38

16.18

3.76

FC: fixed carbon; V: volatile matter; A: ash; M: moisture.

8.0

0.7

SO3 concentration

7.5

Conversion ratio

7.0

0.6

0.5

6.0 5.5

0.4

5.0 4.5

0.3

Conversion ratio / %

SO3 concentration / ppm

6.5

4.0 3.5

0.2

3.0 2.5

5

10

0.1

15

H2 O concentration / % Fig. 3. Effect of H2 O concentration on SO3 formation under oxy-fuel atmosphere (inlet SO2 concentration: 3000 ppm, O2 concentration: 6%; CO2 : balance; temperature: 600 ◦ C).

7.0

0.5

SO3 concentration

6.5

Conversion ratio

6.0 0.4

5.0 4.5

0.3

4.0 3.5

Conversion ratio / %

SO3 concentration / ppm

5.5

0.2 3.0 2.5 2.0

2

4

6

8

10

12

14

16

18

0.1

O2 concentration / % Fig. 4. Effect of O2 concentration on SO3 formation under oxy-fuel atmosphere (inlet SO2 concentration: 3000 ppm; H2 O concentration: 10%; CO2 : balance; temperature: 600 ◦ C).

176

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5.2

0.5

SO3 concentration Fitting curve of the SO3 concentration Conversion ratio

5.1

0.4

4.9

0.3

4.8 0.2

4.7

Conversion ratio / %

SO3 concentration / ppm

5.0

4.6 0.1 4.5 4.4

2000

2500

3000

3500

4000

4500

0.0

5000

SO2 concentration / ppm Fig. 5. Effect of SO2 concentration on SO3 formation under oxy-fuel atmosphere (O2 concentration: 6%, H2 O concentration: 10%; CO2 : balance; temperature: 600 ◦ C).

40

gas phase Fe2O3 V2O5 CuO

30

Conversion ratio / %

SO3 concentration / ppm

35

25 20 15 10 5 400

500

600

700

2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2

gas phase Fe2O3 V2O5 CuO

400

500 600 o Temperature / C

o

Temperature / C

(a)

700

(b)

Fig. 6. Effect of metal oxides on the SO3 formation under oxy-fuel condition (O2 concentration: 6%; H2 O concentration: 10%; CO2 : balance).

3.2. Heterogeneous SO3 formation under oxy-fuel conditions Fe2 O3 , V2 O5 and CuO all display clear catalytic effect on SO3 formation under oxy-fuel condition, as shown in Fig. 6. V2 O5 acts as a much stronger catalyst than Fe2 O3 and CuO over the tested

Gas phase ash 1 ash 2

0.4

0.3 Conversion ratio / %

quickly from 3.0 ppm to 4.7 ppm. Further increasing O2 does not lead to significant increase of SO3 formation and the associated SO2 conversion ratio. It is reasonable to expect that SO3 concentrations will be higher when the input SO2 concentration increases, as shown in Fig. 5. SO2 concentration varied from 2000 ppm to 5000 ppm in this study and was chosen to represent the oxy-fuel combustion condition with flue gas recycle. A good linear fit can be achieved for the SO3 concentration over the SO2 concentration. As the O2 concentration (6% in this study) is higher than the SO2 concentration, a pseudo-first-order reaction can describe the SO3 formation very well. It can be also seen that the conversion ratio decreases as the SO2 concentration increases, which corresponds to the conclusion in previous studies (Belo et al., 2014; Fleig et al., 2013).

0.2

0.1

0.0

400

500

600

700

o

Temperature / C Fig. 7. Effect of fly ash on the SO3 formation under oxy-fuel condition (O2 concentration: 6%; H2 O concentration: 10%; CO2 : balance).

L. Duan et al. / International Journal of Greenhouse Gas Control 43 (2015) 172–178

25

10

177

700 600

8

O2 in the furnace inlet O2 in the flue gas H2O in the flue gas

15

6

SO2 SO3

10

4

400 300

SO2 / ppm

500

SO3 / ppm

O2, H2O concentration / %

20

200 5

2 100

0

air combustion

oxy-fue lcombustion

0

0

Fig. 8. SO3 concentration during oxy-fuel CFB pilot test.

temperature range, while Fe2 O3 is slightly stronger than CuO. Kim and Choi (1981) suggested that SO2 appears to be adsorbed essentially on the O lattice of a-Fe2 O3 as a molecular species, while O2 is adsorbed on an O vacancy as an ionic species and the whole catalysis mechanism of Fe2 O3 can be express as: O2 (g) + e−  O2 − (ads) −

−

(4)

−

O2 (ads) + e  2O (ads) SO2 (g) + O

2−

−

(6)

(latt)

(7)

(latt)  SO3 (ads) + e −

−

(5)

−

SO3 (ads) + O (ads) → SO3 + O

2−

V2 O5 , is a widely used catalyst on SO2 oxidation during the sulfuric acid manufacture process, and has been extensively investigated in terms of its catalysis mechanism (Dunn et al., 1999). The active site for the adsorption and coordination of sulfur dioxide on the catalyst surface is usually proposed as the pathway of this catalysis which can be represented as: SO2 (g) + (V5+ ) ↔ (V5+ )SO2 (ads)or(V3+ )SO3 (ads)

(8)

(V5+ )SO2 (ads)or(V3+ )SO3 (ads) → SO3 (g) + (V3+ )

(9)

−

O2 ↔ 2O (ads) −

O (ads) + (V

3+

(10) ) → (V

5+

)

(11)

CuO has also been confirmed as a catalyst in SO2 oxidation (Macken and Hodnett, 2000; Tseng et al., 2003). The effects of the fly ash on SO3 formation at different temperature under oxy-fuel condition are shown in Fig. 7. At 400 ◦ C, ash 1 and ash 2 both exhibit catalytic effects on SO3 formation, and ash 2 has a larger effect than ash 1. This is probably because the Fe2 O3 content in ash 2 is higher than in ash 1. At 500 ◦ C, the two fly ashes still catalyze SO3 formation even though the catalysis effect for ash 2 is minor. With a further increase of the temperature to 600 ◦ C, SO2 conversion ratios with fly ash are even less than those via gas phase reaction, indicating no catalysis takes place or the catalytic effect is dominated by other effects. At the highest temperature, 700 ◦ C, the conversion ratios with the two fly ashes are both much lower than that from the gas phase. The conversion ratio with fly ash 2 is the lowest, which indicates the alkali and alkaline earth metal oxides like CaO can absorb the SO3 and causes the SO3 in the gas phase to decrease. This is in agreement with experience that the sulfur retention rate by CaO increases as the temperature increases.

The results are different from the previous conclusion by Belo et al. (2014), which may be due to the large difference between the compositions of the fly ashes. The materials used in their study contain only a small amount of alkali and alkaline earth metal oxide, so any retention effect is insignificant. However, in our study, both fly ashes have relatively high CaO content, which will retain SO3 rather than catalyze its formation (Marier and Dibbs, 1974). Overall, the effect of fly ash on SO3 formation seems to be largely decided by the ash composition. 3.3. SO3 formation during oxy-fuel CFB pilot test Concentrations of SO3 and some other gases during pilot oxyfuel CFB combustion with wet flue gas recycle are shown in Fig. 8. The overall oxygen concentration during oxy-fuel combustion was kept at around 23.4% to maintain a similar bed temperature and a similar exit oxygen concentration to those in air combustion. SO2 concentration is observed to be higher during oxy-fuel combustion (Ca/S molar ratio = 4) than that in air combustion (Ca/S molar ratio = 2.5) due to the accumulation by gas recycle, even though more limestone has been injected into the furnace during oxy-fuel operation. Due to the amount of the flue gas during oxy-fuel combustion being only about one fourth of that during air combustion, higher limestone injection can largely decrease the total SO2 emission amount rather than decrease the SO2 concentration in the flue gas under oxy-fuel combustion (Duan et al., 2014). Consequently, SO3 concentration during oxy-fuel combustion was 7.9 ppm, about 4.5 times that in air combustion. Furthermore, H2 O concentration in the flue gas during oxy-fuel combustion was 17.4% compared with 7.1% in air combustion, which also favors the SO2 conversion to SO3 under oxy-fuel combustion. For the fly ash, it has been found (Duan et al., 2011b) calcium presents more likely in the form of CaCO3 rather than CaO due to the higher partial pressure of CO2 under oxy-fuel CFB combustion. The absorption reaction between CaO and SO3 is faster than that between CaCO3 and SO3 . The combined influence of these factors causes a higher SO3 concentration during oxy-fuel CFB combustion. This result is also in agreement with the results from our bench scale test, which confirm that SO3 during oxy-fuel CFB combustion is mainly formed via the reactions (2) and (3). The higher SO2 and H2 O concentration together with the low retention ability of fly ash in the flue gas enhances the SO3 formation during oxy-fuel CFB combustion.

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