Effect of steam and sulfur dioxide on sulfur trioxide formation during oxy-fuel combustion

International Journal of Greenhouse Gas Control 43 (2015) 1–9

Contents lists available at ScienceDirect

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

Effect of steam and sulfur dioxide on sulfur trioxide formation during oxy-fuel combustion Xiaopeng Wang, Xiaowei Liu ∗ , Dong Li, Yu Zhang, Minghou Xu ∗ State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China

a r t i c l e

i n f o

Article history: Received 1 July 2015 Received in revised form 22 September 2015 Accepted 7 October 2015 Keywords: Oxy-fuel combustion SO2 SO3 Temperature Steam

a b s t r a c t The purpose of the present study is to clarify the effects of temperature, initial SO2 concentration and steam addition on SO3 formation under oxy-fuel combustion. The experiments consisted of two parts. The first part was the homogeneous experiment conducted with the simulated flue gas in a horizontal tube furnace. In order to further study the formation characteristics of SO3 in the heterogeneous gas atmosphere, the other part of the experiments was carried out by using a high-sulfur coal and a lowsulfur coal respectively in a drop tube furnace. The experiment results indicated that SO3 concentration increased first but then decreased with the temperature increasing, and the maximum SO3 concentration appeared at around 950 ◦ C. The increase of initial SO2 concentration could significantly enhance SO3 formation. But the injected steam inhibited SO3 formation in all test cases obviously, as the injection of steam could increase the kinds of the radical pool compositions in the flue gas, which might involve more complex interactions with the formed SO3 during wet recycling. Moreover, compared to the former homogeneous experiment, the latter presented a more intense inhibiting behavior, which was probably attributed to the more diversification of the radical pool compositions in pulverized-coal combustion. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, more and more anthropogenic CO2 emission causes serious global warming, and the increasing coal consumption of coal-fired power plants in the world is one dominant cause (Li et al., 2015). CO2 capture and storage (CCS) technologies have been proposed to reduce CO2 emission from coal-fired power plants, and the oxy-fuel combustion is one of the most promising technologies in terms of the CO2 emission reduction (Chakroun and Ghoniem, 2015). The main principle of the oxy-fuel combustion is that the fuel is burned with the mixed gas of nearly pure oxygen and the recycled flue gas, instead of air (Stanger et al., 2015). Under oxy-fuel combustion, due to the absence of the diluting airborne-N2 , SO2 concentration would be much higher by a factor of 3–6 times compared to conventional air-fired conditions (Weller et al., 1985; Woycenko et al., 1994; Kiga et al., 1997; Croiset and Thambimuthu, 2001; Tan et al., 2006; Kakaras et al., 2007; Monckert et al., 2008). To a large extent, the amount of generated SO3 mainly depended on SO2 concentration in the flue gas. Moreover, SO3 concentration under oxy-fuel combustion is generally

∗ Corresponding authors at: State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China. E-mail addresses: [email protected] (X. Liu), [email protected] (M. Xu). http://dx.doi.org/10.1016/j.ijggc.2015.10.002 1750-5836/© 2015 Elsevier Ltd. All rights reserved.

several times higher than that under air-fired combustion conditions (Croiset and Thambimuthu, 2001; Ahn et al., 2011; Kenney et al., 2011; Fleig et al., 2013; Spörl et al., 2014). As for the oxy-fuel combustion with the wet recycling, high concentrations of SO3 and steam cause a relatively higher acid dew-point with an increase of 20–30 ◦ C (Fleig et al., 2009), which would lead to more serious low-temperature corrosion risks for the cold parts of coal-fired power plants. Currently, the method of diminishing this corrosion harmfulness is mainly by keeping the operation temperature of the low-temperature zones above the acid dew-point temperature. And this is realized at the expense of a lower utilization of the flue gases’ sensible heat and the power plant efficiency would be lower (Spörl et al., 2014). Therefore, it is essential to perform the fundamental research on SO3 formation under oxy-fuel combustion. The formation behavior of SO3 under oxy-fuel combustion has been extensively studied in recent years. Zheng and Furimsky (2003) concluded that SO3 concentration under oxy-fuel combustion was higher than that of conventional air-fired combustion by chemical equilibrium calculations, only relying on oxygen concentration. Fleig et al. (2011a,b, 2013) evaluated the influence of different combustion parameters on SO3 formation with a detailed gas-phase model. Moreover, they also discussed the sulfur chemical migration characteristics after completing the combustion of lignite at a pilot-scale oxy-fuel test facility (Fleig et al., 2011a,b). The auxiliary data measured by Tan et al. (2006) at a vertical combustor

2

X. Wang et al. / International Journal of Greenhouse Gas Control 43 (2015) 1–9

Fig. 1. Schematic diagram of the homogenous experimental system.

research facility showed that SO3 concentration under oxy-fuel combustion was about three times higher than that under the airfired combustion. Ahn et al. (2011) compared the concentrations of the formed SO3 concentrations at different temperature sampling points with three different coals at the pulverized-coal and circulating fluidized bed test facilities. Spörl et al. (2014) studied the effects of initial SO2 concentration and alkali/alkaline earth metals on SO3 concentration in the flue gas in a vertical tube furnace with coal as fuel. There is no doubt that the existing research results have important guiding significance to further analyze the influence mechanism of SO3 formation. Nevertheless, steam in the furnace is expected to be extensive enrichment because of the recycling of a large portion of the flue gas under oxy-fuel combustion, which has a considerable impact on the distribution characteristics of trace elements in particulate matter (Wang et al., 2014). It can be assumed that steam in the flue gas may also affect SO3 formation. Therefore, this field is worth to perform some research to reveal the influence mechanism of steam on SO3 formation. Glarborg et al. (1996) demonstrated that steam would greatly inhibit SO3 formation once the mixed flue gas contained CO in a quartz flow reactor. Fleig et al. (2012) found that the injected steam had positive impacts on SO3 formation by using propane as fuel at a pilot-scale oxy-fuel test facility. But the inhibiting phenomenon could be observed apparently at a temperature range of 1000–1400 K with the injection of CO in a vertical quartz glass tube furnace (Fleig et al., 2013). In conclusion, the effect of steam on SO3 formation is very complicated, and some disagreements occur in the above mentioned conclusions about the effect of steam on SO3 formation. Moreover, the available results are mainly based on the homogeneous gas-phase atmosphere. Until now, almost no available researches about the effect of steam on SO3 formation in pulverized-coal combustion have yet been explored, so the influence mechanism of steam on SO3 formation should be further studied.

To improve the full understanding on the formation mechanism of SO3 under different gas atmospheres, it is necessary to investigate the potential impact of each combustion parameter on the SO3 formation characteristics at the pulverized-coal test facility, especially for the case of the wet recycling with high concentration of steam. In the present study, a horizontal tube furnace was applied to perform the homogeneous experiments. Simultaneously, the pulverized-coal experiments were conducted in a drop tube furnace. Through the above different types of experiments, the potential effects of temperature, SO2 concentration and steam addition on the formation of SO3 for oxy-fuel operation were extensively clarified, together with the formation mechanism being comprehensively analyzed. 2. Experimental 2.1. Sampling and measurement methodology The SO3 measurement method is another critical factor to the accuracy of the experimental data. The method used in the present study is the controlled condensation method (CCM) according to the German standard method VDI2462, sheet 7. As detailed in Figs. 1 and 2, the SO3 measurements were performed at the outlet of the two reactors in all cases by linking a petty quartz glass tube to the inlet of the spiral tube. The spiral tube was placed in the water bath to maintain a suitable temperature. The water temperature (denoted as T1 ) was always maintained between 75 ◦ C and 85 ◦ C, and it was set to 80 ◦ C in all experiments of the present study. Therefore, the flue gas temperature could be kept above the water dew-point temperature but below the acid dew-point temperature. As a result, SO3 and the gas sulfuric acid would condense effectively onto the surface of the spiral tube without the condensing of SO2 and steam. The process design could make SO3 as well as the gas

Fig. 2. Schematic diagram of the pulverized-coal experimental system.

X. Wang et al. / International Journal of Greenhouse Gas Control 43 (2015) 1–9

sulfuric acid selectively separate from the mixed flue gas. Moreover, in order to ensure the reliability of the collection, the gas temperature was monitored continuously at the inlet and outlet of the spiral tube during all test cases. The fluctuation range of the inlet temperature (denoted as T2 ) was in 215 ± 15 ◦ C and that of the outlet temperature (denoted as T3 ) was in 61 ± 3 ◦ C, which meant a rather perfect SO3 absorption could be achieved by this method. Then the remaining flue gas passed through four impingers. The first three impingers in the ice bath contained the 3% hydrogen peroxide solution to oxidize and capture SO2 , and the last impinger was filled with silica gel to eliminate the residual moisture prior to the flue gas entering the dry gas flow meter. The spiral tube and the first three impingers were respectively flushed with the deionized water after each sampling sequence. The volume of both solutions was measured by the volumetric flask. Finally, the sulfur oxide in the flue gas was completely transformed into sulfates, and then measured by the ion chromatography system (ICS-90, DIONEX). It is worth mentioning that the instrument would be regularly calibrated with the GSB 04-1773-2004 sulfate ion standard solution, and all samples were analyzed at least three times to ensure the precise results. Moreover, the mean value calculated from three single measurements was the desirable result to be used. 2.2. Homogeneous experiment test facility The homogenous experiments were conducted in an electrically heating horizontal tube furnace (shown in Fig. 1). A quartz glass tube with an inner diameter of 50 mm and a length of 1500 mm was inserted in the furnace to be the reactor, and the maximum temperature of the furnace was 1100 ◦ C. The non-corrosive gases (O2 and CO2 ) and corrosive SO2 from tanks were all fed into the furnace by the anticorrosion mass flow meter (MFC), and steam was generated and supplied by the control-evaporation-mixing (CEM) system. The above mixed gases were used to simulate the oxy-fuel gas atmosphere with a constant total gas volume rate of 2 L/min. Table 1 lists the experimental test conditions of all experiments in detail. Of particular note is that due to the restriction of the gas purification technology, the non-corrosive gases (O2 and CO2 ) from tanks are not entirely pure gases with a purity of about 99.9%, and the concentrations of steam in the tanks are all about 0.03%. In order to make the experiment standard, the quartz glass tube should be full of the mixed flue gas but without SO2 before beginning each sampling procedure. Then when SO2 was imported into the tube, the sampling began and each sampling time was set to 30 min. Once the sampling procedure was completed, SO2 was immediately closed, but other gases were still introduced into the tube to guarantee the rest SOx could be collected completely. All test cases were performed at least three times to ensure the repeatability. 2.3. Pulverized coal test facility The pulverized-coal experiments were carried out in a drop tube furnace (shown in Fig. 2), essentially similar to an electrically heating vertical tube furnace. A disc vibration coal feeder was used as the coal feeding system, with a coal feeding rate of 0.2 g/min and a size distribution range of about 55–90 ␮m. The coal feeder also should be calibrated before the coal particles were pneumatically entrained into the reactor. The reactor was a single quartz glass tube with an inner diameter of 60 mm and a length of 1500 mm. The length of the isothermal reaction zone is designed to 1000 mm, which made it possible to simulate the combustion of the pulverized coal under the post-flame conditions in coal-fired power plants. There is a water-cooled quartz injector connected with the top of the reactor, and the primary gas stream conveys

3

Table 1 Inlet gas compositions of all homogeneous test cases. Test

T (◦ C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

350 350 350 550 550 550 750 750 750 750 750 850 850 850 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 1050 1050 1050

Inlet gas composition SO2 (ppm)

O2 (%)

H2 O (%)

CO2 (%)

500 1500 2500 500 1500 2500 500 700 1500 2500 3000 500 1500 2500 500 700 1500 2500 3000 500 700 1500 2500 3000 500 700 1500 2500 3000 500 700 1500 2500 3000 500 1500 2500

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 15.03 15.03 15.03 15.03 15.03 25.03 25.03 25.03 25.03 25.03 35.03 35.03 35.03 35.03 35.03 0.03 0.03 0.03

95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 80 80 80 80 80 70 70 70 70 70 60 60 60 60 60 95 95 95

the pulverized coal to the combustion zone by passing through the injector. In an individual experiment, the mixed flue gas entered the sampling setup after the fly ash was efficiently captured by a baghouse filter at the downstream of the reactor. The total gas volume flow of all experiments was maintained to 4 L/min. Table 2 shows the test conditions of all the experiments in detail. A low-sulfur Huo Lin He (HLH) lignite coal and a high-sulfur Ping Ding Shan (PDS) bituminous coal were selected to be used for the experiments in the drop tube furnace. The properties of these two kinds of coals were listed in Tables 3 and 4, and the molar alkaline and alkaline earth metals to sulfur were presented in Table 5. 2.4. Sulfur mass balance In order to evaluate the accuracy of all the performed experiments, the sulfur mass balance was calculated on the basis of the coal’s sulfur and the injected SO2 . After each experiment, the measured sulfur was almost included into the flue gas, condensed water and the fly ash. The measured data could be used to calculate the total sulfur mass balance. During the homogenous campaigns, the measured sulfur was almost included into the flue gas and condensed water due to the absence of fly ash. Hardly, the amount of sulfur in the condensed water could be negligible. Table 6 lists the sulfur mass balance of each test case. The deviations of measured and theoretical value were in a range between −18 and −2%. A significant difference between the pulverized-coal experiment and the homogenous experiment was whether the fly ash existed, which had the capability of capturing the formed SO3 in

4

X. Wang et al. / International Journal of Greenhouse Gas Control 43 (2015) 1–9

Table 2 Inlet gas compositions of all pulverized-coal test cases. Test

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

T (◦ C)

950

Table 5 Molar alkali and earth-alkali to sulfur ratios for HLH and PDS coals.

Inlet gas composition SO2 (ppm)

O2 (%)

H2 O (%)

CO2 (%)

500 700 1500 2500 3000 500 700 1500 2500 3000 500 700 1500 2500 3000 500 700 1500 2500 3000 500 700 1500 2500 3000

27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27

0.03 0.03 0.03 0.03 0.03 5.03 5.03 5.03 5.03 5.03 15.03 15.03 15.03 15.03 15.03 25.03 25.03 25.03 25.03 25.03 35.03 35.03 35.03 35.03 35.03

73 73 73 73 73 68 68 68 68 68 58 58 58 58 58 48 48 48 48 48 38 38 38 38 38

the flue gas. Therefore, the amount of sulfur self-retention by the coal ash should be quantified. However, it was found that no difference existed between the dry flue gas recycling and the wet flue gas recycling, which meant that the sulfur content in the fly ash samplings from the two types of the flue gas recycling had no obvious difference with the presence and absence of steam. Table 7 presents the sulfur mass balance of these two coals, and the deviation ranged from −9 up to +18%. Based on the above discussion, the sulfur mass balance was quite good by comparing the measured and theoretical sulfur masses, and the deviations were in the acceptable range for all the test cases. 3. Results and discussion 3.1. Influence of temperature on SO3 formation Fig. 3 shows the mean SO3 concentrations at different operating temperatures during the simulated dry flue gas recycling and three different initial SO2 concentrations are 500 ppm, 1500 ppm and 2500 ppm separately. As shown in Fig. 3, there is an obvious increasing first and then decreasing trend for the measured SO3 concentrations at different temperatures. When the temperature was below 750 ◦ C, the measured SO3 concentration was very small and almost beyond the level of detection in some cases. At the

Coal

Na/2S

K/2S

Mg/S

Ca/S

HLH PDS

0.17 0.05

0.11 0.04

0.36 0.10

0.95 0.25

elevated temperatures, SO3 concentrations were relatively sensitive to the temperature and the maximum concentration reached at about 950 ◦ C. But SO3 concentration decreased roughly at higher temperatures. Moreover, the trend of SO3 concentration change with the temperature was almost alike in relation to different initial SO2 concentrations. The published results obtained by Belo et al. (2014) also provided the similar variation tendency with a maximum value at 700 ◦ C in the heterogeneous experiments, the influence of different combustion parameters on the conversion of SO2 to SO3 was studied by performing the experiments in an electrically heated horizontal furnace, concluding that the catalytic components of the fly ash (particularly Fe2 O3 ) had obvious impact on the conversion of SO2 to SO3 . SO3 mainly formed during the cooling process of the flue gas in the post-flame region. The formation and decomposition mechanism involves a great many interactions of SOx with the radical pool compositions. Moreover, the flue gas components under different combustion conditions may have some impacts on certain radicals, thus indirectly affecting SO3 formation. Obviously, the Ocycle was of great significance for the formation and consumption of SO3 under the present conditions based on the following reaction mechanism. SO2 + O(+M)  SO3 (+M)

(1)

SO3 + O  SO2 + O2

(2)

Under the investigated conditions, O radical derived mostly from the decomposition of oxygen itself. O2  O + O

(3)

SO3 formation absolutely depended on the temperature and O radical concentration, and the temperature could affect the SO2 oxidation indirectly by influencing the amount of O radical (Merryman and Levy, 1979). The rate constants of Reaction (1) of different temperatures obtained from previously published investigation (Mueller et al., 2000), as well as Reaction (2) (Alzueta et al., 2001), are shown in Table 8. It was obvious that the rate constants of Reaction (1) did not change significantly as the temperature increased. However, the rate constants of Reaction (2) were apparently sensitive to the variation of the temperature. The consumption of SO3 was mainly realized via the decomposition of Reaction (2), and its reaction rate escalated as the temperature increased, which meant that a faster decomposition rate would appear at higher temperature. Because the O radical

Table 3 Ultimate and proximate analyses of HLH and PDS coals [wt.%, ad basis]. Coal

Moisture

HLH PDS

7.71 4.11

a

Ash 21.32 35.03

Volatile matter

Fixed carbon

C

H

Oa

N

S

34.54 28.85

36.43 32.02

49.59 44.47

5.54 3.17

14.19 8.80

1.05 0.73

0.60 3.69

Calculated by difference.

Table 4 Ash analysis of HLH and PDS coals. Coal

Na2 O

MgO

K2 O

CaO

Al2 O3

SO3

Fe2 O3

SiO2

P2 O5

MnO

TiO2

HLH PDS

0.90 1.12

1.27 1.12

0.93 1.17

4.68 4.57

18.04 29.81

4.69 8.23

3.42 3.99

64.39 48.82

0.95 0.26

0.07 0.02

0.48 0.64

X. Wang et al. / International Journal of Greenhouse Gas Control 43 (2015) 1–9

5

Table 6 Sulfur mass balance of all homogeneous test cases. Test 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Balance 0.86 0.85 0.90 0.90 0.91 0.88 0.94 0.93 0.93 0.83 0.87 0.86 0.95 0.98 0.93 0.84 0.84 0.82 0.86 0.91 0.96 0.88 0.93 0.86 0.92 0.93 0.85 0.89 0.88 0.87 0.92 0.91 0.87 0.84 0.84 0.86 0.93

Fig. 3. Measured SO3 concentration at different operating temperatures in the homogeneous experiments. Table 8 Rate constants of Reactions (1) and (2) under different temperature conditions.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

k1 (cm3 mol−1 s−1 )

k2 (cm3 mol−1 s−1 )

350 550 750 850 950 1050 1500

5.81 × 1010 6.5 × 1010 6.95 × 1010 7.13 × 1010 7.28 × 1010 7.41 × 1010 7.83 × 1010

2.15 × 105 1.06 × 107 1.14 × 108 2.72 × 108 5.63 × 108 1.04 × 109 7.11 × 109

concentration is very low before 750 ◦ C, the rate and the extent of SO2 oxidation were consequently very small. Therefore, minor SO3 could be measured at the downstream of the tube furnace. When the temperature increased to 950 ◦ C, O radical concentration increased drastically, and SO2 oxidation rate became faster to form more SO3 . Conversely, when the temperature rose above 950 ◦ C, SO3 decomposition rate might accelerate roughly, and this would make for SO2 formation, so SO3 concentration decreased. The discrepancy of SO3 formation between Reactions (1) and (2) gradually became smaller, but the former always had the relative advantage, so the concentration of the formed SO3 was still fewer compared to its concentration at 950 ◦ C.

Table 7 Sulfur mass balance of all pulverized-coal test cases. Test

T (◦ C)

3.2. Influence of initial SO2 concentration on SO3 formation

Balance HLH

PDS

0.92 1.00 0.94 1.02 0.99 1.00 1.01 0.99 0.99 1.00 1.01 0.98 0.96 0.99 0.94 0.98 0.93 0.96 0.97 0.98 1.10 0.91 0.91 0.98 0.95

1.18 1.03 1.08 1.06 1.04 1.16 1.06 1.09 0.97 0.99 1.10 0.99 1.04 0.98 0.98 1.08 1.03 1.03 1.00 0.98 1.17 1.06 1.09 1.02 1.00

Fig. 4 shows that the measured SO3 concentrations varied with initial SO2 concentration during the temperature range from 750 ◦ C to 950 ◦ C in the homogenous experiments with the simulated

Fig. 4. Measured SO3 concentration with different initial SO2 concentrations in homogeneous experiments.

6

X. Wang et al. / International Journal of Greenhouse Gas Control 43 (2015) 1–9

Fig. 5. Measured SO3 concentration with different initial SO2 concentrations in pulverized-coal experiments.

dry flue gas recycling. It can be observed that SO3 concentration increased with an increasing SO2 concentration under these two temperature points, and a smaller amount of the formed SO3 was found under the lower temperature. Fig. 5 shows the impact of initial SO2 concentration on SO3 formation at the pulverized-coal test facility. It can be seen from Fig. 5 that the concentration of the formed SO3 in the pulverizedcoal experiments has the similar variation trend with that in the homogenous experiments. It is noted that for the same initial SO2 concentration, a larger concentration of SO3 formed in the experiments with the high-sulfur coal (PDS) while minor SO3 concentration was found in the experiments with the low-sulfur coal (HLH). When initial SO2 concentration changed from 700 ppm to 1500 ppm, the formed SO3 concentration of the low-sulfur increased sharply from 0.914 ppm to 24.189 ppm, while the formed SO3 concentration of the high-sulfur coal increased smoothly from 20.597 ppm to 36.076 ppm. This phenomenon was probably associated with the ash properties. As shown in Table 5, the molar alkaline and alkaline earth metals to sulfur ratios of the low-sulfur were comparatively higher than those of the high-sulfur coal, which meant that the low-sulfur coal had more advantage on capturing SO3 in the flue gas. For the cases with low initial SO2 concentrations, the formed SO3 might effectively be captured by the entrained fly ash of the low-sulfur coal. Only when the initial SO2 concentration increased above a certain level, there would be distinct measured SO3 in the flue gas, which was probably linked to the saturation limitation of the alkaline and alkaline earth compounds. Therefore, the uncaptured gas-phase SO3 would be collected by the condensing device. The above results illustrated that the SO3 formation was not only related to the combustion condition, but also influenced by the experimental apparatus and coal type.

Fig. 6. Influence of steam on SO3 concentration in the homogeneous experiments.

with the injection of steam, which indicated steam could inhibit SO3 formation. When the injected steam accounted for 15% of the total simulated flue gas, the inhibiting effects on the formation of SO3 was very obvious. Moreover, for these three cases with different high concentrations of steam, the difference on SO3 concentrations with the same initial SO2 concentration was relatively small. The formed SO3 concentration increased with an increasing initial SO2 concentration as before, which indicated that the injection of steam did not change the influencing effect of SO2 concentration on SO3 formation. Figs. 7 and 8 show the outlet SO3 concentrations in the pulverized-coal experiments. The variation trend of SO3 formation was consistent with the homogenous cases, but the more inhibiting effect of steam was found. It can be seen that SO3 concentration can be reduced greatly by the very low concentration of steam, especially for the low-sulfur coal, and the SO3 concentration became very low with the highest steam injection. As for the high-sulfur coal, the outlet SO3 concentration of each test case was always higher than the low-sulfur coal, and the amount practically had no difference under the higher steam conditions. As for these two coals, the inhibiting effect would be more strengthened as the steam concentration gradually increased. Moreover, if the concentration of the formed SO3 was below the capture limitation of the ash, SO3 concentration in the flue gas would be inevitably very small, which was more visible for the

3.3. Influence of steam on SO3 formation The flue gas in the wet recycling oxy-fuel combustion generally contains high concentration of steam, even above 30% (Hecht et al., 2012). Steam can affect the flame combustion characteristics, and also can react abundantly with the unburned carbon in the inflame zone. Furthermore, steam has a significant influence on the concentrations of certain radicals, thus affecting the SO3 formation indirectly. Also, the diversification of the radical pool compositions is correspondingly richer than the case of the dry recycling. Fig. 6 shows the measured SO3 concentrations in the homogenous experiments with the simulated wet flue gas recycling. It was noteworthy that the outlet SO3 concentration decreased extremely

Fig. 7. Influence of steam on SO3 concentration in HLH pulverized-coal experiments.

X. Wang et al. / International Journal of Greenhouse Gas Control 43 (2015) 1–9

7

Table 9 Rate constants of Reaction (4) under different temperature conditions.

Fig. 8. Influence of steam on SO3 concentration in PDS pulverized-coal experiments.

low-sulfur coal. Compared with the homogenous experiments, the stronger inhabiting effect of steam in the pulverized-coal experiments was also probably linked to the complex flue gas atmosphere. In the homogenous experiments, steam would react with O radical to form OH radical (Fleig et al., 2011a,b). H2 O + O  OH + OH

(4)

Simultaneously, the self-decomposition reaction of steam also happened (Wine et al., 1984). H2 O  OH + H

(5)

During the wet recycling, besides Reaction (1), SO3 is also mainly formed via the intermediate of HOSO2 (Hindiyarti et al., 2007). SO2 + OH(+M)  HOSO2 (+M)

(6)

HOSO2 + O2  SO3 + HO2

(7)

Apart from Reaction (2) described above, the combustion process is also accompanied by the consumption of SO3 through the following reactions. SO3 + H  SO2 + OH

(8)

SO3 + HO2  HOSO2 + O2

(9)

SO3 + OH  SO2 + HO2

(10)

In conclusion, the radical pool compositions in the simulated wet flue gas recycling is evidently critical to SO3 formation, the major consumption source of SO3 not only derived from Reaction (8) (Hindiyarti et al., 2007), but also from Reaction (9) (Glarborg et al., 1996; Mueller et al., 2000). Also, Reaction (2) played an important role in the consumption of SO3 (Glarborg et al., 1996), and minor consumption of SO3 resulted from Reaction (10) (Hindiyarti et al., 2007). Through Reactions (4) and (5), the injected steam could produce plenty of OH radical, as well as H radical, making the OH/O ratio increase sharply. The rate constants of Reaction (4) at different temperatures are listed in Table 9 (Mueller et al., 2000). It can be found that the rate constant of Reaction (4) is several orders of magnitude higher than that of Reaction (1) at the same temperature range. Also, the steam concentration was far greater than the SO2 concentration in the flue gas under the present conditions. What is more, steam was obviously superior to SO2 during the competitive processes for O radical. As a result, O radical was mostly converted to OH radical, and the SO2 oxidation directly by Reaction (1) was diminished. Therefore, Reactions (6) and (7) were considered to be the main paths to generate SO3 . As for Reaction (4), two OH radical

T (◦ C)

k (cm3 mol−1 s−1 )

350 550 750 850 950 1050 1500

9.88 × 1010 3.25 × 1011 7.39 × 1011 1.03 × 1012 1.37 × 1012 1.78 × 1012 4.37 × 1012

would be produced when one single O radical was consumed, and this would produce a large amount of OH radical in the flue gas, which indirectly affected the SO3 formation and consumption. While OH radical enhanced the SO3 formation, it could also inhibit SO3 formation by consuming SO3 directly through Reaction (10). The enriched O radical in the flue gas could not only consume SO3 through Reaction (2), but also react with OH radical from Reaction (11): O + OH  HO2

(11)

To some extent, this reaction was largely dependent upon O and OH radical levels in the flue gas. Besides enhancing the reduction of SO3 by the reverse direction of Reaction (7), the great amount of HO2 generated from Reactions (10) and (11) was also involved in the recombination reaction with OH radical from Reaction (12). And this reaction further lowered OH radical levels. OH + HO2  H2 O + O2

(12)

In addition, H radical coming from the dissociation of steam not only consumed SO3 by Reaction (8), but also reacted with O2 to form HO2 according to Reaction (13). H + O2  HO2

(13)

This reaction reduced oxygen partial pressure in the flue gas, and the generated HO2 radical decreased SO3 formation directly or indirectly through the above relevant radical reactions. The overall superimposed combined reactions weakened the contribution of Reactions (6) and (7) to the formation of SO3 . Furthermore, Glarborg et al. (1996) found that HOSO2 was very unstable and decomposed rapidly from the reverse reaction of Reaction (6), and only about 1% of the consumed HOSO2 participated in Reaction (7) to be oxidized. They also found the SO3 /SO2 ratio generally dropped as steam levels increased and demonstrated that steam could inhibit SO3 formation, which was in good agreement with the results of the present study. In a word, the injected steam help increase OH radical levels sharply, but the OH-cycle did not increase SO3 concentration. On the contrary, the formation of OH radical was mainly by steam competing O radical with SO2 , which prevented SO2 being oxidized directly. Moreover, the contribution of steam to the formation of SO3 was particularly small, so steam could inhabit SO3 formation obviously. In the pulverized-coal experiments, SO3 concentration in the flue gas decreased apparently and steam inhibited SO3 formation, in accordance with the homogeneous experiment. Therefore the inhibition mechanism for the homogeneous experiments was also applicable to the pulverized-coal experiments. The flue gas components during the pulverized-coal combustion were more complex than those in the experiments with homogeneous atmosphere. A large amount of CO, NOx and other gases produced from coal combustion could affect the formation of SO3 by reacting with certain radicals. As a consequence, the inhibition mechanism of steam on the formation of SO3 was much more complicated during pulverized-coal combustion. Besides, the injected steam had minor impact on the sulfur captured efficiency of the fly ash because there

8

X. Wang et al. / International Journal of Greenhouse Gas Control 43 (2015) 1–9

was no obvious difference emerging among the sulfur contents of the fly ash under different conditions. As for the experiments with the simulated wet flue gas recycling, steam had an influence on the reactivity of gasification, and the activation energy for the coal char gasification with steam was generally lower than that with CO2 . In addition, the gasification rate of coal char with steam is several times higher than that with CO2 (Liu et al., 2009). Consequently, CO concentration was much higher in the wet recycling than the dry recycling. The recombination of CO with OH came up according to Reaction (14). CO + OH  CO2 + H

higher than that from the low-sulfur coal, which indicates that coal type is also a critical factor in terms of SO3 formation. 3. The injected steam can significantly decrease the concentration of the formed SO3 . With the increase of steam concentration, the inhibiting effect caused by steam tends to be stronger. Furthermore, the inhibiting effect is more pronounced in the pulverized-coal experiments, due to the complex components of the flue gas. Acknowledgments

(14)

This reaction could significantly reduce the concentration of OH radical in the flue gas, thereby the amount of SO3 generated through Reactions (6) and (7) would be reduced. Furthermore the H radical formed from Reaction (14) also reacted with certain relevant radical pool compositions to further inhibit the SO3 formation. Because the recycled flue gas contained large amount of SO2 , the presence of SO2 could affect both the induction time and the post-induction oxidation of CO. The induction time might be prolonged even more than an order of magnitude. Consequently, the rapid oxidation of CO would be inhibited (Glarborg et al., 1996). Due to the weaker competitive ability of SO2 for OH radical than CO, the recombination reaction between SO2 and OH radical hardly happened, which possibly facilitated the inhibition of CO on SO3 formation indirectly (Gleason et al., 1987). In the flame zone, NOx was also produced besides the main gas product of CO, and NO might had some impacts on the SO3 formation by affecting the radical levels. In recent years, great attention have been drawn to the interactions between sulfur chemistry and nitrogen chemistry (Glarborg, 2007). During the pulverizedcoal combustion, the following radical pool reactions probably occurred: NO + HO2  NO2 + OH

(15)

NO + O(+M)  NO2 (+M)

(16)

NO + OH(+M)  HONO(+M)

(17)

Some fractions of HO2 was consumed to form OH radical through Reaction (15), and this would enhance SO3 formation, but Reactions (16) and (17) competed with Reactions (1) and (6) for the O and OH radicals, which were critical to the formation of SO3 . Therefore, to a certain extent, NO could inhibit the formation of SO3 to some extent. 4. Summary and conclusions In this paper, the homogeneous and the pulverized-coal experiments are separately carried out at the horizontal tube furnace and the drop tube furnace test facilities under laboratory conditions. The effects of different combustion parameters on SO3 formation are systematically studied to further explore the influence mechanism of temperature, initial SO2 concentration, steam concentration and the ash property on the formation of SO3 . The following conclusions are obtained from the results of the present study. 1. As increasing the experimental temperature, SO3 concentrations with different initial SO2 concentrations have the same variation tendency of increasing first and then decreasing, achieving a maximum value at around 950 ◦ C. 2. With an increasing SO2 concentration, the formed SO3 concentrations at both experimental facilities increase significantly. Moreover, the generated SO3 from the high-sulfur coal is much

The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China under Award Numbers 51476064 & U1261204. We also express our appreciation to the Analytical and Testing Center at the Huazhong University of Science and Technology for the testing of samples. References Ahn, J., Okerlund, R., Fry, A., Eddings, E.G., 2011. Sulfur trioxide formation during oxy-coal combustion. Int. J. Greenh. Gas Control 5, S127–S135. Alzueta, M.U., Bilbao, R., Glarborg, P., 2001. Inhibition and sensitization of fuel oxidation by SO2 . Combust. Flame 127, 2234–2251. Belo, L.P., Elliott, L.K., Stanger, R.J., Spörl, R., Shah, K.V., Maier, J., Wall, T.F., 2014. High-temperature conversion of SO2 to SO3 : homogeneous experiments and catalytic effect of fly ash from air and oxy-fuel firing. Energy Fuels 28, 7243–7251. Chakroun, N.W., Ghoniem, A.F., 2015. High-efficiency low LCOE combined cycles for sour gas oxy-combustion with CO2 capture. Int. J. Greenh. Gas Control 41, 163–173. Croiset, E., Thambimuthu, K.V., 2001. NOx and SO2 emissions from O2 /CO2 recycle coal combustion. Fuel 80, 2117–2121. Fleig, D., Alzueta, M.U., Normann, F., Abián, M., Andersson, K., Johnsson, F., 2013. Measurement and modeling of sulfur trioxide formation in a flow reactor under post-flame conditions. Combust. Flame 160, 1142–1151. Fleig, D., Andersson, K., Johnsson, F., Leckner, B., 2011a. Conversion of sulfur during pulverized oxy-coal combustion. Energy Fuels 25, 647–655. Fleig, D., Andersson, K., Johnsson, F., 2012. Influence of operating conditions on SO3 formation during air and oxy-fuel combustion. Ind. Eng. Chem. Res. 51, 9483–9491. Fleig, D., Andersson, K., Normann, F., Johnsson, F., 2011b. SO3 formation under oxyfuel combustion conditions. Ind. Eng. Chem. Res. 50, 8505–8514. Fleig, D., Normann, F., Andersson, K., Johnsson, F., Leckner, B., 2009. The fate of sulphur during oxy-fuel combustion of lignite. Energy Proc. 1, 383–390. Gleason, J.F., Sinha, A., Howard, C.J., 1987. Kinetics of the gas-phase reaction HOSO2 + O2 → HO2 + SO3 . J. Phys. Chem. 91, 719–724. Glarborg, P., Kubel, D., Dam-Johansen, K., Chiang, H.M., Bozzelli, J.W., 1996. Impact of SO2 and NO on CO oxidation under post-flame conditions. Int. J. Chem. Kinet. 28, 773–790. Glarborg, P., 2007. Hidden interactions—trace species governing combustion and emissions. Proc. Combust. Inst. 31, 77–98. Hecht, E.S., Shaddix, C.R., Geier, M., Molina, A., Haynes, B.S., 2012. Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char. Combust. Flame 159, 3437–3447. Hindiyarti, L., Glarborg, P., Marshall, P., 2007. Reactions of SO3 with the O/H radical pool under combustion conditions. J. Phys. Chem. 111, 3984–3991. Kakaras, E., Koumanakos, A., Doukelis, A., Giannakopoulos, D., Vorrias, I., 2007. Oxyfuel boiler design in a lignite-fired power plant. Fuel 86, 2144–2150. Kenney, J.R., Clark, M.M., Levasseur, A.A., Kang, S.G., 2011. Emissions from a tangentially-fired pilot scale boiler operating under oxy-combustion conditions. In: IEAGHG Special Workshop on Oxyfuel Combustion, London, UK. Kiga, T., Takano, S., Kimura, N., Omata, K., Okawa, M., Mori, T., Kato, M., 1997. Characteristics of pulverized-coal combustion in the system of oxygen/recycled flue gas combustion. Energy Convers. Manage. 38, 129–134. Li, W., Zhang, X., Lu, B., Sun, C., Li, S., Zhang, S., 2015. Performance of a hybrid solvent of amino acid and ionic liquid for CO2 capture. Int. J. Greenh. Gas Control 42, 400–404. Liu, H., Zhu, H., Kaneko, M., Kato, S., Kojima, T., 2009. High-temperature gasification reactivity with steam of coal chars derived under various pyrolysis conditions in a fluidized bed. Energy Fuels 24, 68–75. Monckert, P., Dhungel, B., Kull, R., Maier, J., 2008. Impact of combustion conditions on emission formation (SO2 , NOx ) and fly ash. In: 3rd Meeting of the Oxy-Fuel Combustion Network, Yokohama Symposia, Yokohama, Japan. IEA Greenhouse Gas R&D Programme. Merryman, E.L., Levy, A., 1979. Enhanced SO3 emissions from staged combustion. In: Symposium (International) on Combustion, pp. 727–736. Mueller, M.A., Yetter, R.A., Dryer, F.L., 2000. Kinetic modeling of the CO/H2 O/O2 /NO/SO2 system: implications for high-pressure fall-off in the SO2 + O(+M) = SO3 (+M) reaction. Int. J. Chem. Kinet. 32, 317–339.

X. Wang et al. / International Journal of Greenhouse Gas Control 43 (2015) 1–9 Spörl, R., Walker, J., Belo, L., Shah, K., Stanger, R., Maier, J., Scheffknecht, G., 2014. SO3 emissions and removal by ash in coal-fired oxy-fuel combustion. Energy Fuels 28, 5296–5306. Stanger, R., Wall, T., Spörl, R., Paneru, M., Grathwohl, S., Weidmann, M., Scheffknecht, G., McDonald, D., Myöhänen, K., Ritvanen, J., Rahiala, S., Hyppänen, T., Mletzko, J., Kather, A., Santos, S., 2015. Oxyfuel combustion for CO2 capture in power plants. Int. J. Greenh. Gas Control 40, 55–125. Tan, Y., Croiset, E., Douglas, M.A., Thambimuthu, K.V., 2006. Combustion characteristics of coal in a mixture of oxygen and recycled flue gas. Fuel 85, 507–512. Wang, C., Liu, X., Li, D., Wu, W., Xu, Y., Si, J., Zhao, B., Xu, M., 2014. Effect of H2 O and SO2 on the distribution characteristics of trace elements in particulate matter at high temperature under oxy-fuel combustion. Int. J. Greenh. Gas Control 23, 51–60.

9

Weller, A.E., Rising, B.W., Boiarski, A.A., Nordstorm, R.J., Barrett, R.E., Luce, R.G., 1985. Experimental evaluation of firing pulverized coal in a CO2 /O2 atmosphere. Argonne National Laboratory Report: ANL/CNSV-TM-168.342. Wine, P.H., Thompson, R.J., Ravishankara, A.R., Semmes, D.H., Gump, C.A., Torabi, A., Nicovich, J.M., 1984. Kinetics of the reaction OH + SO2 + M → HOSO2 + M. Temperature and pressure dependence in the fall-off region. J. Phys. Chem. 88, 2095–2104. Woycenko, D., Ikeda, I., van de Kamp, W.L., 1994. Combustion of pulverised coal in a mixture of oxygen and recycled flue gas. International Flame Research Foundation, Technical Report IFRF Doc F98/Y/1. Zheng, L., Furimsky, E., 2003. Assessment of coal combustion in O2 + CO2 by equilibrium calculations. Fuel Proc. Technol. 81, 23–34.