AC catalyst-sorbent for simultaneous SO2 and NO removal

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 35, Issue 3, June 2007 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2007, 35(3), 344−348

RESEARCH PAPER

NH3 regeneration of SO2-captured V2O5/AC catalyst-sorbent for simultaneous SO2 and NO removal GUO Yan-xia1,2, LIU Zhen-yu1,3,*, LI Yun-mei1, LIU Qing-ya3 1

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

2

Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

3

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Abstract: Activated coke (AC) supported V2O5 (V2O5/AC) is an efficient catalyst-sorbent for simultaneous SO2 and NO removal from flue gas at 150–200 °C. It was found that this type of catalyst-sorbent can be effectively regenerated in the presence of NH3 at temperatures much lower than required by AC under an inert atmosphere. Systematic experiments were performed in this study to understand the effect of regeneration conditions on SO2 and NO removal activities in the subsequent cycles, including regeneration temperature (200–380 °C), NH3 concentration (0–5%), and regeneration time (10–80 min). Results indicate that the optimal regeneration conditions of SO2-captured V2O5/AC are: 300 °C, 3–5 % NH3 in an inert atmosphere, 60 min. With the aid of elemental analysis and FT-IR characterization, it was found that the regeneration serves three roles: desorption of the adsorbed sulfur species from the catalyst-sorbent, surface modification by NH3, and adsorption (storage) of NH3 on the surface. The roles of NH3 are mainly in surface modification (chemical effect) and storage of NH3 on the surface (physical effect). The former occurs at relatively higher temperatures and favors SO2 removal and the latter is more significant at lower temperatures and favors NO removal. Key Words: V2O5/AC; SO2 and NO removal; regeneration; NH3

SO2 and NOx in flue gas are major air pollutants, causing acid rain and photochemical smog. Controlling their emissions is important for the clean use of energy and protection of environment in China. Activated coke/carbon (AC) has been applied industrially for SO2 removal under dry conditions at around 100 °C and selective catalytic reduction (SCR) of NOx by NH3 at relatively higher temperatures[1–3]. Loading V2O5 on activated coke (V2O5/AC) improves the activities of SO2 and NOx removal of AC and elevates the operational temperature to 200 °C, which makes the technology more economical[4]. The SO2 removal by AC or V2O5/AC is actually storage of sulfur species in the catalyst/adsorbent, which is different than NOx removal by SCR reaction to harmless N2. SO2 adsorbed on the AC or V2O5/AC surfaces is oxidized to SO3 first, and then converted to sulfuric acid with the aid of H2O or to ammonium sulfate with the aid of NH3 used for NO removal and store in the pores. With the decrease in pore volume available for sulfuric acid or ammonium sulfate storage, SO2 removal activity gradually decreases. To a

certain degree, the sulfur species must be removed from AC or V2O5/AC through regeneration to recover SO2 removal activity and simultaneously obtain sulfur-contained resources. A common regeneration method for AC-based catalyst-sorbent is thermal regeneration under inert gases, during which AC is heated to above 400 °C and sulfuric acid and ammonium sulfate are reduced by carbon and produce concentrated SO2[5,6]. The previous study[7] found that the SO2-captured V2O5/AC can also be efficiently regenerated by thermal regeneration at 400 °C. However, when a small amount of NH3 is introduced to the inert atmosphere during regeneration (call NH3-regeneration in this study), the regeneration temperature may decrease to 300 °C and the SO2 released is simultaneously converted to ammonium-sulfur salts by reaction with NH3. Apparently, NH3-regneration is superior to the thermal regeneration in the operation cost and the simplified sulfur recovery process. Theoretically, many types of reactions would occur during NH3-regeneration of the SO2-captured V2O5/AC, such as the

Received: 2006-12-16; Revised: 2007-03-14 * Corresponding author. Tel/Fax: +86-351-4053091; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (90210034) and the Knowledge Innovation Programs of the Chinese Academy of Sciences. Copyright©2007, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

GUO Yan-xia et al. / Journal of Fuel Chemistry and Technology, 2007, 35(3): 344−348

1.1

Experimental Preparation of V2O5/AC

A coal-derived commercial granule AC from Xinhua Chemical Plant (Taiyuan, China) with particle sizes of 30–60 meshes (0.23–0.45 mm) was used as the matrix. The V2O5/AC was prepared by pore volume impregnation using an aqueous solution of ammonium metavanadate in oxalic acid. After the impregnation, the samples were dried at 110 °C for 5 h, followed by calcination in Ar at 500 °C for 5 h and pre-oxidation in air at 250 °C for 5 h. The V2O5 loading of the V2O5/AC is about 2%. For convenience, the catalyst-sorbent is denoted as V/AC in general, the fresh sample is denoted as V/AC-F, and the SO2-captured sample is denoted as V/AC-S in this study. The NH3 regenerated samples are denoted as NH3-T-c-t, where T is the regeneration temperature (°C), c is the NH3 concentration (%), and t is the regeneration time (min). 1.2

Activity test and regeneration in NH3/Ar

Simultaneous SO2 and NO removal experiments were performed in a fixed-bed reactor of 12 mm in diameter at 200 °C and 7800 h−1. The feed gas contains 0.15% SO2, 0.05% NO, 0.05% NH3, 5% O2, 2% H2O, and remaining Ar. The concentrations of SO2, NO, and O2 in the inlet and outlet were measured online by a Flue Gas Analyzer (KM9106). SO2 removal efficiency of the V/AC was 100% initially, but decreased with time on stream after a breakthrough in effluent SO2 concentration. When the SO2 removal efficiency decreased to 80%, the mass of SO2 captured on V/AC was calculated and termed SO2 capture capacity (SC80 in short). The time experienced when the SO2 removal efficiency decreased to 80% was termed t80. The SO2 captured V/AC was then in situ regenerated in Ar or NH3/Ar stream at a flow rate of 100 mL/min at the desired temperatures for a certain time. Afterward, the temperature was lowered to 200 °C with an Ar purge before the next SO2/NO removal-regeneration cycle.

Elemental analysis

Contents of sulfur in the V/AC were measured on a Vario EL Analyzer (Elementar Analysensysteme GmbH Co.). The regeneration efficiencies are calculated based on the sulfur contents of V/AC before and after regeneration, which is expressed as: ( m ⋅ S − m2 ⋅ S 2 ) η= 1 1 ×100% (m1 ⋅ S1 − m0 ⋅ S0 ) where η denotes the regeneration efficiencies (%); S0 and S1 are sulfur contents of V/AC before and after SO2 adsorption (%); and S2 is the residual sulfur content in the regenerated V/AC; m0, m1, m2 denote the masses of V/AC before and after SO2 adsorption and the regenerated V/AC (g), respectively.

2

Results and discussion

2.1

Effect of the regeneration temperature

Fig. 1 shows the SO2 and NO removal activities of V/AC-S after regeneration in 5% NH3/Ar at various temperatures. SO2 conversion x / %

1

1.3

100

NO Conversion x / %

reduction of sulfuric acid by carbon or NH3, the oxidation of AC itself, and the interactions of NH3 with sulfuric acid, and so on. Understanding these behaviors is important for the catalyst/adsorbent improvement and the process optimization. However, little information can be found in this regard in the published report. For this reason, based on the previous studies[4,8], the effect of NH3-regeneration conditions on SO2 and NOx removal activities in the subsequent cycles was investigated in this study to understand and optimize the regeneration processes.

100

90 80 70 V/AC-F NH3-200-5-60 NH3-250-5-60 NH3-280-5-60 NH3-300-5-60 NH3-350-5-60 NH3-380-5-60

60 50

90 80 70 60 50 40 0

20

40

60

80

100

Time on stream t / min

Fig. 1 SO2 and NO conversions over V/ACs regenerated at various temperatures (reaction conditions: 0.15% SO2; 0.05% NO; 0.05% NH3; 2% H2O; 5% O2; and remaining Ar, 200 °C, GHSV: 7800 h−1)

It can be seen that t80 slightly increases with increasing regeneration temperature from 200 to 350 °C, but decreases with increasing temperature to 380 °C. Regeneration at 200 and 250 °C yields lower SO2 removal activities, with t80 of less than 55 min (NH3-200-5-60 and NH3-250-5-60), which is much lower than that of V/AC-F (about 77 min). After regeneration at 300 °C (NH3-300-5-60), t80 increases to about 77 min, which almost equals to that of V/AC-F. Regeneration at 350 °C yields the longest t80 of about 92 min (NH3-350-5-60). However, with increasing the regeneration temperature to 380 °C, the SO2 removal activity decreases

GUO Yan-xia et al. / Journal of Fuel Chemistry and Technology, 2007, 35(3): 344−348

instead and t80 (about 81 min) is lower than that of NH3-350-5-60. All the regenerated V/ACs present higher NO removal activities, compared to the fresh V/AC (V/AC-F). However, the SCR activity decreases with increasing the regeneration temperature, which is opposite to the trend of SO2 removal activity. After regeneration at 200–300 °C, the NO conversions are 100% initially and decrease gradually with time on stream. When SO2 conversions decrease to 80%, the NO conversions are still about 70%, which are higher than that of V/AC-F (about 50%). These results are consistent with those drawn by Ma[7]. Considering the recovery of SO2 removal activity and the improvement of the NO removal activity, the optimal regeneration temperature should be 300 °C. Effect of NH3 concentration

SO2 removal x / %

Fig. 2 shows the SO2 and NO removal activities of V/AC-S after regeneration at 300 °C in various NH3 concentrations. 100 V/AC-F NH3-300-0-60 NH3-300-0.5-60 NH3-300-1-60 NH3-300-3-60 NH3-300-5-60

90 80

NO conversion x / %

70 100

2.3

Effect of regeneration time

Fig. 3 shows the effect of regeneration time on the activities of SO2 and NO removal at the regeneration temperature of 300 °C and NH3 concentration of 5%. The activities of SO2 and NO removal roughly increase with an increase in the regeneration time. For the SO2 removal, t80 is about 53 min at the regeneration time of 10 min (NH3-300-5-10). With increasing the regeneration time to 60 and 80 min (NH3-300-5-60 and NH3-300-5-80), t80 increases to about 77 min, the same as that of V/AC-F. For the NO removal, when the regeneration time increases from 10 min to 60 and 80 min, the initial NO conversion increases from 80% to 100%. Apparently, the regeneration time of 60 min is enough to recover the SO2 removal activity and meanwhile, NO removal activity is quite high.

80 60 40 0

20

40

60

80

Time on stream t / min

SO2 removal x / %

2.2

With increasing NH3 concentration to 1% or above, NO conversions increase to the initial 100% and then decrease with time on stream. When the SO2 conversions decrease from 100% to 80%, their NO conversions are still higher than that of V/AC-F. Obviously, SO2 and NO removal activities increase rapidly when the NH3 concentration increases from 0 to 0.5%. However, additional increases in NH3 concentration yield a slow increase in the activities. Considering the recovery of SO2 removal activity, the optimal NH3 concentrations are 3–5% at the regeneration temperature of 300 °C for 60 min.

Fig. 2 SO2 and NO conversions over V/ACs regenerated at various (reaction conditions: the same as those in Fig. 1)

On the whole, the SO2 removal activities increase with increasing NH3 concentration. In the absence of NH3 during regeneration (NH3-300-0-60), t80 was only 52 min, which is much lower than that of V/AC-F (about 77 min). With increasing NH3 concentration to 0.5% (NH3-300-0.5-60), t80 increases to about 65 min. With additional increase in NH3 concentration (1% or above), t80 slightly increases and fluctuates at 75 min, which is close to that of V/AC-F (about 77 min). Like SO2 removal activity, the NO removal activity also increases with increasing NH3 concentration. When NH3 concentration is zero, the NO conversion is about 60%, which is a little higher than that of V/AC-F (about 50%).

V/AC-F NH3-300-5-10 NH3-300-5-20 NH3-300-5-40 NH3-300-5-60 NH3-300-5-80

90 80 70

NO conversion x / %

NH3 concentrations

100

100 80 60 40 0

20

40

60

80

Time on stream t / min

Fig. 3 SO2 and NO conversions over V/ACs regenerated for different periods of time (reaction conditions: the same as those in Fig. 1)

The results presented so far indicate that the SO2 and NO removal activities of V/AC are significantly influenced by the regeneration conditions. Regeneration temperature shows an opposite effect on SO2 and NO removal, whereas both

GUO Yan-xia et al. / Journal of Fuel Chemistry and Technology, 2007, 35(3): 344−348

the linear relation of SC80 with the residual sulfur content (Fig. 4(C)) is possibly related to both regeneration efficiency and surface modification by NH3. 80 60

2.4.1

Regeneration efficiency

5% NH3/Ar, 60 min

380 C

o

250 C

40 80 B: Regeneration Concentration

70

3%

60

o

300 C, 60 min 1%

5%

50 0

40 80

C: Regeneration Time

70

o

300 C, 5% NH3/Ar

60min

60

The main aim of the regeneration is to remove sulfur-containing species from the SO2-captured V/ACs, therefore the residual sulfur contents in the regenerated V/ACs reflect the regeneration efficiencies. Fig. 4 shows the relations of sulfur content with SC80 of the regenerated V/ACs. From Fig. 4(A), it can be seen that after regeneration at 250, 300, and 350 °C, the sulfur contents in V/AC are 1.91%, 0.86%, and 0.42%, respectively, which correspond to regeneration efficiencies of 28%, 79%, and 100%, respectively. Regeneration at 380 °C, as at 350 °C, also yields a regeneration efficiency of almost 100%. On the whole, the smaller the sulfur content, the higher the second SO2 removal activity, except NH3-380-5-60 (Fig. 4(A)). The exceptive behavior suggests that the second SO2 removal activity is influenced not only by the regeneration efficiency, but also by other factors, possibly the surface properties as discussed below. The effect of NH3 concentration on the residual sulfur content in the regenerated V/AC-S is very little. As shown in Fig. 4(B), the residual sulfur contents remain at 0.95–0.86%, corresponding to regeneration efficiencies of 74.4–78.7%. However, the SC80 of V/AC regenerated in the absence of NH3, 40 mg/g, is far lower than those of the others, about 60 mg/g, which suggests that the presence of NH3 during regeneration is important for a higher SO2 removal activity in the subsequent cycle. Obviously, this is not due to the regeneration efficiency, but due to the modification of surface properties by NH3. The effect of the regeneration time on the residual sulfur of V/AC is quite obvious. As shown in Fig. 4(C), at the regeneration conditions of 300 °C and 5% NH3/Ar, the residual sulfur in the regenerated V/AC decreases sharply from 1.33 to 0.90% with increasing the regeneration time from 10 to 40 min, but slightly to 0.86% with regeneration time to 60 min. This is not surprising, because a long regeneration time assures the effective removal of sulfur species. Meanwhile, a long regeneration time may intensify the modification of the surface properties by NH3. Therefore,

A: Regeneration Temperature o

300 C o

50 SC80 w / mg.g

2.4 Relations of regeneration conditions with activities of SO2 and NO removal of the regenerated V/AC

o

350 C

70

−1

NH3 concentration and regeneration time show consistent effects on the removal of SO2 and NO. These suggest that different surface properties are required for SO2 removal and SCR reaction (NO removal) and the influence of regeneration temperature on the surface properties of V/AC is different than that of NH3 concentration. As a conclusion, the optimal regeneration conditions are 300 °C, 3–5% NH3, and 60 min.

40min

50

20min 10min

40 0.4

0.8

1.2 Sulfur content w / %

1.6

2.0

Fig. 4 Relations of V/AC’s sulfur content with subsequent SO2 adsorption capacity

The effect of residual sulfur content on NO removal activity is also very complicated. With increasing the regeneration temperature from 250 to 350 °C, the NO removal activity decreases (Fig. 1), which is consistent with the change of the residual sulfur (Fig. 4(A)). This seems to indicate that the residual sulfur accounts for the improved NO removal activity of the regenerated V/AC, as reported in the published reports[9,10]. However, V/AC-F and NH3-350-5-60 have the same amount of sulfur, but far different NO removal activity. Moreover, the NO removal activities of all V/AC samples in this study decrease with time on stream during the simultaneous removal of SO2 and NO. These indicate that the sulfur on the V/AC surface is not the main reason for a higher NO removal activity for the catalyst/adsorbent in this study. Figs. 2 and 3 show that increasing NH3 concentration and time during regeneration significantly improves the NO removal activity of V/AC in the subsequent cycle, which suggests that the adsorption of NH3 or the surface modification by NH3 affect the removal of NO. This can also be used to explain the effect of regeneration temperature on NO removal activity: the low regeneration temperature results in more adsorption of NH3 and thus higher NO removal activity. 2.4.2

NH3 adsorption during regeneration

To determine the adsorption of NH3 during regeneration, Fig. 5 shows FT-IR spectra of various V/ACs. Adsorption of SO2 on V/AC-F results in the appearance of new bands at 619, 1100, and 1400 cm−1 (V/AC-S). The peaks at 619 and

GUO Yan-xia et al. / Journal of Fuel Chemistry and Technology, 2007, 35(3): 344−348

1100 cm−1 can be assigned to SO42− and 1400 cm−1 to NH4+ (although Ma[7] figured it as the adsorption band of VOSO4), indicating the presence of H2SO4 and/or ammonium sulfate in V/AC-S, as reported in the previous study[11−12]. After regeneration in NH3/Ar, the peaks at 619 and 1100 cm−1 gradually diminish with increasing the regeneration temperature, indicating the decrease in SO42− content. It is interesting to note that the peak at 1400 cm−1 is stronger for V/ACs regenerated at 200, 230, and 250 °C than that of V/AC-S, which indicates NH3 adsorption on the surface of V/AC in the form of NH4+. However, the peak at 1400 cm−1 weakens with increasing regeneration temperature from 200 to 250 °C and even disappears at 300 and 350 °C, indicating that the adsorption of NH3 on V/AC decreases with increasing the regeneration temperature. Apparently, for the regenerated V/AC, the changes of the NH4+ contents (Fig. 5) are consistent with those of the NO removal activities (Fig. 1), suggesting that the storage of NH3 on the surface of V/AC during regeneration accounts for the higher NO removal activity in the subsequent cycle. Due to the gradual consumption of NH3 stored on the surface of V/AC, the NO removal activity decreases with time on stream.

3

Conclusions

The optimal regeneration conditions of SO2-captured V/AC are: 300 °C, 3–5% NH3 in an inert atmosphere, 60 min. Under these conditions, the SO2 removal activity can be recovered effectively and the NO removal activity can be improved. The regeneration efficiency is mainly controlled by the regeneration temperature. The presence of NH3 or not has little effect on it. NH3 may modify the surface properties of V/AC, which promotes the second SO2 removal. Meanwhile, NH3 can be stored on the surface of V/AC in the form of NH4+, which significantly promotes the removal of NO.

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The results presented in this section indicate that the functions of NH3-regeneration can be categorized into three aspects. 1) Removing sulfur species from V/AC and releasing active or storage sites, which is mainly determined by the regeneration temperature, but not by the presence of NH3. 2) The adsorption and storage of NH3 on the surface of V/AC. 3) Modification of the surface properties of V/AC by NH3, which is drawn by the fact that the V/ACs regenerated at 350 and 380 °C have almost the same sulfur contents as that of V/AC-F, but the SC80 of the former were much higher than that of latter.

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