Mercury Adsorption and Oxidation over Cobalt Oxide Loaded

May 29, 2015 - In this work, the Hg0 removal behaviors as well as the involved reactions mechanism were ... Shibo Zhang , Yongchun Zhao , Jianping Yan...
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Mercury Adsorption and Oxidation over Cobalt Oxide Loaded Magnetospheres Catalyst from Fly Ash in Oxyfuel Combustion Flue Gas Jianping Yang, Yongchun Zhao,* Lin Chang, Junying Zhang,* and Chuguang Zheng State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *

ABSTRACT: Cobalt oxide loaded magnetospheres catalyst from fly ash (Co−MF catalyst) showed good mercury removal capacity and recyclability under air combustion flue gas in our previous study. In this work, the Hg0 removal behaviors as well as the involved reactions mechanism were investigated in oxyfuel combustion conditions. Further, the recyclability of Co−MF catalyst in oxyfuel combustion atmosphere was also evaluated. The results showed that the Hg0 removal efficiency in oxyfuel combustion conditions was relative high compared to that in air combustion conditions. The presence of enriched CO2 (70%) in oxyfuel combustion atmosphere assisted the mercury oxidation due to the oxidation of function group of C−O formed from CO2. Under both atmospheres, the mercury removal efficiency decreased with the addition of SO2, NO, and H2O. However, the enriched CO2 in oxyfuel combustion atmosphere could somewhat weaken the inhibition of SO2, NO, and H2O. The multiple capture−regeneration cycles demonstrated that the Co− MF catalyst also present good regeneration performance in oxyfuel combustion atmosphere.

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al.24 studied the Hg0 removal activity over the selective catalytic reduction (SCR) catalyst (V2O5−WO3/TiO2) in oxyfuel combustion conditions, and the enriched CO2 (80 vol %) promoted the Hg0 removal. Spörl et al.25 studied the mercury emissions behavior and removal by ash in oxyfuel combustion system. The results indicated that the mercury capture efficiency of fly ash decreased in oxyfuel combustion conditions compared to that in air combustion conditions, which was likely attributed the higher Hg, SO2 and SO3 concentrations under oxyfuel combustion conditions with flue gas recycling. Regarding activated carbon and other conventional sorbents, the common limitations are10,26−28 (i) the recovery and recycle of spent sorbent, (ii) the disposition of mercury containing in the spent sorbent, (iii) the negative effect on the utilization of fly ash. Therefore, it is extremely attractive to develop recoverable and regenerable sorbent/catalyst to overcome these limitations. In our previous study, the cobalt oxide loaded magnetospheres catalysts from fly ash (Co−MF catalyst) showed good mercury removal capacity and recyclability under air combustion conditions.29 Magnetospheres were first separated from fly ash, and then the Co− MF catalyst was synthesized by loading cobalt oxide. Afterward, the catalyst was injected into flue gas to remove mercury.

INTRODUCTION Carbon dioxide (CO2), as the main greenhouse gas emitted through human activities, has drawn worldwide attention in recent years due to its potential effects on climate change. Coal−fired power plants are considered as one of the main anthropogenic CO2 source. Oxyfuel combustion is being considered as a promising technology used to capture CO2 from coal−fired power plants. However, the elemental mercury (Hg0) present in the flue gas can damage the CO2 compression and purification units through liquid metal embrittlement and amalgam corrosion.1,2 Moreover, mercury is a great environmental concern because of its volatility, persistence, and bio− accumulation.3,4 Coal−fired power plants are one of the biggest anthropogenic source of mercury emissions to the atmosphere.3 The concentration of mercury is generally higher in oxyfuel combustion flue gas compared to that in air combustion because of the recycle of flue gas. Thus, special attention needs to be paid to mercury removal in oxyfuel combustion system. Many technologies have been investigated for Hg0 removal from air combustion flue gas, such as catalytic oxidation,5−11 carbon−based sorbent,12−14 fly ash,15−19 zeolite,20 Ca−based sorbent,21 etc. Limited researches on mercury control in oxyfuel combustion system have been published. Lopez−Anton et al. reported the mercury capture capacity of a series of activated carbons obtained from leather industry waste22 as well as commercial activated carbon with/without sulfur23 in oxyfuel combustion conditions, and high mercury oxidation efficiency was observed in an oxyfuel combustion conditions. Wang et © 2015 American Chemical Society

Received: Revised: Accepted: Published: 8210

March 2, 2015 May 14, 2015 May 29, 2015 May 29, 2015 DOI: 10.1021/acs.est.5b01029 Environ. Sci. Technol. 2015, 49, 8210−8218

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Environmental Science & Technology

Table 1. Experimental Conditionsa, and Accumulation Hg0 Removal Efficiency (ηT−a)

Because of the magnetism of magnetospheres, the spent catalyst that was collected together with fly ash by the particulate control devices could be recovered again using magnetic separation. In this case, the catalyst could be recycled multiple times after regenerated. However, in oxyfuel combustion system, the flue gas components are significantly different compared to that in air combustion system. CO2 is the main components in oxyfuel combustion atmosphere rather than N2 in air combustion atmosphere. The concentration of water vapor (H2O) and SO2 is significantly higher in oxyfuel combustion flue gas,30,31 since the flue gas is recycled to the boiler. Moreover, different proportions of SO2/SO3 are present in the flue gas in two atmosphere.32 Thus, the Hg0 removal performance may be very different in oxyfuel combustion and air combustion atmosphere. In view of these points it is of considerable importance for evaluating the mercury removal performance of Co−MF catalyst in oxyfuel combustion system. Three novel aspects are taken into consideration in this work: (i) the Hg0 removal performance under air and oxyfuel atmospheres; (ii) the involved reaction mechanisms in oxyfuel combustion conditions; (iii) the recyclability of Co−MF catalyst in oxyfuel combustion atmosphere. This work will provide further insight into the development and application of cost−effective recyclable magnetic sorbents for mercury removal from oxyfuel combustion flue gas.

experiments

■

flue gas components

ηT−a

set I

N2 + 4% O2 12% CO2 + 4% O2 70% CO2 + 4% O2 70% CO2 + 4% O2, without catalyst

75.9 ± 2.3 77.5 ± 3.5 85.7 ± 2.8 0.38

set II

12% CO2 + 4% O2 + 300 ppm of NO 70% CO2 + 4% O2 + 300 ppm of NO

65.1 ± 3.4 75.4 ± 3.9

set III

12% CO2 + 4% O2 + 1200 ppm of SO2 70% CO2 + 4% O2 + 1200 ppm of SO2

47.5 ± 3.1 69.2 ± 3.4

set IV

12% CO2 + 4% O2 + 10 ppm of HCl 70% CO2 + 4% O2 + 10 ppm of HCl

91.5 ± 2.7 93.3 ± 2.2

set V

12% CO2 + 4% O2 + 10% H2O 70% CO2 + 4% O2 + 10% H2O 70% CO2 + 4% O2 + 20% H2O

64.1 ± 4.1 65.5 ± 3.1 50.7 ± 3.6

set V

DSFGb DSFG + 10% H2O DSFG + 20% H2O

84.8 ± 4.2 71.6 ± 3.9 67.7 ± 3.6

Set VI

DSFG, five capture−regeneration cycles

84.8 ± 4.2

a

The test time is 60 min; the balance gas is N2. bDSFG: dry simulate oxyfuel flue gas, N2 + 4% O2 + 70% CO2 + 300 ppm of NO + 2000 ppm of SO2 + 10 ppm of HCl.

EXPERIMENTAL SECTION Catalyst Preparation. The previously characterized Co− MF catalyst29 was used as mercury sorbent in this work. The loading value of Co3O4 on the Co−MF catalyst surface was defaulted as 5.8%, which presented optimal Hg0 removal activity in air combustion atmosphere. Characterization of Catalyst. The characteristics of the catalyst was analyzed by X−ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM), which was also described in the Supporting Information (SI). Experimental Apparatus and Procedures. The Hg0 removal performances of Co−MF catalyst in oxyfuel combustion atmosphere were studied using a bench−scale experimental system, which is similar to that used in our previous study29 and is shown in SI Figure S1. Briefly, it consisted of the simulated flue gas feed system, the fixed−bed reactor, and the continuous mercury analyzer. The simulated flue gas (SFG) was a mixture of 4% O2, 12%/70% CO2, 1200 ppm of SO2, 300 ppm of NO, 10 ppm of HCl, balanced in N2, with a total flow rate of 1.2 L·min−1. The Hg0 concentration in the stream was 85 μg·m−3, which was a consequence of a mercury permeation tube (VICI, Metronics Inc., Santa Clara, CA) at a specified temperature as well as gas flow. An online mercury analyzer (VM3000 Mercury Vapor Monitor) was used to monitor the gas−phase Hg0 concentration in the stream. To identify the adsorption and oxidation behaviors, a mercury speciation conversion system was employed in this work. The sampling gas was divided into two parallel streams by the conversion system to measure the concentration of Hg0 and HgT, respectively. The difference between Hg0 and HgT is the concentration of Hg2+. More details about the mercury conversion system are described in SI. The experiments conditions are summarized in Table 1. In Set I, the Hg0 removal performance of the catalyst was studied in an enriched CO2 atmosphere. In Set II to Set VII, the mercury removal performances with the addition of acid flue

gases components (NO, SO2, HCl) and H2O, were studied under oxyfuel combustion conditions and compared to that in air combustion conditions. In Set VIII, the recyclability of Co− MF catalyst in oxyfuel combustion atmosphere was evaluated. To evaluate the variations of mercury removal performance with the addition of acid flue gases and H2O as well as short the experiment time of each tests, 0.1 g Co−MF catalyst sample and 1.9 g quartz sand was used. In this work, the gas hourly space velocity (GHSV) was about 38996 h−1, and the interaction reaction time was approximately 0.09s. Before the performance evaluation, it was confirmed that quartz did not adsorb Hg0 at the reaction temperature (150 °C).29 To identify the possible interaction of flue gas, catalyst, and Hg0, the contents of flue gas components at the inlet and outlet of the reactor were determined by gas analyzer (OPTIMA7, York Instruments Ltd) and Fourier Transform Infrared gas analyzer (FT/IR, DX4000, GASMET), and the concentration of SO42− in the solutions were analyzed using ion chromatography (ICS90, DIONEX). In this case, the concentration of SO3 in the flue gas could be determined through detecting the concentration of SO42− in the condensates. More details about the determination of flue gas contents please refer to the SI. In Set VIII, five mercury removal−catalyst regeneration cycles were performed to investigate the regeneration characteristics of the catalyst. After mercury adsorption at 150 °C for 2 h under simulated flue gas atmosphere, the spent catalyst was removed from the reactor and regenerated by heating at suitable temperature in a air carrier gas. Afterward, the regenerated catalyst was placed back into the same reactor to perform the next cycle of mercury removal. The regeneration temperature was determined by a mercury−temperature-programmed decomposition (Hg−TPD) experiment, which was described 8211

DOI: 10.1021/acs.est.5b01029 Environ. Sci. Technol. 2015, 49, 8210−8218

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Environmental Science & Technology

aggregated when the external magnetic field was removed. Moreover, the flue gas components have no obvious effect on the magnetism. Mercury Removal Performance under Oxyfuel Combustion Conditions. In this work, the mercury removal performance of Co−MF catalyst in oxyfuel combustion atmosphere was preliminary evaluated in a enriched CO2 (70 vol %) atmosphere, and subsequently investigated that in the atmospheres containing acid flue gas (SO2, NO, and HCl) and H2O. Though the effects of these flue gas components have been systematically investigated in our previous study,29 the mercury removal behaviors in the oxyfuel combustion atmosphere with the addition of SO2, NO, HCl, and H2O need to be further investigated. This is mainly because CO2 is the dominant component in the oxyfuel combustion atmosphere rather than N2 in air combustion atmosphere. As shown in Table 1, relatively high mercury removal efficiency was observed in the oxyfuel combustion atmosphere (70% CO2) compared to that in air combustion atmosphere (12% CO 2 ). Within the reaction proceed similar Hg 0 adsorption behaviors were observed in the air and oxyfuel combustion atmosphere, while higher Hg0 oxidation efficiency η was attained in the later atmosphere (Figure 1). Further,

in our previous study.29 The mercury released in the regeneration step was condensed and recovered by a device immersed in ice.29 In this work, the operating temperature is 150 °C, which attained highest Hg0 removal efficiency in air combustion atmospheres. Two different calculation methods for Hg0 removal efficiency were used in this work: instantaneous value (ηi) and accumulation value (ηa). The instantaneous total Hg0 removal efficiency (ηT−i, %), Hg0 adsorption efficiency (ηads−i, %), and Hg0 oxidation efficiency (ηoxi−i, %) were defined as the following equation, respectively: ηT − i =

0 Hg 0in − Hg out

ηads − i =

ηoxi − i =

× 100%

Hg 0in

(1)

Hg 0in − Hg Tout Hg 0in

× 100% (2)

0 Hg Tout − Hg out

× 100%

Hg 0in

Hgin0

(3)

0 Hgout

Where and represents the instantaneous Hg0 concentration at the inlet and outlet of the reactor. HgTout represents the instantaneous total mercury concentration at the outlet of the reactor. The accumulated total Hg0 removal efficiency (ηT, %), Hg0 adsorption efficiency (ηads, %), and Hg0 oxidation efficiency (ηoxi, %) in 60 min were defined as the following equation, respectively: t

ηT − a =

t

0 ∑0 Hg 0in − ∑o Hg out t

∑0 Hg 0in t

ηads − a =

(4) t

∑0 Hg 0in − ∑o Hg Tout t

∑0 Hg 0in t

ηoxi − a =

× 100%

(5) t

0 ∑o Hg Tout − ∑0 Hg out

∑t0Hg0in

× 100%

t

∑0 Hg 0in

Figure 1. Instantaneous Hg0 adsorption and oxidation efficiency in air and oxyfuel combustion atmospheres. (12% and 70% CO2, 4% O2, balanced with N2).

× 100% (6)

∑t0Hg0out

Where and represents the accumulated Hg0 concentration at the inlet and outlet of reactor in the reaction period, and ∑t0HgTout represents the accumulated total mercury concentration at the outlet of the reactor. The accumulated time of each set of experiment (t) was set to be 60 min in this study.

similar Hg0 removal efficiency was attained in the atmospheres without CO2 and with 12% CO2. This implied that the impact of CO2 with low content on Hg0 removal is negligible. However, the enriched CO2 in oxyfuel combustion system would assist the mercury removal. This could probably be interpreted by the interactions between the enriched CO2 and Co−MF catalyst, since the influence of CO2 on Hg0 removal could be negligible if without catalyst. With the Addition of SO2. The Hg0 removal behaviors with the addition of SO2 should be highlighted under oxyfuel combustion conditions, since the concentration of SO2 would be 3−4 times higher than that in air combustion conditions, and also higher proportion of SO3/SO2 are present under oxyfuel combustion conditions.32,33 As shown in Table 1, with the addition of SO2 into the above flue gas (12%/70% CO2, 4% O2, and N2), obvious decrease of mercury removal efficiency was observed in two atmospheres. In our previous study,29 it is demonstrated that the inhibitive role of SO2 in Hg0 removal are attributed the following reactions: (i) SO2 compete with Hg0 for the same adsorption sites, (ii) SO2 interacted with cobalt oxide on the catalyst forming cobalt sulfate species, which is

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RESULTS AND DISCUSSION Characterization of Catalysts. The textural properties of the catalyst, including the BET surface area, the magnetic property, the crystal form of cobalt species on the carrier, and the microstructure, were summarized in our previous study.29 Briefly, the BET surface area of the Co-MF catalyst (6.21 m2· g−1) significantly increased compared to the raw magnetospheres (0.28 m2·g−1), due to a good dispersion of cobalt oxide. The cobalt oxide loaded on the magnetospheres surface is mainly present as Co3O4 in an amorphous state. The catalyst sample presents superparamagnetism with a minimized coercivity and negligible magnetization hysteresis. This makes it possible to separate the spent Co-MF catalyst after mercury removal from fly ash by magnetic separation, but will not be 8212

DOI: 10.1021/acs.est.5b01029 Environ. Sci. Technol. 2015, 49, 8210−8218

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Figure 2. Instantaneous Hg0 adsorption and oxidation efficiency in air- and oxyfuel-combustion atmospheres with the addition of SO2, NO, and HCl. (a)−(d) shows the mercury removal performance with the addition of 2000 ppm of SO2, 300 ppm of NO, 10 ppm of HCl, and 2000 ppm of SO2+300 ppm of NO+10 ppm of HCl. Basic gas: 12% and 70% CO2, 4% O2, balanced with N2.

the Hg0 removal behaviors between air and oxyfuel combustion atmosphere. However, higher adsorption and oxidation efficiency was observed in oxyfuel combustion atmosphere with the reaction continued. With the Addition of HCl. As shown in Table 1, the addition of HCl significantly promoted Hg0 removal under both air and oxyfuel combustion conditions, and similar Hg0 removal efficiency were attained in two atmospheres. In our previous study, it is demonstrated that the enhancement of HCl was attributed to the formation of active chlorine species on the surface in air combustion atmosphere.29 Evidence also suggested that homogeneous mercury oxidation does not occur in atmospheres with 25 ppm of HCl + 4% O2 in different proportions of CO2.39 Thus, the positive effect of HCl on Hg0 removal in oxyfuel combustion atmosphere should also be due to the heterogeneous reaction with gaseous Hg0 on the chlorinated catalyst surface. As shown in Figure 2 (c), similar Hg0 adsorption and oxidation behaviors were observed in two atmospheres, and a high proportion of Hg0 was oxidized to Hg2+ in two atmospheres. This could be interpreted that the formed oxidized mercury interacted with HCl and decompose into Hg0 and Hg2+, and then released in a gas phase.29 With the Addition of SO2, NO, and HCl. The Hg0 removal performances of Co−MF catalyst were studied in the dry oxyfuel combustion simulated flue gas atmosphere with the addition of SO2, NO, and HCl. As shown in Figure 2 (d), the mercury removal including adsorption and oxidation under both air- and oxyfuel-combustion conditions. The total mercury efficiency ηT−i was primarily determined by the adsorption

inert for mercury oxidation. Figure 2 (a) showed that both the adsorption and oxidation efficiency decreased with the addition of SO2 under two atmospheres (comparing with the results in Figure 1). However, the enriched CO2 in oxyfuel combustion atmosphere could somewhat weaken the inhibition of SO2. Within the reaction proceed, the adsorption efficiency of catalyst in the air combustion atmosphere was obviously lower than that in the oxyfuel combustion atmosphere. Moreover, as the reaction progress, higher oxidation efficiency was attained in oxyfuel combustion atmosphere compared to that in air combustion atmospheres. With the Addition of NO. Under both air and oxyfuel combustion conditions, the addition of 300 ppm of NO suppressed the Hg0 removal over Co−MF catalyst (Table 1). NO could interact with the catalyst to form new species, such as NO2, NO+, and NO3−.34 Previous studies evidenced that low concentrations of NO2 (about 30 ppm) could heterogeneously react with Hg0 on the carbon surface.35,36 On the other hand, the formed NO2 could adsorbed on the catalyst surface and then formed Co(NO3)2.37 This could be evidenced by the presence of obvious peak of N 1s corresponding to NO3− (at approximately 399.2ev38) on the surface of spent catalyst after mercury removal experiment under N2 + 4% O2 + 70% CO2+300 ppm of NO atmosphere (SI Figure S2). The formed cobalt nitrate would decrease the oxidation of mercury and the conversion of NO into NO2. However, the inhibition of NO in oxyfuel combustion atmosphere was relatively slight compared to that in air combustion atmosphere. As shown in Figure 2 (b), at the first reaction stage there is no obvious difference of 8213

DOI: 10.1021/acs.est.5b01029 Environ. Sci. Technol. 2015, 49, 8210−8218

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Environmental Science & Technology efficiency ηads−i at the first reaction stage. After that, the adsorption efficiency ηads−i gradually decreased, and the oxidation efficiency ηoxi−i increased with the reaction process. Relative high total Hg0 removal efficiency was observed in oxyfuel combustion conditions compared to that in air combustion conditions. There is no obvious variation of adsorption efficiency under two atmospheres. However, higher oxidation efficiency was observed under oxyfuel combustion conditions. With the Addition of H2O. As shown in Table 1, under both air- and oxyfuel-combustion atmospheres, the introduction of 10% H2O vapor seriously decreased the Hg0 removal efficiency. Previous study showed that higher concentration of H2O was presented in oxyfuel combustion conditions compared to that in air combustion conditions.39 Thus, in this work, 20% H2O was further introduced into the oxyfuel combustion flue gas stream. Apparently, the presence of 20% H2O in the flue gas seriously impedes the removal of mercury, probably due to the occupy of water on the active sites available for mercury adsorption.6,40 With the Addition of SO2, NO, HCl, and H2O. With the further addition of H2O into the dry simulated flue gas (DSFG) atmosphere, serious decrease of total mercury removal efficiency as well as adsorption efficiency were observed (Figure 3). This is mainly due to the competitive adsorption

clarify the involved reactions mechanism, a series set of experiments were carried out. First, to identify the involved reactions mechanism with the addition of enriched CO2, the change of surface characteristics between fresh, CO2-pretreated catalyst as well as the spent CO2-pretreated catalyst after Hg0 removal was investigated. The CO2-pretreated catalyst was pretreated by a 1 L·min−1 gas flow containing 4% O2, 70% CO2, and balanced with N2 for 24 h. The spent CO2-pretreated catalyst was the sample after mercury removal experiment under N2 atmosphere using the CO2-pretreated catalyst. As shown in Figure 4, obvious C 1s

Figure 4. XPS spectra of the fresh, CO2-pretreated, and spent CO2pretreated catalysts over the spectral regions of C 1s.

spike at approximately 286.3ev, corresponding to function group of C−O,41,42 was observed over the CO2-pretreated catalyst. However, the spike of C−O decreased over the spent CO2-pretreated catalyst after Hg0 removal. This implied that the function group of C−O formed from CO2 participate in the reaction of Hg0 oxidation, as demonstrated in Figure 5. Note that the amount of the symbols in Figure 5 represents the concentration of flue gas components. In the oxyfuel combustion system, the amount of symbols representing CO2 in the stream was decreased after passed through the catalyst, since part of CO2 was participated in the formation of C−O. Moreover, the spike CO (C 1s peaks at 287.9 ev41,42) on the spent catalyst surface decreased compared to the fresh and the CO2-pretreated catalyst, indicating that CO might also be involved in the Hg0 oxidation. The following reactions could be employed to interprate this observation:

Figure 3. Accumulation Hg0 adsorption and oxidation efficiency in airand oxyfuel-combustion atmospheres. (12% and 70% CO2, 4% O2, balanced with N2). (DSFG: simulated flue gas, N2 + 4% O2 + 70% CO2 + 300 ppm of NO + 2000 ppm of SO2 + 10 ppm of HCl, the test time is 60 min).

with Hg0 on the same adsorption sites, which also agrees with the conclusions attained in the atmospheres containing only 12%/70% CO2 and 4% O2. However, the Hg0 oxidation efficiency increased from 30.9% to 35.2% when 10% H2O was introduced into the DSFG atmosphere, while the Hg0 oxidation efficiency further increased to 40.8% when the H2 O concentration was increased to 20%. Though the presence of water with high concentration in the flue gas seriously impedes the mercury adsorption, the larger amount of Hg2+ in the flue gas is favorable for the reduction of mercury emission since the Hg2+ could be facilitate captured by the flue gas desulfurization (FGD) system. Identification of Involved Reactions Mechanism. As presented above, the variation of mercury removal behaviors under two atmospheres are mainly caused by the presence of enriched CO2 and the addition of SO2, NO, and H2O. To

Hg 0(g) → Hg 0(ad)

(1)

CO2 (g) → CO2 (ad)

(2)

CO2 (ad) + catalyst → C − O, C = O

(3)

0

8214

2Hg (ad) + 2C − O → 2HgO + C = C

(4)

2Hg 0(ad) + 2C = O → 2HgO + C = C

(5)

DOI: 10.1021/acs.est.5b01029 Environ. Sci. Technol. 2015, 49, 8210−8218

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Figure 5. Involved reactions of mercury removal over Co−MF catalyst under oxyfuel combustion and air combustion atmospheres.

The second set experiment was carried out to understand the variation of Hg0 adsorption and oxidation behaviors in air and oxyfuel combustion atmospheres with the addition of SO2. This could probably be due to the difference of the interaction between SO2 and the catalyst under two atmospheres, as demonstrated in Figure 5. To clarify the hypothesis, the flue gas components at the inlet and outlet of the reactor was detected. As shown in SI Figure S3, the SO2 concentration sharply decreased when the gas stream passed the catalyst, indicating that some of SO2 could interact with the catalyst. With the reaction continued, the interaction between SO2 and catalyst weakened. Further, when the SO2 concentration at the outlet flue gas reached the initial level, the Hg0 removal activity almost disappeared. This is also in line with our previous study,29 where the Hg0 removal activity of catalyst significantly decreased after pretreated by SO2, suggesting that the interaction of SO2 and catalyst would affect the textural properties of the catalyst. However, the catalyst would maintain Hg0 removal capacity for relative long time under oxyfuel combustion atmosphere. A possible explain was that the enriched CO2 could compete the adsorption sites with SO2 on the catalyst surface. This would alleviate the interaction between SO2 and cobalt oxide on the catalyst. Further, previous studies showed a higher proportion of SO2 would be converted into SO3 in oxyfuel combustion conditions.32 In this work, the analysis of the condensates obtained by cooling the outlet gas showed that larger amount of SO3 are formed in oxyfuel combustion atmosphere than that in the air combustion atmosphere (SI Figure S4). This is in line with the former hypothesis that the impact of SO2 on the textural properties of the catalyst was alleviated in the enriched CO2 atmosphere. The SO3 concentration slightly decreased in the presence of Hg0, suggesting the interaction between SO3, Hg0 and catalyst. The following reactions could be employed to interprate this observation: SO2 (g) → SO2 (ad)

SO2 (ad) + Cox Oy → SO3 − Cox Oy − 1

Hg 0 + SO3 + 1/2O2 → HgSO4 (g)

(10)

Hg 0(ad) + CoxOy → HgO − Cox Oy − 1

(11)

HgO − Cox Oy − 1 + 1/2O2 → HgO(ad) + Cox Oy

(12)

HgO(ad) → HgO(g)

(13)

HgO(ad) + SO3(ad) → HgSO4 (ad)

(14)

HgO(g) + SO3(g) → HgSO4 (g)

(15)

However, it is observed that very little amount of SO3 was decreased when Hg0 was added into the stream compared to the overall amout of SO3. This suggested that SO3 originated form SO2 conversion is sufficient for the interaction between SO3, Hg0 and catalyst, which might be restricted by the slow reaction kinetics between them. The third set test of the identification of involved reactions mechanism under two atmosphere with the addition of NO were performed. Similar to SO2, the variation of the mercury removal behaviors under two atmospheres with the addition of NO was also probably due to the difference of interaction between NO and catalyst under two atmospheres. As shown in SI Figure S5, the NO concentration decreased when the gas stream passed the catalyst, indicating that some of NO interacted with the catalyst. The analysis of the outlet gas composition revealed that some of NO was converted into NO2 (SI Figure S6), and higher concentration of NO2 in oxyfuel combustion atmosphere was attained in comparison with that in air combustion atmosphere. However, the NO2 concentration was observed to decrease when Hg0 was introduced to the gas stream. This implied that NO2 participated in the reaction of Hg0 oxidation, and the higher concentration of NO2 favors the Hg0 oxidation. The reactions proposed in our previous study could be employed to interpret this observation:29

(6) (7)

NO + Cox Oy → NO − O − Cox Oy − 1

(16)

NO − O − Cox Oy − 1 → NO2 + Cox Oy − 1

(17)

SO3 − Cox Oy − 1 + 1/2O2 → SO3(ad) + Cox Oy

(8)

Hg 0 + NO2 → HgO + NO

(18)

SO3(ad) → SO3(g)

(9)

Hg 0 + 2NO2 + O2 → Hg(NO3)2

(19)

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fresh catalyst, the spent catalyst after mercury removal, and the regenerated catalyst after five capture−regeneration cycles were characterized by XPS. As shown in Figure 7, compared to fresh

(20)

In oxyfuel combustion atmosphere, a higher proportion of NO2 together with the higher Hg0 adsorption and oxidation efficiency were observed, as demonstrated in Figure 5. A possible explanation is that the enriched CO2 probably assisted the desorption of NO2 from the catalyst surface, restraining the formation of cobalt nitrate. This would be beneficial for the Hg0 oxidation because cobalt nitrate is inert for Hg0 oxidation. Further, with the addition of H2O into the oxyfuel combustion simulated flue gas, higher oxidation efficiency was observed. This could probably due to the reaction in aqueous phase resulting from the formation of sulfate and nitric acid through the following reactions:22,23,39 SO3(aq, g) + H 2O(l, g) → H 2SO4 (aq)

(21)

2NO2 (g) + H 2O(l, g) → HNO2 (aq) + HNO3(aq) (22)

3HNO2 (aq) → HNO3(aq) + 2NO(g) + H 2O(l, g) (23)

NO2 (g) + NO(g ) + H 2O(l, g) → 2HNO2 (aq)

(24) Figure 7. XPS spectra of the fresh, spent, and regenerated catalysts over the spectral regions of Co 2p and O 1s.

Regeneration Performance under Oxyfuel Combustion Conditions. Repeated mercury capture−regeneration cycles were conducted to evaluate the recyclability under oxyfuel combustion atmosphere. In this work, the spent Co− MF catalyst was regenerated at 400 °C in air for 0.5 h, at which the mercury adsorbed on the catalyst could be near-completely released. To confirm most of the captured mercury was desorbed at 400 °C, a comparison test was carried out at temperatures up to 800 °C. The results showed that little residual Hg (less than 5%) was still retained in the sorbent after regenerated at 400 °C. The accumulated total Hg0 removal efficiency ηT−a was selected to evaluate the regeneration performance when the catalyst attained 20% and 50% breakthrough, respectively. As shown in Figure 6, the Hg0 removal efficiency maintained along multiple capture−regeneration cycles under oxyfuel combustion conditions, which is similar to that in air combustion conditions.29 This implied that the catalyst after multiple capture−regeneration cycles did not undergo significant changes. To validate this hypothesis, the

catalyst, obvious variation was observed of the Co 2p spectra on the spent catalyst after Hg0 removal. This might be attributed to the reaction between Hg0 and cobalt oxide, forming adsorbed mercuric oxide (HgO(ad)−CoxOy−1). Moreover, the peaks fitted into chemisorbed oxygen (at 531.0−532.9 ev peaks for XPS43) obviously weakened, indicating that the chemisorbed oxygen consumed in the Hg0 removal performance. However, no obvious variations were observed of Co 2p and O 1s XPS spectra on the fresh and regenerated catalyst. Thus, this implied that after regenerated by heating to 400 °C under air atmospheres for 0.5 h, the status of Co and O on the catalyst was recovered, which agrees with the experimental results attained from repeated mercury capture−regeneration cycles. Moreover, the regeneration time is much shorter compared to the adsorption time, which is benefitial for the application in industial. Although the mercury capacity (about 0.05 mg Hg/g catalyst in 1.5 h, when the catalyst attained 50% breakthrough) of this material is lower than that of some competing technologies, such as S−AC (0.85 mg Hg/g sorbent),44 the spent catalyst after mercury removal could be separated from fly ash to recycle after regenerated. As the byproducts in coal combustion, the cost−effective magnetosphere catalyst would present a huge potential application in Hg0 removal.

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ASSOCIATED CONTENT

S Supporting Information *

Information regarding the experimental apparatus, the determination of the concentration of gas components, Tables S1, Figures S1−S8. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.est.5b01029.

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AUTHOR INFORMATION

Corresponding Authors

0

Figure 6. Hg accumulation removal efficiency of Co−MF catalyst along repeated capture−regeneration cycles under oxyfuel−combustion DSFG atmosphere.

*(Y. C. Zhao) Phone: 86−27−87542417−8312; fax: 86−27−87545526; e-mail: [email protected]. 8216

DOI: 10.1021/acs.est.5b01029 Environ. Sci. Technol. 2015, 49, 8210−8218

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Environmental Science & Technology *(J. Y. Zhang) Phone: 86−27−87542417−8213; fax: 86−27−87545526; e-mail: [email protected].

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Key Basic Research Program (973) of China (No.2014CB238904, 2011CB707301), the National Natural Science Foundation of China (NSFC) (No.51176060, No. 51376074, No. 51206192) and Fund of State Key Lab of Coal Combustion (FSKLCCB1402). Author would like to thank anonymous reviewers for their critical comments.

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