Reduction of SO2 with CO to Elemental Sulfur in Activated Carbon

Aug 1, 2016 - National Engineering Laboratory of Coal-Fired Pollution Reduction, Shandong ... and creates a need for extra facilities for environmenta...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/EF

Reduction of SO2 with CO to Elemental Sulfur in Activated Carbon Bed Tai Feng,† Xiqiang Zhao,† Tao Wang,† Xiao Xia,† Mengze Zhang,‡ Qingchao Huan,‡ and Chunyuan Ma* †

National Engineering Laboratory of Coal-Fired Pollution Reduction, Shandong University, No. 17923 Jingshi Road, Jinan 250061, China ‡ Shandong Shenhua Shanda Energy Environment Company, Limited, No.173 Lishan Road, Jinan 250061, China ABSTRACT: To develop a new process for reducing high-concentration SO2 to elemental sulfur, the reduction of SO2 with CO and activated carbon in a fixed bed experimental system was investigated. The effects of temperature, CO/SO2 molar ratio, and reaction time on SO2 reduction were studied. The results showed that the starting temperature of SO2 reduction with activated carbon was approximately 700 °C and that the addition of CO decreased the starting temperature to 400 °C. Higher temperature led to an increase in SO2 conversion. The S yield also increased initially but then decreased when the temperature exceeded 800 °C, due to the formation of COS. SO2 conversion increased with an increasing CO/SO2 molar ratio, the optimum S yield being achieved at a CO/SO2 ratio of 2. However, the existence of unreacted SO2 in the product decreased the S yield and resulted in an optimum S yield occurring at a low ratio. Lower selectivity of SO2 reduction to elemental sulfur was observed if reaction time was reduced. By prolonging gas−carbon contact time, the S yield increased and gradually approached SO2 conversion at temperatures below 700 °C. The catalytic mechanism over activated carbon conformed to the COS intermediate mechanism, and the rate of the overall CO−SO2 reaction was determined by the Claus reaction between COS and SO2. O2, and H2O.7,14 The presence of S-containing noxious gases adversely affects the elemental sulfur yield and creates a need for extra facilities for environmental pollution control. The reduction of SO2 with reducing gases, such as CO,15−19 H2,19−21 CH4,22−24 and C2H4,25 has also been widely researched. Compared to the other gases, use of CO as a reducing agent has been extensively researched, mainly because CO is available from various industrial sectors. In the studies of SO2 reduction with CO, high SO2 conversion and S yield were achieved at a relatively low temperature (below 500 °C) by taking advantage of appropriate catalysts. Catalysis follows two mechanisms: a redox mechanism and COS intermediate mechanism.17,18 Activated carbon has abundant surface functional groups, which supply active sites for various catalytic reactions. This suggests that SO2 reduction with both CO and activated carbon may result in improved performance than only with either of them. Despite the previous research that has helped in understanding the characteristics of SO2 reduction with CO and with activated carbon, respectively, very few studies have been concerned with the combined effects of CO and activated carbon. Feng14 found that the addition of O2 and H2O could enhance reduction of SO2 in a carbon bed. The main reason for enhancement is that CO has been produced through reactions between carbon and O2 or between carbon and H2O. CO is also the critical component among the volatile components of coals. Ratcliffe and Pap’s11,12 studies of SO2 reduction with lignite and bituminous coal showed that volatile components released from coal pyrolysis could decrease the reaction

1. INTRODUCTION Sulfur dioxide is one of the major contaminants generated from coal-fired power plants. Currently, the limestone−gypsum wet flue−gas desulfurization (WFGD) system is the most widely used method for the removal of SO2. However, this process leads to problems of secondary pollution and environmental damage due to wastewater emission, high consumption of water, limestone mining, etc.1 Dry-type desulfurization technology can avoid high water consumption and water pollution and simultaneously remove many other contaminants such as NOx and SO3. Nevertheless, it still consumes calciumbased sorbents such as limestone and dolomite and cannot achieve the recycling of SO2.2−6 In recent years, much research effort has been focused on SO2 reduction to elemental sulfur, which would provide an effective solution for the abovementioned problems and ease the shortage of elemental sulfur in China. Carbon is low cost, and it is a readily available reducing agent for the reduction of SO2. The main reaction associated with this process is described as follows C + SO2 → CO2 + S

(1)

The reduction of SO2 to elemental sulfur with different types of carbon, such as graphite,7 coke,7−10 charcoal,7 and coal,10−12 has been previously reported. Among this range of carbon types, activated carbon showed the highest reactivity because of its low crystallinity, well-developed pore structure, and abundance of surface active sites.7,13 In most of the studies, a high reaction temperature (above 700 °C) and a long reaction time were necessary conditions, but these would lead to high cost in practical applications.8,11 Sulfur-containing byproducts such as COS, H2S, and CS2 are generated under high temperatures or in the presence of other gases such as CO2, © 2016 American Chemical Society

Received: April 26, 2016 Revised: July 31, 2016 Published: August 1, 2016 6578

DOI: 10.1021/acs.energyfuels.6b01006 Energy Fuels 2016, 30, 6578−6584

Article

Energy & Fuels Table 1. Properties of Activated Carbon proximate analysis (%)

ultimate analysis (%)

Aad

Mad

Vad

FCad

N

C

H

S

bulk density (g/cm3)

apparent density (g/cm3)

14.50

0.42

0.77

84.31

0.68

85.68

0.41

0.60

0.6384

1.142

Figure 1. Experimental system for reduction of SO2 with CO in activated carbon bed. generated by the reaction. SO2 and COS among the gaseous products were analyzed by a GC-MS spectrum analyzer (Thermo ISQ) employing a GC-Carbonplot column. CO2 and CO were analyzed using a refinery gas analyzer (PerkinElmer PE CLAUSE 500 GC). In our research, no other S-containing products were detected except for SO2 and COS. The fractional conversion of SO2 (XSO2) and yield of elemental sulfur (YS) are defined as follows

temperature and complete SO2 conversion within an extremely short time. While reduction of SO2 with lignite occurred at 600 °C in their study, other researchers report that little SO2 was converted at this temperature when volatile components were removed before reaction. As shown in previous studies, the presence of CO presented positive effects on the reduction of SO2 with carbons. In addition, activated carbon could be prepared in situ from fast pyrolysis using coal, and CO could be produced inside the power plant because it is one of the main components of pyrolysis gas released when preparing activated carbon. If successfully managed, this combination would make the process of conversion of SO2 to elemental S sustainable.26,27 Research regarding the reduction of SO2 with CO and activated carbon together is therefore necessary and meaningful. In this work, the influencing factors of SO2 reduction, such as temperature, CO/SO2 ratio, and reaction time, were evaluated and the reaction mechanism was also discussed in detail.

i f i XSO2 = (FSO − FSO )/FSO × 100% 2 2 2

(2)

i f f i YS = (FSO − FSO − FCOS )/FSO × 100% 2 2 2

(3)

is the inlet molar flow rate (mol/min) of SO2 and and where FfCOS are the outlet molar flow rates of SO2 and COS, respectively. The outlet molar flow rate can be finalized by multiplying the volume flow rate (mL/min) of effluent gas by the molar concentrations (mol/mL) of SO2 and COS in final gaseous products. The selectivity of SO2 reduction to elemental sulfur (S) is defined as follows FiSO2

FfSO2

i f f i f S = (FSO − FSO − FCOS )/(FSO − FSO ) × 100% 2 2 2 2

2. EXPERIMENTAL SECTION

= YS/XSO2 × 100%

2.1. Materials. The coal-based activated carbon used in this study was sieved to a particle size of 0.70−0.84 mm (20−25 mesh) and then dried at 105 °C in an oven for 24 h. In order to avoid reactions between volatile components and SO2, the activated carbon was heated to 1050 °C for 30 min under N2 protection to eliminate volatile components and tars before the experiments were performed. Some characteristics of this activated carbon are presented in Table 1. 2.2. Examination of Reactivity. SO2 reduction with CO in an activated carbon bed was studied in a fixed-bed reactor under conventional heating using an electric heating furnace, as shown in Figure 1. The quartz tube with a diameter of 20 mm was fixed vertically in the tubular furnace with an automatic temperature controller, and activated carbon was placed in the center of the tube, so that a uniform and constant temperature could be obtained. The feed gas contained SO2 and CO, balanced with N2 and passed through the carbon bed at a flow rate of 300 mL/min. SO2 concentration in feed gas was maintained at 5 vol % for all runs. The inlet flow rate and the proportion of feed gas were controlled exactly using mass flow controllers (Beijing Sevenstar Co., CS200). The effluent gas passed through a coiled condenser to separate the elemental sulfur that was

(4)

3. RESULTS AND DISCUSSION 3.1. Effect of Temperature on SO2 Reduction and Carbon Balances. Experiments were performed in a fixed bed, which was packed with 3 g of activated carbon. The two experimental conditions used were in the presence of 5 vol % CO and in the absence of CO. The SO2 conversion and S yield at various temperatures are shown in Figure 2. In the absence of CO, SO2 conversion is zero at temperatures below 600 °C, thus illustrating that the reduction of SO2 with activated carbon is impossible under this temperature range. A similar conclusion has already been reported in previous studies using other types of carbon.7,8 Figure 2 also shows that SO2 conversion increases dramatically with the elevation of temperature from 700 to 900 °C and that SO2 conversion and the S yield basically share the same curve at temperatures below 800 °C. At 800 °C, these two curves separated, with the S yield value lower than the SO2 6579

DOI: 10.1021/acs.energyfuels.6b01006 Energy Fuels 2016, 30, 6578−6584

Article

Energy & Fuels

Compared to the reduction of SO2 in the presence of activated carbon only, the starting temperature of SO2 reduction with CO also being present in an activated carbon bed decreases from 700 to 400 °C. The addition of CO obviously promotes SO2 conversion and S yield at temperatures from 400 to 800 °C as shown in Figure 2. This suggests that CO has high reduction reactivity of SO2 at such a temperature range, with the greatest enhancement occurring at 700 °C. The SO2 conversion and S yield in the activated carbon bed without CO are 6% and 5.72%, respectively, while those of reaction with 5 vol % CO addition increase to 54.61% and 52.45%. The activated carbon mass loss of reactions in the activated carbon bed for 30 min is shown in Figure 3, with corresponding

Figure 2. SO2 conversion and S yield as a function of temperature in activated carbon bed without CO and with 5 vol % CO.

conversion value at the same temperature, due to the formation of COS. Considering the C−CO2 reaction prevails at higher temperatures, the formation of COS should therefore be related to the presence of CO. When 5 vol % CO is used, SO2 is rarely converted at 300 °C, and then the SO2 conversion increases as temperature increases from 400 to 1000 °C. Within this temperature range, the S yield is considerably lower than SO2 conversion, particularly at temperatures below 600 °C and above 800 °C. In other words, addition of CO decreases the selectivity of SO2 reduction to elemental sulfur. The large difference observed between SO2 conversion and S yield at higher temperatures occurs for the same reason as the reaction without CO. The reason for the difference at low temperatures is low reactivity. At temperatures below 600 °C, CO is only partially converted, and thus, sulfur mainly exists in the form of COS, as shown in Table 2.

Figure 3. Mass loss rate of carbon as a function of temperature.

carbon balances shown in Table 2. In the carbon balance calculations, the mass of input carbon was calculated according

Table 2. Carbon Balances for the Reaction of SO2 with Activated Carbon and CO output carbon temp. (°C)

CO fraction in inlet gas (%)

total input carbon (g)a

sulfur content (%)b

carbon in solid (g)c

CO (%)

CO2 (%)

COS (%)

carbon in gas (g)

total output carbon (g)

ratio of output and input carbon mass (%)

300

0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5

2.5297 2.7707 2.5297 2.7707 2.5296 2.7710 2.5296 2.7705 2.5303 2.7713 2.5295 2.7710 2.5296 2.7704 2.5300 2.7705

0.56 0.57 0.56 1.37 0.62 2.38 1.16 3.66 3.52 4.16 5.54 5.32 4.38 5.60 2.42 4.26

2.5597 2.5591 2.5594 2.5603 2.5577 2.5499 2.5586 2.4925 2.5297 2.5130 2.3110 2.3864 2.1544 2.1564 1.8927 1.9419

0.00 4.96 0.00 1.27 0.00 0.54 0.00 0.47 0.00 0.00 0.00 0.00 4.22 4.89 11.71 14.97

0.00 0.04 0.00 2.63 0.00 3.61 0.00 4.38 0.24 5.42 4.13 7.73 3.44 5.22 0.19 0.53

0.00 0.00 0.00 1.10 0.00 0.80 0.03 0.18 0.01 0.11 0.10 0.22 1.15 1.92 0.73 2.07

0.0000 0.2406 0.0000 0.2413 0.0000 0.2387 0.0014 0.2425 0.0121 0.2666 0.2039 0.3833 0.4248 0.5800 0.6089 0.8471

2.5597 2.7997 2.5594 2.8016 2.5577 2.7885 2.5601 2.7351 2.5418 2.7796 2.5149 2.7697 2.5791 2.7365 2.5017 2.7890

101.18 101.05 101.17 101.11 101.11 100.63 101.21 98.72 100.45 100.30 99.42 99.95 101.96 98.78 98.88 100.67

400 500 600 700 800 900 1000

The total input carbon mass (Micarbon) was calculated as Micarbon = MiAC × FCad + MiCO, where miAC is the mass of activated carbon before the reaction and MiCO is the mass of carbon contained in the input CO gas over 30 min. bSulfur content was detected by infrared sulfur analyzer (SDS212). cThe mass of carbon in solid after reaction (Mfs,carbon) was calculated as Mfs,carbon = MiAC × FCad − (MfAC × Sfad − MiAC × Siad), where mfAC is the mass of activated carbon remained in the reactor after the reaction and Siad and Sfad are the sulfur content before and after the reaction, respectively. a

6580

DOI: 10.1021/acs.energyfuels.6b01006 Energy Fuels 2016, 30, 6578−6584

Article

Energy & Fuels

Figure 4. SO2 conversion and S yield as a function of CO:SO2 molar ratio: (a) 500, (b) 600, (c) 700, and (d) 800 °C.

results can be obtained at 600 °C, it also shows a decrease in optimum S yield likely due to lower reactivity at lower temperatures. However, Figure 4a and 4d shows obvious differences at 500 and 800 °C, respectively. Significant amounts of CO in feed gas cannot be converted at 500 °C, leading to low SO2 conversion. Meanwhile, a part of elemental sulfur, which has been generated undergoing reaction, is reduced by residual CO to COS. Therefore, the S yield at such a CO/SO2 ratio range is always low, and optimum S yield is only 13%. At 800 °C, decreases in SO2 occur because a considerable amount of SO2 is converted by activated carbon without CO being present. On the other hand, CO2 can react with carbon to form extra CO according to the Boudouard reaction. Both of these can result in a high actual CO/SO2 ratio in the CO−SO2 reaction and result in the maximum yield of S shifting to a low CO/SO2 ratio. This suggests that the optimum S yield is achieved at a CO/SO2 ratio of 2 when the CO and SO2 in feed gas is mostly converted at temperatures below 800 °C in order to avoid extra CO produced. Either unconverted CO or extra CO generated from the C−CO2 reaction would lower S yield at a CO/SO2 ratio of 2 and shift the optimum S yield toward a lower CO/SO2 ratio. 3.3. Effect of Reaction Time on SO2 Reduction. The reaction time (t) varied with carbon mass and is defined as follows

to FCad in proximate analysis of input activated carbon and CO concentration in feed gas. The mass of output carbon in solids was measured using an electronic scale with a precision of 0.001 g after the experiments were completed, and carbon mass in gas was calculated from the concentration data of C-containing gases. As shown in Figure 3, at temperatures above 700 °C, the mass loss increased tremendously because of the increasing conversion of SO2 and the prevailing C−CO2 reaction. At temperatures below 700 °C, the mass loss rates are almost zero or even negative. This is attributed to low SO2 conversion with carbon and sulfur combining within the carbon matrix in the form of C−S compounds.28−30 Thus, it is indicated that the activated carbon bed can perform for a long time at lower temperatures. As shown in Table 2, the change to sulfur content in activated carbon after a 30 min reaction confirms this presumption. The activated carbon mass shows a slight difference in results between reactions with 5 vol % CO in feed gas and without CO. This difference also results from the changes to sulfur content because only a slight difference is observed between the output mass of carbon in the solids with and without CO. The carbon balances are reasonable, considering the mass ratio of output to input carbon was nearly 100%. 3.2. Effect of CO/SO2 Molar Ratio on SO2 Conversion and S Yield. The CO/SO2 molar ratio changed from 0 to 3 as CO concentration was varied from 0 to 15 vol %. As shown in Figure 4, SO2 conversion in the activated carbon bed increases as the CO/SO2 ratio is increased at all temperatures. Figure 4c demonstrates that S yield increases as expected at 700 °C and reaches a maximum at a CO/SO2 ratio of 2, which corresponds with the stoichiometry of the CO−SO2 reaction. The optimum SO2 conversion and S yield are 95.82% and 88.48%, respectively, thus demonstrating superior reactivity. With further increase in the CO/SO2 ratio, a larger amount of COS is generated because of an excess of CO, followed by decrease in the S yield. While Figure 4b shows that similar

t = Vε /qv = mρb (1 − ρb /ρa )/qv

(5)

where V (cm3) is the volume of activated carbon bed, ε is the void ratio of the carbon bed, qv (mL/min) is the flow rate of feed gas, m (g) is the mass of activated carbon, and ρb and ρa (g/cm3) are the bulk density and apparent density of activated carbon, respectively. As determined using eq 5, for each gram of activated carbon added, the reaction time increased by about 0.138s at a flow rate of 300 mL/min through the activated carbon bed. The CO/SO2 molar ratio remained constant at 2:1. As shown in 6581

DOI: 10.1021/acs.energyfuels.6b01006 Energy Fuels 2016, 30, 6578−6584

Article

Energy & Fuels

time except at 800 °C. The decrease in S yield at 800 °C is caused by an increase in CO generated by the C−CO2 reaction after the SO2 conversion is complete. Compared with the SO2 conversion value, the S yield is smaller over a short reaction time. The reasons for low S yield can be explained by Table 3, which includes the SO2 conversion, S yield, and product distribution at 500 °C for various reaction times. It is found that little elemental sulfur is generated at the beginning of the reaction, despite a significant amount of SO2 being reduced. A large amount of COS was detected, and most sulfur existed in the form of COS within 0.42 s. The change in concentration of CO2 illustrates that elemental sulfur has been generated, because CO2 is always generated following with elemental sulfur according to the reactions below

Figure 5, the SO2 conversion increases with increasing reaction time at various temperatures (400−800 °C). The reaction time

2CO + SO2 → 2CO2 + S

(6)

2COS + SO2 → 2CO2 + 3S

(7)

However, excessive CO reduces elemental sulfur to COS and leads to a low yield of S. With an increasing reaction time, the concentration of CO decreases as it is converted completely to CO2 and COS. As the reaction time continues to increase, the concentrations of COS and SO2 both decrease gradually and the S yield approaches SO2 conversion. This suggests that SO2 is reduced through a Claus reaction between COS and SO2, after CO is completely converted. In other words, COS is an intermediate in the CO−SO2 reaction. 3.4. Reaction Mechanism. To determine the reaction mechanism for reduction of SO2 with CO and COS, experiments were carried out in an activated carbon bed and in a quartz sand bed of equal volume. The concentrations of reaction gases, including SO2, CO, and COS, were maintained at 5 vol %. As shown in Figure 7, a small amount of SO2 was reduced by CO or COS in the quartz sand bed at temperatures below 1000 °C. However, at temperatures above 400 °C in the activated carbon bed, SO2 is converted as the temperature increases. The gradual increase in SO2 conversion caused by reaction either with CO or with COS suggests that activated carbon acts as catalyst in the reduction of SO2 with CO or COS at temperatures above 400 °C. The curves of SO2 conversion with two gases almost overlap, suggesting that reduction of SO2 with CO and COS reacts according to the same pathway. Therefore, reduction of SO2 with CO catalyzed by activated carbon follows the COS intermediate mechanism. The high consistency in SO2 conversion also indicates that the COS intermediate mechanism is the dominant reaction pathway for CO and SO2 in an activated carbon bed. The rate of reaction between COS and SO2 determines the rate of overall CO−SO2 reaction, particularly at lower temperatures. The reaction procedure, which includes three steps, is shown in Figure 8. The first step is the generation of elemental sulfur. SO2 reduction by the COS intermediate mechanism cannot occur at the initial reaction, because COS cannot be generated without elemental sulfur or metal sulfide,17,18 and thus, elemental sulfur generated through another reaction path would be required. Considering the C−SO2 reaction can only occur above 700 °C in active site C(1), a small amount of elemental sulfur should be generated from SO2 reduction by the redox mechanism in active site C(2). The second step is the process of elemental sulfur being converted to COS by CO in active site C(3). Residual SO2 would be reduced by COS through a Claus reaction in active site C(4), which is the third step. The COS intermediate mechanism is indicated as follows

Figure 5. SO2 conversion as a function of reaction time at temperatures from 400 to 800 °C.

for complete SO2 conversion decreases with increasing temperature, due to the faster rate of reaction at higher temperatures. SO2 conversion exceeded 90% within 0.42 s at temperatures above 600 °C; however, a longer reaction time is required when the temperature is below 600 °C. For example, SO2 conversion reaches 90% within approximately 1.7 s at 500 °C and requires 4.2 s at 400 °C. The SO2 conversion increases slightly when it was nearly completed at temperatures below 500 °C due to a slow reaction rate and low reactant concentrations. It suggests that an extremely long reaction time would be required for complete SO2 conversion, and this reaction time is difficult to achieve in practical applications operating below 500 °C. Even so, the reaction time for complete SO2 conversion with CO is still shorter than the time required for SO2 reduction with carbon reported by Bejarano,8 in which complete SO2 reduction by oil sands fluid coke required 8 s at 700 °C. The S yield generated under the same operating conditions is shown in Figure 6. S yield increases with increasing reaction

Figure 6. S yield as a function of reaction time at temperatures from 400 to 800 °C. 6582

DOI: 10.1021/acs.energyfuels.6b01006 Energy Fuels 2016, 30, 6578−6584

Article

Energy & Fuels Table 3. SO2 Reduction at 500 °C as a Function of Reaction Time product distribution reaction time (s) 0.138 0.414 0.691 0.967 1.243 1.520 1.796 2.072

CO2 (%)

CO (%)

SO2 (%)

COS (%)

SO2 conversion (%)

S yield (%)

selectivity (%)

2.53 6.65 8.66 9.42 9.76 9.79 9.96 10.47

6.81 1.57 0.00 0.00 0.00 0.00 0.00 0.00

3.97 2.83 1.68 1.16 0.66 0.51 0.41 0.34

1.00 1.86 1.21 0.67 0.38 0.24 0.13 0.06

20.57 43.37 66.50 76.83 86.78 89.79 91.90 93.17

0.56 6.10 42.34 63.53 79.14 85.08 89.37 92.02

2.74 14.07 63.67 82.69 91.20 94.75 97.25 98.77

COS and unconverted SO2 in gaseous products should correspond to the stoichiometry of the COS−SO2 reaction when the CO/SO2 ratio was 2:1 depending on the carbon balance. Nevertheless, the CO/SO2 molar ratio shown in Table 3 does not accord with the above-mentioned law due to partial decomposition of COS to CO and elemental sulfur.

4. CONCLUSIONS The effect of temperature, CO/SO2 molar ratio, and reaction time on reduction of SO2 with CO in an activated carbon bed was investigated in this research. In this system, activated carbon acted as both catalyst and reducing agent. Reduction of SO2 with activated carbon occurred at temperatures above 700 °C, and addition of CO decreased the starting temperature of this reaction to 400 °C. As temperature increases, the SO2 conversion in the activated carbon bed, with or without CO being present, also increases. S yield increases first with increasing temperature but then decreases after the SO2 conversion is complete, due to formation of COS. Carbon balances indicate that the mass loss of activated carbon increases with temperature rising. Addition of CO would not have resulted in increased mass loss. The high CO/SO2 ratio causes SO2 conversion to increase, with optimum S yield achieved at the CO/SO2 ratio corresponding to the stoichiometry of the CO−SO2 reaction (CO/SO2 = 2:1). However, extra CO generated through the C−CO2 reaction or unconverted CO at low temperatures would negatively affect S yield and shift the optimum yield of S to a low CO/SO2 ratio. With increasing temperature, a shorter reaction time is required for complete SO2 conversion. S yield is much lower than the SO2 conversion over a short reaction time. It then increases and gradually approaches the SO2 conversion after CO is converted completely with increasing reaction time. The catalytic reduction mechanism conforms to the COS intermediate mechanism, and COS is generated from the CO− S reaction. The overall reaction rate is controlled by the rate of reaction between COS and SO2. Therefore, increasing the amount of catalytic active sites for the COS−SO2 reaction or loading appropriate catalysts to improve catalytic activity for this reaction are effective pathways to decreasing reaction temperature and reaction time.

Figure 7. SO2 conversion as a function of temperature for 5 vol % CO or COS in activated carbon bed and quartz sand bed.

Figure 8. Reaction procedure.

C(or MO) + xCOS → C(Sx )(or MSx ) + xCO2

(8)

C(Sx )(or MSx ) + CO → C(Sx − 1)(or MSx − 1) + COS (9)

SO2 + 2COS ⇔ 3S + 2CO2

(10)

where C(S) is the surface S-contained compound and MO and MSx are, respectively, the metal oxide and metal sulfide. The specific functional group in carbon matrix or metal oxide in ash may provide catalytic active sites for this step.7,18 Actually, most of the elemental sulfur which reacted with CO is formed by the COS intermediate mechanism in this step. After CO has converted completely, residual SO2 is reduced by COS to elemental sulfur, which, along with CO2, is one of main products from SO2 reduction. Theoretically, the molar ratio of

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 53188399369. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6583

DOI: 10.1021/acs.energyfuels.6b01006 Energy Fuels 2016, 30, 6578−6584

Article

Energy & Fuels

■

ACKNOWLEDGMENTS Financial support of this work by the National Natural Science Foundation of China (no. 51506115), Promotive Research Fund for Excellent Young and Middle-Aged Scientists of Shandong Province (BS2013NJ022), National Key Technology R&D Program (2014BAA02B03), and Young Scholars Program of Shandong University, “YSPSDU”: Basic Research on Thermal Effects of Microwave Applied in the Field of Energy and Environmental Protection, is gratefully acknowledged.

■

REFERENCES

(1) Liu, Y.; Bisson, T. M.; Yang, H.; Xu, Z. Fuel Process. Technol. 2010, 91 (10), 1175−1197. (2) Lupiáñez, C.; Guedea, I.; Bolea, I.; Díez, L. I.; Romeo, L. M. Fuel Process. Technol. 2013, 106, 587−594. (3) Gungor, A. Chem. Eng. J. 2009, 146 (3), 388−400. (4) Scala, F.; Chirone, R.; Meloni, P.; Carcangiu, G.; Manca, M.; Mulas, G.; Mulas, A. Fuel 2013, 114, 99−105. (5) Krzywanski, J.; Czakiert, T.; Blaszczuk, A.; Rajczyk, R.; Muskala, W.; Nowak, W. Fuel Process. Technol. 2015, 137, 66−74. (6) Krzywanski, J.; Czakiert, T.; Blaszczuk, A.; Rajczyk, R.; Muskala, W.; Nowak, W. Fuel Process. Technol. 2015, 139, 73−85. (7) Humeres, E.; Moreira, R. F. P. M.; da Gloria B. Peruch, M. Carbon 2002, 40 (5), 751−760. (8) Bejarano, C. A.; Jia, C. Q.; Chung, K. H. Environ. Sci. Technol. 2001, 35 (4), 800−804. (9) Bejarano, C.; Jia, C. Q.; Chung, K. H. Ind. Eng. Chem. Res. 2003, 42 (16), 3731−3739. (10) Cha, C. Y.; Kim, D. S. Carbon 2001, 39 (8), 1159−1166. (11) Ratcliffe, C. T.; Pap, G. Fuel 1980, 59 (4), 237−243. (12) Ratcliffe, C. T.; Pap, G. Fuel 1980, 59 (4), 244−248. (13) Wang, X.; Wang, A.; Wang, X.; Zhang, T. Energy Fuels 2007, 21 (2), 867−869. (14) Feng, W.; Jia, C. Q. Environ. Sci. Technol. 2005, 39 (24), 9710− 9714. (15) Zhao, H.; Luo, X.; He, J.; Peng, C.; Wu, T. Fuel 2015, 147, 67− 75. (16) Han, G. B.; Park, N.-K.; Lee, J. D.; Ryu, S. O.; Lee, T. J. Catal. Today 2006, 111 (3−4), 205−211. (17) Han, G. B.; Park, N.-K.; Yoon, S. H.; Lee, T. J. Ind. Eng. Chem. Res. 2008, 47 (5), 1427−1434. (18) Park, N.-K.; Lee, T. H.; Lee, T. J.; Baek, J.-I.; Lee, J. B. Catal. Today 2016, 265, 131−137. (19) Paik, S. C.; Kim, H.; Chung, J. S. Catal. Today 1997, 38 (2), 193−198. (20) Paik, S. C.; Chung, J. S. Appl. Catal., B 1995, 5 (3), 233−243. (21) Ishiguro, A.; Liu, Y.; Nakajima, T.; Wakatsuki, Y. J. Catal. 2002, 206 (1), 159−164. (22) Zhu, T.; Dreher, A.; Flytzani-Stephanopoulos, M. Appl. Catal., B 1999, 21 (2), 103−120. (23) Sarlis, J.; Berk, D. Ind. Eng. Chem. Res. 1988, 27 (10), 1951− 1954. (24) Yu, J.-J.; Yu, Q.; Jin, Y.; Chang, S.-G. Ind. Eng. Chem. Res. 1997, 36 (6), 2128−2133. (25) Wang, C.-H.; Lin, S.-S.; Sung, P.-C.; Weng, H.-S. Appl. Catal., B 2003, 40 (4), 331−345. (26) Zhang, Z.; Wang, T.; Ke, L.; Zhao, X.; Ma, C. Energy Fuels 2016, 30 (2), 896−903. (27) Zhang, Z. W. T; Ma, C.; Feng, T.; Ma, T. J. China Coal Soc. 2014, 39 (10), 2107−2113. (28) Rai Puri, B.; Singh Hazra, R. Carbon 1971, 9 (2), 123−134. (29) Humeres, E.; Peruch, M. d. G. B.; Moreira, R. F. P. M.; Schreiner, W. J. Phys. Org. Chem. 2003, 16 (10), 824−830. (30) Humeres, E.; Mendes de Castro, K.; Moreira; Peruch, M. d. G. B.; Schreiner, W. H.; Aliev, A. E.; Canle, M.; Santaballa, J. A.; Fernández, I. J. Phys. Chem. C 2008, 112 (2), 581−589. 6584

DOI: 10.1021/acs.energyfuels.6b01006 Energy Fuels 2016, 30, 6578−6584