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Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal
JES-00346; No of Pages 7 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 5 ) XX X–XXX
Available online at www.sciencedirect.com
ScienceDirect www.journals.elsevier.com/journal-of-environmental-sciences
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Song Ding1,2 , Yuran Li2 , Tingyu Zhu1,2,⁎, Yangyang Guo2
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1. School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China. E–mail: [email protected] 2. National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
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Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal
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AR TIC LE I NFO
ABSTR ACT
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Article history:
To decrease the operating cost of flue gas purification technologies based on carbon-based
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Received 20 October 2014
materials, the adsorption and regeneration performance of low-price semi-coke and
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Revised 8 December 2014
activated coke were compared for SO2 and NO removal in a simulated flue gas. The
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Accepted 28 February 2015
functional groups of the two adsorbents before and after regeneration were characterized
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Available online xxxx
by a Fourier transform infrared (FTIR) spectrometer, and were quantitatively assessed using
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Keywords:
results show that semi-coke had higher adsorption capacity (16.2% for SO2 and 38.6% for
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Functional groups
NO) than activated coke because of its higher content of basic functional groups and
SO2 recovery
lactones. After regeneration, the adsorption performance of semi-coke decreased because
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Adsorption
the number of active functional groups decreased and the micropores increased. Semi-coke
Carbon consumption
had better regeneration performance than activated coke. Semi-coke had a larger SO2
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temperature programmed desorption (TPD) coupled with FTIR and acid–base titration. The
recovery of 7.2% and smaller carbon consumption of 12% compared to activated coke. The
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semi-coke carbon-based adsorbent could be regenerated at lower temperatures to depress
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the carbon consumption, because the SO2 recovery was only reduced a small amount.
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© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
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Introduction
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The combustion of coal and other fossil fuels emits many pollutants, such as SO2 and NOx. To decrease the amounts of these pollutants, many flue gas purification technologies have been developed. Those using carbon-based materials are considered some of the best flue gas purification technologies (Tsuji and Shiraishi, 1997; Zhu et al., 2000; Li et al., 2012; Izquierdo et al., 2003; Guo et al., 2013; Hou et al., 2009). These technologies use activated coke, activated carbon or activated carbon fiber as the adsorbent for SO2 and NOx removal, since these materials possess both adsorption capacity for SOx, NOx, dioxins and other pollutants based on physical-sorption and/ or chemisorption, and catalytic activity for NOx reduction with
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Published by Elsevier B.V.
NH3. Moreover, no wastewater or secondary pollutants are produced, and SO2 can be effectively recovered by the reaction of H2SO4 with carbon on the surface when the adsorbents are heated (Guo et al., 2008; Zhang et al., 2012b). However, these flue gas purification technologies with carbon-based materials have two key problems, namely, high operating cost and high heat consumption. For example, when activated coke (approximately 5000 CNY/ton) is used as the adsorbent, the cost of the activated coke is 50%–70% of the total operating cost (Zhang and Xu, 2012). The regeneration temperature is usually 400°C, requiring a large amount of heat. Therefore, the main research thrust for flue gas purification technologies with carbon-based materials is to find a low-price carbon-based material as the adsorbent in
⁎ Corresponding author. E-mail: [email protected] (Tingyu Zhu).
http://dx.doi.org/10.1016/j.jes.2015.02.004 1001-0742/© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
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2
76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
SBET (m /g)
Vm (mL/g)
Vt (mL/g)
D (nm)
Activated coke Semi-coke
88.400 59.879
0.0366 0.0244
0.0703 0.0579
1.272 1.419
C
1. Experiments
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1.1. Materials and characterization
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Semi-coke from the Sanjiang Coal Chemical Plant (Shanxi, China) and activated coke from the Xinhua Chemical Plant (Taiyuan, China) were used. The two materials were crushed, and the particles were sieved through 2–60 mesh (0.25–0.85 mm). These particles were washed by deionized water and then dried at 105°C for 24 hr after filtration. Elemental analysis was performed on an Analyzer (Vario EL cube, Elementar, Germany), and the results are shown in Table 1. The pore properties of adsorbents were determined at −196°C through nitrogen adsorption (Autosorb-iQ, Quantachrome, USA). The BET (Brunauer–Emmett–Teller) surface area (SBET), micropore volume (Vm) and average pore diameter ( D ) were calculated by the Brunauer–Emmett–Teller equation, Dubinin– Radushkevich equation and Horvath–Kawazoe equation, respectively. The total pore volumes (Vt) were directly calculated. The results are shown in Table 2. Functional groups of adsorbents were characterized by a FTIR spectrometer (Nicolet 6700, Thermo, USA). Absorption was measured over the range from 400 to 4000 cm−1 with a resolution
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t1:1 t1:3 t1:2
120
R
R
Sample
C
N
H
S
t1:5 t1:6
Activated coke Semi-coke
75.73 76.66
0.65 0.72
1.18 1.23
0.75 0.47
121 122 123 124 125 126 127 128 129 130
1.2. Experimental system and reaction conditions
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The experimental system is composed of a fixed-bed reactor, heating temperature control system, simulated flue gas system and detection system. The reactor is a quartz tube with a sintered silica plate in the middle for loading the adsorbents. The diameter of the tube is 20 mm, and the length is 520 mm. The heating temperature control system is a resistance furnace connected to a temperature controller to adjust the temperature. After passing through flow-meters, gases, such as SO2, NO, NH3, O2 and N2, were mixed in a jar and then entered the reactor. The water vapor was produced by evaporating water in a U-tube and was carried by N2. The amount of water vapor can be calculated by the Antoine equation and the flow of N2. The composition of the simulated flue gas was 0.1 mol% SO2, 0.05 mol% NO, 0.05 mol% NH3, 5 mol% H2O, 5 mol% O2, and balance N2. During adsorption, the adsorbent was maintained for 90 min in the simulated flue gas, the temperature was 150°C, the total flow was 300 mL/min, and the gas hourly space velocity (GHSV) was approximately 5700 hr−1. After adsorption, the adsorbent was regenerated by heating. The regeneration
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1000 Solid symbol: activated coke Open symbol: semi-coke
SO2
800
600
NO
400
NH3
200
Table 1 – Elemental analysis of the adsorbents (wt.%).
t1:4
t2:5 t2:6
of 2 cm−1. The background spectrum was obtained by using KBr as a blank. The ratio of sample to KBr was approximately 1:1000, and the samples were scanned 8 times. The functional groups that contained oxygen were analyzed by temperature programmed desorption (TPD) coupled with Fourier transform infrared (FTIR), and the basic functional groups were determined by acid–base titration. The SO2 recovery is the ratio of total SO2 released during regeneration to total SO2 adsorbed during adsorption. Carbon consumption is the total C amount of CO and CO2 emitted during regeneration (mmol/g).
Concentrations (×10-4 mol%)
106
O
105
C
104
t2:4
t2:7 t2:9 t2:8
N
103
U
102
E
99 98
t2:3 t2:2
SBET: BET surface area; Vm: micropore volume; Vt: total pore volume; D: average pore diameter.
T
97
Sample
F
75
t2:1
O
74
2
R O
73
Table 2 – Pore properties of adsorbents.
P
72
order to reduce the cost, improve the adsorption performance by modification, and lower the regeneration temperature to save heat. Activated coke has been used in many plants, because its price is cheaper than that of activated carbon. To improve the de-SO2 and de-NO performance of activated coke, the coke is modified by loading with metal oxides, such as those of V, Co, Mo and Fe (Liu and Liu, 2013; Ma et al., 2003, 2008; Xing et al., 2008). To save energy consumption, the regeneration progress of spent activated coke has also been investigated through changes in the heating method or the gas medium during regeneration (Zhang et al., 2012b; Guo et al., 2007a, 2007b; Xing et al., 2007). Semi-coke (approximately 1000 CNY/ton) is prepared from non-caking coal or weakly caking coal, carbonized at 500– 700°C. Semi-coke is richer in surface functional groups than activated coke, which is treated by oxidization in order to increase the specific surface area. Although the preparation processes are different, semi-coke and activated coke have similar physical and chemical properties. It was reported that low-price lignite semi-coke has an adsorption capacity for mercury (Zhang et al., 2012a) that is the same as that of activated coke. Therefore, to explore the application potential of semi-coke for SO2 and NO removal, the adsorption and regeneration performances of semi-coke and activated coke were compared. The promotional effect of the physical and chemical properties of the materials on SO2 and NO adsorption was investigated. Carbon consumption and SO2 recovery were also studied to compare the regeneration performance.
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40 50 60 Time (min)
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90
Fig. 1 – Breakthrough curves of the adsorbents.
Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
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1435
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0.09
1628
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2853
3433
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0.00 4000
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2. Results and discussion
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2.1. Adsorption performance
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Fig. 1 shows the breakthrough curves of the two adsorbents. The SO2, NO and NH3 breakthrough concentrations of semi-coke were lower than those of activated coke. Through integration and calculation, the de-SO2 and de-NO capacities of activated coke were found to be 11.70 and 0.88 mg/g, respectively, and the de-SO2 and de-NO capacities of semi-coke were 13.60 and 1.22 mg/g, respectively. The adsorption capacities of semi-coke were 16.2% higher for SO2 and 38.6% higher for NO than those of activated coke. The result indicates that semi-coke had better adsorption performance for SO2 and NO than activated coke. From the results of Table 2, the surface area, micropore volume and total pore volume of semi-coke can be seen to be smaller than those of activated coke. Therefore, semi-coke possesses
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3500
2.0
a
2800
2100
1.6
Solid symbol: activated coke Open symbol: semi-coke
Amounts (mmol/g)
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N C O
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Concentrations (×10-4 mol%)
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T
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D
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temperature was ramped from 150 to 400°C at 5°C/min and then was maintained at 400°C for 40 min in N2. The gas flow was 180 mL/min during regeneration. The adsorption experimental parameters for regenerated absorbents were identical to the initial experimental adsorption parameters. The concentrations of components in the inlet and outlet were measured online by a FTIR spectrometer (Tensor 27, Bruker, Germany).
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Fig. 2 – IR absorption spectra of the adsorbents. 152
high removal efficiencies for SO2 and NO due to its more plentiful functional groups or larger pore size than activated coke. The IR absorption spectra of the two adsorbents are shown in Fig. 2. The two adsorbents show a number of absorption peaks in the range from 400 cm−1 to 4000 cm−1. According to previous studies (Ning, 2010; Shafeeyan et al., 2010), the peaks at 3600–3200 cm−1 can be assigned to the O–H stretching vibrations of alcohol, phenol, water and the N–H stretching vibrations of –NH and –NH2; the peaks at 3000–2800 cm−1 are assigned to the C–H stretching vibrations of –CH2 and –CH3; the peaks at 1700–1585 cm−1 are assigned to the C_O stretching vibrations of carbonyl and the C_C stretching vibrations of benzene; the peaks at 1440–1395 cm−1 are assigned to the C_O stretching vibrations of carboxyl; and the peaks at 1200–1000 cm−1 are assigned to the C–O stretching vibrations of alcohol, ether and anhydride. Comparing the spectra of the two adsorbents, the peaks at 1435 and 1628 cm−1 are significantly different. The reason is that semi-coke has more –COO– and less C_O than activated coke. Fig. 3a shows the TPD curves of the two adsorbents. It can be seen that the CO concentration of semi-coke is smaller than that of activated coke, and the CO2 concentration of semi-coke is greater than that of activated coke. Because the thermal stability of different functional groups varies, when the temperature is increased, the active functional groups will decompose into CO and/or CO2. Normally, CO2 derives from carboxyl, anhydride and lactone and CO derives from anhydride, phenol, carbonyl and quinonyl (Figueiredo et al., 1999; Figueiredo and Pereira, 2010). By integrating the CO and CO2 curves of the two adsorbents, the amounts of functional groups can be determined. The results are shown in Fig. 3b. The numbers of functional groups on the two adsorbents vary in decreasing order according to lactone, carbonyl, quninoyl, phenol, carboxyl and anhydride. Between semi-coke and activated coke, the difference is in the amounts of lactone and carbonyl. The lactone content of semi-coke is 92% higher than that of activated coke, but the carboxyl content of semi-coke is lower than that of activated coke. These results are in agreement with the IR analysis. By analyzing the adsorbents by acid–base titration, the result shows that the basic functional group concentrations of semi-coke and activated coke are 1.055 and 0.609 mmol/g,
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Absorption intensity (%)
Activated Coke Semi-coke
CO2 1400 CO 700 0 150
b
Activated Coke Semi-coke
1.2
0.8
0.4
300
450 600 Temperature (°C)
750
900
0.0
Lactone Carbonyl Phenol Quinonyl Carboxyl Anhydride
Fig. 3 – Temperature programmed desorption (TPD) analysis of adsorbents. (a) Desorption curves for CO2 and CO and (b) amounts of oxygen containing functional groups. Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
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Solid symbol: activated coke Open symbol: semi-coke 800
NO NH3
200
70
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90
F
40 50 60 Time (min)
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Fig. 4 – Breakthrough curves of adsorbents after regeneration.
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3500
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2500 2000 1500 Wavenumber (cm-1)
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Semi-coke
Fresh Regenerated
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1435
1628 1435
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Fresh Regenerated
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1628
Activated coke
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0.00 4000
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3000
2500 2000 1500 Wavenumber (cm-1)
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Fig. 5 – IR absorption spectra of adsorbents before and after regeneration.
0.03
0.08 0.06
0.02
0.04 0.02 0.00 0.3
Activated coke
0.6 0.9 1.2 Pore size (nm)
1.5 1.8
0.12
0.04
0.01 0.00
0.10 0.08
Solid symbol: fresh Open symbol: regenerated
0.04 0.03
0.06
0.02
Vm (mL/g)
0.10
0.05 Semi-coke
0.35 nm 0.42 nm
0.12
0.14
0.05
Solid symbol: fresh Open symbol: regenerated
dVm/dD (mL/(nm.g))
0.14
Vm (mL/g)
224
O
223
C
222
N
221
U
220
Absorption intensity (%)
219
respectively. The basic functional group content of semi-coke is 73.2% higher than that of activated coke. Additionally, C–O–C in the lactonic structure plays an important role in SO2 adsorption and may be the active center of the desulfurization on activated semi-coke (Ju et al., 2005). Therefore, the abundant basic and acidic functional groups on semi-coke promoted the SO2 and NO removal.
dVm/dD (mL/(nm.g))
218
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R O
20
The breakthrough curves of adsorbents after regeneration are shown in Fig. 4. Through integration and calculation, the adsorption capacities of activated coke after regeneration are found to be 12.07 mg/g for SO2 and 0.88 mg/g for NO. The adsorption capacities of semi-coke after regeneration are 12.75 mg/g for SO2 and 0.6 mg/g for NO. The de-SO2 capacity of semi-coke after regeneration is 5.6% higher than that of activated coke, but the de-NO amount of semi-coke after regeneration is 31.8% lower than that of activated coke. Comparing the adsorption capacities of semi-coke before and after regeneration, those after regeneration are 6.3% smaller for SO2 and 50.8% smaller for NO than those of fresh semi-coke. It is tentatively proposed that the pore structure or chemical properties of the adsorbents changed during thermal regeneration; for example, the decomposition of lactone and carboxyl possibly took place. Infrared adsorption spectra of adsorbents before and after regeneration are shown in Fig. 5. The spectra display significant differences at 1628 and 1435 cm−1 for the two adsorbents. The carboxyl content of activated coke did not obviously change, but the carboxyl content of semi-coke decreased. Fig. 6 shows the micropore volumes and pore size distributions of the two adsorbents before and after regeneration. After
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2.2. Re-adsorption performance after regeneration SO2
Absorption intensity (%)
Concentrations (×10-4 mol%)
1000
0.04 0.01
0.02 0.00 0.3
0.6 0.9 1.2 Pore size (nm)
1.5 1.8
0.00
Fig. 6 – Micropore volume (Vm) and pore size distributions (dVm/dD) of adsorbents before and after regeneration. Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
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300 240°C
250
500 NH3
200 NO
0 0
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40 50 60 Time (min)
70
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CO2
400
T
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325°C 300
300 240°C
200 0
150 90
250
CO
150
0
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40 50 60 Time (min)
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150 90
O
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450
Temperature (°C)
T
450
Solid symbol: activated coke Open symbol: semi-coke
b
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SO2
400
Concentrations (×10-4 mol%)
1500
600
450
Solid symbol: activated coke Open symbol: semi-coke
a
Temperature (°C)
Concentrations (×10-4 mol%)
2000
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Fig. 7 shows the regeneration curves of the two adsorbents. SO2, NO and NH3 curves are shown in Fig. 7a, and CO and CO2 curves are shown in Fig. 7b. The trends of the SO2, NO, NH3, CO and CO2 concentrations were observed during regeneration, but NO was not detected. This was because NO mainly reacted with NH3 through SCR and formed N2, and SO2 transformed into H2SO4, then some H2SO4 reacted with NH3 and generated ammonium sulfates on the adsorbent surface. During thermal regeneration, H2SO4 and ammonium sulfates can react with
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t3:1
C
258
E
257
R
256
R
255
N C O
254
U
253
Table 3 – Carbon consumptions of the adsorbents during regeneration.
t3:3 t3:2
t3:4 t3:5 t3:6
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2.2. Carbon consumption
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carbon on the surface, releasing SO2, NH3, CO and CO2 with the rising temperature. Fig. 7a shows that with increasing temperature, the SO2 concentrations of the two adsorbents first increase and then decrease. The SO2 concentration is at a maximum when the temperature is approximately 240°C. Through integrating the SO2 concentration curves, the amount of SO2 released for semi-coke was found to be more than that for activated coke. During the regeneration process, the SO2 recovery ratios of semi-coke and activated coke were 69.6% and 62.4%, respectively. Fig. 7b shows that the CO concentration increases with increasing temperature, then the concentration suddenly drops at 400°C. This result indicates that CO is derived from active functional groups that have poor thermal stability. Under 325°C, the trend of the CO2 concentration is similar to that of the SO2 concentration, which increases quickly and then decreases. This result indicates that CO2 is mainly derived from the reaction of C with H2SO4 or ammonium sulfates. When the temperature reaches 325°C, the CO2 concentration gradually increases with increasing temperature and suddenly drops at 400°C. CO2 is mainly derived from the decomposition of active functional groups from 325 to 400°C. The results of the integration of CO, CO2 and SO2 curves during regeneration are shown in Table 3. When 1 mol SO2 is released, the carbon consumption of semi-coke and activated coke is 0.8 and 0.91 mol, respectively. The carbon consumption of semi-coke is 12% smaller than that of activated coke. Fig. 8 shows the SO2 recovery ratios and carbon consumption of the two adsorbents during regeneration at temperatures from 150 to 400°C. The initial decomposition temperature is 200°C. As the temperature rises, the SO2 recovery ratios of the two adsorbents first increase quickly and then slowly, and the carbon consumption increases throughout the entire course.
D
270
251
T
269
regeneration, the micropore volumes of semi-coke and activated coke increased by 25.8% and 15.9%, respectively, because of the opening of closed pores and the expansion of micropores. The range of pore size distribution of activated coke did not change, but the range of pore size distribution of semi-coke widened because of increasing numbers of micropores, the pore size of which is less than 0.42 nm. According to previous studies (Raymundo-Piñero et al., 2000, 2003; Guo et al., 2001; Xu et al., 2006), the SO2 and NO equivalent diameters are in the range of 0.28–0.35 nm and 0.11–0.31 nm, respectively, and the optimal pore size of activated coke for removal of SO2 was reported to be 0.7 nm. Therefore, a wide range of pore size is adverse for the adsorption and oxidation of SO2. The NO adsorption amount of an adsorbent is determined by its surface chemical properties. Although the pore volume of semi-coke increases after regeneration, the increased volume is mostly contributed by micropores. Smaller pore size hinders the dispersion of SO2. Therefore, the adsorption capacities of semi-coke are reduced for SO2 and NO because of the decomposition of active functional groups and the increase of micropores, which are not beneficial to SO2 adsorption.
250
E
249
R O
Fig. 7 – Regeneration curves of adsorbents after adsorption. (a) SO2, NH3, and NO curves and (b) CO2 and CO curves. T: temperature.
Activated coke Semi-coke
CO (mmol/g)
CO2 (mmol/g)
Total carbon (mmol/g)
SO2 (mmol/g)
C/SO2 (mmol/mmol)
0.040 0.050
0.061 0.071
0.10 0.12
0.11 0.15
0.91 0.80
Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
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0.04
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250 300 Temperature (°C)
0.00 400
350
Fig. 8 – SO2 recovery and carbon consumption during regeneration.
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3. Conclusions
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Semi-coke shows better adsorption performance for SO2 and NO than activated coke, because it is richer in basic functional groups and lactones. After regeneration, the adsorption capacities of semi-coke decreased by 6.3% for SO2 and 50.8% for NO. This decrease occurred because the active functional groups on the semi-coke surface decreased and the micropores, which are not beneficial to SO2 dispersion, increased. The adsorption capacity of activated coke did not change, because the micropores, which are beneficial to SO2 dispersion, were increased as a result of thermal regeneration and promoted SO2 adsorption, despite the fact that the number of active functional groups on activated coke decreased. However, the adsorption capacity of semi-coke for SO2 was 5.6% higher than that of activated coke. Semi-coke displayed better regeneration performance than activated coke. After regeneration, the SO2 recovery ratios of semi-coke and activated coke were 69.6% and 62.4%, respectively, and the carbon consumption of semi-coke was less than 12% the carbon consumption of activated coke. When adsorbents are regenerated at a lower temperature, the carbon consumption will be depressed and the SO2 recovery ratio will only be slightly reduced.
348 347
Acknowledgments
349
The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 21207132),
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350
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R
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O
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C
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N
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U
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T
322
The SO2 recovery ratios of semi-coke and active coke at 350°C (64.6% and 54.4%, respectively) are 4.3% and 5.1% smaller than at 400°C (68.9% and 59.5%, respectively). The carbon consumptions of semi-coke and active coke at 350°C (0.069 and 0.054 mmol/g, respectively) are 23.3% and 27.0% smaller than at 400°C (0.090 and 0.074 mmol/g, respectively). Therefore, to depress the decomposition of active functional groups and reduce the carbon consumption, the adsorbents that adsorbed SO2 could be regenerated at lower temperatures, and the SO2 recovery would only slightly decrease.
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Figueiredo, J.L., Pereira, M.F.R., 2010. The role of surface chemistry in catalysis with carbons. Catal. Today 150 (1-2), 2–7. Figueiredo, J.L., Pereira, M.F.R., Freitas, M.M.A., Órfão, J.J.M., 1999. Modification of the surface chemistry of activated carbons. Carbon 37 (9), 1379–1389. Guo, Z.C., Xie, Y.S., Hong, I., Kim, J., 2001. Catalytic oxidation of NO to NO2 on activated carbon. Energy Convers. Manag. 42 (15-17), 2005–2018. Guo, Y.X., Liu, Z.Y., Li, Y.M., Liu, Q.Y., 2007a. NH3 regeneration of SO2-captured V2O5/AC catalyst-sorbent for simultaneous SO2 and NO removal. J. Fuel Chem. Technol. 35 (3), 344–348. Guo, Y.X., Liu, Z.Y., Liu, Q.Y., Sun, D.K., 2007b. Mechanism of carbon burn-off on V2O5/AC for simultaneous SO2 and NO removal during regeneration and NH3 atmosphere. Chin. J. Catal. 28 (6), 514–520. Guo, Y.X., Liu, Z.Y., Liu, Q.Y., Huang, Z.G., 2008. Regeneration of a vanadium pentoxide supported activated coke catalyst-sorbent used in simultaneous sulfur dioxide and nitric oxide removal from gas: effect of ammonia. Catal. Today 131 (1-4), 322–329. Guo, Y.Y., Li, Y.R., Zhu, T.Y., Ye, M., 2013. Effects of concentration and adsorption product on the adsorption of SO2 and NO on activated carbon. Energy Fuel 27 (1), 360–366. Hou, Y.Q., Huang, Z.G., Guo, S.J., 2009. Effect of SO2 on V2O5/ACF catalysts for NO reduction with NH3 at low temperature. Catal. Commun. 10 (11), 1538–1541. Izquierdo, M.T., Rubio, B., Mayoral, C., Andrés, J.M., 2003. Low cost coal-based carbons for combined SO2 and NO removal from exhaust gas. Fuel 82 (2), 147–151. Ju, S.G., Li, Z.L., Li, C.H., 2005. Investigation on activated semi-coke desulfurization. J. Environ. Sci. 17 (1), 91–94. Li, P., Liu, Q.Y., Liu, Z.Y., 2012. Behaviors of NH4HSO4 in SCR of NO by NH3 over different cokes. Chem. Eng. J. 181–182. Liu, Q.Y., Liu, Z.Y., 2013. Carbon supported vanadia for multi-pollutants removal from flue gas. Fuel 108, 149–158. Ma, J.R., Liu, Z.Y., Liu, S.J., Zhu, Z.P., 2003. A regenerable Fe/AC desulfurizer for SO2 adsorption at low temperatures. Appl. Catal. B Environ. 45 (4), 301–309. Ma, J.R., Liu, Z.Y., Liu, Q.Y., Guo, S.J., Huang, Z.G., Xiao, Y., 2008. SO2 and NO removal from flue gas over V2O5/AC at lower temperatures — role of V2O5 on SO2 removal. Fuel Process. Technol. 89 (3), 242–248. Ning, Y.C., 2010. Interpretation of Organic Spectra. Science Press, Beijing pp. 368–380. Raymundo-Piñero, E., Cazorla-Amorós, D., de Lecea, C.S.M., Linares-Solano, A., 2000. Factors controling the SO2 removal by porous carbons relevance of the SO2 oxidation step. Carbon 38 (3), 335–344. Raymundo-Piñero, E., Cazorla-Amorós, D., Linares-Solano, A., 2003. The role of different nitrogen functional groups on the removal of SO2 from flue gases by N-doped activated carbon powders and fibres. Carbon 41 (10), 1925–1932. Shafeeyan, M.S., Daud, W.M.A.W., Houshmand, A., Shamiri, A., 2010. A review on surface modification of activated carbon for carbon dioxide adsorption. J. Anal. Appl. Pyrolysis 89 (2), 143–151. Tsuji, K., Shiraishi, I., 1997. Combined desulfurization, denitrification and reduction of air toxics using activated coke: 1. Activity of activated coke. Fuel 76 (6), 549–553.
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the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB05050502) and the Special Research Funding for Public Benefit Industries from National Ministry of Environmental Protection (No. 201209005).
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0.10 Solid symbol: activated coke Open symbol: semi-coke
Carbon consumption (mmol/g)
SO2 recovery ratio (%)
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Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
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Xing, X.Y., Liu, Z.Y., Wang, J.C., 2007. Elemental sulfur production through regeneration of a SO2-adsorbed V2O5-CoO/AC in H2. Fuel Process. Technol. 88 (7), 717–722. Xing, X.Y., Liu, Z.Y., Yang, J.L., 2008. Mo and Co doped V2O5/AC catalyst-sorbents for flue gas SO2 removal and elemental sulfur production. Fuel 87 (8-9), 1705–1710. Xu, L.S., Guo, J., Jin, F., Zeng, H.C., 2006. Removal of SO2 from O2-containing flue gas by activated carbon fiber (ACF) impregnated with NH3. Chemosphere 62 (5), 823–826. Zhang, X.L., Xu, D.P., 2012. The Preparation and Properties of Activated Semi-coke, and Mechanisms of Flue Gas Desulfurization. Chemical Industry Press, Beijing pp. 10–11.
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Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
Available online at www.sciencedirect.com
ScienceDirect www.journals.elsevier.com/journal-of-environmental-sciences
3Q1
Song Ding1,2 , Yuran Li2 , Tingyu Zhu1,2,⁎, Yangyang Guo2
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1. School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China. E–mail: [email protected] 2. National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
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Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal
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AR TIC LE I NFO
ABSTR ACT
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Article history:
To decrease the operating cost of flue gas purification technologies based on carbon-based
12 17
Received 20 October 2014
materials, the adsorption and regeneration performance of low-price semi-coke and
13 18
Revised 8 December 2014
activated coke were compared for SO2 and NO removal in a simulated flue gas. The
14 19
Accepted 28 February 2015
functional groups of the two adsorbents before and after regeneration were characterized
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Available online xxxx
by a Fourier transform infrared (FTIR) spectrometer, and were quantitatively assessed using
21 32 22
Keywords:
results show that semi-coke had higher adsorption capacity (16.2% for SO2 and 38.6% for
33 23 34 24
Functional groups
NO) than activated coke because of its higher content of basic functional groups and
SO2 recovery
lactones. After regeneration, the adsorption performance of semi-coke decreased because
35 25 36 26
Adsorption
the number of active functional groups decreased and the micropores increased. Semi-coke
Carbon consumption
had better regeneration performance than activated coke. Semi-coke had a larger SO2
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D
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10 9
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C
T
temperature programmed desorption (TPD) coupled with FTIR and acid–base titration. The
recovery of 7.2% and smaller carbon consumption of 12% compared to activated coke. The
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semi-coke carbon-based adsorbent could be regenerated at lower temperatures to depress
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the carbon consumption, because the SO2 recovery was only reduced a small amount.
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© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
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Introduction
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The combustion of coal and other fossil fuels emits many pollutants, such as SO2 and NOx. To decrease the amounts of these pollutants, many flue gas purification technologies have been developed. Those using carbon-based materials are considered some of the best flue gas purification technologies (Tsuji and Shiraishi, 1997; Zhu et al., 2000; Li et al., 2012; Izquierdo et al., 2003; Guo et al., 2013; Hou et al., 2009). These technologies use activated coke, activated carbon or activated carbon fiber as the adsorbent for SO2 and NOx removal, since these materials possess both adsorption capacity for SOx, NOx, dioxins and other pollutants based on physical-sorption and/ or chemisorption, and catalytic activity for NOx reduction with
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Published by Elsevier B.V.
NH3. Moreover, no wastewater or secondary pollutants are produced, and SO2 can be effectively recovered by the reaction of H2SO4 with carbon on the surface when the adsorbents are heated (Guo et al., 2008; Zhang et al., 2012b). However, these flue gas purification technologies with carbon-based materials have two key problems, namely, high operating cost and high heat consumption. For example, when activated coke (approximately 5000 CNY/ton) is used as the adsorbent, the cost of the activated coke is 50%–70% of the total operating cost (Zhang and Xu, 2012). The regeneration temperature is usually 400°C, requiring a large amount of heat. Therefore, the main research thrust for flue gas purification technologies with carbon-based materials is to find a low-price carbon-based material as the adsorbent in
⁎ Corresponding author. E-mail: [email protected] (Tingyu Zhu).
http://dx.doi.org/10.1016/j.jes.2015.02.004 1001-0742/© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
56 57 58 59 60 61 62 63 64 65 66 67 68 69
2
76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
SBET (m /g)
Vm (mL/g)
Vt (mL/g)
D (nm)
Activated coke Semi-coke
88.400 59.879
0.0366 0.0244
0.0703 0.0579
1.272 1.419
C
1. Experiments
100
1.1. Materials and characterization
101
Semi-coke from the Sanjiang Coal Chemical Plant (Shanxi, China) and activated coke from the Xinhua Chemical Plant (Taiyuan, China) were used. The two materials were crushed, and the particles were sieved through 2–60 mesh (0.25–0.85 mm). These particles were washed by deionized water and then dried at 105°C for 24 hr after filtration. Elemental analysis was performed on an Analyzer (Vario EL cube, Elementar, Germany), and the results are shown in Table 1. The pore properties of adsorbents were determined at −196°C through nitrogen adsorption (Autosorb-iQ, Quantachrome, USA). The BET (Brunauer–Emmett–Teller) surface area (SBET), micropore volume (Vm) and average pore diameter ( D ) were calculated by the Brunauer–Emmett–Teller equation, Dubinin– Radushkevich equation and Horvath–Kawazoe equation, respectively. The total pore volumes (Vt) were directly calculated. The results are shown in Table 2. Functional groups of adsorbents were characterized by a FTIR spectrometer (Nicolet 6700, Thermo, USA). Absorption was measured over the range from 400 to 4000 cm−1 with a resolution
107 108 109 110 111 112 113 114 115 116 117 118 Q2 119
t1:1 t1:3 t1:2
120
R
R
Sample
C
N
H
S
t1:5 t1:6
Activated coke Semi-coke
75.73 76.66
0.65 0.72
1.18 1.23
0.75 0.47
121 122 123 124 125 126 127 128 129 130
1.2. Experimental system and reaction conditions
131
The experimental system is composed of a fixed-bed reactor, heating temperature control system, simulated flue gas system and detection system. The reactor is a quartz tube with a sintered silica plate in the middle for loading the adsorbents. The diameter of the tube is 20 mm, and the length is 520 mm. The heating temperature control system is a resistance furnace connected to a temperature controller to adjust the temperature. After passing through flow-meters, gases, such as SO2, NO, NH3, O2 and N2, were mixed in a jar and then entered the reactor. The water vapor was produced by evaporating water in a U-tube and was carried by N2. The amount of water vapor can be calculated by the Antoine equation and the flow of N2. The composition of the simulated flue gas was 0.1 mol% SO2, 0.05 mol% NO, 0.05 mol% NH3, 5 mol% H2O, 5 mol% O2, and balance N2. During adsorption, the adsorbent was maintained for 90 min in the simulated flue gas, the temperature was 150°C, the total flow was 300 mL/min, and the gas hourly space velocity (GHSV) was approximately 5700 hr−1. After adsorption, the adsorbent was regenerated by heating. The regeneration
132
1000 Solid symbol: activated coke Open symbol: semi-coke
SO2
800
600
NO
400
NH3
200
Table 1 – Elemental analysis of the adsorbents (wt.%).
t1:4
t2:5 t2:6
of 2 cm−1. The background spectrum was obtained by using KBr as a blank. The ratio of sample to KBr was approximately 1:1000, and the samples were scanned 8 times. The functional groups that contained oxygen were analyzed by temperature programmed desorption (TPD) coupled with Fourier transform infrared (FTIR), and the basic functional groups were determined by acid–base titration. The SO2 recovery is the ratio of total SO2 released during regeneration to total SO2 adsorbed during adsorption. Carbon consumption is the total C amount of CO and CO2 emitted during regeneration (mmol/g).
Concentrations (×10-4 mol%)
106
O
105
C
104
t2:4
t2:7 t2:9 t2:8
N
103
U
102
E
99 98
t2:3 t2:2
SBET: BET surface area; Vm: micropore volume; Vt: total pore volume; D: average pore diameter.
T
97
Sample
F
75
t2:1
O
74
2
R O
73
Table 2 – Pore properties of adsorbents.
P
72
order to reduce the cost, improve the adsorption performance by modification, and lower the regeneration temperature to save heat. Activated coke has been used in many plants, because its price is cheaper than that of activated carbon. To improve the de-SO2 and de-NO performance of activated coke, the coke is modified by loading with metal oxides, such as those of V, Co, Mo and Fe (Liu and Liu, 2013; Ma et al., 2003, 2008; Xing et al., 2008). To save energy consumption, the regeneration progress of spent activated coke has also been investigated through changes in the heating method or the gas medium during regeneration (Zhang et al., 2012b; Guo et al., 2007a, 2007b; Xing et al., 2007). Semi-coke (approximately 1000 CNY/ton) is prepared from non-caking coal or weakly caking coal, carbonized at 500– 700°C. Semi-coke is richer in surface functional groups than activated coke, which is treated by oxidization in order to increase the specific surface area. Although the preparation processes are different, semi-coke and activated coke have similar physical and chemical properties. It was reported that low-price lignite semi-coke has an adsorption capacity for mercury (Zhang et al., 2012a) that is the same as that of activated coke. Therefore, to explore the application potential of semi-coke for SO2 and NO removal, the adsorption and regeneration performances of semi-coke and activated coke were compared. The promotional effect of the physical and chemical properties of the materials on SO2 and NO adsorption was investigated. Carbon consumption and SO2 recovery were also studied to compare the regeneration performance.
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0
0
10
20
30
40 50 60 Time (min)
70
80
90
Fig. 1 – Breakthrough curves of the adsorbents.
Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151
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J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 5 ) XX X–XXX
0.15
1082
1435
2923
0.09
1628
0.12
2853
3433
0.06
0.00 4000
3500
3000
2500 2000 1500 Wavenumber (cm-1)
1000
F
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500
156 157 158 Q3
2. Results and discussion
161
2.1. Adsorption performance
162
Fig. 1 shows the breakthrough curves of the two adsorbents. The SO2, NO and NH3 breakthrough concentrations of semi-coke were lower than those of activated coke. Through integration and calculation, the de-SO2 and de-NO capacities of activated coke were found to be 11.70 and 0.88 mg/g, respectively, and the de-SO2 and de-NO capacities of semi-coke were 13.60 and 1.22 mg/g, respectively. The adsorption capacities of semi-coke were 16.2% higher for SO2 and 38.6% higher for NO than those of activated coke. The result indicates that semi-coke had better adsorption performance for SO2 and NO than activated coke. From the results of Table 2, the surface area, micropore volume and total pore volume of semi-coke can be seen to be smaller than those of activated coke. Therefore, semi-coke possesses
170 171 172 173 174
C
E
3500
2.0
a
2800
2100
1.6
Solid symbol: activated coke Open symbol: semi-coke
Amounts (mmol/g)
169
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168
R
167
N C O
166
U
165
Concentrations (×10-4 mol%)
164
T
160 159
163
P
155
D
154
temperature was ramped from 150 to 400°C at 5°C/min and then was maintained at 400°C for 40 min in N2. The gas flow was 180 mL/min during regeneration. The adsorption experimental parameters for regenerated absorbents were identical to the initial experimental adsorption parameters. The concentrations of components in the inlet and outlet were measured online by a FTIR spectrometer (Tensor 27, Bruker, Germany).
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Fig. 2 – IR absorption spectra of the adsorbents. 152
high removal efficiencies for SO2 and NO due to its more plentiful functional groups or larger pore size than activated coke. The IR absorption spectra of the two adsorbents are shown in Fig. 2. The two adsorbents show a number of absorption peaks in the range from 400 cm−1 to 4000 cm−1. According to previous studies (Ning, 2010; Shafeeyan et al., 2010), the peaks at 3600–3200 cm−1 can be assigned to the O–H stretching vibrations of alcohol, phenol, water and the N–H stretching vibrations of –NH and –NH2; the peaks at 3000–2800 cm−1 are assigned to the C–H stretching vibrations of –CH2 and –CH3; the peaks at 1700–1585 cm−1 are assigned to the C_O stretching vibrations of carbonyl and the C_C stretching vibrations of benzene; the peaks at 1440–1395 cm−1 are assigned to the C_O stretching vibrations of carboxyl; and the peaks at 1200–1000 cm−1 are assigned to the C–O stretching vibrations of alcohol, ether and anhydride. Comparing the spectra of the two adsorbents, the peaks at 1435 and 1628 cm−1 are significantly different. The reason is that semi-coke has more –COO– and less C_O than activated coke. Fig. 3a shows the TPD curves of the two adsorbents. It can be seen that the CO concentration of semi-coke is smaller than that of activated coke, and the CO2 concentration of semi-coke is greater than that of activated coke. Because the thermal stability of different functional groups varies, when the temperature is increased, the active functional groups will decompose into CO and/or CO2. Normally, CO2 derives from carboxyl, anhydride and lactone and CO derives from anhydride, phenol, carbonyl and quinonyl (Figueiredo et al., 1999; Figueiredo and Pereira, 2010). By integrating the CO and CO2 curves of the two adsorbents, the amounts of functional groups can be determined. The results are shown in Fig. 3b. The numbers of functional groups on the two adsorbents vary in decreasing order according to lactone, carbonyl, quninoyl, phenol, carboxyl and anhydride. Between semi-coke and activated coke, the difference is in the amounts of lactone and carbonyl. The lactone content of semi-coke is 92% higher than that of activated coke, but the carboxyl content of semi-coke is lower than that of activated coke. These results are in agreement with the IR analysis. By analyzing the adsorbents by acid–base titration, the result shows that the basic functional group concentrations of semi-coke and activated coke are 1.055 and 0.609 mmol/g,
O
Absorption intensity (%)
Activated Coke Semi-coke
CO2 1400 CO 700 0 150
b
Activated Coke Semi-coke
1.2
0.8
0.4
300
450 600 Temperature (°C)
750
900
0.0
Lactone Carbonyl Phenol Quinonyl Carboxyl Anhydride
Fig. 3 – Temperature programmed desorption (TPD) analysis of adsorbents. (a) Desorption curves for CO2 and CO and (b) amounts of oxygen containing functional groups. Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217
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Solid symbol: activated coke Open symbol: semi-coke 800
NO NH3
200
70
80
90
F
40 50 60 Time (min)
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30
Fig. 4 – Breakthrough curves of adsorbents after regeneration.
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0.04
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3500
3000
2500 2000 1500 Wavenumber (cm-1)
1000
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Semi-coke
Fresh Regenerated
0.08
1435
1628 1435
0.08
C
Fresh Regenerated
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1628
Activated coke
T
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0.02
0.00 4000
3500
3000
2500 2000 1500 Wavenumber (cm-1)
1000
500
Fig. 5 – IR absorption spectra of adsorbents before and after regeneration.
0.03
0.08 0.06
0.02
0.04 0.02 0.00 0.3
Activated coke
0.6 0.9 1.2 Pore size (nm)
1.5 1.8
0.12
0.04
0.01 0.00
0.10 0.08
Solid symbol: fresh Open symbol: regenerated
0.04 0.03
0.06
0.02
Vm (mL/g)
0.10
0.05 Semi-coke
0.35 nm 0.42 nm
0.12
0.14
0.05
Solid symbol: fresh Open symbol: regenerated
dVm/dD (mL/(nm.g))
0.14
Vm (mL/g)
224
O
223
C
222
N
221
U
220
Absorption intensity (%)
219
respectively. The basic functional group content of semi-coke is 73.2% higher than that of activated coke. Additionally, C–O–C in the lactonic structure plays an important role in SO2 adsorption and may be the active center of the desulfurization on activated semi-coke (Ju et al., 2005). Therefore, the abundant basic and acidic functional groups on semi-coke promoted the SO2 and NO removal.
dVm/dD (mL/(nm.g))
218
226
R O
20
The breakthrough curves of adsorbents after regeneration are shown in Fig. 4. Through integration and calculation, the adsorption capacities of activated coke after regeneration are found to be 12.07 mg/g for SO2 and 0.88 mg/g for NO. The adsorption capacities of semi-coke after regeneration are 12.75 mg/g for SO2 and 0.6 mg/g for NO. The de-SO2 capacity of semi-coke after regeneration is 5.6% higher than that of activated coke, but the de-NO amount of semi-coke after regeneration is 31.8% lower than that of activated coke. Comparing the adsorption capacities of semi-coke before and after regeneration, those after regeneration are 6.3% smaller for SO2 and 50.8% smaller for NO than those of fresh semi-coke. It is tentatively proposed that the pore structure or chemical properties of the adsorbents changed during thermal regeneration; for example, the decomposition of lactone and carboxyl possibly took place. Infrared adsorption spectra of adsorbents before and after regeneration are shown in Fig. 5. The spectra display significant differences at 1628 and 1435 cm−1 for the two adsorbents. The carboxyl content of activated coke did not obviously change, but the carboxyl content of semi-coke decreased. Fig. 6 shows the micropore volumes and pore size distributions of the two adsorbents before and after regeneration. After
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400
10
225
D
600
0 0
2.2. Re-adsorption performance after regeneration SO2
Absorption intensity (%)
Concentrations (×10-4 mol%)
1000
0.04 0.01
0.02 0.00 0.3
0.6 0.9 1.2 Pore size (nm)
1.5 1.8
0.00
Fig. 6 – Micropore volume (Vm) and pore size distributions (dVm/dD) of adsorbents before and after regeneration. Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248
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350
1000
300 240°C
250
500 NH3
200 NO
0 0
20
30
40 50 60 Time (min)
70
80
CO2
400
T
350
325°C 300
300 240°C
200 0
150 90
250
CO
150
0
10
20
30
40 50 60 Time (min)
70
80
150 90
O
10
450
Temperature (°C)
T
450
Solid symbol: activated coke Open symbol: semi-coke
b
F
SO2
400
Concentrations (×10-4 mol%)
1500
600
450
Solid symbol: activated coke Open symbol: semi-coke
a
Temperature (°C)
Concentrations (×10-4 mol%)
2000
271
Fig. 7 shows the regeneration curves of the two adsorbents. SO2, NO and NH3 curves are shown in Fig. 7a, and CO and CO2 curves are shown in Fig. 7b. The trends of the SO2, NO, NH3, CO and CO2 concentrations were observed during regeneration, but NO was not detected. This was because NO mainly reacted with NH3 through SCR and formed N2, and SO2 transformed into H2SO4, then some H2SO4 reacted with NH3 and generated ammonium sulfates on the adsorbent surface. During thermal regeneration, H2SO4 and ammonium sulfates can react with
259 260 261 262 263 264 265 266 267 268
272 273 274 275 276 277 278 279
t3:1
C
258
E
257
R
256
R
255
N C O
254
U
253
Table 3 – Carbon consumptions of the adsorbents during regeneration.
t3:3 t3:2
t3:4 t3:5 t3:6
P
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carbon on the surface, releasing SO2, NH3, CO and CO2 with the rising temperature. Fig. 7a shows that with increasing temperature, the SO2 concentrations of the two adsorbents first increase and then decrease. The SO2 concentration is at a maximum when the temperature is approximately 240°C. Through integrating the SO2 concentration curves, the amount of SO2 released for semi-coke was found to be more than that for activated coke. During the regeneration process, the SO2 recovery ratios of semi-coke and activated coke were 69.6% and 62.4%, respectively. Fig. 7b shows that the CO concentration increases with increasing temperature, then the concentration suddenly drops at 400°C. This result indicates that CO is derived from active functional groups that have poor thermal stability. Under 325°C, the trend of the CO2 concentration is similar to that of the SO2 concentration, which increases quickly and then decreases. This result indicates that CO2 is mainly derived from the reaction of C with H2SO4 or ammonium sulfates. When the temperature reaches 325°C, the CO2 concentration gradually increases with increasing temperature and suddenly drops at 400°C. CO2 is mainly derived from the decomposition of active functional groups from 325 to 400°C. The results of the integration of CO, CO2 and SO2 curves during regeneration are shown in Table 3. When 1 mol SO2 is released, the carbon consumption of semi-coke and activated coke is 0.8 and 0.91 mol, respectively. The carbon consumption of semi-coke is 12% smaller than that of activated coke. Fig. 8 shows the SO2 recovery ratios and carbon consumption of the two adsorbents during regeneration at temperatures from 150 to 400°C. The initial decomposition temperature is 200°C. As the temperature rises, the SO2 recovery ratios of the two adsorbents first increase quickly and then slowly, and the carbon consumption increases throughout the entire course.
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regeneration, the micropore volumes of semi-coke and activated coke increased by 25.8% and 15.9%, respectively, because of the opening of closed pores and the expansion of micropores. The range of pore size distribution of activated coke did not change, but the range of pore size distribution of semi-coke widened because of increasing numbers of micropores, the pore size of which is less than 0.42 nm. According to previous studies (Raymundo-Piñero et al., 2000, 2003; Guo et al., 2001; Xu et al., 2006), the SO2 and NO equivalent diameters are in the range of 0.28–0.35 nm and 0.11–0.31 nm, respectively, and the optimal pore size of activated coke for removal of SO2 was reported to be 0.7 nm. Therefore, a wide range of pore size is adverse for the adsorption and oxidation of SO2. The NO adsorption amount of an adsorbent is determined by its surface chemical properties. Although the pore volume of semi-coke increases after regeneration, the increased volume is mostly contributed by micropores. Smaller pore size hinders the dispersion of SO2. Therefore, the adsorption capacities of semi-coke are reduced for SO2 and NO because of the decomposition of active functional groups and the increase of micropores, which are not beneficial to SO2 adsorption.
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Fig. 7 – Regeneration curves of adsorbents after adsorption. (a) SO2, NH3, and NO curves and (b) CO2 and CO curves. T: temperature.
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C/SO2 (mmol/mmol)
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0.061 0.071
0.10 0.12
0.11 0.15
0.91 0.80
Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
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Fig. 8 – SO2 recovery and carbon consumption during regeneration.
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3. Conclusions
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Semi-coke shows better adsorption performance for SO2 and NO than activated coke, because it is richer in basic functional groups and lactones. After regeneration, the adsorption capacities of semi-coke decreased by 6.3% for SO2 and 50.8% for NO. This decrease occurred because the active functional groups on the semi-coke surface decreased and the micropores, which are not beneficial to SO2 dispersion, increased. The adsorption capacity of activated coke did not change, because the micropores, which are beneficial to SO2 dispersion, were increased as a result of thermal regeneration and promoted SO2 adsorption, despite the fact that the number of active functional groups on activated coke decreased. However, the adsorption capacity of semi-coke for SO2 was 5.6% higher than that of activated coke. Semi-coke displayed better regeneration performance than activated coke. After regeneration, the SO2 recovery ratios of semi-coke and activated coke were 69.6% and 62.4%, respectively, and the carbon consumption of semi-coke was less than 12% the carbon consumption of activated coke. When adsorbents are regenerated at a lower temperature, the carbon consumption will be depressed and the SO2 recovery ratio will only be slightly reduced.
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Acknowledgments
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The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 21207132),
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The SO2 recovery ratios of semi-coke and active coke at 350°C (64.6% and 54.4%, respectively) are 4.3% and 5.1% smaller than at 400°C (68.9% and 59.5%, respectively). The carbon consumptions of semi-coke and active coke at 350°C (0.069 and 0.054 mmol/g, respectively) are 23.3% and 27.0% smaller than at 400°C (0.090 and 0.074 mmol/g, respectively). Therefore, to depress the decomposition of active functional groups and reduce the carbon consumption, the adsorbents that adsorbed SO2 could be regenerated at lower temperatures, and the SO2 recovery would only slightly decrease.
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Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004
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Please cite this article as: Ding, S., et al., Regeneration performance and carbon consumption of semi-coke and activated coke for SO2 and NO removal..., J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.02.004