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Environmental and Carbon Dioxide Issues
NaBr enhanced CaO-based sorbents with a macropore-stabilized microstructure for CO2 capture Yongqing Xu, Haoran Ding, Cong Luo, Qi Zhang, Ying Zheng, Xiaoshan Li, Yingchao Hu, and Liqi Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01327 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on July 2, 2018
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Energy & Fuels
NaBr enhanced CaO-based sorbents with a macropore-stabilized microstructure for
1 2
CO2 capture
3
Yongqing Xu†; Haoran Ding†; Cong Luo∗,†; Qi Zhang‡; Ying Zheng†; Xiaoshan Li†;
4 5
Yingchao Hu†; Liqi Zhang†
6 7
† State Key Laboratory of Coal Combustion, School of Energy and Power
8
Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R.
9
China ‡ School of Energy and Power Engineering, Huazhong University of Science and
10 11
Technology, Wuhan 430074, P. R. China
12
Abstract
13 14 15
Calcium looping process(CaLP) is well considered as an cost-effective scenario for
16
trapping CO2 from flue gas. However, the CO2 capture capacity of natural CaO-based
17
sorbents spoiled rapidly over the long-term cycles. In this work, NaBr was introduced
18
to enhance the cyclic CO2 capture capacity of CaO sorbents. The NaBr modified CaO
19
showed an improved activity and durability for carbonation. After 100 cycles, the
20
“NaBr/CaO-10/100” maintained a capacity of 0.202 g-CO2/g-sorbent, which was
21
about 185% higher than that of unmodified CaCO3 precursor. The mechanism of ∗
C. L: tel,+86-27-87542417-301; fax,+86-27-87545526; e-mail,
[email protected].
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22
enhancement was analyzed by Simultaneous Thermal Analyzer(STA), In-situ X-Ray
23
Powder Diffraction(In Situ XRD), Inductively Coupled Plasma Optical Emission
24
Spectrometer(ICP-OES), Field Emission Scanning Electron Microscope Coupled
25
Energy Dispersive X-ray Spectrometer (FSEM-EDS) and N2 physical absorption, and
26
the results showed that the modified sorbent formed a well-linked macro-pore
27
structure, which was relatively stable at high temperature reactions; besides, NaBr
28
incorporated inside the CaO crystal lattice promoted the durability of pore structures
29
and cyclic CO2 capture capacity. NaBr is an effective promoter that has the ability of
30
enhancing the cyclic CO2 capture capacity of CaO-based sorbents.
31 32
Keywords: carbon dioxide, calcium looping process, NaBr, doping
33 34
Introduction
35 36
The global climate is outlining unequivocal signals of warming, resulting in
37
sensible ascent in average ocean and air temperatures, as well as the alarming growth
38
in the average sea level.1 The enormous anthropogenic emissions of greenhouse gases
39
are the major contributory elements to the global warming and CO2 is accepted as the
40
largest contributor.2 CO2 capture and storage(CCS)3,
41
technologies that have the potential to isolating CO2 from power plants and some
42
industrials. Among them, calcium looping process(CaLP)5 is considered as one of the
43
most promising scenarios for CO2 capture, as show in Fig. 1 (schematic diagram).
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4
is serials of developing
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44
This process has several intrinsic advantages such as the high theoretical absorption
45
capacity, abundance of raw materials (limestone6, carbide slag7, steel slag8 and so on)
46
and wide applicability of the calcium looping process (no need to scale-up existing
47
power station boilers) and so on. It is based on the reversible carbonation and
48
decarbonation reactions between CaO(g) with CO2(s) as show in Eq. (1).
49
CaO( s) + CO2 ( g ) ↔ CaCO3 ( s)
∆H r ,298 K = −178 kJ/mol
(1)
50 51
Fig. 1 schematic diagram of calcium looping process(CaLP).
52 53
The carbonation process is exothermic and the counterreaction process, calcination
54
process, is endothermic, which means that calcination is favored by higher
55
temperatures9. The calcination process will occurred only if the partial pressure of
56
ambient CO2 is below the decomposition pressure of carbonate product, which is
57
controlled by equilibrium thermodynamics10. Based on enough experiments, a thermal
58
equilibrium diagram of the reaction temperature versus CO2 partial pressure was
59
recorded and then a typical expression for equilibrium decomposition pressure Peq
60
with ambient temperature is proposed11, 12, as shown in Eq(2).
61
20474 Peq = 4.137 × 107 exp − atm T
(2)
62
Nevertheless, the primary demerit of this process is sintering13, 14 occurred at the
63
repeated high temperature reactions, accordingly, the CO2 capture capacity of the
64
CaO-based sorbents decayed drastically during the long-term cycles. Grasa and
65
Abanades et al.15 have presented a semi-empirical formula to analysis the attenuation
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of carbonation conversion of CaO in long-term cycles, as show in Eq. (3).
XN =
67
1 1 + kN 1− Xr
+ Xr
(3)
68
Where, X N is the carbonation conversion achieved after Nth carbonation, k is
69
the de-activation constant and X r represents the residual carbonation conversion
70
after infinite number of cycles.
71
Sintering is the main reason for the drastic decay of carbonation conversion of CaO.
72
Specifically, the pore structure and specific surface area of CaO are destroyed within
73
the duration of high temperature reaction process.
74
On the basis of a large amount of experimental data, German and Munir et al.16
75
have modeled the decay rate of specific surface area at high temperatures, as
76
illustrated in Eq. (4)
(
77
S0 − S γ ) = K st S0
(4)
78
Where γ is a mechanism-derived parameter and K s represents a sintering
79
constant which grows-up exponentially with temperature ( min −1 ), t shows the
80
sintering time (min), S0 and S represent specific surface areas before and after
81
sintering.
82 83
84
85
Coble et al.17 put forward a model to estimate the decay of porosity of CaO at long-term high temperature sintering as illustrated in Eq. (5).
ε 0 − ε = k p ln(
t+a ) t0 + a
(5)
Where k p represents a constant on diffusion, ε is the porosity at time t , ε 0
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and t0 represent initial porosity and time at which grain shrinkage begins, a is a
87
constant which is associated with the property of materials.
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The carbonation process is generally initiated with a rapid chemical reaction
89
controlled stage which was followed by a precipitate transition to a product layer
90
diffusion controlled stage with a very slow reaction rate18, 19. There is a critical
91
thickness of product layer20, which is well regarded as 49nm (±19% of standard
92
deviation), when the reaction kinetics of carbonation shifted to a slow product
93
controlled stage from the fast chemical reaction controlled stage.
94
During a typical calcium looing process(CaLP), the CaO sorbent was reutilized in
95
long-term cycles. However, due to the drastic decay of CO2 capture capacity of CaO
96
sorbents21, a mass of fresh CaO stocks were still needed, hence increasing the
97
operating cost of the system22. Whereupon, recycling the waste calcium precursor23, 24
98
has been well presented; besides, a great number of technologies have also been
99
proposed for enhancing the CO2 capture capacity of CaO-based sorbents, including activation25,
hydration26,
organometallic
calcium
precursor27,
28
100
steam
,
101
acid-pretreatment29, templating with pore creating materials30,
102
supporting33-35 with other oxides, improving the reaction conditions36, 37, employing
103
some advanced preparation methods such as vapor-phase deposition38, 39, combustion
104
synthesis40 and so on41, 42.
31
, doping32 or
105
Among these techniques, doping43 has been considered as a cost-effective method
106
to enhance the CO2 capture capacity of CaO. However, alkali salt is well considered
107
as low melting material which cannot enhance the cyclic CO2 capture behavior of
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108
CaO sorbents. There still a few researchers44-46 suggested that HBr can effectively
109
enhance the cyclic CO2 capture capacity of the sorbents, but the HBr is an strong
110
corrosive acid that not well suited for industrial CaL process. In our previous work,
111
we have found that sea water hydrated lime sorbents kept a CO2 capture capacity of
112
0.31 g of CO2/g of sorbent after 40 cycles, which was about 130% higher than that of
113
original limestone precursor. However, that work failed to explore the enhancing
114
mechanism due to the fact that limestone and sea salt were all mixtures. By means of
115
DFT calculation, Wang et al.
116
capacity of CaO, but the doping effect has not been detected by experiments yet. In
117
this work, NaBr was doped to study the mechanism of Na+ on CO2 capture capacity
118
enhancement of CaO sorbents.
47
stated that Na+ doping can boost the CO2 capture
119 120
2. Experimental section
121
2.1 Materials and sorbent preparation
122
The CaCO3 precursor and NaBr used in this paper were analytical grade purchased
123
from Sinopharm Chemical Reagent Co., Ltd, P. R. China. The preparation process is
124
based on the method which has been reported in our previous work48. 10 g of CaCO3
125
precursor was calcined at 850 oC, then it was poured into 50 ml of salt solution which
126
had a given mass of NaBr dissolved. After that, the milk-like mixture was agitated
127
continuously at 80 oC by a magnetic stirrer for 45 min, and then the mud-like mixture
128
was dried at 100 oC for two days. At last, the lumpy product was ground and sieved to
129
grains in 0.2-0.3 mm. For conveniently describing, the obtained doped sorbents were
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named as “NaBr/CaO-xx/100” (“xx/100” is the mole ratio of NaBr to CaO in the
131
obtained sorbents).
132 133
2.1 Experimental methods
134
The cyclic carbonation/decarbonation performance of these sorbents were
135
examined by a simultaneous thermal analyzer (STA 2500 Regulus, Netzsch), and the
136
testing principle was illustrated in Fig. 2. Loading of about 8mg of samples, a
137
measure crucible was held on the end of the measure holder, while another empty
138
crucible was placed on the end of reference holder. A thermal analysis unit coupled
139
with high sensitive analytical balance system was linked with these two holders. Prior
140
to the calcium looping process, the samples were heated to 850 oC at a rate of 20
141
o
142
complete precalcination at 850 oC for 10min, 50 carbonation/decarbonation repeated
143
tests were conducted continuously under the alternating of atmospheres as show in
144
Table 1.
C/min under an N2 flow(high pure, 99.999%) of 170 ml/min in the STA. After
145 146
Fig. 2. Schematic diagram of the simultaneous thermal analysis system.
147 148
Table 1. Reaction conditions for calcium looping.
149 150 151
The Nth and accumulative CO2 capture capacity are calculated as Eq. (6) and Eq. (7).
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CN =
152
mN − m0 m0
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(6)
n
C = ∑ CN
153
(7)
k =1
154
Where C N and mN are the CO2 capture capacity and the mass of CaO sorbent
155
after Nth carbonation, m0 is the mass of CaO sorbent after completely calcination, C
156
is the accumulative CO2 capture capacity.
157 158
3 Results and discussion
159
3.1 cyclic CO2 capture behavior of NaBr enhanced CaO-based sorbents.
160
The cyclic CO2 capture performance of these modified samples was examined by
161
the STA for 100 cycles, and the CO2 capture capacity C N of the samples were
162
calculated by mass change, assuming that the mass change were only resulted from
163
the carbonation and decarbonation of CaO. As the results shown in Fig. 3, the CO2
164
capture capacity of CaCO3 precursor decayed dramatically over 100 cycles, from 0.59
165
g-CO2/g-sorbent during the 1st carbonation to 0.071 g-CO2/g-sorbent during the 100th
166
carbonation. But all of the NaBr modified sorbents showed continuously increase of
167
capture capacity in the initial 4 cycles, after that the rate of loss-in-capacity of this
168
modified sorbents were much slower than that of CaCO3 precursor with the increase
169
of cyclic numbers. Over the long-term cycles, the stability of CO2 capture activity of
170
the modified sorbent was enhanced with the increase of doping ratios. After 100
171
cycles, the “NaBr/CaO-10/100” held a capacity of 0.202 g-CO2/g-sorbent(about 184%
172
higher than that of CaCO3 precursor), which was tightly followed by the
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“NaBr/CaO-5/100”, “NaBr/CaO-1/100” and “NaBr/CaO-0.5/100”, with roughly
174
178%, 147%, 101% higher than that of CaCO3 precursor. Hence, it can be speculated
175
that the cyclic CO2 capture capacity became more stable as the NaBr doping ratio
176
increase.
177 178
Fig. 3. Cyclic CO2 capture behavior of the NaBr enhanced sorbents.
179 180
The CaO sorbents would be reused to cyclic carry off CO2 from flue gases. Thus
181
the accumulative CO2 carry capacity over the long-term cycles should be evaluated
182
and the results were listed in Table 2. After 100 repeated cycles, the
183
“NaBr/CaO-5/100” captured 24.370 g CO2(about 108% higher than that of CaCO3
184
precusor),
185
“NaBr/CaO-1/100” and “NaBr/CaO-0.5/100” , with about 24.370, 22.460 and 21.320
186
g of CO2 carrying capacity.
which
was
closely
followed
by
the
“NaBr/CaO-10/100”,
187 188
Table 2. Accumulative CO2 capture capacity over 100 calcium looping process.
189 190
The mechanism of cyclic CO2 capture capacity enhancement of doped CaO should
191
be further investigated. The “NaBr/CaO-50/100” was examined by the STA to analyze
192
the effect of NaBr on doped CaO, and the result as shown in Fig. 4 revealed that the
193
sample underwent a severe weight loss over 750 oC, which illustrated that NaBr
194
would sublimate rapidly when the temperature of the sample higher than 750 oC. A
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195
marked endothermic peak appeared at 750 oC, which suggested a drastic sublimation
196
rate at this high temperature.
197 198
Fig. 4 Thermoanalysis of NaBr doped CaO by a STA at high temperature.
199 200
To analyze the ratio of NaBr remained inside the sorbent, the “NaBr/CaO-10/100”
201
was tested by an Inductively Coupled Plasma Optical Emission Spectrometer
202
(ICP-OES). Before the test, the samples were calcined at 850 oC, and then dissolved
203
by dilute nitric acid. The result was listed in Table 3 that the atom ratios of Na to Ca
204
inside the sample decreased from 10:100 to 3.7:100 after calcination at 850 oC, which
205
illustrated that there was still NaBr remained inside the doped sample.
206 207
Table 3. Mole ratio of Na:Ca (tested by ICP-OES)
208 209
To characterize the crystal structure change of the doped samples, the
210
“NaBr/CaO-10/100” was examined by an In-situ XRD apparatus(D8 Advance,
211
Bruker), and the interplanar spacing was calculated by Braggs law as shown in Eq. 8.
212
As the result illustrated in Fig. 5, the interplanar spacing of (200), (311) and (222)
213
became larger when the temperature gone up, while the spacing changed back to its
214
original state when the temperature reduced. The lattice expansion was the results of
215
the high temperature, accordingly trapped sodium atoms inside the grains. Besides,
216
the (311) and (222) revealed convincing additional diffraction peaks, which
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represented some potential doping atoms inside the crystal structures. The enhanced
218
cyclic CO2 capture capacity of the CaO may be resulted from the Na+ inside the CaO
219
crystal lattice.
220
2d sin θ = nλ , n=1,2......
221
(8)
222 223
Fig. 5 In situ XRD patterns of “NaBr/CaO-10/100” at different temperatures (step
224
size= 0.020481) (1#, room temperature ; 2#, heating up to 650 oC; 3#, heating up to
225
850 oC; 4#, cooling back down to 650 oC; 5#, cooling back down to room
226
temperature)
227 228
To analyze morphology of the calcined sorbents after long-term cycles, a Field
229
Emission Scanning Electron Microscopic Coupled Energy Dispersive X-Ray
230
Spectroscopy (FSEM-EDXS) was employed and the topographies of the various
231
calcined CaO were displayed in Fig. 6. After 50 cycles, the topography of calcined
232
“CaCO3 precursor” seemed coarse, as illustrated in Fig. 6(a) and (b), which were
233
slightly fluffier than that of the “NaBr/CaO-1/100” as shown in Fig. 6(c) and (d).
234
While the “NaBr/CaO-10/100” showed much more porous than “NaBr/CaO-1/100”
235
after the same 50 cycles, as represented in Fig. 6(e) and (f).
236
It is well stated that porous structure49 is beneficial to the diffusion of CO2 into the
237
CaO pore walls, accordingly enhance the CO2 capture capacity of the sorbents. The
238
“NaBr/CaO-10/100” held a host of uniform macro-pores (about 1 um) inside the CaO
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239
particle, and the well inter-connected micro-scale pores should be good depressor that
240
inhibited the agglomeration of CaO grains.
241
Fig. 6 FSEM images of calcined sorbents after 50 cycles: (a) and (b) CaCO3
242 243
precursor; (c) and (d) “NaBr/CaO-1/100”; (e) and (f) “NaBr/CaO-10/100”
244 245
To investigate the uniformity of NaBr doping inside the CaO particles, the
246
“NaBr/CaO-10/100” was further tested by EDXS, and the elemental mapping
247
(calcined at 750 oC) in Fig. 7 proved the well-disperse Ca, O, Na and Br inside the
248
particles, although the peak intensity of Na and Br were relatively weak. After higher
249
temperature calcination process and 50 cycles, the signal of Na and Br were much
250
weaker as shown in Fig. S1 and S2 (Supporting Information).
251
Fig. 7 Elemental mapping of images (Na, Br, Ca, O).
252 253 254
It is regarded that the sintering of CaO sorbents are linked with the specific surface
255
area and porosity of the calcined sorbents. The BET surface areas of the calcined
256
sorbents after different cycles were examined by nitrogen physical absorption at -196
257
o
258
samples were degassed under vacuum condition for 180 min, and the specific surface
259
areas were analyzed by Brunauer Emmett Teller (BET) model. As the results listed in
260
Table 4, the fresh CaO derived from “CaCO3 precursor” held the largest BET surface
C (3H-2000PS, Beishide Instrument Technology Co., Ltd). Before the test, the
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area, with 10.719 m2/g, which was about 3 times that of fresh CaO from
262
“NaBr/CaO-10/100”. After 30 cycles, the BET surface area of calcined CaO derived
263
from “CaCO3 precursor” dropped to 6.368 m2/g, but the CaO derived from
264
“NaBr/CaO-10/100” soared to 7.844 m2/g.
265
The isothermal N2 adsorption/desorption profiles of the two CaO sorbents after
266
various cycles were plotted in Fig. 8. The fresh CaO derived from “CaCO3 precursor”
267
displayed Ⅳ isotherm sorption curves with a marked H1 type hysteresis loop in high
268
relative pressure zone, which were related to the capillary condensation when filling
269
and emptying of nitrogen molecular on the well-interlinked macro-pore walls. After
270
30 cycles, the isotherm sorption/desorption curves of “CaCO3 precursor” was so
271
smooth
272
“NaBr/CaO-10/100” held a Ⅲ isotherm sorption curves, which was similar with that
273
of “CaCO3 precursor” after 30 cycles with low porosity; however, after 30 cycles, a
274
noteworthy H1 type hysteresis loop was observed in high relative pressure zone,
275
which was extremely similar with the fresh CaO of “CaCO3 precursor”. The porosity
276
of
277
“NaBr/CaO-10/100” was low in porosity after the first calcination, but over the
278
long-term cycles, it formed well-linked macro-pore structures, which was relatively
279
stable at high temperature reactions. Hence it can be speculated that the NaBr may
280
enhance the stability of microstructure of CaO sorbents.
that
“CaCO3
illustrated
precursor”
low
sank
porosity.
drastically
The
fresh
during
30
CaO
originated
cycles;
by
281 282
Table 4. BET specific surface area of calcined samples after various cycles
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from
contrast,
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283 284 285
Fig. 8. N2 isotherm adsorption/desorption curves of the calcined samples after various cycles.
286 287
Conclusion
288 289
In this study, NaBr was introduced to enhance the cyclic CO2 capture capacity of
290
CaO sorbents. Results indicated that after 100 cycles, the “NaBr/CaO-10/100”
291
achieved a capacity of 0.202 g-CO2/g-sorbent, which was about 185% higher than that
292
of unmodified CaCO3 precursor, and the modified CaO sorbent captured about
293
24.370g of CO2 over the repeated 100 cycles, which was about 107% higher than that
294
of unmodified CaCO3 precursor; besides, the NaBr enhanced sorbent formed a
295
well-linked macro-pore structure, which illustrated relatively stable activity at high
296
temperature reactions. The mechanism of NaBr enhancement on doped CaO was
297
analyzed by STA, In-situ XRD, BET, ICP-OES and SEM. It revealed that NaBr inside
298
the CaO grains underwent drastically sublimate under high temperature (higher than
299
750 oC), but the interplanar spacing of CaO crystal cell became larger when the
300
temperature gone up, accordingly trapped sodium and bromide ion inside the grains,
301
and the sodium and bromide ion doped inside the CaO crystal lattice boosted the
302
durability of micropore structures and cyclic CO2 capture capacity. In conclusion,
303
NaBr is an effective promoter that has the ability of enhancing the cyclic CO2 capture
304
capacity of CaO-based sorbents.
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Acknowledgements
307
The financial supports from National Natural Science Foundation of China (No.
308
51606076) and the Foundation of State Key Laboratory of Coal Combustion
309
(FSKLCCB1705) are sincerely acknowledged. The authors are also grateful for the
310
support from the “Analytical and Testing Center” at Huazhong University of Science
311
& Technology.
312 313
References:
314 315
(1) Pachauri, R. K.; Allen, M. R.; Barros, V. R.; Broome, J.; Cramer, W.; Christ, R.;
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Church, J. A.; Clarke, L.; Dahe, Q.; Dasgupta, P., Climate change 2014: synthesis
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report. Contribution of Working Groups I, II and III to the fifth assessment report of
318
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Table 1. Reaction conditions for calcium looping. Temperature(°C)
N2(ml/min)
CO2(ml/min)
Duration(min)
carbonation
850
50
120
5
calcination
850
170
0
5
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Table 2. Accumulative CO2 capture capacity over 100 calcium looping process. Samples
Accumulative CO2 capture capacity(g)
CaCO3 precusor
11.768
NaBr/CaO-0.5/100
21.320
NaBr/CaO-1/100
22.460
NaBr/CaO-5/100
24.522
NaBr/CaO-10/100
24.370
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Table 3. Mole ratio of Na:Ca (tested by ICP-OES) Samples
mole ratio of Na/Ca
NaBr/CaO-10/100 before calcination
10:100
NaBr/CaO-10/100 after calcination
3.7:100
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Table 4. BET specific surface area of calcined samples after various cycles Sample
BET specific surface area(m2/g)
CaCO3, after 0 cycles
10.719
NaBr/CaO-10/100, after 0 cycles
3.724
CaCO3, after 30 cycles
6.368
NaBr/CaO-10/100, after 30 cycles
7.844
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Fig. 1 schematic diagram of calcium looping process(CaLP).
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Fig. 2. Schematic diagram of the simultaneous thermal analysis system.
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Fig. 3. Cyclic CO2 capture behavior of the NaBr enhanced sorbents.
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Fig. 4 Thermoanalysis of NaBr doped CaO by a STA at high temperature.
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Fig. 5 In situ XRD patterns of “NaBr/CaO-10/100” at different temperatures (step
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size= 0.020481) (1#, room temperature; 2#, heating up to 650 oC; 3#, heating up to
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850 oC; 4#, cooling back down to 650 oC; 5#, cooling back down to room
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temperature)
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Fig. 6 FSEM images of calcined sorbents after 50 cycles: (a) and (b) CaCO3 precursor; (c) and (d) “NaBr/CaO-1/100”; (e) and (f) “NaBr/CaO-10/100”
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Fig. 7 Elemental mapping of images (Na, Br, Ca, O).
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Fig. 8. N2 isotherm adsorption/desorption curves of the calcined samples after various cycles.
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