| 3
SUN ET AL .
TABLE 1
Chemical components of LM, PLM, and limestone
Sample
CaO MgO SiO 2
Al 2 O 3
Fe 2 O 3
SO 3
Ti 2 O K 2 O Na 2 O Cl
Others
LOI
LM
52.39 0.7
2.52 1.49
0.29
0.31 0.056 0.013 0.14
0.88 0.049
41.16
PLM 52.52 0.73
2.64 1.71
0.27
0.3
0.066 0.013 0.044 0.30 0.051
41.36
Limestone 52.08 1.32
3.32 0.53
0.03
-
-
-
0.02
-
0.47
42.23
2.2 | Carbonation kinetics performance of the sorbent
analysis. The crucible containing 10 ± 0.1 mg sample was firstly put into the furnace of the TGA. The reaction tempera- ture was increased to 850°C at a heating rate of 30°C/min, and then, the temperature was constant for 10 min in pure N 2 . The sample was completely calcined in this period. Then, the temperature decreased to 700°C at 30°C/min in pure N 2 . The gas mixture was switched to the carbonation atmosphere (15% CO 2 balanced with N 2 ) at the moment when the tem- perature reached 700°C, and the temperature was constant at 700°C for 30 minutes for carbonation reaction. One data were collected per 3 second during the carbonation process. The gas flow was 120 mL/min. The carbonation rate of the sam- ple was calculated according to Equation (2).
The cyclic calcination/carbonation tests of the LM, PLM, and limestone were accomplished in a dual-fixed bed reactor (DFR), as shown in Figure 2. The internal diameter of the DFR is 30 mm, and the constant temperature zone of the re- actor is about 300 mm, which can make sure that the samples stay in the constant temperature zone at any operating mode. The N 2 and CO 2 feed were controlled by mass flow control- lers and introduced into the reactor. The sample was firstly calcined for 10 minutes at 850°C in pure N 2 and then was carbonated at 700°C for 20 minutes. The sample mass after calcination and carbonation was measured by a delicate elec- tronic balance, and the carbonation conversions of the sorb- ents were calculated according the mass change during the carbonation and calcination stage, as shown in Equation (1).
d X
N =
N
(2)
r
d t
where r N is carbonation rate of the sample at t (carbonation time) in the Nth carbonation, s −1 . t denotes the reaction time, s.
N −
M
m
m
N =
CaO
cal
⋅
X
(1)
m
A
M
0
CO 2
2.3 | Kinetics analysis model A surface reaction-controlled kinetic model with a Boltzmann equation was employed to describe the reaction of CO 2 and CaO during the chemical reaction-controlled stage, the curve of which is an S-type. The fitting equation is shown in Equation (3):
where X N is the carbonation conversion of the sample after N cycles. m 0 is the initial mass of the sample. m N is the mass of the carbonated sample after N cycles. m cal is the mass of the completely calcined sample (the mass of the sample after each calcination is the same). M CaO and M CO2 are molar masses of the CaO and CO 2 , respectively. A is the content of CaO in the initial sample. A thermogravimetric analyzer (TGA) was employed to in- vestigate the carbonation kinetics of the sorbent during mul- tiple cycles. After multiple calcination/carbonation cycles, the sorbents were sampled from the DFR and sent for TGA
X
N, t =
u −
u 1 + exp [
0 ) u ] k
X
u )
(0 ≤ X
X
X
N, t ≤
( t − t
(3)
X
FIGURE 2
Schematic diagram of
dual-fixed-bed reactor (DFR)
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