4 |
SUN ET AL .
where X u is the ultimate carbonation conversion in the chemical reaction controlled stage, mol/mol; k is the reaction rate con- stant in the chemical reaction controlled stage, s −1 ; t 0 is the time at which the carbonation achieves highest carbonation rate, s. The carbonation rate r N, t is described in Equation (4)
conversion at the point of maximum carbonation rate. Then, the carbonation process of the sorbent can be divided into chemical reaction controlled stage and diffusion controlled stage by the value of X u . Here, this surface reaction-controlled kinetic model was employed to discuss the carbonation kinetics of the LM and PLM during the chemical reaction controlled stage. 2.4 | Microstructure analysis The chemical components of the LM, PLM, and limestone were examined by X-ray fluorescence (XRF). The pore vol- ume and pore area distributions of the calcined LM, PLM, and limestone after different cycles were examined by a ni- trogen adsorption analyzer (Micromeritics, ASAP 2020-M). The pore volume and pore size distribution of the sample were computed by BJH (Barrett-Joyner-Halenda) model, the BET surface of the sample was calculated by BET model. 3 | RESULTS AND DISCUSSIONS 3.1 | Carbonation kinetics of LM in multiple cycles Figure 3 shows the carbonation conversions of the LM and limestone in multiple calcination/ carbonation cycles. The LM exhibits more stable CO 2 capture capacity with cycle number, while it shows lower carbonation conversions in the initial cycles compared with the limestone. The carbonation conversions of the LM were higher than those of the lime- stone after 15 cycles. However, this result was obtained in a dual-fixed bed reactor with a relatively long carbonation time. Figure 4A,B present the carbonation conversions and carbonation rates of the LM and limestone with carbonation
u (
u )
d X
X
X
N, t
N, t
= k ×
1 −
N, t =
N, t
(4)
r
d t
X
X
perform the differential calculation according to Equation (4), and Equation (5) is obtained:
u (
N, t )
d 2 X
d X
d r
= k
N, t
N, t
N, t
u − 2
=
(5)
X
X
X 2
d t 2
d t
d t
when d rN ,t /d t = 0, it is related to the point of maximum carbon- ation rate. Here, X N ,t is equal to half the value of X u . According to this, the value of X u can be obtained by 2 times of the
FIGURE 3
Carbonation conversions of LM and limestone during
multiple calcination/carbonation cycles
(a)
(b)
t /s
t /s
FIGURE 4
Carbonation conversions and rates of the LM and limestone with carbonation time during multiple cycles. A, Carbonation
conversion and B, carbonation rate
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