4.6 Effect of heating and cooling rate
4.6.1 Effect of cooling rate on the CO 2 absorption process at
4.6.1 Effect of cooling rate on the CO2 absorption process at 20°C/min and 60°C/min
Figure 4-9 Effect of cooling rate at 20° C/min and 60 °C/min in the CO2 cyclic absorption. Heating rate at 40°C/min, absorption time 10 min at
700°C , desorption time 1min 850°C.
Figure 4-9 shows the effect of the cooling rate over the absorption process. It can be observed that the absorbed amount at 60°C/min is slightly higher than that of 20°C /min, but the great advantage is the time saved during the process when utilize the faster rate.
4.6.2 Effect of heating rate in the CO2 absorption process at 40 °C/min, 50 °C/min and 60° C/min
In Figure 4-10 it can be observed that the effect of the higher heating rate, which produce more sintering effect, decreasing the absorbed amount. The sintering effect present at higher heating rates is confirmed in the study of Borgwardt (1989) which found that the heating rate, rather than final temperature, was a more critical factor in terms of sorbent sintering.
Figure 4-10 Effect of heating rate on the cyclic absorption process at 40 °C/min, 50
°C/min and 60 °C/min. Cooling rate 60°C/min, absorption temperature 700°C, desorption temperature 850°C
700
Figure 4-11 shows the absorption curves for three cycles at 700°C during 10 min, desorption temperature 850°C for one minute, for two different conditions. With the purpose to use more efficiently the time that it takes for the desorption temperature to reach the carbonation temperature, in this case from 850 °C to 700 °C, the CO2 was allowed to pass during this period. It is well known that the CO2 absorption start around 770°C with a slow rate, gradually increasing until 700 °C, this condition provides an improvement in the absorbed amount. In Figure 4-11 it observed that when CO2 was loaded at the beginning of cooling process the absorbed amount increases compared to the normal process.
Figure 4-11 Effect of passing CO2 in the cooling process from 850 to 700 °C for absorption. Desorption 1 min at 850°C. Heating rate 40°C/min,
cooling rate 60°C/min
4.8 Mixture of CaCO3 (95%) and MgCO3 (5%)
One of the objectives of this work was to find a method for the production of CaO-based sorbents that could be able to prevent the sintering effect, which causes the CO2 capture capacity of the material to deteriorate rapidly after a few cycles of utilization. Here we show a mechanical mixing method which consists of the mixture of CaCO3 (95%) and MgCO3 (5%) precipitates from sea water. The purpose of adding MgCO3 to the CaCO3 was to improve the stability of the sorbent by inhibiting sintering. This enhancement could be due the melting point of the MgO which is higher than that of the CaOand acts as a physical barrier to prevent sintering and agglomeration as is suggested in the study of Fang et al. (2009). Liu et al. (2010) also suggested that the sintering resistant nature of the sorbent is believe to be owing to the inert support material effectively separating the CaO particles.
0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28
1 2 3 4 5 6 7 8 9 10
Absorbed amount (CO2 g/g Sorbent)
Cycle number
Ca‐Mg CaO
Figure 4-12 CO2 cyclic absorption test for Ca-Mg mixture and pure CaO sorbent.
Absorption time 10 min at 700 °C, calcination time 1 min at 850°C, heating rate 40°C/min, cooling rate 60°C/min
Figure 4-12 shows the result of the cyclic absorption test using TGA system with a fixed carbonation temperature of 700°C for 10 min, desorption temperature for 1 min at 850°C heating rate of 40°C/min and cooling rate of 60°C/min. It can be seen that the absorbed amount and stability are higher for the CaO-MgO mixture sorbent precipitates compared with the pure CaO precipitates. The absorbed amount increased in a 10%, compared to the pure CaO precipitates from sea water and decayed to 85 % after 10 cycles.
0.15 0.17 0.19 0.21 0.23 0.25 0.27 0.29
1 2 3
Absorbed amount (g CO2/g Sorbent)
cycle number
CaO‐MgO (CO2) CaO (CO2)
The MgO which does not absorb CO2 under the same condition as that of CaO has a structure stabilizing effect, as can be seen in the MgCO3
which present higher cycle stability compare to CaO is show in Crhissafis et al. (2009b). Li et al. (2009) also found that adding an inert material has a critical effect on the long term stability of the CaO-based absorbent.
Chrissafis (2005a) indicated that mixing high melting point compounds such Al2O3 or MgO can avoid the effect of sintering. In Albrecht et al.
(2008a) a mixture of 20%wt of MgO was produced, showing an improvement in the absorbed amount of 45% greater capacity than the original material. Without the MgO the absorbed amount was very small after several cycles.
Figure 4-13 CO2 cyclic absorption test for CaO-MgO mixture and pure CaO sorbent. Passing CO2 during the cooling time, absorption time 10
min at 700 °C, calcination time 1 min at 850°C, heating rate 40°C/min, cooling rate 60°C/min.
0.1
Figure 4-13 shows a cyclic absorption test of CaO-MgO mixture sorbent and the pure CaO at 700°C for 10 min, desorption temperature 850°C for 1 min, at heating rate 40°C/min, cooling rate 60 °C/min. During this process for both of the sorbent the CO2 was allowed to flow thought the cooling time. It can be observed that at this condition provided a better stability compared to the normal process.
Figure 4-14 (a) cyclic absorption test for CaO-MgO mixture sorbent with and without passing CO2 in the cooling time and pure CaO. Carbonation time 10 min at 700°C, calcination time 1 min at 850°C at heating rate
of 40°C/min and cooling rate of 60°C/min
Figure 4-14 (a) shows the result of the pure CaO and the CaO-MgO mixture sorbent with and without passing CO2 at 700°C for 10 min, desorption temperature 850°C for 1 min, at heating rate 40°C/min, cooling rate 60 °C/min. It can be observed that in the case of CaO-MgO mixture without passing CO2 during the cooling time, the absorbed amount increased in a 10%, compared to the pure CaCO3 and decayed to
0.1
85 % after 10 cycles. When passing CO2 during the cooling time the absorbed amount increased 17%, and decayed 94 % after 10 cycles.
Using the pure CaCO3, the absorbed amount decays in 53% after 10 cycles. It is shown in these results that passing CO2 in the cooling time the stability of the sorbent is more stable along the cycles, and also the method of MgO introduction plays an important role in producing stable absorbents.
Figure 4-14 (b) cyclic absorption test for CaO-MgO mixture sorbent passing CO2 in the cooling time and pure chemical limestone. Carbonation time 10
min at 700°C, calcination time 1 min at 850°C, heating rate of 40°C/min and cooling rate of 60°C/min
Figure 4-14 (b) shows the result of the absorption process for the pure chemical Limestone and the CaO-MgO mixture sorbent, with a fixed carbonation temperature of 700°C and a cooling rate of 60°C /min. It can be seen that the performance of the Limestone has a higher initial absorbed amount (0.53 CO2g/g sorbent) compare to the CaO-MgO mixture sorbent (0.28 CO2g/g sorbent) for the first cycle, but the
0.1
deterioration of the pure Limestone sorbent is more remarkable than the mixture sorbent. For the 6th cycle the absorbed amount of both sorbent are almost similar (0.26 g/g) and for the end of the last cycle the absorbed amount of the limestones was 40% of the original capacity, and the mixture sorbent the absorbed amount was 94% of the original capacity.
Figure 4-15 shows the result of the absorption process with a fixed carbonation temperature of 700°C and a cooling rate of 60°C /min, varying the heating rate at 60°C /min and 40°C /min, with a special condition in which the CO2 was allowed to flow throughout the cooling time region. First an improvement is observed at heating rate of 40°C/min, for the stability and absorbed amount of the mixture sorbent. But as comparing the two different heating rates, it can be seen that higher heating rate of 60°C/min has more severe effect in terms of sorbent sintering, similar to what was observed by Borgwardt et al. (1989). Thus the CO2 absorbed amount continuously decreased.
Figure 4-15 CaO-MgO mixture sorbent absorbed amount at heating rate of 40
°C/min and 60°C /min passing CO2 in the cooling time. Carbonation temperature 700°C, calcination temperature 850°C
4.9 SEM images CaCO3 after CO2 23 cycles at different temperatures
Figure 4-16 (a) shows the SEM image of the fresh sea water precipitates CaCO3 sample. It is observed that the grain structure of the parent CaCO3 fresh sample has three particle phases, a quasi spherical shape, a slides shape and square shapes.
Figure 4-16 (b) shows the carbonation effect at 650°C after 23 cycles, in which the sample suffer some sintering effect compared to the fresh one. At 650°C a poor CO2 capture performance was observed.
Figure 4-16 (c) and (d) gives the SEM images from 700°C and 730°C after 23 cycles, both samples presented also the sintering effect, a growth in the particles is observed being this more remarkable at 730°C. A inter granular cracking along the grain boundaries in the carbonated samples can also be observed. However, at these temperatures the absorbed amount is higher than 650°C. Base on this a temperature of 700°C is consider to be a optimal having a good absorbed amount and avoiding the more sintering effect.
Figure 4-17 shows the EDX analysis of the CaCO3 fresh sample it was detected different distribution of the elements in the CaCO3.
a) Fresh sample b) After 23 cycles at 650°C
c) After 23 cycles at 700 °C d) After 23 cycles at 730 °C
Figure 4-16 SEM images CaCO3 precipitates from se water. a) Fresh sample b) 650°C c) 700°C d) 730°C. After 23 cycles.
Figure 4-17 EDX diagram of CaCO3 precipitates from se water fresh sample. a) square shape b) slide shape c) Particle shaped
4.10 SEM images Mg-Ca mixture after CO2 after
carbonation/calcination cycles at 700°C using TGA
Figure 4-18 (a) shows SEM images of the fresh CaO-MgO mixture sorbent. Figure 4-18 (b) shows the sample CaO-MgO mixture sorbent after 10 cycles. In which the MgO particles are adhered in the surface of the CaO, also confirm by the Energy-dispersive X-ray spectroscopy (EDX). Because of this, an improvement in the absorbed amount was observed in the mixture sorbent compared to the pure one, both after 10 cycles. A certain level of heterogeneous mixing appears to have benefit, probably as a result of providing stable segregation of small CaO particles (Li et al. 2009).
a) CaO sample b) CaO-MgO mixture sorbent
Figure 4-18 (a) fresh CaO-MgO mixture sorbent (b) SEM images CaO-MgO mixture after 10 cycles . Both at 700°C using TGA at heating rate
40k/min and cooling rate 60k/min
Figure 4-19 shows the SEM images of CaO-MgO mixture sorbent at 700°C. Figure 4.-19 (a) at heating rate of 40°C/min. (b) at heating rate of 60°C/min. It can be observed that the sample with a heating rate of 40°C/min present less sintering effect than at 60°C/min, where there is more agglomeration of the particles, affecting the absorbed amount along the carbonation/calcination cyclic test. As a result of the higher heating rate, poor absorbed amount was observed.
a) Heating rate of 40°C/min b) Heating rate of 60°C/min Figure 4-19 SEM images Mg-Ca Mixture at 700°C after 10 cycles using TGA. (a)
At heating rate 40°C /min. (b) At heating rate 60°C /min. Both samples with a fixed cooling rate of 60°C/min passing CO2
4.11 Packed column absorption test
The CO2 absorbed amount of the MgO-CaO mixture sorbent and original CaO sorbent during cyclic absorption are shown in Figure 4-20.
It can be seen from Figure 4-20 that the MgO-CaO mixture sorbent showed higher CO2 capture efficiencies than that of original CaO sorbent.
65.00 70.00 75.00 80.00 85.00 90.00
1 2 3 4 5
Absorbed amount ( mg CO2/g Sorbent)
Number of cycles
Mg‐Ca Pure CaCO3
This phenomenon is similar to that in TGA system, confirming that the MgO acted as an anti sintering sorbent. Figure 4-21 shows the CO2
removal efficiency of the cyclic column test, it can be seen that the CaO-MgO mixture sorbent have a better removal efficiency than the pure sample after the last cycle.
Figure 4-20 Cyclic CO2 capture efficiency by packed column test. (Flow rate: 500 ccm, 15% CO2, 6% O2, 79% N2, absorption at 700°C for 10 min, desorption at
850°C for 30 min, heating rate 36°C/min, cooling rate 6°C/min)
0 0.2 0.4 0.6 0.8 1 1.2
0 2 4 6 8 10 12
CO2removal efficiency 1‐C/Co
Time (min)
cycle 1 CaO‐MgO
cycle 1 CaCO3
cycle 5 CaO‐MgO
cycle 5 CaCO3
Figure 4-21 CO2 removal efficiency by packed column test. (Flow rate: 500 ccm, 15% CO2, 6% O2, 79% N2, absorption at 700°C for 10 min, desorption at 850°C for 30 min, heating rate 36°C/min, cooling rate
6°C/min)
Figure 4-22 shows the results on cyclic CO2 absorbed amounts for both TGA and packed column test. At absorption temperature of 700°C and desorption temperature 850°C. The absorption time was 10 min for both system and the desorption time in packed column was 30 min, whereas it was only 1 min in the TGA test for all cycles. The absorbed amount of the CaO-MgO mixture sorbent measured in packed column system is much lower than that measured in the TGA apparatus. This is partly attributed to the effect of the heating rate in the packed column which is approximately 36°C/min without a constant rate, it means in the first minute increase from 76°C, for the second minute increase 35°C, the
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
1 2 3 4 5
Absorbed amount ( mg CO2/g Sorbent)
Number of cycles MgO‐CaO TGA
CaO‐MgO Packed column
third minutes increased 24°C so on. Also the time for the cooling rate is much longer in the packed column being this at 6°C/min exposing more the sorbent at highest temperatures. In addition, the absorption time in column test is not enough for the sorbent to reach the maximum absorbed amount in each cycle so it leads lower absorbed amounts. As well as the desorption process, this should be studied in detail, to confirm a completed desorption for every cycle, the desorption time could affect the results in the absorbed amount.
Figure 4-22 Cyclic CO2 capture amounts CaO-MgO sorbent by both TGA and packed absorption column tests. The absorption time for both was 10 min.
CHAPTER FIVE CONCLUSIONS AND RECOMMENDATION
5.1 Conclusions
In this study, a TGA system was employed to find the optimal conditions for the CO2 absorption using CaCO3 precipitates as well as a mixture of CaO-MgO precipitates from sea water. From the results of this study the following conclusions can be derived:
1. The CaCO3 has a range of absorption for CO2 from 650 °C to 760°C, having a better performance at 700°C using a heating rate of 40 °C /min and cooling rate 60 °C /min.
2. Different parameters of cooling rate and heating rate were tested. If the cooling rate is lower 20°C/min the absorbed amount will increase when the CO2 is allowed to pass since 850°C to room temperature compare to at higher cooling rate because the absorption time is longer than at 60°C/min and a the saturation of the sorbent occurs fast. The cooling rate at 20°C/min and 60°C/min seems does not affect the performance of the sorbent when there is a fixed absorption temperature of 700°C. The heating rate affects the absorption capacity of the sorbent significantly.
3. Two different conditions were tested for the absorbed amount; one is the normal process of carbonation in which the CO2 is allowed to flow at the fixing temperature of 700°C. The other is that the CO2 was allowed to pass during the cooling time at 850°C. It was found that more CO2 can be adsorbed for the later one.
4. Using a mechanical mixing method between CaO (95%) and MgO (5%), a good anti- sintering sorbent can be produced. The absorbed amount was enhanced by 17% and 10% respectively, with and
without the flowing of the CO2 during the cooling time as compared to the original CaO sorbent, while maintaining a stability of 94%
and 85% after 10 cycles.
5. For the sea water precipitated CaO pure sorbent the absorbed amount was 240 mg/g for the first cycle with passing CO2 in the cooling time, decreasing to 128mg/g after 10 cycles. For the CaO-MgO mixture sorbent, the absorbed amount was 281mg/g for the first cycle, where the CO2 was allowed to pass during the cooling time it decreased to 266 mg/g after 10 cycles. In the case in which the CO2 was introduce after the fixing temperature of 700°C, the absorbed amount was 266 mg/g for the first cycle, it decreased to 228 after 10 cycles.
5.2 Recommendation
1. The portion of the mixture of the CaCO3 (95%) and MgCO3
precipitates from sea water can be changed in order to find an optimal ratio of CaO/MgO mixture for better absorbed amount.
2. Different parameters can still be changed such as the carbonation and calcination time to study the effect in the deactivation of the sorbent.
3. At temperature programmed cooling/heating rates for the packed column can be employed to further investigation of the effect of cooling and heating on the sintering of the CO2 absorption by CaO-MgO mixture.
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