| CO2 is considered to be the major greenhouse gas contributing to global warming which may be disastrous to the global environment. Worldwide emission control and reduction of CO2 has increasingly become a global consensus. Investigators around the world hold interests in various CO2 capture options. Alkali metal-based solid sorbents have been identified to be one of the promising candidates for CO2 capture. The primary advantage of using sodium carbonate is its lower price, easier accessibility, and lower CO2 capture costs, while its main drawback is the low CO2 adsorption reactivity, slow reaction rate and unsatisfactory reaction time. The development of the technology is hindered.To solve the problems above, several key issues were investigated in this paper. First, carbonation behaviors of analytical reagent Na2CO3 and Na2CO3 obtained from NaHCO3 precursor were investigated. In comparison with analytical reagent Na2CO3, Na2CO3 obtained from NaHCO3 precursor shows much better carbonation behavior, its carbonation conversion increases to 73.9%, and carbonation products are NaHCO3 and Na2CO3·3NaHCO3. It is illuminated that the generation of Na2CO3·3NaHCO3 can be attributed to the poor heat dispersion of microscopic structure. The differences above were explained by microscopic structure differences of the two samples, and the reaction principle was analyzed.Second, the structure and carbonation characteristics of several sodium-based sorbents supported on diatomite, SiO2, ZrO2, TiO2 and γ-Al2O3 were thoroughly investigated. Na2CO3 supported on γ-Al2O3 shows excellent carbonation characteristic. Na2CO3 obtained from NaHCO3 precursor was also loaded on γ-Al2O3 and the carbonation characteristic got improved analogously. Carbonation conversions of the two sorbents increase to around 85%. The improvement of the two sorbents is due to the developed microscopic structure of γ-Al2O3. The effect of Na2CO3 loading amounts on carbonation characteristic of Na2CO3/Al2O3 was analyzed. The optimized loading amount is about 20% because the sorbent exhibits quite high carbonation capacity and the reaction time is short.Third, behaviors and characteristics of carbonation and regeneration of Na2CO3/Al2O3 were investigated systematically. Effects of temperature and vapor concentration on carbonation behavior of Na2CO3/Al2O3 were studied. The temperature window is narrow for carbonation. Carbonation capacities vary as the temperature changes, from 10 mg CO2/g sorbent at 80 ℃ to 85 mg CO2/g sorbent at 60 ℃. Carbonation conversion increases significantly with the increasing vapor concentration. Regeneration behavior of Na2CO3/Al2O3 was studied. The regeneration process of Na2CO3/Al2O3 is similar to the decomposition process of NaHCO3.The regeneration conversion of Na2CO3/Al2O3 reaches nearly 100% at 200 ℃.And then, TiO2 was presented for the first time as dopant for the CO2 capture behavior modification of Na2CO3/Al2O3. The effect of TiO2 and the loading amounts on carbonation is significant. With 1% TiO2 loading, CO2 capacity increases as much as 26%, and percent conversion gets a 25% increase at least, absorption rate is faster and reaction time is shortened. The carbonation products are NaHCO3 and Na2CO3·3NaHCO3. Surface characterizations of TiO2 were characterized by XPS and FTIR. The modification mechanism was revealed by analyzing the molecular polarity and surface hydroxyls of TiO2. The forming reason of two products was analyzed. Based on the modification mechanism of TiO2, TiO(OH)2 was proposed as another dopant. The effect of TiO(OH)2 and the loading amounts on carbonation is significant. Na2CO3/Al2O3 with 5% TiO(OH)2 loading also exhibits similar carbonation characteristic with that of 1% TiO2-doped sorbent. The carbonation products are the same with those of TiO2-doped sorbent. SEM and XRD characterizations indicate that TiO2 and TiO(OH)2 are stable within the temperature range. Regeneration behaviors of the two sorbents were studied. The doping of TiO2 and TiO(OH)2 doesn’t affect the regeneration behaviors and the two carbonation products can be completely decomposed in 200 ℃.And then, MgO was doped on Na2CO3/Al2O3 to modify CO2 capture behavior for the first time. It is found that MgO in the sorbent can also absorb CO2 at low temperature of 60 ℃ in the presence of vapor forming Mg6Al2CO3(OH)16·4H2O in carbonation, which increases the carbonation capacity of sorbent. The effect of MgO and the loading amounts on carbonation is significant. Na2CO3/Al2O3 with MgO loading around 5% exhibits higher CO2 capacity and percent conversion and faster capture rate. The promotion mechanism of Mg6Al2CO3(OH)16·4H2O on carbonation of Na2CO3 was analyzed. It is verified that Mg6Al2CO3(OH)16·4H2O can decompose into basic compounds in the temperature range 310 ℃-400 ℃. Nevertheless, because of the promotion of Mg6Al2CO3(OH)16·4H2O on carbonation of Na2CO3,200℃ is still chosen for regeneration.Finally, behaviors and characteristics of carbonation, regeneration and multi-cycle carbonation-regeneration of Na2CO3/Al2O3 and the three composited sorbents were investigated systematically in a fluidized-bed reactor. It is found that the three composited sorbents exhibit higher CO2 capture rate, especially in the first 10 min of carbonation, and shorter carbonate reaction time, In the fourteen-cycle tests, regeneration conversions of the four sorbents are almost close to 100% and the decays of carbonation capacities of the four sorbents are not obvious. The surface area and pore volume of the four sorbents are stable showing excellent durability performance. |
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