| Growing concerns for global warming and climate change in recent years have motivated research activities toward developing more efficient and improved processes for carbon dioxide capture and storage (CCS) from large point sources, especially coal/gas-power plants. The conventional technology for CCS with aqueous solution of amine not only required a great amount of energy but also has serious corrosion issues with its equipment. Therefore, as an alternative approach, CO2 capture from flue gas with solid sorbents has attracted much attention in recent years. In this work, industrial grade multi-walled carbon nanotubes(IG-MWCNTs) were functionalized with tetraethylenepentamine(TEPA). The sorbents were measured by examing the performance of CO2 adsorption. And we selected the adsorbent with high adsorption performance, obtained the adsorption/desorption kinetic parameters, investigated the influence of flue gas contaminants. In addition, as a contrast research, we also investigated the CO2 adsorption with MIL-lOl(Cr) from flue gas.We systematically examined the performance of IG-MWCNTs impregnated with TEPA, which have high CO2 adsorption capacity comparable to that of TEPA impregnated purity multi-walled carbon nanotubes(P-MWCNTs), but IG-MWCNTs, at one-tenth the price of P-MWCNTs. IG-MWCNTs impregnated with polyethylenimines(PEIs) were investigated the performance of CO2 adsorpion, and found that ethylenediamine end-capped PEI(PEI-EC) exhibited a significantly higher adsorption capacity than others. After impregnation, the morphology and structure of IG-MWCNTs were retained, the BET surface area and pore volume decreased with the increase of impregnation. The CO2 adsorption capacity first increased and then decreased with the increase of impregnation, and got the maximum when the impregnation amount of TEPA and PEI were 50 wt% and 40 wt%, respectively. CO2 adsorption capacity also first increased and then decreased with the increase of temperature. TEPA and PEI-EC impregnated IG-MWCNTs reached its maximum adsorption capacity at 343 K, as 3.088 mmol/g and 2.538 mmol/g, respectively. These adsorption values were all greater than 2.5 mmol/g in the flue gas environment of 313- 343 K for TEPA impregnated IG-MWCNTs. The Freundlich equation was used to fit the isotherm data of TEPA impregnated IG-MWCNTs, and the adsorption heat decrease with the increase in CO2 adsorption capacity.The adsorption/desorption kinetics of CO2 on TEPA impregnated IG-MWCNTs was investigated to obtain insight into the underlying mechanisms on the fixed bed. After evaluating four kinetic models for CO2 adsorption at various adsorption temperatures, CO2 partial pressure, and amine loadings, it was found that Avrami’s fractional order kinetic model provided the best fitting for the adsorption behavior of CO2. In order to find the optimal regeneration method, three desorption methods were evaluated for the regeneration of solid sorbents. The activation energy Ea of CO2 adsorption/desorption was calculated from Arrhenius equation and used to evaluate the performance of the adsorbent. The Ea decreased with increasing CO2 concentration, indicating that CO2 adsorption of amine-functionalized IG-MWCNTs is possibly the intra-particle controlled. Meanwhile, because of the energy input of vacuum pump, Ea for vacuum swing regeneration method was less than that for temperature awing regeneration. Considering the economic and technical feasibility, the best way for the regeneration of adsorbent was vacuum temperature swing adsorption (VTSA) regeneration. Activation energy Ea/Ea(des) of CO2 adsorption/desorption were calculated as-14.622/88.337 kJ/mol and-7.353/46.931 kJ/mol for PEI-EC and TEPA impregnated IG-MWCNTs, respectively. The absolute value of Ea/Ea(des) of former was larger than that of later, which indicates the higher potential barrier and thus the difficult adsorption/desorption process for PEI-EC impregnated IG-MWCNTs.We examined the performance of TEPA impregnated IG-MWCNTs in trace amounts of flue gas contaminants such as H2O, NO, and SO2. It was observed that H2O and NO had a minimal impact on CO2 adsorption capacity, while the effect of SO2 on CO2 adsorption was influenced by adsorption temperature and SO2 concentration. Compared with silica-based adsorbents, i.e., TEPA-impregnated MCM-41, amine-functionalized IG-MWCNTs shows significantly better tolerance to H2O and SO2. In addition, we examined the variation of CO2 adsorption with and without SO2 with various experimental methods and molecular simulation. Experimental results show that irreversible sulphate/sulphite species deposited into the adsorbent contributes to the decrease on CO2 adsorption. While the results from simulation studies reveal that the enthalpy difference of between the isolated TEPA with SO2 and TEPA…SO2 (△ H(TEPA-…SO2)) is larger than that of CO2 (△(TEPA…CO2)), indicating that SO2 has a stronger reaction activity with TEPA than CO2. The increase of ratio of △ H(TEPA…SO2)/AH(TEPA…CO2) with increasing temperature illustrates that the difference of CO2 adsorption capacity with and without SO2 increases with elevated temperatures.MIL-101(Cr) has drawn much attention due to its high stability compared with other metal-organic frameworks(MOFs). In this study, three trace flue gas contaminants (H2O, NO, SO2) were each added to a 10 vol% CO2/N2 feed flow and found to have a minimal impact on the adsorption capacity of CO2. In dynamic CO2 regeneration experiments, complete regeneration occurred in 10 min at 328 K for temperature swing adsorption-N2-stripping under a 50 cm3/min N2 flow and at 348 K for vacuum-temperature swing adsorption at 20 KPa. Almost 99% of the pre-regeneration adsorption capacity was preserved after 5 cycles of adsorption/desorption under a gas flow of 10 vol% CO2,100 ppm SO2,100 ppm NO, and 10 RH%, respectively. However, MIL-101(Cr) has a lower CO2 adsorption capacity, the maximum of which is 0.495 mmol/g at 298 K. Therefore, MIL-101 (Cr) can not effectively and economically capture CO2 from flue gas streams. |
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