The rapid increase in atmospheric CO2 has become a major environmental concern in recent years. Coal-fired power plants, releasing flue gas containing CO2, account for approximately 30% of total CO2 emissions worldwide. Solid amine sorbents such as amine immobilized on silica (SiO2) has gained significant consideration for capturing CO2 from flue gas due to its lower operation cost and equipment corrosion compared to existing liquid amine process. The -NH2 functional group of these solid amine sorbents binds CO2 through acid-base interactions, allowing CO2 to adsorb and desorb at temperatures in the range of the flue gas operating conditions.
Studies have shown that the solid amine sorbents can initially adsorb CO2 at the economical level compared to that of liquid amine processes. The CO2 capture capacity of solid amine sorbents reduce over the period of time due to the thermal instability and contaminant poisoning of the amine. Our current development focuses on improving the sorbent stability and mechanistic study of the interactions between the amine, CO2, and contaminants present in the flue gas. This dissertation presents a study of the use of polyethylene glycol (PEG) to enhance the stability of amine-immobilized silica. Long term stability of tetraethylenepentamine-immobilized on silica (TEPA/SiO2) and PEG-enhanced TEPA/SiO2 (PEG/TEPA/SiO2) were evaluated by performing multiple cycles of CO2 capture on the sorbents under the constant monitoring of in situ infrared and mass spectrometers. PEG/TEPA/SiO2 shows slower degradation than TEPA/SiO2. The IR absorbance spectra reveal that the accumulation of the strongly adsorbed CO2 species as bicarbonates and carboxylates is the cause of sorbent degradation. The IR absorbance spectra further suggested that the presence of PEG decreased the formation of these strongly adsorbed CO2, reducing the degradation of the sorbent.
The interactions between the -NH2 groups, CO2, and other electron acceptor species present in the flue gas govern the CO2 capture capacity and long term stability of the sorbent. The flue gas from coal-fired power plants contains 40-250 ppm of SO2 and 5-7 vol% of H2O. Although the presence of these species in the flue gas is expected to influence the performance of the sorbent, the extent of the interaction between the amine groups and these species has not been studied. CO2 capture under the presence of 250 ppm in the CO2 adsorption stream was performed on TPSENa sorbent (29 wt% TEPA, 18 wt% PEG, 49 wt% SiO2, 3.8 wt% EPON, and 0.2 wt% Na2CO3). The initial CO2 capture capacity of TPSENa was 1.195 mmol-CO2/g-sorb. which decreased to 0.532 mmol-CO2/g-sorb. after 24 cycles of CO2 capture under presence of 250 ppm SO2. The CO2 capture capacity of TPSENa showed a slight reduction from 0.869 to 0.764 mmol-CO2/g-sorb. under the absence of SO2. The IR absorbance spectra indicate that formation of both strongly-adsorbed CO2 species and SO2-adsorbed species accelerated the degradation of the sorbent in the presence of SO2.
The development of high stability solid amine sorbent requires an in-depth understanding of the interaction between CO2 and the amine groups. The mechanism of CO2 adsorption on amine groups follows acid-base type interaction where the CO2 acts as the acidic species and amine groups are the basic site. The products of the reaction between CO2 and the amine are ammonium ion (NH3+), carbamate, and bicarbonates. The evidence of the formation of carbamate and bicarbonates are commonly available in literatures while that of the ammonium ion is scarce. The in situ injection of HCl on TEPA/SiO2 was performed under constant infrared spectroscopic monitoring to elucidate the acid-base reaction. The IR spectra of TEPA/SiO2 after HCl injection shows similar absorption features to those of TEPA/SiO2 during CO2 adsorption, evidences for the formation of NH3+. IR spectra also suggests that HCl is likely to react with primary amine (-NH2) of TEPA and then further reacts with secondary amine (-NH), resulting in the decrease in the available amine sites for CO2 adsorption. The in situ injection of H2O on TEPA/SiO2 caused the removal of TEPA from SiO2.
The results of this study have provided the several key information of which should prove to be helpful in the development of highly stable solid amine sorbent. The study of PEG-enhanced TEPA/SiO2 has shown that the stability can be improved by addition of chemical stabilizers which slows down the formation of the carboxylate species. The SO2 poisoning of the sorbent is caused by (i) accelerating the formation of strongly adsorbed CO2 species and (ii) depositing of SO2-adsorbed species on the amine sites. The further studies should focus on development of the sorbent with high resistance to HCl and SO2. Additional of aromatic amine to the alkyl amine on silica support may reduce the HCl and SO2 poisoning as the aromatic amine has a strong reactivity toward the acidic gaseous species. The addition of these compounds requires optimization to ensure that the sorbent resistance to SO2 while stability and capture capacity are not significantly affected.