| Summary: | Research Doctorate - Doctor of Philosophy (PhD) The development of cost-effective carbon dioxide (CO 2 ) capture strategies to achieve significant reduction in atmospheric CO 2 levels is one of the main global challenges for the near future. Post combustion capture (PCC) of CO 2 offers a short-term option to help the reduction of CO 2 emissions from fossil fuel power plants. Employing aqueous ammonia (NH 3 ) based solutions has been recognised as a promising option because of its key advantages for PCC, compared to the current benchmark solvent - monoethanolamine (MEA). Advantages include higher CO 2 absorption removal capacity, lower regeneration energy, as well as the chemical stability of NH 3 in the presence of SO 2 , NO 2 and oxygen in the flue gas. There are, however, two major challenges limiting the large scale deployment of aqueous NH 3 -based PCC technology; the low CO 2 absorption rate and the high solvent loss via evaporation. Therefore, this research is focused on these areas to further advance and accelerate the commercialization of aqueous NH 3 -based PCC processes. This thesis focuses on improving the physio-chemical properties of aqueous ammonia based solvents through the addition of additives to reduce ammonia loss via evaporation and/or to increase the CO 2 absorption rate. In addition, this work lead to the development of a direct way to determine the Henry Coefficient of CO 2 in aqueous amine solutions and this was validated in the MEA-CO 2 -H 2 O system. 1. Attempts to suppress ammonia loss by additional of metal ions (M(II) = Zn(II), Ni(II) Cu(II)) was experimentally measured and theoretically simulated with a comprehensive model of the M(II)-NH 3 -CO 2 -H 2 O system. The theoretical and experimental investigations both suggest that complexation of ammonia by metal ions effectively reduces the ammonia loss via evaporation. However, because of the reduction of concentration of free ammonia in solution, the reactivity towards CO 2 also decreased. Simple reduction of the total NH 3 concentration to the same concentration of free NH 3 concentration as that obtained by the addition of M(II) showed a similar reduction in ammonia loss and CO 2 absorption rate. 2. Piperazine (PZ) has been reported as an effective rate promoter in the aqueous ammoniabased solvent process for the post combustion capture (PCC) of CO 2 . The wetted-wall column technique was employed to examine the mass transfer coefficient of PZ-promoted aqueous ammonia solutions. Results showed that PZ is an effective promoter for both pure and CO 2 -loaded aqueous ammonia solutions. Stopped-flow spectrophotometry was used to further investigate the promotion mechanism of PZ on CO 2 absorption into aqueous ammonia solutions. A comprehensive reaction mechanism for the PZ-NH 3 -CO 2 -H 2 O system revealed that the simple combination of all reactions involved in the individual PZ-CO 2 -H 2 O and NH 3 -CO 2 -H 2 O systems can quantitatively explain the reaction mechanism between CO 2 and blended PZ/NH 3 solutions. As suggested from the modelled results, the reactive piperazine mono-carbamate species (PZCO 2 /PZCO 2 H), in high CO 2 -loaded blended PZ/NH 3 solutions, acts as the dominant rate promoter, enhancing CO 2 absorption into aqueous ammonia solutions. 3. It was noted from previous publications, that the family of cyclic amine absorbents i.e. piperazine (PZ), 3- and 4-piperidinealkanols (3-PM, 4-PM), had the properties of fast kinetics and high absorption capacity. As such they were recognised to be an attractive family of absorbents for CO 2 absorption on their own and also blended solvents with aqueous ammonia solutions. Herein we investigate and report for the first time the complete temperature dependent kinetic and equilibrium behaviour of a new heterocyclic amine 4- aminomethyltetrahydropyran (4-AMTHP), with CO 2 , in aqueous solutions at 25, 35 and 45 oC. Furthermore, a comprehensive model describing all reactions in the 4-AMTHP-NH 3 -CO 2 -H 2 O system was also developed to rationalise the experimental kinetics results. The combination of kinetics and enthalpy properties of 4-AMTHP showed this amine to be a promising candidate for CO 2 absorbance on its own and as a solvent blend with aqueous ammonia. 4. The Henry's Coefficient of CO 2 is a fundamental property and crucial for the reliable simulation of the absorption and desorption of CO 2 in amine/ammonia solutions related to post combustion capture (PCC). CO 2 is a reactive gas. When mixing with aqueous amine solutions, several species are formed including the carbamic acid/carbamate species, carbonate, bicarbonate, carbonic acid, and CO 2(aq) . In this chapter, a direct way to determine Henry's Coefficient of CO 2 in amine solutions was proposed as a superior alternative to the well-known "N 2 O analogy" method. The method requires published vapour liquid equilibrium (VLE) measurements of the amine-CO 2 -H 2 O system and MEA was chosen as it is the most extensively studied amine and there are abundantly published results on the VLE data of the MEA-CO 2 -H 2 O system. The free CO 2 concentration in solution can be computed using the total MEA concentration, loading and temperature, which allows the determination of the Henry’s Coefficient from the CO 2 partial pressure. A 10-parameter polynomial is used to approximate the Henry's Coefficient as a function of the total MEA concentration, loading and temperature.
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