Post-combustion Capture Of CO₂ Using Advanced Solvents And Novel Regeneration Process

Presently, post combustion CO2 capture technology using chemical absorption methods was considered to be the most promising technology to limit CO2 emission from existing coal-fired power plants. However, the biggest challenge for post combustion CO2 capture by chemical absorption was the high capital cost and energy consumption. Hence, the development of efficient and stable absorbents with low regeneration energy and capture system modification and integration were the crucial aspects to reduce the cost and energy requirement. In this thesis, the development of ammonia based blended absorbents, reaction kinects between solvents and CO2, ammonia volatile loss control and novel direct steam stripping system development were studied.This work focused on the development of advanced ammonia absorbents by mass transfer studies, kinetic studies and novel ammonia based system studies aiming to solve two major issues of ammonia as low CO2 absoption rate and high ammonia volatile loss. Due to its low absoption rate, eight additives were added into ammonia and the mass transfer coefficients were studied using wetted wall column. We found 0.3 M sarcosinate, PZ and 1-MPZ additives could increase the overall mass transfer coefficients of 3 M ammonia by 127%-258%. We further investigated the effect of temperature, sarcosinate concentration, and CO2 loading on the mass transfer coefficients of CO2 absorption in a sarcosinate-promoted aqueous ammonia solution and considered sarcosinate was a good ammonia additive.For the selected blended NH3/SAR-(sarcosinate) absorbent, we further investigated the reaction kinetics in the solution. Firstly, we developed a detailed reaction scheme including all possible reactions in SAR--CO2-H2O system. We investigated the temperature-dependent rate and equilibrium constants of the reaction between aqueous CO2 and SAR- using stopped-flow spectrophotometry and NMR technology. All unknown rate and equilibrium constants were obtained by global data fitting. The rate constant k9 for the reaction between CO2 and SAR- to form the carbamate at 25.0℃ is 18600 M-1 s-1. The temperature denpendence of k9 could be fitted by Arrhenius equation as k9=9.5×1014exp(-7348/T). Similarly, we have obtained all unknown rate and equilibrium constants in NH3-CO2-H2O system. The rate constant for the reaction between CO2 and NH3 to form the carbamate at 25.0℃ is 431M-1 s-1. We further investigated the kinetics of the reaction between CO2 and a blended NH3/SAR- absorbent using stopped-flow spectrophotometric techniques. We did not observe any synergistic or catalytic effects between NH3 and SAR- in the blended solution, the mechanism of the reaction of CO2 with the NH3/SAR- mixture was the simple combination of the individual reactions of NH3 and SAR- with CO2.In this work, we investigated the effect of total pressure on CO2 absorption and ammonia vaporization in ammonia solutions on a wetted-wall column. We found that the elevated pressure absorption process was an effective way to increase CO2 absorption rate and suppress ammonia vaporization at the same time. Hence, the elevated pressure absorption process would be a viable direction for the development of ammonia based capture technologies especially for high pressure gases. We found the mass transfer mechanism at elevated pressure was different from that at atmospheric pressure. The overall mass transfer coefficients of CO2 absorption in solutions obtained at elevated pressure were lower than that under atmospheric pressure. We thought the decrease of the overall mass transfer coefficients was due to the decrease of the gas side mass transfer coefficients. Besides, a novel ammonia separation method from low concentration ammonia solution by membrane vacuum regeneration was developed which could recover ammonia gas from low concentration ammonia solution and the separated water could be reused in the wash column. Hence, the wash water requirement and solvent loss in ammonia based CO2 capture system could be reduced. We found the solvent handling capacity and removal efficiency could be significantly increased by using serial membrane vacuum regeneration facilities.In this work, we built a lab-scale stripping platform to validate the idea of direct steam stripping process and seek the potential problems. The solvent used was standard MEA for comparison with standard energy value from literature. We investigated the direct steam stripping and the conventional stripping mode in terms of energy consumption and steam condensation. The results showed, for the direct steam stripping mode, the optimum energy consumption was 2.98 MJ/kg CO2,23.2% lower than that of the conventional stripping mode. Steam condensation in the column was an issue for the direct steam stripping process. We found higher feeding solvent temperature and carrier steam superheating temperature were beneficial to reduce steam condensation in the column. However, a higher feeding solvent temperature resulted in an increase of energy consumption as well. In addition, a equilibrium model in Aspen Plus has been developed for studying the novel direct steam stripping system based on detailed experimental study. The modeling results were found in good agreement with the experimental results. By the comparison between modeling and experimental results, we think the energy consumption in the experimental system could be reduced by stripper structure modification and mass transfer promotion. Besides, the energy consumption could be further reduced by reducing the temperature pinch of the condensation-evaporization heat exchanger. In this case, a 44% reduction of energy consumption compared to the standard process could be achieved.