1 Center for Energy Resources Engineering, Center, Technical University of Denmark2 Department of Chemical and Biochemical Engineering, Technical University of Denmark3 CERE – Center for Energy Ressources Engineering, Department of Chemical and Biochemical Engineering, Technical University of Denmark4 Department of Chemistry, Technical University of Denmark
By implementing carbon dioxide (CO2) capture, it is possible to clean the flue gas from coal-fired power plants, so that it is almost completely free of this greenhouse gas. The most advanced techniques towards practical application are based on chemical absorption, where CO2 in the flue gas is chemically bond by a solvent, usually an aqueous solution of amines, resulting in a cleaned gas being released to the atmosphere. Heating of the solution, desorbs the CO2 and regenerates the solvent, which is then ready for a new round of absorption. The captured CO2 can then be compressed and sent to storage. Typical solvents for the process are based on aqueous solutions of alkanolamines, such as mono-ethanolamine (MEA), but their use implies economic disadvantages and environmental complications. Amino acid salt solutions have emerged as an alternative to the alkanolamines, partlybecause they are naturally occurring substances, and partly because they have desirable properties, such as lower vapor pressures and higher stability against oxidative degradation. One important feature of these new solvents is the formation of solids upon CO2 absorption, which happens especially at higher CO2 loadings and/or amino acid salt concentrations. The formation of solids poses challenges, but it also holds the promise for improving the efficiency of the capture process. This project focuses on phase equilibrium experiments of five systems CO2 + amino acid salt + H2O, at conditions relevant for the CO2 capture process. Also, attention is given to the chemical compositions of the precipitations, which forms as a result of CO2 absorption into the five amino acid salt solutions. Phase equilibrium data are needed to develop safe and economically viable capture processes. Two different experimental apparatuses were used. One was developed specifically for this project and is based on an analytical semi-flow method. The other has recently been developed in another project, and is based on a synthetic method. The semi-flow method was used for measurements at 10 kPa CO2 partial pressure, total pressure equal to the atmospheric pressure and temperatures of 298 K, 313 K and 323 K; these conditions cover the absorber part of the capture process. The apparatus based on the synthetic method was used to obtain data at different pressures and temperatures up to 353 K (80 ºC), which is the maximum operation temperature of some of the components in the apparatus. Ideally, measurements in the full temperature range for desorption, which usually needs approximately 393 K (120 ºC), would be desirable. Using the 2 apparatuses, CO2 solubility in aqueous solutions of MEA and the potassium salts of taurine, glycine, L-alanine, L-proline and L-lysine were examined. It was found that all of the tested amino acid salt solutions have the potential to replace MEA. Nevertheless, solutions of the potassium salt of L-lysine showed the best properties. The chapters of this thesis are organized as follows: Chapter 1 is a brief introduction to the central issues of this work, setting the subject of the project in perspective to the issues of global warming and CO2 capture and storage (CCS). Chapter 2 explains the principle behind the chemical absorption of CO2 from flue gas, emphasizing on the issues involving the solvent. Chapter 3 is dedicated to the concept of using amino acids salt solutions as solvents in CO2 capture, it includes a description of general amino acids chemistry and an outline of the chemical reactions involved in CO2 absorption into amino acids salt solutions. Chapter 4 deals with the description and validation of the new apparatus for measuring of CO2 solubility based on the semi-flow method. A validation study of CO2 solubility in aqueous solutions of MEA is presented. Chapter 5 focuses on the determination of the chemical compositions of the precipitations, which arise in the five amino acid salt solutions upon CO2 absorption. The solutions were saturated with CO2 at 298 K, using the apparatus based on the semi-flow method. The precipitates were isolated and analyzed with XRD. Chapter 6 concerns a CO2 solubility study using the apparatus based on the semiflow method; CO2 solubility were examined in aqueous solutions of MEA and the five amino acid salts, with concentrations between 2 and 8 molal and temperatures around 298 K, 333 K and 323 K. Chapter 7 describes a CO2 solubility study using the apparatus based on the synthetic method; CO2 solubility in aqueous solutions of the potassium salts of Llysine and L-proline, with concentrations of 3.5 molal and 7 molal respectively, were studied at CO2 partial pressures up to about 760 kPa, and temperatures around 313 K, 333 K and 353 K. Similar data were obtained with solutions of 7 molal MEA, at the same temperatures and at CO2 partial pressures up to around 2.3 MPa Chapter 8, concludes on the accomplishments of the project, and finally addresses considerations concerning future work.