Optimizing the Chilled Ammonia Process Technology for Post-Combustion CO2 Capture by Controlling Solid Precipitation

Abstract

The Chilled Ammonia Process enables the separation of CO2 from flue gas of large-scale, stationary CO2 emitters. Such capture of CO2 is a crucial step in CO2 Capture and Storage, a system of technologies that has the potential to reduce global CO2 emissions rapidly in order to mitigate climate change. In typical CO2 capture processes, the flue gas is contacted with a solution that has a high affinity to CO2. After loading the solution with CO2 in the absorption section of the process, the solvent is heated in the regeneration section and almost pure CO2 evaporates. In the case of the Chilled Ammonia Process, an aqueous ammonia solution is recycled between the absorption and the regeneration section. Ammonia and CO2 undergo a complex interaction in the aqueous phase, including a series of chemical reactions and at high concentrations also the formation of several different solid compounds. The formation of solids may lead to clogging of process equipment and it has been recognized as an important challenge for the controlled operation of the process. In this thesis, the thermodynamics of the CO2-NH3-H2O system was analyzed systematically with a focus on solid formation. Based on a detailed thermodynamic model, phase diagram tools were developed with the objective of supporting process development and optimization. The CO2 capture process was investigated and critical streams and unit operations that may be concerned with solid formation were identified based on an exemplary process simulation and on ranges of operating conditions typically reached according to the literature. Implications for process design and energy consumption are discussed and ternary phase diagrams mapping the composition regimes of solid formation and the typical operating conditions are presented. It is shown that the energetic optimization of the capture process pushes the composition of several process streams towards the solubility limit. In the following, a new ammonia-based process for CO2 capture from flue gas was developed, which utilizes the formation of solid ammonium bicarbonate to increase the CO2 concentration of the solution before the regeneration. Precipitation, separation, and dissolution of the solid phase are realized in a dedicated process section, while the packed absorption and regeneration columns remain free of solids. A rigorous performance assessment based on the integration of the capture process with a coal-fired power plant and on the specific primary energy consumption per CO2 avoided was implemented and the new process was compared to a standard Chilled Ammonia Process without solid formation in detail. An optimization algorithm was developed to screen a wide range of operating conditions in equilibrium-based process simulations. The new process shows a significant, i.e. greater than 10% reduction of the specific energy requirement. Favorable conditions under which the improvement would further increase are discussed. The further investigation of the new process requires a detailed design of the solid handling section based on rate-based process simulations. Due to a lack of growth data in the literature, the growth kinetics of ammonium bicarbonate was investigated experimentally. A setup has been developed that allows to measure the liquid composition in a pressurized batch-type reactor on-line. In addition, an injection system that enables the addition of seed crystals has been designed. Upon seeding of a supersaturated solution, the trend of the decreasing solute concentration with time was used to regress the growth rate parameters applying a population balance model. Besides the work on the Chilled Ammonia Process, this work provided an important contribution to the knowledge transfer from academia to the general public in the field of CO2 capture and storage. A physical model for the geological storage of CO2 was developed and built and successfully demonstrated at numerous exhibits and events.