Ammonia as an Absorbent of Carbon Dioxide in Post-Combustion Capture - an Experimental, Technical and Economic Process Evaluation

Doctoral thesis, 2014

Carbon capture and storage is vital to facilitating the transition from our current fossil fuel-based energy system to a sustainable energy system. The concept of post-combustion capture is based on the selective chemical absorption of carbon dioxide (CO2). The highly concentrated CO2 stream that is suitable for storage is generated by heat-induced regeneration of the solvent. In this work, ammonia is evaluated as an absorbent of CO2 in post-combustion capture processes. The major goals of this thesis are to define favorable applications and local conditions for ammonia-based post-combustion capture processes. The current benchmarking absorbent, monoethanolamine (MEA), has inherent problems is associated with specific drawbacks, such as a high heat requirement for regeneration and a tendency to undergo degradation during operation. Several novel absorbents, including ammonia, have been proposed as being superior to MEA in these aspects. The major disadvantage of ammonia is its volatility, which results in loss of absorbent (slip). This thesis reveals the results of experimental, technical and economic performance analyses. To evaluate the capture process, validation of thermodynamic models of the NH3-CO2-H2O system is required. These thermodynamic models rely on a comprehensive experimental data on the solid-liquid-gas equilibrium for all operating conditions. This work presents new gas-liquid equilibrium data for temperatures that are relevant to the absorber (<25°C), which is an operating range of no prior industrial interest. This work presents the results of simulations of the absorption, desorption and ammonia recovery processes, for both the individual component level and the aggregated capture cycle. The performance of ammonia-based post-combustion capture is evaluated under various operational and local conditions. The design of a full-scale, ammonia-based capture process is described and the cost of capture is estimated for this process integrated with a coal-fired power plant. Special attention is focused on the design of the absorber system. In this thesis, a dual absorber with intermediate cooling is proposed as an essential measure to reduce ammonia slip. Preferable operating conditions for the absorber system are defined as: lean and rich CO2 loadings of 0.25 and 0.50, respectively; and a concentration of ammonia (CO2-free) of 14.3wt%. The thesis concludes that ammonia-based absorbents represent a feasible alternative to the first- and second-generation absorbents for post-combustion capture of CO2. The specific total heat requirement for ammonia-based capture is calculated to be approximately 3,000 kJ/kg CO2. The most serious problem identified with respect to the ammonia process is the cost of reducing the ammonia slip to allowable levels. This concern limits the locations at which the present process could be implemented with commercial success. Successful application of ammonia-based post-combustion capture is expected to be achieved at locations that are characterized by access to low-temperature cooling water (~5°C) and a high concentration of CO2 (~15vol%).