Ammonia-Based Post-Combustion Capture of Carbon Dioxide

Licentiate thesis, 2013

Post-combustion capture of carbon dioxide is one of the measures to reduce emissions of carbon dioxide from large point source emitters. In post-combustion capture the carbon dioxide is absorbed from the flue gas by means of a liquid absorbent. In this thesis, ammonia is evaluated as absorbent of carbon dioxide in a post-combustion capture application. The major energy penalty for capturing carbon dioxide is, in the case of post-combustion capture, the heat required to release the carbon dioxide from the absorbent, also called the heat of regeneration. The overall aim of this thesis is to determine the heat requirement of ammonia regeneration and to evaluate the consequences of integrating post-combustion capture with an existing coal-fired power plant. The primary evaluation tool in this work is equilibrium-based thermodynamic modeling. Three thermodynamic models proposed in literature for the NH3-CO2-H2O system are evaluated. A literature review in this work revealed missing data for equilibrium partial pressures of carbon dioxide and ammonia at 0 - 20°C, which is the typical temperature range for post-combustion capture with ammonia. An experimental methodology was therefore established in Paper I in which the aim was to determine the partial pressures of ammonia and carbon dioxide at 10 and 20°C. The experimental setup consisted of an equilibrium cell connected to a gas chromatograph. The experimental conditions included 5.7 and 10.7 wt% NH3 and CO2-loadings between 0.15 - 0.75. The results provide additional data points in the partial pressure range 0.1 - 20 kPa and 0.01 - 10 kPa for ammonia and carbon dioxide, respectively. In Paper II the CO2 capture cycle is simulated in the process simulation software Aspen Plus. The thermodynamic models form the foundation for the flow sheet process modeling. The heat requirement for regeneration is evaluated for NH3 concentrations ranging from 2 to 20% and the lean CO2-loading is varied between 0.2 - 0.5. The specific heat requirement was determined to be 2500 kJ/kg CO2 captured. There is a minimum of 2100 kJ/kg CO2 captured at a CO2-loading of 0.5, just where solid precipitation starts. However, at these conditions the equilibrium sets a limit to the capture efficiency of 50%. Furthermore, it is concluded that the major reason for the low heat requirement for ammonia regeneration is not a low heat of reaction but a low heat of vaporization, due to the pressurized desorption. In Paper III, the ammonia-based post-combustion technology “Chilled Ammonia Process” is thermally integrated with a coal-fired power plant. It is concluded that the electric efficiency of the power plant will decrease with 9.2%-points. Yet, this figure depends strongly on the available cooling water temperature. Access to low temperature cooling water greatly enhances the performance of the capture process. A 10°C higher cooling water temperature could increase the energy penalty of the power plant with almost one percentage point.