Chemistry and Process Design of Integrated Removal of Nitrogen and Sulfur oxides in Pressurized Flue Gas Systems
Doctoral thesis, 2019
Carbon capture and storage is vital to reach the climate goals to limit the increase in the global temperature. Among the carbon capture technologies, oxy-fuel and chemical looping combustion produce a stream mainly containing carbon dioxide (CO2) and water but also contaminants such as nitrogen oxides (NOx) and sulfur oxides (SOx). The carbon dioxide is compressed and separated from water and impurities to achieve suitable conditions for transport and storage. In addition to storage and transport system requirements, separation of NOx and SOx during the CO2-conditioning is required to avoid corrosion issues in various parts of the process. In addition to the emission control measures available for conventional power plants, there are new opportunities for control of NOx and SOx in pressurized flue gas systems of oxy-fuel and chemical looping combustion plants.
The work presented in this thesis evaluates the chemistry and process design of NOx and SOx removal during CO2-conditioning in oxy-fuel and chemical looping combustion systems. The primary goals of this thesis are to expand the current understanding of NOx and SOx chemistry and identify favorable conditions for achieving cost-effective control processes. Analysis of the reaction system by a detailed chemistry model, uncovers the importance of pH level in the liquid for the selectivity of the integrated NOx and SOx removal process. Moreover, a reduced mechanism is proposed for engineering calculations of the pressurized flue gas systems that captures the effect of pH and describes the relevant gas and liquid-phase chemistry. Process simulations, which utilize the reduced mechanism, enable evaluation of design of the integrated removal of NOx and SOx in pressurized flue gas systems. Technical evaluation of the integrated process reveals that removal rates of >98% for SO2 and >90% for NOx may be achieved. Moreover, the efficiency of the NOx removal can be improved by the presence of SOx and increased concentration of O2 in the flue gas. A comparison of the economic performance of the integrated removal process and the conventional emission control measures, i.e., selective catalytic reduction and wet flue gas desulfurization with limestone, shows significantly lower costs of removal by the integrated process.