Evaluation of Operating and Design Parameters of Pressurized Flue Gas Systems with Integrated Removal of NO_x and SO_x

Abstract

This study investigates the operating and design parameters of product gas compression and integrated control of nitrogen oxides (NO x ) and sulfur oxides (SO x ) in large-scale oxy-fuel and chemical looping combustion processes. A process model that includes a comprehensive description of nitrogen and sulfur chemistry and mass transfer is developed. The results show that the fraction of NO oxidation into NO2 will be 10–50% in a multistage compressor to 30 bars (1–4% O2 in the gas) depending on the residence times in intercoolers and pressure levels. At lower O2 concentrations (>0.1% O2 in the gas), the oxidation is limited but still active. Nitric acid formation in the compressor condensate is, thus, inevitable, although limited, as most water is condensed in the early stages, whereas the acid gases are formed in the later stages. The NO2/NO x ratio has an important effect on the total amount of NO x absorbed and extra residence time should be added after the compressor to increase this ratio. Evaluation of the process behavior in relation to simultaneous absorption of SO2 and NO x revealed that increased SO2/NO x ratio and bottom liquid recycling enhanced the total NO x absorption. In addition, maintaining the pH in the absorbing solution above 5 improves the removal efficiencies of NO x and SO2. NO x removal rates of up to around 95% can be achieved for SO2/NO x > 1 in the flue gas with appropriate design of the absorber. For SO2/NO x < 1, increasing the packing height or addition of S­(IV) solutions could enhance the NO x removal rates to 95% or more. The model predictions are compared with the experimental data from a laboratory-scale absorber. The process model developed in this work enables design studies and techno-economic evaluation of absorption-based NO x and SO x removal concepts.

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