Exploring design options for pressurized chemical looping combustion of natural gas

Abstract

Chemical Looping Combustion (CLC) is a low-cost CO2 capture technology highly competitive for industrial and energy applications. Up to date, CLC has been mostly developed at atmospheric pressure. Pressurized CLC may bring additional advantages due to the intensification of the process. The main objective of this work was to determine whether an increase in the operating pressure could improve the fuel conversion in CLC systems of gaseous fuels when ilmenite was used as oxygen carrier. Fluid dynamics considerations were evaluated to determine a window of suitable design parameters at pressurized conditions. A theoretical model previously validated at atmospheric pressure was modified to consider fluid dynamics and reaction kinetics at pressurized conditions, both in the bubbling and turbulent regimes, using ilmenite as oxygen carrier. The conversion of natural gas was estimated as a function of several design and operating parameters, such as the gas velocity, particle size and reacting temperature. Uncompleted combustion of natural gas was predicted at atmospheric conditions, and a limited improvement was estimated at pressurized conditions. A discussion about the required reactivity of the oxygen carrier to achieve complete combustion was performed. Furthermore, an energy analysis of a CLC system operating at atmospheric or pressurized (0.5 MPa) conditions with fuel reactor designed under the bubbling or turbulent regimes was carried out using the Aspen HYSYS process simulation software for two different applications of heat generation: production of domestic hot water and process steam. From this energy balance, it could be concluded that the energy efficiency achieved at pressurized conditions was slightly higher than that obtained at atmospheric pressure, essentially because at pressurized conditions it was possible to use the latent heat of condensation of the water generated in the fuel reactor. In addition to the results obtained in this work, a thorough assessment of technical risks and a techno-economic analysis would be required in order to make a final decision about the feasibility of operating at pressurized conditions in CLC systems.