Process configuration options for handling incomplete fuel conversion in CO2 capture: Case study on natural gas-fired CLC

“Innovative reactors” (based on chemical looping, membrane separation or sorption-enhanced processes) in power processes for CO2 capture appear to behave in such a way that some unconverted fuel could remain in the captured CO2-rich stream. This has the double drawback of adding impurities to the CO2 and of reducing the efficiency of the power cycle from which the CO2 was captured. The purpose of this paper is to illustrate the implications of some options for handling fractions of unconverted fuel in a captured CO2-rich stream. The selected case study is a chemical looping combustion (CLC) reactor integrated in a gas turbine cycle. Different process layouts are presented for the CO2 processing to reduce the amount of fuel components in the CO2 and process simulations are performed for fuel conversion rates in the range of 90-100%. Three different assumptions for the composition of the unconverted fuel were investigated (mixture$ of hydrocarbons, methane only, mixture of H2 and CO). Also the impact of the presence of fuel-derived N2 in the CO2 was investigated. Three different process solutions were studied for handling of the unconverted fuel: Separation of non-condensable gases in a distillation column, either for recirculation to the CLC reactor (fuel recovery method C) or for combustion in a duct burner (fuel recovery method C/B), or final oxidation of the unconverted fuel with oxygen from a cryogenic ASU (fuel recovery method O). Generally speaking, recovery of unconverted fuel for recirculation to the CLC reduction reactor through the use of a distillation column could have the double benefit of increasing the cycle efficiency and reducing the amount of impurities in the compressed CO2. It is however difficult to quantify the efficiency improvement, since it cannot be determined with the applied CLC reactor model how the reactor would respond to the introduction of a recirculation stream. The presence of H2 in the CO2 as well as the presence of N2 (or other inerts) in the fuel are found to be issues that merit further investigation. Also increasing rates of unconverted fuel in the CO2 due to oxygen carrier degradation could be an issue that merits further attention for CLC.

Authors: Jordal, Kristin; Gunnarsson, Jonas ;Full Source: Journal of Greenhouse Gas Control 2011, 5(4), 805-815 (Eng) ;