Fate of Fuel Nitrogen in Chemical Looping Combustion: Influence of Homogeneous and Heterogeneous Reactions

Abstract

Chemical looping combustion is a novel combustion technology that can be used to reduce CO2 emissions. However, with nitrogen containing fuels, NOx is expected to form. The mechanism of NOx formation and reduction due to fuel bound nitrogen in chemical looping combustion is not well established. As a step in understanding this mechanism, this thesis aims to investigate the influence of homogeneous, gasphase, reactions and heterogeneous, gas-solid, reactions causing NOx formation and reduction in chemical looping combustion. This was done by combining modelling and experimental endeavours. Gas-phase modelling of important homogeneous reactionswasperformedusingCHEMKIN.Furthermore,experimentswerecarriedout with thermal gravimetric analysis to study the important heterogeneous reactions, for which kinetics were derived. Activated, Norwegian ilmenite was used as oxygen carrier, as this is currently a bench-mark oxygen carrier. The proposed reaction pathway that the modelling work and the experiments are based on suggests that the NOx precursor NH3 can be catalytically decomposed to N2 or oxidised to NO by the oxygen carrier and that the formed NO can be reduced to N2, either by the catalyticreactionwithNH3 orbyoxidationofthereducedOC.Theresultsindicated that the heterogeneous reactions are of large importance and that the environment in chemical looping combustion cannot be modelled using only gas-phase chemistry. The main conclusions are that the heterogeneous reactions seem to dominate the NOx formation and reduction pathways and that NOx formation is larger than NOx reduction with respect to the heterogeneous reactions when utilising ilmenite as oxygen carrier.