THE MECHANISM OF OXYGEN UPTAKE ON CARBON AS A KEY STEP IN LOOPING COMBUSTION

CarboLoop is an alternative method for chemical looping combustion of solid carbons, proposed by Salatino and Senneca [1,2]. It pursues the simple idea that the carbon-based fuel can extensively uptake oxygen and act itself as oxygen carrier as it is cycled between an Oxidizer and a Desorber. Looping of the carbon fuel between the two reactors enables stepwise conversion of carbon and yields a nearly pure stream of CO2 at the exhaust of the Desorber. The mechanistic basis of CarboLoop is represented by the semilumped carbon oxidation scheme proposed by Haynes and coworkers: -Cf + O2-->-C (O2)* R1 -C (O2)*+O2-->-C( O )+ CO2, CO R2 -C (O2)*-->-C (O) + CO2, CO R3 -C (O)-->-Cf + CO2, CO R4 where a key role is played by the mild chemisorption step R1. This step takes place irreversibly at moderate temperature and results in extensive oxygen uptake on carbon, with formation of a "metastable" surface oxide. The mechanism and energetics of mild chemisorption of oxygen on carbon has been investigated with a combination of techniques. Thermochemical (Temperature Programmed Desorption coupled with Differential Scanning Calorimetry) and microstructural techniques have been applied to either raw or oxidized carbon samples to elucidate the extent and energetics of oxygen uptake and the chemical nature and stability of surface oxides of carbon. Model computations have been performed to assess the relevance of the extent and energetics of oxygen chemisorption/desorption on the performance of the looping combustion process. Results are analysed and discussed with a focus on the optimization of the oxidation step of the CarboLoop process.