Assessment of the thermochemistry of oxygen chemisorption and surface oxide desorption during looping combustion of coal char

Abstract

Carbons have a pronounced tendency to chemisorb oxygen at moderate temperatures and to desorb surface oxides as combustion products (CO, CO2) once the oxidized fuel is heated up under inert conditions. Based on this feature, a novel process of looping combustion of carbons (CarboLoop) has been recently proposed, and a proof-of-concept given. The process is based on alternated oxygen chemisorption on carbon and desorption of surface oxides carried out in dual bed reactors. One of the reactors, operated with an air feeding, acts as the fuel oxidizer. The second reactor, operated with partly recycled gaseous effluents (CO2+impurities), acts as the fuel desorber. Further development of the CarboLoop concept requires the knowledge of the thermochemistry associated with the individual steps of carbon oxidation and of surface oxide desorption, which dictates the energy integration between the reactors. The present study moves along this path and provides a separate assessment of the thermochemistry of carbon oxidation and desorption stages. The procedure is based on complementary thermoanalytical techniques (TG, DSC, TPD) as well as on calorimetry. Experimental results obtained with a bituminous coal char indicate that both oxygen uptake (ΔH≅−4kJ/gc) and desorption (ΔH≅−31kJ/gc) are exothermic. The strong exothermicity of the desorption stage, at odds with published data on CO and CO2 abstraction from oxidized carbons, is explained in the light of the cocurrent progress of the stabilization of metastable surface oxides and of abstraction of CO and CO2 from oxidized carbon.