Prompt considerations on the design of chemical-looping combustion of coal from experimental tests

Abstract

The Chemical-Looping Combustion of coal in the reactor system has been proposed as an interesting option to process a solid fuel in a CLC system. In this process, a solid fuel is directly fed to the fuel reactor in a CLC system. Solid fuel pyrolysis, char gasification and oxidation of gaseous products by reaction with the oxygen-carrier are the main chemical processes happening in the fuel reactor. The aim of this study is to analyze the performance of ilmenite as oxygen-carrier for CLC of coal regarding to the conversion of gaseous products from char gasification. Successive reduction-oxidation cycles were carried out in a fluidized bed using bituminous coal char as reducing agent. The changes on chemical and physical properties of ilmenite particles were determined. An activation process of ilmenite through the redox cycles was evidenced which was justified by an increase of porosity. The results showed that the activation for ilmenite reduction reaction was completed after 7 redox cycles. However, the oxidation reaction rate was increasing still after 16 redox cycles because the porosity was not fully developed. The gasification reaction rate and the ilmenite reactivity were analyzed. The effect of ilmenite itself and the influence of the gasification agent, i.e. H2O, CO2 or H2O/CO2 mixtures, and temperature on the gasification rate were evaluated. Limited use of CO2 in the fluidizing gas was identified in order to maintain high gasification rates. Higher temperature improved the char gasification rate, mainly using steam as gasification agent, and the combustion efficiency of the gasification products. Nevertheless, the effect of temperature on the combustion efficiency was of lower relevance than that on the gasification rate. Finally, a theoretical approach was developed to easily evaluate the conversion of char in the fuel-reactor by gasification.