Iron-based materials as tar depletion catalysts in biomass gasification: Dependency on oxygen potential

A study has been performed using experimental iron based granules as a tar breakdown catalyst in a biomass gasification gas. Previous examinations established that metallic iron located in a separate catalytic bed reactor has a stronger influence on the tar content and composition in the product gas than their corresponding iron oxides. The results from the present study show that tar diminution in the product gas is dependent on temperature, catalyst material and oxygen potential. Typically, values of 50–75% tar reduction were achieved when varying the catalytic bed temperature between 750 and 850 °C. Also, the oxidation state of the catalyst material has an influence on the tar content and gas composition in the gas. When changing the gasification temperature from 800 °C to 850 °C the oxygen potential in the producer gas also changes, resulting in a transition from oxidative to reductive conditions in the gas. This implies that when the gasification temperature is 800 °C, the catalyst is transformed from its metallic state to the iron oxide, wustite. Consequently, the tar reduction capacity of the catalyst is reduced by approximately 20%. In view of the overall results it can be concluded that the catalysts in their metallic states in general exhibits a better tar cracking capacity than their corresponding oxides. The iron material used is sintered iron powders manufactured at Höganäs AB, Sweden. The iron materials were dispensed in the metallic state.