Biomass gasification in ABFB: Tar mitigation

Biomass gasification may be an attractive alternative for meeting future energy demand. Although gasification is a mature technology, it has yet to be fully commercialised due to tar formation. This study focuses on the tar mitigation in gas produced in an atmospheric bubbling fluidised bed (ABFB) gasification system. Previous studies indicated significant tar variability along the system. In this work the experimental procedure has been improved for reliable results and better understanding of tar variability in the producer gas. After having introduced a new sample point for tar analysis to the system, experimental results indicated tar reduction in the gasifier, probably due to continuous accumulation of char and ash in the bed, as well as in the ceramic filter owing to thermal and catalytic effects. Iron-based materials, provided by Höganäs AB, were applied in a secondary catalytic bed reactor for tar decomposition in the producer gas. It was found that tar concentration depends on catalytic and gasification temperatures and catalyst material. When changing the gasification temperature from 850 °C to 800 °C the conditions in the producer gas also changed from reductive to oxidative, transforming the initial metallic state of catalyst into its oxidised form. It may be concluded that the catalysts in their metallic states in general exhibit a better tar cracking capacity than their corresponding oxides. Due to the low reactivity of petroleum coke, an alternative may be to convert it in combination with other fuels such as biomass. Co-gasification of petroleum coke and biomass was studied in this work. Biomass ash in the blends was found to have a catalytic effect on the reactivity of petroleum coke during co-gasification. Furthermore, this synergetic effect between biomass and petcoke was observed in the kinetics data.