Recirculation of reactive fines – an optimization strategy for existing dual fluidized bed gasification systems

In industrial dual fluidized bed gasifiers, flue gas and producer gas ashes are generally re-injected in the system with the aim of recovering bed material and unconverted carbon. The impact of such recirculation on the gas quality has however not been investigated before. Some of the ash components found in these fly ashes are known to positively affect the gas composition, char gasification and tar content of the producer gas and, as a consequence, their recirculation is expected to impact these parameters. In order to investigate these effects, recirculation of coarse fly ashes was carried out in the Chalmers 2-4MWth gasifier, by first artificially creating a substantial amount of fly ashes by injection of fine particles of untreated olivine. The fly ashes collected were recirculated at several occasion and the gas quality assessed. It was found that recirculation of reactive fly ashes enhances the gas quality, in particular in term of tar concentration. This improvement was also observed to be lasting, suggesting change in the bed material composition. Very high bed activity was reached after two days during which only the combustor was functioning, and this despite an extensive regeneration of the bed