Effect of ash circulation on the performance of a dual fluidized bed gasification system

During gasification of biomass, ash forming elements are released from the fuel and some of these elements can have a positive impact on the quality of the gas produced. In a dual fluidized bed (DFB) gasifier, a significant amount of these components are found in the fly ash from the gasification and combustion reactors. In order to increase carbon conversion and bed material recovery, these streams are generally circulated back to the combustor for the raw gas fly ash and in some cases to the gasifier for the flue gas fly ash. The impact on the gasification performance has, however, not been investigated. Circulation of flue gas coarse ash was carried out in the Chalmers gasifier, with the aim of assessing the impact on gas quality, in particular in term of tar yields, and how it relates to the flow and properties of ash streams. The coarse ash was first enhanced by an injection of untreated olivine in a fine particle size and was then recirculated, yielding a direct decrease in tar concentration. This effect persisted after the recirculation and the bed activity was seen to increase with time, at a higher rate than a reference aging experiment. Both the internal bed material cycle and the external fly ash loop were found to get enriched in ash components, which was linked to the activity gains observed. These results show the potential of continuous fly ash recirculation as an activity enhancer in industrial dual fluidized bed gasification systems.