Dual fluidized bed steam gasification: Change of product gas quality along the reactor height

The impact of the counter-current column of the gasification reactor of a 100 kWth dual fluidized bed
steam gasification pilot plant on the product gas quality was investigated. Through the advanced design
of the gasification reactor by operating the lower part as bubbling bed and the upper part as countercurrent
column, the gas-solid interactions between downward flowing hot bed material particles with
upwards flowing product gas could be enhanced. This was realized by equipping the counter-current
column with constrictions, which increase the residence time and the bed material hold-up. Thus, the
conversion efficiency of the fuel including the tar was improved. For the investigations three different
experimental campaigns converting softwood pellets using a mixture of olivine and limestone (50/
50 wt.-%), a mixture of feldspar and limestone (50/50 wt.-%), and 100 wt.-% quartz as bed materials were
conducted. Higher H2 contents and lower contents of higher hydrocarbons could be detected along the
height of the counter-current column. Especially heavy tar compounds could be reduced significantly.
These two effects are explained by enhanced water gas shift and steam reforming reactions. In case of
catalytically inactive quartz, only thermal effects are available and therefore lower effects on tar
reduction could be obtained.