The mechanism of bed material coating in dual fluidized bed biomass steam gasification plants and its impact on plant optimization

The bed material and especially its catalytic activity plays an important role in biomass steam gasification in dual fluidized bed gasifiers. The bed material is modified by interaction with biomass ash during operation of the gasification plant forming layers at the particles which are induced by the biomass ash. Optimization of dual fluidized biomass steam gasification will have significant influence on the process variables such as temperatures, inorganic composition and product gas composition. The influence of these changes on layer formation is still unknown. This paper summarizes results of investigations about bed material characteristics taken from the industrial-scale biomass steam gasification plant in Güssing where woody biomass is used as fuel. Analyses of the surface and the crystal structures of the bed material particles treated in gasification and combustion atmospheres were carried out. The thermal behavior of used olivine and fresh olivine in different atmospheres was analyzed. A suggestion for the mechanism of formation of the layers is presented and the influence of possible optimization measures is discussed. A change in the elemental composition of the surface was not detectable but a slight change in the crystal structure. Thermal investigations show a weak endothermic weight loss with used olivine in a CO2-rich atmosphere which could not be determined with fresh olivine. The formation of layers at the olivine particles is considered to be caused by the intensive contact with burning char particles in the combustion reactor.