Experimental fact-finding in CFB biomass gasification for ECN's 500 kWth pilot plant

CFB biomass gasification has been studied by experimentation with ECN's pilot facility and a cold-flow model of this plant. Data obtained by normal operation of this plant and the results of some special experiments have provided new insight into the behavior of circulating fluidized bed reactors and CFB biomass gasifiers in particular. It has been found, for instance, that char gasification reactions and reform reactions of lower hydrocarbons do not proceed in an atmospheric CFB. Moreover, it appears that the axial and radial mixing of gas and solids is not ideal and that the coupling between the prevailing solids circulation rate, the gas velocity (profile), and the solids hold-up does not provide optimal reactor conditions. As a consequence, problems are observed in practice, such as insufficient thermal efficiency and an excessive concentration of contaminants (e.g., tars) in the product gas of CFB gasifiers. A two-dimensional engineering reactor model has been developed, partially on the basis of the results obtained from measurements with the pilot plant and the cold-flow model. The predictions of this model are in good agreement with the observed overall performance of the plant, as well as the measured axial and radial gas concentration profiles inside the riser. A novel multistage fluidized bed reactor concept has been tested (at laboratory and pilot scale) to investigate whether the problems encountered in conventional CFB gasifiers could be resolved. In this novel reactor, a separate char combustion zone is created that results in a significant increase of the thermal efficiency.