Defluidisation of fluidised beds during gasification of biomass

Defluidisation and agglomeration during fluidised bed gasification of biomass is analysed and discussed. It is argued that the agglomeration and defluidisation processes, in principle, closely resemble those that determine the behaviour of glass during glass processing. Crucial properties for working with glass melts are the viscosity, stickiness, surface tension, etc. It is, however, (very) difficult to theoretically quantify these properties through thermodynamics or other theoretical means. Hence it will be problematic to theoretically predict agglomeration and defluidisation. Models for predicting defluidisation must therefore probably be of an empirical nature. As a consequence of this, a number of fluidised bed gasification tests were empirically analysed with respect to defluidisation. In total 145 tests were evaluated; of these 51 defluidised or exhibited some kind of bed disturbance. A number of fuels and bed materials were included in the analysis using a multivariate statistical approach.Based on the analysis an empirical regression equation for predicting the defluidisation temperature during fluidised bed gasification is suggested.

► An empirical regression equation for predicting the defluidisation temperature is suggested.
► Alkali and pressure lowers the defluidisation temperatures, whilst Ca and Mg has the opposite effect.
► Magnesite as bed material relative to quartz or olivine sand increase defluidisation temperatures with more than 100 °C.