Computational modelling of the impact of particle size to the heat transfer coefficient between biomass particles and a fluidised bed

Abstract

The fluid–particle interaction and the impact of different heat transfer conditions on pyrolysis of biomass
inside a 150 g/h fluidised bed reactor are modelled. Two different size biomass particles (350 ?m and 550 ?m
in diameter) are injected into the fluidised bed. The different biomass particle sizes result in different heat
transfer conditions. This is due to the fact that the 350 ?m diameter particle is smaller than the sand particles
of the reactor (440 ?m), while the 550 ?m one is larger. The bed-to-particle heat transfer for both cases is
calculated according to the literature. Conductive heat transfer is assumed for the larger biomass particle
(550 ?m) inside the bed, while biomass–sand contacts for the smaller biomass particle (350 ?m) were
considered unimportant. The Eulerian approach is used to model the bubbling behaviour of the sand, which
is treated as a continuum. Biomass reaction kinetics is modelled according to the literature using a two-stage,
semi-global model which takes into account secondary reactions. The particle motion inside the reactor is
computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used
as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of
User Defined Function (UDF).

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