The fluid - particle interaction inside a 41.7 mg s(-1) fluidised bed reactor is modelled. Three char particles of sizes 500 mu m, 250 mu m, and 100 mu m are injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. Due to the fluidising conditions and reactor design the char particles will either be entrained from the reactor or remain inside the bubbling bed. The particle size is the factor that differentiates the particle motion inside the reactor and their efficient entrainment out of it. A 3-Dimensional simulation has been performed with a completele revised momentum transport model for bubble three-phase flow according to the literature as an extension to the commercial finite volume code FLUENT 6.2. (C) 2009 Elsevier Ltd. All rights reserved.