In the present work, the fundamental mechanism between bed-to-membrane water-walls in the riser column of a circulating fluidized bed (CFB) combustor is presented. The bed-to-membrane water-wall heat transfer depends on the contributions of particle heat transfer, dispersed phase heat transfer and radiation heat transfer. The fundamental mechanism of particle heat transfer and the effect of fraction of wall exposed to clusters and gas gap thickness between cluster and wall on particle heat transfer coefficient and bed-to-wall heat transfer coefficient are investigated. The influence of operating parameters like cross-sectional average volumetric solids concentration and bed temperature on particle and bed-to-wall heat transfer are also reported. The present work contributes some fundamental information on particle heat transfer mechanism, which is responsible for increasing the bed-to-wall heat transfer coefficient (apart from dispersed phase convection and radiation heat transfer). The details on particle heat transfer mechanism will enable to understand the basic heat transfer phenomena between bed-to-membrane water-walls in circulating fluidized bed combustors in a detailed way, which in turn will aid for better design of CFB combustor units. The particle heat transfer mechanism is significantly influenced by the fraction of wall exposed to clusters and gas gap thickness between clusters and wall. Copyright (C) 2003 John Wiley Sons, Ltd.