In this paper, an experimental study of the natural gas-air mixture combustion in a fluidised bed containing sand particles with 350-mum mean diameter and operating at temperatures lower than the critical temperature (less than 850 degreesC) has been presented. A particular attention has been given to the freeboard zone where the main part of the reaction rate takes place at such temperatures. The experimental results obtained at different operating conditions has shown the essential role played by the projection zone in the global thermal efficiency of the reactor working between 600 and 800 degreesC. Parallel to this experimental study, a model of natural gas-air combustion taking in consideration the interaction between dense and dilute regions of the reactor has been proposed. The proposed model takes into account thermal exchange by conduction and radiation in both between gaseous and particular phases, and gas-particle suspension and reactor walls. The predictions of this model agree well with the experimental data.