Ž . Powder Technology 120 2001 49–54 www.elsevier.comrlocaterpowtec Natural gas combustion in fluidised bed reactors between 600 and 850 8C: experimental study and modelling of the freeboard S. Dounit, M. Hemati ) , D. Steinmetz Laboratoire de Genie Chimique, U.M.R 5503, E.N.S.I.G.C, 18 Chemin de la Loge, 31078, Toulouse Cedex, France ´ Abstract In this paper, an experimental study of the natural gas–air mixture combustion in a fluidised bed containing sand particles with Ž . 350-mm mean diameter and operating at temperatures lower than the critical temperature less than 850 8C 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 8C. 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. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Fluidised bed; Combustion; Natural gas; Modelling 1. Introduction Industrial fluidised bed furnaces operating with natural gas are used in certain specific applications such as foundry sand reconditioning, incineration of sludge with a high moisture content, cleaning of metallic parts and calcination of solid particles. Given the ecological benefit of using natural gas in fluidised bed furnaces, it appears of interest to describe the combustion process in fluidised beds by an experimental and a theoretical approach. In the previous work undertaken in our laboratory in collaboration with Gas de France, the mechanism of natural gas combustion wx in fluidised bed has been investigated 2 . The experimen- tal measurements of methane concentration in the solid project zone, as well as those of the gas temperature in the freeboard zone highlighted the existence of a critical bed temperature ranging between 750 and 800 8C, beyond which, the gas mixture burns inside the bed. At tempera- tures above 900 8C, methane conversion in the bed is total. Here, we note that combustion occurs very gradually over the entire height of the bed. Furthermore, a model based on the hypothesis of the bubble assemblage model was ) Corresponding author. Tel.: q 33-561-2523-41; fax: q 33-561-2523- 18. Ž . E-mail address: Mehrdji.Hemati@ensigct.fr M. Hemati . built. This model takes into account the heat exchange between phases, the variation in gaseous flow rate along the bed and the coupled conductive and radiative heat transfer between the bed and the reactor walls. This model coupled with a two-stage kinetic scheme of natural gas wx combustion proposed by Drayer and Glassman 7 gives a very good prediction of the axial profile of species concen- trations in dense bed at temperatures exceeding 850 8C. Below 850 8C, the major part of the reaction occurs in projection zone. Thus, it is necessary to include the phe- nomena taking place in this zone in the established model. In this paper, we present essentially the experimental results concerning the evolution of natural gas combustion process in dense fluidised bed and in the freeboard at Ž temperatures lower than the critical value less than 850 . 8C . The experimental data were used to compare the species concentration and the temperature profiles along the bed to the simulation results. The simulation model couples the phenomena occurring in the dense region and that taking place in the projection zone. 2. Description of the plant A sketch of the experimental setup used is given in Fig. 1. The reactor consists of a heat resistant steel pipe 180 mm in diameter and 1400 mm in height, above which, a 0032-5910r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0032-5910 01 00346-1