Modelling and experimental validation of a fluidized-bed reactor freeboard region: Application to natural gas combustion

A theoretical and experimental study of natural gas–air mixture combustion in a fluidized bed of sand particles is presented. The operating temperatures are lower than a critical temperature of 800 °C above which the combustion occurs in the vicinity of the fluidized bed. Our study focusses on the freeboard zone where most of the methane combustion takes place at such temperatures. Experimental results show the essential role of the projection zone in determining the global thermal efficiency of the reactor. The dense bed temperature, the fluidizing velocity and the mean particle diameter significantly affect the thermal behaviours.A model for natural gas–air mixture combustion in fluidized beds is proposed, counting for interactions between dense and dilute regions of the reactor [P. Pré, M. Hemati, B. Marchand, Study of natural gas combustion in fluidised beds: modelling and experimental validation, Chem. Eng. Sci. 53 (1998) (16), 2871] supplemented with the freeboard region modelling of Kunii–Levenspiel [D. Kunii, O. Levenspiel, Fluidized reactor models: 1. For bubbling beds of fines, intermediate and large particles. 2. For the lean phase: freeboard and fast fluidization, Ind. Eng. Chem. Res. 29 (1990) 1226–1234]. Thermal exchanges due to the convection between gas and particles, and due to the conduction and radiation phenomena between the gas-particle suspension and the reactor walls are counted. The kinetic scheme for the methane conversion is that proposed by Dryer and Glassman [F.L. Dryer, I. Glassman, High-temperature oxidation of CO and CH4, Proceedings of the 14th International Symposium on Combustion, The Combustion Institute, Pittsburg (1973) 987]. Model predictions are in good agreement with the measurements.