The role of two-stage combustion in the development of oscillations duringfluidized bed combustion of gases

Unstable combustion of methane-air mixtures in a bubbling fluidized bed of catalytic material has been observed and analyzed. With the reactor continu ously supplied with the substrates, oscillations of state variables, especi ally the bed temperature and the degree of conversion of methane can take p lace. It was found that the process either oscillates, with different oscil lation periods, or the system attains a steady state. These dynamic phenome na are represented in terms of time and phase plots. A physical model has been proposed, treating the whole system as a cascade of two chemical reactors, in which conversion of the substrates into the pr oducts can take place. The bubbling, fluidized bed, containing most of the solids is the first, catalytic reactor. The freeboard, with few solid parti cles, constitutes the second reactor, with homogenous combustion dominant. Mass transport from the first to the second reactor and the heat transport from the second to the first reactor, make up a feedback system. This can l ead to the oscillations observed. Using this simple model, a mathematical description of the whole system has been developed. On the basis of the quantities measured, some of the model parameters were estimated and comparison between numerical calculation and experimental results have been carried out. This confirmed thar the assump tions about the physical structure of the reacting system were basically co rrect. (C) 2000 Elsevier Science Ltd. All rights reserved.