The role of two-stage combustion in the development of oscillations during fluidized 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 continuously supplied with the substrates, oscillations of state variables, especially the bed temperature and the degree of conversion of methane can take place. It was found that the process either oscillates, with different oscillation periods, or the system attains a steady state. These dynamic phenomena are represented in terms of time and phase plots. ical model has been proposed, treating the whole system as a cascade of two chemical reactors, in which conversion of the substrates into the products can take place. The bubbling, fluidized bed, containing most of the solids is the first, catalytic reactor. The freeboard, with few solid particles, 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 lead to the oscillations observed. 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 that the assumptions about the physical structure of the reacting system were basically correct.