Mathematical modelling of fluctuations in fluidized bed combustion

Bubbling fluidized beds are characterised by the random action of bubbles passing through the bed. The effect of this randomness on the performance of bubbling fluidized bed combustors is investigated here, first through the analysis of pressure fluctuations, then the analysis of bed expansion and the division of gas flow, and finally the incorporation of these results into a two-phase reactor model with fluctuations.;The pressure signals were measured on two cold models with square cross-section, one 15 cm wide, the other 60 cm wide. The profiles of the standard deviations, the power spectra, ARMA models and cross-correlation functions are all used in the analysis. It is shown that the ARMA(2,2) model gives a good representation for the broadband power spectra characteristic of bubbling beds, and that the bed can be treated as a stochastic damped oscillator.;The bed expansion is determined from time average axial pressure profiles. A flow model with increased flow in the dense phase due to the presence of bubbles and expanded dense phase is shown to give a good prediction of the data and data from literature.;The flow model and the fluctuations are used to derive a stochastic two-phase reactor model. The model is steady-state with stochastic coefficients. The solution requires the numerical transformation of a random variable. Sample calculations on two simple models for carbon oxidation shows that the effect of the fluctuations in flow quantities on the gas concentrations is small.