Development of stochastic model of particulate coal fluidized bed expansion and axial structure

Abstract

Particulate solids are in the state of fluidization at many stages of preparation and treatment of solid fuels. An effective drag force coefficient Cd is used to describe a mechanical contact between gas stream and an individual particle. The evaluation of the effective drag force coefficient is not limited by the force of hydraulic resistance but also includes a set of different forces. This set of forces is rather indeterminate, and a lot of empirical equations for effective drag force coefficient calculations can be found in the scientific papers. Choosing the applicable formula for calculation is often difficult. In addition, it requires taking into account the flow patterns around an individual particle. Thus, a comparative study to examine the most well-known drag force models with a uniform approach to account the flow patterns around an individual particle is important. The Markov chain approach is used as a mathematical basis for modeling of the flow patterns in a fluidized bed. The identification of the model parameters is completed and the complementation of transition matrices with the current physical properties of substances involved into the flow makes the model non-linear. The comparative study of the results obtained with using of the two correlations for drag force coefficient is performed. The stochastic model of fluidized bed expansion and axial structure has been proposed. The comparative analysis of two different scenarios of fluidized bed expansion using different drag force models has been performed. The authors developed and tested the model to describe fluidized bed expansion and axial structure on the basis of the Markov chain approach. The low certainty of physical drag coefficients models under conditions of flow patterns around an individual particle has been shown. The conducted research proves that consistent description of structure inhomogeneity of fluidized bed is possible using the nonlinear mathematical models based on mesoscale level of the object decomposition. Predictive efficiency of similar models is limited by low reliability of formula for calculation of drag force coefficient. Thus, it is possible to state the importance of further comparative research to check different formulas for calculation of gas-particle drag force coefficient in order to provide a reliable forecast of the fluidized bed height.