May 22-27, 2016
The purpose of the present study is to evaluate the possibilities to estimate the fluctuation characteristics of gas concentrations in a CFB from the characteristics of velocity and voidage fluctuations. In CFB combustion, the local concentrations of reactant gases strongly fluctuate which affects the reaction rates. In CFD modeling, the effect of turbulent mixing on reaction rates is commonly taken into account by means of mixing descriptions based on fluctuation time scales. For fast reactions the mixing rate can be the rate limiting step. Similar to gas phase reactions, for heterogeneous combustion of solid fuel, fluctuations in gas concentrations should also be taken into account.
One estimate for gas species mixing is obtained by analyzing velocity fluctuations on the basis of which mixing time scales can be determined (1). This approach does not account for the actual correlation between velocity and concentration fluctuations. In a combustion chamber, air enters from riser bottom while combustible gases are released in dense suspension from fresh fuel. There are regions of high oxygen concentration and regions with high concentration of volatiles. The paper shows by means of a simplified CFD simulation how the characteristics of the concentration fluctuations are related to the fluctuations in velocities and voidage. In the simulation, air enters from riser bottom and CO that reacts with oxygen is added to dense suspension. The time-averaged fluctuation characteristics show a similarity to velocity fluctuations and that the effect of the uneven distribution of gas components should be accounted for in the modeling.
1. J. Peltola, S. Kallio, Estimation of turbulent diffusion coefficients in a CFB on basis of transient CFD simulations, Proceedings (Vol. 2) of 21st International Conference on Fluidized Bed Combustion, 2012, pp. 595 – 602
Timo Niemi, Sirpa Kallio, and Juho Peltola, "Analysis of fluctuations in velocities, voidage and gas concentration in cfb conditions" in "Fluidization XV", Jamal Chaouki, Ecole Polytechnique de Montreal, Canada Franco Berruti, Wewstern University, Canada Xiaotao Bi, UBC, Canada Ray Cocco, PSRI Inc. USA Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/fluidization_xv/14