May 22-27, 2016
Operational conditions, such as the fluidization velocity or the solids cross-flow, have been found to affect both lateral and axial mixing of fuel particles in a fluidized bed (1, 2). The fuel axial mixing has been shown to influence the heat and mass transfer between the bed and the fuel particles, which in turn affect the char conversion rate (3). Furthermore, the fuel lateral mixing affects the residence time of the fuel inside the fluidized bed reactor. The aim of this work is to investigate the effect of the operational conditions on the resulting char conversion in the gasification chamber of an indirect gasifier, while accounting for and assessing the role played by lateral and axial fuel mixing in the residence time and conversion kinetics, respectively.
A 1-dimensional model for indirect gasification (4) was used in the present study. The model includes empirical inputs regarding the influence of the operational parameters on the fuel mixing in the lateral and axial directions. The axial mixing of fuel undergoing pyrolysis is described by results from previous large-scale experiments (2), while that of char particles is characterized in laboratory-scale experiments under cold conditions carried out in this work. Regarding the kinetics, previous findings on the char gasification rate resulting from fuel conversion at different axial locations in the bed (3) were used as empirical input data to the modeling.
The experiments conducted in this work showed that the fluidization velocity significantly influences the axial mixing of char. Furthermore, results from the modeling showed that the fluidization velocity has a strong effect on the degree of char conversion in the gasification chamber: increasing the excess velocity from 0.05 m/s to 0.20 m/s resulted in a 10-fold decrease in the degree of char conversion. This is due to the decrease in the fuel residence time caused by the enhanced fuel lateral mixing. In comparison to this, the fuel axial mixing and its effect on the char conversion kinetics played a smaller role: disregarding the axial mixing of fuel resulted in char conversion degrees up to 1.2 times higher than those obtained when accounting for it.
- E. Sette, D. Pallarès and F. Johnsson. Influence of bulk solids cross-flow on lateral mixing of fuel in dual fluidized beds. Fuel Process. Technol., 140:245-251, 2015.
- E. Sette, T. Berdugo Vilches, D. Pallarès and F. Johnsson. Measuring fuel mixing under industrial fluidized bed conditions – a camera-probe based fuel tracking system. To be submitted, 2015.
- L. Lundberg, P.A. Tchoffor, D. Pallarès, R. Johansson, H. Thunman and K. Davidsson. Influence of Conversion Conditions on the Gasification Rate of Biomass Char in a Fluidised Bed. Submitted to Fuel Process. Technol., 2015.
- L. Lundberg, D. Pallarès, R. Johansson and H. Thunman. A 1-dimensional model of indirect biomass gasification in a dual fluidised bed system. 11th International Conference on Fluidized Bed Technology, CFB 2014. Beijing: Chemical Industry Press, 607-612, 2014.
Louise Lundberg, Antonio Soria-Verdugo, Robert Johansson, and David Pallarès, "The role of fuel mixing on char conversion in a dual fluidized bed gasifier" 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/126