Modelling and optimization of fuel conversion in an indirect bubbling fluidized bed gasifier
Examensarbete för masterexamen
The aim of this thesis was to improve the fuel conversion in Chalmers bubbling fluidized bed (BFB) gasifier. A mathematical model has been developed and experiments in a fluid-dynamically scaled reactor have been conducted. To increase fuel conversion the use of horizontal tubes and partitioning walls was studied. The mathematical model is 2-dimensional and accounts for fluid dynamics, heat transfer and fuel conversion. The model was also used for determining the dispersion coefficient, D, and velocity slip factor, α, from experimental data. Lateral bed material mixing was evaluated through an indirect method by tracing iron powder in the bed material. The lateral fuel mixing was evaluated through a direct method using digital image analysis. A new indirect method to investigate the fuel lateral fuel dispersion and the convective bed material flow influence was developed and evaluated. The method was easy, quick and robust, thus showing great potential. The resulting up-scaled values of dispersion coefficients for the bed material were in a range of 2.44 - 4.77·10-3 and 2.15 - 6.17·10-3 m2/s for fuel particles. The dispersion coefficients at tube banks for bed material and fuel particles were 1.08 - 1.34·10-3 and. 1.07 - 1.79·10-3 m2/s respectively. This shows that tube banks reduced the lateral dispersion of bed material and fuel by around 70% whereas the partitioning walls had little impact on the modelled char conversion. Tube banks also reduced the influence of the convective flow of bed material on fuel, α, with 40% to 100% thereby increasing the residence time of the fuel. Through simulations of the tuned model for different arrangements of internals, it was found that the residence time of the fuel is the main parameter for improved fuel conversion. The modelled char conversion was increased from 6% for a bed without internals to 22% when two thirds of the beds were covered with a properly placed tube bank.
Energi , Hållbar utveckling , Energiteknik , Energy , Sustainable Development , Energy Engineering