Hydrodynamics of a Cold Model of a Dual Fluidized Bed Gasification Plant
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Biomass energy is increasingly used to reduce the dependence on fossil fuels and reduce the impact of greenhouse gas emissions on global warming. Fluidized bed gasification converts solid biomass into gaseous fuels that can be used for combustion or liquid fuels synthesis. The efficiency of biomass gasification is directly affected by the fluidized bed hydrodynamics. For example, the solids recirculation rate through the system is an important parameter that affects the heat and mass transfer rates. In this study, a cold model of a dual fluidized bed (DFB) biomass gasification plant was designed using scaling laws, and was constructed to investigate the hydrodynamics of industrial DFBs. A DFB consists of a bubbling fluidized bed (BFB), where biomass is gasified to produce syngas, and a circulating fluidized bed (CFB) where the residues of gasification are combusted. The investigation was divided into Phase I and II. In Phase I, an operational map was developed for the CFB to define operational boundaries for steady state operation of the plant. An empirical model was developed to predict the solids mass flow rate out of the CFB riser, which is an empirical function of the exit opening width, the CFB diameter, and a newly introduced aerodynamic factor. The correlation coefficient, R2 for the empirical function was 0.8327. The aerodynamic factor accounts for the particle inertia and clustering effects at the exit of the CFB riser. Results from Phase I also showed that increasing the fluidizing velocities increased the solids circulation rate and affected the pressure drop over various points in the CFB plant due to redistribution of solids with the system. A critical assessment was performed on published correlations found in the literature to determine how accurately they predicted the hydrodynamics in the CFB riser. By comparing predicted and experimental results, the correlations were found to be inaccurate for the conditions and configuration of the CFB tested in this study. For example, the solids velocity was not accurately predicted by published correlations due to unaccounted particle clustering effects. The main issue with the published correlations was a lack of generality, so that the correlations only applied for predicting fluidizing behaviour in the equipment they were developed in. In Phase II, an operational map was developed for the DFB, which incorporated both the CFB and the BFB. Experiments with a binary mixture representing sand and char in an industrial gasifier showed a blocking effect in the connecting chute between the CFB and BFB by the material representing char, which was larger and less dense than the material representing sand. A computational fluid dynamics (CFD) based design tool for modelling the cold model CFB cyclone was developed and validated by comparing the predicted and experimental cyclone pressure drop. The correlation coefficient for the CFD pressure drop prediction was 0.7755. The design tool contained information about the grid resolution and the time step required for modelling the cyclone accurately.