Increase in particle size occurs during several processes that use fluidized beds. While it is desirable in some processes such as granulation and pellet-making, its occurrence causes undesirable deposition problems in combustion and gasification processes. Particle size increase or agglomeration occurs when particles that are covered by a binder-liquid stick on colliding with one another. The binder may be externally added or can form during phase transformations of the particles depending on the operating temperature and/or chemical composition of the particles. The study of particle growth or deposit formation involves a detailed understanding of both the thermodynamics of phases that lead to the liquid binder formation or the binder-liquid chemistry, and also the fluid dynamics of particles in the system. As particle growth progresses and the particle size distribution changes, the bed hydrodynamics such as particle collision frequencies also change continuously. This progression makes it challenging to predict the kinetics of particle growth, since the chemistry-, and physics-based parameters are interlinked.
Existing models consider either of these two aspects, while predicting the increase in particle size. Models that can incorporate interdependencies and variations in both bed hydrodynamics and binder chemistry are required for more accurate predictions. Understanding the kinetics of growth in particle size will help in the granulation, drying, catalytic cracking industries. This knowledge is also critical to avoid deposition problems in the petroleum, fluidized bed gasification and combustion industries.
In fluidized bed gasification and combustion systems, agglomeration occurs by sticking of fuel ash particles that are wetted by slag-liquid. The non-uniform temperature conditions and heterogeneity in ash chemical composition need to be accounted for in the prediction of the slag-liquid binder chemistry. Additionally, the polydispersity of bed ash, particle velocities and collision frequencies are other important physics-based factors that affect particle growth rate. This study attempted to develop a model that combines the effect of ash chemistry and changing bed hydrodynamics to accurately predict agglomeration in fluidized bed gasification and combustion systems.
Penn State has developed a unique two-particle collision based model that combines the two effects mentioned above. The model is being applied to polydispersed fluidized bed combustion and gasification systems. Every two-particle collision in the system is tested for sticking using the Stokes’ criterion. According to the Stokes’ criterion, the particles will remain stuck after collision, if the viscous dissipation due to the presence of binder, exceeds the kinetic energy of particles. The extent of viscous dissipation depends upon the chemistry of the slag-liquid formed, while the particle kinetic energy depends on the bed fluid dynamics. In the Penn State model, thermodynamic equilibrium calculations are used to estimate the amounts of slag-liquid in the system, while the evolution of particle collision frequencies is accounted for by tracking the number density of differently sized particles. Computational fluid dynamics modeling is used in conjunction to obtain initial inputs to the model. The model helped to gain an understanding of low-temperature ash agglomeration in these systems.
In addition to combining the effects of the chemistry and physics based parameters, it is also important to include the effects of particle-level non-uniformities in chemistry that affect the slag-liquid properties. In this study, the heterogeneities in chemical composition of fuel ash were studied by separating the bulk fuel into particle classes that are rich in specific minerals. The mineral matter behavior of these constituent classes was studied. Each particle class undergoes distinct transformations of mineral matter under fluidized bed operating temperatures, as determined by using High temperature X-ray diffraction, Thermo-mechanical analysis, Scanning electron microscopy with Energy dispersive X-ray spectroscopy (SEM-EDX) and Thermogravimetric analysis. The heaviest gravity fraction (>2.6 g/cc) indicated the occurrence of ternary and multi-phase eutectics involving CaO, Fe2O3, SiO2 and Al2O3 phases and a low slag onset temperature of 850 oC at equilibrium. Thus, significant slag-liquid levels were observed to form around certain particles even at fluidized bed combustor operating temperatures. Even at those relatively low temperatures, initiation of ash agglomeration is found to occur around such particles that tend to become sticky. This novel approach provides a method to account for the heterogeneous nature of ash in the development of models for accurate prediction of fluidized bed agglomeration.
Using the Penn State Ash agglomeration model, attempts are made to predict the operating conditions (in terms of both ash chemistry and particle physics) under which the occurrence of ash agglomeration is more likely to occur in fluidized bed gasification and combustion systems. This model can further be extended to predict the rate of particle size growth and deposition issues in allied industries.
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