Bubbling fluidization coal combustion

Coal particle fragmentation and attrition during combustion has been extensively studied. For bubbling fluidized bed combustion, the attrition rate has been typically assumed proportional to the excess gas velocity above minimum fluidizing conditions, carbon... 

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

BFB [Bubbling Fluidized Bed] A coal combustion process for developing steam for power generation, developed by Foster Wheeler. It is particularly suitable for burning fuels of low heating value and high moisture and ash content. 

Figure 42 shows that the availability of calcium for desulfurization in coal combustion in a bubbling fluidized bed decreases with increasing particle diameter and is less than 20% for particles larger than 1 mm. On the other hand, fly ash collected from flue gas shows a calcium availability of not more than 10%, possibly due to its short residence time in the combustor. 

FBC offers a technology that can be designed to burn a variety of fuels efficiently and in an environmentally acceptable manner with various forms. In FBC units, coal is combusted in a hot bed of sorbent particles that are suspended by combustion air that is blown in from below through a series of nozzles. Depending on the gas velocity in the bed, fluidized bed can be classified into bubbling fluidized bed (BFB) and circnlating fluidized bed (CFB). The fluidized bed can be operated at atmospheric (AFB) and elevated pressure (PFB). 

Fluidized-bed combustion It accomplishes coal combustion by mixing the coal with a sorbent such as limestone or other bed material. The fuel and bed material mixture is fluidized during the combustion process to allow complete combustion and removal of sulfur gases. Atmospheric FBC (AFBC) systems may be bubbling (BFBC) or circulating (CFBC). Pressurized FBC (PFBC) is an emerging coal combustion technology. 

Almsedt AE. Measurements of bubble behavior in a pressurized fluidized bed burning coal, using capacitance probes—Part II. Proceedings of the 8th International Conference on Fluidized Bed Combustion. Houston, 1985, pp 865-877. 

Naruse I, Ohtake K, Koizumi K, Kuramoto K, Lu G. Gas exchange between the bubble and emulsion phases during bubbling fluidized bed coal combustion elucidated by conditional gas sampling. 25 S5miposium on Combustion. Comb Inst 1994, pp 545-552. 

Zhou HS, Flamant G, Gauthier D, Flitris Y Simulation of coal combustion in a bubbling fluidized bed by distinct element method, Chem Eng Res Des 81 1144—1149, 2003. 

Let us consider a shallow fluidized bed combustor with multiple coal feeders which are used to reduce the lateral concentration gradient of coal (11). For simplicity, let us assume that the bed can be divided into N similar cylinders of radius R, each with a single feed point in the center. The assumption allows us to use the symmetrical properties of a cylindrical coordinate system and thus greatly reduce the difficulty of computation. The model proposed is based on the two phase theory of fluidization. Both diffusion and reaction resistances in combustion are considered, and the particle size distribution of coal is taken into account also. The assumptions of the model are (a) The bed consists of two phases, namely, the bubble and emulsion phases. The voidage of emulsion phase remains constant and is equal to that at incipient fluidization, and the flow of gas through the bed in excess of minimum fluidization passes through the bed in the form of bubbles (12). (b) The emulsion phase is well mixed in the axial... 

---