Fast fluidization characteristics

CO2 capture characteristics of dry sorbents in a fast fluidized reactor... 

One of the advanced concepts for capturing CO2 is an absorption process that utilizes dry regenerable sorbents. Pure sodium bicarbonate from Dongyang Chemical Company and spray-dried sorbents were used to examine the characteristics of CO2 reaction in a flue gas environment. The chemical characteristics were investigated in a fast fluidized reactor of 0.025 m i.d., and the effects of several variables on sorbent activity, including gas velocity (1.5 to 3.5 m/s), temperature (40 to 70 °C), and solid concentration (15 to 25 kg/m /s)], were examined in a fast fluidized-bed. Spray-dried Sorb NX30 showed fast kinetics in the fluidized reactor. 

The fluidized reactor can be a bubbling fluidized type or a fast fluidized type, depending on the gas velocity and the reactivity of the sorbents. We adopted a fast fluidized bed type reactor for carbonation and regeneration reactions in order to identify the chemical characteristics of sorbents in a fast fluidized reactor of 0.025 m i.d. 

General Characteristics. Energy addition or extraction from fast fluidized beds are commonly accomplished through vertical heat transfer surfaces in the form of membrane walls or submerged vertical tubes. Horizontal tubes or tube bundles are almost never used due to concern with... 

Figure 16, General heat transfer characteristics of fast fluidized bed. (Front Kiartg el at., 1976.)...

As noted in 10.3.1, the transport velocity, Uti, marks the lower limit of the gas velocity for fast fluidization operation. The characteristics and the prediction of Uti are given in the following discussion. 

Another operational limit in the CFB system involves gas suppliers. Three types of gas suppliers, i.e., a reciprocating compressor, a blower with throttle valve, and a compressor, are commonly used in the CFB system. For blower operation, as the gas flow rate decreases, the pressure head of the blower increases. For compressor operation, the pressure head of the compressor can be maintained constant with variable gas flow rates. The interactive behavior between a CFB system and a blower can be illustrated in Fig. 10.9, where dashed curves refer to the blower characteristics and solid curves refer to the riser pressure drop. At point A, the pressure drop across the riser matches the pressure head provided by a blower thus, a stable operation can be established. Since the pressure drop across the riser in fast fluidization increases with a decrease in the gas flow rate at a given solids circulation rate, a reduction in the gas flow rate causes the pressure drop to move upward on the curve in the figure to point B with an increase in the pressure drop of Spr. In the case shown in Fig. 10.9(a), with the same reduction in the gas flow rate, i.e., SQ, the increase in the pressure drop, Spr, from point A to point B is greater than that which can be provided by... 

Fast fluidization is a regime intermediate between bubbling fluidization and pneumatic transport, possessing many of the advantages of both, but divorced from many of their disadvantages. It has the following unique characteristics ... 

Dry, R. J. Radial concentration profiles in a fast fluidized bed, Powder Technol. 49,37-44(1987). Dry. R. J., and White, C. C. Gas residence time characteristics in a high-velocity circulating fluidized bed of FCC catalyst, Powder Techn. 58, 17 (1989). 

To describe the complex phase structures in fast fluidization as shown in Fig. 1, a system of parameters is shown in Table I. These parameters can be grouped into three types independent, dependent and characteristic, as will be described in this section. 

To account for the intrinsic characteristics of particle-fluid two-phase flow in fast fluidization, the particles and the fluid are considered to interact with each other on both a micro-scale and meso-scale level to produce local or meso-scale heterogeneity (phases), and the overall fluid-particle system interacts with the equipment boundaries on a much larger scale to produce macro-scale heterogeneity (regions). 

Although many processes are reported for biomass gasification, BCL (Battelle-Columbus Laboratory) Process is applied for this analysis [5]. According to the report, the system is fast-fluidized bed and indirectly heated. The temperature is 863 C and the operating pressure is 0.101 MPa. As for product gas characteristics, the yield of (kmol/ton-dry feed) is 45.8, and HHV (MJ/Nm3 raw gas) is 15.19. Cold gas efficiency is 80.1%. 

Xia et al. (1992) applied this signal analysis method to study the oscillatory behavior of light output signals in a fast fluidized bed. Figure 4-23 shows the typical power spectral density of optic output signals in the fast fluidized bed. The oscillatory behavior of the optic output signals has no characteristic time scale, or a deterministic frequency response, but forms fractal time characteristics. 

Zheng CG, Tung YK, Li HZ, Kwauk M. Characteristics of fast fluidized beds with internals. In Potter OE, Nicklin DJ, eds. Fluidization VII. Engineering Foxmdation, New York, 1992, pp 275-284. 

Another hydrodynamic complication found in fast fluidized beds is the tendency for particles to aggregate into strands or clusters, as reported by Horio et al. (1988) and Chen (1996). The concentration of solid particles in such clusters is significantly greater than in the bed itself, and it increases with increasing radial position and with increasing total solid flux (see Soong et al., 1993). This characteristic also directly affects heat transfer at walls. 

The above is a very condensed discussion of the hydrodynamic characteristics of fast fluidized beds. As presented below, heat transfer is strongly dependent on the time-averaged local concentration of solid particles and is therefore influenced by these hydrodynamic characteristics. Almost all the heat transfer models require information on solid concentration. The reader is referred to Chapter 19 of this book for more detailed discussion of the hydrodynamics, and... 

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