(270c) CFB Flow Regimes with Application to Fluidized Bed Combustion

Based on more than 25 years of fluidized bed (FB) research on mixing and fluid-dynamics with application to FBs in fuel conversion, we give an overview of the characteristics of gas and solids flow in fluidized beds for combustion and gasification. In particular, we discuss the flow and mixing characteristics of the four main regions identified in FB units for fuel conversion, with focus on CFB furnaces; the dense bed, the splash zone, the transport zone and the exit zone. We discuss the main characteristics of these zones and present recent results on how they relate to the net solids flux (G_s), i.e. the externally recirculated solids flux, which is a key parameter for CFB boilers and other dual bed systems, such as applied in chemical looping combustion for CO₂ capture and indirect biomass gasification. We draw our conclusions based on measurements and modeling of cold models as well as large commercial FB boilers and gasifiers.

CFB furnace characteristics

The furnace of fluidized beds for fuel conversion - combustion and gasification â??typically have the following characteristics:

• Aspect ratio of the furnace (H₀/D_eq) of the order of or less than 10;

• Aspect ratio of the settled bed, i.e., the bed formed if the solids are not fluidized, H_b,settled/D_eq of less than 1;

• Solids belonging to Group B in the Geldart classification; and

• Net solids flux that typically ranges from 0.5 to 20 kg/m²s (CFB boilers).

Especially when it comes to circulating fluidized beds (CFB), experimental data on fluid dynamics and mixing reported in the literature were often carried out for other chemical engineering applications than fuel conversion, such as cracking, coating and polymerization. Such data may therefore not be used to draw conclusions of relevance for fuel conversion systems. In contrast to the characteristics listed above, the latter systems typically employ Geldart Group A particles and operate at net solids fluxes often far exceeding 20 kg/m²s and for riser inventories which, if operating at low velocities, correspond to a tall bed operating in slugging regime - a regime which is not found in commercial-scale FBC boilers and gasifiers.

CFB furnace mixing and fluid-dynamics - an overview

In short, the fluid dynamics and mixing of the main fluid dynamic regions in a CFB furnace can be characterized as follows.

The dense bed: The dense bed is present provided there is a sufficiently high solids inventory and it is characterized by strong fluctuations in the gas flow due to the bubble passages. The dense bed can be maintained also when the superficial velocity exceeds the terminal velocity of a major part of (or all of) the bed solids (under CFB conditions). This is mainly due to that the primary gas passes through the dense bed in a heterogeneous way, partially in the form of large bubbles, which, at high velocities, create a shortcut of gas between the gas distributor and the region above the dense bed. Thus, this flow results in bubbles with oxidizing conditions and an emulsion phase under reducing conditions. Although the amplitude of the gas flow as induced by the bubble flow is drastically reduced above the dense bed, the gas flow fluctuations have important implications for the characteristics of the gas-solids flow in the freeboard.

The splash zone:  Above the dense bed, there is a so-called splash zone that consists of solids, which is characterized by strong back mixing mainly due to ballistic movement of clustered solids thrown up by the bubble eruptions.

The transport zone: Under CFB conditions, the gas velocity is sufficiently high to cause a significant portion of the solids to be entrained up through the furnace. Thus, above the splash zone, a transport zone is formed which is characterized by that most of the back-mixing occurs at the furnace walls. The transport zone flow forms a core-annulus structure, with the up-flow mainly in the core region and with a continuous back-mixing at the furnace walls, thereby forming downward flowing wall-layers of solids. Such continuous back-mixing represents a solids size segregation up through the transport zone, i.e. the coarser the solids the higher the probability that they are separated at the furnace walls. Therefore, there is a continuous decrease in average solids size with height in the freeboard of a CFB furnace.

The exit zone: Near the inlet to the cyclone, an exit zone is formed which is governed by gas and solids flow properties and the design of the fluidized bed furnace. A certain portion of the solids which reach the level of the inlet to the cyclone are internally recirculated due to back-flow effects. Thus, whereas the above described flow characteristics of the bed, the splash and the transport zones seem to only depend on furnace (riser) pressure drop and fluidization velocity given a certain set of gas and solid properties, the externally recirculated solids â?? the net solids flux (G_s) â?? depend on exit geometry and the configuration and operation of the cyclone side of the CFB loop (cyclone dipleg and particle seal).