Fluidized Bed Combustion Primer

A fluidized bed consist of a bed of particles suspended above a grid (e.g., perforated plate) by a fluid (air) moving upward through holes in the grid. The velocity of the fluid must be sufficient to suspend the particles so that they move freely in the bed region. If the velocity is too low, the particles are immobile in a "fixed bed"; at velocities greater than the particle's terminal velocity, the particles are blown completely out of the bed, i.e., elutriated. In the fluidized bed regime, particles remain in the bed (hence good residence time) yet move about freely (hence good mixing). These characteristics, i.e., good mixing and long residence time, are the primary reasons for the numerous benefits of using fluidized beds.

Fluidized bed combustion (FBC) is the technique whereby a fluidized bed of particles is used as a medium for the combustion process. The bed particles themselves are not usually consumed in the combustion process but may play important roles in emissions control, for example, limestone particles are used to capture sulfur dioxide (forming calcium sulfate, i.e., anhydrite). The fuel particles typically comprise a very small fraction of the bed, e.g., less than 1% for burning coal. Good particle mixing results in nearly uniform bed temperatures. This together with high thermal inertia due to the considerable mass of particles in the bed allows for virtually isothermal reaction at the optimum operating temperature. This confers upon FBC a number of advantages over conventional combustion systems. Some of FBC's advantages can be summarized as follows.

Fuel particles are rapidly heated to stable combustion temperatures.

Almost any fuel can be handled, including nonhomogeneous fuels with low volatility or high moisture content.

The requirements for fuel preparation are minimal; washing, drying, and pulverization are generally unnecessary.

Judicious selection of bed material can result in the in-bed capture of gaseous pollutant species. Low combustion temperatures (typically 800-950°C) ensure that the formation of nitrogen oxides from air nitrogen is negligible.

A schematic diagram of the FBC process is shown in Figure 1. A blower supplies air to the windbox, which then discharges it through the distributor plate into the bed. The particle bed is traversed by heat exchanger tubes to provide for heat extraction from the bed. Above the particle bed a "freeboard" region is provided to allow particles ejected from the bed to settle back and to allow gas-phase reactions to proceed to greater completion. The cyclone separator removes all but the finest particles carried by the exhaust gases. The cyclone product can be either recycled to the bed for further reaction or removed from the system. The gases leaving the cyclone are passed through a fabric filter "baghouse" to remove fly ash.

The type of FBC unit shown in Figure 1 is generally referred to as a bubbling bed because there exists a distinct bed through which air in excess of that required for fluidization passes as "bubbles." Superficial fluidizing velocities are typically 1-4 m/sec. If the fluidizing velocity



Figure t Schematic diagram of FBC process.


Figure t Schematic diagram of FBC process.

is increased substantially, i.e., to 4-8 m/sec, the bed particles become entrained and are carried out of the combustor. These particles can, however, be captured by a "hot" cyclone and returned to the bed. A hot particle circulation loop is thus established that allows particle reactions to continue throughout the complete cycle. This type of system offers some advantages for capture of pollutant species and combustion of low-reactivity fuels due to the considerably increased residence time of fine particles in the bed. The action of this system is referred to as circulating fluidized bed combustion (CFBC).

This section is included only as a basic introduction to FBC. The literature on FBC is quite voluminous, a recommended source being the American Society of Mechanical Engineers' series of conferences, the most recent of which is the 12th International Conference on FBC held in San Diego in May 1993.

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