The basic principle of a fluidized bed combustor consists of distributing air homogeneously across a combustion area where a bed of fuel and inert mineral particles is fluidized, meaning that the air velocity achieves the mass suspension of the bed particles. This is accomplished by blowing air at relatively high pressure, (around 0.15 bar relative with respect to grate firing where five times lower pressures are considered) through a distribution plate on top of which the bed of fuel and inert is positioned. A fluidized bed burner is therefore a relatively high cylindrical or square sectioned vessel that hosts on the bottom part a perforated plate positioned to divide the furnace from a pressurized air-box.

The inert material, which usually represents up to 98% of the bed material, acts as a grate because its supports the fuel which floats in it while also contributing to abrasion of the fuel particles and also acting as a refractory by radiating heat and providing conduction thanks to the continuous turbulent collision with fuel particles. The high turbulence optimizes combustion hence low excess air is required (from 10 to 30%) and the mixing effect may be used to reduce emissions as formed by injecting reactants such as urea for NOX control and limestone for acid gases abatement, directly into the bed.

The plate and holes diameter and geometry is of primary importance to achieve an efficient air distribution which will prevent short-circuiting through a portion of the bed (channeling) and avoid the ejection of the bed (plugging) for missed fluidization. Temperature control is also particularly critical to avoid ash melting and slagging within the bed which would cause the inert particles to agglomerate and the bed to collapse. Typical combustion temperatures therefore will be kept in the 650-900°C range (Obernberger, 1998).

During startup of a fluidized bed combustor air velocity increases until it reaches the terminal velocity where the aerodynamic drag force on the particles equals its buoyancy and weight. The particles within the agglomerated bed start to disengage and the bed swells, increasing its height within the reactor, accordingly the pressure drop across the bed increases until a maximum value is achieved, corresponding to the maximum bed height. Further increases in the air velocity (within a certain range) from this value, named minimum fluidization velocity, will not modify significantly either the bed height or the pressure drop across it.

A fluidized bed working slightly above the minimum fluidization velocity is called a stationary fluidized bed or Bubbling Fluidized Bed (BFB, Fig. 5.20) because the air flowing in the bed forms bubbles that are dragged by the air flow and create typical movements and ruptures of the bed surface, when they reach the top layer, that resemble a boiling liquid. The bed material is usually silica sand of about 0.5-1.0mm in diameter; the fluidization velocity of the air varies between

1.0 and 2.0 m/s. The secondary air is introduced through several inlets in the form of groups of horizontally arranged nozzles at the beginning of the upper part of the furnace (called the freeboard). BFB furnaces are usually considered for plants with a nominal boiler capacity of over 10-20 MWt.

If the air velocity is further increased up to 5-10 m/s, and smaller particles of sand are used (0.2-0.4 mm in diameter), a point is reached when pressure drops across the bed plunge dramati­cally and pneumatic transport of bed particles is achieved. This working condition is characterized by mass extraction from the bed which must be recuperated from the flue gases, through solid separation devices such as cyclones, and continuously returned to the bed. A burner working in this manner is called Circulating Fluidized Bed (CFB, Fig. 5.21).

Figure 5.20. Schematic of a Bubbling Fluidized Bed (BFB).

The sand particles are carried with the flue gas, separated in a hot cyclone and fed back into the combustion chamber. The bed temperature normally is comprised between 800-900°C and it is controlled through an internal heat exchanger. The fuel amounts only to 1-2% of the bed material and the bed has to be heated before the fuel is introduced.