Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 263

A bubbling fluidized bed (BFB) consists of fine, inert particles of sand or alumina, which are selected based on their suitability of physical properties such as size, density, and thermal characteristics. The gas flow rate is chosen to maintain the bed in a fluidization condition, which enters at the bottom of the vessel.

The dimension of the bed at some height above the distributor plate is increased to reduce the superficial gas velocity below the fluidization velocity to maintain inventory of solids and to act as a disengaging zone. A cyclone is used to trap the smaller size particle that exit the fluidized bed, either to return fines to the bed or to remove ash rich fines from the system.

Biomass is introduced either through a feed chute to the top of the bed or deep inside the bed. The deeper introduction of biomass in to the bed of inert solids provides sufficient residence time for fines that would otherwise be entrained in the fluidizing gas. The biomass organics pyrolytically vaporize and are partially combusted in the bed. The exothermic combustion provides the heat to maintain the bed at temperature and to volatilize additional biomass.

The bed needs to be preheated to the startup temperature using hydrocarbon resources such as natural gas, fuel oil, either by direct firing or by indirect heating. After the bed reaches the biomass ignition temperature, biomass is slowly introduced into the bed to raise the bed temperature to the desired operating temperature which is normally in the range of 700 to 900°C (1,290 to 1,650°F). Bed temperature is governed by the desire to obtain complete devolatilization versus the need to maintain the bed temperature below the biomass ash fusion temperature. The advantages of the fluidized bed gasifiers are (i) yield of a uniform product gas, (ii) able to accept wide range of fuel particles sizes, including fines, (iii) a near-uniform temperature distribution throughout the reactor, and (iv) a high rate of heat transfer between inert material and biomass, aiding high conversion, with low tar. The disadvantage is formation of large bubbles at higher gas velocities, which bypass the bed reducing the high rate of heat /mass transfer significantly.

See also: Biomass – Gasification.