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Wood Ash Database

– Wood for Energy -

Combustion

Fluidized bed combustorBabcock&Wilcox. Fluidized BedU.S Dept. of Energy. Fluidized Bed Technology - Overview

In FBCs (Fluidized bed combustors) solid fuels are suspended on upward-blowing jets of air, and as a result gases and liquids are turbulently mixed during the combustion process. This tumbling action enhances the chemical reactions and provides more effective heat transfer.

The technology of FBCs originated from the desire to find a process of combustion able to control pollutant emissions internally, rather than externally (e.g. with scrubbers). In FBCs, the fuel is burned at between 760 to 930 degrees Celsius. This temperature level has certain advantages, one being that it is well below the level where nitrogen and oxygen would otherwise combine to form nitrogen oxide pollutants (that would occur around 1370ºC).

FBC technology also reduces the amount of sulfur pollutants by bringing a sulfur-absorbing chemical, such as limestone or dolomite, into contact with the flue gases. Another advantage of FBCs is that they are greatly fuel flexible, and almost any combustible materials can be burned; from coal to municipal waste.

Because of the positive aspects of the fluidized bed technology, it is often seen as the preferable choice when building a new combustion plant. Also, companies have been known to upgrade their existing grate type boilers to either BFBs or CFBs.

BFB

BFBs (Bubbling Fluidized Bed) are first generation pressurized fluidized bed combustors (PFBCs). During combustion, a relatively stationary fluidized bed is established using low air velocities to fluidize the fuel and a heat exchanger generation steam from inside the bed. Cyclone separators are used to remove particles (fly ash) from the flue gas.

BFBs are regarded as being a fitting choice when using wet fuels and fuels of varying quality. It is imperative, however, that the fuel is added at a steady pace. Otherwise, uneven streams, gas- or temperature profiles might arise within the boiler. Eriksson, L., Ingman, R. Anvisningar för konvertering av rosterpannor till fluidiserad bäddteknik. Värmeforsk.

By having a well defined fuel bed and a less complex design, a BFB combustor is somewhat more similar to a grate boiler compared with a CFB combustor. In general, this means that an existing grate boiler is easier converted to a BFB combustor than to a CFB combustor

CFB

A CFB (Circulating Fluidized-Bed) combustor is part of the second generation PFBCs. In CFBs, higher air flow velocities are used to move the bed material. Furthermore, nearly all of the bed material is recirculated, thus the name "Circulating Fluidized-Bed".

CFBs are considered to have the highest flexibility regarding fuel alternatives. It is also regulatory faster than the bubbling FB, as there is less fuel stored in the fuel bed. Ibid.

CFB

The 12-MWth FBC boiler at Chalmers University of Technology: (1) combustion chamber; (2) fuel feed chute; (3) air plenum; (4) secondary air inlet at 2.1 m; (5) hot primary cyclone; (6) loop seal; (7) cyclone exit duct; (8) cold secondary cyclone; (9) bag house filter; (10) flue gas recirculation fan; (11) fuel bunkers; (12) sand bin; (13) sludge pump; (14) hydrated lime bin; (15) bed ash sampling hole; (16) measurement spot (before convection pass) IACM instrument, deposit probe and FTIR; (17) measurement spot (after convection pass) FTIR; and (18) measurement spot (stack) FTIR.Pettersson, A., Zevenhoven, M. et al. 2008. Application of chemical fractionation methods for characterisation of biofuels, waste derived fuels and CFB co-combustion fly ashes.