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What are the main characteristics of

fluidised bed combustors?

Combustion File no.: 87 Version no.: 1 Date: 23-07-2001

Author: Jen Kovcs Referee: Peter RobertsSource:[1] Basu P. editor (1984): Fluidised Bed Boilers: design and application. Pergamon Press.[2] Technical Brief from Residua & Warmer Bulletin: Fluidised bed combustion.http://www.residua.com/wrftbfbc.html[3] Elliott T.C., Chen K. & Swanekamp R.C. (1998): Standard Handbook of Powerplant Engineering.

McGraw-Hill.

1. Background

In a fluidised-bed boiler, the fuel is fed into a solid bed, which has been fluidised, i.e.,lifted off a distribution plate by blowing air or gas through the plate. The amount of bedmaterial is very large in comparison to that of the fuel. The general principle offluidisation and the main fluidised bed combustion phenomena are detailed in separatecombustion files.

Fluidised bed combustors have a variety of advantages, including their simplicity ofconstruction, their flexibility in accepting solid, liquid or gaseous fuels (in combinationand with variable characteristics), and their high combustion efficiency at a remarkablylow temperature 750-950 oC which minimises thermal NOx generation and enhance theefficiency of SO2 absorption from the products of combustion. A major advantage offluidised bed combustion is the possibility of in-bed removal of SO2 using added

limestone or dolomite. The unburned carbon loss can be under 2 %. However, the ash ismixing with sorbent, limiting disposal options and reuse. The environmental aspects offluidised bed combustion are examined in more detail in a separate combustion file.

Fluidised bed units are eminently suitable for intermittent operation. The fluidised bed(FB) boilers provide good possibility to burn several different fuels in the same boiler:coal, peat together with biomass, waste, recycled/recovered fuel (REF) or refusederived fuel (RDF).

The combustion may take place under atmospheric or high pressure either in bubbling(BFB) or circulating fluidised bed (CFB) boiler, see Figure 1. FB boilers are wellcontrollable because of the fluid like bed and are reliable in operation. The well-mixed

bed resists rapid temperature changes, gives a large margin of safety in avoidingtemperature runaway with highly exothermic reactions. The heat exchange betweenthe bed and immersed heat exchanger is high, and a relatively small surface area forheat exchangers is required. FB boilers are suitable for large-scale operation, up to 600MWth in CFB and 300 MWth in BFB.

2. Bubbling Fluidised Bed Combustors

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The bubbling fluidised bed (BFB) is an excellent solids mixer, capable of ensuring ahomogeneous operating temperature and a good contact between the fuels and waste tobe fired, and the gas phase, although part of the gas tends to short-circuit the bedcontent. To minimise this undesirable feature it is possible to enhance the bed height,which, however, also increases the pressure drop over the bed.

In BFB, the bed expands to only one or two meters above the furnace floor. The bedtemperature is controlled by the bed area stoichiometry. Gasification takes place in thebed, while the final combustion is above the bed, both in nearly adiabatic conditions. The

fuel fuel

fuelfuel

ash ash

ashash

air air

airair

BFBCFB

PCFBPBFB

Figure 1: Bubbling fluidised bed (a), circulating fluidised bed b),pressurised bubbling bed (c) and pressurised circulating fluidised bed (d).

BFB is well suited for solid biomass and waste co-combustion. A BFB combustor, seeFigure 2a for an example, is composed of various parts. From the windbox the primarycombustion air is introduced into the bed, by means of a suitable distributor. The latterinfluences the quality of fluidisation and the circulation patterns of bed material. Thewindbox, or plenum chamber, generally consists of a refractory-lined chamber, in whichthe primary air is preheated with the aid of auxiliary burners. Preheating is important instarting-up a cold combustor and in improving the thermal efficiency of the unit. Thepreheat temperature is limited by the maximum operating temperature of thedistributor, which in turn is dictated by material selection.

The distributor has the dual function to support the bed and distribute the primary air.In its most conventional design it consists of a refractory arch, pierced with calibratedholes, or of a plate, made from refractory steel, and fitted with perforated airdistribution nozzles. The distributor may be covered by refractory material or a layerof coarse gravel to shield it from the temperature of the bed.

The freeboardregion is mainly used as a disengagement zone, in which material carried-over from the bed can settle and return to the bed. It also serves as a post-combustion

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chamber, in which secondary air is introduced to create turbulence and promote gasmixing. The height required to complete the combustion of volatiles is often larger thanthe disengagement height required for the settling and flow-back of entrained particles.

a) b)

Figure 2: Typical atmospheric BFB (a) and CFB (b) units: 1 limestone chute, 2 spreader feeder, 3 coal-limestone feeder, 4 air distributor, 5 primary air inlet, 6 secondary air nozzle, 7 fluidised air, 8 hot gas generator, 9 evaporator, 10 superheater, 11 economiser, 12 water wall, 13 circulator, 14 bed drain pipe.

3. Circulating Fluidised Bed Combustors

A circulating fluidised bed (CFB), see Figure 2b for an example, operates at velocities,which correspond to a regime of pneumatic transportation. The particles are collectedand recirculated, possibly after passing through a conventional bed, cooled by boilerinternals. Since they operate at high linear velocities circulating fluid bed units arecomparatively high, in order to allow for comparable residence times of the gas.

In CFB, the bed material circulation and the high turbulence in the combustion chamberensure good mixing and long residence time for fuel particles, providing good combustion

and emission control. The circulating bed acts as a heat carrier, stabilising the bedtemperature.

The high heat capacity of the bed allows burning high calorific value fuels withoutproblems in bottom bed temperature control, as well as burning high moisture content (