1. Background

In a fluidised-bed boiler, the [GLOSS]fuel[/GLOSS] 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 bed material is very large in comparison to that of the fuel. The general principle of [GLOSS]fluidisation[/GLOSS] and the main fluidised bed combustion phenomena are detailed in separate combustion files.

[GLOSS]Fluidised bed combustors[/GLOSS] have a variety of advantages, including their simplicity of construction, their flexibility in accepting solid, liquid or gaseous fuels (in combination and with variable characteristics), and their high [GLOSS]combustion efficiency[/GLOSS] at a remarkably low temperature 750-950°C which minimises thermal NOx generation and enhance the efficiency of SO2 absorption from the products of combustion. A major advantage of fluidised 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 is mixing with sorbent, limiting disposal options and reuse. The environmental aspects of fluidised 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 [GLOSS]biomass[/GLOSS], [GLOSS]waste[/GLOSS], recycled/recovered fuel ([GLOSS]REF[/GLOSS]) or refuse derived fuel ([GLOSS]RDF[/GLOSS]).

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 well controllable 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 avoiding temperature “runaway” with highly exothermic reactions. The heat exchange between the bed and immersed heat exchanger is high, and a relatively small surface area for heat exchangers is required. FB boilers are suitable for large-scale operation, up to 600 MWth in CFB and 300 MWth in BFB.

2. Bubbling Fluidised Bed Combustors

The bubbling fluidised bed (BFB) is an excellent solids mixer, capable of ensuring a homogeneous operating temperature and a good contact between the fuels and waste to be fired, and the gas phase, although part of the gas tends to short-circuit the bed content. 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 bed temperature is controlled by the bed area stoichiometry. Gasification takes place in the bed, while the final combustion is above the bed, both in nearly adiabatic conditions.

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

The BFB is well suited for solid biomass and waste co-combustion. A BFB combustor, see Figure 2a for an example, is composed of various parts. From the windbox the primary combustion air is introduced into the bed, by means of a suitable distributor. The latter influences the quality of fluidisation and the circulation patterns of bed material. The windbox, or plenum chamber, generally consists of a refractory-lined chamber, in which the primary air is preheated with the aid of auxiliary burners. Preheating is important in starting-up a cold combustor and in improving the [GLOSS]thermal efficiency[/GLOSS] of the unit. The preheat temperature is limited by the maximum operating temperature of the distributor, 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 calibrated holes, or of a plate, made from refractory steel, and fitted with perforated air distribution nozzles. The distributor may be covered by refractory material or a layer of coarse gravel to shield it from the temperature of the bed.

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

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 [GLOSS]pneumatic transportation[/GLOSS]. The particles are collected and recirculated, possibly after passing through a conventional bed, cooled by boiler internals. Since they operate at high linear velocities circulating fluid bed units are comparatively 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 chamber ensure 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 bed temperature.

The high heat capacity of the bed allows burning high [GLOSS]calorific value[/GLOSS] fuels without problems in bottom bed temperature control, as well as burning high moisture content (< 60%) biomass, even liquid fuels. Rapid change between different fuels is also possible. The cyclone separates the solid phases (unburned fuel, bed material and sorbent) from the flue gas and returns those to the bottom of the bed. The circulation gives more residence time for the limestone used for sulphur removal.

4. Internally Circulating Fluid Bed Combustors

The internally circulating fluid bed (ICFB) features a pattern of internal circulation, with centrally a downward and externally an upward current of bed material. This effect is obtained by applying differential airflows in the centre, with a gently bubbling bed, and in the external zones, where vigorous bubbling takes place. The bed material there is projected upward and deflected towards the centre by the furnace roof; then it falls in lapping waves over the central part of the bed. The ICFB units have been tested successfully for highly calorific waste, such as plastics and rubber. Preliminary shredding is unnecessary, although refuse bags are opened and some size reduction take place.

5. Pressurised Fluidised Bed Combustors

Increasing the operational efficiency of the fluidised bed boilers maybe achieved, among others, by modifying the bed geometry or introducing differential air pressure, resulting in pressurized fluidised bed combustion (PFBC). Operating efficiencies for PFBC depend on fuel characteristics (i.e. sulphur content, ash content, caloric value), type of PBFC system (i.e. combined cycle versus turbo-charged, see in Figure 3, circulating versus bubbling), and peak temperature of the gas turbine. Further improvement can be reached by integrating the gas and steam cycles.

In PFBC, coal is injected into a pressurized (10 – 20 bar), 800 – 950°C bed of material containing 90 – 95% coal ash and a desulphurisation sorbent. The coal rapidly combusts, and steam is generated within in-bed tubes. Particulates are removed from the hot flue gases, which are subsequently expanded through a gas turbine. The exhaust gases can be cooled, generating more steam for power production. The steam turbine produces 80 – 90% of the generated power, and the gas turbine 10 – 20%.

Combustors of the size 70-80 MW have been in operation for a number of years. A larger 350 MW combustor is under construction (2001) in Japan. The expected efficiency is 41% (HCV) on black coal with the potential to improve efficiency up to 43% in future plants. The PFBC has not been demonstrated to be economic compared to plants using the conventional cycle. With load reduction to 50% of rated output the efficiency is estimated to reduce by 9%.

Figure 3: Schematic structure of a turbo-charged pressurised fluidised bed boiler. [3].

6. Advantages and Disadvantages of Fluidised Bed Combustors

Advantages

1. Simplicity in construction.
2. Because of the low operating temperature around 850 °C,
   – little evaporation of salts takes place, with concomitant fouling of heat exchange surfaces
   – the thermal NOx generation is minimised
   – temperature is ideal for removal of SO2/SO3 by means of limestone or dolomite
3. High flexibility in accepting solid/liquid/gaseous fuels even with low calorific value (waste) and high moisture content (biomass).
4. Fluidised bed technology can be useful for gasification and pyrolysis.

Disadvantages

Fluidised bed combustion has some disadvantages and limitations. These include:

1. the need for fuel particle size to be less than 300mm
2. a relatively high pressure-drop is required to fluidise a bed of granular particles. The pressure drop is proportional to the weight of the bed, but after incipient fluidisation rises only slowly with the gas velocity
3. the flue gas carries a high dust load
4. although the operation of a bed is basically stable and the evolution of temperature is slow the problem of fluidised bed regulation and control is not straightforward
5. the possibility of sintering of bed material limits the maximum operating temperature, generally to a value of 850-950 °C, but sometimes more;
6. the operating experience with fluidised bed combustors is still limited. Wear upon submerged surfaces, the occurrence of attrition and elutriation upon bed particles, the evolution of the particle size distribution and of the composition of the bed material cannot be predicted with confidence.

Keywords

Fluidisation, fluidised bed combustion, bubbling fluidised bed, circulating fluidised bed, atmospheric fluidised bed, pressurised fluidised bed, combustion.

Source

[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.