1. Combustion

The feed particles introduced in the fluidised bed are dried, heated and charred almost instantaneously, since they are heated to bed temperature in a matter of seconds. The evolution of [GLOSS]volatile matter[/GLOSS]
makes them buoyant and drives them to the bed surface. Charred particles move around in the bed and burn at a temperature, which may rise substantially (100-200°C) above the average temperature of the bed. The evolving volatile matter burns partly in the bed, partly above it. Hence the temperature in the freeboard zone is generally well above that of the bed when firing biomass, organics, refuse-derived fuel (RDF) or plastics. Plastics melt, coat the individual bed particles and are rapidly distributed over the bed.

The bubbling bed is a virtually isothermal, well-mixed reactor, capable of dispersing solid, liquid or gaseous waste streams over the entire bed area and volume. It operates as a gas/solids contactor in which the solids are permanently mixed by the effect of fluidisation. The residence time of the gas and volatile matter is fairly limited. Thus, it can be stated that solid particles burn in the bed (unless they become airborne), whereas the volatile matter often largely burns above the bed especially when the latter is relatively shallow.

2. Bed Structure and Fuel Feed

Normally the carbon content of the bed is low, ranging from about 0.5 to a few weight percent. Since the conversion of carbon in the bed is a factor of time, temperature and turbulence, the turbulence of the fluidised bed keeps the percentage low. Under steady-state conditions there is a balance between the generation of charred particles by devolatilising the feed material; the elimination of these particles by gasification and combustion.

Bed particles are in vigorous motion and continuously exposed to attrition and elutriation. Generally the bed material has to be replenished periodically. In other cases the bed material is gradually augmented by the
[GLOSS]ash[/GLOSS], derived from the fuel fed. In that case material has to be extracted to limit bed height and pressure drop.

It is important to distribute the feed continuously and homogeneously over the bed. Accumulated feed material in one corner of the bed, which is suddenly distributed over the bed, causes incomplete combustion and uncontrolled emissions.

It is necessary to select the feed point(s) and method with care. The simplest is to drop the feed through a lock, so that it falls onto the bed where it is dispersed over the surface or throughout the bed mass, depending on the nature of the ascending/descending bed currents at the point of impact. Top or sidewall gravity feeding systems are simpler to build and operate than in bed-feeding systems, using a lateral screw, piston or pneumatic feeder. In-bed feeding is desirable when the particles to be fed are small and would immediately be entrained by the gas stream instead of burning in the bed. The feeding system operates under overpressure and heat conduction along the system may create specific problems, such as the charring of the feed.

The bed wall consists of refractory lined steel or of vertical water-tube panels, the lower part of which is often lined to reduce both erosion and corrosion. Bed internals serve to enlarge the tube area, available for heat transfer, and in some cases to organise the gas bubble flow in the bed. The tubes may consist of individual submerged tubes of tube banks in an in-line or a staggered design, or of tube vanes. In all cases the internals are exposed to erosion and corrosion and vigorous bubbling beds are considered destructive especially for large linear velocities, as used in most plants.

3. Bed Temperature

At lower operating temperatures and for low reactive chars the rate of consumption of carbon is also lower and hence the amount of carbon present in the bed tends to higher values. Thus a lower reactivity may be partly compensated by a larger inventory of the carbon.

The temperature of combustion is affected by:

• the feed rate of the fuel
• the moisture content of the fuel
• the feed rate of the primary or fluidisation air
• the calorific value of the fuel
• the preheat temperature of the primary air
• the heat extraction rate.

From the previous considerations it follows that a high bed temperature (typically 800-850 °C) is desirable, since it accelerates combustion, increases the capacity of the unit, limits the carbon inventory and reduces the air requirements and the flow of the flue gases.

The maximum operating temperature is severely limited, however, by the possible occurrence of
[GLOSS]sintering[/GLOSS], which is self-accelerating and leads to unexpected and sometimes catastrophic loss of
[GLOSS]fluidisation[/GLOSS]. When sintered together the bed must be cooled down and loosened with pneumatic hammers. Sintering is caused by the softening of ash particles, which adhere to bed particles, form an outer coating and – when sintering occurs – cement them together.

This leads to the formation of coarser particles, which are no longer fluidised or – hence – cooled adequately. Thus it is possible that relatively large aggregates are formed in a brief period of time, or even that the entire bed solidifies to one large block. Sometimes arches are built at the interface between bed and freeboard, namely when a high freeboard temperature occurs.

The development of temperature gradients in the bed and abrupt rise in pressure drop may serve as an advanced warning of loss of fluidisation. A close control on the fuel particle size distribution is necessary. It is also to be recommended to eliminate from the bed at regular intervals any fine, reactive ash particles as well as coarse, coated particles.

4. Sintering, Slagging, Fouling and Agglomeration

Sintering problems mainly occur when firing biomass, chemical sludge or after a surge in the calorific value of the feed. With municipal solid waste (MSW) or refuse derived fuels (RDF) sintering problems may not occur, not even with glass particles as a nucleus, [2]. Such particles are coated with sand, but do not lead to operating problems.

In a full-scale fluidised bed boiler several ash related problems such as [GLOSS]slagging[/GLOSS],
[GLOSS]fouling[/GLOSS], [GLOSS]bed agglomeration[/GLOSS] and even corrosion may occur during combustion of a recycled/recovered fuel (REF) if the REF is co-combusted with a wrong type of primary fuel, or at an unsuitable mix composition. [3]

5. Flue gas

The flue gas arising in a [GLOSS]fluidised bed combustor[/GLOSS] has high dust content, typically 20-50 g/m3 (at standard temperature and pressure), or a factor 10 higher than in a conventional, mechanical grate incinerator.

Cyclones or electrostatic precipitators are sometimes used to reduce the dust load of the gases leaving the bed. The separated particles can be recycled into the bed, since they contain a sizeable amount of carbon.

Keywords:

Fluidisation, bed temperature, sintering, slagging, agglomeration, fouling.

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] Zevenhoven, R., Skrifvars, B-J., Hupa, M. & Frankenhaeuser, M.: Characterisation of Ashes from Co-combustion of Refuse-Derived Fuel with Coal, Wood and Bark in a Fluidised Bed.