AFBC / Bubbling Bed Boiler

Atmospheric classic Fluidised Bed Combustion System AFBC / Bubbling Bed:

            In AFBC, coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type of fuel feed and fed into the combustion chamber. The atmospheric air, which acts as both the fluidization air and combustion air, is delivered at a pressure and flows through the bed after being preheated by the exhaust flue gases. The velocity of fluidising air is in the range of 1.2 to 3.7 m /sec. The rate at which air is blown through the bed determines the amount of fuel that can be reacted.

            Almost all AFBC/ bubbling bed boilers use in-bed evaporator tubes in the bed of limestone, sand and fuel for extracting the heat from the bed to maintain the bed temperature. The bed depth is usually 0.9 m to 1.5 m deep and the pressure drop averages about 1 inch of water per inch of bed depth. Very little material leaves the bubbling bed – only about 2 to 4 kg of solids are recycled per   ton of fuel burned. Typical fluidised bed combustors of this type are shown in Fig - 1 and fig -2.

            The combustion gases pass over the super heater sections of the boiler, flow past the economizer, the dust collectors and the air pre heaters before being exhausted to atmosphere.

            The main special feature of atmospheric fluidised bed combustion is the constraint imposed by the relatively narrow temperature range within which the bed must be operated. With coal, there is risk of clinker formation in the bed if the temperature exceeds 950°C and loss of combustion efficiency if the temperature falls below 800°C. For efficient sulphur retention, the temperature should be in the range of 800°C to 850°C.

General Arrangements of AFBC Boiler

AFBC boilers comprise of following systems:

i) Fuel feeding system

ii) Air Distributor

iii) Bed & In-bed heat transfer surface

iv) Ash handling system.

Many of these are common to all types of FBC boilers.

1. Fuel Feeding System:

For feeding fuel, sorbents like limestone or dolomite, usually two methods are followed: under bed pneumatic feeding and over-bed feeding.

Under Bed Pneumatic Feeding

If the fuel is coal, it is crushed to 1–6 mm size and pneumatically transported from feed hopper to the combustor through a feed pipe piercing the distributor. Based on the capacity of the boiler, the number of feed points is increased, as it is necessary to distribute the fuel into the bed uniformly.

Over-Bed Feeding

The crushed coal, 6–10 mm size is conveyed from coal bunker to a spreader by a screw conveyor. The spreader distributes the coal over the surface of the bed uniformly. This type of fuel feeding system accepts over size fuel also and eliminates transport lines, when compared to under-bed feeding system.

2. Air Distributor:

The purpose of the distributor is to introduce the fluidizing air evenly through the bed cross section thereby keeping the solid particles in constant motion, and preventing the formation of defluidization zones within the bed. The distributor, which forms the furnace floor, is normally constructed from metal plate with a number of perforations in a definite geometric pattern. The perforations may be located in simple nozzles or nozzles with bubble caps, which serve to prevent solid particles from flowing back into the space below the distributor.

The distributor plate is protected from high temperature of the furnace by:

i) Refractory Lining

ii) A Static Layer of the Bed Material or

iii) Water Cooled Tubes.

3. Bed & In-Bed Heat Transfer Surface:

a) Bed

The bed material can be sand, ash, crushed refractory or limestone, with an average size of about 1 mm. Depending on the bed height these are of two types: shallow bed and deep bed. At the same fluidizing velocity, the two ends fluidise differently, thus affecting the heat transfer to an immersed heat transfer surfaces. A shallow bed offers a lower bed resistance and hence a lower pressure drop and lower fan power consumption. In the case of deep bed, the pressure drop is more and this increases the effective gas velocity and also the fan power.

b) In-Bed Heat Transfer Surface

In a fluidised in-bed heat transfer process, it is necessary to transfer heat between the bed material and an immersed surface, which could be that of a tube bundle, or a coil. The heat exchanger orientation can be horizontal, vertical or inclined. From a pressure drop point of view, a horizontal bundle in a shallow bed is more attractive than a vertical bundle in a deep bed. Also, the heat transfer in the bed depends on number of parameters like (i) bed pressure (ii) bed temperature (iii) superficial gas velocity (iv) particle size (v) Heat exchanger design and (vi) gas distributor plate design.

4. Ash Handling System:

a) Bottom Ash Removal

In the FBC boilers, the bottom ash constitutes roughly 30 – 40 % of the total ash, the rest being the fly ash. The bed ash is removed by continuous over flow to maintain bed height

and also by intermittent flow from the bottom to remove over size particles, avoid accumulation and consequent defluidization. While firing high ash coal such as washery rejects, the bed ash overflow drain quantity is considerable so special care has to be taken.

b) Fly Ash Removal

            The amount of fly ash to be handled in FBC boiler is relatively very high, when compared to conventional boilers. This is due to elutriation of particles at high velocities. Fly ash carried away by the flue gas is removed in number of stages; firstly in convection section, then from the bottom of air preheater/economizer and finally a major portion is removed in dust collectors.

            The types of dust collectors used are cyclone, bagfilters, electrostatic precipitators (ESP’s) or some combination of all of these. To increase the combustion efficiency, recycling of fly ash is practiced in some of the units.

Features of bubbling bed boiler:

Fluidised bed boiler can operate at near atmospheric or elevated pressure and have these essential features:

• Distribution plate through which air is blown for fluidizing.

• Immersed steam-raising or water heating tubes which extract heat directly from the bed.

• Tubes above the bed which extract heat from hot combustion gas before it enters the flue duct.