2. INTRODUCTION 2
In 1921, the first fluidized bed being used successfully in
Germany.
Fluidized bed combustion has emerged as a feasible
alternative and has significant advantages over
conventional firing system and offers multiple benefits –
compact boiler design, fuel flexibility, higher combustion
efficiency and reduced emission of noxious pollutants
such as SOx and NOx.
The fuels burnt in these boilers include coal, washery
rejects, rice husk, wood chips & other agricultural wastes.
The fluidized bed boilers have a wide capacity range-
0.5 T/hr to over 100 T/hr.
3. 3
MECHANISM
Fluidisation of solids
Sand particles resting on a mesh (left) become
fluidised when air is blown through (right) and take on
the appearance and some of the properties of a
boiling fluid.
Granular solids remain in layers when one is poured on
to another (left), but rapid mixing occurs on fluidisation
(right).
A bed of stationary particles supports objects
whatever their density (left). On fluidisation, an object
of lower density (the green ball) floats while the higher
density (red ball) sinks.
In a bed of stationary particles (left), heat is
transferred slowly and there are big differences in
temperature. In a fluidised bed (right), rapid mixing
ensures uniformity of temperature.
4. CHARACTERISTICS OF FLUDIZED BED
4
Fluidization depends largely on the particle size and the air velocity.
Combustion process requires the three “T”s that is Time, Temperature and
Turbulence. In FBC, turbulence is promoted by fluidisation.
If sand particles in a fluidized state is heated to the ignition temperatures of
coal, the coal will burn rapidly and bed attains a uniform temperature.
The fluidized bed combustion (FBC) takes place at about 840OC to 950OC.
Since this temperature is much below the ash fusion temperature, melting of
ash and associated problems are avoided.
The lower combustion temperature is achieved because of high coefficient
of heat transfer due to rapid mixing in the fluidized bed.
Limestone is used as particle bed, control of sulphur dioxide and nitrogen
oxide emissions in the combustion chamber.
5. Types of Fluidised Bed Combustion Boilers 5
There are three basic types of fluidised bed
combustion boilers:
1. Atmospheric classic Fluidised Bed Combustion System
(AFBC)
2. Atmospheric circulating (fast) Fluidised Bed Combustion
system(CFBC)
3. Pressurised Fluidised Bed Combustion System (PFBC).
6. 6
1.AFBC / Bubbling Bed
Coal is crushed to a size of 1 – 10 mm depending on the rank of
coal.
The atmospheric air, which acts as both the fluidization air
and combustion air.
The velocity of fluidising air is ranges from 1.2 to 3.7 m /sec.
Almost all AFBC/ bubbling bed boilers use
in-bed evaporator for extracting the heat
from the bed to maintain the bed temperature.
The bed depth is 0.9 m to 1.5 m deep.
Air
Distributor
7. 7
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
8. 1. FUEL FEEDING SYSTEM 8
For feeding fuel, sorbents like limestone ,
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.
feed pipe
9. 9
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
coal bunker
screw
conveyor
10. 10
2. Air Distributor
Purpose is to introduce the fluidizing air
through the bed cross section, keeping the
solid particles in constant motion.
The distributor, is constructed from metal
plate having a number nozzles or nozzles
with bubble caps.
The distributor plate is protected from high
temperature of the furnace by:
i) Refractory Lining
ii) A Static Layer of the Bed Material
iii) Water Cooled Tubes.
11. 11
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.
b) In-Bed Heat Transfer Surface
In a fluidized 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.
12. 12
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.
Ash from the boiler furnace outlet falls into
the crusher, where large ash particle is
crushed to small size. Finally ash is carried by
high velocity air to delivery point.
13. 13
b) Fly ash removal
The amount of fly ash to be handled in FBC boiler is relatively
very high, when compared to conventional boilers.
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, bag filters,
electrostatic precipitators (ESP’s) .
To diminish the SOx, recycling of fly ash
is practiced in some of the units.
14. 2. Circulating Fluidised Bed Combustion (CFBC) 14
Taller boiler structure
Coal is crushed to a size of 6 –12 mm depending on
the rank of coal.
The fluidising velocity in circulating beds ranges from
3.7 to 9 m/sec.
Combustion efficiency as high as 99.5%.
The combustion takes place at 840-900oC, and the
fine particles (<450 microns) are elutriated out of the
furnace with flue gas velocity of 4-6 m/s.
The particles are then collected by the solids
separators and circulated back into the furnace.
There are no steam generation tubes immersed in
the bed.
15. Advantage over bubbling bed boiler 15
Higher combustion efficiency.
Higher sulphur retention degree.
Better limestone utilization.
Lower emission level of NOx and SOx.
More economic produces 75 – 100 T/hr of steam .
Capacity range is about 400-500 MW & that of FBC boiler
is10-300 MW.
No. of feeding point less, because lateral mixing is done
by high velocity fluidizing air.
16. 16
3.Pressurised Fluid Bed Combustion
Combined cycle i.e Rankine & Brayton cycle using steam & gas turbine.
Operating temp. & pressure is 860OC & 16-18 bars.
The fuel is fed along with the sorbent and is maintained in
fluidised condition in the pressurized combustion chamber.
The pressurized flue gases are cleaned off & are expanded
into a gas turbine.
In addition, the excess air requirements of the boiler are
met by the gas turbine compressor.
Power generated by steam cycle and that generated
by gas turbine which is of the order of 80:20
17. 17
Advantages
Improved Cycle Efficiency & is estimated to
be 4-5 % more than conventional steam plant.
Lower fluidising velocities (around 1m/sec) which
reduce the risk of erosion for immersed heat
transfer tubes.
Reduced Emissions & Improved Combustion.
Reduced Boiler Size.
18. Advantages of FBC boiler over Conventional boiler
1. High Efficiency.
Combustion efficiency of over 95%
Overall operating efficiency is 84%
2. Fuel Flexibility.
3. Ability to Burn Low Grade Fuel.
4.Pollution Control.
SOx formation is minimised by addition of limestone for high sulphur coals.
CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) + CO2 (gas)
SO3 + CaCO3 = CaSO3 + CO3
Low combustion temperature eliminates NOx formation.
NOx formation takes place around1500o C
5. Easier Ash Removal – No Clinker Formation
19. 6. Simple Operation, Quick Start-Up
7. No Slagging in the Furnace-No Soot Blowing
8. Provisions of Automatic Coal and Ash Handling System
9. Provision of Automatic Ignition System
10.High Reliability and low maintenance costs.
20. Disadvantages of FBC boiler over Conventional boiler
1. The only disadvantage is that, a large amount of power will
be required to lift up the silica surface. So motor of force draft
fan will b twice bigger than that of other conventional system.
