FBC BOILER

FLUDIZED BED COMBUSTION BOILER

1

INTRODUCTION 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.

2

MECHANISMFluidisation 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.

3

CHARACTERISTICS OF FLUDIZED BED 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.

4

Types of Fluidised Bed Combustion Boilers

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).

5

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.

6

Air Distributor

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

7

1. FUEL FEEDING SYSTEM

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 sizeand pneumatically transported from feed hopper to the combustor through a feed pipe piercing the distributor.

8

feed pipe

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

9

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.

10

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.

11

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.

12

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.

13

2. Circulating Fluidised Bed Combustion (CFBC)

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.

14

Advantage over bubbling bed boiler

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.

15

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

16

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.

17

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

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.

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.