Fluidized bed combustion boilers -

Fluidized bed combustion boilers (FBC) is a combustion technology, based on a layer of sand or a sand-like media, where the fuel is introduced into and combusted. The combustion air blows through the sand layer from an opening in the bottom of the boiler.

A bed of solid particles is said to be fluidized when the pressurized fluid (liquid or gas) is passed through the medium and causes the solid particles to behave like a fluid under certain conditions.

Fluidization causes the transformation of the state of solid particles from static to dynamic.

Fluidization in fluidized combustion boilers depends largly on the particle size and velocity. The mean solids velocity increases at a slower rate than the the gas velocity. The difference between the mean solid velocity and mean gas velocity is called as slip velocity. Maximum slip velocity between the solids and the gas is desirable for good heat transfer and intimate contact. If sand particles in a fluidized state is heated to the ignition temperatures of coal, and coal is injected continuously into the bed, the coal will burn rapidly and bed attains a uniform temperature. The fluidized bed combustion (FBC) takes place at about 840°C to 950°C for coal. Since this temperature is much below the ash fusion temperature, melting of ash and associated problems are avoided.

Main reasons for consideration of Fluidized bed combustion Boiler :-

Liberty of fuel choice – Normally fuel burns at 700 to 900 Deg C. Please note 1400 Deg C is threshold for the formation of nitrogen oxide.
a low emission of nitric oxides and the possibility of removing sulfur in a simple manner by using limestone as bed material, Lime or Dolamite CaMg(CO3)2 can be added to capture more than 95% of Sulphur pollutant.

Benefit of FBC boilers:-

Fuel flexibility; even low-grade coal such as sludge or refuse can be burned.
Lower combustion temperature lowers the problem related to melting of ash.(low sintering process)
Low NOx emission due to low temperature combustion.
Wide range of acceptable fuel particle sizes; pulverizing of the fuel is unnecessary.
Control of SOx emission by desulfurization during combustion; this is achieved by employing limestone as a bed material or injecting limestone into the bed.
Relatively small installation, because flue gas desulfurization and pulverizing facilities are not required.
Less area is required for FBC due to high coefficient of convective heat transfer.
Higher efficiency than conventional fired boiler as low excess air requirement.
Faster response to load fluctuation.

There are three basic types of fluidized bed combustion boilers:

1. Atmospheric Fluidized Bed Combustion System (AFBC)

2. Circulating (fast) Fluidized Bed Combustion system(CFBC)

3. Pressurised Fluidized Bed Combustion System (PFBC).

1. Atmospheric Fluidized Bed Combustion System (AFBC)

Atmospheric Fluidized bed combustion boilers are most common and simple FBC boiler where combustion takes place at layer of sand or sand like material. Bubbling of sand bed is created with the help of fluidizing air of velocity 1.2-3.7 m/sec. Coal of size 0-10mmis spread over bed (typically 1-6 mm size for under bed firing and 6-10mm for overbed firing), as soon as it reaches bed combustion takes place. In bed tubes are used in AFBC boiler for maximizing heat transfer. In this boiler 30-40% of ash is collected though bed ash.

2. Circulating (fast) Fluidized Bed Combustion system(CFBC)

Circulating (fast ) fluidized bed combustion system are designed to overcome drawbacks associated with AFBC boiler. Fluidizing velocity in this type of boiler is relatively very high 3.7-9m/sec, The combustion takes place at 840-900 °C, 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. Solid recycle is about 50 to 100 kg per kg of fuel burnt.

Inbed tubes are not used in this type of boiler. The circulating bed is designed to move a lot more solids out of the furnace area and to achieve most of the heat transfer outside the combustion zone – convection section, water walls, and at the exit of the riser. This type of boilers designs are economical in generally above 100 TPH capacity.

Comparison between of AFBC and CFBC boiler:-

SN.	Description	AFBC Boiler	CFBC Boiler
1	Fuel Size	0-10 mm	6-10 mm
2	Velocity of fluidization	1.2-3.7 m/sec	3.7 to 9 m/sec
3	Inbed evaporator tubes	Almost all AFBC boilers uses inbed evaporator tubes	There are no steam generation tubes immersed in the bed
4	Bed depth	.9 to 1.5 mtr	NA
5	Pressure drop	about 1 inch of water per inch of bed depth	NA
6	material leaves the bubbling bed	about 2 to 4 kg of solids are recycled per ton of fuel burned	Solid recycle is about 50 to 100 kg per kg of fuel burnt
7	Combustion temperature	There can be fluctuation in combustion temp	Combustion temperature can be maintained relatively constant.
8	calcium to sulphur utilisation	3.2 to 1	1.5 to 1
9	combustion air is supplied at	3–5 psig	1.5 to 2 psig
10	Combustion efficiency		Higher than AFBC
11	Turndown ratio		Better than AFBC

3. Pressurised Fluidized Bed Combustion System(PFBC).

Pressurised Fluidised Bed Combustion (PFBC) is one of type of FBC boiler where combustion takes placed at very high pressure of about 16 Kg/cm2. Off gas is used to drive gas turbine and makes compressed air available for combustion process. Steam is generated at Fluidized bed reactor. This type of FBC boilers are highly efficient and can operated with low grade coal. 70-350MW is the normal range of PFBC boilers.

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