Transcript CFBC: CIRCULATING FLUIDIZED BED COBUSTION

Fluidized Bed Combustion System
Fluidized Bed Combustion System
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FLUDIZED BED COAL FIRED BOILERS
 INTRODUCTION
When a air or gas is passed through an inert bed of solid
particles (supported on mesh) will initially sleek upward
through sand.
With Further increase in velocity the air starts bubbling
through bed and Particles attain a state of high turbulence.
Under such conditions Bed attains the appearance of fluid
and exhibit the properties of Fluid.
PRINCIPLE OF FBC
MECHANISM OF FLUIDIZED BED
COMBUSTION
Temperature of bed should be at least equal to ignition
temperature of coal.
Bed temperature must not increase melting point of Ash.
Equilibrium temperature achieved through transfer
tubes immersed in bed and walls of combustor.
Gas velocity must be maintained between fluidization
velocity and the particle entrainment velocity.
FIXING, BUBBLING AND FAST FLUIDIZED BEDS
As the velocity of a gas
flowing through a bed of
particles increases, a
value is reaches when the
bed fluidizes and bubbles
form as in a boiling
liquid. At higher
velocities the bubbles
disappear; and the solids
are rapidly blown out of
the bed and must be
recycled to maintain a
stable system.
TYPES OF FBC
I.
AFBC : Atmospheric Fluidized Bed Combustion
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
Bubbling fluidized bed combustion.
Circulating fluidized bed combustion.
II. PFBC : Pressurized fluidized bed combustion.
BUBBLING FLUIDIZED BED COMBUSTION
FEATURES OF B.F.B
• 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.
CIRCULATING FLUIDIZED BED
REACTOR
FEATURES OF CIRCULATING F.B.C
• At high fluidizing gas velocities in which a fast
recycling bed of fine material is superimposed on a
bubbling bed of larger particles .
• The combustion temperature is controlled by rate of
recycling of fine material.
• Hot fine material is separated from the flue gas by a
cyclone and is partially cooled in a separate low
velocity fluidized bed heat exchanger, where the heat
is given up to the steam.
PRESSURIZED FLUIDIZED BED COMBUSTION
IMPORTANT POINTS
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Coal is crushed to a size of 6mm
The air is blown inside by a high pressure fan
The velocity of air is 3-10 (ft/s)
The ignition takes place ,solid densities are
reduced
• A temperature of about 1500-1600f is produced
IMPORTANT POINTS
• Heat energy is utilized by water in the water walls
and gets converted into steam
• Flue gases are collected by a cyclone separator.
• It separates the mixture into pure flue gas, ash &
unburnt coal particles.
• The unburnt coal particles are again re-circulated.
HORIZONTAL FIRE-TUBE PACKAGE
*Integral water cooled
rectangular furnace
is`used
*A carbon reinjection is
mounted ,which entraps
Large particles ad
re-circulate
WATER TUBE BOILER FBC
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•
•
•
•
•
•
•
•
Has a water cooled
Membrane wall panel
Flue gas pass through a
Super heater ad a
Convective Bank
A grit arrestor placed
In the flue gas path
Collects the particulate
Matter for recirculation
VERTICAL FIRE TUBE PACKAGE
• It has three pass multitubular construction.
• Cooling water tubes are
used within the bed to
Obtain Equilibrium.
CFBC: CIRCULATING FLUIDIZED BED
COBUSTION
A FLUIDIZED BED THAT IS OPERATED AT
VELOCITIES IN THE RANGE OF 13 – 22 FEET
PER SECOND IS REFERRED TO AS CFBC.
CFBC: CIRCULATING FLUIDIZED BED
COBUSTION
 Primary air is introduced into the lower portion of
combustor.
 Secondary air is introduced at higher levels in the
combustor to ensure complete combustion and to
reduce NOx emissions.
 combustion chamber of a CFB unit consists of
membrane type welded water walls.
FUEL FEED SYSTEM
Gravimetric feeders with rotary lock valves.
Provision of negative pressure in the combustor
at the fuel feed points to eliminate the
rotary lock valves.
Pneumatic fuel feed system can also be used.
FUEL REQUIREMENTS
Low volatile fuels such as Anthracite must be crushed to
smaller top size than higher volatile fuels
( approx 1/6 inch or less)
Larger fuel size are acceptable for higher volatile fuels.
SORBENT REQUIREMENTS:
Lime stone is crushed to a top size of 1000 microns
with average size 150 microns.
TECHNICAL PERFORMANCE.
# CFBC boiler can exhibit high efficiency.
# CFBC boilers are simple to operate and maintain.
# CFBC boilers offer better dynamic response.
ENVIRONMENTAL PERFORMANCE
# Emissions
* In case of CFBC technology NOx production is
reduced substantially.
* Production of SOx can be controlled.
PFBC: PRESSURIZED FLUIDIZED BED
COBUSTOR
UTILITY
Its utility began in 1960 to express purpose of
Increasing cycle efficiency in coal fueled
plants.
GENERAL CONFIGRATION
Classification done in 2 types
I. First generation units.


Turbocharged units.
Combined cycle units.
II. Second generation units.
PFBC TURBOCHARGED
• Combustion gas from
PFBC boiler is reduced to
750 F.
• Little net Gas Turbine
output.
• Major part of electricity is
produced by Steam
Turbine.
PFBC COMBINED CYCLE UNIT
• Combustion gas at
1500F-1600F is fed to
gas turbine fro PFBC.
• 20% of plant electrical
output is provided by
Gas Turbine.
• Increase of 2%-3% of
thermal efficiency
compared to
turbocharged unit.
SECOND GENERATION UNIT
• Low btu-gas produced from pyrolyzed coal in carbonizer
and gas produced from char are mixed in Topping combustor.
• Combustion of these gases in topping combustor increases
temperature of gas up-to 2300 F.
• Higher input gas temperature leads to increase of gas turbine
efficiency.
• Heat is recovered from gas turbine exhaust in a heat recovery
steam generation called as HRSG.
SECOND GENERATION UNIT
• Power generated by S.T
is approximately equal
to G.T.
• Thermal Efficiency
up-to 45% & more is
achieved.
PFBC
# PFBC combustor can either use
* Circulating bed combustor
* Bubbling bed combustor
# Fluidizing velocity is kept up-to 3 feet/sec.
It is kept low to minimize combustor erosion.
# Fuel and Sorbent can be send to combustor in 2
forms:
* Dry form
* Pump able paste
PFBC
# Coal in dry form:
• Coal is dried and grounded to an average size of 50
microns.
• Carried to combustor or carbonize through lock
hoppers.
# Sorbent in dry form
• Limestone is used as sorbent.
• Grounded to an average size of 100 microns.
• Usually feed with coal or can feed in separate stream.
PFBC
# Paste Feed System
• Coal is crushed to ¼ inch top size and mixed with
water to about 25 % of water.
• Paste is pumped into combustor through positive
displacement pup.
# Sorbent
• Sorbent can be mixed with coal or can be feed
separately as with dry feed system.
PFBC PERFORMANCE
# Overall performance of PFBC units in terms of:
* Heat transfer
* Combustion efficiency
* Emission
Is better than as compared to AFBC.
PFBC
# Higher operating pressure ( 12 to 13 atm) of
PFBC units results in higher heat transfer rate
and combustion.
# NOx emissions are reduced considerably as
compared to AFBC due to pressurization.
DESIGN,OPERATION AND MAINTENANCE
OF FBC BOILERS
• FUEL FLEXIBILITY
FBC boilers can be operated efficiently with a variety
of fuels.
• COAL QUALITY
Coals with high moisture, high ash can be efficiently
burnt.
DESIGN,OPERATION AND MAINTENANCE
OF FBC BOILERS
• FUEL INJECTION
The coal is evenly distributed by judicious location of
the feed points
• FREE BOARD
Due consideration is given for provision of adequate
free board space and design of the convective bank
DESIGN,OPERATION AND MAINTENANCE
OF FBC BOILERS
• PRESSURE DROP AND BED HEIGHT
A shallower fuel bed allows over bed feeding, whereas a
deeper fuel bed allows bottom feeding
• LOAD CONTROL
Steam o/p is controlled by manipulating the principal bed
parameters:
• Height
• Temperature
• Inventory and
• Fluidization velocity
DESIGN,OPERATION AND MAINTAINANCE
OF FBC BOILERS
• Bed temperature:
FBC normally operates at a lower temperature range i.e.
750 to 950ºc
• Heat transfer:
The overall heat transfer coefficient decreases with increase
in particle size and increases with increase in fluidization
velocity
ADVANTAGE OF FBC BOILERS
Considerable reduction in boiler size is possible due to
high heat transfer rate.
Low combustion temp. of the order of 750-900 C
facilitates burning of coal with low ash fusion temp.
,prevent NOx- formation, reduces high temp.
corrosion and erosion.
High Sulphur coals can be burnt efficiently without
much generation of Sox by feeding limestone
continuously.
The unit can be designed to burn a variety of fuels.
High
turbulence of the bed facilitates quick startup
and shut down.
Efficiency of 80% and above can be achieved.
Lower coal crushing cost due to higher particle size.
High thermal inertia helps to overcome the problem
of flame stability.
DISADVANTAGES OF FBC
• The need for fuel particle size to be less than 300mm.
• A relatively high pressure-drop is required to fluidize a bed of
granular particles. The pressure drop is proportional to the weight
of the bed, but after incipient fluidization rises only slowly with
the gas velocity.
• The flue gas carries a high dust load.
• Although the operation of a bed is basically stable and the evolution
Of temperature is slow the problem of fluidized bed regulation and
control is not straightforward.
• The possibility of sintering of bed material limits the maximum
operating temperature, generally to a value of 850-950 °C, but
sometimes more.
• The operating experience with fluidized bed combustors is still
limited. Wear up on submerged surfaces, the occurrence of attrition
and elutriation upon bed particles, the evolution of the particle size
distribution and of the composition of the bed material cannot be
predicted with confidence.
CURRENT RESEARCH AND TRENDS
• Most current research aims to quantify and explain the
behavior of the phase interactions in the bed.
• Specific research topics include particle size distributions,
various transfer coefficients, phase interactions, velocity and
pressure effects, and computer modeling.
• The aim of this research is to produce more accurate models of
the inner movements and phenomena of the bed.
Thank You