CFBC Boiler Operation

Bajaj Energy Pvt. Ltd.
2×45 MW IPP
Unit: Barkhera
WELCOME
BEPL Barkhera
“CFBC Boiler Operation”
Present by
Subodh kandwal

BOILER DEFINITION
A Boiler is a device used for generating steam for
Power generation , process use or heating purposes.
According to IBR Act 1923,a boiler is a closed pressure
vessel with capacity exceeding 22.75 Liters used for
generating steam under pressure.

BOILER CONFIGURATION
• Natural Circulation, Single Drum, Top Supported, Balanced Draft, Tower
Type, Membrane Panel Constructed, Outdoor Unit Equipped with Two
Cyclones, having Economizer and Tubular Airpreheater as back end heat
traps and 2x 60% Draught Plant (Primary Air fan, Secondary Air Fan and
Induced Draught Fan).

Water & Steam Circuit (Schematic Diagram)

In general the heat absorbing parts subject to internal pressure in a boiler will
be called as pressure parts. The main pressure parts in a boiler are as follows
:-
– Drums
– Water walls
– Super heaters
– Economizers
– Evaporators
– Screen Tubes
• First pass:-The Drum, Down comers, evaporators, economizer, water
wall headers and water walls will form the circulation system and cover
the furnace zone, in which the water is converted into steam-water
mixture at the same temperature, by absorbing the radiant heat in the
furnace zone. The furnace walls form the enclosure to the first pass.
• Second pass:-The second pass enclosures, in which the temperature of
the water is raised to the required level by a combination of radiation &
convection process. Economiser coils are located in the second pass. Along
with Tubular type Air Pre Heater.
Copyright © 2014 Bajaj Energy Pvt. Ltd. 6

• The down comers carry the water from the drum to the bottom ring
headers. The water at high temperature flows through the furnace
walls from the headers. Heat transfer takes place through the
water wall tubes and the fluid raises to the top headers from where
the steam-water mixture is transmitted to the drum.
• The drum provides the necessary space for locating the steam
separating equipments for separation of steam from mixture of
steam and water. It also serves as a reservoir for the supply of water
to circulation system to avoid possible starvation during operation.
• Super heaters are used to raise the steam temperature above the
saturation temperature by absorbing heat from flue gas to increase
the cycle efficiency. For utility boilers which are meant for power
generation, the super heater outlet temperature is limited to 535 ̊C
due to metallurgical considerations.
• Economizers are used to absorb heat from the flue gases and add
this as sensible heat to the feed water before water enters the
evaporation circuit of the boiler.

Types of water circulation in boiler
The water circulation in the boiler may be
natural circulation due to density difference or
by force circulation with the help of pumps.
• Natural circulation
• Forced circulation

Natural Circulation type Steam Generator (Basic Circuit
Diagram)

pressure
density
Critical pressure
221.2bar
water
steam
Density difference
Density vs pressure curve

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. Pressurised Fluidised Bed Combustion System (PFBC).
3. Circulating (fast) Fluidised Bed Combustion system(CFBC)
Deepak Rana, Bajaj Energy Ltd - Barkhera

Circulating (fast) Fluidised Bed Combustion system(CFBC)
This CFBC technology utilizes the fluidized bed principle in which crushed (6 –
12 mm size) fuel and limestone are injected into the furnace or combustor.
The particles are suspended in a stream of upwardly flowing air (60-70% of
the total air), which enters the bottom of the furnace through air distribution
nozzles. The fluidising velocity in circulating beds ranges from 3.7 to 9 m/sec.
The balance of combustion air is admitted above the bottom of the furnace as
secondary air.
Since limestone is used as particle bed, control of sulfur dioxide and nitrogen oxide
emissions in the combustion chamber is achieved without any additional control
equipment. This is one of the major advantages over conventional boilers.

Circulating Fluidized Bed
At the bottom of the boiler furnace
there is a bed of inert material. Bed is
where the coal or fuel spreads. Air
supply is from under the bed at high
pressure. This lifts the bed material and
the coal particles and keeps it in
suspension. The coal combustion takes
place in this suspended condition. This
is the Fluidized bed.
Special design of the air nozzles at the
bottom of the bed allows air flow
without clogging. Primary air fans
provide the preheated Fluidizing air.
Secondary air fans provide pre-heated
Combustion air. Nozzles in the furnace
walls at various levels distribute the
Combustion air in the furnace
Circulating Fluidized Bed
At the bottom of the boiler furnace there is a
bed of inert material. Bed is where the coal
or fuel spreads. Air supply is from under the
bed at high pressure. This lifts the bed
material and the coal particles and keeps it
in suspension. The coal combustion takes
place in this suspended condition. This is the
Fluidized bed.
Special design of the air nozzles at the
bottom of the bed allows air flow without
clogging. Primary air fans provide the
preheated Fluidizing air. Secondary air fans
provide pre-heated Combustion air. Nozzles
in the furnace walls at various levels
distribute the Combustion air in the furnace

Circulation
Fine particles of partly burned coal, ash and bed material are carried along with the
flue gases to the upper areas of the furnace and then into a cyclone. In the cyclone the
heavier particles separate from the gas and falls to the hopper of the cyclone. This
returns to the furnace for recirculation. Hence the name Circulating Fluidized Bed
combustion. The hot gases from the cyclone pass to the heat transfer surfaces and go
out of the boiler.
Bed Material
To start with the bed material is sand. Some portion is lost in the ash during the
operation and this has to be made-up. In coal fired boilers the ash from the coal itself
will be the makeup material. When firing bio fuels with very low ash content sand will
be the makeup bed material. For high Sulphur coals Limestone addition to the bed
material reduces SO2 emissions.
CFBC uses crushed coal of 3 to 6 mm size. This requires only a crusher not a
pulverizer. From storage hoppers Conveyer and feeders transport the coal to feed
chutes in the furnace. Start up is by oil burners in the furnace. Ash spouts in the
furnace remove the ash from the bottom of the furnace.

• Type of Cyclone:-
• 1. Hot cyclone ( Temp around 800-850 degree)
• 2. cold cyclone (Temp around 400-450 degree)
• 3. U Beam cyclone (Temp around similar to bed temp.)
• Hot Cyclone:- Hot cyclone generally use for low volatile matter and high
ash content coal. It helps during low reactivity as heat absorbed by hot
cyclone helps to burn this type of fuel.
• Cold Cyclone:- Cold Cyclone generally use for high volatile matter and low
ash content coal.
• U Beam Cyclone:- This type of cyclone is generally used for High volatile
matter and low ash content coal.

In general the heat absorbing parts subject to internal pressure in a boiler
will be called as pressure parts. The main pressure parts in a boiler are as
follows :-
– Drums
– Water walls
– Super heaters
– Economizers
– Evaporators
– Screen Tubes
• The Drum, Down comers, evaporators, economizer, water wall headers
and water walls will form the circulation system and cover the furnace
zone, in which the water is converted into steam-water mixture at the
same temperature, by absorbing the radiant heat in the furnace zone.
The furnace walls form the enclosure to the first pass.
• The second pass enclosures, in which the temperature of the water is
raised to the required level by a combination of radiation & convection
process. Economiser coils are located in the second pass. Along with
Tubular type Air Pre Heater.

• The drum provides the necessary space for locating the steam separating
equipments for separation of steam from mixture of steam and water. It
also serves as a reservoir for the supply of water to circulation system to
avoid possible starvation during operation.
• The down comers carry the water from the drum to the bottom ring
headers. The water at high temperature flows through the furnace walls
from the headers. Heat transfer takes place through the water wall tubes
and the fluid raises to the top headers from where the steam-water
mixture is transmitted to the drum.
• Super heaters are used to raise the steam temperature above the
saturation temperature by absorbing heat from flue gas to increase the
cycle efficiency. For utility boilers which are meant for power generation,
the super heater outlet temperature is limited to 535 ̊C due to
metallurgical considerations.
• Economizers are used to absorb heat from the flue gases and add this as
sensible heat to the feed water before water enters the evaporation circuit
of the boiler.

 Induced Draught (ID) Fan :

Starting PA Fans:
For starting of the PA Fan, SA fan should be running for the last
5min with SA flow more than 6kg/cm2.
• Ensure CO Vent is Open, Before starting PA fan, check the physical
condition of the valve in the field also.
• Ensure supply and control power is on, all bearings with proper
lubrication, free movement of the dampers and cleanness of the
area.
• Ensure fluidizing air line to siphon are is open condition.
• Ensure proper oxygen and CO level at Economiser inlet. Before
starting of PA fan CO level at economiser inlet shell be less than
300ppm and oxygen at PA duct should be grater than 18%
• Drum level is normal and boiler air protection is ok.
• Ensure inlet and outlet dampers of PA Fans are closed condition.
• Start one PA fan and instruct site operator to observe any abnormal
sound / vibration / abnormal heating / rubbing of the rotating parts.

Operation of HGG
Pre- Start-Up Checks For Burner Start-Up
• Opening/closing of air slide damper for all the burners are smooth with
complete travel.
• Verify cooling air to scanners and peep holes are available.
• Verify the operation of fuel oil control valve opening and closing.
• All instruments for measurement of oil pressure, flow and temperature are
in service.
• Burner Start Up Procedure:
• Starts any one of the oil pump and maintain the rated oil pressure >10
kg/cm2
• Before starting the burner, the pre-interlocks, main interlocks purge
interlocks & oil firing permissive are to be satisfied as per BMS logic like as:
• LDO pressure > 6kg/cm2
• Atomising air pressure >7kg/cm2
• LPG Pressure >1kg/cm2
• Combustion air flow > 15TPH
• Differential across the HGG <25 mmWC

• Burner Start PB Operated
• Following Cycle will be initiated:
• Step – 1
• LPG Gas valve will open and LPG Timer 35 secs count started Atomizing
Air Valve will Open, Igniter Energized and Igniter Timer 15 secs count
started. Oil valve will Open 5 secs after Start command And Flame is ON.
• Step – 2
• Ignition Timer Over and Igniter De-Energized, LPG Valve closes after 35
sec timer is over.
• The Main burner is lighted up keeping oil flow around 150 kg/hr.
• In case the Oil does not catch fire during this period the whole sequence
is to start again.
• The combustion air damper is to be adjusted to the predetermined “air-
fuel oil ratio” for efficient combustion and to keep the flame healthy.
• Keep dilution air quantity sufficient to maintain HGG outlet gas
temperature at around 350 ̊C initially

• start raising the oil flow till it reaches around 900 kg/hr. Open
dilution air damper such that HGG outlet gas temperature rises
gradually and does not exceed 850 ̊C in any case
• Both HGGs should not be started together. After approximately 15
minutes of stabilised operation of one HGG, second should be
started.
• Start Boiler feed pump for maintaining Drum level.
• Initially, Superheater drain valves are kept open, along with drum
and Superheater vent valves. When drum pressure exceeds 2
kg/cm², drum vent valve and Superheater vent valves should be
closed.

• Once drum pressure reaches approx. 5 kg/cm², Superheater
drain valves are kept partially open for allowing steam flow
through the Superheaters.
• At approximately 8 to 10 kg/cm² drum pressure, close all S/H
drains and partially open start up vent valves such that drum
pressure increase is kept within permissible gradient 2 kg/cm²
(g) per minute.
• This will allow steam flow through the super-heaters and final
steam temperature will also increase as per requirement.

• During pressure raising the water level will
normally rise to the top of gauge glasses as steam
pressure is increased. This is due to expansion in
volume of water due to heat transfer and
circulation.
• The excess water from system should be removed
by blow down.
• While giving the blow-down, do not keep the
blow-down valves open for more than 20
seconds. The blow-down normally should be
given with quick open/close operation

• Main fuel (coal) is charged after temperature of bed material exceeds
ignition temperature(550 deg. c) of the fuel and all other boiler
interlocks are fulfilled.
• Due to cold fuel, initially there is slight "dip" in bed temperature. If
coal combustion has been established. Take another coal feeder into
service adjust PA and SA as per Requirement. after the bed
temperature exceeds 700°C, gradually reduce the oil flow to about
250 kg/hr and then smoothly trip the HGGs one after another. Now
both coal feeder in service and maintain bed temperature<800deg.
And raise main steam pressure.
• Once the fuel firing through both the streams is established and HGGs
switched off. The HGG Bypass damper should be opened gradually.
combustion and dilution damper closed.

• When main steam pressure came 25kg/cm2. First,
open by-pass valve of main steam stop valve.
• Allow steam to flow for sufficient time to warm up
main steam piping.
• Open main steam stop valve and subsequently
close by-pass valve.
• Now steam charged up to CSDH.

Boiler Trip Logics
Protection conditions Alarm Value Trip value Remarks
1. Furnace Pr very HI >50 mmwc >100 mmwc PA,SA, CF TRIP, selected ID trip.
2. Furnace Pr very low <-80 mmwc <-150 mmwc Both ID fans trips if both are
running
3. Furnace pr. Very low <-80 mmwc <-250 mmwc After selected ID trips and
another
ID fan running.
4. Drum level Very HI >64% >74% ONLY PA TRIP, selected ID trip.
5. Drum level Very low <30% <10% ONLY PA TRIP, selected ID trip.
6. Total PA flow< minimum <20 Kg/sec <15 Kg/Sec ONLY DCF TRIP
7. Bed height very low <800 mm <500 mm ONLY DCF TRIP
8. Bed height very high >1200 mmwc >1400 mmwc ONLY DCF TRIP
9. Both ID fans tripped
10. MFT operated ONLY DCF TRIP
NOT IN GRAPHICS
Remark if any
• Total PA flow once reach above 20 kg/sec then it reduced to below 15 kg/sec then only DCF will trip.
•Bed height once reaches above 1000mm then it reduced to below 600mm then only DCF will trip.

CFBC Boiler Operation

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