The document discusses traditional pulverized fuel firing systems and circulating fluidized bed combustion (CFBC) boilers. It provides details on the principles and types of CFBC boilers, as well as their advantages over traditional systems, including greater fuel flexibility, lower emissions, and easier desulfurization. CFBC boilers allow for in-furnace reduction of NOx and SOx through low-temperature combustion and the addition of limestone, providing an inherently more environmentally friendly combustion system compared to pulverized fuel firing.

2.  Traditional Fuel Firing systems
 CFBC Boiler and Types
 Advantages of CFBC boiler
 Environmental friendliness of CFBC
 NOx Control
 SOx Control
 Potential Failure problems and Prevention.

3.  The Traditional modes of burning solid fuels like
coal or lignite are
 Static Mode
 Suspension Mode

4.  Mass burning (MB) and traveling
grate (TG) stokers burn solid fuel in
static mode with the fuel resting on
a grate.
 Most Traditional power plants use
pulverized fuel (PF) firing which
burns the fuel in suspension mode in
transport condition.
 Between these two extremes of
burning, as static and suspension
modes, the intermittent one is the
fluidized mode.

5.  PF firing Technology has a long history, the roots way back to 200 years, when it is
started with Cement industry and next by steel industry. Then Finds its way to
power generation.
 PF firing technology shifted the Power generation from 15 MW to as high as 1300
MW in early 1970.
 When a solid fuel such as coal is reduced to the consistency of talcum powder and
fired in an open furnace, the resulting combustion is almost equal to oil or gas
firing—in
 Speed
 controllability, and
 Heat release.
 A coal particle burns out between 1 and 2 s, depending on its volatile content,
similar to oil, and the combustion is most complete at >99% carbon burn up.
 For high-volatile matter (VM) coals and lignites, the combustion efficiency can be
as high as 99.7%.

6. The limitations relate to the inability of PF to
 Deal with variation in fuel without risking
fouling and slagging of boilers
 Provide multi-fuel firing
 Fire fuels with >40% total moisture unless
there is enough VM such as in brown coals
 Burn very low-volatile fuels such as petcoke
 Combustion is very poor in fuels with
 Gross calorific value (GCV) <2000 kcal/kg
 A burden >65% (ash and H2O)
 Very high S
 Have step less load turndown of more than
—70 to 100%

7. PF firing has been the preferred
method for solid fuel firing in large
quantities for utilities since last 8
decades.
However, in the last few years,
circulating fluidized bed combustion
(CFBC) boilers have begun to disturb
this equilibrium by offering reliable
solutions in the areas not served well
by PF firing.
The limitations of PF boilers were
recognized decades ago. In the
absence of a better alternative, they
have been accepted.

8. Principles of Operation
Types of FBC
Advantages and
Limitations

9. With the advent of the CFBC technology, remedial measures
have since been found to some extent for the limitations of
PF boilers. Circulating fluidized bed combustion boilers
 Can address all the above issues with PF firing
 Are nearly as efficient for conventional fuels
 Are more operator friendly, with very few moving parts and
controls
 Have lower O&M costs if erosion issues are not encountered
 Offer better environmental friendliness and ensure against
emerging requirements
 Present little danger of explosion

10.  Traditional methods of firing failed to
address the emerging emission norms
early and late 70’s in a cheaper way and
addressed these with a secondary gas
cleaning system
 The more and more stringent pollution
limits, pushed the cost of secondary gas
cleaning system to a new height.
 FGR – Flue Gas recirculation or
 SCR – Selective Catalytic reduction for
NOx
 FGD – Flue Gas De-Sulphurisation for
SOx
 Pollution limits of SOx and NOx Played a
vital role in a new way of combustion.
 A cleaner and cooler combustion was
the only way to meet strict levels of SOx
and NOx.

11.  Fluidized bed combustion (FBC) is
burning of various solid fuels in the
fluidized state—a condition where a gas-
solid mixture behaves like a free-flowing
fluid.
 With the right proportioning of air
pressure and the proper sizing of fuel,
the air-solid mixture behaves like a fluid.
The fluid bed experiences progressively
more turbulence, as air velocity is
increased.
 FBC is the combustion in this state—
bubbling FBC (BFBC) at the lower end
and circulating FBC (CFBC) at the higher
end.

12.  Based on the fluidized region of operation, the FBC
boilers emerged in following types
 BFBC or AFBC – Bubbling/Atmospheric Fluidized bed combustion
boilers
 CFBC – Circulating fluidized bed combustion boilers
 PFBC – Pressurized Fluidized bed combustion boilers
 In the 1980s and early 1990s, both technologies were
developed simultaneously, BFBC boilers in the United
States and CFBC boilers in Europe. Both shared the
fluidization principle. As the contours and the limits of
the technologies grew sharper, it became clearer that
they were more complementary than competing.

13. The classical CFBC Boiler operates at the higher end of the fluidized bed regime,
just lower than the transport phase. It includes a fluid bed expanded all the way to
the roof of the combustor instead of restricting the bed to the lower part as in a
BFBC boiler.

14.  In the Classic 3 Ts of perfect
combustion
 Time
 Turbulence
 Temperature
 PF fired boilers makes the
combustion more efficient by
keeping all the 3 Ts in optimized
condition.
 With resident time of 1-2 seconds,
more turbulence and very high
temperature burning causes the
combustion more effective.
Temperature
TurbulenceTime

15.  As the FBC operates lower temperature range, from 800°C to
950°C., where as the PF fired boilers operates more than
1200 to 1500° c.
 The Negative effect of the lower temperature of FBC range of
boilers in Combustion (3 Ts) has been over come by
increasing the other 2 Ts, Turbulence and resident time.
 The Combustion efficiency in fact increased drastically
because of high turbulence in the bed and longer residence
time, despite being operated in lower combustion
temperature.

16.  High fan power for fluidization reduces net output per unit
fuel by 1% compared to PF, if deNOx and deSOx units are not
large enough or are absent.
 Tube and refractory erosion issues are not fully resolved in
CFBC Boilers.
 Single unit sizes of 1000 MWe and above are proven in PF,
whereas in CFBC, units >300 MWe are still under initial
operation.

18.  Developed in 1970s and commercialized in 1980s in Europe
 Pulverized fuel boiler supremacy has been challenged
seriously after its reign of more than half a century by the
CFBC boiler.
 CFBC boilers replaced PF fired boilers literally under 100
MW category.

19. Besides the expensive hot cyclone, the second-generation designs such as
cold cyclone, compact, and U-beam are available in industrial range.
For large utility boilers, full — circulation types give a more compact
arrangement. The expanded bed designs can also meet the utility boiler
requirements.
Present-day designs can be broadly categorized as follows:
1. Full circulation
A. Hot cyclone design
B. Compact design
2. Expanded bed
A. Cold cyclone design
B. U-beam/no-cyclone design

20. Classic CFBC boiler PF fired Boiler

21. CFBC boilers are gaining widespread use at least in the sub-
100 MW sizes, due to
 Fuel flexibility-Capability to burn almost any fuel
 Excellent multi-fuel flexibility
 High combustion efficiency
 Environmental friendliness
 In situ and very convenient desulfurization
 Very low NOx generation
 Low O&M costs
 No slagging and fouling of tubes
 Good to excellent load response
 Simpler ash handling

22. One of the major advantages of a CFBC boiler is its fuel flexibility, ability to operate
in vast range of fuels. The Fig. shows the fuel range in which the CFBC can operate.

23. Heat Release (MW/H)
Evaporation (TPH)
100 200 300 400 500 600 1800

24. DeNOx and DeSOx
Or
De-nitrification and De-Sulfurization

25.  In FBC boilers operating at ~850°C, there is an inherent
lower production of NOx auto-matically, as the combustion
temperature is low.
 Even In PF boilers, Flame temperature reduction from
1480-1500 to 1250° C will impact NOx generation by 10
fold.
 These boilers also offer a very convenient way of reducing
the SOx emission within the furnace enclosure by the
reaction with lime stone
 This desulfurization is adopted only for coals and other
solid fuels with medium to high sulphur.
 CFBC Boilers keeps an Upper hand on environmental
friendliness with cheaper way of DeNOx and DeSOx
capabilities.

26. Nox Formation
NOx Types
De-Nitrification - DeNOx

27.  During Combustion in Furnace, At elevated temperatures, oxygen
combines with nitrogen to form nitrogen oxides and other complex
compounds collectively called NOx.
 Nitrogen oxides are of environmental concern because they initiate
reactions that result in the formation of ozone and acid rain, which can
cause health problems, damage buildings, and reduce visibility.
 NO reacts to form NO2, which reacts with other pollutants to form
ozone (O3).
 Three Kinds of NOx formed during Fuel Combustion
 Fuel NOx
 Prompt NOx
 Thermal NOx

28. Nitrogen is present in fuel and combustion air.
 Fuel NOx forms when Nitrogen in fuel reacts with oxygen in combustion air.
 These fuel bound Nitrogen accounts for 50% of total NOx emission from coal and oil
combustion.
 Prompt NOx results when fuel hydrocarbons break down and recombine with
nitrogen in air (this reaction generally takes place before the flame tip).
 Accounts for 15-20% of total NOx Emission.
 Thermal NOx forms when Nitrogen in air reacts with Oxygen along with intense
heat. These kind of Nox Rate of formation increases,
 Exponentially with Temperature
 And Directly Proportional to Oxygen (O2) concentration.

29.  Much higher temperatures of >1200°C (2200°F) are needed to
Form Thermal NOx, which are fortunately not feasible for FBC.
As all the FBC boilers generally operates at 800° C to 950°C.
 In PF fired boilers which operates in this temperature Range the
NOx emission will be generally higher than CFBC due to higher
combustion Temperature.
 Thermal NOx <2000 mg/NM3with Normal burners
 NOx 600 mg/Nm3 with low NOx burners
 Low NOx is achieved in PF boilers with Low NOx burners with
lower flame temperature and better air staging

30.  Fuel NOx is formed in FBC boilers also as equal to PF, the
analysis results data says that, almost all the nitrogen in fuel
may be converted to NOx.
 For 1% of Nitrogen in the fuel, the possible potential NOx
emission will be approximately 3800 Mg/Nm3.
 This Fuel NOx then, Largely reduced to Elemental Nitrogen
again by the Presence of the strong reducing agents in the
form of
 Char (C – Carbon) and
 CO (Carbon Monoxide) in the bed

31.  In CFBC boilers the combustion happens in two stages,
 The primary Combustion in Furnace bed with primary air and secondary
combustion in free board area with Secondary and Tertiary air.
 55 to 60 % in Furnace Bed – Primary Combustion
 40 to 45 % in Free board area - Secondary Combustion
 Due to this staged combustion, the furnace bed will be in Sub-
Stoichiometric conditions, this will increase the active Carbon (char) and
Carbon Monoxide (CO) in furnace bed.
 The Final resulting NOx emission in CFBC will be almost 50% that formed
in BFBC boilers.

32.  In CFBC boilers, PA forms only
50 to 60% of total air required
for combustion. Remaining part
of air is taken care by SA
 Figure, captures the effect of
Secondary Air on the NOX
emissions.
 Higher the SA air %, lower the
Nox emission.
 NOX emissions effort-lessly stay
at <200 mg/N m3 on 6% O2
when 30% or more SA is given.
Secondary air %

33. De-Sulphurisation
Limestone Requirement
(Dis)Advantages

34.  In-bed desulfurization is a breakthrough in CFBC boilers.
 Lime stone-sulphur as SO2 reaction within the furnace bed,
along with the combustion reaction and the resultant gypsum
exit with ash is simplification personified.
 A good understanding of the mechanics of this reaction and
the limitations is very necessary for ensuring realistic emission
of Sox.
 It also helps to correctly set up the limestone and ash handling
systems that meet the present and future requirements.
 5 to 6 % Sulphur in fuel (typically pet coke) can lead to 10000
mg/NM3 of SOx in exit flue gas.

35.  Sulphur Capturing in CFBC has been done by adding
Limestone (CaCO3) or dolomite (MgCO3) along with
bed material as sulphur absorbent.
 The absorbent dosed along with fuel.
 The Capturing process is done in two different
reactions, the reactions are
 Calcination
 Sulphation

36. The Limestone (Calcium Carbonate – CaCO3) added into the
furnace along with fuel decomposes as Calcium oxide (CaO) is
called Calcination. The Reaction is,
During this Process of Calcination Limestone generates 44% of
Carbon dioxide (CO2).
100 Kg of Pure Limestone will decomposes and gives
 56 Kg calcium oxide and
 44 kg Carbon dioxide.
CaCO3  CaO + CO2

37. In case of Dolomite addition, it decomposes as
Magnesium Oxide and emits Carbon dioxide.
During this process 84 kg of pure dolomite
decomposes as,
 40 kg of Magnesium oxide and
 Emits 44 kg of Carbon Di Oxide
MgCO3  MgO + CO2

38.  During Combustion, the Sulphur in fuel reacts with
Oxygen in combustion air and gives SO2 (sulphur
dioxide).
 32 Kg of Sulphur Reacts with 32 kg of Oxygen and gives
64kg of SO2.
 Each Kg of sulphur in fuel will give 2 kg of Sulphur
dioxide.

39. The sulfation reaction is the reaction of solid CaO with gaseous SO2 in the
presence of oxygen (O2). The sulfation reaction is normally referred to as the
reaction which yields solid CaSO4 as the final product
 56 Kg of Calcium oxide Reacts with
 64 kg of SO2 along with 16 Kg of Oxygen and
 gives 136 Kg of Calcium Sulphate – CaSO4.
 This By product of CaSO4 is also called as dry Gypsum.

40.  The Theoretical requirement of
Lime vs sulphur is 1 mol of Ca per
1 mol of Sulphur (Ca/S ratio is 1)
in molar units.
 The trend shows the DeSox
efficiency vs the molar ratio. At
2% molar ratio the SOx capturing
efficiency is >95%.
 In Mass fractions, 1 kg of suphur
requires 1.25 kg (40/32 kg) of
Calcium (Ca). Ca/S Molar Ratio
 In terms of CaO or calcium oxide the mass requirement is 1.75 (56/32) kg
CaO/kg of Sulphur.
 As, the availability is in Limestone as CaCO3, the mass requirement of
Limestone is equal to 3.125 Kg / kg of Sulphur (100/32).
 So, every 1% of sulphur in Fuel requires 3.125% of pure Limestone.

41.  Both sulfation and calcination
reactions start at ~700°C
(1300°F) and are optimum at
840-850°C
 Bed temperature vs DeSOx
efficiency trend shows that, the
Limestone Consumption is
lowest at this temperature range.
 At the Ca/S molar ratio of 4 or 3
the DeSOx efficiency is >95%.
Bed Temperature ° C

42.  The sulfation takes place on the surface of the lime particle in the bed and so
the core of the particle fails to participate.
 Some sulfur in fuel, which is inorganically bound, does not oxidize to SO2.
 Some SO2 escapes when sorbent is less or accompanies the volatile matter
(VM) of fuel.
 Available Limestone purity is lower than optimum—typically —92%.
 Maintaining the bed temperature around 800-900 ° C is important for de-
sulphurisation because
 Calcination is not complete at Temperature <800° C
 Sulfation reaction falls off rapidly beyond 850°C because CaSO4 formed on the
sur-face of CaO melts due to high temperature and forms a coating on CaO, and
isolates that for further reaction.
 Within the residence time SO2 gas molecules do not encounter reactive solid
CaO particles despite high bed turbulence.

43.  Bed Temperature
 Particle Resident Time
 Bed Quality
 Gaseous Environment (O2%)
 Furnace Pressure
 Chemical Composition (purity of limestone)
 Porosity of Limestone
 Surface area
 Particle Size

44. CaO
• Calcium Oxide
SO2
• Sulphur Dioxide
O2
• Oxygen

• -8974.8 KJ/kg
CaSO4
• Calcium Sulphat
CaCO3
• Calcium Carbonate

• +1783 KJ/kg
CaO
• Calcium Oxide
CO2
• Carbon DiOxide
The Calcination process of Limestone is an Endothermic reaction. As shown in the
figure below, each kg of CaCO3 will consume 1783 kJ/kg heat from furnace.
The Sulphation process of CaSO4 formation is an Exothermic reaction in which
during the reaction like combustion this process will yield 8974.8 kJ/kg of heat per
kg of Calcium Oxide.
So Each Kg of CaCO3 gives 3243 kJ/kg of Heat energy as final result.
This is an UnTapped advantage of In Bed Sulphur Capturing

45.  The calcination and sulfation reactions in the bed, which are endo
— thermic and exothermic, respectively, alter the heat marginally.
 With a Ca/S ratio >2, there is a net loss,
 whereas at <2, there is a net gain in heat.
 Sensible heat loss. With coals having high ash and employing
desulfurization, the bed ash discharge at —850°C can represent
loss as high as 5%. As the bed ash formed will be 20-25% of total
ash
 Fan credits. Forced draft (FD)/primary air (PA) fans consume a lot of
power in fluidizing the combustion air.
 The churning of air in the fan casing to produce such a high
pressure heats the air, the power for which is provided by the fan.

46.  Cyclone radiation loss. Radiation loss taken from the standard
American Boiler Manufacturers Association (ABMA) chart does not
account for the losses of cyclones.
 The CaO formed during De-sulphurisation process is equal of cement
and excess limestone dosing can solidify in bed during shutdown and
leakage conditions.
 It also can block bed ash drain pipes after getting cooled.
 Limestone Dosing in bed increases the Nox emission.
 If the fuel has higher moisture which can condense in the APH and
allow the CaO to settle and solidify. Which can reduce the heat
transfer performance of APH.
 Tube Leakage in first pass can make the bed as cemented concrete
due to the CaO content.

47. Failure Potentials in CFBC boilers
Furnace Explosions
PA duct explosions
HGG explosions

48.  A few CFBC Boilers have suffered furnace explosion
in the past.
 As regards CFBC boilers, these are comparatively
newer generation of technology and explosion
avoidance measures are not clearly understood by
the operating engineers.

49.  There are three necessary elements
which must occur simultaneously to
cause a fire:
 Fuel
 Heat and
 Oxygen.
 These elements form the three legs of
the fire triangle.
 By removing any one of these elements, a fire becomes impossible.
 For example,
 if there were very little or no oxygen present, a fire could not occur regardless of
the quantities of fuel and heat that were present.
 Likewise, if insufficient heat were available, no concentrations of fuel and oxygen
could result in a fire.
Fuel
Oxygen
Ignition source
Fire

50. For an explosion to occur, In any
case, there are five necessary
elements which must occur
simultaneously:
 Fuel
 Heat
 Oxygen
 Suspension
 Confinement
Fuel Confinement
Suspension
Heat
Oxygen
Explosion

51.  These form the five sides of the explosion pentagon. Like the fire
triangle, removing any one of these requirements would prevent an
explosion from Confinement propagating.
 For example,
 if fuel, heat, oxygen, and confinement occurred together in proper
quantities, an explosion would still not be possible without the suspension
of the fuel.
 However, in the above case, a fire could occur and the explosion will not
be possible.
 If the burning fuel were then placed in suspension by a sudden blast of air,
water or steam, all five sides of the explosion pentagon would be satisfied
and an explosion would be imminent.
 This is where, the CFBC boiler is prone to failure in some critical cases

52.  Furnace explosions in CFBC boilers are rare when both bed and free board
temperatures are above 760 °C. Chances of explosions are very high when
these temperatures are below 540 °C.
 Though not fully established, yet chances of explosions cannot be ruled out
when bed temperature remains between 540 °C and 760 °C.
 All Fluidized bed boilers are exposed to this risk of explosion when they are
stopped/tripped on loaded condition.
 The explosions mostly occur in CFBC when
 Boiler is restarted after a trip out.
 Boiler is restarted after a short period of Hot stoppage
 Fast cooling of boiler is resorted to following a tube leakage especially when the
leaking water falls on the bed
 The risk is very high when the leakage water wet the bed partially one sided and
other side of the bed is very hot.

53.  During hot stoppage/tripped condition…
 It has good amount of fuel along with hot bed.
 A Typical 100 TPH boiler will have 40 to 50 MT of hot bed material with ~5% of coal
at 850°C and Confined in the furnace.
 It meets the 3 legs of explosion pentagon,
 Heat – above 850 ° C
 Fuel – Sufficient enough
 Confined
 Such cases when ever the system gets other 2 legs of explosion pentagon Fuel
Suspension (instant) and Oxygen, it explodes immediately.
 There is a possibility of these to happen when the boiler is restarted after hot
stoppage with any abnormality likes of tube leakage.

54. There are cases where, the PA duct has been exploded during a boiler start up.
 When ever there is trip out on CFBC boiler,
 The total fluidizing bed comes to static condition.
 During this, the PA/FD air passed thru’ PA nozzle and entered into the fluidized
zone will be pushed back thru’ the same nozzle to wind box due to the heavy
weight of bed material.
 These phenomenon causes the combustion air with high temperature rushes to
wind box taking along some amount of fine fuel with CO as the burning process
is not complete.
 These un burnt ready to fire fuel (CO and Char) will cause more CO formation in
the wind box and remain in the PA duct.
 When ever the boiler is started after the trip, PA duct will get good amount
of Oxygen as fresh air.

55.  Our Analysis after every hot tripping says that, the O2% in PA duct and wind
box will go down as low as to 12% during hot trip which directly indicates that,
there is some amount other gases formation during tripping – expected to be
Carbon monoxide.
 This will close all the legs of the Explosion pentagon
 Heat
 Fuel
 Oxygen
 Confined (PA duct enclosed in all direction)
 Suspended fuel (CO)
 With PA/FD has been started, the Wind box is exposed to high heat of bed
material will cause a explosion.

57.  This is mainly applicable for Under bed burners or Hot gas generators
of cold cyclone boilers.
 HGG explosion occurs due to consecutive start up failures of HGG in
Flame failure condition.
 This start up failure on flame failure will lead to a accumulation of
sprayed Liquid fuel in HGG, when the fire catches the accumulated fuel
causes a explosion.
 Fuel shut off valve passing also causes HGG explosion. The failed valves
can allow the HSD to pass thru’ and get dumped in the HGG chamber.
When the actual firing starts this accumulated fuel get exploded.

58.  Fuel should never be fed into the furnace continuously for more than 12
seconds when there is no fire.
 Furnace is completely purged of the explosive mixture and then fired.
 Fuel supply is stopped immediately if fire/flame is not established and re-
purging is surely done before restart.
 Correct air fuel ratio is maintained so that dust concentration within
explosive limits is never achieved.
 Do not start PA/FD fan once the boiler is stopped due to tube leakage.
 Continue to run ID and SA fans. Once the SA fan is tripped do not restart
during leakage condition. It can give required Oxygen to the gases formed in
bed due to water injection which leads to an Furnace explosion.

59.  Provide a vent in PA duct for venting out the CO formed during Sudden tripping of
boilers.
 Interlocking with an O2 analyser will further reduce the potential PA duct explosion.
 Don’t start the Under bed burners or HGG’s without sufficient purging of furnace.
 Continuous tripping of HGG on Flame failure during start up can lead to explosion.
 Avoid restarting of HGG before ensuring that the HSD/Fuel sprayed inside HGG
chamber without burning is totally drained out or purged out.
 Isolate fuel from HGG immediately after burner cut off.
 Always take oil burner support when ever the bed temperature drops down below
540°C.