CFBC, preferred Technology for lignite fuel - Experiences - Infraline

CFBC, preferred Technology for lignite fuel - Experiences
Dr.Srikrishna, S.
M.Raghuram
NTPC – CW, Noida
ABSTRACT
CFBC (Circulating Fluidised Bed Combustion) is a class of Fluidized Bed Combustion
(FBC) technology. FBC initially used in the chemical and process industries was applied
to the electricity industry because of its perceived advantages over widely used
combustion technologies. CFBC technology has advantages of low NOx generation due
to low combustion temperature, low SOx generation due to in-situ capturing of Sulphur
in fuel. This technology has become most preferred when fuel is lignite. Gujarat
Industries Power Company (GIPCL), Neyveli Lignite Corporation (NLC), Gujarat
Mineral Development Corporation (GMDC), Rajastan Rajya Vidyut Utpadan Nigam
Limited (RRVUNL) and Raj West Power Ltd opted for this technology. GIPCL is
operating two boilers since 2000 and is expanding with another two units of 125 MW
capacity boilers. GMDC is operating two units of 125 MW capacity at Akrimota in
Gujarat, GSECL is setting up two units of 75 MW capacity in Kutch District of Gujarat
RRVUNL is operating one unit of 125 MW capacity at Giral , Rajasthan and second unit
of 125 MW capacity is under commissioning. Rajwest power is erecting/commissioning
8 units of 130 MW capacity in Barmer District of Rajasthan. NLC is erecting two units
of 125 MW capacity at Barsinghsar, Rajasthan. M/s KSK Power Company is also setting
up power generating unit with CFBC technology boiler for using lignite fuel. There are
more IPPs opting for CFBC technology steam generators using lignite fuel. 250 MW
capacity units are being considered by NLC and GIPCL for their expansions.
This paper presents availability of lignite in India, power generation using lignite as fuel
with CFBC technology. Advantages & disadvantages of CFBC over conventional
Pulverized Fuel (PF) Combustion boilers and its inherent nature in protecting the
environment are discussed. CFBC technology boiler has many advantages compared to
pulverised fuel boilers in the perspective of operational ease, fuel preparation, in situ
capturing of sulphur in the fuel. At the same time these boilers have long start up and shut
down times, and are difficult to maintain with longer down time, maintenance time and
higher maintenance cost. CFBC technology boiler demands very high quality erection
with respect to combustor water wall straightness and super fine finish quality of weld
joints. Problems faced at different sites in operating and maintaining CFBC boilers,
important points to be taken care during erection stage based on the operational
experience are dealt in detail.
At the end, this paper advocates the use of such green power technologies and stresses the
need to encourage them by the government by relaxing the norms or laying down

separate norms in respect of Auxiliary Power Consumption, reimbursement of complete
fuel additive charges, if necessary.
Introduction
India has total estimated usable lignite reserves of 97,000 MMT , concentrated in the
states of Tamilnadu, Gujarat and Rajasthan. Tamilnadu has 70,000 million Tones,
Rajasthan has15,000 million Tones, Gujarat has 12,000 million Tones. All these states
are far away from coal reserves located in India. It is economical to use lignite for power
generation in these areas. These are more than 30% of coal reserves available in India.
Estimated power generation capacity addition with this lignite reserves is around
3,00,000 MW for 50 years. India has not exploited lignite reserves available for power
generation as it has to be considering the available resources. Performance of units with
PF technology using lignite has been low. Performance of Units with CFBC
technology using lignite is encouraging.
CFBC operates with combustor temperatures in the region of 850 – 900 deg C. These
combustor temperatures are much lower than ash fusion temperature. So, in CFBC
boilers problem of any clinker formation is not encountered with lignite as primary fuel.
General quality of lignite fuel available for using in boilers in India is as follows:
Parameter Range
Total Moisture 40 to 52%
Ash 8 to 20 %
VM 21 to 34%
FC 11 to 20 %
CV
2500 to 4000 Kcal/kg
Sulphur 1 % to 6%
Chloride 0.20 % to .35%
ADVANTAGES OF CFBC TECHNOLOGY
Environment Friendly
Sulfur emissions are effectively controlled by adding lime stone to the fluidized bed,
eliminating the need for an external desulfurization process such as SO2 scrubbers,
which are costly. Because of reduced combustion temperatures, Nox emissions are
inherently low or negligible.
Fuel Flexibility
FBC units were touted as being “fuel flexible”, with the capability of firing a wide
range of solid fuels with varying heating value, ash content, and moisture content.
Combustion in FBC units takes place at temperatures below the ash fusion
temperatures of most fuels. Consequently, tendencies for slagging and fouling are
greatly reduced with FBC.

Simplified Fuel Preparation And Feeding
Normally crushers are sufficient for fuel preparation. A top size, in the range of 5 –
12 mm is required for sub-bituminous coals. Pulverizes and associated maintenance
are eliminated.
Compact Plant Design
CFBC boilers require less floor space than conventional steam generators with down
stream flue gas cleaning equipment.
Sustainability Under Cyclic Loading
Minimizes stress on boiler tube material with low variation of combustor
temperatures under cyclic loading of boiler particularly under varying grid
frequencies.
High Reliability
Low flue gas velocities in combustor minimizes erosion and hence the boiler tube
failures leading to very high reliability of the boiler.
Comparison of CFBC technology with PF technology on some parameters
Description CFB Boiler PF Boiler Benefits of CFB
Fuel Size 12-6 mm >70%

CFBC boiler is operational friendly. This boiler is called dark boiler because of
absence of any flame in the combustor. Hence, there is no boiler tripping due to flame
failure/ furnace disturbance. As this boiler has huge refractory and very large amount
of bed ash in circulation, there is no need to trip the turbine in case of all fuel cut off
to boiler for about 20 minutes. Boiler continues generating steam within specified
parameters for sustaining turbine at reduced loads. CFBC combustor is a reservoir of
heat energy. This energy is used in case of all fuel failure for short duration before
restoration of one of the fuel available (lignite or oil). CFBC boilers do not
experience large sharp fluctuations in boiler pressure due to very stable firing system.
Oil support is not required even at part loads of about 25 % and combustor
temperatures, steam parameters are maintained. CFBC boilers have almost zero
combustibles in bottom ash with share of bottom ash of about 30-40%. The intense
heat storage capacity of this boiler makes it useful for quick hot re-starts.
One of the disadvantages of CFBC boiler is longer cold start-up and shut down time.
Cold start-up time is about 10-12 hours as boiler is to be heated up slowly at specified
heating rates as per refractory heat up curves. Similarly during shutdown also, gradual
cooling is required. A typical boiler start-up is given Fig: 1. As lime is added in
CFBC boiler for capturing Sulfur, the units with CFBC boilers have more number of
auxiliary equipments such as air blowers, limestone handling, milling and
transporting system. Hence, auxiliary power consumption is on higher side for lower
capacity units.
Figure-1: Typical Boiler start-up curve from light-up to Synchronization of Unit
700
600
Temperature in deg C
500
400
300
200
100
0
30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600
Time in Minutes
Lower bed Temp Middle bed Temp FG temp after Cyclone FG temp after Economiser FG temp after APH
Using Lignite as fuel with CFBC boilers, following environmental parameters can be achieved.
SOx (ppm) NOx (ppm) CO (mg/N.Cu.M) SPM(mg/N.Cu.M)
< 50 < 20

Other General operation problems being experienced are:
• Back sifting of bed ash in to primary air wind box trough combustor nozzle
grate
• Pulsation in cyclones at lower loads, bed ash/bed materials hang up in
cyclones.
• Agglomeration of ash in combustor to cyclone ducts.
• Deposition of ash on back-pass coils.
• Clinker formation in Start up burners ,Bed lances
Maintenance Experiences:
CFBC boilers experience higher erosion rates in water wall tubes at refractory
transition zone in general. CFBC combustor has refractory lining up to about 9 meters
height to protect water wall against erosion. But erosion problems experienced at
refractory transition zone due to bed material sliding back to combustor along water
wall from different heights. Erosion of water wall at this zone is due to scooping
action of bed material. Some designs use kick-out tubes to overcome this problem.
To protect the water wall tube from erosion, protective coating is being used in some
units and is giving satisfactory results. Area requiring metal spray can be identified
with the help of tube thickness survey of water wall and identifying erosion pattern.
Life of water wall tubes in these areas extended to three times with metal spray.
Due to large quantities of refractory and bed ash in circulation, CFBC boilers require
more down time to repair as it takes longer periods to cool down boiler to working
temperature. A typical cooling curve is shown Fig 2. This is results in increased down
time and decreased availability. These boilers also demand major refractory repairs
particularly in combustor water wall refractory, ash cooler refractory and refractory
along fuel path.
In case a tube leakage in combustor or Fluidized bed Heat Exchangers (FBHE) in
some designs requires immediate evacuation of all bed material to avoid solidification
of the material (due to presence of Calcium sulfate).
Non Metallic expansion Joints (NMEJs) in ash cycle path also require very special
attention. Any ingress of hot bed material in to it will damage the NMEJ.
Combustor grate nozzle is another major maintenance area. Severe erosion is
observed in the nozzles due to bed material. About 10% of nozzles require
replacement every year.
Lignite chain feeders, Bead ash handling equipment also require more maintenance
due to corrosion and erosion.

As lignite has high sulfur content, in absence of proper Sulfur capture, chances of
cold end corrosion in tubular Air Pre-heater are more and may require frequent
replacement of entire cold banks. Presence of chlorides in lignite will further
aggravate the problem.
Figure-2: Cooling curve of CFBC Boiler at SLPP
1000
900
800
700
Furnace Bottom Temp
600
500
400
300
200
100
0
1 4 7 101316192225283134374043464952555861646770737679828588919497
Time in Hrs
Max
Min
CONCLUSIONS
For a developing nation planning to meet the rising demand with capacity additions, use
of technologies, which are environmental friendly and minimize the impact on
environment, is necessary. CFBC is one of such Technologies that operates under low
combustion temperatures and has facility to add sorbent. It effectively minimizes the Sox
and NOx emissions. It can also use poor quality fuels for effective, efficient combustion
minimizing emissions and impact on environment. Advantages of CFBC technology,
special commissioning activities of this environmental friendly technology, experiences
have been briefly discussed. Several power generating companies have adopted this
technology. Problems experienced by power generators using CFBC technology have
been presented along with special care to be taken during erection stage. To encourage
the adoption of such environment friendly technology, government or the beneficiary
should help the generating companies by relaxing the normative operating/commercial
norms such as Auxiliary power consumption and reimbursement of cost of fuel additive.

India has good reserves of lignite in Gujarat, Rajastan & Tamilnadu parts of the country.
This has already been in use for power generation. But using lignite in conventional PF
boilers has several problems of clinkering. CFBC is most suitable technology for lignite
fuel as has been experienced by power generating companies. Important thing that has to
be taken care is improvisation of this technology for Indian working conditions. Some of
the elements of this technology may need to be developed for suiting Indian conditions.
NTPC may also plan for adopting this technology for coal with higher sulphur or lignite.