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Trouble shooting of Refractory problem in CFBC Boiler
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1
RELIABILITY AND MAINTENANCE
IMPROVEMENTS IN LATEST
GENERATION FLUIDIZED-BED
BOILERS
Eero Hälikkä
Foster Wheeler Energia Oy
Finland
2
1 ABSTRACT
The fuel flexibility of circulating fluidized-bed (CFB) technology has made it attractive in
power generation where the fuel quality is not constant or where the capability to burn
different types of fuels is crucial for economical operation. This environmentally friendly
technology is also used in utility-scale coal-fired boilers to a wider extent because of its
increased efficiency and lower emissions.
During the last few years CFB technology has faced new challenges. One of those challenges
is the requirement to achieve high levels of life-cycle profit, which emphasizes reductions in
overall operating costs. The fuel scale has to be expanded and CFB unit sizes increased as the
operators aim to use cheaper fuels and achieve more efficient steam generation processes. As
a result, maintaining boiler availability has become a challenge when more difficult fuels are
utilized and higher steam values are applied.
New, more reliable boiler designs have been developed and existing boilers have been
modified to achieve greater reliability. This development of enhanced boiler designs has been
based on actual experiences from boiler operation. Only a long cumulative operation history
of a large number of boilers can provide feedback data reliable enough for design evaluation
and development.
This paper describes how new boiler designs and design modifications have answered the
challenging reliability issues and increased boiler operation reliability while decreasing
annual maintenance costs. The reliability and performance figures of the latest generation of
fluidized-bed boilers are discussed based on actual feedback and operation experiences from
operating CFB units. The operational and defects statistics are used in the evaluation of the
design improvements. The paper also illustrates how to prepare for forthcoming challenges
and how to maintain high unit reliability in an ageing boiler fleet.
3
2 INTRODUCTION
Increasing competition in power markets is setting new demands for cost reduction to deliver
increased profit over a boiler’s operating lifetime. As the price of electricity is set by the
power markets the main focus falls on lifetime costs. From the power producers’ point of
view this means a trend towards more efficient O&M activities as the purchase costs are
already addressed and minimized in the equipment purchase phase.
It is said that over 80% of all the lifecycle costs are defined/determined in the equipment
design and development phase. This means in practice that the main opportunity, or
responsibility, for reducing the lifecycle costs of a power production unit is on the equipment
manufacturer. Only 20% of the lifetime costs are under the management of the power
producer. This sets demands for the development of equipment that can be operated
efficiently and reliably and can be maintained efficiently and cost-effectively.
2.1 Boiler efficiency
In order to maximize total power plant efficiency in utility-scale units, it is necessary not only
to deliver high boiler efficiency but also higher steam values. Higher steam values increase
boiler material costs in the purchasing phase. They also increase the need for maintenance
because they tend to cause reliability problems in boiler tubes, especially final superheaters.
This in turn tends to increase lifecycle costs. In practice this means that the reduction in
lifecycle costs on the higher steam values are marginal in traditional CFB technology and the
future trend is towards supercritical once-through CFB technology.
2.2 Low-cost fuels
Efficient operation also increases pressure to move towards low-cost fuels. Low-cost fuels set
challenges for boiler manufacturers in two ways; increasing the demand for multi-fuel boilers
and presenting the recycled fuels for co-combustion or alone for utility and industrial boilers.
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The low-cost fuels are, or contain to some extent, recycled materials like plastics, paper, wood
etc. Burning these materials, especially mixed plastics with PVC, recycled material such as
demolition wood which contains wood preservatives, or materials with a relatively high
chlorine content, together with high steam values, tend to cause very aggressive corrosion.
This requires either more expensive tube materials in the final superheaters or new boiler
designs, where the tubes of the final superheater are not exposed to highly corrosive flue
gases. As the high corrosion-resistant materials increase, the boiler purchase costs and
lifecycle costs increase too, and so new designs have been studied to avoid corrosion
problems.
2.3 Improved maintenance
As already stated, with the majority of the lifecycle costs set during the design phase of the
plant, as the reliability and maintainability depend on the plant design, the maintenance costs
once the plant is in operation are important for the power producers as they are the only
source of costs the owner of the plant can truly affect. This cost optimization can be done by
selecting equipment that has high maintainability i.e. can be maintained easily and cost-
effectively. The maintenance method selection should be adaptive so that the experiences
gained during operation are evaluated and the method selections are reassessed and
questioned at regular intervals. This process should be continuous, particularly when the plant
is ageing and the wearing of the equipment parts starts to appear.
3 RELIABILITY IMPROVEMENTS
The driving forces for several major design changes during the history of CFB technology
have been the call for higher efficiency and dependability and the changes in the boiler
operation environment. Boiler manufacturers will have to focus their efforts to develop more
reliable boilers to remove design-based reliability bottlenecks encountered in former designs
and earlier generations of boilers. The development will have to be based on the actual
experiences gained from operating boiler units. This means that manufacturers will have to
have a constant and continuous contact with the customer in order to get effective feedback.
5
Based on feedback, boiler manufacturers have been focusing on the reliability and
maintainability of their boilers, especially the most critical parts: boiler pressure parts and
refractory-lined structures. There have been two major steps in CFB boiler design
development that can be used as landmarks to separate CFB boiler generations:
- development of modern water- or steam-cooled separators to replace heavy
refractory-covered hot cyclones in order to minimize refractory problems
- development of INTREXTM superheaters inside solids flow in bed material
return loop to minimize superheater erosion and corrosion and to enable firing
of more challenging fuels with high corrosion potential.
The effects of development steps towards the latest generation of CFB boilers discussed in
this paper are based on the actual experiences and statistics of CFB units of the large
industrial size and smaller utility-scale from 120 MWth up to 400 MWth with both fossil and
bio fuels and their mixtures as the main fuel. Units with recycled fuels as the main fuel
components (>20%) are excluded from this analysis.
3.1 Solids separator development
In the early years boilers had heavy refractory-covered cyclone separators. The refractory
structures experienced cracking and from time to time also sustained major damage as the
anchoring could not hold the refractory tiles especially in the cyclone roof area. Annually this
meant approx. 56 hours of unavailability on average, varying from near 0 to over 100 hours.
The total share of refractory damage was almost 20% of all the unavailability. The heavy
refractories also required significant maintenance during scheduled maintenance outages.
The damage in heavy refractories identified the need to improve the refractory materials and
to minimize the amount of refractory in a boiler. This lead to the invention of cooled separator
structures, where the separator walls are water- or steam-cooled and covered with only
relatively thin cast refractory material for erosion protection. Also the structures of the sand
return leg are cooled.
Figure 1 below compares the average annual unavailability hours of hot cyclone design and
modern cooled separator design boilers. The hours for each are presented in two categories:
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unavailability hours caused by cyclone/separator refractory damages and the unavailability
hours caused by sand return system and the expansion joints. The hours are presented as a
function of unit nominal thermal power.
Figure 1: Average annual unavailability hours of hot cyclone design and modern cooled
separator design in smaller utility-scale boilers
0.025.050.075.0
100.0125.0150.0175.0200.0
0 50 100 150 200 250 300 350 400
Boiler thermal power [MWth]
Hot cyclone refractories
Sand return leg and exp. joints
Cooled separator refactories
INTREX sand return
Even though there had been some minor damage due to installation errors in the refractories
of cooled separators, the total amount of refractory and expansion joint damage and the
unavailability due to these issues has decreased dramatically. The average annual unavailable
hours in smaller utility-scale CFB boilers caused by refractory damage decreased from over
55 hours to less than 3 hours with the use of cooled separators, which corresponds to the
decrease from 20% to 2 % of the total unavailability.
In large utility-scale boilers the statistics are less detailed due to the relatively limited number
of reference boilers, but the improvement in unavailable hours seems to be even bigger, as
Figure 2 shows.
Figure 2: Average annual unavailability hours of hot cyclone design and modern cooled
separator design in large utility-scale boilers.
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0.050.0
100.0150.0200.0250.0300.0350.0400.0
0 100 200 300 400 500 600
Boiler thermal power [MWth]
Hot cyclone refractoriesSand return leg and exp. jointsCooled separator refactoriesINTREX sand return
At the same time the maintenance costs of refractories decreased, as there was no damage
during operation, and the amount of refractory replacement and the other refractory
maintenance needed at every outage decreased to near zero. The percentage of refractories
replaced annually in an approx. 300 MWth unit with a hot cyclone design can be as high as
10% of the total refractory amount, with an average of 5%. The share of the total annual
maintenance costs accounted for by refractory repairs could be as high as 40%, with an
average of 25% as can be seen in Figure 3.
Figure 3: Annual maintenance costs for refractories and for boiler plant in total, both in
annual costs and related to boiler thermal power.
0.0
250.0
500.0
750.0
1000.0
1250.0
1500.0
0 50 100 150 200 250 300 350 400Boiler thermal power [MWth]
Ann
ual c
osts
[kE
UR
/a]
0.0
2.5
5.0
7.5
10.0
12.5
15.0
Cos
ts/M
W [k
EU
R/M
W,a
]
Total maint. costs [kEUR/a]Refractory maint. costs [kEUR/a]Total maint. costs [kEUR/MW,a]
The hot cyclone design also requires large expansion joints, which can also experience some
damage and although they are not as critical component as the refractories, they also require
quite a lot of maintenance. The annual average unavailability caused by expansion joints and
bed material return system has decreased from 4 hours to 3 hours.
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3.2 Superheater development
Early CFB boilers had different types of superheaters in addition to traditional convective
superheaters. Typically they were located in the flue gas flow before the boiler separator, such
as Omega superheaters. Omega superheaters are horizontal superheaters inside the furnace
penetrating the furnace front and rear walls. The penetration points form a discontinuity point
in solids flow and tended to cause erosion in nearby furnace wall tubes. Leaking steam from
furnace wall tubes caused further erosion in superheater tubes. After such tube damage the
area of tubes that needed repair was large and the repairs were quite slow as the penetration
points had to be rebuilt and the repair required scaffolding. The lowest tube rows are equipped
with erosion shields to protect against direct erosion by bed material. Also the tube shields
experience some problems, especially with shield hangers causing the shields to drop off,
exposing tubes to erosion. The erosion damage in Omega superheaters caused an average of
15 hours of unavailability, which is almost 10% of the total annual unavailability. In some
boilers even 70-90% of the annual unavailability was connected to Omega superheaters. That
corresponds to 190-300 annual unavailable hours. As the trend continues towards the use of
cheaper, lower-quality and recycled fuels with high corrosion potential in high temperatures
together with higher steam values, this increases the corrosion potential in the other
convective superheaters, mainly the final superheaters. For this reason, new superheater
designs were studied.
The first commercial INTREX superheater in industrial-scale boilers with recycled fuels was
built in 1997, and in 1999 this technology was applied to utility-scale plants. It is a tube
bundle superheater which is located in a separate chamber in the bed materials circulation
loop in the loop seal as a final superheater. Its tubes are covered in fluidized bed material with
no contact with the flue gases and fly ash flow and therefore has a low corrosion potential. As
the fluidization velocity in the chamber is moderate or small the erosion effect is minimal or
non-existent. Using an INTREX superheater as a final superheater enabled the combustion of
recycled materials, even waste like refuse-derived fuel (RDF) in boilers with relatively high
steam values.
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Figure 4 compares the average annual unavailability hours of superheaters in boilers with
Omega superheaters with the unavailability hours of modern design boilers with a INTREX
superheater. The data for each is presented in two categories: unavailability hours caused by
Omega/ INTREX leakages and the unavailability hours caused by leakages in convective
superheaters. The hours are presented as a function of unit nominal power.
Figure 4: Average annual unavailability hours for superheaters in boilers with Omega
superheaters compared with modern design boilers with INTREX superheater.
0.0
10.0
20.0
30.0
40.0
50.0
0 50 100 150 200 250 300 350 400
Boiler thermal power [MWth]
Conv. SHs in Omega design
Omega SHs
Conv. SHs in INTREX design
INTREX SHs
With INTREX superheaters the annual forced outage hours due to superheater tube failures
decreased as there was no need for Omega superheaters. The average annual unavailability
hours in utility-scale CFB boilers caused by superheater leakages decreased with the INTREX
design from approximately 15 hours to less than 5 hours. In addition to INTREX tube failures
the annual unavailability hours of convective superheaters decreased from 9 hours to
practically zero. As the final superheater, the INTREX enables lower steam temperature after
the last convective superheater stage and therefore decreases the corrosion in convective
superheaters.
3.3 Availability improvement due to new designs
With design improvements the overall availability of CFB units has increased in CFB boilers
burning bio and fossil fuels. The graph in Figure 5 presents the overall development of boiler
availability in CFB units. Data are sorted by the commissioning year of the boiler.
Figure 5: Overall development of boiler availability in CFB units.
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90.0 %
92.0 %
94.0 %
96.0 %
98.0 %
100.0 %
1988 1990 1992 1994 1996 1998 2000 2002 2004 2006Year
Hot cyclone separator
Cooled separator, no INTREX
Cooled separator with INTREX
Recently fuel has become more and more challenging also in utility-scale units as the co-
combustion of several fuels has increased. Even though the fuels are more challenging,
availability has been maintained at the same level as 20 years ago and has even slightly
increased. The effect of more challenging fuels can be seen in Figure 6 where the annual
availability figures are presented sorted by relative fuel difficulty on the x-axis. The relative
fuel difficulty is normalized to the most difficult fuel in the reference boiler group.
Figure 6: Annual availability related to relative fuel difficulty
90.0 %
92.0 %
94.0 %
96.0 %
98.0 %
100.0 %
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Relative fuel difficulty
Hot cyclone separator
Cooled separator, no INTREX
Cooled separator with INTREX
The trend lines in graph show that the advantages of modern CFB design become more
significant when the fuel becomes more challenging.
This paper is focusing on the utility-scale units which in practice means that the percentage of
recycled fuels in the analysed units is relatively low, less than 20%. In CFB units with more
than 20% of recycled fuels the availability has been only slightly lower. In recycled wood
units the decrease in availability due to fuel has been only less than one percent and even in
entirely waste-fired boilers the availability has been near 90%. This may seem like a low
availability (Should this be unavailability?), but without the latest INTREX design CFB
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boilers would not in practice be suitable for burning recycled fuels, especially 100% waste
fuels, with today’s requirements for high steam values.
4 MAINTAINABILITY IMPROVEMENTS
Reliability is the key issue in optimizing boiler lifecycle profit, but maintainability
development is today becoming a more important tool towards achieving a low-maintenance
CFB boiler. Along with the design changes targeted at improved reliability of the CFB boiler,
the boiler maintainability is also taken into account in CFB boiler development. In CFB boiler
pressure part and layout design the maintainability improvements are mainly focusing on the
accessibility of the boiler parts, but also the ease of dismantling, repairing and reassembling,
as well as the working positions and overall safety.
In CFB boilers the maintainability is mostly taken into account in superheater placement in
order to maximize the accessibility of the tubes for case of emergency repair. The INTREX
design itself improves boiler maintainability as the superheaters are not located inside the
furnace at a high level requiring scaffolding for even minor maintenance or repair. The tube
packages are located at a relatively low level and are designed so that they can be removed
from the chamber easily, enabling fast repairs outside of the dark and dirty conditions of the
boiler furnace. The location of the chamber can be equipped with appropriate space, hoists
and fixed scaffolding for the repair works or even total replacements.
In the latest CFB designs, especially for corrosive fuels like wastes, the location and design of
convective superheaters is also optimized for the fast maintenance action and tube bundle
replacements. Also the convective superheaters are brought to a low level and there is enough
free space reserved for lifting up the entire tube bundles for repair or replacement. A hoist can
be also provided at the location for the task.
The maintainability of even normally “maintenance-free” equipment, like valves, may be
crucial for the entire plant availability in case of emergency repairs and therefore must be
considered in the design phase. In this task 3D design programs have provided a new
approach as the operators and maintenance staff of a new plant can “walk through” the virtual
3D model of the unit in order to find the bottlenecks in equipment accessibility and other
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maintainability bottlenecks. Together with a range of maintainability analysis methods this
kind of approach has expanded the ability to design for maintainability far beyond the
traditional equipment and layout design methods.
5 AGEING
5.1 CFB boiler ageing
In CFB boiler availability there is clearly the early phase with so called “teething problems”
or “infant mortality”. In later phases of the boiler lifetime, after a “constant availability”
period, the availability tends to decrease as the equipment tend to “wear out” when the boiler
is ageing. Depending on the equipment the phase when ageing starts to show in availability
statistics varies a lot as do the ways of ageing. Based on the statistics the first signs of
decrease in total boiler availability can be seem after 15 to 17 years as can be seen in Figure 7.
Figure 7: CFB unavailability over time
0.0 %
1.0 %
2.0 %
3.0 %
4.0 %
5.0 %
0 5 10 15 20
Boiler age [years]
Hot cyclone separator
Cooled separator
The availability starts to decrease because of the following reasons:
- Some equipment or structures simply wear out due to erosion, corrosion, abrasion or
some other slowly developing defect types. In CFB boilers these defects are typically
erosion in economizer and furnace wall tubes, corrosion in ESP plates, and increased
erosion defects in fuel feeding lines, ash and sand systems and air nozzles.
- Some equipment exposed to mechanical or thermal cycling starts to develop cracking or
creeping as the material reaches the limit of such a cycles. The effects of mechanical
13
cycling can be seen in the increased incidence of fan impeller damage after 18-20 years.
Thermal cycling also increases boiler tube damages outside the flue gas flow.
- Electrical components usually have very evenly distributed lifetimes, but in larger
electrical systems, like fan frequency converters, the ageing behaviors can be seen after
15-18 years of operation due to dust, excessive heat and other conditions that shorten
the lifetime of electrical components.
- The increasing number of failures in the equipment causes abnormal operation
situations like fast dynamic changes which further increase the ageing of the other
equipment.
5.2 Counter actions against ageing
The plant owners or managers have already, at the beginning of a unit’s operation, selected a
maintenance strategy such as total productive maintenance or reliability-centered
maintenance, based on the economic or technical limits set by the operating environment. In
the early phase of the operation, when most of the equipment is as good as new, the
maintenance can be mainly proactive, either time-based or condition-based. For some
equipment the selected maintenance strategy may even be reactive while the other equipment
are maintained according to a schedule or based on simple condition measurements like
bearing vibration levels. As the boiler is ageing the probability of an equipment failure tends
to increase, which calls for the re-evaluation of the maintenance strategy. Re-evaluation of the
maintenance strategy should be done even before the plant ageing is visible in the availability
statistics. There is a need for re-evaluation if:
- the number of failures in an equipment is increasing;
- there are clear signs of wear-out in visual inspections;
- the condition of equipment in inspections is otherwise worse than before; or
- the consumption of an input (oil, water, electricity) has increased dramatically.
As the type of ageing and the age when it starts to show are strongly dependant on the
equipment itself, the re-evaluation should be done individually for each piece of equipment.
This may require quite a lot of work, but will finally pay for itself.
The uncritical equipment which can be replaced quite easily at low cost may still be
maintained reactively, but for the critical equipment that may stop the unit operation if a
14
failure occurs, the re-evaluation should be done. For some of the most critical components
like boiler pressure parts the need for maintenance increases as the probability for tube
failures increases with age. In practice this means that the condition of an equipment item
should be monitored constantly even though the actual maintenance actions or inspections
may not be performed more frequently than during the early phase of the boiler lifetime. This
sets pressures for moving towards predictive maintenance. Instead of replacing the pressure
parts when the tube thickness is worn below the change limit, or dealing with tube leakage
repairs, the predictive maintenance strategy is trying to predict when a certain part should be
replaced.
Predictive maintenance requires that the equipment condition is monitored constantly “on-
line” or at least regularly. The level of condition monitoring should be based on the
maintenance strategies and the decisions for applying monitoring procedures for each item of
equipment should be based on the results of the maintenance strategy re-evaluation. The
condition monitoring alone does not provide the means to decrease the effects of equipment
ageing, but will provide means for the maintenance improvements. The investment in
monitoring systems or equipment is not the critical issue; the most critical issue is the
commitment of maintenance and the operational staff to frequently monitor the condition of
the equipment and the commitment to extend the equipment lifetime with effective
maintenance. Usually this calls for moving towards more sophisticated maintenance strategies
and methods.
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Primary contact and author:
Mr Eero Hälikkä
Development Engineer
Foster Wheeler Energia Oy
P.O. BOX 201
FIN-78201 VARKAUS
Tel +358 (0)10 393 7492
Fax +358 (0)10 393 7743
Co-authors:
Mr Dariusz Lewinski
Technical Director
CHP ELCHO (Elektrocieplownia ELCHO Chorzów)
tel. +48-32-771-40-00
fax. +48-32-771-40-20
Mr Kazimierz Szynol
Operating Director
Południowy Koncern Energetyczny S.A.
Ul. Lwowska 23
40 - 389 Katowice
Tel.: +32 731 22 11
Faks: +32 731 22 12
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