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Index
1.
Cycling
Operation of Circulating Fluidized Bed Boilers
2.
Fluidized Bed Grid Nozzle Designs (coming soon!)
3.
Petroleum Coke Firing in Circulating
Fluidized Beds (coming soon!)
This article is based on a presentation made at “The Sixth Foster Wheeler Fluidized Bed Customers’ Conference” on August 18, 2000, in San Diego, CA.
The
advent of deregulation of the U.S. power industry is leading to significant
changes in the way circulating fluidized bed boilers (CFB’s) are being
operated. Prior to deregulation, most
CFB’s were operated in a base-loaded mode.
The turbulent variations in power prices brought about by deregulation,
however, are forcing many of these plants to carefully evaluate their plant
operation and in many cases to operate in a cycling mode to maximize
profits. The key to survival for many
of these plants is to “match generation to power pricing.”
CFB’s
have generally been operated in a base-loaded mode because they typically
represent the state-of-the-art power generation technology in a system. CFB boilers provide excellent combustion and
emissions performance and typically replace or displace older, less efficient
and dirtier boilers. For any power
generating system to reach its maximum performance it is necessary to ensure
that the best technology is fully utilized at all times. Older technology is only used as standby or
auxiliary generation to make up requirements that the fully utilized “higher
technology” cannot meet.
Another
main reason for the traditional operation of CFB’s in a base-loaded mode is
that many of the power contracts for independent power producers were written
in compliance with governmental regulations meant to encourage development of
the independent power industry. In most
cases, the contracts were long-term and at a relatively stable price. In addition, the utilities generally had to
accept nearly all of the generated power.
Many of these long term contracts are expiring and/or are undergoing
changes due to the deregulation of the power industry.
So
although most CFB operating experience has been with base-loaded operation,
this not the whole story. While the
independent power producers were perfecting base load operation, the industrial
and university power sectors were gaining experience with CFB cycling
operation. Both sectors have power and
steam loads which can vary significantly - in university applications, boiler
loads vary regularly during the day, as well as seasonally, while industrial
operations tend to have varying boiler loads during the day as production
increases or decreases with shift operation and also may have very rapid load
swings if high energy consuming machinery is brought quickly on or off
line. Much of the knowledge needed to
convert CFB’s to cycling operation comes from these two sources.
Conversion
of CFB boiler operation from base-loaded to cycling requires significant
changes for both the boiler operating staff and the boilers themselves. Boiler operators of a base-loaded unit
typically get the boiler set-up for full load operation early in the life of
the boiler and then cruise at this altitude.
Boiler transient load operation - when boiler operation tends to be most
problematical - generally provides only a minor inconvenience, because it does
not occur frequently. When operating in
a cycling mode, however, the inconvenience of transient load operation becomes
a major consideration, because of the increased frequency of the
transients.
With
the introduction of cycling operation a boiler operator needs to understand the
dynamics of transient load operation in his particular plant. This includes the response of steam
conditions, combustion and perhaps most importantly fluegas emissions to
transient and part load operation, so that operation of the boiler is not
jeopardized. In most cases, operators
will require training aimed at teaching them how to effectively manage the
technical and financial aspects of CFB operation brought about by the
conversion from base-load to cycling operation.
Most
CFB’s were not specifically designed for cycling operation. As a result, low or transient load is generally
“on the edge” of the originally designed operating envelope. Without control system or mechanical
equipment modifications boilers converted to cycling operation may be unable to
meet long term operating requirements.
In particular, the combustion and emissions processes are most affected
by transient or low load operation.
Each
CFB installation is unique, because of the almost unlimited combinations of
solid fuels, sorbents (for emissions control), regulated plant emissions
levels, and steam conditions. As
multi-boiler CFB owners know, even identical boilers with identical fuels and
sorbents tend to operate differently.
Operation of an already “unique” boiler at the edge of it’s operating
envelope, magnifies it’s differences from other boilers. What this means is that, although general
principles apply (these will be explained later), every CFB boiler requires a
unique approach when converting to cycling operation.
When
converting from base-loaded to cycling operation, the 3 most important issues
are:
How Low Can I
Go? – What is the lowest operating load that can be reliably maintained and
what can be done to reduce that minimum?
When the price received for selling power is less than what it costs to
generate it, the lower the minimum operating load, the better.
How
Fast Can I Get There? – Once the minimum operating load is defined, the faster
it can be reached, the more money can be saved. This is probably less of a concern when cycling for cost savings
than when trying to meet specific load requirements (as in industrial and
university applications), but it is still important.
What
Is The Long Term Effect On My Boiler? – Thermal cycling of materials of
construction affects the life cycle of all mechanical equipment.
Although
it has already been said that most CFB’s operate in a base-load mode at or near
MCR, nearly all CFB boilers have been designed to operate as low as 35% MCR
without auxiliary fuel firing. Despite
being designed for operation down to 35% MCR, most boilers control systems have
not been tuned to operate frequently at or to transition smoothly to that
load. Experience with tuning CFB’s for
part load operation down to 35% MCR indicates that although sometimes time
consuming, the process is generally pretty painless. The changes generally involve minor air curve modifications and
proper setting of control loop parameters, and load indexing of gain and
integral functions.
Several
CFB’s have demonstrated the ability to operate satisfactorily at boiler loads
down to 25% MCR. Much more aggressive
work with the control systems is typically required to reach these lower loads
on a regular basis. In many cases,
operation this low will not be possible without modifications to the mechanical
equipment. In many CFB’s it may be
possible to reach loads less than 25% MCR with physical modifications to the
mechanical equipment.
Furnace
temperature is the main issue when trying to define minimum operating load of a
CFB. In particular, insufficient
furnace temperature is usually the problem.
At MCR load, the furnace temperature is typically between 1550°F and
1650°F with a relatively homogeneous temperature profile throughout the furnace
and cyclone. As load is reduced below
MCR, the furnace temperature begins to sag with the temperature high in the
furnace dropping more quickly than the temperature in the lower, refractory
lined zone. Somewhere between 50% - 70%
MCR, the combustion airflow no longer decreases and becomes constant. Decreasing the load further without
decreasing airflow increases dramatically the rate of furnace temperature
reduction.
Furnace
temperature at part load primarily affects or depends on the following:
·
Combustion Stability. Because of
the even bed temperatures and lack of a “flame” in a CFB, the furnace
temperature needs to stay high enough to ensure that the fuel can reliably
“auto combust”. Fuel characteristics
significantly affect the combustion process and thus the temperature required
for stable combustion. In general, at
low temperatures high volatile fuels burn better than low volatile fuels and
low ash fuels burn better than high ash fuels.
At the same time though, some low volatile fuels may actually burn
better than high volatile fuels at low load, because of the specific combustion
(and temperature) profile that they produce in the furnace. The control system is typically set-up to
define the minimum operating temperature as 1250°F, which is conservative for
just about all. In specific cases,
proper testing and modification may allow for reduction in the minimum
temperature.
·
Emissions. Emissions generation
and reduction reactions from a CFB are significantly affected by furnace
temperature. In general, the various
fluegas emissions interact with each other and the interactions change with
different furnace temperatures.
-
CO increases with
decreasing furnace temperature and boiler load. Although CO generally is higher at lower loads, the increase above
full load operation levels is usually minimal, because nearly all CO is
combusted anyway due to the excess oxygen and high combustion residence
time. Low load CO emissions are fuel
dependent with some fuels causing problems at low load.
-
NOx
typically decreases with decreasing furnace temperatures. NOx emissions are also sensitive
to excess oxygen and tend to increase with increasing excess air (which
typically increases as boiler load decreases).
If the boiler is equipped with a selective non-catalytic reduction
(SNCR) system for NOx reduction, furnace temperature plays an even
more important role, because the reduction reaction requires a higher
temperature to be effective. To
maintain emissions at an acceptable level, the uncontrolled NOx
emissions must drop off rapidly enough that the uncontrolled emission is less
than the regulated level when the SNCR reaction becomes ineffective. An additional complication is that NOx
emissions tend to increase during transient load operation, because of a transiently
high Ca/S ratio.
-
SO2
control generally is optimized at moderate temperatures. At high furnace temperatures (>1650°F) SO2
capture is reduced, because the limestone pores plug quickly with reactants
preventing effective utilization of the entire surface area of the limestone
particle. At lower temperatures
(<1400°F), the reduction reaction slows down also making SO2
removal less effective. Both high and
low furnace temperatures result in reduced SO2 removal efficiency,
which can both fortunately usually be made up for by increasing the Ca/S
ratio. The difficulty is that
increasing the Ca/S ratio has the unwanted effect of increasing NOx
emissions.
·
Excess Combustion Air. Excess
combustion air is typically constant from MCR down to 50%-70% MCR. At that point, the combustion airflow
generally stays constant as load decreases to maintain adequate fluidizing
velocities and to minimize the chances of localized bed defluidization and bed
agglomeration. As the excess air
increases, the furnace temperature drops rapidly because the ratio of cooling
air (the excess air) to energy in the fuel (fuel feed rate) increases
continuously. The minimum airflows are
typically set quite conservatively during boiler design and start-up. Site specific testing and modifications have
the potential to significantly reduce the minimum airflows and thus maintain a
higher furnace temperature at minimum load.
·
Fluidizing Grid Performance.
Performance of the fluidizing grid at low load is closely tied to excess
air. The tendency of a grid to plug or
develop uneven airflow resulting in bed agglomerations ultimately determines
minimum airflow (and thus excess air) that must pass through the nozzles. Different nozzles have different pluggage
characteristics. Nozzles with a reduced
tendency to plug can operate at lower airflows resulting in lower excess air
and higher furnace temperatures (at low load).
Fuel ash characteristics also have an effect on grid performance, because
uneven bed fluidization can be tolerated at low load, if a particular fuel has
a lower tendency toward bed agglomeration or sintering.
·
Steam Conditions. Furnace
temperature and excess air work together to control the heat transfer to the
superheaters. Under certain conditions
main steam temperatures may not be able to be met at part load. This may or not be a concern for the turbine
and boiler operation, but it needs to be taken into consideration.
In summary, the
minimum stable operating load will be that at which the furnace temperature is
adequate to maintain stable combustion while simultaneously maintaining fluegas
emissions compliance and avoiding grid nozzle pluggage and bed agglomeration.
In some cases, the
best course of action when trying to operate at low loads for more than about 4
hours may be to shut the boiler down hot.
There are typically no emissions problems when shutdown, and the
operating costs are about as low as they can go without turning out the lights
and sending everyone home. The concept
behind shutting down the boiler hot is to maintain as much heat in the boiler
(both gas/solids and steam sides), so that a hot or warm restart can be quickly
accomplished. A CFB provides
significant advantages over other combustion technologies in terms of hot or
warm restarts, because the bed ash and relatively large refractory mass hold a
large amount of heat and effectively insulate the boiler from heat loss.
Typically hot
restarts can be accomplished quickly if initiated within 8 hours after shutting
off the boiler. In many cases, all that
is needed is an air purge (safety mandated) and the furnace temperature will
still be high enough for immediate introduction of solid fuel. In other cases a short period of time on
auxiliary fuel will be required to bring the furnace temperature back up to the
solid fuel firing permissive. Shutdowns
longer than about 8 hours will generally take longer to restart, because more
heat will have been lost from the bed and have to be replaced during the
restart process. Twelve to eighteen
hours is about the longest the boiler can be shutdown and still effectively
perform a hot or warm restart of the boiler.
Hot shutdowns
require operators that are specifically and well trained in shutting down and
restarting the boiler hot. All the operating
decisions that typically can be made over a 12 hour period need to be made
simultaneously when shutting down and restarting hot. Every moment of indecision during a shutdown or restart means
more heat lost and a longer restart time.
It is very important that clear operating guidelines be defined for the
operators and that they fully know them and understand the reasoning behind the
various operations.
The main technical
issues with shutting down and restarting hot are:
·
Greater Thermal Cycles on Refractory and Pressure Parts. When shutting down and restarting the
boiler, the thermal cycles of the mechanical equipment will be deeper than
during simple cycling operation. Some
consideration, needs to be given to life of the refractory and pressure parts.
·
Fluidizing Grid Pluggage.
Certain types of fluidizing grid nozzles are prone to pluggage during
shutdown and restart operation. In these
cases, the nozzles may need to be replaced with a non-plugging type if hot
shutdowns/restarts are done on a frequent basis.
·
Bed Sintering. Certain fuels may
have a tendency to result in sintered bed ash when the boiler is shutdown
hot. High alkali and/or high sulfur
fuels have the greatest tendency to agglomerate. High ash fuels on the other hand generally have a lower tendency
to bed sintering. Most fuels will not
have a sintering problem when shutdown hot, but bed sintering should be
considered when making a decision to go to hot shutdown/restart operation.
Once the minimum
load has been defined, the next question is how quickly the minimum load can be
reached. Although boiler load ramp rate
is not as important when cycling for cost-reduction as when tracking an external
load, it is still an important consideration.
There are many ways to define boiler load ramp rate, but unless they
include some definition of allowable steam condition fluctuations they are
relatively meaningless. For the
purposes of this discussion, the load ramp is defined as the maximum boiler
load rate of change that results in maximum steam temperature and pressure
fluctuations that will be considered acceptable to most IPP boiler owners. The typical “out of the box” maximum ramp
rate is about 4-5% MCR/min. A load ramp
rate of 2-3% MCR/min is more likely to meet “utility acceptable” criteria for
maximum steam condition fluctuations.
The main concerns
when determining the maximum load ramp rate are steam conditions and fluegas
emissions. As the boiler load demand is
changed, it’s heat input needs to respond quickly to maintain the main steam
conditions, especially pressure. Proper
control of steam conditions depends on proper tuning of the steam temperature
and pressure control loops as well as the interface loop(s) with the turbine and
extraction steam systems. The shape of
the CFB’s air curves and the tuning of the combustion air system also have a
significant effect on the response of the boiler to load changes. In general, boiler load changes are most
dramatically affected by grid airflow changes.
The tuning of the airflow control loops has to be optimized to maintain
the desired airflows when boiler load is changing.
As the boiler
responds to load demand changes by changing the firing rate, it affects the
combustion conditions that generate emissions.
It is to be expected that fluegas emissions will experience significant
transient variations during boiler load changes. During a load reduction SO2 emissions will normally
drop while NOx emissions will typically increase. This is primarily a result of a high
effective Ca/S during the load reduction as well as a lag in reduction of the
excess air. Conversely, when load is
increased, SO2 will typically increase and NOx decrease,
because of a low effective Ca/S.
Transient CO emissions are generally not a problem, unless the airflow
control system is so poorly tuned that excess air drops to very low
levels. Set-up and tuning of the
combustion air control system is essential for optimizing emissions performance
during transient boiler operation.
Depending on the specific emissions requirements, various operating
schemes can be developed to allow more rapid load changes by allowing certain
emissions to climb for short periods and then being averaged with subsequently
lower emissions to meet whatever time-averaged emissions are required.
Control system
set-up and tuning for frequent transient operation is highly site
specific. Some of the things that will
significantly affect the tuning are:
fuel characteristics, damper control, combustion air fan
characteristics, turbine characteristics, emissions reduction systems and
allowable emissions levels.
It is clear that
cycling operation will have some detrimental effect on the life of a
boiler. Thermal cycling of nearly all
materials of construction (primarily pressure parts and refractory in a boiler)
results in transient differential thermal expansion causing undesirable stress
in the materials. In the case of
pressure parts, the most significant stresses occur where different components
are connected together. At these
connections, differential thermal expansion will tend to pull the connections
apart or push them together deforming the metal. In the case of refractory, either thin layers of refractory are
expanding at a different rate than the pressure parts they are attached to or a
thick refractory lining is expanding at different rates at different depths in
the refractory. In either case, the
refractory will tend to develop cracks.
It is generally
accepted that normal cycling operation of a boiler has some effect on the
boiler life, but in most properly designed boilers the reduction in life should
be insignificant. When operating in a
hot shutdown/restart mode, the effect on boiler life will be more significant,
because of the greater thermal changes experienced. In either case, especially the hot shutdown/restart mode, it is
recommended that the boiler manufacturer perform a review of the pressure part
design to minimize the chance of experiencing any severe mechanical problems
from frequent cycling.
As deregulation of
the power industry continues the conversion of CFB boilers from base-loaded to
cycling operation will become more common.
Although CFB’s have not traditionally been operated in a cycling mode,
they have demonstrated very good performance under cycling conditions. With the proper control system set-up and
tuning and possibly some mechanical equipment modifications, nearly every CFB
boiler can meet the owners expectations for efficient, clean and economical
operation when operated as a load cycling plant.