special print (2021) - powerplant chemistry 2010, 12(4
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PPCHEM JOURNAL ▪ SPECIAL PRINT (2021)
PPCHEM SPECIAL PRINT
PPCHEM® Journal ▪ www.ppchem.com ▪ SPECIAL PRINT (2021) ▪ PowerPlant Chemistry 2010, 12(4), 246–251
The Journal of All Power & Plant Chemistry Areas
S P E C I A L P R I N T
ANALYTICAL INSTRUMENTS
Water Steam Cycles
Conductivity monitor with pH value and alkalizing reagent concentration calculation
AMI Deltacon DGAutomatic and continuous measurement of total, cation and degassed cation conductivity. Re-boiler according to Larson-Lane (ASTM D4519-94).
Swan Analytische Instrumente AGCH-8340 Hinwil ∙ [email protected]
Sampling, Monitoring, Analytics
Effects of Steam Sample Degassing on CCGT Station Start-up ProfilePeter J. ClarkPowerPlant Chemistry 2010, 12(4), 246–251
PPCHEM JOURNAL ▪ SPECIAL PRINT (2021)
PPCHEM SPECIAL PRINT
© 2021 by PPCHEM AG. All rights reserved.
246 PowerPlant Chemistry 2010, 12(4)
PPChem Effects of Steam Sample Degassing on CCGT Station Start-up Profile
INTRODUCTION
This paper is designed to address the effect of carbon
dioxide upon steam cation conductivity and to ascertain
whether or not the actual conditions of the steam entering
the steam turbine can be assessed more reliably when
using the degassed cation conductivity monitoring. The
theory and conclusions gained from this study can be
applied to both base and peak load power plants,
although peak load power plants shut down and start up
their steam turbines more often than base load stations,
so the conclusions may be of more benefit to those partic-
ular sites.
The research and practical investigation was carried out at
Centrica Energy South Humber Bank (SHB), a 1 260 MW
combined cycle gas turbine (CCGT) power station, utiliz-
ing the start-up of the second phase of the plant after a
planned outage.
The current method of analysing steam sample conductiv-
ity in operation at the station is cation (after-cation-
exchange) conductivity monitoring. This technique is not
capable of taking into account the contribution of carbon
dioxide dissolved in the sample, so during start-up, the
time delay to wait for steam cation (after-cation-exchange)
conductivity to go below a certain level (0.2 µS · cm–1) is
elevated due to the presence of carbon dioxide in the
sample. In removing the dissolved carbon dioxide, the
degassed cation (after-cation-exchange) conductivity
measurement should supply more reliable information
about the actual steam quality and possibly allow the
steam to be transferred to the steam turbine earlier. The
scope of the project was to identify the levels of cation
(after-cation-exchange) conductivity that account for dis-
solved carbon dioxide and to determine whether these
levels have a significant impact upon the start-up profile of
a CCGT power station.
ABSTRACT
Many power stations dose feedwater with oxygen scavengers such as carbohydrazide; these compounds remove
the dissolved oxygen but release inorganic carbon dioxide into the water. The effect of carbon dioxide upon corro-
sion levels is a controversial subject and as such is not within the scope of the work discussed in this paper. The
effect of carbon dioxide upon conductivity measurements is the major consideration.
Degassed cation conductivity (DGCC) is a widely used technique to remove dissolved gases from high purity water. A
typical DGCC instrument consists of a reboiler which raises the temperature of the sample water above its saturation
temperature, thus reducing the solubility of gases, such as carbon dioxide, effectively boiling the gas out of the water
sample stream.
The present method used for measuring water or steam purity is cation (or acid) conductivity, often denominated as
after-cation-exchange conductivity. This technique should indirectly assess levels of anions such as chloride,
sulphate, formate and acetate for corrosion avoidance purposes. However, due to the presence of carbon dioxide
dissolved in the sample, the monitoring results are not appropriate for this purpose. The degassed cation conductiv-
ity technique can be applied to power station start-ups when the steam conditions have to be monitored closely. By
removing the dissolved carbon dioxide from the sample stream, more accurate information about the actual purity of
the water or steam is given. This paper will give the results and economic benefits when this monitoring technique is
applied to a cold start on a combined cycle gas turbine (CCGT) power station.
Effects of Steam Sample Degassing on CCGT Station
Start-up Profile
Peter J. Clark
© 2010 by Waesseri GmbH. All rights reserved.
SAMPLING, MONITORING, ANALYTICS
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PPCHEM JOURNAL ▪ SPECIAL PRINT (2021)
PPCHEM SPECIAL PRINTSAMPLING, MONITORING, ANALYTICS
247PowerPlant Chemistry 2010, 12(4)
PPChemEffects of Steam Sample Degassing on CCGT Station Start-up Profile
There are different methods of degassing water (or con-
densed steam) samples; the most popular ones are pre-
dominantly heating and stripping (gas and membrane) [1].
Boiling was chosen for this investigation as it is a reliable,
proven technology for this application. Membranes such
as those used in total organic carbon monitoring technol-
ogy do not provide such a simple, robust and cost effec-
tive analysis.
South Humber Bank is a triple steam pressure plant pro-
ducing
– high pressure (HP) steam at 91.6 bar
– intermediate pressure (IP) steam at 19.3 bar
– low pressure (LP) steam at 3.8 bar
The degassed cation (after-cation-exchange) monitoring
equipment was put onto the high pressure steam sample
line at a point before the HP steam enters the steam tur-
bine.
INSTRUMENT USED
The instrument used throughout this investigation was the
Swan AMD Degassed Cation Conductivity Monitor. A
schematic of the monitor is shown in Figure 1.
The unit has a reboiler positioned after a cation exchange
column. There are three measurements obtained from the
unit: specific, cation (after-cation-exchange) and degas -
sed cation conductivity. Specific conductivity is the meas-
urement of the condensed steam sample straight from the
sample line, i.e., before a cation exchanger. The cation
conductivity measurement is taken after the ion exchange
column, and degassed conductivity is taken after the
reboiler positioned downstream of the ion exchange col-
umn.
PRELIMINARY INVESTIGATION
Several tests were conducted to test the resilience of the
instrument against differing flow rates and water qualities.
Manual calibration of the monitor was conducted to
establish a baseline of results that data taken from the
monitor could be compared against. The baseline for
degassed cation (after-cation-exchange) conductivity was
established at 0.18 µS · cm–1, in line with the purity of the
feedwater (0.2 µS · cm–1) required for the steam conditions
at plant start-up.
The results of the preliminary investigations were conclu-
sive in that the degassed cation (after-cation-exchange,
Figure 1:
Instrument schematic (courtesy of Swan UK Analytical Instruments).
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PPCHEM JOURNAL ▪ SPECIAL PRINT (2021)
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248 PowerPlant Chemistry 2010, 12(4)
PPChem Effects of Steam Sample Degassing on CCGT Station Start-up Profile
ACE) conductivity returned to the baseline result after 15
minutes for a cold start and 5 minutes for a hot start. The
reboiler has proven its high efficiency.
PRIOR INVESTIGATIONS
A similar experiment was conducted by Pedro Wuhrmann
of Swan Analytical Instruments to prove the effects of
using degassed cation (ACE) conductivity as compared to
cation (ACE) conductivity as a method of monitoring start-
up water purity.
The results from this investigation provide the ideal pat-
tern that the results from the project at South Humber
Bank should mirror and are shown in Figure 2 [2].
The chart shows the marked difference between
degassed cation (ACE) conductivity and classic cation
(ACE) conductivity during start-up. These results were
taken from a newly built power station in Portugal on the
first day of commissioning.
PRIMARY INVESTIGATION
The primary investigation was centred on the cold start of
two of the 160 MW gas turbines after a 2-month planned
outage. The degassed cation (ACE) conductivity monitor
was linked into the high pressure steam sample line corre-
sponding to that of the shut-down HRSG's and inline
results were established throughout the start-up period.
The serial tags of the two turbines monitored were GT21
and GT22 and they will be referred to in this way for the
remainder of the paper.
GT21 Start-up
The commissioning profile for GT21 was in two parts, the
first consisting of a full speed no load followed by three
days of stepped load tests. The second period occurred a
week later and was a ramped profile until sustained high
load (110 MW). Figure 3 shows the start-up profile for the
gas turbine combined with the data from the degassed
cation (ACE) conductivity monitor.
The steam turbine was synchronized at 15:58:00 BST, with
the steam conditions shown to be ready hours before the
steam turbine synchronization. The dashed box shows the
time frame when the degassed cation (ACE) conductivity
(DGCC) instrument was recording results, and this period
is shown in greater magnification in Figure 4. The readings
taken by this instrument clearly give more reliable informa-
tion than the classic cation (ACE) conductivity instrument
with respect to the actual steam purity. (Cation conductiv-
ity levels in the presence of carbon dioxide are sometimes
more than twice the levels measured after carbon dioxide
removal.) The cation (ACE) conductivity readings are also
affected by the addition of dosing chemicals such as car-
bohydrazide that do not affect degassed readings; this
effect is shown on Figure 3 just after 13:12:00 BST when
the cation (ACE) conductivity reading takes a sudden
increase. An assumption can be made that an increase in
the levels of dissolved carbon dioxide (due to decomposi-
tion of carbohydrazide) is the cause as the degassed
cation (ACE) conductivity readings do not report a similar
pattern.
The difference in conductivities seems slight but the
degassed cation (ACE) conductivity reaches the pre-
determined steam condition set-point of 0.5 µS · cm–1 by
2.5
2.0
1.5
1.0
0.5
0
–1
Co
nd
uctivity [
µS
cm
]·
Time [h:min]
09:30
250
200
150
100
50
0
Lo
ad
[M
W]
10:00 10:30 11:00 11:30 12:00 12:30 13:00
10:08 Gas turbine
synchronized 13:03 Degasser off
9:30 Flame on
full speed no load
Acid Degassed Gas turbine load
Figure 2:
Ideal results (courtesy of Swan UK
Analytical Instruments) [2].
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249PowerPlant Chemistry 2010, 12(4)
PPChemEffects of Steam Sample Degassing on CCGT Station Start-up Profile
12:51:00, whereas the classic cation (ACE) conductivity
measurement does not even show such low levels of con-
ductivity. Furthermore, degassed cation (ACE) conductiv-
ity values show fewer fluctuations in the results than the
cation (ACE) conductivity values do.
The large initial spike in both conductivities is the
response to pressurization of the water-tubes in the boiler
and the high concentrations of ions present in these tubes
before start-up.
The steam conditions are at their pre-determined level
much sooner, so theoretically HP steam can be sent to the
steam turbine earlier; however, as discussed at the start of
the this paper, the economic benefit lies with the gas tur-
bine and the analysis of this will be shown below.
GT22 Commissioning
Although sufficient results were obtained from the start-up
profile of GT21, results from GT22 prove the reliability of
both the degassed cation conductivity instrument design
and the technology used. The GT22 commissioning profile
didn't include steam turbine synchronization so only the
patterns of conductivity during the gas turbine start could
be monitored.
Figure 3:
GT21 start-up profile.
ST steam turbine
Figure 4:
GT21 – Difference between cation
(ACE) and degassed cation (ACE)
conductivity of the steam sample.
–1
Co
nd
uctivity [µ
Scm
]·
20
18
16
14
12
10
8
6
4
2
0
Lo
ad
[M
W]
120
100
80
60
40
20
0
Time [h:min]
10:48 12:00 13:12 14:24 15:36 16:48
Degasser switched off
ST28
synchronized
at 15:58
Acid Degassed ST28 GT21
–1
Co
nd
uctivity [
µS
cm
]·
20
18
16
14
12
10
8
6
4
2
0
Lo
ad
[M
W]
60
50
40
30
20
10
0
Time [h:min]
11:31 11:45
Time taken for DGCC to show
lower conductivity than after-
cation techniques
12:00 12:14 12:28 12:43 12:57 13:12 13:26 13:40 13:55 14:09
GT21 set to load
Acid Degassed GT21
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250 PowerPlant Chemistry 2010, 12(4)
PPChem Effects of Steam Sample Degassing on CCGT Station Start-up Profile
This data was collected from Day 3 of the commissioning
program for GT22 so the profile and results are more suit-
able for a warm start-up.
It is observable that the levels of degassed cation conduc-
tivity reach a required steam purity at much earlier times
than the classic cation (ACE) conductivity levels (Figure 5).
Using this in conjunction with Figure 4 gives the time
saved by the use of the degassing technique to be on
average 1 hour after a gas turbine (GT) start. However,
steam conditions are not the only property that deter-
mines the time difference between GT start-up and steam
turbine (ST) synchronization. At South Humber Bank sta-
tion, the other conditions normally take around two hours
to be at the correct specification, but this time frame dif-
fers between sites. Using the degassed cation conductiv-
ity helps in reducing the time gap between GT start-up
and ST synchronization.
BENEFITS AND COST ANALYSIS
A cost analysis for this project is very difficult to determine
as there are differing factors that affect a start-up and as a
consequence the costing. Initially it seemed viable to set
the ST to base load earlier, thus trading power at base
load for a longer time. On closer inspection of the start-up
profile it is only the amount of time the GT is held at
80 MW that can be decreased. This has a direct effect on
the EOH (estimated operating hours) of the gas turbine as
the number of gas turbine operating hours producing
80 MW decreases so the amount of gas turbine operating
hours at base load increases. At 160 MW (base load) the
power station is trading electricity for a higher price for
each operating hour than at 80 MW, increasing the profit
margin.
Another cost analysis method would be to look at the dif-
ferences in efficiency between running the gas turbine for
longer at 80 MW than for a shorter time at 160 MW. At
160 MW the CCGT uses more natural gas but the spark
price (the difference in profit between selling the gas and
converting it into electricity and selling the power) is
higher.
Savings in carbon credits can also be accomplished. One
credit is equivalent to one tonne of carbon; if an industrial
producer is below its credit quota, the credits can be sold
as a commodity to other producers who may have pro-
duced more than their credit quota. By running a gas tur-
bine for a shorter time at start-up the amount of carbon
produced per year decreases so the excess carbon cred-
its can be sold for a profit; this benefit may also be applied
to the planned NOx credits. From an environmental per-
spective a decrease in the amount of carbon produced
every year by reduced running of the gas turbine during
start-up is a large advantage of this monitoring technique.
This economic analysis gives a qualitative perspective on
the benefits of using degassed cation conductivity moni-
toring. The data in this paper shows that this technique
can save operating time during a gas turbine start-up,
which as shown in the economic analysis has several
other benefits when the number of gas turbine start-ups
performed every year is taken into account. A quantitative
–1
Co
nd
uctivity [
µS
cm
]·
Lo
ad
[M
W]
120
100
80
60
40
20
0
Time [h:min:s]
08:24:00 08:38:24 08:52:48 09:07:12 09:21:36 09:36:00 09:50:24 10:04:48
6
5
4
3
2
1
0
Acid Degassed GT22
Figure 5:
GT22 start-up profile.
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251PowerPlant Chemistry 2010, 12(4)
PPChemEffects of Steam Sample Degassing on CCGT Station Start-up Profile
analysis depends upon the type of turbine and differs
between stations and load profiles, so this has not been
discussed in this report.
CONCLUSION
The application of a degassed cation conductivity monitor
allows an earlier start-up of the steam turbine. However,
whilst the steam conditions are ready for steam turbine
synchronization earlier, this doesn't mean the steam tur-
bine can be started earlier; conversely, the gas turbine can
be fired up later preserving its life and allowing trading for
a shorter time at an output of 80 MW. This is a very broad
conclusion gained from the investigation, but it gives an
overall experimental outcome.
Looking more closely at the results, it is clear that the
degassed cation conductivity is a more reliable method of
indicating the purity of the steam sample. In the start-up
graphs, Figures 3–5, the degassed cation conductivity
curves not only show a large decrease in the time taken to
reach acceptably low levels of conductivity, but they also
indicate that degassing of the sample after cation
exchange reduces the amplitude of the oscillations in the
curves. This reduction in amplitude signifies that the use
of a degassed cation conductivity monitor also has the
effect of reducing extreme values in the start-up profile.
The economic benefits of decreasing the time spent wait-
ing for correct steam conditions in a station start-up pro-
file are astounding. Not only is the gas turbine primary
mover held at lower power output for a shorter time,
increasing the time spent with the primary mover held at
higher efficiencies, but the risk of corrosion of plant cycle
components is reduced due to more reliable and precise
determination of the possible presence of corrosive con-
taminants in the plant cycle. The annual turbine operating
hours are also reduced as the gas turbines may be ignited
later in the start-up profile.
REFERENCES
[1] Drew, N., PowerPlant Chemistry 2004, 6(6), 343.
[2] Wuhrmann, P., Cation and Degassed Cation Con -
ductivity, 2008. Paper presented at the Second
International Conference on the Interaction of
Organics and Organic Cycle Chemicals with Water,
Steam and Materials, November 4–6, 2008 (Lucerne,
Switzerland). PowerPlant Chemistry GmbH,
Neulussheim, Germany.
[3] Jonas, O., Machemer, L., Proc., Eighth International
Conference on Cycle Chemistry in Fossil and
Combined Cycle Plants with Heat Recovery Steam
Generators, 2006 (Calgary, Alberta, Canada). Electric
Power Research Institute, Palo Alto, CA, U.S.A.,
1014831, 9-2.
ACKNOWLEDGEMENTS
The author would like to thank Paul Kelk, the South
Humber Bank Station chemist, and the BIAPWS (in partic-
ular Richard Harries, BIAPWS Secretary) for the appoint-
ment as the undergraduate award student 2009, as well as
for the invaluable help and support during and after the
work placement. Thanks also go to Swan Analytical
Instruments (Joern Boedeker, Swan UK) for technical sup-
port during the work period and consent for the use of all
Swan information given in this paper.
THE AUTHOR
Peter J. Clark is a third year undergraduate studying
Chemical Engineering at the University of Birmingham. He
has been in higher education since 2007. He was awarded
the BIAPWS Undergraduate Award placement 2009 in co-
operation with Centrica Energy, the main focus of the
placement being on degassed after-cation conductivity
with a side-interest in total organic carbon measurement
techniques. Peter Clark has been awarded the University
Nash Prize 2009 for excellence in both academia and
extra-curricular activities in the second year. He is a cur-
rent student member of the Institute of Chemical
Engineers.
CONTACT
Peter J. Clark
The Knoll
Thornbury Hill
Alveston (Bristol)
BS35 3LG
United Kingdom
E-mail: [email protected]
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