effects of steam sample degassing on ccgt station start-up ...ppchem.net/free/ppchem...

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 carbondioxide upon steam cation conductivity and to ascertainwhether or not the actual conditions of the steam enteringthe steam turbine can be assessed more reliably whenusing the degassed cation conductivity monitoring. Thetheory and conclusions gained from this study can beapplied to both base and peak load power plants,although peak load power plants shut down and start uptheir 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 atCentrica Energy South Humber Bank (SHB), a 1 260 MWcombined cycle gas turbine (CCGT) power station, utiliz-ing the start-up of the second phase of the plant after aplanned 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 notcapable of taking into account the contribution of carbondioxide dissolved in the sample, so during start-up, thetime delay to wait for steam cation (after-cation-exchange)conductivity to go below a certain level (0.2 µS · cm–1) iselevated due to the presence of carbon dioxide in thesample. In removing the dissolved carbon dioxide, thedegassed cation (after-cation-exchange) conductivitymeasurement should supply more reliable informationabout the actual steam quality and possibly allow thesteam to be transferred to the steam turbine earlier. Thescope 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 theselevels have a significant impact upon the start-up profile ofa CCGT power station.

ABSTRACT

Many power stations dose feedwater with oxygen scavengers such as carbohydrazide; these compounds removethe 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. Theeffect 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. Atypical DGCC instrument consists of a reboiler which raises the temperature of the sample water above its saturationtemperature, thus reducing the solubility of gases, such as carbon dioxide, effectively boiling the gas out of the watersample stream.

The present method used for measuring water or steam purity is cation (or acid) conductivity, often denominated asafter-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 dioxidedissolved 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. Byremoving the dissolved carbon dioxide from the sample stream, more accurate information about the actual purity ofthe water or steam is given. This paper will give the results and economic benefits when this monitoring technique isapplied 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.

Heft2010-04:Innenseiten 18.04.10 19:30 Seite 246

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 suchas 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) monitoringequipment was put onto the high pressure steam sampleline at a point before the HP steam enters the steam tur-bine.

INSTRUMENT USED

The instrument used throughout this investigation was theSwan AMD Degassed Cation Conductivity Monitor. Aschematic of the monitor is shown in Figure 1.

The unit has a reboiler positioned after a cation exchangecolumn. There are three measurements obtained from theunit: specific, cation (after-cation-exchange) and degas -sed cation conductivity. Specific conductivity is the meas-urement of the condensed steam sample straight from thesample line, i.e., before a cation exchanger. The cationconductivity measurement is taken after the ion exchangecolumn, and degassed conductivity is taken after thereboiler positioned downstream of the ion exchange col-umn.

PRELIMINARY INVESTIGATION

Several tests were conducted to test the resilience of theinstrument against differing flow rates and water qualities.Manual calibration of the monitor was conducted toestablish a baseline of results that data taken from themonitor could be compared against. The baseline fordegassed cation (after-cation-exchange) conductivity wasestablished at 0.18 µS · cm–1, in line with the purity of thefeedwater (0.2 µS · cm–1) required for the steam conditionsat 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).




ACE) conductivity returned to the baseline result after 15minutes for a cold start and 5 minutes for a hot start. Thereboiler has proven its high efficiency.

PRIOR INVESTIGATIONS

A similar experiment was conducted by Pedro Wuhrmannof Swan Analytical Instruments to prove the effects ofusing degassed cation (ACE) conductivity as compared tocation (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 HumberBank should mirror and are shown in Figure 2 [2].

The chart shows the marked difference betweendegassed cation (ACE) conductivity and classic cation(ACE) conductivity during start-up. These results weretaken from a newly built power station in Portugal on thefirst day of commissioning.

PRIMARY INVESTIGATION

The primary investigation was centred on the cold start oftwo of the 160 MW gas turbines after a 2-month plannedoutage. The degassed cation (ACE) conductivity monitorwas linked into the high pressure steam sample line corre-sponding to that of the shut-down HRSG's and inlineresults were established throughout the start-up period.The serial tags of the two turbines monitored were GT21and GT22 and they will be referred to in this way for theremainder of the paper.

GT21 Start-up

The commissioning profile for GT21 was in two parts, thefirst consisting of a full speed no load followed by threedays of stepped load tests. The second period occurred aweek later and was a ramped profile until sustained highload (110 MW). Figure 3 shows the start-up profile for thegas turbine combined with the data from the degassedcation (ACE) conductivity monitor.

The steam turbine was synchronized at 15:58:00 BST, withthe steam conditions shown to be ready hours before thesteam turbine synchronization. The dashed box shows thetime frame when the degassed cation (ACE) conductivity(DGCC) instrument was recording results, and this periodis shown in greater magnification in Figure 4. The readingstaken by this instrument clearly give more reliable informa-tion than the classic cation (ACE) conductivity instrumentwith respect to the actual steam purity. (Cation conductiv-ity levels in the presence of carbon dioxide are sometimesmore than twice the levels measured after carbon dioxideremoval.) The cation (ACE) conductivity readings are alsoaffected by the addition of dosing chemicals such as car-bohydrazide that do not affect degassed readings; thiseffect is shown on Figure 3 just after 13:12:00 BST whenthe cation (ACE) conductivity reading takes a suddenincrease. An assumption can be made that an increase inthe levels of dissolved carbon dioxide (due to decomposi-tion of carbohydrazide) is the cause as the degassedcation (ACE) conductivity readings do not report a similarpattern.

The difference in conductivities seems slight but thedegassed cation (ACE) conductivity reaches the pre-determined steam condition set-point of 0.5 µS · cm–1 by

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Acid Degassed Gas turbine load

Figure 2:

Ideal results (courtesy of Swan UKAnalytical Instruments) [2].




12:51:00, whereas the classic cation (ACE) conductivitymeasurement does not even show such low levels of con-ductivity. Furthermore, degassed cation (ACE) conductiv-ity values show fewer fluctuations in the results than thecation (ACE) conductivity values do.

The large initial spike in both conductivities is theresponse to pressurization of the water-tubes in the boilerand the high concentrations of ions present in these tubesbefore start-up.

The steam conditions are at their pre-determined levelmuch sooner, so theoretically HP steam can be sent to the

steam turbine earlier; however, as discussed at the start ofthe 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-upprofile of GT21, results from GT22 prove the reliability ofboth the degassed cation conductivity instrument designand the technology used. The GT22 commissioning profiledidn't include steam turbine synchronization so only thepatterns of conductivity during the gas turbine start couldbe 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.

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GT21 set to load

Acid Degassed GT21



This data was collected from Day 3 of the commissioningprogram 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 timesthan the classic cation (ACE) conductivity levels (Figure 5).Using this in conjunction with Figure 4 gives the timesaved by the use of the degassing technique to be onaverage 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 steamturbine (ST) synchronization. At South Humber Bank sta-tion, the other conditions normally take around two hoursto 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-upand ST synchronization.

BENEFITS AND COST ANALYSIS

A cost analysis for this project is very difficult to determineas there are differing factors that affect a start-up and as aconsequence the costing. Initially it seemed viable to setthe ST to base load earlier, thus trading power at baseload for a longer time. On closer inspection of the start-upprofile it is only the amount of time the GT is held at80 MW that can be decreased. This has a direct effect onthe EOH (estimated operating hours) of the gas turbine asthe number of gas turbine operating hours producing80 MW decreases so the amount of gas turbine operating

hours at base load increases. At 160 MW (base load) thepower station is trading electricity for a higher price foreach operating hour than at 80 MW, increasing the profitmargin.

Another cost analysis method would be to look at the dif-ferences in efficiency between running the gas turbine forlonger at 80 MW than for a shorter time at 160 MW. At160 MW the CCGT uses more natural gas but the sparkprice (the difference in profit between selling the gas andconverting it into electricity and selling the power) ishigher.

Savings in carbon credits can also be accomplished. Onecredit is equivalent to one tonne of carbon; if an industrialproducer is below its credit quota, the credits can be soldas 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 carbonproduced per year decreases so the excess carbon cred-its can be sold for a profit; this benefit may also be appliedto the planned NOx credits. From an environmental per-spective a decrease in the amount of carbon producedevery year by reduced running of the gas turbine duringstart-up is a large advantage of this monitoring technique.

This economic analysis gives a qualitative perspective onthe benefits of using degassed cation conductivity moni-toring. The data in this paper shows that this techniquecan save operating time during a gas turbine start-up,which as shown in the economic analysis has severalother benefits when the number of gas turbine start-upsperformed every year is taken into account. A quantitative

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Figure 5:

GT22 start-up profile.




analysis depends upon the type of turbine and differsbetween stations and load profiles, so this has not beendiscussed in this report.

CONCLUSION

The application of a degassed cation conductivity monitorallows an earlier start-up of the steam turbine. However,whilst the steam conditions are ready for steam turbinesynchronization earlier, this doesn't mean the steam tur-bine can be started earlier; conversely, the gas turbine canbe fired up later preserving its life and allowing trading fora shorter time at an output of 80 MW. This is a very broadconclusion gained from the investigation, but it gives anoverall experimental outcome.

Looking more closely at the results, it is clear that thedegassed cation conductivity is a more reliable method ofindicating the purity of the steam sample. In the start-upgraphs, Figures 3–5, the degassed cation conductivitycurves not only show a large decrease in the time taken toreach acceptably low levels of conductivity, but they alsoindicate that degassing of the sample after cationexchange reduces the amplitude of the oscillations in thecurves. This reduction in amplitude signifies that the useof a degassed cation conductivity monitor also has theeffect 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 primarymover held at lower power output for a shorter time,increasing the time spent with the primary mover held athigher efficiencies, but the risk of corrosion of plant cyclecomponents is reduced due to more reliable and precisedetermination of the possible presence of corrosive con-taminants in the plant cycle. The annual turbine operatinghours are also reduced as the gas turbines may be ignitedlater 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 SecondInternational Conference on the Interaction ofOrganics 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 InternationalConference on Cycle Chemistry in Fossil andCombined Cycle Plants with Heat Recovery SteamGenerators, 2006 (Calgary, Alberta, Canada). ElectricPower Research Institute, Palo Alto, CA, U.S.A.,1014831, 9-2.

ACKNOWLEDGEMENTS

The author would like to thank Paul Kelk, the SouthHumber 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 asfor the invaluable help and support during and after thework placement. Thanks also go to Swan AnalyticalInstruments (Joern Boedeker, Swan UK) for technical sup-port during the work period and consent for the use of allSwan information given in this paper.

THE AUTHOR

Peter J. Clark is a third year undergraduate studyingChemical Engineering at the University of Birmingham. Hehas been in higher education since 2007. He was awardedthe BIAPWS Undergraduate Award placement 2009 in co-operation with Centrica Energy, the main focus of theplacement being on degassed after-cation conductivitywith a side-interest in total organic carbon measurementtechniques. Peter Clark has been awarded the UniversityNash Prize 2009 for excellence in both academia andextra-curricular activities in the second year. He is a cur-rent student member of the Institute of ChemicalEngineers.

CONTACT

Peter J. ClarkThe KnollThornbury HillAlveston (Bristol)BS35 3LGUnited Kingdom

E-mail: [email protected]


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