VGB PowerTech - Autorenexemplar - © 2015>>> VGB DIGITAL <<<

VGB

Pow

erTe

ch -

Aut

oren

exem

plar

- ©

201

5

78

Measurement of sulphuric acid in CCGT VGB PowerTech 3 l 2016

Authors

Kurzfassung

Schwefelsäure-Messungen im ppb-Bereich in gas-befeuerten Kombikraftwerken

Abhitzekessel (AHK) sind Teil von Kombikraft-werken. Aufgrund der schwefelhaltigen Ein-satzbrennstoffe kommt es zu Korrosion im AHK und in Folge dessen zu sauren Emissionen und Immisisonen (saure Partikel in direkter Um-gebung der Kraftwerkstandorte). Vor diesem Hintergrund wurden im Rahmen eines Projek-tes SO2, SO3/H2SO4 sowie der Säuretaupunkt im Rauchgas eines AHK gemessen. Das Projekt hat gezeigt, dass SO2 und SO3 im Reingas im ppb-Bereich gemessen werden können. Das glei-che gilt für den Säuretaupunkt im Reingas, der unterhalb 80 °C ermittelt wurde. l

Sulphuric acid measurements at ppb levels at a natural gas-fired CCGTChristine Koczab, Frans van Dijen, Chetan Chothani and Mark Lauwers

Christine Koczab, Engineer MSc.Dr. Frans van Dijen, Engineer MSc.Laborelec/ENGIE Linkebeek, Belgium Chetan Chothani, Engineer BSc.Breen Energy Solutions Carnegie, Pennsylvania, USAMark Lauwers, Engineer MSc.Electrabel/ENGIE, Esch-sur-Alzette, Luxemburg

Introduction

Sulphuric acid corrosion is commonly ob-served in CCGT-HRSG systems. This is also true for CCGTs operated by ENGIE and other companies in the industry. Sulphu-ric acid corrosion results in emissions of acid corrosion products like particles that are entrained together with flue gases via stack. Immissions of these acid particles are observed by surrounding neighbour-hoods and are a cause of concern for neigh-bours and relevant authorities alike.Corrosion of the HRSGs and the accompa-nying emissions triggered actions within ENGIE, including studies. One of these studies was related to measurements of ADP (acid dew point) and SO2 and SO3/H2SO4 emissions with the flue gases. The sulphuric acid originates from sul-phur present in natural gas. One source of sulphur (S) is the organic compound added to natural gas to give it an odour. Other potential sources of S are H2S from natural gas and H2S from biogas and/or bio-methane, which is injected into natu-ral gas. Corrosion by sulphuric acid occurs both during operation and stand-still. The iron sulphate formed on the tubes at about 100 °C is very hygroscopic and acidic, with a pH of about 1. During stand-still, corro-sion products absorb water and corrosion proceeds. The sulphuric acid corrosion of HRSGs is described in literature [1]. With the CCGT in Luxemburg, the maximum S content of the natural gas, as guaranteed, is 30 mg/Nm3. Based on the S content of the natural gas of about 8.1 mg/Nm3 as meas-ured, the SO2 content of the flue gases of the CCGT was estimated to amount to ap-proximately 580 µg/Nm3. The deposition of H2SO4 on the surface of the tube bundles reduces the concentration of SO3/H2SO4 in the flue gases at the stack.If a DENOX plant (selective catalytic reduc-tion, SCR) is available in the HRSG for the reduction of NOx emissions, ammonia slip also has to be considered. SO3 may react with ammonia, forming ammonium sul-phate (AS) and/or ammonium bisulphate (ABS). With moisture, AS and especially ABS may cause corrosion of steel. For our case of a HRSG and natural gas with low S-content, AS and ABS may deposit on the steel surfaces at temperatures of about be-tween 150 and 200 °C, depending on the concentrations of ammonia and SO3. The slip of ammonia down-flow of the SCR is

usually less than 5 mg/Nm3. The formation of AS and/or ABS also reduces the concen-tration of SO3/H2SO4 in the flue gases at the stack. Cleaning of the tube bundles and corro-sion prevention are important. Corrosion of the HRSG at the flue gas side during stand-still is prevented or limited by heat-ing the HRSG to about 30 °C and keeping the relative humidity inside below some 30 %. Heating of the HRSG during stand-still to about 30 °C also prevents or limits corrosion at the air side (under the insu-lation) by water condensation. Much lit-erature is available about corrosion pre-vention of HRSGs at the water-steam side during operation and stand-still. Laborelec is involved in several studies regarding pre-vention of emissions of corrosion products and corrosion prevention.

Flue gas measurements

A study was performed regarding meas-urements in the flue gases of the ADP and the SO2 as well as SO3 concentrations. Ob-viously, the standards for manual meas-urement of SO2 and SO3 in flue gases are not made for concentrations below about 1 ppm, down to some ppb. Also, methods for measurement of the ADP of flue gases are (usually) not made for ADPs of about 80 °C and lower. For the manual measurement of SO3/H2SO4, both the condensation method and the salt method seemed suitable [2]. From the SO3/H2SO4 content and the moisture content of the flue gases, the ADP can be calculated. For the measurements, Labore-lec selected the condensation method, which was adapted for measuring at ppb level. The ADP was measured using the Breen Energy AbSensor – ADM measure-ment system. This method is based on the acid condensation between 2 electrodes embedded within a polished glass surface whose temperature is controlled by modu-lating cooling air flow. The AbSensor-ADM system utilises a kinetic algorithm to meas-ure ADP, which sets it apart from prior in-struments utilising similar measurement methodologies. The AbSensor ADM cools the probe surface down until some conden-sate is detected (formation temperature) and then heats it up until the condensate is fully evaporated (evaporation tempera-ture), and monitors the temperature and current between these end points. Heuris-

VGB PowerTech - Autorenexemplar - © 2015>>> VGB DIGITAL <<<

VGB

Pow

erTe

ch -

Aut

oren

exem

plar

- ©

201

5

79

VGB PowerTech 3 l 2016 Measurement of sulphuric acid in CCGT

tic algorithms are used to determine the peak current position where the formation and evaporation rates are in equilibrium and the corresponding temperature is re-corded as the ADP. F i g u r e 1 shows a typi-cal measurement cycle.The Breen Energy AbSensor device was installed upstream of the tube bundles of the evaporator and condensate preheater, which are situated in the upper part of the vertical HRSG (F i g u r e 2). These tube bundles showed most sulphuric acid cor-rosion. Also the measurements of SO2 and SO3/H2SO4 were installed upstream of these tube bundles. The same holds true for the measurement of the moisture con-tent. The H2O measurement was based on the condensation method, standard EN 14790. The SO2 measurement was based on ab-sorption in an aqueous H2O2 solution, standard EN 14791. The SO3 measurement was based on the controlled condensation, standard VDI 2462, 2011, and similar. The methods were adapted by increas-

ing the sampling times to 6 and 14 hours. With the SO3 measurements, the cooling temperature was reduced to about 55 °C as well. The ADP was also measured us-ing the LAND probe ADM220 at different positions. The measuring principle of the LAND probe is similar to that of the Breen Energy probe, but it uses a different algo-rithm, equilibrium as opposed to kinetic, and is not adapted for measuring an ADP below 125 °C.

The manual measurements were per-formed during a week, and the load of the CCGT varied not much with time, with between 80 and 100 %. The results are presented in Ta b l e 1. The ADP was also calculated based on the SO3 and moisture content of the flue gases. Nm3 is expressed dry and actual vol.% O2 (13.6 vol.%). With the Breen Energy AbSensor device, the probe was running for about 6 weeks, with start-up and shutdown of the CCGT each week. With the Breen Energy Ab-Sensor device, the measured ADP varied between 55 and 65 °C (F i g u r e 3 and Ta b l e 2). Sometimes the ADP increased with time to 75 °C. Solid deposits formed on the probe with time. These deposits were recovered and analysed with the scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS). Most likely, these deposits consist of “normal” dust and corrosion products. Some particles found on the probe were high in Fe and/or S indeed.

Discussion

When studying sulphuric acid-induced corrosion of steel surfaces, it soon became clear that it is not the ADP of the flue gases which is important, but the ADP of the sur-face. Due to the hygroscopic character of corrosion products and capillary conden-sation, the moisture content of the surface is elevated, compared to the flue gases [3 to 5]. This increases the ADP. Besides, dust on the surface may act as nuclei for the formation of sulphuric acid droplets. It is also important to consider the tem-peratures of the steel surfaces instead of the temperature of the flue gases. This all explains very well why sulphuric acid-induced corrosion is observed at (surface) temperatures between some 80 and 100 °C, which is well above the flue gas tempera-ture at the stack of about 80 °C and well above the ADP of the flue gases of about 70 °C. It also explains why the ADP as measured increases with time, when start-ing measuring with a clean Breen Energy AbSensor device, which experiences foul-ing with time. According to our knowledge, there is no standard method for measuring SO2 and SO3 in flue gases at ppb levels. Therefore, the quality of measurements as performed

Probe temperature

Probe current

Condensation detectedformation temperature

Equilibrium dew point ADP

Material vaporisedevaporation temperature

Fig. 1. A typical measurement cycle with the Breen AbSensor device.

Breen AbSensorADM installation

Fig. 2. Breen AbSensor installation.

Tab. 1. Results of manual flue gas measurements: ADP, SO2 and SO3/H2SO4.

Unit Average Standard deviation

Moisture vol.% 7.45 0.46

SO3 ppbv 6 3

SO3 µg/Nm3 21 10

ADP, calculated °C 70 4

SO2 ppbv 223 24

SO2 µg/Nm3 636 67

Conversion SO2 to SO3 % 3 1

VGB PowerTech - Autorenexemplar - © 2015>>> VGB DIGITAL <<<

VGB

Pow

erTe

ch -

Aut

oren

exem

plar

- ©

201

5

80

Measurement of sulphuric acid in CCGT VGB PowerTech 3 l 2016

are not guaranteed. To guarantee the re-sults, it is required to validate and adapt the methods at these operating conditions, e.g. with calibrated gas with ppb levels of SO2 and SO3.Breen Energy postulated that deposition of solids on the probe tip mirrors similar deposits found on the HRSG tube bundles and therefore not only the surface ADP of the AbSensor device increases over time, but is also most representative of condi-tions on the tube surfaces as well. The ADP, as measured by the AbSensor device and increasing over time, stabilised in the 70 to

Breen Energy – AbSensor – Data PlotFlue gas temperature ADP

Several weeks of continuous operation with start-ups and shut-downs

Acid dew point (ADP) measured is in the range of 55 to 75 °C

Max. ADP measured seemsto have stabilised in the70 to 75 °C range

Unit shutdown – no condensable detected

Date, time

Tem

pera

ture

in °

C

300

250

200

150

100

50

0

93.3

65.5

37.8

10

2/19/15 12:00 AM 2/24/15 12:00 AM 3/1/15 12:00 AM 3/6/15 12:00 AM 3/11/15 12:00 AM 3/16/15 12:00 AM

From start-up to full load,from water dewpoint toH2SO4 ADP increases over time

Fig. 3. AbSensor device, increase of the measured ADP with time.

75 °C range. Laborelec suggests that due to the fact that deposits/corrosion products, both on the AbSensor device and the tube bundles, are hygroscopic, the ADP of the surface of the probe is increased compared to the ADP of the flue gases. Capillary con-densation of sulphuric acid on the depos-its of the probe increases the surface ADP as well. The measured ADP of the flue gas using the AbSensor device, of between 55 and 75 °C, proved difficult to link with vari-ation of the operating of the CCGT, howev-er, as the CCGT went through its start-up/shutdown cycles, thereby passing through the water dew point, additional solids were collected on the AbSensor device resulting in higher surface ADP. A similar effect can be predicted for the tube surfaces as well.According to the documentation of the supplier, the LAND probe is not suited for measuring ADP of less than 125 °C. This was confirmed by the efforts of Laborelec

regarding measuring the low ADP in this case anyway, which proved to be in vain.

Conclusions

The measured concentration of SO2 in the flue gases could correspond to the antici-pated theoretical concentration, based on the S-content of the natural gas. There is less conversion of SO2 to SO3 than anticipated, with about 3 % as measured. The ADP of the flue gases as calculated gives the same order of magnitude as the ADP measured with the Breen Energy Ab-Sensor device.Measurement of SO2 and SO3 at ppb lev-els in “clean” flue gases seems possible. It seems also possible to measure the ADP of “clean” flue gases of below 80 °C.Corrosion of HRSGs by sulphuric acid dur-ing operation and standstill needs more attention, especially preventing emissions and immissions of acid particles, and pre-venting sulphuric acid corrosion.Data collected during these tests provides clues to the phenomenon where tubes ex-perience corrosion activity at temperatures well above the bulk flue gas ADP.

References [1] Craig, B.: Identify, correct HRSG gas-side cor-

rosion problems, heat-recovery steam genera-tors. 2006 Outage Handbook, Combined Cy-cle Journal 2005, 3rd Quarter 2005, 14-20.

[2] Koczab, C., and van Dijen, F.: SO3/H2SO4 measurements in flue gases – Difficulties and solutions. VGB PowerTech, 3, 2014, 52-56.

[3] Badea, G.E., Cret, P., Lolea, M., and Setel, A.: Studies of carbon steel corrosion in atmos-pheric conditions. Acta Technica Corvinien-sis, Bulletin of Engineering, Tome IV (2011).

[4] Morcillo, M., Otero, E., Chico, B., and de la Fuente, D.: Atmospheric corrosion studies in a decommissioned nuclear power plant. Nu-clear Power, Pavel Tsvetkov (ed.), Chapter 12, InTech, 2010, www.intechopen.com.

[5] The role of relative humidity in corrosion, Munters Corporation, presentation, 2001. l

Tab. 2. Results of flue gas ADP measurements with the Breen Energy ABSensor de-vice.

Unit Average

ADP measured °C 55 to 65, up to 75

 

 

 

 

 

 

 

FIND & GET FOUND! POWERJOBS.VGB.ORG

ONLINE–SHOP | WWW.VGB.ORG/SHOP

JOBS IM INTERNET | WWW.VGB.ORG

International Journal for Electricity and Heat Generation

Please copy >>> fill in and return by mail or fax

Yes, I would like order a subscription of VGB PowerTech.The current price is Euro 275.– plus postage and VAT.Unless terminated with a notice period of one month to the end of the year, this subscription will be extended for a further year in each case.

Return by fax to

VGB PowerTech Service GmbHFax No. +49 201 8128-302

or access our on-line shop at www.vgb.org | MEDIA | SHOP.

Name, First Name

Street

Postal Code City Country

Phone/Fax

Date 1st Signature

Cancellation: This order may be cancelled within 14 days. A notice must be sent to to VGB PowerTech Service GmbH within this period. The deadline will be observed by due mailing. I agree to the terms with my 2nd signature.

Date 2nd Signature

Vo lu me 89/2009 · ISSN 1435-3199

K 43600

In ter na tio nal Edi ti on

Focus: Power Plants in Competiton

New Power Plant Projects of EskomQuality Assurance for New Power PlantsAdvantages of Flexible Thermal Generation

Market Overview for Imported Coal

In ter na tio nal Jour nalfor Elec tri ci ty and Heat Ge ne ra ti on

Pub li ca ti on ofVGB Po wer Tech e.V.www.vgb.org

Vo lu me 89/2009 · ISSN 1435-3199

K 43600

In ter na tio nal Edi ti on

Focus: VGB Congress

Power Plants 2009

Report on the Activities

of VGB PowerTech

2008/2009

EDF Group Reduces

its Carbon Footprint

Optimising Wind Farm

Maintenance

Concept for Solar

Hybrid Power Plants

Qualifying Power Plant Operators

In ter na tio nal Jour nal

for Elec tri ci ty and Heat Ge ne ra ti on

Pub li ca ti on of

VGB Po wer Tech e.V.

www.vgb.org

Con gress Is sue

Vo lu me 89/2009 · ISSN 1435-3199

K 43600

In ter na tio nal Edi ti on

Focus: Furnaces, Steam Generators and Steam TurbinesUSC 700 °C Power Technology

Ultra-low NOx Combustion

Replacement Strategy of a Superheater StageEconomic Post-combustion Carbon Capture Processes

In ter na tio nal Jour nalfor Elec tri ci ty and Heat Ge ne ra ti onPub li ca ti on ofVGB Po wer Tech e.V.www.vgb.org

Vo lu me 90/2010 · ISSN 1435-3199

K 43600

In ter na tio nal Edi ti on

Fo cus: Pro Quality

The Pro-quality

Approach

Quality in the

Construction

of New Power Plants

Quality Monitoring of

Steam Turbine Sets

Supply of Technical

Documentations

In ter na tio nal Jour nal

for Elec tri ci ty and Heat Ge ne ra ti on

Pub li ca ti on of

VGB Po wer Tech e.V.

www.vgb.org

V

00634 K

9913-5341 NSSI · 5002/58 emulo

International Edition

Schwerpunktthema:

Erneuerbare Energien

Hydrogen Pathways

and Scenarios

Kopswerk II –

Prevailing Conditions

and Design

Arklow Bank

Offshore Wind Park

The EU-Water

Framework Directive

International Journal

for Electricity and Heat Generation

Publication of

VGB PowerTech e.V.

www.vgb.org

Vo lu me 89/2009 · ISSN 1435-3199

K 43600

In ter na tio nal Edi ti on

Focus: Maintenance

of Power Plants

Concepts of

IGCC Power Plants

Assessment of

Generators for

Wind Power Plants

Technical Data for

Power Plants

Oxidation Properties

of Turbine Oils

In ter na tio nal Jour nal

for Elec tri ci ty and Heat Ge ne ra ti on

Pub li ca ti on of

VGB Po wer Tech e.V.

www.vgb.org

PowerTech-CD/DVD!Kontakt: Gregaro Scharpey Tel: +49 201 8128-200mark@vgb.org | www.vgb.org

Ausgabe 2014: Mehr als 1.100 Seiten Daten, Fakten und Kompetenz aus der internationalen Fachzeitschrift VGB PowerTech

(einschließlich Recherchefunktion über alle Dokumente)Bruttopreis 98,- Euro incl. 19 % MWSt. + 5,90 Euro Versand (Deutschland) / 19,90 Euro (Europa)

Jetzt auch als

Jahres-CD 2014

mit allen Ausgaben

der VGB PowerTech

des Jahres: nur 98,– €

Fachzeitschrift: 1990 bis 2014

Diese DVD und ihre Inhalte sind urheberrechtlich geschützt.© VGB PowerTech Service GmbH

Essen | Deutschland | 2015

· 1990 bis 2014 · · 1990 bis 2014 ·

© S

erge

y N

iven

s - F

otol

ia

VGB PowerTech DVD 1990 bis 2014: 25 Jahrgänge geballtes Wissen rund um die Strom- und Wärmeerzeugung Mehr als 25.000 Seiten Daten, Fakten und Kompetenz

Bestellen Sie unter www.vgb.org > shop