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Effect of sulphur containing additive on initial corrosion of superheater tubes in waste fired boiler
Swerea KIMAB AB, Stockholm, Sweden.
Outokumpu Stainless AB, Avesta, Swden.
Vattenfall Power Consultants AB, Göteborg, Sweden.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
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2009 (English)In: Corrosion Engineering, Science and Technology, ISSN 1478-422X, E-ISSN 1743-2782, Vol. 44, no 3, 234-240 p.Article in journal (Refereed) Published
Abstract [en]

The major drawback to generating electricity from waste fired boilers is the rapid corrosion of superheaters which increases the maintenance costs. Within the last few years, it has been shown that additions of ammonium sulphate to biomass fired boilers decrease the corrosion tendencies. This paper reports on the effects of ammonium sulphate on corrosion in a waste fired CFB boiler. Air cooled probes were exposed at a position corresponding to the one of superheater tubes. The probe temperature was 500 degrees C, corresponding to a steam temperature of similar to 450 degrees C. Both the austenitic steel EN1.4301 (Fe-18Cr-9Ni) and the low alloyed ferritic steel EN1.7380 (Fe-2.25Cr-1Mo) were tested. During exposure, the concentration of alkali chlorides in the flue gas was measured and a decrease was observed when adding ammonium sulphate. After 4 h of exposure, the probes were removed for detailed analysis with SEM-EDS, TOF-SIMS and XRD. The sides of the tubes facing the flue gas were covered with a calcium rich deposit, while relatively more sodium and potassium were present on the lee side. The results also show that ammonium sulphate shifted the deposit composition from chloride rich and highly corrosive, to one significantly less corrosive and dominated by sulphates of sodium, potassium and calcium. Metallography shows a marked difference in corrosion attack between the two steels. Iron chlorides accumulate at the metal/oxide interface of the ferritic steel, while the amounts of iron chlorides were significantly lower in the austenitic steel. These results indicate that ammonium sulphate has the potential to reduce corrosion in waste fired boilers and that austenitic stainless steels are more likely to resist corrosion in these environments than low alloyed ferritic steels.

Place, publisher, year, edition, pages
2009. Vol. 44, no 3, 234-240 p.
Keyword [en]
Waste fired boilers, Superheaters, Power generation, High temperature corrosion
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-32408DOI: 10.1179/174327809X419203ISI: 000269201500010OAI: oai:DiVA.org:kth-32408DiVA: diva2:410544
Note

QC 20110414

Available from: 2011-04-14 Created: 2011-04-14 Last updated: 2017-12-11Bibliographically approved
In thesis
1. High temperature corrosion during waste incineration: characterisation, causes and prevention of chlorine-induced corrosion
Open this publication in new window or tab >>High temperature corrosion during waste incineration: characterisation, causes and prevention of chlorine-induced corrosion
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Waste-fired boilers suffer severely from corrosion of critical components such as superheater tubes. In this work the high temperature corrosion of candidate superheater alloys have been investigated by detailed laboratory studies and controlled field exposures in full-scale boilers.

In a laboratory study the detrimental effect of gaseous hydrochloric acid (HCl) on three  different ground surface and preoxidised austenitic stainless steels was investigated. Exposures were conducted in an environment comprising N2-10O2-5H2O-0.05HCl at both 400 °C and 700 °C. A positive effect of preoxidation is evident when the alloys are exposed at 400 °C. Oxide layers formed during preoxidation effectively suppress chlorine ingress and lower the corrosion rate for all three materials while accelerated corrosion and chlorine accumulation at the metal/oxide interface is detected for ground surface specimens. The positive effect of preoxidation is lost at 700 °C and corrosion resistance is dependent on alloying level. At 700 °C metal chloride evaporation contributes significantly to the material degradation. Based on the results, high temperature corrosion in the presence of gaseous HCl is discussed in general terms.

 In two different waste-fired boilers measures for counteracting superheater corrosion were investigated. In a grate-boiler the deposit formation and high temperature corrosion of some candidate superheater materials were studied. Metal loss measurements showed unacceptably high corrosion rates for the lower alloyed ferritic steels 13CrMo44 (Fe-1Cr-0.5Mo) and HCM12A (Fe-11Cr-2W), as well as for the austenitic Super 304 (Fe-18Cr-9Ni-3Cu). The corrosion attack for these alloys was manifested by the formation of mixed metal chloride/metal oxide scales. A different type of behaviour was seen for the higher alloyed austenitic steels and nickel-base alloys, which were able to form a chromium-enriched oxide next to the metal. However, the alloys suffered from localised pitting attack. Since analyses of the deposit revealed appreciable amounts of low melting salt mixtures such as ZnCl2-KCl, PbCl2-KCl, FeCl2-KCl and NaCl-NiCl2, oxide dissolution in these molten salts is the probable reason for pitting attack. In a waste-fired boiler ammonium sulphate solution was added to the flue gas and the effect on flue gas and deposit composition was evaluated. It was evident that the sulphur-rich additive reduced the amount of alkali chlorides in both the flue gas and the deposit. Results also indicated that the initial corrosion rates were lowered with the use of ammonium sulphate. It was concluded that using the additive could be a possible strategy for changing the flue gas chemistry so that superheater corrosion is mitigated.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. 50 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2011:18
Keyword
high temperature corrosion, waste incineration, superheater tubing, steel, alkali, chlorine, suphur
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-32412 (URN)
Presentation
2011-04-08, Ytkemiska Institutet, Drottning Kristinas väg 51, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20110414

Available from: 2011-04-14 Created: 2011-04-14 Last updated: 2013-11-07Bibliographically approved
2. Superheater corrosion in biomass and waste fired boilers: Characterisation, causes and prevention of chlorine-induced corrosion
Open this publication in new window or tab >>Superheater corrosion in biomass and waste fired boilers: Characterisation, causes and prevention of chlorine-induced corrosion
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomass and waste fired boilers suffer severely from corrosion of critical components such as superheater tubes. In this work high temperature corrosion of superheater alloys, and methods to mitigate the problem, have been investigated by laboratory studies and controlled field exposures in commercial boilers.

In Paper I, laboratory work investigated the detrimental effect of gaseous hydrochloric acid (HCl) on austenitic stainless steels at two different temperatures and two different surface treatments. At a lower temperature, a positive effect of preoxidation was apparent, effectively suppressing chlorine ingress and lowering the corrosion rate for all three materials. Chlorine accumulation at the metal/oxide interface was observed only on the ground surface specimens. At a higher temperature, the beneficial effect of preoxidation was lost and corrosion resistance depended on the alloying level. In this case, chloride evaporation contributed significantly to the material degradation. Based on the results, high temperature corrosion in the presence of HCl(g) is discussed in general terms.

In Papers II and III, corrosion during waste incineration was investigated for a number of candidate superheater alloys. Laboratory and field exposures revealed that lowalloyed steels/carbon steels are more vulnerable to metal chloride formation and accelerated attack than candidate stainless steels. Boiler exposures showed unacceptably high corrosion rates for the lower alloyed ferritic steels and austenitic stainless steels. The corrosion attack for these alloys was manifested by the formation of mixed metal chloride/metal oxide scales with poor protective properties. Different behaviour was seen for the higher alloyed austenitic steels and nickel-base alloys, which developed a chromium-enriched oxide next to the metal and metal chloride formation was suppressed. However, these alloys suffered from localised pitting attack. Deposit analyses revealed substantial amounts of low melting salt mixtures such as zinc chloride-potassium chloride and lead chloride-potassium chloride. Molten mixtures of corrosion products and deposit compounds such as iron chloride-potassium chloride and sodium chloride-nickel chloride were also observed. It was evident that oxide dissolution in molten salts limits the performance of these alloys in waste-to-energy plants.

In Papers IV-VI, the use of additives to avoid condensation of alkali chlorides on the tube surfaces was investigated and promising results were obtained by injecting ammonium sulphate into the flue gas stream. In more detail, the work investigated effects of the sulphate additive while firing the boiler at different air excess ratios (λ- values) showing a beneficial effect of increased air excess with faster sulphation reactions and less corrosion attack. Furthermore, in a comparison between ammonium sulphate and mono ammonium phosphate different behaviour was observed for the two additives. While ammonium sulphate captures alkali both in the gas phase and in the solid phase, mono ammonium phosphate reacts only in the gas phase. These findings explain why flue gas measurements and deposit measurements do not always correlate. Finally, in a study injecting ammonium sulphate in a waste-to-energy plant it was shown that the additive could also be used to significantly reduce alkali chlorides in the flue gas and deposit in waste fired boilers.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 55 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2013:7
Keyword
high temperature corrosion, waste, biomass, superheater tubes, steel, alkali, chlorine, sulphur
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-120438 (URN)978-91-7501-645-0 (ISBN)
Public defence
2013-04-19, F3, Lindstedtsvägen 26, KTH, Stockholm, 13:00 (English)
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Supervisors
Note

QC 20130408

Available from: 2013-04-08 Created: 2013-04-05 Last updated: 2013-04-08Bibliographically approved

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