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Superheater corrosion in biomass and waste fired boilers: Characterisation, causes and prevention of chlorine-induced corrosion
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
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 [en]
high temperature corrosion, waste, biomass, superheater tubes, steel, alkali, chlorine, sulphur
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-120438ISBN: 978-91-7501-645-0 (print)OAI: oai:DiVA.org:kth-120438DiVA: diva2:614735
Public defence
2013-04-19, F3, Lindstedtsvägen 26, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20130408

Available from: 2013-04-08 Created: 2013-04-05 Last updated: 2013-04-08Bibliographically approved
List of papers
1. HCl-induced high temperature corrosion of austenitic stainless steels under thermal cycling conditions and the effect of preoxidation
Open this publication in new window or tab >>HCl-induced high temperature corrosion of austenitic stainless steels under thermal cycling conditions and the effect of preoxidation
2011 (English)In: Oxidation of Metals, ISSN 0030-770X, E-ISSN 1573-4889, Vol. 76, no 1/2, 111-126 p.Article in journal (Refereed) Published
Abstract [en]

Gaseous HCl released during combustion is one reason for the severe materials degradation often encountered in power generation from waste and biomass. In this study, three stainless steels (the low alloyed EN 1.4982, the standard EN 1.4301 and the higher alloyed EN 1.4845) were tested by repeated thermal cycling in an environment comprising N2–10%O2–5%H2O–0.05%HCl at both 400 and 700 °C. The materials were exposed with ground surfaces and preoxidised at 400 or 700 °C. A positive effect of preoxidation is evident when alloys are exposed at 400 °C. Oxide layers formed during preoxidation effectively suppress chlorine ingress for all three materials, while chlorine accumulation at the metal/oxide interface is detected for surface ground 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 chlorinating environments is discussed in general terms.

National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-120439 (URN)10.1007/s11085-010-9227-1 (DOI)000294691800008 ()2-s2.0-79960315001 (Scopus ID)
Note

QC 20130408

Available from: 2013-04-05 Created: 2013-04-05 Last updated: 2017-12-06Bibliographically approved
2. Materials performance in simulated waste combustion environments
Open this publication in new window or tab >>Materials performance in simulated waste combustion environments
2008 (English)In: Corrosion Engineering, Science and Technology, ISSN 1478-422X, E-ISSN 1743-2782, Vol. 43, no 2, 123-128 p.Article in journal (Refereed) Published
Abstract [en]

Combustion of waste for power generation gives increased fouling and corrosion compared to fossil fuels, and leads to higher operating and maintenance costs. New materials solutions to increase lifetime include austenitic stainless steels, nickel base alloys, coatings and weld overlaying. In this work a simulated waste incineration environment with regularly renewed deposits of KCl - ZnCl(2) has been used to evaluate the performance of candidate materials and to elucidate the operative degradation mechanisms.

Keyword
high temperature corrosion, chloride, deposits, power generation
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-120448 (URN)10.1179/174327808X286365 (DOI)000257881900005 ()
Note

QC 20130408

Available from: 2013-04-08 Created: 2013-04-08 Last updated: 2017-12-06Bibliographically approved
3. Corrosion of superheater materials in a waste-to-energy plant
Open this publication in new window or tab >>Corrosion of superheater materials in a waste-to-energy plant
Show others...
2013 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 105, no SI, 106-112 p.Article in journal (Refereed) Published
Abstract [en]

A major drawback when generating electricity from waste-fired boilers is the rapid corrosion of critical components such as superheater tubes. In this work a number of commonly-used superheater materials have been exposed on internally cooled probes in a waste-fired grate boiler. The investigated materials are the ferritic steel 13CrMo44, the ferritic-martensitic steel HCM12A, the austenitic steels Super 304, 317L and Sanicro 28, and the nickel-base alloys Hastelloy C-2000 and Inconel 625. Short-term exposures (3 h) for analysis of deposit composition and initial corrosion, as well as long-term exposures (1550 h) to investigate corrosion rates and corrosion characteristics have been made. Analysis revealed a deposit dominated by CaSO4, KCl and NaCl, but also appreciable amounts of low melting salt mixtures such as ZnCl2-KCl, PbCl2-KCl, FeCl2-KCl and NaCl-NiCl2. Metal loss measurements showed unacceptably high corrosion rates for 13CrMo44, HCM12A and Super 304. 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, these alloys suffered from some localised pitting attack. The behaviour is explained by oxide dissolution in the molten salts that are present in the deposit.

Keyword
high temperatur corrosion, waste incineration, deposits
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-32407 (URN)10.1016/j.fuproc.2011.06.017 (DOI)000312414400015 ()2-s2.0-84869089947 (Scopus ID)
Note

QC 20130110

Available from: 2011-04-14 Created: 2011-04-14 Last updated: 2017-12-11Bibliographically approved
4. Deposit chemistry and initial corrosion during biomass combustion: The influence of excess O2 and sulphate injection
Open this publication in new window or tab >>Deposit chemistry and initial corrosion during biomass combustion: The influence of excess O2 and sulphate injection
(English)Article in journal (Other academic) Submitted
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-120450 (URN)
Note

QS 2013

Available from: 2013-04-08 Created: 2013-04-08 Last updated: 2013-04-08Bibliographically approved
5. Effects of sulphate and phosphate additives on mitigating superheater corrosion
Open this publication in new window or tab >>Effects of sulphate and phosphate additives on mitigating superheater corrosion
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-120451 (URN)
Note

QS 2013

Available from: 2013-04-08 Created: 2013-04-08 Last updated: 2013-04-08Bibliographically approved
6. Effect of sulphur containing additive on initial corrosion of superheater tubes in waste fired boiler
Open this publication in new window or tab >>Effect of sulphur containing additive on initial corrosion of superheater tubes in waste fired boiler
Show others...
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.

Keyword
Waste fired boilers, Superheaters, Power generation, High temperature corrosion
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-32408 (URN)10.1179/174327809X419203 (DOI)000269201500010 ()
Note

QC 20110414

Available from: 2011-04-14 Created: 2011-04-14 Last updated: 2017-12-11Bibliographically approved

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