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High temperature corrosion in a biomass-fired power boiler: Reducing furnace wall corrosion in a waste wood-fired power plant with advanced steam data
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. (Högtemperaturkorrosion)ORCID iD: 0000-0003-0184-7601
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The use of waste (or recycled) wood as a fuel in heat and power stations is becoming more widespread in Sweden (and Europe), because it is CO2 neutral with a lower cost than forest fuel. However, it is a heterogeneous fuel with a high amount of chlorine, alkali and heavy metals which causes more corrosion than fossil fuels or forest fuel.

A part of the boiler which is subjected to a high corrosion risk is the furnace wall (or waterwall) which is formed of tubes welded together. Waterwalls are made of ferritic low-alloyed steels, due to their low price, low stress corrosion cracking risk, high heat transfer properties and low thermal expansion. However, ferritic low alloy steels corrode quickly when burning waste wood in a low NOx environment (i.e. an environment with low oxygen levels to limit the formation of NOx). Apart from pure oxidation two important forms of corrosion mechanisms are thought to occur in waste environments: chlorine corrosion and alkali corrosion.

Although there is a great interest from plant owners to reduce the costs associated with furnace wall corrosion very little has been reported on wall corrosion in biomass boilers. Also corrosion mechanisms on furnace walls are usually investigated in laboratories, where interpretation of the results is easier. In power plants the interpretation is more complicated. Difficulties in the study of corrosion mechanisms are caused by several factors such as deposit composition, flue gas flow, boiler design, combustion characteristics and flue gas composition. Therefore, the corrosion varies from plant to plant and the laboratory experiments should be complemented with field tests. The present project may thus contribute to fill the power plant corrosion research gap.

In this work, different kinds of samples (wall deposits, test panel tubes and corrosion probes) from Vattenfall’s Heat and Power plant in Nyköping were analysed. Coated and uncoated samples with different alloys and different times of exposure were studied by scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), X-ray diffraction (XRD) and light optical microscopy (LOM). The corrosive environment was also simulated by Thermo-Calc software.

The results showed that a nickel alloy coating can dramatically reduce the corrosion rate. The corrosion rate of the low alloy steel tubes, steel 16Mo3, was linear and the oxide scale non-protective, but the corrosion rate of the nickel-based alloy was probably parabolic and the oxide much more protective. The nickel alloy and stainless steels showed good corrosion protection behavior in the boiler. This indicates that stainless steels could be a good (and less expensive) alternative to nickel-based alloys for protecting furnace walls.

The nickel alloy coated tubes (and probe samples) were attacked by a potassium-lead combination leading to the formation of non-protective potassium lead chromate. The low alloy steel tubes corroded by chloride attack. Stainless steels were attacked by a combination of chlorides and potassium-lead.

The Thermo-Calc modelling showed chlorine gas exists at extremely low levels (less than 0.1 ppm) at the tube surface; instead the hydrated form is thermodynamically favoured, i.e. gaseous hydrogen chloride. Consequently chlorine can attack low alloy steels by gaseous hydrogen chloride rather than chlorine gas as previously proposed. This is a smaller molecule than chlorine which could easily diffuse through a defect oxide of the type formed on the steel.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , 55 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2013:24
Keyword [en]
High temperature corrosion, Waterwalls, Power plant corrosion, NOx reducing enviroments, Biomass, Waste wood, Thermodynamic calculation modelling, corrosion-resistance alloy, Furnace wall corrosion
Keyword [sv]
Högtemperaturekorrosion, Eldstadsväggar, kraftverks korrosion, låg Nox mijöer, biomassa, returträ, Termodynamisk modellering, korrosionsbeständighet legering, eldstadskorrosion
National Category
Corrosion Engineering
Identifiers
URN: urn:nbn:se:kth:diva-121155ISBN: 978-91-7501-741-9 (print)OAI: oai:DiVA.org:kth-121155DiVA: diva2:617187
Presentation
2013-06-10, Rum 3, SP, Drottning Kristinas Väg 45, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130423

Available from: 2013-04-23 Created: 2013-04-21 Last updated: 2013-06-12Bibliographically approved
List of papers
1. The analysis of furnace wall deposits in a low-NOx waste wood-fired bubbling fluidised bed boiler
Open this publication in new window or tab >>The analysis of furnace wall deposits in a low-NOx waste wood-fired bubbling fluidised bed boiler
2012 (English)In: VGB PowerTech Journal, ISSN 1435-3199, Vol. 92, no 12, 96-100 p.Article in journal (Other academic) Published
Abstract [en]

Increasing use is being made of biomass as fuel for electricity production as the price of natural wood continues to rise. Therefore, more use is being made of waste wood (recycled wood). However, waste wood contains more chlorine, zinc and lead, which are believed to increase corrosion rates. Corrosion problems have occurred on the furnace walls of a fluidised bed boiler firing 100 % waste wood under low-NOx conditions. The deposits have been collected and analysed in order to understand the impact of the fuel.

Place, publisher, year, edition, pages
VGB Power Tech, 2012
Keyword
High Temperature Corrosion, Waterwalls, biomass, FBF boiler, corrosion
National Category
Corrosion Engineering Materials Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-108163 (URN)2-s2.0-84873677836 (Scopus ID)
Funder
StandUp
Note

QC 20130226

Available from: 2013-03-15 Created: 2012-12-19 Last updated: 2013-04-23Bibliographically approved
2. The effect of a nickel alloy coating on the corrosion of furnace wall tubes in a waste wood fired power plant
Open this publication in new window or tab >>The effect of a nickel alloy coating on the corrosion of furnace wall tubes in a waste wood fired power plant
2014 (English)In: Materials and corrosion - Werkstoffe und Korrosion, ISSN 0947-5117, E-ISSN 1521-4176, Vol. 65, no 2, 217-225 p.Article in journal (Refereed) Published
Abstract [en]

The use of waste wood as a fuel in power plants is becoming more widespread in Europe, because it is a renewable energy source with a lower cost than forest fuel. However it is more corrosive than coal and corrosion problems have arisen in the furnace wall area of a low NOx heat and power boiler. The furnace walls are made of a low alloy steel which has been coated in some parts with a nickel alloy to reduce corrosion. In this work, furnace tubes coated with a nickel alloy were compared to the uncoated tubes of the low alloy steel 16Mo3 after 3 years of exposure in the boiler. The nickel alloy coating and uncoated material were also compared with more controlled testing on a corrosion probe lasting for about 6 weeks. The corrosion rates were measured and the samples were chemically analysed by SEM/EDS/WDS and XRD methods. The corrosive environment was also modelled with Thermo-Calc software. The corrosion rates measured from the probe and tube samples of 16Mo3 agreed well with each other, implying linear corrosion rates. The results also showed that the use of nickel alloy coatings changes the corrosion mechanism, which leads to a dramatic reduction in the corrosion rate. The results are discussed in terms of the corrosion mechanisms and thermodynamic stability of the corrosion products.

Place, publisher, year, edition, pages
Wiley: , 2014
Keyword
16Mo3, alloy 625, furnace wall, high temperature corrosion, nickel coating, waste wood
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-121180 (URN)10.1002/maco.201307118 (DOI)000331999600014 ()
Note

QC 20140404

Available from: 2013-04-22 Created: 2013-04-22 Last updated: 2017-12-06Bibliographically approved
3. Initial Corrosion of Waterwalls Materials in a Waste Wood Fired Power Plant
Open this publication in new window or tab >>Initial Corrosion of Waterwalls Materials in a Waste Wood Fired Power Plant
(English)Manuscript (preprint) (Other academic)
Keyword
high temperature corrosion; furnace wall; waste wood; nickel coating; 16Mo3; Alloy 625
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-121191 (URN)
Note

QS 2013

Available from: 2013-04-23 Created: 2013-04-23 Last updated: 2013-04-23Bibliographically approved

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