Furnace Wall Corrosion in a Wood-fired Boiler

Yousef Alipour

Doctoral Thesis KTH Royal Institute of Technology School of Chemical Science and Engineering Division of Surface and Corrosion Science SE-100 44 Stockholm

ii

TRITA CHE-Report 2015:52 ISSN 1654-1081 ISBN 978-91-7595-695-4 Denna avhandling är skyddad enligt upphovsrättslagen. Alla rättigheter förbehålles.

 2015 Yousef Alipour All rights reserved. No part of this thesis may be reproduced by any means without permission from the author. The following items are printed with permission: PAPER II: Wiley PAPER III: Maney Publishing Paper IV: Maney Publishing Paper V: Elsevier Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan framlägges till offentlig granskning för avläggande av teknologie doktorsexamen torsdagen den 27 november 2015 klockan 10:00 i hörsal F3, Kungliga Tekniska Högskolan, Lindstedtsvägen 26, Stockholm. Printed at Universitetsservice US-AB

iii

“Science is a way of life, science is a perspective. For those lucky enough to experience it, that is empowering and emotional.”

Brian Greene

iv

v

Abstract The use of renewable wood-based fuel has been increasing in the last few decades because it is said to be carbon neutral. However, wood-based fuel, and especially used wood (also known as recycled wood or waste wood), is more corrosive than virgin wood (forest fuel), because of higher amounts of chlorine and heavy metals. These elements increase the corrosion problems at the furnace walls where the oxygen level is low. Corrosion mechanisms are usually investigated at the superheaters where the temperature of the material and the oxygen level is higher than at the furnace walls. Much less work has been performed on furnace wall corrosion in wood or used wood fired boilers, which is the reason for this project. Tests are also mostly performed under laboratory conditions, making the results easier to interpret. In power plants the interpretation is more complicated. Difficulties in the study of corrosion processes are caused by several factors such as deposit composition, flue gas composition, boiler design, and combustion characteristics and so on. Therefore, the laboratory tests should be a complement to the field test ones. This doctoral project involved in-situ testing at the furnace wall of power boilers and may thus contribute to fill the gap. The base material for furnace walls is a low alloy steel, usually 16Mo3, and the tubes may be coated or uncoated. Therefore tests were performed both on 16Mo3 and more highly alloyed materials suitable for protective coatings. Different types of samples exposed in used-wood fired boilers were analysed by different techniques such as LOM (light optical microscopy), XRD (X-ray diffraction), SEM (scanning electron microscopy), EDS (energy dispersive spectroscopy), WDS (wavelength dispersive spectroscopy), FIB (focused ion beam) and GD-OES (glow discharge optical emission spectroscopy). The

vi

corrosion rate was measured. The environment and corrosion processes were thermodynamically modelled by Thermo-Calc®. The results showed that 16Mo3 in the furnace wall region is attacked by hydrogen chloride, leading to the formation of iron chloride and a simultaneous oxidation of the iron chloride. The iron chloride layer appeared to reach a steady state thickness. Long term exposures showed that A 625 (nickel chromium alloy) and Kanthal APMT (iron-chromium-aluminium alloy) had the lowest corrosion rate (about 25-30% of the rate for 16Mo3), closely followed by 310S (stainless steel), making these alloys suitable for coating materials. It was found that the different alloys were attacked by different species, although they were exposed in the boiler at the same time in the same place. The dominant corrosion process in the A 625 samples seemed to be by a potassium-lead combination, while lead did not attack the APMT samples. Potassium attacked the alumina layer in the APMT samples, leading to the formation of a low-protective aluminate and chlorine was found to attack the base material. The results showed that stainless steels are attacked by both mechanisms (chlorideinduced attack and potassium-lead combination). Decreasing the temperature of the furnace walls of a waste wood fired boiler could decrease the corrosion rate of 16Mo3. However, this low corrosion rate corresponds to a low final steam pressure of the power plant, which in not beneficial for the electrical efficiency. The short term testing results showed that co-firing of sewage sludge with used wood can lead to a reduction in the deposition of K and Cl on the furnace wall during short term testing. This led to corrosion reduction of furnace wall materials and coatings. The alkali chlorides could react with the aluminosilicates in the sludge and be converted to alkali silicates. The chromia layer in A 625 and alumina in APMT were maintained with the addition of sludge. Keywords: High Temperature Corrosion, Used-wood Fired Boilers, Furnace Wall Corrosion, Cl-induced Corrosion, 16Mo3, Nickel-based Alloys, FeCrAl Alloys, Fuel Additives, Sewage Sludge, Thermodynamics Modelling

vii

Sammanfattning Förnybara träbaserade bränslen har ökat i användning under de senaste decennierna, eftersom det är koldioxidneutrala. Emellertid är träbaserade bränslen, och i synnerhet använt trä (även känt som återvunnet trä, returträ eller träavfall), mer korrosivt än skogsbränsle, på grund av högre halter klor och tungmetaller. Dessa ökar korrosionsproblemen på eldstadsväggarna, särskilt på platser där syrehalten är låg. Korrosionsmekanismer undersöks vanligtvis på överhettare dvs. på områden där materialets temperatur och syrenivån är högre än vid eldstadsväggarna. Färre arbeten har utförts på eldstadskorrosion i returträ pannor, vilket är motiveringen till detta projekt. Normalt sätt så görs endast i laboratorietester där resultaten är lättare att tolka. I kraftverk är tolkningen mer komplicerad. Undersökningar av korrosionsprocesser försvåras av flera faktorer såsom panndesign, förbränningsegenskaper, rökgassammansättning, beläggningskemi och så vidare. Därför bör laboratorietester kompletteras med fältförsök. Detta doktorandprojekt kan således bidra till att fylla denna brist. Eldstadsväggarna är uppbyggda av flera rör som svetsas samman och de består vanligtvis av 16Mo3 stål. Rören kan vara belagda eller obelagda. Tester har därför genomförts på 16Mo3 samt på höglegerade material vilka är lämpliga som skyddande beläggningar. Olika typer av prov som exponerats i förbränningspannor av returträ analyserades med olika tekniker såsom SEM (svepelektronmikroskopi), EDS (energidispersiv spektroskopi), WDS (våglängd dispersiv spektroskopi), FIB (fokuserad jonstråle) LOM (ljusoptisk mikroskopi), XRD (röntgendiffraktion), och GDOES (glimurladdning med optisk emissionsspektroskopi). Miljön samt korrosionsprocesser har modellerats termodynamiskt med mjukvaran TC (Termo-Calc®).

viii

Resultaten visade att 16Mo3 i eldstadsväggen angrips av väteklorid, vilket leder till bildning av järnklorid och en samtidig oxidation av järnkloriden. Järnkloridskiktet verkade nå ett stationärt tillstånd vad avser tjocklek. Sex veckors prov visade att A 625 (nickelkromlegering) och Kanthal APMT (järnkromaluminiumlegering) hade den lägsta korrosionshastigheten (ca 25-30% av korrosionshastigheten för 16Mo3), följt av 310S (rostfritt stål). Vi har funnit att de olika legeringarna angrips genom olika mekanismer, även om de var exponerade i pannan samtidigt på samma plats. Den dominerande korrosionsmekanismen för legeringen A 625 verkar i huvudsak bero på kalium och bly, medan bly inte attackerar Kanthal APMT. Kalium angriper aluminiumoxidskiktet på Kanthal APMT, vilket leder till bildning av icke-skyddande aluminat medan klor i sin tur attackerar basmaterialet. Resultaten visar att rostfritt stål attackeras genom klor-inducerad korrosion samt kalium och bly i kombination. Reducering av temperaturen kan minska korrosionshastigheten hos 16Mo3. Men denna lägre korrosionshastighet motsvarar ett lågt slutligt ångtryck hos kraftverket, vilket inte är fördelaktigt för elverkningsgraden. De kortare exponeringarna visade att samtidig förbränning av avloppsslam med returträ kan leda till minskad avsättning av kalium och klor i form av alkaliklorider på eldstadsväggarna. Detta ledde till korrosionsminskning av alla studerade material. Dessa alkaliklorider skulle kunna reagera med aluminiumsilikaterna från slammet och omvandlas till alkalisilikater. Detta verkar minska den alkali-inducerade korrosionen på A 625, APMT och 310S. Den aluminiumoxid som bildades på APMT och det kromoxidskikt som bildades på A 625 upprätthölls med tillsats av slam. Nyckelord: Högtemperaturkorrosion, Returträ Förbränningspannor, Eldstad Korrosion, klor-inducerad korrosion, 16Mo3, Nickelbaslegeringar, FeCrAllegeringar, Bränsletillsatser, Avloppsslam, Termodynamik Modellering

ix

Preface This doctoral thesis concerns corrosion occurring at furnace walls in power station boilers combusting 100% waste wood or used wood. The aim of this PhD project was to lead to a better understanding of the corrosion processes occurring at the furnace walls area during waste wood firing, thereby being able to suggest some solution to the problems. The studies involved visits to power plants, in-field exposure, examination of real components and test specimens from plant using a number of sophisticated microscope & spectroscope-based techniques and thermodynamics calculations. The content of this thesis is schematically illustrated in Figure 1.

Figure 1. Summary of the thesis

Stockholm, November 2015

Yousef Alipour

x

List of appended papers I.

II.

Corrosion of the low alloy steel 16Mo3 in the furnace region of used-wood fired boilers Y. Alipour, C. Davis, P. Szakalos, P. Henderson Submitted (2015) Effect of temperature on corrosion of furnace walls in a waste wood fired boiler Y. Alipour, P. Henderson, P. Szakalos Materials at High Temperatures, 32 (2015) 188-196

III.

The effect of a nickel alloy coating on the corrosion of furnace wall tubes in a waste wood fired power plant Y. Alipour, P. Henderson, P. Szakalos Materials and Corrosion, 65 (2014) 217-225

IV.

Corrosion of furnace walls materials in wastewood fired power plant Y. Alipour, P. Henderson Corrosion Engineering, Science and Technology (British Corrosion Science), 50 (2015) 355-363

V.

VI.

The effect of co-firing sewage sludge with used wood on the corrosion of an FeCrAl alloy and a nickel-based alloy in the furnace region Y. Alipour, A. Talus, P. Henderson, R. Norling Online, Fuel Processing Technology (2015) http://dx.doi.org/10.1016/j.fuproc.2015.07.014 Initial corrosion of 16Mo3 and 310S when exposed in a used wood fired boiler with and without sewage sludge additions A. Talus, Y. Alipour, R. Norling, P. Henderson Submitted (2015)

xi

Papers and reports not included in the thesis VII.

VIII.

The analysis of furnace wall deposits in a low-NOx waste wood-fired bubbling fluidised bed boiler Y. Alipour, P. Viklund, P. Henderson Journal of VGB PowerTech, 12 (2012) 096-100

Furnace wall corrosion in biomass-fired boilers at higher steam temperatures and pressures – KME 508/515 P. Henderson, Y. Alipour, M. Mattsson, A. Stålenheim, J. E Ejenstam, P. Szakalos, M. Glazer, A. Hjörnhede, A. Talus, R. N R. Norling, C. Davis, A. Johansson, S. Enestam, P. Cho Consortium Materials Technology, (2014)

xii

Conferences presentations based on this thesis IX.

X.

Reducing furnace wall corrosion by coating the furnace tubes in a waste wood fired power plant Y. Alipour, P. Henderson Energy and Materials (2012) Spain Short-term corrosion of furnace wall materials in a waste-wood fired power plant Y. Alipour, P. Henderson Gorodon Conferences on High Temperature Corrosion (2013) USA

XI.

Effect of temperature on the corrosion of furnace walls in a waste-to-energy boiler Y. Alipour, P. Henderson Microscopy of Oxidation (2014) UK

XII.

The effect of co-firing of sewage sludge with waste wood on furnace wall corrosion Y. Alipour, P. Henderson International Symposium on High Temperature Oxidation and Corrosion (2014) Japan

XIII.

Discussing reasons for high temperature corrosion in a waste wood power plant Y. Alipour Corrosion in WtE Plants (2015) Germany

xiii

Contribution The author’s contributions to the included papers are listed below: I.

All the experimental work except for the tube failure sample analysis. Major part in planning and evaluation of the experimental work. Writing the manuscript. M.Sc. Annika Talus at Swerea Kimab AB performed the GD-OES raw data, PhD Colin Davis at E.ON Technologies Ltd. Performed the tube failure analysis.

II.

All the experimental work and evaluation of the experimental work. Writing the first draft of manuscript.

III.

All the experimental work and major part of planning and evaluation. Writing the first draft of manuscript.

IV.

All the experimental work and major part of planning and evaluation. Writing the first draft of manuscript.

V.

VI.

Major part of experiments, evaluation and writing. GD-OES measurement was done by M.Sc. Annika Talus at Swerea Kimab AB.

f

All the sample preparation and deposit analysis and part of the writing. The evaluation of the results and major part of the writing was done by M.Sc. Annika Talus at Swerea Kimab AB.

xiv

Summary of papers In paper I, 16Mo3 tube material samples were exposed in a waste wood fired boiler with the same temperature but different times of exposure (15h and 1075h). The results were compared to a life time sample (32000h). The analyses along with Thermo-Calc results revealed that corrosion of 16Mo3 in the furnace wall area of a recycled wood fired boiler is caused by HCl leading to formation of a continuous layer of iron chloride. The iron chloride simultaneously is oxidised by water vapour and releases more HCl. The iron chloride layer appears to reach a steady-state thickness under the conditions investigated. In paper II, the effect of material temperature on the corrosion of furnace walls in a recycled wood boiler was investigated. The corrosion fronts of the samples were analysed by EDS. The deposits were analysed by XRD and EDS. The environment was modelled by Thermo-Calc. The amount of Cl and K in the deposits decreased with decreasing temperature. Corrosion rate decreased with decreasing temperature, but the boiler pressure needs to be reduced to a low level of around 30 bar which not beneficial for electricity production. The corrosion attack was found to be similar at the lowest and highest temperatures, which was chloride attack. In paper III, a furnace tube coated with a nickel-based alloy was compared to the uncoated tubes of 16Mo3, the reference material, after three years of exposure in the boiler. The coating material and the low alloy steel 16Mo3 were also compared with more controlled testing on a fin wall air cooled probe exposed for about 6 weeks in the used wood firing boiler. The corrosion rates were measured and the samples were chemically analysed by EDS, WDS and XRD. The thermodynamic stability of the corrosion product was also modelled with Thermo-Calc. The results showed that the

xv

use of nickel alloy coatings changes the corrosion mechanism, which leads to a dramatic reduction in the corrosion rate. In paper IV, three coating materials were evaluated. The samples were exposed at the furnace wall in a power boiler burning used wood. The Alloy 625 (nickel-based alloy) and Kanthal® APMT (an FeCrAl alloy) had the lowest corrosion rates closely followed by stainless steel 310S. The 16Mo3 low alloy steel, which the walls are constructed from, had the highest rate. Different corrosion mechanisms were found to occur according to the alloy type. In papers V & VI, the effect of a type of fuel additive, digested sewage sludge, on the corrosion of 16Mo3 (as the reference material), 310S, Alloy 625 and APMT has been studied. The short term results showed that the co-firing of sewage sludge with used wood reduces the corrosion. The results showed the alumina layer in the APMT was attacked by K during burning 100% waste wood but co-firing of sewage sludge with the waste wood can reduce this attack. In the nickel based-alloy Alloy 625, when burning 100% waste wood, the chromate was attacked by K-Pb combination, but this attack was suppressed by co-firing of sewage sludge in 14.25h exposure. In stainless steel 310S attack by both alkali and chloride was seen. In the 16Mo3 the chloride attack was the dominant mechanism in both cases.

xvi

Acknowledgements This thesis would not have been possible to produce without the inspiration, support and assistance from many people to whom I am greatly indebted. To those who are not mentioned here I would like to say that everyone I have encountered has contributed a lot to my development both as a person and as a scientist. For this you have my great appreciation. A few persons I wish to mention by name are: Pamela Henderson, I wish to express my sincere gratitude to you for your constant encouragement and willingness to share your time, knowledge and experience in creative discussions during the progress of this work. Apart from being an inspiring supervisor, you also are great person and a friend. Peter Szakalos, I wish to thank you for your constant readiness to help. You are an excellent teacher and your guidance in the area of thermodynamics is deeply appreciated. Rikard Norling, Annika Talus, Peter Viklund and Colin Davis, I would like to thank all my co-authors for stimulating discussions and cooperation. Annika Talus, I would like to thank you also for all the help with the GD-OES measurement. Mattias Mattsson and Annika Stålenheim, this work would not be done without your help and support in field-exposures. Swedish Energy Agency and Vattenfall AB are greatly thanked for the financial support. Inger Odnevall Wallinder, Rachel Pettersson, Christopher Leygraf, Mark Rutland and Mats Lundberg, I never forget the highly interesting discussions and all the help during my PhD.

xvii

Surface and Corrosion Science, I want to thank all the present and former members. The friendly atmosphere and stimulating working environment created by them were really encouraging for my research and “after hours” activities. I wish to alphabetically name Eleonora Bettini, Erik Landberg, Jesper Ejenstam, Majid Sababi, Mattias Forslund, Neda Mazinanian, Olga Krivosheeva, Rasmus Bodvik, Ruben Alvarez and Zahra Besharat. Thank you for all we had in common. Golrokh Heydari, Maziar Sedighi and Golsa Sedighi my hat is off to you for your endless support. My mother, my parents in law, my sisters and their families, I wish to thank you who live far away and still have given me support and love from a distance. My dad, you are always in my heart. Ghazal, my brilliant and beautiful wife without whom I would be nothing, you always comfort and console. I dedicate not only this book, but also my heart to you. Don't know much about history, Don't know much biology, Don’t know much about medicine book, Don't know much about the French I took, But I do know that I love you, And I know that you love me too, What a wonderful world this would be.

xviii

Abbreviations LOM SDS SE BSE EDS WDS FIB GD-OES TC CHP BFB

Light Optical Microscopy Scanning Electron Microscopy Secondary Electrons Back Scattered Electrons Energy Dispersive spectroscopy Wavelength Dispersive spectroscopy Focused Ion Beam Glow Discharge Optical Emission Spectroscopy Thermo-Calc® Software Combined Heat and Power Bubbling Fluidised Bed

xix

Table of contents Abstract ............................................................................................. v Sammanfattning .............................................................................. vii Preface ............................................................................................. ix List of appended papers ..................................................................... x Papers and reposts not included in the thesis .................................... xi Conferences presentations based on this thesis ................................ xii Contribution ................................................................................... xiii Summary of papers ......................................................................... xiv Acknowledgements......................................................................... xvi Abbreviations ................................................................................xviii Table of contents ............................................................................ xix 1

Introduction ............................................................................... 1 1.1 Objectives.................................................................................. 2 1.2 Methodology ............................................................................. 3

2

Background ................................................................................ 5 2.1 The used-wood fired plant ........................................................ 5 2.2 High temperature corrosion ..................................................... 7 2.2.1 Chlorine/Chloride corrosion ............................................... 7 2.2.2 Alkali corrosion ................................................................. 10 2.2.3 Molten salt corrosion ....................................................... 11

3

Techniques ............................................................................... 15 Air-cooled probe testing ......................................................... 15 3.1 3.2 LOM ......................................................................................... 16 3.3 SEM/EDS/WDS/FIB .................................................................. 16 3.4 XRD .......................................................................................... 17

xx

3.5 3.6

GD-OES .................................................................................... 18 Thermo-Calc modelling ........................................................... 19

4

Corrosion mechanism of furnace wall tubes .............................. 21 Background .............................................................................. 21 4.1 4.2 Microscopic and chemical analyses ........................................ 22 4.3 Thermodynamics modelling .................................................... 29 4.4 Mechanistic formulation ......................................................... 31

5

Reducing corrosion problems ................................................... 37 The effect of temperature ....................................................... 37 5.1 5.2 Alloys for coating..................................................................... 40 5.3 The effect of a fuel additive .................................................... 48 5.3 Practical implications of the results ........................................ 58

6

Conclusion ............................................................................... 59

6

Future work ............................................................................. 61

7

References............................................................................... 63

Introduction|1

1

Introduction

The use of biomass as a fuel is increasing in Sweden (and Europe). Global warming and the resulting climate change have led to strict restrictions on greenhouse emissions in energy production [1]. 40% of the total emissions in the world are caused by the energy sector [2]. This resulted in the higher use of renewable energy sources and CO2 neutral fuels, such as biomass. There are more than 2800 biomass firing boilers throughout the world [3]. The number of biomass firing boilers is increasing, because the share of needed energy produced by renewable energy sources, such as biomass, should be raised to 20% by year 2020 in the EU [4, 5]. Biomass boilers can utilize different types of fuels, e.g. wood, forest residues and used wood (also called recycled wood or waste wood). Used wood comes from construction and demolition waste. The use of waste wood as a fuel is of great interest, because it is less expensive than virgin wood and the deposition of waste is forbidden. However waste wood comes from construction and demolition waste and contains high amounts of chlorine, alkali metals (potassium, sodium and calcium) and heavy metals (zinc and lead) [6]. Zn and Pb are both used as stabilisers in PVC and Pb is used in additives for drying wood and in paint. These corrosive elements are released to the flue gas during combustion and deposited inside the boiler. This leads to corrosion attack inside the boiler and furnace walls area (so-called waterwalls). It is estimated that corrosion accounts for 70% of power plant shutdowns and the corrosion-related maintenance costs are as high as 10% of the annual turnover [7]. Corrosion rates of up to 1.5 mm a year have been measured on low alloy steel waterwalls giving a lifetime of only three years if no action is taken, Figure 2.

2|Introduction

Figure 2. Wall thickness measurements in the Idbäcken plant at the furnace walls area during operation with different fuels

Figure 2 shows that the average corrosion rate in 1994 to 2003 was found to be 60-70 µm per year. In the period 2003-2006 the average corrosion rate of 300 µm per year was found in the furnace walls area. The furnace walls were completely replaced in 2008 and the corrosion rate continues to be high when firing 100% waste wood at moderate to low oxygen levels. Locally, corrosion rates of 1.5 mm per year have been measured in the worst affected areas. A new furnace wall for a 100 MWth boiler costs around EUR 2.5 million.

1.1

Objectives

The thesis has two main objectives: • Obtain a better understanding of corrosion processes in the furnace walls area of used-wood plants where the oxygen level is low. • Suggest some ways to reduce the corrosion problems by analysing alloy performance and the corrosive environment in the used-wood plants. Furthermore, there is also an alternative, but not less important, objective which comprises the fact that this thesis is meant to represent a stepping stone for further research in the field of

Introduction|3

biomass firing power generation that may further increase the power efficiency of these plants.

1.2

Methodology

In order to achieve the aforementioned objectives, different alloys were exposed in real boiler environments and their corrosion rates measured. The samples were analysed by several microscopic and spectroscopic techniques, including LOM, SEM with EDS and WDS, FIB with EDS and GD-OES. The results were compared to the laboratory tests and thermodynamically modelled by TC. The research work was funded by the Swedish Energy Agency (KME 508 & KME708) and Vattenfall AB. This thesis begins with a first introductory chapter, in which the background, the objectives and the method of investigation are presented. Chapter 2 explains the used-wood fired heat and power plant where the samples are exposed and some theories behind high temperature corrosion. Chapter 3 explains the instruments and techniques which were used in this thesis. Chapter 4 presents the obtained results for the validation of the proposed corrosion mechanisms at the waterwalls area of used wood boilers. Chapter 5 suggests three investigated solutions to reduce the corrosion. Chapter 6 concludes the work in this thesis. Finally, in Chapter 7 suggestions for future work is given. The papers included in this thesis are appended. Some results and discussions, mainly in chapter 4 and subchapter 5.2, are not presented or published elsewhere.

4|Background

Background|5

2

Background

2.1

The used-wood fired plant

Biomass power plants can be defined as heat engines (boiler with superheaters) that convert heat energy into rotational energy which turns the turbine generator and produces electrical energy. The excess heat can be used to heat the water of the districtheating network [8]. The low pressure steam is again condensed to water. This cycle is called the Rankine cycle and is dependent on steam data [9]. The field exposure in this work was mostly done in the Idbäcken power plant which is owned and operated by Vattenfall AB. Idbäcken CHP is located in Nyköping, Sweden. It consists of three boilers. Boiler 1 and 2 are now used only in summer, during winter peak loads and when the third boiler is shut down. Boiler 3 which generates both electricity and hot water is a BFB boiler. Table 1 shows the boiler information. The samples tested in this thesis are exposed in this boiler. In a BFB boiler air mixes the fuel and bed material, which is often coarse sand, and keeps them in a suspended fluidised state. Bed and fuel particles are surrounded by hot gas. Bed material is heated by the burning fuel which is the heat source for new fuel particles. Special design of the air nozzles at the bottom of the bed allows air flow without clogging [10]. It should be noted that the temperature should not become higher than the fusion point of the bed material and ash produced during combustion. BFB boilers reduce the amount of SOx emissions and production of NOx which is beneficial [10]. Table 1. Boiler 3 Facts Maximum Capacity Designer Boiler type Steam data Production Fuel

97.5 MWth EcoEnergy Outokumpo OY BFB 540 ˚C and 140 bar 35 MW of electricity and 69 MW of heat 100% used wood

6|Background

The plant has been in operation since the end of 1994. The boiler 3 originally operated on a mixture of biomass and coal, but over the years the amount of coal has been reduced and the amount of waste wood increased. Since the summer of 2008, the plant operates on 100% waste wood. As mentioned in the introduction, corrosion has severely increased over the years of changing the fuel, (see Figure 2 ). Table 2. Typical chemical composition of used fuels in wt% Coal[11] Forest Waste Waste wood wood[6] wood[6] spread [6]

Moisture (as) Ash (as) Carbon (daf) Hydrogen (daf) Oxygen (daf) Nitrogen (daf) Sulphur (daf) Chlorine (daf) Potassium Sodium Zinc Lead as: as received daf: dry ash-free

3.0 10.3 75.5 4.4 2.5 1.2 3.1 0.02 0.06 0.03 ---

44.0 2.6 51.0 6.0 40.6 0.4 0.04 0.02 7.2 0.7 0.2 0.006

23.0 5.8 52.0 6.3 40.3 1.2 0.1 0.06 2.0 1.4 1.03 0.05

11.0-39.0 3.2-15 50.0-56.0 6.1-6.9 36.2-42.0 0.12-1.5 0.04-0.3 0.04-0.22 1.0-2.6 0.6-1.9 0.24-18.4 0.014-2.86

The flue gas chemical composition was measured at the position where the probe is exposed at the back wall of the boiler with a distance of 10 cm from the wall (see Table 3). The amount of oxygen fluctuates greatly, sometimes lower than 0.2% and sometimes up to 4.6%. Table 3. Average flue gas chemical composition at the boiler wall H2O CO2 CO CH4 O2 NH3 NOx NO SO2 NaCl+KCl % % % % % % ppm ppm ppm ppm 18 3 1.7 0.9 0.7 0.3 51 50 24 23(a) (a)

this value is measured at superheater area

HCl HF ppm ppm 8 1

Background|7

Corrosion problems have been reported in other wood-fired boilers as well. A field test during 80’s [12] showed high corrosion rates in the low-alloy steel waterwalls and the tubes were replaced just in two years. Another field test during 90’s[13] in three shredder wood-firing boilers have shown corrosion rates of 8-13 mm/firing season in the carbon steel waterwalls. The corrosion was reported to occur evenly over the surface. The corrosion was higher above the grate and around the secondary air.

2.2

High temperature corrosion

Metals are thermodynamically unstable with respect to the surrounding environment and depending on the environment can form oxides, sulphides, carbides, etc. or mixtures of products. The atmosphere is very complex in the combustion zone. Therefore different types of corrosion are thought to occur in the waterwalls of a wood firing boiler. The variation of temperature and flue gas composition is large when burning waste wood. At the waterwalls area, between secondary and tertiary air, the combustion is not complete and the oxygen level is low. It can be as low as 0.2% [14], the oxide layer formed is of poor quality and thus corrosion is more severe. This section summarizes some mechanisms proposed by others in the similar corrosive environment on the base furnace wall material as well as some other metals and alloys. 2.2.1 Chlorine/Chloride corrosion Chlorine exists in a waste wood-fired boiler environment from both chloride-rich deposits and flue gas. Chlorine is well-known as one of the most reactive elements. Tests on pure Fe [15] and pure Cr [16] in 100% HCl showed that at a specific temperature of around 500 ˚C a steady state layer of chloride forms (i.e., the thickness of chloride is constant over the time) which means a continuous corrosion attack.

8|Background

By the addition of other elements such as oxygen in the environment the mechanism gets more complicated. One suggestion is active oxidation [17] which means the formation of a porous non-protective oxide layer. In this mechanism gaseous chlorine can diffuse through the cracks and pores of the oxide layer and react with the metal substrate at a low oxygen level and form metal chloride (step I). The metal chloride formed volatises and diffuses outward (Step II). At high oxygen partial pressures the metal chloride is oxidised, leading to the release of gaseous chloride (Step III). The gaseous chloride can then again participate in the corrosion process. This process is called the chlorine cycle [18, 19] . Figure 3 shows the proposed mechanism including the steps and reactions.

Figure 3. A schematic diagram of the chlorine cycle, M stands for metal.

Alternatively HCl can diffuse as the corrodant. This is a small molecule which could diffuse easily through a defect oxide [20]. It was suggested that the metal (for example, iron) dissolves in the metal chloride closer to the substrate and metal ions diffuse to the oxide side, where they accelerate oxidation [20]. It has been shown [21] that the higher concentration of HCl can increase the corrosion attack of stainless steels 310S and 304L at 500 ˚C and

Background|9

600 ˚C exposed for 200h in a gas composition of 40%H2-30%CO20%CO2-10H2O. It has also been proposed [22] that chloride ions can diffuse towards the metal substrate along the oxide grain boundaries. The iron chloride formed at the grain boundaries increases the rate of transport of oxygen and iron ions. However, other work [23] has suggested that chloride ions can become incorporated in the closepacked oxide lattice and increase the ionic mobility. The corrosion of five commercial steels containing 0-19 wt% Cr (see Table 4) has been studied [24] in reducing conditions (H2HCl-CO2) at 400 ˚C with and without a ZnCl2-KCl deposit. Results showed that the presence of the ZnCl2-KCl salt increases the corrosion rate and formation of porous scales. Some Cl was found at the interface of metal/oxide, indicating that Cl-containing species could go through the porous oxide and attack the steel. The authors suggested that the corrosion is mainly by Cl- anions in the salt [24] and described the corrosion as active oxidation. Table 4. Chemical composition of material tested in wt% with iron as balance

CS20 B2.25CrMo NF616 12CrMoV SS304

Cr 0 2.21 9,10 11.20 19.28

Other alloying elements C0.2 Mo0.9, Mn0.43, Si0.31, S0.01 W1.7, Mn0.5, Mo0.4, V0.2, Si0.11, C0.1 Mo1.0, Mn0.5, Ni0.5, Si0.3, V0.3, C0.2 Ni8.83, Mn1.67, Si0.45

Even though the corrosion rates decreased with increasing Cr content, the highest-Cr stainless steel was still unable to provide a good corrosion resistance against the ZnCl2–KCl deposit. It should be mentioned the ZnCl2 is volatilized continuously and consequently the amount of KCl will be enriched in the salt. This leads to an increase of the melting point of ZnCl2- KCl [25] and the corrosion attack can be reduced with time. Internal chlorination has also been suggested as a mechanism of attack. Tests on 800H alloy (Ni30Cr20Fe40) in an air+2% Cl2

10|Background

environment at 900 ˚C showed the formation of chromium chloride within an internal corrosion zone [26]. It has been reported by several authors [27-30] that chlorine has a negative effect on the adherence of the oxide layer to the substrate and the oxide layers formed in a chlorination condition spall easily. It can happen even during an exposure with metal temperature of 400 ˚C [31]. 2.2.2 Alkali corrosion Alkali products exist at the waterwall’s deposits and also in the flue gas of waste wood-fired boilers and can attack the metal. KCl has been found as a strong corrodant for many alloys and metals [32-35]. It has been suggested that KCl (g, s) can directly attack the chromia in nickel-base alloys or stainless steels and produce an unprotective chromate [36-39]. The breakdown of the chromia scale can be shown according to Equation (1) [39]: 2𝐾𝐶𝑙 (𝑔, 𝑠) + 1�2 𝐶𝑟2 𝑂3 (𝑠) + 𝐻2 𝑂 + 3�4 𝑂2 → 𝐾2 𝐶𝑟𝑂4 (𝑠) + 2𝐻𝐶𝑙 (1)

It has been shown that NaCl [40, 41] or K2CO3 [38] can also attack chromia. The corrosion of pure Cr was tested under synthetic air with KCl or K2CO3 [42]. The results showed that Cr is attacked by KCl while K2CO3 did not react with Cr. On the other hand it was found that Cr2O3 is not reactive with KCl and is reactive with K2CO3. The authors concluded that the reaction between pure Cr and K2CO3 starts but does not continue, while the reaction with KCl is sustained [42]. The authors also showed that [43] all the alkali chloride deposits (LiCl, KCl and NaCl) are corrosive towards pure Cr if the temperature is high enough (higher than 500 ˚C), but the alkali earth metals (BaCl2, CaCl2 and MgCl2) are not corrosive towards pure Cr. These observations indicate that the presence of chloride ions alone cannot initiate the corrosion and the properties of cations are also important [43].

Background|11

Other studies [41, 44, 45] on pure Cr showed that as long as the temperature is below the melting points of BaCl2 and CaCl2 salts, the corrosion was initiated but did not continue. However, a sustained attack was observed for NaCl well below its melting point. It can be concluded that both the alkali and halogen component of the salt are important and the alkali alone cannot initiate the corrosion. NaCl on superheater tubes was equally as corrosive as KCl for practical applications [46]. However the stainless steel Sanicro 28 showed more internal degradation with NaCl deposits than with KCl deposits [46]. The deposition of alkali chloride is shown [47] to be reduced by co-combustion of sewage sludge with wood at the superheater tubes with a temperature of 500 ˚C. KCl is also reported to attack the alumina layer in alumina forming alloys [48] and a study showed that the initial corrosion of a low alloy steel deposited with KCl starts at 355 ˚C [49]. 2.2.3 Molten salt corrosion It has been shown that low melting point chlorine-containing alkali metals can accelerate the corrosion at elevated temperatures [22, 40]. This type of corrosion can have rapid kinetics due to the rapid transport of ions in the liquid phase [50]. Fluxing mechanism and dissolving the metal oxide into the salt melt is another explanation of the attack [51]. The amount of these compounds is higher in wood compared to coal [52]. The corrosion by low melting point salts is often called molten salt corrosion or synergistic hot corrosion [53, 54]. It has been shown that a mixture of NaCl and KCl can attack chromium oxide and nickel oxide to a lesser extent, and iron oxide to a much greater extent [55]. Some mixtures of chlorides can even have melting points as low as 230 ˚C [56]. Table 5 lists the melting points of some compounds and eutectic mixtures that may cause molten salt corrosion in biomass-fired boilers. In practice even more complex mixtures may be present in the deposits.

12|Background

Table 5. Melting points of compounds or lowest melting point of mixtures from some salts expected in waste fired boilers [56-58]

Single compound ZnCl2 PbCl2 FeCl2 KCl NaCl CrCl2 PbO K2SO4 PbSO4

Melting Point ˚C 318 501 677 771 801 845 886 1076 1170

Salt mixture KCl-ZnCl2 ZnCl2-FeCl2 NaCl-FeCl2 KCl-PbCl2 NaCl-PbCl2 PbCl2-FeCl2 NaCl-Na2CrO4 KCl-NaCl KCl-K2CrO4

Melting Point ˚C 230 300 378 406 408 421 592 657 658

Long-term testing of four different NiCrMo alloys in a waste incineration plant (burning 50% domestic and 50% industrial refuse) showed [59] the presence of metal chloride and alkali chloride which resulted in molten salt corrosion. All the alloys suffered from pitting. A test panel test [60] with single overlay weld of five Ni-based alloys and one stainless steel (Table 6) was exposed in a circulating fluidised bed boiler burning a mixture of industrial and household waste. The water temperature was around 470 ˚C and exposure time was above 7700 h (one operating season). Table 6. Chemical compositions of the tested materials as a coating in wt% [60]

Alloy A 625 A 625mod A59 A650 A22 310S

Ni 64.40 64.79 59.90 51.60 57.50 20.70

Cr 22.23 22.62 24.75 19.56 22.27 25.80

Mo 8.70 9.98 14.85 11.55 14.09 0.04

Fe 0.27 0.14 0.11 13.70 2.31 51.08

Nb 3.66 0.01 0.24 -

W 3.07 3.07 1.50 3.01 -

C 0.011 0.006 0.006 0.0005 0.003 0.120

Results [60] showed that nickel based alloys can suffer from pitting attack. Pitting attack in A 650 was relatively small

Background|13

compared to other Ni-based alloys. This alloy is a Ni-based alloy which also contains iron. The stainless steel 310S exhibited a higher corrosion rate but in a uniform front which means that monitoring the corrosion is easier. The corrosion in all alloys was suggested to be the formation of a molten salt layer which results in a fluxing mechanism. Field studies have shown that the combustion of biomass (60% industrial waste and 40% of household waste) results in locally high concentrations of heavy metal salts (e.g., PbCl2, ZnCl2) on waterwalls with large variations in deposit compositions, which may explain the local corrosion attack, i.e., pits. [61]. In a laboratory study it was observed that stainless steels exposed to PbCl2 at 400 ˚C showed accelerated corrosion due to the formation of lead chromate whereas ZnCl2 was found to have only a marginal effect on the corrosion rate and no chromate was detected. Both PbCl2 and ZnCl2 increased the corrosion rate on a low alloyed steel, but PbCl2 was far the more aggressive [62, 63]. It should be noted that vapour-condensation of PbCl2 and ZnCl2, and also ZnSO4 and PbSO4 is only possible at temperatures lower than 400 ˚C. This is the actual temperature of waterwall tubes , so the presence of these salts is expected to increase the fireside corrosion of furnace walls [64, 65]. It should be mentioned that the presence of ZnCl2 can increase the corrosion rate of low alloy steel 10CrMo9-10 (Cr2.5Mo1) at temperatures as low as 300 ˚C [66], but this attack decreased with increasing temperature. ZnCl2 is oxidised to ZnO at temperatures above 300 ˚C but PbCl2 is stable at 400 ˚C (the actual temperature of waterwalls) [62]. In addition, it is known that lead oxide attacks nickel–chromium alloys and reduces the corrosion resistance by the formation of lead chromate [67]. Copper as another heavy metal in the deposit is also shown to increase the corrosion rate of nickel base and iron base alloys in waste incineration plants [68]. The melting point of the copper-containing salt was found to be lowered by more than 70 ˚C. This resulted to a more fluid salt and easier dissolving of the metal in to the melt [68].

14|Background

Techniques|15

3

Techniques

3.1

Air-cooled probe testing

An air-cooled probe was designed by Vattenfall AB for long-term or short-term corrosion testing and deposit collection. Figure 4 shows the wall corrosion probe before exposure. It contains four specimens with the dimensions of 48 mm length, 7 mm width and 6 mm thickness. Air can flow in the probe to control the temperature. The temperature is measured by a thermocouple inserted centrally at the back of each specimen.

Figure 4. A wall probe before exposure, specimens are mounted in positions

The probe is vertically inserted into slits made in the fins between two tubes at the furnace walls area, Figure 5.

16|Techniques

Figure 5. a) Idbäcken boiler from outside with the insulation removed, showing the slit in the back wall where the probe is inserted. b) the slit from the inside at the fin wall between two tubes

3.2

LOM

The LOM is a type of microscope which uses visible light (usually defined as the wavelengths between 390 and 700 nm of the electromagnetic spectrum) in an arrangement of optical lenses that generates magnified images of sample surfaces, giving the viewer an erect enlarged virtual image [69]. The microscope used in this study was a Leica DM2700M instrument.

3.3

SEM/EDS/WDS/FIB

SEM was the main technique in this work for chemical and imaging analysis. SEM produces images of conductive surfaces in vacuum by scanning a focused beam of electrons. The electrons are generated by an electron gun and interact with atoms at the surface, emitting new electrons that can be collected and used to characterise the sample. Some of these responses are: secondary electrons and back-scattering electrons [70]. Secondary electrons (SE) are generated through ionisation of atoms at the surface layer and can only escape from a shallow region. This generates surface topography.

Techniques|17

BSE is basically an elastic scattering of the electron beam, which means that the incident electrons are scattered back with the same kinetic energy by nuclei at the surface. They are generated from a larger region than SE. The BSE intensity is increased with increasing atomic number. Thus, BSE generates images with chemical contrast [71]. SEM can be combined with a number of different techniques, such as EDS, WDS and FIB. The EDS generates relative quantitative chemical information from the surface and the precision of the analysis increases with atomic number. The EDS detector measures the energy of radiating photons which is characteristic for each element. By WDS the photons are diffracted by a crystal and only X-rays with a specific wavelength fall onto the detector. The WDS is usually used to distinguish the elements which have an energy overlap, such as Pb/S/Mo. FIB is a technique to make fresh cross sections and facilitates analysing deposit, corrosion front and substrate at the same time. The SEM instruments used in this study were a JEOL 7001 equipped with EDS, a JEOL 6400 equipped with WDS, a Quanta 3D FEG equipped with FIB and EDS. For analyses an acceleration voltage of 20 keV was used.

3.4

XRD

A heated tungsten filament generates electrons in XRD. When the electron beam hits the anode it gives rise to X-rays, which are filtered by foils or crystal monochrometers to produce monochromatic X-rays. A beam of monochromatic X-rays is aimed at the specimen surface at an angle of ϴ. When Bragg’s law [72] is satisfied constructive interference occurs. Each crystalline phase has its own characteristic X-ray diffractions according to the lattice spacing (dhkl). This technique is suitable for detecting different compounds in the deposit. The deposits formed on each specimen were separately scraped off and were powdered by hand, and then the powder was investigated under a D8 from Bruker [73] with parallel beam by

18|Techniques

Goebbel mirror, 1.2 mm divergence slit, and long Soller slit on the detector side. The X-ray source was Cu Kα with energy dispersive detector, SolX (Bruker) which avoids fluorescence. The Goniometer set up was in θ-θ arrangement with fixed sample. Single crystal Si sample holder with no diffracting planes in this set up was used for powders. Diffraction patterns were matched against ICCD´s ”PDF 4+” (version 2010) using element filter allowing H, C, N, O, Na, Mg, Al Si, S, Cl, K, Ca, Cr, Fe, Ni and Pb. A semi-quantitative measure, S-Q, was obtained by comparing intensities with intensity correlation factors, I/Ic, stated in the PDF 4+ data base.

3.5

GD-OES

GD-OES in this work is used to provide an elemental depth profile from deposit into the substrate. This instrument consists of a noble gas-filled vacuum vessel and a separated surface as anode and specimen as cathode. The electric field between anode and cathode leads to plasma formation and the sample is subsequently eroded by the bombarding ions. Whenever sputtered ions diffuse into the plasma a process of emission begins. Collisions with electrons take place leading to excitation of ions and atoms to higher energy levels. This excitation gives rise to an optical emission that can be detected by a spectrometer. A spectrum with characteristic wavelengths is generated from species originating from the eroded specimen [74]. A GDS 850A from Leco was used in this work. A circular area with a diameter of 2mm or 4mm was continuously sputtered using Ar plasma at a potential of 700 V and a current of 20 mA. At the beginning of this project, the device employed did not have a potassium detector. Since potassium has been shown to attack the material in a high temperature environment, a CCD detector (charge coupled device) was used to ascertain the distribution of potassium. The GD-OES instrument was run to get all data from the possible spectra (except potassium) and the CCD detector was

Techniques|19

run at the same time in order to capture the results. The K and Cl data from the CCD was manually collected every 15-20 seconds. Thus it is possible to study Cl (λ= 726nm) and compare the intensity data from CCD with the intensity data from GD-OES. If the Cl data from CCD is similar compared to the GD-OES result it is possible to obtain comparable data for K as well. The potassium has a wavelength of λ= 770nm. Later the device was equipped with the K detector.

Figure 6. Comparison of the data for Cl in CCD and GD-OES results for Alloy 625 when burning waste wood. The intensities of Cl are similar in either detector which means that the CCD results are usable

Figure 6 shows that the chlorine signals in CCD follow the results from GD-OES; the peaks are occurring at the same time. It should be noticed that the GD-OES and CCD have different units, so an undefined unit was used to compare them.

3.6

Thermo-Calc Modelling

Thermo-Calc modelling is based on thermodynamics law and Gibbs free energy for phases and components. TC software [75] is used in different stages of this work to:

20|Techniques

1) Predict the stable phases or component in a specific alloy, environment and temperature. 2) Draw ternary phase diagrams 3) Model the corrosive environment and corrosion processes in a waste wood-fired boiler 4) Predict different stable layers on the furnace walls from the flue gas to the substrate The databases used were TCFE7 (iron-based alloys), SSOL4 (general alloys) and SSUB5 (compounds).

Corrosion mechanism of furnace wall tubes|21

4

Corrosion mechanism of furnace wall tubes

One of the most important steps to reduce corrosion in the furnace wall region of the waste wood-fired boiler is to understand the thermodynamics that govern corrosion processes in that area. It can be realised from subchapter 2.2 that there is no universal mechanism to account for the attack occurring in the furnace and depending on the flue gas chemical composition, deposit chemistry, material and material temperature different corrosion mechanisms can dominate. In this chapter the literature survey results are collated and results from Paper I and Paper II further discussed with the help of thermodynamics laws to propose a corrosion mechanism that occurs in furnace walls when burning wood and waste wood. Some mechanistic discussions have not been concluded in any of the papers.

4.1 Background The furnace wall tubes and membranes are made of 16Mo3 because of its high heat transfer properties, low thermal expansion, low stress corrosion cracking and a low price. The final steam data in the boiler is 140 bar and 535 ˚C which gives the water inside the waterwalls a temperature of 343 ˚C. A rule of thumb says that the temperature of the tube material is 50 ˚C higher that water temperature, which gives us the temperature of 393 ˚C. Based on these two points, two coupons of 16Mo3, Table 7, were attached to the air-cooled probe and exposed in the Idbäcken boiler when burning 100% waste wood. The temperature was controlled to 390 ˚C. One sample was inside the boiler for 15 h (short term testing) and the other one was exposed for 1075 h (medium term testing).

22|Corrosion mechanism of furnace wall tubes

Table 7 Typical chemical composition of 16Mo3 steel in wt%

Mo C Mn Si P S Cu Fe 0.3 0.16 0.55 0.22 0.02 0.02 0.3 balance

During these tests, a waterwall tube failure occurred after 32000 h in another boiler with similar size, similar fuel (chlorine content of 0.04 wt% versus 0.07 wt% for the Idbäcken plant) and the same final steam pressure. Thus, this tube was taken out at the crown and was used as a life time testing sample.

Figure 7. The failed waterwall tube after four firing seasons (around 32000 h) in service

The rupture length was 265 mm. The original thickness of the tube was 6.1 mm and after the failure the thickness near the rupture was 1.3 mm which gave a corrosion rate of 156 μm per 1000 h.

4.2 Microscopic and chemical analyses (Paper I) One part of each sample was cut, polished and analysed under LOM. No indication of thermal degradation, cracking or swelling was found in the samples, Figure 8.

Corrosion mechanism of furnace wall tubes|23

Figure 8. LOM images of a) the tube sample (32000 h) b) the probe sample (1075 h) c) the probe sample (15 h). Normal ferritic-pearlitic microstructure was observed in the samples.

Some deposits were scraped off the wall near the tube failure and analysed under EDS. The deposits on the top of the probe samples were also analysed under EDS. Table 8 shows the average results of the deposit compositions. Table 8. Average chemical composition of samples’ deposits in wt% Sample Probe 15 h

Cl K Ca Ti Mn Fe 15.0 8.4 6.7 1.1 0.8 5.4

Cu Zn Pb 0.1 3.5 4.4

Probe 1075 h 25.5 6.2 0.4 0.2 0.5 0.1 3.8 21.6 17.9 0.3 0.2 0.9 9.7

1.2 5.0 3.2

Tube(a)

O Na Mg Al Si P S 31.8 7.9 1.1 1.7 1.6 0.4 6

33.3 3.7 0.2 0.3 0.7 0.1 5.9 8.8 14.6 0.6 0.1 0.5 22.9 0.7 2.6 1.4

(a):

results on deposits near the failure Carbon content has been removed from the results. But the amount was around 5 wt%

Table 8 shows that the amount of chlorine in the deposit is more on probe samples compared with on the tube sample, which can be related to the higher chlorine content of the fuel ( 0.07 wt% and 0.04 wt% respectively). The iron content is increasing with time which shows a migration from the metal substrate to the deposit as corrosion proceeds. To study which elements or compounds (salts) are participating in the corrosion process, one edge of each probe sample was polished at 45 ˚ without water (and, thus, the various layers in yaxis are enlarged 1.4 times more in thickness). Note that polishing in this way can facilitate analysing substrate, corrosion front and

24|Corrosion mechanism of furnace wall tubes

oxide (or deposit) at the same time, Figure 9. The results are presented in Figures 10 -11.

Figure 9. Manual polishing of one edge of probe samples. Note that chloride is soluble in the water, so the polishing was done without water.

For the tube sample the membrane parts (3 o’clock and 9 o’clock) are mounted in cold curing, glass filled epoxy, the samples were cut dry using a bandsaw, finally samples were ground and polished using non-aqueous lubricants and oil based diamond suspensions. The results are presented in Figure 12-13.

Figure 10. Elemental mapping of the 15h probe sample. The 16Mo3 substrate is on the left side of each mapping.

Figure 10 shows that Cl is not in combination with K at the interface between substrate/oxide. Cl is mainly in connection with Fe at the interface. Some deposits have fallen off during sample preparation.

Corrosion mechanism of furnace wall tubes|25

Figure 11. Elemental quant-mapping of the 1075h sample.

The corrosion product in the 1075 h sample is also the same as in the short term testing (15 h) sample. Figure 11 shows that the corrosion product contains mainly Fe, O and Cl.

Figure 12. Elemental mapping of tube sample at membrane (3 o’clock) showing the substrate and corrosion products. The EDS analyses results are shown in the figure.

26|Corrosion mechanism of furnace wall tubes

The EDS analyses in Figure 12 confirm the deposit contains alkali and heavy metals, but Cl penetrates through to the metal surface.

Figure 13. Elemental mapping of the tube sample with a thick scale at membrane (9 o’clock). Although the tube has been in service for 32,000h, the chloride layer is only around 10 μm thick.

Figure 10 to Figure 13 show that Cl closer to the metal substrate is mainly associated with Fe and O which could indicate iron oxychloride. However the ternary plot of these three elements at 400 ˚C, Figure 14 , does not show any iron oxychloride at low oxygen partial pressure.

Corrosion mechanism of furnace wall tubes|27

Figure 14. The ternary phase diagram of Fe, Cl and O. The oxygen partial pressure in the flue gas at the furnace walls region is shown with an arrow.

It should be mentioned that the samples were exposed to ambient air after cross sectioning and before EDS analysing. So the corrosion product is possibly iron chloride which has been oxidised or hydrated when exposed to the air. Iron chlorides are strongly hygroscopic and absorb moisture from the air. To examine this further, a section was made to the 1075 h probe sample by FIB and the in-situ EDS analysing were done directly without exposing the sample into the air. The line mapping result is shown in Figure 15.

28|Corrosion mechanism of furnace wall tubes

Figure 15. a) sectioning results on 1075 h probe sample after tilting by 52˚. The line mapping was done along the 4 μm line shown in the figure with in-situ EDS. b) results of the line mapping from the oxide to the substrate. The results are related to wt% but not calibrated.

Figure 15 does not show any overlap between Cl, O and Fe. The results show a continuous layer of iron chloride, about 2 μm, right next to the substrate with iron oxide above it. Although the laboratory studies have shown alkali metals such as Na or K and heavy metals such as Pb and Zn are very corrosive, none of these elements were present in the corrosion front. The 15 h sample was prepared according to Figure 16 and the GD-OES results for Cl, Fe and K signals are shown in Figure 17. The deposit has fallen off from the sample during the preparation and the spectroscopy device was calibrated for the substrate.

Figure 16. sample preparation in GD-OES a) the probe coupon after drycut to make a 2 mm sample b) the sample after ultrasonic moulding in Sn-Bi c) the area after sputtering

Corrosion mechanism of furnace wall tubes|29

The results show that the chloride layer is around 1-2 μm (see Figure 17). K is not in association with Cl.

Figure 17. sputtering results for the 15 h probe samples near the substrate at corrosion front

4.3 Thermodynamics modelling (Papers I and II) Thermodynamics modelling was done based on the EDS results, Figure 10 to Figure 13, and average compositions of the deposits, Table 8, by TC using SSUB and SSOL databases. The oxygen amount is increasing from 10-40 bar (to simulate the layer close to the substrate under a deposit) to 0.01 bar (oxygen partial pressure at the flue gas in the furnace walls area). The temperature was set to 400 ˚C as the temperature of the furnace wall material. HCl and H2 are assumed to move freely through the porous oxide. The amount of Fe, Mn, Si, Mo and C was set by the 16Mo3 chemical composition, Table 7. K, Pb and S levels have a gradient from zero at the metal to their average amount in the deposits, Table 8. The total amount of species was set to 1 mol with N as balance. Figure 18 shows all the assumptions.

30|Corrosion mechanism of furnace wall tubes

Figure 18. Diagram showing the thermodynamics assumptions based on main corrosion products from bulk metal (left) to flue gas (right). The amount of Fe, Mn, Si, Mo and C was set by the 16Mo3 chemical composition, Table 7. The smallest gas molecules, i.e. H2 and HCl are assumed to move freely, so H and Cl have flat gradients. Nitrogen is balancing specie in the modelling as the total amount of species is always one mole.

The calculated amounts of HCl, H2, O2, H2O and Cl2 in the gas phase are shown in three points. The gas phase contains mainly N2, H2O, H2 and HCl.

Corrosion mechanism of furnace wall tubes|31

Figure 19. Modelling of pure iron at 400 ˚C in furnace walls area. The equilibrium amounts of HCl, H2, H2O, O2 and Cl2 are shown in 3 points. The substrate 16Mo3 is on the left side and flue gas is on the right side as described in Figure 18.

The calculated results at the flue gas in Figure 19 are close to the actual measurements (Table 3).

4.4 Mechanistic formulation (Paper I) Table 8 shows that the deposits on the samples contain chlorine, alkali and heavy metals. Deposits taken from the Idbäcken boiler [76] showed a wide spread in chemical composition, although Cl and K was found in all the deposits. Zn was present in most of the deposits, Pb was found in some of them at low average concentrations but high concentrations locally. However, the EDS mapping in this work, Figure 10 to Figure 13, showed that the 16Mo3 furnace wall material is mainly attacked by Cl. The fresh sectioning, Figure 15, showed a continuous FeCl2 layer under an

32|Corrosion mechanism of furnace wall tubes

iron oxide layer. It was believed [17-19] that Cl2 can diffuse through the cracks and pores of the iron oxide and attack the substrate and form FeCl2. The iron chloride produced can diffuse out and react with water vapour and the Cl2 formed as a result of this reaction is able to participate again in the corrosion process. But Cl2 is a large molecule with the bond length of 199 pm [77] and cannot diffuse easily through the oxide layer. On the other hand HCl is a smaller molecule (bond length of 127 pm [77]). The thermodynamics calculation, Figure 19, also shows that the Cl2 is unstable at the furnace wall area under the deposit. Instead, it is predicted that HCl and H2O are participating in corrosion process. The unstable Cl2 is thermodynamically expected to react with H2 or H2O and form HCl. 𝐻2 + 𝐶𝑙2 → 𝐻𝐶𝑙 (2)

Alkali chloride in the deposit or in the flue gas can also react with water vapour and form HCl [78]. The HCl participates in the corrosion process. 𝐾𝐶𝑙 (𝑠, 𝑔) + 𝐻2 𝑂 (𝑔) → 𝐾𝑂𝐻 (𝑔) + 𝐻𝐶𝑙 (𝑔) (3)

The KOH formed will be released into the flue gas. Figure 19 shows that iron chloride is already oxidised at very low oxygen partial pressures. The chlorine released is not stable and is converted to HCl (g). FeCl2 formed in the short term test (Figure 17), the medium term test (Figure 15) and the failed tube sample (Figure 13) is present as a thin layer of 1-5 μm FeCl2 under the oxide in all cases. This means that a steady state and continuous corrosion process includes the formation of FeCl2 as an intermediate step which facilitates the overall oxidation. The microscopy and thermodynamics results can be summarized in Figure 20.

Corrosion mechanism of furnace wall tubes|33

Figure 20. A schematic figure of the corrosion product on the furnace wall tubes in a waste wood fired power plant. The equilibrium gas phase at the iron chloride/magnetite interface consists of H2, H2O and HCl and not by O2 and Cl2 .

The proposed mechanism is shown in two steps, Figure 20. The chloride activity is constant in different layers and in step  (which is an intermediate step) iron chloride is formed, Equation (4). The transportation of HCl through the oxide scale will stop when the iron chloride layer reaches its steady state thickness. 𝐹𝑒 + 𝐻𝐶𝑙 → 𝐹𝑒𝐶𝑙2 + 𝐻2 (𝑔) (4)

The iron chloride formed is not protective and it seems to accelerate the magnetite formation (the main corrosion product). In step  the water vapour molecules (the career of oxygen) which exist in the interface of iron chloride and magnetite reacts with the formed iron chloride and releases HCl. 3𝐹𝑒𝐶𝑙2 + 4𝐻2 𝑂 → 𝐹𝑒3 𝑂4 + 6𝐻𝐶𝑙(𝑔) + 𝐻2 (𝑔) (5)

34|Corrosion mechanism of furnace wall tubes

The released HCl will then participate again in the first step. It is proposed, therefore, that waterwalls (made of 16Mo3) in the furnace region area of a waste wood-fired boiler are attacked by HCl (as HCl cycle) and H2O as an active corrosion mechanism. Figure 21 schematically shows how iron chloride forms iron substrate with time. The mechanism involves the formation of iron chloride at the interface of iron and oxide.

Figure 21. Schematic views of the iron corrosion in the combustion environment at different times (t4>t3>t2>t1)

At time=t1 in Figure 21 the oxygen partial pressure is high enough to avoid iron chloride formation by HCl and instead a porous iron oxide layer is formed. Then with the increasing of oxide scale (t=t2), the oxygen activity decreases and iron chloride forms. When the iron chloride reaches a steady state thickness at t=t3, i.e., the thickness does not change with time, the corrosion process continues by an oxygen or water molecule inward flux. Since the overall reaction is magnetite formation another possibility to the HCl-cycle would be that iron ions are transported through the iron-chloride layer. A simple calculation of corrosion kinetics based on Fe+2 diffusion through the FeCl2 layer can be performed by Einstein’s random walk, Equation (6).

Corrosion mechanism of furnace wall tubes|35

𝑙 2 = 2𝐷𝑡 (6)

where 𝑙 is the effective Fe-diffusion distance, 𝐷 is diffusion coefficient and 𝑡 is time. This distance is set to around 2 μm (the steady state thickness of FeCl2 after 1000h) according to Figure 15. Data on the solubility and transport of ions in iron chloride is difficult to find, but information of the diffusion coefficient (D) for the cation in caesium chloride is available. CsCl has a fairly close melting temperature to FeCl2 which are 645 ˚C and 677 ˚C [56] respectively. The value of D, in caesium chloride at 400 ˚C is around 10-10 cm2sec-1 [79]. Using these values in Equation (6) gives a short time of around 100 seconds and a rough estimation of corrosion thickness would be several hundred microns after 1000h. The average corrosion rate of 16Mo3 is reported to be ~115 μm/1000h, Figure 26. This means that iron ion diffusion through the iron chloride layer is not rate-limiting. The influx of oxygen (water molecule) must thus be the corrosion rate limiting step. It could also be speculated that the iron chloride probably has a weak adhesion and low strength and can spall if getting too thick. The Cl-activity should thus be kept low enough to avoid systematic spallation of the semiprotective and rate limiting magnetite layer. Based on the diffusion data the reactions in step  and step  may thus be described as: 3𝐹𝑒 → 3𝐹𝑒 2+ + 6 𝑒 − (7) the anodic reaction

6𝑒 − + 3𝐹𝑒 +2 + 4𝐻2 𝑂 → 𝐹𝑒3 𝑂4 + 4 𝐻2 (8) the cathodic reaction The overall reaction may thus be summarised as:

4𝐻2 𝑂 + 3𝐹𝑒 → 𝐹𝑒3 𝑂4 + 4 𝐻2 (9)

36|Corrosion mechanism of furnace wall tubes

The corrosion mechanism can be described as active oxidation by HCl and H2O where FeCl2 is an intermediate step, i.e., this process requires the presence of iron chloride phase as a catalyst and corrosion continues by oxidation with H2O. This process is mainly dependent on oxygen supply (primarily as water molecule) through a highly defective oxide, resulting in a high corrosion rate in the furnace walls of wood-fired boilers.

Reducing corrosion problems|37

5

Reducing corrosion problems

In chapter 4 we realized the corrosion process on 16Mo3 walls in a wood-fired boiler. According to the process and also the literature synopsis in Chapter 2 some suggestions for reducing the problem are decreasing the temperature of the walls, coating the walls with more protective alloys and the use of the fuel additives. In this chapter all these solutions are studied and discussed.

5.1 The effect of temperature (Paper II) In general, a decrease in temperature decreases the rate of chemical reactions due to the decrease of the diffusion rate [80, 81], so the corrosion can be reduced by reducing the temperature [82]. To test the effect of temperature four coupons of 16Mo3 were exposed in the furnace wall at the boiler of Idbäcken, Figure 5. The average temperatures of the samples were 285, 358, 398 (the actual temperature of the furnace walls) and 409 ˚C with a ± 20 ˚ deviation. The exposure time was 1075 h. The thickness of the samples before exposure was measured by micrometer at four equally spaced positions along the centre line. After exposure the samples were cut at the measuring point, then the thickness of the samples were measured at 5 points, according to Figure 22. One part of each sample was left un-mounted for advanced analyses. The corrosion rate results are shown in Figure 23.

Figure 22. a) A diagram of the sample after exposure. The points 1-4 are the measuring points before exposure, the black dots are showing the measuring points after exposure. b) four parts of the sample 409 ˚C after cutting to measure the corrosion. The top part of each sample was left unmounted.

38|Reducing corrosion problems

Figure 23. Thickness loss of the four samples according to the temperature. Average, minimum and maximum of 20 measuring points are shown.

Figure 23 shows that increasing the temperature increases the corrosion rate and even at the lowest temperature (285 ˚C) the corrosion rate is 100 μm per 1000 h (relates to about 0.5 mm per firing season) which is still high. Also in order to reduce the temperature of the furnace wall, the pressure of water/steam should be reduced. This reduces the temperature of the water inside the furnace wall tubes and consequently the temperature of the furnace walls. To reach 285 ˚C the final steam pressure should be 26 bar. This results in a large drop in electrical efficiency. The deposit of the un-mounted part of each sample was analysed under EDS and the results are shown in Figure 24. Results show that the deposition of the Cl increases on the samples by increasing the temperature. The amount of K also increased with increasing temperature. Cl is shown to attack the 16Mo3 in the previous chapter. K is also known as a corrosive agent. KCl is shown to be corrosive in various laboratory studies [22, 83, 84].

Reducing corrosion problems|39

Figure 24. The amount of key elements in the deposit of the 16Mo3 samples exposed to 100% waste wood according to temperature

It can be concluded that increasing the temperature leads to a more corrosive environment with higher K and Cl levels. In oxidising conditions and at higher temperatures, however, it has been shown that the amount of Cl decreases with increasing temperature [85]. Thermodynamics calculations of the corrosive environment in the furnace walls area show the FeCl2 is more stable at higher temperatures. The amounts of S and O are on the other hand decreasing with the temperature. This can lead to less formation of sulphate which is less corrosive. Lead was present in all deposits. The mixture of PbCl2 with the alkali chloride can have a low melting point of 400 ˚C [86] and can increase the transportation of ions and the corrosion rate. The amount of Pb was found to be higher in 358 ˚C sample which can be the reason of high corrosion rate in some points of that sample. XRD results from the deposit of this sample (358 ˚C) showed that Pb can be in pure form or oxide form.

40|Reducing corrosion problems

According to this work, lowering the temperature is not a viable solution to reduce the corrosion rate of furnace walls made of 16Mo3 in the furnace region area when burning waste wood.

5.2.

Alloys for coating (Papers III and IV)

Coating of the waterwalls with a corrosion-protective alloy is a popular method to reduce corrosion. Different types of materials are tested and analysed in this work. Table 9 shows all the tested materials. Table 9. The alloys tested in the waste wood-fired boiler at the waterwalls area and the nominal chemical composition in wt% [87, 88]. Iron is as balance Alloy Ni Cr Mo Other alloying elements 16Mo3 0.3 Mn0.55, Si0.22, Cu0.3, C0.16 APMT(a) 21.0 3.0 Al5.0, C0.08, Si0.7, Mn0.4, A 625 63 21.0 9.0 Nb+Ta3.5,Mn0.35,Si0.2,Ti0.25, Al0.19, 310S 19 25.4 0.11 Mn0.84, Si0.55, C0.046, Cu0.08, N0.04 304L 8.1 18.1 0.29 Mn1.63, Si0.35, C0.02, Cu0.32, N0.07 153MA 9.4 18.7 0.22 Mn0.62, Si1.26, C0.05, Cu0.25, N0.139 253MA 11 20.7 0.17 Mn0.43, Si1.61, C0.088, Cu0.1, N0.169 (a)

APMT sample was pre-oxidised by material supplier. The heat treatment was for 8 h in air and at 1050 ˚C to form a one μm protective alumina layer.

The samples were exposed in the boiler of the Idbäcken plant (Figure 25) using the air-cooled probe. Four samples (304L, 16Mo3, 153MA and 253MA) were exposed at the right wall and four samples (APMT, 16Mo3, A 625 and 310S) were exposed at the back wall. A coupon of 16Mo3 was used in both positions as a reference material. The exposure time was 934 h and the temperature was controlled to 400 ˚C.

Reducing corrosion problems|41

Figure 25. Schematic figure of the right wall of the Idbäcken boiler. The positions of the test panels and slits on the right and back walls are shown in the figure.

The thickness of each sample was measured before and after the exposure according to Figure 22 and thickness loss results are shown in Figure 26. Since the Ni-based alloy is a popular coating alloy, a test panel with 16Mo3 low alloy steel was also designed and welded inside the right wall of the Idbäcken boiler. Some of the tubes were arc-weld overlay coated with A 625 and some left uncoated to use as the reference tube. The front part (facing the combustion gases) were dry-cut out after 3 years in

42|Reducing corrosion problems

operation (about 20000 h) and the thickness of the tubes were measured at 8 equally spaced positions after mounting and polishing. The average thickness of A 625 welded coating layer on the tube before the test was about 4 mm. After the exposure the average thickness was about 3.8 mm with the minimum thickness of 3.4 mm. The thickness of 16Mo3 tube before exposure was about 7 mm and after the 3-year test the average thickness was about 5.35 mm with the minimum thickness of 5.0 mm.

Figure 26. The thickness loss per 1000 h (corrosion rate) of all the tested probes and tube samples. The results are normalised to 1000 h. The first four samples were exposed in the back wall and the rest in the right wall. (a)Tube samples

Comparing results on 16Mo3 coupons exposed in the back wall and right wall of the boiler shows that corrosion is higher in the back wall. This agrees well with the reports from the plant. There are many parameters that can affect the higher corrosion rate. The flue gas temperature has been measured for 8 minutes at 5 seconds interval at the places where the probes were exposed. Flue gas temperature at the back wall was between 855 to 895 ˚C while the flue gas temperature at the right wall varied between 667 to

Reducing corrosion problems|43

766 ˚C. This temperature difference can be the reason for the corrosion rate difference. It should be noted that the fuel feeds from back and front wall in Idbäcken. The corrosion rate of 16Mo3 probe sample (exposure time 934 h) and 16Mo3 tube sample (exposure time 20000 h) are similar. This means that the corrosion rate of 16Mo3 is linear and no protective oxide forms on 16Mo3 steel. It is more difficult to draw such conclusions for the A 625, since there was not any probe testing on the right wall. Also being a welded structure of A 625 tube sample, gave a non-accurate initial thickness measurements on that sample. However, the very low average corrosion rate for the tube exposed for 20000 h may be representative and indicate a parabolic corrosion rate and a semi-protective oxide layer formation. The results, Figure 26, showed that APMT FeCrAl alloy can also be a good alternative to Ni-based alloy. Nickel is more prone to be attacked by lead solution [89]; so Fe-based alloys may show better results specially when burning high lead-containing waste wood. To further investigate the deposits, chemical composition of the samples were analysed by EDS at 3 different areas (with the size of 2 mm × 2.5 mm) at the middle of each specimen and Table 10 shows the results. Carbon content is not mentioned in the table; however the value was between 5-10 wt%. Table 10. Average chemical compositions of the deposits wt% Sample

Wall O

Na

Mg S

Cl

K

Ca Cr

APMT

Back

A 625

Back

310S

Back

50.1

40.3 9.6

---

4.8

2.1

3.4

0.9 14.8 3.9 ---

1.5

33.3 4.6

0.7

7.9

2.8 8.1

4.7 1.0

3.2 ---

10.6 ---

0.6

3.1

1.2

16Mo3

Back

37.0 4.2

1.1

9.2

6.2 9.6

3.3 ---

304L

Right 43.2 6.3

0.7

0.6

2.2 0.4

0.3 15.5 8.9 7.9

0.8 1.1

153MA

Right 23.5 1.3

1.6

5.1

2.8 0.8

2.8 8.9

---

253MA

Right 33.3 2.5

2.5

16Mo3 Right 35.5 4.0 (a) 16Mo3 Right 38.2 4.0 (a) A 625 Right 38.0 0.6 (a)Tube

samples

4.5

Fe

Ni

0.7 13.1

13.4 2.5 2.2 5.1

---

6.3 5.6

Cu Zn Mo Pb 4.1 0.3 0.7

--10.5

---

1.4 ---

4.4

---

2.5 ---

12.9

4.1

3.5

2.8 8.9

8.1

3.9

1.0 2.4

1.4

11.4 7.2 10.6 1.2

3.1

0.9 4.1

6.0 9.1

4.1

---

4.9 ---

---

2.5 ---

10.1

0.7

3.1

6.0 11.2 4.6 ---

9.1 ---

---

2.1

7.1

2.4

11.8 0.2 1.26 9.1

1.7

---

2.2 7.1

0.7

2.5

10.1 6.2 1.0

10.4

44|Reducing corrosion problems

The chemical compositions of the deposits vary between the specimens, depending on the alloy, however all the deposits contained Cl and alkali metals. To study the corrosion front some samples were polished according to Figure 9 (for the probe samples) and Figure 27 (for the tube samples) and the results are shown in Figure 28 to Figure 32.

Figure 27. A Schematic diagram of manual polishing of tube sample without water

Figure 28. Reference tube elemental mapping under EDS. The sample was exposed at the right wall of boiler for 3 firing seasons.

Figure 28 shows that the corrosion product is mainly iron, oxygen and chlorine. This is in good agreement with the previous chapter. Chlorine was not found in the corrosion front for the A 625 probe and tube samples, (see Figure 29 and 30) but K and Pb were present in the interface between substrate and deposit.

Reducing corrosion problems|45

Figure 29. SEM analyses of A 625 probe sample shows concentration of lead and potassium. The interface is free of Cl (apart from an isolated area under the oxide)

Figure 30. Quantmapping of A 625 coated tube sample exposed for 3 firing seasons in the Idbäcken plant. The results show migration of Ni and Cr into the deposit.

Some deposits were taken from A 625 coated tube sample, then manually ground and analysed under XRD, Table 11. Table 11. XRD results on deposits from A 625 tube sample

Strong intensity Medium Intensity

Crystalline compound KCl NiO, NaCl, K3Na(SO4)2, K2Pb(CrO4)2

46|Reducing corrosion problems

The results showed the presence of K2Pb(CrO4)2. It was reported that K can attack the protective chromia and form unprotective chromate [90]. Lead has also been shown to have similar effects on Ni-Cr alloys [67]. Results from this work imply that K together with Pb can attack the chromia. It can be the combination of K-Pb that attacks the oxide. Another explanation to form K2Pb(CrO4)2 is in two steps: 1. K attacks the chromia and forms low-protective chromate according to Equation (1) 2. The acidic potassium chromate can dissolve lead and form the potassium lead chromate. Results on APMT sample are present in Figure 31 and Table 12 accordingly. Some laboratory testing for 1 week at 600 ˚C showed that the attack of chromia by K is reduced by pre-oxidising [91, 92], but this result showed that the alumina layer was not retained after 6 weeks at 400 ˚C. The results presented here showed that chlorine was present at the corrosion front together with potassium and zinc. It seems likely that potassium and zinc attacked part of the protective oxide and that chloride corrosion also occurred, but at a low rate.

Reducing corrosion problems|47

Figure 31. A cross section through the corrosion product in the APMT probe sample with 13 marked analysis points. The results are given in Table 12. Table 12. Chemical composition of 13 spots related to Figure 31 in wt%. WDS was employed to distinguish among S, Pb and Mo Spectrum 1 2 3 4 5 6 7 8 9 10 11 12 13

O 22.4 28.6 29.4 29.9 28.0 28.6 29.4 27.4 27.6 28.6 31.8 29.2 28.1

Na --------------------6.73 8.97 8.38

Al 4.1 2.5 3.7 4.8 1.8 0.8 0.8 0.6 0.6 2.3 6.0 4.1 3.9

S 0.4 1.4 1.6 1.8 1.5 1.6 1.8 2.2 1.0 2.3 5.1 3.4 2.3

Cl 6.2 2.5 2.3 4.0 2.3 1.8 1.4 1.6 0.8 2.4 2.5 2.5 1.5

K 1.7 0.6 0.8 1.3 0.6 0.5 0.6 0.9 0.4 1.2 4.8 2.8 1.4

Cr 19.0 13.1 11.9 11.4 12.2 13.8 16.0 14.6 5.7 16.4 13.8 18.1 13.2

Fe 30.4 45.9 43.9 38.6 45.0 41.3 37.2 31.0 55.2 31.1 19.9 19.5 32.0

Zn --4.4 6.1 7.1 8.6 10.9 12.6 11.8 8.7 15.3 8.9 11.1 9.2

Mo 2.3 0.3 1.5 3.0 2.1 3.4 3.1 2.0 1.0 1.3 0.5 3.1 1.2

Pb ---------------------------

SEM point analyses on stainless steel 310S are presented in Figure 32 and Table 13.

48|Reducing corrosion problems

Figure 32. Point analyses on the corrosion front of 310S stainless steel probe sample with the 5 marked spots. The results are shown in Table 13 Table 13 Results in wt% from Figure 32 O Na S Cl K Ca Cr 1 39.6 0.8 0.8 2.2 1.0 --- 16.1 2 22.6 1.5 3.7 --- 4.7 --- 14.9 3 39.6 --- 1.6 2.2 0.6 --- 30.1 4 21.2 0.8 1.7 9.5 --- 1.6 24.1 5 29.2 --- 0.8 --- 4.9 0.8 15.1

Mn ----1.0 --0.6

Fe 30.7 18.4 9.3 29.9 9.3

Ni 8.0 12.1 4.3 5.0 5.0

Pb --15.5 ----24.4

Figure 32 and Table 13 show the presence of Cl, K and Pb at the interface between oxide/substrate. Pb and K are in association together, but not in reaction with Cl. It seems that 310S is attacked by two mechanisms: Cl attack (which is dominant in 16Mo3 samples) and a K-Pb combination (which is dominant in A 625 samples). Figure 26 shows that the corrosion rate of 310S is in between that of 16Mo3 and A 625.

5.3

The effect of a fuel additive (Papers V and VI)

Heat and power plant owners have successfully used fuel additives to reduce the corrosion attack by changing the flue gas and deposit chemistry [93-95]. One of the fuel additives is sewage sludge. Sludge has a negative gate fee which is advantageous. Firing of sludge is only a small part of the capacity of the power plant and therefore the process is not adversely affected. Sewage

Reducing corrosion problems|49

sludge has been used as an additive with coal for several years [96]. It also has been shown [47, 97, 98] that co-firing sewage sludge with biomass can reduce the corrosion attack in the superheaters area. In this work this effect is studied at the furnace wall region. APMT, A 625 and 310S were chosen to be tested along with 16Mo3 as the reference material. According to the previous chapter, these alloys showed low corrosion rates. Two air cooled probes were exposed at the Idbäcken furnace walls area. One probe was exposed when firing 100% waste wood and the second one when co-firing 8.4 wt% (1.7 vol%) sewage sludge with the waste wood. The sludge used was digested municipal sewage sludge and contained high amounts of aluminosilicates and sulphate was also added to the sludge during the treatment process. The exposure time was 14.25 h. The metal temperature of the 16Mo3 and 310S samples was controlled to 350 ˚C and the metal temperature of the APMT and A 625 samples was controlled to 400 ˚C. The average chemical composition from three different areas at the middle of each specimen is given at Table 14. Table 14. Average chemical composition of the probe deposits in wt%, without- and with the addition of sewage sludge O 28.6 46.1 35.6 310S 43.7 40.4 APMT 44.7 40.8 A 625 42.1

16Mo3

Values in

Na Mg Al Si 7.7 3.2 2.8 4.2 6.5 3.7 4.6 4.0

0.9 1.8 1.2 1.4 1.4 1.6 1.6 1.4

1.1 2.4 1.6 3.5 1.5 2.7 1.0 4.5

2.0 4.7 2.9 3.6 1.6 3.5 1.9 3.5

P

S

Cl

K

Ca

Ti Mn Fe Cu Zn Pb

0.4 1.7 0.6 1.8 0.5 1.8 0.7 1.8

4.3 5.3 7.9 6.6 11.3 7.1 10.6 6.6

12.2 3.7 4.6 2.7 5.0 2.3 4.0 3.1

3.8 3.3 5.1 4.4 7.5 5.0 8.5 5.0

8.6 14.1 14.5 11.2 8.9 12.3 10.7 10.8

2.5 3.9 4.3 3.0 2.6 3.2 3.2 2.9

0.7 0.3 0.5 0.2 0.2 0.2 0.2 0.2

15.1 4.3 3.7 4.9 1.6 4.4 1.1 4.3

0.8 0.0 0.6 0.3 0.4 0.0 0.2 0.3

3.3 3.2 5.4 3.7 2.5 3.6 4.2 3.3

area are waste wood+ sludge results.

Table 14 shows that the co-firing of sewage sludge with used wood reduced the amount of K and Cl in the deposits of the samples. The amount of Pb was also decreased. This led to a

8.0 1.9 8.6 4.7 8.0 3.3 6.6 6.0

50|Reducing corrosion problems

reduction in the corrosion of the tested materials at the furnace walls area. The use of sewage sludge in tests on superheaters has also shown to reduce the Cl deposition and reduce the corrosion rate [99, 100]. The sulphation of the alkali chlorides had the main role in the superheaters to reduce the corrosion. However, in furnace walls tests in Table 14 little or no increase in the amount of sulphur of sewage sludge samples was seen, although the sludge contained sulphur. This lack of sulphation is thought to be due to the low level of oxygen in the gas. Although some S is converted to SO2, the sulphation reaction does not widely occur in the furnace walls region. It should be noted that the tests are done at furnace walls in the combustion zone, between secondary and tertiary air where the combustion is not complete. The formation of the oxides of carbon is more favoured than the oxidation of sulphur at the prevailing gas temperature of 1000 ˚C, i.e., more negative Gibbs free energy of oxidation of C than of S. Therefore, any oxygen in that area will react with C before reacting with S. The high CO levels (0.8%) witness to the incomplete combustion. Therefore it is suggested that sulphation is not the main cause of the reduction in Cl in the deposits. Figure 33 shows all the samples after exposure. The sewage sludge and waste wood samples appear to have a rougher deposit than 100% waste wood samples.

Reducing corrosion problems|51

Figure 33. The sample after 14.25 h exposure in the furnace a) when burning sewage sludge with waste wood b) when burning waste wood. The materials are mentioned in the figure

The deposits on waste wood samples (Figure 33 b) had more tendency to spall during preparation and analyses than the deposits on co-firing sewage sludge samples (Figure 33 a). An adherent deposit may act as a barrier to the corrosive flue gas and reduce the corrosion. XRD results on the deposits of the individual samples are presented in Table 15. The diffraction patterns were very complex and it was impossible to solve all peaks but a few more obvious phases are suggested.

52|Reducing corrosion problems

Table 15. Crystalline compounds formed in the deposits of the tested samples

16Mo3

Strong intensity

Medium intensity

Weak intensity

Fe2O3, Fe3O4, NaCl

KCl, CaSO4 Fe2O3, Fe3O4, KCl, NaCl, K2Ca2(SO4)3 KCl, Fe2O3, Ca(SO4), K2Ca2(SO4)3 Fe2O3, Fe3O4, NaCl, KCl, K2Ca2(SO4)3 K2Al2O4 K2Al2Si2O8 (Na,K)2SO4 Unidentified peaks

K2Ca2(SO4)3

CaSO4 Fe3O4, NaCl

310S CaSO4 APMT A 625

KCl, CaCl2 KCl NiO K2Ca2(SO4)3, Cr2O3

Compounds mentioned in

Unidentified peaks Cr2O3 Cr2O3 K2PbO2 K2Ca2(SO4)3 K2SO4, K2Pb(CrO4)2 Fe2O3

area are waste wood+ sludge results.

K2Al2O4 is formed in the deposit of the APMT sample when burning 100% waste wood, Table 15. It shows that K can attack the protective alumina layer and form poorly protective potassium aluminates, Equations (10) and (11). 𝐾2 𝑂 + 𝐴𝑙2 𝑂3 → 𝐾2 𝐴𝑙2 𝑂4 (10)

2𝐾𝐶𝑙 + 𝐻2 𝑂 + 𝐴𝑙2 𝑂3 → 𝐾2 𝐴𝑙2 𝑂4 + 2𝐻𝐶𝑙 (11)

The latter equation is also found in superheater areas [101]. But co-firing sewage sludge could suppress this attack. Potassium is captured by aluminosilicates according to Equation (12) [97] . 𝐴𝑙2 𝑂3 ∗ 2𝑆𝑖𝑂2 + 2𝐾𝐶𝑙 + 𝐻2 𝑂 ⇄ 𝐾2 𝑂 ∗ 𝐴𝑙2 𝑂3 ∗ 2𝑆𝑖𝑂2 + 2𝐻𝐶𝑙 (12)

This agreed well with EDS elemental mapping results, Figure 34 and Figure 35.

Reducing corrosion problems|53

Figure 34. Elemental mapping of APMT short testing probe sample when burning 100% waste wood

Figure 35. Elemental mapping of APMT short testing probe sample when co-firing sewage sludge with waste wood

Figure 34 shows migration of substrate into the deposit during this test. K and Al are found in association together which means the K can attack alumina while in Figure 35 the initial alumina layer is found which means that the attack is suppressed. The aluminium peak in the sewage sludge sample can also be seen with GD-OES results, Figure 36.

Figure 36. Depth profile of APMT sample when co-firing sewage sludge

54|Reducing corrosion problems

Potassium lead chromate is found in the deposit of A 625 when burning 100% waste wood, Table 15. This compound was also present in the long term A 625 sample, Table 11. It can be concluded that chromia can be attacked by the Pb-K combination even in a short time exposure. This attack was suppressed when adding sewage sludge and the chromia layer was retained, Table 15. The effect of co-firing sewage sludge is also presented in the GDOES results of the A 625 samples, Figure 37.

Figure 37. GD-OES results at oxide and deposits of A 625 samples a) when burning 100% waste wood b) when co-firing sewage sludge with waste wood. The K signal is from CCD and according to Figure 6. The instrument was calibrated for the substrate but not for the deposit as deposit layer is porous. So, the depth results are semi-quantitative

Figure 37 shows the amount of Cl and K is lower in the deposits and oxide of A 625 sample when co-firing sewage sludge, while the amounts of Al and Si (indication of aluminosilicates) are higher. This means that aluminosilicates can capture the corrosive agents. The effect of co-firing sewage sludge on the 310S corrosion front is shown in Figure 38 and Figure 39.

Reducing corrosion problems|55

Figure 38. EDS mapping of 310S stainless steel when burning 100% waste wood

Figure 39. EDS mapping of 310S stainless steel when co-burning sewage sludge with waste wood

Comparing Figure 38 and Figure 39 shows that the KCl area is thinner in the 310S probe sample when co-firing sludge. KCl can attack the chromia and form a poor-protective chromate layer according to Equation (11). Fe and Cr have migrated less into the deposit with co-firing sludge. Nickel is enriched when burning 100% waste wood and it can be seen close to the metal surface and also further out in the corrosion product (Figure 38). The nickel enrichment appears to follow the growth of the oxide. Iron and chromium were oxidised, while nickel was not reacting and the lack of iron and chloride lead to nickel enrichment.

56|Reducing corrosion problems

Another explanation of Nickel-enrichment is as a result of nickel plating out from a reaction of NiCl2 forming CrCl2 and FeCl2 [102]. Nickel is less prone to react with chlorine. This may explain the enrichment close to the substrate surface. The nickel enrichment is stopped by co-firing sewage sludge and cannot be seen in Figure 39. The corrosion process in 16Mo3 is already discussed in the previous chapter. The results here also show that the closest layer to the substrate is FeCl2 under the oxide and deposit, Figure 40 and Table 16.

Figure 40. FIB cross section on the 16Mo3 sample when burning 100% waste wood along with the 6 marked points. The EDS results are shown in Table 16. Table 16. Chemical composition of 6 spots related to Figure 40 in at% O Na Si S Cl K Mn Fe Zn Pb 1 --- 0.6 0.4 33.2 -0.5 65.2 -- 0.1 2 24.4 -- -- 1.4 8.3 --65.7 -- 0.2 3 24.4 -- -- 0.8 2.1 1.1 -66.0 -- 0.2 4 29.4 -- 0.4 0.4 1.9 0.8 -69.8 -- 0.2 5 26.9 -- -- 0.6 23.3 20.1 0.6 42.7 1.2 0.3 6 7.0 4.2 -- 0.3 0.9 0.6 -70.5 -- 0.2

Low alloy steel 16Mo3 corrodes quickly, but in the sample exposed to sewage sludge and waste wood (Figure 42) the iron chloride formed is less compared to the sample exposed to 100% waste wood (Figure 41).

Reducing corrosion problems|57

Figure 41. Elemental mapping of 16Mo3 probe sample when burning waste wood

Figure 42. Elemental mapping of 16Mo3 probe sample when burning waste wood + sewage sludge

In both samples Cl is not in combination with K. It should be mentioned that the samples are exposed to air after making the cross section, so the iron chloride has been oxidised or hydrated. The results from this chapter showed that co-firing digested sewage sludge with waste wood can reduce the corrosion in the furnace walls area and the coating materials. It showed that this special fuel additive can change the corrosion process at least in a short exposure time.

58|Reducing corrosion problems

5.4 Practical implications of the results Concluding remarks from this chapter showed that furnace wall corrosion in recycled-wood fired boilers can be decreased by coating the wall with the nickel-based alloy A 625. Stainless steel coatings also worked well and Kanthal APMT (FeCrAl alloy) showed potential. The use of digested sewage sludge changed the chemistry of the deposits and decreased corrosion during short-term testing.

Conclusion|59

6

Conclusion

Furnace walls (waterwalls) are made of 16Mo3 which corrodes very rapidly when burning used wood. The corrosion was found to be caused by chloride attack (governed by HCl) leading to the formation of a continuous and steady state thin layer of iron chloride under a thick iron oxide. Similar corrosion rates were measured from tests lasting 1000 hours and 20000 hours, indicating a linear corrosion rate. The corrosion mechanism is described as active oxidation by HCl and H2O where HCl plays a catalytic role in the process. The deposition of the Cl and K and the corrosion rate of 16Mo3 increases with increasing temperature in the furnace walls area of a wood-fired boiler. The corrosion mechanism was found to be similar at low and high temperature. Even at a low metal temperature (285 ˚C) the corrosion rate of the furnace wall exceeded 0.5 mm per firing season which implied 16Mo3 without a coating is not a suitable material for waterwalls when burning used wood. Testing different coating materials showed that the nickel-base alloy A 625 can reduce the corrosion rate drastically. The FeCrAl alloy APMT also showed a very low corrosion rate. Stainless steel 310S showed a moderate corrosion rate and could be a lessexpensive alternative to nickel-based alloys. The A 625 samples were attacked by a combination of potassium and lead leading to the formation of non-protective potassium lead chromate. APMT samples were mainly attacked by K and Cl. Pb was not found in the corrosion product of APMT. Stainless steel 310S was found to be corroded by both mechanisms: combination of K-Pb and chloride-induced corrosion. Preliminary results from the use of digested sewage sludge which was mixed with the waste wood indicated that levels of Pb, K and Cl were reduced on the furnace wall deposits of 16Mo3, A 625, 310S and APMT. This led to a reduction in the corrosion rate

60|Conclusion

during short-term tests and suppressed the formation of potassium-lead chromate in A 625 as the protective chromia layer was maintained. The alumina layer was also retained in the APMT sample when co-firing sewage sludge with waste wood in the short-term tests. Thermo-Calc is a very strong tool to predict the corrosion product. The thermodynamic calculations are in good agreement with metallographic results.

Future work|61

7

Future work

The results showed the very complex environment of the boiler when burning waste wood. More fundamental and engineering studies are needed to elucidate the proposed corrosion mechanism. One year test panel samples and more probe tests with exposures between 15 and 1000 h will enable a more thorough investigation of the iron chloride layer. Controlled laboratory studies can help us to understand the transportation and influx of oxygen across the oxide scale. Investigation of additional alloys and coatings as an alternative to the expensive nickel-based A 625 coated tubes is needed. For example FeCrAl alloy and the stainless steels 310S, 304L, 153 MA and 253 MA which also showed good results. The FeCrAl alloy APMT has not previously been used in the furnace wall area or as a coating material and further studies are needed to test the coatability of this alloy. APMT should be evaluated on a furnace wall tube for at least one firing season. The co-firing of digested sewage sludge could be a useful means of reducing corrosion and should be further investigated. Longterm testing with this fuel additive and testing of higher amounts of fuel additive is imperative. Thermodynamics modelling were mainly based on the low alloys steel 16Mo3. Further studies are interesting in modelling other alloys such as Ni-based alloy A 625 and FeCrAl alloy APMT in the actual chemical composition of fuel. Thermodynamic modelling of co-firing of sewage sludge with waste wood is also interesting.

62|References

References|63

8

References

1.

Pachauri R.K. and Reisinger A., Climate change 2007: Synthesis Reort, 2007, IPCC: Geneva. p. 1-104. Foster V. and Bedrosyan D., Understanding CO2 emissions from the global energy sector, 2014, World Bank Group: Washington DC. Döing M., Siebertz M., Loenicker J., Schneider K., Kupper L. and Ersoy S.R., Biomass to power (Analyst version); The World Market for Biomass Power Plants 2014/2015, 2014, EcoProg GmBH: Cologne. p. 1-770. Key EU targets for 2020. Access on 2015 July, 08th; Available from: http://ec.europa.eu/clima/policies/brief/eu/index_en.htm. Chrusciak M., European waste to energy plant market, 2013, Frost & Sullivan: California, USA. Strömberg B. and Svärd S.H., The Fuel Handbook 2012, 2012, Värmeforsk: Stockholm, Sweden. Valdez B., Eliezer A., Alvarez L., Carrillo M. , Schorr M., Stoytcheva M., Rosas N. and Zlatev R., Corrosion control in industry2012: INTECH Open Access Publisher. Steingress F.M., Frost H.J. and Walker D.R., High Pressure Boilers1994: American Technical Publishers. Drescher U. and Brüggemann D., Fluid selection for the Organic Rankine Cycle (ORC) in biomass power and heat plants. Applied Thermal Engineering, 2007. 27(1): p. 223228. Kerr K. and Probert S., Fluidised bed combustion: Improved system design leading to reduced pollutant emissions. Applied Energy, 1986. 23(4): p. 233-267. Vassilev S.V., Baxter D., Andersen L.K. and Vassileva C.G., An overview of the chemical composition of biomass. Fuel, 2010. 89(5): p. 913-933.

2.

3.

4.

5. 6. 7.

8. 9.

10.

11.

64|References

12.

13.

14.

15.

16.

17.

18.

19.

Odelstam T., Performance of composite furnace tubes in recovery boilers. T. Odelstam, Pulp & Paper Industry Corrosion Problems-, 1983. 4: p. 64-67. Hudson J.C., Stanners J.F. and Hooper R.A.E, Low-alloy Steels, in corrosion control, Shreir L.L, Jarman R.A. and Burstein G.T., Editors. 1994, Butterworths Heinemann: Oxford. p. 23-33. Alipour Y., Henderson P. and Szakálos P., The effect of a nickel alloy coating on the corrosion of furnace wall tubes in a waste wood fired power plant. Materials and Corrosion, 2014. 65(2): p. 217-225. Ihara Y., Ohgame H., Sakiyama K. and K. Hashimoto, The corrosion behaviour of iron in hydrogen chloride gas and gas mixtures of hydrogen chloride and oxygen at high temperatures. Corrosion Science, 1981. 21(12): p. 805-817. Ihara Y., Ohgame H., Sakiyama K. and K. Hashimoto, The corrosion behaviour of chromium in hydrogen chloride gas and gas mixtures of hydrogen chloride and oxygen at high temperatures. Corrosion Science, 1983. 23(2): p. 167181. Grabke H.J., Reese E. and Spiegel M., The effects of chlorides, hydrogen chloride, and sulfur dioxide in the oxidation of steels below deposits. Corrosion Science, 1995. 37(7): p. 1023-1043. Reese E. and Grabke H.J., influβ von chloriden auf die oxidation des 2¼ Cr-1Mo-stahls. Werkst korros, 1992. 43: p. 547-557. Reese E. and Grabke H.J., Einfluß von natriumchlorid auf die oxidation von hochlegierten Chrom- und ChromNickel-Stählen. Materials and Corrosion, 1993. 44(2): p. 41-47.

References|65

20.

21.

22.

23.

24.

25.

26.

27.

28.

Szakalos P., Henderson P. and Pettersson R., Mechanisms of chlorine induced corrosion and effect of sulphur additions in superheater corrosion in biomass- and waste fired boilers, in 16th international 'Corrosion Congress'2005: Beijing, China. Paper 13-36. Viklund P., Juma S., Eriksson C. and Engvall K., Corrosion resistance of 304L, 310S and APMT in simulated biomass gasification environments. in European Corrosion Congress, EUROCORR 2011. Stockholm. Folkeson N., Jonsson T., Halvarsson M., Johansson L-G., and Svensson J-E., The influence of small amounts of KCl(s) on the high temperature corrosion of a Fe-2.25Cr1Mo steel at 400 and 500°C. Materials and Corrosion, 2011. 62(7): p. 606-615. Hossain M. and Saunders S., A microstructural study of the influence of NaCl vapor on the oxidation of a Ni-Cr-Al alloy at 850° C. Oxidation of Metals, 1978. 12(1): p. 1-22. Lu W., Pan T., Zhang K., and Niu Y., Accelerated corrosion of five commercial steels under a ZnCl 2–KCl deposit in a reducing environment typical of waste gasification at 673–773°K. Corrosion Science, 2008. 50(7): p. 1900-1906. McNallan M., Liang W., Kim S. and Kang C., Acceleration of High Temperature Oxidation of Metals by Chlorine. High Temperature Corrosion NACE, 1983. 316. Stott F., Prescott R., Elliott P. and Al'Atia M., Assessment of the degradation of metals and alloys in Air--2% chlorine at high temperature. High Temp. Technol., 1988. 6(3): p. 115-129. Han G. and Cho W.D., High-Temperature Corrosion of Fe3Al in 1% Cl2/Ar. Oxidation of Metals, 2002. 58(3-4): p. 391-413. Cho W.D. and Han G., High-temperature corrosion of Ydoped Fe3Al in environments containing chlorine and oxygen. Journal of Materials Engineering and Performance, 2006. 15(5): p. 558-563.

66|References

29.

30.

31.

32.

33.

34.

35.

36.

37.

Schwalm C. and Schütze M., The corrosion behavior of several heat resistant materials in air + 2% Cl2 at 300 to 800 °C. Part 3 – Alumina formers and intermetallics. Materials and Corrosion, 2000. 51(3): p. 161-172. Strafford K.N., Datta P.K., and Forster G., The high temperature chloridation of iron, nickel and some ironnickel model alloys. Corrosion Science, 1989. 29(6): p. 703-716. Zahs A., Spiegel M. and Grabke H.J., Chloridation and oxidation of iron, chromium, nickel and their alloys in chloridizing and oxidizing atmospheres at 400–700°C. Corrosion Science, 2000. 42(6): p. 1093-1122. Cha S. and Spiegel M., Local reactions of KCl particles with iron, nickel and chromium surfaces. Materials and Corrosion, 2006. 57(2): p. 159-164. Cha S.C. and Spiegel M., Fundamental studies on alkali chloride induced corrosion during combustion of biomass. in Materials science forum. 2004. Trans Tech Publ. Li Y. and Spiegel M., Internal oxidation of Fe–Al alloys in a KCl-air atmosphere at 650°C. Oxidation of Metals, 2004. 61(3-4): p. 303-322. Li Y.S., Sanchez-Pasten M. and Spiegel M., High temperature interaction of pure Cr with KCl. in Materials science forum. 2004. Trans Tech Publ. Pettersson C., Pettersson J., Asteman H., Svensson J-E., and Johansson L-G., KCl-induced high temperature corrosion of the austenitic Fe–Cr–Ni alloys 304L and Sanicro 28 at 600°C. Corrosion Science, 2006. 48(6): p. 1368-1378. Karlsson S., Pettersson J., Johansson L-G., and Svensson J-E., Alkali induced high temperature corrosion of stainless steel: the influence of NaCl, KCl and CaCl2. Oxidation of Metals, 2012. 78(1-2): p. 83-102.

References|67

38.

39.

40.

41.

42.

43.

44.

45.

46.

Pettersson J., Svensson J-E., and Johansson L-G., Alkali induced corrosion of 304-type austenitic stainless steel at 600 C; comparison between KCl, K2CO3 and K2SO4. Materials science forum. 2008. Trans Tech Publ. Segerdahl K., Pettersson J., Svensson J-E., and Johansson L-G., Is KCl (g) Corrosive at Temperatures Above its Dew Point? Influence of KCl (g) on Initial Stages of the High Temperature Corrosion of 11% Cr Steel at 600°C. Materials science forum. 2004. Trans Tech Publ. Karlsson S., Pettersson J., Johansson L-G., and Svensson J-E., Alkali induced high temperature corrosion of stainless steel: The influence of NaCl, KCl and CaCl2. Oxidation of Metals, 2012. 78(1-2): p. 83-102. Shinata Y. and Nishi Y., NaCl-induced accelerated oxidation of chromium. Oxidation of Metals, 1986. 26(34): p. 201-212. Lehmusto J., Lindberg D., Yrjas P., Skrifvars B.J. and Hupa M., Thermogravimetric studies of high temperature reactions between potassium salts and chromium. Corrosion Science, 2012. 59: p. 55-62. Lehmusto J., Skrifvars B-J., Yrjas P. and Hupa M., High temperature oxidation of metallic chromium exposed to eight different metal chlorides. Corrosion Science, 2011. 53(10): p. 3315-3323. Shinata, Y., Accelerated oxidation rate of chromium induced by sodium chloride. Oxidation of Metals, 1987. 27(5-6): p. 315-332. Shinata Y., Hara M. and Nakagawa T., Accelerated oxidation of chromium by trace of sodium chloride vapor. Materials Transactions, JIM, 1991. 32(10): p. 969-972. Enestam S., Bankiewicz D., Tuiremo J., Mäkelä K. and Hupa M., Are NaCl and KCl equally corrosive on superheater materials of steam boilers? Fuel, 2013. 104: p. 294-306.

68|References

47.

48.

49.

50.

51.

52.

53.

54.

55.

Åmand L-E., Leckner B., Eskilsson D. and Tullin C., Deposits on heat transfer tubes during co-combustion of biofuels and sewage sludge. Fuel, 2006. 85(10–11): p. 1313-1322. Israelsson N., High Temperature Corrosion of FeCrAl Alloys, The influence of water vapour and alkali salt, in Chemical and Biological Engineering2013, Chalmers: Gothenburg, Sweden. p. 52. Jonsson T., Folkeson N., Svensson J-E., Johansson L-G. and Halvarsson M., An ESEM in situ investigation of initial stages of the KCl induced high temperature corrosion of a Fe–2.25 Cr–1Mo steel at 400°C. Corrosion Science, 2011. 53(6): p. 2233-2246. Kofstad P., High temperature corrosion. Elsevier Applied Science Publishers, Crown House, Linton Road, Barking, Essex IG 11 8 JU, UK, 1988. Li Y., Niu Y. and Wu W., Accelerated corrosion of pure Fe, Ni, Cr and several Fe-based alloys induced by ZnCl2–KCl at 450 C in oxidizing environment. Materials Science and Engineering: A, 2003. 345(1): p. 64-71. Kassman H., Strategies to reduce gaseous KCl and chlorine in deposits during combustion of biomass in fluidised bed boilers2012: Chalmers University of Technology. Luthra K.L. and Shores D.A., Mechanism of Na2SO4 induced corrosion at 600–900°C. Journal of The Electrochemical Society, 1980. 127(10): p. 2202-2210. Hwang Y.S. and Rapp R.A., Synergistic dissolution of oxides in molten sodium sulfate. Journal of The Electrochemical Society, 1990. 137(4): p. 1276-1280. Ishitsuka T. and Nose K., Stability of protective oxide films in waste incineration environment—solubility measurement of oxides in molten chlorides. Corrosion Science, 2002. 44(2): p. 247-263.

References|69

56. 57.

58. 59.

60.

61.

62.

63.

64.

65.

Levin E., Robbins C. and McMurdie H., Phase diagrams for ceramists: 1969 SUPPLEMENT. 1969. Robelin C. and Chartrand P., Thermodynamic evaluation and optimization of the (NaCl+ KCl+ MgCl2+ CaCl2+ ZnCl2) system. The Journal of Chemical Thermodynamics, 2011. 43(3): p. 377-391. Chase M.W., NIST-JANAF thermochemical tables, 1998, Joint Army Navy Air, USA, Montgomery, M., Hansson A.N., Jensen S.A., Vilhelmsen T. and Nielsen N.H., In situ corrosion testing of various nickel alloys at Måbjerg waste incineration plant. Materials and Corrosion, 2013. 64(1): p. 14-25. Pettersson R., Storesund J. and Nordling M., Corrosion of overlay weld cladding in waterwalls of waste fired CFB boiler. Corrosion Engineering, Science and Technology, 2009. 44(3): p. 218-226. Storesund J., Sund G., Pettersson R., Nordling M. and Högberg J., Countermeasures to corrosion on water walls, Report number 1023, 2007, Värmeforsk: Stockholm. Enestam S., Corrosivity of Hot Flue Gases in the Fluidized Bed Combustion of Recovered Waste Wood, 2011. Academic Dissertation, Åbo, Finland, 2011. Bankiewicz D., Enestam S., Yrjas P., and Hupa M., Experimental studies of Zn and Pb induced high temperature corrosion of two commercial boiler steels. Fuel Processing Technology, 2013. 105: p. 89-97. Otsuka N., A thermodynamic approach on vaporcondensation of corrosive salts from flue gas on boiler tubes in waste incinerators. Corrosion Science, 2008. 50(6): p. 1627-1636. Stålenheim A. and Henderson P., Materials for higher steam temperature in biomass and waste fired plant. - A review of preent knowledge, reposrt number 1174, 2011, Värmeforsk: Stockholm.

70|References

66.

67.

68.

69. 70.

71.

72. 73.

74.

75.

Enestam S., Mäkelä K., Backman R. and Hupa M., Occurrence of zinc and lead in aerosols and deposits in the fluidized-bed combustion of recovered waste wood. Part 2: Thermodynamic considerations. Energy & Fuels, 2011. 25(5): p. 1970-1977. Chatterji D., McKee D., Romeo G. and Spacil H., The effects of lead on the hot corrosion of nickel‐base alloys. Journal of The Electrochemical Society, 1975. 122(7): p. 941-952. Galetz M., Bauer J., Schütze M., Noguchi M., Takatoh C. and Cho H., The influence of copper in ash deposits on the corrosion of boiler tube alloys for waste‐to‐energy plants. Materials and Corrosion, 2014. 65(8): p. 778-785. Systems T.K., The IIT Foundation Series - Physics Class 8, 2/e: Pearson Education India. Goldstein J., Scanning electron microscopy and X-ray microanalysis : a text for biologists, materials scientists, and geologists1981, New York: Plenum Press. xiii, 673 p. Goodhew P.J., Humphreys J. and Beanland R., Electron Microscopy and Analysis, Third Edition2000: Taylor & Francis. Hammond C., The Basics of crystallography and diffraction2001: Oxford University Press. Lueven K., X-Ray diffraction Bruker D8. 2010 , Access on July 2015, 5th; Available from: https://fys.kuleuven.be/iks/nvsf/experimental-facilities/xray-diffraction-2013-bruker-d8-discover. Marcus P. and Mansfeld F.B., Analytical Methods In Corrosion Science and Engineering2005: Taylor & Francis. Andersson J.O., Helander T., Höglund L., Shi P. and Sundman B., Thermo-Calc & DICTRA, computational tools for materials science. Calphad, 2002. 26(2): p. 273-312.

References|71

76.

77. 78.

79.

80. 81. 82. 83.

84.

85.

Alipour Y., Viklund P. and Henderson P., The analysis of furnace wall deposits in a low-NOx waste wood-fired bubbling fluidised bed boiler,. VGB PowerTech, 2012. 12: p. 96-100. Stevenson D., The Strengths of chemical bonds. Journal of the American Chemical Society, 1955. 77(8): p. 2350-2350. Pettersson J., Folkeson N., Johansson L-G., and Svensson J-E., The effects of KCl, K2SO4 and K2CO3 on the high temperature corrosion of a 304-type austenitic stainless Steel. Oxidation of Metals, 2011. 76(1-2): p. 93-109. Crawford J.H. and Slifkin L.M., Point defects in solids: General and ionic crystals2013: Springer Science & Business Media. Naval Facilities Engineering Command, Corrosion control, 1992, Public Works Centre, USA. Fontana M.G., Corrosion engineering, 2005: Tata McGraw-Hill. Atkins P., Physical chemistery, ed. 6, 1998, New York: Freeman. Pettersson C., Pettersson J., Asteman H., Svensson J-E., and Johansson L-G., KCl-induced high temperature corrosion of the austenitic Fe–Cr–Ni alloys 304L and Sanicro 28 at 600°C. Corrosion Science, 2006. 48(6): p. 1368-1378. Pettersson C., Svensson J-E. and Johansson L-G., Corrosivity of KCl (g) at Temperatures above Its Dew Point-Initial Stages of the High Temperature Corrosion of Alloy Sanicro 28 at 600ºC. in Materials science forum. 2006, Trans Tech Publ. Henderson P., Szakalos P., Pettersson R., Andersson C. and Högberg J., Reducing superheater corrosion in woodfired boilers. Materials and corrosion (1995), 2006. 57(2): p. 128-134.

72|References

86.

87. 88. 89. 90.

91.

92. 93.

94.

95.

Viklund P., Superheater corrosion in biomass and waste fired boilers : Characterisation, causes and prevention of chlorine-induced corrosion, 2013, KTH Royal Institute of Technology: Stockholm. p. 55. Chemical composition products. Access on July 2015, 21st; Available from: www.sandvik.com. Chemical composition products. Access on July 2015, 21st; Available from: www.outokumpu.com. IAEA, Lead as a coolant, 2002. Pettersson J., Asteman H., Svensson J-E. and Johansson LG., KCl induced corrosion of a 304-type austenitic stainless steel at 600 C; the role of potassium. Oxidation of Metals, 2005. 64(1-2): p. 23-41. Israelsson N., Engkvist J., Hellström K., Halvarsson M., Svensson J-E. and Johansson L-G., KCl-induced corrosion of an FeCrAl alloy at 600 °C in O2 + H2O environment: The effect of pre-oxidation. Oxidation of Metals, 2015. 83(1-2): p. 29-53. Svensson J-E., FeCrAl alloys for superheaters in biomass and waste fired boilers, 2009, Elforsk: Stockholm. Vainio E., Yrjas P., Zevenhoven M., Brink A., Laurén T., Hupa M., Kajolinna T. and Vesala H., The fate of chlorine, sulfur, and potassium during co-combustion of bark, sludge, and solid recovered fuel in an industrial scale BFB boiler. Fuel Processing Technology, 2013. 105(0): p. 59-68. Pettersson J., Pettersson C., Folkeson N., Johansson L-G., Skog E. and Svensson J-E.. The influence of sulfur additions on the corrosive environments in a wastefired CFBboiler. in Materials science forum. 2006. Trans Tech Publ. Viklund P., Pettersson R., Hjörnhede A., Henderson P. and Sjövall P., Effect of sulphur containing additive on initial corrosion of superheater tubes in waste fired boiler. Corrosion Engineering, Science and Technology, 2009. 44(3): p. 234-240.

References|73

96.

97.

98.

99.

100.

101.

102.

Bemtgen J., Hein K. and Minchener A., APAS clean coal technology programme 1992-1994.-2: Combined combustion of biomass/sewage Sludge and coals: Final Reports1995. Aho M., Yrjas P., Taipale R., Hupa M. and Silvennoinen J., Reduction of superheater corrosion by co-firing risky biomass with sewage sludge. Fuel, 2010. 89(9): p. 23762386. Karlsson S., Åmand L-E. and Liske J., Reducing hightemperature corrosion on high-alloyed stainless steel superheaters by co-combustion of municipal sewage sludge in a fluidised bed boiler. Fuel, 2015. 139: p. 482493. Jonsson T., Pettersson J., Davidsson K., Johansson L-G. and Svensson J-E., Sewage sludge as additive to reduce the initial fireside corrosion caused by combustion of shredder residues in a waste-fired BFB boiler. in 9th Liège Conference on Materials for Advanced Power Engineering. 2010. Gyllenhammar M., Svärd S.H., Davidsson K., Hermansson S., Liske J., Larsson E., Jonsson T. and Zhao D., Additive for reducing operational problems in waste fired grate boilers, in WR, W. 47, Editor 2013, Waste Refinery: Stockholm. Israelsson N., High temperature oxidation and chlorination of FeCrAl alloys, 2014, Chalmers University of Technology. Jonsson T., Folkeson N., Halvarsson M., Svensson J-E. and Johansson L-G., Microstructural investigation of the HClinduced corrosion of the austenitic alloy 310S (52Fe26Cr19Ni) at 500° C. Oxidation of Metals, 2014. 81(5-6): p. 575-596.