A Review of Current and Planned Smooth Surface Technologies for Fouling Resistance in Boiler

MATERIAL- OCH KEMITEKNIK 1232

Smooth Surfaces: A review of current and planned smooth surface technologies for fouling resistance in boiler

Robert Corkery, Linda Bäfver, Kent Davidsson, Adam Feiler

Smooth Surfaces: A review of current and planned smooth surface technologies for fouling resistance in boiler.

Glatta ytor: En genomgång av glatta ytor som teknik för att stå emot bildning av beläggningar på värmeöverförande ytor.

Robert Corkery, Linda Bäfver, Kent Davidsson, Adam Feiler

M08-838

VÄRMEFORSK Service AB 101 53 STOCKHOLM · Tel 08-677 25 80 Februari 2012 ISSN 1653-1248

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Abstract

Here we have described the basics of boilers, fuels, combustion, flue gas composition and mechanisms of deposition. We have reviewed coating technologies for boiler tubes, including their materials compositions, nanostructures and performances. The surface forces in boilers, in particular those relevant to formation of unwanted deposits in boilers have also been reviewed, and some comparative calculations have been included to indicate the procedures needed for further study. Finally practical recommendations on the important considerations in minimizing deposition on boiler surfaces are made.

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Sammanfattning Det höga innehållet av alkali (kalium och natrium) och klor i avfall och biobränsle leder till drift- och underhållsproblem genom bildning av påslag och efterföljande korrosion på överhettare i förbränningsanläggningar. Detta projekt handlar om interaktioner på överhettartubers ytor vid förbränning av avfall och biobränsle. En genomlysning av tekniker som kan användas för att skapa glatta tubytor i pannor presenteras. Glatta ytor skall förhindra eller fördröja bildning av påslag.

Påslag består av askelement från bränslet. Vid överhettare är kaliumklorid särskilt kritiskt och vid avfallsförbränning även natriumklorid. Både kalium- och natriumklorid är korrosiva. Vid avfallsförbränning kan även ämnen som bly och zink vara betydelsefulla. Hög koncentration av svavel i bränsle och därpå följande rökgas är istället positivt eftersom alkalisulfater kan bildas och de är inte lika klibbiga och korrosiva som motsvarande klorider. Både gasformiga ämnen och partiklar kan bilda påslag. De grundläggande mekanismerna för bildning av påslag är: tröghetsimpaktion (partiklar > 1 µm), diffusion (gas eller partiklar < 1 µm), termofores (partiklar < 1 µm), kondensation (gas) och kemiska reaktioner (gas eller partiklar). Vid överhettare (rökgastemperatur 800-1000 °C) är kalium- och natriumklorid i gasfas. Närmast överhettarytorna blir dock temperaturen lägre och kalium- och natriumklorid övergår till fina partiklar eller kondenserar direkt på ytorna. Ett sådant påslag kan vara tunt och homogent, medan andra påslag kan bli 10-15 cm tjocka och vara heterogent sammansatta.

Man kan minska påslag genom att belägga en ståltub med en beläggning som är keramisk eller med keramisk kompositbeläggning, så att ytan blir glatt. Den stora utmaningen vid keramisk beläggning av ståltuber är skillnader i termisk utvidgning av keramiskt och metalliskt material vid uppvärmning. Det leder till sprickor eller termisk chock med avskalning och andra problem. Cykling av tubtemperaturer vid drift kan också påverka gränsytor mellan ståltub och beläggningsmaterial. Olika keramiska material kan skräddarsys för att minimera termisk utvidgning (CTE - Coefficient of Thermal Expansion) och medföljande problem, men än så länge är de inte tillräckligt inerta i förbränningsmiljöer.

Termisk chock är oundviklig, men konsekvenserna kan minimeras genom att sprida ut spänningar. Det kan göras genom att med hjälp av legeringslager binda keramiskt material till tubmaterialet. Man kan använda komposit-keramer vilka innehåller partikulärt material med modifierad CTE och mer nanostrukturerade system som har mycket fina fibrer för homogen spänningsöverföring, mineralfibernätverk för flexibilitet och styrka, och till slut få en viss nivå av porositet som klarar volymförändringar utan att krackelera. I beläggningsmaterial som står emot termisk chock väl har nano- och mikrostrukturer en avgörande betydelse.

När man gör keramiska beläggningsmaterial kompatibla med stål uppstår oundvikligen porositet, nano- och mikrostrukturer som gasformig alkaliklorid kan diffundera in i. Det kan orsaka problem om struktursystemen är sammankopplade, vilket sannolikt är fallet i dagens material. Det kan också uppstå problem med porer och ledning av alkali-ånga vid tillverkning av keramiska beläggningsmaterial med slamgjutning.

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Fördelningen av porer i en keramisk beläggning som står emot termisk chock innebär sannolikt nanoporös struktur. Kelvin-ekvationen säger att KCl kondenserar in i dessa porer, väl under mättnadstryck för KCl i gasbulken, så vätskeformig KCl stabiliseras även vid högre temperaturer. Vidare så stabiliseras dessa vätskor av små porer. På så sätt stabiliseras vätskor vid mycket lägre temperaturer i den keramiska beläggningen på panntuben. Sammanfattningsvis så kondenserar KCl i porer med hjälp av kapillär kondensation, trots att KCl är i gasfas vid den aktuella rökgastemperaturen och de små porerna kommer också hjälpa till att transportera KCl mellan keramisk beläggning och ståltub.

Ytkrafter gås igenom utgående från beskrivna utmaningar för att belägga ståltuber med keramiskt material. Slutsatsen är att bästa metoden för att minimera påslag och korrosion med hjälp av keramisk beläggning, och upprätthålla livslängden för en keramisk beläggning är att använda keramisk nanokomposit som är resistent mot termisk chock med ”tie layer” till ståltub. Mer arbete behövs för att förstå den exakta ytkemin för alkaliklorid och interaktioner i sprickor och mikrostrukturer med hänsyn till olika keramiska lager. Ett idealt material skulle innebära en keram med modifierad CTE, ett ”tie layer” för kohesion, stängd porositet och termisk och kemisk inert i överhettarmiljö.

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Executive Summary This project has been predominantly focused on surface interactions related to boiler deposits in superheater zones of Waste to Energy (WTE) and biomass boilers. We have reviewed the literature and mechanisms of deposit formation, the nature of the deposits, in particular the initial stages of deposition. It is not a classical thin boundary layer deposit, but can be 10-15 cm thick and with a very heterogeneous chemical and physical nature. The corrosion problems are complex, the fluid dynamics and thermodynamics are likewise. In order to simplify the understanding of the problem, we have reviewed the composition of fuels, and the composition of deposits relevant to the initial stages of deposition. While many factors are involved in this initial stage too, it is considered that the increase in roughness is brought about through condensation of volatile chlorides, in particular alkali chlorides which are abundant in biomass and waste streams fed in as fuel. While it is true that more complex chemistry may occur in the earliest stages, the importance of KCl seems clear from the literature as both a key early depositor on surfaces, and also a key source of aggressive chloride for corrosion.

From reviewing the literature, and from interpreting the literature with the aid of some calculations we can make some conclusions and recommendations, and possibly some new observations. In superheater zones with fireside temperatures between 800-1000°C, KCl can exist in the gaseous state until close to the cooler surfaces of the boiler tubes, at which time it can condense. This is well enough known that additives are used by plant operators to reduce alkali chloride concentrations in the vapour, giving rise to HCl(g) which is less harmful on account of their higher volatility and can be scrubbed.

Other interventions to reduce boiler tube damage by flue gas include the coating of steel boiler tubes with thin ceramic and ceramic composite coats. The role of these is to block corrosive chemical attack from the fireside without compromising heat transfer and to minimize deposits. One great challenge with ceramic coatings of ferrous and nonferrous alloy boiler tubes is differential thermal expansion leading to cracks or thermal shock leading to wholesale delamination and other failure modes. Various ceramics can be tailored to minimize coefficient of thermal expansion mismatch and yet these are not necessarily of the best chemical composition to remain inert with respect to the fireside chemistry.

Inevitably, thermal shock damage in the surface coatings on boiler tubes can be reduced by spreading stresses, for example, using bond layers or through nanocompositing and by maintaining a certain level of porosity that can accommodate volume changes. Cycling of tube temperatures during operations can affect the interfaces in these materials and inevitably, even in the most thermally shock resistant coatings, nano- and microfractures will play the role of stress distributors.

In making ceramic coatings compatible with steels there is invariably porosity or nano and microfractures for gaseous KCl to diffuse into. These will offer a barrier only as long as these fracture systems remain disconnected, and this is probably not the case in

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today’s materials. Secondly the porosity formed during slip casting and green body firing of sol gel ceramics will remain to also be a conduit for KCl gas.

Pore sizes in thermally shock-resistant ceramic coats are likely to be in the nanometric range. The Kelvin equation tells us that KCl will condense into these pores below the saturation pressure of KCl gas in bulk, so KCl liquid is stabilized even at high temperatures. Furthermore these liquids can be stabilized by small pore diameters due to the freezing point depression of liquids in small pores. These liquids can then migrate to the ceramic-alloy interface and cause harmful corrosion. Even worse are the eutectic melts such as can form from KCl and FeCl2 which can be stable in the boundary layer down to almost 200°C.

In order to investigate the role of physical adhesion mechanisms such as van der Waals forces in the formation of boiler deposits this review focuses on the initial layer of deposition that leads to further build up. In this respect, we concentrate on the role played by alkali chloride compounds, primarily KCl. Surface forces are reviewed in considerations of the above challenges, including effects of surface roughness on adhesion forces, the effect of interfacial wetting by fireside liquids and interparticle forces operating in the fireside flue gases.

The conclusion is that the best method to minimize corrosion, maintain fewer build ups and to maintain the life of a ceramic coating is to use a thermally shock resistant nanocomposite ceramic with a bond layer to the boiler tube alloy while at the same time using additives to lower the alkali chloride load in the flue gas.

Further work needs to be done on the exact surface chemistry of KCl liquid vapour solid equilibrium in respect to interactions with silica bed-material in fluidised bed boilers and also the general surface and capillary science of cracks and microfractures with respect to various condensates in the ceramic layers. An ideal material would include a ceramic that is composed for CTE matching, has a bond layer for cohesion and has closed porosity and thermal and chemical inertness to a range of fireside chemistries.

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Table of contents

1 INTRODUCTION ...... 1 1.1 BACKGROUND ...... 1 1.2 AIM AND TARGET GROUP ...... 1 2 BOILERS AND FUELS ...... 2 2.1 BOILERS ...... 2 2.1.1 Grate-fired ...... 2 2.1.2 Fluidised beds ...... 2 2.1.3 Pulverised combustion ...... 2 2.2 FUELS ...... 2 2.2.1 Biomass ...... 3 2.2.2 Waste ...... 4 2.3 COMBUSTION ...... 4 2.3.1 Fundamental combustion phases ...... 4 2.3.2 Release of alkali, chlorine and sulphur ...... 5 3 FORMATION OF DEPOSITS ...... 6 3.1 FLUE GAS COMPOSITION ...... 6 3.1.1 Ash elements ...... 6 3.1.2 Chlorine ...... 6 3.1.3 Sulphur ...... 6 3.2 MECHANISMS ...... 6 3.2.1 Inertial impaction ...... 6 3.2.2 Diffusion ...... 7 3.2.3 Thermophoresis ...... 7 3.2.4 Condensation ...... 7 3.2.5 Chemical reaction ...... 7 4 TYPICAL TUBING MATERIALS AND CORROSION ...... 8 5 SURFACE CHEMICAL PROPERTIES OF A DEPOSIT ...... 9 5.1 LIQUID DEPOSITS ...... 9 5.1.1 Condensation ...... 9 5.1.2 Chemical reaction ...... 10 5.2 SOLID DEPOSITS ...... 11 5.3 EXAMPLES OF SOLID DEPOSITS FOUND ON SUPERHEATERS ...... 11 6 COATING MATERIALS...... 13 6.1 COATING MATERIALS FOR PRESENT AND FUTURE BOILER TUBES ...... 14 6.2 AN EXAMPLE OF A COMMERCIAL NANOCOMPOSITE COATING ...... 15 6.2.1 Review of a boiler tube coating patent ...... 16 6.2.2 Comments on preparing boiler tubes for coating ...... 20 6.2.3 Other commercial systems for coating boiler tubes ...... 20 7 ASPECTS OF SURFACE FORCES IN BOILERS ...... 22 7.1 INTRODUCTION ...... 22 7.1.1 Van der Waals and dispersion forces ...... 22 7.1.2 Relevance of optical data to boiler deposits ...... 23 7.1.3 Dielectric response functions ...... 24

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7.1.4 Optical data for KCl at high temperature ...... 26 7.1.5 Hamaker constants for alkali chlorides ...... 26 7.1.6 Dielectric response functions of ceramic and inorganic materials ...... 32 7.1.7 A comment on negative Hamaker constants ...... 34 7.2 ADHESION, CAPILLARY FORCES AND SMOOTH SURFACES ...... 35 7.2.1 Forces between surfaces ...... 35 7.2.2 Rough surfaces and adhesion ...... 37 7.2.3 Thin films of liquid alkali chlorides on surfaces ...... 38 7.2.4 Van der Waals forces and KCl aerosols ...... 39 7.2.5 Adsorption of alkali gas phases on clays ...... 40 7.2.6 Direct condensation of KCl(g) into surface pores ...... 41 7.2.7 Capillary forces and KCl liquids in pores ...... 42 8 CONCLUSIONS ...... 48 9 RECOMMENDATIONS AND USE ...... 50 10 SUGGESTIONS FOR FURTHER RESEARCH ...... 51 11 LITERATURE REFERENCES ...... 52

APPENDICES

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1 Introduction

1.1 Background The high content of alkali (sodium and potassium) and chlorine in waste and biomass leads to operational and maintenance problems through the formation of deposits and subsequent corrosion of the heat transfer surfaces, particularly on superheaters. Operation is in itself also a problem because deposits affect the heat transfer. To remove deposits, soot blowing is used, but it leads to wear on the heat exchange surfaces and eventually also to increased material loss of the tubes. Thus, it is desirable to avoid the formation of deposits in the first place.

Formation of deposits during combustion and corrosion of heat transfer surfaces has been widely studied in several Värmeforsk projects. Only one Värmeforsk study about smooth tubing surfaces, i.e. surfaces with a non-stick effect, has been performed [1]. There are more experiences in this field, and a need to compile them. The purpose of this work is to review state of the art smooth surface research and technology for fouling resistant surfaces in boiler tubes, with emphasis on superheaters. To achieve this, we combine expertise in both surfaces and boilers. Specifically we utilize expertise in coatings, surface chemistry, nanotechnology and surface adhesion presently at YKI (Ytkemiska Institutet, part of the SP group). We combine this with expertise in alkali chemistry and deposit formation in boilers and formation and emission of ash particles relevant to boilers currently at SP Technical Research Institute of Sweden.

1.2 Aim and target group The aim of this work is to present a comprehensive state-of-the-art review of deposit- resistant smooth surfaces for boilers. It has a special focus of superheaters. The target group of this work are owners of heat and power plants, designers in this field, those responsible for operation and maintenance, and researchers.

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2 Boilers and fuels

2.1 Boilers Different techniques are used to combust solid fuels. They differ mainly with respect to the furnace; i.e. where most of the conversion of the fuel takes place. The major techniques are grate-fired, fluidised bed and pulverised fuel firing. All techniques may be used for power production, which requires water to be vaporized and superheated. The steam tubes in which this takes place are called superheaters. They are usually placed downstream of the furnace and are therefore exposed to the stream of flue gas.

2.1.1 Grate-fired In a grate-fired boiler, the fuel is combusted on a grate at the bottom of the furnace. Air is supplied through nozzles in the grate. Usually, the grate is moving so that the fuel enters from one side of the furnace and is transported during conversion. Finally, bottom ash is removed from the other side of the grate. The grate-fired technique is relatively insensitive to the fuel quality as for the size of fuel particles and moisture. However, the mixing of fuel and air may become uneven.

2.1.2 Fluidised beds A fluidised bed consists of fluidised sand in a reactor. The sand is usually quartz-based. Air for fluidisation and combustion is supplied through nozzles at the bottom of the furnace. Above a certain air velocity the sand becomes fluidised, which means that it attains the characteristics of a liquid. The contact between sand and fuel implies high heat transfer, and the turbulent characteristics of the fluidised bed provide good mixing. The high heat transfer to the fuel particles allows the bed temperature to be relatively low, which implies lower formation of thermal NOx compared with other combustion techniques. In a circulating fluidised bed the air velocity is so high that the sand particles are lifted by the gas stream. They are later separated from the gas in a cyclone and returned to the furnace. Most solid fuels can be fired in a fluidised bed as long as the fuel particles are not so large that they are not fluidized. Some alkali-rich fuels may cause the particles of the bed sand to agglomerate and eventually cause de-fluidisation.

2.1.3 Pulverised combustion In pulverised fuel firing, the fuel is milled before combustion. The small fuel particles are introduced together with air to the furnace by burners on the furnace wall. The fuel has to be dry and possible to mill. This technique is suitable for coal and some pretreated biomass (e.g. torrified biomass).

2.2 Fuels This report focuses on solid biomass and has a Swedish focus. Consequently it has the fuels biomass and waste as starting points. Typically, fuel related problems like formation of deposits vary from minor for wood to more challenging for straw, agricultural residues and waste.

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2.2.1 Biomass Biomass includes all kinds of materials that were directly or indirectly formed through photosynthesis, not too long ago. The main components of plant matter include cellulose, hemicelluloses and lignin. The concentration of each component in plant matter depends on species, type of plant tissue, stage of growth, and growing conditions. Three main classes of biomass are:

1. Wood and woody materials 2. Herbaceous and other annual growth materials 3. Agricultural wastes and residues

In Table 1 compositions and heating values for a number of biomass fuels are presented. For comparison waste fuels are included as well. With respect to the formation of deposits, the ash content and its composition are crucial, but also the concentrations of sulphur and chlorine are important. The least problematic biomass wood pellets contain less than 1 % ash and negligible amounts of sulphur and chlorine, whereas, wheat straw is known for causing problematic deposits because of its high content of chlorine and ash.

Table 1. Fuel analyses of a number of fuels. Composition in mass-% of dry fuel, and low heating value (LHV) in MJ/kg dry fuel. MSW = Municipal Solid Waste. IW = Industrial Waste. Tabell 1. Bränsleanalyser för ett antal bränslen. Sammansättning i mass-% torrt bränsle, och lägre värmevärdet (LHV) i MJ/kg torrt bränsle. MSW = Municipal Solid Waste. IW = Industrial Waste.

Fuel a C H O N S Cl Ash LHV

Wood pelletsb 50.6 6.0 43.1 0.08 <0.01 <0.01 0.3 19.12 Willow woodb 49.9 5.9 41.8 0.61 0.07 <0.01 1.71 18.40

Wheat strawc 44.9 5.5 41.8 0.44 0.16 0.23 7.02 16.83 Switchgrassc 46.7 5.8 37.4 0.77 0.19 0.19 8.97 16.89

Almond shellsd 49.3 6.0 40.6 0.76 0.04 <0.01 3.29 18.28 Oat graind 47.6 6.6 40 2.2 0.19 0.05 3.1 18.49

Mixed papere 48.0 6.6 36.8 0.14 0.07 - 8.33 19.45 60% MSW/40% IWe 44.3 5.6 26.4 1.1 0.4 0.5 21.7 15.86 a Fuel analyses are from reference [2] cited by reference [3], except for wood pellets [4] oat [5] and 60%MSW/40% IW [6] bWoody fuels c Herbaceous and other annual growth materials dAgricultural wastes and residues eWaste

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2.2.2 Waste Waste fuels are more heterogeneous than biomass. They also vary with respect to chemical content depending of their origin, e.g. plastics, paper and wood. Compared to biomass, waste often has higher ash content (Table 1) leading to severe formation of deposits on superheater tubes.

2.3 Combustion A fuel particle that enters the furnace undergoes the fundamental combustion phases drying, devolatilisation, and char combustion. Depending on size and shape, these phases can take place at the same time in different positions in a particle. In the following description it is assumed that the particle is so small that the processes are consecutive. During these phases, alkali, chlorine, sulphur and other species important for deposit formation are released to gaseous phase.

2.3.1 Fundamental combustion phases Drying starts as soon as the particle gets heated. The temperature increases to about 100 °C where it remains until the drying is complete.

Devolatilisation starts as the temperature reaches about 200 °C. The thermal energy causes chemical bonds to break and thus volatile compounds are released. In biomass, the first molecule to break is hemicellulose, and thereafter cellulose. Lignin is devolatilised over a wider temperature range, but at 500 °C, most of the devolatilisation reactions have been completed. Waste consists of many different materials. The major difference between waste and biomass is the presence of plastics, but a major part of those also devolatilise in the range 200-500 °C. The devolatilisation products are lumped into gas and tar. Tar consists of condensable organic compounds of different molecular size. The condensation temperatures differ between these compounds. Most of them condense below 300 °C, but some at higher temperatures. The gases are compounds that are not condensable at room temperature. They comprise CO, CO2, CH4, H2 and others. As the tar and gas meet oxygen, an oxidation process takes place. The radiated heat is visible as a flame. The heat sustains the devolatilisation process. The phenomenon usually referred to as "fire" is actually devolatilisation of e.g. wood, and the heat for the devolatilisation is provided by the flames.

Char combustion begins as the rate of devolatilisation decreases, since then oxygen can reach the char surface. The char is the remaining solid of the fuel after devolatilisation, and it has a high content of carbon. When the oxygen meets the carbon, the reaction produces CO2, and the heat released in the process results in a visible temperature rise of the char surface; usually referred to as glow. The solid remains after char combustion are ash. Some of the ash forms particles small enough to be carried away by the gas flow and is therefore called fly ash. The rest is bottom ash. A small fraction of the ash is also transferred to gaseous form.

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2.3.2 Release of alkali, chlorine and sulphur The release of alkali, chlorine and sulphur to the gaseous phase occurs by devolatilisation and/or evaporation. A small fraction of the alkali and a substantial fraction of the sulphur in the fuel is organically bound, which means that alkali and sulphur are released during the devolatilisation. Evaporation of inorganic compounds increases with the temperature of the fuel because of the rising vapour pressure. Chlorine is released as HCl in the same temperature range as the devolatilisation. Additional chlorine is released above 600 °C as the vapour pressure of alkali chlorides increases. At higher temperatures alkali sulphates may be evaporated. To summarise, alkali chlorine and sulphur are expected to be released in two temperature ranges. HCl, SO2 and a small amount of alkali are released at typical devolatilisation temperatures (200-500 °C), and alkali chlorides and sulphates evaporate during char combustion (>600 °C).

In the literature it has been found that the release of chlorine during thermal conversion of straw appears in two temperature ranges [7]; about 60% was released between 200 and 400 °C, i.e. during devolatilisation, and the rest between 700 and 900 °C. Likewise, SO2 and alkali have been found to be released during devolatilisation and at higher temperatures [8, 9]. However, the release of alkali during the devolatilisation constitutes only a small fraction of the total release [10,11].

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3 Formation of deposits The main content of severe deposits is typically alkali chloride, i.e. potassium chloride and sodium chloride. However, firing waste may also lead to a more complex deposit composition, e.g. lead and zinc may be crucial. The formation of deposits depends on the flue gas composition and by what mechanisms some of its content may deposit on steel tubes.

3.1 Flue gas composition The flue gas consists of a mix of gaseous compounds and particulate material. Most important for formation of deposits are ash elements, chlorine and sulphur.

3.1.1 Ash elements During combustion, ash elements are released from the fuel (c.f. 2.3.2). Alkali, i.e. potassium and sodium, is the major problem. In biomass, potassium dominates, and in waste both potassium and sodium are important. Potassium and sodium may cause low- melting compounds, which lead to agglomeration of the fuel bed or formation of deposits on heat transfer surfaces. The detailed ash reactions in a specific boiler are mainly determined by temperature, the ash’s composition and concentration, and possible bed sand. For example, the release of alkali and subsequent formation of alkali chloride increases with temperature. By decreasing the temperature in the fuel bed one can decrease the formation of deposits in the superheater region [12]. The released ash elements often deposit on superheater surfaces, and if they form alkali compounds with low melting points there is a risk for corrosion, especially in cases with high material temperatures.

3.1.2 Chlorine When chlorine is present, the released alkali easily forms alkali chloride. Therefore, high chlorine content in the fuel is problematic with respect to formation of deposits when the concentration of alkali in the fuel is high. High content of chlorine also means a high formation of acid HCl, but that is another problem.

3.1.3 Sulphur When the sulphur concentration in the flue gas is high alkali sulphates may be formed. Sulphates are less sticky and less corrosive than chlorides so in this sense, sulphur has a positive effect on deposits. It is well known that problems with superheater deposits can be reduced by adding sulphur to the process [13, 14].

3.2 Mechanisms

3.2.1 Inertial impaction Inertial impaction refers to deposition of particles. Particles carried by the flue gas generally follow the gas stream. However, when the gas reaches a steam tube it has to make a sharp bend. Some of the particles may be heavy enough (have enough inertia) to

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continue in the previous direction, i.e. not following the gas stream. This means that they are liable to hit the surface of the tube. Depending on the structure of the surface or the particle, it can then become stuck on the surface, thus building up a deposit. Inertial impaction is especially important for deposition on the windward side of a tube.

3.2.2 Diffusion Diffusion refers to the deposition of particles (mainly size < 1 µm) and gaseous molecules. It occurs if there is a concentration gradient with respect to particles or the molecule of interest. Then, for statistical reasons, a net movement of particles/molecules occurs from higher to lower concentration. If the gradient is present close to the surface of the steel tube, the molecules can condense on, or chemically react with, the surface and thus build up a deposit.

3.2.3 Thermophoresis Thermophoresis refers to the deposition of small particles. In a sharp temperature gradient of the flue gas, the side of a particle that faces the warmer gas will experience harder and more frequent collisions with the flue gas molecules than the opposite side. This results in a force that moves the particle from warm towards colder flue gas. Because the surface of the steel tubes is typically at 450 °C and the flue gas may be 600-800 °C, there is a temperature gradient that can move particles to the surface.

3.2.4 Condensation Condensation is a phase transition from gaseous to liquid form. It occurs when a gas reaches a partial pressure above the saturation pressure, which is temperature dependent, why cooling of the gas may initiate condensation. Condensation may occur by homogeneous nucleation in the bulk of the gas, but more likely on a surface. Surfaces in the flue gas are provided by the particles carried by the flue gas. These particles then get a partly liquid surface, which increases the probability that it will get stuck if it reaches the surface of the steel tube e.g. by inertial impaction or thermophoresis. Condensation may also occur directly on the surface which is usually much colder than the gas. In such a case, if diffusion is the transport process, the condensation maintains the concentration gradient in the gas phase, and is thereby a condition for deposition by diffusion.

3.2.5 Chemical reaction Chemical reactions may occur between species in the flue gas and the steel surface or deposits on the steel surface. If the product is liquid or solid it builds up the deposit. Chemical reactions can also take place on particles in the flue gas. If the product is liquid it can enhance deposition in the same way as condensation on particles.

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4 Typical tubing materials and corrosion Superheater corrosion usually occurs due to high material temperatures in combination with alkali compounds with low melting points and a reducing atmosphere. Different tubing materials are used in different positions of a boiler, depending of the need of corrosion resistance, with respect to temperature and surrounding flue gas. The steel grades used in a boiler can be sorted into five groups, see table 2 [15]. Most simple is carbon steel/low-alloyed steels, which contains less than 3 % Cr. An example of this is the commonly used superheater material 15Mo3. Low-alloyed steel forms iron oxide during combustion. The other steel groups contain higher amounts of chromium, which leads to formation of a continuous layer of a chromium-rich oxide that protects against corrosion. In boilers firing biomass or waste the most commonly used tubing materials for superheaters are austenitic stainless steels and high-alloyed Fe-based steels [15]. These steels provide good value for money with respect to corrosion resistance. Most resistant against corrosion are the alloys with highest Cr/Fe value, i.e. high alloyed Ni- based alloys. However, such materials are also most expensive.

Table 2. Common alloys used for high temperature applications (after [15]). Tabell 2. Vanliga legeringar som används vid högtemperaturapplikationer (efter [15]).

Steel/alloy Fe Cr Ni Mn Si Mo Cr/Fe Carbon steels/Low alloyed steels 15Mo3 99 - - 0.6 - 0.3 0

13CrMo44 98 1 - - - 0.5 0.01 T22 96 2 - 0.5 - 1 0.02 Ferritic chromia formers P91 89 9.5 0.1 0.5 0.3 0.9 0.11 X20 85 12 0.5 1 - 1 0.14 433 77 22 0.1 0.6 0.2 - 0.29 Austenitic stainless steels Esshete 1250 67 15 9 6 0.5 1 0.22 316L 67 17 12 1 0.4 2.5 0.25 347H 68 17 11 1.7 0.6 0.3 0.25 304L 68 19 10 1.4 0.6 0.5 0.28 253MA 66 21 11 0.4 1.5 - 0.32 High alloyed Fe-based steels 310 51 26 18 1.6 0.9 - 0.51 AC66 40 28 32 - - - 0.70 353MA 36 26 33 1.4 2.4 - 0.72 Sanicro 28 35 27 31 2 0.7 3.5 0.77 High alloyed Ni-based alloys Inconel 690 9 30 60 - - - 3.33 Inconel 625 5 22 58 0.5 0.5 9 4.40 Sanicro 63 2 22 65 - - 9 11.00

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5 Surface chemical properties of a deposit The present work concerns the initial build-up of deposits on the surface of a steel tube and how it can be minimized through materials engineering. It can be assumed that liquid compounds that reach the surface stick to the surface. For solid substances, the attractive forces between the molecule and the surface have to be compared with other forces affecting the molecule.

5.1 Liquid deposits The liquid substances can reach the surface by diffusion coupled with condensation or a chemical reaction. Liquid substances can also be formed on particles in the flue gas and be transported to the surface by impaction, diffusion or thermophoresis.

5.1.1 Condensation The process of condensation implies, by definition, that a liquid is formed on the surface from a gaseous species. To judge whether condensation of a certain substance will occur, the concentration/partial pressure in the gas phase and the temperature of the surface have to be known. This is compared with the saturation pressure at that temperature. If the partial pressure is above the saturation pressure, condensation can occur. As will be seen later, condensation can occur on surfaces when the partial pressure is lower than the saturation pressure at which the liquid condenses under certain conditions of surface geometry. This sub-saturation condensation can occur if the condensed liquid normally wets the surface, and if the surface has nanoscale roughness or accessible nanoscale porosity. We will return to this in chapter 7.

As an example for the case of smooth surfaces, the saturation concentration of gaseous KCl as a function of temperature is given in Figure 1. It shows that for condensation to occur on a smooth surface at say 800 °C, the concentration must reach around 2000 ppm, which is an unlikely high concentration. If, instead, the concentration is around 100 ppm at 800 °C, condensation should occur if the gas were cooled to around 700 °C. Considering that superheater tubes seldom are warmer than 500 °C, it can be concluded that there would be risk for condensation as the flue gas approaches the tube surface.

The flue gas is convectively cooled by passing the steel tubes. Condensation may then occur on particles in the flue gas. These particles may thereafter come in contact with the surface of a steel tube and stick there.

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Figure 1. Saturation concentration of KCl versus temperature [16, 17]. Figur 1. Mättnadskoncentration av KCl mot temperatur [16, 17]

5.1.2 Chemical reaction A liquid deposit may result from the chemical reaction between a gaseous species and a solid species on the surface, or between a gaseous species and the surface itself. The prerequisite is that the product is liquid. For example, the reaction

G + S → L, where G is a gaseous species, S is a solid species on the surface, and L is the liquid product, shows such a process. In practice, however, it seems unlikely that this process would be of great importance, since it requires firstly that a heterogeneous reaction take place, and secondly that the product thereof be liquid at the temperature of the surface.

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5.2 Solid deposits Solid deposits may build up simply from adhesion of particles in the flue gas. The particulate matter may consist of pure mineral species, soot, oxides or complex aggregates of various species. The size range of particles may vary from nm to microns. The adsorption of particles to the surface governed by a force balance between attractive forces including van der Waals forces, and electrostatic interactions and repulsive forces including viscous drag of the gas around the particles. The solid deposit therefore reflects to some extent the distribution of species in the flue gas but more accurately which type of particulate matter that has highest affinity for the surface.

5.3 Examples of solid deposits found on superheaters Jensen [18] examined deposits from straw-based biomass boilers in Denmark. The two plants were Ensted and Masnedø of 100MWth and 33MWth, respectively. The Ensted superheater runs slightly hotter than the Masnedø superheater, with final steam temperatures of 542 °C and 520°C respectively. Deposits at Masnedø were 2-15 cm thick, while at Ensted they had a maximum thickness of a few centimetres. Both were comprised of three layers, with one relatively thick intermediate layer.

The thick middle layer of deposits at Masnedø was formed when molten KCl reacted with a calcium-rich silicate ash-derived phase. The reaction formed a chlorine-depleted layer due to conversion of chlorine to gas. The inner most layer of the deposit contained many sublayers of mainly iron oxide with KCl and K2SO4. The Ensted intermediate layer comprised KCl with melt textures, with these having inclusions of Ca- and Si-rich particles. The innermost layer at Ensted was an iron oxide layer next to a potassium sulfate layer. Jensen [18] also found that the mature deposits were different in composition compared with that of experimental probes exposed for a short time, suggesting that reactions continue after deposition. The mature deposits form dense layers next to the boiler tube surfaces that are rich in KCl and K2SO4. The same authors postulate that the innermost layer continues to expand by condensation of gaseous KCl that has diffused into the preformed deposits, giving rise to a more and more dense and K-rich deposit as time proceeds. They noted that in the case of straw, potassium was not present in the sulfate form, which implies that the potassium sulfate deposits seen on the boiler tubes must have come from the reaction with sulfurous gases, such as may occur by catalytic oxidation of SO2 on iron oxide. The oxidized SO2 species could then react with KCl to yield K2SO4 plus chlorine gas. Their thermodynamic calculations also indicate that in the case of excess chloride, the potassium species will be gaseous KCl up to about 1200°C, and if sulfate is present in the gaseous phase above about 780°C, potassium sulfate will be thermodynamically more stable than the chloride.

KCl is considered far more corrosive than K2SO4 – the reason why studies of additives that favour the formation of sulfates have been performed. These studies include research on additions of sulfur sources and additions of clay minerals such as kaolinite (see Wu et al. (2011) and references therein) [19]. These interventions are Al, Si, S, P and Ca-based and lead to predominantly more potassium sulfate and less alkali chlorides depositing on boiler surfaces. The chlorine in alkali chlorides is converted to gaseous HCl, which can escape deposition. These interventions can reduce NOx output

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and decrease corrosion rates, but can also increase sulfurous gas outputs. In any case the use of additives will not be considered further here, other than to say that if coatings are used on boiler tubes then the chemistry of the flue gas plus additives should be taken into account as they will have different reaction pathways compared with burning of the raw fuel.

In the case study of Liu et al. [20], the ternary mixture KCl- K2SO4- Na2SO4 as components of biomass-fired boiler deposits was examined. The samples were collected from various super-heater zones of a 30 MW boiler. The deposits discussed here are mainly from the secondary superheater zone (SH2) where flue gas temperatures were intermediate at 550-750°C and steam temperatures of 430-470°C. The fuel was cotton stalk. The higher temperature zones formed deposits rich in SiO2 and Ca-(Fe,Mg)Si2O6 pyroxenes. Any salts formed at this temperature were probably evaporated and carried to lower temperature surfaces. In the SH2 zone, the initial layers were < 2mm thick oxide scales, as determined by elemental analysis. Iron oxide was present as Fe2O3 as determined using X-Ray diffraction (XRD).

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6 Coating materials

It is apparent that the problem of producing a fuel-independent coating technology for boiler alloys is a problem of high-temperature chemistry and physics, and low- temperature solution nanochemistry and corrosion chemistry. Ideal anti-soil coatings must resist the creep, thermal expansion and oxidation of boiler tubes in their normal operations, and the chemical attacks. Nor should they offer additional thermal resistance. In addition they should be resistant to inertial impacts that erode, pit crack and generally fatigue the surface coating in ways that lead to non-smooth surfaces that can lead to formation of deposits. They should of course be affordable and practical to apply.

The interface and the elastic/plastic behaviour of a high performance coating will provide a natural gradient over which adverse solid-state interfacial tensions between various materials can be alleviated allowing well-bonded, semi-elastic ceramic layers with extremely smooth surfaces and high temperature chemical resistance. Various approaches to achieving highly adherent coatings include the use of intermediate layers (e.g., bond layers, solid solutions) at the metal/metal oxide ceramic interface (for both alloy and ceramic coatings). For example these can be formed during sintering of ceramics or by thermal spray of appropriate compositions [21-25]. The mechanics of stressed ceramics is relevant to these approaches has been examined in Sweden, for example by Odén et al. [26].

Next generation boiler tubes will need to be designed with extreme care considering the plethora of interfaces in these highly engineered systems, involving nano-scale structures with high specific surface areas. The primary purpose of this section is to point to other reviews on the subject of coating compositions, nano-compositing and coating methods, and to identify some illustrative cases from the academic literature and industrial patent literature.

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6.1 Coating materials for present and future boiler tubes This section briefly reviews coating materials for protecting boiler tubes from formation of deposits in combustion facilities. It briefly introduces several classes of oxides, ceramics and other crystal classes found in the literature survey and then gives some examples of present and future potential materials from academic and commercial sources.

The definition of the term “refractory” is key to most materials relevant here. Formally it means that the material does not evaporate readily. The elements that form refractory oxides generally are found in left corner of the d-block transition metals on the periodic table (Figure 2).

Figure 2. Basic periodic table of the elements [27]. Figur 2. Periodiska systemet [27].

Elements forming chemically resistant refractory oxides are from the left side of the d- group transition metals and include Ti, Y, Zr, Nb, Hf, Ta, W and also from the p-group including Al. Of these alumina is the cheapest and it appears in several patented nanocomposites for boiler tube coatings reported below, and is probably the most studied of the refractory oxides. Titania in the high temperature form is rutile and is also found in several recent patents for boiler tube coatings. Oxides of Y (yttria) are extremely expensive, but are extremely useful because of their chemical inertness at high temperature and extreme resistance to temperature, so much that it is often used in foundries as a ceramic for casting superalloys for jet engine turbine blades and other performance materials. Yttria, amongst other additives is also useful as a stabilizer of the tetragonal phase of ZrO2. Zirconia is the last of the relatively affordable elemental refractory oxides, and for this reason it is also to be found in the nanocomposites disclosed in several patent applications. Nb, Hf, Ta and W oxides are only considered likely to be practically useful when used as minor additives, or as solid solutions components in other oxides such as rutile, for which Ta and W oxides share the same crystal structure.

In addition to the elemental oxides, more complex crystalline refractory materials appear to be useful enough for companies to be actively patenting these. In these

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materials, other elements become useful such as B, C, N, Si, Sn, alkaline earth metals, several transition metals (Cr, Fe etc.), and more, but usually (for complex oxides) when Al, Ti, Y and Zr occur with them in the structure. They are often doped with other elements to tune their properties and form equilibrium compounds with other refractory materials in their multi-elemental phase diagrams. They are also candidates for coating of SiC as discussed below. Examples often include layers or composite layers of various spinels (CuAl2O4, MgCr2O4, yttria-doped-MgAl2O4, (Fe,Zn)Cr2O4), doped-rutiles, yttrium aluminium garnets (YAG), pyrochlores, perovskites (YCrO3, CaTiO3), mullite . (nAl2O3 mSiO2), various zirconates and titanates (Al2TiO5/Al2O3 composites, ZrTiO4, BaZrO3, Al2TiO5 and CaTiO3), Gd2Zr2O7, V2O5, silicon carbides, boron carbides, silicon nitrides, aluminium nitride/silicon carbide composites and C-CSi composites [28-30].

Boss [30] provides a comprehensive review of potential composite materials for SiC. They conclude that no other material beside SiC could provide the combination of resistance to thermal creep, thermal shock and oxidation necessary for constructing externally-fired combined cycle (EFCC) power systems. For example, Si3N4 was considered to have too low a coefficient of thermal expansion (CTE). They also concluded that corrosion by coal combustion products severely degraded SiC so their study was aimed at coating SiC. A large number of materials and their properties were considered.

6.2 An example of a commercial nanocomposite coating A nanocomposite material called Nanocomp PP has been tested on the metal tube surfaces on boilers tube surfaces in Sweden [1]. Nanocomp PP, according to an early article in German [31], is a nanocomposite of nanoparticulate, hexagonal boron nitride (“white graphite”) and ceramic nanoparticles nominally zirconia (judging by their patent application – [21]), but could also be Al2O3, AlO(OH), ZrO2, Y-ZrO2, TiO2, Fe2O3 and/or SnO2. While it stayed adherent to the boiler tubes, it provided significant benefits in terms of corrosion and soil resistance [1]. However the retention of the coating was significantly impacted in a relatively short time.

The patent application for Nanocomp PP technology from ItN Nanovation AG is for coating surfaces in Waste to Energy (WTE) and other boiler based plants. The patent number is EP1386983B2 (in German) and refers specifically to boron carbide (BC) composites with a fairly brief teaching of its disclosure [21]. However it is possible that another set of patents is relevant, i.e., WO 03/93195 and US 20090253570 A1 [32], also from ItN Nanovation.

Even if this patent is not directly relevant to the studies reported in Värmeforsk report 1034 [1], a detailed look at this patent will help with understanding a wider range of materials and coatings for boiler tube application. Therefore a review of this patent is used to drive a broader understanding of ceramic coatings in general. When this review was written we were not aware of the US filing so translations from German are referred to below. In this mini-review, the patent technology is described along with simultaneous comments and observations of significant points.

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6.2.1 Review of a boiler tube coating patent US 20090253570 A1 (and WO 03/93195) [32] is a patent application, and describe the sintering of a composition comprising particles of component A with particles of component B and binder particles C. A and B form a spinel phase at the same time C is used as a binder during sintering at 900-1000°C, which is much lower than for most similar ceramics. This is standard operating procedure because of the choice of a eutectic mix, which has a lower temperature than otherwise achievable for the individual components. This lowering of the melting temperature through eutectic formation was first described in ItN Nanovations patent on boron nitride nanocomposites for boiler tubes, and was the first topic in the body of the spinel-based patent applications being reviewed here, such is its centrality to the invention.

“A composition is provided for producing high-temperature-stable ceramic …layers, coatings and moulded bodies and particles of an inorganic component A and particles comprising an inorganic component B. This may together form a eutectic mixture and reacted at … at least partially with each other, where it is formed at least a ternary chemical compound (i.e. a combination of 3 elements), and in particular the spinel type.”

Coating ingredients, A and B Spinels of the disclosed composition are of the type AB2X4. The A component being oxides of the transition metal, rare earth metal and p-group metals of the periodic table, with Cu, Fe, Co and Zn oxides are particularly preferred, in particular copper, and the B component chosen from trivalent metal oxides, most preferably Al2O3. Component A is added in proportions, in particular between 5-15 weight %, but as wide as 1-40%. Component B is added in proportions in particular in the range 70-90%, but as low as 50%. The preferred spinels so formed are copper aluminate spinels and chromium iron spinels. It is possible that for boiler tubes, iron chromium spinel nanocomposites might also be useful from a compositional similarity to the oxide layers of the tube alloy. This is not disclosed in their patent. They did disclose that the casting slip for forming a copper spinel/titania nanocomposite described below was applied to boiler tubes and tested for a very short time under nominal combustion in a power station boiler.

Binder ingredients, C (or excess B) Component C, the effective binder, can be one of A and B, and also can be chosen from at least one member from the group alumina, boehmite, zirconium oxide with yttrium- stabilized zirconia, chromium oxide, cerium oxide, iron oxide, silica, tin dioxide and more preferably titanium dioxide. The exact ingredients are chosen to respond best to the ternary spinel formed by A and B at the sintering temperature, in particular chrome iron spinel (Fe(Cr,Fe)2O4). As a side note, chromite spinels are used in the production of ferrochromium master alloys in arc furnaces. In addition, the binder can be the component B added in a quantity where the spinel forms by an excess of component B remains unreacted, and can form the binder phase with no addition of a third component. This is a particularly preferred embodiment of their invention. Component C is added according to the other ingredients in the range of 1-40% and in particular between 5-15 wt %.

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Other ingredients Other ingredients include suspending agents for the obvious dispersion stability of a rheological fluid with the right viscosity and other fluid properties for spreading and wetting the surface, but the exact amount is said to be not overly critical, thus lending some flexibility according to application. However, organics such as polyelectrolyte dispersants, often used for ceramics might be best used at lowest amounts to achieve homogenous suspensions to enable least final carbides and/or porosity. Fillers can also be used, such as mineral fibres, coarse particles etc. to add strength and other properties. It is said to be important that alkali and alkali earth elements are essentially absent from the mix as these can negatively interfere with formation and stability.

Example of a ceramic casting slip for coating boiler tubes To make up the casting slip nitric acid was made up at pH 2 in a bath and used as a suspension solution for the oxides. Next, α-Al2O3 (89.5 wt %) nano/microparticles (100 nm to 1 micron) were added to the nitric acid solution and homogenized. Nanoscale rutile was then added at a rate of 6.5 wt% with high energy stirring. Next, CuO is stirred in at 4 wt%. The solids content of the solution after homogenizing should be 80 wt%. The casting slip is then ready to work with and can be applied to a boiler tube. When this composition was sintered their example disclosed a phase mixture of 87% elongate nanorods of corundum (α-Al2O3), rutile (TiO2) and CuAl2O4-spinel. Formation of CuAlO2 was avoided by staying below 1000°C.

The nanoparticulate size of the ingredients should dramatically reduce equilibration times of the spinel formation. Jost et al. [32] indicate that the initial choice for the crystal habit of corundum is important for successful growth on a single crystal axis yielding fibres during sintering and for these same alumina crystals to seed the growth of copper oxide deposits in grain boundaries leading to a eutectic melting at the phase boundary at 90:10 alumina to copper oxide. The addition of nano-titania apparently aids the eutectic formation, possibly through nucleating low temperature melting.

Though the authors did not say, it is likely that in addition, nitric acid will also passivate the surface and will induce roughness enabling better physical interlocking with the ceramic. It is also likely that during acid treatment defects (in electrochemically reduced states) are oxidized in and on the existing porous oxide extreme outer layers. Dehydration probably also occurs, with iron hydroxides possibly converted back to oxides, making them denser phases and homogenizing the average oxide phase composition at the interface. In the case of iron chromium spinel coats, this will ensure a chemically cohesive and seamless transition to the iron chromium spinels in particular. A look at the literature on solid solutions for chromium iron oxides from the oxide layer of the tube alloy (plus or minus nickel) with these spinels will likely indicate the favourability of such solid solutions as bond layers between the steel and spinel coats.

Advantages of the nanocomposite solution coating methods described above include its high temperature corrosion resistance, relative inexpensiveness, the use of novel binding and sintering methods, the use of Cr and Fe oxides in the ceramic, and its impact resistance. The latter advantage is owed to the extremely high strength and hardness of the coat, having a bending modulus in the range 200-300 MPa, determined according to

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DIN ISO 60 672. The low chemical reactivity of the coat will slow the formation of its roughening during combustive operation, concomitantly slowing formation of nucleation sites for deposit growth. This decreases the potential for capillary related liquid wetting of the surface (see chapter 7) by KCl. Degradation of the smooth fluid dynamic aspects of the circular cross-section of the tube will also therefore be less of an issue as is the all important heat exchange performance. Of course these are all benefits assuming the patented composite lives up to its claims.

Given the importance of CuAl2O4 spinels in their patent disclosure, it is worthwile documenting its thermal expansion coefficient, for the record. The temperature dependence of CuAl2O4 spinels can be found very accurately described in O’Neill et al. [33]. They used neutron diffraction to investigate the temperature dependence of the cation distribution in the spinel. A graph of the unit cell parameter of spinel as a function of temperature is shown in Figure 3 below. As shown in figure 3b the linear -6 coefficient of thermal expansion of CuAl2O4 is around 8-10 *10 /ºC at temperatures typical of the boiler conditions. Steel has a linear CTE of around 13*10-6/ºC and Austenitic stainless steel has CTE in the range 10 to 17 *10-6/ºC . Typical boiler tube materials supplied by Sandvik for example have CTE in the range of 12-16 *10-6/ºC at temperatures of 400 ºC. The similarity in CTE of CuAl2O4 compared with steel and Austenitic stainless steel provides indication that this spinel should have favourable physical properties for tube coating. As a reminder most ceramics have very much lower CTE in the range 2-6 (e.g .SiC = 4*10-6/ºC ) which leads to thermal shock and cracking problems as mentioned earlier.

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b) Figure 3. Thermal expansion of copper aluminium spinel. Figur 3. Termisk expansion av koppar-aluminium-spinell.

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6.2.2 Comments on preparing boiler tubes for coating Pre-cleaning of tubes prior to solution coating Another important consideration is good pre-cleaning. Mild sand, CO2 or water blasting of appropriately chosen and stored steel tubes should remove loose rust. A carefully selected non-ionic surfactant and co-solvent system should be used to remove any grease or other organic pollutant or soil films. Residues from the surfactants cannot be beneficial. So rinsing and careful choices of cleaning agent should be used.

Pre-passivation An important consideration for maximum adhesion of the ceramic layer is to have a high-strength, high density oxide layer to bond to at the steel surface. This could be comprised of fine-grained, interlocked crystals formed by classic nitric acid passivation. To induce a uniform, high-density oxide layer at the boiler tube surface, it could be pretreated. Hydrogen build up should be carefully avoided during any pre-passivation as it could weaken the steel. If pre-passivation is not done, hydrogen bubbles could form from during various reactions of any exposed metal and cause defects during coating. Therefore it is recommended that native metal, accessible through surface pores, defects and surface cuts and scratches, be pre-passivated with nitric acid before coating.

Coating When the coating composition is to be prepared, high quality ceramic coating practices will help. These can be relatively cheap and repeatable and preferably complemented by use of colloidal science principles in preparing the surfaces. The skill is in fine-tuning the colloidal properties of the coating suspension (e.g. the slip) including the suspension homogeneity and rheology so it forms a uniform ceramic precursor (green body) layer. Most likely a spray coat and/or a vertical or horizontal dip coat is possible too. The description of these is beyond the scope of this report. Avoidance of alkali and alkali earths is important when preparing the boiler tube coatings. It would be recommended here that not only the coating composition be free of these during application, but also the steel surfaces be substantially free from these contaminants at the time of coating.

6.2.3 Other commercial systems for coating boiler tubes A number of commercial and institutional organizations have expertise, intellectual property and/or products in metal corrosion or ceramic coatings as solutions for materials protection in power plants. The most common ceramic coatings amongst these are alumina and fireclay-based (mainly kaolin as a precursor) which are more alkali resistant for biomass applications. Silicon carbide is also a key material in the burn zone as it has a larger thermal conductivity than most ceramics, and can thereby reduce the relative deposition rate for a particular fireside situation compared with a surface that cannot conduct heat away so readily. However it is not preferred for high alkali biomass fuels due to its reactivity as stated earlier. Most commercial manufacturers also try to keep the coating thickness thin in order to keep the tube surface-ceramic system close to isothermal. Judging by manufacturers comments, thin (perfect) coatings minimize differential thermal expansion to the point where it is not a problem. These coatings can be applied to tubes or to shells. Manufacturers are less prone to disclosing exact

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compositions and structures for obvious reasons. However as has been highlighted, nanostructured systems are more and more common and are expected to give higher all round performances per unit cost in current and future coatings. In use, whether coated or not, the manufacturers recommend that the operator keep the coatings as cool as possible for avoiding deposits under the given conditions. This can be for several reasons, the most important probably to reduce the “stickiness” of adhesive particles.

For particulars please see the reference list, noting some of these have been described above [21, 22, 31, 34-43]. It is also noted that thin (0.8 mm) corrosion coatings produced by arc spraying with bond strength of 25N.mm-2 are commercially available and have been tested in certain commercial labs [44].

A number of other studies exist that have examined boiler tubes, their coatings and their resistance to corrosion and dirt build up. The interested researcher is encouraged to go to these for valuable representative information pertaining to the current subject [1, 30, 45-47].

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7 Aspects of surface forces in boilers

7.1 Introduction

In the various surfaces of concern in current and future boiler tube materials, van der Waals forces are becoming more relevant. This is primarily because nano-scale structural engineering of interfaces and coatings is inherently dealing with higher surface areas. Nanoscale roughness, capillaries and nanoscale cracks and nanoparticles in the ceramic coating composites and in the aerosolized and condensing and abraded fireside atmosphere are heading quickly to ubiquity. Since this is the realm where van der Waals forces are operative (along with other forces), these are becoming more important to deal with. In this respect the reviewers recognized that in order to make progress in understanding the literature, some mathematical language relating to van der Waals (vdW) forces relevant to ceramics and liquids would have to be decoded and interpreted.

It was decided on the basis of known problems with alkali chlorides in biomass (and other) fuels that any new materials would have to be directly designed to deal with the major constituent of alkali chlorides, namely potassium chloride, and its interactions with as many possible constituents of present and future boiler tube surfaces as possible. In reviewing the literature, little detail was present on van der Waals interactions relevant to boilers. This next section deals with this. For those readers less interested in the mathematical details of calculation of dispersion forces, please skip to section 7.2. However more recent theoretical approaches suggest that current theories of interaction of macroscopic bodies, such as the modern dispersion theory used and reviewed here, are inadequate from many perspectives and need to be totally replaced with a new theory or theories [48]. These advances must be kept in mind while reading this review.

7.1.1 Van der Waals and dispersion forces The modern dispersion theory of interactions of macroscopic bodies relevant to colloids may be used to gain some understanding of the interactions of fireside particulates and gases with boiler tubes in burners. Dispersion forces are the component of the van der Waals force that arise from interactions between spontaneously formed dipoles and induced dipoles. The other components that involve permanent dipoles are the Keesom and Debye forces (dipole-dipole and dipole-induced dipole forces). London dispersion forces are mediated through exchange of electromagnetic radiation, and are therefore an optical phenomenon. A full analysis is not offered here, but the principles are given with a few simplified examples to pave the way for a more sophisticated approach, requiring further experimental data and probably sophisticated computer calculations.

Ninham and Parsegian [49] describe the (London) van der Waals force as “founded on the recognition that spontaneous, transient electric polarization can arise at a center due to motion of electrons, molecular distortion or molecular orientation. This polarization will act on the surrounding region to perturb spontaneous fluctuations elsewhere”.

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The modern theory of dispersion forces between macroscopic bodies is owed to Lifshitz [50], and has its roots in the presence of attractive forces between neutral atoms noted first by London [51] and its method based on the ideas of Casimir and Polder [52]. The interaction between macroscopic objects is regarded “as occurring through the medium of the fluctuating electromagnetic field which is always present in the interior of any absorbing medium, and also extends beyond its boundaries, - particularly in the form of travelling waves radiated by the body, partially in the form of standing waves which are damped exponentially as we move away from the surface of the body” [50]. Lifshitz’ theory overcame previous difficulties of calculating dispersion forces for materials other than dilute gases by including all many-body forces using a continuum approach, retaining all interaction frequencies and handling intervening dielectric media other than a vacuum. Importantly, Lifshitz showed that the information required for the calculation of surface forces were contained within the dielectric properties of the medium and interacting condensed matter.

In many cases, Ninham and Parsegian have shown that knowledge of the full optical spectrum of a material system is unnecessary for calculation of van der Waals forces using full Lifshitz theory [49, 53]. Instead, they demonstrated that a damped oscillator approach requiring only the average characteristic absorption frequencies in the UV and IR and indices of refraction at visible frequencies and the static dielectric constants were sufficient.

Hough and White [54] also discuss the Hamaker’s constant in relation to wetting of liquid films in an atmosphere against a surface. The condition for wetting is that the Hamaker constant for the system atmosphere/liquid film/solid should be negative. The Hamaker constant is calculated in the non-retarded regime (less than 5 nm separation) to give a reliable indication if a liquid film will wet a surface. If the film is wetting then one can apply the concepts of capillary condensation to liquid films in ceramic pores, cracks and corrosion pits, between ceramic nanocomposite components, and fireside solid and liquid aerosols such as KCl and its interactions with fluidized bed media and so on. The next section explores these interactions in more detail.

7.1.2 Relevance of optical data to boiler deposits It seems anti-intuitive that optical data, such as refractive indices and optical absorption coefficients can be relevant to boiler surfaces and deposits. But the London or dispersion part of van der Waals force which governs many interactions in the formation of boiler deposits, is mediated by fluctuations of the electromagnetic (optical) field in the vicinity of interacting bodies across all frequencies. This dynamic electromagnetic interaction involves light at various frequencies outside the visible range, and is dominated by the UV, and in decreasing significance, the IR and microwave [55]. Materials from metals to ionized gases are subject to dispersion forces. The numerous interactions where it dominates includes in the formation of colloidally stable rheological fluids for ceramic processing, through the formation of intergrain boundaries in ceramic nanocomposites and between metals and ceramics, and in the

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interactions of aerosols of fireside liquid and solid particles with each other and with ceramics and oxide coatings and layers on tubes. The mathematical constructions here were necessary in order to give an opinion on the significance of new material designs on dispersion force interactions, both from an inherent property point of view and from interaction during operations of boiler tubes. Below is a description on how full Lifshitz theory has been applied in the literature, and how it can be used to get new useful information on the interactions of fireside components with the ceramics and oxide coatings of the boiler tubes.

7.1.3 Dielectric response functions Non-retarded, full Lifshitz theory, involving frequency dependent spectral data subject to various Kramer-Kronig transforms can be used for construction of dielectric response functions of various materials [55]. The full, non-retarded Lifshitz theory can be applied to obtain the interaction parameter known from colloid science as the Hamaker constant, from which the London component of the vdW energy are derived.

In order to calculate these various frequency dependent dielectric response functions, Hough and White [54] showed that Parsegian and Ninham’s damped oscillator model could be written as follows:

(1)

where is the frequency dependent dielectric response function for a solid or liquid, CUV, CIR are the oscillator strengths in the UV and IR respectively, and ωUV, ωIR are the UV and IR oscillator damping frequencies, respectively.

Under the full non-retarded Lifshitz theory, calculation of Cauchy plots from refractive index can be made with limited spectral data, using a damped oscillator model (e.g., Hough and White) [54]. This requires the frequency dependent refractive index, to be known in a relatively slowly and smoothly changing part of the refractive index curve (in the visible range), and preferably in a frequency range where there is negligible absorbance. For most solid non-conductors the absorption strengths in the UV and IR can be derived from two equations. The first is the Cauchy equation, from which a Cauchy plot is constructed:

(2)

2 2 2 2 A linear plot of (n (ω)-1) versus (n (ω)-1)ω ) will have slope, 1/ ω UV and will intersect the ordinate axis at CUV. CIR can then be determined from:

CIR = ε(0) - CUV -1 (3) where ε(0) is the zero-frequency, static dielectric constant of the material.

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This gives a set of parameters ωUV, CUV, CIR, so the only missing parameter is the ωIR, the IR oscillator damping frequency, and is taken as the characteristic absorption frequency in the IR region. In practise for many materials this is a minor contributor to the overall dispersion energy. In the range of discrete Matsubara frequencies over which each contribution to the total dispersion energy is summed, only a few IR values occur, while there are hundreds in the UV part of the complex spectrum. Figure 4 below shows the first step in the procedure, using solid KCl as an example.

1.62 n KCl 1.6

1.58

1.56

1.54

1.52

refractive index, KCl index, refractive 1.5

1.48

1.46 200 400 600 800 1000 1200 1400 1600 1800 w avelength KCl Figure 4. Wavelength dependence of the real part of the complex refractive index for solid KCl [56]. Figur 4. Våglängdsberoende hos den reella delen av komplext brytningsindex för fast KCl.

The corresponding Cauchy plot is shown in Figure 5, constructed using the KCl refractive index data above. It shows excellent linearity in the range and allows a very accurate determination of the Cauchy parameters.

1.6 n^ 2-1 KCl

1.5

1.4

1.3 n^ 2-1 KCl

1.2

y = 1.1691 + 4.1175e-33x R= 0.99971 1.1 02 1031 4 1031 6 1031 8 1031 1 1032 (n^2-1)omega^2 KCl Figure 5. Cauchy plot for solid crystalline KCl. Figur 5. Cauchy-diagram för fast kristallin KCl

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Bergström [59], Faure et al.[62], Lee and Sigmund, Ackler et al., French et al., all go into some detail of calculations of Hamaker constants using this method. The authoritative works however are by Hough and White [54] and by Ninham and Parsegian [53]. Hough and White [54] and Bergström [59] calculated many of these Cauchy plots from reliable optical data, so further examples will not be given. In this review calculations have been included in order to better interpret literature on present and future boiler tube materials.

7.1.4 Optical data for KCl at high temperature Optical data for KCl at high temperature is rather limited. In the solid state, we can use the room temperature data, given the weak dependency of the Hamaker constant on temperature. For the liquid state, where the ionic liquid behaviour sets in, the frequency dependence will change, making a new set of data important to have, but none appears to be available after extensive searching. More information is given in Appendix 1.

7.1.5 Hamaker constants for alkali chlorides Hamaker constants are the scalar factor used to relate molecular interaction forces as a function of the separation between the bodies. Hamaker constants are calculated from imaginary frequency dependent dielectric response functions using full Lifshitz theory [50] using a limited damped optical oscillator model [49, 53] for dielectric materials interacting across air and liquid according to validated methods [54]. Please see Appendix 2 for more detail related to the calculation of Hamaker constants relevant to alkali chlorides in boiler deposits.

Calculations of Hamaker constants between interacting materials follow the protocol of a layer model of material 1 interacting with material 2 through a medium (material 3) and is thus for a three layer model given the terminology A132. For the systems discussed in this report material 1 relates to the boiler tube material, the medium 3 is liquid KCl ; material 2 is air (tables 5 and 6) or alternatively a ceramic coating, or KCl

Air (table 5,6) KCl (table 5,6,7)

Figure 6. Layer model for determination of Hamaker constants A132.

Figur 6. Skikt-modell för bestämning av Hamaker-konstanter A132

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Ninham and Mahanty [55] published the following relationship for the Hamaker constant in terms of a damped oscillator model:

4 where,

(5)

(and similarly for ) where , the imaginary frequency is given by:

(6) where the prime in equation (4) instructs that the first term in m(=0) should be halved, k is Boltzmann’s constant, T is the temperature in Kelvin, m is an integer (here around 4000), s is the integer number of higher order terms, and is the frequency dependent dielectric response function for material one. Figure 7 shows the variation in dielectric response function as a function of the complex frequency iξm.

The equations above show that the Hamaker is a sum over certain frequencies of relationships between the imaginary frequency dependent dielectric spectra of different materials. The frequencies are discrete and temperature dependent as seen from equation (6) and otherwise depend remarkably only on Planck’s constant. A notable fact arises from the scaling of both equations (4) and (6) by kT. This temperature dependency of the Hamaker constant nearly vanishes because the discrete Matsubara frequencies, over which the vdW interaction energies are summed, shift as the temperature changes in a way that reduces the overall temperature effect.

Since KCl is an ionic liquid, and has a finite electronic conductivity, dielectric response functions are also calculated using Drude oscillators to allow estimates of Hamaker constants from combined dielectric spectra of bound and free electrons as has been done for graphite [57].

In calculating the various Hamaker constants for systems of the type substrate-KCl- substrate with many different materials here, it is assumed that oscillators in the UV and IR make the largest contributions [49, 53].

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Table 3. Optical oscillator models for KCl with bound electrons use in equation (1) Tabell 3. Optiska oscillatormodeller för KCl, med bundna elektroner,i ekvation 1.

Model Material state n0 ε(0) CUV CIR ωUV ωIR 1016 rad.s-1 1014 rad.s-1

KCl-xtal KCl (s) cubic 1.473 4.4 1.17 2.23 1.58 0.27 KCl-IR KCl (l) liquid 1.37 5.5 1.17 3.33 1.58 0.27 KCl-IR-LO KCl (l) “ 1.37 5.5 1.17 3.33 1.58 1.198† KCl-UV KCl (l) “ 1.37 5.5 2.27 2.23 1.58 0.27 KCl-UV-LO KCl (l) “ 1.37 5.5 2.27 2.23 1.58 1.198† KCl-split KCl (l) “ 1.37 5.5 1.72 2.78 1.58 0.27 KCl-split-LO KCl (l) “ 1.37 5.5 1.72 2.78 1.58 1.198† † the “LO” term here refers to the longitudinal optical phonon mode which is observed in liquid molten salts. In this case we have used the value for molten LiF [58].

The static dielectric constant of crystalline solid KCl (“KCl-xtal”) has been reported as 4.4. [59] We know it is approximately 5.5 for bulk KCl in the molten state from measurements. [60] Using equation (1) above, and the data of Table 3, a range of dielectric response functions for KCl liquid can thus be determined and bounded by the range of possible UV and IR terms. The damping frequencies were held constant (except the “-LO” models) and the values of CIR (2.23 and 3.33) and CUV (2.27 and 1.17) were varied in respective calculations.

3.5

KCl xtal KCl split 3 KCl liq I R hi KCl liq UV hi

2.5 ) m

ξ i ( ε 2

1.5 Dielectric response function response Dielectric

1 1014 1015 1016 1017 1018 -1 complex frequency, iξ rad.s m Figure 7. Dielectric response functions for crystalline and liquid KCl Figur 7. Dielektriska responsfunktioner för kristallin och vätskeformig KCl

Somani [61] performed ab initio simulation of liquid KCl to obtain a static snapshot of K and Cl atomic positions in the liquid state and used Born-Huggins-Meyer pair potentials for interactions of the ions. The dielectric response function of liquid KCl rescaled to eV according to Somani [61] is shown in figure 8a. Figure 8b shows the variation in dielectric response functions, in eV, calculated according to a Cauchy plot [54] derived from optical spectral data. Somani’s solution appears to be between the results obtained here for KCl-IR and KCl-split case.

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3.5 KCl 3 KCl ir-high KCl uv-high KCl split ) 2.5 m

ξ (i ε 2

dielectric response 1.5

1 0 1020304050 eV a) b)

Figure 8. Dielectric response functions for KCl. a) Simulated dielectric response function of Somani [61] and b) Dielectric response functions derived here for bound electrons in KCl liquid and crystalline KCl (labelled “KCl”). Functions with different contributions from UV and IR damped oscillators are included as separate curves. The functions were derived from optical spectral data and the formalism of Hough and White [54] according to a Cauchy plot. Figur 8. Dielektriska responsfunktioner för KCl. a) Simulerad dielektrisk responsfunktion av Somani [61] och b) Dielektriska responsfunktioner för bundna elektroner i vätskeformig KCl och kristallin KCl (märkt “KCl”). Funktioner med olika bidrag från UV- och IR-dämpade oscillatorer är inkluderade som separata kurvor. Funktionerna har härletts från optiska spectral-data och formalismen från Hough och White [54], enligt ett Cauchy-diagram.

The Clausius-Mosotti equation (equation 7) can be used to approximate the melt-phase dielectric spectrum using the known crystalline spectrum

(7) where ρmelt and ρXtal are the respective densities of KCl given in the table below. Equation (7) can be used to transform the dielectric response function of the crystalline solid to that of the melt. It assumes the molecular polarizability of KCl(l) is the same as the solid, which is not true (but it is assumed here to approximate the effect of bound electrons on the dielectric response). It accounts for the density change in polarizable entities upon melting. The result of calculations will now be shown on the above systems as a comparison. First it is necessary to know the density of KCl at various temperatures (Table 4).

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Table 4. Physical properties of KCl liquid at various temperatures. Tabell 4. Fysikaliska egenskaper hos vätskeformig KCl vid olika temperaturer. Temperature State Conductivity, σ Density, ρ Viscosity,η (K) (ohm-1.cm-1) (cm3.g-1) (mPa.s-1) 273 crystalline solid small 1.987 Small 1060 Liquid 2.203 1.5178 1.14 1130 Liquid 2.374 1.4770 0.92 1200 Liquid 2.517 1.4362 0.80

3 KCl xtal KCl CM1060K KCl CM1130K 2.5 KCl CM1200K KCl split ) m ξ 2 i (

1.5 Dielectric response function response Dielectric

1 1014 1015 1016 1017 1018 -1 com plex frequency, iξ rad.s a) m 3.5

KCl xtal 3 KCl split KCl CM1060K KCl liq I R hi 2.5 KCl-CM+ DL1.05 )

m Kcl-split LO

ξ i ( ε 2

1.5 Dielectric response function response Dielectric

1 1014 1015 1016 1017 1018 -1 complex frequency, iξ rad.s b) m

Figure 9. a) Dielectric response functions, ε(iξm) for KCl liquid using low temperature optical data and two different transforms to the high temperature form. b) Combined free bound model, including contribution from a Drude-Lorenz oscillator, ε(iω) with a cut- off frequency (in the IR) of 1.198 x 1014 rad.s-1 and ε(i∞) = 1.05 and ε(i0) (KCl- CM+DL1.05). .

Figur 9. a) Dielektriska responsfunktioner, ε(iξm) för vätskeformig KCl, framtagna genom optiska lågtemperaturdata och två olika omvandlingar till högtemperaturdata. b) kombinerad fri bindningsmodell, inclusive bidrag från en Drude-Lorenz oscillator, ε(iω) med en cut-off frekvens (i IR-området) på 1.198 x 1014 rad.s-1 och ε(i∞) = 1.05 och ε(i0) (KCl-CM+DL1.05).

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Figure 9a shows a range of predicted dielectric response functions for KCl. The use of the Clausius-Mosotti equation above yields the overlapping, lower three curves and does not return the measured zero-frequency dielectric constant of 5.5 for liquid KCl (not shown in graph). This is partly due to the false assumption that polarizability remains constant on melting the solid KCl. The curve for the crystalline case and the liquid case are shown in both Figure 9a and 9b for comparison.

The dielectric response functions shown in Figure 9b here show the range of models, including both bound and free electrons. The “split” case was used in Hamaker constant calculations for materials interacting with KCl because it is consistent with the known experimentally determined dielectric constant, has the fewest assumptions of all the models here and is the most critical of all models here in respect to determining the sign of the Hamaker constant for KCl interacting with other materials. In this respect the “split” model best points out which materials interactions with KCl are least qualititatively predicted assuming the applicability of Lifshitz theory.

The uncertainty in the sign of Hamaker constants is probably not high for many materials that KCl can interact with in a boiler environment, as given in Tables 5, 6 and 7, particularly ceramics. This is because most materials have much higher dielectric constants than KCl, which to a first order means their response functions will also be larger and the sign of their Hamaker constants well determined (assuming applicability of Lifshitz theory). If the sign is negative for certain types of Hamaker constants (i.e., solid-air-liquid), a liquid is predicted to form a wetting film on a solid surface. This will be explained using examples below. When the dielectric function of a material is approximately the same as that for KCl, considerable uncertainty of the sign of the Hamaker constant exists. The most critical materials in this sense are the fluidized bed sands used in CFBs, and clays such as kaolin added as alkali scavengers. Both silica and kaolin have relatively low dielectric contrast with KCl, and thus interactions of these solids with KCl (solid or liquid) is subject to large errors using current data and theory. The error in the magnitude of the Hamaker constants determined here in Tables 5 and 7 are probably better than 5%, except in the case of KCl liquid interactions, where the uncertainty is estimated as perhaps as high as 50-100%.

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7.1.6 Dielectric response functions of ceramic and inorganic materials Figure 10 shows plots of dielectric response functions for a large range of ceramic and inorganic materials calculated using oscillator values of Bergström [59] and Faure [62].

6 alumina silica KCl 5 quartz mica NaCl magnetite 4 maghemite hematite rutile yttria-hex 3 3Y-zirconia-tet MgAl2O4 SrTiO3 6H SiC 2 beta-SiC diamond I I a

1 1014 1015 1016 1017 1018

Figure 10. Dielectric response functions calculated here using the oscillator values of Bergström [59] and Faure [62]. Figur 10. Dielektriska responsfunktioner, här beräknade genom att använda oscillator-värden från Bergström [59] och Faure [62]

Table 5. Non-retarded Hamaker constants for use in boilers, A132, where KCl liquid is the medium between materials 1 and air. Tabell 5. Icke-retarderade Hamaker-konstanter för användning i pannor, A132, där vätskeformig KCl är mediet mellan material 1 och luft. Material 1 Static dielectric constant Material 1 Hamaker constant 10- ε(0) structure

α-Al2O3 10.1 hexagonal -2.73 Fe3O4 20 cubic 4.375 γ-Fe2O3 20 cubic 2.374

α-Fe2O3 12 hexagonal 0.543 Y2O3 hex 11.8 hexagonal -2.1334 3Y-ZrO2 18 tetragonal -5.264 MgAl2O4 8.3 cubic -1.6133 SrTiO3 311 cubic -2.917 6H-SiC 10.2 hexagonal -7.0818

SiO2 3.82 amorphous 1.772 TiO2 rutile 114 tetragonal -3.078 SiO2quartz 4.25 cubic 0.3832 KCl(s) 4.25 cubic 2.385 Olivine 8 Clay 3.55 monoclinic -0.080 muscovite (mica)

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Table 6. Hamaker constants for materials 1 and air separated by KCl liquid (CM-1060). Tabell 6. Hamaker-konstanter för material 1 och luft, separerade genom vätskeformig KCl (CM- 1060). Material 1 Material  Material 1 Hamaker ε(0) structure constant, A 10-20 J

α-Al2O3 10.1 hexagonal -3.934 SiO2 3.82 solid -1.503 amorphous quartz 4.29 trigonal -2.153 Mica clay 3.55 monoclinic -2.420

Table 7. Non-retarded Hamaker constants [59, 62] for materials 1 and 2 separated by either liquid KCl or air. Tabell 7. Icke-retarderade Hamaker-konstanter [59, 62] för material 1 och 2 separerade genom vätskeformig KCl eller luft. Material 1 Material 2 Hamaker Hamaker Literature constant 10- constant 10- value 20 J 20 J 10-20 J “split air/vacuum vacuum model” KCl(l) 3Y-ZrO2 Silica -1.03 11.42 11.4 rutile Alumina 0.48 14.24 14.2 KCl crystal Mica -0.04 7.24 7.31 NaCl crystal Silica 0.31 6.45 6.45 quartz Alumina 0.04 11.59 11.6 yttria Silica 0.05 9.24 9.24 NaCl crystal Alumina -0.54 9.77 9.77 yttria Alumina 0.48 14.05 14.0 6H-SiC Silica 1.42 12.58 12.6 SrTiO3 Silica -0.76 9.44 9.44 MgAl2O4 Silica -0.24 9.05 9.05 diamond Silica -1.45 13.72 13.7 alumina Alumina 1.074 15.20 15.2 quartz Quartz 0.119 8.86 8.86 rutile Rutile 2.74 15.26 15.3 diamond Diamond 8.18 29.54 29.6 6H-SiC 6H-SiC 5.78 24.82 24.8

γ-Fe2O3 γ-Fe2O3 2.78 6.64 6.8 α-Fe2O3 α-Fe2O3 1.92 9.15 9.2 Fe3O4 Fe3O4 3.76 4.08 4.3 α-Fe2O3 Quartz -0.30 7.96 α-Fe2O3 3Y-ZrO2 -0.03 12.49 KCl(l)split KCl(l) split 9.36 KCl(l)UV high KCl(l) UV high 4.34 KCl(l)IR high KCl(l)IR high 0.50 KCl(l)CM1060 KCl(l)CM1060 0.11 KCl(l)CM1130 KCl(l)CM1130 0.14 KClCM-DL1.05 KClCM-DL1.05 5.76 KCl-split-LO KCl-split-LO 9.43

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7.1.7 A comment on negative Hamaker constants

One of the consequences of negative values of the Hamaker constant itself equating to a repulsive force, is that the intervening medium has a tendency to wet the surface if it is a liquid [54]. Furthermore there is a tendency to drive this liquid to become thicker. This is the basis of capillary wetting, and the appearance of negative values in the table 7 above for liquid KCl implies that KCl liquid will not form films on fluidized bed media made of quartz or silica, but will form on clay (mica) and perhaps kaolin added to the fireside mix to control alkali chlorides (this may be the reason it works) and also onto many of the higher valued dielectrics expected to be used in new coatings of boiler tubes. This wetting of ceramics and oxides by liquid KCl however is conjectured here to be only the case below the saturation pressure, p0 of KCl for very small pores where according to the Kelvin equation, for example, the condensation pressure can fall way below the saturation vapour pressure and films can form at very low p/p0. This will be discussed briefly below in relation to capillary condensation according to the Kelvin equation, that can be written in terms of the Hamaker constant through surface energies.

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7.2 Adhesion, capillary forces and smooth surfaces

7.2.1 Forces between surfaces It has been noted that in the size-range of fly ash particulates, “van der Waals forces are generally considered the most fundamental of the interparticle forces. Other forces are usually subsidiary; that is they become important after the van der Waals forces have attracted particles close enough together for the shorter-range forces to participate in the adhesion. At this point the subsidiary forces may become dominant, as in the case of capillary forces, for example.” [63]

The total force acting between two neutral parallel plates (infinite half-spaces ) can be expressed as:

(8) where Aab is the Hamaker constant, Bab is the constant arising due to Born repulsion or Pauli exclusion, zm is the distance of maximum vdW attractive force, where , where σ is the equilibrium separation where Born repulsion is balanced by the dispersion attraction (see figure 11 below).

Figure 11. Force-distance curve for a general Lennard Jones style interaction between surfaces. When the curve is above zero on the y-axis the force is repulsive, while below it is attractive. The sum of the repulsive and attractive forces yields the familiar minima, synonymous with a low energy potential well, with closest approach at zm. Figur 11. Kraft-avstånds-kurva för interaktion mellan ytor, enligt generell Lennard Jones stil. När grafen är ovanför noll på y-axeln är kraften repulsive och när den är nedanför är kraften attraktiv. Summan av de repulsiva och attraktiva krafterna ger minima, vilket är synonymt med en lågenergipotentialkälla, närmast vid zm.

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The total force acting between a spherical particle and a half space can be expressed as:

(9) where R is the sphere radius.

1/6 In this case zm = 2 σ = 0.4 nm.[64] Note that the exponent on the separation distance r, has a different integer value depending on the geometry (e.g., plate-plate, sphere-plate, sphere-sphere, cylinder-cylinder etc.)

The pull-off force or detachment force, which is a measurable quantity [65, 66] is usually defined as the value of the vdW force at rm. We will calculate the detachment force (normalized by sphere radius) for the Hamaker constant combinations above to illustrate the force using the left side of the sphere-plate relation. For a smooth sphere size of R=10 μm and rm = 0.41 nm and which reduces to:

(10)

The Hamaker constants derived here and by others can be used as a basis for further work regarding adhesion and detachment forces, capillary forces and capillary condensation and the effect of smooth/rough surfaces in the context of efficient boiler tube coating and in-use optimization.

Various theories exist for adhesion of particles at surfaces, including sophisticated theories based on the incorporation of van der Waals forces to Hertzian contact mechanics. [64] The latter are used to calculate the increased surface area on deformation of two surfaces under a load. Part of the normal force arises from the attraction between the particles. These theories have been developed by Krupp; Tabor; Johnson, Kendall, and Roberts (JKR); Derjaguin, Muller and Toporov (DMT); and Muller and Yushchenko (MYD); Maugis and Pollock and others. [64] The choice of which theory to use for predictions in certain situations depends on parameters like the size and elasticity of the particles. For example, JKR theory is more applicable to large, compliant particles, while DMT is more applicable to small, highly elastic particles.

In boiler deposition, the late stages of thermophoresis and or diffusion/condensation leading to fireside liquid KCl droplets approaching and solidifying near the surface, JKR analysis could be done quite rigorously. The load term would be close to zero, and the elastic term that of the solid. For inertial impaction mechanistics the load term becomes very high for solid impactors such as condensed KCl or quartz or other fluidized bed media. For liquid KCl inertially impacting the surface at relatively high velocity, it may not be fully crystallized during impact, despite crossing a thermal gradient, in which case the elastic contribution would be lower and the deformation of the droplet would rapidly enhance the contact area, increasing the adhesion force

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mediated by the Hamaker constant. For identical ideal quartz grains of equal mass inertially impacting the surface at the same angle but different velocities, adhesion theories would predict a larger detachment force for the higher velocity impactor simply because the contact area is larger the deeper the indentation depth.

7.2.2 Rough surfaces and adhesion The smoothness of the surface in adhesion/detachment force studies of particles against surfaces was also the subject of much research, but only two papers will be mentioned here. Rumpf used Derjaguin’s approximation of separating the contact from non-contact forces for assessing the detachment/adhesion forces. [67] In this case the model has particles in contact with idealized rough surfaces having asperities in contact and troughs or low points in the roughness that are not in contact. In Rumpf’s model roughness was defined in terms of the average asperity radius, and the force term calculated using that simplest of roughness parameters, the particle radius, the closest approach distance of 0.3 nm and the Hamaker constant. Rabinowich [67] used a modification of Rumpf’s model and included a root mean squared (rms) roughness (single-scale), wavelength of the average roughness feature and its maximimum height and a coefficient relating rms roughness and the maximum peak height.

Figure 12 shows a plot of predicted total force of adhesion for a particle to a rough surface according to Rabinowich [67] and the contributions to adhesion force from the contact and non-contact terms for the same system (without an rms term) according to a modified Rumpf treatment. The force of adhesion in Figure 12 is normalized by the radius of the adhering particle. Here the particle radius R = 10 μm, peak-peak roughness, λ=0.250 μm, closest approach = 0.3 nm and Hamaker constant of 10-19 J.

100 Total force Normalized non-contact force Normalized contact force

10 -1 mN.m 1 Total adhesion force

0.1 10-10 10-9 10-8 10-7 rms roughness, nm Figure 12. Adhesion force of particles to rough surfaces as a function of the rms roughness. Figur 12. Adhesionkraft till en skrovlig yta för partiklar, som funktion av rms-skrovlighet.

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An important, but not too surprising conclusion here is that rough surfaces decrease the adhesion interaction in idealized systems by orders of magnitude when the particle radius is much larger than the roughness. Conversely, smooth surfaces will be ideal for adhesion of larger particles since they offer more actual contact surface area than a rough surface. Because the adhesion force is proportional to the Hamaker constant high dielectric ceramics will have a larger interaction force with adhering particles. However for solid particles, the interaction energies will be low due to roughness of the particles themselves, and because the tough, elastic surface of ceramics will not allow appreciable deformation during impaction to increase the area of contact.

The exception is molten material that spreads before solidifying on surface forming a large contact area approaching the theoretical limit on smooth surfaces. Adhesive forces for molten KCl (or low melting mixed compositions e.g., KCl+K2SO4) on boiler tube ceramics and wettable fluidized bed media will be very adherent on this basis, and will be even more adherent the higher the dielectric of the surface on which it spreads. Because surface forces are much lower than across air (see Table 7 above), then we can safely conclude that the stickiness of liquid KCl-coated grains or boiler tube surfaces is not a direct result of vdW forces, but because the vdW forces are effectively idealized by its ability to flow. This maximization of the vdW forces will also enhance the viscous drag and capture effect on glancing collisions from other particles. This will be worse the higher the solid-KCl liquid-air Hamaker constant (for example olivine sand, with relatively high ε(0)=8) , and may not be operative at all for quartz sand since its solid-KCl liquid-air Hamaker constant is positive, and thus not wet by KCl under the assumption of the applicability of Lifshitz theory to liquid KCl wetting. Spontaneous wetting of KCl liquid on various surfaces will be considered in a little detail in the next section.

7.2.3 Thin films of liquid alkali chlorides on surfaces Alkali chlorides are used industrially for various applications because they can form thin films on ceramic materials. For example, molten alkali chlorides have been used for separation of ceramics from metals during recycling of metal-matrix composites [68]. Film formation of molten alkali chlorides on ceramics has been useful in the separation of molten metals from molten alkali chlorides using ceramic filters in the recycling of aluminium. [69] These technologies are enabled because of the better wettability (contact angle less than 90°) of ceramics such as alumina, mullite, zirconia and phosphate sintered aluminas and porous carbons and carbon glasses by molten salts such as KCl in the presence of metals. Sol-gel based Al2O3 coats have been used to “seal” the micropores and fissures at the surface of YSZ ceramic coatings (with CoNiCrAlY bondcoats on a 6061-nickel superalloy), which was otherwise wet by the molten salt (in that case melts of 95wt%Na2SO4 + 5wt%V2O5 at 950°C) [70]. This preferential (partial) wetting of alkali chlorides for refractory metal oxides over metal is obviously of use in metal and ceramic processing (see Appendix 3) but could also lead to problems on, in and under ceramic coatings of boiler tubes unless coatings are designed with such imbibition in mind.

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Table 8. Table of published experimental interfacial tension values for molten salts and substrates. Tabell 8. Tabell över publicerade experimentella värden på gränsytspänningar för smältor och substrat. Molten Salt Material Interfacial Reference tension mJ.m-2 KCl(l) 780-970 °C air 110.3- [71] 85.7** NaCl (800°C) air 115* [72] KCl (800°C) air 96* [72] 0.5 KCl(l) + 0.5 NaCl(l) Al2O3 1185 From salt-Al-Al2O3 system[73] LiCl-KCl Silica 123 [74] glass LiCl-LiF-LiBr Silica 80 [74] glass *critical surface tension – for an uncharged surface, the interfacial tension on a substrate is thought to be that of the surface tension at the liquid-air interface. See below. **

7.2.4 Van der Waals forces and KCl aerosols Since KCl forms a molecular gas, it can form a capillary condensate in the manner of other (non-ideal) vdW gases in that a meniscus forms at the condensate boundary with a curvature set by the equilibrium according to the Young-Laplace relation. The radius of curvature relates to physically real structures in at least two cases relevant here – for nucleation and growth of alkali chloride aerosols, and for the condensation in small pores in the rough surfaces or pores of boiler tubes. In both cases the curvature of the menisci is related to the vapour pressure through the gas-liquid surface tension.

Models for the process of condensation of alkali chloride aerosols in flue gases that are ideally representative of combusted straw fuel have been discussed by Christensen and Livbjerg (2000). [75] (Also please see Appendix 2). Their paper serves as an excellent example here of how boiler operations and particle, HCl, SO2 and deposit formation are critically dependent on surface tension, and thus surface forces.

Kristensen and Livbjerg [75] give evidence that their model is a realistic simulation with respect to the influence of chemical reactions, particle formation by homogeneous nucleation of and particle growth by condensation and nucleation. Of significance is their conclusion that homogeneous sulfate nucleation is the primary source of condensation nuclei for sulfate and KCl aerosols confirmed earlier by Jensen et al. [7]. Here we note their theories established a significant precedent in favour of our (non- unique) thesis here, that surface forces are critically operative in the formation and understanding of boiler deposits, particularly for biomass fuels.

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Once formed, aerosols themselves can interact with other substrates via vdW and other surface forces. We do not illustrate agglomeration of aerosolized particles here other than to note that the dielectric response functions in the tables above can be used to calculate the London dispersion contributions to these.

Other literature that elludes to aerosol formation leading to deposition in boilers are numerous. For example, photochemical aerosols of sulfates have also been shown to be accurately modeled when London forces are included, [76] and also establish that these forces have a significant role in the earliest stages of sulfate aerosol formation in boilers. Closer to the mark are the studies of dry scrubbing tools for flue gas desulfurization depend on Lifshitz theory for their effective design and utilization. [63]

7.2.5 Adsorption of alkali gas phases on clays Additives such as clays, with the ability to influence the nucleation and growth of sulfate/chloride aerosols are best engineered with the surface forces discussed here taken into account. Kaolinite, Al2Si2O5.(OH)4, a sheet silicate clay, as noted earlier, has been used as an effective scavenger of K (e.g., as KOH - the precursor in flue gas to KCl). [75] However it is considered too expensive to implement.

Several authors have discussed alkali gas adsorption by kaolin [5, 77, 78] in the context of Na scavenging. The mechanism appears to be sorption of KOH prior to KCl formation.[75] The solid product of reaction between kaolinite and K is the high melting island silicate kaliophilite, KAlSiO4 (and nephelinite, (K,Na)AlSiO4). These condensed phases (and others) can then render K ineffective in forming corrosive KCl molecules.

The use of additives such as high calorific value clay waste from edible oil production is possibly a cost effective method to reduce K in the flue gas. [19] Like kaolin, the various clays used in oil bleaching can potentially react in the same way as kaolin. That is, by clay dehydrating to their highly activated, high specific surface area, meta-clay forms followed by reaction with K to form alkali aluminosilcates, leaving HCl as the “lesser evil” carrier of the chloride anion. However, spent bleaching earths, and related Fuller’s earths, attapulgites, palygorskites (acid activated bentonites etc.) used in edible oil processing represent a wide class of clays, all rich in silica, but varying in their Mg and Al content. The presence of Mg over Al would not only change the reaction chemistry but would require different interaction parameters to determine agglomeration before injection (in the hydrated forms) and during firing and deposition through surface force mediated adhesion.

A last word here is that the adsorption of KOH on metakaolin is highly likely dependent on surface adsorption in porous materials. Kaolinite dehydrates in the boiler to metakaolin. Metakaolin has a relatively massive specific surface area of the order of 150-250 m2.g-1, compared with kaolinite, which has less than 5-20 m2.g-1. The high surface area arises from the opening up of the original laminated clay layers to yield micro and mesoporosity. This thermal dehydroxylation mechanism opens up other clays like expanded perlite and pyrophyllite. Gas adsorption via London forces is enhanced in microporous materials [79] and the high surface also leads to very fast kinetics for

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rapidly K scavenging. The tuning of surface area for a given clay therefore, is probably the most important structural determiner for adsorption. This suggests that careful preliminary research should be performed to ensure maximum potential for generating internal surface area on the metaclay at the K(Na) scavenging stage.

7.2.6 Direct condensation of KCl(g) into surface pores In addition to the condensation to form aerosols, typical high alkali chloride straw flue gases could condense directly in the pores or rough surfaces of the tubes, or their coatings and deposits, According to Kelvin’s equations, condensation in small pores can occur at partial pressures (p/p0) below its saturation vapour pressure, p (dew point), according to the following:

(11)

-5 where is the liquid-vapour interfacial tension, Vm is the molar volume = 4.9 x 10 3 -1 -5 3 -1 3 - m .molecule , (cf. NaCl(l)=3.764.9 x 10 m .molecule ) based on ρ1060Κ = 1.518cm .g 1[80] R is the universal gas constant, (8.314 J.mol-1.K-1), r is the mean radius of a pore in the boiler tube surface or coating, and T is the temperature in K. Segerdahl et al. [81] showed that gaseous KCl is corrosive above its dew point, and while this is probably likely, it is also interesting to understand if their observations also involved a component due to liquid KCl formed by capillary condensation below its dew point. For this we need some assessment of the interfacial tension.

It has already been established here that a negative Hamaker constant of the type substrate-liquid-air is enough to ensure a wetting film can form on many ceramic substrates, a pre-requisite to having capillary condensation in a pore. However, the adsorption of dipolar gases like KCl is probably more dominated by Keesom and Debye forces than London forces. It has been shown that reasonable values of interfacial tension for molten salts can be obtained using simulations, by combining the separate contributions to interfacial tensions due to Coulombic, repulsive, dispersive and polarizable components. [82] The simulations of Aguado [82] show that for KI(l), the relative contributions of the dispersion terms are from 60-75% of the total predicted interfacial tension. In their models for the liquid-vapour interface, a strong drive for Coulombic clustering is found, maximizing coordination number for both cations and anions of 6 and inducing a thick low density gradation zone with a capillary wave structure, with an increasing amplitude with temperature.

It is of note that we obtain similar values, i.e., ca. 60-70% of the observed surface tensions using only the induced-dipole/induced dipole London dispersion interactions. These interfacial tension were calculated using the work of adhesion determined from the Hamaker constants for a liquid-air-liquid systems (see Appendix 5). Aguado et al. [82] appear to use only Keesom and Debye vdW forces in their rigid ion, hard sphere model without polarization to obtain similar values.

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In any case and very importantly here, we have found in the literature model- independent, experimentally-determined surface tension values for KCl liquid (Table 8 above) and will use these for various calculations relating to surface interactions of KCl below.

In the case of Kelvin-type condensation in pores, Figure 13, gaseous components may condense into surface pores and cracks and nanoporosity in the chromium or metal oxide and ceramic coatings and any delamination zones between either oxide surfaces and ceramics, metal and ceramics, or the constituents of nanocomposite coatings, bond- layers or within the deposits themselves, the capillarity and wetting tendencies are significant and van der Waals forces are central to these phenomena.

10000

1000 1100K (ppmV) 100 (g) 1050K

10 1000K

950K 1 900K

0.1 850K partial pressure KCl 800K 0.01 100 10 1 0.1 pore radius nm Figure 13. Critical partial pressure for capillary condensation of KCl as a function of pore radius or equivalently droplet size. Calculated using equation (11). Figur 13. Kritiskt partialtryck för kapillärkondensation av KCl, som en function av por-radie eller ekvivalent droppstorlek. Beräknad från ekvation 11.

7.2.7 Capillary forces and KCl liquids in pores When liquid molten salts such as KCl condense in pores from the gas phase, they can move if subject to capillary pressures, as would be the case in a narrowing crack in a ceramic coating, at rates that can be estimated using various expressions. The Lucas Washburn equation [83] is derived from the Hagen-Poiseuille equation relating volumetric flow to pressure drop along a cylindrical tube, and including a capillary pressure term. The Washburn equation involves an interfacial tension, a contact angle term and viscosity (and others structural parameters). Here we note the viscosity of molten KCl at 1060-1100 K is about that of water (Table 4) and will not by itself significantly hinder imbibition into pores. It is beyond the scope of this report to offer a detailed discussion of flow rates of molten salts through porous deposits or ceramic coats, partly through lack of adequate theory of interfacial phenomena for ionic liquids. [48, 84] The interested reader is pointed to the reference list for a detailed look at traditional capillary flow in porous media and capillary uptake of molten salts. [83-87] For now a few examples illustrate the uptake of molten salts into porous media.

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Baumli et al. [88] find a finite contact angle for liquid KCl against graphite surfaces, and also find a spontaneous imbibition into the pores. Wade et al. [89] noted that molten alkali carbonate salts (γSL∼200 mN/m) have a zero degree contact angle with oxides, are imbibed by capillary action into porous yttria-stabilized zirconia, gadolonium doped ceria and alumina membranes. Kim et al. [90] observed and measured the capillary imbibition of various molten salts at high temperature during the high temperature processing of radioactive waste into glass at the Hanford site in Oregon, USA. These are a few examples of the penetration of molten salts into pores above their melting temperature. Forcheri and Berlin [87] reported the electrophoretic transport of molten salts in porous Al2O3, MgO and ZrO2 and derived diffusion coefficients and suggested that the pore radius became critically slowing to diffusion below about 10 nm. Nowok [91] related viscosity and surface tension to study the transport properties of molten salts (including molten silicates) in capillaries.

Here we show, using the Gibbs Thomson equation, that molten salts can have reduced melting temperatures in small pores with implications for the deposition processes in boilers. The classical Gibbs-Thomson equation for a cylindrical pore relates melting point depression to the radius of curvature of its pores through a scalar, K, comprised of the product of interfacial tension, molar volume, bulk enthalpy of fusion and its reciprocal bulk melting temperature:

(12) where d is the pore diameter, and

(13)

Furo et al. [92] observed freezing point depression in a range of molten salt hydrates, which is a low temperature ionic liquid proxy for molten salts here. For a given system, the melting point depression is simply a constant divided by the pore diameter.

By way of illustrating the melting point depression of salts in porous materials we give an example for molten KCl as a function of pore size shown in figure 14. The analysis is not performed here for many salts on account of the lack of high temperature interfacial tension data for salts on various substrates. Here we find the following values for K using equation 12 and the data presented in Tables 4 and 8 and the data from Liu et al. [20] for the melting point of KCl. We repeat the determination of K (in equation 13) for water and Zn(NO3)2.6H20 as performed by Furo et al. [92] as an internal check of the method, and to illustrate the magnitude of the effect for these ionic liquids compared with water and hydrated molten salts.

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Table 9. Data used for melting point depression curves of KCl. See Figure 13 below. Tabell 9. Data för smältpunktssänkningsgraf på KCl, se figur 13

Liquid-Substrate Melting Enthalpy of Interfacial Solid Gibbs- Point Fusion tension, density Thomson -1 (K) (kJ.mol ) (mN.m-1) of frozen coefficient liquid K (g.cm-3) (K.nm)

H2O-CPG 273.2 6.01 30 0.92 53.5 Zn(NO3)2.6H20- 309.4 39.9 46 2.067 99.2 (116) # CPG *KCl(l)-Al2O3 777 26.95 1183* 2.012 3425 NaCl** 801 28.2 NA 2.17 - *KCl(l)-Al2O3 777 26.95 150*§ 2.012 435 *KCl(l)-Al2O3 777 26.95 100*§ 2.012 290 *KCl(l)-Al2O3 777 26.95 50*§ 2.012 145 *Note: the surface tension terms used here is approximated by that of the mixed NaCl-KCl system, but is considered an accurate proxy. ** NaCl is included here to show that its enthalpy of fusion is almost the same as KCl. # the terms in parentheses is the calibrated to the known pore size used in their experiments. § The values here are representative of surface tensions determined experimentally and rounded to 100 mN.m-1. A value of =50 mN.m-1, is considered too low, but is included as an estimate of an extreme lower bound for the KCl-alumina interfacial tension

The immediately obvious conclusion is that KCl and thus other salts will have significantly lower melting (freezing) temperatures compared with their bulk values in sub-micron pores. The implications would be wide in all parts of the boiler, not the least of which is for transport mechanisms of corrosive KCl in and on boiler tubes.

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800

750

700

650 (°C)

600

melting point ofKCl KCl-Al2O3 550 KCl 150 mN/ m KCl 100 mN/ m KCl-50 mN/ m 500 1000 100 10 1 pore diameter (nm) Figure 14. Freezing point curves for molten salts as a function of pore diameter. These curves are based on the range of experimentally determined surface tensions for alkali chloride melts (Table 9) and calculated using equations 12 and 13. Data for the steepest curve were derived from the contact angle of molten metal on alumina in molten salt liquid. Figur 14. Fryspunktskurvor för smälta salter som funktion av pordiameter. Dessa kurvor är baserade på experimentellt bestämda ytspänningar för alkalikloridsmältor (Table 9) och beräknade genom ekvationerna 12 och 13. Data för den brantaste kurvan härleddes från kontaktvinkeln hos smält metal på aluminium i en smält saltlösning.

Figure 15 presents melting temperatures of key alkali metal, chloride, sulfate and silica components in the layers of ash close to the boiler surface. Figure 16 shows likely freezing temperature depression effects of various minerals salts and sulphates typically found in boiler deposits as a function of pore size.

It is our opinion here that the experimentally determined interfacial tension measurements for alumina (Table 9) may be less realistic for estimates of capillary behaviour than those for glass and molten metals (with a range of 100-150 mN/m). A correction is probably needed in the surface tension to make an estimate, which accounts for the known lowering of interfacial tension with surface charge (e.g., the Helmoholtz-Perrin equation). This depends on the structure of the so called double layer at the interface. [60]

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KCl+ FeCl2 KCl-K2SO4-Na2SO4 KCl+ K2SO4+ Fe2O3 KCl + K2SO4 K2O.4SiO2+ CaO.SiO2 K2O+ SiO2 KCl NaCl Na2SO4 K2SO4 CaSO4 SiO2 0 500 1000 1500 2000 minimum bulk melting temperature °C Figure 15. Melting temperatures of key alkali metal, chloride, sulfate and silica components in the layers of ash close to the boiler surface. Data is from Jensen et al. [18] and Liu et al. [20] and references therein. Figur 15. Smält-temperaturer hos nyckelkomponenter; alkalimetaller,klorider, sulfater och kiselkomponenter i beläggningar på värmeöverförande ytor. Data från Jensen et al. [18] och Liu et al. [20] och referenser i dessa artiklar.

1800 Freezing SiO2

1600 Freezing CaSO4 Freezing K2SO4 1400 Freezing Na2SO4

1200 Freezing NaCl

Freezing KCl 1000 Freezing K2O+ SiO2

800 Freezing K2O+4SiO2+CaO.SiO2

Freezing KCl+ K2SO4 600 KCl+K2SO4+Fe203 Freezing temperature°C Freezing 400 Freezing KCl+ K2SO4+ Na2SO4

Freezing KCl+ FeCl2 200 1000 100 10 1 pore diameter nm Figure 16. Indication of the freezing temperature depression of various minerals salts and sulphates typically found in boiler deposits in model pores. The calculated curves here are only approximated for a very conservative K value of 50 in equations 12 and 13 and Table 9. Figur 16. Indikation på frystemperatursänkning hos olika mineralsalter och sulfater, som är typiska i beläggningar på värmeöverförande ytor, i modellporer. Beräknade kurvor här är en approximation för ett mycket konservativt K-värde på 50 i ekvationerna 12 och 13, samt tabell 9.

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The structure of KCl liquid at interfaces has been discussed in the context of surface forces against various substances (e.g., ceramics metals, vapour). [60, 82, 84, 86, 93] The picture that emerges regarding the dielectric constant of molten salts close to a surface is close to unity. Further Kunz, [84] in a rather new theory, states that at very high ionic strengths (here calculated as 408 mol.dm-3 for KCl), Coulombic interactions are totally screened, but not the dispersion forces. This has interesting implications for the prediction of KCl liquid behaviour. First let us look at some more traditional approaches, which tend to fail in one way or another, but can give approximations nonetheless.

Goodisman et al. [86], extend a standard model of ion distribution functions to include the surface tension versus surface charge density. They use calculations of neutral surfaces (at their points of zero charge), as in successful models for the molten salt-air interfacial tensions. They calculate interfacial tensions using an electrocapillary model as a function of surface charge by using a correction for charge. Their results however do not agree with Guouy-Chapman nor Lippman equations, testing their differential surface tension with charge density against known capacitances. Nonetheless, their predicted surface tensions for charged surfaces are as expected, i.e., lower than the pzc value, which is equivalent to the surface tension of liquid KCl in air of close to 100 mN/m. Their results are close to those determined using surface-air work of adhesion determined interfacial tension values for molten salts against Pb, In and Bi -1 metals with nominally charged surfaces (γSL = 139, 134, 105 mN.m , respectively).

According to Ninham et al. [48, 84], current theories of ion-surface interactions are wrong for many reasons, not the least of which is their neglect of polarizability terms and/or dispersion interactions that are often not calculated in a correct way. It is likely that the curves presented above for melting point depression are too steep, owing to the short-range electrostatic interactions that Ninham et al. [48, 84] state dominate as the dispersion forces between the ions and interface bring them close together. The potential at short range is given by Ninham [84] as an exponential function. In any case the melting point depression is easily tested using calorimetric models. [94]

Given that alkali chlorides, in particular KCl are very corrosive and can stay in the molten state below their bulk freezing temperature in smaller pores, their liquid transport to the metal interface can be enhanced. This is perhaps even a secondary effect compared with the condensation of gaseous KCl into small pores in a similar way except according to the Kelvin equation that relates the capillary condensation pressure to the pore radius, and can very significantly reduce the liquid-gas transition temperature. Given that coatings need to be porous to some extent to moderate thermal cycling and shock in cooled/heated boiler tubes, micron and nanometer sized pores and cracks are inevitable in a cohesive coating. Therefore the gases can condense at lower temperature in the pores and can also then stay as liquids particularly if they form low melting eutectics to very low temperatures, as low as 260 °C for the FeCl2-KCl system.

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8 Conclusions

Alkali chlorides are a threat to boilers as a corrosive agent particularly in burners using biomass or waste fuels. Analysis of alkali chloride interactions in the molten state with surfaces allows generalization of many of important surface forces occurring in boilers. However, no theories are adequate at this stage for a full understanding. Traditional theories are probably special limiting cases, and have been applied here to make some qualitative conclusions in respect to mechanisms and materials approaches against deposition.

Nanocomposite materials such as alpha alumina and silicon carbide and their composites appear to be the most effective, least expensive and high performing, chemically resistant options for protection against deposition and corrosion processes on boiler tubes. Other materials are emerging that might also be used when affordable like nanocomposited aluminas with yttria stabilized zirconias. Thermally sprayed alloy coatings also appear to have very good high temperature corrosion-resistant properties and can form very adhesive bonds to the metal tubing. Ceramic coatings applied to boilers form better corrosion barriers and are more adhesive when bond layers are used.

Surface forces are critical to design and understanding of deposition in boilers and the development of effective materials and methods to deal with deposition. This is because the processes of alkali chloride control, deposition and dynamics are critically dependent on knowledge of surface forces. From a materials point of view, the trend to using nanocomposite coats introduces an exponential rise in the surface area of interfaces within these systems. As a consequence more and more surfaces will become vulnerable to KCl attack if systems are not designed well. This could lead to false negative results for promising materials that are poorly applied, for example.

Of all the physical forces operating in the deposition process in boilers, surface forces are key and in particular the van der Waals forces, yet they are rarely incorporated in modelling. These are operative in the condensation nucleation and growth of alkali sulfates and chloride aerosols and in their agglomeration to larger particles and adhesive interactions with the boiler tube walls during the actual deposition. This also goes for the other components such as silicates. They are also operative after deposition in the capillary interactions of fireside fluids (gases and liquids) within pores in the deposits or the coatings or tube materials themselves. For example we have discussed the capillary condensation of gases in pores and cracks in boiler tubes or their coatings and the possibility of melting point depression within the pores in cracks and fissures down to 1-5 nm within the pores, particularly for eutectic melts.

The Hamaker constant provides a quantitative description of the sum of the intermolecular forces acting between surfaces. For the case of surface coatings relevant for boiler tube coatings the integrity and potential bonding strength of a ceramic coating on boiler surfaces can be predicted by knowledge of the Hamaker constant of the system.

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For the case of deposit build up it was shown that liquid KCl displays a negative Hamaker constant against many ceramics and alloy surfaces in air which predicts wetting of theses surfaces. This report has shown that wetting of boiler surfaces by KCl is the main cause of deposit build up. Thus, calculations of Hamaker constants for the complete system of boiler tube-liquid KCl-gas enable material properties of novel coatings to be designed to limit the deposit and build up of alkali chloride soils.

Finally, returning to the concept of smooth surfaces, we can state that ceramic surfaces are often smooth because these are formed from fine-grained slurries with interfaces flattened by surface tension prior to sintering. These stay smooth during boiler operations by resisting inertial impact based erosion, and are thus more resistant to formation of capillary condensates in surface defects compared with less tough materials. Roughness of a surface is a relative term. Rough surfaces have been shown to be less adhesive for particulates with size much larger than the scale of the roughness. In this respect smooth surfaces are not preferred, at least for smooth particulate aerosols and other suspended solids on the fireside. Fortunately, it is likely that the particulates themselves are quite rough, so the smooth ceramic or alloyed surface receiving a rough depositing particle has little adhesive force.

The case for liquids is the opposite. These can condense in the troughs of the rough surface, and/or can spread over the rough surface to form a perfect contact, guaranteeing strong adhesion simply because of the relatively high surface area of interaction. In this respect smooth surfaces made of ceramic or specialty alloys must remain porosity free, at least at their surfaces, otherwise they will quickly attract condensing alkali chloride vapours. Also in respect of stickiness at surfaces, the tendency of the alkali chlorides to form reaction products with the coatings is obvious. SiC, which has excellent thermal conductivity is reactive, particularly to alkali chlorides and must be protected by secondary coatings like alumina.

Interfacial tensions of liquid alkali chlorides are very poorly constrained. Predictions of the interfacial tensions of alkali chlorides before, during and after deposition are not yet possible by any single theory and therefore must be studied using other methods. Our findings suggest that capillary forces may be key to future studies of alkali chlorides, for example in designing new technologies against boiler deposition and corrosion.

Lastly, on a practical note, if coatings are to be used, the study of surface forces here suggest these be applied using very well thought out protocols with the goal to remove all possible defects and surface porosity. This includes extra passivation steps after particle blasting and removal of all traces of organic contamination and other sources of gases that could compromise the coating. Very careful thermal annealing and or sintering/bonding would also be ideal. This would lead to a coating with very tight control on thickness and homogeneity of composition. It would also keep the coating as cool as possible to reduce thermal shock induced fractures and compromise of nanocomposite internal adhesive bonds and to reduce the stickiness by freezing approaching otherwise liquid particulates in a steeper thermal gradient. To this end silicon carbide is the most attractive material but it must be chemically protected and doing this is best implemented by both testing in real boilers in combination with theoretical modelling as far as it is can be taken with current knowledge.

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9 Recommendations and use

From a practical point of view using ceramic coatings, nanocomposites are recommended. These can be tailored to have superior mechanical, thermal and chemical properties compared with monolithic ceramics.

We recommend that during application of coatings to boiler surfaces or shells, strict surface preparation and cleaning protocols be used that minimize both the free metal at the surface and eliminate any trace of organic compounds and other sources of gas that could be harm to either interfacial bonding during sintering of a ceramic nanocomposite and introduce unwanted porosity into the ceramic film. The wet ceramic nanocomposite precursor, in the case of sol-gel style application, is best applied with strict attention to the wetting and spreading to form as thin a uniform layer as possible and phase diagrams and trials should be consulted and run to optimize the sintering/annealing and to avoid formation of unwanted phases.

The use of NiCrAlY alloy bond layers for nanocomposite ceramic coats can be useful if affordable for decreasing the corrosive reactions and gaseous diffusion of corrosive substances to the tube surface. These also allow better adhesion of the ceramic layer to the tube/shell surface.

Thermally sprayed alloy coatings on boiler tubes have excellent adhesion, are becoming cheaper and can reduce the corrosion of boiler tube surfaces and can form superior durable coatings to ceramic nanocomposites.

From an overall performance standpoint, coatings made predominantly of nanocomposited SiC, alumina nanofiber with a thin protective nanostructured alumina top-coat would provide the ideal combination of materials for thermal, mechanical and chemical performance at the lowest cost. This could be used in combination with bond coats for ensuring higher performance adhesion.

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10 Suggestions for further research

On the more scientific side, we recommend a full experimental study of the surface forces reviewed here. In particular, much would be gained through experimental studies on the condensation of KCl and other relevant gases in pores of various materials such as ceramics, and a study of the wetting and spreading of KCl liquid in capillaries, rough and smooth surfaces. Furthermore longer-term benefit would come from a thorough combined study of theoretical and experimental surface forces operative in biomass burners where KCl is present in high concentration. This would be from condensation and nucleation, alkali scavenging, particle growth, agglomeration, deposition and wetting of various interfaces described here.

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11 Literature references

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Appendices

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A.1 Optical properties of KCl

Approximately 30% reduction in density on melting of KCl can be used to estimate a simple concomitant reduction in refractive index across all frequencies, followed by use of an approximate, non-full Lifshitz treatment such as the Tabor approximation [95]. However a more detailed discussion is required. Spectral changes in KCl due to melting are discussed elsewhere [58, 96, 97]. There are enhanced interactions of photons with ion density fluctuations. The IR reflectivity of alkali chlorides is strongly reduced and the edge shifted and the spectrum broadened, thus changing the IR contribution to the Hamaker constant. The UV component will be affected through the delocalization of electrons in the more conductive molten salt, and will also contribute to a change in the Hamaker constant relative to the solid. Brillouin scattering is much stronger and sharper in the liquid than the solid and the lines are strongly polarized in the (vacuum) frequency range up to 10 cm-1, this will have a secondary effect. Somani et al. [61] performed ab initio simulations on the structure of the molten salts to determine the complex frequency dielectric spectrum and to compare it with high temperature refractive index measurements. Their simulated refractive indices for KCl at 1000°C are approximately 1.5x higher than the actual measured value of 1.37. They calculate a -21 Hamaker constant, A121 value for SiO2-KCl(l)-SiO2 of only 3-4 zJ (10 J). The refractive index (RI) of solid KCl is about 1.473. The reduction on melting reflects the lowering in density, but does not capture the whole story. The static dielectric value of a molten salt can be estimated at 5.5 – see Kizsa [93]. This shows that other contributions to the Hamaker constant not indicated by the visible wavelength refractive index must be accounted for by changes in the UV and IR. Full spectral data is needed for a more precise determination. In this review the dielectric response functions for non-lossy dielectrics (i.e., dielectrics with bound electrons) are used for Hamaker constant calculations in line with the “modern theory”. This is, however not ideal for a thorough understanding surface forces for liquid KCl, because vdW forces due to dipoles and charges and electronic conduction are unaccounted for. Nonetheless, near a surface KCl liquid becomes highly structured, almost like in the solid. Therefore Hamaker constants determined using dielectric response functions for non-conductors can still be instructive for making qualitative statements about surface forces (e.g., predicting wetting of surfaces by condensation of KCl gas to liquid). Hamaker constants used for predicting surface force interactions in boilers are valid and quantitative for solid-state interactions in particular, e.g., adhesion forces of solid alkali salts against ceramics.

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A.2 Condensation of KCl from flue gases Kristensen and Livbjerg [75] model aerosol nucleation by a classical theory with the surface tension modified for molecular clusters instead of monomers, and give examples where both KCl and K2Cl2 vapour molecules are involved in chloride salt seed formation, as is representative of real flue gases (where these are present in roughly equimolar ratios).

They model growth by condensation followed by agglomeration using different multicomponent models. In their models, the steady state particle velocity scales initial particle numbers proportionally to the sum of the loss to coagulation and wall (surface) deposition due to turbulent diffusion, impaction and thermophoresis. They use integration axially and a discontinuous set of size classes that can change axially in their sums.

Gas to particle conversion is modeled using growth equations for particle size involving gas mass flow and gas density and area using a model of condensation rate of species onto certain diameter particles. This model is valid for normal alkali flue gas concentration and involves a surface tension term that linearly reduce the scale of partial pressure at the particle surface relative to the gas partial pressure at that temperature and particle composition due to the presence of other species. This is in accord with the Kelvin effect.

They study 4 limiting cases of gas to particle conversion that modify these scalars:

1) Segregated surface layer via heterogeneous initiation of nucleation involving an increase in the vapour pressure according to the Kelvin effect of surface tension of a meniscus, and a unitary scaling of the curvature factor 2) Perfectly mixed particles where the mole fraction in each particle scaling the curvature term and a surface tension terms changed according to the mixture 3) Perfectly mixed surface layer, they point out that if mixing in the particle is slow the surface chemistry is not the same as the bulk and thus the partial pressure is surface composition dependent. Here the surface composition is analytically approximated using the flux from the flue gas to the particle in an iterative fashion. This case was also modified for a mixed monomeric and dimeric KCl gas. 4) Growth of pure crystals by condensation onto intermixed, separated regions of pure crystalline phases, in a model that was not implemented for certain reasons.

They make the important point that the reduction in surface tension for clusters may increase the rate of nucleation by several orders of magnitude compared with the bulk surface tension, with obvious implications for deposition. More information is available in the publication of Kristensen and Livberg [75]

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A.3 Intergranular film formation in ceramics

100 Disjoining (ordering) pressure dispersion pressure Net pressure 50

-50 Disjoining (ordering)pressure Disjoining

-100 01234567 thickness (h/ xi) Figure 17. Disjoining pressure, dispersion pressure and total pressure of an intergranular film in between alumina grains of a polycrystalline ceramic (calculated after Clarke). [98] In this case a liquid silica film wets the interface between two ceramic grains in a system where the Hamaker constant is positive and = 22 zJ, and the disjoining pressure (due to liquid structure) is negative and opposes this pressure, yielding a minima in the net pressure, indicative of a stable film. The calculations here assume a molecular correlation length, xi=0.3 nm. The disjoining pressure is calculated from the heat of melting of cristobalite, a polymorph of quartz (SiO2). Figur 17. Separering kraft, dispersion kraft and totalt kraft av en interkristallin film mellan aluminiumoxid korn av en polykristallin keramisk (beräknas efter Clarke). [98] I detta fall väter en vätskeformig kisel-film gränsytan mellan två keramiska korn I ett system där Hamaker-konstanten är positiv och = 22 zJ, och separering kraft (pga vätskestruktur) är negativt och ställs mot detta tryck, vilket ger ett minimum i netto- tryck, som indikerar en stabil film. Vid beräknignarna här antas en molekylär korrelationslängd, xi=0.3 nm. Separering kraften beräknas från värmen av att smälta kristobalit, en polymorf i kvarts (SiO2).

Clarke [98] discussed the importance of the balance of forces in determining if intergranular films will adopt an equilibrium thickness in polycrystalline ceramic materials. Clarke summed the total contributions from repulsive disjoining forces (due to the structure of the intergranular liquid) and attractive van der Waals dispersion forces arising due to positive Hamaker constants to conclude thin liquid films of finite equilibrium thickness (< 1 nm) can form in polycrystalline ceramics. This approach could also be applied to intergranular films of alkali chloride liquid and other molten phases sourced from flue gas that become interstitial phases through capillary condensation, capillary flow, chemical reactions or otherwise. As Clarke points out, such intergranular films are expected to be of different thicknesses dependent upon the dielectric constants of the adjacent grains, and the intervening liquid. In the case of alkali chlorides, the balance of forces will also involve attractive and repulsive forces including those due to ions and dipoles and their interactions. [48]

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Film thicknesses of molten silica between other materials can be assessed by combining the disjoining pressure curve above (figure 17) and using Hamaker constants for various ceramics that are wet by silica (generally all materials with higher dielectric constants). Table 10 below gives some examples using the dielectric response function of solid silica as an approximation of a fused silica (as done by Clark). [98] These Hamaker constants are also relevant for understanding film formation between particles in the flue gas.

Table 10. Table of Hamaker constants for materials separated by fused silica Tabell 10. Tabell på Hamaker-konstanter för material som separerats av kvartsglas Material 1 Material 2 liquid Hamaker constant comment (10-20 J)

Al2O3 Al2O3 SiO2 2.26 beta-SiC beta-SiC SiO2 8.41 Will predict very thin films if any Fe2O3 (hematite) Fe2O3 (hematite) SiO2 2.55 TiO2 (rutile) TiO2 rutile SiO2 4.49 Al2O3 MgAl2O4 (spinel) SiO2 1.67 Asymmetric case Mica (clay) Mica (clay) SiO2 0.39 Kaolinite mimic Na2SO4 crystal Na2SO4 crystal SiO2 ??? Na2SO4 dielectric constant = 7.9

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A.4 Thin film formation Hough and White [54] discuss spontaneous thin film formation on surfaces according to the dispersion forces interactions. The condition for spontaneous formation of a thermodynamically stable wetting layer on a substrate is given by:

(14) where in this case, ESLA is the dispersion interaction free energy per unit area of a liquid film of thickness L between planar half-spaces of the substrates and air.

For film thicknesses less than 5 nm the following expression is useful here:

(15) where ASLA is the Hamaker constant for the system substrate-liquid-air (SLA).

We immediately see that a negative valued Hamaker constant means that ESLA(L) is a continuous function, decreasing with increased L, and as such with a continuously increasing value of its first derivative, thus satisfying (14) above. Thus thickening the film increases the energy of the system. They state “a stable film thickness is obtained when the energy balances the entropy loss upon condensation from the vapour.” If the Hamaker constant is positive the system can gain energy by thinning a film to dryness by evaporation. No wetting film can exist, only droplets of finite contact angle. The surface tension can be estimated as [54, 95]:

(16)

2 where ALAL is the liquid-air-liquid Hamaker constant, and L c is a cut-off term introduced to prevent divergence of (15) at the limit of decreased separation.

2 A value L c = 0.165 nm is used here. [95] From these calculated values we can say KCl can therefore wet the surface of many of the materials listed in the tables here. Retardation effects of screening will limit the validity of the use of London forces to determining film thicknesses because the Debye screening lengths in ionic liquids is typically 1-4 nm. [99]

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A.5 Work of adhesion and interfacial tension One of the simplest interaction energy representations of two surfaces is

(17) where D is the separation distance, A is the Hamaker constant, and k is the cohesion energy within the materials. The second term is the surface energies, 2γ of the pair, arising from mismatches in lattices alignments and sizes etc. that leave bonds unsatisfied. So a simple representation of surface energy, γ, of two identical media separated by some distance can be expressed as half the right hand side of (17):

(18) where D0 is the interfacial contact separation usually taken as 0.165 nm because of its reliability in predicting surface energies. The surface energy is half the work required to separate the surface from D0 to infinity.

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