The use of waste (or recycled) wood as a fuel in heat and power stations is becoming more widespread in Sweden (and Europe), because it is CO2 neutral with a lower cost than forest fuel. However, it is a heterogeneous fuel with a high amount of chlorine, alkali and heavy metals which causes more corrosion than fossil fuels or forest fuel. A part of the boiler which is subjected to a high corrosion risk is the furnace wall (or waterwall) which is formed of tubes welded together. Waterwalls are made of ferritic low-alloyed steels, due to their low price, low stress corrosion cracking risk, high heat transfer properties and low thermal expansion. However, ferritic low alloy steels corrode quickly when burning waste wood in a low NOx environment (i.e. an environment with low oxygen levels to limit the formation of NOx). Apart from pure oxidation two important forms of corrosion mechanisms are thought to occur in waste environments: chlorine corrosion and alkali corrosion. Although there is a great interest from plant owners to reduce the costs associated with furnace wall corrosion very little has been reported on wall corrosion in biomass boilers. Also corrosion mechanisms on furnace walls are usually investigated in laboratories, where interpretation of the results is easier. In power plants the interpretation is more complicated. Difficulties in the study of corrosion mechanisms are caused by several factors such as deposit composition, flue gas flow, boiler design, combustion characteristics and flue gas composition. Therefore, the corrosion varies from plant to plant and the laboratory experiments should be complemented with field tests. The present project may thus contribute to fill the power plant corrosion research gap. In this work, different kinds of samples (wall deposits, test panel tubes and corrosion probes) from Vattenfall’s Heat and Power plant in Nyköping were analysed. Coated and uncoated samples with different alloys and different times of exposure were studied by scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), X-ray diffraction (XRD) and light optical microscopy (LOM). The corrosive environment was also simulated by Thermo-Calc software. The results showed that a nickel alloy coating can dramatically reduce the corrosion rate. The corrosion rate of the low alloy steel tubes, steel 16Mo3, was linear and the oxide scale non-protective, but the corrosion rate of the nickel-based alloy was probably parabolic and the oxide much more protective. The nickel alloy and stainless steels showed good corrosion protection behavior in the boiler. This indicates that stainless steels could be a good (and less expensive) alternative to nickel-based alloys for protecting furnace walls. The nickel alloy coated tubes (and probe samples) were attacked by a potassium-lead combination leading to the formation of non-protective potassium lead chromate. The low alloy steel tubes corroded by chloride attack. Stainless steels were attacked by a combination of chlorides and potassium-lead. The Thermo-Calc modelling showed chlorine gas exists at extremely low levels (less than 0.1 ppm) at the tube surface; instead the hydrated form is thermodynamically favoured, i.e. gaseous hydrogen chloride. Consequently chlorine can attack low alloy steels by gaseous hydrogen chloride rather than chlorine gas as previously proposed. This is a smaller molecule than chlorine which could easily diffuse through a defect oxide of the type formed on the steel. / <p>QC 20130423</p>

The use of renewable wood-based fuel has been increasing in the last few decades because it is said to be carbon neutral. However, wood-based fuel, and especially used wood (also known as recycled wood or waste wood), is more corrosive than virgin wood (forest fuel), because of higher amounts of chlorine and heavy metals. These elements increase the corrosion problems at the furnace walls where the oxygen level is low. Corrosion mechanisms are usually investigated at the superheaters where the temperature of the material and the oxygen level is higher than at the furnace walls.  Much less work has been performed on furnace wall corrosion in wood or used wood fired boilers, which is the reason for this project.    Tests are also mostly performed under simplified conditions in laboratories, making the results easier to interpret.  In power plants the interpretation is more complicated. Difficulties in the study of corrosion processes are caused by several factors such as deposit composition, flue gas composition, boiler design, and combustion characteristics and so on. Therefore, the laboratory tests should be a complement to the field test ones. This doctoral project involved in-situ testing at the furnace wall of power boilers and may thus contribute to fill the gap. The base material for furnace walls is a low alloy steel, usually 16Mo3, and the tubes may be coated or uncoated. Therefore tests were performed both on 16Mo3 and more highly alloyed materials suitable for protective coatings. Different types of samples exposed in used-wood fired boilers were analysed by different techniques such as LOM (light optical microscopy), XRD (X-ray diffraction), SEM (scanning electron microscopy), EDS (energy dispersive spectroscopy), WDS (wavelength dispersive spectroscopy), FIB (focused ion beam) and GD-OES (glow discharge optical emission spectroscopy). The corrosion rate was measured. The environment was also thermodynamically modelled by TC (Thermo-Calc ®). The results showed that 16Mo3 in the furnace wall region is attacked by HCl, leading to the formation of iron chloride and a simultaneous oxidation of the iron chloride. The iron chloride layer appeared to reach a steady state thickness.   Long term exposures showed that A 625 (nickel chromium alloy) and Kanthal APMT (iron-chromium-aluminium alloy) had the lowest corrosion rate (about 25-30% of the rate for 16Mo3), closely followed by 310S (stainless steel), making these alloys suitable for coating materials. It was found that the different alloys were attacked by different species, although they were exposed in the boiler at the same time in the same place. The dominant corrosion process in the A 625 samples seemed to be by a potassium-lead combination, while lead did not attack the APMT samples. Potassium attacked the alumina layer in the APMT samples, leading to the formation of a low-protective aluminate and chlorine was found to attack the base material.  The results showed that stainless steels are attacked by both mechanisms (Cl- induced attack and K-Pb combination). Decreasing the temperature of the furnace walls of a waste wood fired boiler could decrease the corrosion rate of 16Mo3. However, this low corrosion rate corresponds to a low final steam pressure of the power plant, which in not beneficial for the electrical efficiency. The short term testing results showed that co-firing of sewage sludge with used wood can lead to a reduction in the deposition of K and Cl on the furnace wall during short term testing. This led to corrosion reduction of furnace wall materials and coatings. The alkali chlorides could react with the aluminosilicates in the sludge and be converted to alkali silicates. The chromia layer in A 625 and alumina in APMT were maintained with the addition of sludge. / Förnybara träbaserade bränslen har ökat i användning under de senaste decennierna, eftersom det är koldioxidneutrala. Emellertid är träbaserade bränslen, och i synnerhet använt trä (även känt som återvunnet trä, returträ eller träavfall), mer korrosivt än skogsbränsle, på grund av högre halter klor och tungmetaller. Dessa ökar korrosionsproblemen på eldstadsväggarna, särskilt på platser där syrehalten är låg. Korrosionsmekanismer undersöks vanligtvis på överhettare dvs. på områden där materialets temperatur och syrenivån är högre än vid eldstadsväggarna. Färre arbeten har utförts på eldstadskorrosion i returträ pannor, vilket är motiveringen till detta projekt. Normalt sätt så görs endast i laboratorietester där resultaten är lättare att tolka. I kraftverk är tolkningen mer komplicerad. Undersökningar av korrosionsprocesser försvåras av flera faktorer såsom panndesign, förbränningsegenskaper, rökgassammansättning, beläggningskemi och så vidare. Därför bör laboratorietester kompletteras med fältförsök. Detta doktorandprojekt kan således bidra till att fylla denna brist. Eldstadsväggarna är uppbyggda av flera rör som svetsas samman och de består vanligtvis av 16Mo3 stål. Rören kan vara belagda eller obelagda. Tester har därför genomförts på 16Mo3 samt på höglegerade material vilka är lämpliga som skyddande beläggningar. Olika typer av prov som exponerats i förbränningspannor av returträ analyserades med olika tekniker såsom SEM (svepelektronmikroskopi), EDS (energidispersiv spektroskopi), WDS (våglängd dispersiv spektroskopi), FIB (fokuserad jonstråle) LOM (ljusoptisk mikroskopi), XRD (röntgendiffraktion), och GD-OES (glimurladdning med optisk emissionsspektroskopi). Miljön samt korrosionsprocesser har modellerats termodynamiskt med mjukvaran TC (Termo-Calc®). Resultaten visade att 16Mo3 i eldstadsväggen angrips av väteklorid, vilket leder till bildning av järnklorid och en samtidig oxidation av järnkloriden. Järnkloridskiktet verkade nå ett stationärt tillstånd vad avser tjocklek. Sex veckors prov visade att A 625 (nickelkromlegering) och Kanthal APMT (järnkromaluminiumlegering) hade den lägsta korrosionshastigheten (ca 25-30% av korrosionshastigheten för 16Mo3), följt av 310S (rostfritt stål). Vi har funnit att de olika legeringarna angrips genom olika mekanismer, även om de var exponerade i pannan samtidigt på samma plats. Den dominerande korrosionsmekanismen för legeringen A 625 verkar i huvudsak bero på kalium och bly, medan bly inte attackerar Kanthal APMT. Kalium angriper aluminiumoxidskiktet på Kanthal APMT, vilket leder till bildning av icke-skyddande aluminat medan klor i sin tur attackerar basmaterialet. Resultaten visar att rostfritt stål attackeras genom klor-inducerad korrosion samt kalium och bly i kombination. Reducering av temperaturen kan minska korrosionshastigheten hos 16Mo3. Men denna lägre korrosionshastighet motsvarar ett lågt slutligt ångtryck hos kraftverket, vilket inte är fördelaktigt för elverkningsgraden. De kortare exponeringarna visade att samtidig förbränning av avloppsslam med returträ kan leda till minskad avsättning av kalium och klor i form av alkaliklorider på eldstadsväggarna. Detta ledde till korrosionsminskning av alla studerade material. Dessa alkaliklorider skulle kunna reagera med aluminiumsilikaterna från slammet och omvandlas till alkalisilikater. Detta verkar minska den alkali-inducerade korrosionen på A 625, APMT och 310S. Den aluminiumoxid som bildades på APMT och det kromoxidskikt som bildades på A 625 upprätthölls med tillsats av slam. / <p>QC 20151015</p>