Melanie Montgomery

Field test corrosion experiments in Denmark with biomass fuels. Part 1: Straw‐firing

In Denmark, straw and other types of biomass are used for generating energy in power plants. Straw has the advantage that it is a “carbon dioxide neutral fuel” and therefore environmentally acceptable. Straw combustion is associated with corrosion problems which are not encountered in coal-fired plants. The type of corrosion attack can be directly ascribed to the composition of the deposit and the metal surface temperature. A series of field tests have been undertaken in the various straw-fired power plants in Denmark, namely Masnedo, Rudkobing and Ensted. Three types of exposure were undertaken to investigate corrosion: a) the exposure of metal rings on water/air cooled probes, b) the exposure of test tubes in a test superheater, and c) the exposure of test tubes in existing superheaters. Thus both austenitic steels and ferritic steels were exposed in the steam temperature range of 450–600°C. The corrosion rates were assessed by precision measurements of material loss and internal corrosion. The corrosion products and course of corrosion for the various steel types were investigated using light optical and scanning electron microscopy. Corrosion mechanisms are discussed in relation to temperature and deposit composition. Korrosionsfeldversuche in Danemark mit Brennstoffen aus Biomasse. Teil 1: Strohfeuerung In Danemark werden Stroh und andere Arten von Biomasse zur Energieerzeugung in Kraftwerken eingesetzt. Stroh hat den Vorteil, dass es ein „kohlendioxidneutraler Brennstoff“ und daher umweltfreundlich ist. Die Strohverbrennung ist aber mit Korrosionsproblemen verbunden, denen man in kohlebefeuerten Anlagen nicht begegnet. Die Art des Korrosionsangriffs kann direkt der Zusammensetzung der Ablagerungen und der Metalloberflachentemperatur zugeschrieben werden. Eine Serie von Feldversuchen wurde in den verschiedenen strohbefeuerten Kraftwerken in Danemark durchgefuhrt: Masnedo, Rudkobing und Ensted. Zur Untersuchung der Korrosion wurden drei Auslagerungsarten eingesetzt: a) die Auslagerung von Metallringen auf Wasser-/Luft-gekuhlten Proben, b) die Auslagerung von Testrohren in einem Versuchsuberhitzer und c) die Auslagerung von Testrohren in existierenden Uberhitzern. Sowohl austenitische als auch ferritische Stahle wurden in dem Dampftemperaturbereich von 450–600°C ausgelagert. Die Korrosionsgeschwindigkeiten wurden durch Feinmessungen des Massenverlustes und der inneren Korrosion beurteilt. Die Korrosionsprodukte und der Verlauf der Korrosion fur die verschiedenen Stahltypen wurden mittels lichtoptischer und Rasterelektronenmikroskopie untersucht. Die Korrosionsmechanismen werden bezuglich Temperatur und Zusammensetzung der Ablagerungen diskutiert.

In-situ corrosion investigation at Masnedø CHP plant – a straw-fired power plant

Various austenitic and ferritic steels were exposed on a water-cooled probe in the superheater area of a straw-fired CHP plant. The temperature of the probe ranged from 450–600°C and the period of exposure was 1400 hours. The rate of corrosion was assessed based on unattacked metal remaining. The corrosion products and course of corrosion for the various steel types were investigated using light optical and scanning electron microscopy. The ferritic steels suffered from corrosion mainly via material loss. The austenitic steels suffered from predominantly selective corrosion which resulted in depletion of chromium from the alloy. A clear trend was observed that selective corrosion increased with respect to the chromium content of the alloy. In-situ Korrosionsuntersuchungen an der Masnedo CHP-Anlage – ein strohbefeuertes Kraftwerk Verschiedene austenitische und ferritische Stahle wurden auf einer wassergekuhlten Probe in dem Uberhitzerbereich einer strohbefeuerten Warme- und Energieanlage ausgelagert. Die Temperaturen der Probe lagen zwischen 450 und 600°C bei einer Auslagerungszeit von 1400 Stunden. Die Korrosionsgeschwindigkeit wurde an Hand von verbliebenem, nicht angegriffenem Metall beurteilt. Die Korrosionsprodukte und der Verlauf der Korrosion fur die verschiedenen Stahlsorten wurden mittels lichtoptischer und rasterelektronenmikroskopischer Techniken untersucht. Die ferritischen Stahle erlitten im wesentlichen Korrosion durch Materialverlust. Die austenitischen Stahle wiesen dagegen hauptsachlich selektive Korrosion auf, was sich in einer Chromverarmung der Legierung bemerkbar machte. Ein klarer Trend wurde beobachtet, das die selektive Korrosion mit dem Chromgehalt der Legierung zunahm.

Field test corrosion experiments in Denmark with biomass fuels. Part 2: Co-firing of straw and coal

In Denmark, straw is used for generating energy in power plants. However during straw combustion, potassium chloride and SO2 are released in the flue gas and through condensation and deposition processes they will result in the formation of superheater deposits rich in potassium chloride and potassium sulphate. These components give rise to varying degrees of accelerated corrosion. This paper concerns co-firing of straw with coal to reduce the corrosion rate from straw to an acceptable level. A field investigation at Midtkraft Studstrup suspension-fired power plant in Denmark has been undertaken where coal has been co-fired with 10% straw and 20% straw (% energy basis) for up to approx. 3000 hours. Two types of exposure were undertaken to investigate corrosion: a) the exposure of metal rings on water/air cooled probes, and b) the exposure of a range of materials built into the existing supertheaters. A range of austenitic and ferritic steels was exposed in the steam temperature range of 520–580°C. The flue gas temperature ranged from 925–1100°C. The rate of corrosion was assessed by precision measurement of material loss and measurement of oxide thickness. Corrosion rates are lower than for 100% straw-firing. The corrosion products and course of corrosion for the various steel types were investigated using light optical and scanning electron microscopy. Catastrophic corrosion due to potassium chloride was not observed. Instead a more modest corrosion rate due to potassium sulphate rich deposits was observed. Corrosion mechanisms include sulphidation, oxidation and hot corrosion. Korrosionsfeldversuche in Danemark mit Brennstoffen aus Biomasse. Teil 2: Gemeinsame Stroh- und Kohlefeuerung In Danemark wird Stroh zur Energieerzeugung in Kraftwerken verwendet. Wahrend der Strohverbrennung wird jedoch Kaliumchlorid und SO2 im Abgas freigesetzt und durch Kondensation und Abscheidungsprozesse fuhrt dies zur Bildung von Kaliumchlorid- und Kaliumsulfat-reichen Ablagerungen am Uberhitzer. Durch diese Komponenten kommt es zu verschiedenen Graden an beschleunigter Korrosion. Diese Arbeit behandelt die gemeinsame Feuerung von Stroh mit Kohle, um die Korrosionsgeschwindigkeit von Stroh auf ein vertretbares Mas zu reduzieren. Am Suspensions-befeuerten Kraftwerk Midtkraft Studstrup in Danemark wurde eine Felduntersuchung durchgefuhrt, bei der Kohle gemeinsam mit 10% und 20% Stroh (% Energiebasis) bis zu ca. 3000 Stunden eingesetzt wurde. Um die Korrosion zu untersuchen, wurden zwei Arten von Auslagerung durchgefuhrt: a) die Auslagerung von Metallringen auf Wasser-/Luft-gekuhlten Proben und b) die Auslagerung von verschiedenen Werkstoffen, die in die existierenden Uberhitzern eingebaut wurden. Eine Reihe von austenitischen und ferritischen Stahlen wurden im Dampftemperaturbereich von 520–580°C ausgelagert. Die Abgastemperatur variierte zwischen 925 und 1100°C. Die Korrosionsgeschwindigkeit wurde durch Feinmessungen des Massenverlustes und Messungen der Oxiddicke beurteilt. Die Korrosionsgeschwindigkeiten sind niedriger als bei 100% Strohfeuerung. Die Korrosionsprodukte und der Verlauf der Korrosion fur die verschiedenen Stahltypen wurden mittels lichtoptischer und Rasterelektronenmikroskopie untersucht. Katastrophale Korrosion durch die Kaliumchloride wurde nicht beobachtet. Stattdessen wurde eine geringere Korrosionsgeschwindigkeit durch die Kaliumsulfatreichen Ablagerungen beobachtet. Die Korrosionsmechanismen beinhalten Sulfidierung, Oxidation und Hot Corrosion.

Experiences with Inconel 625 in Biomass and Waste Incineration Plants

Inconel 625 is utilised in both biomass and waste incineration plants in Denmark. In both cases, the performance is good however the morphology of corrosion attack is different which indicates different corrosion mechanisms. In waste incineration plants there is general attack and shallow pitting, and in some cases dendritic attack especially on the fins of waterwalls. The dendritic attack is in the dendritic core. The presence of pits or dendritic attack is linked to the temperature of the metallic surface and the molten salt composition. In a woodchip biomass plant, chromium depletion was observed on the surface of the weld overlay leaving behind a nickel and molybdenum rich porous structure. The corrosion attack was not related to the dendritic microstructure of the weld. In two straw-fired biomass plants, co-extruded Sanicro 63 (alloy 625 type) as well as Inconel 625 weld overlay revealed the same type of attack, again chromium depletion. This indicates that the corrosion mechanism in woodchip and straw power plants are similar. Another interesting result in straw-fired boilers was that Nibas welds (alloy 625 composition) could provoke excessive corrosion in adjacent 18-8 stainless steels indicating a galvanic reaction. The corrosion mechanisms observed in each case are discussed in relation to temperature and corrosive environment.

Long term steam oxidation of TP 347H FG in power plants

Abstract The long term oxidation behaviour of TP 347H FG at ultra supercritical steam conditions was assessed by exposing the steel in test superheater loops in a Danish coal-fired power plant. The steamside oxide layer was investigated with scanning electron microscopy and energy dispersive X-ray diffraction in order to reveal the effect of oxidation time and temperature on the microstructure. A double layered oxide formed during steam oxidation. The morphology of the inner Cr-containing layer was influenced by the oxidation temperature. At temperatures below 585°C, it consisted of regions of Fe–Ni–Cr spinel surrounded by Fe–Cr oxide. At higher temperatures almost the entire inner oxide layer was composed of Fe–Cr oxide. Possible mechanisms for the oxide growth are discussed and it is suggested that faster Cr transport within the alloy at higher temperatures explains the change in morphology. The thickness of the inner oxide layer appeared to be independent of oxidation time and temperature for exposes less than 30 000 h; however, after 57 554 h, the thickness had increased significantly at the lowest temperatures.

Steam Oxidation of TP 347H FG in Power Plants

The long term oxidation behaviour of TP 347H FG at ultra supercritical steam conditions was assessed by exposing the steel in test superheater loops in a Danish coal-fired power plant. The steamside oxide layer was investigated with scanning electron microscopy and energy dispersive X-ray diffraction in order to reveal the effect of oxidation time and temperature on the microstructure. A double layered oxide formed during steam oxidation. The morphology of the inner Cr-containing layer was influenced by the oxidation temperature. At temperatures below approx. 585oC, it consisted of regions of Fe-Ni-Cr spinel surrounded by Fe-Cr oxide. At higher temperatures almost the entire inner oxide layer was composed of Fe-Cr oxide. Possible mechanisms for the oxide growth are discussed and it is suggested that faster Cr transport within the alloy at higher temperatures explains the change in morphology. This hypothesis is supported by thermodynamic calculations and kinetic data. The thickness of the inner oxide layer did not change significantly with oxidation time and temperature for exposures less than 30000 h; however after 57554 h the thickness had increased significantly at the lowest temperatures.

Field Investigation of Steamside Oxidation of TP347H

The steamside oxide formed on two TP347H superheater tubes was compared. The two specimens investigated were exposed in situ in power plants in Denmark, one specimen was coarsegrained and the other was fine-grained. Parts of both the coarse-grained and fine-grained specimens were turned (machined on the inner side) to give a constant metal thickness so more precise wall thickness measurements could be undertaken. Machined and non-machined areas were investigated using light optical microscopy, and scanning electron microscopy with EDS analysis. The oxide on the fine-grained specimen was also investigated with grazing incidence X-ray diffraction. Results from coarse-grained and fine-grained specimens (machined and non-machined) show that grain size influenced oxide thickness and morphology. The oxides from non-machined specimens had an outer iron rich oxide and an inner iron chromium oxide. However a thinner oxide had grown on the fine-grained steel. The machining of fine-grained and coarse-grained specimens resulted in a thin chromium rich oxide layer. The presence of surface deformation of the inner metal surface was evident on the coarse-grained specimen but not on the fine-grained specimen. However other indications that the fine-grained specimen had been deformed were observed. The influence of temperature, grain size, surface finish and exposure duration is discussed.

High temperature corrosion during biomass firing: improved understanding by depth resolved characterisation of corrosion products

Abstract The high temperature corrosion of an austenitic stainless steel (TP 347H FG), widely utilised as a superheater tube material in Danish power stations, was investigated to verify the corrosion mechanisms related to biomass firing. KCl coated samples were exposed isothermally to 560°C, for one week, under conditions simulating straw-firing. Thorough characterisation of the exposed samples was conducted by the analysis of sample cross-sections applying microscopy and spectroscopy based techniques. Cross-section analysis revealed the microstructure, as well as chemical and morphological changes within the near surface region (covering both the deposit and the steel surface). Such cross-section analysis was further complemented by plan view investigations (additionally involving X-ray diffraction) combined with removal of the corrosion products. Improved insights into the nature of the corrosion products as a function of distance from the deposit surface were revealed through this comprehensive characterisation. Corrosion attack during simulated straw-firing conditions was observed to occur through both active oxidation and sulphidation mechanisms.

Phase Identification and Internal Stress Analysis of Steamside Oxides on Plant Exposed Superheater Tubes

During long-term, high-temperature exposure of superheater tubes in thermal power plants, various oxides are formed on the inner side (steamside) of the tubes, and oxide spallation is a serious problem for the power plant industry. Most often, oxidation in a steam atmosphere is investigated in laboratory experiments just mimicking the actual conditions in the power plant for simplified samples. On real plant-exposed superheater tubes, the steamside oxides are solely investigated microscopically. The feasibility of X-ray diffraction for the characterization of steamside oxidation on real plant-exposed superheater tubes was proven in the current work; the challenges for depth-resolved phase analysis and phase-specific residual stress analysis at the inner side of the tubes with concave surface curvature are discussed. Essential differences between the steamside oxides formed on two different steels typically applied for superheaters, ferritic-martensitic X20CrMoV12-1 and lean austenitic stainless steel TP347H, respectively, are revealed by X-ray diffraction.

Development of the inner oxide zone upon steam oxidation of an austenitic stainless steel

Abstract The oxidation behaviour of TP 347H FG in mixtures of water, oxygen, and hydrogen was investigated in the temperature range 500–700°C for a fixed oxidation time of 336 h. The samples were characterised using reflective light and electron microscopy methods. Thin discontinuous double-layered oxide scales developed during oxidation at 500°C, whereas continuous double-layered oxide scales covered the entire sample surface after oxidation at 600 and 700°C. The major part of the scale grew into the former alloy grains, whereas Fe—Cr spinel grew along the former alloy grain boundaries. TEM and EELS investigations revealed that the part of the scale that grows into the alloy grains consists of particles of Fe—Cr spinel embedded in a metallic Fe—Ni matrix, which indicates that this part of the scale grows by an internal oxidation mechanism. Growth of the internal oxidation zone at high humidity (46%) is not significantly affected by the type of carrier gas used.