The effect of digested sewage sludge as a fuel additive to reduce corrosion of furnace walls has been studied. The nickel base alloy Alloy 625 and the iron-chromium-aluminium alloy Kanthal APMT™ were exposed for 14.25. h at the furnace wall in a power boiler burning 100% used (also known as waste or recycled) wood. The test was then repeated with the addition of sewage sludge to the waste wood. The samples were chemically analysed and thermodynamically modelled and the corrosion mechanisms were investigated. The results showed that the co-firing of sewage sludge with recycled wood leads to a reduction in the corrosion. Attack by a potassium-lead combination appeared to be the main corrosion mechanism in Alloy 625 during waste wood combustion, while attack by alkali chloride was found to be dominant in APMT alloy.
This work examines the difference in behaviour between two austenitic steels, 304L and 310S, in a simulated environment with carbon and oxygen activities typical of the syngas cooler of a steam blown gasifier. The cyclic exposures were performed with and without addition of 500 ppm HCl for a total duration of 960 h. The oxide layers (200-600 nm) were studied using TEM, a focused ion beam microscope with EDX (FIB-EDX), electron back scattered diffraction (EBSD), SEM and X-ray diffraction (XRD). For 304L, a Cr and Ni depleted zone with a ferritic structure was observed below the oxide layer and a thin, manganese rich layer at the metal/oxide interface. For 310S, circular oxide patches were observed on the surface after exposure with HCl. These displayed elevated concentration of Al, Ca and Si, typical of inclusions. Results indicate that the addition of HCl results in a difference in oxide quality and structure but that HCl at the level of this study has little impact on the sound metal thickness.
In this work, three different stainless steels (304L (CrNi-18-8), 253 MA®(CrNiSi-21-11), Kanthal® A-1 (FeCrAl)) and a reference low-alloyed ferritic steel (16Mo3 (Fe0.3Mo)) were exposed in a commercial biomass gasifier for three periods of 9 min, 580 and 1054 h in the temperature range 350–500 °C. Biomass is a fuel with generally higher amounts of chlorine and lower amounts of sulphur compared to coal and there is a current lack of data on materials performance in such environments. A high level of zinc sulphide was observed on the surfaces of all materials after exposure. It is argued that zinc plays a key role in capturing sulphur in this environment, thus preventing iron from sulphidation. Some incorporation of sulphur in the oxide scale was observed for Fe0.3Mo and CrNi-18-8. CrNiSi-21-11 showed some internal oxidation and pitting was observed for the FeCrAl material. All four materials showed acceptable performance with low total metal loss.
Aggressive corrosion can occur when firing waste or bio-based fuels, due to the presence of high concentrations of heavy metals, alkali metals, and chlorides. These deleterious compounds deposit on furnace walls and can form mixtures that can rapidly accelerate corrosion. The effect of salts containing lead had not been studied extensively at temperatures lower than 400 °C in nickel-based materials. This study investigates the effect of the individual salts PbCl2 and KCl and their mixture on the high temperature corrosion of alloy 625 at 340 °C and 380 °C. Samples of alloy 625 were covered with individual salts or a salt mixture and exposed to high temperatures in an atmosphere of synthetic air, 20-vol% H2O, and 100 ppm HCl. The results show that the presence of individual salts does not induce observable corrosion attack on alloy 625 after 168 h at any tested temperature. The salt mixture did cause a severe corrosion attack at 380 °C, observed after 24 h of exposure. It is suggested that the salt mixture induces the formation of lead chromates that may prevent or disrupt the formation of a protective chromia scale. It is believed that a key part of the mechanism is the formation of eutectic melts by the interaction of the scale with the salt mixture. Thermodynamic equilibria calculations show that the first melting temperature of PbCl2 and KCl salt mixture after reaction with oxygen can be as low as about 382 °C, and even lower (357 °C) if chromates are present.
This work encompasses the evaluation of deposit and material related limitations on boiler performance and the lifetime of CO2-free, oxyfuel operated power plants. The aim of the study was to determine the reliability of advanced and conventional boiler materials. Six alloys, varying from state-of-the-art ferritic-martensitic and austenitic Fe-based steels to an advanced Ni-based superalloy, were selected for the corrosion tests. The impact on corrosion of oxy-firing of two fuels is considered in the study: S-lean bituminous coal from Indonesia and S-rich bituminous coal from Venezuela. Fireside corrosion tests were performed by two laboratories using varying methodologies: exposures in a combustion test rig using cooled corrosion probes followed by laboratory exposures and isothermal laboratory exposures only. The tests result in a similar ranking of the alloys' corrosion resistance, but also revealed some differences in the corrosion products formed. © 2013 Published by Elsevier Ltd.
This work encompasses evaluation of deposit and material related limitations on boiler performance and lifetime of CO2-free, oxyfuel operated power plants. The aim of the study was to determine the reliability of advanced and conventional boiler materials. Six alloys, varying from state-of-the-art ferritic-martensitic and austenitic Fe-based steels to an advanced Ni-based superalloy, were selected for the corrosion tests. Impact of oxy-firing of two fuels is considered in the study: S-lean bituminous coal from Indonesia and S-rich bituminous coal from Venezuela. Fireside corrosion tests were performed by two laboratories using varying methodologies: exposures in a combustion test rig using cooled corrosion probes followed by laboratory exposures and isothermal laboratory exposures only. The tests result in a similar ranking of the alloys' corrosion resistance, but also revealed some differences in the formed corrosion products. © 2013 Published Elsevier Ltd.
With an expanding use of low quality bio fuels, corrosion problems on water wall tubes are increasing. In this study, the possible corrosion reducing effect when adding digested sewage sludge to the fuel in a used wood (also known as waste or recycled wood) fired furnace has been evaluated. The low alloyed steel 16Mo3 and the stainless steel 310S were exposed for 14.25 h at the furnace wall position when firing only used wood and used wood with sewage sludge additions. The exposures were performed in a bubbling fluidized bed boiler and the metal temperature of the test samples was controlled to 350 °C. Chemical analysis of the deposits and microscopic evaluation of the metallic samples showed reduced amount of alkali metals and chlorine in the deposit together with reduced initial corrosion for both materials when co-firing with digested sewage sludge. In the corrosion process, metal chlorides were formed for both materials when firing only used wood, iron chlorides for the low alloyed steel, and chromium chlorides for the stainless steel. When co-firing with sewage sludge, this behavior was suppressed.
High amounts of lead in waste/recycled wood fuel are known to be a contributing factor to the increased corrosion often related to this type of fuel. In combination with potassium, usually present in the fuel, low-melting point salt mixtures between lead chloride (PbCl 2) and potassium chloride (KCl) are expected to form. The purpose of this study is to investigate reactions in the mixed salt of PbCl 2 and KCl and its interactions with carbon steel P265GH and its oxide. Laboratory exposures were performed in an isothermal tube furnace with a salt mixture of PbCl 2/KCl (50/50 mol%) put on steel samples. The test duration was 24 hr at either 300°C or 340°C in an atmosphere of 100 ppm HCl and 20 vol% H 2O in synthetic air. After exposure, the salt mixture consists of distinct areas of KCl and PbCl 2 but also the compounds K 2PbCl 4 and KPb 2Cl 5. A general observation is that the oxide thickness increases with temperature and that areas with Pb/K-mixed salt are frequently found in close connection to more corroded areas. Often the more lead-rich phase KPb 2Cl 5 is located closest to the corrosion product indicating its importance for the corrosion.
Corrosion of furnace wall tubes is a problem often caused by the use of corrosive fuels. The relatively high contents of lead, zinc, alkali metals and chlorides in these fuels are believed to contribute to the corrosion. Initial corrosion as a function of lead content in a wood-based fuel was studied for three materials: 16Mo3, 304L and Alloy 625. The materials were exposed for 8 h in a laboratory combustion test rig at a position resembling furnace wall conditions. The metal temperatures investigated were 350 and 400 °C. Increasing the lead content in the fuel or the temperature accelerated the corrosion rate of 16Mo3. It is proposed that lead and lead oxides in deposits react with iron chloride to form lead chloride, which when combined with alkali chlorides results in a very corrosive deposit containing low melting salt mixtures. Negligible corrosion was observed for 304L and Alloy 625.
Gas turbines operating under fuel-rich conditions may suffer from material degradation and metal dusting.To evaluate this cyclic exposures have been done at 700 °C during 5000 h in two synthetic environments having a carbon activity of 0.26 and above unity. It was found that the common stainless steel 304L is incapable of withstanding either of the environments, while the stainless steel 253MA performs well because a protective silica layer is formed. The ferritic alumina formers Kanthal APM and Kanthal APMT perform well, together with several commercial chromia forming Ni-base alloys. As a general trend the material degradation is slower in the environment with the higher carbon activity, but pre-oxidised samples of chromia forming alloys did experience sudden and rapid carburisation after scale failure. Also a TBC system failed earlier in this environment, because graphite formation at the top coat/bond coat interface caused spalling of the top coat. Further the MCrAlY bond coat cracked and caused carburisation of the underlying Ni-based substrate. A silicon modified aluminide coating showed good degradation resistance, but stimulated excessive carbon deposition in the environment of high carbon activity. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.