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

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In biomass fired power plants, deposition of alkali chlorides on superheaters, as well as the presence of corrosive flue gas species, give rise to fast corrosion of superheaters. In order to understand the corrosion mechanism under this complex condition, the influence of the flue gas composition on high temperature corrosion of an austenitic superheater material under laboratory conditions mimicking biomass firing is investigated in this work. Exposures involving deposit (KCl)‐coated and deposit‐free austenitic stainless steel (TP 347H FG) samples were conducted isothermally at 560 °C for 72 h, under both oxidizing and oxidizing‐chlorinating atmospheres, and the resulting corrosion products were comprehensively studied with scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS), and X‐ray diffraction (XRD) techniques. The results show that deposit‐free samples suffer grain boundary attack only in an oxidizing‐chlorinating atmosphere, otherwise corrosion results in formation of a duplex oxide. Corrosion attack on deposit‐coated samples was higher than on deposit‐free samples irrespective of the gaseous atmosphere. Specifically, severe volatilization of alloying elements occurred on deposit‐coated samples under oxidizing‐chlorinating atmosphere due to enhanced impact of KCl and HCl.

Section: Introductionmentioning

confidence: 99%

Effect of flue gas composition on deposit induced high temperature corrosion under laboratory conditions mimicking biomass firing. Part I: Exposures in oxidizing and chlorinating atmospheres

Okoro

Kiamehr

Montgomery

et al. 2016

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“…Previous work at Studstrup [5] showed that at 20% straw co-firing at higher metal (600 8C) and gas temperatures, formation of large pits were observed for high alloy steels. This indicates a more corrosive environment in some localised regions.…”

Section: Hot Corrosion Mechanismsmentioning

confidence: 99%

“…Thus, co-firing was conducted in a pilot plant at Studstrup [5]. Various material types were exposed for 3000 h to 10-20% (energy basis) straw þ coal.…”

Section: Introductionmentioning

confidence: 99%

Potential high temperature corrosion problems due to co‐firing of biomass and fossil fuels

Montgomery

Vilhelmsen

Jensen

2008

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Over the past few years, considerable high temperature corrosion problems have been encountered when firing biomass in power plants due to the high content of potassium chloride in the deposits. Therefore, to combat chloride corrosion problems cofiring of biomass with a fossil fuel has been undertaken. This results in potassium chloride being converted to potassium sulphate in the combustion chamber and it is sulphate rich deposits that are deposited on the vulnerable metallic surfaces such as high temperature superheaters. Although this removes the problem of chloride corrosion, other corrosion mechanisms appear such as sulphidation and hot corrosion due to sulphate deposits. At Studstrup power plant Unit 4, based on trials with exposure times of 3000 h using 0-20% straw co-firing with coal, the plant now runs with a fuel mix of 10% straw þ coal. Based on results from a 3 years exposure in this environment, the internal sulphidation is much more significant than that revealed in the demonstration project. Avedøre 2 main boiler is fuelled with wood pellets þ heavy fuel oil þ gas. Some reaction products resulting from the presence of vanadium compounds in the heavy oil were detected, i.e. iron vanadates. However, the most significant corrosion attack was sulphidation attack at the grain boundaries of 18-8 steel after 3 years exposure. The corrosion mechanisms and corrosion rates are compared with biomass firing and coal firing. Potential corrosion problems due to co-firing biomass and fossil fuels are discussed.

“…Many field investigations have been undertaken into corrosion due to straw combustion in Denmark over the past 15 years, and details can be found in these publications [2][3][4][5][6][7][8][9][10][11][12][13]. This paper will describe more recent results from Maribo Sakskøbing (MSK) and Avedøre 2 bioboiler (AVV2 bio) and compare them to results from previous investigations.…”

Section: Introductionmentioning

confidence: 99%

Experiences with high temperature corrosion at straw‐firing power plants in Denmark

Montgomery¹,

Jensen²,

Borg³

et al. 2011

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By the end of 2009, there will be eight biomass and five biomass co‐firing plants in Denmark. Due to the steep increase of corrosion rate with respect to temperature in biomass plants, it is not viable to have similar steam data as fossil fuel plants. Thus for the newer plants, Maribo Sakskøbing, Avedøre 2 biomass boiler, Fyn 8 and Amager 1 (Fyn 8 and Amager 1 are under commissioning), the steam temperature of the final superheaters are approximately 540 °C and the steel type used is an 18–10 stainless steel, (TP347H). However there is still a need to monitor corrosion rates, and to collate data to enable better lifetime prediction of vulnerable components in straw‐firing plants since the corrosion rates are so much faster than in coal firing plants. Therefore, there are continued investigations in recently commissioned plants with test tubes installed into actual superheaters. In addition temperature is measured on the specific tube loops where there are test tube sections. Thus a corrosion rate can be coupled to a temperature histogram. This is important since although a superheater has a defined steam outlet temperature, there is variation in the tube bundle due to variations of heat flux from the flue gas. This paper will describe the corrosion investigations for tube sections removed from Maribo Sakskøbing and Avedøre 2 biomass boiler which have been exposed for up to 30 000 h. In addition to monitoring the corrosion rates of actual components, there is a need to measure corrosion rates at higher temperatures to assess if there is a possibility to increase the outlet temperature of the plant, thus making the plant more cost effective. For this purpose Avedøre 2 biomass boiler has a test superheater loop fabricated in TP347H FG (the same material as the final superheaters). Some results from this test superheater will also be described. Effects of flue gas temperature and flue gas direction on corrosion rates are also discussed.