51315-5550-Utilization of On-line Corrosion Monitoring in the Flue Gas Cleaning System

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Product Number: 51315-5550-SG

ISBN: 5550 2015 CP

Author: Melanie Montgomery

Publication Date: 2015

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Utilisation of online corrosion monitoring in the flue gas cleaning systemMelanie Montgomery (COWI A/S/ DTU Mekanik) Lars V. Nielsen (Metricorr) Michael B. Petersen (Metricorr)Amager unit 1 is a 350 MW multifuel suspension-fired plant that was commissioned in 2009 to fire with biomass (straw and wood pellets). Increasing corrosion problems in the flue gas cleaning system have been observed since 2011 in both the gas-gas preheater (GAFO) and the booster fan. The GAFO constructed in CORTEN steel has corroded by 2 mm in 2 years and due to the substantial oxide growth the 316 bolts holding the sealing strips have suffered from stress corrosion cracking. In addition pitting corrosion of the booster fan and flue gas duct has also been apparent. A root cause analysis concluded that corrosion occurs due to corrosion products/deposit formed during operation however it was unclear when corrosion actually occurs. In all cases the chlorine content in the flue gas results in the presence of chloride containing salts such as KCl or chlorine containing corrosion products FeOCl2/FeCl3. There are two probable corrosion mechanisms:a) Dewpoint corrosion at operation temperature of approx 60-85C where the presence of hygroscopic salts and corrosion products has resulted in corrosion.b) Downtime corrosion caused when the dewpoint of hydrochloric acid and condensation of water occurs when the plant temperature is reduced. Hygroscopic salts will also make this effect worse.Since these two mechanisms are a type of dewpoint corrosion it is difficult to differentiate between the extent to which they contribute to corrosion Without knowing this it is difficult to take reasonable measures to reduce corrosion. In order to gain an improved understanding of the corrosion problem an on-line corrosion measurement system provided by Metricorr was established before the booster fan. This system uses resistance measurements of an exposed probe compared to a reference probe and thereby measures the actual decrease of thickness of a corroding element. The corrosion rates measured from this sensor with respect to time were correlated to on-line plant data such as load temperature gas composition water content and soot blowing. From these results it is clear that many shutdown/start-ups influence corrosion and therefore result in decreased lifetime of components and increased maintenance. More data is being collected to assess what would be the optimal way to shut-down/start-up the flue gas cleaning system to minimise the burden of corrosion.