Reducing superheater corrosion in wood-fired boilers

Reducing superheater corrosion in wood-fired boilers 

P J HENDERSON, C ANDERSSON, H KASSMAN and J HOGBERG, Vattenfall Research and Development, Sweden and P SZAKALOS and R PETTERSSON, Corrosion and Metals Research Institute, Sweden 

In the last few years, there has been a move away from burning fossil fuels through the co-utilisation of biomass and coal and finally to 100% biomass such as wood and waste wood products. Unfortunately, burning of biomass causes widespread fouling of superheater tubes and corrosion can occur rapidly under the sticky alkali chloride deposits. Even at today s maximum steam temperatures of 500 to 540°C there are some severe corrosion problems when burning 100% wood-based fuel [1]. It is also desirable to be able to burn other environmental fuels such as straw, demolition wood or other waste wood products, to reduce production costs and avoid dumping waste at landfill sites. This, however, makes the corrosion and fouling problems even more serious [2, 3]. 

A complete set of superheaters for a 100 MW combined heat and power boiler costs in excess of lm. The durability of superheaters is thus an important factor in determining the long-term production costs. Unplanned outages due to leaking superheaters are also very expensive. As well as causing corrosion problems, the build-up of deposits reduces the heat uptake to the superheaters which leads to lower efficiency. Consequently, ways are being sought to reduce superheater corrosion. 

This study reports on measures taken to reduce superheater corrosion in two fluidised bed boilers burning wood-based fuels, using the ChlorOut additive to control the KCl levels and more corrosion-resistant steels. 

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Reducing superheater corrosion in wood-fired boilers 429... 

Superheater probe testing showed that ammonium sulphate reduced the deposit growth rate and halved the corrosion rate of ferritic/martensitic steels in a wood-fired boiler. Under normal conditions (without sulphate) iron chloride particles were present at the metal/oxide interface. With the addition of the sulphate, iron sulphides were formed within the oxide, which are believed to... 

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