In Denmark, biomass such as straw or woodchip is utilized as a fuel for generating energy. Straw is a "carbon dioxide neutral fuel" and therefore does not contribute to the greenhouse effect. When straw is combusted, potassium chloride and potassium sulfate are present in ash products, which condense on superheater components. This gives rise to specific corrosion problems not previously encountered in coalfired power plants. The type of corrosion attack can be directly ascribed to the composition of the deposit and the metal surface temperature. A test superheater was built into the straw-fired Masnedo combined heat and power (CHP) plant to investigate corrosion at temperatures higher than that of the actual plant. The highest steam temperature investigated was 570 deg C. Various alloys of 12-22 percent chromium content were welded into this test loop. Their corrosion rates were similar and increased with temperature. The mechanism of attack was grain boundary attack as a precursor to selective chromium depletion of the alloy. In addition welds coupling various tubes sections were also investigated. It was seen that there was preferential attack around those welds that had a high nickel content. The welds which had a similar composition to the tubes did not incur this type of corrosion. It is suggested that high temperature galvanic corrosion occurs due to the formation of molten chloride mixtures which serve as the electrolyte.
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