In Denmark, straw is used for generating energy in power plants. However during straw combustion, potassium chloride and SO2 are released in the flue gas and through condensation and deposition processes they will result in the formation of superheater deposits rich in potassium chloride and potassium sulphate. These components give rise to varying degrees of accelerated corrosion. This paper concerns co-firing of straw with coal to reduce the corrosion rate from straw to an acceptable level. A field investigation at Midtkraft Studstrup suspension-fired power plant in Denmark has been undertaken where coal has been co-fired with 10% straw and 20% straw (% energy basis) for up to approx. 3000 hours. Two types of exposure were undertaken to investigate corrosion: a) the exposure of metal rings on water/air cooled probes, and b) the exposure of a range of materials built into the existing superheaters. A range of austenitic and ferritic steels was exposed in the steam temperature range of 520-580°C. The flue gas temperature ranged from 925-1100°C. The rate of corrosion was assessed by precision measurement of material loss and measurement of oxide thickness. Corrosion rates are lower than for 100% straw-firing. The corrosion products and course of corrosion for the various steel types were investigated using light optical and scanning electron microscopy. Catastrophic corrosion due to potassium chloride was not observed. Instead a more modest corrosion rate due to potassium sulphate rich deposits was observed. Corrosion mechanisms include sulphidation, oxidation and hot corrosion.
|Journal||Materials and Corrosion|
|Publication status||Published - 2002|