Field test corrosion experiences when co-firing straw and coal: 10 year status within Elsam

In Denmark, straw is utilised for the generation of energy and district heating in power plants. Combustion of straw gives rise to high contents of potassium chloride and some sulphur dioxide in the flue gas. These compounds can lead to deposits with high content of potassium chloride and potassium sulphate on superheater tubes resulting in increased corrosion rates.

From field experimental results this paper show, that by co-firing straw with coal, corrosion rates can be brought down to an acceptable level. This paper firstly deals with the results from a demonstration program co-firing coal and straw at the 150 MW pulverized coal fired boiler Studstrup unit 1.

Two exposure series lasting 3000 hours each were performed for co-firing 10 and 20% of straw (% energy basis) with coal. Using built in test tubes in the hot end of the actual superheaters and air/water cooled corrosion probes, the corrosion during these experiments was monitored. Various ferritic and austenitic materials were investigated at steam temperatures ranging from 520 to 580 degrees C and flue gas temperatures ranging from 925 to 1100 degrees C.

The results obtained in the demonstration program led to the rebuilding of the 350 MW pulverized coal fired boiler, Studstrup unit 4, into a co-firing boiler with straw in 2002. During the rebuilding, test tube sections of X20CrMoV12 1 and TP347H FG were built into the superheater and the reheater loops.

The temperature ranges during these exposures was for the steam from 470 to 575 degrees C and for the flue gas from 1025 to 1300 degrees C. All these test tubes have been removed during the last three years at one year intervals for corrosion studies. The corrosion studies performed on all investigated tubes included measurements of the corrosion attack, light optical microscopy and scanning electron microscopy of the corrosion products.