Corrosion in thermal waste treatment plants
Corrosion in the area of the heat exchangers of boilers, which are fired with waste or substitute fuel, not only increases servicing and maintenance costs but also the plant inspection and stoppage times. They therefore cause high operating and economic costs.
Dust deposited on the superheater tubes results in scale deposits, which are related to the cause of corrosion. The key reaction for the corrosion is the sulphating of particles in the scale deposits that contain chloride. The corrosion rate, among other things, depends on the arrival of new dust particles. Knowledge about the particle size distribution and composition of the particles is important in order to understand the process.
Developement of flue gas diluting sampling system
In cooperation with the Helmholtz centre in Munich (Helmholtz Zentrum München), bifa developed a flue gas diluting sampling system for largely unadulterated particle sampling in the boiler and used it in three incineration plants. There was no identifiable, significant difference in dust concentration and gas phase between the highly and less corroding plants.
The particle size distribution in all plants shows two clear maxima at approx. 0.5 μm and at > 20 μm. While the fine particles, which have a low probability of impinging on the tube, mainly consist of alkali chlorides, the chlorine content of the coarse dust is low. Particles > 20 μm have a high impact probability. But pure ash particles have poor adhesion properties; they therefore hardly contribute to the build-up of the scale deposit.
Apart from ash particles, highly adherent, salt-rich agglomerates of finer particles can be detected in the coarse dust, which were far more prevalent in the plant with high corrosion rates. Evaluation of the design and operating data with the corrosion rates showed that short residence times within the temperature range 850—700 °C (condensation of gaseous alkali chloride) can have a corrosion-promoting effect. The large difference in corrosion rates in the various plants (factor > 5) cannot be linked to a single parameter; instead the interaction of several factors appears to be the cause.
The current model of the scale formation and the superheater corrosion as the knowledge base for process-integrated reduction of the corrosion was confirmed or rather extended. The results show that in existing plants a reduction in corrosion rates will more likely be achieved by modification of the plant than by changing the operating regime. The effect of the residence time in the temperature window between 850 °C and 700 °C can be taken into account directly in new build plants.
Apart from the very cost-intensive material solutions to the corrosion problems of superheaters of waste and substitute fuel incineration plants, the knowledge acquired can be used to develop (primary) measures to reduce superheater corrosion in new build plants and in the modification of existing plants. Successful implementation helps to improve the economic efficiency of thermal waste treatment plants.