Corrosion studies in biogas waste combustion
One postulation is that the enormous temperatures, pressures and alkali chlorine content of gases cause the accelerated corrosion in metallic tubes by reacting with metallic oxides. It was thought that these elements react in the presence of oxygen to form complex alkali-transition metal oxides, such as alkali ferrites, and chlorine. To verify the theory, a laboratory study put corrosive scenarios through their paces. Thermodynamic analysis performed on the "active oxidation" theory showed that corrosive activity may not occur. The presence of alkali ferrites (NaFeO2, KFeO2) therefore did not explain or prove they were responsible for corrosion. However, the presence of complex mixture phases of alkali, alkali oxides and iron oxides was thought to play a more important role in accelerated corrosion. Experimentally derived corrosive products on low allow steels could be cross-examined using available phase diagrams. It was shown that the alkali metal activity together with the presence of chloride in the super-heater could be the responsible agent of corrosion. So to, in the super-heated areas, calcium may play an important role in the active oxidation of heat transfer surfaces especially when water vapour content is high and sulphur dioxides are low. Having identified these critical elements of accelerated corrosion, it may now be possible to develop methods by which they can be effectively addressed. In doing so, they can positively affect their marketability, by enhancing performance values, and as well as providing longevity of reactor life.
