Corrosion in biomass-fired installations

Firing with biomass presents considerably greater corrosion-related challenges than firing with fossil fuels, partly due to the different chemical composition of biomass, in particular its higher content of chlorine and potassium.

Purely in terms of corrosion, biomass- and waste-fired boilers can be divided up into three zones: the fuel feed zone (funnels etc.); the high-temperature zone (combustion chamber, superheater etc.); and the low-temperature zone (convection and economiser section etc.). Each of these zones is susceptible to different corrosion mechanisms, but most corrosion occurs in the high-temperature zone, particularly around the superheaters. These are often made of alloy steel containing chromium, which forms a protective chromium oxide layer on the surface. If chlorine and potassium are present in the flue gas, reactions can occur between the flue gas and the metal surfaces, as a result of which the protective oxide layer is broken down and the surface protection is lost.

In the high-temperature zone, three main corrosion mechanisms are common: chlorine corrosion, alkali corrosion and molten salt corrosion. In the course of their work as a consultant and through studies of damage observed in biomass-fired installations, FORCE Technology has built up a wealth of experience of these and other corrosion mechanisms affecting all boiler areas, particularly in relation to temperature and flue gas flow, fuel type and alloy type.

Using this experience, FORCE Technology is making a targeted effort to improve the predictability of corrosion conditions with the aid of a newly developed corrosion probe that has been used in tests at the Danish Test Centre for Bioenergy to measure corrosion conditions in the burning of wood pellets. A number of other tests are also being developed.