Corrosion Mechanisms - Active Oxidation of Iron by Chlorine Attack
The primary biomass fuels, bark, wood chips, sawdust, and other waste products originating from these contain low concentrations of chlorine and sulphur that can result in severe corrosion if the moisture, temperature, and pressure of the gas are withing specific activation range. The active chemicals attack the boiler tubes and downstream heat exchanger tubes.
The enrichment of biomass combustion chlorine has two primary reasons:
Significant concentrations of alkali chlorides are formed from the biomass fuel and are mixed in the flue gas during the combustion process since Potassium (K), Sodium (Na) and Chlorine (Cl) are relatively volatile elements.
A large portion of these alkali chlorides may result from pollution, especially from "salting" the biofuel in winter, to keep it from freezing during transport or from low levels of salt in the soil. Another source is from the mist drifting off the oceans to the forest and soil.
Salinity in dry lands can occur when the water table is between six to twelve feet from the surface. The salts from the groundwater are raised by capillary action and absorbed by tree roots and other plants.
These alkali chlorides will condense on the tube surfaces and will react with the flue gas under formation of sulphates and the release of chlorine according to these two reactions.
The free chlorine generated from these products in the fuel can now attach to the iron in the tubes. Even stainless steel is not exempt. Any scratches from ash impingements remove the micro thin layer of chromic oxide (Cr2O3) allowing the chlorine to attack the iron below the oxide layer forming (FeCl2). Here are the surprising corrosion mechanisms that are extremely harmful.
This is the reason Chlorine in the fuel, even small amounts are very corrosive. The corrosion begins prior to the formation of hydrochloric acid (HCL). Once the FeCL2 is formed in the presence of iron oxides Fe3O4, the Chlorine combines with iron oxide to form 2Fe3O3 and CL2. At this point, the CL2 again attacks the iron and the “Oxidation Fire” is burning as long as the fuel supplies chlorine and the combustion processes supply the temperature and moisture conditions.
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