Low-temperature corrosion in co-combustion of biomass and solid recovered fuels

Lowering flue gas temperatures to recover more energy is of great interest. One of the limiting factors in lowering the flue gas temperature is low-temperature corrosion. In this work the causes of low-temperature corrosion was studied in a full-scale bubbling fluidized bed boiler combusting mixtures of bark, sludge, and solid recovered fuel. The presence of sulfuric acid in the flue gases may lead to severe low-temperature corrosion if the temperature of the pre-heaters and the flue gas duct parts are below the acid dew point. Measurements of H2SO4(g) were performed in the second pass and stack with a technique where SO3 or H2SO4 is captured in a KCl salt plug located in the tip of a sampling probe. Additionally, short-and long-term corrosion probe measurements were conducted to study the corrosion rate of carbon steel at a material temperature of 100 degrees C, under different operational conditions of the boiler. The windward, leeward, and side deposits on the corrosion rings were collected and analyzed by means of scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray fluorescence spectroscopy (XRF). The work revealed that the risk of acid dew point corrosion was minimal; the H2SO4(g) concentration was below 0.3 ppmv, both in the second pass and stack. This implies that any SO3 formed in the furnace was captured by ash constituents such as calcium and alkali. Interestingly, the deposits on the corrosion rings showed high calcium and chlorine contents, suggesting the presence of calcium chloride (CaCl2). Calcium chloride is an extremely hygroscopic salt and absorbs water from the flue gas to form a wet deposit at a significantly higher temperature than the pure water dew point. Corrosion was observed in the cases when the flue gas water vapor concentration was high and could be explained by water uptake by the hygroscopic deposit and subsequent wet corrosion.