The role of potassium in the corrosion of superheater materials in boilers firing biomass

Over the last few decades, there has been intense debate about the role played by anthropogenic drivers in global warming and the ensuing climate change. Carbon dioxide (CO₂) is considered to be the most important anthropogenic greenhouse gas (GHG), originating primarily from the use of fossil fuels, mainly coal and oil. Inhibition or at least deceleration of global warming has become one of the major goals for the coming decades. A key strategy will be to replace fossil fuels with more sustainable fuels, which has generated growing interest in the use of waste-derived fuels andof renewable fuels such as biomass.From the environmental point of view, the main advantage of biomass is that it can be considered a CO₂ neutral fuel. In addition, large and reasonably stable supplies of biomass can be found in most countries, although the transportation of these fuels is seldom cost-efficient. However, from the chemical point of view, biomass is an inhomogeneous fuel, usually with a high concentration of water and containing considerable amounts of potassium and chlorine, all of which are known to affect the durability of superheater tubes. To slow down or reduce corrosion, power plants using biomass as fuel have been forced to operate at lower steam temperatures, which reduces power production efficiency: every 10°C rise in the steam temperature results in an approximate increase of 2% in power production efficiency. More efficient ways to prevent corrosion are needed so that power plants using biomass and waste-derived fuels can operate at higher steam temperatures.The aim of this work was to shed more light on the alkali-induced corrosion of superheater steels at elevated temperatures, focusing on potassium chloride, the alkali salt most frequently encountered in biomass combustion, and on potassium carbonate, another potassium salt occasionally found in fly ash. The mechanisms of the reactions between various corrosive compounds and steels were investigated. The tests were performed in the laboratory using different types of furnaces and analytical equipment (DTA-TGA, SEM-EDX). To simplify the evaluation of the reactions, single salts rather than complex authentic mixtures were used to mimic deposits, and the reaction mechanisms of these salts with pure chromium were studied using thermogravimetry. Test coupons of commercial steels were exposed in a tube furnace to assess the actual corrosion.Potassium chloride (KCl) and potassium carbonate (K₂CO₃) were placed on a 10CrMo-type ferritic steel, a 304L-type austenitic stainless steel, and an Alloy 625-type nickel-based alloy, and a comparison showed that the presence of potassium is sufficient to initiate accelerated oxidation. Underneath each of the salts, the protective oxide layer of the steel was clearly damaged, and the steel started to oxidize in an uncontrolled manner. However, the thicknesses and compositions of the oxide layers formed depend not only on the temperature and atmosphere, but also on the potassium compound.The presence of water vapor in the furnace had a clear effect on the oxide layer thickness in the temperature range 500°-600°C. With no salt added, 304L and Alloy 625 withstood oxidation well under wet conditions. In the case of 10CrMo, the presence of water vapor resulted in accelerated oxidation. When dry and wet conditions were compared, it was observed that, if KCl was placed on the steel, thinner oxide layers were formed under wet conditions. The effect of the presence of water vapor on the oxide layer thickness is less clear in the case of K₂CO₃ than in that of KCl. This might be because the reaction between K₂CO₃ and water vapor cannot produce any reasonable volatile compounds.Based on the results, the potassium-induced accelerated oxidation of chromia protected steels appears to occur in two consecutive stages. In the first, the protective chromium oxide layer is destroyed through a reaction with potassium, from either potassium chloride or potassium carbonate, leading to the formation of potassium chromate (K₂CrO₄) and/or potassium dichromate (K₂Cr₂O₇) and depleting the chromium in the protective oxide layer. As the chromium is depleted, chromium from the bulk steel diffuses into the oxide layer to replenish it. In this stage, the ability of the material to withstand corrosion depends on the chromium content (which affects how long it takes the chromium in the oxide layer to be depleted) and on external factors such as temperature (which affects how fast the chromium diffuses into the protective oxide from the bulk steel). For accelerated oxidation to continue, the presence of chloride appears to be essential. However, accelerated oxidation did not occur with all the chlorides studied, so although the presence of chlorine as a chloride seems to be required for this reaction, it is not in itself sufficient to initiate it. While chlorine gas is known to cause accelerated corrosion, the reaction seems to proceed via another mechanism in the case of solid chlorides. The reactivity and properties of the cation play a role. Nonetheless, it could not be confirmed that the ability of the cation to form chromates is fundamentally responsible for the initiation of accelerated oxidation. Another possibility could be the manner in which the cation releases chlorine, which could enable the formation of chlorinecontaining intermediates, such as metal chlorides.Results of the work can be used to develop solutions for the superheater corrosion either by controlling the chemical behavior of the alkalis or by finding better superheater materials.