Potential high temperature corrosion problems due to co‐firing of biomass and fossil fuels

Abstract

Over the past few years, considerable high temperature corrosion problems have been encountered when firing biomass in power plants due to the high content of potassium chloride in the deposits. Therefore, to combat chloride corrosion problems cofiring of biomass with a fossil fuel has been undertaken. This results in potassium chloride being converted to potassium sulphate in the combustion chamber and it is sulphate rich deposits that are deposited on the vulnerable metallic surfaces such as high temperature superheaters. Although this removes the problem of chloride corrosion, other corrosion mechanisms appear such as sulphidation and hot corrosion due to sulphate deposits. At Studstrup power plant Unit 4, based on trials with exposure times of 3000 h using 0-20% straw co-firing with coal, the plant now runs with a fuel mix of 10% straw þ coal. Based on results from a 3 years exposure in this environment, the internal sulphidation is much more significant than that revealed in the demonstration project. Avedøre 2 main boiler is fuelled with wood pellets þ heavy fuel oil þ gas. Some reaction products resulting from the presence of vanadium compounds in the heavy oil were detected, i.e. iron vanadates. However, the most significant corrosion attack was sulphidation attack at the grain boundaries of 18-8 steel after 3 years exposure. The corrosion mechanisms and corrosion rates are compared with biomass firing and coal firing. Potential corrosion problems due to co-firing biomass and fossil fuels are discussed.

Fireside corrosion of superheaters: Effects of air and oxy-firing of coal and biomass

“…At present power plants burning biomass alone as a fuel have issues with fouling, deposition, corrosion and agglomeration, which restrict the steam temperatures that can be used and so reduce the efficiencies of these power plants. However these issues can be significantly reduced by the co-firing of biomass and coal [4,[9][10][11]22]. Co-firing coal-biomass mixtures in air-or oxy-firing conditions and producing higher temperature steam for electricity generation all support the meeting of CO 2 reduction targets.…”

“…The allowable ratio of coal and biomass in co-firing is still under investigation; however, reports show that up to 20% of selected biomass can be used in co-firing with particular coals without having any severe corrosion and depositions issues [8,10,22].…”

Corrosion in biomass combustion: A materials selection analysis and its interaction with corrosion mechanisms and mitigation strategies

“…In this context, the composition of the fly ash and its corresponding deposit on the surface of the superheater material determine if molten or solid phases will be present at a specific temperature. A survey of melting temperatures of relevant compounds and eutectic mixtures for biomass combustion can be found in [20,69]. The prediction of the melting properties of different fly ashes is an important step toward the proper control of molten salt corrosion of superheater tubes [59].…”

Complementary Methods for the Characterization of Corrosion Products on a Plant-Exposed Superheater Tube

“…Although combustion of biomass provides a 'CO2 neutral' means of energy generation, this fuel gives operational challenges during combustion in power plants [1][2][3]. The high alkali chloride content in biomass causes fast corrosion of superheaters due to the formation of corrosive deposits (rich in KCl) as well as the presence of corrosive species in the flue gas (HCl, SO2, etc.)…”