Managing corrosion in biomass boilers: Benefits and limitations of coatings

Oxidation and corrosion of materials in highly demanding applications is critical for longterm performance of materials used in power generation. Energy production in biomass fired boilers is increasing rapidly due to the advantages of CO2 neutrality and renewability. Fluidised bed combustors and grate fired boilers have proved the most reliable technologies for burning biomass since its fuel characteristics are variable. The fly ash of biomass is relatively corrosive due to its composition containing high concentrations of chemically active compounds of alkali, sulphur and chlorine, and it also may contain erosive components especially in the case of fluidised bed boilers. Due to the potential for severe chlorine induced corrosion, plant operating temperatures and efficiency must be limited. To overcome material wastage more alloyed materials are needed. The high cost and sometimes hard to manufacture bulk materials could be replaced with coatings, which can be applied on the more economic low alloy steel having the proper mechanical properties. The paper discusses the benefits and limitations of thermal sprayed coatings in biomass boilers. The applied examples have included iron and nickel based HVOF and arc sprayed coatings subjected to verification field testing in boiler testing under in aggressive biofuel conditions. The coatings have shown good corrosion resistance in long-term field tests.