Coating solutions against high temperature corrosion - performance validation and feasibility at biomass fired boilers

To overcome severe corrosion problems of heat exchanger surfaces, especially superheaters, in biomass fired boilers protective thermal spray coatings can be applied. Fouling and corrosion in biomass boilers originate from used fuels, which contain alkali metals, chlorine and other corrosive elements, and can lead to tube failure and leakage in the worst case. Different HVOF (high velocity oxy-fuel) and arc sprayed coatings were exposed to biomass co-combustion conditions in a CFB boiler using peat, coal, SRF and biomass as fuel. VTT is conducting a programme of material exposure trials at the Alholmens Kraft power plant in the city of Pietarsaari. The exposure campaign involves the deployment of one corrosion probe in a 550 MWth circulating fluidised bed boiler. Duration of the measurement campaign was about 1300 hours. The corrosion probe measurement was performed for samples with five coatings: NiCr, IN625, Diamalloy 4006, SHS9172 and NiCrTi. Samples were exposed at two metal temperature ranges of 550 and 750°C, which correspond to both todays and future steam temperatures of a high-efficiency boiler. Used reference tube materials were ferritic steel T92 and nickel super alloy A263. Performance validation of the coatings was performed as well as an indicative analysis of the effects of fuels, deposit properties and boiler process parameters on corrosion phenomena. Preliminary economic feasibility of the use of coatings to extend lifetime of a superheater was included. Overall at 550°C the coatings had negligible corrosion and at the higher temperature clear differences emerged: NiCr and NiCrTi being the top performers.