Fireside corrosion of heat exchanger materials for advanced solid fuel fired power plants

To address the challenge of climate change, future energy systems need to have reduced greenhouse gas emissions and increased efficiencies. For solid fuel fired combustion plants, one route towards achieving this is to increase the system’s steam temperatures and pressures. Another route is to co-fire renewable fuels (such as biomass) with coals. Fireside corrosion performance of two candidate superheater/reheater alloys has been characterised at higher heat exchanger surface temperature. Samples of the alloys (a stainless steel, Sanicro 25 and a nickel-based alloy, IN740) were exposed in fireside corrosion tests at 650 °C, 700 °C and 750 °C, in controlled atmosphere furnaces using the ‘deposit recoat’ test method to simulate superheater/reheater exposure for 1000 h. After exposure, the samples were analysed using dimensional metrology to determine the extent and distributions of corrosion damage in terms of surface recession and internal damage. At 650 °C, the stainless steel and nickel-based alloy performed similarly, while at 700 °C and above, the median damage to the steel was at least 3 times greater than for the nickel-based alloy. Optical and electronic microscopy studies were used to study samples’ damage morphologies after exposure. Intergranular damage and pits were found in sample cross sections, while chromium depletion was found in areas with internal damage. For high-temperature applications, the higher cost of the nickel-based alloy could be offset by the longer life they would allow in plant with higher operating temperatures.European Union funding RFCS-2015/70995

Fireside performance of different coatings in biomass power plant

The energy sector will need to employ novel strategies to reduce greenhouse gas emissions, such as the increase of steam temperatures/pressures or the use of low carbon fuels (i.e. biomass). Both cause heat exchanger materials’ degradation issues, due to the formation of more/different corrosive deposits, which requires the use of expensive nickel-based materials or coatings. This paper focuses on the behaviour of three different coatings (HVOF NiCrFeSi, laser clad FeCrAl and Laser Clad NiCrFeSi) deposited on TP347HFG, at 700°C (up to 1000 h exposure). Tests were performed using the ‘deposit recoat’ method (KCl deposit) in simulated biomass combustion environments. Cross-sections were analysed using dimensional metrology, to determine distributions of metal loss and internal damage. Intergranular damage and pits were identified using SEM/EDX. A ‘diffusion cell’ behaviour was observed, which led to depletion of alloying elements from the coating and consequent increase in damage. The results suggested a severe degradation of all coatings.European Union funding: RFCS-2015/70995

High temperature corrosion of HVOF coatings in laboratory-simulated biomass combustion superheater environments

This study examines the fireside corrosion of FeCrAl, NiCr, NiCrAlY and A625 coatings applied by ‘high velocity oxy fuel’(HVOF) and exposed to simulated biomass firing conditions (gas composition CO2, N2, SO2 and HCl). The coatings and a typical base steel alloy (T92) were exposed to simulated conditions at 600 °C for 1000 h in a laboratory scale furnace. Samples were coated with a potassium chloride deposit. Samples were then cold mounted in a low-shrinkage epoxy resin and then cross-sectioned. Corrosion was assessed by dimensional metrology comparing the coating thickness change of the samples. The cross-sections of the ‘worst’ and ‘best’ coatings were examined. Results show that all but one coating (HVOF NiCr) outperformed the T92 alloy. No coating composition or method was conclusively better. Evidence of Cr depletion as well as the formation of a sulphidation layer have been found in the exposed samples with coatings. The formation of a K2SO4 layer has also been observed on all coated specimens.Innovate UK: 10116