Fireside Corrosion of Heat Exchanger Materials for Advanced Solid Fuel Fired Power Plants

Abstract

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.

Fireside corrosion and deposition on heat exchangers in biomass combustion systems

“…These differences in chemical composition will alter the chemistries of the respective flue gases, and therefore the chemistry and quantity of any species condensing onto the heatexchanger surfaces as deposits [10]. The fireside corrosion of power plant heat exchangers is known to be very dependent on the combustion environment chemistry, as already shown [13,14], with the corrosion damage exhibiting a well-known 'bell shape' distribution as function of metal surface temperature [13,14]. Another aspect when considering the extent of fireside corrosion is the amount of Cl present in the combustion gas environment.…”

“…Another aspect when considering the extent of fireside corrosion is the amount of Cl present in the combustion gas environment. It was seen that the Cl content could have a significant impact on the extent of degradation on heat-exchanger materials [14]. This is due to the ability of Cl to react via an ion exchange mechanism [15] that enhances corrosion damage.…”