High temperature corrosion in waste-to-energy plants.

High temperature corrosion of the heat exchanger materials is the important factor that limits the efficiency of various energy systems. The problem becomes more serious when fuels containing alkali metal, heavy metals, chlorine, and sulphur are used. In combustion systems utilizing biomass and municipal solid waste, the steam temperatures are kept lower than 450°C in order to avoid the corrosion problems. This results in low overall plant efficiencies (e.g. 25 - 30%). Therefore, methods to prevent or control high temperature corrosion in these plants must be investigated. The overall objectives of this PhD research study were to investigate (i) the factors affecting the high temperature corrosion in waste to energy plants and, (ii) to explore and test corrosion control methods in the real furnace conditions. The experimental program was carried out at a UK waste to energy plant. Two air-cooled sampling probes were designed and placed at different locations in the furnace in order to simulate the corrosion process taking place on the superheater tubes and also to collect the deposits of combustion residues. Sampling probe sections were fabricated from different types of superalloys and were equipped with two corrosion control methods; sacrificial baffle and aluminide coated alloy. After approximately 800 hours of exposure to hot flue gas having temperature range of 730 - 813°C, each probe was carefully disassembled and analysed. Our study showed that both ‘temperature’ and ‘particle deposition’ had great effects on the high temperature corrosion inside this plant. Damages due to hot corrosion were significantly magnified when the metal surface temperature range (modelling results) increased from 363 - 440°C to 404 - 495°C. Tests showed that sulphates and chlorides of alkali metals (namely calcium, sodium, and potassium) and heavy metals (namely zinc, lead, and arsenic) were the main contributors to the hot corrosion. In this particular environment, tubes made of nickel based alloys were found to have higher corrosion resistance than iron based alloys. Test showed that l aluminide coating on the tube surfaces could significantly improve their corrosion resistance. In addition, mathematical modelling using FLUENT code was carried out in order to simulate the flow characteristics and heat transfer inside the furnace and the region around the air cooled sampling probes. Results from the modelling corresponded with the plant information and explained the experimental results very well. This PhD study has yielded valuable information that can be used by the operators of waste-to-energy plants. Our study showed that ‘aluminide coating’ is a promising corrosion control technique for superheater materials in the waste-to-energy plants. The coating is relatively cheap and simple but it can significantly increase the corrosion resistance of materials.