Boiler material optimisation with corrosion probe measurements

Combustion of different biofuels and waste generates highly corrosive gases and heavy metals chlorides as well as ashes containing alkaline chlorides and sulphates. Using such fuels in power and CHP boilers can pose a challenge for materials performance, and the pressure to maintain high efficiency increases the challenge further. Heat exchanger tubes may experience substantial thinning leading to premature failure as a result of fireside corrosion during the operation. Such failures cause unpredicted overhauls and availability losses. The change of tube materials in the most critical components is one option to extend the lifetime of components. This paper summarises the results of exposure tests carried out in a CFB boiler using various wood, paper and plastic-based recycled trade and industry fuels suitable for energy use together with peat. The aim of the study was to find alternative tube material for superheater, in location where the current type 347 austenitic stainless steel tube material has experienced fireside corrosion up to 2 mm/a. The exposure test was carried out with air cooled probe exposed in the boiler for a time up to 1400 hours at nominal material temperature of 550 °C. Materials selected for the study were austenitic stainless steels types TP347H (UNS 34709), HR3C (TP310HCbN) and Sanicro 28 (UNS N08028). Following exposure, a section was taken from each specimen and examined using light microscopy and scanning electron microscopy. The results showed chlorine and sulphur species penetrating through grain boundaries into alloy structures. The higher alloying provides better corrosion performance, however all alloys suffered corrosion attack.