Microscopy of fireside corrosion on superheater materials for oxy-fired pulverised fuel power plants

The current pressures for increased worldwide electricity supplies coupled with reduced environmental emissions, are leading to a revolution in the operating conditions within pulverised fuel fired boilers to improve their generating efficiencies. This paper reports results from a series of 1000 hour 'deposit recoat' laboratory tests that are assessing the effects of increasing heat exchanger surface temperatures (600, 650 and 700 degrees C) on the fireside corrosion resulting from the combustion of a biomass/coal mix using oxy-firing (with hot flue gas recycle before desulfurisation). The results presented focus on two materials: a ferritic steel (T92) and an austenitic stainless steel (TP347HFG), to illustrate the effects of alloy Cr content. After exposure, the samples were examined using scanning electron microscopy/energy dispersive X-ray analysis to evaluate: surface morphological changes (formation of nodules and whiskers) on bare samples; microstructures of scaleydeposit/metal in cross-sections; and, elemental distributions. The performances of the samples were determined using dimensional metrology: pre-exposure contact metrology and post-exposure optical microscopy image analysis measurements. The metal loss data generated is being used to develop statistical models to predict the lifetimes of candidate materials for use in superheaters/reheaters in advanced power plants.