Experimental and numerical characterisation of materials for biomass co-firing
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As Europe continues to decarbonize its electricity generation capacity, operators of thermal power plants face challenges to which they must adapt including increasingly transient operation and co-firing with biomass. This research focuses on peat-fired power plants and the response of superheater tube materials to the altered deposit composition formed during peat-biomass co-firing. Both experimental and multi-physics modelling studies on the effects of biomass deposit compositions on boiler tube material properties are presented. The addition of biomass to peat has been shown to result in increased levels of corrosion on heat exchanger tubes in plants. An experimental methodology has been developed at NUI Galway for representative corrosion testing of power plant materials for different levels of biomass-peat mixtures. Synthetic salts, representative of deposits formed during biomass co-firing are produced and their compositions are compared to compositions of deposits obtained from operational plants. These salts are applied to candidate materials and exposed to temperatures in the range of 540 °C to 600 °C for up to 1 month. The corrosion layer of each sample is then measured using scanning electron microscopy (SEM) and optical microscopy. Energy dispersive X-ray spectroscopy (EDX) elemental analysis is also carried out; this provides information regarding chemical composition at different depths. A novel physically based corrosion model (PCM) to describe the accelerated corrosion process active oxidation which occurs beneath alkali-halide-containing deposits, which form on heat exchanger tubes during biomass firing was developed. This model uses measurements of porosity and pore radius, coupled with a physically based corrosion mechanism, to predict corrosion rates. Results from this model are validated with experimental results and published models that make use of a fitting factor. A finite-element (FE) methodology which combines corrosion effects with creep damage in pressurised tubes is presented. Experiments are carried out to obtain a corrosion rate for P91 steel. This corrosion rate is then used to simulate corrosion in the FE model via adaptive meshing. This is combined with creep damage models to investigate the effect of corrosion tube wall loss and creep damage on tube stresses and creep rupture life. Experimental results allowed initial characterisation of the complex, multi-layered nature of oxides formed during the active oxidation of P91. Computational results quantify the detrimental effect of uniform corrosion on creep rupture life for a range of internal steam pressures. These results will be of significant interest to plant operators who are concerned with the impact of corrosion on creep life of plant components. An in-depth experimental microstructural characterisation of P91 and 347SS following exposure to salts representative of deposits formed during biomass co-firing is presented. Samples have been etched allowing for a comprehensive evaluation of the microstructural degradation of samples. Several key processes have been identified which would contribute to a reduced life expectancy of tubes. The detrimental effect of impurities in materials, such as inclusions, on the corrosion process is identified. Pitting corrosion is found to initiate at sites of near-surface inclusions and its mechanism is discussed. Measurements of pits taken from in-situ tubes are compared with experimental measurements and the results are found to agree. Localised intergranular corrosion was detected at numerous locations across the samples. Results indicate that the corrosion mechanism and rate at which it occurs depend on the Cl and K content of the salts applied to the samples. The surface topography of samples has been analysed to determine the effect of corrosion on surface finish. Results from testing on P91 and 347SS are compared to determine the material with a greater corrosion resistance. 347SS is found to offer significantly higher corrosion resistance than P91. This is primarily attributed to the higher Cr content in 347SS, which results in more protective oxide scales forming closer to the substrate surface. FE models have been developed which investigate the effect of differential thermal expansion coefficients between material impurities, such as inclusions and base metal. The detrimental effect of such inclusions, which lead to localised stress concentrations, is shown. FE models which investigate the effect of pitting corrosion on stress levels in pressurised tubes are also presented. Stress levels are shown to increase with increasing pit depth, size and frequency
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