@phdthesis{Abang2016, author = {Abang, Roger Atini}, title = {Thermodynamic modeling and experimental investigations of high temperature corrosion and oxidation due to increased power plant cycling}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-40032}, school = {BTU Cottbus - Senftenberg}, year = {2016}, abstract = {With the rapid growth of renewable energy sources (RES) in the power generation mix in accordance with the German energy transition policy ('Energiewende'), fewer baseload coal power plants will be required. Future power generation will be supplied through decentralized power utilities such as off-shore wind parks and also through high operational flexibility of existing conventional coal power units. High operational flexibility means conventional power plants have to increase cyclic operations to cope with feed-ins from variable-RES such as wind and solar. Unlike medium and peak load power plants that can react quickly to load changes and power ramps, baseload power plants are not suited for such operations. Important technical requirements for flexible operation include among others; frequent start-ups and shut-downs, a minimum downtime, shorter startup time and short operational periods. Baseload coal power plants however do not meet these requirements. This increased cyclic mode of operation can have severe impacts on vital power plant components such as superheater and reheater tubes resulting in high temperature cyclic oxidation/corrosion especially because these plants were not designed for frequent cyclic operations. To optimize plant operations, minimize material damage and reduce operational and maintenance cost, it is therefore important to understand the oxidation and corrosion risk to plants materials associated with this flexible mode of operation. In this context, thermochemical modeling in FactSage 6.4ᵀᴹ as well as experimental investigations were carried out. For the experimental investigations, five commercial coal boiler superheater and reheater materials, namely T91, VM12-SHC, TP347-HFG, DMV304 HCu and DMV310 N were exposed for 1000 hours under discontinuous isothermal oxidation conditions and 1000 hours thermo-cyclic oxidation conditions at a metal surface temperature of 650 °C. The synthetic corrosive flue gas consisted of a mixture of CO₂, O₂, SO₂, N₂ and H₂O. The test material samples were partly covered in fly ash to investigate the effect ash deposits on the corrosion and oxidation behavior of the test materials. After exposure metallographic analysis by means of light microscopy and scanning electron microscopy (LOM and SEM-EDS) were carried out to study the oxide morphology and micro-structural properties of the materials. The oxidation kinetics (weight change) results showed significant oxide growth rates (weight gain) under cyclic oxidation conditions especially in the martensitic alloys - T91, VM12-SHC. Furthermore, metallographic analysis revealed severe oxide spallation in the ash covered sections of these alloys. The austenitic materials (TP374-HFG, DMV310 N) with the exception of DMV304 HCu showed good oxidation behavior with minimal oxide growth both under isothermal and thermal cyclic conditions. However, severe grain boundary attack and internal sulphidation were found in these alloys. DMV310 N showed the best corrosion and oxidation performance. The thermochemical modeling calculations supported the experimental results.}, subject = {Thermo-chemical modeling; Oxidation; High temperature corrosion; Cycling; Flexible generation; Oxidation; Hochtemperaturkorrosion; Lastwechsel; Flexible Kraftwerke; Thermochemische Modellierung; Kraftwerk; Hochtemperaturkorrosion; Lastwechselverhalten; Oxidation; Thermodynamik; Modellierung}, language = {en} }