Effect of creep lifetime on geometric optimization of boiler tubes for thermal power plants

In this study, thermomechanical and creep analyses of boiler tubes are performed and a procedure for cost-based optimization of boiler tube geometries is presented. First, analytical thermal convection and conduction heat transfer under ultra-supercritical (USC) and advanced ultra-supercritical (A-USC) conditions as well as numerical evolution of creep stresses in the boiler tube are obtained. Subsequently, the results are used to calculate the thermal efficiency and creep lifetime of austenitic stainless steel Super 304H and Ni-based alloy Nimonic alloy 80A. The geometric parameters of the boiler tube are then optimized to minimize the total cost related to the boiler tubes including capital investment and operational costs. Comparing with the conventional approach which assumes a fixed creep lifetime for simplicity, the present results also showed that creep lifetime assessment is important and should be included in the optimization. This optimization procedure can also be applied to other boiler tube materials under various operating conditions of thermal power plants.