Abstract
Supercritical once-through utility boilers are increasingly common in flexible operations in China. In this study, the tube temperature changes at a vertical water-cooled wall are analyzed during a fluctuating flexible operation. There are considerable differences in the temperatures of the parallel tubes at the minimum load, and the resulting thermal stress distributions at a front water-cooled wall are established using structural calculation software ANSYS 17.1, USA. A wide thermal stress distribution occurs among the parallel tubes, and the local cyclic stress amplitudes under flexible operation are higher than those under cold, warm, hot, or load-following operations. Because of the water wall expansion structure at the furnace, the higher tube temperature areas suffer from compressive stress, while the lower tube temperature areas suffer from tensile stress. During flexible operation, combustion uniformity and a two-phase flow distribution can improve the safety of vertical water-cooled wall operation. The minimum load of the utility boiler should be set as a limitation, and the tube temperature is an important parameter affecting the thermal and cyclic stresses.
Highlights
Because of the penetration of renewable energy in the power grid, the fluctuations in the residual load of the power grid have been increasing for fossil fuel power plants
To measure the uniformity among parallel tubes in flow circuits, the tube temperature thermocouples are installed at the outer wall of the spiral and vertical water-cooled wall every three thermocouples are installed at the outer wall of the spiral and vertical water-cooled wall every three to seven parallel tubes
The temperature difference (ΔT) remains between 50 °C and 160 °C when the load is less than 300 MW, i.e., a large temperature difference occurs at loads of less than 50% boiler maximum continuous rate (BMCR); this temperature difference exceeds the allowable parallel adjacent tube temperature difference (50 °C) according to the utility boiler water wall design rules
Summary
Because of the penetration of renewable energy in the power grid, the fluctuations in the residual load of the power grid have been increasing for fossil fuel power plants. In most of the literature, the thermal stresses in thick-walled components such as headers and turbines are considered critical for flexible operation at large-scale power plants, but the tube metal at the heating surface of the water-cooled wall, superheater, and reheater is important for safe and reliable operation [4,5,10,11,12]. No thermal stress and cyclic stress evaluation has been performed on a water-cooled wall at minimum load in a utility boiler that is operating under uneven combustion and flow distribution conditions because of the lack of data from a power plant under flexible operation [14,15,16,17,18,19]. The simulation provides a reference for the safe operation of a peaking power plant under low fluctuating loads
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