Study on the effect of flame offset on water wall tube temperature in 600℃ and 700℃ ultra-supercritical boiler

摘要

The distribution of water wall temperature in the ultra-supercritical (USC) boilers was obtained by establishing a coupled heat transfer model. The reliability of this model has been validated through the comparisons of simulated and measured water wall temperatures along different dimensions in the furnace of reference USC boiler. Then, the effect of flame offset on water wall tube temperature in 600 degrees C and 700 degrees C USC boilers was investigated by importing the flame offset variables into the coupled model. The water wall temperature distribution is significantly influenced by flame offset, and both the fluctuation and growth rate of temperature are increased with the ascent of flame offset distance, especially on the furnace walls that flame deviates toward. The radiative and convective heat flux to water walls is strengthened simultaneously during the flame offset process, resulting in the local overheating of water wall. In the 600 degrees C USC boilers, when the distance of flame offset exceeds 5 m, multiple peak distribution of wall temperatures appears, which can increase the burst risk of water wall tubes because of shear stress inside the tube material. The maximal distance of flame offset should be limited to 3 m to avoid the tube burst accidents. In the 700 degrees C USC unit, the variation tendency of water wall temperature is resemble with that in 600 degrees C USC unit, but the fluctuation of wall temperature is larger. As the flame offset distance approaches 3 m, the maximal water wall temperature reaches 595 degrees C, which greatly exceeds the material allowable temperature in the 600 degrees C USC unit. The material of water wall tubes with allowable temperature of 605 degrees C is recommended for the 700 degrees C USC unit. Based on the thermal security of metal material, the maximal distance of flame offset should be yielded to 2 m.