Analytical and numerical investigations on the high temperature upgrading solution of subcritical boilers

Abstract

Currently there still exist a large base of subcritical coal-fired boilers remaining in operation worldwide. Improving their thermal efficiency is critical for these units to reduce the cost of electricity and meet the increasingly stringent efficiency regulations. This paper presented an economic high temperature upgrading retrofit solution for these subcritical units in which the final superheat and reheat steam temperatures are elevated from 540 °C to 600 °C level while the steam pressures remain at subcritical such that the retrofit can be made with minimum modifications to the boiler to reduce retrofit cost and technical risk. A systematic analysis was first conducted to deduce the best boiler heating surface modification option to achieve such significant increase of boiler steam temperatures. The analysis shows that replacing part of the furnace water wall below the furnace exit with wall superheater is the most viable and economic boiler modification design. A three-dimensional computational fluid dynamics (CFD) simulation was then conducted for a typical 320 MW subcritical boiler design to verify the feasibility of this design and determine the area of furnace wall that needs to be modified to achieve the target steam temperatures. The results show that with only 3.6 m height of furnace wall replaced with wall superheater both the superheat and reheat steam temperatures can reach the target 600 °C level even without any modification made to the reheaters. With this heating surface modification design, the efficiency of the existing subcritical power plants can be improved considerably with minimum boiler retrofit work. More importantly, this technology is going to help the plant owners meet the increasingly stringent efficiency and emission regulations mandated by the government and place them in a better position to continue operating these units.