Causes and mitigation of gas temperature deviation in tangentially fired tower-type boilers

Gas temperature deviation is a common problem for tangentially fired boilers due to the residual swirling at the furnace exit. It can induce overheating or even explosion of superheaters and/or reheaters tubes. Tower design of tangentially fired boilers is expected to abate the deviation. However, large gas temperature deviation still exists in tower-type boilers. In this study, a numerical method that is validated by experimental data and designed value is used to investigate the gas temperature deviation in a 1000 MWe tangentially coal-fired tower-type boiler, from which the two main causes of the deviation are identified and a new method to mitigate the deviation is proposed. First, the traction of the induced draft fans skews the flue gas flow towards the rear wall, on which the furnace exit is located. Such gas flow skewness becomes aggravated along with the furnace height. Second, the dense panel heaters obstruct the gas flow, which further aggravates the gas flow skewness. Both in combination result in a highly non-uniform gas flow in the panel heaters zone and thus a large gas temperature deviation. In the 1000 MWe tower-type boiler under investigation, the gas temperature deviation between the left side and the right side on the entrance cross-section of the final superheater reaches 120 K. A new method, i.e., adding an arch nose under the furnace exit, is proposed to mitigate the deviation, in which the height of the arch nose is a decisive factor and needs to be carefully determined. A too low arch nose cannot eliminate the skewness of flue gas towards the rear wall while a too high arch nose skews the flue gas towards the front wall. The new mitigation method has been proven effective.