Simulation Study on Gas-Solid Flow and Combustion Characteristics of Subcritical CFB Unit During Banked Fire

With the rapid development of renewable energy, the flexibility requirements of traditional thermal power units in load regulation and emergency response were increasing. Aiming at the problems of smooth start-up and safe operation in the process of near-zero load regulation of CFB boilers in many power plants, the simulation study of banked fire and ignition restart was carried out. The laws of gas-solid flow, temperature change and oxygen concentration change were analyzed in depth, and the influence of these factors on combustion stability and safety was discussed. Results show that the furnace is still accompanied by a gradually weakened combustion reaction during the banked fire period. The bed material and unburned particles settle to the bottom of the furnace after stopping the air and coal, resulting in the loss of fluidization capacity of the CFB boiler, the significant decrease of oxygen concentration at the bottom of the furnace, and the obvious decrease of temperature at the upper part of the furnace and slow change of the temperature at the bottom. Before the banked fire operation, the concentration of NO shows a distribution of high at the bottom and low at the upper part. During the banked fire period, NO and CO accumulate in the bed and the surrounding area. During the ignition, the accumulated NO is brought to the upper part of the furnace by the air flow, resulting in instantaneous excessive emission. Although the airflow can be restored during the banked fire process, the recovery of oxygen concentration at the bottom is delayed, and local oxygen deficiency may still lead to incomplete combustion and CO release. The bed temperature prediction error is small, and the bed temperature limit for banked fire can be flexibly adjusted through real-time monitoring, which effectively reduces the safety risk in the operation process. The research provides a model reference for optimizing the operation strategy and safety management of CFB boiler under flexible operation mode.