Combustion reconstruction strategy for ultra-low-load stability in a 1000 MW ultra-supercritical tangentially fired boiler

Abstract

Coal-fired power plants are increasingly required to operate with high flexibility to accommodate renewable energy integration. However, stable combustion of supercritical large-scale units at ultra-low loads (e.g., <20% rated load) is difficult because conventional air-staged strategies designed for emission reduction are incompatible with the weak aerodynamic field under low-momentum conditions. This study develops a combustion organization strategy focused on aerodynamic maintenance for a 1000 MW tangentially fired boiler operating at 20% load, based on an analysis of instability mechanisms and systematic parameter optimization. The results indicate that instability in the base air distribution is caused by an aerodynamic conflict between the over-fire air (OFA) and the main combustion flow, which prevents the formation of a stable tangential vortex and flame core. The proposed strategy addresses this through three modifications. The prerequisite is reversing the OFA to a co-rotating direction with sufficient velocity (≥33.5 m/s), this creates a unified macroscopic flow field and establishes an aerodynamic shield to isolate the core flame from upper furnace recirculation. To further enhance combustion stability, the flame is intensified by optimizing the main zone excess air ratio to 1.0, maximizing the heat release by balancing oxygen availability against flame cooling. Meanwhile, the integrity of tangential vortex is maintained by an optimal primary-to-secondary air mass ratio of 0.5 balances fuel jet penetration with swirl stability. This strategy offers a framework for constructing a stable combustion, offering guidance for the deep peak shaving operation of large-scale boilers.