The co-influence of operation parameters on fly ash deposition onto heating surfaces: A numerical study based on the CFD-DEM coupling model

Abstract

Coal power plays a crucial role in peak shaving for the power grid, and coal-fired power plants must operate flexibly, and even under ultra-low operational conditions that deviate significantly from the design specifications. Consequently, the flow velocity becomes very low, which may lead to increased ash particles deposition on the heat transfer tubes, severely affecting the performance of coal boiler. The phenomenon of fly ash deposition on the cross-flow heating surfaces of the coal-fired boilers was simulated with the CFD-DEM coupling method in the study, which considers the interactions between particles, particles and tube walls, and consider the influence of fluid flow on particle behavior. The mechanisms and processes governing the deposition of ash particles were examined through the lens of particle dynamics, with a focus on the particle viscosity, flow velocity, and particle size on the ash deposition characteristics. This analysis aims to enhance the understanding of the ash deposition mechanisms. The primary mechanisms of deposition are identified as inertial impaction on the windward side and turbulent diffusion on the leeward side. As the deposition process continues, the scouring action of the fluid, combine with the impact of the high-velocity particles, leads to the disruption of numerous P-P contact forces, resulting in fracture and detachment of the deposited ash layer. In the early stages of deposition, high-viscosity fly ash particles show a maximum deposition efficiency of approximately 24% at a flow velocity of 5 m/s, whereas low-viscosity fly ash particles achieve a maximum deposition efficiency of around 20% at a flow velocity of 3 m/s. The particles deposition is simultaneously influenced by two variables that exert opposing effects: collisional momentum and St number. his indicates that specific velocities and particle sizes will inevitably result in maximum deposition. This work provides some guidance on the ash deposition patterns and fouling behavior of boiler heating surfaces and facilitating more reliable predictions of ash particle deposition and mitigating trends in deposition and fouling.