Deposition behaviour of molten alkali-rich fly ashes—development of a submodel for CFD applications

Abstract

In this work, an advanced CFD-based ash deposition model is presented that allows for the quantitative prediction of the transport of fly ash particles near furnace walls and heat exchanger surfaces, combined with a detailed description of the thermochemical properties of the particles that consist of salt mixtures melting over a wide temperature range rather than at one specific melting point. The physical and chemical phenomena considered in this approach are: particle melting, particle impaction, particle sticking as a function of particle melt fraction, restriction of deposit layer growth by melt flow, and particle rebounding from the furnace wall or superheater surface. The model is validated with entrained flow reactor experiments at well-defined conditions. Three model substances consisting of mixtures of sulphates and chlorides are fired in the reactor with particle sizes typical for biomass combustion in fluidised bed combustors, ranging from smaller than 75 to 250 μm. The results indicate that while the proposed deposition model is capable of predicting deposit buildup on cooled surfaces, a clear distinction between physical and chemical effects contributing to deposit buildup for different particle size classes has to be made. For fly ash particles smaller 75 μm, all particles containing 15% molten phase or higher will stick to cooled surfaces as soon as they are transported to them and as long as the deposit does not grow above the steady state thickness. Larger particles may follow the same trend, but the high probability of them rebounding is a function of the impact angle and needs to be considered in the model.