Engineering model of ash deposition and its effect on heat transfer to superheater tubes

A generally applicable theory is presented which attempts to explain how ash deposits form, with or without the presence of sticky particles and surfaces. Some experimental and modeling results are described. Ash deposition due to inertial impaction and thermophoresis was calculated. Boiler design specification, loads, coal properties, boiler operation choices, and locally-entrained ash characteristics are input in terms of superheater tube diameter, steam temperature, flue-gas temperature, particle velocity, gas and particle density, gas velocity, multiple time increments, particle diameter distributions, and dust loading with elemental analyses for multiple narrow- range particle-diameter bins plus calculated viscosity for impacting particles. The total heat flux of the superheater tube is calculated. Calculated two- dimensional deposition resulting from the inertial impaction and the resulting total heat flux are compared with measurement data from the +20MWt magnetohydrodynamic (MHD) facility at the University of Tennessee Space Institute (UTSI) and the Fuels Evaluation Facility (FEF) at the Pittsburgh Energy Technology Center (PETC). The numerical and experimental results are in the good agreement. The influence of several variables including time, tube diameter, gas velocity, particle density, particle size distribution, and dust loadings on ash deposition are discussed.