Deposition of ash on heat exchanger tubes is an important phenomenon associated with pulverized coal fired boilers. In the present study, a combined approach of field observation, sample collection, laboratory testing and computational modeling was employed to study this phenomenon.

Field survey of five boilers indicated that ash deposition is limited to high temperature zones of first pass and horizontal pass. The deposits are high in iron oxide content and much lower alumina content as compared to bulk ash composition. High iron oxide and heterogeneous distribution of iron oxide among ash particles are the prime cause of ash deposition. Mixing of high alumina coal could reduce slagging/clinkering tendencies of coal with high iron content. Further, there is a possibility that removal of the heaviest ash fractions by coal washing will help in reducing slagging due to heterogeneous distribution of iron content.

Turbulent ash-laden flow past an inline array of three tubes with dimensions and parameters associated with platen super heater, front reheater and rear reheater of a typical boiler was modeled in two dimensions using CFD code FLUENT®. The simulations showed that for lateral spacing of 4 to 15 diameters, the first tube in the row is most susceptible to particle collisions. The collisions on 2nd and 3rd tubes are substantially less even with turbulence. With the objective of mitigating this aspect, modeling was done by replacing the front circular tube with elliptical tubes of major-to-minor axis ratios of 1.2, 1.5 and 2. The elliptical shape of the front tube reduces inertial impaction and reduces ash deposition on it. The data from CFD simulation was combined with a deposition model to show that deposition is very sensitive to iron oxide content, temperature and heterogeneity of ash particles in composition.

Key words: Boiler, Inertial impaction, Ash deposition, Turbulent dispersion, Particle-laden flow, Elliptical tube, Ash composition.