SHU-MU HSIEH

Date of Award

Winter 1982

Project Type

Dissertation

Program or Major

Engineering

Degree Name

Doctor of Philosophy

Abstract

This study predicts the nature of condensation ash formed from direct combustion of coal. Three coal-fire combustors, a cyclone-fired (CF) boiler, a pulverized-fuel (PF) and a drop tube (DT) furnace were considered.

Predictions were based on the hypothesis that condensation ash is formed through two consecutive steps: vaporization and recondensation. Metal-containing species in coal are believed to vaporize during high temperature combustion. These vapors then diffuse and condense at the bulk gas phase.

The amount of metal-related vapor transport to the bulk gas phase is dictated by char surface composition which is, in turn, determined by the temperature history of a burning coal particle. This temperature-burnout relationship was obtained through an energy balance around the burning particle.

The char combustion rate (involved in energy balance) was calculated using a combustion model developed by Simons et al. Condensation and growth were predicted employing Ulrich's coagulation model with modifications to accommodate the formation of condensation ash.

According to the predictions, condensation ash is aggregated in both the CF and PF boilers. Their average primary diameters are 50 and 180 nm for CF and PF boiler, respectively. For the drop tube furnace, particles were predicted to be single particles with average size of 34 nm in diameter. Laboratory data agree quite well with size predictions except that particles also appear to be aggregates in the drop tube furnace (29).

Predicted condensation ash compositions are in poor agreement with published data. Poor activity coefficient data, the heterogeneous distribution of mineral ash, and heterogeneous condensation are believed to be major factors for these discrepancies. Further studies are recommended.

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