Masters Theses

Date of Award

5-1994

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

Roy J. Schulz

Committee Members

John Foote, Lloyd Crawford

Abstract

This research addresses the strength of coal fly ash deposits on simulated superheater tubes in an U.S. Department of Energy, pilot-scale, coal-fired testing facility called the Coal-Fired Flow Facility (CFFF), operated under magnetohydrodynmamic (MHD) power generation conditions by the University of Tennessee Space Institute. The governing mechanism that develops fly ash deposit strength is sintering. Because deposits on superheater tubes inhibit heat transfer from the flue gas to the tubes, sootblowing is used to remove the ash from the tubes. It is of critical importance to learn more about the relationship between sintering and ash deposit strength, so that sootblower or other ash removal systems may be designed more effectively.

An experimental method was used to examine the relationship between fly ash strength and the sintering process. The method consisted of forming ash pellets at temperatures comparable to those experienced in the exhaust flue gas duct, sintering the pellets at uniform temperature for approximately 15 hours, and crushing the pellets in a compression testing machine. This determined the maximum stress the pellets would withstand, when cold, when they were formed at the given temperature. The research was carried out for two different potassium carbonate seeded coals, an eastern coal, Illinois #6, and a western coal, Montana Rosebud. Seeded coals are necessary for the coal-fired MHD power generation process.

This research also included an independent derivation of J. Frenkel's model for particle bonding and an application of the model to the specimens used for the crush testing. The size of the bond between ash particles is related to the particle liquidus viscosity and surface tension forces, as well as their sizes. Data required for this part of the research was obtained by photographing the structure of the sintered pellets using a scanning electron microscope (SEM) and by using computer image analysis software to measure the size of the particles and the bonds between them.

Relationships between the strength and the sintering temperature were successfully determined for both eastern and western coal ashes. However, although SEM/computer analysis of the particle bonding was attempted, it was found that, for the time used to heat and sinter the pellets, slagging occurring on the surface and inside the pellet prohibited accurate measurements of bonding, and thus also prohibited finding a reliable correlation between the strength and the sintering rate parameter.

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