Date of Award

1-21-1998

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Douglas K. Ludlow

Abstract

Pressurized fluidized bed combustion (PFBC) offers the ability to reach high system efficiencies while controlling both SO$\sb2$ and NO$\rm\sb {x}$ emissions. However, current technology does not allow the capture of alkali to the recommended level of 24 ppb set by turbine manufacturers. Aluminosilicate minerals have the potential to capture alkali, especially sodium and potassium, under conditions typical of the fluid bed (750 to 1000$\sp\circ$C). When alkalis and alkaline-earth elements react with aluminosilicate minerals, phases have been observed which result in the formation of sodalites, such as hauyne and nosean, that have cubo-octahedral structure capable of incorporating sulfate and chlorine anions.Thermal gravimetric analysis was used to study the mechanism of alkali capture using sodalite. Further testing was performed on a 3$\sp\prime\sp\prime$-diameter pressurized fluid-bed reactor. Early results indicated that simultaneous removal of alkali and sulfur and/or chlorine was not practical under the conditions for commercial PFBC operations. Therefore, the focus of the later part of this work was on sorbents that have been shown to capture alkali in other systems. The effectiveness of bauxite and kaolinite to reduce vapor phase alkali concentrations was determined. In addition to studying the gettering capability of the sorbent, the impact of the getter on operational performance was evaluated. This evaluation included examining potential agglomeration of bed particles, deposition on heat-transfer surfaces, and the bridging and blinding of ceramic candle filters.The overall conclusion from this work is that in-bed alkali getters can effectively reduce the vapor phase alkali concentrations. This reduction is of a magnitude great enough to control ash deposition and agglomeration and filter blinding, but not to a level low enough to meet current turbine manufacturer's recommendations for vapor phase alkali. Bauxite was the best getter tested based on its ability to control all of the above mentioned problems. Kaolinite is less effective because of its tendency to form sintered deposits from its fine fraction. Finally, although sodalite and nosean can be formed and result in combined sodium and chlorine or sulfur capture, they do not form at a rate high enough to make them effective getters under PFBC conditions.

Download

Share

COinS