Characterization of open-cycle coal-fired MHD generators. Quarterly technical summary report No. 3, January 1--March 31, 1977. [Negative ion formation, electron/slag interaction, and alkali/slag interaction

Abstract

The purpose of this contract effort is to understand how nonfuel components of coal will affect the electron and alkali seed chemistry in a high temperature coal combustion system like that envisioned for direct fired MHD generators. Three specific problems are being considered during this contract period. The first problem area is to characterize the formation of negative ions due to electron attachment processes in the combustion flow. While some stable negative ions may be formed from hydrocarbon combustion species (OH/sup -/), the bulk of the stable negative ions are expected to be formed from oxidized inorganic coal slag constituents (BO/sup -//sub 2/, PO/sup -//sub 2/, AlO/sup -//sub 2/, etc). Negative ion formation can reduce the conductivity of the MHD plasma, particularly at the low temperature end of the MHD channel, thus decreasing the efficiency of power generation. The second problem area involves the role slag condensation may play in determining the electron density through recombination, also adversely affecting conductivity in the core flow. The competitive balance between thermionic emission from slag droplets and electron/ion recombination on the droplet surfaces may be severely tipped in favor of electron loss processes, depending on the slag properties. The third problem area is the heterogeneous interaction of alkali seed with particles formed by slag condensation in the generator channel. Alkali seed material can be chemically bound into the molten slag particles tightly enough that seed recovery becomes prohibitively expensive. The loss of significant amounts of alkali seed with the slag could have a serious economic impact on proposed MHD systems. A coupled approach, involving both theoretical modeling and experimental measurements, has been devised to explore the negative ion formation, the electron/slag interaction, and the alkali/slag interaction problems. Research progress is reported.