Suppression of fine ash formation in pulverized coal flames. Quarterly technical progress report No. 5, October 1, 1993--December 31, 1993
Laboratory work and studies of full-scale coal-fired boilers have identified two general mechanisms for ash production. The vast majority of the ash is formed from mineral matter that coalesces as the char burns, yielding particles that are normally larger than 0.5{mu}m. Flagen and Friedlander proposed a simple model for this residual ash, called the breakup model. The second major mechanism is the generation of a submicron aerosol through a vaporization/condensation mechanism. When the ash size distribution is plotted in terms of number density, the submicron mode generally peaks at about 0.1 {mu}/m. When plotted in terms of mass, this mode is sometimes distinct from the residual ash mode, {sup 13} and sometimes merged into it. Although these particles represent a relatively small fraction of the mass, they can present a large fraction of the surface area. Thus, they are a preferred site for the condensation of the more volatile oxides later in the furnace. This leads to a layering effect in which the refractory oxides are concentrated at the particle core and the more volatile oxides reside at the surface. This also explains the enrichment of the aerosol by volatile oxides that has been noted in samples from practical furnaces. These volatile metal oxides include the majority of the toxic metal contaminants, e.g., mercury, arsenic, selenium and nickel. Risk assessment studies suggest that toxic metal emissions represent a significant portion of the health risk associated with combustion.
- Research Organization:
- Washington Univ., Seattle, WA (United States). Dept. of Mechanical Engineering
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- FG22-92PC92548
- OSTI ID:
- 10133519
- Report Number(s):
- DOE/PC/92548--T5; ON: DE94008331; BR: AA1525050
- Country of Publication:
- United States
- Language:
- English
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