Suppression of fine ash formation in pulverized coal flames. Quarterly technical progress report No. 3, April 1, 1993--June 30, 1993
One of the major obstacles to the economical use of coal is managing the behavior of its mineral matter. Ash size and composition are of critical importance for a variety of reasons. Fly ash size and emissivity affect radiant furnace heat transfer. Heat transfer is also affected by the tendency of ash to adhere to heat transfer surfaces, and the properties of these deposits. Removal of ash from flue gas by electrostatic precipitators is influenced by both particle size and particle resistivity. The efficiency of fabric filter-based cleaning devices is also influenced by ash size. Both types of devices have reduced collection efficiencies for smaller-sized particles, which corresponds to the size most efficiently retained in the alveolar region of the human lung. 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. The second major mechanism is the generation of a submicron aerosol through a vaporization/condensation mechanism. 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:
- 10180715
- Report Number(s):
- DOE/PC/92548-T3; ON: DE93041263
- Resource Relation:
- Other Information: PBD: 23 Jul 1993
- Country of Publication:
- United States
- Language:
- English
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Suppression of fine ash formation in pulverized coal flames. Quarterly technical progress report No. 2, January 1, 1993--March 31, 1993
Suppression of fine ash formation in pulverized coal flames