Suppression of fine ash formation in pulverized coal flames

Technical Report · Thu Apr 29 04:00:00 EDT 1993

DOI:https://doi.org/10.2172/6562068· OSTI ID:6562068

Kramlich, J C; Hoffman, D A; Butcher, E K

The second major ash producing 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, and sometimes merged into it. During diffusion-limited char combustion, the interior of the particle becomes hot and fuel-rich. The non-volatile oxides (e.g., Al[sub 2]O[sub 3], SiO[sub 2], MgO, CaO, Fe[sub 2]O[sub 3]) can be reduced to more volatile suboxides and elements, and partially vaporized. These reoxidize while passing through the boundary layer surrounding the char particle, thus becoming so highly supersaturated that rapid homogeneous nucleation occurs. This high nuclei concentration in the boundary layer promotes more extensive coagulation than would occur if the nuclei were uniformly distributed across the flow field. The vaporization can be accelerated by the overshoot of the char temperature beyond the local gas temperature. 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 resideat 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 systems.

• Research Organization: Washington Univ., Seattle, WA (United States). Dept. of Mechanical Engineering
• Sponsoring Organization: DOE; USDOE, Washington, DC (United States)
• DOE Contract Number: FG22-92PC92548
• OSTI ID: 6562068
• Report Number(s): DOE/PC/92548-T1; ON: DE93015053
• Country of Publication: United States
• Language: English

Related Subjects

01 COAL, LIGNITE, AND PEAT
010600 -- Coal
Lignite
& Peat-- Properties & Composition
014000* -- Coal
Lignite
& Peat-- Combustion
AEROSOLS
ARSENIC
ASHES
CARBONACEOUS MATERIALS
CHEMICAL COMPOSITION
CHEMICAL REACTIONS
COAL
COLLOIDS
COMBUSTION
COMBUSTION PRODUCTS
DISPERSIONS
DOCUMENT TYPES
ELEMENTS
ENERGY SOURCES
EVAPORATION
FOSSIL FUELS
FUELS
HAZARDS
HEALTH HAZARDS
MATERIALS
MERCURY
METALS
NICKEL
OXIDATION
PARTICLE SIZE
PHASE TRANSFORMATIONS
PROGRESS REPORT
REDUCTION
RESIDUES
RISK ASSESSMENT
SELENIUM
SEMIMETALS
SIZE
SOLS
SURFACE AREA
SURFACE PROPERTIES
THERMOCHEMICAL PROCESSES
TRANSITION ELEMENTS
VAPOR CONDENSATION