Page 55 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

278 Aerosols

to this complexity, research has been centered on those coal-based synthetic fuels, research on coal combustion
areas considered to be the most important for practical has surged ahead. As with fuel sprays, a modern descrip-
applications. tion of the particle burning process is complicated by the
A combustion aerosol differs from a premixed, com- interactions of diffusion and chemical kinetics.
bustible gaseous system in that it is not uniform in com- The process of particle combustion depends on the
position. The fuel is present in the form of discrete parti- physical and chemical nature of the solid as it heats and
cles, which may have a range of sizes and may move in burns. Coal is a complex material of volatile and non-
different directions with different velocities than the main volatile components which becomes increasingly porous
stream of gas. This lack of uniformity in the unburned during volatilization of low-boiling constituents in burn-
mixture results in irregularities in the propagation of the ing. The crucial practical questions for boiler design con-
flame through the spray and, thus, the combustion zone is cern whether pulverized fuel combustion is controlled by
geometrically poorly defined. oxidizer diffusion or by chemical kinetics.
The process of particle combustion can be illustrated by An important by-product of the combustion of liquid
the simplest case, that of a one-dimensional laminar flame or vapors is soot particles. Carbonaceous particle produc-
moving at low velocity. The flame can be considered to tion is also a serious limitation in the use of diesel engine
be essentially a flowing reaction system in which the time technology for transportation, because of air pollution.
scale of the usual reaction rate expression is replaced by However, the limitation is of “benefit” in another indus-
a distance scale. As the unburned particle approaches the try. The production of carbon black is a major industry,
flame front, it first passes through a region of preheating, with widespread use of the product for binders and dyes.
during which some vaporization occurs. As the flame zone The use of fine powders for industrial applications has
is reached, the temperature rapidly rises and the particle become an increasingly important factor in aerosol tech-
burns. The flame zone can thus be considered a localized nology. Finely divided powders now are used for the rein-
reaction zone sandwiched between a cold mixture of fuel forcement of materials, surface coatings, and laminated,
and oxidant on one side and hot burned gases on the other; polycomponent materials.
if the gas flow through the flame is one-dimensional, the One of the more interesting applications of aerosol
flame front is planar. The nature of the burned products particle technology is in the rapidly expanding field of
depends on the properties of the spray in its unburned microelectronics. The production of electronically active
state. If the particles are large, combustion may not be surface films on substrates by the deposition of semi-
complete in the main reaction zone and unburned fuel conductor particles is a rapidly advancing technology.
penetrates well into the burned gas reaction. If the particles A potentially important extension of this technology is
are small, a state of affairs exists that approximates very the production of nanoparticles in the 0.01-µm diame-
closely the combustion of premixed gaseous flame. Here, ter range. Surface films imbedding these particles can
theparticlesarevaporizedinthepreheatzone,andreaction have unique microelectronic properties that offer opportu-
after that is between the reactants in their gaseous state. nities for making molecular-level semiconductor “quan-
The other factors that determine the time to reach complete tum” dots or wires imbedded in other semiconductors, or
combustion (that is, the length of the combustion zone) are “quantum well pyramids” that have special luminescence
thevolatilityoftheliquidfuel,theratiooffueltooxidantin or optical properties of practical interest.
the unburned mixture, and the uniformity of the mixture.
In most practical systems such as a furnace or a rocket
D. Environmental Influence
engine, the combustion process is much more complicated
due to two important factors. First, to a large extent the Aerosols have an adverse effect on human health and cre-
mixing of the fuel and oxidant takes place in the combus- ate hazards to public safety. Particle suspensions are in-
tion chamber and thus the mechanics of the mixing pro- volved in catalyzing respiratory disease in the workplace
cess plays an important role. Second, the flow patterns are andhomeorthroughpollutionofambientair.IntheUnited
complicated by turbulence or recirculation and frequently States, other environmental disturbances, including poten-
cannot be represented by simplified theories. tial effects on biota, accelerated material deterioration, and
A fair number of dusts are capable of sustaining flames; visibility degradation, are attributed to pollution aerosols.
the number exceeds more than 100. However, the one of Particle suspensions also can affect safety because of their
principal technological importance is coal dust. The last potential for inflammability and explosion. Dust explo-
quarter of the twentieth century saw a major resurgence of sions have occurred in a variety of industrial situations in-
coal as a stable energy source in many nations. Because cluding grain storage and manufacturing areas where par-
of the large capital investment in coal-fired systems, the ticle suspensions are produced. The hazards of toxic dust
pressures of air pollution emission control, and interest in and fume release are well documented in the work-place,