Soot

Description

The production of soot in a flame is a complex process consisting of several chemical reactions taking place in series. In the fuel-pyrolysis zone of the flame, typically clear or blue, the fuel molecules are broken down into various fragments, including carbon-ring structures, acetylene (C₂H₂), the radical C₃H₃ (and higher order), as well as monatomic and diatomic hydrogen. As the combustion process continues the radicals quickly combine into new structures, giving off heat. These precursors polymerize into larger "pre-soot" chains then gather into formations of hydrogen-rich spheres in the soot-inception zone. In the soot-growth zone these spheres give up their hydrogen gas through diffusion, resulting in solids consisting of several of the formerly liquid spheres stuck together into larger chains. It is this portion of the flame that has the bright yellow color. Hydrogen-rich examples then further oxidize, releasing more heat. In perfect combustion the soot would break down into almost pure CO₂ and H₂O, it is only in incomplete combustion that the soot is able to form and escape the flame.^[1]

Soot normally forms at about 1400 C, forming an excellent blackbody radiator of colors in the yellow to red spectrum. The typical yellow color of a candle flame or wood fire is produced primarily by the hot soot forming inside.

The energy being radiated from the soot is an important contributor to the ongoing combustion process, cooling the flame above the soot-growth zone and feeding energy back into the fuel-pyrolysis zone. In "pool fires" of open liquid fuel this process can feed as much as 50% of the flame's energy back into the liquid fuel below, which vaporizes it and keeps the reaction going; it would otherwise burn much more slowly. ^[2] The same release of energy is responsible for quickly cooling the flame above the soot-growth region, limiting its further combustion into lighter molecules, and explaining why these fires release so much soot.^[1] A canonical example is the 2005 Hertfordshire Oil Storage Terminal fire, which released massive amounts of soot and covered the skies over a large portion of the London area.

The separation of flame into zones of different chemical reactions due to convection forcing the hot reactants upward. In microgravity or zero gravity convection no longer occurs, and such flames tend to become more blue and more efficient, producing much less soot.[1] Experiments by NASA reveal that diffusion flames in microgravity allow more soot to be completely oxidized than in conditions on Earth, because of a series of mechanisms that differ from those in normal gravity conditions. [2]

Production

Lampblack has been used since prehistoric times as the source for carbon black, collected by holding a cold surface over a cool flame. Candles or lamps using animal fats or waxes generate considerable amounts of soot that can be collected and then mixed with a lubricant to produce ink. This process can be easily duplicated today by passing some noncombustible surface, such as a tin can lid or glass, closely through a candle flame. Lampblack produced in this way is among the darkest and least reflective substances known^{[citation needed]}.

Lampblack is also used to coat lacquer. Similar coatings were used in direct recording pendulum seismometers. While not a sensitive instrument, these were capable of directly recording the direction of significant horizontal shocks upon a smoked glass plate.

Hazards

Soot is in the general category of airborne diesel engines, while composed mostly of carbon soot, is considered especially dangerous owing to both its particulate size and the many other chemical compounds present.^{[citation needed]}

Soot can stain clothing and can possibly cause illness if inhaled. Breathing common urban air pollution (containing soot) is much more deadly than previously thought, according to a major study and an editorial published in New England Journal of Medicine on February 1, 2007.

See also

References

1. ^ ^{a b} Soot: Giver and Taker of Light, American Scientist, May-June 2007, pp.252-239
2. ^ CR Shaddix, etal. (2005), " ", International Journal of Heat and Mass Transfer 48: 3604-3614