Soot

Emission of soot from a large diesel truck, obviously without particle filters.

Soot (pronounced /ˈsʊt/) is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres, charred wood, petroleum coke, etc. that may become airborne during pyrolysis and which are more properly identified as cokes or chars. The gas-phase soots contain polycyclic aromatic hydrocarbons (PAHs).^[1] The PAHs in soot are known mutagens and probable human carcinogens.^[2] They are classified as a "known human carcinogen" by the International Agency for Research on Cancer (IARC).^[3]

Soot, as an airborne contaminant in the environment has many different sources but they are all the result of some form of pyrolysis. They include soot from internal combustion engines, power plant boilers, hog-fuel boilers, ship boilers, central steam heat boilers, waste incineration, local field burning, house fires, forest fires, fireplaces, furnaces, etc. These exterior sources also contribute to the indoor environment sources such as smoking of plant matter, cooking, oil lamps, candles, quartz/halogen bulbs with settled dust, fireplaces, defective furnaces, etc. Soot in very low concentrations is capable of darkening surfaces or making particle agglomerates, such as those from ventilation systems, appear black. Soot is the primary cause of “ghosting”, the discoloration of walls and ceilings or walls and flooring where they meet. It is generally responsible for the discoloration of the walls above baseboard electric heating units.

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.^[4]

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.^[5] 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.^[4] 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 is 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]

Role in global warming

Emissions of soot, including massive emissions from cookstoves in Asia and Africa, as well as diesel engines and coal plants there, is estimated to account for 18% of global warming. Airborne for weeks, often settling on glaciers, or on ice in arctic regions, black carbon absorbs heat directly.^[6]

Hazards

Soot is in the general category of airborne particulate matter, and as such is considered hazardous to the lungs and general health when the particles are less than five micrometres in diameter, as such particles are not filtered out by the upper respiratory tract.^{[citation needed]} Smoke from 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 deadlier than previously thought, according to a major study and an editorial published in New England Journal of Medicine on February 1, 2007.^{[citation needed]}

Diesel exhaust (DE) is a major contributor to combustion derived particulate matter air pollution. In several human experimental studies using a well validated exposure chamber setup DE has been linked to acute vascular dysfunction and increased thrombus formation.^[7][8] This serves as a plausible mechanistic link between the previously described association between particulate matter air pollution and increased cardiovascular morbidity and mortality.

See also

• Activated carbon
• Bistre
• Black carbon
• Carbon
• Carbon black
• Colorant
• Fullerene
• Indian ink

References

1. ^ Rundel, Ruthann, "Polycyclic Aromatic Hydrocarbons, Phthalates, and Phenols", in INDOOR AIR QUALITY HANDBOOK, John Spengleer, Jonathan M. Samet, John F. McCarthy (eds), pp. 34.1-34.2, 2001
2. ^ Rundel, Ruthann, "Polycyclic Aromatic Hydrocarbons, Phthalates, and Phenols", in INDOOR AIR QUALITY HANDBOOK, John Spengleer, Jonathan M. Samet, John F. McCarthy (eds), pp. 34.18-34.21, 2001
3. ^ Soots (IARC Summary & Evaluation, Volume 35, 1985)
4. ^ ^{a b} Soot: Giver and Taker of Light, American Scientist, May-June 2007, pp.252-239
5. ^ CR Shaddix, etal. (2005), "Soot graphitic order in laminar diffusion flames and a large-scale JP-8 pool fire", International Journal of Heat and Mass Transfer 48: 3604–3614, doi:10.1016/j.ijheatmasstransfer.2005.03.006 
6. ^ "Third-World Stove Soot Is Target in Climate Fight" article by Elizabeth Rosenthal in The New York Times April 15, 2009
7. ^ Diesel exhaust inhalation increases thrombus formation in man† Andrew J. Lucking1*, Magnus Lundback2, Nicholas L. Mills1, Dana Faratian1, Stefan L. Barath2, Jamshid Pourazar2, Flemming R. Cassee3, Kenneth Donaldson1, Nicholas A. Boon1, Juan J. Badimon4, Thomas Sandstrom2, Anders Blomberg2, and David E. Newby1
8. ^ Persistent Endothelial Dysfunction in Humans after Diesel Exhaust Inhalation Ha°kan To¨rnqvist1*, Nicholas L. Mills2*, Manuel Gonzalez3, Mark R. Miller2, Simon D. Robinson2, Ian L. Megson4, William MacNee5, Ken Donaldson5, Stefan So¨derberg3, David E. Newby2, Thomas Sandstro¨m1, and Anders Blomberg1