Soot

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For other uses, see Soot (disambiguation).
Emission of soot from a large diesel truck, 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.

The formation of soot depends strongly on the fuel composition.[4] The rank ordering of sooting tendency of fuel components is: naphthalenes > benzenes > aliphatics. However, the order of sooting tendencies of the aliphatics (alkanes, alkenes, alkynes) varies dramatically depending on the flame type. The difference between the sooting tendencies of aliphatics and aromatics is thought to result mainly from the different routes of formation. Aliphatics appear to first form acetylene and polyacetylenes; aromatics can form soot both by this route and also by a more direct pathway involving ring condensation or polymerization reactions building on the existing aromatic structure .[5] [6]

Contents

[edit] 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.[7]

Soot normally forms at about 140 °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.[8] 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.[7] 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]

[edit] Role in global warming

Soot is found worldwide, but its presence and impact are particularly strong in Asia.

One of the most complex effects on albedo is from black carbon (BC) particles. All aerosols are capable of scattering incoming radiation back to space, BC is no exception. A portion of incoming solar radiation is scattered back to space by BC particles. A study done by Conant et.al (2003)[9] determined the single scattering albedo (ω) of a black carbon particle at 500 nm wavelength and 80% relative humidity to be 0.226. The low single scattering albedo (ω) demonstrates the strong absorption properties of BC particles. While a portion of incoming radiation is scattered back to space, a larger portion of the incoming solar radiation is absorbed by BC containing particles in the air. The absorbed fraction of solar radiation results in surface cooling because the solar radiation that would have reached the Earth is absorbed in the atmosphere. Although surface warming is seen under these conditions, atmospheric warming is also observed because the incoming radiation is trapped in the atmosphere.[10]

There are two primary sources that contribute to BC emission on the global scale: fossil fuel emissions and biomass burning. The AR-4 published by the IPCC in 2007 suggests the raditaive forcing from these two combined sources is +0.44 ± 0.13 Wm-2. Carbonaceuos aerosol emissions inventories are currently (as reported by the IPCC) claiming that 34-38% of emissions is generated from biomass burning, the remainder from fossil fuel burning. [11]

BC also has an indirect effect on climate change as it relates to the albedo effects of snow, ice, and land use changes. There is potential for BC particles to decrease the albedo of snow and also affect the rate of snowmelt. Hansen and Nazarenko (2004) reported a radiative forcing value of +0.15 Wm-2 in AR-4 with respect to changes in snow and ice albedo; however, this estimate has high uncertainty levels. (Forster, et al., 2007) [12]

[edit] 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.[13]

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.[14][15] This serves as a plausible mechanistic link between the previously described association between particulate matter air pollution and increased cardiovascular morbidity and mortality.

[edit] See also

[edit] 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. ^ Seinfeld, John H. ; Pandis, Spyros N. Atmospheric Chemistry and Physics - From Air Pollution to Climate Change (2nd Edition).. John Wiley & Sons.
  5. ^ Graham, S. C, Homer, J. B., and Rosenfeld, J. L. J. (1975) "The formation and coagulation of soot aerosols generated in pyrolysis of aromatic hydrocarbons", Proc. Roy. Soc. Lond. A344, 259-285.
  6. ^ Flagan, R. C., and Seinfeld, J. H. (1988) Fundamentals of Air Pollution Engineering, Prentice-Hall, Englewood Cliffs, NJ.
  7. ^ a b Soot: Giver and Taker of Light, American Scientist, May-June 2007, pp.252-239
  8. ^ 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 
  9. ^ Conant, W. C., Seinfeld, J. H., Wang, J., Carmichael, G. R., Tang, Y., Uno, I., et al. (2003). A model for radiative forcing during ACE-Asia derived from CIRPAS Twin Otter and R/V Ronald H. Brown data and comparision with observations. Journal of geophysical research , 8661.
  10. ^ Seinfeld, J. H., & Pandis, S. N. (2006). Aerosols and Climate. In J. H. Seinfeld, & S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change (pp. 1054-1088). New York: John Wiley & Sons.
  11. ^ Forster, P. V., Ramaswamy, P., Artaxo, T., Bernsten, T., Betts, R., Fahey, D., et al. (2007). Changes in Atmospheric Constituents and in Radiative Forcing. In S. D. [Solomon (Ed.), In: Climate Change 2007:The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA.: Cambridge University Press.
  12. ^ Forster, P. V., Ramaswamy, P., Artaxo, T., Bernsten, T., Betts, R., Fahey, D., et al. (2007). Changes in Atmospheric Constituents and in Radiative Forcing. In S. D. [Solomon (Ed.), In: Climate Change 2007:The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA.: Cambridge University Press.
  13. ^ "Long-Term Exposure to Air Pollution and Incidence of Cardiovascular Events in Women" article by Kristin A. Miller, M.S., David S. Siscovick, M.D., M.P.H., Lianne Sheppard, Ph.D., Kristen Shepherd, M.S., Jeffrey H. Sullivan, M.D., M.H.S., Garnet L. Anderson, Ph.D., and Joel D. Kaufman, M.D., M.P.H. in New England Journal of Medicine February 1, 2007
  14. ^ 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
  15. ^ 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
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