"Smokeless" solid fuel combustion?

Technical questions that one would like posed to experts
(scientists) in fields related to particulate pollution.

"Smokeless" solid fuel combustion?

Postby Wilberforce » Thu May 08, 2008 6:40 pm

"Smokeless" solid fuel combustion?

There is no difference in particle counts of visible smoke and invisible smoke

The study cited here shows measurements of particle counts being roughly in equal amount, no matter
at what temperature a biomass (wood) fire burns. That is, a relatively low temperature smoldering fire
creates a lot of visible smoke. But a high temperature, seemingly "smokeless" wood fire, creates
a lot of smoke as well. But this smoke cannot be seen, because the particles are far too small to
effectively scatter (the relatively long wavelength) visible light radiation.

Image

note: 1 nanometer = 0.001 microns = 1 millionth of a millimeter

Interpreting the chart: Particle counts of up to 3 million particles per cubic centimeter (vertical axis) are
present at any wood combustion temperature. Particle sizes (horizontal axis) in the visible light wavelengths
are larger, hence scatter light effectively, thus they can be seen. There are slightly higher counts in the short
end of the visible spectrum, hence smoke sometimes has a bluish tint (think of blue smoke coming from the
exhaust pipe of an oil-burning car) There is an abundance of these sizes of particles with a relatively cool,
smoldering wood fire.

As the fire temperature increases, the particle sizes decrease to a much smaller size, therefore can no
longer be seen. This may be consistent with a more rapid shearing effect of the generation of the particles
from the hard substrate. Another reason may be the high temperature effluent imparts sufficient kinetic
energy to the larger (clumped) particulates to readily disperse them, since the cohesion forces are relatively
weak in the (many) nonpolar chemicals present. A third reason, mentioned in the text, is partial oxidation
(burning off) of the larger particles, rendering them into reduced size. (This is like burning a log; the log
gets smaller as it is burned away)


Combustion timing (in short microseconds) may also be a factor: A too-rapid of airflow can result in lower
combustion efficiency, thus many fuel particles escape, unburned.

The higher the burn temperature, the finer the particle. Such particles could be photographed under short-
wavelength ultraviolet light. We cannot visually detect the particles, since their average size falls way outside
of the lower limit of human vision.

Thus a "smokeless" wood fire is never really "smokeless" at all.

The chart appears in this article (from the PDF file)
Generation and Characterization of Hardwood Smoke Inhalation Exposure Atmospheres
Jacob D. McDonald; Richard K. White; Edward B. Barr; Barbara Zielinska; Judith C. Chow; Eric Grosjean
http://www.informaworld.com/smpp/conten ... a748861283

REFERENCES

Characteristics of Particles - Particle Formation
http://www.epa.gov/apti/bces/module3/fo ... ormate.htm

Incinerators
http://www.epa.gov/ttn/oarpg/t1/reports/sect5-5.pdf

Partial oxidation
http://en.wikipedia.org/wiki/Partial_oxidation
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Postby turning_blue » Tue Jul 22, 2008 6:14 pm

Thanks Woodnyet for posting this. Somehow I missed it when it was first posted.

I think this is so important. Should it also be on the highlights page or with wood smoke 101? I'm going to print this so I can easily have it at hand.
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