A cloud of dust with a high enough concentration of coal dust could cause a
flashover. Do I know what the concentration would have to be? No. Smoke
from brush fires in South Africa has been documented as a cause for
flashovers on transmission lines there. Coal dust is probably more
conductive than smoke particles.
Charles Perry P.E.
Dust clouds I think have been known to actually explode under the right
conditions. Coal I suspect in a fine enough dispersion might be a possibility.
Coal is pretty dirty carbon, so it's reasonable to assume it'd be somewhat
conductive. If that's so, I can imagine a discharge propagating through it.
After all, there are lots of sharp edges, and that leads to high field
gradients. When one has a bit of a corona discharge it effectively reduces the
air gap, and cascading could happen.
Or not.
a
Smoke
possibility.
it.
the
Coal dust has been known to explode. I would suspect that if a spark occurs
and the dust was dense enough, there would be explosive combustion. Flame
stability is a major concern with coal fired steam generators using fine
coal (as dust in an air suspension) - if the flame goes out and fuel is
still being blown in there is a chance of a hot spot igniting the coal or
sudden explosive ignition when the flame returns - bye bye boiler.
It has also happened with flour as well due to static or other sparks. Any
combustable material in a fine enough suspension can explode.
I took a chemistry course back in 1964, where the introduction
to the class was the prof providing a little demonstration.
This was done in an auditorium, where he had a 20-30 foot high
ceiling. He had a 1 gallon paint can modified to have two items
inside it. One was a candle holder on the bottom. Half way up
the side was a small container about 1/2" diameter, maybe 5/8"
high, with a small tube running from the bottom of the container
out through the side of the can, connected by rubber tubing to a
squeeze bag.
He put plain flour into the container, lit up a candle and set
it on the bottom, and the put the lid securely on the paint can.
And squeezed the bag. Air rushes into the bottom of the
container full of flour, which is blown in up in a cloud of dust
inside the can, and explodes. The top of the can didn't reach
the ceiling in the demonstration I saw, which clearly
disappointed the professor! He said on good days it hit hard.
Given the right "coal dust", there could be one *big* explosion!
over? Thanks
Carbon is a conductor... coal dust is largely carbon in one
of the more combustible forms... the issues are voltage in the
lines, distance between the lines, density of dust cloud,
relative humidity at the time. There are charts on the
density of coal dust necessary for a explosion... called the
LEL... lower exposive limit. etc.
You can search that on google, but measurements would have to
be taken regardless.. my guess..if the power lines are within
50' of the emmisions source and you have a dense emmision, the
risks are higher than if the power lines are 100' or more
away.
the higher the voltage, the denser the dust cloud and the
higher the humidity within limits, the more likely the cloud
will ignite. the force of the blast determined by size of
the dust cloud, and if it is pyrotechnically linked to any
relatively closed in spaces ...such as a building or large bin
or hopper containing part of the cloud.
Uncontained...the blast will for the most part create a lot of
noise... not destruction as with a bomb. Contained within a
building it will take out the walls or maybe worse.
The odds of ignition?
fairly slim if its a 6,000 volt line probably not a major
issue over 50'...but its still an issue.
If its high voltage lines, well over the above number, into
the tens of thousands of volts...its a much more serious
issue..particularly if its old wiring with transformers in the
cloud area and close to the source of coal dust.
What can the company do? they can put knock out water sprays
at the source of the carbon dust emmission at the very least..
not a legal solution. but it would reduce the hazard IF
applied well.. that would be a mist created from a 1500 lb
pressurized water source (search misting equipment on google)
and sufficient to cover the emmisssions opening with a 3'
thick mist...that will take maybe 20 spray nozzels.. my wild
guess a 5 gpm flow rate total or 0.2 gpm per nozzle...
controls would be to turn the mister on when dust was detected
(broken light beam across the stack)..and turn it off when the
area cleared.
A scrubber is the legal solution. (contained water spray mist
over ceramic fill, it washes the air so to speak)
The above is not a recommendation as I have not seen the
site... just an opinion. get a qualifed local PE to review the
situation... chemical or mechanical...take that persons
advice.
Phil Scott
Mech engr.
Coal dust explosions are common.. even powdered metals not
normally considered combustambe when disbursed in the air can
burn.. even steel.
In fact you can buy a fine steel wool pad and set it on fire
with a match..it will go up in flames (leaving a small pile of
rust).
Phil Scott
'
There's at least one case of someone having this happen in
a shopping bag, where a steel wool cleaning pad came into
contact with a small 9V battery's terminals.
wrote:
| In fact you can buy a fine steel wool pad and set it on fire
| with a match..it will go up in flames (leaving a small pile of
| rust).
I saw a lightning arrestor made with steel wool stuffed into a PVC pipe
along with the coax it was protecting ... after it took a lightning
strike. The steel wool didn't just burn ... it exploded, shredding
the PVC pipe. I suppose you could call it an "expulsion arrestor".
But it had the desired effect of protecting the radio transmitter from
the lightning apparently by giving the lightning a "more interesting"
way to dissipate the energy.
--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
On 10/27/04 6:10 PM, in article clp9t0$ntq$ snipped-for-privacy@pita.alt.net, "Checkmate"
Many times with deadly consequences. Elevator explosions are another
thing that can be reduced with the use of modern sensors etc. I guess there
is some sort of chemical that can be released just as an explosion starts
that will lessen the explosive force.
http://www.fireworld.com/magazine/grainperil.html
Dean
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