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#1
Old 03-18-2012, 11:18 AM
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Why does sawdust burn so poorly?

Take a log and set if afire and you get a nice flame. Break it down into kindling first, and it'll burn even faster. Reduce it to sawdust, however, and at best you get a weak, smoldering flame. Why?

(I realize sawdust can create an explosion when mixed with air and ignited, but I'm talking about when it just sits in a pile.)
#2
Old 03-18-2012, 11:30 AM
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I would think that the saw dust doesn't allow the proper mix of fuel and oxygen. You need three things for a fire- oxygen, fuel and heat. The dust does not allow for good airflow. I bet if you poked tunnels in the dust, it would burn slightly better.

Last edited by Ca3799; 03-18-2012 at 11:31 AM.
#3
Old 03-18-2012, 12:09 PM
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Sawdust is a good insulator

My wag is that sawdust is a good insulator (all that trapped air) and prevents the heat from getting inside to the bulk. Also, as posted above, it prevents air circulation.
#4
Old 03-18-2012, 12:10 PM
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Perhaps all that trapped air in there is causing the pile to act like an insulator? Good question though because at first glance you'd think sawdust would burn quite well.

Darnit, beaten by 5 seconds!

Last edited by billfish678; 03-18-2012 at 12:12 PM.
#5
Old 03-18-2012, 12:25 PM
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I think it has to do with particle size.
If you look at a how a piece of wood burns, the fibers tend to curl, and pull away from each other, which exposes unburned wood to the air. When sawdust burns, the outside chars, and the tiny particles pack so close together that the unburned dust beneath them never gets any air.
#6
Old 03-18-2012, 12:32 PM
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Quote:
Originally Posted by Vicullum View Post
I realize sawdust can create an explosion when mixed with air and ignited
Homemade fuel-air explosives? They didn't teach us THAT in woodshop.
#7
Old 03-18-2012, 12:40 PM
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My combustion chem class was a long time ago but I will give it a stab.

Sawdust IIRC tends not to burn due to lack of airflow (too rich to burn). It can also trap alot of heat and have a fire smolder in it for weeks or even months waiting for the right conditions to flare up.

However, given the right conditions sawdust (or almost any particulate matter) can make for some wicked fires and explosions.

http://en.wikipedia.org/wiki/Dust_explosion

With the right dispersion of material and an ignition source you effectively get

http://en.wikipedia.org/wiki/Thermobaric_weapon
#8
Old 03-18-2012, 12:57 PM
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pellets, which can be made by compressing sawdust, burn well. i agree it's air and airflow.
#9
Old 03-18-2012, 01:24 PM
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nm

Last edited by Reply; 03-18-2012 at 01:24 PM. Reason: Undid threadshitting.
#10
Old 03-18-2012, 02:15 PM
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Pellet stoves are really popular in rural areas. Pellet fuel is cheaper than propane or electric. A great use for very fine sawdust that would have been thrown out.

As the OP noted a pile of sawdust doesn't burn well. It needs to be compressed into a log or pellet.

Quote:
Pellet fuel is a renewable, clean-burning and cost stable home heating alternative currently used throughout North America.
Our pellet fuel is made from clean sawdust, much of which comes from our own lumber operations in the heart of the Appalachian Region of the United States in West Virginia.

Why Choose Hamer's Hot Ones? The dust is compressed into pellets that produce clean, reliable heat with low emissions and an extremely small amount of ash.
http://hamerpellet.com/

Last edited by aceplace57; 03-18-2012 at 02:18 PM.
#11
Old 03-18-2012, 02:21 PM
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And yet curiously enough, the chief ingredient in firelogs is sawdust, either compacted by itself or compacted with paraffin.
#12
Old 03-18-2012, 03:08 PM
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Quote:
Originally Posted by Vicullum View Post
...
(I realize sawdust can create an explosion when mixed with air and ignited, but I'm talking about when it just sits in a pile.)
Quote:
Originally Posted by Reply View Post
Homemade fuel-air explosives? They didn't teach us THAT in woodshop.
It has to a very dry and very fine sawdust (besides having the proper air/sawdust ratio) in order to react explosively.
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#13
Old 03-18-2012, 03:21 PM
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Yeah, it's all about the fuel/air ratio. Sawdust of the right grain size and packed at the right density is incredibly flammable. In pellets and fake logs it's actually compacted to reduce the flammability and create a slower burn.
#14
Old 03-18-2012, 03:30 PM
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Quote:
Originally Posted by Reply View Post
Homemade fuel-air explosives? They didn't teach us THAT in woodshop.
Hell, they never taught us any of the really useful stuff in school.

For instance, I took 5 years of Spanish in middle and high school. I move over to Spain, and I find that I desperately need to inform someone that he's the product of a union between a syphilitic goat and a dimwitted dog. However, despite getting excellent marks in my Spanish vocabulary tests, I am unable to express myself adequately.

Now, if you'll excuse me, I have to dig up some sawdust and do some practical science experiments.
#15
Old 03-18-2012, 03:36 PM
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Originally Posted by Lynn Bodoni View Post
Hell, they never taught us any of the really useful stuff in school.

For instance, I took 5 years of Spanish in middle and high school. I move over to Spain, and I find that I desperately need to inform someone that he's the product of a union between a syphilitic goat and a dimwitted dog. However, despite getting excellent marks in my Spanish vocabulary tests, I am unable to express myself adequately.

Now, if you'll excuse me, I have to dig up some sawdust and do some practical science experiments.
An easy one is to fill about 2 inches of a soda straw or similar tube with very fine sawdust. Blow hard through the straw and direct the bloom of sawdust at a flame about 2 feet away. Then repeat using coffee creamer powder for a better effect.
#16
Old 03-18-2012, 05:40 PM
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The general topic is "dust explosion". Basically anything which will burn can become explosive if dispersed into the air as fine particles. I remember being taught about this as a kid using flour - stick a candle in a coffee can with a small amount of flour and a tube to blow through. Generates a nice little "whoomp", and blows the lid off the coffee can.

The Mythbusters did, IIRC, 500 pounds of coffee creamer after confirming a youtube video of someone's "sawdust cannon". It was quite impressive.

I'm actually moderately surprised that most people didn't learn about this as kids.

Wiki on dust explosions:

http://en.wikipedia.org/wiki/Dust_explosion
#17
Old 03-19-2012, 09:27 AM
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Quote:
Originally Posted by Lynn Bodoni View Post
Hell, they never taught us any of the really useful stuff in school.

For instance, I took 5 years of Spanish in middle and high school. I move over to Spain, and I find that I desperately need to inform someone that he's the product of a union between a syphilitic goat and a dimwitted dog. However, despite getting excellent marks in my Spanish vocabulary tests, I am unable to express myself adequately.
Have a look at this book to round out your language skills
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