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Treebeard's Stumper Answer
10 November 2000

Burning Bogs and Exploding Pancakes

An unusual peat bog fire has been burning at nearby Vandenberg Air Force Base for nearly 2 months now, resulting in health concerns and traffic accidents caused by smoke-enhanced fog. How can a wetland burn, despite recent rains and constant watering by fire fighters? Even dry, there's no air to feed it, like trying to burn a thick newspaper. Another unusual kind of fire is a dust or silo explosion. You can demonstrate this by (carefully!) blowing or sifting ordinary flour over a candle flame. I'll have a more impressive demo at the Open House tonight! These different kinds of burning are stumpers by themselves, but they also have an interesting connection. Explain!

I got to talk about this stumper at the Dunn Middle School Open House on Friday night, so I suppose I can say more here.

The Harris Fire started in mid-September in Santa Barbara County in Central California. The big fire was soon extinguished, but not before it spread into the Barka Slough near the mouth of San Antonio Creek on the edge of Vandenberg Air Force Base. The summer-dry wetland caught fire, and it's still smoldering two months later. There's a good record of news reports on the fire in the 30th Space Wing News Flash from Vandenberg AFB. Local newspapers have covered this story, but the best reports have disappeared from their websites. Google still has a few stories in their cache: L.A. Times (23 Sep 2000), Santa Barbara Newspress (20 Sep 2000, 21 Sep 2000, 22 Sep 2000), and Lompoc Record (6 Nov 2000). The best report I read was in the Santa Barbara Newspress (8 Oct 2000), but I can't find it online. A new update (17 Nov 00) is here.

     
One of the four water cannons - hoses with huge nozzles
mounted atop trailors - pouring water around the clock
onto the smoldering bog fire at Barka Slough. We've also
had five inches of rain at my house nearby. This wetland
is critical habitat for the endangered red-legged frog.

     

Smoke from the smoldering bog fire has created unusually
dense fog that has caused traffic accidents on Highway 101.
We haven't seen any sign of the fire at Dunn Middle School
in nearby Los Olivos.

We don't usually think of wheat or dust as an explosive substance, but grain elevators and silos really can explode with deadly results. All it takes is grain dust in the air and a careless spark
     
Historic photo from 1913 shows the Erie Elevator in Buffalo,
New York completely engulfed in flames after an explosion
that killed five men. Photo from Grain Elevators - A History.

     

This photo is from Firefly AB, a site that sells industrial
safety equipment. They point out that "Anything that
has been transformed into finely granulated particles
can explode. Unfortunately, all too often it does."

You can make an impressive fireball with nothing more than ordinary wheat flour, the main ingredient of pancake mix! An easy demo is to blow or sift fine flour or corn starch over a candle. You can make a real explosion by blowing flour or almost any dust over a candle inside a container with a tight lid. This will make a fireball that will blow the the lid into the air! There are directions for this impressive demo on the Web here and here, though my design is slightly different. The DMS Weird Science club had the best results with a 2:1 mix of flour and corn starch, using a two foot high metal popcorn can as the "silo". Be careful kids, this is a real explosion. Don't singe your beard!
Gabe sets off a beautiful flour explosion in the DMS Science room. The
lid is visible hanging flat in the air on the right. I reckon the fireball gets
its classic mushroom shape from pushing up against the lid.

Bog fires and exploding pancakes are stumpers by themselves. They also have an interesting connection. Explain!


Bog fires and grain silo explosions seem very different, but they both involve a fine, dusty fuel with a potentially huge surface area. Fire needs fuel, oxygen, and heat, the classic fire triangle. A mass of peat or flour can only smolder because the air can't penetrate. But that same fuel can literally explode if it is blown into the air to increase the surface area exposed to oxygen. Even steel can burn if it is first shredded into steel wool. Bog fires can smolder for years because of the fine-textured fuel, and grain silos can suddenly explode for the same reason!

Notes:

I don't know how to put out the Vandenberg bog fire, but I do know why steel wool burns and flour can explode. It's because of a chemistry fact and some geometry.

The fact is that most things react with oxygen in the air to oxidize. When this happens slowly, iron rusts and avocados turn brown. Burning results from very fast oxidation. Things oxidize by combining with oxygen, so they oxidize mostly on their surfaces that are in contact with air. Things with lots of surface area burn better. That's why we crumple newspaper to start twigs, and hope they will start the big logs burning in the fireplace.

Here's where geometry comes in. There is a difference between surface area and volume. Volume occupies space, how much stuff there is inside. Surface area is how much outside there is to a thing. Volume depends on size3 and area depends on size 2, so the ratio of volume to surface area changes as size changes. Geometry effects how things burn.

Consider a cube that is two blocks on each side. It's volume is 2 x 2 x 2 = 8 cubic blocks. Each of its six sides has an area of 2 x 2 = 4 square blocks, so its total surface area is (2 x 2) x 6 sides = 24 square blocks.
What happens to the area and volume if I cut the cube apart into 1 x 1 blocks? The volume is still the same, 8 blocks. Each little cube has a surface area of (1 x 1) x 6 sides = 6 square blocks. There are eight blocks total, so the total surface area is now 6 x 8 = 48 square blocks. That's exactly twice what it was before.

If I cut the blocks again, the math (and graphics!) is a little harder because of fractions, but the result is the same. Now I have 4 x 4 x 4 = 64 blocks, each of which is 1/2 block on a side. So the volume is still the same, since 1/2 x 1/2 x 1/2 = 1/8, and 1/8 x 64 = 8 blocks. Each block has a surface area of (1/2 x 1/2) x 6 sides = 3/2. With 64 blocks, the total surface area is 3/2 x 64 = 96 square blocks. Again, that's twice what it was before.

And if I cut the blocks again and again and again? The volume always stays the same, since we have the same amount of stuff. But the surface area doubles with each cut. As I cut the blocks finer and finer, the surface area becomes very large without limit. Flour is just wheat that has been ground so fine that it has a huge surface area. Packed in a pile, those small particles keep the air out. Confined in a limited space and dispersed into the air, they can burn so fast that an explosion results.

Graybear's answer helped me clarify my thoughts on this stumper, and I stole some of his words for my answer:

Fire needs three things; fuel, oxygen, and heat. My guess is that peat has enough oxygen in the form of certain carbohydrates that little outside oxygen is needed for combustion. Therefore, flooding the bog only helps by absorbing heat. Dust explosions are just the opposite - the finer the dust, the higher the amount of surface area exposed to oxygen. Steel wool will burn because of its surface area - I've ignited steel wool with flashlight batteries (an old Boy Scout trick), with a magnifying glass, and with a spark from a flint and steel. Heavy steel will burn in high-oxygen atmospheres, that's why the Apollo I disaster occurred. This is why the shovels used in grain (and other dusty) storage are made from wood or a non-sparking metal like bronze.

This stumper also reminds me of the underground fires, such as the ones that have been burning in (underneath) Pennsylvania for hundreds of years. The fuel in these cases is coal, and the fires have started by various means from lightning to mining mistakes.

Good answer, though I don't know about peat providing its own oxygen for combustion. My understanding is that wet peat bogs are quite anoxic, which is why they sometimes preserve bodies and ancient artifacts so well. I guess diffusion can bring in enough fresh air for a fire to smolder in a dry bog.

The connection between smoldering and exploding is more than academic. The Augusta Chronicle for August 3, 1999 gives this report on a silo explosion that injured 10 workers in Monetta, South Carolina a few days before:

"They had a smoldering fire in that bin over the weekend, and they brought in several (fire) departments to try and contain it. They flooded it with water and sealed it up to smother it and let it cool over the weekend," Mr. Quinn said.

After fire departments from Ridge Spring, Monetta and Batesburg-Leesville contained the smoldering fire, officials sealed the corn silo with cardboard to "smother the smoldering remains of the fire" and keep any more corn from spilling out, Mr. Quinn said.

On Monday, a cleanup crew began clearing the mess left by firefighters on the outside by raking, hosing down the area and hauling off debris. None of the workers went into the bin. "But when they were around back, there was an explosion, and some of the top blew off and a back door blew out right where they were cleaning up," Mr. Quinn said.

The explosion blew off the top of the corn silo, twisting metal and spewing burnt kernels a quartermile. It caused several cracks in the silo's 8-inch-thick concrete wall and was enough to scare forklift operator Robert Aull, who was working nearby for Monetta Peach Packers. He said the explosion wasn't very loud, but smoke billowed from the silo's top.

A smoldering fire can fill an enclosed space with dusty fuel particles, and the slightest spark can set it off. This may be happening on a small scale at the Vandenberg bog fire. The Santa Barbara Newspress reported in October that plumes of fire would suddenly emerge from hot caverns below the surface, making the fire particularly dangerous. These fire plumes sound like small dust explosions. The newpaper quotes Tony Wilder, a fire management officer with Fish & Wildlife:
A peat fire can appear to be dormant and then leap out onto dry ground, burning in three dimensions. It's like looking into the gates of hell. You're kind of at its mercy.

Wetlands can burn when they dry out, and this is an important part of their long-term ecology. There have been huge fires in the Florida Everglades in recent years. Hundreds of thousands of acres of peat swamp forest burned in Indonesia in 1977, caused by a combination of greedy logging and agricultural practices, and dry El Niño weather conditions. The same thing happened here. The Barka Slough was drained over recent decades by greedy wells for Vandenberg AFB and private farms and vineyards. There's no mystery why this wetland dried out. Vandenberg at least has closed its wells. The irony is that fighting this fire uses even more ground water.

Dry peat is hydrophobic, it actually repells water. I used to use peat moss as part of my potting mix for seedlings. I would flood it with water in a wheelbarrow and kneed it with my hands for a long time to get it to take up water. Dry pockets would remain even then. If peat dries out in a pot, it shrinks away from the sides, and it's very hard to re-wet. I no longer use it because of this, and because peat moss is "mined" from wetlands around the world that should be left alone. It's a habitat, and we need that peat for proper whisky!

Our bog fire is still smoldering after three months. The newest proposal is to excavate the bog with bulldozers. But this is also critical habitat. The Santa Barbara Newspress (17 Nov 00) quotes Lt. Col. Scott Westfall, VAFB commander of the base environmental department about the complications:

A "cornucopia of environmental laws are at play" in trying to end the fire, Westfall said. For instance, the wetland is covered by the Clean Water Act. The endangered red-legged frog habitat and population are protected by the Endangered Species Act. Native American cultural resources are managed by the National Historic Preservation Act. Smoke from the peat brings the Clean Air Act into play.
This reminds me of the still-undecided (as I write this) Presidential election: "What a mess!"

Here are a few starting links for further research.

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Copyright © 2000 by Marc Kummel / mkummel@rain.org