small explosion - a cautionary tale

Yesterday I was making a batch of biochar with the barrel-in-a-barrel method.  My inner barrel has holes every 2-3 inches around the bottom and a hole in the center of the lid on top.  The fire had been going for 15-20 minutes and was starting to put off a large amount of heat when I had a small explosion that blew the lid off the inner barrel.  When the lid blew off, it blew flaming chunks of wood for 15 feet or so in every direction, starting a number of small fires.  I was using mostly 2x4 cutoffs about 4 inches long that I get from the local truss company.  Apparently one of the blocks fell down and covered the top central hole and the weight of all the other blocks on top of it caused enough pressure to build up to blow the lid off.  I guess the pressure builds up much higher at the top, so the gases couldn't vent thru the bottom holes.  I had a hose nearby and put everything out quickly, but on a dry day without water readily available, the situation could have been much worse.

I have seen steam do that.  How dry would you say the wood was? Also I see a lot of inverted inner barrels with no top so the gas would build but lift the inner chamber used in the 2 barrel design. And locking rings on the 55. Did you have a ring? It sounds loose. If it was syngas I could imagine that caused a bit of a mushroom cloud in the backyard. Steam not so much.

The 55 gal drum doesn't have a lid at all.  The wood was fairly dry.  The wood they use at the truss company is marked heat treated and kiln dried.

Todd Parr wrote:  I guess the pressure builds up much higher at the top, so the gases couldn't vent thru the bottom holes.

Huh??!?  That's not how gases behave. To get that much pressure build-up, you need to block all of the vents.  Even a 1/4" diameter vent hole is not going to allow much more than 15 psi of over-pressure to build up.  Take a look at an ordinary pressure cooker -- the vent on it is smaller than 1/4", and it is generally limited to 1 atmosphere of pressure.  I'd say something was effectively blocking what you thought were vent holes.  There is a lot of movement of creosotes and tar during a biochar burn.  Gassing off from wood sources and re-depositing wherever there is a cooler surface.  That might generate internal blockages.

I can't tell you how it happened, but I can tell you what happened, and it's exactly what I said.  The vent holes aren't what I think are vent holes, they are holes I drilled myself in the barrel.  I forget the exact size, but I believe they are 1/2" and they are every 2"-3" all the way around the bottom of the barrel, as well as the hole in the center of the top that is the same size.  I'll try to remember to take some pictures of it tonight.  I suppose it's possible that a wood block covered every vent hole on the bottom of the barrel, as well as one blocking the top hole, but that seems almost impossible to me.  I haven't even removed the smaller barrel from the larger yet, so I can take a picture of it still sitting in the wet charcoal mess.

Glad you are safe.

Maybe in the future you could strap the lid down but with some slack or something so that if it could release the gases without flying off? Just pondering a way to avoid this situation in the future.

Thanks Gene.  There was very little chance for any kind of injury, because my inner barrel is so much smaller then the outer one.  Even if it managed to explode the inner barrel into shrapnel, it could only go up and out of the outer barrel.  The danger of starting a fire was very real however.

I'm working on a much better system right now, it just isn't done yet.  Meantime, I'll just be very careful that top vent isn't covered in any way, and maybe I'll make it a little larger

Some pictures.

Wow.  I don't even have holes around my inner barrel bottom or one in the top at all.  My inner barrel just sits, and the gasses escape out the crack between it and the bottom of my outer barrel.  My outer barrel has plenty of large holes in the bottom, allowing oxygen in and keeping fire ripping hot.  I've never had what you describe happen.  

With angular blocks of wood as you have, you should not have been able to create a seal of your holes, or to trap gasses.  It doesn't make sense.     ??    

My only guess is that you had one chunk of truss material that had a big sappy knot with too much water trapped in it.  When that water turned to steam, it exploded outwards, possibly igniting the gases around it.  Only a guess.

I agree with Roberto. With that many holes and the square shaped wood I don't think think you could have had a slow build up of pressure. The only logical reason for blowing the lid off like that was a very quick build up in pressure, which means indeed something exploded inside: instant pressure rise. Trapped water turning to steam would be the most obvious reason. In open fires it's the same, lots of cracking noises and the occasional little or not so little bang that throws the sticks around. Even dried wood can still hold pockets of moisture.

I just wanted to clarify that my system's inner barrel does not have a removable top.  It is a capped cylinder. The top is welded shut with a disk. The bottom is open and rests on the inside bottom of the outer barrel.  I have no holes whatsoever in my inner barrel, besides the crack that forms naturally between the bottom end of the inner barrel, and the inside bottom of the outer barrel both of which are not perfectly flat.  The inner barrel is made out of a heavy gauge steel pipe and is unlikely to fail for a long time, or to lift off in the event of a minor explosion of gases.  That said, I will be watching my unit more closely, to ensure that if something does happen that I am there to stop any fires.  Thanks for posting about this, Todd.

 The small explosion that you experienced was not caused by a build up of pressure that was then released.  It was caused by the sudden ignition of the right mixture of wood gas and air in a confined space.  You should not have holes on the sides or the top of the interior can.  By doing that you created a path for oxygen to get into the inner vessel during the process.  When you get the right mix of wood gas, air, and heat you generate a sudden explosion.  By putting holes only on the very bottom of the can, gas can find it's way out but air won't get back in.  It works a little like a check valve.  Those are the conditions that we want in order to make biochar.  Heat in.  Gas out.  No air in to the inner can.

It works a little like a check valve.  Those are the conditions that we want in order to make biochar.  Heat in.  Gas out.  No air in to the inner can.

 Yes.  This is an excellent point.  I should have thought of that and brought that up.  Thanks Bob; that is probably more likely what happened.  

Bob Wells wrote:  The small explosion that you experienced was not caused by a build up of pressure that was then released.  It was caused by the sudden ignition of the right mixture of wood gas and air in a confined space.  You should not have holes on the sides or the top of the interior can.  By doing that you created a path for oxygen to get into the inner vessel during the process.  When you get the right mix of wood gas, air, and heat you generate a sudden explosion.  By putting holes only on the very bottom of the can, gas can find it's way out but air won't get back in.  It works a little like a check valve.  Those are the conditions that we want in order to make biochar.  Heat in.  Gas out.  No air in to the inner can.

As I said before, I'm not certain what happened.  I have made biochar this way perhaps a dozen times.  Every time it worked the same way:  As soon as the heat in the barrel reached a high enough temperature, wood gas exits the hole in the lid and ignites.  I can't tell what is happening with the holes near the bottom of the can because there is still wood in the way that blocks being able to see them.  My assumption is that wood gas is being vented out those as well, but isn't ignited til it gets up near the flames.  When the wood in the outer barrel burns down far enough, you can clearly see flames shooting out those holes as well.  That's how I know when the charcoal is done.  As soon as the flames stop coming out the holes, I take the inner barrel out and put it into another 55 gal drum that is airtight.  I leave it in that barrel overnight to cool, and take the charcoal out.  It may be that it worked as you outlined in your post, but it's odd it hasn't happened in any of the previous attempts.  Either way, thanks for your input, I may try a different inner barrel with openings only in the bottom as you said.

That s a similar result to when you run a wood /solid fuel burning stove if you throw in a load of fresh fuel on top of embers and it smoulders for awhile from the bottom up the heat starts to cook the volatile oils out of  unburnt timber/fuel  into a gas, if the air coming in is closed down it often just hiccups into a flame at some stage , but if the air inlet is wide open and ignition point suddenly  happens ---theres a big belching woof which blows top lids off and ash out the loading door vent --most stoves only can fit a small load so the bang is contained better. Glad  you lost nothing , one of my experiences took my fringe and eyebrows , the scrap timber i was using was contaminated with linseed oil.

Pottery explodes for the same reason. Water is chemically bound until you hold at 700 F for an hour. so bring it up to 250 and let it sit for 30 minutes, then let the temp rise at 200 deg per hour until you get to 700, then hold one hour, then you can go up as fast as you want because water will no longer be chemically bound to the wood.