Thursday, January 16, 2014

Pocket Rocket Stove

One nice thing about combustion of various fuels this time of year is that it produces heat.  Heat, it turns out, is very useful for increasing the temperature of things, like bacon on a grill, a fermentation vessel currently developing a batch of wild grape wine, or the air in a garage or house.

Many wood stoves and fireplaces used for those purposes have two distinct drawbacks with regard to that last one: they lose a lot of heat out the chimney and the don't completely burn the wood.  Those drawbacks in turn lead to the problems of unnecessary wood cutting and air pollution (enjoyment in the exercise and the charming scent of dilute woodsmoke notwithstanding).

Enter the rocket mass heater, RMH, rocket stove, or half-dozen other names by which they may be called.  These heaters are designed with two characteristic features in mind.  First, they naturally draw sufficient air to completely combust the fuel (usually wood).  That is, there is enough oxygen flowing through the burn chamber that volatile gases from fuel pyrolysis can ignite and burn completely.  Second, maximal heat is extracted from those hot combustion gases because the 'chimney' (more accurately called an exhaust tube) takes a tortuous path out of whatever building the heater is heating, and doesn't just allow the heat to go straight up.  This tortuous exit path is often surrounded by high-heat capacity materials, such as cob, to absorb the heat while the stove is burning and radiate the heat later on after the stove is snuffed.

Anyway, we decided that it was time for us to dabble in the art of rocket stove construction because we wanted to 1. heat the integral part of the Lab commonly identified by outsiders as 'the garage,' and 2. have an extremely clean-burning device on hand for making some biochar from leftover butchering waste (i.e., something that wouldn't smell like burning meat).

We recently had the opportunity to browse the classic Rocket Mass Heater book, which describes the theory, operating principles, and construction of a variety of this class of heaters, and decided that a Pocket Rocket stove would be a good place to start.  We came to that conclusion primarily because it's simple, portable, and we already had most of the necessary components on-hand.  Note: if you do an internet search for this model of heater, make sure to include 'stove' in your search terms, or you'll just end up with lots of motorcycle pictures (or other impertinent stuff). 

Caveat: This project has the potential to cause lots of damage to you and your surrounding area if you aren't careful.  If you try this, make sure you do test burns in a well-ventilated space away from flammable buildings and trees so the dern thing won't kill you with carbon monoxide, burn you like the Nazis in Raiders of the Lost Ark (albeit by a different mechanism), tip over and start your garage on fire, or put you on Smokey the Bear's naughty list. If you do it on a concrete surface, insulate the floor from the bucket so the concrete underneath doesn't crack and crumble.  Do more research than just this blog post.  Don't fire this thing up during drought conditions.  Don't accidentally think of other ways to cause a disaster with it, and above all, don't try to sue is if you do! (Trust us, it's not worth the effort.)

Anyway, let's get started!

We started with a steel bucket and steel lid, marked off where the inlet and outlet holes would go, and cut a hole out of the middle of those.  We used a regular 3.5" hole saw, and were surprised it worked so well.  Then we marked off where we wanted to cut tabs out to the lines we marked off...
...and cut the tabs.  A big wire cutters would have probably worked well, but so did the knife, and we were looking for an excuse to bring it back to Scary Sharp (which we'll have to do if we want to cut anything with it after this project).  The original plans call for a 6" stove pipe as the inlet, but an old paint can is also right about 6".  Other tools we found useful in this step were leather gloves, a vice grips, and a hatchet for bending the tabs back.
The exit pipe is a 4" diameter steel pipe that formerly supported our precarious mailbox.  It's too heavy to alight upon the lid like the plans said, so we cut a section out of the bottom and let it rest on the bottom of the bucket.  We pounded those flaps in to make it easier to stick through the hole in the lid.
Here it is fully assembled and ready for a test burn.  Right now, all the hot gases will still go out the top, but we're planning to change that before using it as a garage heater.  It turns out that the gases coming out the stack are hot enough to burn a piece of wood, so they would presumably have the same effect on a garage roof.  We'd like to avoid that.
To light the stove, we made a few origami snowballs out of newspaper, junk mail, or whatever, and put them down the chimney.  It helps if they're small enough to fall all the way to the bottom, but it's not completely necessary because they'll get smaller and fall down as they burn.  (Whoa, far out!)  We Lit the last one and dropped it in the chimney on top of the others.  This should get the draft going the right direction. (It did for us, in surprisingly easy fashion.)
Once the draft is going through the chimney, we put some more paper in through the intake such that it would catch fire from the snowballs, and slowly built up with small sticks, then larger sticks, scraps of wood, etc., until we had a roaring blaze like this going.  Any coating on the bucket should start decomposing in short order.  It was kind of neat to see a lot of the fumes being sucked into the intake.  Holy smokes (heh), it's working like it's supposed to!
Also, if there's a plastic handle on the bucket, it might be a good idea to remove it before starting the fire.  But check out the radiant heat flow on this puppy!
If the path to the chimney isn't quite clear enough, or the wood not quite dry enough, it would burn a little like this (picture shows worst event).  One correction we might make with our design is to make a bigger hole in the bottom of the chimney so coals and ashes don't block it as easily.  Our chimney is also a little shorter than it should be, which doesn't help.
But if we keep the path clear and don't overload the intake, the exhaust is nice and clear.  Time for a more challenging test!
The black thing in the middle is part of a chicken carcass, post roasting and de-meating.  As long as we kept the wood-to-chicken ratio high enough (which wasn't hard), there was no smoke and no odor.  Hypothesis supported!  Woo hoo!  Plus it makes some high-phosphorus ashes/biochar.  

Now we just need some extra 4" stove pipe to direct the exhaust back toward the ground and along the floor so we can heat the garage with it.  Fortunately, we've got IFTTT on post if any pops up on Craigslist for the right price.

Have you constructed a rocket heater for a particular space?  Have you used it to make biochar before?  Let us know in the comments section below!


  1. I can't believe we never built one of these as kids.

    1. I'm starting to wonder if the distinct shortage of steel 5-gallon buckets around our place was on purpose...