Advice for a rocket stove in a community garden

Hi,

as you may know, we have a small community garden without water. Participation is at its lowest right now, so I'm thinking we could revitalize the community with some bbq days. It may also be a perfect excuse for building a rocket stove, but I don't know where to start.

My building materials are basically dirt and stones. My budget for this project is zero. I was thinking in building some sort of ´table´ out of dirt, using a pvc pipe as a mold for the chimney. At the exhaust I think a flat stone could be good for storing heat and cooking on it. It doesn't have to be fancy. What I want most is the stove being able to do the work, a.k.a. cooking without burning my stuff, and second to inspire people with a demonstration of this technology.
As I said, I want to lure people back, and showing a new feature is the bait.

If your dirt is clayey, you can make a fine rocket stove with it. If you don't know the answer to that, take some dirt, dampen it and work it until you can make a ball you can squish around. Form that into a "snake" shape, and try to bend it. If you can bend it around your finger without cracking apart, you probably have enough clay to use.

You can make a barbecue heater with some iron grate over the heat riser, or support a flat steel/cast iron plate above the riser to cook on. A stone is most likely going to crack quickly, and certain kinds of stone will explode if heated that way.

The simplest and most foolproof style of rocket stove is the L-tube: a horizontal burn tunnel where wood is fed, with a sharp angle to a vertical heat riser. I have found that around 2/3 to 3/4 meter for both lengths works well, and around 10-15 cm diameter. Larger would give a very intense fire, probably too much to cook on.

You can try more advanced styles once you have a basic L-tube working well.

Thank you, Glenn.

Yes, my dirt is clayey. I didn't know rocks would break, I'll try to find that iron.
Am I reading well? 66 to 75 cm for both pipes?

I don't fully understand where it would be the place for the iron grate. Let me try some ASCII art.

               ============      <-- Iron grate?
                 I     [       ]      I
***********[        ]**********
***********[        ]**********
***********[        ]**********
***********[        ]**********
===========[        ]**********
 wood                     ]**********
===============***********
**************************

Yes, your "sketch" is exactly right.

With a longer burn tunnel, I would make that a bit taller, like 10 x 15 cm or 15 x 20 cm, so there is room for coals to build up without choking the airflow. A long piece of sheetmetal under the wood feed would allow fresh preheated air to get to the back of the firebox, though I haven't found that to be necessary with the added height inside. The smaller the wood, the more likely it is to collapse into coals before burning up and choke the bottom coals so they can't get air and burn. Larger wood mostly burns while log-shaped, so the back of the firebox does not get choked.

One thing that I have found is that food as a reward for volunteers helps bring out the volunteers. I've been thinking along the same lines however making a rocket wok and/or pizza oven rather than a BBQ. Since it is a community garden and fresh veggies abound, roasted veggies or stir fries would be an easy way to use items on hand. Throw the idea out and see what the community can scavenge for your project. Make it a volunteer event to draw out interested parties. It could be a dual draw for those not only interested in the community garden but for others interested in rockety things.

Hello, I think I need help again.

I've built the stove, and it sounds like a rocket, so it should be good. However, I don't know how to operate this. I've managed to bring some sticks to fire, but when I try to load more sticks, they just don't burn, it's like the fire is going inwards and not touching the new sticks.
What am I doing wrong?

From the angle of the photo, it is hard to tell what the actual proportions are. Is it 100 mm spiral duct? Is that a flexible duct curved to form both horizontal and vertical?

My dimensions were for a permanently constructed core with smooth surfaces. The corrugated surface you have would make it more difficult to feed wood, especially at the smaller end of the size range. You may need some slightly larger wood than the twigs and vegetable stalks I see. You have to be able to put new fuel at least partially on top of what is burning. My feeling is that it would not be good to have the length of the horizontal more than four or five times its diameter. You wouldn't be able to see in to know what the fire was doing.

Digging deep into the ground is fine for an experimental stove, but I think a community setup would work better if mostly built above ground. How flat is the ground in your garden? Can you make a place to stand and feed the fire (and tend cooking things on top of the riser) that will drain away in case of rain without making a puddle? I think people will be considerably more interested if they can look at all parts of the stove working without getting down on their knees.

@Glenn
It's 150mm spiral duct. It goes 70cm in horizontal and 70 cm in vertical. I tried larger wood but they didn't catch fire. In an open fire it usually works, but here flames were heating inwards, making it difficult to heat new wood.
Don't worry about it beind dug, I made use of a jump, and it looks like a table in one of the sides.

I am wondering if making a J-tube of dirt would improve things.

When you put a grate and a pot on the exhaust, will that slow down the draft enough for fresh wood to catch fire?

Douglas Alpenstock wrote:When you put a grate and a pot on the exhaust, will that slow down the draft enough for fresh wood to catch fire?

This could be it. It was very windy yesterday and I had no pot at hand to test.

That is curious, dry wood laid on top of burning wood generally catches fire, even more when fanned by a draft. How dry is your larger wood?

Glenn Herbert wrote:That is curious, dry wood laid on top of burning wood generally catches fire, even more when fanned by a draft. How dry is your larger wood?

Can't say for sure. I picked them from the ground. Some came from a recent pruning, some are from the last year one.
But it looks like all the heat went into the chimney and very little remained to heat the new fuel, as if it had too much draft. It was very windy.
I'll try again in a more peaceful weather.

This is a rough and ready installation. Better too much draft than not enough!

As an experiment, you could adjust the draft by pushing a chunk of broken brick/rock into the tunnel. Or you could put big pieces of slow burning wood further into the tunnel. Why not?

It works!
Thank you all.

I've added that piece of marble, to give it a feeder shape. Now I think of the marble as if it was the bottom of a common hearth; small sticks touching the stone, bigger wood closer to the chimney. It's cool to see the flames going horizontally.
We made some pop-corn as a test.

Well, now we have done several tests and we were able to even prepare a meal for seven people.

Things learned so far...
-It cooks with flame. Big branches that turn to cinder are good for bbq but not for pan cooking. We need to cut them into smaller sticks.
-Sticks need to be rather straigh if we want them to go down as they burn. Not too long or they risk burning outside the chamber.
-The metal corrugated tube does not stand heat. I planned to use it as a mould, but the dirt is not compacted yet (not enough rain). The result is that some dirt was falling down as we cooked, blocking the intake. We need to compact the dirt first before using it again.
-Wet wood works (that's all we had), but temperature is lower and the cooker still is smelling like smoke.
-We can't let the fire unattended. Someone has to keep feeding the thing for maintaining the flames. Flames collapse very fast and turn into cinder if unattended. (Sadly, the cooker was often required to attend conversation in the lounge area, only to find the fire extinguished when he was back to the 'kitchen').
-A pan lid would have been welcomed, especially for vegs.
-It's worth to invest time in the leveling stones. Once the pan is hot it is hard to maneuver.
-It is easy to overfed the tube, blocking ventilation. We had better results by placing the fire just outside the mouth of the tube, allowing the flames to be absorbed by the draft. It has to be within 5 cm of the entrance.