Rocket Stove with Cob

Hi,

First of all thank you for this forum. I learned a lot about Rocket Stove concept here.

I have created a Cob Rocket Stove of 4" system (Dia of pipe).
With ratio formula 1:2:4, dimensions are 6'' feeder, 12" burner and 24" riser.
 



This was my second attempt and overall it worked well. I could see the temperature more than 500 deg C at riser end.



I have one or two questions regarding the feeding.



How much fuel should be fed in feeder at a time for good efficiency? Can we fill it full so that there is no empty space left? I think some empty space is required to suck the fresh air inside.
And if some empty space is required then at what side? I mean in feeder hole, wood sticks should be kept near right edge (Figure 1) or left edge (Figure 2)?

Figure 1 - sticks left side of feeder(x - wall, S- stick)
                      x        x
                      x        x
                      x        x
                      x        x
                      x        x
    SS             x        x
  xSS   x         x        x
  xSS   x         x        x
  xSS   x x x x x        x
  xSS                        x
  x x x x x x x x x x x x


Figure 2 - sticks right side in feeder (x - wall, S- stick)
                         x        x
                         x        x
                         x        x
                         x        x
                         x        x
         SS           x        x
  x     SS x         x        x
  x     SS x         x        x
  x     SS x x x x x        x
  x     SS                      x
  x x x x x x x x x x x x x

Another question is how to run stove for longer duration, how to keep on feeding without blocking (by ash) the burner area? Or what is proper procedure and precaution for continues feeding?

Thanks,
Gagan

Hello Gagan,  
Your stove looks very nice.  :)

In regards to your questions: I would agree that some air space would be required for good efficiency but not too much so that the heat is not dissapated too quickly by the incoming air. Have you heard of the "P" channel? ...or seen any of the modifications to the J tube rocket stove that a fellow named Peter Van den Berg has come up with to help mix the gasses for the most optimal combustion?
Without a gas analyzer, I would think its impossible to tell how efficient and clean a stove is really burning other than noticing if it produces smoke or not.

For reducing ash buildup for longer burn times, from my experience has been to let the fire burn down to coals occasionally to allow the ash to be naturally swept away in the exhaust stream before adding more wood. Jiggling the wood ends also will help loosen some of the ash below and send it on its merry way.

Figure 1 leads to less smoke.  I believe this has been studied.

Figure 2 has all the incoming air passing between sticks instead of bypassing them. This can lead to better combustion. I always lean the sticks against the far side of the feed tube, closest to the riser. A P-channel, which for a 4" system especially can be made from a flattened tin can, will keep a secondary air channel open, and shoots a jet of fresh air down into the middle of the fire. The cross sectional flow area of a P-channel should be about 5% (1/20th) of the total. For a 4" square opening, that would be one fifth of an inch by four inches (say 1/4" to allow for irregularities). Adjust to do the same for a round feed, and you might try a flat plate that gives 1/2" maximum thickness of opening.

Here's the thread where the results of this being studied are discussed http://donkey32.proboards.com/thread/355/small-scale-development

the air which is sucked into the feed tube had to be unrestricted on the side of the burn tunnel. In other words, letting the fuel lean to the burn tunnel side resulted in a dirty burn, letting it lean to the other side it did clear up again. All according to the Testo 330-2 gas analyzer. In order to minimize the chance of going the wrong way I've mounted a plate in the feed tube, leaving approximately a gap of 1/4" which can't be filled with fuel at all.

Right, that's the P-channel. Without that, if the sticks were in a dense enough mass I could see airflow being choked off and not enough oxygen getting to the fire. I think it depends on the character of your typical fuel - if you usually have crooked sticks, you can't avoid a lot of gaps to allow airflow; if you have very straight wood, like milled lumber cutoffs, choking is a serious possibility.

I observe that when my logs are half burned away on the burn tunnel side so there is a large free channel with a pile of hot burning wood on the near side (and a cover over 2/3 or so of the top to regulate airflow and eliminate flames out of the top), it will sometimes start to pulse, going "huff-huff-huff" with flames wanting to come out of the top. When I move the logs to the burn tunnel side, the fire evens out and brightens up. Of course, all this time the P-channel is doing its job supplying preheated air to the heart of the fire in the burn tunnel.

There are lots of different combinations one has to deal with.  In the original Winiarski L tube, it works better with a grate under the wood with the explanation that incoming air is heated by the coals that fall through the grate.  But in the batch box, a grate under the wood causes smoke.  I guess it's because in some situations the incoming air cools the fuel so lowering the combustion temperatures more than it preheats the air leading to increased combustion temperatures.

That's why the latest iteration of the batch box has the secondary air routed in a steel tube in the floor of the firebox; it gives preheating without disturbing the coals. Also, in this location, it is easily replaceable when the steel corrodes as it will in the environment. The injector pipe rising from the steel tube can be swapped out quickly.

Thank you @GerryParent for your feedback. Regarding the ash after long duration burn, now I have one doubt on my question itself. If the burning is efficient then the ash content should be very minimal. I think my burning of wood was not efficient and that resulted in unburnt wood (looks like tiny pieces of coal and ash). Any comment on this.  

A highly insulated core will be much hotter and will produce very little ash.

Thank you @Graham and @Glenn for your feedback and discussion. I learned a lot with this about the inflow of air in burn chamber.

I was thinking of Bernoulli's principle, consider Figure 1 where stick are at left of feeder (away from burner), as the air after burning from stick is hotter and moving towards riser, the pressures reduce on the left of burning sticks and now opening is on the left so fresh air would be sucked in automatically. And that may help in better burning.

Regarding P channel (thanks @Glenn for dimensions) I have to see how to design in cob rocket stove while casting.

Cob wall of riser are 3 inches thick, hope that's quite a good insulation.

You need refractory materials for your core. Cob will eventually crack and that will further lower your temperatures.

I have built wood-fired pottery kilns out of cob, firing sometimes as high as cone 02 (about 2000F), and the inner face of the cob will show shrinkage cracks but not fall apart, in fact it becomes pottery. The more straw/grass/organic material is in the cob, the more tiny voids there will be and the better insulating.

Gagan Bansal wrote:Cob wall of riser are 3 inches thick, hope that's quite a good insulation.

Cob does not make for good insulation. Your generally looking for light weight materials that can withstand high heat with exception to the feed tube which needs to withstand the scraping and gouging of the wood. Cob will absorb the heat from the fire taking much longer to get a cleaner hot burn and eventually crack as Graham mentioned due to the thermal shock it goes through with each burn cycle.

Gagan Bansal wrote:Thank you @GerryParent for your feedback. Regarding the ash after long duration burn, now I have one doubt on my question itself. If the burning is efficient then the ash content should be very minimal. I think my burning of wood was not efficient and that resulted in unburnt wood (looks like tiny pieces of coal and ash). Any comment on this.

From my experience, each type of wood will produce more or less ash than another but no matter what the species of wood you burn, unburnt wood is unburnt wood. Have you noticed that most of the ash gets blown up into the exhaust stream as your burning? I would think that some minerals present in the wood can only be broken down so far if optimal high temperatures for them to do so are present so I would say there's no such thing as an ashless fire, only a fire that has efficiently burned as much as it possibly could due to optimal conditions.

Graham Chiu wrote:You need refractory materials for your core. Cob will eventually crack and that will further lower your temperatures.

My objective is to make rocket stove for villagers who are already using three wall wood stove made up of cob. Refractory material would be quite costly.

I have to improve my cob rocket stove only, as I am in learning phase.

Glenn Herbert wrote:The more straw/grass/organic material is in the cob, the more tiny voids there will be and the better insulating.

I got your point. I have used local clay and rice husk with proportion around 2 part clay and 1 par part husk (volume wise). The final cob after adding water and mixing was quite heavy. Next time I am thinking to increase the husk proportion. Any idea what should be the ideal proportion?

Gerry Parent wrote:Have you noticed that most of the ash gets blown up into the exhaust stream as your burning?

In total I had done around 30-40 min burning, total ash had not blown up. I'll try next time for longer duration.

I can't tell you what proportions of clay and rice husks will work well, you have to test your local materials. I would start with 1:1, and also some larger proportions of rice husks. All the insulating material has to do is stand up on its own; for strength, you would add a more clay-rich layer around that.

Strength and insulation are inversely related: the stronger it is for given raw materials, the less it will insulate, and the more it insulates, the weaker it will be.

Thank you Glenn for valuable input.