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The Rocket Stove Title - Is It TOO Broad Reaching?  RSS feed

 
pollinator
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I just received an email about a new product, and it brought back several questions that I have had about rocket stoves.  I have been studying rocket stoves for many years, and there is quite a discrepancy betweh
The main stove that I view as a rocket stove has a drum above the main burn area, so that the vapors can combust again.  This makes it efficient, and reduces the smoke and emissions from the stove.  Within the past few years, people have proposed that their J style stoves are rocket stoves.  I can indeed agree that these use the same primary burn method, the use of thermodynamics to increase air flow over the burn area.  However, I fail to thint that these are actual rocket stoves.  Am I being closed minded?  Am I applying my own definition to it, where it does not apply?

The newest stove that I have seen listed as a rocket stove has been made by biolite.  https://www.bioliteenergy.com/products/homestove  I am impressed, because this is a 150 dollar stove that has the same charging ability as their smaller stove.  I also see an ability to shove a stovepipe down into the top, to allow this to be used in tents.  Granted, this would inhibit the cooking ability, but it would allow it to provide heat inside.  This is a larger unit, so it doesn't compare with my Seek Outside stove, but I am interested none the less.  I am a techy at heart, and I absolutely love biolite's mission.  

So, do you view this as a rocket stove?  Do you think rocket stoves need a reburn?  Or, do you think that companies are latching onto the rocket stove name, and that this is a marketing ploy?  I am interested, because we have many rocket stove afficianados here.  
 
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I would say that the J style tube would be known as a Rocket Stove, But the design with the barrel and cobing of some sort would be known as the Rocket Mass  heater.
 
William Wallace
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I don't think the barrel is what makes it a rocket mass heater, but that is just my own personal opinion of it.  The part that makes it the mass heater, is the cob or other heat soaking material around it.  This is the mass that gets heated and helps to radiate it through the room, and holding much more heat over time.  Of course, I have run into the J style stoves quite often, on youtube and such, and they just don't seem as efficient.  

Perhaps there needs to be some terminology for the one with the reburn barrel, to distinguish it from the normal J tubes.  I understand the point that the J tubes are rocket stoves, because the air is accelerated through the burn chamber ..... just for some reason I see the need for the reburn barrel.  This is likely my own view, and not shared by others, for the reburn does not affect the rocket-ability of the stove, just the efficiency of it.  Perhaps the barrel makes it a "HE" rocket stove, standing for high efficiency.  
 
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I'm pretty certain that combustion is "supposed to be" complete by the time it leaves the insulated riser.
The barrel can be replaced with a lot of current things, and the burn should still be clean and complete.
 
William Wallace
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I'm not sure this is the case.  Regular J tube rocket stove are made out of metal, and have uninsulated risers.  The barrel type have the same lower portion as these claimed rocket stoves.  If the barrel then is able to recombust the gases, this means that the J tube styles must be leaving some combustibles unburned.  Otherwise, there would be no gases to burn in the barrels.  This is one of my major problems between the two different stove types.  Also, the amount of combustible material left is hard to measure, so the normal J tube could make the claim that it's efficient, when it's unable to be measured.  
 
William Wallace
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It's entirely possible that it's just my preconception that the small J style rocket stoves are not efficient.  Has there been any research done, like tests side by side for fuel consumption and BTU output over time?
 
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I have a traditional masonry stove.

It has a big firebox ( 4ft deepx2ft widex 32 in high)  and serpentine vaulted brick chambers that equal about 15 feet of afterburn chamber before any smoke enters the chimney. The whole stove is 8 feet high.

This unit also gives complete combustion of both wood and smoke/gasses.

Not a rocket stove by any means, but does operate on the same principle.

Only difference being size. I can put a garbage can full of wood at a time and lengths of 3 ft. So its great for a big house like mine, in a cold climate like mine.

In Siberia , big masonry stoves are often made with a sleeping platform on top!

The chimney top in photo is the one for the big masonry stove plus one other flue.
SANY1677.JPG
[Thumbnail for SANY1677.JPG]
Big house to heat. Note chimney top (3x4ftsquare)
 
William Wallace
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How can you gauge "complete combustion" ?  Surely, one can't look just at the lack of deposits left on a stove, because some of the volatile chemicals wouldn't leave deposits, as they would be expelled as gaseous chemicals.  It is the creosote and similar materials that leave the visible markers.  

Is complete combustion an opinion based factor, or do people actually capture the off gases and try to combust them?  
 
Mark Deichmann
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In my case I can hear the afterburning roaring up through the chambers.
Also the lack of creosote in the stove and chimney.
Also very little smoke coming out of the chimney.
Thats about as scientific as it gets,  I figure wood should always be burned with signigicant air flow /draft and only reduced draft when there is a glowing pile of embers.

I guess any stove can be burned hot and with a high rate of combustion.

For that reason I think maybe the "rocket stove" title should only apply to the specific stove type being promoted as such.

Certainly " complete combustion" or "clean burning" is not something that can be "owned" by any specific design , but is more likely related to how the stove is used and the dryness of the fuel.

In the stove pictured here, the smoke/gasses circulate in the tall chamber above the firebox and that is where the afterburning occurs in this case. A person can stand upright inside this unit. It provides both radiant heat, tons of light and hours of re-radiated , stored heat.
SANY0485.JPG
[Thumbnail for SANY0485.JPG]
High mass fireplace style masonry stove with large combustion chamber
 
William Wallace
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Indeed, that is a difficult stove to measure the gases from, because your chimney is so far above the burn chamber.  In a small rocket stove, I wonder if there is an easy method for a combustion check.  I've been thinking about these things, as I ponder the efficiency of stove designs.  It's one of the topics that interests me, ever since I saw that one can create a propane alternative as a by-product of biochar.  

In that Biolite stove, they help propel the fire with the use of a fan that is powered by the fire itself.  I'm not quite sure that this stoking of the fire really makes it a rocket stove.  Granted, one of the main traits of a rocket stove is the increased draft airflow over the fire, but is an L shaped burn chamber all that is needed to capitalize on the "rocket stove" fad?  I'm not suggesting that rocket stoves are something that are "in style" and will go out of style, but that the term itself "rocket stove" brings some marketing and sales power along with it.  

I figure there is some expensive vapor analyzer units that can give readouts of volatile components exiting stoves, but I've never seen reports done for different stove types.  If someone says that rocket stoves are 80% more efficient (a number that I just made up), is that really a scientific number, or is it opinion?  Are they basing that on things that can actually be measured, and can those measurable things actually determine efficiency?  

As you point out, there are many variables in stove use.  The wood plays a factor, so does external temperature and things like humidity.  Different species of trees burn significantly different, and different sizes of stoves burn very differently.  Do we know of any scientific side by side comparisons on stove models?  I've seen lots of people talk about their experiences, but it seems as if they "feel" that one works better than the other, or they gauge it based on the amount of wood that they go through in a season.  I agree that the volume of wood burned is an indication of burning efficiency, but I question whether it's a trademark of combustion efficiency.  

Would one place a flame above the stove, such as a bunsen burner?  Or would you need a device such as the reburn barrel so that you create cavitation with oxygen to get those excess volatiles to reburn?  I really don't know.  I haven't seen this topic breached before.

I appreciate all of your responses.
 
Mark Deichmann
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Certainly alot of different modifications have been tried/ are in use.

Some stove owners  pipe in outside cold dense air and blow it into the firebox. This increases effeciency as the cold air burns better than warm and it also doesn't create negative air pressure inside the house or effect it.

Certainly the government agencies that rate stoves perform tests on effeciency, here in Canada its CSA that rates them. They do all the scientific testing you are looking for. In the US , not sure but maybe a division of the EPA or whatever safety standards authority. Their stamp will be on any stove sold for public use.

Further to the use of woodsmoke/gasses as a propane substitute I found the cars that were converted to burn wood by the Germans during ww2 very interesting.  They had a large container at the rear of the vehicle where wood was kept smoldering(not burning) and the smoke was piped to the engine and ran the engine which I believe had to be started on gas.  YOu might be able to look that up. Lots of possiblities with wood fuel !

I agree that peoples experiences are variable and not scientific.  The testing labs are probably what you are looking for , they rate the stoves to a BTU output ., which again is related to size and will vary with dryness and species of wood. Fans and intake modifications such as cold air intakes will add further efficiency.

In my experience , keeping wood as dry as possible and burning hot with lots of airflow will give the greatest amount of heat, and keep the flue clean. Smoldering fires with less than dry or sappy wood will give a "cold fire" and creosote build up.
 
William Wallace
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Have you seen the PDF put out by FEMA, on how to convert your tractor to wood burning?  It is a fascinating read, and a valuable resource.  I appreciate you reminding me of it, because I was compiling all of my favorite SHTF pdfs (and this is one that shouldn't be forgotten).

http://soilandhealth.org/wp-content/uploads/0302hsted/fema.woodgas.pdf

I'm also not sure if those stove regulators are looking at complete combustion, or if they are just looking to see if it is efficient enough to not produce a dangerous amount of buildup.  Also, I'm not quite sure if BTUs is the right way to determine if all volatiles have been burned, because different stove types will put off different BTUS, and the heat will be dispersed in different ways.  Think of the way the rocket stove really gets roaring.  To me, that increased draft over the burn area would also pull more heat up and out of the stovepipe.  Would a less efficient stove (or at least one with less airflow) then create more BTU?  I'm not quite sure ..... I just know that something doesn't feel right with the preconceptions of stove use.  
 
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A rocket stove can be called so because it has an L or J shaped combustion core, with a vertical riser to increase draft and allow combustion to complete. Small or metal ones may not have as good efficiency as bigger ones, but if the core design gives proper turbulence and dwell time, combustion will still be good. In any case, I don't think that a well-configured rocket stove would actually have more combustion in a surrounding barrel where the temperature is much lower.

When you get to the little tin can sized units, they can't have enough heat concentration or dwell time to achieve full combustion. It used to be that DIY designs for small units still emphasized insulation around the core; if ones are being sold with no insulation, I would hesitate to call them rocket stoves with or without a barrel.

There is one professional emissions testing unit I know of, the Testo 320 I believe, that Peter van den Berg uses for his stove development. It can give levels of oxygen, carbon dioxide, carbon monoxide, efficiency, temperature (usually of the exhaust to test how much heat is being lost in spent gases), and a filter that gives a relative gauge of particulate emission by how clean or dirty it looks at the end of a run. Unfortunately it costs a couple-few thousand dollars.

A core designed according to published proportions and sizes should have no volatiles left after leaving the riser. The only real way to test this, aside from gross differences in output, would be with a Testo. You might be able to get a decent relative approximation by actually sniffing the (cooled) exhaust; this would only be practical among relatively clean designs. I once made a firebrick mockup of a 4" batch box core, and could stand a few feet above the riser with my face fully in the exhaust and comfortably breathe. The flames in that little free-flowing core hardly reached more than halfway up the riser, but obviously burned clean.
 
Glenn Herbert
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There is such a thing as too much airflow; more than is needed for full combustion, counting excess to compensate for inevitable less than perfect mixing, will only cool the fire. A rule of thumb is that you need around three times theoretical stoichiometric volume for best results. Batch box designs have been calibrated to use about 15-20% of system size for primary plus secondary air. I find my J-tube burns best with the top about 2/3 to 3/4 covered. This creates a jet of air blowing on the fire as well as limiting the volume to what is needed.
 
Mark Deichmann
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THe airflow question is easy as I  posted above, Open draft during combustion, closed draft when the is a heap of Glowing Embers . Not much cooling or heat loss then!

Also wanted to add on a historical note: The Romans built  heating systems similar to some  rocket stove designs with horizontal flue under the floors(heated benches) and then up walls and out. The firebox was outside the house wall though , another system that has been brought back into use in recent years.

The Romans developed this heating system for their villas in England (Brittania) and called it a "Hypocaust" to make life in the cooler climate more comfortable. Probably some of the first floor heating systems constructed.

 
William Wallace
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Glenn Herbert wrote:A rocket stove can be called so because it has an L or J shaped combustion core, with a vertical riser to increase draft and allow combustion to complete. Small or metal ones may not have as good efficiency as bigger ones, but if the core design gives proper turbulence and dwell time, combustion will still be good.



Glenn, would you call that Biolite stove a rocket stove?  
 
I agree. Here's the link: https://richsoil.com/wood-heat.jsp
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