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Rocket Stoves

 
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My First Rocket Stove Fabrication!

Hello! I have completed my First Prototype Rocket Stove. It is capable of heating a 2000 square foot home and burns about 1.25 pounds of wood pellet fuel an hour. It is designed to run for hours at a time without fussing with it and requires no electricity to operate. It has unique feature that uses supersaturated steam to help with a cleaner burn and regulate the intense burn chamber temperature. The refractory acts as a mini thermal mass and stays warm for about 3 hours after the pellets burn off. The stove is started up in the morning using a propane torch and is shut down at nighttime. This stove has run for over two years now. I originally posted the build on Instructables for a metal build contest and it got 10k views in the month I had it on their site. I decided to take it down because I didn’t feel it was the appropriate site. I know that wood pellet isn’t exactly “salt of the earth “ homesteading but it is using a renewable resource. One would have to make a larger burn chamber for split wood.
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Steam addition and ash reburner
Steam addition and ash reburner
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Main assembly
Main assembly
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Overview
Overview
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Burner swirl
Burner swirl
 
pollinator
Posts: 316
Location: Victor, Montana; Zone 5b
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That is pretty cool. Looks like something out of a steampunk novel.
 
Peter Chauffeur
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Thank you! Pink is our favorite color!  The addition of steam morphed the design into a steam punk look.
 
pollinator
Posts: 434
Location: Penticton, Canada
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building woodworking rocket stoves
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Peter,    I agree with Daniel, totally steampunk!...I could see "Doc" from Back to the Future running it.  I would love to see the inner workings on this stove. Do you have it re-posted somewhere on permies?  Thanks Gerry
 
Peter Chauffeur
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Here is a diagram of the “general idea “. Basically it is a standard J tube construction where there are ash trays below each 90 degree bend. The hardest part of the design is the delicate balance where embers fall to the secondary burn chamber ( the stainless steel pot and lid) Remember, you have to utilize the coals to add to the heat input so they become part of the heat equation. I originally called this rocket stove “Frankenstove” as the refractory was cast in a box holding it all together with threaded rod.
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Schematic
Schematic
 
pollinator
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Impressed...

I would be very interested to know if you have the PDF of this when it was on instructables...

I have been trying to get an automatic feed for wood chips, but have not found a working design as of yet...

Thanks for sharing.
 
Gerry Parent
pollinator
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Location: Penticton, Canada
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Thank you for the diagram Peter  
A few questions for you....

1) The Primary air cover...can't quite make out what it is made from? Is it adjustable for different burn rates?...or does the primary air come through those slits?
2) "It has unique feature that uses supersaturated steam to help with a cleaner burn and regulate the intense burn chamber temperature." Here's what I see: The brownish upside down bottle is the source of the liquid (water?) which keeps the stainless flask topped up. The liquid in the flask gets heated through the side contact with the stove. The steam produced travels through the copper pipe which then looks like it penetrates through the stove right after the burn chamber. So if any of this is correct, I'm at a loss as to how cooling down the fire with steam produces a cleaner burn? Could you elaborate?
3) What are you using as a burn pot?  (The cage that holds the pellets while their burning)
 
Peter Chauffeur
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Hello Gerry,

I hope these answers satisfy your questions.

A1) The Primary cover is a repurposed glass pen holder with holes thru it that just so happen to give optimal burn and air mix that helps keep the burn chamber clear of ash (ash ends up being pushed down into the stainless steel burn pan directly under the primary burn chamber. There is no air adjustment per say, but I have found that sliding the pellet basket forward or backward in the burn chamber does allow for some heat output adjustment. Designing the pellet basket to allow for just the right size of ember to fall thru the grate down into the stainless burn pot.

A2) steam addition to a combustion chamber is common in industrial incinerators and where temperatures are high enough, the vapor dissociates into free radicals to react with unburined fuels facilitating a cleaner burn fort her down the burn chamber. Steam also displaces nitrogen in the secondary air which helps lowers the NOx  issues. I had a chemical engineer explain all this to me in fascinating detail but what I got out of it was; if the flame is hot enough superheated steam helps protect the burn chamber going in and further down the stream the water dissociates and reacts with all the unburined vapours. That is all I can explain, other than it works! Below are two versions of pellet baskets I have made. Each has a variation on it to allow burning of the pellets. The basket on the left is a fall  and the one on the right is used in winter.  It’s the difference in spacing of the grates in the picture below

A3). The burn pot is a square metal tube that slides into the burn chamber. Top is cut open to allow pellets to fall into it. The bottom has a series of metal rods to allow embers to fall down into the secondary burner / ash bucket. Making the burn pot is one of the trial and error things that anyone building a pellet stove will have trouble wrestle with.
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Pellet baskets
Pellet baskets
 
Gerry Parent
pollinator
Posts: 434
Location: Penticton, Canada
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Thank you for your explanations Peter.
 
Peter Chauffeur
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Hello fellow Permies!  

Well, it is bittersweet feelings that I add to my post on Rocket Stoves.

As I mentioned in the beginning of this thread, my prototype Rocket Stove / Thermal Mass Heater powered by sustainable wood pellets has finally outlived its purpose and has been scrapped.

There is a sadness to see the decommissioning of the Rocket Stove that I will always fondly refer to as “Frankenstove”. However, this stove had taught me a lot about Rocket Stove operation and I will use some parts to manufacture a “Mark 2” version that will resolve some of the issues of the prototype “Frankenstove”.

So without further wording, here is the “gently demolished” remains of “Frankenstove”.
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Frankenstove autopsy
Frankenstove autopsy
 
Peter Chauffeur
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So, my fellow Permies, you can see the above photo shows my attempt to take apart a cast cell of refractory material. You should note that there is a clean fault at the back of the cube. This crack happened the first time I fired up the stove. It takes time for refractory cement to cure and unfortunately, I did not wait long enough. I think I waited less than a week before I grew impatient and fired it up. I used high heat silicone to seal the crack after I noticed and it held for the entire three years of operation so I’m impressed with the red silicone.
The next thing I would like to comment on was the extreme heat that the 3” riser had to endure. The riser was fabricated from automotive muffler pipe and was held in place with “L” brackets that were also welded to the metal pipe that held the form for the refractory cube of the burn chamber / thermal mass. The pipe riser became very brittle and you can see the jagged break on the bell housing. Which was also welded to the riser.  I think the continuous expansion and contraction of the cold to hot back to cold cycle every day for three fall, winter and spring cycles were just too much for Frankenstove. It could also be that  the part of the metal riser in the top bell housing was not insulated and that caused there to be heat stresswhere the bell housing was welded to the riser. Not exactly sure, just an observation.
I would also like to point out that the thermometer usually read an operating temp of 600 degrees Fahrenheit so I think that there was insufficient bell surface area to dissipate the heat from the riser.
Ok, so these are my observations on a Rocket Stove that I built which ran well and kept our 2000 square foot two story house cosy at roughly one pound of wood pellets per hour running 10 to 12 hours during the day and shut down at bedtime. I am very happy with the performance of this design and will consider a few modifications for a second design. I will capitalize on using the proven pellet burner and secondary ash burner but will discard the glass ash jar immediately below the riser. I will let you know how that turns out. I will also continue to pursue the steam addition as I think the temps are high enough to allow steam reformation to react with any free carbon in the combustion stream. I know that many will argue the effectiveness of steam addition but it appears to work with my design.
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Riser deterioration
Riser deterioration
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Refractory autopsy
Refractory autopsy
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Riser to bell weld
Riser to bell weld
 
master pollinator
Posts: 543
Location: Ashhurst New Zealand
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I loves me a good postmortem, Peter. Eagerly awaiting the next build and I hope you post early and often as it happens.

Where in the world are you? How cold are your winters?
 
Peter Chauffeur
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Hello, thank you for your interest!

I live on an island on the West coast of Canada. Our weather here is much like that of Seattle, Washington. Basically it is damp, rainy and cold from October until April every year with light snowfalls December and January.

The pellet stove runs about 10 to 12 hours a day and the thermal mass still generates heat for a couple of hours afterwards. The refractory block is still slightly warm to the touch in the morning though.

The pellet feed and burner assembly work very well and can go hours without flaming out or pellet feed jams so when it comes time to make the “Mark 2”, I will probably salvage the burner assembly from the prototype. That way it can serve as a benchmark.

Changes I see for the “Mark 2” will include:
1) a more sleek look for the refractory burner and heat bell.
2) a better secondary ash / secondary air system
3) a way to “change out” burner tubes
4) a better stand where ash clean up is easier
5) a vertical pellet feed
6) a larger pellet hopper
7) better aesthetics
8) addition of a Pink Salt Block as a decorative thermal mass ( a want, not a need)
 
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The steam idea might work, as things like cavitation only work with liquids.
 
steward & bricolagier
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So sorry your Frankenstove died ... Nice autopsy and analysis :) I look forward to seeing your mark 2! :D
 
Peter Chauffeur
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Hello Ted,

I have carefully been monitoring the burn characteristics of this pellet fed Rocket Stove and I have found that it needs the additional moisture to burn clean. I have found that if I let the stainless steel water bottle boil dry the flue gas has wisps of black smoke which is in burned fuel. It also has a hint of an acrid smell. When I fill the container and reservoir and it reaches the slow boil to feed the secondary air behind the flame, the smoke and smell cease and the clear exhaust returns.

According to the label on the wood pellets I purchased, the moisture guarantee is around 6%. Now,  I have no way to accurately figure out if this is true but I have researched that wood that is too dry is also problematic for combustion. If you go to the website woodheat.org, the ideal moisture content for wood is 15 to 20%. This gives me reason to believe that moisture is actually necessary to feed the secondary air at atmospheric pressure.

Remember that this stove operates without any electric input ( the circulation fan is not necessary) so the stove draws only what it needs by draft alone.

Again, these are observations from my prototype stove of my own design where I have combined many designs off the internet and then “bastardized them” with my own ideas and experiences. I would like to encourage anyone that decides to build such a stove to glean what I have figured but I am certainly not interested in arguing the theories.
What I have built, this particular design, burns just slightly over a pound of wood pellets an hour and heats a 2,000 square foot home without having to babysit it. We originally had electrical heat and according to our money grubbing electricity utilities company, we have documented our power consumption is down 33% compared to when we didn’t have the stove.
I have put these ideas on this site to help those that want to pursue an off-grid lifestyle and whose mindset is open to unconventional thinking. I have had the fortune of working around engineers whose sole occupation was to figure out how to incinerate nasty chemicals so they wouldn’t draw attention of the neighbors with sensitive noses!
 
Peter Chauffeur
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hello Fellow Permies!

I spent some time over the past few days reading some posts on this site and I have to admit that I actually have a design flaw in my Rocket Stove.  That flaw is a Metal Riser. Now, I did not know that a metal riser was technically a "no-no" in the design and I did so out out of ignorance! That being said, my observations of the compromised riser in the autopsy of  Frankenstove. This does bear witness to the extreme heat generated in a Rocket Stoves and the need for high heat resistant materials to be used to prevent this from happening to your creation is necessary. So, if you are contemplating building something along the lines that I have, use something non metal or after about three years of operation, you will have the same issues I have had.  Now, here is the kicker,  I have made reference to a "Mark 2" design  with a list of things I would like to change from the original Frankenstove prototype.  Well, that stove does exist and it is currently heating our home and actually doing a better job than the original design.  The problem is, I overlooked the issue that this design utilizes a metal riser like the original; which does not qualify it to be a Rocket Stove by definition.

So, basically I have a pellet fed wood stove that makes a rocket noise as it burns but it is not a Rocket Stove. For that error in ignorance, I apologize and I ask for forgiveness among the permies community. I do, however, maintain that there is merit in the design of my stove and the next variant will incorporate a cast refractory riser to satisfy the full definition of a Rocket Stove. As we are all here to help each other to lessen our need of the "Conventional and Convenient Grid" it is also important to notify each other when we have overlooked a design parameter that may cause someone else grief { or catastrophic failure } down the road.  For those of you that have expressed a wish to see the Mark 2 please standby as I am preparing a write up with an instruction to use a refractory riser as opposed to a metal one. If we do not learn from History, we are destined to repeat it!

Thank you for reading my post
 
pollinator
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Perhaps you might want to consider a ceramic fibre riser, more efficient in many ways and easier to build than a refractory riser.
They are commonly termed “5 minute risers” due to ease of construction.
 
Peter Chauffeur
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OK then, all is not lost.  Where do I find this "Ceramic Fibre Riser " material.... Does it come in a can? Can I apply it to the inside diameter of my riser tube in situ? Please help!
 
gardener
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Hey Peter;
Here is a link to my post on ceramic blanket. https://permies.com/t/95849/Working-Morgan-Superwool-ceramic-blanket
Truly a pleasure to work with and it works outstanding!
 
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There are many types of ceramic fibre blanket available but Morgan seems to be the only manufacturer that claims to have low biopersistence.  This means that if you breathe the fibres into your lungs they eventually dissolve.  Whereas persistent ceramic fibres just stay in the lung and potentially cause inflammatory lung disease.  There's no good evidence yet that any of them act like asbestos fibers though caution is advised.

Their HT product is rated to 1300 C.

http://www.morganthermalceramics.com/media/1141/english-superwool_brochure_single_pages.pdf
 
Peter Chauffeur
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Thank you Graham,

I will keep the ceramic blanket route for another stove. Sadly my current "Mark 2" has the same 3' riser so I am investigating a ceramic refractory spray like the kind that is used in Automotive Manifolds to prevent the metal muffler pipe from becoming ruined.  Currently the Mark 2 is operating better than expected with lower bell temperatures due to the more than double surface area and the consumption of pellets is just around a pound per hour. So at $8 / bag for a 40 pound bag, I am heating my home for less than $0.20 an hour or less than $2.50 a day. That's still a win in my pocketbook
 
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frankenstove sounds vaguely familliar somehow.  cool build, very steampunk indeed.  i've never seen a 3" riser before!  although i do like the exhaust pipe riser.  one of my recent stoves uses a 5" diesel stack lol.  3" seems kinda small for a rocket riser but if it heats 2000 sq ft then something is working!  my first RMH hybrid burns stick but also pellets unassisted.  it's nice to be able to burn pellets if you dont want to split the wood, or other way around if you dont want to buy pellets :)  im in ontario canada :D
 
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