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Thread: Wood Stoves Double Wall vs Single Wall

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    Scout Vendor zelph's Avatar
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    Default Wood Stoves Double Wall vs Single Wall

    Is a double wall really needed for gasification?

    A quote from my website bplite.com:

    ""Arjuna02 , read some of this information and see if it changes your mind in some little way:

    http://www.mb-soft.com/juca

    http://mb-soft.com/juca/print/315.html Theory of Design of a Functional Secondary Air System

    http://www.alternative-heating.com/best_wood_stove.html

    http://www.bioenergylists.org/stoves...rinciples.html

    Larry Winiarski's Rocket Stove Principles

    Dear Friends,

    I was typing up Larry's latest simple stove principles for Aprovecho's

    newsletter, "News From Aprovecho", (two to three times a year, $30/year,

    describes activity, I'm editing number 59) and thought I'd send it along.

    Reflecting on the Rocket I might point out a interesting point: no secondary

    air. I've tried adding heated secondary air into the top end of the internal

    chimney above the combustion chamber but haven't noticed an improvement in

    amount of smoke or in fuel efficiency. I ended up thinking that enough

    primary air is left at the top of the combustion zone anyway. Adding air may

    just reduce temperatures. I'll test this further with better equipment.

    The Rocket stove is trying to create supportive conditions for complete

    initial combustion which seems to pretty much work when the right amount of

    fuel is introduced. The added draft created by the insulated chimney above

    the fire pulls in lots of air, which like a fan, makes a hotter, vigorous

    burn.

    ----------------------------------------------------------------------

    Now I have a question for you, do you think Dr. Winiarski would feel the same way about our DIY "woodgas stoves"

    Do you think he would say don't introduce secondary air above the burn chamber?

    Did you see him say introducing air may reduce temperatures. That's the big guy's opinion.

    I say the double wall is not necessary. I think Larry would agree with me No need to introduce warm secondary air for combustion.

    .................................................. .................................................. .......................................


    Theory of Design of a Functional Secondary Air System

    The JUCA design does not depend on air-starvation (air-tight) operation so the problems of excessive creosote production and carbon monoxide production do not represent the great problem existing in most wood burners on the market. For this reason, it was unnecessary for us to try to arrange a functioning secondary air system even when that was the current rage. We quietly said for years that it was unlikely that any of the secondary air systems worked very well, if at all. Independent researchers eventually showed that we were right all along. With this preface, we herein will give the reasoning why all existing secondary air systems don't work well; and the design considerations necessary to make a system that does work as intended (again, for AIRTIGHT products where it could be important).

    See other sheets of ours for a description of the overall theory of airtight products and that of products like the JUCA (Sheets 128, 120, 310, 314 and others).

    Since the lack of enough oxygen in the fire's vicinity is the cause for the incomplete combustion in an air-starvation burner, it would be advantageous to supply air to it later on to complete the combustion. The hitch is that the primary reaction that needs to occur (carbon monoxide plus oxygen gives carbon dioxide) will only occur above about 1200°F. If it didn't happen while it was still in the flame tips, we may have trouble keeping it hot enough for the reaction to go.

    Let's consider an example. The actual flame temperature of a wood fire can range from about 900°F to 2500°F. An "average" fire will commonly be around 1900°F. Almost instantly on leaving the flame tip the smoke mixes with other air or smoke, quickly reducing the temperature. The amount of this temperature reduction is dependent on many variables, some of which are not yet fully understood. For argument's sake, let's say it is at 1400°F. In order to permit substantial secondary combustion to occur, it will be necessary to supply a decent amount of secondary combustion air, generally on the order of the primary air supply.

    This is necessary so that the statistical probability of CO molecules being able to "bump into" O2 in the hot zone is high, preferably at least 90%. The molecules will only be in this environment a very short time, but we want the great majority of them to have the opportunity to combine with the oxygen atoms. These conditions are mandatory to ensure substantial and consistent secondary combustion over a wide range of firing conditions.

    Some currently available products do seem to be able to support secondary combustion SOME OF THE TIME and TO A LIMITED EXTENT. Under optimal conditions maybe 1/3 of the available fuel is recovered. Under most other conditions, less. The amount of air supplied is too small to allow high probability of the CO and O2 reacting. Just do a molal analysis to see the lop-sided proportion of many CO to few O2 molecules. Actually if pure oxygen was fed, it would work fairly well. Air being 80% Nitrogen just reduces the probabilities of reaction.

    And it represents more material that must be pre-heated so as not to chill the smoke to below 1200°F. Getting back to our example, if we mix our 1400°F smoke with an equal amount of room temperature secondary air, the resultant temperature of the mixture will be less than 800°F, far less than the necessary 1200°F. No reaction. Poor efficiency. A lot of creosote. A lot of pollution. Bad. You can probably see that you are going to need a source of secondary combustion air at about 1000°F or higher under these conditions. A pre-heater will be necessary to boost the room air to 1000°F.

    Unfortunately, there are some conditions of low fire (severely held back) where the smoke itself is under 1200°F within inches of the logs. In that case secondary combustion is almost out of the question. It is ironic that in the situation of a severely suffocated fire that most needs the effect of secondary combustion, it is most difficult to obtain. When the fire is burning relatively freely (and therefore cleanly), that is when it is easiest to initiate secondary combustion.

    Again let's get back to the example at hand. We need to pre-heat air to 1000°F. It will be necessary to use a heat exchanger to do this. Some current products have a 6" long tube to pre-heat the air as it passes through. We'll see that this isn't even close to enough exchanger surface. Assuming that the stove consumes 35 CFM of primary air for the fire, we will also need 35 CFM of secondary air as described above. To heat 35 CFM from 70°F to 1000°F will take about 35 x (1000-70) x 0.24 / 28 * 60 or approximately 17000 Btu/hr. The 0.24 is the air's specific heat; the 1/28 is the air's specific volume at the mean temperature; 60 is the number of minutes in an hour.

    When we are talking about a unit that is only going to develop 20,000 or 30,000 Btu/hr, you can see that we are going to have to use more than half of the capability for pre-heating. The secondary combustion might add 25% to the output (maybe 7000 Btu/hr) but you use 17000 to do it. A losing proposition. Except for the safety considerations, it would be foolish to consider.

    Conventional heat exchange analysis (see other sheets in the 300 series) will give the necessary areas of heat exchange for this pre-heater. We will avoid the math here. A two stage boost heater is most logical and effective here, where the first stage heats the air to (600°F in our example). The necessary area in a 700°F part of the stove for this exchanger is 1.6 sq. ft. The air then passes to the second exchanger to be heated further (to 1000°F) in a hotter part of the firebox right over the flame tips. The necessary area of this exchanger is 1.5 sq. ft., assuming the smoke temp is 1300°F in that part of the firebox.

    If the supply tube is 2" in diameter, the first exchanger must be nearly 9 feet long (wrapped around inside the firebox) and then the second will also be about 9 feet long. The secondary combustion air supply would have to pass through a total of 18 feet of specially located heat exchanger to ensure good secondary combustion. There would not be much room left in the firebox in most stoves for any exchangers for USEFUL heat. And remember, even then there are conditions when secondary combustion still won't occur. Is there any wonder why currently available products with a stub tube pre-heater don't work?

    Home Page of JUCA

    http://mb-soft.com/juca/index.html
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    Guide Bush Class Basic Certified teb_atoz's Avatar
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    People that say you don't need a double wall chamber really don't understand thr gasification process. I have seen people claim to have built a gasification stove but you can see by the flame, yellow, that is not achieving pyrolysis where H2 and CO are produced and burned, a blue flame. No claim to authority but a degree in chemistry helps but not required as I have learned a lot from many people. You just have to want to learn.

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    Quote Originally Posted by teb_atoz View Post
    People that say you don't need a double wall chamber really don't understand thr gasification process. I have seen people claim to have built a gasification stove but you can see by the flame, yellow, that is not achieving pyrolysis where H2 and CO are produced and burned, a blue flame. No claim to authority but a degree in chemistry helps but not required as I have learned a lot from many people. You just have to want to learn.
    If you will please, show us a DIY stove that meets your requirements.
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    Quote Originally Posted by teb_atoz View Post
    People that say you don't need a double wall chamber really don't understand thr gasification process. I have seen people claim to have built a gasification stove but you can see by the flame, yellow, that is not achieving pyrolysis where H2 and CO are produced and burned, a blue flame. No claim to authority but a degree in chemistry helps but not required as I have learned a lot from many people. You just have to want to learn.
    Doesn't Hydrogen burn to create an orange flame? Blue or blue/green would be copper burning. Yellow is salt if I remember correctly. Unless you mean the burning of the hydrocarbons given off when burning wood? Which cant happen in pyrolysis because that needs oxygen and pyrolysis is how you make charcoal and is done in the absence of oxygen.

    Better explanation than I could do: http://www.ars.usda.gov/Main/docs.htm?docid=19898

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    Pyrolysis

    From Wikipedia, the free encyclopedia

    Jump to: navigation, search


    Not to be confused with Pyrrolysine.





    Simplified depiction of pyrolysis chemistry.
    Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen (or any halogen). It involves the simultaneous change of chemical composition and physical phase, and is irreversible. The word is coined from the Greek-derived elements pyro "fire" and lysis "separating".

    Pyrolysis is a type of thermolysis, and is most commonly observed in organic materials exposed to high temperatures. It is one of the processes involved in charring wood, starting at 200–300 °C (390–570 °F).[1] It also occurs in fires where solid fuels are burning or when vegetation comes into contact with lava in volcanic eruptions. In general, pyrolysis of organic substances produces gas and liquid products and leaves a solid residue richer in carbon content, char. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization.

    The process is used heavily in the chemical industry, for example, to produce charcoal, activated carbon, methanol, and other chemicals from wood, to convert ethylene dichloride into vinyl chloride to make PVC, to produce coke from coal, to convert biomass into syngas and biochar, to turn waste into safely disposable substances, and for transforming medium-weight hydrocarbons from oil into lighter ones like gasoline. These specialized uses of pyrolysis may be called various names, such as dry distillation, destructive distillation, or cracking. Pyrolysis is also used in the creation of nanoparticles,[2] zirconia[3] and oxides[4] utilizing an ultrasonic nozzle in a process called ultrasonic spray pyrolysis (USP).
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    Curmudgeon. B.O.F. Supporter 45jack's Avatar
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    This seems like a nice explanation and maybe even a fun father/son project or father daughter....

    http://people.morrisville.edu/~balla...sCampStove.pdf
    ~ Jack
    "Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other."
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    Yeah that is basically what I linked from the USDA Research division. My only point was that you wouldn't get a blue flame because the hydrocarbons given off don't combust in pyrolysis. There would have to be a secondary chamber with oxygen fed into it to combust the byproducts from pyrolysis. So looking at the color of the flame wouldn't help.


    Edit: That is a cool stove, I just saw your comment. I might have to try that.

    Also, I think I see where the confusion is, he stated:
    "People that say you don't need a double wall chamber really don't understand thr gasification process. I have seen people claim to have built a gasification stove but you can see by the flame, yellow, that is not achieving pyrolysis where H2 and CO are produced and burned, a blue flame.

    I think he meant to say combustion of pyrolysis gases.
    Last edited by Vanitas; 01-23-2015 at 02:19 PM.

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    Quote Originally Posted by 45jack View Post
    This seems like a nice explanation and maybe even a fun father/son project or father daughter....

    http://people.morrisville.edu/~balla...sCampStove.pdf
    That stove design is very difficult to ignite and burn properly. It is a version of the "beaner" stove that produces a lot of charcoal and is also difficult to operate properly.

    A quote from that site:

    These woodgas stoves are batch loaded, easy to build, fun to use, and nearly smokeless when properly fired. Our design is an adaptation of the "batch-loaded, inverted down-draft gasifier" described by Ray Garlington (http://www.garlington.biz/Ray/WoodGasStove/) and based on Reed and Larson’s 1996 paper “A wood-gas stove for developing countries”. For more “woodgas” information, see: http://www.woodgas.com/ . Note: any references to or images of commercial products or brands does not constitute endorsement of any particular product or brand by the RETC; they are simply for illustration purposes.
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    I can not claim any scientific understanding of the gasification process, although I understand the concept and have observed it in progress many times.

    I have several wood stoves, I think they are all more efficient in terms of cooking than an open fire. Although I love camp fires and used to cook with them all the time.

    Single wall:

    1. Sierra zip stove with a fan & 9V battery I bought 20 years ago. Works great!
    2. Emberlit & another one...can't remember the name.
    3. DIY coffee can with computer fan & 9V battery

    Double wall:

    1. Solo wood gas stove
    2. DIY TLUD using a coffee can & a Progresso soup can with a stainless coal grate, I have made about 30 of these and learned a lot. Based loosely On Ray Garlington's design.

    My double wall, wood-gas stoves burn very hot with little smoke once you get the stove heated and learn how to feed the stove the right type & size of wood.
    They do have nice secondary combustion via the holes (jets) at the top of the fuel chamber, the secondary flames generally start out orange-ish and once the stove really gets going they are bluish like my alcohol stoves.

    It is a cool process to observe & onlookers always seem intrigued by it.

    However, in terms of sheer performance and the time it takes from start to finish to cook a meal I would have to give the edge to the fan powered stoves, although they have moving parts, a battery, etc. So, they are not stupid simple which I prefer in theory.

    So - my experiences with these stoves leads me to wonder how a fan powered TLUD (wood gas stove) would perform. Obviously you don't want to push the air so fast through the double walled section that it doesn't get super heated as it rises and exits the jets at the top of the fuel chamber. A very low volume fan maybe?

    Anyone tried this?
    Is there a commercial stove like this?

    I hope I explained this in a way that is understandable, I will be happy to clarify if not!

    Thoughts, criticism, or advise welcome, feel free to speak your mind!
    Last edited by Mike G.; 01-23-2015 at 05:00 PM.

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    Quote Originally Posted by zelph View Post
    That stove design is very difficult to ignite and burn properly. It is a version of the "beaner" stove that produces a lot of charcoal and is also difficult to operate properly.

    A quote from that site:

    These woodgas stoves are batch loaded, easy to build, fun to use, and nearly smokeless when properly fired. Our design is an adaptation of the "batch-loaded, inverted down-draft gasifier" described by Ray Garlington (http://www.garlington.biz/Ray/WoodGasStove/) and based on Reed and Larson’s 1996 paper “A wood-gas stove for developing countries”. For more “woodgas” information, see: http://www.woodgas.com/ . Note: any references to or images of commercial products or brands does not constitute endorsement of any particular product or brand by the RETC; they are simply for illustration purposes.
    Is the difficulty a function of the small size and air volumes?

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