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the science behind the rocket mass heater

j cornelissen


Joined: Aug 04, 2009
Posts: 7
Hi I hope I'm putting my post on the right spot, if not pls let me know I'll start a new thread.

Like so many people on here, reading about the various types of stoves I am getting very exited and want to build one!

My reasons for building one are slightly different tho'. I am a science teacher here in Melbourne and a colleague and myself are developing a sustainability unit for the highschool we're working.

Building them won't be an issues seeing all the wonderful info around, I'd like to know a bit more about the science behind it.

I understand the "butt warmer" concept with the combustion chamber, that serves for a more complete combustion of the fuel, I'm not quite clear on the rocket-stove science though and have a number of questions.

Why does the elbow shape help in the more efficient use of the fuel?
Is it simply because you direct the heat towards the cooking area (if so why doesn't every gas cooker come with rings around the heating points!)

Or- does the length of the chimney-like vertical part of the stove accommodate a 2nd stage combustion?

It would be great if I could get some answers that could explain the positive sides of these stoves.


Btw we're looking at including the biochar story into the unit and the idea is to combine the rocket stoves with pyrolysis of wood and hence biochar production (in short the Anila stove, probably got that name wrong), and with a bit of luck demonstrate the positives of biochar as well (to give you the full picture).

Cheers, Jan
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15230
Location: missoula, montana (zone 4)
    ∞
Why does the elbow shape help in the more efficient use of the fuel?


What makes you think that it does? 

I think there are two basic kinds of feeds:  J-tube and L-tube - both are the way they are to manage fuel stuff.

Is it simply because you direct the heat towards the cooking area


Are we still talking about a rocket mass heater (which generally isn't hot enough for serious cooking) or a rocket stove (like a lorena)?


Or- does the length of the chimney-like vertical part of the stove accommodate a 2nd stage combustion?


My impression is that the height of that part is really important.  And second stage combustion is involved.  (hopefully somebody will be able to have a better answer to your question)

Btw we're looking at including the biochar story into the unit and the idea is to combine the rocket stoves with pyrolysis of wood and hence biochar production (in short the Anila stove, probably got that name wrong), and with a bit of luck demonstrate the positives of biochar as well (to give you the full picture).


Hmmm .... can you make charcoal with a rocket mass heater?  I would think it wouldn't get hot enough.  Or maybe you have an idea for a different design?





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Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
paul wheaton wrote:
What makes you think that it does? 

I think there are two basic kinds of feeds:  J-tube and L-tube - both are the way they are to manage fuel stuff.



Good questions Jan.  I'll try to hit all of them, but bear with me, it's the Rambling Hour.

Paul - the feed tube shape does affect burn, not just fuel handling.  The right angles splash the flames, increasing mixing.  That's also why brick works better than metal pipe in the burn chambers: it increases turbulence in the critical area where fuel and air are first combining.

The void shape at the top of the heat riser (where the gas fountains up against the barrel surface) also aids mixing, splaying the flame/exhaust mixture into a torus.  Secondary burn occurs at the top of the column, along this torus, and in some systems can extend about a third of the way down the barrel during a hot burn session.

This does give you a hot surface to cook on, but that's a bonus.  The gap and torus can be tweaked to focus heat up top for cooking, or for more heat down along the barrel (better delivery into the room as a radiant heater).

Rocket Stoves configured for cooking generally lack this torus, and instead place the pot directly in the stream of combustion gases.

Secondary combustion does also occur in the top of the heat riser.  Heat riser height and insulation are both important components of a successful system - the hotter its interior surface gets, the better the burn and the better the draft.

Any stove that burns wood produces charcoal temporarily.  Rocket mass heaters  promote complete combustion, and there's not much charcoal left at the end of a burn session. 
  Charcoal is the result of a partial burn.  Partial burns tend to be dirty, and separating a partial burn from a secondary burn is a tricky design feature that would interfere with other design considerations.

Regarding BioChar:
   I'm skeptical about biochar as a carbon sequestration or soil amendment solution - wouldn't it affect soil microbes like activated charcoal, and tend to sterilize them?  Or reduce nitrogen availability like wood chips?
  Don't living trees sequester carbon just as well?  If we could cultivate some 1000-year old-growth to replace what we've lost, it seems like that would be more biomass and more carbon locked away overall. 
  I'm sensitized to this issue by a scandal we had here in Oregon a few years back.  Forests regenerate quicker after burning if unburned matter is left in situ - http://www.eugeneweekly.com/2006/05/11/news2.html
Agribiz and forestry currently seem to prefer tactics which deplete soil and biomass, with petrochemicals supplying the deficit.  Natural breakdown and growth of plant matter works great in our region for restoring soil fertility. 

  It's possible that there are regions where the critical, missing component in soils is charcoal.  But I wouldn't expect it to be the case worldwide, and certainly I'd hesitate to burn biomass for no other purpose than to sequester carbon. 
   I'm afraid students might see biochar as a system of "burn a tree and bury the charcoal," which would not be an appropriate global solution at all. 

I'm all for stacking functions.  If you're going to have charcoal available at the end of the wood-distillation unit, burying it as "carbon" is one way to honor it.  But doesn't it have other uses too?  'Sequestering' it as art (use as pigment or pencils, make tempera paint or crayons), save it for medicine or water filtration.  Maybe see if the biology teacher wants to do a "before and after" microscope examination of pond-water filtered through activated charcoal.  Use the dirty charcoal to smelt some rusty iron for recycling.  Then bury any remains once it's been thoroughly "used up."

The lesson I'd want them to take home is not that charcoal=carbon=bad, but that every part of the cycle is useful in its own way.  Our job is to turn waste from one process into resources for another.  Things that we "bury" (or burn) are being returned to the parts of the cycle that we don't control. In years or centuries, someone will come back and find roses growing on a grave, or trees sprouting from a burnt-out stump, feeding on the rich exhalations of rotting ancestors.  Give the rot a place of honor - far away from us!

Sometimes a 3-part cycle helps break them out of binary thinking.

The main advantages of a rocket stove, from an appropriate design perspective:
1) Uses less fuel, preserving live biomass for sustainable local production.  No need to cut trees- dropped limbs will do.
2) Burns fuel completely, avoids wasting any part of it, and avoids putting out smoke pollution.  Short hot fires instead of smouldering ones.
3) Provides for most human heating needs in temperatre climates:
a) immediate heat as a reward for lighting the fire, from the radiant barrel.
b) the kind of all-night, next-day heat we want, from a kind of activity we enjoy: sitting by the fire in the evening.  Avoids the need for a smouldering fire all night.
4) Converts waste into resources: can be built from scrap metal, fill dirt, and some sand and rocks and bricks.  Can be fueled on waste: orchard trimmings or (clean) construction scrap.
5) Delivers heat efficiently and comfortably: you get warm for a long time, instead of too hot or too cold.  Contact is more efficient than convection or radiation for comfort warming.
6) Safety factors: fire is hidden, no hearth for embers to fall onto, less toxic materials, less smoke, lower surface temperatures for safe contact, low center of mass for stability (compared to chimney heaters), fire does not need to be left unattended for home to stay warm.

Tradeoffs:
1) "cheap" - you can spend a few weeks or months with a junkpile and muddy living room, instead of paying to install something quickly.

Drawbacks:
1) Aesthetics: you can't see the fire, and the barrel is funky.  Cob is pretty, tho.
2) Heat storage: not suitable for hot-season cooking, and takes time to "prime" the heat pump in the autumn. 
3) So unusual it's not easy to get a building permit.
4) Pushing the efficiency envelope means every design decision is a tradeoff: vertical chimney means less heat storage, more heat storage means horizontal exhaust and designing around prevailing winds. 
5) Big heavy thing in your living room, commitment of time, space, and effort.
6) Recycled materials means an on-site junkpile; requires learning to differentiate materials as-is from eclectic sources. 

You're welcome to look at our http://www.ernieanderica.info/firescience Fire Science page, which includes some play-by-play for teaching clean fire.  I include other kinds of stoves and fireplaces, so people can see that if they want a summer cookstove, or a quick-heating cabin fireplace for a weekend vacation, or visible flames and soot, there are excellent designs that meet each of those needs from traditional materials.  When someone attends a workshop with us, I want them to come away well-informed about appropriate choices for their situation, even if it may not be a rocket stove.

Yours,
Erica Wisner
http://www.ErnieAndErica.info

Play with nature, make nifty stuff:
www.ErnieAndErica.info
rose macaskie


Joined: May 09, 2009
Posts: 2134
I like what Erica say about everything  only want to complain about s detail about what she says about living trees sequestering carbon.
    trees use carboon as a building blocks, all food stuffs and woods etc., are full of  molecules with carbon atoms in them carbon but if we decide the answer is to have a thousand year old trees we have to wait a thousand years, while if we plant lots and lots of small trees we can have lots of carbon sequestered soon and we need it now, until wind and solar energy have kicked in and we can relax and not mind how many trees there are to gobble carbon because as we wont be producing incredible quantities of carbon, so it wont matter.
    If we must cut down these trees that are full of carbon them if we use them for building with the carbon will stay sequestered. As news papers are soon to be out, we can use the wood to have really wood greedy houses like log cabins and so sequesters lots of carbon in our homes. it wont matter if we knock down an dburn the homes in a hundred years we will have resolved the problem by then we wont be producing enormous quantities of carbon gases.

    If on top of planting lots of little trees we  have cover crops or green fertilisers on land left fallow there will also be lots more plants gobbling up carbon were before there was bare ground and in this case wondering how to be sure more carbon from these green fertilisers that get fixed in the soil becomes humus of the type that does not degrade is important, it is a way of making sure organic matter that is good for the soil provides a maximum guarantee of sequestering carbon at the same timenas improving crops.
    Green manures or cover crops usually get ploughed into the land before planting a new cereal crop when the years rest, the year left fallow is over, gets fixed in the ground instead of turned back into carbondioxide is important because green fertilisers are for the ground.
      Maybe cover crops could they could be dried and used as insulation for our houses and in that way sequester into our walls but then soils would not build up with the organic matter that they need and farming would be harder.
      I don't know how to fix more vegetable matter in the soil and make sure less of the matter fed in gets broken down and turned back into gaseses.

    Does manure produce less methane, a more potent global warming gass than carbon dioxide, if it is kept dry instead of in pools?
      Thats another one.
      In factory farms i have heard it is kept in pools.
      In some poor but advanced countries, as far as stopping global warming goes, they store manure in a shut vessel and use the methane  it produces for cooking.  A pot of manure buried in your yard means free cooking fuel.

        The Indians from India, dry little cakes of cowpatties mixed with staw for fuel, less methane, also less vegetable matter to better soils and help farming in poor rural India. Its all intricate.
    Cow patty ash is good for soils fertility but ash does not work to increase the water and nutrient absorption and retention of soils, that is so important in dry countries, like organic matter does.
      Cow patties are full of heavy metals so they aren't good for soils. Why are they full of heavy metals? i haven't read why yet, lead from our exhausts would be one reason.
      It seems cow patties need Paul Stamet to mycoremediate them to get  the heavy metals out of the manure, though the incredible thing his fungi do is to break down poisonous molecules that were non biodegradable, until Stamets trained fungi to do it,  like vx toxins and herbicides and pesticides, while phytoremediation, plants that absorb heavy metals which can be recovered from the harvested plants would be more to the point to get the metals out of manure.
 
  being biodegradable or not as I remember it, originally referred to whether a nociviouse molecule like a herbicide one got broken down by natural processes or went on unchanged though absorbed by plants that later rotted or passed into the body of an animal that ate the plant and passed out again, with out changing their molecular structure, as poisonouse as ever. It does not mean your amphora will be here a thousand years later, the answer to which is, so what?
   
  Water vapor is a global warming gas. Do we now need deserts that dry our atmosphere or jungles that absorb global warming gases? Stopping global warming is an intricate business. agri rose macaskie.
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15230
Location: missoula, montana (zone 4)
    ∞
I once saw something where somebody welded a j-tube setup.  Since everything was square, it would seem that that would be a smarter way to go - more mixing than if you had a round innter tube.  Plus, there would then be some metal near the initial burn spot. 

But then I got to thinking:  wouldn't it be better still if the inner layer was moderately thin metal?  After all, moderately thin metal surrounded by insulation (in the combustion chamber) would get much hotter.

paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15230
Location: missoula, montana (zone 4)
    ∞
See the fire:  I wonder if we can put a little glass in just the right spot so we can see the fire ....
j cornelissen


Joined: Aug 04, 2009
Posts: 7
Hi all, thanks for the useful replies, I think I've got my head around the science side of things.

Coming back to the biochar side of things, a couple of things;

Biochar is a proven improvement to soil structure as was observed in the Amazon basin. It works by absorbing both water and nutrients, giving much higher crop yields. It offers a structure for microbes to prosper.

Production of biochar through pyrolysis yields both gases that can be burned as well as biochar that can be used to improve soil structure.

Here in Oz there's a plant that uses the energy released by burning wood gas to produce :
-electricity
-as well as cause more pyrolysis hence more gas, more energy
-the very useful by product is biochar.

It is by far the most simple, feasible to improve our current predicament. Granted, growing more 1000 year old forests is preferable, the drawback is it takes a 1000 yrs............

Here's a plan for the anila stove
http://www.bioenergylists.org/files/images/Biochar_Anila_s_Page_13.preview.jpg

that produces biochar, which is part of a secondary process in the double wall of the burner, quite ingenious.

Got some interesting sites on biochar if anyone is interested

http://terrapreta.bioenergylists.org/taxonomy/term/253
http://www.theoildrum.com/node/4522 this one is particularly good, watch the you tube clips

cheers, Jan

Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
j_cornelissen wrote:
Hi all, thanks for the useful replies, I think I've got my head around the science side of things.

Coming back to the biochar side of things, a couple of things;

Biochar is a proven improvement to soil structure as was observed in the Amazon basin. It works by absorbing both water and nutrients, giving much higher crop yields. It offers a structure for microbes to prosper.

Production of biochar through pyrolysis yields both gases that can be burned as well as biochar that can be used to improve soil structure.
...
http://www.theoildrum.com/node/4522 this one is particularly good, watch the you tube clips

cheers, Jan



Thanks, Jan, for responding to my questions about biochar.  That's definitely a good site.

I found this quote on the site you suggested:
"In tropical or depleted soils ECOSS fertilizer sustainably improves soil fertility, water holding and plant yield far beyond what is possible with nitrogen fertilizers alone."

Nitrogen fertilizers alone tend to be water-soluble, easily rinsed away or evaporated into a crystalline crust on the soil.  My understanding is that most nitrogen fertilizers tend to deplete soils over time.  Any organic (life-based) amendment, with some biomass to it, is going to build soils better than nitrogen fertilizers.  If they're using mesquite briquets, I don't suppose they also compared manure instead of just soluble "plant food."

The site also described the bacteria in the soil as a key component of its ongoing fertility:
"Not far from Painted Rock Cave is a 300-acre area with a two-foot layer of terra preta quarried by locals for potting soil. The bottom third of the layer is never removed, workers there explain, because over time it will re-create the original soil layer in its initial thickness. The reason, scientists suspect, is that terra preta is generated by a special suite of microorganisms that resists depletion."

Amazonian soils lack biomass and minerals.  The rain leaches everything soluble out.  So a nutrient-capturing, water-locking charcoal ingredient could turn the soil back into living stuff.

Does it also work for drylands soils?  If I recall, Oz has both, but more drylands.

Basically, anything that helps rebuild soils with significant organic content helps fertility (as compared to depleted soils whose fertility depends on soluble, and therefore transient, minerals).  Soil biomass is a good thing, and as significant as plant biomass for sequestering carbon.

It seems like biochar is likely to help restore the carbon released from all the soil depletion of the last few centuries, but I agree with the author that it seems unlikely to completely offset the fossil fuel carbon load. 

Have you seen biochar production that's completely self-fueled - no fossil inputs?
Seems like if it gets popular enough to impact global carbon, it will also be tempting to mass-produce the "valuable" product without collecting the "by-products," and ship it all over the place, turning a multi-functional harvesting process into a destructive industrial process.

Another piece of the fossil fuel / soil puzzle:
Manure and methane.  In the US, we have feedlots so the animal waste cycle is separate from plant crop cycles.  Instead of manure being dropped or spread on fields near where it's produced, it ends up in piles at the end of the feed lots.  Meanwhile, N-P-K fertilizers are used in place of the manures to keep the soil producing (if not actually healthy) and the manure mostly rots and offgasses methane and ammonia.   Both the manure and the soluble N-P-K fertilizers wash into rivers and create dead zones on the coast.   Biomethane plants are being built, but it's still a drop in the bucket.

We've disconnected all the parts of this thing called "life on earth," and are inefficiently running each part of the machine separately, with bits spilling out on the floor. 

Connecting those parts of the cycle seems like the critical thing to teach.  Good luck with your students. 

Are you still considering using rocket stoves in your class? 
I'm curious to see how you tie the two together.

Yours,
Erica Wisner
http://www.ErnieAndErica.info
j cornelissen


Joined: Aug 04, 2009
Posts: 7
Hi Erica,

yeah you're right the way things are done at the moment is completely insane. Bit by bit people realise the soil has been depleted further and further with the conventional way of fertilizing.

There are people now that use animal manure on a large scale to fertilize their land (duh!). They find they can increase the P levels, much higher and for longer than with the conventional way of fertilizing. On top of that they'll accomplish improved water retention.

Once you get the process going (you need an initial amount of E input) the energy release of pyrolysis is enough for further pyrolysis as well as produce electricity so that's all great. I agree with you that the danger lies in people starting to produce charcoal without using the woodgas released in the process thus increasing the amount of greenhouse gas.

My lessons will concentrate on a 3rd wrld setting for the stoves and all the advantages for the people using them (health, time gained, safety for those collecting wood, less deforestation etc etc)

Will let you know how things go.

Cheers, Jan
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
j_cornelissen wrote:
Hi Erica,

yeah you're right the way things are done at the moment is completely insane. Bit by bit people realise the soil has been depleted further and further with the conventional way of fertilizing.

There are people now that use animal manure on a large scale to fertilize their land (duh!). They find they can increase the P levels, much higher and for longer than with the conventional way of fertilizing. On top of that they'll accomplish improved water retention.

Once you get the process going (you need an initial amount of E input) the energy release of pyrolysis is enough for further pyrolysis as well as produce electricity so that's all great. I agree with you that the danger lies in people starting to produce charcoal without using the woodgas released in the process thus increasing the amount of greenhouse gas.

My lessons will concentrate on a 3rd wrld setting for the stoves and all the advantages for the people using them (health, time gained, safety for those collecting wood, less deforestation etc etc)

Will let you know how things go.

Cheers, Jan


Best of luck with your class.  I sometimes envy you full-time teachers the opportunity to blow open tender young minds

Funny how much easier it is to conceive of improvements for the third world, than for our own tangled web of inefficiencies, overconsumption, and regulations.  (Speaking as an American, you understand.)

I've been struggling to make change within my own culture, for the most part, rather than blundering into unfamiliar territory.  Slow going, but sometimes I see a glimmer of progress.

Do let me know how it goes, and feel free to use me as a resource for student questions.

Yours,
Erica Wisner
http://www.ErnieAndErica.info
 

 
 
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