Sawdust fired Rocket Mass Heater?

Hi,

Newbie to the forums here, but I wanted to throw out some ideas.  I'm the farm manager for a small, non-profit urban farm in New Mexico.  We grow seedlings to sell as a fundraiser in the spring, and I'd like to build a rocket mass heater to heat a bench for germinating plug trays.  I understand that a few people have done this successfully.

HERE'S THE QUESTION:  Could a RMH be built that uses sawdust?  I have been looking at designs for sawdust stoves, and they essentially mimic the heat riser on a RMH.  Would it create enough draft to take the exhaust through the pipe?  Would it burn cleanly enough to avoid creosote buildup?  We have access to 5-6 barrels of clean, dry hardwood sawdust every week.  It would be great to be able to use it.  Here's a link to the type of sawdust stove that I had in mind:

http://www.treesearch.fs.fed.us/pubs/19308

Any input would be greatly appreciated.  Thanks!

Matt
Growing Awareness Urban Farm

this is simple and elegant



i think that it is in need of secondary air.
but i don't like to be bothered by knowledge or experience 

Noraa,

Yeah, that's the kind of sawdust burner that I've found...the problem is I don't want to heat my whole greenhouse- just the bench for growing seedlings (and maybe the foot above it).  I may end up adding on a burn chamber to this design (like the fellow mentions above) and run it as sort of a dual-fuel unit. 

Matt

ha ha caught me not paying attention.

i am imagining that you have a large volume of fuel to heat a small area of work bench.

if the bench was held up by 55 gallon drums of water (and stone) then you could store the heat of one burn in the mass of the stones and use the water to circulate the heat below the germination table.

From looking at the links given above, I would suggest the the rocket part might not be what you need. The burner as described seems to use much of the rocket principles of secondary combustion at high heat already, the fuel acts as insulation to keep the burn tube hot and the tight packing only allows the surface of that tube to burn. Note they show there is little or no smoke.

So, take this burner and mate it to the RMH mass storage unit for what you want. to generate less heat to begin with, shrink everything. I have seen sawdust cook stoves made with brick:

http://tilz.tearfund.org/Publications/Footsteps+41-50/Footsteps+46/Alternative+fuels.htm

So small is ok too. If you don't wish to heat the room while heating your bench, then don't leave any of the drum exposed. The top would be the hard part as it has to be removed to recharge with fuel. However the sides could be part of your heat storage/bench just by bringing the cob right up to the top. As the top is removable, it is probably best to insulate it with the lightest material you can find to keep the heat in. Light weight so you can lift it easily.

More thoughts:

-Greater mass means fewer firings.

-Shorter firings can be achieved by using three barrels... an inner barrel with insulation around it will limit the amount of sawdust charge while still leaving the operating characteristics the same just a shorter time.

-A more "rocket like" performance could be had by putting an insulated "donut" on top of the sawdust charge giving a small hot pipe for burning any vapor/gas not yet burned. This would also limit the amount of sawdust charge.

-I like the idea of using a barrel of water to store some of that heat as mentioned above.

Another thing you can try, if you want to heat a small space, is the "Kandle Heeter" (http://www.heatstick.com/_KanHeet01.htm).  While it was originally designed to use container candles, it shows how you can use a 50W Infrared bulb to get the same effect.

? Could you use some type of basket or holder to use saw dust in a RMH
in the RMH I built I used a 1/2"mesh SS basket to hold wood chips fed from a hopper, works OK but still tweaking it. (wood chips free by the truck load from local tree trimmers)

Tarkus wrote:
? Could you use some type of basket or holder to use saw dust in a RMH
in the RMH I built I used a 1/2"mesh SS basket to hold wood chips fed from a hopper, works OK but still tweaking it. (wood chips free by the truck load from local tree trimmers)

What would the water content of that be (sounds like green chips)? Or do you dry it first? Chips could dry a lot faster than cord wood... or take a lot longer depending on how it is stored. I would think that if it is just stored in a pile or bin, it could take a long time (years?) Wet chips will burn, but a lot of the energy will be used up heating the water... A taller vertical tube might allow the top end to get warm enough to finish burning all the gases. Or storing the chips on screen flats with 2 or three inches of chips on each may dry it out better.

Len: I'm in So Cal spring and summers here dry a large truck load of chip pretty fast, turn it a couple of times with tractor during the dry season. I started getting the chips for the garden walk ways and have a pile 2years old (rather large pile) so when I started playing with the RMH it was an obvious fuel.

Tarkus wrote:
Len: I'm in So Cal spring and summers here dry a large truck load of chip pretty fast, turn it a couple of times with tractor during the dry season. I started getting the chips for the garden walk ways and have a pile 2years old (rather large pile) so when I started playing with the RMH it was an obvious fuel.

Ah, makes sense. I live in the Vancouver Island Rain forest... Well not quite, that would be the other (less inhabited) side of the Island. Still, it is raining as I type, and will probably keep doing so till next summer. I hear we may get more snow this year, last year there was none to stay on the ground for 24hrs. Still, I work outside and love it. It makes me feel alive. I spent 22 years inside... I won't be going back that i can see. Any of the tree trimmers I have talked to already have a use for their chips, but any garbage day there are lots of bundles of sticks, so I am thinking of focusing on those for a fuel. Gotta get some bricks and try some things (outside first). There is already a flu from an old wood burner that was removed when the box rotted. The people before us put in a gas fireplace which was next to useless (I hear they thought the same). I switched gas off and save hundreds going all electric. Adding any wood heat at all will lower that some more.

My understanding and observations are the RMH or at least the rocket part will burn any combustible you can get in it. Makes sense to utilize the "waste" in any given area as a fuel taking in to consideration toxicity. My personnel goal is to never pay a utility bill ever, without giving up comfort or convenience.
Various fuels I believe would require different feed and handling techniques/methods but could supply lots of free fuel if were willing to work on it.
Just a thought
Try to stay dry Len and good luck with the RMH

David 

I'm trying one of these.  I've got my 55 gallon drum with snap ring lid.  I had some trouble finding a 30 gallon drum so am using the top half-minus of a water heater tank.  I think I've got everything I need.  I'm doing this a bit outside the box as I'm having the ash drawer and drawer sleeve (not using a false bottom) made at the local sheet metal shop.  I'll run everything up and have our son tack weld the sleeve onto the bottom of the barrel.

I opted to go with a sleeve instead of a false bottom because it will help preheat the air for the burn.  In theory . . . but then you know what they say about theories.

The smaller diameter of the water heater tank will allow me to insulate the burn tank.  I see this as a plus.

ElfN

Here's a smaller version:
http://www.motherearthnews.com/Do-It-Yourself/1974-11-01/How-To-Sawdust-Stove.aspx

That's a cooking stove, not a heater.  A heater has a radiating tank and a stove pipe.  A cooking stove is just a can packed with sawdust with legs or set on bricks and a flue down the center and through the bottom.  There's usually offsets at the top for holding the pan up so the heat can rise around the pan.

Using your RMH to burn sawdust sounds like a feasible idea my only question is in the loading of it. The fuel inlet for a standard RMH is where the draw for the fire  happens there is a tremendous amount of sucking force to this once it gets going real well my concern is two fold one would this force of suction actually suck chips up into the chamber unburned and cause a clog or unnecessary buildup of unburned material and 2. you'd have to really watch your RMH and be constantly filling the burn chamber as loose sawdust will combust in this type of stove rather quickly and overfilling would choke off the air inlet and kill your fire.

That said have you thought about making biomass briquettes out of your sawdust? this would solve two problems one they would be compacted and thus increase the burn time and the chips wouldn't be loose and flying up into the unit. Youtube has many various videos on Briquette making.

I kinda asked myself the same question you posited and the briquettes were my solution. Fun to make and easy to deal with and handle DIY presto logs without all the glue and binders. Just a thought for you

I think it's very doable.  I've been thinking about it for a while.

If, instead of thinking how to turn a sawdust stove into a rocket mass heater, I turned it around and worked through turning a rocket mass heater into a sawdust stove.

Using a water heater tank as the core, it would be possible to put a layer of insulation on it, though it wouldn't have to be very thick as the sawdust does the insulating during the burn right down to nearly the end.

I'm very interested in burning sawdust to heat my wood shop/barn. been working out there pretty steady for the last two weeks without any heat and it's wearing on me. while I'm building, I make a good amount of sawdust (20 gallons/day?), so it makes sense to use it for heating.

so the standard two-barrel sawdust stove looks doable, but I'm wondering how it could be used to heat a mass. looks like the typical design relies on the vertical chimney for draw, so a horizontal run through a mass might not be feasible. my space is pretty big and very well ventilated, so heating the air doesn't make much sense.

what about adding an insulated riser and outer heat exchange barrel from the RMH design above the sawdust? it would make the stove fairly tall, and would complicate loading, but it seems like a reasonable way to get the draw without the hot vertical chimney.

any other ideas? I'm sure I can find other ways to use the sawdust, but this would solve two problems at once.

oooh. what about compressing a charge in a similar fashion to the sawdust stove, but that would fit in the feed tube of a rocket mass heater? use two sticks to create an L-shaped route for air and combustion products. the whole thing could be wrapped in butcher paper to keep it together. or a metal cartridge could be fabricated that would fit perfectly in the feed chamber. the cartridge would need a removable lid to meter air supply.

could it work?

If you want to convert your sawdust into standardized fuel briquettes, take a look at the Holey Briquette

Andrew Parker wrote:If you want to convert your sawdust into standardized fuel briquettes, take a look at the Holey Briquette

that could be just the ticket. looked around for something like that a bit, but wasn't having any luck. thanks.

the holey briquette got me searching, and I found these plans for a press to make them. I'm not sure that I'll need the hole in the middle, but I'll probably try it both ways to see which works best. my guess is that a relatively small diameter without a hole will work well for a rocket mass heater.

as soon as a couple of other projects are finished, a friend and I will be building one or more of these presses to try out. I'm excited.

If you are making small briquettes, the size of commercial barbecue briquettes, you probably won't need holes, but if you make briquettes as used in the Holey Rocket, the same diameter as the fuel inlet, you will most definitely need the hole the same size as shown. What binder material are you thinking of using -- paper pulp, starch, clay, manure?

Check out the Peterson Press. It is a homemade hydraulic press using inexpensive dimensioned lumber. It uses the same molds as the other holey briquette presses. Check out the youtube videos referenced in the document.

well, I was going to see if I could get away without using any binder at all. the cedar sawdust I make most frequently is fairly resinous, so I'm hoping that with enough pressure it will stick together on its own.

if that fails, I'll try whatever's easy to get. I would guess that newspaper pulp would be relatively easy, and there's a fair amount of manure around here.

as far as size, I was thinking I would go for smaller diameter than the holey briquettes, but rather longer. maybe two or three inches wide, and eight or so inches long. with a smaller cross section, I ought to be able to give them more force with the same press than the larger holey briquettes, so that may change things. but again, I'll try different methods out to see what works better.

I won't be using these for a cook stove like the holey briquettes seem to be designed for, so there will be some experimentation involved to find the best fit.

have you made any of these, Andrew? I would be really interested to hear some first hand experience.

No first hand experience briquetting as of yet. I have a flypaper memory and like to research, so I throw out ideas when I feel it appropriate and do some quick googling to see if there are any relevant web pages that would help in a thread.

My only experience with wood stoves, other than campfires, firepits and fireplaces, is trying out my brother's StoveTek stove, making a little modified rocket stove out of pavers and bricks, and an old weber kettle grill with the bottom cut out that I use on top of the rocket stove for grilling.

The Holey Briquette and the Holey Rocket have really caught on in the last couple of years. I think it should work well in the J-rocket stoves like the RMH.

Another design that ought to work well with an RMH is the Pasifier (a modification of the Dasifier) by Alex English. It is a natural draft cross-draft burner that can burn pellets or chips fed through a hopper. It looks very promising. I don't know what modifications would have to done to burn sawdust, but I suppose it is possible. Mr. English also specializes in moving biomass, so he ought to know if it can be done without a lot of complication. The current solution for using sawdust in stoves not built specifically for that purpose appears to be to press it into pellets or logs, which requires special machinery, or make it into briquettes, which is doable on the cheap.

well, first hand experience or not, I think you pointed me in the right direction. thanks.

use a bit of horse manure for binder. if you are going to use pressure make the press out of steel cause i keep breaking the wood ones and am sure other folks will as well.

I would do an iron plate and pipe with a hydraulic jack myself then i could put a few thousand pounds of pressure on the briquette. also you might think about the dura log type briquette as it may work better than what we where working with. (the ones with the hole in them.) make the briquette either small or to the height of the feed tube.

Ernie Wisner wrote:use a bit of horse manure for binder. if you are going to use pressure make the press out of steel cause i keep breaking the wood ones and am sure other folks will as well.

I would do an iron plate and pipe with a hydraulic jack myself then i could put a few thousand pounds of pressure on the briquette. also you might think about the dura log type briquette as it may work better than what we where working with. (the ones with the hole in them.) make the briquette either small or to the height of the feed tube.

hmm. steel sounds good. my friend has some stout oak he wants to build the levers and frame out of. that should be pretty darn strong, but using steel for the pressing parts makes sense and shouldn't be a problem.

my vague plan was to match the height of the feed tube, and without the donut hole, so you've confirmed that course of action for me. I've got horse manure to spare, so that sounds good. I'll still try a sawdust only mix, just to see what happens.

have you used these in a rocket mass heater, Ernie?

Morning Tel
yep We built a model for pressed briquettes it needed a bit more tinkering before it was ready for prime time but it worked OK. the donut hole fuel tended to smoke up through the hole and that made me try a few presto logs those worked OK.

What the fuel was saying is we needed to press it harder than we could get manually. We had to move before i was ready to build the next phase with a better press.

Hello I have tried making one of these today in a smaller version. The outer drum is a 30-gallon drum, the inner one a 20-litre drum. The smaller one is approx 6 ins lower when in place than the larger one and there is roughly 3ins difference in diameter between the two sizes. The sawdust has slightly bigger particles, its not as fine as that used in the video. Everything else was pretty much the same, and it all went together OK, just a smaller version. However, I just cannot get it to stay alight As soon as I seal the drum up, it goes out. My feelings are that theres not enough oxygen getting to it? But then there doesnt seem to be any allowance for that in the video either . Oh the other thing thats different is I havent flued it. I just placed it on a wall so that the outer drum edge hangs over and creates an outlet for the 'smoke' Could the particles of sawdust be too big, could the flue thing make a difference, could the sawdust be damp, although its been inside? Or could it be not enough air? What is the most likely problem here, does anyone know?

hot gases only rise, and a sawdust stove needs a draugh to get the combustion air. It sounds as if a flue is the first thing you may need. Have you tried just running the inner drum filled with sawdust and no lid? If that works then the sawdust is ok, it's just likely to need a flue.

Yesterday I cobbled together my first attempt at a rocket-wood-dust-stove for purposes of mass heating & dry steam generation:
50g steel outer exhaust down flow drum,
30 g steel dust charge drum ot (wood dust compressed in place),
vertical burn hole formed with a 4.5"OD plastic pipe set in a
steel air supply & fire starting tube 7" ID, with rough fit sealed with earth false floor.
Assembly set on water leveled & stabilized ground (puddled foundation earth)

Test fired last night. Burned well initially without the lid on.
With the lid on the lid did not get hot enough to support secondary combustion of the flue gas and so power the pump of flue gasses downward.
I somewhat expected that since there is a 3" gap between the top of the 50g barrel and the open top of the 30g drum.
That space is needed for pipes to superheat and dry the steam (open flow system arising from a pressure cooker on top of the 50g drum lid) .
A bit of hot steel to facilitate the secondary burn appears necessary.
Just to test that I dropped a bit of screen over the top of the saw dust's burn hole - now the dust burnt clean and hot.
But the burn was not given its secondary burn by the lid so that the down flow of flue gas would be pumped ram-jet style.
Draw was not nearly enough to produce the burn rate my intended use required - even with the lid off.

A right-gapped lid on on the inner 30g drum may do the job of starting and concentrating the burn so that the outer rim of the lid and upper edge of the 50g drum will get hot enough to complete the secondary burn powering the down draft pump. Sound a bit like wishful thinking. I might test that tonight.

My compressed-to-form dust charge collapsed at about 2/3 burnt. That may have been prompted by weight of the screen I used.
I suspect that my saw dust also needs to be drier.

I would sure like to get it right for this evening test burn. Any insights or guesses will be greatly appreciated. TIA.

Since the burn face and its tube grows and the dust charge is consumed it seems like it should continually work better until its form remains collapse.
Does anyone know if this is true?

Too easy! I'm in Thailand. At a local uni open day i observed an innovations expo / competition. The winner was a sawdust stove. You can make it any size and it is FREE.
1. Get a 5 gal tin. Cut a hole in the side near the bottom. Cut another in the centre. The hole should be the diameter of some metal or plastic pipe you have.
place one bit of pipe from the side into the centre of the drum meeting the second bit of pipe coming down the middle of the drum. They used 2" pipe.

Now fill that drum with sawdust, pressing it VERY tightly a number of times. They used a frame and a car jack to force the metal lid of the drum down on the sawdust as they added more sawdust. When crammed full withdraw both pipes. Use a twisting motion and make sure the pipe is smooth all the way around - you don't want to collapse the chimney you've just made.

What you have is an air intake at the bottom, and a chimney in the middle. The sawdust on the sides of the chimney burns - and burned CLEAN - I was pretty amazed.

If you think about it, this has the 2 essential elements of a rocket stove - turbulent burn at the angle and insulation - the sawdust was incredibly good as insulation. I saw this stove for about 3 hours - and that was about 2 hours after it was lit. You could actually see a blue flame at times indicating that gasses were being burned. The chimney widened as the sawdust burned but the sides of the metal can never even got warm.

They told me it could burn for 20 hours. They use these for pasturising mushroom growing media, and for cooking big pans of corn cob. Anything where a long burn modest heat is needed. They could put the fire out by placing a lid on top.

You could probably make a mini version in a a coffee can for your heat output needs. The thing burns totally safely - so you can walk away and it will burn all night. They also demonstrated an innovation that uses rice husk as fuel (actually the rice husk never burned) It was a top down fire that burnt the gas given off from the charring rice husks I think. That one used a computer fan to suck the air down and somehow the flame came out a rim around the outside of the 2 chamber pot. I must go visit Mr Jai again and take some pics. Again it was smokeless. Maybe there is a youtube vid of this type of burner.

Sometimes low heat is better than high heat. For my cooking needs (I often cook outside using a dutch oven) a wood fire burns too hot and needed too frequent tending. I found using dried cowpats was PERFECT once alight from a tiny bit of kindling they smolder for hours. I used to roast my veges on cow dung - I found 2 pats under my 3 brick supported dutch oven was just right. I did place an old metal can that the bottom had rusted out of around it - made a good wind shield and kept the heat in and the cold out.