Reverse engineering Riley pellet burner

[rectifier]

EDIT: Attention any newcomers to the thread! I consider this drawing to be no longer valid. If you read the thread you will notice a steady stream of modifications. You can build a working burner from this drawing - but it will likely experience issues such as jamming, flame creep and possible detonation of wood gas, resulting in a shower of flaming pellets. It WILL NOT operate safely overnight unattended. You have been warned!

I was doing some research into gravity-feed pellet burners today, when I remembered the original one I really wanted when I first got interested in tent camping. Riley Stoves make a small gravity-feed pellet burner that can run 12-14 hours on a bag of pellets.

The only problem is the cost, $600 for the burner unit ALONE (no stove). I have always wanted to tear one apart and see how it runs.

Today it turned out someone was selling one on Ebay. (For $500.) But for the first time, I got to see it taken all the way apart and photographed from many angles.

Hours later, I have made a scale drawing from the photos. I have tried my best to keep it accurate despite to the limited resolution of the source photos. I believe you could build a functioning burner from this drawing and scrap steel, and I will when I get back to my shop.

Edit: the angle of the feed is right on 60 degrees. Forgot to write that on the drawing.

For now I thought I would post it so any other pellet enthusiasts can get a view inside this mystery burner.

And I tried to keep the size of my image down this time

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[rectifier]

Some notes on operation and design theory:

As noted by the manufacturer, the lid MUST be kept tightly on the hopper at all times other than when you are refilling it. Otherwise, I'm sure the fire will creep up and eat the hopper. Pellets are very dry and will take any excuse to burn.

Throttling is accomplished by pushing the throttle body (in blue) in and out. It is drawn at 100% throttle and is pushed in to decrease power. I find it odd that it sticks into the hopper at full throttle - but according to the photos, that's the only way it can be capable of closing the hopper completely for shutdown (which it is designed to do). Maybe this is to cause a break in flow to discourage flame creep. I don't think I would ever reduce power as part of a RMH system, that would just slow the whole system down.

Starting is done by filling the hopper, setting to full throttle and squirting starter gel through a hole in the side directly below the upper air intake (the hole is then closed.)

Speaking of air, the theme here is basically LOTS OF AIR. With a slotted ramp as well as 4 separate air tubes to deliver air to every part of the pellet heap, the goal is obviously to keep it well aerated, blow away ash and charred pellets, and keep it hot enough to completely consume even the lower pellets, allowing more to drop. The manual notes NO dampers/sparkguards/opening the stove door/anything in the smoke path. Also, it won't run above 8,000' elevation - my theory is as it will run too rich, charcoalled pellets will not be consumed readily enough and the feed will back up. Charcoalled pellets are a serious problem in any pellet feeder designs I have worked on - once the volatiles are gone, you have to get a lot of air to them and keep them hot, or they will just sit and smolder for hours, especially in naturally aspirated gasifiers. Thus the fans in home stoves.

Obviously powerful draft is very important to proper operation. That's why I think this will work great as a firebox for a rocket stove, the hardest-pulling naturally aspirated stove around.

This is a 4" firebox by design, the largest on the market, and I'm not sure if it will have to be scaled up to work with a 6" or greater riser, or if the 6" riser will simply pull extremely hard on the firebox and get it glowing red hot. In the RMH book, the firebox is said to be the tightest part of the system - which it definately would be. Only experimentation will tell, and perhaps some bypass air will be required to keep it cool.

Also, the lifespan will be intereresting to see. This was designed as part of a camp stove, and thus to run a relatively low number of hours, despite the 5-year warranty. With oxygen-laden inlet tubes passing directly into the heart of the fire, oxidation should be a serious concern unless built out of stainless steel or thick heavy iron.

Finally there is no damn way this thing should cost $600. It even looks cheaply made in the photos. My goal is to build the test model for under $20 and I see no reason it would cost more.

Only 5 more weeks until I get back to my shop... sigh

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[rectifier]

The guts... never before seen!

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[stork]

We've got the same project going , be good to have someone to share with .... I guessed the feed angle at 45* ... I built the outside already ( 3" & 4" ) , cut some wood to act as the inside with a piece of 1/2 dia. fitted to clone the down pipe . both have a 3" feed pipe ... I'm useing metal vent pipe for the hopper ... I've ran several tests just dropping pellets , a friend guessed the angle about same as you , so we whacked the other end of the wood off at 62* ; tappered the corners in more tests .. after a case of millerlite , loading a 2' hopper 30 times , pulling the pellets out as they fell, measuring and marking , we came to the conclusion that everything we did worked close to the same ....
I plan on chopping up an old hot water heater tank to make a cylinder stove (w/flat top) .. I've milled some flanges to bolt the different burners to bolt to the stove ( plan on trying some other designs of burners down the road) ... I'll keep you posted as things get done ..
the more I look at the jpg's of the reily burner with the scabby welding job, I keep wondering if this is their product or just bait for us DIY guys

[canyon]

Nice job posting rectifier, thank you for sharing those photos/drawing! It took me a bit to wrap my head around the thing but I get it now. Kind of a brilliant design but a bit of work to build. Please keep sharing too stork! I am wondering how to adapt that to chips or a sawdust/chips blend so as to avoid the trouble of pelletizing or buying pellets. I wonder if making the horizontal burn tunnel on a downslope passing over a deep removable coal tray would allow some time to get a decent burn length with challenging fuel blends. I could see stainless as a good choice of materials for this kind of thing. Please keep us posted ya'll.

[rectifier]

Great to see another guy working on this concept! I think this burner combined with insulated risers has great potential in usability and convenience over the traditional 'rocket' that must be fed constantly. Sounds like you are going to try to replicate the 3" stove, I figured to try the 4" since a bigger feed is less likely to bridge and jam up, and more power will make it draw better.

It's actually quite simple to build I think. You just need to bend a piece of plate to 120 degrees, tack some pipe to it and slide it into a piece of 4x4 square. Zipcut to make the slots. Knock a hole in the top, tack on feed pipe and you're basically good to go. Like stork says it's probably a case of beer's worth of work on Saturday. Dropping pellets will feed for sure (i tried some designs like this before, not knowing the internal structure) but the real test is how the pellets drop when under flame, and does the coal bed stay together enough to light them, does it choke or back feed. Can't wait to see some results from your experiments.

Canyon, I have experimented with sawdust and chips in some other stoves before. Chips gasify very nicely in a NA stove but feed terribly without a powered feed due to their assorted sizes and rough edges. If you are looking for a design to use in the home that would be fine, with an auger. My goal with this is to build a portable unit so power is out of the question.
Sawdust... I found sawdust simply refuses to sustain a burn unless burnt in a packed batch gasifier (pretending it's a big chunk of wood basically) or with forced air or massive draft. Its air requirements are too finicky and it likes to smoulder or just go out if it packs itself too dense. My mother's farmhouse when she was young was heated by a sawdust stove - she told me she would never think of heating with sawdust again after all the time spent unclogging the thing, relighting it, dealing with fire creep back in to the hopper etc... and that was with a powered feed too! Chopping wood is less work!

[jonmhenderson]

Any updates on the pellet burner? I would love to have someone build me one!

[rectifier]

Sorry all, I am back at my shop but between a surplus of work, shortage of local scrap steel (we are selling it all to the chinese as soon as it comes into the yards... hard to get a hold of good scrap around here now), gratuitous prices on non-scrap steel and trying to get ready for an early season bow hunting trip that won't require a stove, the pellet burner is on the 'back burner' for now. I don't even have time for paid side jobs let alone hobby stove development!

I'll be back on it in September, promise.

[stork]

I've got a burrner built , but need to build a stove to test it ...
scrounging up the steel is a challange around here too..
Too many projects not enuf time , lol

[rectifier]

Cool, you don't really need a stove though, my plan is to just put a 90 coming out of the burner and then up a 4' stack (to mimic the draw of a rocket chamber) for testing. Get enough draft to pull on it and get that sucker burning!
Back from my hunting trip, didn't get my deer, but it was cold enough at night I wish I'd brought a stove. Going to start getting serious about this burner now and get it together.

[jackvegas]

Hi all,
Just saw this thread. Hope I didn't arrive too late to play :-)

The Riley unit looks interesting but a much simpler design, and I think better, is the one in the Clarry Stove. It is somewhat similar to the Riley, but I think easier to build, less complex, and more efficient.

Their web site with instructive videos is here:
www.clarrypelletstove.com/

To get detailed drawings of the guts, certainly adequate for an experimenter to copy, check their patent application # US 20090038603. You can download a pdf of this free from Google's patent web side:

www.google.com/patents/about?id=z5OzAAAAEBAJ&dq=clarry+pellet+stove

I'm still about 3 months from being able to try building one in my shop. We just moved and all my wife's cloths are currently stored in the shop. How the heck did I allow THAT to happen?!!

[rectifier]

I've seen the Clarry before, I was concerned about grate burnout and pellet loss through the grate, so I looked at the Riley for my inspiration. That and good solid grate is a lot harder to come by than steel tubing. That is a handy set of drawings though and it's good to welcome another pellet enthusiast on board!

That said, a machinist friend of mine sliced me up some scrap steel. After tonight's frantic fabrication activity, the stove is 90% complete minus the feed and hopper. I guess those are the important parts, but I still wanted to test what I had to see how the pellet pile was aerated.
So, I sat it vertically, stuffed in pellets to fill the burn chamber, plopped on a 2' tall chimney aka piece of "big ol' pipe" that was laying about the shop, and let it rip. Let me tell you there is so much frickin' airflow to that pellet pile that the throttle mechanism better work. It ate those pellets in about half an hour, gave off a good loud roar and heated up the big ol' pipe so hot that all the oily gunk on the pipe burned off and left it clean (not that impressive unless you saw the pipe before... heh.)

Small amount of blue smoke for exhaust but the big ol' pipe is about the opposite of the rocket chamber, being thick walled and having tons of thermal mass, I'm impressed it started as well as it did. Pellet feeding worked pretty well as towards the end of the burn it slumped and dumped all the glowing pellet embers out through the air tubes onto the floor, sitting vertical as it was. That probably would have resulted in the hopper feeding plenty of new pellets into the chamber.

My friend is cutting the hopper elbow tomorrow, hopefully it will get put together and tested for real within the week!

[rectifier]

Hopper on - test success! I burned it about an hour with no interference other than observation as an initial test. The hopper will get cut and turned in the middle for a more vertical feed, hopefully before the next test.

Stove configuration: 5' chimney with 90 at base, loose connection to horizontal stove box
Fuel: Enough pellets to fill the chamber and the bottom of the hopper
Burn time: ~1hr to consume all pellets

Burned hot and clean with no exhaust other than the occasional fly ash. Burn mode was mostly gasification - a bottom bed of coals is kept hot by the bottom air feed. New pellets land on this bed and are gasified - the gas then flashes to flame when it hits the air coming from the top nozzle. These pellets then replace the bottom coals as they crumble to ash - thus maintaining a continuous bed.

Flame intensity does fluctuate somewhat as new pellets land on the bed and initially quench it, but no smoke is produced and the fluctuations are minor.

Only issue: The hopper is capped, but towards the end of the burn when the hopper is clear, pyrolysis gas creeps up into it and condenses on the walls. This heats it, drawing further gas to rise into it. Shouldn't cause flame creep, but maybe can be solved by adding an extra air wash at the front of the hopper to keep the gas from sneaking up there.

Future plans:
- fill hopper completely and make sure it burns the whole thing
- feed a 4" rocket with it and design the heat exchanger for the wall tent heater
- scale up to 6" system size to see if it can feed a household rocket

Very happy with it and it looks like it could really work great for applications where you don't want to feed the wood by hand... I'm thinking institutional type stoves, continuous flow water heating and maybe even a steam engine boiler due to the consistency of the power level.

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[rectifier]

Staring down the air tubes from the back of the stove.. you can see the bed of coals roasting over the lower air feeds

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[jackvegas]

Hey rectifier!

Good stuff. Thanks for the update and pictures. It looks like you are making steady progress. Your idea of adding extra air wash to keep gas from sneaking up int the hopper sounds promising. You seem to be getting such good draft that you could conceivable set up the air wash such that it pulls a slight negative pressure in the hopper. Also, the potential for adding air wash validates your choice of the Riley type burner since it lends itself well to custom gas flow distribution.

For myself, I'm thinking of moving from welded to sheet metal fabrication concepts. Two reasons. First, my welding gear is trapped in the rear of my shop for the next few months, and second, I suspect sheet metal fab could offer an alternative path for people without welding equipment or skills. The concept I'm "noodling-on" would be a simple feeder/burner assembly that could be inserted into the mouth of common brick/cob type rocket mass heaters. Fabrication would require only simple hand tools like tin snips, pliers, hand drill, and maybe a pop-rivet tool. Will keep you posted.