Reverse engineering Riley pellet burner

[gooch]

Your reports are very interesting and much appreciated. Keep them coming!

[rectifier]

I'm a bit frustrated as I cannot seem to achieve a longer than 6 hour burn without some kind of interference (move the throttle, kick the stove etc...).
Yesterday I thought I was good to go after a few mods:
- increasing stack height to 7' with a piece of flex out the window instead of the 90 (a 3' stack is really far too short) increased power to >30Kbtu at full throttle
- agressive porting of the 'throttle body' to target charred pellet buildup in certain areas, which caused an over-rich burn due to too much gas and not enough secondary air thus...
- increasing the 'nose' length to poke out of the pellet heap and provide a real secondary air source. This results in the burn leaning out, the flame burning on the 'nose' instead of on the end of the burner where air leaked in, and a great pulsejet roar at full throttle
but ran out of fuel after 2hrs at 30Kbtu... however I was encouraged as the shutdown was 100% clean, burning every single pellet to fine white ash.
Today, when fueling up the stove I banged the stove cement seal on the elbow, which shattered, allowing excess air into the rocket chamber again. It operated OK for 6hrs but stalled again due to a buildup of charred pellets, receiving a complaint from an irritating neighbor about smoke from my shop (it's an industrial area guys... we usually make more smoke, it just goes out the stack, not the front window)

Therefore, that stupid elbow is going due to its poor fit, weakness, and loss of heat before the heat riser. As it is, the stove needs to be running at 30kbtu or greater to have the flame in the heat riser, and when the active pellets are burned out and needing to be consumed to make room for more, the power drops enough that the flame drops out of the riser and there is not enough draft to consume the charred pellets.

Mods for this weekend:
- 4" square downpipe instead of the elbow, welded to the stove below the riser, and flanged on the side to mate directly to the burner.
A couple hopes from this one:
- flame will go straight up the riser from the burner, without 6" of extra horizontal travel and cooling in the elbow, better at lower power
- square edge will create much more turbulence, allowing richer mixture
- downpipe welded to base of stove and attached w/flange will make the stove one sturdy unit instead of several pieces + fragile cement
- 4" downpipe will continue past the burner to provide an ash pit, so I don't have to take it apart all the time to remove ash
- Grind/polish the edges of the hole where the hopper meets the firebox! I can't believe I never thought to clean up the rough torch-cut edges. They definitely produce rough surfaces where pellets can hang up, and everything helps
- Maybe reintroduce the slots at the front of the hopper. They helped remove pellets from the front edge, allowing better dropping. Maybe not necessary if the edges are polished.

[gooch]

I built the smaller 3x3 and the unit works fine but doesn't throw as much heat as I'd like it to. It takes about 30 minutes to get up to full burn. The pipes are 3/4 inch and so it the square tube. They just fit inside the hss. The square tube seems to be blocked by new pellets and I don't get the air turbulence that I should be. Did you find that extending the center bottom tube helped? I also added another tube in front of the hopper as they show on the smaller unit. I'll post some pics later. I don't need more than 20,000 BTU as this is for my ice shack.

[rectifier]

I didn't extend the center bottom tube, I extended the center top tube (the 'nose' if you get my drift). My center bottom tube doesn't even stick through the ramp, but it has a little overhang welded above the hole to keep it from getting stuffed with pellets. I'm not quite sure what it's doing for me, anyways... maybe blowing away ash? It doesn't do that very well.
My throttle body now looks different, I added more porting, and the burner is welded/flanged together much more tightly. I'm all set up for a +8hr test tomorrow if all goes as planned and I'll post some pics.

How long of a burn time have you got so far before blockage, and what kind of stove/stack are you driving the burner into? I hope to throttle back to around 15kbtu eventually, but I am working on getting it running at full throttle since it is more forgiving of blockages. I want to heat a medium-sized wall tent or a small house. But in a ice shack you are just sitting there anyways, no problems with giving it the occasional kick, right? heh.

I noticed that small tube, I think it may be for primer fuel for lighting the stove? Or it could be air wash, to keep the hopper cool. That is still one of my issues, gasification in the hopper.

[rectifier]

Stop the presses - this entire theory was wrong. Purely by accident today, I discovered how this stove is supposed to operate. I feel a bit humbled by how simple the stove really is, yet brilliant.

It is not supposed to work by pellets dropping into the burner. The burner, which I'm sure everyone is focused on, is almost completely irrelevant. Size of pipes, shape of pipes, porting... It is only used for creating the initial conditions.

This drawing shows what I was thinking for theory when I built the stove, and what I'm sure is roughly other peoples' idea of the theory of operation. However, this only holds true for the initial burn of the primer fuel and the first few times the pellets slump into the firebox.

This design would be considered a poor version of an updraft gasifier. The gas is not clean as it doesn't pass through a coal bed and contains heavy volatiles. It burns sooty, but that is OK if we feed it into a rocket chamber. Also, the flame burns directly on the fuel surface, not much of a gasifier. But it makes sense to the human brain, familiar with campfires - pellets burn and are replaced by more. The embers on the bottom are attacked by gas, slump into ash, and are hopefully blown away. This is not how the stove works.

This will only work for a few hours before ash builds up and it stalls. I got 8 hours today by slowly pulling the throttle out every 2 hours.

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[rectifier]

I stayed after work to work on an engine and see what would happen. Stalling seemed inevitable. However when I walked back over to the stove half an hour later, the power level had increased instead of decreased. Good news, so I left it alone to see what would happen.

Half an hour later and it was in thermal runaway, the roar audible from across the shop. What had happened? Looking through the 'nose', all lower air slots were blocked with ash, but it was running fine. Hot pellet embers were dropping in front of the 'nose' and being shredded in the blast. For some reason the stove was working again, and better than ever.

Wait a second, hot embers? Shouldn't they be fresh pellets?

On closer inspection, I found that one of the guys shutting down the shop must have bumped the lid on the hopper, opening it just a crack. Removing the lid caused the stove to roar like an angry bear. Closing the lid silenced it. Cracked open, it ran smoothly. Suddenly, some of the odd parts of the Riley design made sense.

The Riley stove is actually a downdraft gasifier.

All this time I had been trying to stop the hopper from gasifying pellets. On the contrary, that is what it is built to do. The burner at the bottom is more of a carburetor, to mix the gas with air and burn it.

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[rectifier]

It ran for 3 hours as a gasifier with no interference other than topping up the hopper, at which point I wanted to go home and let it run out of fuel. Total burn time today: 11 hours, 20lbs of pellets. Average of 17Kbtu/hr and enough to keep the gas-guzzling overhead heater from switching on at the shop - not bad.

So, here's how it actually works:

The initial startup fuel burns hot and fast, well aerated by the pipes. It creates the draft necessary to pull the gas downwards in the hopper, and lays an insulating layer of ash over the ramp. The feed then jams on purpose and pyrolysis begins to creep into the hopper. Pellets above gasify, and the gas is drawn through the embers where heavy tars are cracked into clean wood gas. This wood gas burns when it hits the preheated secondary air coming in through the 'nose'.

The 'throttle' adjusts both how easy it is for the bridge to break and drop old embers, and the amount of area for wood gas to flow through. It has to be pushed in by an inch at full throttle because otherwise, the feed won't jam and it won't operate as a gasifier at all. A mystery explained.

The slots at the base of the hopper are supposed to be there after all. They provide secondary air to attack the embers and oxidize them rapidly so that the embers will drop and not build up.

As the icing on the cake, as the last pellets turn to embers in the hopper, oxygen directly hits the embers without passing through the pyrolysis zone. They then heat up enough to get the hopper walls gently glowing, burning off any condensed tars and leaving it clean for the next run. Surely not intentional but a handy feature nonetheless.


I have to think the original inventor discovered this gasifying mode by accident as well, as it seems impossible to design on purpose. A stove like the Clarry is a much more likely design for someone to come up with. Otherwise it is brilliant - a downdraft gasifier with no fans and no grating to burn out, as the pellets support themselves in the gap - no wonder they can offer a 5 year warranty!

In any case, I take back the remark I made about sawdust jamming the stove - a gasifier like this should easily eat sawdust. I plan on trying it on sawdust myself tomorrow as we get big bags for free to mop up spills.

Edit: coarse sawdust. Not the fine, dusty kind - no airflow. Fortunately that's the kind we have.

[gooch]

AWESOME!

[peterberg]

WOW!
I'd follow this thread since May. From the very first article I've got the idea "Maybe, just maybe, a burner like that could be adapted to a rocket mass heater with a vertical feed".
Looks like I've got some work to do when spring comes in.
And, just to be sure, WOW! again.

[rectifier]

Took it for a spin on sawdust today. Started on a hopper full of pellets, gasification mode was established after half an hour and 1/4 hopper of pellets were burnt. After 3/4 of the hopper had been consumed, I stuffed it full of sawdust, a mix of coarse and fine.

The remaining 1/4 hopper of pellets were completely consumed. At that point, the first of the sawdust to ignite formed a charcoal bridge that didn't break, the feed stalled and it went out.
So...
- sawdust too light? it is less than half the density of pellets. Maybe even a quarter. Light and fluffy.
- sawdust too wet? it had been kicking around the shop in a pail for a year, the shop is near 100% humidity much of the year
- not enough air to the bridge area? There is only air attacking one side as the other was welded better. Going to grind it out anyways.

Those were most likely the issues. I don't plan on looking into it much as I would need a huge hopper to make it through a day with the low density available. It's much easier to use a controlled fuel like wood pellets - or perhaps 1/2" or smaller wood chips? My goal is not really heating my shop, it's heating a hunting tent/cabin, and I'm not bringing sawdust into the woods.

Thanks for the words of encouragement, I am out of fuel again, but going to pick up more tonight and see how long of a burn I can get in gasifier mode tomorrow. Hopefully it should make at least 8hrs without any prodding, but I expect the pyrolysis region to try to move about in the hopper - getting it to remain stable and not creep up too high will be the trick. However it did not get above the elbow in 3hrs of burning, so I doubt it will be a significant problem.

[rectifier]

As a gasifier, the burn rate is very stable once set. I increased the hopper size to hold 15lbs and be fully sealed, then cut primary air slots closer to the burner. With the throttle set at the same setting (a score mark on the tail piece) it burned 5lbs at 2.2lbs/hr, 10lbs at 2.2lbs/hr and 15lbs at 2.1lbs/hr. This corresponds to roughly 22kBTU/hr fuel consumption, and is sufficient to keep my shop at a good 'working temperature' (55-60F) when it is 30F outside.

The pyrolysis region managed to sneak up all the way to the primary air intakes I cut, 6" up the hopper, but didn't do anything bad when it got there. The increased air attacked the embers and stabilized it there. However, I don't need that many embers to burn at the end (takes roughly 1hr, running with a CO flame). I reduced primary air by 1/3 and then closed up some of the excess porting on the front tubes and decreased the airflow through the 'nose' - this causes it to pull harder on the gasifier, increasing the flow through the bridge slots and hopefully attacking the bridge pellets more.

There is a gas outage today, so I can't use my gas furnace to keep the chill off overnight as I usually do. So the pellet rocket is (hopefully) keeping the chill off the shop overnight unattended! I left it with 15lbs of pellets in the hopper and the throttle set significantly lower than its usual setting. However due to the changes in the porting, it seemed to be running with a flame I estimated at 15kBTU, and the ramp mostly clear of embers due to the faster airflow.

Hope it burned all 15lbs come the morning and the shop didn't end up full of smoke because the throttle was set too low and it went out... the hopper is well sealed though, so any smoke should all go up the stack.

[gooch]

Once again another excellent report. I'll get some pics of mine today and post them. I'm thinking of putting in a "Y" pipe with a diverter as it seems that a lot of heat goes out the exhaust that would be really nice in the shack.

[rectifier]

Went out after burning 10 out of 15lbs last night, best theory is that the flame fell out of the riser and then kept shrinking until it simply ran out of draft to continue. This gives a definite minimum power level, which so far seems to be defined at maintaining an exhaust temperature of roughly 290F on the pipe. Anyways, I pulled out the throttle, gave it a thump, more diesel and re-lit it to keep the shop warm today, and it worked fine.

Gooch, remember that any wood fire exhaust, no matter how clean it looks or smells, contains enough CO to kill you quickly. Never, ever, vent it into an enclosed space. It may be safe to run a gas flame indoors with a window cracked, but not wood!

[gooch]

Thanks. I was thinking more of running a second exhaust pipe around the shack for some extra heat. Here are the pics of mine.

[rectifier]

Lookin' good gooch! I suggest more porting to develop more power. Don't see any slots in your lower 2 pipes? The main differences are mine has a longer nose, slots, and I have the top 2 slots at the base of the hopper that you have welded on solidly.
Edit: I welded caps on the bottom pipes to make all the air come out the slots to blast the middle. Yours are open at the end and just blow air into the flame zone. I found the slots do a lot of work burning the pellet embers.

I like the blast gate, does it work well to cut off the pellets?

As I see it those slots are essential to proper gasifier operation, so yours may not be running as a gasifier, explaining the low power. Also with the barrel stove you are losing a lot of heat up the stack. I was losing probably 80% whereas the rocket stove probably retains 80% or more. So the flue diversion around the shack is a good idea, or a downdraft heat exchanger like the rocket barrel.

I have a 6' piece of 3" aluminum flex in my stack, running at about 30 degrees upward slant. Its temperature decreases from 300F at the start to 190F at the end - a lot of heat gets extracted from it. Note that aluminum flex is only rated to 600F before it is damaged.