Ash problem

[milegas]

Hi guys, few years ago i have built all metal rocket with external riser for quick heat. It is working surprisingly well but nowdays i have tested it in a all day fire and noticed a problem.
For fuel i am using leftovers from saw mill and it is mostly basswood and cottonwood. In a long term fire i heats greatly but it produces a large amount of ash and tends to clog 14cm(6 inch) j tube and smokes and fire back to feeder.

Now i am planing to build cast core using Peter's upgraded plans but i fear that it will make the same problem since it doesn't have ash pit.
My question is how will it afect burning if i add grill under the feeder for ash to fall down. It would be sealed of course. Tnx in advance.

[Forsythe]

Mar 13, 2022 3:17:36 GMT -8 milegas said:

[...] few years ago I have built all metal rocket with external riser for quick heat.

All-metal rockets can't burn hot enough for complete combustion of the tars and carbon in the woodsmoke. Steel carries away too much heat from the burn chamber and results in an inefficient burn. The fact that you built it several years ago and it hasn't burnt through the metal yet it is a dead-giveaway that its burning dirty. (Unless you built it out of inconel 600 or 330 stainless steel... which would be outrageously expensive.) Those dirty burns have probably filled up the riser with soot and fly ash, and potentially creosote. Need to take that thing apart and make sure it still has the top gap needed for proper airflow... and that it's not going to catch fire from a creosote buildup.

It is working surprisingly well but nowdays i have tested it in a all day fire and noticed a problem.
[...]In a long term fire [...] it produces a large amount of ash and tends to clog 14cm(6 inch) j tube and smokes and fire back to feeder.

Well... This is several problems.

1) rocketstoves like this aren't meant to be fired all-day long. The whole concept is that you burn a relatively short fire and store the heat in thermal mass (like masonry, cob, stone, etc.) and that thermal mass then radiates comfortable heat into your living space all-day-long. If you use the heat output calculators on Peter's batchrocket.eu site, you'll be able to size the heater to your living space's needs, and then you should only need to fire 2 short, (1-hour) fires a day — morning and evening.

2) the fact that its clogging and smoking back is a sure sign that it's not burning cleanly... and is probably full of soot and creosote, which poses a huge, uncontrolled-fire hazard.

3) the fact that you're needing to burn it all day long is not only a sign that it's not producing enough heat to burn cleanly, it also means it's not burning efficiently. You should be able to burn a total of 2 hours a day and rely on the thermal mass for heat. If you're burning for 10-12 hours continuously, then you're burning 500-600% more wood than you need to be. No wonder you're concerned about ash build-up. You're producing 500-600% more ash than you should be.

Now i am planing to build cast core using Peter's upgraded plans but i fear that it will make the same problem since it doesn't have ash pit.
My question is how will it afect burning if i add grill under the feeder for ash to fall down. It would be sealed of course. Tnx in advance.

Please don't add a grate like that. The batch rocket won't burn properly if you do. I promise that if you follow the instructions which Peter very generously laid out on his website, including the parts that warn against building with metal, the need to use refractory materials like firebrick or castable refractory, etc. *and* the parts about storing heat in thermal mass like a masonry "bell" then you will have a very satisfactory result that will save you time, money, and a potential house fire.

And you also won't have anywhere near the amount of ash that you currently are producing.

[milegas]

Well, the thing was built to heat up selected space quickly. I built it from steel because it would give the heat away fast and the heat riser is not inside the bell. It is designed to heat up fast, make fire vortex in heat riser and burn as clean as possible. It is actually genuine Frankenstein stove.

I was aware that it can't reach temps for complete combustion but the compromise was necessary. Despite of all cons of steel fabrication it burns clean, no smoke, no tar, i can tell by the condition of flue pipe. Only some ash can be found iside flues and that's it.
Outside temp at the top of heat riser is around 650C at the end of burn tunel and bottom of heat riser it comes to around 800C that is no enough to burn CO but everything else is pretty much burned.
And last thing is it is designed to burn 5-6 hours a day max.

Now, i have tested it with beech and it produces far less ash than basswood but i don't think i will change fuel since this is cheap and perfect for j rocket, 1 meter long and 3-5cm i diameter.

If you could explain why and how is grate changing the burn process i would be grateful.

I don't know if you have understood me, i want to build core from cast refractoy, i foud a company here in Serbia that makes refractory insulating cement mix that is rated to withstand up to 1300C and with
Insulating coefficient of around 0.3

[Forsythe]

Mar 13, 2022 12:14:15 GMT -8 milegas said:

Well, the thing was built to heat up selected space quickly. I built it from steel because it would give the heat away fast and the heat riser is not inside the bell. It is designed to heat up fast, make fire vortex in heat riser and burn as clean as possible. It is actually genuine Frankenstein stove.

I was aware that it can't reach temps for complete combustion but the compromise was necessary.

I would highly encourage you to read more of the info here and on Peter's site about rocketstove design. There is never a necessity of robbing heat from the burn chamber by making it out of steel — especially when you're going to the trouble of making an insulated riser. The earliest, cheapest, and easiest form of rocketstove to make are the "Rocket Mass Heaters" which feature a steel drum over the heat riser... which rapidly delivers heat to the room, before entering the thermal mass, and happens immediately downstream of the complete and clean combustion.

Getting a clean, complete burn —and then spreading the exhaust gasses over that wide internal surface area of the steel drum— will net you a whole lot more instant heat than robbing heat prematurely from the burn chamber with a steel J tube:





There's also a design featuring this steel-barrel rapid heat-exchange method on Peter's site: https:/batchrocket.eu

If you could explain why and how is grate changing the burn process i would be grateful.

I don't know if you have understood me, i want to build core from cast refractoy, i foud a company here in Serbia that makes refractory insulating cement mix that is rated to withstand up to 1300C and with
Insulating coefficient of around 0.3

These snapshots are also from Peter's batchrocket.eu site:





... I guess... well... I'm not 100% sure what you meant by:

[...]planing to build cast core using Peter's upgraded plans but i fear that it will make the same problem since it doesn't have ash pit.
My question is how will it afect burning if i add grill under the feeder for ash to fall down.[...]

Did you mean the Peter's Batch Box rocket? If so, The one with the P-channel above the firebox? Or the one with the floor channel?




...or did you mean a different set of upgraded plans for something else? ...'cause the batch box rockets don't have "feeders" like J-tube or L-tube rockets do. They have, well, batch boxes with doors that close completely instead of feed tubes.

Those batch box doors then admit a very carefully calculated amount of primary and secondary air into the burn chamber and port, respectively, at very specific points with regulated velocities and flow directions — which are consciously designed to function in very specific ways. As Peter repeatedly cautions against, deviating from those design parameters will very often cause negative impacts to the burn cycle.

Adding an ash grate under the floor of the firebox in one of these would probably disrupt the flow of air over and through the fuel — because of the air pressure gradient created by putting a grate with a hollow cavity beneath it into the firebox floor — and would thus disrupt the vortex created by the mix of gasified fuel and secondary air via the P-channel/floor-channel-tube and port into the heat riser.

...And adding an ash grate would be impossible with the floor-channel design's location, anyhow... (that floor-channel being a more recent "upgrade" to the design... one that's easier to fabricate and replace, at least... which I initially assumed you were referring to.)

He also mentions that the ash is only about 1% of the wood content, which is not going to clog up the firebox. Not when you build one to spec and feed it dry wood.

Even that aside: The velocity and flow direction of the batch box design sucks most of the tiny amount of ash through the riser and into the barrel exchanger or heat storage bell, which you then only have to clean out very rarely. (Many masonry stove designs only have to have their ash cleanout ports removed every 10 years for a cleaning of the flue gas passages.)

I would almost guarantee that at least some the excessive amounts of ash that you're producing with your metal rocketstove are due in large part to the incomplete combustion. That ash has probably got a lot of carbonaceous combustable material in it that would burn if your burn chamber were allowed to get hot enough.

[milegas]

I wanted to build j rocket and house configuration does not allow too big thermal mass. Maybe if i use side walls for heat storage. But mailny the plan was to distribute hot air to distant rooms. That is why i need longer burn periodes and mass should keep house warm overnight.

[milegas]

Another thing, hydronic addition could be the solution. Maybe water heating inside a rocket and radiators for heat distribution. Main problem for mine configuration is heat distribution since heater place ii in the hallway in house center and left and right are rooms to be heated.

[Forsythe]

Mar 13, 2022 22:47:17 GMT -8 milegas said:

I wanted to build j rocket and house configuration does not allow too big thermal mass.

Ah, okay, this is where our confusion came from. Peter's designs are "batch boxes." The insulated refractory J-Tubes have been around for much longer.

A steel grate in the floor of a J-Tube tunnel will burn out *very* quickly. In order to get clean combustion and use all of the energy stored in the wood, these rocketstove cores have to be made of refractory materials, which make the internal temperatures of the combustion zone around 900º-1100ºC (sometimes even higher)

Please see this thread over on Permies.com regarding steel in rocketstove cores, which has a collection of input from a lot of this forum's users, too, including Peter and Satamax: permies.com/t/52544/metal-burn-tunnel-heat-riser/

one of many examples: this is a grate after just 30 days in a rocketstove core, and this one was even made of high-speed steel (HSS) which is meant to take the "high heat" created by high-speed tooling friction:




You can (and should) include clean-out ports in your RMH design to facilitate ash clean-out. but cleaning out ash should NOT happen while the system is hot — and especially not when it's actively burning.

Please look through the forum materials and use the search bar for J-Tube designs. 

Maybe if i use side walls for heat storage. But mailny the plan was to distribute hot air to distant rooms. That is why i need longer burn periodes and mass should keep house warm overnight. [...]Main problem for mine configuration is heat distribution since heater place ii in the hallway in house center and left and right are rooms to be heated.

Longer burn periods are not the solution here. You could build two J-tube rocket mass heaters —each with only a small masonry mass— put one heater in each room, and still be burning less than half the wood than you are currently doing with an all-day or 5-6 hour burn.

And a narrow hallway is not a safe place for the high-heat of an instant-heat exchanger. I don't know what fire codes are like in Serbia, but in North America, you need a minimum of 3 feet (~92cm) of clearance between combustable surfaces (walls) from a "firebox opening" and 18 inches (~46cm) of clearance from the high heat of a metal stove body. ... but if you use a masonry mass to diffuse that heat, the required clearance from the masonry mass to a wall is only 4 inches (aprox. 10cm)

Plus... in a hallway... it's only a matter of time until a person gets burned on it or catches their clothing on fire, while trying to brush past an actively-burning J-tube.

Another thing, hydronic addition could be the solution. Maybe water heating inside a rocket and radiators for heat distribution.

You could certainly try hydronic heating pumped from one room to another... but placing the hot water heating element inside the rocket will cause a flash steam explosion followed by interior flooding of the house and the melting down of your heat exchanger coil. People have tried it. It has often failed. Some have even died days later in the hospital from 3rd degree steam burns.

Even if you were able to force enough water through the exchanger at a high enough volume to avoid flash-boiling to steam; that amount of heat robbed from inside the rocket would make for a disastrously smoky and creosote-laden burn. The fire needs to be up around 900ºC.... and you'd be actively cooling the contact points in the burn chamber to below 100ºC just to avoid a steam explosion.

A safer way to do hydronic heating is to use the rocketstove to heat a whole tank of water located outside the combustion zone, at the heat exchange barrel.


There is a lot of info here that already covers all of this information for you. Please read it.

[milegas]

Now we understand each other. I certainly would like better to use masonry for heat storage and the walls are closest to what i can get to work in my case. Walls are double full brick and could be used as heat storage.

About grid at the bottom of feeder, i don't expect too high temps since the air mass should be cooling it a bit but anyway i was planing to make it easily replaceable since i can make them on dayly base or even from firebrick with holes or bars from refractory material.

About placing furnace in the hallway, in my hallway there is a place for a heater that is not in the pathway around 2 by 0.6 meters with two sidewalls common with two rooms that i want to heat. I was thinking about metal box cover for the whole heater with fan that blows air through it to side holes into these rooms for faster heating and better distribution. It could be double wall barrel or something like that.

I'm sorry if i bore you but i am searching for a solution.

Thank you.

[martyn]

The basic concept with a j tube rocket stove is a bit like a pair of bellows used to superheat a black smiths forge …. Fresh air is being pulled over the fuel to encourage a super hot burn and therefore offering a complete burn of the wood = very little ash.
For this effect to work efficiently you need to build to the best know specification including insulation in the right places.
However rocket stoves operating like the design should, will not offer long slow burning but fast hot burning and that is why we try to capture the heat and store it.
For radiant heat only a J tube rocket stove will need feeding every 10-20 minutes, although some variants can go 30minutes or so.

[Forsythe]

Mar 14, 2022 3:57:50 GMT -8 milegas said:

I certainly would like better to use masonry for heat storage and the walls are closest to what i can get to work in my case. Walls are double full brick and could be used as heat storage.[...] About placing furnace in the hallway, in my hallway there is a place for a heater that is not in the pathway around 2 by 0.6 meters with two sidewalls common with two rooms that i want to heat. I was thinking about metal box cover for the whole heater with fan that blows air through it to side holes into these rooms for faster heating and better distribution. It could be double wall barrel or something like that.

Hey, this is all great stuff to work with! Sounds like you have an opportunity to do something I wish *I* could do, but my walls are made of wood slat and the one wall I'd like to do this with is structural and can't be touched. ...I'll come back to this in just a sec.

About grid at the bottom of feeder, i don't expect too high temps since the air mass should be cooling it a bit but anyway i was planing to make it easily replaceable since i can make them on dayly base or even from firebrick with holes or bars from refractory material.

Well, as Martyn reiterated, there will be a whole lot less ash when you're getting complete combustion — added to the fact that you won't need to burn for extended periods, so you'll produce even less ash that way, too. Much less than you're used to getting with the metal J-Tube. I promise.

If you wanted to try out a [sealed ashbox] grate made of firebrick or castable refractory at the bottom of the J's feed-tube, that should be okay. Some ceramic kilns use this method because steel doesn't last. if you use castable refractory, don't make them thinner than 1" (25mm) to avoid fragility and breakage.

I would highly discourage you from allowing steel to keep burning out and continuously replacing it, though. One of the things we have to worry about with wood ash is its ability to "flux" aluminosilicate refractory materials (like both the castable and the firebricks.) Fluxing slags in the ash can lower the softening-point temperature of the aluminosilicate and weaken the vitrified bonds holding the material together... which can eventually cause the refractory/brick to crumble at lower and lower temperatures once the flux absorbs into the refractory face.

Normally, Potassium oxides are the biggest slag culprits in woodash for doing this to aluminosilicate refractories, but it's usually not a problem with dense refractory.

Calcium oxides are a big percentage of woodash, too, but calcium normally doesn't soften or "wet" the aluminosilicate enough at rocketstove temps to penetrate very deeply into the refractory... that is, unless there is also a fair amount of iron oxide mixed into it. When powdered iron oxides co-mingle with the calcium oxides, (like can happen from many replacement steel grates burning away repeatedly into the woodash residue,) the iron then fluxes the calcium to melt at lower temperatures — causing the calcium to wet the refractory surface and *both* the iron and calcium absorb deeply into the refractory body. Once that happens, the bricks or castable start to undergo softening expansion upon each firing, followed by uneven shrinking upon cooling... which causes cracking that will erode the refractory away below the rated temperature threshold.

So like... if you want to give the steel grate a try once, and if it corrodes away... then just don't keep replacing it, because you'll then have to replace the whole refractory / firebrick core sooner, too. (This type of refractory corrosion is a slow process that's hard to notice until your bricks or cast core suddenly start spalling and crumbling when you thought they were fine.)

...okay enough warnings... back to this:

...About placing furnace in the hallway, in my hallway there is a place for a heater that is not in the pathway around 2 by 0.6 meters with two sidewalls common with two rooms that i want to heat.

So, it sounds like you have
1) an alcove in the hall,
2) both rooms needing heat on the same side of the hallway
3) double-withe brick walls with no combustable material between the alcove and the two rooms — which share a wall with each other and the alcove?

Sweet. maybe something like this, yeah?



If so... This a pretty cool chance to install a pair of bells like "hinterladeröfen" or "rear-fueling stoves" AKA "Back-loading ovens"



I'm thinking along these lines:



with the alcove and brick wall there, you could simply put a 6" stainless flue pipe connector (insulated from the structural brick to avoid cracking the mortar joints) through that wall into the first room, then a connector up high and one down low between the two bedrooms, then a final one down low again going back out to the alcove and exiting up the chimney. (Those stainless flue pipes are what people use in their rocket mass heaters to run the flue gas through benches or bells.)

The upper and lower connectors through the rooms' shared wall (between Bell 1 and Bell 2) would effectively make it like a single shared bell between the two rooms, allowing them to both get even heat:



...Please forgive these crude attempt at rendering this concept in image format...

Here's a kinda-sorta 3-D cutaway view looking over the top of that alcove wall and down into the bells from the top of the hallway area. (Note that the lower connector between the two bells would be below viewing level from this angle over that wall.)




The premise is that those connector locations would allow stratification of the gasses vertically... and they would thus both be heated evenly from the output flue gasses after the barrel heat exchanger, rather than Bell 1 (and Room 1) taking all the heat first, exhausting only cooled gas into Bell 2 — which would leave room 2 much colder.

[Forsythe]

and... a funny sidenote:

That German wikipedia article about Hinterladeröfen only shows 2 picture examples of "rear-loaders" ..."designed to take wood from behind." ...of which one is ...uh.. this one. 




... Which I find hilarious.

I can't imagine that the double-entendre is created purely by the English translation.

[milegas]

I hope you ment 1 inch (2.5cm) 25cm is more like 1 foot. I was thinking about my cast walls to be between 2 and 3 inches thick.

Now, that your idea is perfect but i have a little different position here. Heater place is located at the end of the hallway and bac wall of it doesn't have to be heated, behind it is wardrobe that does not require a lot of warmth. I was thinking of insulating that wall from inside of the bell to force as much heat to remaining three walls that should be used for heating rooms.

If this could be possible i could make bb rocket core and surround it with magnesite brick bell. The front wall would be heating hallway and side walls two side rooms. We keep room doors open most of the time and it should help spread warmth evenly.

There is one room at the beginning of the hallway, it doesn't require to be too warm so it could be helped a bit by adding hot air tubing and a silent and slow fan that blows air it from the pipe that would pe located at the top of the bell fron inside.

Now, sidewalls are around 1 foot(25cm) thick, full brick, and the surface area of the side of bell could be 4 or 5 by 2 feet (1.25m x 0.5m). Do you think this might work? Side rooms are around 200 sq ft (20m2) medium insulation.

[Forsythe]

I hope you ment 1 inch (2.5cm) 25cm is more like 1 foot. I was thinking about my cast walls to be between 2 and 3 inches thick.

Sorry, the "25cm" was a typo that I didn't see until after I posted it. I edited to fix that to 1in (25mm), but apparently not before you had already click to quote and reply.

You were talking about using castable refractory bars as a fire grate, and that's what I was referring to:



Another option for an ash trap would be to use 2 firebrick splits sloped at an inward angle towards a 1/2inch slot that the ash then funnels itself into, like Trev uses in the Vortex stove. (thread is HERE donkey32.proboards.com/thread/703/vortex-stove)




But I still maintain that you don't need an ash grate at all in a J-tube, regardless. The castable grate bars or sloped firebrick floor would be my recommendation ONLY IF you're dead-set on using an ash grate as a far better alternative to a *steel* grate in the firebox floor.


As far as the rest of your question...I'm having a hard time visualizing the house's floorplan and wall layout you're working with, so I can't quite follow what you're wanting to attempt. If you'd like to upload some sketches, folks can do a better job of troubleshooting and making recommendations.

To post pictures in the forum, just use a third party image host (I use imgur, personally.) and then you can grab the BB code for quick embeds in the body of your posts.

[milegas]

ibb.co/KwQb40q

This is a free hand mode. Doors mark three rooms, red are walls, pink is place for heater and black is the chimney.

It is not in scale but it is pretty close. What i was thinking is to use side walls of heater place to heat two side rooms.

[Forsythe]

Damn. That makes this trickier with a J-Tube.


If I understand correctly, the space behind the wall at the end of the hall is a wardrobe... meaning it's an enclosed closet area and won't dissipate heat the way the rooms will. That wall may be brick, but that means it will accumulate heat in that closet and be a fire hazard, so it will need to have a 4" open-air gap on the hallway-side between that rear brick wall and the heater body.

... that only leaves you 15.6 inches to work with...

with a 6 inch J Tube, you'll have a burn tunnel and riser that's least 10-12 inches wide.... leaving only 3-5 inches for the riser cover... which puts the steel barrel out of the question. And I think it's gonna make a brick riser / bell nearly impossible, too.

You're probably going to need to consider scaling this down to a 4inch J-Tube, I think. Just to be able to fit it in that narrow space with a closet on the opposite side.

I'm gonna have to think on this one.