Reverse Burning and charcoal choking in ultralight miniature tent stoves with inverted J-burners

This post describes the events of charcoal choking and reverse burning that can occur in ultralight tent stoves with inverted J-burners.

“The charcoal choking can slow the combustion performance of the stove and stimulate reverse burning. Unfortunately, this makes the incoming fuel tube into the defacto flue pipe, within the tent!!!!”

Post note (20/07/2020 during the COVID-lockdown):

In the current post, I prematurely declare the end of the road for my inverted J-burners.

Now, with the wisdom of hindsight, I provide a link to a new post (In the first addendum.) that opens the roadblock for my beloved inverted J-burner. It can again burn brightly and even hotter and safely without charcoal choking or reverse burning.

Although the current post may now seem redundant, I consider that it still holds valuable lessons for ‘fellow stovies’.

It also will remain as my reference text to define these two important small stove combustion characteristics.

Introduction

Wood and charcoal combustion

Understanding the vast difference between the COMBUSTION OF WOOD AND CHARCOAL is key to understanding the reverse burning phenomenon in an inverted J-burner. Briefly, when wood is heated sufficiently, wood gas or smoke can be made. It will combust if suitably hot enough (600-700C) and ignited to makes a hot flame/gas cloud that can move to find more oxygen and move to a flue pipe to create a strong stove draft. The draft, in turn, can support continued combustion (hopefully balanced) of both the wood and residual charcoal component of the burning fuel.

My crude description of the chemistry of the combustion of wood via pyrolysis. The two linked hydrocarbon units represent a little part of the long hydrocarbon of solid wood that has been gasified by pyrolysis. They react with three oxygen molecules to produce two molecules of carbon dioxide and two of water. The number of gas molecules increases during the reaction. This expanding hot gas/flame can move to find more oxygen and provide heat to the distant flue pipe to sustain the stove ‘draft’. “This is hot chemistry in motion.”

In contrast, the charcoal burns much hotter (1,000C) with plenty of oxygen. It can also easily burn slowly at much lower temperatures (150C). However, it must wait for the oxygen to come to it for the reaction to proceed.

Furthermore, in most natural draft stoves, the charcoal is shielded by the shroud of evolving wood gas that gets priority access to the available oxygen. This means that the charcoal accumulates until the wood gas evolution subsides.

When the combustion is essentially fuelled by charcoal the chemistry changes so that there is minimal new gas produced in its combustion and this alone can not sustain good flue draft in a stove with an inverted J-burner.

My crude description of the chemistry of charcoal burning. Two molecules of carbon (solid) react with two molecules of oxygen (gas) and make two molecules of carbon dioxide (gas). There is no increase in the number of gas molecules in this reaction to drive a strong flue pipe draft. In my stoves that have inverted (or upside down) burners, this means that if the wood ‘feed-in’ stalls, then the combustion regresses to charcoal only (with no flame) and the fuel tube will become the undesirable substitute flue pipe. *“This is very hot chemistry, but it is largely confined to the charcoal surface, but wait till I use a* *USB fire blower* *on it.”*

Definitions

Inverted J-burner

To explain what I mean by an inverted J-burner please see the crude sketch below or some of my many previous TENT STOVE POSTS.

A sketch of a section of my micro tent stove. The section is cut down the middle of the length of the stove along the dividing wall of the heat exchanger. The exit to the flue pipe is hidden behind the wider fuel/burner tube — A crude sketch of a section of my early micro tent stoves that exhibited the dual problem of **charcoal choking** and **reverse burning**. The section is cut down the middle of the length of the stove along the dividing wall of the heat exchanger. The exit from the heat exchanger to the flue pipe is hidden behind the wider fuel/burner tube. The primary air port directs the inflowing air into the charcoal bed that rests on an insulating ash bed on the bottom of the heat exchanger. The flow rate and velocity of this airflow can be increased, by covering/blocking the alternative secondary air port. This can be done to preferentially burn charcoal to reduce charcoal choking. It also will have the effect of restricting fuel stick pre-ignition and reverse burning in the fuel tube. The long walled cap can be simply made from a tall aluminium drink can with the top end cut off. I call this a snuffer can. It can still cover the fuel tube opening even when there are fuel sticks protruding from the opening. It can also be used to cover the top of the flue pipe, to keep out snow and rain, when the stove is not in use.

The photo below shows an example of an ultralight tent stove with an inverted J-burner.

My Slim Oval Tent Stove set up in a snow pit inside a small tent. The stove has a big heat exchange surface for comfort and cooking, a tall fuel stick/burner tube for convenient refuelling and a very short (~40mm deep) ‘formal’ combustion zone at the bottom of the fuel tube. The oval heat exchanger/cooktop is divided down the middle by a long baffle that makes the flame and hot gas from the burner tube flow turbulently around a long path to get to exhaust through the flue pipe (fitted behind the burner tube). Light from the intense gas combustion is showing at the top of the fuel stick/burner tube.

Charcoal choking and reverse burning

In a small backpacking inverted J-burner stove, for example, my Slim Oval Stove, (shown above) the wood sticks convert to charcoal ‘sticks’ in the short burn zone. Ideally, the charcoal snaps off in time to let more wood feed downwards to sustain the gas flame. Ideally, the residual broken off charcoal chunks below the burner will be burnt quickly enough to allow more wood to feed in.

The charcoal may be strong enough to prevent its collapse and thus stall the entry of the ‘new wood’. This stops the production of wood gas and the hot moving flame/gas cloud. Then the stove draft can fail and the charcoal can burn under and within the fuel/burner tube and reverse burn. Eventually, the rising temperature will ignite the remaining stalled wood sticks.

A similar fuel stick stall can occur with broken of chunks of packed charcoal that may build up within the burn chamber below the incoming fuel sticks. This can come about because the wood gas gets the first usage of the incoming oxygen and the charcoal is deprived of enough oxygen to burn enough to keep it in balance.

These wood-feeding stall situations reduce the flue pipe draft to such an extent that the opposing draft up the very hot fuel tube will eventually be stronger and the stove will start reverse burning.

“And it gets worse.” Without any intervention, any wood fuel in the fuel tube will be combusted and accelerate the reverse burning.

Long fuel/burner tube. It is also tempting to have a tall fuel feed tube, as in the photo above. This allows a convenient or lazy big fuel stick load while keeping them tidy. However, the taller this tube is, the more it will invite reverse burning. The quantity of reverse burning wood also adds to the complexity of resolving the problem when it happens.

“What do you do with a big bundle of burning sticks inside a tiny tent? Even if you drop them in the snow they will make a lot of smoke.’

A short fuel tube or no fuel tube. A short fuel tube helps to prevent the reverse burning problem. An ultra-short or ‘none-at- all’ can be used to correct a reverse burn and burn the excess charcoal.”

Thick fuel sticks. Thick fuel sticks make another lazy fuelling option. When the sticks are damp and frozen, as they often are, the inner core of wood within the charcoal can be resistant to pyrolysis.

“I call them pencil-points.” The wood inside prevents the collapse and the surface that is exposed to combustion is only charcoal. This becomes charcoal only combustion and will probably reverse burn. Further splitting of such sticks makes them a suitable fuel once more.

[Photo of pencil-point fuel sticks]

Fuel stick hang-up. Even perfectly combustible but irregular shaped bush fuel sticks can ‘hang-up’ or jam in the fuel/burner tube. For example, it could be splintering, stubs of lateral branches or simple wedging, Whatever the cause, it will have the same outcome and eventually catch fire with radiant heat from the charcoal. “Luckily, jiggling the fuel sticks will usually prevent or resolve this reverse burning.”

Fuel deprivation. Even when the stove is deliberately or accidentally allowed to run out of fresh wood fuel. For example, when packing up to go to sleep for the night. The residual charcoal WIIL go into the reverse burning mode if not properly managed.

“Yes, carbon monoxide is likely to be in the exhaust mix! Very good for putting you to sleep forever! Use of a ‘snuffer can over the fuel tube is a way of stopping this.”

A photo of a reverse burn is shown below. There is also a series of photos at the bottom of the post that shows the progression toward this reverse burn.

Reverse burning in an inverted stick stove burner 4 fully developed phase. — Reverse burning in an inverted stick stove burner in fully developed phase. The flames have progressively moved back up through the incoming fuel sticks. This is because the residual charcoal on the ‘sticks’ is strong enough to prevent fresh wood fuel from falling down to replenish the production of wood gas. This results in an inadequate supply of wood gas and flame in the burn chamber or fire dome. Then there is a resultant loss of flue pipe draft such that it can not oppose the reverse draft up the fuel entry port. **“Don’t feel bad if at first, you don’t understand this sequence of events, as it has taken me years to do so!”**

Stove heat exchange and reverse burning. If the stove efficiently extracts more heat out of the combustion (eg by having a big and efficient heat exchanger with internal baffles that cause turbulence, (As in the two photos above) then this reduces the flue pipe temperature and the resultant draft. This increases the risk of reverse burning.

“The more heat energy that is taken for comfort and cooking, the less heat energy is left to drive the stove burner robustly in the correct direction. It is a critical balancing act”

No significant thermal mass. Lastly, my ultralight inverted J-burner stoves deliberately have a minimal weight for backpacking. This means that there is also minimal thermal mass and no heat riser to ‘keep the draft going’ during fuelling glitches.

“They can be thought of as Rocket Mass Heaters with no mass. So little mass that they cool ready to be packed up within a minute of emptying out the hot charcoal!”

Discussion and conclusion

My criticism of inverted J- burners for ultralight backpacking stoves such as mine should not be taken as criticism of such burners in general. In fact, I consider them to be some of the finest most efficient, clean, safe and environmentally sensitive forms of heating that are within easy reach of DIY backyard tinkerers such as me.

Exemplar ‘downdraft stoves’ are described in great detail in the Aprovecho Research Center PDFs. These are; The ‘Down Draft Pole Burning Stove’ on page 43 and ‘Down-Feed Heating Stove With High Mass Bench’ on page 27 DESIGNING IMPROVED WOOD BURNING HEATING STOVES.

Unlike my ultralight tent stoves, these have great thermal mass and a tall and dedicated ceramic lined heat risers. These insulate the flames and causing very hot combustion of the wood gas. The heat riser also creates such a powerful stove draft that the flue gas can be ‘pushed’ through the long heat exchanger in the ‘high mass bench’ and not stall the flue draft.

“The heat energy to maintain stove draft is used first and then the rest of the heat is available for human comfort.

Unlike my stoves, these stoves no longer require a draft to be ‘created’ in the flue pipe. Also, the great thermal mass of the heat riser stops the reverse burning during fuelling glitches and in the end phase when the last wood is burnt or there is a fuelling ‘hangup’ and only charcoal remains burning.”

The end of the road for my ultralight J-burner tent stoves?

(Note: Please see the correction to this section in Addendum 0 below.)

I have developed various strategies (a long list) to make my beloved inverted J-Burners operate effectively and safely. These include; the thin splitting of fuel sticks, separating the combustion of charcoal from that of wood, prioritizing airflow into the charcoal, adding blowers to burn charcoal, very short fuel tubes, no fuel tube and snuffer cans to block the top of the fuel tube.

The bottom line to all this is that a simple and safe tent stove should not have these inverted J-Burner shortcomings and requirement for complex management. “It should just work!”

Instead, it should facilitate simple, robust and safe combustion that can be managed by someone with minimal fire skills even if the stove is a little less efficient. A single ‘ever upwards’ path for the combustion gases is key to this simplicity. “So I thought.”

An ode to stove design to prevent reverse burning;

Preventing draft reversal is complex it’s true,
That is, products of combustion only up the flue,
With only one gas path generally up from the hearth,
In a tent, it’s a smart strategy that’s not entirely new.

Miniature Dome stove, in tent mode showing its glowing cooktop at night. A cheery sight for cold weary winter snow trekkers. It is free of the above inverted J-burner **reverse burning** issues and is a simple delight to use even if it may be a little less efficient. “Any fool could use it.”

Addendum 0: A very important correction about the end of the road

I have slipped this addendum in as zero. My breakthrough with stopping reverse burning in an improved inverted J-burner has changed my conclusion.

The KISS Stove (Link below) and other stoves with similar burners have beaten the reverse burning problem and produced the absolute highest stove temperatures and best heat distribution from a very simple stove.

“So, it is not the end of the road for my beloved inverted J-burner, just the start of a new journey after little setback. “

“I don’t make a habit of putting odes in an addendum, but this is a deserving exception. Having declared the end of the road for the inverted J-burner. It may sound a little triumphal but that is the feeling that can come from seizing victory from certain defeat.

With great heating potential, this stove was a farce,
Without choking or reverse burning would be top class,
Reverse burning dissected then thoroughly corrected,
The KISS Stoves performance is ready to safely kick-ass.

The KISS Tent Stove for alpine tent heating

Addendum 1

I have added a series of photos of the various phases of reverse burning that may help you (and me) to better understand the nature of this pesky reverse burning phenomenon.

“It took some time to ‘catch it out’ after a few false starts. I felt that I needed to turn my back on the stove to have it happen.

Preparing dinner did the trick. The camera was ready, on the tripod at dusk, in the fading light, to capture the elusive event. You don’t want this to happen to your stove when you ‘duck out’ of a tent for a pee in the snow and to gather a few more sticks for the evening”

Reverse burning in an inverted stick stove burner 0 incipient phase. — **Reverse burning** in an inverted stick stove burner 0 incipient phase. The reverse burn nearly occurred, but it corrected itself when the charcoal sticks spontaneously collapsed.

Reverse burning in an inverted stick stove burner 1 incipient phase. — **Reverse burning** in an inverted stick stove burner 1 incipient phase. This was a second reverse burn that spontaneously corrected

Reverse burning in an inverted stick stove burner 2 developing phase. — **Reverse burning** in an inverted stick stove burner 2 developing phase. Here reverse burning was established. Note the red colour of the fuel tube connector cone. It indicates that it has reached above 500C. “It is not where we want such heat!”

Reverse burning in an inverted stick stove burner 3 developing phase. — **Reverse burning** in an inverted stick stove burner 3 developing phase. Reverse burning is increasing as the flue pipe draft is deprived of hot combustion gases.

Reverse burning in an inverted stick stove burner 5 recovering phase. — **Reverse burning** in an inverted stick stove burner 5 recovering phase. The reversing flame is slowly pulled down the fuel tube to restore normal combustion after my intervention and the flue pipe draft steadily increases.

Reverse burning in an inverted stick stove burner 6 recovered phase. — **Reverse burning** in an inverted stick stove burner 6 recovered phase. The flame is contained within the fire dome once more. All flames and gas are being sucked down the fuel/burner tube. The exposed portion of the fuel/burner tube has cooled down from red hot.

Tim

Author

Hi Brian, Thanks for such great questions. In my answers, I will deal with them under separate headings.
Reverse burning.
Your understanding is not correct. A longer or wider stove pipe can increase the stove power, but will not stop the reverse burning (and will steal cooking space). The reverse burning does not happen that often, but when it does, it is because there is not enough heat going to the pipe, so making it bigger will not help: When it happens:
It requires intelligent human intervention to correct,
Is very inconvenient in a small tent,
Is Downright dangerous if it is unattended when you fall asleep, as you may never wake up from CO poisoning.

Consequently, a stove design such as the Kiss Stove that prevents the problem is the best solution, but it was a long time coming and I nearly gave up trying to solve the problem.

If we start with a stove made from a simple stove pipe with an air hole and a fire in the bottom it would never reverse burn. The heat and hot air and combustion gas from the wood or charcoal combustion will just safely rise. Even when the wood gas is exhausted and there is no visible flame, the hot gas will still safely rise up through the pipe. My stoves with inverted burners must first pull hot air and gases downwards to keep the combustion going. Then it must travel horizontally to the stovepipe before it can rise up to create a draft to drive this process.

For example, my Bento Box stove and Kiss Stove both do this but there is a big difference. They both work their best when there is a good mix of charcoal and ‘fresh’ wood to make plentiful wood gas. The charcoal makes intense heat that does not move off as a flame front or make extra gas molecules. Instead, it is there on the glowing coals (1,000C+) and contributes little to draft and lots of heat to the tent by irradiating its heat to the stove body and the bottom of the incoming wood and air. In contrast, the wood gas flame is less hot, but it can move toward the stovepipe while still reacting with the oxygen from the hot air (burning). Some of the flames will finish burning in the bottom of the stovepipe. Both are critically important sources of heat and work best as a team in their mutual destruction! The gas-phase of the combustion is what drives the strong stove draft. In the Bento Stove, the confined area below the fuel tube can become choked with charcoal that stops fresh wood from feeding down. Then with minimal stove pipe draft, the burner tube becomes the stove pipe inside the tent!!!!!!! In contrast, the more open area of the Kiss Stove allows the charcoal to fall away and burn preferentially and thereby prevents the stall of the wood feed.

Two little experiments can demonstrate that this theory is probably true. Take a charcoal stove that is stalled and shift some charcoal sideways into the floor of the Bento, the sticks fall down and the burn will start again. Even dropping a piece of wax-soaked cardboard down into the charcoal give enough gas to stop the stall or sometimes it needs a small ignition flame to light the smoke. The small hole on the bento stove is used for this.

Stovepipe size
Yes, my Kiss stove pipe diameter is optimized to fit inside the fire dome and everything else is optimized to work with it. That was my backpacking priority.
This pipe can also run larger stoves, but they progressively make a less intense and satisfying heat for cold campers, without a commensurate increase in both the stovepipe and burner diameter. Such a comparison is in:
Biscuit or cookie tin stoves

Most of my stoves have had a burner tube of a similar diameter to the stovepipe. That makes a good rule of thumb. I have experimented with enlarging the area of the burner by 44% with some success. It does not make the stove run noticeably hotter, but it makes lazier fuelling with larger sticks easier. However, enlarging pipes and burners can ‘steal’ the cooking space.

Fuel burner tube enlargement

No stovepipe taper
Lastly, my telescopic stove pipe is not tapered (The same standard rings top to bottom and everywhere else, otherwise they would fall down.). The sacrificial adaptor that connects between the stove and the pipe is slightly tapered so that the pipe can be fitted over the outside of the top of the adaptor.

A guard tube as a stovepipe adaptor

I hope this helps,
Tim

3 Comments

Hi Tim-
I regards to reverse burning. If I”m reading correctly, some of this could be solved by an increase in draft(pull thorugh the stove) from the flue pipe–am I understanding this correctly? If so, could a longer stove pipe, or even larger diameter help eleviate the problem? I understand more of the heat may end up shooting into the sky instead of the tent, but there may be a happy medium. I understand you have settled on a pipe diameter based upon what can pack into the stove when rolled up, but I dont think I saw where you tested performance based on size(though based on your thoroughness I”m sure you did!) SInce you’ve gone to the spiral/pipe that gets smaller towards the top what start diameter is your pipe(at the stove) and what is the end diameter(where the smoke releases)?

Thanks!
-Brian

Brian

February 11, 2022


1. Author
  
  Hi Brian, Thanks for such great questions. In my answers, I will deal with them under separate headings.
  Reverse burning.
  Your understanding is not correct. A longer or wider stove pipe can increase the stove power, but will not stop the reverse burning (and will steal cooking space). The reverse burning does not happen that often, but when it does, it is because there is not enough heat going to the pipe, so making it bigger will not help: When it happens:
  It requires intelligent human intervention to correct,
  Is very inconvenient in a small tent,
  Is Downright dangerous if it is unattended when you fall asleep, as you may never wake up from CO poisoning.
  
  Consequently, a stove design such as the Kiss Stove that prevents the problem is the best solution, but it was a long time coming and I nearly gave up trying to solve the problem.
  
  If we start with a stove made from a simple stove pipe with an air hole and a fire in the bottom it would never reverse burn. The heat and hot air and combustion gas from the wood or charcoal combustion will just safely rise. Even when the wood gas is exhausted and there is no visible flame, the hot gas will still safely rise up through the pipe. My stoves with inverted burners must first pull hot air and gases downwards to keep the combustion going. Then it must travel horizontally to the stovepipe before it can rise up to create a draft to drive this process.
  
  For example, my Bento Box stove and Kiss Stove both do this but there is a big difference. They both work their best when there is a good mix of charcoal and ‘fresh’ wood to make plentiful wood gas. The charcoal makes intense heat that does not move off as a flame front or make extra gas molecules. Instead, it is there on the glowing coals (1,000C+) and contributes little to draft and lots of heat to the tent by irradiating its heat to the stove body and the bottom of the incoming wood and air. In contrast, the wood gas flame is less hot, but it can move toward the stovepipe while still reacting with the oxygen from the hot air (burning). Some of the flames will finish burning in the bottom of the stovepipe. Both are critically important sources of heat and work best as a team in their mutual destruction! The gas-phase of the combustion is what drives the strong stove draft. In the Bento Stove, the confined area below the fuel tube can become choked with charcoal that stops fresh wood from feeding down. Then with minimal stove pipe draft, the burner tube becomes the stove pipe inside the tent!!!!!!! In contrast, the more open area of the Kiss Stove allows the charcoal to fall away and burn preferentially and thereby prevents the stall of the wood feed.
  
  Two little experiments can demonstrate that this theory is probably true. Take a charcoal stove that is stalled and shift some charcoal sideways into the floor of the Bento, the sticks fall down and the burn will start again. Even dropping a piece of wax-soaked cardboard down into the charcoal give enough gas to stop the stall or sometimes it needs a small ignition flame to light the smoke. The small hole on the bento stove is used for this.
  
  Stovepipe size
  Yes, my Kiss stove pipe diameter is optimized to fit inside the fire dome and everything else is optimized to work with it. That was my backpacking priority.
  This pipe can also run larger stoves, but they progressively make a less intense and satisfying heat for cold campers, without a commensurate increase in both the stovepipe and burner diameter. Such a comparison is in:
  Biscuit or cookie tin stoves
  
  Most of my stoves have had a burner tube of a similar diameter to the stovepipe. That makes a good rule of thumb. I have experimented with enlarging the area of the burner by 44% with some success. It does not make the stove run noticeably hotter, but it makes lazier fuelling with larger sticks easier. However, enlarging pipes and burners can ‘steal’ the cooking space.
  
  Fuel burner tube enlargement
  
  No stovepipe taper
  Lastly, my telescopic stove pipe is not tapered (The same standard rings top to bottom and everywhere else, otherwise they would fall down.). The sacrificial adaptor that connects between the stove and the pipe is slightly tapered so that the pipe can be fitted over the outside of the top of the adaptor.
  
  A guard tube as a stovepipe adaptor
  
  I hope this helps,
  Tim
  
  Tim
  
  February 12, 2022
  
  
  1. Tim-
    That was incredibly helpful. Thank you for the detailed response. Sorry it took so long for me to mull it over, wanted to go back and read more of the theory you have posted to really understand it all. And thank you for clarifying that your pipe is not tapered, for some reason I thought it was based upon some the the images I had seen. Thanky ou!
    
    Brian
    
    February 16, 2022

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