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efficient RMH combustion: theory and questions  RSS feed

 
Posts: 161
Location: S. Ontario, Canada
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It has often been pointed out on this forum that 3 factors are involved in achieving an efficient burn in an RMH: time, temperature and turbulence. Let me briefly summarize these 3 (as I understand them) and then add a possible 4th factor for consideration and then ask some questions about applying these factors in our designs.
(1) TIME: The longer the gaseous form of the fuel (smoke or wood gases) can be kept in the combustion zone (the flame), the more efficient will be the burn and the hotter the temperature.  Most bonfires are very smoky because the smoke can quickly rise out of the combustion zone without being fully consumed.  This is also why candles, oil lamps and wood stoves produce smoke and soot. But when fuel is forced to travel horizontally through the flame (as in a J-tube, L tube or batch box) rather than being allowed to rapidly rise out of the fire, it is retained longer within the combustion zone and thus burns more completely and produces more heat.

(2) TEMPERATURE: The higher the temperature of the fuel and oxygen mixture is in the combustion zone, the better it will burn and thus the hotter the temperature of the flame. As fuel needs to be in a gaseous form to burn, a solid fuel such as wood needs to be heated, or a liquid fuel such as diesel fuel or furnace oil needs to be vaporized and heated before it will turn to gas so it will ignite. Then, the hotter that gas is, the more efficiently it will burn and the hotter will be the flame.                                                                                                        

(3) TURBULENCE: The higher the turbulence is between the oxygen molecules and the molecules of the gaseous fuel (i.e. the better they are mixed together before they reach the flame and while they are within the flame), the higher will be the temperature of the flame and the more efficient the burn. This is why liquid fuel is often forced through a nozzle to make it a fine spray, why oxygen and acetylene gas are forced through a nozzle to mix them for the hot flame of a cutting torch or why propane is forced through tiny holes in the burner of a furnace or barbecue so the fuel and oxygen of the surrounding air are thoroughly mixed.

Here’s a possible 4th  “T” factor for consideration….

(4) TOTAL TRANSMISSION OF OXYGEN THROUGH THE COMBUSTION ZONE; The more oxygen molecules there are passing through the combustion zone, the higher the temperature of the flame and efficiency of combustion. The “TTO” is determined by at least 3 factors:
(a) PRESSURE: As atmospheric air pressure increases, the number of oxygen molecules increases in any volume of air. (That is why a fire will burn more efficiently at sea level than on Mt Everest, why runners can perform better at lower altitudes than at higher altitudes because their bodies need oxygen and more oxygen is available at lower altitudes. Also the reason why hypobaric (high pressure) chambers are used to speed healing by getting more oxygen into a patient’s blood.)
(b) VELOCITY: As velocity (draft) of the air passing through the combustion zone increases, the number of oxygen molecules passing through that same space increases and so the temperature increases. (- as a blacksmith uses a bellows to raise the temperature of his forge and wind makes a forest fire burn hotter and more rapidly.)
(c) CONCENTRATION: As the concentration of oxygen in the total fuel-air mixture increases the number of oxygen molecules passing through the combustion zone increases and thus the temperature of the combustion increases. Pure oxygen fed to a fire always produces a much hotter flame than just air. (i.e. an oxy-acetylene torch).

Some of these factors we may not change but how may we increase some of these factors in our designs to achieve the hottest, most efficient flames in our heaters?
1. Re. TIME elapsed from production of wood gas until that gas can rise above the flame: It would seem to me that the longer we can keep the wood gases travelling horizontally and thus within the flame, the better.  Here’s an unorthodox question….If flames are reaching and ascending in our heat risers, is it possible that we may be allowing unburned fuel to rise up out of the flame? Should not our burn tubes then be at least as long as the flames produced within them? Has anyone built an RMH  (J-tube, L-tube or batch box ) with the burn tube this long? What effects were observed? I’m considering trying this.
2. Re. TEMPERATURE:  When starting a fire in a cold heating appliance of any kind the burn is always somewhat smoky until temperature of the fuel-air mixture and that of the surrounding combustion zone is raised by the heat of the fire. As the temperature of the fuel-air mixture rises, combustion is much more efficient and as the temperature of the burn tube increases, it radiates heat back into the flame to make the combustion temperature even higher. (I understand that this is the rationale behind insulating burn tubes and I don’t deny it. I am simply wondering how much the combustion temperature (the actual flame temperature) is raised by an insulated burn tube in contrast to a non-insulated one.) Can the temperature of combustion also be raised in other ways?  Pre-heating incoming air? Pre-heating wood in a closed air-tight container which feeds hot wood gas into the burn tube? Has anyone tried this?
3. Re. TURBULENCE: Since thorough mixing of oxygen and the hot wood gases is essential to en efficient burn, how can we most effectively mix the two in our designs? The venturi and “P channel” in the batch box is one effective means of accomplishing this. A baffle in the feed tube of a J-tube RMH to insure unrestricted air flow into the burn tube is another as are a number of design innovations in the shape of cast burn tubes. Are there other possible ways of thoroughly mixing unburned fuel gases with oxygen in our designs? Has anyone tried forcing air through a “manifold” of tiny holes in the sides of the burn tube to mix it with the wood gases there (similar to forcing natural gas or propane into a BBQ to mix it with the surrounding air)?
4. Re TOTAL TRANSMISSION OF OXYGEN:  we cannot change the atmospheric air pressure in our particular locations! But are there means by which we can improve the draft?
As the draft in an RMH (a reverse siphon) is affected by the same principles that the flow of water is affected in a siphon… our draft is increased by                                                                                                                 (1) the temperature differential between our inside air and the outside air (hot air rises while cooler air falls and makes pressure pushing hotter air up)                                                                                                   (2) the size i.e. height and cross-sectional area CSA of our chimneys (the greater the volume of heated air rising the greater the draft)
(3) the height and CSA of our heat risers                                                                                                                                                                                                                                                                  (4) the temperature differential between the flue gases inside the core and outside of it The combination of these two factors (super-heated air ascending in the heat riser to be cooled, contracted and fall from the barrel or bell) is the “main engine” which drives the draft of an RMH. 

So increasing the height of chimney or heat riser as much as is practical and anything which can be done to increase the temperature of the burn inside the RMH will beneficially affect the draft. So too, any reduction in the height of chimney or heat riser or any restriction or reduction in the CSA of any part of the system, any horizontal flue path or any elbows inserted in the path of the flue gases from core to chimney will also reduce the draft by friction.

If a system has a good natural draft, it may be increased by adding a blower for a more efficient burn. But never build a system which is dependent on that powered draft as it will surely draw back with smoke or even flame in event of a power failure!

Can we increase the concentration of oxygen in our burn tubes or boxes?     Many have experimented with electrolysis of water to produce hydrogen and oxygen. Has anyone tried introducing oxygen produced in this way into the fuel mix of an RMH?  As hydrogen is quite flammable and explosive, it likely would not be a good idea to send your rocket into “orbit” by introducing hydrogen into the mix! Are there other practical and inexpensive ways by which the concentration of oxygen could be increased in our burn tubes or batch boxes?

 
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Your item 4, total oxygen throughput, is not an unlimited feature. Since mixing will never be perfect, there needs to be some excess oxygen to allow all of the fuel gases to combust, but beyond this point, more oxygen will just cool the combustion zone without increasing combustion. I have heard a figure of approximately 3x excess oxygen for a RMH-type system; the correct figure for any given system may or may not be somewhere near this.

The temperature factor is relevant in that certain components of the fuel gases need higher temperatures to burn; once the available and desirable components are burnt, there is no particular benefit to higher temperatures. Beyond a certain temperature, around 2370F, nitrogen starts to burn and produce NOx, nitrogen oxides, which are a harmful pollutant. (EPA technical bulletin on nitrogen oxides, p. 14.) So hotter beyond that point is not beneficial.

Introducing supplemental air through grates in the sides of the burn tunnel would expose those grates to very high temperatures which would quickly destroy most metals (all common ones). Refractory vents might work, as long as they are not exposed to mechanical damage from wood.

The J-tube is a continuous combustion zone, and making the burn tunnel longer would not improve its characteristics. The common ratios are given because they have been found to give the best results. Combustion continues in the heat riser, and making it longer improves the total combustion up to a point, beyond which it just makes the system more unwieldy.
 
Glenn Herbert
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An air-cooled burn tunnel will obviously mean the walls of the burn tunnel are cooler than in an insulated burn tunnel, and the flames will radiate more heat to the walls and be cooler than the identical insulated configuration. You can try lots of techniques to increase the flame temperatures, but the simplest, cheapest and most effective will likely be insulating the combustion zone.
 
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Bruce Woodford wrote:TOTAL TRANSMISSION OF OXYGEN THROUGH THE COMBUSTION ZONE; The more oxygen molecules there are passing through the combustion zone, the higher the temperature of the flame and efficiency of combustion.


As Glenn already said, this is rather limited. Combustion takes place when combustible gases and oxygen are mixed at the right temperature. When there's more oxygen than is needed to react with all the combustibles, it is heated up and escapes through the chimney without any contribution to efficiency. Rather the other way around, it'll lower the efficiency by cooling the fire. On top of that, air contains 21% oxygen at whatever elevation you are, it's only thinner air when higher above sea level. The rest of the gases, 79% of it, doesn't react at all with the combustibles and thus lowers efficiency. In order to achieve the highest possible combustion efficiency, we need to look at the oxygen residue in the exhaust gases.

If there's nothing left and no more combustibles in the exhaust gases, this is called stoichiometric combustion. Almost impossible to achieve outside a laboratory, so normally at the top of the burn there's still some oxyen left, together with the ballast gases. This is called the excess air, something between 20 and 1 times the necessary amount, noted down as lambda. Years ago, values of lambda 3, which is equivalent to about 10% oxygen was the goal of most stove builders, provided they were taking their work seriously, of course. In a well-tuned batch box system lambda 1.5 (5% oxygen) is achievable for a short span of time. But one would need a gas analizer to get there due to variables as chimney stack, fuel and method of heat extraction, sometimes hampered by sheer coincidence. What iinterests me the most is what the rest of the burn was doing so a full burn need to be recorded complete with minimum,maximum and average values.

Concentrating on the highest possible temperatures doesn't do the trick and I'll second Glenn about the chance of producing NOx, you certainly don't want that.

 
Bruce Woodford
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Thanks Glenn and Peter! I'm looking for input from those of you who know your stuff well on these matters.

Appreciate your comments.
Bruce
 
Peter van den Berg
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Edit: Lambda 1.5 oxygen value as stated above isn't correct, this should be 7%.
The statement of lambda 3 is wrong as well, this should be 14%. And to conclude it: lambda 2 is equivalent to 10.5% of oxygen, for many years this was regarded as the lowest workable level of excess air.
Today I ran a test on a new experiment, the lowest level happened to be lambda 1.32, oxygen level 5.1% during a stable run, although the CO happened to stay too high for my liking. No smoke to be detected during the burn, once heating up time was over in 8 minutes.
 
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What kind of system was that running on Peter? Is it a new modification on your batch box?
 
Peter van den Berg
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Yes it is, can't tell much about it at this stage. For every succesful implementation there's at least half a dozen of failures, this might just be one of those. But don't worry, when the results are promising, I'll publish the development together whith do's and don'ts.
 
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