Wood Burning Stove Advice and

[DAN-Y]

When using the Bushbuddy in winter and no dry wood is available and you want to melt snow, you can use alcohol in the Companion Burner also known as the Mega Starlyte.

https://backpackinglight.com/forums/topic/mega-starlyte-with-lid/

[KRS]

I have a Trail Designs Sidewinder with a 900ml pot I added the inferno kit for wood burning and have been very happy with the burn times. Ill light it and let it burn down without the pot until i have some nice coals. When the fire dies down ill put the pot on and get a full 900ml pot to boil on just the burning embers. I mainly use it with Esbit but to save fuel I burn wood when i can. Ads per other stoves, I don’t have any experience with the ones mentioned.

 

[DAN-Y]

Kurt, do you store the stove in the plastic storage containers sold by TD?

a few videos showing an easy DIY wood burning stove and how I stack wood in it:

[DAN-Y]

Theory of Design of a Functional Secondary Air System

The JUCA design does not depend on air-starvation (air-tight) operation so the problems of excessive creosote production and carbon monoxide production do not represent the great problem existing in most wood burners on the market. For this reason, it was unnecessary for us to try to arrange a functioning secondary air system even when that was the current rage. We quietly said for years that it was unlikely that any of the secondary air systems worked very well, if at all. Independent researchers eventually showed that we were right all along. With this preface, we herein will give the reasoning why all existing secondary air systems don’t work well; and the design considerations necessary to make a system that does work as intended (again, for AIRTIGHT products where it could be important).

See other sheets of ours for a description of the overall theory of airtight products and that of products like the JUCA (Sheets 128, 120, 310, 314 and others).

Since the lack of enough oxygen in the fire’s vicinity is the cause for the incomplete combustion in an air-starvation burner, it would be advantageous to supply air to it later on to complete the combustion. The hitch is that the primary reaction that needs to occur (carbon monoxide plus oxygen gives carbon dioxide) will only occur above about 1200°F. If it didn’t happen while it was still in the flame tips, we may have trouble keeping it hot enough for the reaction to go.

Let’s consider an example. The actual flame temperature of a wood fire can range from about 900°F to 2500°F. An “average” fire will commonly be around 1900°F. Almost instantly on leaving the flame tip the smoke mixes with other air or smoke, quickly reducing the temperature. The amount of this temperature reduction is dependent on many variables, some of which are not yet fully understood. For argument’s sake, let’s say it is at 1400°F. In order to permit substantial secondary combustion to occur, it will be necessary to supply a decent amount of secondary combustion air, generally on the order of the primary air supply.

This is necessary so that the statistical probability of CO molecules being able to “bump into” O2 in the hot zone is high, preferably at least 90%. The molecules will only be in this environment a very short time, but we want the great majority of them to have the opportunity to combine with the oxygen atoms. These conditions are mandatory to ensure substantial and consistent secondary combustion over a wide range of firing conditions.

Some currently available products do seem to be able to support secondary combustion SOME OF THE TIME and TO A LIMITED EXTENT. Under optimal conditions maybe 1/3 of the available fuel is recovered. Under most other conditions, less. The amount of air supplied is too small to allow high probability of the CO and O2 reacting. Just do a molal analysis to see the lop-sided proportion of many CO to few O2 molecules. Actually if pure oxygen was fed, it would work fairly well. Air being 80% Nitrogen just reduces the probabilities of reaction.

And it represents more material that must be pre-heated so as not to chill the smoke to below 1200°F. Getting back to our example, if we mix our 1400°F smoke with an equal amount of room temperature secondary air, the resultant temperature of the mixture will be less than 800°F, far less than the necessary 1200°F. No reaction. Poor efficiency. A lot of creosote. A lot of pollution. Bad. You can probably see that you are going to need a source of secondary combustion air at about 1000°F or higher under these conditions. A pre-heater will be necessary to boost the room air to 1000°F.

Unfortunately, there are some conditions of low fire (severely held back) where the smoke itself is under 1200°F within inches of the logs. In that case secondary combustion is almost out of the question. It is ironic that in the situation of a severely suffocated fire that most needs the effect of secondary combustion, it is most difficult to obtain. When the fire is burning relatively freely (and therefore cleanly), that is when it is easiest to initiate secondary combustion.

Again let’s get back to the example at hand. We need to pre-heat air to 1000°F. It will be necessary to use a heat exchanger to do this. Some current products have a 6″ long tube to pre-heat the air as it passes through. We’ll see that this isn’t even close to enough exchanger surface. Assuming that the stove consumes 35 CFM of primary air for the fire, we will also need 35 CFM of secondary air as described above. To heat 35 CFM from 70°F to 1000°F will take about 35 x (1000-70) x 0.24 / 28 * 60 or approximately 17000 Btu/hr. The 0.24 is the air’s specific heat; the 1/28 is the air’s specific volume at the mean temperature; 60 is the number of minutes in an hour.

When we are talking about a unit that is only going to develop 20,000 or 30,000 Btu/hr, you can see that we are going to have to use more than half of the capability for pre-heating. The secondary combustion might add 25% to the output (maybe 7000 Btu/hr) but you use 17000 to do it. A losing proposition. Except for the safety considerations, it would be foolish to consider.

Conventional heat exchange analysis (see other sheets in the 300 series) will give the necessary areas of heat exchange for this pre-heater. We will avoid the math here. A two stage boost heater is most logical and effective here, where the first stage heats the air to (600°F in our example). The necessary area in a 700°F part of the stove for this exchanger is 1.6 sq. ft. The air then passes to the second exchanger to be heated further (to 1000°F) in a hotter part of the firebox right over the flame tips. The necessary area of this exchanger is 1.5 sq. ft., assuming the smoke temp is 1300°F in that part of the firebox.

If the supply tube is 2″ in diameter, the first exchanger must be nearly 9 feet long (wrapped around inside the firebox) and then the second will also be about 9 feet long. The secondary combustion air supply would have to pass through a total of 18 feet of specially located heat exchanger to ensure good secondary combustion. There would not be much room left in the firebox in most stoves for any exchangers for USEFUL heat. And remember, even then there are conditions when secondary combustion still won’t occur. Is there any wonder why currently available products with a stub tube pre-heater don’t work?

Home Page of JUCA

[Tony Campana]

I have a Bushbuddy, an Element, and a Mini-E wood stove. I don’t know the science of it, but that Bushbuddy burns clean! And a top down fire in it works well. The Element is fun to watch as it is full of embers.  Add a Zelph alky stove and you have a dual system.

Efficiency? Who cares? Not me. These stoves are just fun and add to the experience.

 

[DAN-Y]

Lots of information on wood burning stoves at bplite,com

Here is some information that relates to the burning of wood.

http://www.gekgasifier.com/forums/showthread.php?t=9

Gasification Basics Explained (with graphics)
What it is

Gasification is the use of heat to tranform solid biomass, or other carbonaceous solids, into a synthetic “natural gas like” flammable fuel. Through gasification, we can convert nearly any solid dry organic matter into a clean burning, carbon neutral, gaseous fuel. Whether starting with wood chips or walnut shells, construction debris or agricultural waste, the end product is a flexible gaseous fuel you can burn in your internal combustion engine, cooking stove, furnace or flamethrower.

Sound impossible?

Did you know that over one million vehicles in Europe ran onboard gasifiers during WWII to make fuel from wood and charcoal, as liquid fuels were largely unavailable? Long before there was biodiesel and ethanol, we actually succeeded in a large-scale, alternative fuels redeployment– and one which curiously used only cellulosic biomass, not the oil and sugar based biofuel sources which famously compete with food.

This redeployment was made possible by the gasification of waste biomass, using simple gasifiers about as complex as a traditional wood stove. These small-scale gasifiers are easily reproduced (and improved) today by DIY enthusiasts using simple hammer and wrench technology. The goal of the GEK project is to show you how to do it, while upgrading the engineering and deployment solutions to something relevant for contemporary users.

How it Works

Gasification is most simply thought of as a process of staged or choked combustion. It is burning solid fuels like wood or coal without enough air to complete combustion, so the output gas still has combustion potential. The gas produced by this method goes by a variety of names: “wood gas”, “syngas”, “producer gas”, “suction gas”, etc.

You might think of gasification as burning a match, but interrupting the process by piping off the clear gas you see right above the match, not letting it mix with oxygen and complete combustion. Or you might think of it as running your car engine extremely rich, creating enough heat to break apart the raw fuel, but without enough oxygen to complete combustion, thus sending burnable gasses out the exhaust. This is how a hot rodder gets flames out the exhaust pipes.

The input to gasification is some form of solid carbonaceous material– typically biomass or coal. All organic carbonaceous material is made up of carbon (C), hydrogen (H), an oxygen (O) atoms– though in a huge variety of molecular forms. The goal in gasification is to break down this wide variety of forms into the simple fuel gasses of H2 and CO– hydrogen and carbon monoxide.

Both hydrogen and carbon monoxide are burnable fuel gasses. We do not usually think of carbon monoxide as a fuel gas, but it actually has very good combustion characteristics (despite its poor characteristics when interacting with human hemoglobin). Carbon monoxide and hydrogen have about the same energy density by volume. Both are very clean burning as they only need to take on one oxygen atom, in one simple step, to arrive at the proper end states of combustion, CO2 and H20. This is why an engine run on syngas can have such clean emissions. The engine becomes the “afterburner” for the more dirty and difficult early stages of combustion that now are handled in the gasifier.

How it Works (again): The 4 Processes of Gasification

Now let’s complicate things slightly. “Proper” gasification is a bit more than just the “choked combustion” summary above. It is actually a series of distinct thermal events put together in serial steps, creating an interdependent chain of thermal-chemical events. Simple incomplete combustion is a dirty mess. The goal in gasification is to take control of the discrete thermal processes usually mixed together in combustion, and reorganize them towards desired end products. In digital terms, “Gasification is the operating system of fire”. Once you understand its underlying code, you can pull fire apart into its constituent parts, then reassemble it into a wide range of processes and end products.

Gasification is made up for 4 discrete thermal processes: Drying, Pyrolysis, Combustion and Reduction. All 4 of these processes are naturally present in the flame you see burning off a match, though they mix in a manner that renders them invisible to eyes not yet initiated into the mysteries of gasification. Gasification is merely the technology to pull apart and isolate these separate processes, so that we might interrupt the “fire” and pipe the resulting gasses elsewhere.

Two of these processes tend to confuse all newcomers to gasification. Once you understand these two processes, all the others pieces fall in place quickly. These two non-obvious processes are Pyrolysis and Reduction. Here’s the quick cheat sheet.

Pyrolysis:

Pyrolysis is the application of heat to raw biomass, in an absence of air, so as to break it down into charcoal and various tar gasses and liquids.

Biomass begins to “fast decompose” once its temperature rises above around 240C. The biomass breaks down into a combination of solids, liquids and gasses. The solids that remain we commonly call “charcoal”. The gasses and liquids that are released we collectively call “tars”.

The gasses and liquids produced during lower temp pyrolysis are simply fragments of the original biomass that break off with heat. These fragments are the more complicated H, C and O molecules in the biomass that we collectively refer to as volatiles. As the name suggests, volatiles are “reactive”. Or more accurately, they are less strongly bonded in the biomass than the fixed carbon, which is the direct C to C bonds.

Thus in review, pyrolysis is the application of heat to biomass in the absence of air/oxygen. The volatiles in the biomass are “evaporated” off as tars, and the fixed carbon-to-carbon chains are what remains– otherwise known as charcoal.

Reduction:

Reduction is the process stripping of oxygen atoms off completely combusted hydrocarbon (HC) molecules, so as to return the molecules to forms that can burn again. Reduction is the direct reverse process of combustion. Combustion is the combination of an HC molecule with oxygen to release heat. Reduction is the removal of oxygen from an HC molecule by adding heat. Combustion and Reduction are equal and opposite reactions. In fact, in most burning environments, they are both operating simultaneously, in some form of dynamic equilibrium, with repeated movement back and forth between the two states.

Reduction in a gasifier is accomplished by passing carbon dioxide (CO2) or water vapor (H2O) across a bed of red hot char (C). The hot char is highly reactive with oxygen, and thus strips the oxygen off the gasses, and redistributes it to as many single bond sites as possible. The oxygen is more attracted to the bond site on the C than to itself, thus no free oxygen can survive in its usual diatomic O2 form. All available oxygen will bond to available C sites as individual O, until all the oxygen is gone. When all the available oxygen is redistributed as single atoms, reduction stops.

Through this process, CO2 is reduced to CO. And H2O is reduced to H2 and CO. Combustion products become fuel gasses again. And those fuel gasses can then be piped off to do desired work elsewhere.

Combustion and Drying:

These are the most easily understood of the 4 Processes of Gasification. They do what we think by common understanding, though now they do it in the service of Pyrolysis and Reduction.

Combustion is what generates the heat to run reduction, as well as the CO2 and H2 to be reduced in Reduction. Combustion can be fueled by either the tar gasses or char from Pyrolysis. Different reactor types use one or the other or both. In a downdraft gasifier, we are trying to burn the tar gasses from pyrolysis to generate heat to run reduction, as well as the CO2 and H2O to reduce in reduction. The goal in combustion in a downdraft is to get good mixing and high temps so that all the tars are either burned or cracked, and thus will not be present in the outgoing gas. The char bed and reduction contribute a relatively little to the conversion of messy tars to useful fuel gasses. Solving the tar problem is mostly an issue of the reaction dynamics in the combustion zone.

Drying is what removes the moisture in the biomass before it enters Pyrolysis. All the moisture needs to be (or will be) removed from the fuel before any above 100C processes happen. All of the water in the biomass will get vaporized out of the fuel at some point in the higher temp processes. Where and how this happens is one of the major issues that has to be solved for successful gasification. High moisture content fuel, and/or poor handling of the moisture internally, is one of the most common reasons for failure to produce clean gas.

http://bplite.com/viewtopic.php?f=49&t=4303&sid=e2951c13b436371edd002a51da11a90b&start=40

[James Marco]

Dan, this mostly applies to the production of wood-gas in a closed container and using the resulting tars/etc to power an internal combustion engine.

These are not about a back packing, wood burning gassifier stove. Most do not understand enough to know the difference. A BP wood stove is NOT a gassifier. One of the conditions for gassificatiion is that it is enclosed with no oxygen. In BP stoves, most of the wood gas is consumed in the fire of a wood stove when air(oxygen) is present. Most of the smoke is actually water vapor, at least initially. Once water is removed, then gassification can proceed where it usually bursts into flames in the presence of air/oxygen. Everything commonly burned, must first be ionized, ie turned into a gas. Even charcoal(usually fairly pure carbon) does this.

I would investigate a Liter SS bottle with a tube to a remote canister stove for clean burning, using the carbon from the previous day to produce a new batch of gas. I think this will produce enough gas for a cup or two, but you might need a larger bottle. Again, wood fuel is usually ubiquitous. Plugging up from the tars(creosote like) will be a constant cleaning chore, though.Perhaps a cleanable trap of some sort.

[DAN-Y]

Eric Blumensaadt wrote:

Many do not understand the “gassifier” concept of the Bushbuddy and Inferno equipped Sidewinder & Tri Ti stoves. Both versions force unburned combustion gasses to recirculate through the combustion chamber for more complete and hotter combustion. Typically wood burned in any gassifier stove becomes mostly white ash, the sign of very hot combustion.

The development of this concept was done by two American professors who decided not to patent it so that poorer people in 3rd world countries could use the technology “free” to conserve fuel.

James Marco….that is correct. I present to the group a little bit of everything related to the burning of wood. :-) My research has been extensive ;)

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