stumpjumper83
ArboristSite Operative
anyone here have any experience with this process of burning wood? My early experiments seem pretty interesting.
wood gasification
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habanero
ArboristSite Guru
I've built a few little gasifiers. It's a highly efficient way of burning wood, if you get everything to work right. I have a little junk B&S engine that I'm itching to see if I can get to run on wood gas, when I get time to mess with it.
avalancher
Arboristsite Raconteur
I am not sue about this, wood gasification? sounds interesting. Please explain a bit more!
stumpjumper83
ArboristSite Operative
wood gassification is the process of burning wood with limited or no oxygen. The process starts at about 350 degrees. Once it gets dried out and that hot the wood emits a flamable gas. When the process is over you are left with charcoal and some tar. Here are some pics of a gassifier that I made from some standard plumbing parts. For a better more indept understanding google it.
The flame lasted for almost 20 minutes at the intesnsity in the picture. The black stuff in my hand is the charcoal left after the reaction.
The gassifier is a 6" long piece of 2" steel pipe with a cap on one end and the other end reduced to a 6" x 1/4" nipple.
If a moderator can modify this so the pics apear with the post that would be nice.
The flame lasted for almost 20 minutes at the intesnsity in the picture. The black stuff in my hand is the charcoal left after the reaction.
The gassifier is a 6" long piece of 2" steel pipe with a cap on one end and the other end reduced to a 6" x 1/4" nipple.
If a moderator can modify this so the pics apear with the post that would be nice.
Last edited by a moderator:
habanero
ArboristSite Guru
Basically any wood combustion happens by gasification. Solid wood doesn't burn, only the vapors given off as the wood pyrolyzes (sp?) burn. A gasifier just moderates this process such that you control the oxidation of the wood to give off the vapors in a controlled fashion. The vapors are then either burnt at or near the point of the pyrolysis (generally called close-coupled gasification) or alternatively the gases are moved somewhere else for combustion. The gases can even be used to fuel an internal combustion engine. In fact, coupling a gasifier to an engine generally streamlines the design of the gasifier, as it uses the vacuum from the engine to pull air through the gasifier.
As stumpjumper said, do a google search on the subject. There's a ton of information and examples of gasifiers out there.
Marc
Addicted to ArboristSite
Right... what he said. As a mater of fact, no matter in solid or liquid state can actually combust. In a solid state combustion reaction, there are two distinct families of reactions happening at the same time: pyrolysis and oxidation (combustion). The oxidation is an exothermic reaction, meaning it releases heat, while the pyrolysis reaction is endothermic, meaning it absorbs heat. The combustion reaction feeds heat energy to the wood to sustain the endothermic pyrolysis reaction. Unfortunately when wood pyrolyses, the compounds are often complex and contain several families of molecules that will not combust unless given more heat to break down further. This is why most traditional wood boilers, including the vast majority of OWB on the market today are not efficient. By surrounding the firebox with a water jacket, a huge thermal mass, much of the fire box never gets hot enough to transfer the energy to these big molecules to break them down further.
Gasifiers remove the heat sink from close proximity to the combustion reaction, and supply an over abundance of oxygen to ensure relatively complete combustion. The big molecules liberated by pyrolysis, that actually require further heat to break down still release much more heat in the combustion process than it takes to get them in a state in which they can combust.
The two basic methods for accomplishing this in a "gasifying" boiler: either cook the wood in a primary chamber and mechanically force (via blower) the volatiles through a refractory lined chamber, which is the style of the boilers made by HS Tarm, Econoburn, Eko and Woodgun (as well as the Wood Doctor Converter); or to retain a single fire box design and line the whole thing with refractory to ensure the fire box as a whole stays at a very high temperature... this style is made by brands like Greenfire, Greenwood, Garn...
In general, I've read accounts from people switching from conventional wood boilers to gasifiers and reducing their wood consumption by 40 - 50%, and producing almost no visible smoke (just during start up). The down side is, of course, they're significantly more expensive, require more skill to run, absolutely require seasoned wood, and are generally much more convenient to operate in conjuction with a large thermal mass buffer (hot water storage tank).
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Marc
Addicted to ArboristSite
In WWII... the Swedes and Norwegians, and maybe even the Germans at one point, made troop carriers that ran on wood gas. They had a wood cooker that they pulled around out back that fed gas up to the carburetor.
I actually that about making something along those lines to feed cooktop burners... it might be tough to supply enough O2 to get a smokeless burn though, without introducing a chemcial oxidizer though...
habanero
ArboristSite Guru
I know you're talking about specific application of gasification in an OWB and I'm talking about gasification in general, so what I'm going to say may not fully apply. But, research has shown gasifiers in general work best with a good amount of moisture present during combustion. The theory is that extra moisture lends itself to more steam, which in turn helps to crack the complex, long chain hydrocarbons in the volatiles down to carbon monoxide and hydrogen. Ideal woodgas is almost 100% composed of those two compounds, though you'll never acheive that.
Of course soaking wet wood probably is not the best, but "green" wood is generally just fine for gasification. I know my little gasifiers worked best with freshly chipped green branches. When I used chips from dead branches the pyrolysis zone would get too big on me and I'd walk right on the fine line of a thermal runaway.
Probably proper design could optimize a gasifier for virtually any moisture content, though.
Marc
Addicted to ArboristSite
Well, CO, hydrogen and methane... I've heard before that some moisture is necessary, but I've also heard that the ideal moisture content is somewhere around 20% or below... which I believe is about where 2 year seasoned hardwood is. I don't know, I'm not an expert on that type of thing. My formal training was not actually even in controlled combustion, but I know enough about it to spout off
I'd still be skeptical of the benefits of the presence steam without a detailed explanation... all I can see it doing is adding thermal mass to the process and absorbing heat to change phase and then raise its temperature, which takes away from the available heat the more complex molecules need to break their bonds and reduce.
The big problem I see with gasifiers and moisture is, after a bit a gasifier is running at a relatively steady state. There's a minimum rate of heat generation needed to sustain the complete combustion process. The only thing the I see that presence of water vapor does is absorb heat and bring the process back towards that minimum. After all, half of your combustion products are already water vapor...
habanero
ArboristSite Guru
Well, CO, hydrogen and methane... I've heard before that some moisture is necessary, but I've also heard that the ideal moisture content is somewhere around 20% or below... which I believe is about where 2 year seasoned hardwood is. I don't know, I'm not an expert on that type of thing. My formal training was not actually even in controlled combustion, but I know enough about it to spout off
I'd still be skeptical of the benefits of the presence steam without a detailed explanation... all I can see it doing is adding thermal mass to the process and absorbing heat to change phase and then raise its temperature, which takes away from the available heat the more complex molecules need to break their bonds and reduce.
The big problem I see with gasifiers and moisture is, after a bit a gasifier is running at a relatively steady state. There's a minimum rate of heat generation needed to sustain the complete combustion process. The only thing the I see that presence of water vapor does is absorb heat and bring the process back towards that minimum. After all, half of your combustion products are already water vapor...
Duh, I forgot about methane. It's been a while since I read much formal research on gasification. I'll have to look back at some of my old notes and see if I can find the references that dealt with moisture content of the material.
I agree with what you're talking about with the water adding thermal mass, but the only real energy penalty you're paying is the energy it takes to turn liquid water into vapor which admittedly isn't insignificant. But if, that water vapor improves the efficiency of the hydrocarbon cracking-thus resulting in less tar by-product being removed by the filtration-then the penalty isn't nearly as bad.
I know we did some work when I was at Ok. State looking at gasification by-products but I think most of the starting materials had a pretty uniform moisture content. I don't know if the my former boss still has any of that data or not, but I bet she does somewhere. I'll have to give her a call sometime and see.
Marc
Addicted to ArboristSite
Just out of curiosity, what was/is your field of study/profession?
habanero
ArboristSite Guru
I'm trained as an analytical chemist, but have worked mostly in academic research labs studying soil chemistry, oilseeds and oilseed by-product utilization, and wheat proteins. My wife's a veterinary surgeon, so her training has moved us around the country quite a bit is the reason for all the different jobs. Now that we're settled semi-permanently, I'm finally working as an analytical chemist doing gas chromatography on polymers and polymer precursors.
My gasification work came while I was at Oklahoma State where I worked in the oilseeds research lab. The PI was trying to find a use for peanut and pecan shell by-products and one of her ideas was to make activated carbon out of them. The pecan shells showed promise but the peanut shells were hopeless. Of course the funding was provided by the peanut commission, so that project died a quiet death. The PI sort of had professional ADD, so if a project didn't show results pretty quickly it got canned and she moved on to something else.
windthrown
361 Junkie
Scandinavian fireplace
Amusing read this. One thing that gets overlooked on the subject of wood gassification; there are simpler systems out there that are far cheaper. In particular, the Russian or Scandinavian style fireplace. Those systems have no moving parts, nor any need for a water storage mass or pumps to move the heat around. They burn more completely, and have almost no emissions. They are also very efficient.
One simple system is to use two types of bricks, one inside that expands and contracts and has high heat resistance, and an outer layer that stores and radiates the heat out into the house. Small burn chamber to burn wood in and burn it all fast, hot and completely. If they are built right they have little or no creosote buildup, and very little heat escapes out the chimney. I was down at a friend's house near Crater Lake last week, and was given a tour of his fireplace. They use 1/3 of the wood that they did when they had a standard indoor wood stove for heat.
If I were building a house (and I may be soon) I wood use this type of system for heating the house with. I wood add an outside door to the burn chamber as well, so I could add wood from outside the house, and clean the ashes out from outside as well.
Amusing read this. One thing that gets overlooked on the subject of wood gassification; there are simpler systems out there that are far cheaper. In particular, the Russian or Scandinavian style fireplace. Those systems have no moving parts, nor any need for a water storage mass or pumps to move the heat around. They burn more completely, and have almost no emissions. They are also very efficient.
One simple system is to use two types of bricks, one inside that expands and contracts and has high heat resistance, and an outer layer that stores and radiates the heat out into the house. Small burn chamber to burn wood in and burn it all fast, hot and completely. If they are built right they have little or no creosote buildup, and very little heat escapes out the chimney. I was down at a friend's house near Crater Lake last week, and was given a tour of his fireplace. They use 1/3 of the wood that they did when they had a standard indoor wood stove for heat.
If I were building a house (and I may be soon) I wood use this type of system for heating the house with. I wood add an outside door to the burn chamber as well, so I could add wood from outside the house, and clean the ashes out from outside as well.
Marc
Addicted to ArboristSite
You're talking about something similar or the same thing as a masonry heater, correct? That is an efficient way to make use of wood, however, it too is not without its downsides. Like you mentioned, they can be installed without too much additional cost in new construction but don't lend themselves to a retrofit in existing construction very well.
Generally the layout of the house is designed around these as well. Then there are some other advantages that are just inherent to a hydronic system that really move the ball to the court of the boiler type gasifiers for me... that is... you can heat your DHW with a boiler; thermal storage isn't necessary if one doesn't want the expense, but it does allow one to use wood to heat DHW in the summer as well as the convenience of firing the boiler at your leisure in the winter. It also lends itself to other "green" add ons, most notably solar hot water, or a ground source heat pump. It also allows one the freedom to heat multiple separate structures, or little conveniences like say a hot tub loop (which I plan on incorporating). There's also the fact that a boiler can be put in the basement or an outside shed, eliminating the mess of wood from the living space.
There are pro's and con's to any heating choice, you just have to choose what suits you and your situation best.
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