Wood Gasifier Design

When it comes to designing a wood gasifier that produces the less amount of tar, what parameters are considered the most important? Type of wood? The design of the reactor? The airflow in the system? The cleaning system?

What design parameters can be flexible, while others are pretty set?

what do you plan on fueling with it. feedstock. then making the gasifier’s work together to have a favourable result.

Welcome Martin,
I have only built one gasifier but with all my hearth changes and modifications have proven to me that it is the hearth design that is most important. The hearth and reduction zone (grate also) is where the action is. That is where ultimately the rubber meeting the road is proven. The hearth design will give you the best results while other areas like cooling and filtering are also important but more secondary concerns. Wayne’s design is the best for driving on wood. Choose a good hearth design first. Hearth and “Heart” are almost the same word. Have fun always,Gordon

again, i’m gonna go back to what do you plan on fueling with it. feedstock.

a gasifier designed for a large truck probably won’t work as well if what you want to run is a small genset on softwood chips.

Martin, what do you wish to fuel with your gasifier?

Thanks for the info. Building a gasifier with solid hearth makes sense. Does a solid hearth design provide a board (wide) operating scale for airflow velocity and reactor temperature? Or does this have to be closely managed to provide “tar free” gas?

At this point, looking at using pine or fir chips to fuel a stationary engine for running a generator. Maybe down the road a truck.

Martin; ArvidO is correct.
Until you can lay out your woodgas personal intended USE goals; your wood fuel impute stocks; your capability to source, process, DRY and uniformly supply that wood fuel, then all talk is vaporware.

However your core question of " . . a wood gasifier that produces the least amount of tars . . . parameters?" can be answered.
What are these tars? Wood fuel hydrogen and carbon (and oxygen) long chained molecules that have NOT yet been properly ripped apart, siplified, and recombined back into into motor grade energy potentualized gaseous fuels.
Takes a balanced combination of Time (in process), Temperature (of processes), and the Turbulence (within the process) With Control to achieve this. The simplest to make and control gasifier able to do this are downdraft flow types. With the Time and Turbulence floating governed by the produces gas demands. With the Temperatures ALWAYS maintained up to materials edge of destruction high if you truly want it to be as complete as possible tars converting effective; therefore tars passed out minimum.

Read much in the historical articles and books and this is all layed out.
Or, build 3-4-5 systems and once you are internally melting down it will be tars free. Then the design tricks are to make it survive the found process efficient heats/temperatures.
And then IF you operate it poorly, with too wet of fuel wood, under too low of a system load it will internal temperature drop and still make tars passes out.
Why WayneK says proper wood gasification is 25% the mechanical system and 75% the system operator experience.

Steve Unruh
(halleluiah ChrisKY! No more 403’s!)

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look at Stephen Abbadess’ Victoria, good place to start for softwood chips. and it is quite elegant in it’s simplicity.

Thanks Arvid, will take a look.

Thanks for the info Steve. Did not realize that application was so important to proper reactor temperatures and complete burn. Is this due in part to each application draws a certain amount of air, which largely controls the correct temperatures in the hearth area? Reminds me of the three bears story from childhood. Not to hot, not to cold, but just right.

The three Ts. Seem logical in theory. I gather this is where the experience comes in to maintain the sweet spot.

Hi back Martin.
Your intended woodgas application goal IS the most important criteria to start put with. Here is why.
A woodgasifier operates by heat energy. First produce heat energy out of the wood and air oxygen. That that heat energy then converted and stored back as a chemical energy in the reactive woodgas components sent to the engine.
Stationary electrical generation (wood makes electricity - me) and some of ArvidO’s work, then it is easy from the get-go to be putting engine/motor reject waste heat directly/indirectly into the gasifier system as a gain. This work best designing from the get-go with this in mind.
Vehicle systems if close coupled like WayneK’s pick up trucks he is able to add-in a percentage of engine waste heat from the engine exhaust system. If that vehicle woodgasifier system is hung out way back on a tow behind trailer I’ve yet to see anyone able to put any engine heats back into the gasifier system.
Road vehicle systems usually can benefit from the travel moving air stream for produced gas cooling purposes. Stationary engine systems then you must always design in a forced air cooling capabilty.

Importance of your wood species, size prepping wiliness/ability and dryness also relates to base the gasifier design.
Some designs will ONLY operate on wood pellets. Some only on screened wood chips. Some only on chunked up wood fuel. A very few on different percentage mixes of these.
And these more fuel flexible will have provisions designed in to allow for this flexibility of fueling. E.g. grating: passive? or active?

Some designs will only work with very dried fuel wood.
Other designs very capable of to a certain extent dehumiding in-place their input fuel wood stocks.
This ability really must be a from the get=go design priority. Not nearly as effective if just added on later. Non-dehumidifing types are rellly not bad. Mine is not a dehumidifing hearth/hopper type. I use the engine/gen-head waste heats to dehumidify and warm my input fuelwood just-in-time. Sweet n’ simple.

Don’t over focus yet too much on oxygen input control. This is easily corrected later. Thermal control is not easily corrected later.
The velocity and internal time residence controlling designs are very dependent on internal physical sizing proportions matched into an expected engine/produced gas range of usage.
And the actual engine/woodgas-use loading volume will determine the thermal range the gasifier itself must endure.
A temperature controlling design not so internal velocity/residence time range limited.
The simplest temperature controlling designs just always maintain reduction zone temperatures above 800-850C. And let internal zones temperatures float up. It is these temperatures spiking up swings that can and do kill theses early unless designed for. And better yet, operator perceived and understood. REDUCE the engine load!
Sophisticated temperature controlling designs use active internal zones temperature measuring with logic controllers. These eat temp probes.

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Thank you Steve for the detailed reply. You have given me much to think about and ponder. Looks like I have more studying to do concerning the matters that you stated. On a side note, is there a gathering of wood gas guys in the North West (Montana, Idaho … )?

I’ll try not to put words in Steve’s mouth, but judging from his past comments, I would think the next best step might not be so much studying, but to go out to the shop and build something based on what you already know, with the hope of gaining a first-hand feel for the peculiarities of these things. As Dutch John says, “gasification is female” - we’ll never understand it all, but it’s fun trying . . .

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Martin, do you have a genset you are willing to try wood gas on? do you have pics of the wood chips you are hoping to use?

I suspect that with that knowledge in hand there are people here that would be willing ot offer some guidence

Alex, I understand your comment. There is a lot of unspoken things that are learned while in the processing of building the device. Things that would be hard to learn on paper.

Arvid, I really like the willingness of those of this forum to help out those starting in this field.

Currently I am looking at a old 6 cylinder to turn a generator head at this moment. Looking at the possibility of this motor at the moment.

what size of 6 cylinder engine and what rpm do you expect it to spend most of its life at on woodgas

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a 72 225 slant 6 out of an old duster. Not sure about the rpms at this moment.

Hi Martin, A slant 6 is a great motor. Indestructable from all my efforts to do so. I have put them under great strain. A friend of mine said he drained the oil and put a brick on the gas pedal and said he couldn’t blow it up. (so his dad would buy him a V8…) I’m not sure I believe that but I used them hard. A slant 6 has good torque at low rpms. Steve Unruh (pronounced Unroo) will give you better tech data on such a common engine.
Your hearth to grate is where your going to make your temps for good gas so get your demensions right there first and then start welding. Don’t waste too much time studying everyone’s different designs. It will make you crazy. Wayne’s works great. It’s not easy making good gas, but it’s also NOT THAT HARD! I made my mistakes but have since corrected them and my gas is good. . I am making another gasifier soon. My blog is “my first gasifier”. I got great help here from everyone. have fun always, Gordon

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Thanks for the heads up concerning the slant 6. Not much power, but a dependable motor.

Will take a look at your thread “my first gasifier”. It is nice to see everyone so helpful.