Question about downdraft systems

Hi, I’ve been doing a lot of reading of wood gasifiers for some time now. I’m sure I’m completely missing something here, but I’m wondering why air cannot be introduced lower down in the system? It would seem to fix any issue of the grate clogging up as most of the char would turn to ash, much like an updraft system does.

Like I said I’m sure I’m missing something here but i would really like to know the answer. Perhaps In the setup I’ve illustrated the zones would not actually be where I imagine they would be. Thanks for any input!

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Hello Justin and welcome to the DOW.

The gases have to reach a certain temperature before the gases are converted to wood gas . The further the gases travel through glowing hot charcoal the hotter and better they are .

The drawing above will serve as a heater and burn the wood and char but will not make good wood gas .

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Hi wayne, thanks for the reply! Wouldn’t the pyrolysis gases in this system still have to flow through the hot coal bed?

Yes , but not enough glowing char to get the job done.

Remember we need time , temperature and turbulence

The above drawing would be short on time and oxygen would be remaining past the grate . The gasifier would be consuming any gas it makes .

You are right about the grate staying clean.


Thanks Wayne! I’d like to build a downdraft system but I worry about the grate getting plugged up and have a buildup of volatile gases in the upper chamber (hopper).

Hello Justin.

There is a huge learning curve with gasification one has to tackle . ( 75 % operator 25% machine )

I have been lucky enough to log somewhere
near half million miles and no big issues on plugging or volatiles in the hopper that I can think of .


That’s great to hear Wayne and definitely puts my mind at ease more! My fabrication skills are not great so for a time I was thinking of a simple fire charcoal system but I know the wood systems tend to have better efficiencies and power output.

Let me tie the bell to the cat…
No, it is not …

Move the reduction below the reduction and you’r good to go

But remember some wise words i ever picked up here on DOW…
" wood gasifying is nothing more then pulling/sucking smoke thru a pile of glowing charcoal "

Now to get it smokeless and useful after the glowing charcoal…

You are right to start with a simple fire system, in reference to your technical skills and knowledge so far.

But you also could build a charcoal unit similar as in your drawing, just have the reduction between combustion zone and grate…

Any good downdraft system is actual a downdraft charcoal unit with a charcoal maker on top.
Just to find/build a balance between the speed of production /consumption of raw wood to charcoal to gas…


Cant argue with the last statement. It allso explains perfectly why l like to hack the wood gasifier by adding some charcoal in the wood mix.


Oh wow I never thought about it that way but that makes complete sense. In a way in a downdraft wood system it really is just a charcoal maker. However would you not gain a bit more energy from the wood systems because of the pyrolysis gases? In a straight charcoal system the pyrolysis gases have already been removed.

Definitely would make things easier to get going for sure! Is there a certain engine H.P./size where it would be better to switch from a charcoal to a wood system? I did read somewhere that charcoal units are typically only suitable for smaller engine sizes.

The easy way:

Ad a wood condensate drip at the nozzle inlet and balance out the effect of extreme hot glowing to sufficient glowing.

Ask a 100 people here and you will probably get 100 answers. For me, any mobile application shuld be charcoal based, or at least primary charcoal based (mix fuel).
Its faster, lighter and safer! And done right, charcoal can even mean less work thain preparing wood chunks.

However for a stationary system l dont think l wuld ever consider fueling it with charcoal. Maybee only for a small portable generator that you need runing imediatly and for a short time, other thain that l wuld always choose wood.


What happens when you add carbon to the wood?
Is it the heat that increases or is it because the water decreases that the effect would be better?
Why firewood instead of coal in stationary?

Yes to both questions.

The second question complements the first. Wood gasifiers do not like changes. In real world, there are a lot of changes forced upon a gasifier mounted on a vehicle. Draw difference, different vibrations, outside temperature (wind)… a charcoal gasifier is more forgiveing.
Problem becomes when the char produced vs char used ratio in a wood gasifier gets destroyed and for some reason you burn more thain you use. Culd be overdrawing, a bridge… adding some charcoal can help, along with other benefits it has.

Statinary sistems are more or less constant on the gas demand.


Wood that help a FEMA build too or just cause more problems? And has anyone tried a air intake mode to a fema at the combustion area to increase heat and prevent migration of combustion zone, but still having the majority of airflow coming through the top to prevent gases from rising through the top and reaching a combustible level causing puff backs?

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Yes. Yes. Yes. And yes.
These have all been tried.
Some work for a limited amount of time.
Think about it . . . .
That deep upper bed pack in FEMA controlling the air flow down;
and in an updraft charcoal system cooling and soots/ash dropping the produced gas up and out;
gets used up/consumed. Shallow flow beds. Then you lose control.
Then your insistent make-it-work folks usually then try to auger dribble feed in to constantly maintain the deep consistent, bed pac . . .
Auger feeds require constituent texture, and sizing and, particle durability to be able to smoothly auger feed. So then feed stocks pre-prepetition’s complications . . .
The auger drive needs a slow high torque gear drive electric motor. That motor an electrical power supply. An activation sensing on-off system . . .
Still not quite good enough.
Then rings levels of air jets to follow a moving hot zone . . . The zone moving as affected by different gas demands loading.
CPC systems. IISc systems. Others. Gotten complicated. Many steps expensive to manufacture. Many single-point steps to fail at.

The Tail then wagging the Dog.
Just face it: F.E.M.A.s suck.
An American embarrassment, so says this American.
Steve Unruh


Oh. Apologies JuliusH.
Welcome to the DOW.
Use the magnifying glass search tool above and search out these systems:
CPC Community Power Corporation
IISc India Institute of Science; their CGPL Combustion Gasification and Propulsion Laboratory
DriZzler systems

For a true inner guts look at; and see in-use pictures of the many built Wayne Keith systems (evolved and modified off of early FEMA beginnings) you have to Premium pay and sign up.

Again Regards, and welcome to the Drive On Wood.
Steve unruh


“The tail wagging the dog” lol I love it! So what your tell me is that my 6” dia. 3/16thick SS 24” long with 8” 16 gauge SS 12” long tig welded with donuts with air intakes on both sides vacuumed sealed 500 microns) to keep heat where I want it to control migration and reach tar cracking temps while allowing adrino to control ESP (External Static Pressure) to maintain right mixture while monitoring leaving gas temp to make sure I’m getting high temps in combustion chamber while reusing the heat off cooling coil to heat top draft air supply… hummm. Well that was my thoughts anyways. What would you advise again? Thanks for your response. I have a plan but it might be a bad idea. Simple fire?!

Good. Good.
I did not hurt your feeling causing you to go silent.

My advice is to fire it up. Experience and learn.
That is the very best way.

Using thick SS and able to TIG it; all failures and step-backs, and retries salvages nearly 100% of materials to work forwards.
Much more to learn by failing and try, try again then by armchairing for perfections.

With these beginnings really, really search up the works of Stephen Abbadessa at Northern Self-Reliance. (just type in Stephen in the magnifying glass here)
Not charcoal. Is downdraft.
But small diameter systems, tall proportioned systems.
Steve unruh