Micro Gasifiers
- Disclaimer
- Introduction
- DJ-2 Lawnmower
- DJ-3 AC generator
- Fuel
- Dimensioning
- Measuring
- Scrap parts
- Startup pump
- Gas generator
- Cooling and filtration
- Mixer
- Reheater
- Engine adaptions
- Tar and impurities
- Summary

Micro Gasifiers


The information on this internet site has been composed with the largest possible care. The writer rejects liability for possible loose ends, inadequacies and implications. Moreover he is not responsible for damage, loss, injuries or death, caused by DIY builders or producers, who have used the information on this internet site.


On this page I want to tell you, my dear reader, about my experiences with micro woodgasifiers. Note that I mention my and wood. Others can have different experiences and normally micro gasifiers are fueled with charcoal. In the following report, I often state how NOT to do things and point out what COULD work. I am only explaning how to prevent some of your hairs from growing prematurely grey…

The language used, askes for a broad basic knowledge of wood gasification. If you are a total newbee and a micro is the first one you want to build, then I have to disappoint you. For a micro literally everything needs to be perfect. Read the woodgas “bibles” and gain experience with a larger, car sized gasifier.

I’d like to name the Volvo gasifier DJ-1. The next one to be fed with woodgas was an old, but sturdy ATCO lawnmower. That’s DJ-2. Despite it’s sometimes gruesome character and not very clean gas, it is my favorite. It is compact, light, runs an hour on a few handfull woodchips and did only cost 20 euro. It asks for delicacy and patience on startup, but when started it runs like there is no tomorrow.

In order to have emergency power and when I say “emergency” I mean besides no electrical power also no petrol nor batteries, I built DJ-3. It powers a 6 kVA Europower generator.

Contrary to the spotless and without any concessions built stainless Volvo gasifier, I used mostly scrap materials for both little ones. Junk has some advantages: it’s easy and fast to get, cheap, and no worries if you mess up with some parts. Excellent to explore unknown area’s. And the builder gains some artistic qualities.

As you will read, micro gasifiers tend to mess up engines and grow grey hair on the operators head. Like a wild unsettled thing needing much patience, understanding and a steady hand. Do not start building a micro, when your are wild and unsettled yourself…

DJ-2 Lawnmower

Nicknamed “Tiny”. My ATCO lawnmower celebrated its thirtiest birthday and I suprised it with a new engine. The old Tecumseh was very tired and needed more oil than petrol. Bypassers often thought it was on fire.

The new engine was a Chinese clone of a 5.5 hp Honda GX160.  Little money, but I recommend the original. It took me a long time to find out that the average power consumption of the mower is only about 1 hp. And that, my dear friends, is crucial to know and understand. On the other hand it is possible to walk an other route, as I will explain later.

DJ-3 AC generator

Nickname “Dinyfier”, a tribute to my wife Diny.

How nice it is to have emergency power. For electric tools or to avoid a cold house. To keep refridgerators running or to charge the battery packs when sun and wind are down for several days. With the experience gathered on Tiny’s new engine, I chose a sturdy petrol AC generator. Europower has a very good reputation, because they use best quality parts. Have a look at their website:

Goal was to have about 3 kWe on woodgas. The Europower EP6000 is able to deliver 5.4 kWe continuously and 6 kWe peak. It’s Honda GX390 can even handle 7 kWe peak. An excellent fit, because, as you will discover, on woodgas generally only a maximum 60% of the power on petrol can be achieved. Keeping the restrictive carburettor, reheating the mixture and no extra ignition advance loweres this 60% further yet. 

DJ-3 delivers easily 3 kWe continious on woodgas. 4 kWe peak. Above this number the voltage drops below an unacceptable 200 Volt.


Both your best friend and worst enemy. Woodgasification asks for a narrow range of sized fuel. Micro gasification asks for perfectly sized, shaped and dried woodfuel. Small bins and hearths tend to bridge faster than large ones. Before starting a woodgas project, one should be sure of  a source that brings perfect shaped wood. Bad wood makes bad gas, even when the gasifier is superb.

Moist content of the wood is best below 20%. Despite what often is stated, 0% apparent moisture would be ideal, since no physical water is needed for the highest hydrogen yield. The chemically bound water is adequate. Average climates allow wood to dry to about 25% seasonally, so you can gain some by storing wood inside or use waste heat from for example the exhaust to dry it.

The wood I use at the moment: sieved wood chips, made by a large drum chipper. It is amazing how uniform the chips become, when sieved, although the fraction with the large particles, also contains too many flat pieces. Flat pieces act in the combustion zone like small particles and steal too much oxygen, starving the bigger particles. Flat pieces also cause local bigger interstitial space. This space collapses when the particle gets charred, causing an intermittent solid fuel down flowing. Resulting in tarry gas.

The large fraction measures about 25x25x15 mm. The medium fraction about 20x15x8 mm. The small fraction about 20x8x5 mm. Tiny likes the small fraction, but also deals with the middle sized wood. DJ-3 eats both middle and large. It is forbidden to put these fractions together, because the larger particles can remain only partly decomposed.

Small fuel bins hate edgy, sharp fuel, like chips. They tend to bridge. Not really severe hollow burns, but intermittent dropping down instead of an ideal constant flowing movement.  Adding a stirring mechanism allows a steady flow, but complicates the system.

Despite the flaws, I still use chips, because it is all I have at the moment. I recently (spring 2010) planted coppice willow and have high hopes to harvest quality fuel within a few years. Willow is not very dense, but yields high gains. The bark causes relatively more ash, but a well designed and positioned grate can deal with that. A chunker and drum sieve are planned to be built. Wood billets (short round length cut sections) do not result in flow problems and are easy to dry and sieve.  As a purist, I like the wood to be grown locally, preferably insitu.

On a maiden startup, the hearth has to be filled with charcoal. Micros are very sensitive on type and size of this charcoal. Dust is absolutely forbidden! Too big and hard of char pieces and they will remain inert in the reduction zone for too a long of time. Use pieces between 5 and 12 mm. The smaller beginning down on the grate with the larger stacked on top to 30mm above the air nozzles.


When an engine is not supposed to run full power most of the time, the calculations mentioned in the car part of this website do not hold. You need to know the asked power of the engine and do the calculation in Appendix 1 of FAO 72 backwards.

Right…… here some sizing you will find no-where else. Some gasifier designers will want to shoot me for revealing this information:

The nozzletip ring diameter is partly determined by the fuel size. But the other way around too. Average fuel size should be around 15% of this diameter. Volume of the oxidation zone is between 6% and 10% of the calculated cold gas volume per second, depending on the woodfuel size. Reduction volume one-third to half of the oxidation volume. The opening area of the grate is about the same as the restriction opening area.

The air flow through the gasifier nozzles can be taken at a low 20-25 m/s, because of the small size of the micro’s. A produced gas flow Superficial Velocity of 2.0 m/s instead of the usual 2.5 in the restriction worked well for me, but depends on the fuel/char size. Note that when char is very fine, it tends to pack, causing gases flow restriction. That is why I like both the reduction and the oxidation a bit wider, but shorter than usual. For tarfree operation you want it narrow and long. It is a balance between low pressure drop and tarfree gas. It is difficult to have both on micro gasification. It’s a long proces of trial and error to find best possible settings.

Be sure that all sizes can be adjusted fast and easy. Your calculations are just a starting point. Running the gasifier at full power and measuring the pressure drop over the nozzles and over the hearth will teach you where to make adjustments. I like a vacuum of about 8 cm H2O over each of them. When lower, you get poor, partly reduced gas. When higher, the cylinder filling degree of the engine gets lower, resulting in less shaft power. Make adjustments and test, test, test. Make several tests on one setting. Sometimes a cooling down and restarting gives other results. Without any apparant reason. Over time, you will agree with me that woodgasification is female…

Building the gasifier takes a relatively small part of total to invest time. Testing and optimising consumes the greater part.

The Dinyfier has a restriction of 42 mm. The gas is tarfree, but contains lots of soot, indicating that tars are cracked, but only just. Tiny has a restriction of 22 mm. I stepped over a boundary there. The gas is often not tarfree. It could be better if the gasifier was better insulated, primairy air preheated and ideal coffeebean shaped fuel was used. Nevertheless, the gas is clean enough not to stick any valves. Some impurities settle in the restrictive carburettor, now used as a throttle block. Tiny hasn’t run on gasoline for a year now. It doesn’t even have a gas tank anymore. It runs on a weekly basis.

Now the other, possible (I didn’t test it) route to walk: micro’s have a small internal volume compared to their external surface area, making them not very efficiënt. The relatively large outer surface causes lots of heat radiation losses. A bigger gasifier has relatively less losses and therefore a higher minimum to maximum good fuel gas production turndown ratio. Fuel flow in a bigger gasifier is better. So if you were building a larger gasifier, where the engine demanded fuel gas supply more easily remains within this wider turndown ratio, clean gas can be produced.
On a micro sized gasifier it depends however on the developing skills of the builder to design a super insulated gasifier that puts as much waste heat as possible back into the primary air and the woodfuel to widen the narrow turndown ratio. 


You are obliged to measure vacuum. Having a thermometer in the gas outlet of the generator is also helpful. Not in absolute sense, but to have an impression in different circumstances. Therefore it is not so relevant where the actual position of the thermometer is. An ordinary central heating gauge can be used, but needs to be further downstream, or you will overheat it.

Vacuum measuring is important, because it is your teacher. Without measuring the pressure drop, you have absolutely no idea what is going on in the gasifier. It is very easy to construct a dead simple, but very accurate manometer gauge. All you need is a U shaped clear hose filled with coffee colored water. Actually you need at least three of them. The first one has one side attached to the fuel bin. The other side is open to the air. This loop measures the pressure drop over the nozzles.

The second loop is attached somewhere between the generator bottom and the cleaning train. The other end of the U is Teed in on the bin line. The second loop measures the pressure drop over the hearth.

The third loop is connected over the cleaning train.

The second loop is the most important one. It tells you what is going on in the hearth. Often the vacuum is high on startup, but drops once everything is settled. Like I told before: I like the vacuums over the nozzles and over the hearth to be both about 8 cm coffee.

Note that a high pressure drop over the hearth (plugging) lowers the pressure drop over the other two loops. Hint: an extreme low pressure drop over the hearth, high gas temperature and loss of power tells you either urgently need to refill the bin or kick the gasifier in order to deal with a hollow burn.
A high pressure drop over the system will restrict gas flow supply and ask for more choking of the engines secondary air supply, causing an even higher pressure drop over the system. You will need to experience yourself how everything is related.

So please invest in some hoses, coffee and a slick brain. It is the best value-for-money-teacher you can get.

Scrap parts

How refreshing it can be to use junk when one is surrounded by shining new stainless steel! It also challenges the creative mind, saves money and is morally a right thing to do. Drawback is the concessions one has to make. Sizes are hardly ever optimum. Building the ideal dry filtering drum is nearly impossible with junk parts. Wet filtering is about the only way to go. Effective heat recapturing is also hard.

If you want a perfect design, scrap is not the way to go. But in order to gain experience it is good enough. In case of an emergency or a low budget, junk is often the only available material source.

You will recognise several used parts: propane tank (fill with water or inert welding gas before cutting!), fire extuingishers, cooking pots, kitchen door hinge, egg-box clamp, central heating tubing and fittings.

Startup pump

Sometimes things are so simple that you look beyond them. Micro’s can easily be started with a double action airmatress pump. Either blow in primary gasifier air or have it at the end of the cleaning train or even past the mixer. For pumping in primary air, it will have a long life expectancy, since it does not gum up by dirty startup gas.

Pumping is best without much pulsation, which can be done when the vacuum gauge hoses are in sight. Do pump long enough. Having the gas lighting is not enough. I always like it to get an orange flare appearance, which comes after purple.

Starting the gas producer generator is very easy: while pumping you hold a cigarette lighter or small torch near a nozzle for a few seconds. The flame is sucked in and lights the char instantly.

Do not overpull the system and DO tap or poke the fuel stack a few times during startup pumping. The absence of a vibrating engine leads to a possible hollow burn and total combustion of the char in the hearth.

Starting the engine: have the secondary engine air control valve nearly closed while cranking and slowly open it. Once it starts, it will need a bit fiddling to overcome the always present “weak moment”, where the engine wants to stall.

Gas generator

I attached a pencil drawing of the gas generator hearth of DJ-3. It’s design is compromised by the available parts. Some general, but important remarks; of which some are not taken care of in my design:

-The hearth parts should always be made of thin walled stainless steel, with good ash-insulation around it. Keep the heat inside.

-The outer shell of the gasifier should be insulated. Keep the heat inside.

-Put as much heat back into primairy air and the fuel. Keep the heat inside.

-Avoid heat sinks. Thick materials need time to heat up and allow dirty gas to be produced long after the engine has started.

-A natural insulation ash cone for the oxidation zone does not work for micro’s. They need the slick and fixed surface of a stainless (or ceramic) cone.

Make it widely adjustable:

-Nozzles made of bolts. Easy to adjust length or change them.

-Oxidation/reduction assembly in height adjustable.

-Grate in height adjustable.

The grate has a shaker. Few elbows made of threaded bars. Seal and bearing is a copper compression fitting. Replace the compression cylinder by graphite cord. You may need to drill the “chamber” a bit wider in order to obtain a longer sealing surface.

Make neat welding seams. Any leaks will allow air to be drawn in, combusting part of the hot gas. If you seen any white spots in the otherwise black area’s downstream of the grate, you have an air leak.

Cooling and filtration

Tiny has one cooler, between cyclone and wet chip filter. It’s is custom made of junk rectangular tubing. It’s top can be removed in order to wipe out the tubes. In first 2/3 of the vertical pipes the gas goes up, in the other 1/3 down. So even the up-part is flushed by condensate.

Note that there always is a point where condensation starts. This point is prone to accumulate a dust and condensate mixture, choking the system over time. On hot dry filtering this point is after filtering, so no accumulation. On wet filtering this point is best in the filter or, less good, in the cooler. But never somewhere in tubing!

The first used junk cooling spirals act like tubes and plug. The near horizontal tube is not scrubbed because of the flat angle and because of the high gas speed.

The Dinyfier has two coolers. One pre-cooler. The poor heat exchange in the generator leaves the gas too hot for the wet filter. So it needed some cooling before entering the filter. The second cooler on top of the filter allows condensate to drip over the wet filter medium. Either woodchips or clay granulate can be used as medium. Wet filtering is however never as good as hot dry filtering by means of heat resistant fabric. Make its volume bigger than I did.

The cooling tower can be made more efficiënt by making it parachute shaped or tilting the package, although the 24 narrow tubes do well enough in our Dutch climate. A 2” oil drum cap on top allows access to rag the tubes. In one of the tubes is a 5 mm rod to shake up the aggregate in the filter without the need of dismantling. Soot and dust is flushed by condensate to a container.

Both small ones have cyclones, Tiny one, Dinyfier three. Very basic: a 1” main tube without cone, a 1/2” delivery tube, flatened where welded to the main tube (carefully grind the internals). And a 1/4” tube as central outtake tube. Stupid simple, but very effective.

An oil bath filter works well as an after cleaner. Unfortunatly it also collects condensate. Easily draining and filling of the oil must be possible, because it has to be done frequently.


The mixer is plain. It is a T fitting (cross on DJ-3), with the gas entrance on the side. Secondairy air input on one end of the T and mixture output on the opposite side. Extending the air entrance by a short narrow tube into the mixture output makes the mixer acting like a venturi and creates a kind of selfadjusting character. Upstream of the mixer are a ball valve and an air filter. This secondary engine air choke valve needs some adjustment on heating up after engine starting, but after that you only may need to lean the mixture on refilling of the fuel bin. Increasing pressure drops ask for re-adjustments. Something will be wrong upstream.

When the gasifier is in balance and the protubing venturi tube in the mixer is well dimensioned, there will be no adjustment necessary between idle and full power.


In order to prevent condensate entering the engine or mess up the tubing between mixer and engine with a mix of unfiltered carbon black and water condensate, you need to reheat the gas, the secondary air or the mixture. When the gas leaves the cooler, it got rid of a lot of condensate while cooling down. But it is still saturated. The pressure drop after the mixer and after the throttle valve sweeps condensate out. With an engine nearby, it is simple to make a reheater. Just have hot exhaust gas flush around with some tubing.

Efficiëncy maniacs can try to use exhaust gas to pyrolyse the fuel. Exhaust gas contains much more energy than newly made woodgas.

Engine adaptions

I made no adaptions to the engine, except for a small tube on the airfilter support to attach the mixture hose on. Ignition advance will bring you additional 10-15% power. Replacing the restrictive carburettor and airfilter support by a straight wide tube helps too.

I did not choose for this, because I wanted DJ-3 to be able to run on petrol too. Tiny is underpowered anyway, so it doesn’t ask for adaptions.

Note that we in Western Europe use 50 Herz. Most electrical generators are 1,500 rpm or 3,000 rpm. 3,600 rpm (60 Hz) is more difficult to obtain on woodgas. It surely askes for extra ignition timing advance . Running it on slow, longer combusting woodgas at full power without an extra ignition advance head-start for long intervals, could overheat and burn the exhaust valve from the still burning gases exiting. Re-key the crankshaft or flywheel.

If the woodgasgenerator is not fixed to the engine, it might be necessary to connect a horizontal (!) vibration rod to avoid bridging in the fuel bin. When the grate is designed and positioned well, it will also take over the job of the grate shaker. Tiny has no shaker and doesn’t need one.

Dusty gas will ask for a more frequent oil change and head cleanout. It’s soft non-abrasive character does not wear out the engine.

Tar and other impurities

Oh yes, you will make tar. Probably you start with tarry gas. Or you had luck with your first gasifier and make tar on experimenting with other woods and dimensioning. Don’t be ashamed. One that claims never to make tar, either lies or never achieved anything but that single lucky incident. Sometimes, with a poor design or bad wood, good motor grade fuel gas can never be produced.

This is the route from bad to good:
If you find very liquid, sticky, beige tar which glues an intake valve within 10 minutes, you have your engine running on pyrolysis gas. Yes, very powerfull, but not for long. Either the cracking temperatures are way to low, or an internal leak allows pyroylis gas to bypass the hearth.

Then there is tertiairy tar: the black goo. Sticks intake valves after time. Wrong or wet wood, wrong dimensioning.

Soot. Now you are near. Soot is partially cracked tar.

Dust. Or carbon black. This is cracked soot. No worries, that is good. “Freezing” (rapidly cooling to stabilize) the hot gas after the grate can prevent soot and dust to some extent, but using the waste heat energy out of the produced gases to put back into primairy air and fuel is not really freezing these chemical reactions.

Every gasifier makes a certain amount of tar. The best ones only make a small amount of non-condensable tars.

Your wife will tell you when your gasifier is operating tar-free.


Well, dear reader, I guess you come to the same conclusion as I do: micro woodgasification is difficult to make perfect. It is also contradictional: the need for micro gasification is high for common people like you and me. But we have no luxury materials, nor exclusive equipment. While, contrary to larger and more forgiving gasifiers, a micro needs everything to be perfect.

Nevertheless it is possible to design and build a lowcost small gasifier that does not ruin a good engine. Gas can be clean enough, not leading to sticking valves. Although a carbon cleanout of the cylinderhead may be needed from time to time.

Micro woodgasification is what it always has been: an emergency methode.

I do not consider myself a teacher or a master. I am still a student. One that wants to learn. So please send me your experiences to Both good and bad. Both similar and opposite. Do not be shy or ashamed. One cannot learn whithout stepping over borders and mess up from time to time.
Thanks for your attention, I hope you enjoyed the ride.
Dutch John


Copyright © 2008-2011 All rights reserved.