Introduction: A Home-Built Biomass Gasifier for Producing Wood Gas

Picture of A Home-Built Biomass Gasifier for Producing Wood Gas

I've built a lot of alternative energy projects over the years. See my web site at and my other instructables to see my home-built solar panels, wind turbine, improvised generator, and other projects. I've always wanted to build a wood or biomass gasifier too. Why? Well, the internal combustion engine is really an important part of our society and the basis of a lot of our transportation and portable power technology. It isn't going to be going away any time soon. I've mastered making my own electricity from the sun and wind, but that doesn't help my truck go down the road, power the lawn mower, or run my generator on cloudy, windless days. Those all have internal combustion engines, and they all need fuel to run. I finally decided it was time to master making my own fuel. Why pay the Arabs for it if I can make a working substitute myself?

So what is a biomass gasifier? Basically is a chemical reactor that converts wood, or other biomass substances, into a combustible gas that can be burned for heating, cooking, or for running an internal combustion engine. Gasifiers are an old, but generally overlooked alternative energy technology. Few people these days realize that gasifiers were used extensively by both sides during WWll to power cars, trucks and buses during fuel shortages. Gasifier technology rapidly evolved and matured during the war.

Gasification is achieved by partially combusting the biomass in the reactor, and using the heat generated to pyrolyse or thermally break down the rest of the material into volatile gasses. A well built gasifier will convert wood or other cellulosic biomass into the flammable gases Carbon Monoxide and Hydrogen.

My goals here were to build a gasifier using easy to obtain materials, that would run on readily available fuels, and would produce enough gas to at least run a small generator or other machine powered by an internal combustion engine. In this instructable I am presenting the finished product (so far) that has resulted from many months of experimentation and modification. To see the entire long and winding road I went down to get here, please visit the gasifier section of my web site at

Step 1: How Does the Gasifier Work?

Picture of How Does the Gasifier Work?

There are no pretty pictures or informative diagrams in this step. This is the stuff you need to read before attempting to build this project. Please don't skip it.

A word of warning here. This project is dangerous. The operation of a biomass gasifier produces lots of heat, also  lots of flammable and poisonous gases. Never operate the gasifier indoors. The gases produced are flammable and potentially explosive if allowed to accumulate in an enclosed space, like a building. Also, the Carbon Monoxide the gasifier produces is lethal! Only operate the gasifier outdoors and try to stay up wind of the unit when it is running. Treat the gas coming out of the gasifier with the same respect as you would for the natural gas that you may have piped into your house. It is just as potentially explosive and deadly.

As I said in step 1, a biomass gasifier is a chemical reactor that converts wood, or other biomass substances, into a combustible gas. The formula is simple. Biomass + Heat = Pyrolysis Byproducts. Pyrolysis is a fancy-pants word that chemists use to describe the process of heat breaking down big molecules into smaller ones. In the gasifier we want to break big biomass molecules (mainly cellulose) down into smaller ones like Hydrogen and Carbon Monoxide.

Where does the heat come from? We get heat by partially combusting some of the biomass with a limited supply of Oxygen. The heat produced by the combustion then drives the pyrolysis reaction. A well built reactor will also convert combustion byproducts like CO2 and water vapor into flammable CO and H2 by passing them over a bed of hot charcoal, left over from the partial combustion, where they will get reduced.

Thus the gasifier converts most of the mass of the wood (or other biomass feedstock) into flammable gases with only some ash and unburned charcoal (bio-char) residue. That is the theory anyway. This is an extreme over-simplification of how the gasifier really works. Wood and other biomass is made of incredibly complex macro-molecules like Cellulose and Lignin that break down into hundreds or thousands of different smaller molecules as the reaction proceeds. There are thousands of different complex chemical reactions going on inside the reactor. The overall result though, if the gasifier is working well, is lots of clean, flammable gas.

Ideally, the gasifier would break down biomass into nothing but Hydrogen and Carbon Monoxide. Here in the real world though, things rarely work ideally. The dirty (literally) little secret about biomass gasification is tar production. Above I said that the macro-molecules that make up biomass get broken down into smaller molecules. Some of those smaller molecules are still pretty big though. If the gasifier is working well, these big breakdown by-products will be further "cracked" into smaller molecules. If the gasifier isn't working so well, these big molecules will wind up in the gas being produced. They will condense out of the gas as a thick, sticky, black, semi-liquid that very closely resembles roofing or road tar, but is even stinkier. Even a well-built gasifier produces a small amount of tar. Most real-world applications (like engines) can't handle much, or even any, tar. My struggle to design and build a working biomass gasifier could actually be accurately described as an ongoing battle to reduce tar production. The first few iterations of this gasifier produced more tar than gas. The complete history of this design can be found on my web site at Below is the most important of all chemical reactions a novice gasifier builder needs to know.

Biomass + Poorly Designed Gasifier = Tar!

I strongly recommend that anyone interested in gasifier technology do some research and read up on it. There is a lot more to this technology than I can present in an instructable.

Step 2: Schematic of the Gasifier

Picture of Schematic of the Gasifier

I know, it is a busy and confusing drawing. Don't panic though. It isn't as bad as it looks. There is actually more to the finished design than this, but this is a good place to start. More complexities will be introduced in later steps. I strongly suggest you read through the entire instructable before attempting to build your own gasifier.

Dimensions are not critical for most things. I do give a few dimensions here and there where it is important. However, you don't have to worry about slavishly copying my unit in every minute detail in order to make a gasifier that works. I improvised my unit from found materials and made it work by tweaking it over time. You could almost certainly do as well (maybe better) yourself with completely different materials. Feel free to experiment.

Back to the schematic. Basically, the outer shell of the gasifier is just a 5-gallon steel drum. The drum must be steel because of the extreme heat generated during the gasification process. The drum collects the gas that is generated. The gas exits through one of the drum's bungs. Ash and char produced collect in the bottom of the drum. An access door cut in the side of the drum allows for periodically cleaning out the ash and char. The solid by-products remaining in the bottom of the drum is known as bio-char. It is often used to improve poor soils.

A stainless steel tube 4 1/4 inches in diameter serves as both fuel hopper and reaction tube. There is a constriction in the tube near the bottom. A perforated stainless steel grate at the bottom of the tube prevents the contents from just spilling out the bottom. The grate can move, and is shaken periodically during operation to allow ash and char to exit the reaction tube and allow fresh fuel to fall into the reaction zone. The gases produced travel downward and exit through the shaker grate. So this type of gasifier is called a downdraft gasifier.

Six j-shaped copper tubes pipe air into the reaction zone at the base of the stainless steel tube. Since the outer drum is full of hot gas during operation, the long j-tubes also pe-heat the air before it goes into the reaction tube.

The entire core assembly consisting of the reaction tube, j-tubes, and shaker grate can be removed from the outer drum for easy maintenance and for making modifications.

In operation, air enters the reactor through the j-tubes and is pre-heated as it moves downward. Combustion of the biomass occurs in the flame zone between the j-tubes. Radiant heat from the combustion drives pyrolysis reactions in the pyrolysis zone. Lower-grade heat further up dries the fuel in the drying zone. A constriction just below the flame zone forces gases produced in the pyrolysis zone to pass through, or at least very near the flame zone to exit the reactor. This helps thermally crack the tars. Hot charcoal exiting the flame zone moves down into the reduction zone between the constriction and the shaker grate. Combustion gases leaving the flame zone get reduced in the hot charcoal. Shaking the grate periodically allows ash and spent char to sift through the holes in the grate, and drop off the edges of the grate. This allows fresh fuel to move downward through all the zones.

Originally I used a motorized blower attached to the gas outlet pipe to pull air through the gasifier. Later I built a manifold connecting the tops of all the j-tubes and used compressed air to run the gasifier. Ideally the gasifier would be driven by the manifold vacuum from an engine it is supplying fuel gas to.

More on all this later. Go to the next step to start building the gasifier.

Step 3: The Structure of the Gasifier

Picture of The Structure of the Gasifier

I tried to use inexpensive and easy to find parts to build the gasifier. At the beginning of this project I did not have regular access to a welder, so I tried to minimize the amount welding necessary. It should be fairly easy for any well-equipped, backyard tinkerer to build a gasifier like this.

1st Photo:
The basic structure of the gasifier is built around a 5 gallon steel drum, and a stainless steel tube 4 1/4 inches in inside diameter, and 14 inches long. These dimensions are not really critical. The tube could be a little longer or shorter, and a little wider or narrower in diameter. I got the drum at work. We use a variety of chemicals that come in small metal drums like these, and we always have a lot of empties around. The stainless steel tube came from a scrap yard. It was a little pricey. I have since discovered that many fire extinguishers have stainless steel bodies that are about the right size for use in a gasifier. Old fire extinguishers are easy to find and cheap.

The purpose of the drum is to be the main body of the gasifier unit. It contains everything and collects all the gas, ash and char the unit will produce. The smaller of the two bungs on the drum will be the gas outlet. The stainless steel tube serves several purposes. The bottom of the tube will be the reactor where the gasification takes place. The remainder of the tube is a hopper for holding un-reacted fuel. The tube will be subjected to very high temperatures and corrosive gasses. Stainless steel is the obvious choice here.

2nd Photo:
I started by cutting a large hole in the top of the drum so the stainless steel reaction tube can be inserted. The hole was made very oversized so that the entire core of the gasifier will be easily removable. The hole is offset to the side of the drum opposite the small bung. The large bung was sacrificed, since I wasn't planning on using it.

3rd Photo:
Next I cut a flange from a piece of 1/8 in steel for mounting the reaction tube into the drum. The flange is large enough to completely cover the large opening in the top of the drum. The hole in the flange is just the right size for the reaction tube to slip snugly through.

4th Photo:
I installed clip nuts on the corners of the hole in the top of the drum, and drilled mating holes in the above flange. This would allow me to bolt the flange down to the top of the drum. My idea here was to make the core of the gasifier easily removable for service and modification.

5th Photo:
Here is the access door in the side of the drum. I cut a rectangular hole in the side of the drum just big enough for me to get my hands inside to clean out the ash and char. then I cut a larger rectangular piece that would overlap the opening on all sides out of another drum to serve as the door. The door is held in place with six more clip nuts and bolts and sealed with lots of high-temp silicone gasket material.

Step 4: Mounting the Reaction Tube

Picture of Mounting the Reaction Tube

1st Photo:
I made some angle brackets out of aluminum and used them to bolt the reaction tube to the flange. I left 6 1/2 inches of the reaction tube sticking up above the flange. The rest protrudes down into the drum. At this point in the project I did not yet have access to a welder. Even if I had one, I'm not sure I could have welded the mild steel flange to the stainless steel reaction tube anyway. Here the unit is being test fit on top of the drum. The holes in the ends of the angle brackets align over the clip nuts in the top of the drum.

2nd Photo:
This is my new best friend. I went through several tubes of this high temperature silicone gasket material. I used it to seal every crack, crevasse, joint, seam and bolt hole in the gasifier. It works great.

3rd Photo:
Here I have used the gasket material to seal the gap between the flange and the reaction tube. I put a bead of the silicone around both the top and underside of the gap.

4th Photo:
Here I am doing another test fit to make sure all the bolt holes line up with the clip nuts in the top of the drum. When mounted for real, a bead of the high-temp silicone gasket material between the drum and flange will seal it air-tight.  I have also installed a ball valve on the small bung. The valve ensures that I can completely seal off the gasifier when it is shut down.

5th Photo:
The top of the reaction tube needs to be capped with an air-tight cap during gasifier operation. This is a rubber cap for PVC pipe I found at a hardware store. It is actually meant for a slightly smaller OD pipe than the reaction tube. So it is a really tight fit, even without using the clamp that was included with the cap. The cap can be easily removed for loading fuel.

Step 5: Installing the J-Tubes

Picture of Installing the J-Tubes

The design of the gasifier evolved over time. The j-tubes were not part of the original design, and were added later when it was discovered they were needed for the gasifier to work better. So that is why the gasifier looks well cooked and encrusted with lots of red silicone in these photos. The complete evolution of this gasifier can be seen on my web site at

1st Photo:
Here I have installed the six j-tubes. They are made of 3/8 inch copper tubing. They are called j-tubes because they are shaped like the letter J. I used a large hose clamp cinched down tight to hold the tubes in place. The opening in the top of the drum needed to have a few notches cut in it to accommodate a couple of the j-tubes that stuck out too far.

This photo also shows the chains that suspend the shaker grate. More on that later.

2nd Photo:
Here is a look up the bottom of the reaction tube at the business ends of the j-tubes. Copper is probably not the ideal material to use to make them, since at least in theory, the temperature at the point the air is injected could be high enough to melt them. So far my gasifier doesn't seem to get anywhere near that hot, and the copper is holding up well. However, in my next gasifier, I will probably make at least the tips of the air inlets out of steel. Copper is just so much easier to bend and work with compared to steel tubing.

3rd Photo:
Here is a photo of the top of the partially assembled gasifier showing the tops of the j-tubes poking out of a sea of red silicone gasket material. It's a little messy, but to me it was a thing of beauty. If you are following in my footsteps and trying to build your own gasifier based on my design, I recommend you extend the j-tubes at least a couple of inches above the flange. This is a change that will make your life easier in later steps when it is time to make a manifold to connect the tops of all the j-tubes together. I did it the hard way, but you don't have to.

Step 6: The Restrictor Plate

Picture of The Restrictor Plate

We aren't talking NASCAR racing here. This is a different kind of restrictor plate. The gasifier has a restriction just above the bottom of the reaction tube, immediately below the j-tubes. The purpose is to force all the gases exiting the gasifier to pass through he hottest part of the reaction zone. This will cause tars to be cracked by the heat. My first restrictor plate was too big. I made it smaller after doing some testing.

1st Photo:
Here is a view of the original constrictor plate I made. By this point in the project I had my own welder (Yahoo!) and was getting somewhat proficient at using it. To make the plate I cut a circle out of 1/8 inch sheet steel that would fit in the bottom of the reaction tube. Then I cut a 2 1/2 inch diameter hole in the center of the circle. To mount the constrictor in the reaction tube, I welded three 1/4-20 nuts to the plate, and drilled passage holes in the reaction tube for three 1/4-20 bolts.

2nd Photo:
Here is a view of the constrictor plate installed in the bottom of the reaction tube. Building it this way made it easily removable. I had a feeling I might need to modify it. Turns out I was right.

3rd Photo:
After some testing I reduced the size of the constrictor plate. Now the opening is only 1 1/2 inch in diameter. The theory here is that by making the restriction smaller, the tar has to pass through the hottest part of the reaction zone and gets cracked. My original larger opening was allowing tar to sneak out without passing through the hottest zone just below the j-tubes. The complete evolution of this gasifier can be seen on my web site at

Step 7: The Shaker Grate

Picture of The Shaker Grate
The shaker grate hangs about 1/2 inch under the bottom of the reaction tube. It prevents material from just falling out of the bottom of the tube. It also holds hot char exiting the bottom of the reaction tube. Gases exiting the bottom of the reaction tube have to pass through this layer of hot charcoal and get reduced. Shaking the grate periodically allows fine char and ash to sift out through the holes in the grate. Since the grate is hung about 1/2 inch below the bottom edge of the reaction tube, larger pieces of char can also exit off the sides of the grate. By the time char makes it to the edge of the grate it is too cool to further reduce the gases, so it can be ejected by shaking to make room for fresh, hot char.

1st Photo:
Note that the core assembly is upside-down in this photo. Here I have installed the shaker grate. I made it by cutting the bottom out of a stainless steel colander I bought cheap at a yard sale. The colander already had a lot of holes in it, but I drilled quite a few more in it as well. If I had it to do over again, I'd make even more and bigger holes. The holes should be just small enough that the fuel pieces can't fall through until they have been well cooked and reduced in size. The grate is suspended under the bottom of the reaction tube by four chains. This allows the grate to move with respect to the reaction tube so I can shake it from time to time to promote flow through the system.

2nd Photo:
This photo shows how the other ends of the chains are attached to the bolts on the bottom of the flange. I used ring terminals crimped onto the ends of the chains. When the core assembly is inverted and inserted into the drum, the shaker grate will hang from the chains below the reaction tube.

3rd Photo:
Here is a look down the reaction tube from the top. Past the ends of the j-tubes and the constrictor plate is the shaker grate at the bottom.

4th Photo:
I attached a length of stainless steel wire to one of the chains of the grate and ran it outside the drum through a tiny pinhole drilled in the side of the drum just above the access door. The hole is so tiny that not much gas escapes the drum. Tugging on the wire makes the grate shake and twist. It is not an ideal system, but it seems to work.

5th Photo:
I put a ring on the outside end of the wire to make it easier to grip so I can easily tug on it to shake the grate. I normally shake the grate every few minutes during operation.

Here is a brief video of the grate in action.

Step 8: Making the Air Manifold

Picture of Making the Air Manifold

Here is another drawing showing the gasifier in its final (so far) form. If I had been thinking in the beginning, I would have made the tops of the j-tubes stick up enough so that they could all be connected together with standard copper pipe fittings to make a manifold. Unfortunately, I wasn't thinking that day. So later, when I came to the realization that I needed to connect all the j-tube inlets together, I had to come up with another idea. Those of you trying to build your own gasifier should just extend the j-tubes a little and plumb them together. Don't do it the way I did unless you are a glutton for punishment. My improvised manifold is always springing leaks, and tends to break loose when I remove the core assembly from the drum.

Ignore the part of the drawing that says "extended reduction zone" since that is an experiment I tried that didn't really produce any benefits.

1st Photo:
Here is the manifold I made to cover the inlets of all six j-tubes. It was cut from a 6 in to 4 in steel AC duct reduction fitting. It slips down over the reaction tube and gets siliconed to the top of the flange. A single air inlet fitting will be installed on the side of the manifold.

2nd Photo:
Here is the new single air inlet on the side of the manifold. I used a Tee fitting. One leg of the Tee goes into the manifold. One leg has a hose fitting installed that I can use to inject compressed air. The third leg of the tee is plugged for now. My idea here was that I could start the gasifier on compressed air, then once it was running, I could unscrew the plug, and let engine vacuum pull air through the gasifier (from whatever engine the gasifier eventually gets connected to).

3rd Photo:
Here is the manifold in place on the gasifier. Everything is all buttoned up and sealed with yet more great gobs of red silicone gasket material.

Step 9: The Fuel

Picture of The Fuel

I failed to do a lot of basic research about gasification before just diving in and trying to build a gasifier. So I wound up repeating a lot of the same mistakes other people made before me. So this gasifier went through many versions, re-designs and re-builds before getting to this state. You can see the complete evolution of this project, including the spectacular failures, on my web site at

One early mistake I made was poor initial choice of fuel. It turns out gasifiers are finicky about what you feed them. They like fuels that are uniform in both particle size, shape and composition. Non-uniform fuels produce lots of gunky tars. My first choice of fuel was about as bad as it gets. I tried to run the first version of the gasifier on wood mulch.

1st Photo:
My dream fuel for the gasifier in the early days was free wood chips and mulch available from lots of places nearby. I know of at least three places I pass on a regular basis that have signs offering free wood chip mulch to anyone who would come and haul it away. There are probably dozens of other sources I could find with a little research. So I got myself a bag of wood chip mulch. The chips were very wet. So here I am drying them with a fan. After 2 weeks under the fan, they were bone dry and ready to burn in the gasifier. I realized that if this worked, I'd have to find a less energy intensive way of drying the wood chips in the future. But it didn't work. The gasifier didn't really work at all on wood chips. The non-uniform size and shape of the chips, combined with their mystery composition led to terrible problems. The chips didn't feed right, didn't burn right, didn't pyrolize right, and often wouldn't even burn at all. When the gasifier was running on these chips I got far more tar than gas out if it. Out of frustration I hit the books to try and figure out what the problem was. That's when I learned about gasifier fuel needing to be uniform to work well. So I started groping around for a better fuel option.

2nd Photo:
I thought wood pellets would be a good fuel. Unfortunately I live in Florida, and nobody burns wood pellets here. They are essentially impossible to obtain here. So I settled on hay pellets. I could get them from feed stores. They are more expensive than I would have liked, but they didn't break the bank for testing purposes. with their uniform shape, size and composition, they seemed like a reasonable substitute for wood pellets. The gasifier worked much better on hay pellets. There was more gas and less tar. The pellets fed nicely through the reaction tube and exited as little beads of char. I could start seeing the potential of this machine.

3rd Photo:
I finally found some wood pellets. On one of my trips to my Arizona property, I bought back two 40 pound bags of wood pellets. They were dirt cheap too. Less than $6 per bag. I couldn't find them to save my life in Florida. Every hardware and homecenter store in Arizona seems to carry them though. Later I also found them on a trip to the North Georgia mountains, and brought back some more. Now I have plenty of high quality fuel for test running the gasifier. Fortunately I drive out to Arizona twice a year. So bringing a few 40lb bags of wood pellets back home on each trip in my big truck was not a problem.

4th Photo:
A close-up of some wood pellets. The gasifier runs even better on the wood pellets than on the hay pellets. The wood pellets are designed to be a fuel after all. The more or less uniform size, shape and composition of the pellets is just what is needed for good gasification. Wood pellets are also dirt cheap if you can find them.

Step 10: Getting Air Through the Gasifier

Picture of Getting Air Through the Gasifier

Air must flow through the gasifier for it to operate properly. In normal operation, the manifold vacuum of whatever engine the gasifier is connected to will pull air through the gasifier and keep the reaction going. When first starting up a cold gasifier, or testing a unit not attached to an engine, there must be another method of forcing air through the gasifier.

1st Photo:
Most gasifier projects I have seen use a blower to pull air through the unit. It is usually used to start the gasifier, then once running, the vacuum from the intake of the engine the gasifier is meant to power keeps the gas flowing. I initially chose this method too. This blower is a little under powered. However, it was the only all metal blower I could find at the time. Most blowers these days are full of plastic parts. The plastic would melt at the temperatures the gasifier operates at. So I made do with my undersized blower. I actually had some success with it.

2nd Photo:
This photo shows the jet of gas coming out of the blower ignited and burning nicely. The blower worked, but it was very undersized. the gasifier wasn't heating up to optimum running temperature because it was starved for oxygen. To get good performance out of the gasifier I knew I was going to have to increase the air flow. I looked into buying a larger blower, but powerful all-metal models were rare and very expensive.

3rd Photo:
Then I had a brainstorm. I have an endless supply of compressed air in my workshop. So why not blow compressed air through the gasifier, rather than using a blower to pull it through? So I scrounged up a pressure regulator, a valve and some hose and hooked it all up. This is the idea that forced building the manifold to connect the inlets of all the j-tubes together several steps back.

4th Photo:
This photo shows the air hose from the regulator attached to the air inlet of the gasifier. I am using a piece cut from a silicone cooking sheet as a heat shield to protect the hose from melting where it touches the hot drum.

The compressed air really worked great. The gasifier starts up almost instantly, and gets much hotter than before. The quality of the gas has greatly improved.

Step 11: The Flare Stack

Picture of The Flare Stack

Eventually I had the gasifier working well. It was making lots of gas and hardly any tar. Everything was working great. The gasifier was producing a huge volume of gas. The problem is that this gas is flammable and toxic. I needed to burn or flare off the gas to keep it from being an explosion or health hazard. Just lighting the jet of gas coming out of the outlet pipe of the gasifier didn't really work very well because the wind would quickly blow out the flame. I decided I needed a better way of flaring off the gas. So I bodged together a quick and dirty gas burner.

1st Photo:
After some experimenting, I determined that the gas needed to be mixed with the air to properly burn, and a flame holder was needed to prevent the wind from blowing out the flame. My flare stack is made from an old tin can and a stainless steel vegetable steamer. I just drilled a bunch of holes in the bottom of an 18 ounce steel can, and bolted it on top of the gas outlet pipe. I then put an old stainless steel vegetable steamer over the open top of the can. It works great as a burner. The flame doesn't blow out even in very strong wind gusts. I increased the stack height to prevent the heat from the burner from cooking the rubber and silicone parts on top of the gasifier.

2nd Photo:
Here you can see the bottom of the burner. It is just an old 18 ounce bean can with lots of air holes punched in the bottom. It sits on top of the outlet pipe of the gasifier and the gas enters through a large hole in the center of the can bottom. The top of the can is open. The gas and air mix inside the can.

3rd Photo:
This photo shows the stainless steel vegetable steamer sitting upside-down on top of the can. There are three screws that go into the can. The steamer is wired to these screws with stainless steel wire to keep it in place. This setup works great as a gas burner. It didn't cost anything and took almost no time to build. The burner holds the flame even in very strong winds.

Step 12: Starting the Gasifier

Picture of Starting the Gasifier

My method of starting up the gasifier takes a few minutes, but it starts producing relatively clean gas quickly.

1st Photo:
I begin with hardwood lump charcoal. This sort of charcoal starts easily and burns very hot. Briquettes might work, but I haven't tried them. They would need to be broken up into small pieces. The lump charcoal is soft and breaks up easily.

2nd Photo:
I break the charcoal up into small bits. Here the bits have been placed in a stainless steel vegetable steamer and I am using a propane torch to start the charcoal burning. The small bits of charcoal rapidly catch and quickly heat up. Once the charcoal is good and hot, I dump enough down the reaction tube to fill it from the shaker grate up to the j-tubes.

3rd Photo:
Then I fill the reaction tube up the rest of the way with wood pellets and put the cap on the end of the tube. The bed of hot charcoal in the bottom of the reaction tube really jump starts the gasification process.

At this point I can open the valves and start the air flow. Within a couple of minutes the gas coming out the flare stack will support combustion.

Step 13: Operating the Gasifier

Picture of Operating the Gasifier
This first video shows a timed run of my home-built biomass gasifier to see how long it takes to use up a full load of wood pellets. The gasifier ran for about 45 minutes before all the pellets were used up. The video though is only a few minutes long. This is an earlier version of the gasifier with the larger constriction opening. Notice the bright yellow flame. That is a sign of tar production. The next video shows the gasifier after making the constriction smaller.

This second video is  of a night test of the final version (so far) of the gasifier with the smaller constriction. I am very happy with this run. There is much less tar. Running at night allowed me to see the true color of the flame, and see that there is much less tar now, since tar makes the flame bright yellow.

This video is another night test run of my home-made biomass gasifier, but this time it is running on charcoal. The gasifier produces a nice, clean gas, and a tar-free flame running on charcoal. I wanted to try running the gasifier on charcoal only and no wood just to see how well it works, even though the gasifier isn't really designed for running on charcoal.

You can see the entire evolution of this project and more videos of the gasifier in operation on my web site at

Step 14: Future Plans and Modifications

Picture of Future Plans and Modifications

The gasifier is not "done" yet. It may never be done because there are so many modifications I want to try out. There are a few big ideas I have for the not too distant future. Here are some of my future plans, not in any particular order.

#1. I still have the goal of using the gasifier to run an engine. I have come a long way in reducing the amount of tar the gasifier produces, but will need to further clean the gas before feeding it into an engine. The remaining tar and any particulates will need to be removed from the gas. So I need to build a scrubbing system, or cyclone separator. The gas should also be cooled before feeding it into an engine. So a radiator or other cooler will need to be added to the system.

#2. I am very seriously considering rebuilding the central core of the gasifier using ceramic materials. The hotter the reaction zone can get, the better the gasification process works. The stainless steel reaction tube absorbs a lot of heat and conducts it away from the reaction zone. Ceramic materials don't conduct heat like metal, and can handle much higher temperatures without melting.

#3. I want to build an automatic shaking system for the shaker grate.

#4. I want to build a moisture removal system for the fuel hopper section. As the fuel moves down the reaction tube, toward the reaction zone, it heats up and moisture is driven out of it. This moisture then condenses on the cool upper section of the tube, and makes the fuel there damp. Damp fuel doesn't burn or gasify well.

#5. I have an idea for an auger feed system to keep the fuel hopper topped off from a larger pellet bin. And maybe another auger system to remove accumulated ash and char from the bottom of the drum. With these in place, the gasifier could run indefinitely.

#6. I am considering installing diesel engine glow plugs in the reaction zone so the gasifier can be started electrically.

#7. I'd like to someday build a larger version of the gasifier that could power a vehicle like a car or truck.

These are just a few of my ideas. I probably have more ideas than I can try out in 10 lifetimes. You can follow the future evolution of this project, and my many other projects, on my web site at


b_said (author)2016-01-31

This is a great intractable but I have one question, in the air manifold for the J-tubes are the J-tubes connected with pipe which is connected to the T-fitting or does the T-fitting just pipe air directly into the manifold which goes into the J-tubes.

devran3232 (author)2016-01-30

I guess heat is going to waste can it be hook up with central heating or some sort of boiler ?

JohannD4 (author)2015-12-20

I'm thinking about trying a scaled up version of this to produce gas not for use in an engine, but to generate steam.

EricS83 (author)2015-10-07

Great post! I figure that as like attracts like, and with experience of the amazing oil bath Land Rover air filter, my intended build will use a light oil bath filter with a load of steel turnings to break up the coarser bubbles to remove the tars. A 1 meter long 75mm pipe crammed with steel turnings thereafter should take care of the minuscule amount of hydrogen sulfide.

tylerkat (author)2015-04-05

I always thought you could take the exhaust from the engine you were trying to run and run the hot gases the light your gasifier by using a pipe. I honestly don't know all the running temperatures here and if it would even be hot enough. But I think I'll spend some time looking into it.

thisdudeisnotin (author)2015-03-19

I saw an article in Mother Earth News about running the "Producer gas" through a barrel of loose hay to catch the tar b4 feeding it to a pickup truck engine.

experimenter4life (author)2014-09-23

I love your design and am at awe in how painstaking you've been documenting your discoveries. You inspired me to attempt my own, working from your successes. You will need to remove sulfur compounds if you wish to run an engine. I've been reading up on early sulfur-removal techniques for gas streams....might consider iron oxide impregnated wood chips ie. iron sponge...many mulch companies use iron oxide as an 'all natural' dye in their red curious to know if it would work to convert hydrogen sulfide into iron sulfide....anyway great work!

tpoulton (author)2014-07-01

awesome setup. Let me know when I can come visit and have a larger model installed by you on my Jeep Wrangler (I'm willing to pay for parts and booze). Seriously, nice build, I'm jealous of your skills.

Bruce Ratcliffe (author)2012-05-06

The old FEMA article about gassifiers from the 1980's lacks the J-tubes, making it easier to build. But it never states where in the fuel column the flame is. I'd think you'd want the flame at the top, so that the heated air would do pyrolysis on the wood below as the sucking air pulls the flames down. Does anybody know if this is actually how a simple biomass gassifier works?

If I remember correctly, the FEMA version lets the air flow directly down through the biomass that's to be burned? That does work, but by modifying the design to use air inlet tubes, you can have a better burn.

The burn point in the FEMA version, as well, is at the bottom of the tube, rather than the top. What you want is for the biomass to self-feed into the fire, rather than for the fire to simply burn randomly. Having the fire right about at the constrictor plate makes for the hottest burn, letting the ash fall through the grate, while gravity feeds the biomass down to continue the burn at it's most efficient point. The J-tubes used in this particular instructable also help to keep the burn at (or near) it's most efficient location. If you were to use this model to provide fuel for an engine, you would have the option of stopping the engine after awhile to add more fuel, without significantly damaging the burn - you could simply close it back up, and start the engine again. The suction from the carburetor would keep fresh air flowing into the gasifier.

There are also a lot of really neat gasifier projects that people have put up on YouTube. Some run lawnmowers, some run pickup trucks. I've even seen a small one that runs a motorcycle.

rjbatc (author)2012-07-18

Wouldn´t a simple water and maybe charcoal filtering system help clean your gas? I think a simple water bubbler takes out a lot of tars. The question is, what to do with the stinky, dirty water then.

surelyujest71 (author)rjbatc2014-04-12

As I understand it, water is definitely helpful at removing tars from the air via a bubbler system. However, doing so would mean putting a lot of water vapor into the system by way of evaporation. The hottest point in the combustion process in the gasifier should take care of cracking (that's actually the technical term!) the tars, making them into combustible gasses. If it's not hot enough, then perhaps a narrower restriction plate might help. Also, simple air filters (old-fashioned hay or dry pine needles work well) will also absorb a lot of the tars while letting the combustibles flow through.

freak0vnature (author)2013-10-16

i like your design a lot but do you think i could run a steam engine off of this

It would be fairly inefficient to use this to run a steam engine on - it's main purpose is to produce flammable gasses that are clean enough to run a vehicle's internal combustion engine with. While you could use the simple burner on the end of the pipe to provide heat for the boiler on a steam engine, you'd also have to provide air flow (via air pressure or an in-line blower motor) to keep the air moving through the system.

It's actually more efficient (for steam engines, at least!) to simply burn your combustibles so they heat the boiler directly.

One offshoot of gasifiers is called a "rocket stove." You could use a rocket stove to more efficiently provide heat for your boiler.

korlich (author)2013-12-30

So, dumb question here. What keeps that rubber cap on top of the hopper from bursting into flames? Isn't there a blazing inferno just a couple feet under it? Very intriguing and simple design. First "doable" one I've seen so far. Thanks!

sullymonster01 (author)2013-12-25

If you welded the pipe fittings and other points than it could get pretty much as hot as you wanted although I see you dont have access to a welder.You can usually find a fabricator shop that will weld it for free or at a low price.

TheWilks1 (author)2013-07-15

Wood gasifiers are confusing to me but I was looking at your design and was wondering if you could put a valve where the rubber piece is to allow easier access? And just to make sure that where you put your fuel right?

AtomRat (author)2012-12-06

Hi there! This is by far the best gasifier design I have seen for home use! I too have had a journey creating them, and I love how you have mixed so many designs into yours! It is definitely open for more modification, I would like to suggest some further ideas:

I am now on my way to attempting methanol creation from bio-gas which requires a fluid bed reactor, and from what I know now, these can be made and simplified just as easy as we can do with these wood gas burners.

Once the gas leaves the unit, you can further purify it by using:
- Filters: several can be placed along the gas process. I know of cyclone, water, fluid bed and fiberglass. ( sorry, only just heard that in the vid )

Clean the gas further by condensing it to a liquid, easier to separate by adding a simple condensing unit.

Peltier or Stirling modules for energy production off the burner.

Download a .pdf book you can google called: Wood Gas As Engine Fuel
It provides an extreme amount of info on our beloved burners. Hope any of this helps.
p.s - unfortunately I became a victim of lack of information and ended up in emergency from Carbon Monoxide poisoning. As you can guess, I stay clear of the smoke these days :(

MTtoo (author)2012-06-06

You have written a very informative and well written i9nstructable. You have touched every detail and told of the consequences of doing that operation some other way. Excellent reading. Very easy to follow. Like all the rest of of the things you have built over the years (on your website). I have followed your work for a long time. Would like to ask a question. Many years ago (about the gas shortage of 1980) I was readung up on the wood gasifiers. Some knowledgeable person was explaining the different wood types used as fuels. I was thinking that a hardwood that gave off a lot of heat and little smoke would be the best. According to this fellow, the best fuels were the ones that produced a lot of smoke. Smoke = carbon monoxide. I am sure you have studied the wood types. What are your suggestions?
Thanks again for your great work.

shabir.khan84 (author)2012-04-16

hi. i am shabir khan.i am from pakistan.i saw this gas maker today and i am going to make it.because here is gas shortage to spread quicly.i am very than full to you for make best wishes with you make it in a car injine we intence need here for that.i am very thank full to you again.

Razanur (author)2012-03-25

Has anybody ever tried to build a externally heated Gasifier? Using e.g. some of the produces gas. It would have three benefits: It's easy to "ignite" (just using some leftover gas from last run), you don't get CO² in the produced gas and temperature is easy to control.
What are reasons to not do it like this?

Noodles For Free (author)2012-03-24

I'm making a ceramic gasifier that will compress the gas instead of using a blower for use and possibly for later ignition of the gasifier, and I'm waiting for it to come out of the kiln, but in the meantime, I was wondering if you think it would be possible to ignite the thing electronically, or would i have to clean it out each time i want to start it? Also, do you think that a ceramic gasifier might work temperature wise (ceramics are fiered at around 2500 degrees the last time).

Uptonb (author)2012-03-02

Congrats on being a finalist, and good luck!

Ogg1 (author)2012-02-02

Kudos for a wonderful Instructable! It does however prompt the following question: How does a gasifier compare to a small boiler/steam engine for stationary installations like turning a generator? What I am thinking about here is a comparison of the efficiecy, safety and practicality for those of us with plentiful supplies of wood. Steam power, while not so common today certainly has a long history and is a mature technology.

frankcox (author)Ogg12012-02-17

Steam is superior but prohibitively expensive.
A steam engine to drive a generator would run you 30k + . They are wonderfully efficient but not cheap.

tgothan (author)frankcox2012-02-24

A modern, large boiler and turbo-alternator set would definitely have a superior efficiency to a dustbin sized gasifier and converted petrol engine, but we must compare "apples with apples". A small scale boiler and reciprocating steam engine would probably have a poorer overall energy conversion efficiency, as well as being considerably more expensive.

Ogg1 (author)tgothan2012-02-24

That is what I was really fishing for. I have all this wood that is currently going to waste and I am searching for a practical approach to using it. Heat is an obvious choice and I have acquired two wood stoves to that end but as electricity is becoming more expensive I would like to address that as well.

mdavis19 (author)Ogg12012-02-02

Well, like you said, steam power is not so common today. Internal combustion engines are everywhere though, and cheap and easy to obtain. A gasifier will power them from wood or other waste biomass. If you don't happen to have a steam engine, your steam efficiency is zero. If you need to turn a generator, which is easier to build, a steam engine, or a gasifier?

frankcox (author)mdavis192012-02-17

The technology to build a steam engine is light years ahead of a wood gasifier.
A small steam engine will cost 30 - 40K , there is a reason for it.
They are incredible machines , the speed of an old steam powered wooden launch would amaze you. Internal combustion replaced steam because it was cheaper and safer , not because it was better.

The licences to run a steam engine cost more than the gasifier.

Ogg1 (author)mdavis192012-02-02

You do make some very good points. As to which one is easier to build that is debatable. It kind of depends on your personal talents and inclinations. A small steam engine can be driven from either a boiler or a solar concentrator and that may present certain operational advantages.

I was more hoping to stir up a little input from the engineers out there than you personally. Once again I would like to commend you on an excellent instructable. I can only hope to someday contribute something of such elegance.

tgothan (author)2012-02-24

A good instructable. I have built several larger gasifiers, which used waste wood from drymills to generate around 40 kW(E). I have several suggestions, the most useful for quick startups is fitting a 1" tube from outside the gasifier to just above its hot zone. You then simply push a lit, kerosene-soaked wad on the end of a wire into the burn zone. An old hair dryer blowing down this tube would get things going in under 15 minutes (you can see this down the tube). The test was to light the pilot nozzle on the gas outlet. If it burned steadily, you could remove the wire and blower, seal the starter tube with a screw cap seal and start the alternator engine. There was no need to unpack the hopper to add hot coals.

junkhacker (author)2012-02-05

Would it remove much tar if you sent the output gas through a tube tightly filled with charcoal? The idea being that the tar would become deposited on the charcoal as it pushed through, and as it cooled. This tar heavy charcoal could be reused as fuel,where the tar would have a second chance to be cracked, and fresh charcoal put in the tube.

tinkerist (author)2012-02-02

i just read your website write-up. that's some excellent experimental engineering you got going on there. i would like to drop you a suggestion on creosote and sulfur removal (the excess sulfur in syn-gas can be a problem for engines due to it's corrosive qualities).
for the creosote/tar, i have seen designs where the gas is percolated through a water reservoir. this would be much simpler than your spray design and potentially more manageable than a cyclone separator, as maintenance would largely consist of changing out the water regularly. of course, the more and smaller holes the gas goes through at the bottom of the percolator, and the more time it's in contact with the water the more effective it will be. you might consider a condenser down circuit from this to remove excess water.
as to the sulfur, the most common method i've seen is running the gas through a column full of steel wool. the steel bonds with the sulfur and provide a surface for it to crystallize, making little yellow florettes. there's a way to remove the sulfur from the steel wool and reuse it a few times before it rusts to bits, but i can't remember how to do that right now. i don't think it takes very much steel wool to remove a significant amount of sulfur but a longer column with more steel wool would probably be more effective and require maintenance less often.

as an added bonus, you might be able to find uses for the creosote and sulfur. they're both flammable and, thus, potentially usable as fuel. they have other uses as well.

tinkerist (author)tinkerist2012-02-02

also, it occurred to me that you might get less tar by making your j-tubes more nearly flush to the inside of the flame zone, or building up the constriction zone to a more conical shape toward the air induction points. my logic on this is that some of the gasses and tar might be bypassing the flame zone through the areas outside of the air induction, where your highest heats would be. does that make sense? i know what i mean but i'm not sure if i'm stating it clearly.

mdavis19 (author)tinkerist2012-02-03

A conical constriction is used in some gasifier designs. I had the idea of building that originally. The problem is, the section below the top of the cone is a dead leg. Material (mostly ash and char) gets trapped there and cannot move down. Once that space fills up with inert material, there is no real difference between a conical constriction and a planar constriction. The only real advantage that I can see to the conical design is that some of the liquids produced that aren't thermally cracked can be trapped in the dead space below the top of the cone. In the end, I decided it wasn't worth the extra effort to make a cone.

tinkerist (author)mdavis192012-02-04

what about cutting back the j-tubes a little, increasing the diameter of the flame zone?

tinkerist (author)tinkerist2012-02-02

one more thing. did you notice any increase in performance when you extended the tube below the flame zone? if so, would it be due to airflow somehow?

mdavis19 (author)tinkerist2012-02-03

No, I did not see any improvement in performance. My idea was to extend the length of time that gas and tar exiting the reaction zone would be in contact with hot charcoal in hopes of cracking more tar and getting better reduction of combustion by-products. It didn't work because (as I realize now) the char exiting the reaction zone cools quickly. Only a short distance below the reaction zone it is already too cold for any further reactions to occur.

bennelson (author)2012-02-03

Here's a basic one some friends and I were working on.
he neat thing about gasifiers is that they can almost be as simple or complicated as you want them to be.

{Havoc}.Goliath (author)2012-02-03

Maybe i missed something about it, but is the compressor or any other forced air system only used on startup or for the whole burn? Thanks

mdavis19 (author){Havoc}.Goliath2012-02-03

If the gasifier is being used to produce gas to power an engine, then the forced air system would only be used for startup. After that, the manifold vacuum of the engine would take over driving the system.

ALogan97 (author)2012-02-03

How much more efficient would this be than using pyrolysis to break down algae to produce gas fuel? I'm doing a project for the Google Science Fair about it, but I want to know if I should switch to gasification instead of pyrolysis.

prince-of-weasels (author)2012-02-03

There are 'pellet' stoves for sale that burn either wood pellets or corn!
Also there are off the shelf usable parts for your gassifier......restricter plate and shaker etc.....
Corn ,in some years, goes currently for 643.00 cents per bushel/50lbs bag or better yet grow your own^_^ Gives you something to do with all that 'franken corn' {genetically modified}nobody wants to eat

metalarts (author)2012-02-02

Is it possible that the "tar" that you mentioned is in fact uncrystalized creosote?If so,you might allow it to harden and use it as a heat source in your gas generator.

Liceo Piovesan (author)2012-02-02

Nice project! This technology has been used in Brazil and do not know why it was abandoned
In the 301s and 40's years, in Brazil has used a very similar system to feed the engines of transport trucks and tractors. You can see something on the links below

lorenojr (author)2012-02-02

It isn´a a "gasogen" manu use at 2aWoldrWar when the petrolium was use just for 2WW??
Excuse me my Inglish,ist´s so poor
I think it was used a lot during the 2WW
If you have more infs about to incres the performance ( energy rendiment) PLEASE keep up to date


MTJimL (author)2012-02-02

What an intriguing project! I'm waiting for some bright engineer to build a gasifier that would be acceptable to the neighbors in an apartment block, both from the sensory and the safety standpoints. Seems folks highly approve of alternative energy as long as they don't have to see, smell or hear the means to produce it.

rosec (author)2012-02-02

Just a thought, what if you put a large diameter metal funnel to direct the flare more. You could harness the "wind" from the flare and spin a nice metal wind turbine and run your generator on cloudy and windless days. I'm not as technical as you but I've burned my hand with tea kettle steam enough times to realize there is energy there.

Javin007 (author)2012-02-02

This is made that much cooler by looking like a steampunk coffee maker.

rlacoma (author)2012-02-02

Wow, I just read a good Wired Magazine article of a startup using a Gasifier and plasma arc incinerator combo in Oregon.

Here is the link:

Thanks for sharing, this definitely wants me to look more into this form of trash removal.

About This Instructable




More by mdavis19:How to build a sturdy workbench inexpensivelyCasting a Large, Light-Weight Telescope Mirror from Recycled GlassA Home-Built Biomass Gasifier for Producing Wood Gas
Add instructable to: