I've built a lot of alternative energy projects over the years. See my home-built solar panel and
wind turbine pages. 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?
My original goals with this gasifier project, were to build a compact and simple gasifier, that used inexpensive feedstock (like wood chips or mulch that is available very inexpensively, or even free), and produced high-quality gas. Little did I know in the beginning that these goals appear to be largely incompatible. Simple gasifiers don't produce good gas, and inexpensive fuel is the most difficult to work with. Only after working away at the project for a while, and going through several major redesigns of the gasifier and changes of fuels, did I achieve a system that works reasonably well. So this web site will chronicle the evolution of the gasifier, from early failure, to ultimate success. I will point out the mistakes I made and blind alleys I went down, so that you won't have to make those same mistakes.
As I said above, my original goals were to produce high quality gas from a compact, simple and easy to fabricate design. My research showed that the downdraft gasifier
design generally produced the best quality gas. However, there are a bewildering number of variations on the downdraft design. Some quite complex
and difficult to fabricate, others much simpler. So naturally I gravitated toward the simpler designs. I originally aimed for a simple open core
design, like the one on the far left of the bottom row of the diagram.
I chose the open core stratified downdraft gasifier design because it was by far the simplest of all the designs I could find. Everything I read about
it (at the time) said it should work great. I saw vague references to people in India having great success with this design. So I thought I couldn't fail.
Turns out this design sucks. It is really good at producing tar, but not so great at making high quality gas. Unfortunately I had to build it before I
figured it out.
As I said above, I made a lot of early mistakes with this project. I was fortunate though in starting with an good foundation that I was able to
modify and build on to ultimately make a working gasifier. In building another unit, even knowing what I know now, I would start out exactly the same way.
I started by cutting a large hole in the top of the drum so the stainless steel flame tube can be inserted. The hole was made very oversize, a fortuitous decision
as it turned out. 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.
Next I cut a flange from a piece of 1/8 in steel for mounting the flame tube into the drum.
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.
Next, I made some angle brackets out of aluminum and bolted the flame tube to the flange. I left 6 1/2 inches of the flame 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 flame tube anyway. Here the unit is being test fit on top of
the drum. The holes in the ends of the angle brackets are over the clip nuts in the top of the drum.
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.
Here I have used the gasket material to seal the gap between the flange and the flame tube.
Another test fit to make sure all the bolt holes line up with the clip nuts in the top of the drum. I have also installed a ball valve on the small bung.
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 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 silicone gasket material.
It was time to tackle the problem of what to do for a grate at the bottom of the flame tube. At a total loss for ideas, I spotted a stainless steel
vegetable steamer at a yard sale. Inspiration struck. I decided to try using the steamer as a grate. I wired the petals together with stainless steel
wire to keep the steamer in this bowl shape.
Here the steamer is suspended on chains a little bit below the bottom of the flame tube. The steamer has been formed into a bowl shape a little larger
than the diameter of the flame tube. I suspended the steamer from the bottom of the flange with chains so that it could move back and forth a little.
I next tied a length of stainless steel wire (not shown) to the steamer and ran the wire outside the drum through a tiny hole drilled in the drum.
Tugging on the wire made the steamer shake and twist. I wasn't thrilled with the result, but I figured it would work well enough for a test run or two
of the gasifier.
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.
With the internal parts all sorted out, it was time to button up the gasifier. Here I have mounted to central core assembly (flame tube, flange and grate)
and used liberal amounts of the high-temp silicone gasket material to seal up the unit. I could hardly wait for the silicone to cure so I could start the
Here is the air blower I used to pull air through the gasifier. This blower last saw use in
my home-built jet engine project. 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. This blower is a little under powered. However, it is the only all metal blower I could find. 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, after I modified the design of the gasifier.
I used a flexible metal natural gas line like the ones used on gas ranges and dryers to connect the gasifier to the blower. The gas line is a little undersized, and probably contributed to the poor performance of the pump.
My dream fuel for the gasifier is 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 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 chips. 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.
Now it was time to give some thought to how to start up the gasifier. A lot of people build in a port in the side of their gasifier so they can stick a
blowtorch inside and start the fire. I decided to take a different approach. I decided to pre-load the reduction section of the gasifier with hot charcoal, and
then I'd fill the hopper up the rest of the way with whatever fuel I was going to burn. This starting procedure actually works quite well, and I have continued
to use it, even through all the redesigns and fuel changes the gasifier has gone through. With the reduction section full of hot charcoal, the reaction
starts almost immediately, and the gasifier (in its final design below) is producing good gas in only a couple of minutes.
Here I am lighting the charcoal with my propane torch. The pulverized charcoal is in yet another stainless steel vegetable steamer. These things
are just so amazingly useful for so many non-vegetable steaming tasks. The small pieces of hardwood charcoal light easily and burn very hot. Only a
few quick passes from the torch gets them going. Once
the charcoal is good and hot, I dump it into the flame tube of the gasifier, and fill it up to the top of the reduction section. Then I fill the
flame tube up the rest of the way with fuel.
Here is a photo of the completed Mark I gasifier in operation. Ignore the hopper extension at the top of the flame tube. It was an experiment I tried
to increase the fuel capacity of the gasifier. I no longer use it. The aluminum foil is on top of it because it was raining a little bit, and I was
trying to keep the fuel dry.
After doing some more research, (research I probably should have done up front), I came to realize that wood chips are a difficult fuel. The non uniform size
and shape of the chips makes them difficult to burn in a simple gasifier. So I switched fuels. I wanted to use wood pellets, but they are hard to find here
in Florida. So I settled on hay pellets. I could get them from feed stores, and they were reasonably priced. They seemed like a reasonable substitute for
So I sat down and made a list of the problems I was having with the gasifier, in order of their severity. My day job involves use of a lot of statistical analysis.
One tool we use a lot at work is Pareto Charting. With a Pareto Chart, you list the problems or defects in your process in order from highest to lowest percentage.
Then you start trying to fix the problems at the top of the list because fixing them first will produce the biggest improvement in your product. I had a
list of defects with the gasifier, and so I listed them in what seemed like the most logical order.
Looking at the list, it was obvious to me that the zone migration problem was the biggest problem I was facing. The fuel was being mostly converted into charcoal, rather than being properly gasified. Solving that problem would make a big improvement in the operation of the gasifier. I also realized looking at the list that solving the poor air flow problem would also probably improve or even eliminate other problems on the list like low temperature and tar production. So I decided to tackle the top two problems first, and tackle the others as opportunities presented themselves.
So it was back to the drawing board. I knew that the easiest way to solve the zone migration problem was to cap the top of the flame tube. The flame was
simply moving up-wind toward the source of oxygen. Since the original design required air to move through the whole fuel column, the flame simply went
up the column toward the source of air and created the zone migration problem. Capping the tube meant I had to find a new way to get the air into the
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.
Here is a look up the bottom of the flame tube at the business ends of the j-tubes. Copper is probably not the ideal material to use here, 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 darn easy to bend and work with compared to steel tubing.
Here is a view of the constrictor plate I made. By this point in the project I had my own welder (Yahoo!) and was getting 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 flame tube. Then I cut a 2 1/2 inch diameter hole in the center
of the circle. To mount the constrictor in the flame tube, I welded three 1/4-20 nuts to the plate, and drilled passage holes in the flame tube for three
Here is a view of the constrictor plate installed in the bottom of the flame tube.
Here I have installed the new 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. It is suspended under the flame tube by the same four chains that held the
original steamer grate. I used the same system of attaching a length of stainless steel wire to one corner of the grate and running it outside the drum.
Tugging on the wire makes the grate shake and twist. It is not an ideal system, but it seems to work. I put a ring on the outside end of the wire to
make it easier to grip.
Here is a photo of the top of the re-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.
Here is a look down the flame tube of the new and improved design.
After a few runs in this configuration, and burning through a lot of hay pellets, I became convinced that the next big problem that needed solving was
further improving the air flow through the unit. I had tried a few tests where I used a vacuum cleaner I had bought cheap at a yard sale in series with the
blower to increase the draw through the gasifier. I found that the temperature of the gasifier shot up with the increased air flow, and the quality of
the gas being produced seemed to increase. I quickly tarred up the vacuum cleaner, but I didn't care. I only paid a few bucks for it, and it served its
purpose in confirming my suspicion that I needed better air flow through the gasifier.
Back to the drawing board again. While brainstorming on how to get compressed air into the gasifier, I noticed a steel air duct reduction fitting I had previously
used as an aborted attempt at a fuel hopper extension earlier in the project. I realized I could cut it down and make it a manifold that would cover the tops of all
six j-tubes. That would allow me to inject air into the system at only one point on the manifold and feed all six j-tubes. Brilliant, again, and this time it looked
foolproof. The manifold came together quickly.
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
flame tube and gets siliconed to the top of the flange. A single air inlet fitting will be installed on the side of the manifold.
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).
Here is the new and improved (again) gasifier, all buttoned up and sealed with yet more great gobs of red silicone gasket material, ready for a test
run. I couldn't wait to try it. So even though it was threatening rain, I set up the gasifier outside and fired it up.
I finally found some wood pellets. On one of my trips to Arizona, I bought 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.
Now I have plenty of high quality fuel for testing out the new and improved (again) gasifier. Fortunately I drive
out to Arizona twice a year. So getting 80 pounds of wood pellets back home in my big truck was not a problem.
At last! The gasifier was working well. It is making lots of gas and hardly any tar. Everything was working great. The gasifier was
producing such a huge volume of gas, I decided I needed a better way of flaring it off. So I bodged together a
quick and dirty gas burner. 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 but the old stainless steel vegetable steamer I originally
used as a shaker grate 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'll need to increase the stack height since the heat from the burner is starting to cook the rubber and silicone parts a little.
I was just as pleased as could be with the performance of the gasifier. It was working great. The volume of gas it could produce was amazing. As I cranked up the flow of compressed air to the unit, the flow of flammable gas to the burner increased until I had a real flame thrower on my hands. The heat from the burner was so bad that it became difficult to approach the unit to make adjustments or shake the grate. I was frankly amazed and thrilled at the volume of gas my little gasifier could produce. This small unit should be able to power even a huge engine. I had succeeded far beyond my wildest dreams.
So now I had a working gasifier. Time to hook it up to an engine and start producing power, right? Well, not yet. The bright yellow color of the burning gas tells me that there is still a lot of tar in the gas. There is not nearly as much as in the beginning, with the older design, when tar would just run out of the pump and puddle around it, but there is still some tar in the gas, and tar is bad for engines. So I was determined to reduce the amount of tar being produced. Further research led me to believe that reducing the size of the constriction in the hearth might reduce the tar production. Most highly efficient gasifiers seem to have hearth constrictions that are on the order of about 1/3 the diameter of the reactor. Mine was closer to 1/2 the diameter.So I tore down the gasifier and welded in a new 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.
There is still some tar in the gas when burning wood pellets. I also see some ash
and the occasional spark come out of the burner. A look at the burner after a run shows some (not a lot) soot and coke-like material inside. All
this stuff will need to be filtered out before the gas gets piped into the intake of an engine, or the engine probably won't survive for long.
The valves would get gummed up and the cylinder walls would be scored. It would also be nice to cool the gas before sending it to the engine.
Cool gas is denser, and that means more gas could be pulled into the cylinder on each intake stroke. So I need to build a scrubber and cooler
for the gas.
So here is my initial rough sketch of of a scrubber cooler system. My idea here is to use a water spray to clean up and cool the gas.
The gas would move up a column packed with either stones or golf balls, against the flow of water. The purpose of the packing material
is to increase the wet surface area the gas is exposed to as it passes up the scrubber column. The more surface area, the easier the gas
can give up its heat and suspended particles to the water.
The results are in. And the answer is... It didn't work :-(
My future goals for the gasifier include of course getting it attached to and running an engine. Preferably an engine that would do something useful
like run a generator.
© Copyright 2010-2012 Michael Davis, All rights reserved.