Generating Wood Gas

Let's start with the combustion process hardware first: The simplest device is a tank shaped like an inverted cone (a funnel). A hole at the top which can be sealed allows the user to load sawdust into the tank.

There is an outlet at the top to draw the wood gas off. At the bottom the point of the "funnel" is opened and this is where the burning takes place. Once loaded (the natural pack of the sawdust will keep it from falling out the bottom) the sawdust is lit from the bottom using a device such as a propane torch. The sawdust smoulders away.

The combustion is maintained by a source of vacuum applied to the outlet at the top, such as a squirrel cage blower or an internal combustion engine. Smoke is drawn up through the porous sawdust, being partly filtered in the process, and exits the burner at the top where it goes on to be further conditioned and filtered. The vacuum also draws air in to support the fire. This burner is crude and uncontrollable, especially as combustion nears the top of the sawdust pile.

This can happen rapidly since there is no control to assure that the sawdust burns evenly. "Leads" of fire can form in the sawdust reaching toward the top surface. Once the fire breaks through the top of the sawdust the vacuum applied to the burner will pull large amounts of air in supporting full combustion and leaning out the value of the producer gas as a fuel.

This process depends on the poor porosity of the sawdust to control the combustion air so chunk wood cannot be used since its much greater porosity would allow too much air in and you would achieve full combustion at very high temperatures rather than the smouldering and the the partial combustion you want. Such a burner is unsatisfactory for prolonged gas generation but it is cheap to build and it will work with a lot of fiddling.

For prolonged trouble free operation of a wood gas generator the burner unit must have more complete control of the combustion air and the fuel feed. There are various ways to do this. For example, if the point of our original funnel shaped burner is completely enclosed then control over the air entering the burner can be achieved.

This configuration will successfully burn much larger pieces of wood. One of the most widely known burners was developed by Mother Earth News magazine. They produced a complete set of plans for their burner and featured its construction and evaluation in several issues of their magazine.

Destructive Distillation Units

If the type of wood gas you would like to produce is to be obtained by destructive distillation then all you need is a canister about the size of a 55 gallon drum that can be loaded with wood and sealed air tight. It must also have an outlet pipe and a source of heat which can be applied to it from the outside. A simple way to do this would be to just build a wood bonfire under the canister. While this has the advantage of being "quick and dirty" it is the equivalent of the simple sawdust burner I described above. It allows little control and requires a lot of time and fussing to make anything work.

The use of a fossil fuel to heat the wood gives good control over the temperature of distillation but defeats the purpose of producing a fuel from wood. An ideal solution to this control problem is to use a combustion type wood gas generator to produce fuel gas, which is then burned to achieve destructive distillation, which produces a higher heat content fuel; a fuel that can then be liquified into methanol.
It is also desirable to mount the canister in such a way that it can be rotated while it is being heated so that the contents inside are evenly "cooked".

Conditioning And Filtration

However you produce the wood gas you are certainly not home free when your fuel exits your burner. Far from being nice clean chemical reactions, the fuel gas you will actually produce will contain ash, gum, heat, water, creosote, acids, and a lot of other weird things which will render any device you try to burn the fuel in dead on arrival. The fuel must be conditioned and filtered before it can be used.

Cyclone separator
Probably the best first step to achieve gas conditioning is a cyclone separator to get rid of water and particulate matter. This device is again an inverted cone but the bottom is sealed. After the contaminated fuel gas leaves the burner unit it enters the top on a tangent to the circumference of the side.

The exit for the fuel is a pipe which runs up the center of this cone from just above the point at the bottom. Gas coming into the cyclone separator enters with some velocity. Because it has weight and therefore inertia it tries to continue in a straight line even though the sides of the cone are curved. To exit the cone the gas would have to turn 90° toward the bottom and then 180° up the exit pipe if it could take the shortest path.

However under the influence of its own inertia the dirty fuel gas is held against the curved outside wall of the cone, where it takes a circular path toward the bottom and the exit pipe. It circles around and around this wall, all the time moving in tighter and tighter circles toward the point on the bottom, thus speeding up its velocity as the circles get smaller nearer the point. The fuel gas is spinning quite fast near the exit pipe. This rotary gas flow is where the separator gets its name.

Within this stream of rapidly spinning gas the ash and water vapor, weighing more than the fuel gas, are thrown outward against the walls of the separator by centrifugal force. When the heavier components in the fuel gas come into contact with the relatively cool sides of the separator the water condenses, wetting the inside of the separator down and flowing downward to a drain at the bottom.

Ash and particulate matter are also thrown out against the now wet walls where they get washed out of the gas stream by the condensing liquid. All of this gunk winds up running out the bottom of the separator and into a holding tank, which can be drained at intervals.

To help the water condense out of the gas stream the walls of the cyclone separator may be cooled by water or an air stream to raise the efficiency of the condensation and scrubbing process. Upon its exit from the cyclone separator the fuel gas will have to be cooled further. A simple way to do this is to place a vehicle radiator into the gas flow and draw the fuel gas through this.

Water vapour
In a perfect world there wouldn't be any water left in the fuel gas after it left the cyclone separator. Unfortunately there is apt to be quite a bit of uncondensed water vapour still left in the hot fuel gas as it enters the gas cooler.

As the gas cools the water vapour will condense out further. It may be necessary to use multiple stages to completely cool the fuel gas. That is more than one radiator, with the outlet of one connected to the inlet of the next, and so on.

Cooling air can be blown through each radiator by an electric cooling fan and each radiator will need a sealed drain canister to carry off condensed liquid.
Once the cool dry fuel gas leaves the cooler it is ready for the last stage of conditioning. It must be well filtered to keep fine ash and particulate matter from being carried into the engine or burner which the gas will be used in.

Gas Storage
An alternative to immediate use of fuel gas is storage. The gas can be stored at low pressures, 3 to 5 psi, in a simple tank of the type developed to store biogas from methane/solid waste digester. This is an open tank placed upside down in water. When empty, the tank sinks in the water but as gas is forced in the tank rises out of the water. The weight of the tank can be augmented or counterbalanced to control the pressure of the gas inside.

Fuel gas is pulled from the gas generator by a simple centrifugal bower, which forces it through the conditioning stages and into the storage tank under pressure. It is desirable (but not necessary) to have a storage tank even if the gas is to be immediately used as fuel. Such a reservoir maintains a buffer supply of gas to even out any fluctuations in supply and provides an even, controllable supply of pressure to the end device.

It also acts as a cooler and a sink for any impurities which remain in the fuel gas after conditioning. Such a storage tank could pay for itself by saving money on conditioning the fuel gas since contaminants will cool, condense, and settle out in the storage tank and water seal.

Two tanks, used alternately, may reduce the need to condition the gas, requiring only the use of the cyclone separator. In this arrangement one tank can be filled and cooling off while the user device is run from the second tank.

If your choice is to produce methane and methyl gas rather than producer gas, then the conditioning process will not be so demanding. You will not have a lot of ash and carbon to deal with. The products of destructive distillation will exit the generator canister under their own pressure.

If a blower is used to fill a gas storage tank it should be placed after the gas cooling stages are held at a partial vacuum as condensation occurs, this will improve their efficiency. The final particulate filter can be eliminated, especially if gas storage tanks are utilized.

If your goal is to produce methyl alcohol (methanol) then you will have to catalyze the methyl gas and methane into liquid alcohol. In addition, you will not want to use a cyclone separator since the condensed liquid contains the methanol you are seeking.

Gas coming from the generator canister will pass through a catalyzer unit. The alcohol vapor produced by catalyzation is then routed to the gas cooling stages which condense it to liquid methanol. In this case the drains from the cooler carry off the raw methanol distillate rather than waste condensate. This raw methanol will undoubtably require further distillation to purify it and extract excess water.

Catalyzing the methane and methyl gas
This can be accomplished crudely but simply. The gas which comes out of the generator canister is routed through a copper pipe with a copper pot scrubber inserted in it. A one inch copper pipe about twenty feet long attached directly to the generator outlet and having the pot scrubber inserted into it loosely should do the job.

The inlet of the pipe should be above the outlet so that any methanol condensing in the catalyzer can drain out. Insulating the catalyzer to keep the temperature up will help greatly. Soft copper pipe formed into a large coil makes a relatively compact catalyzer. The outlet from the catalyzer is attached to the cooling (condenser) stages.

The use a copper catalyst (the pot scrubberl) suggested, because of its availability and cost but it is not the most efficient catalyst. It may well be that a platinum catalyst would be much more efficient but a custom built platinum catalyst would be very expensive. With the common availability of platinum based automotive catalytic converters, use of one of these converters may be a better alternative than that described.