Wood Gasification for Power Production

Power Producing Hardware

The fuel gases generated from wood can be used as any other gaseous fuel; either burned in an external combustion burner or used as fuel for an internal combustion engine.

Two further considerations are whether the fuel will be used directly as it is produced from the generator or whether the gas will be stored and then used.

Producer gas is totally unsuitable for domestic indoor gas use. If you intend to use fuel gas for this purpose the warning must be reeated about the deadly nature of producer gas due to its carbon monoxide content.

Use of fuel gas in an external burner:
Gas produced through destructive distillation is useful for this purpose. Since it is methane it is similar to natural gas but unlike natural gas it does not have fractions of the heavier petroleum gases such as propane and butane to fortify the heat content.

Being a relatively low heat content fuel Fuel gas obtained from destructive distillation requires larger quantities to do the same job. Because of this the burner has to be designed to flow larger quantities of gas.

In some cases regular gas ovens and burners will work satisfactory since these are usually adjustable so that they can burn either dense LP gas or lighter natural gas. The range of this adjustment may include that necessary for wood gas. Since wood gas is similar to biogas, the user will find useful information on burners in biogas litreature, which is more generally available than that on wood gas.

Gas from destructive distillation burns very cleanly, so clean in fact you may not realize it is burning. In daylight the flame will be almost invisible under the right conditions, The user is cautioned to consider this as a possible safety hazard.

The Internal Combustion Engine
Wood gas is probably most valuable as a fuel for the internal combustion engine and here both types of wood gas will work well.

On the plus side wood gas burns cleanly in a regular internal combustion engine and as a fuel it has a high "octane" rating or resistance to preignition inside the cylinder. On the minus side it is lower in heat value than other fuels.

The significance of this is that wood gas can be burned in almost any engine from small generator or pump engines, car engines and even diesel engines.

The technique for using gas directly from a generator is considerably different from what's required to use fuel from a storage tank. ;;;This is a special case of the general information.

The high "octane" rating may only have significance to the dedicated wood gas user so I will come back to that after covering the disadvantages of the lower caloric value of the fuel.

EDIT BELOW

An internal combustion engine is basically an air pump.

The physical dimensions of the inside of the engine limit the maximum amount of air that can move through it.

There is no such limitation to the amount of fuel that can be added to the air.

So the trick to getting more power out of an engine is to get it to pump more air.

However the air and fuel must burn to produce any power and this will only happen within a narrow range of air and fuel mixture ratios (by weight).

With a gasoline engine the air/fuel ratio is about 14 1/2 to 1, air to fuel. Reduce the quantity of fuel and ignition of the mixture in the cylinder will soon stop.

If you increase the fuel beyond the ideal air/fuel ratio all of the air in the cylinder will burn before all of the fuel can burn. This will waste fuel.

Thus an internal combustion engine is limited in output by the amount of air it can "swallow" and this also limits the amount of fuel that can be taken in.

When burning wood gas there is also an ideal ratio of air to fuel but the limiting factor is still the amount of air the engine can pump.

You have a fuel that is lower in heat value than other fuels and cannot produce as much power per pound.

The net result is that an engine running on wood gas produces less power at the same speed (pumping the same amount of air). It is a loss of about 20%.

The engine will only produce about 80% of the power it would if run on gasoline.

As an example, if we were burning wood gas in a 3 1/2 horse power engine we could only count on getting 2.8 horse power out of that engine.

You would need an engine capable of developing 4.2 horse power on gasoline to give you the same 3 1/2 horse power output.

For practical purposes its an easy matter to use a 5 horse power engine instead of a 3 1/2 horse power engine, but the necessity for this must be kept in mind.

Another "drawback" of wood gas is its lack of lead.

I consider this an asset but some older engines won't.

The advantage of fuel gas is that its a relatively safe low tech fuel that can be fed to any old cheap chunk of junk yard iron to develop power.

But this older iron was developed with soft valves that depended on tetra-ethel lead to lubricate them.

Use with fuel gas will likely cause these valves to burn and the engine will lose compression. The solution here is to try to find an engine designed for low lead gasoline or for natural or LP gas fuels.

These engines will have hardened valves and seats.

Wherever possible the wood gas user is advised to use already existing controls to adapt an engine to wood gas use.

Gas supplied at pressures of up to 5 psi is close to the pressures used to distribute natural gas.

Where it's available natural gas has been in use for many years running industrial and irrigation engines.

Controls for supply and mixture of natural gas are readily available and can be adapted with very little trouble.

This presupposes that the fuel gas is held in a tank and supplied to the engine at a constant density and pressure.

In the case where producer gas is consumed directly from the gas generator the pressure and density are controlled by the engine itself through engine vacuum.

Typically, in a direct installation, gas flow from the burner to the intake manifold is sustained by the running engine.

Engine manifold vacuum pulls the "smoke" through the burner and draws combustion air in, it draws circulating fuel gas through the cyclone separator, through the gas cooling stages, and through the final filter into the engine.

Because of this the engine must be developing vacuum (pumping air) before the gas generator can work.

But the engine must have fuel before it can be run to produce a vacuum.

The solution to this "chicken or egg" quandary can be solved several ways.

An electric powered blower can be installed in the induction tubing just after the burner outlet.

This will provide vacuum to start the burner and will feed fuel gas to the engine so that it can be started and run.

The disadvantages are that the blower will be subject to the high heat and unconditioned gas leaving the generator.

This will destroy the blower.

Moving the blower downstream to the other side of the gas cooler might seem to be the solution here but you will find that the low vacuum which a standard squirrel cage blower can generate is not going to be enough to efficiently draw fuel gas through the obstructions of the cyclone separator and the gas cooling stages.

Use of a high speed, high efficiency centrifugal blower of the type used on a blacksmith's forge will partly overcome these restrictions. This is the type of blower you will need if you want to fill a storage tank.

Of course, the user can try and get the fuel in the gas generator burning and then just crank the engine over with the starter until the draw from it pulls enough fuel gas in to start it. You will find that this takes quite a while though.

It will require a large battery capacity and will result in the eventual destruction of the electrical starter through overheating.

Another method, used successfully , requires starting the engine on gasoline and then switching over to fuel gas on the fly.

One way that this can be done is by piping the fuel gas into the top of the carburetor on the running engine.

Gasoline is shut off to the carburetor and fuel gas, mixed with air, takes over running the engine.

This sounds easy but getting the transition to occur smoothly between fuels (to occur at all) is very difficult.

The Mother Earth method of accomplishing this was to place a "T" in the induction system.

The carburetor was mounted on one side of the "T" and fuel gas was piped to the other.

A flapper valve was placed at the junction of the "T" so it could select one fuel system or the other.

This method works smoothly but requires a throttle body in the fuel gas system in addition to the one in the carburation system.

The single advantage to piping the wood gas into the top of an already existing carburetor is that speed regulation of the engine is done with the existing hardware of the carburetor itself, thus it could be used with an existing machine (generator, pump, tractor, etc.) without modification.

All of these starting methods have specific drawbacks.

The Pony Engine
The method I have finally adopted to get around these disadvantages is to use a small gasoline engine which can be connected and disconnected to the main engine with a clutch.

The drawback here is that it is not a compact method and is more useful with a stationary engine where space and weight are unimportant. Using this method the small engine can be started and used to bring the main engine up to speed with the ignition shut off.

This has the advantage of prelubing the engine with oil before a load is applied and the main engine can be motored over for as long as it takes to light the burner, draw in fuel gas, and allow the temperature and density of the fuel to stabilize.

A flick of the ignition switch will then start the main engine and the "pony" engine is disengaged.

I used a gasoline engine as a "pony" engine because I was experimenting with a 350 cubic inch Chevy V8. If the engine is small enough it could be motored over using an AC or DC motor and either power line or battery current.

The pony engine method of starting works well with a stationary engine or where space or weight are not a problem. Where these are considerations the dual fuel approach is worth the effort.

A diesel engine can also be run off wood gas with almost no modification. In this case the engine will be started and run up on diesel fuel.

The fuel gas supply will require the usual air mixer and a throttle body before entering the induction of the engine. If constant speed is your goal you will will also have to add an external governor to control the fuel gas throttle body.

Once started and switched over to wood gas the diesel throttle is set at idle and the small amount of fuel injected is used to ignite the air/fuel gas mixture in the cylinders. Speed is controlled by the fuel gas throttle body.

I mentioned earlier that wood gas had a high resistance to "knock" or a high "octane" rating. This has some interesting ramifications for someone who is not content to put up with a reduction of power while burning wood gas.

Modern gasolines will tolerate a compression ratio of around 9 to 1 before the engine starts to knock and self destruct.

Wood gas and alcohol will tolerate compression ratios up to 13 to 1 before this happens.

So it would be relatively easy to build an engine based on a high output modern design, using racing parts, which would recapture all of the lost horse power that wood gas costs through higher efficiencies.

The efficiency figure of 28% for a gasoline I gave earlier is not immutable.

A crude gasoline engine might only get 20% efficiency while some all out racing engines would get as high as 50%. The typical modern engine with overhead cam, fuel injection, and electronic ignition and control systems will get in the neighborhood of 32%.

If your philosophy of power production justifies the expenditure this may be a productive path.

Methyl alcohol is the last fuel I will discuss.

Racing engines have been run on alcohol for a long time.

The now obsolete Indianapolis 500 engine known as the Meyers Drake "Offy" was a four cylinder engine of about 155 cubic inches which routinely produced 750 horse power on alcohol.

An engine burning alcohol fuel has a very high knock resistance and so can use the highest compression ratios to improve efficiency.

On the other hand methanol requires a lot of heat to evaporate it into a vapor, this makes an engine very hard to start. In addition methanol has much less heat content than gasoline so it requires much more fuel to do the same work.

This means custom jetting carburetors for alcohol -- beyond the range which their manufacturers designed them for.

On top of that alcohol is very corrosive and will damage metals, gaskets, fibre parts in carburetors, and hoses not specifically designed to handle it.

It is also one of the cleanest burning fuels available. It is quite possible to have a roaring alcohol fire and not even be able to see the flames in the daylight.

So methanol is a mixed bag.

It's available in quantity but you will need a lot of fairly sophisticated mechanical knowledge to make use of it reliably; more than this article can cover.

The most direct applications for wood gas are straight-forward.

For example, running a small gas powered generator to charge your battery bank. But consider some other ideas:
A removable flexible hose carrying wood gas to the intake of your farm tractor so that PTO-driven implements like saw mill, chippers, composting hammer mills, and large electric generators could be driven on wood gas without any change in the farm tractor.

A large 350 cubic inch V8 engine running at 1900 to 2000 rpm could be used to directly drive a large three phase motor.

The imposed 60 hertz line frequency would make this overdriven motor act like a synchronous generator, feeding electricity back into the grid and earning income as a small power station.

Installation of a large V8 engine and truck transmission in a bulldozer chassis, along with the wood gas generator and filters.

By running in low gear this engine will directly replace the slow speed diesel which normally powers such a machine. I have already built this machine and it is practical.

And of course, as Mother Earth News proved, a wood gas generator can be used to power a car or truck. In fact a lot of Europe ran on wood gas during World War II fuel shortages.
With modest mechanical skills this is a technology you can use now, and it's far safer than steam.