Wood gasification
- Introduction
- What is gasification?
- Woodgas and engines
- Pros and cons
- The Imbert generator

The project

Wood gasification


The use of biomass for energy applications is hot. Numerous fermentation and gasification projects have been started recently, mostly stationary units to produce heat and power. For mobile applications the world has been already indulged with alternative fuels such as ethanol and biodiesel. Indulged, because we are talking about liquid alternative fuels that are as easy and convenient to use as petroleum products are.

No fuel can be stored or transported as easy and contains so much energy as a liquid fuel. This, until now, wide and cheap availability of fossil fuels and the dictatorship of big-oil, conspiring with the car manufacturers, have led to an enormous mobility and economic development in the world. An increase which can, and probably already is, turning against us.

After this short, cynical and philosophical sidetrack (I will annoy you again on several occasions), back to the business at hand.

Methane, or biogas as it is also called, is produced and compressed in a central fermentation or digester factory. Or made and used individually in a rural setting.  Fermentation is a slow process and can not be regulated for “on demand” use. For this reason it is difficult to use in mobile applications. Gasification of biomass, or wood, can be produced on demand on the vehicle. The internal combustion engine itself regulates the quantity of produced gas and the installation is relatively compact, at least compared to fermentation.

However, compared to modern gasoline cars, driving on woodgas is not very user-friendly. Start-up demands time. Driving requires the necessary knowledge and skills, because the engine power on woodgas is considerably lower and the gas quality never constant. It often needs refueling and the fuel size must fit the design of the gasifier. After the second world war, wood gasifiers  disappeared in a short time, because petroleum fuels once again became widely available and were much more convenient to use.

In the current era, mobile gasification will not be widely used. Not only because there is less wood available for drastic up-scaling, but the modern consumer does not want to give up his luxury and comfort. He will like the fact of cheap woodgas, but leaving his bed ten minutes earlier to fill up the tank, remove ash and condensate and start the generator will be too much to ask. Rather, the motorist pays considerably more for the convenience of fossil fuel, than give up his current lifestyle and status.

Status and the social rat race, fed by the mass bombardment of the media, paralyze the mental ability of the individual. Only when this individual can shake off status and excessive consumption, can he open his eyes for alternatives. Then he is ready for something like wood gasification. In a certain way real woodgassers are individualists and little anarchists. Like artists, at the same time admired and discredited.

Now you will start to understand why I have gone into this project. Also several personal items came together: idealism, interest, background and skills. What me and those few fellow-gassers find interesting, is the fact that a raw, but very accessible fuel can be used directly, without an extensive refinery. Dream about a journey around the world with only a hacksaw and an axe to produce the fuel .....

This is also a not unimportant aspect of wood as a fuel: self-sufficiency. The accessibility makes the user independent of monopolistic suppliers. 

Pictures of modern Finnish installations made of stainless steel, professionally built by amateurs, were enough to push me over. These units are more efficient and user-friendly than the installations from the second world war and manage chipped and relatively wet wood.

Despite the fact that most woodgassers are loners, it is not possible to manufacture everything by yourself. Motivation of other people, to successfully finish the project with their aid, without a financial catastrophe, appeared to be surprisingly simple. Driving on wood interests many people. When their eyes start to shine, it is clear that another soul is won…

Mobile gasification will not save the world. The installation is too complex, too large and because of that too expensive. Driving demands a mind-turn, knowledge and skill of the driver. Moreover most countries have insufficient wood to provide a national fleet with fuel.

It has always been, is now and will remain a method for crisis times and for idealists. 

What is gasification?

Gasification is a chemical process, where heat converts solid biomass into flammable gas. Thereby the gas can be burned immediately by adding secondary air, like in heating applications. The gas can also be fed to an IC (internal combustion) engine, when first cleaned and dewatered.

Gasification consists of four processes:
1. Drying. By using heat, water evaporates from the wood. This is good, because a surplus of water reduces the oxidation temperature, which doesn’t provide clean gas.
2. Pyrolis. Above 270 degrees Celsius the wood structure falls apart. Long molecules are made smaller. Charcoal and tarry gases appear.
3. Oxidation. Under supply of a measured quantity of air, a part of carbon oxidizes (burns) to carbon dioxide and hydrogen oxidizes to water. A lot of heat is released. This heat is necessary for:
4. Reduction. In the reduction area the most important conversions take place. Most of these reactions however ask energy. This energy has just been released in the oxidation zone, which reaches a temperature of 1400 degrees Celsius. Carbon reacts with carbon dioxide and converts it to carbon monoxide. Carbon also reacts with water, stealing an oxygen atom to convert it to carbon monoxide and hydrogen. These are the most important reactions. Furthermore carbon binds with hydrogen to create methane and carbon monoxide reacts with hydrogen to methane and water.

Oxidation, produces energy:

C + O2 <==> CO2

H2 + 0.5 O2 <==> H2O

Reduction, takes away energy:

C + CO2 <==> 2CO

C + H2O <==> CO + H2

CO2 + H2 <==> CO + H2O

C + 2H2 <==> CH4

CO + 3H2 <==> CH4 + H2O

Roughly woodgas consists of:
20% carbon monoxide CO
18% hydrogen H2
4% methane CH4
8% carbon dioxide CO2
50% nitrogen N2

The carbon dioxide and the nitrogen do not contribute to the combustion of the gas. The nitrogen is as a superfluous component led in by primary air for the oxidation. Large stationary installations partly use superheated steam in the process. This also brings in hydrogen, while useless nitrogen does not dilute the gas. For mobile installations steam is no realistic option, because the construction is complicated and therefore too heavy.

Woodgas and internal combustion engines

Woodgas is a low calorific gas, with little energy content  The nitrogen from the air does not contribute to combustion, and carbon monoxide is a slow burning gas. For combustion engines that means several disadvantages. Because the high proportion of nitrogen the engine gets an insufficient fuel. The fact that the fuel is present before the intake manifold as a gas and engine vacuum is necessary to create this gas, reduces the filling degree of the cylinders. All woodgas engines are asthma suffering patients with lung cancer.
These facts cost approximately 40% engine power. Moreover high engine speed is not possible, because the gas burns slowly. Ignition advance is necessary to allow time for complete combustion because woodgas has a slow flame front. At too high rpm’s, combustion has not yet finished, when the exhaust valve is already opening. The effective engine speed for car engines is limited to 3,000 rpm, despite of the fact that the rpm can go up higher on low loads.

Literature often shows a power decline of 40%. This would mean that an engine which provides 100 hp on petrol, should have a remaining 60 on woodgas. This is not correct. The engine has 60% left at 3000 rpm and therefore perhaps only 40 hp!

Ignition advance on modern engines with ECU’s is not easy for a layman. The engine control management can get confused and go into limp mode or stop entirely. It’s not just in society that managers do more harm that good…

Big, slow running engines with an old-fashioned, but electronic distributor, are best for woodgas.

Pros and cons of the different types of gas generators

We distinguish four types of gasifiers:
1. The fixed bed gasifier; simple for small applications such as camp-site burners. Some power plants work according to this principle.
2. The fluidized bed gasifier; generally very large installations. Mostly applied in electricity power plants.
3. The updraft and crossdraft gasifier; generally used to gasify charcoal or coals. Not applicable for IC engines, because of the high tar output. Frequently applied in central heating systems.
4. The downdraft gasifier. Very suitable for IC engines, because they are both compact and produce little tar. Tar has to pass the hot oxidation and reduction area and is cracked into useful gas. Tar is undesirable in woodgas, because it plugs filters, pollutes piping and sticks valves in the engine. Tar is cumbersome to remove and can cause damage, therefore prevention is preferred above healing.

We distinguish two types of downdraft gasifiers. The old faithful  Imbert, such as used in the second world war. And the Stratified Downdraft, a development from the eighties. The stratified is a very simple construction. It can consume different fuel sizes and compositions. One would be tempted to choose the stratified. However, an important disadvantage sticks to this gasifier. There are, as it happens, no nozzles for supply of primary air. By lacking a nozzle position, the oxidation and reduction area are not fixed, like in the Imbert. Too dry fuel can lead to burning up the bunker stock. Too wet fuel slows down the process, the result being that the reduction area ceases to exist and the production of flammable gas is interrupted. With water injection the process can be controlled. The tar production is high, unless charcoal is used as fuel. But the stratified can be made very simply. FEMA has plans on the internet, how to manufacture a gasifier with scrap materials such as oil drums, trashcans and simple tools. Nevertheless serious builders leave the stratified principle after time and embrace the Imbert.

I show a picture of a statified downdraft, but will spend no further time on it. The stratified downdraft is for me the proof that it is simple to make woodgas, but very difficult to produce tar free woodgas.

The Imbert gas generator

A bit of history:

The Imbert is an invention of Georges Imbert. He developed this principle in the 1920’s. During the second world war more than one million woodgas generators there were driving the European roads, practically all according to the Imbert principle. After the war petrol became widely and cheaply available again and motorists said farewell to the woodgasifiers. Knowledge and experience are mainly lost since that time.
During the cold war the Swedes developed the Imbert further, but besides documentation, little has been done with it. Sweden has virtually no fossil fuels, but sufficient wood, so wood gasification could solve their shipping problems in crisis times.
The energy crises in the seventies had again contributed to a small bounce, which sank as rapidly as it came.
Only in Finland has mobile wood gasification  never disappeared. However not on a large scale, or subsidized by the government, but kept alive by a group devotees. They refined the improvements of the Swedes.

The principle

The drawing shows how a World War 2 Imbert works.

The gasfier unit is filled with wood blocks. The nozzles meter the primary air. Engine vacuum ensures flow in the complete system. The engine speed regulates the quantity of produced gas and with that the quantity of primary air. At the shown generator, the gas is taken high out of the generator to allow dust to fall down. From there the gas passes through a cooler for dewatering and reducing the gas volume. By volume reducing and disposal of water, the energy content of the gas increases. After the filter train the gas is mixed with secondary air and fed to the engine. A blower is necessary to start up the gasification process.

Modern Imbert units work somehow different.

The generator has a condensing mantel around the fuel bunker in order to dry the fuel stock and remove redundant water. A cyclone removes the coarse ash and char particles. Glass-fiber socks or envelopes filter the fine dust out. The cooler drains the water from the gas. During start up, the fan blows the gas out before the filter train to prevent constriction of the filter by tar and water mist from the still cold reduction zone.
Compared to the original Imbert principle, many details are changed. Much attention has been given on preheating primary air, to raise the oxidation temperature. This however, creates new problems, because not all materials can resist the high temperatures. All in all rather more complex than in former days, but it results in tar free, clean gas.

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