Wood gas (also called producer gas) is an alternative fuel, which can be extracted from any biomass with the help of a device called gasifier or wood gas generator. The wood gas can have multiple applications ranging from cooking food to driving an internal combustion engine and electricity generation. This article will concentrate on the explanation of the processes involved in the generation of the wood gas, as well as on the it’s possible uses, while providing references and information that can be used to build a wood gas generator.
The first wood gasifier was built during the 19th century and in the beginning of the 20th century the wood gas was distributed to homes for the purpose of lighting, heating and cooking. After being gradually replaced by the natural gas, the wood gas production and usage faded until the World War II, when the shortage of petrol and it’s derivatives increased the interest in wood gas as an alternative fuel for internal combustion engines. Sweden and Finland are the countries where wood gas powered vehicles were most widely spread. After the war, Sweden has continued it’s research in the feasibility of using wood gas as a substitute of petrol, while trying to limit it’s dependance on the imported oil.
Apart from transportation, the wood gas was successfully used in some factories for electricity generation. Results have show that such installations have a good reliability and are able to produce electricity at very low cost, especially when the owner of the installation has access to cheap wood by-products or scraps. Other more basic uses of the wood gas include heating and cooking food in the so called wood gas stoves. It has to be noted though that wooden gas contains high concentrations of carbon monoxide which is poisonous and therefore caution and strict safety measures should be exercised at all times.
Try to light a match and observe how it burns from a close distance. You will notice that it is not the actual wood that is burning, but instead some kind of gas is released, which is actually the thing that is burning. This single match demonstrates the whole process:
- When lighted, the head of the match burns providing intense heat
- This heat is applied to the wood stick, which releases the wood gas
- The wood gas burns with flame which in turn heats the rest of the wood, providing more wood gas
- At the end the whole wood stick is converted to ashes
The actual chemical process that takes place has the following phases:
- Drying – At this phase additional moisture in the wood is vaporized.
- Pyrolis – When heated to temperatures around 300 C and above, the wood starts to transform into charcoal while releasing hydrogen and tarry gases.
- Oxidation – Parts of the released carbon and the hydrogen oxidize to carbon dioxide and water (C + O2 => CO2; 2 H2 + O2 => 2 H2O). In the process a lot of energy is released in the form of heat which usually reaches temperatures above 1000 C.
- Reduction– This process uses the heat energy generated during the previous phase and the following chemical reaction occur:
- Carbon reacts with carbon dioxide resulting in carbon monoxide (C + CO2 => 2 CO)
- Carbon also reacts with water resulting in carbon monoxide and hydrogen (C + H2O => CO + H2)
- Carbon reacts with hydrogen resulting in methane (C + 2 H2 => CH4)
- Carbon monoxide reacts with hydrogen and converts to methane and water (CO + 3 H2 => CH4 + H2O)
During the process, the nitrogen contained in the air does not react and stays unchanged. The end result of the process is a heated gas that has the following composition:
- 4% Methane (CH4)
- 8% Carbon dioxide (CO2)
- 18% Hydrogen (H2)
- 20% Carbon monoxide (CO)
- 50% Nitrogen (N2)
While many different designs have seen the light of the day, the following major types exist:
Updraft – In this type of gasifier the air is introduced in the bottom part, where the partial oxidation also occurs. The air flow is opposite to that of the fuel, which leads to the following pros and cons:
- The gas has a significant amount of tar and therefore is unsuitable for use in internal combustion engine
- The gas contains more energy, partially due to the increased content for tar in the raw gas
- The produced gas is cooler at the exit of the gasifier compared to other design, as it passes through the fuel and reduces it temperature
- While the produced gas passes through the fuel it also dries it out and reduces the percentage of water contained inside
- This type of gasifier is generally harder to load with fuel, as it has to be loaded from the same zone where the gas exits the gasifier
Downdraft – In this type of gasifier the flow of air is in the same direction as the flow of the fuel, and this is downwards. This design has the following features:
- While tar is not completely eliminated, it is contained in much lower quantities in the produced gas.
- The temperature of the produced gas is much higher compared to the updraft design, because the gas exits the gasifier right after it has been heated during the oxidation phase.
- The generated gas can be used in an internal combustion engine, however it needs additional cooling to bring it to temperatures at around 70 C and filtration to free it from ash and tar. The recommended process is to have the gas cleaned through a water filter, then cooled and then additionally filtered through a finer mechanical filter.
- The resulting gas contains less energy when compared to the updraft design and also tends to contain more moisture.
There are other designs such as the twin-fire and crossdraft gasifiers, however they are either derived from the designs described above or they are used in much larger scale of electricity or thermal energy production, which is outside the scope of this article. We have to mention though the the downdraft gas generator has two distinct designs suitable for building at home and these are the Imbert and the Startified downdraft gasifiers. The Imbert gasifier was the design that was used with great success during the World War II and therefore is extensively tested and has proven to be reliable.
The Startified downdraft gasifier is developed by the Federal Emergency Management Agency in the US and is much easier to build than the Imbert gasifier, therefore ideally suitable for fuel generation in a crisis. Still, the Startified downdraft design has one major disadvantage and that is the lack of a fixed oxidation and reduction zones, which is a consequence of the lack of fixed air nozzles. This can lead to the expansion of the oxidation zone when the fuel is to dry, or to the disappearance of the reduction zone when the fuel has too much moisture, which will essentially cut the gas production.
Plans, links and additional information
While there is much information about the wood gas and the gasifiers in the Internet, it is rarely organized and easy to understand. Below you can find different research documents and plans for building gasifiers, which will help you to further extend your knowledge about wood gas and gas generators, as well as useful sites with more information:
- Federal Emergency Management Agency’s Startified downdraft gasifier plans (PDF)
- The Forestry Department at the Food and Agriculture Organization at the United Nations, Wood Gas as Engine Fuel research paper (PDF)
- The Word Bank Technical Paper Number 296, A Global Review of Small-Scale Biomass Gasifiers for Heat and Power (PDF)
- The Gasifier Experimenters Kit Wiki Page – http://wiki.gekgasifier.com
Time and effort saving ideas
- Using a fraction of the wood gas for cooking, only when that is necessary
- Running a gas turbine, which powers a generator which generates electricity for lighting and other electronics devices, and charges batteries when there is no demand
- Uses the exhaust heat for providing hot water for washing, bathing and heating during the winter
- Also uses the exhaust heat in an absorption refrigerator for chilling food and air conditioning during the summer
Doesn’t get any better than that, does it?