Gasification is a process where substances such as wood, corn cobs, coal, cow manure, peach-pits, and other dried biomasses are reacted (burned). The process takes place in a device called a gasifier or gas producer, which can range in size, shape, and design, depending on application or design year. Instead of burning fast and bright like a campfire, the fuel in the gasifier has limited oxygen and separates the combustion of the fuel in multiple stages. The lack of oxygen and slow burn rate encourage the fuel to release flammable gases like methane, carbon monoxide, and hydrogen. Yes that's right, Hydrogen. Many of us think of hydrogen fuel as a new technology, but in reality its origins can be found as far back as the seventeenth century. Nonflammable gases such as carbon dioxide and nitrogen are also released. The syngas is then treated, and treatment methods vary depending on use. Typically the syngas is filtered, cooled, and moisture content is condensed out, but the order in which this is executed varies. The cleaned syngas can then be used in a number of ways:
- ICE (internal combustion engine), possibly installed in an automobile or
- Gas Turbine, predominantly used for medium to large power plants
- Open flame, possibly used for cooking or heating appliance.
A well designed and fueled gasifier will have efficiencies ranging from 40-80%, which is a good value, but this percentage only factors "energy-in versus energy-out" of the gasifier itself. The device which combusts the syngas for its end use has its own efficiency, and combining this efficiency with that of the gasifier results in a lower overall efficiency number. Evaluating the energy-in versus energy-out of your total system is very important, especially when searching for areas in need of improvement. A common technique to improve efficiency is cogeneration (CHP, or "combined heat and power"). Cogeneration is the reclamation of what would otherwise be wasted heat, taking the excess heat from one or more of the following components: the gasifier itself, cooling components, the internal combustion engine, the gas turbine, or the electric generator. This reclaimed thermal energy can then be used to dry the fuel, for space heating, or even for cooling and refrigeration.
Syngas in Internal Combustion Engines (e.g.- automobiles, tractors, generators):
Here we will go through the basic operation of a gasification system for an engine. Syngas can be created by any gasifier of your choice; for a passenger vehicle, one would typically use an imbert style gasifier.
In a normal imbert gasifier, the fuel is loaded into a bunker section which may feed the fuel by gravity or, alternatively, deliver it to the gasifier with a feed system (such as an auger). The gas exiting the gasifier may immediately enter a cyclone filter. This a very effective way to eliminate large particulates. Gas cooling removes moisture from the gas by condensation, and gas temperatures below 100 degrees Fahrenheit are important for adequate engine performance. Also, cooler gases mix more readily with outside air which is important when you need to adjust the fuel-air mixture.
Gas filtering is vital to protect your engine. Some filters clean hot gas, while others clean the gas after it has cooled. There can be single or multiple stages but when it comes to gas filtering, cost and simplicity are very important. Some common filters use hay or wood chips as the filter material. The syngas is usually pulled out of the gasifier and through the system by the engine's own vacuum. This vacuum provides the negative pressure for the gasifier's inlet air.
Before the syngas line reaches the engine, it is mixed with outside air to obtain the correct fuel air ratio. Typically, the optimal ratio is about 50:50. This mixture is closely regulated by the operator to maintain an ideal ratio of combustible gasses to outside air, as a mixture too rich or too lean will in either case stall the engine. Some of the more modern systems adjust the fuel air mixture automatically with a control system, typically using an oxygen sensor. After it is mixed, the gas/air combination enters the engine and is ignited by the spark plugs just as gasoline vapors would be. Usually the ignition timing of the engine is advanced to take advantage of the higher octane fuel. Engines running on syngas have a reduced power output compared to gasoline by about 30-50%. This reduced power can be avoided and power can even be gained through the used of forced induction methods like turbocharging and supercharging.
A few types of Gasifiers
-Down draft or Imbert
There have been many variations on these designs, some with very surprising results. The main reason for these gasifier modifications is the need for optimal results with a particular fuel or gas requirement.
For internal combustion use, the Imbert gasifier is ideal, as it produces the least amount of tar and particulates. The operation of the Imbert will be described in detail here, although other gasifier types work on similar principles.
Operation process of an Imbert:
A. The upper section is the Fuel Bunker section, which receives some residual heat from the lower zones and also serves as a drying zone (FAO).
B. Then comes the Pyrolysis Zone where fuel begins to burn, break down and produce small amounts of combustible gas (FAO), ie: methane and hydrogen, along with inert gases (mostly nitrogen). This mixture is then pulled down to the next process.
C. The main “gasification” component happens in the Oxidation Zone; this is where outside air containing the key component of oxygen is introduced. This is usually done through the use of nozzles, sized to match the engine and to obtain the right intake velocity. This process typically takes place at temperatures ranging from 1832°-2732°F (1000°-1500°C)(FAO). The resulting hot gases are then pulled through to the next zone.
D. The Reduction Zone: this happens just as the name implies, with a restricted opening at the bottom of the hearth, this is sized to the engine and/or fuel types. The charcoal is converted into gas and breaks down into ash and smaller coals.
E. The ash and coals drop down to the bottom of the gasifier. This area usually employs a grate to help further break down any coals that pulled through the reduction zone.
F. The resulting gas from the processes explained above are pulled up and around the inner section/tank to a gas outlet, typically located toward the top. By extracting the gases higher up, results in a larger quantity of particulates that settle to the bottom and it also helps preheat the inner tank.
Forestry Department. “Woodgas as engine fuel.”, FAO 1986
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For more information on Wood Gas and how it can run your car, please check out these links:
Wikipedia: Wood Gas
Mother Earth News: Woodburning Truck
The Gengas Page
Around Sweden with Wood in the Tank
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Casey's Woodgas Truck Videos
Living Off the Grid in Texas