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Wood/Biomass Gasifier? Answered

I have always been interested in the principles behind gasification, but I have had some problems fully understanding it due to some differences in the devices I have seen.  I have noticed two real differences which I can only describe as closed and open systems.  Closed systems have some heat source which heats a closed container of wood, this wood is directly fed to an engine or flame.  No air can enter and their is no combustion prior to reaching a torch or the engine.  The open systems I have found to be more common but I can't really grasp how they work.  The fuel source is lit and then the smoke is captured, but this captured smoke is then somehow flammable.  I just find it difficult to understand how this can work in an open air-fed system.  Does anyone have more details on the differences between these two styles?


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Best Answer 7 years ago

If you understood how the closed system works, you are lucky... Because it is similar.

Immagine, you start with a closed system... The flammable gas goes to an engine...
Now the first alteration: You dont feed the gas to an engine but torch it as open flame.
OK. Now the second alteration: You bend the pipe to point under your closed container containing the wood. The gases are still burning.
Now you can switch of the first flame you used in the stage of the motor... Now the gases produced in the closed container heat the closed container.

Voilà! You just have opened the closed system. The main difference is the solving of the problem that you have to have a second and indipendent flame in the beginning with the constraption we made above.
This gets solved by the real open system: You fill a can with wood. you light it at the top and it starts to burn. Normally, you start a small fan which creates a underpressure in the can by the venturi-effect. Now the flames and heat get sucked down the can with wood and heating the wood underneth. Normally and if correctly done, the flames use up all the oxygen in the air so the heat which gets sucked down will be oxygen-less and thus gasify the wood instead of only burning it.
By gasification, you turn the wood to charcoal. This is you pyrolysis-zone.
As the charcoal at the top comes into contact with oxygen from the fresh air it will catch fire, produce heat for the underlying gasification and convert to Co2 and other byproducts. This is your burning-zone.

Now what happens with all the flamable gas you pull out off the bottom of the can? it normally gets fed upwards around the can and then ontop into the can. This, to use it partially as heating for the wood (gets sucked down, Burning) and partially to use it as output-heat upwards.

See http://www.envirotoons.com/page6.html for a good imaginery.


7 years ago

An air fed system is a fire. That's what burns in your fire place if you watch carefully - The wood is heated to the point where the flammable gas is released, this then burns. The wood has to be maintained above this temp to keep supplying the flammable gas otherwise it goes out and is reduced to glowing embers.

In a gas producer the wood should be cooked in a sealed compartment as you describe and the gas produced piped away for use.

From wikipedia

"Wood gas is a syngas fuel which can be used as a fuel for furnaces, stoves and vehicles in place of gasoline, diesel or other fuels. During the production process biomass or other carbon-containing materials are gasified within the oxygen-limited environment of a wood gas generator to produce hydrogen and carbon monoxide. These gases can then be burnt as a fuel within an oxygen rich environment to produce carbon dioxide, water and heat. In some gasifiers this process is preceded by pyrolysis, where the biomass or coal is first converted to char, releasing methane and tar rich in polycyclic aromatic hydrocarbons."