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The Process of Gasification

This picture gives an overview of the entire process.

Ignition

Beginning with a 'start-up' charge of charcoal in the combustion zone of the hearth and air dry wood fuel piled on top, gasification proceeds as follows:

Along with a restricted flow of primary air drawn laterally into the hearth, a momentary flame enters to ignite the charcoal and within a minute, combustion begins.

Drying, Distillation and Carbonization

Heat radiating upward from the hearth drives moisture from the wood at a temperature of 100 C. and, as the charcoal beneath is consumed, the dried wood sinks closer to the source of heat. At this point, the temperature is between 250 and 450 C.

The carbon and volatile substances, distilled from the wood, may ignite, raising the temperature further.

At 600 C. the charred wood ignites, rapidly boosting temperature.

Oxidation and Combustion

A great deal of heat is produced in the system at this point; when the ambient air (oxygen) enters the combustion zone; contacts and reacts with incandescent charcoal to produce carbon monoxide, a "fuel gas" which diffuses back into the gasified spaces between the charcoal pieces where it oxidizes and burns to form carbon dioxide, a "flue gas"

As the carbon dioxide increases in the gasified spaces, a counterbalancing decrease in the free oxygen occurs in the same spaces and concentrations of both these gases approach equilibrium.

The carbon monoxide produced in the initial charcoal/air oxygen reaction quickly falls to a low concentration at which it is held in dynamic stability.

Simultaneously, steam from the drying wood has joined with the ambient air (oxygen) also to react with the incandescent charcoal where it decomposes to carbon monoxide and hydrogen, which together also diffuse back into the gasified spaces between the charcoal pieces where they, too, are oxidized and burn to form carbon dioxide and steam respectively.
The process temperature during the above reaction is 800 C. plus.

At this point, the heat requirements of the process balance the heat created. The flame temperature is at its maximum; around 1,100 C. and the oxygen is almost exhausted.

Reduction

At this stage in the process, the carbon dioxide and the steam in the gas spaces diffuse to, and react with, the hot charcoal in the "reduction zone" to produce carbon monoxide and hydrogen, the main fuel constituents of wood derived, 'Producer Gas'. These reactions consume both heat and charcoal.

As process temperature continues to decrease, the rates of these reactions also decrease, until, when the temperature falls to 700-800 C. and/or the hot charcoal supply is exhausted, the gasification process stops.

The main product gases; carbon monoxide and hydrogen, can be later piped away for use as fuel, producing flame temperatures of 2,100 and 1,960 C., respectively.

Summary

Although some stages of the process occur sequentially and others simultaneously, they are continuous and in equilibrium, while wood charcoal and limited "primary" ambient air (oxygen) are fed to the 'hearth' to sustain the process.

Except for the wood fuel drying stage, the other gasification processes occur within the reaction vessel called the 'hearth'.

Because of the high temperature plus the oxidizing and reducing atmospheres which must be developed and sustained within it, the 'hearth;' incorporates special refractory materials, surface treatment and thermal insulating materials, selectively placed to withstand the rigor of the main gasification processes for a prolonged period.