How Gas Is Made

 

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

Along with the restricted supply of primary air admitted laterally into the charcoal bed, through 8 nozzles, a momentary flame enters to ignite the charcoal and within one minute, combustion is established.

Heat radiating upward from the combustion zone, drives moisture from the wood fuel and as charcoal is consumed beneath, the dried wood sinks closer to the source of heat. At this point, the higher temperature promotes the distillation of volatile substances from the wood which then carbonises to charcoal. In the lower levels of this carbonisation zone some of the volatile substances may ignite, increasing the temperature further and at about 600°C the charcoal ignites, producing a much more rapid temperature rise.

Much heat is produced by the process at this level when oxygen in the air admitted to the combustion zone, contacts and reacts with the incandescent charcoal surfaces to produce carbon monoxide. Carbon monoxide diffuses back into the gas-filled spaces between the charcoal pieces where it oxidises to form carbon dioxide. This reaction all takes place in the oxidation zone.

As the amount of carbon dioxide in the gas spaces builds up, there is a corresponding reduction in the amount of free oxygen available in the gas spaces and concentrations of both these gases will approach constant levels. At this point in the system, the carbon monoxide concentration reaches a constant low level which is maintained.

Simultaneously, steam also contacts and reacts with the incandescent charcoal surfaces to produce carbon monoxide and hydrogen. A little carbon dioxide may be produced. The additional carbon monoxide and the hydrogen diffuse back into the gas spaces where they are oxidised to form carbon dioxide and steam respectively. The net effect of the steam reaction is to supplement the carbon-oxygen reaction. Provided it is not excessive, the presence of steam confers the same result as an increase in the available oxygen. (Air dried wood waste and crop residues generate enough steam internally by chemical breakdown to promote the benefits of the steam reaction. Additional moisture can only be detrimental).

As the reactions continue, the amount of oxygen available for combustion decreases until eventually the amount of heat produced at this level in the system balances the amount of heat produced needed to run it. Here the flame temperature reaches its maximum and the amount of oxygen remaining is very small. This is the throat of the hearth and is where residual tar is cracked.

Much heat is consumed by the process below this level which is referred to as the reduction zone. After the maximum flame temperature has been reached and practically all the oxygen consumed, the temperature in the system begins to fall because less heat is being produced than is required to run it. At this stage, the carbon dioxide and steam in the gas spaces penetrates to and reacts with the hot charcoal surfaces to produce carbon monoxide and hydrogen, drawing heat from the charcoal bed in the course of the reactions. As the system temperature continues to decrease, the rates of these reactions also decrease. until when the temperature falls sufficiently or when the charcoal supply is exhausted, the gasification process stops.

Except for the wood fuel drying phase, the other gasification processes described occur within the reaction vessel which we call the 'hearth module' and because of the high temperature, oxidising and reducing atmospheres which must be developed and sustained within it, the hearth module incorporates special refractory materials, surface treatment and thermal insulating materials, selectively placed to withstand the rigours of the main gasification processes.

The Gas. The product gas leaves the gasifier hot, dirty and moist to undergo the degree of cleaning, cooling and drying appropriate to the end-use intended for it. Wood derived producer gas from the Pacific Class gasifier is comprised as follows:

Combustible fuel gases 20% carbon monoxide
19% hydrogen
1% methane
0.1% ethylene
Non combustible gases 9% carbon dioxide
51% nitrogen


For practical purposes the gas is given an average calorific value of 5.03MJ/Cu.m (135 BTU/Cu.ft). Calorific value is subject to wide variations due to the moisture content of the fuel, and its correct size preparation.

Remember: The drier the fuel, better the gas.
  Correct fuel preparation, higher operating efficiencies.