What are the Criteria for Complete Combustion?

In order to achieve "complete combustion" of any fuel, several important criteria must be met.  Among these combustion criteria are time, temperature, turbulence, and correct fuel/air ratio.  In other words, fuel must be changed to a gaseous form, raised to an optimum temperature for the type of fuel used, and mixed with sufficient oxygen to produce complete oxidation and destruction of any intermediate compounds which may be harmful or undesirable.  The non-combustible portion of the fuel (ash) must also be separated and prevented from being discharged out the exhaust stack.  The combined quantity of combustible and non-combustible matter released from the exhaust stack is defined as the "particulate level", expressed as grains/DSCF or mg/m3.  If the particulate level is to be kept to a minimum, then the combustible portion must be eliminated by achieving 100% oxidation.

How Does the Wood Gun™ Wood Gasification Process Work?

Initial pyrolysis

In the Wood Gun™, the bottom of the fuel chamber is lined with sections of uniquely shaped refractory castings that are made of unique materials which create a combustion zone linked to the fuel chamber by a series of openings.  The wood gases produced from the pyrolysis of the fuel charge are drawn down into the combustion chamber by the patented draft-inducer.  Heat generated in the combustion chamber radiates throughout the refractory mass, heating the fuel charge above.

The Time Element in Wood Gasification

The time element is met by a combustion chamber large enough to provide sufficient residence time and turbulence to ensure thorough mixing of the combustibles with oxygen.  This characteristic is referred to as "detention" time, and is defined as the time necessary for the flame to pass from point of entry of secondary air until it contacts the water wall of the boiler.

Proper Oxygen Mix in Wood Gasification

In order to ensure that all combustible particles come in contact with an oxygen molecule, there must be an excess of oxygen present.  The amount of oxygen over and above that which is needed to combine with the combustible molecules is defined as "exess" air.  For proper wood combustion, 50% - 75% excess air is considered ideal.  Levels less or greater than this can adversely affect combustion.

The Result

100% oxidation of the pyrolized material with absolute minimum particulates, which are trapped before emission, yielding:

• Longer boiler life with minimum maintenance

• Complete combustion and energy conversion

• Many advantages over standard updraft boilers including:  longer burn times; uses up to 1/3 less wood; less chimney cleaning required; and fewer ashes are produced

• Sustainable efficiency with variations in fuel

• No hassles getting the equipment permitted if permitting is even required

• Minimizing potential environmental impacts leading to expensive fines and system changes

Heating Value of Wood at Various Moisture Contents

The chart below shows how the heating output of wood varies with the moisture content.  The moisture content range is shown on the bottom line.  Note that as the moisture content increases, more of the heat that is produced is required to simply "cook off" the extra moisture that is in the wood resulting in an increasing loss of heating capacity.  Wood at 20-30% moisture content is optimal for use in the AHS Gasification Wood Boiler.

Source:  Tillman, David A., Wood as an Energy Resource