Firewood

Quality fuel for your woodburning appliance is crucial to successful operation.

The process of combustible is complex and technical. We can break combustion down simply like this:

Fuel, air, and heat must be present for combustion to occur regardless of what fuel is used or what appliance is used. In a solid fuel burning heater, (woodstove), heat breaks up wood molecules causing carbon and hydrogen to mix with oxygen from the incoming air turning it into heat, chemicals and gases.

If any of the three required elements (fuel, air, heat) is removed combustion stops.  If all three occur in the proper ratio combustion is self-sustaining.

If complete combustion occurs only water vapor (H2O), carbon dioxide, heat and ashes are produced.
If incomplete combustion occurs carbon monoxide, volatile hydrocarbons (creosote) as well as H2O and ashes are produced.

Stages of Combustion

For the purpose of woodburning there are four main stages of combustion:

Stage I: Drying
As the fuel is heated any moisture it contains is evaporated. Evaporation absorbs heat – the greener the wood (more water content) the more heat it takes for evaporation to occur. This is heat that is lost.

Stage II: Pyrolysis
When the wood is dry enough the temperature will rise to the point were the molecules making up the chemical structure of the wood breakdown and vaporize (turn to hydrocarbon gases). This mixture of tar fog and combustible gases are not yet hot enough to burn and heat is still being absorbed that at this point.

Stage III: Gas Burning
As the temperature increases the tar fog and hydrocarbon gases reach ignition temperature in the presence of oxygen, oxidizing and burning.

Stage IV: Charcoaling
At the point during combustion when everything except carbon has burned off, charcoal remains. This carbon must come into direct contact with oxygen before it burns – burning with little or no flame. Water vapor continues to the produced because of the mixture of hydrogen and oxygen from the combustion air.

Start with good wood, end with good wood

Since the first two stages of combustion actually absorb heat we can see how crucial good dry wood is to heat production, not to mention the buildup of creosote in the flue and potential chimney fires. Water vapor condensing in the flue and unburned volatile hydrocarbons aka creosote, sticking to it creates glazed creosote the main cause of all chimney fires.

Good logs, not rotting and punky, should be cut, split and stacked and allowed to season for at least a year. Wood should be stacked where it will get good long days of sun and good air flow through the stack. Wood stacks should be covered on top or kept out of the weather. Keep in mind that stacks should never be wrapped with tarps or plastic as this will not allow moisture to escape and will shield the stack from good airflow.

Seasoned wood should have under 20% moisture content.

Example images:

Debarked firewood that looks dry

This barkless log looks dry

Dry looking firewood

The end of the log appears dry

The moisture meter with the probes only halfway inserted reveal over 20% moisture

The moisture meter with the probes only halfway inserted reveal over 20% moisture

When we probe the middle of the log we get 30% moisture.

When we probe the middle of the log we get 30% moisture.

 

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