The obvious advantage of wood pellets over conventional chopped wood logs is the consistency of the fuel both from a size and moisture content. Nature has done a great job of making field corn a palletized fuel that can have even more energy than wood pellets. These palletized fuels burn more efficiently than their wood log counterparts. The heat generated in the burn process, once combustion is achieved, sustains the process as more fuel is added. As the gases burn, the pellet stove’s firebox temperature can achieve temperatures exceeding 1,000o Fahrenheit. The list below gives a sample of ignition temperatures for some of the carbon and volatile matters released when wood is burnt:

 

COMBUSTIBLE

FORMULA

IGNITION TEMP (O F)

 

Fixed Carbon

C

 

765 – 1115

 

 

Volatiles

 

 

 

Hydrogen

H2

1076

– 1094

Methane

CH4

1202

– 1382

Ethylene

C2H4

1008

– 1018

Benzene

C6H6

1364

 

Ethane

C2H6

968

– 1166

Carbon Monoxide

CO

1191

– 1216

Note that all ignition temperatures fall between 950o F through 1400o F for volatile matter. All visible flames emitted during a wood pellet fire come from the ignition of volatile matter. Good combustion occurs when the fuel vapors are thoroughly mixed with the combustion air. Mixing ensures that fuel molecules come in contact with oxygen fuel molecules. There must be an abundant amount of oxygen molecules available to react with the fuel molecules or an incomplete burn will occur. NOTE: The fuel- air mixture must at all times exceed the combustion temperature as well as exceed the ignition temperature if a burn is to be sustained. There must be adequate time for the reaction to proceed to completion (complete burn.) It is commonly stated that good conditions for combustion are defined as the THREE T’s:

Turbulence

Temperature

Time

It requires turbulence in the fuel-air path to ensure that fuel and air molecules are thoroughly mixed to sustain the burn process. In addition, it takes a certain amount of temperature, defined as Ignition Temperature to sustain the burn. It takes a certain combustion temperature to ensure volatile and carbon matter is released from the fuel that will then combine with the oxygen in the air further adding to the heat. This entire combustion process takes time to occur and is self-sustaining only if all conditions are met for the fuel being burnt.

Heat is obtained from a stove by the thermal radiation that occurs from the burning wood pellets. Although a fairly complex phenomena to define, the heat radiated from the fire is a very strong function of the local (burn pot) temperature and the overall combustion temperature that is maintained within a stove’s burn chamber. A relatively minor combustion temperature increase in a pellet stove can produce a significant amount of increased heat output. NOTE: The reverse of the preceding statement is also true. That is, if too much heat is pulled from the combustion chamber via the heat exchanger tubes, the combustion temperature can drop rapidly thus causing a “burn out.” Burn out occurs once the combustion chamber temperature drops below the critical “ignition temperatures” required of the different fuel components. Wood pellets, because of their low combustion temperature ensure an ample supply of volatile and carbon matters available for continued ignition. However, for other types of fuel, such as corn, it is important that the homeowner understand the complexity of this principle and how environmental variables can affect the combustion chamber, combustion temperature, and ignition temperatures. Once a burn chamber’s temperature begins to fall below critical ignition temperatures, the heat required to sustain the combustion temperature can also be lost and the fire will go out.

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