The obvious advantage of wood pellets over conventional chopped wood logs is the consistency of the fuel both from a size and moisture content. Thus, wood pellets burn more efficiently than their 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 reach up to 2000o Fahrenheit. The obvious advantage of corn fuel is it is Nature’s palletized fuel. As customer demand for wood pellets increase, corn fuel will be readily available across America. The list below gives a sample of ignition temperatures for some of the carbon and volatile matters released when wood is burnt:

COMBUSTIBLEFORMULA

IGNITION TEMP (O F)

 

Fixed Carbon

C

765 – 1115

Volatiles

 

1076

– 1094

Hydrogen

H2

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 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 if volatile and carbon matter and 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. All of this 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” once the combustion chamber 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 service technician 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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