Country Flame CF-04 - Manual (Page 53)

Version 2.0e 53
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 1,000
o
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 H
2
1076 – 1094
Methane CH
4
1202 – 1382
Ethylene C
2
H
4
1008 – 1018
Benzene C
6
H
6
1364
Ethane C
2
H
6
968 – 1166
Carbon Monoxide CO 1191 – 1216
Note that all ignition temperatures fall between 950
o F through 1400 o 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.