Combustion

Temperature: The temperature required to ensure the release of flammable vapors. To sustain combustion, the heat evolved must maintain this temperature to provide a constant supply of vapors.

Excess Air: Any air added to the combustion process above the theoretical value defined as 100% efficiency. A combustion system operated exactly at the peak burn efficiency ratio has a high probability that some of the oxygen in the combustion air will not get paired up with the fuel. Adding excess air ensures that a percentage of additional air is available to ensure complete burn of all fuel. Excess air reduces efficiency but ensures complete fuel burn as environmental variables change. In addition, a little excess air provides a cushion against the fuel-air ratio drifting over into the rich (excess fuel) range. With time, all combustion systems will tend to burn richer (that is more fuel than air.) The big enemy that creates a richer burn is dirt. Dirt is more likely to accumulate in the air system rather than in the fuel system. The combustion and makeup air fans, in addition to everything else within a stove, are giant vacuum cleaners, sweeping in any dust, dirt and vapors that happen by. In time, the systems air path becomes partially obstructed, starving the system for air and causing the fuel-air ratio to become rich. Excess air ensures that complete combustion can occur at all times regardless of changes in environmental variables.

Ignition

Temperature: The temperature at or above which a substance will combust.

Pyrolysis: The second stage of ignition during which energy causes gas molecules given off by a heated solid fuel to vibrate and break into pieces.

Room Air: Air that is passed through a stove’s heat exchanger tubes which elevates the room air temperature and thereby delivers heat to a living space.

WOOD PELLET COMBUSTION BASICS

In a normal wood pellet combustion process, water is first boiled out of the pellet. Then volatile matter and fixed carbon matter are released in the combustion process and later burned to create heat. Any remaining product left behind in the burn process is defined as ash. FIGURE 18 illustrates the combustion process of wood.

Moisture

 

 

Gas Phase Volatile

 

Volatiles

 

 

Oxidation

 

 

Solid Particles

Heat

Pyrolysis

 

 

 

 

 

Carbon Phase

Carbon

 

Oxidation

 

 

 

 

 

FIGURE 18: WOOD PELLET COMBUSTION PROCESS

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