Pre-ignition

Pre-ignition is the heat absorbing (endothermic) phase of combustion where heat is applied to fuel resulting in vaporization of water and other substances providing the gases that sustain flames in the next phase of combustion, flaming combustion. During pre-ignition, the temperature of the fuel is raised to at least 100° C by radiation, convection and/or conduction. Heat of pre-ignition is the amount of heat required to raise fuel to ignition temperature. The first major product of pre-ignition fuel heating is water vapor converted from liquid water contained within cells and loosely tied to cell wall structure. Fuel temperatures will not rise above 100° C until this water is volatilized. During this phase, low temperature waxes, oils and other extractives are also degraded, yielding volatile gases such as lipids and other highly combustible hydrocarbons. This process is called pyrolysis.

Pyrolysis is the thermal degradation of chemicals in which the bonds that hold complex molecules together are broken, reducing molecule size and releasing smaller molecules from the material as gases (Pyne et al. 1996). It generally occurs in an oxygen-deficient environment. Pyrolysis occurs at different rates for different compounds. Pyrolytic degradation of polymers such as cellulose is exothermic in the presence of oxygen and endothermic in the absence of oxygen. Both processes produce char that can react with oxygen in an exothermic reaction. Char is the “black solid residue of a variable aromatic nature remaining after the end of the initial rapid weight loss upon heating of a polymer” (Ohlemiller 1985). Volatilization involves the pyrolysis of large compounds in the fuels to a gaseous state. Some resins, waxes and oils of woody materials volatilize to a gaseous state at relatively low temperatures. These volatiles are often released in the highest concentrations during the pre-ignition phase.

Pyrolysis temperatures vary for different organic compounds; hemicellulose begins degradation at 200 to 260^oC, cellulose at 240 to 350^oC, and lignin at 280 to 500^oC (Roberts 1970). Pyrolytic decomposition of cellulose also varies in different temperature conditions. At low temperatures pyrolysis of cellulose produces a variety of products including char, carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), hydrogen (H₂), ethane C₂H₄, water, and low-molecular weight acids (Shafizadeh and DeGroot 1976); higher temperatures (280^oC and above) produce levoglucosan, a volatile fuel that supports a gas-phase flame.