2.2. Stages in heavy fuel combustion

1. Volatile release: After injection, fractional distillation occurs as the fuel droplets are heated in hot air or oxidising atmosphere. Gases cause the ejection of tiny droplets away from the droplet surface, although sometimes boiling may become violent enough to cause the total disruption of the original droplet. Evaporation of the low boiling point components carries on with swelling until the onset of ignition.

2. Ignition: Vapour from the droplets and the hot air mix, causing an increase in the mixture temperature. Ignition occurs when the mixture reaches its flammability limits. The time taken for ignition is named Pre-ignition Delay (t_i), which has been frequently related to an Arrhenius-type expression like the following (Malik (1986)) :
   

   where E_act represents an overall activation energy and T_F is the temperature of the combustion environment. Ignition time depends upon the chemical structure of the hydrocarbons, as it has to facilitate gaseous reactions.

   The onset of combustion causes slight thermal decomposition. The heat released produces further fuel evaporation from the fuel droplet. Then the viscosity of the residue increases as large paraffins are broken down, side chains are stripped from asphaltenes and similar molecules, undergoing condensation to form carbon-like structures.

3. Coke formation: The evolution of the volatile matter ends abruptly and the droplet collapses forming a rigid carbonaceous residue, known as cenosphere. Oils with higher asphaltene content show the least contraction and form large thin-walled coke shells approximately the size of the original droplet. Experimental observations indicate that these materials become rigid while still evolving decomposition vapour, because their aromatic structures are very suitable for rapid carbon formation (see section "4.4.3. Coke").

   The final amount of oil mass remaining in the solid residue represents between 0.5 and 10 % of the original drop mass.

4. Coke structure and burn-out: Coke particles thus formed are spherical and hollow. The carbonaceous residue then undergoes slow heterogeneous combustion at a rate of one-tenth of that of the initial droplet, representing as much as 50 % of the total burning time of the droplet. In large particles at high temperature, diffusion of oxidant species control the reaction rate, whereas in small particles at low temperature the surface reactions control the reaction rate (Williams (1979)) .

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