Soot Formation Stoichiometry

From equilibrium considerations soot will appear when oxygen is not  sufficient to oxidize carbon even to carbon monoxide, i.e., C/O atomic ratio in the mixture is greater than unity. Using the following stoichiometric reaction for a hydrocarbon, C_xH_y ;

(2.38)

when x is greater than 2a i.e. C/O is greater than unity,  solid carbon, C_s or soot is produced during combustion.

The fuel –air equivalence ratio for the above reaction is;

(2.39)

 For practical diesel fuels H/C ratio (y/x) ≈ 2.  Hence, for the critical C/O = 1, the fuel-air equivalence ratio, = 3. However, in practical systems the soot has been observed to form at C/O ratio of 0.5 to 0.8 indicating that soot formation is a kinetically controlled process. It may be noted that for methane (CH4), theoretical critical is equal to 4.  The critical C/O ratio for soot formation increases with increase in temperature. Pressure has a strong influence, higher pressures yielding higher soot formation at the same value off . In other words, increase in pressure results in lowering of  critical value of f at which soot is formed.

Conceptual Models of Soot Formation

 Two conceptual models of soot formation in spray combustion have been suggested;

• One model suggests that the soot is formed in a narrow zone of rich mixture at the spray boundaries close to the diffusion combustion region.
• Another conceptual model based on laser imaging studies of diesel spray combustion in a supercharged engine at Sandia Laboratories has been proposed. Fig 2.23 shows schematically a diesel spray jet. It is seen in these studies that the liquid jet is relatively short and the fuel ahead of liquid jet is in vapour phase.  It was seen that the soot appeard for the first time just downstream of liquid jet in the rich premixed combustion region.  The concentration of soot increases and particle size grows as soot flows downstream towards the spray boundary. The highest soot concentration and largest particle size are in the region forming head or leading edge of the jet. The model suggests that the formation of soot precursors and consequently generation of soot particles takes place in the rich premixed flame where fuel-air equivalence ratio is in the range, =2 to 4. The soot particles grow in size as they pass through the spray towards the spray leading edge. The soot finally gets oxidized in the diffusion flame at the spray boundaries by OH radical rather than the molecular oxygen, O₂.

The Sandia model varies from the earlier models as it suggests that the formation of soot is the result of rich premixed combustion rather than the diffusion combustion phenomena. Further studies however, are required to confirm validity of this hypothesis for all diesel engine combustion conditions.