Soot formation from a distillation cut of a Fischer–Tropsch diesel fuel: A shock tube study

Abstract

The kinetics of soot formation from Fischer–Tropsch (FT) fuels was studied in a heated shock tube under homogeneous conditions. Soot induction delay time and soot yield were measured between 10 and 17atm using a distillation cut at 403K of a Fischer–Tropsch fuel diesel. Two fuel concentrations were investigated in pyrolysis: 0.2% and 0.4% FT in Ar. Equivalence ratios (Φ)=18 and 5 were also investigated for the highest fuel concentration. During this study, a second growth of the soot volume fraction profile was observed with the highest fuel concentration in pyrolysis and at Φ=18. It was shown that this second growth appears only at temperatures higher than the temperature at which the soot yield is maximum. Under the conditions investigated, the soot induction delay time was found not to be very sensitive to the fuel concentration. A careful analysis of the soot volume fraction profiles showed that this finding was linked to the measurement method usually adopted. Nevertheless, this method was found adequate for a systematic comparison between different fuels or for an investigation of the oxygen concentration effects. The addition of oxygen to the mixture promotes soot formation in its early stages by decreasing the soot induction delay time. A shift of the soot yield curve toward lower temperatures was also observed. Moreover, oxygen addition reduces the amount of soot produced. This reduction is proportional to the O2 concentration. Comparisons with literature data showed that a Fischer–Tropsch fuel primarily composed of n-paraffins can be correctly represented by an n-paraffin with a molecular size comparable to the average molecular size of the Fischer–Tropsch fuel. The maximum soot yield of the Fischer–Tropsch distillation cut studied was not significantly different from that of a diesel fuel surrogate previously studied (Mathieu et al., Combust. Flame 156 (2009) 1576–1586).