Effect of propylene glycol ether fuelling on the different physico-chemical properties of the emitted particulate matters: Implications of the soot reactivity

Abstract

The research work presented here focuses on the particulate matter (PM) number concentration and its physico-chemical properties from the combustion of di and tri-propylene glycol methyl ether/diesel blends (D20 and T20 respectively). Exhaust PM were characterised using a Scanning Transmission electron microscopy equipped with energy dispersive X-ray spectroscopy (S/TEM-EDX) to quantify the nanostructure, morphology and elemental composition of the agglomerates, Raman spectroscopy (RS) to further examine the particulate graphite like structure and thermogravimetric analyser (TGA) to analyse the oxidative reactivity. The increase in the fuel oxygen content reduces both, exhaust PM levels and NOx emissions. TGA analysis confirms that the oxygenated blends enhance the particulates oxidative reactivity and increase their volatile fraction in the following order T20>D20>Diesel. EDX analysis shows that the combustion of both D20 and T20 lowers the PM carbon fraction and ash precipitations but increases its oxygen functional groups in the same order. Furthermore, a notable reduction in the primary particles size was recorded, whose carbon layers were found to be more tortuous than diesel, but no significant modification was shown in their length. Unconventionally, smaller separation distance was seen between the carbon layers, and higher graphitisation order was seen from the RS analysis. It was finally concluded that concerning the nanoscale parameters, the initial curvature of the carbon layers present a stronger influence in dictating the particles reactivity compared to the graphitisation order or the initial length and separation distance of the carbon layers. As for the macroscale, primary particle size and the portion of oxygen in the particle could be another possible reason for the better soot reactivity seen from the propylene glycol ethers combustion. The significance of the reduced exhaust particles' level and modified PM's physical and elemental properties can improve Diesel Particulate Filter (DPF) regeneration and allows engine calibration that can favour NOx emissions reduction (i.e. lower NOx -PM trade-off lines).

Combustion, gaseous emissions and PM characteristics of Di-Methyl Carbonate (DMC)-gasoline blend on gasoline Direct Injection (GDI) engine

“…Consequently, formation of bond with chemisorbed oxygen is much easier compared to basal plane atom with greater accessibility of oxygen [61,62]. Thus, in this study D8 shows to possess higher number of carbons located on the edge site, it is believed be another contributing factor to enhance soot reactivity [50].…”

“…Higher soot reactivity was reported to be linked to the smaller primary particles as it increases the specific surface area, SSA (surface area/volume) ratio. Higher SSA means more surface area available for oxygen contact during oxygen [49,50]. Thus, the improvement of soot oxidation explained in previous section is thought that is partially due to the influence of smaller primary particles formation by D8.…”

“…Combustion of oxygenated fuels tend to form soot with high oxidation reactivity in both GDI and compression ignition engines [48,49,50]. D8 soot is more reactive by displaying the start of oxidation temperatures for D8 and gasoline at approximately 300 ˚C and 315 ˚C, respectively (Figure 9a).…”

“…The prime aim of devolatilisation stage in TGA is to remove VOMs to prevent interferences with soot oxidation results. However, this stage has been reported to increase the porosity of particles being more vulnerable to oxygen penetration which in turn, leads to higher soot oxidation rate [50]. Thus, particles formed with D8 blend are related to more rapid oxidation activity.…”

“…Higher amorphous concentration is linked to wider distribution and higher mean value of fringe tortuosity, Ft [60]. According to Serhan et al [50], Ft has major effect on soot oxidation. The latter was attributed to the weaker C-C bonds (breakdown easily) due to the effect of strain imposed on the bond which causes atomic orbitals overlap and lowered the resonance stabilization.…”