In-situ laser diagnostic and numerical investigations of soot formation characteristics in ethylene and acetylene counterflow diffusion flames blended with dimethyl carbonate and methyl formate

Abstract

Oxygenated fuels have emerged as promising energy carriers for storing renewable energy given their compatibility with current energy systems and their potential of reducing pollutant emissions when burning in blends with traditional fossil fuels. In particular, dimethyl carbonate (DMC) and methyl formate (MeFo) have raised increasing interest for their use as gasoline substitutes in recent years. However, the effect of fuel blending on soot formation remains unclear, which prevents the utilization of the full potential of these fuels. In this work, soot formation of ethylene-based and acetylene-based fuel blended with DMC and MeFo in laminar counterflow diffusion flames was investigated by laser-induced incandescence (LII) in combination with laser-induced breakdown spectroscopy (LIBS) and spontaneous Raman scattering (RS). When substituting the base fuels with DMC or MeFo, the flame temperature profiles remain unchanged, allowing to exclusively focus on the chemical kinetic effect of fuel blending on soot formation. The soot formation in the ethylene-based flames can be enhanced by blending a small amount of DMC, while the soot volume fraction begins to decline at high substitution rates of DMC. Blending DMC with acetylene shows no soot enhancement effect. In comparison, the blending of ethylene with MeFo leads to a direct reduction of soot production. This soot formation behavior was further examined using a reaction pathway analysis based on chemical kinetic modeling. The propargyl recombination pathway plays a more important role than the HACA pathway in the formation of benzene and naphthalene, especially for ethylene-based flames. Different from MeFo, DMC can decompose to generate CH3 radicals and subsequently enhance propargyl formation. Therefore, blending DMC means a reduction of C2H2 and an increase of CH3, which results in the soot formation characteristics of DMC blends. Our kinetic analysis also implies the need for improving the decomposition kinetics of C2H4 to C2H2.