An experimental and modeling study of NOₓ and soot emissions from biodiesel and its surrogates

2011-11-17The use of biodiesel is attracting substantial attention currently as a potential effective and environmentally friendly solution to the world’s dependence on conventional petroleum resources. As with conventional diesel, concerns exist with the use of biodiesel in terms of its emissions, especially NOₓ emissions and particulate matter (PM) due to their potential health effects. Although there is strong incentive for using biodiesel to substitute for petroleum fuels, the NOₓ and soot emissions of those fuels are not yet well characterized and understood. In this study, the NOₓ emissions of a class of model biodiesel fuels, namely fatty acid esters (FAME) were studied systematically in both premixed as well as non-premixed flames. The soot emissions of these model biodiesel fuels were also studied systematically in non-premixed flames. In addition, a diesel surrogate was also studied in order to compare its soot emissions with those of the model biodiesels. ❧ The experiments were conducted in the counter-flow configuration for different fuel/oxidizer ratios (for premixed flames) or fuel/N₂ molar ratios (for non-premixed flames) at atmospheric pressures. The NOₓ measurements were carried out using the quartz microprobe sampling method. The soot volume fractions were determined using the laser extinction method. The experimental data were compared against those obtained in flames of n-alkanes with similar carbon numbers. Additional experiments were carried out, in order to assess the effects of carbon number, type of ester group (methyl or ethyl), and the extent of saturation (presence of double bonds) on the NOₓ emissions and sooting propensity of these model biodiesel fuels. ❧ Recently developed chemical kinetic models were utilized in order to simulate the flames and to investigate the kinetic pathways controlling the formation of key soot precursors, such as C₂H₄, C₂H₂ and C₃H₃, aiming to provide insight into the experimentally observed differences in the sooting propensity among the various fuels. In simulating the NOₓ emissions, a recently developed NOₓ sub-kinetic model was incorporated into the aforementioned fuel combustion mechanisms in order to investigate the relative contributions of the four different reaction pathways to the total NOₓ formation in these flames. Overall, this study constitutes a systematic effort to investigate the fundamental emission characteristics of biofuels, and to provide significant new insight into fatty acid ester combustion leading eventually to the detailed study of real biodiesels