Tailor Made Biofuels: Effect of Fuel Properties on the Soot Microstructure and Consequences on Particle Filter Regeneration

Abstract

In recent years a lot of effort has been made to understand the phenomena of Diesel Particulate Filter (DPF) regeneration processes but less attention has been paid to understand the influence of fuel properties on soot reactivity and its consequence on the DPF regeneration behavior. Within the Cluster of Excellence “Tailor-Made Fuels from Biomass (TMFB)” at RWTH Aachen University, the Institute for Combustion Engines carried out a detailed investigation program to explore the potential of future biofuel candidates for optimized combustion systems. These new biofuels are being developed to realize partially homogeneous low-temperature combustion, in order to reduce the emission and fuel consumption to meet future requirements. The chemical structure of these new fuels may impact the thermal decomposition chemistry and hence the in-cylinder particulate formation conditions. This work fundamentally focusses the influence of fuel properties on particulate matter reactivity and, thereby, the regeneration behavior of the diesel particulate filters (DPF). The experiments for particulate measurements and analysis were conducted, under constant engine operating conditions, on a EURO 6 compliant High Efficiency Combustion System (HECS) fuelled with petroleum based diesel fuel as baseline and today’s biofuels like FAME and Fischer Tropsch fuels as well as potential biomass derived fuel candidates being researched in TMFB. Several different methods were used for analysis of mass, composition, structure and spectroscopic parameters of the soot. The graphitic microstructure visible with high resolution transmission electron microscopy (HRTEM) was compared to the results of X-Ray diffraction (XRD), optical light absorption measurement and elementary analysis of samples. The results indicate that combustion with increasing fuel oxygenation produces decreasing engine-out particulate emissions. The ranking of activation energies of soot oxidation analysis from LGB experiments correspond well with the ranking of the soot physico-chemical properties. In comparison to petroleum based diesel fuel, the reduction of engine out soot emission by a factor of five with the use of the future biomass derived fuel candidate was accompanied by ten times reduction of the soot volume based absorption coefficient and two times reduction of carbon to hydrogen ratio. As a result of it, the activation energy of soot oxidation in DPF reduced by ∼ 10 KJ/mol. The reduced engine out soot emission and increased reactivity of the soot from the future biomass derived fuel candidate could cause a significant reduction of thermal DPF regenerations.

Utilization of HVO Fuel Properties in a High Efficiency Combustion System

“…It is very apparent from the curves that all the soot samples possess two significant characteristics peaks. The first peak at ~ 2theta = 23° corresponds to 002 planes and the second peak at ~ 2theta = 44° corresponds mainly to the 100 plane (21) . The main results derived from XRD analysis are summarized in Table 3.…”

“…On the other hand, the benefits in PM emissions with HVO are mainly attributed to its aromatic free composition as well as its lower distillation range as compared to the petroleum diesel (14) . Previous studies have reported significant variations in the structure and oxidative reactivity of the soot derived from different petroleum based diesel fuels and biofuels (11)(12)(13)(17)(18)(19)(20)(21)(22)(23) . Mikkonen et.…”

“…This suggests that carbon structures in the soot vary from highly structured graphitic sites to the highly disordered amorphous hydrocarbons. To regard the impact of background generated by amorphous hydrocarbons, in addition to the evaluation of the lattice spacings and of crystallite size using Scherrer formula, a new parameter named as "Structure toAmorphous Ratio" abbreviated as StA was evaluated(21) :StA ratio = A s / A a Where A s is the indication of the area of peak under structured hydrocarbons and A a is indication of the area of base under amorphous hydrocarbons as depicted in theFigure 8…”

Fuel formulation effects on the soot morphology and diesel particulate filter regeneration in a future optimized high-efficiency combustion system

“…Based on opacity measurement of the engine exhaust with the photoacoustic device by modulated laser at 808 nm downstream DOC, the exhaust-related extinction coefficient (k ext ) was calculated by Lambert-Beer's law. In order to focus on the soot properties, the influence of un-burnt fuel and oil was reduced by calculating the light absorbing soot amount based on the measured NSOF of the soot 43,44 k ext = ln…”

“…[47][48][49][50] In case of 2-MTHF/DBE, its higher soot reduction potential could be attributed to three main favorable properties such as (1) absence of aromatics, (2) improved local air-fuel ratio due to higher oxygen content and favorable functionality of oxygen atom within the fuel molecular structure, and (3) the enhanced mixing due to high volatility and longer ignition delay. 1,44,47 Soot oxidation kinetics in DPF Figure 10 depicts the magnified view of the Arrhenius plots of all the investigated fuels with soot-O 2 reaction. The results indicate that the activation energy of 2-MTHF/DBE soot oxidation for soot-O 2 reaction decreases ;9 kJ/mol as compared to EN 590 diesel soot.…”