Investigation Of The Effects Of Biomass Oxygenated Additives On Soot Emission With Optical Diagnosis

Diesel blending with oxygen biomass fuels is an important way for substituting partial diesel by bio-fuels, and is also an effective method to achieve efficient and clean combustion by fuel characteristics optimization and emissions reduction, especially soot, in diesel engine. It was high focused at home and abroad in recent years. In this thesis, methods of laser induced incandescence(LII), laser induced fluorescence(LIF) and two-color high-speed imaging are the main diagnostic methods. For revealing the effects on soot and polycyclic aromatic hydrocarbon(PAH) formation, alcohols structure, oxygen groups, blending ratio and butanol isomers structure were investigated in partially premixed flames and an optical engine, respectively. And the soot in the process of diesel combustion in cylinder was preliminary diagnosed by LII. This research has important theoretical significance for revealing different oxygenated fuels soot formation mechanism. and important engineering application value for blended fuels characteristics optimization.Firstly, a LII image doubler images calculation model was established by author. In this model, the soot volume fraction distribution was quantified by the two color LII method, while the soot particle size distribution was measured by the time-resolved LII method. The fluorescence emission spectra were researched for each aromatic hydrocarbon and mixtures by Laser induced fluorescence(LIF). Results showed that the ring number determined the fluorescence emission spectra. 1~4 rings aromatics could be discriminated to some extent by band acquisition. In order to revealing the chemical characteristic parameters influence of fuels, a boundary controlled, flame steady, partially premixed burner was designed and the burner can use the liquid fuel in this work. T20, the mixture of 80% n-heptane in v/v and 20% toluene in v/v, was chosen as the basic fuel, which is also alternative fuel. The flame structures, temperature measurement, laser induced fluorescence(LIF) imaging and spectra of polycyclic aromatic(PAHs), and two-color laser induced incandescence(2C-LII) measurements were all close to those of similar partially premixed flames, which showed that the designed burner, the co-flow partially-premixed flame and the diagnostic methods were reliable. Results of PAH-LIF aiming to different rings’ aromatics and soot 2C-LII revealed the soot and PAH formation and oxidation process under different heights of the flame, so the flames and two diagnostic methods can be used for liquid fuels soot formation and oxidation mechanism research in rich mixture condition.To explore the soot and PAH reduction mechanism by using the blends of diesel and oxygenated fuels. T20 was used as diesel surrogate in the current research. Methanol, ethanol, n-butanol, methyl butyrate and 2,5-dimethylfuran(DMF) were blended with T20, respectively. Results show that PAH fluorescence spectrum can measure the formation and growth of PAH in different flames. With the oxygenated additives, the content of toluene is reduced in the T20 fuel, which is the major reason for the reduction of PAH fluorescence spectra intensity and soot volume. Compared to T20 flame, the total soot volume fraction in diagnostic area is reduced nearly 500 ppm in n-butanol blending flame, and the soot size is reduced nearly 25 nm. Comparing the oxygen content consistent and toluene dilution experiments, PAH-LIF intensity and soot volume of n-butanol/T20 blending fuel was lower than those of fuels with methanol, ethanol, methyl butyrate and DMF additives. Therefore, in terms of fuel oxygenated structures, n-butanol is more favorable to reduce PAHs and soot emissions in the combustion processes of diesel surrogate of T20 fuel. The results also found that Four-ring aromatics(A4) have the similar formation and oxidation trends to that of soot, which demonstrates that A4 is a good precursor to estimate the soot formation and oxidation processes.Furthermore, to explore the soot and PAH reduction mechanism of isomers, four butanol isomers were chosen as research objects. The oxygen content of four isomers is the same, so the difference of soot formation and oxidation mechanism can be revealed in four blends. Results show that butanol isomers has significant impacts on soot and PAH formation. Branched chain(iso-butanol and tert-butanol) produced more soot and PAH than linear chain(n-butanol and sec-butanol); when the blending ratio was at 20%, the total soot volume fraction of branched chain butanol in diagnostic area is higher than linear chain butanol nearly 100 ppm. With blending ratio increased, the influences of soot and PAH reduction by blending butanol are weaken. Comparing soot and PAH results, it showed that soot reductions were more significant than PAH reductions by blending butanol, this suggests that adding butanol not only can inhibit the PAH formation for soot reduction, and can facilitate soot oxidation.Last, to investigate the soot formation and oxidation mechanism in cylinder, an optical engine for laser diagnostics was designed, machined, fixed and debugged. First, 20% of methyl butyrate, n-butanol and DMF were blended with diesel by volume, respectively, referred to as MB20, B20 and DMF20. Two-color high-speed imaging was used to investigate the influences of oxygen goups on combustion and soot in diesel engine. The results show that the oxygenated fuel viscosity and volatility had obvious influence in injection. The ignition delay all of these 3 blends are longer than diesel, and obvious “blue flame” from chemiluminescence of combustion process is observed for all of these 3 blends. The ability for soot reduction of these three oxygenated fuels in descending order is DMF20, MB20 and B20. Longer ignition delay plays the most important role in soot reduction in the combustion process of DMF20, while for MB20 and B20, oxygen content leads. In order to get more accurate results of soot in cylinder, preliminary diagnosed by LII was carried. It was founded that influenced by temperature, pressure and chemiluminescence, soot laser diagnostics in cylinder needs advanced, larger gate and large bandwidth.