Study On Gas/Particle Phase Distribution Of Polycyclic Aromatic Hydrocarbons During Diesel Combustion With PTV Method

The gas/particle phase distribution of polycyclic aromatic hydrocarbons (PAHs) during diesel combustion process was investigated with advanced instrument analysis methods, and all samples were obtained from the total cylinder dumping system. The particle-phase PAHs were trapped with a glass fiber filter, and the gas-phase PAHs were captured passing through a glass cartridge packed with XAD-2 resin supported by two polyurethane foam (PUF) plugs. Since the PAHs for a single combustion cycle was at a ultra-trace level, the analysis of ultra-trace PAHs were carried out by gas chromatography with mass spectrometric detection (GC–MS) coupled with large-volume programmed temperature vaporization (PTV) injection. The large-volume PTV injection settings were optimized using a statistical design of experiments, and the results indicated that the requirement of analyzing PAHs at the ultra-trace levels can be well met by this method. In light of the method above, the influence of exhaust gas recirculation (EGR), which was efficiently simulated with the CO₂ addition, on the distribution of the gas/particle phase PAHs during the combustion process was studied. The major achievements are listed as follows:1. The method of selecting a cartridge packed with XAD-2 resin and a glass fiber filter for collecting gas-phase and particle-phase PAHs was reliable, with the PAHs recoveries in the range of 71.83% - 98.64% for the cartridge and 86.41% - 118.35% for the fiber filter. The large-volume PTV injection settings for 50μL injection and 25μL injection were all optimized using two-level full factorial designs and central composite designs (CCD) of experiments. The final injection volume was chosen as 25μL, and the optimized factor settings for 25μL injection provided a composite desirability of 0.83. The large-volume PTV injection method simplified sample analysis steps without affecting the separation effect. For 25μL injection, an increase in sensitivity was 3.8 - 59.2 times as compared with the conventional splitless injection.2. With the presence of EGR, the total PAHs (PAHs on filter+ cartridge) increased at first, and then decreased as the combustion went on; with the absence of EGR, the total PAHs decreased at the beginning and then increased later, but decreased again at the end of the combustion process. At the end of the combustion process, the minimum of the total PAHs could be obtained at the EGR rate of 19.5%.3. Throughout the combustion period, gas-phase PAHs acted as a contribution of more than 95.15% in the total PAHs, and naphthalene was the most abundant gas-phase PAHs, exceeding 97.7% of gas-phase PAHs. Of the total analyzed PAHs, the two- and three-ring PAHs such as naphthalene, acenaphthylene, acenaphthene, fluorene and phenanthrene were the dominant species during the combustion process. At the end of the combustion, the PAHs with three-ring arrangement accounted for 33.5% - 49.3% of the total particle-phase PAHs, and fluoranthene and pyrene were the most abundant. In gas-phase PAHs, naphthalene was found to prevail, which accounted for more than 97.7%.4. During the combustion process, the more the molecular weight of PAH, the lower the proportions of in the gas-phase. The most volatile PAHs were mainly found in the gas-phase, whereas four-ring PAHs and higher-ring PAHs were predominantly absorbed into the particles. Most of three- and four-ring PAHs exhibited an intermediate behavior. A negative correlation was also found between EGR rate and the PAHs distributions in gas-phase at the end of the combustion process.