Impact of Oxygen Enrichment on Soot Properties and Soot Oxidative Reactivity

Open Access
- Author:
- Seong, Hee Je
- Graduate Program:
- Energy and Geo-Environmental Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 17, 2010
- Committee Members:
- Andre Louis Boehman, Dissertation Advisor/Co-Advisor
Andre Louis Boehman, Committee Chair/Co-Chair
Angela Lueking, Committee Member
Randy Lee Vander Wal, Committee Member
Richard A Yetter, Committee Member - Keywords:
- oxygen enrichment
soot properties
soot oxidative reactivity
diesel engines
diffusion flame burners
DPF - Abstract:
- Heavy duty diesel engines have been key power generators in transportation and power plants around the world. However, the hazardous pollutants emitted from diesel engines have driven their emissions to be regulated in increasingly stringent ways. Among the pollutants, particulate matter (PM) has long been studied as to why it is generated during the combustion process and how it can be effectively reduced via fuel injection strategy, fuel modifications and after-treatment systems. In order to meet current and future standards, diesel particulate filter (DPF) systems have become the primary means to remove PM. DPF systems involve particulate filtration from the exhaust and subsequent burn-off of the filtered particulates. Consequently, the effects of engine operation and fuel formulation on particulate oxidation have been of great interest because of the relevance to the application of DPF systems. Previous studies showed that physical and chemical properties of soot, which are represented by the volatile organic fraction of the particulate matter, and the crystalline structure, O and H content and surface oxygen functional groups of the primary soot particles, are significantly affected by engine operations and fuel formulations. Furthermore, the soot properties are observed to have a great impact on soot oxidative reactivity. In spite of the efforts to elucidate certain soot properties affecting soot oxidative reactivity, there are few investigations made on the influence of the combustion process on soot oxidative reactivity. Since oxygen enrichment has a great effect on the combustion process, various oxygen concentrations were examined in a diesel engine and a diffusion flame burner to determine the impact of oxygen enrichment on soot properties. Oxygen addition to the engine was carried out by intake oxygen enrichment and by fuel oxygenation at low load and high load. The analyses of heat release rates and cylinder temperatures indicate that the effect of oxygen enrichment is more appreciable at high load than at low load. Correspondingly, there are more noticeable changes in crystalline structure and oxidative reactivity of soot from high load than of soot from low load. However, surface O content and oxygen functional groups are shown to have mixed results without any consistency at low and high loads with oxygen enrichment. In addition, soot generated from high load with intake oxygen enrichment is observed to contain some metallic species, while soot from low load has little or no metallic species. Although the higher temperature induced by higher oxygen concentration was thought to be a main factor affecting the oxidation of lubricating oil, it is shown that there should be a synergistic effect of high oxygen concentration and high temperature in order to accelerate the oxidation of lubricating oil. There might be an effect of soot crystalline structure on soot oxidative reactivity, but the major influence on the reactivity is shown to be closely related to the amounts of metallic species present in soot due to the catalytic role of metallic species in soot oxidation. In order to better understand the effect of soot properties on soot oxidative reactivity, a diffusion flame burner was employed at various oxygen concentrations. There is a clear inverse relation observed between adiabatic flame temperature and soot oxidative reactivity. However, detailed experiments suggest that the primary factors impacting soot oxidative reactivity are the soot inception time and the soot oxidation process. The earlier soot is formed, the less reactive soot becomes. Accordingly, ethylene-derived soot is shown to be less reactive than n-heptane-derived soot under the same carbon flow rate and the same adiabatic flame temperature, because soot is formed at an earlier stage with ethylene than with n-heptane. In addition, increasing oxygen concentration in the oxidizer stream made soot less reactive, because not only is the soot inception time shortend by the increase in the flame temperature, but also the soot oxidation process is enhanced by increased abundance of oxidizing gases during the combustion process. From these results, it is concluded that the reason why soot derived from oxygenated fuel (a mixture of 70 vol. % n-heptane and 30 vol. % monoglyme) is more reactive than soot from n-heptane fuel is related to the lengthened soot inception time caused by reduced soot precursor particles. Raman spectroscopy, x-ray diffraction (XRD), electron energy loss spectroscopy (EELS) and near edge x-ray absorption structure spectroscopy (NEXAFS) showed that the crystalline order of soot is strongly related to soot oxidative reactivity. However, the trend in the crystalline order of soot is observed to be opposite for soot from the diffusion flame burner and soot from the diesel engine with increasing oxygen concentration. Interestingly, engine soot from high load becomes less ordered in its crystalline structure with increasing oxygen concentration in spite of increasing cylinder temperature, which is contradictory with others’ results. From the study with the diffusion flame burner, it is suggested that since soot precursor particles are oxidized by abundant amounts of oxidizing gases at high temperature, there is a limitation for soot precursor particles to transform to soot particles, which delays soot inception time, resulting in reducing the crystalline order of soot. It is difficult to evaluate the effect of surface O content on soot oxidative reactivity in the case of diesel soot, but the study on flame soot clearly indicates that surface O content correlates with soot oxidative reactivity. A detailed XPS study suggests that C-O groups in alcohol, and carboxylic anhydride and/or ester groups are the most abundant on the soot surface, and upon oxidation C=O groups and COOH groups increase gradually in their relative ratios, whereas C-O groups, and carboxylic anhydride and/or ester groups decrease. Also, it is suggested that soot oxidative reactivity depends more on the active sites that remain in the soot structure than on surface oxygen functional groups.