Soot Abatement Using Oxygenated Additives
Open Access
- Author:
- Wu, Juntao
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 22, 2004
- Committee Members:
- Thomas Litzinger, Committee Chair/Co-Chair
Robert John Santoro, Committee Member
Harold Harris Schobert, Committee Member
Richard A Yetter, Committee Member - Keywords:
- soot
oxygenated additive - Abstract:
- A study of fundamental issues of soot abatement by using oxygenated additives is conducted systematically by both experiment and modeling studies. The overall technical objective is to develop fundamental understanding of the complex interactions of oxygenated additive with the processes that lead to particulate matter emissions. Experiments were performed on a premixed ethylene/air flame. Ethanol and dimethyl ether were chosen as oxygenated fuel additives. The experiments were conducted at two equivalence ratios,  = 2.34 and  = 2.64, and two levels of ethanol or DME, 5% and 10% oxygen in mass of the fuel, were added to the fuel at each equivalence ratio. The experimental results show that Ethanol has evident effect on soot suppression. With the addition of ethanol, the concentration profiles of small PAH, large PAH and soot all show obvious reductions. The magnitude of reduction seems to increase following the sequence of small PAH, large PAH, and soot. Also, the more the oxygen content in the fuel, the more the reduction observed. By comparing the reduction ratios at two equivalence ratios, ethanol seemed to be more effective at  = 2.34 than at  = 2.64. DME addition also shows soot suppression effect. At §¶ = 2.34, DME shows an obvious advantage over ethanol on PAH and soot reduction. However, at §¶ = 2.64, there seems to be no detectable difference between the effective of ethanol and DME on PAH and soot suppression. In order to reveal the chemical processes leading to the effect of ethanol and DME on PAH and soot reduction, a chemical model, Howard-DME-Ethanol (HDE) mechanism, was used to provide possible clues. A crucial clue to understand the effect of DME and ethanol on PAH and soot reduction is how much carbon in the ethanol and DME participates in the aromatic species formation reactions. The reaction flux analysis shows that the effect of DME and ethanol on the reduction of PAH and soot results from the removal of carbon from the pathway to aromatic species formation. DME was more effective than ethanol in reducing soot precursors, because more carbon in DME (68.7%) was removed from the participation of aromatic formation than that in ethanol (45.5%). This difference should be attributed to the molecular structure difference between ethanol and DME. To check the role of temperature in the effectiveness of DME and ethanol addition in reducing PAH, modeling calculations were carried out using temperature profiles identical to the baseline fuel. The results show that temperature is unlikely to be a significant factor in reducing PAH production. Another part of present study is to reveal the detailed soot reduction mechanism of NO2 through modeling investigation. To be consistent with formal ethanol and DME studies, in present modeling study, 5 wt% oxygen in the fuel by NO2 addition was chosen. The equivalence ratios were also set to be ¦µ =2.34 and 2.64. In order to reveal the chemical processes leading to the effect of NO2 on PAH and soot reduction, a chemical model, Howard-NO2 (HN) mechanism, was used to provide possible clues. HN mechanism comprises 256 species and 1043 reactions. The model combines Howard PAH model and NO2 mechanism. Detailed mechanism analysis shows that the addition of NO2 increases the level of OH radicals in the flame, through reactions of NO2+HNO+OH, HO2+NONO2+OH. Then the increased OH decreases the level of H2 by reactions H2+OHH2O+H. The lower level of H2 increases the reaction rate of H2CCCH+HC3H2+H2, and results a lower level of H2CCCH. Since all C6H6 comes from H2CCCH through reaction 2H2CCCHC6H6, low level H2CCCH leads to a reduction of C6H6. As a key element during the PAH growth, low level of C6H6 leads to suppression of all PAH formation and growth. The effect of temperature on PAH reduction are also investigated. Under high temperature environment, the production of C8H6, is reduced due to the increase of C6H5 consumption by two competing reactions: C6H5C6H5(L) and C6H5HCCHCCH+C2H2. More C6H5 is consumed by these two reactions under higher temperature environment, resulting a reduction of C8H6 production through reaction C6H5+C2H2C8H6+H. Because C8H6 is the key radical for PAH growth, reduction of C8H6 directly results a reduction of C10H8, A3, pyrene, and thus soot.