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Kinetic modelling of autoignition phenomena
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
2007 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

To fully understand the elementary reactions behind the ignition of automotive fuels the interaction between the fuel components must be known. The ignition initiation is most often caused by loss of an H radical from a reactive fuel molecule, for example n-heptane. The formed alkyl radical is prone to react with oxygen under lean conditions. However, it can also abstract hydrogen from other fuel molecules, hence activating more unreactive species. This type of reactions is called cooxidation reactions and including it in combustion mechanisms improve ignition delay predictions in a wide range of experiments, for Primary Reference Fuel mixtures and toluene/heptane mixtures. Example of such reactions are

C7H15• + C8H18 = C7H16+ C8H17•

C7H15OO• + C8H18 = C7H15OOH+ C8H17•

Adding cooxidation reactions also significantly improves prediction of the general trend of auto-ignition phasing as function of operating conditions in Homogeneous Charge Compression Ignition, HCCI, engine combustion.

The effect of NO addition on engine combustion has also been studied in this work. A novel strategy to control ignition onset in HCCI engines is to retain exhaust gases in the cylinder to control the cylinder temperature. While this not only controls the engine temperature it also introduces NOx in the cylinder. The NO will advance ignition onset by several crank angle degrees at concentrations below 10 ppm. This is because NO activates HO2 in the reaction: HO2• + NO = OH• + NO2. At higher concentrations the ignition onset is not as advanced and in the PRF case even retarded. This is because NO has cool flame-inhibiting effects.

Kinetic modelling can also be used to predict combustion efficiency in catalytic combustors for power generation. It was shown that at high pressures the number of free sites decreases which limits the combustion efficiency. Thus a hybrid concept could be used where only a fraction of the air and fuel is burned catalytically.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , 53 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:65
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4516ISBN: 978-91-7178-778-1 (print)OAI: oai:DiVA.org:kth-4516DiVA: diva2:12640
Presentation
2007-11-15, 591, KTH, Teknikringen 42, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101109Available from: 2007-11-06 Created: 2007-11-06 Last updated: 2010-11-09Bibliographically approved
List of papers
1. Cooxidation in the auto-ignition of primary reference fuels and n-heptane/toluene blends
Open this publication in new window or tab >>Cooxidation in the auto-ignition of primary reference fuels and n-heptane/toluene blends
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2005 (English)In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 140, no 4, 267-286 p.Article in journal (Refereed) Published
Abstract [en]

Auto-ignition of fuel mixtures was investigated both theoretically and experimentally to gain further understanding of the fuel chemistry. A homogeneous charge compression ignition (HCCI) engine was run under different operating conditions with fuels of different RON and MON and different chemistries. Fuels considered were primary reference fuels and toluene/n-heptane blends. The experiments were modeled with a single-zone adiabatic model together with detailed chemical kinetic models. In the model validation, co-oxidation reactions between the individual fuel components were found to be important in order to predict HCCI experiments, shock-tube ignition delay time data, and ignition delay times in rapid compression machines. The kinetic models with added co-oxidation reactions further predicted that an n-heptane/toluene fuel with the same RON as the corresponding primary reference fuel had higher resistance to auto-ignition in HCCI combustion for lower intake temperatures and higher intake pressures. However, for higher intake temperatures and lower intake pressures the n-heptane/toluene fuel and the PRF fuel had similar combustion phasing.

Keyword
HCCI, homogeneous charge compression ignition, auto-ignition, fuel chemistry, primary reference fuels, n-heptane, toluene, co-oxidation, CHEMKIN
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7570 (URN)10.1016/j.combustflame.2004.11.009 (DOI)000227865300003 ()2-s2.0-14744268733 (Scopus ID)
Note
QC 20101109Available from: 2007-11-06 Created: 2007-11-06 Last updated: 2010-12-20Bibliographically approved
2. High-pressure catalytic combustion of gasified biomass in a hybrid combustor
Open this publication in new window or tab >>High-pressure catalytic combustion of gasified biomass in a hybrid combustor
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2005 (English)In: Applied Catalysis A: General, ISSN 0926-860X, E-ISSN 1873-3875, Vol. 293, no 1-2, 129-136 p.Article in journal (Refereed) Published
Abstract [en]

Catalytic combustion of synthetic gasified biomass was conducted in a high-pressure facility at pressures ranging from 5 to 16 bars. The catalytic combustor design considered was a hybrid monolith (400 cpsi, diameter 3.5 cm, length 3.6 cm and every other channel coated). The active phase consisted of 1 wt.% Pt/gamma-Al2O3 With wash coat loading of total monolith 15 wt.%. In the interpretation of the experiments, a twodimensional boundary layer model was applied successfully to model a single channel of the monolith. At constant inlet velocity to the monolith the combustion efficiency decreased with increasing pressure. A multi-step surface mechanism predicted that the flux of carbon dioxide and water from the surface increased with pressure. However, as the pressure (i.e. the Reynolds number) was increased, unreacted gas near the center of the channel penetrated significantly longer into the channel compared to lower pressures. For the conditions studied (lambda = 46, T-in = 218-257 degrees C and residence time similar to 5 ms), conversion of hydrogen and carbon monoxide were diffusion limited after ignition, while methane never ignited and was kinetically controlled. According to the kinetic model surface coverage of major species changed from CO, H and CO2 before ignition to O, OH, CO2 and free surface sites after ignition. The model predicted further that for constant mass flow combustion efficiency increased with pressure, and was more pronounced at lower pressures (2.5-10 bar) than at higher pressures (> 10 bar).

Keyword
catalytic combustion, gasified biomass, platinum, high pressure, catalytically stabilized combustion, CHEMKIN
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7571 (URN)10.1016/j.apcata.2005.07.003 (DOI)000232436200014 ()2-s2.0-24644491216 (Scopus ID)
Note
QC 20100916Available from: 2007-11-06 Created: 2007-11-06 Last updated: 2014-04-09Bibliographically approved
3. The Influence of NO on the Combustion Phasing in an HCCI Engine
Open this publication in new window or tab >>The Influence of NO on the Combustion Phasing in an HCCI Engine
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2006 (English)In: SAE 2006 World Congress & Exhibition Technical Papers, 2006, no 2006-01-0416Conference paper, Published paper (Refereed)
Abstract [en]

In this work the influence of NO on combustion phasing has been studied experimentally in a single cylinder HCCI engine. A isooctane/n-heptane blend (PRF), a toluene/n-heptane mixture (TRF) and a full boiling range gasoline were tested at two different operating conditions with NO concentrations ranging from 4 up to 476 ppm in the fresh intake air. All three fuels had the same RON of 84. The first operating condition had a high intake pressure (2 bar absolute) and low intake temperature (40 °C), where low temperature chemistry is relatively prominent. The other operating condition had a high intake temperature (100 °C) and atmospheric intake pressure with significantly lower cool flame reactivity. Additionally the effect of NO at two different engine speeds, 900 and 1200 rpm were studied.

The combustion phasing, represented by CA50 was advanced up to 12.5 CAD by the influence of NO. In the cases with the TRF and the full boiling range gasoline the combustion phasing advanced with an increasing NO concentration. The combustion phasing in the PRF case also advanced at low concentrations of NO, but retarded beyond the baseline case at high concentrations in the high-pressure case. Such effects on combustion phasing are explained in terms of reaction kinetic theory from the literature. At low concentrations NO provides extra branching pathways, but as NO concentration increases termination reactions take over. The interaction of NO and aromatic fuels has not been theoretically examined to the same extent in the literature and more work in this area is needed.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7572 (URN)10.4271/2006-01-0416 (DOI)2-s2.0-84864407492 (Scopus ID)
Conference
SAE 2006 World Congress & Exhibition, April 2006, Detroit, MI, USA,
Note
QC 20101109Available from: 2007-11-06 Created: 2007-11-06 Last updated: 2011-07-06Bibliographically approved

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