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Wall Related Lean Premixed Combustion Modeled with Complex Chemistry
KTH, Superseded Departments, Chemical Engineering and Technology.
2002 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Increased knowledge into the physics and chemistrycontrolling emissions from flame-surface interactions shouldhelp in the design of combustion engines featuring improvedfuel economy and reduced emissions.

The overall aim of this work has been to obtain afundamental understanding of wall-related, premixed combustionusing numerical modeling with detailed chemical kinetics. Thiswork has utilized CHEMKIN®, one of the leading softwarepackages for modeling combustion kinetics.

The simple fuels hydrogen and methane as well as the morecomplex fuels propane and gasified biomass have been used inthe model. The main emphasis has been on lean combustion, andthe principal flow field studied is a laminar boundary layerflow in two-dimensional channels. The assumption has been madethat the wall effects may at least in principle be the same forlaminar and turbulent flames.

Different flame geometries have been investigated, includingfor example autoignition flames (Papers I and II) and premixedflame fronts propagating toward a wall (Papers III and IV).Analysis of the results has shown that the wall effects arisingdue to the surface chemistry are strongly affected by changesin flame geometry. When a wall material promoting catalyticcombustion (Pt) is used, the homogeneous reactions in theboundary layer are inhibited (Papers I, II and IV). This isexplained by a process whereby water produced by catalyticcombustion increases the rate of the third-body recombinationreaction: H+O2+M ⇔ HO2+M. In addition, the water produced at higherpressures increases the rate of the 2CH3(+M) ⇔ C2H6(+M) reaction, giving rise to increased unburnedhydrocarbon emissions (Paper IV).

The thermal coupling between the flame and the wall (theheat transfer and development of the boundary layers) issignificant in lean combustion. This leads to a sloweroxidation rate of the fuel than of the intermediatehydrocarbons (Paper III).

Finally in Paper V, a well-known problem in the combustionof gasified biomass has been addressed, being the formation offuel-NOx due to the presence of NH3 in the biogas. A hybridcatalytic gas-turbine combustor has been designed, which cansignificantly reduce fuel-NOx formation.

Keywords:wall effects, premixed flames, flamequenching, numerical modeling, CHEMKIN, boundarylayerapproximation, gasified biomass, fuel-NOx, hybrid catalytic combustor.

Place, publisher, year, edition, pages
Stockholm: Kemiteknik , 2002. , 73 p.
Series
Trita-KET, ISSN 1104-3466 ; 164
Keyword [en]
Wall effects, premixed flames, flame quenching, numerical modeling, chemkin, boundary layer approximation, gasified biomass, fuel-NOx, hybrid catalytic combustor.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:kth:diva-3455ISBN: 91-7283-391-2 (print)OAI: oai:DiVA.org:kth-3455DiVA: diva2:9256
Public defence
2002-12-17, 00:00 (English)
Note
QC 20100504Available from: 2002-12-11 Created: 2002-12-11 Last updated: 2010-05-04Bibliographically approved
List of papers
1. Wall Effects of Laminar Hydrogen Flames over Platinum and Inert Surfaces
Open this publication in new window or tab >>Wall Effects of Laminar Hydrogen Flames over Platinum and Inert Surfaces
2000 (English)In: AIChE Journal, ISSN 0001-1541, E-ISSN 1547-5905, Vol. 46, no 7, 1454-1460 p.Article in journal (Refereed) Published
Abstract [en]

Different aspects of wall effects in the combustion of lean, laminar and stationary hydrogen flames in an axisymmetric boundary-layer flow were studied using numerical simulations with the program CRESLAF. The importance of the chemical wall effects compared to thermal wall effects caused by heat transfer to a cold wall was investigated in the reaction zone by using different combustion systems at atmospheric pressure. Surface mechanisms include a catalytic surface, an inert surface that promotes radical recombinations, and a completely inert wall used as reference was the simplest possible boundary condition. The analysis of the results show that for the richer combustion case ( = 0.5) the surface chemistry gives significant wall effects, while the thermal and velocity boundary layer gives rather small effects. But for the leaner combustion case ( = 0.1) the thermal and velocity boundary layer gives more significant wall effects, while surface chemistry gives less significant wall effects compared to the other case. As expected, the overall wall effects were more pronounced for the leaner combustion case.

Keyword
MODEL; CHEMISTRY; OXIDATION
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12245 (URN)10.1002/aic.690460718 (DOI)000088372000016 ()
Note
QC 20100504Available from: 2010-03-30 Created: 2010-03-30 Last updated: 2017-12-12Bibliographically approved
2. Numerical studies of wall effects with laminar methane flames
Open this publication in new window or tab >>Numerical studies of wall effects with laminar methane flames
2002 (English)In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 128, no 1-2, 165-180 p.Article in journal (Refereed) Published
Abstract [en]

Wall effects in the combustion of lean methane mixtures have been studied numerically using the CHEMKIN software. To gain a deeper understanding of the flame-wall interaction in lean burn combustion, and in particular the kinetic and thermal effects, we have simulated lean and steady methane/air flames in a boundary layer flow. The gas-phase chemistry is modeled with the GRI mechanism version 1.2. Boundary conditions include an inert wall, a recombination wall and catalytic combustion of methane. Different pressures, wall temperatures and fuel-air ratios are used to address questions such as which part of the wall effects is most important at a given set of conditions. As the results are analyzed it can be seen that the thermal wall effects are more significant at the lower wall temperature (600 K) and the wall can essentially be modeled as chemical inert for the lean mixtures used. At the higher wall temperature (1,200 K), the chemical wall effects become more significant and at the higher pressure (10 atm) the catalytic surface retards homogeneous combustion of methane more than the recombination wall because of product inhibition. This may explain the increased emissions of unburned fuel observed in engine studies, when using catalytic coatings on the cylinder walls. The overall wall effects were more pronounced for the leaner combustion case (phi = 0.2). When the position of the reaction zone obtained from the boundary layer calculations is compared with the results from a one-dimensional premixed flame model, there is a small but significant difference except at the richer combustion case (phi = 0.4) at atmospheric pressure, where the boundary layer model may not predict the flame position for the given initial conditions.

Keyword
INERT SURFACES, COMBUSTION, HYDROGEN, PLATINUM, MODEL, AIR
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12249 (URN)10.1016/S0010-2180(01)00342-X (DOI)000173661900011 ()
Note

QC 20100504

Available from: 2010-03-30 Created: 2010-03-30 Last updated: 2017-12-12Bibliographically approved
3. A Numerical Study of Sidewall Quenching with Propane/Air Flames
Open this publication in new window or tab >>A Numerical Study of Sidewall Quenching with Propane/Air Flames
2002 (English)In: Proceedings of the Combustion Institute, ISSN 0082-0784, E-ISSN 1878-027X, Vol. 29, 789-795 p.Article in journal (Refereed) Published
Abstract [en]

The head-on (i.e., stagnation) configuration has generally been used to numerically and experimentally characterize the flame-wall interaction with complex chemistry and multicomponent transport. Other studies have treated the transient case of a flame propagating toward a wall, and combustion in a boundary layer has also been dealt with. In this paper, a two-dimensional stationary model has been used to study the sidewall quenching,of laminar propane/air flames in a boundary-layer flow. This geometry may be described as a flame parallel to the wall that is swept away with a laminar boundary-layer flow while propagating toward and interacting with the wall. The main purpose has been to examine the extent to which the flame can propagate toward the cooled wall for lean flames compared to stoichiometric flames. A detailed kinetic model is used to examine the oxidation of both the fuel and the intermediate hydrocarbons (IHCs). For stoichiometric and near stoichiometric mixtures, the thermal coupling between the flame and the wall is small but significant. However, for very lean flames, the thermal coupling between the flame and the wall is found to be very significant. The intermediate hydrocarbons are the dominant emissions for stoichiometric and near-stoichiometric flames in contrast to the leaner flames in which the fuel becomes more significant. This implies that the IHCs are very important for the overall hydrocarbon emissions from flame quenching; as a result detailed kinetics of complex fuels should be used when determining the unburned hydrocarbon emissions.

National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-12273 (URN)10.1016/S1540-7489(02)80101-8 (DOI)000182866100097 ()
Note
QC 20100505Available from: 2010-04-01 Created: 2010-04-01 Last updated: 2017-12-12Bibliographically approved
4. Kinetic and Transport Effects of Pressurized Methane Flames in a Boundary Layer
Open this publication in new window or tab >>Kinetic and Transport Effects of Pressurized Methane Flames in a Boundary Layer
2003 (English)In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 133, no 4, 503-506 p.Article in journal (Refereed) Published
Abstract [en]

We have recently modeled combustion of lean methane-air [1] mixtures in a boundary layer flow using the program CRESLAF [2, 3 and 4]. A uniform fuel-air mixture above the auto-ignition temperature was introduced at the inlet edge of the wall, which gave an arrested flame zone, propagating in the upstream direction with a local flame speed that is equal and opposite to the local flow velocity. We compared the interaction of this flame with three model wall materials representing three idealized cases from a chemical point of view, a completely inert wall, a radical recombining wall, and a wall supporting catalytic combustion.

Here we report on an analogous study of a flame geometry that may be considered a combination of a one-dimensional flame propagating towards a wall and the combustion of a uniform fuel-air mixture in a boundary layer flow. In contrast to our previous work [1] where we had a uniform inlet flow composition consisting of unburnt gas, here there is only unburnt gas close to the walls while there is burnt gas in the center of the channel. The present study concerns lean pressurized methane flames propagating toward hot isothermal walls where chemistry on the wall is considered important.

The main purpose is to compare the results with those obtained in Ref. [1], which enables us, for the same flow field (boundary layer flow), to compare the effect of flame geometry on the wall effects. We have found and have been able to explain theoretically that such subtle changes of the flame geometry, which would be rather difficult to study experimentally, may have surprisingly significant effects on the combustion process.

Keyword
SURFACE; COMBUSTION; REACTOR
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-12274 (URN)10.1016/S0010-2180(03)00041-5 (DOI)000184168400012 ()
Note
QC 20100505Available from: 2010-04-01 Created: 2010-04-01 Last updated: 2017-12-12Bibliographically approved
5. A Design Concept to Reduce Fuel NOx in Catalytic Combustion of Gasified Biomass
Open this publication in new window or tab >>A Design Concept to Reduce Fuel NOx in Catalytic Combustion of Gasified Biomass
2003 (English)In: AIChE Journal, ISSN 0001-1541, E-ISSN 1547-5905, Vol. 49, no 8, 2149-2157 p.Article in journal (Refereed) Published
Abstract [en]

A reactor concept was studied to reduce the fuel NOx at conditions relevant to catalytic combustion of gasified biomass containing ammonia. A hybrid reactor is modeled with passive and active channels, where only part of the fuel is combusted catalytically in the active channels. The completion of the reactions is carried out in the subsequent homogeneous zone. The air-fuel ratio is found to be the most important parameter for the NOx emission level. When the primary zone is operated fuel-lean, no favorable conditions are established for selective noncatalytic reduction reactions in the homogeneous zone, and the fuel nitrogen is largely oxidized to NO. However, if the air supply to the monolith is staged rich-lean, a 95% reduction in NO is possible. The NO reduction is facilitated by the remaining fuel components, CO and H-2.

Keyword
MODELS; OXIDATION; MONOLITH; BIOGAS; FLOW; NH3
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-12276 (URN)10.1002/aic.690490822 (DOI)000184795500021 ()
Note
QC 20100505Available from: 2010-04-01 Created: 2010-04-01 Last updated: 2017-12-12Bibliographically approved

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