MAE 5310: COMBUSTION FUNDAMENTALS

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MAE 5310 COMBUSTION FUNDAMENTALS

• Overview of Some Important Chemical Mechanisms
• September 24, 2009
• Mechanical and Aerospace Engineering Department
• Florida Institute of Technology
• D. R. Kirk

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INTRODUCTION TO IMPORTANT CHEMICAL MECHANISMS

• Purpose
• Outline elementary steps involved in a number of
  chemical mechanisms of significant importance to
  combustion and combustion generated air pollution
• Fundamental ideas developed in chemical kinetics
  directly applicable to understanding complex real
  systems
• Precautionary note
• Complex mechanisms are evolutionary products of
  chemists thoughts and experiments, and may
  change with time as new insights are developed
• Therefore when we discuss a particular mechanism,
  we are not referring to the mechanism in the same
  sense that we might refer to the first law of
  thermodynamics

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THE H2-O2 SYSTEM

• Hydrogen-oxygen system is important in rocket
  propulsion and also important subsystem in
  oxidation of hydrocarbons and carbon monoxide
• Depending on the temperature, pressure, and
  extent of reaction, reverse reactions may be
  possible
• In modeling the H2-O2 system as many as 40
  reactions can be taken into account involving 8
  species H2, O2, H2O, OH, O, H, HO2, and H2O2
• Consider detailed mechanism shown on the next
  slide
• Consider explosive behavior of H2-O2 system
• Comments
• Understanding of detailed chemistry of a system
  is very useful in understanding experimental
  observations
• Such an understanding is essential to development
  of predictive models of combustion phenomena when
  chemical effects are important

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THE H2-O2 SYSTEM
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H2-O2 EXPLOSION CHARACTERISTICS f1.0
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H2-O2 EXPLOSION CHARACTERISTICS

• Follow a vertical line at, say 500 C
• 1 1.5 mm Hg there is no explosion
• Lack of explosion is a result of the free
  radicals produced in the chain initiation step
  (H.2) and chain sequence (H.3-H.6) being
  destroyed by reactions on the wall of the vessel
• Wall reactions break the chain, preventing
  build-up of radicals that lead to explosion
• Note that the wall reactions are not explicitly
  included in the mechanism since they are not
  strictly gas phase reactions. Symbolically
• 1.5 50 mm Hg there is an explosion
• Direct result of gas-phase chain sequence H.3-H.6
  prevailing over radical destruction at wall
• Remember that increasing the pressure increases
  the radical concentration linearly, while
  increasing the reaction rate geometrically
• 50 3,000 mm Hg there is no explosion
• The cessation of explosive behavior can be
  explained by the competition for H atoms between
  the chain branching reactions, H.3, and what is
  effectively a chain-terminating step at low
  temperatures, reaction H.11.
• Reaction H.11 is chain terminating because the
  hydroperoxy radical, HO2, is relatively
  unreactive at these conditions, and because of
  this, it can diffuse to wall where it is
  destroyed
• Above 3,000 mm Hg there is an explosion
• At these conditions reaction H.16 adds a
  chain-branching step with opens up the H2O2 chain
  sequence

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CARBON MONOXIDE OXIDATION

• Hydrocarbon combustion can be characterized as a
  two-step process
• Breakdown of the fuel to carbon monoxide
• Oxidation of carbon monoxide to carbon dioxide
• CO is slow to oxidize unless there is some
  hydrogen containing species present
• Small quantities of H2O (often called moist CO)
  or H2 can have a tremendous effect on the
  oxidation rate
• This is because the CO oxidation step involving
  the hydroxyl radical is much faster than the
  steps involving O2 and O
• Assuming that water is primary hydrogen
  containing species, following steps describe
  oxidation of CO
• First reaction is slow and does not contribute
  significantly to formation of CO2, but rather
  serves as the initiator of the chain sequence
• The 3rd reaction is the actual CO oxidation step
  (chain propagating step), producing H atoms to
  react with O2 to form OH and O (in reaction 4)
• These radicals, in turn, feed back into the
  oxidation step (reaction 3) and the first chain
  branching step (reaction 2). The CO OH ? CO2
  H (reaction 3) is the key reaction in the overall
  scheme

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OXIDATION OF HIGHER PARAFFINS

• Paraffins or alkanes, are saturated,
  straight-chain or branched chain, single bonded
  hydrocarbons with the general molecular formula
  CnH2n2. We will consider cases where n gt 2
• No attempt is made to explore or list many
  elementary reactions involved
• Strategy instead will be to
• Present an overview of oxidation process
• Indicate key reactions steps
• Discuss multi-step global mechanisms approaches
  that have had some success
• For further discussion of paraffins, olefins,
  etc., see Chapter 3, Section E
• The oxidation of paraffins can be characterized
  by three sequential processes
• Fuel molecule attacked by O and H atoms and
  breaks down, mostly forming olefins (double
  carbon bonds, CnH2n) and H2. H2 oxidizes to
  water, based on available oxygen
• Unsaturated olefins further oxidize to CO and H2.
  Essentially, all of H2 is converted to water
• The CO burns out via reaction CO OH ? CO2 H.
  Nearly all of heat release associated with the
  overall combustion process occurs in this step

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ILLUSTRATION OF PROCESS WITH PROPANE

• Step 1 A single carbon-carbon (C-C) bond is
  broken in the original fuel molecule. The C-C
  bonds are preferentially broken over
  hydrogen-carbon bonds because the C-C bonds are
  weaker
• Example C3H8 M ? C2H5 CH3 M
• Step 2 The two resulting hydrocarbon radicals
  break down further, creating olefins
  (hydrocarbons with double carbon bonds, CnH2n)
  and hydrogen atoms. The removal of an H atom from
  the hydrocarbon is termed H-atom abstraction. In
  the example for this step, ethylene and methylene
  are produced.
• C2H5 M ? C2H4 H M
• CH3 M ? CH2 H M
• Step 3 The creation of H atoms from Step 2
  starts development of a radical pool
• Example H O2 ? O OH
• Step 4 With new radicals, new fuel-molecule
  attack pathways open up
• Examples
• C3H8 OH ? C3H7 H2O
• C3H8 H ? C3H7 H2
• C3H8 O ? C3H7 OH

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ILLUSTRATION OF PROCESS WITH PROPANE

• Step 5 As in Step 2, the hydrocarbon radicals
  again decay into olefins and H atoms via H-atom
  abstraction
• Example C3H7 M ? C3H6 H M
• And following the b-scission rule
• Rule states that the C-C or C-H bond broken will
  be the one that is one place removed from the
  radical site (the site of the unpaired electron)
• The unpaired electron at the radical site
  strengthens the adjacent bonds at the expense of
  those one placed removed from the site
• For the C3H7 radical created in Step 4, two paths
  are possible
• C3H7 M ? C3H6 H M
• C3H7 M ? C2H4 CH3 M
• Step 6 The oxidation of the olefins created in
  Steps 2 and 5 is initiated by O-atom attack,
  which produces formyl radicals (HCO) and
  formaldehyde (H2CO)
• Examples
• C3H6 O ? C2H5 HCO
• C3H6 O ? C2H4 H2CO
• Step 7
• a Methyl radicals (CH3) oxidize
• b Formaldehyde (H2CO) oxidizes
• c Methylene (CH2) oxidizes
• Each of these steps produces carbon monoxide, the
  oxidation of which is Step 8
• Step 8 Carbon monoxide oxidizes following the
  moist CO mechanism discussed above

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GLOBAL AND QUASI-GLOBAL MECHANISMS
One step mechanism
See parameters A, Ea/R, m and n on next
page Chosen to provide best fit agreement
between Experimental and predicted flame
temperatures, as well as flammability limits
Four step mechanism
See parameters x, Ea/R, a, b, c on next
page This particular mechanism assumes that
ethylene (C2H4) is the intermediate hydrocarbon
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GLOBAL AND QUASI-GLOBAL MECHANISMS
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GRI MECH METHANE COMBUSTION (1-46)
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GRI MECH METHANE COMBUSTION (47-92)
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GRI MECH METHANE COMBUSTION (93-137)
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GRI MECH METHANE COMBUSTION (138-177)
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GRI MECH METHANE COMBUSTION (178-219)
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GRI MECH METHANE COMBUSTION (220-262)
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GRI MECH METHANE COMBUSTION (263-279)
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CH4 MOLECULAR STRUCTURE DIAGRAM HIGH T

• Consider the following molecular structure
  diagram which shows a linear progression of CH4
  to CO2 with several side loops originating from
  the methyl (CH3) radical
• Direct Pathways Linear Progression, called the
  backbone
• The linear progression starts with an attack on
  the CH4 molecule by OH, O and H radicals to
  produce the methyl radical
• The methyl radical then combines with an oxygen
  atom to form formaldehyde (CH2O)
• CH3 O ? CH2O H
• The formaldehyde is attacked by OH, H, and O
  radicals to produce the formyl radical (HCO)
• The formal radical is converted to CO by a trio
  of reactions
• HCO H2O
• HCO M
• HCO OH
• Finally CO is converted to CO2, primarily by
  reaction with OH
• Indirect Pathways
• CH3 radicals also react to form CH2 radicals in
  two possible electronic configurations, which are
  shown in the left side pathway
• The singlet electronic state of CH2 is designated
  as CH2(S) (does not stand for solid)
• On the right side-loop, CH3 is first converted to
  CH2OH, which in turn is converted to CH2O
• Other less important pathways complete the
  mechanism, which have reaction rates of less than
  1x10-7 mol/cm3 s, and are not shown in the
  structure diagram

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CH4 MOLECULAR STRUCTURE DIAGRAM HIGH T
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CH4 MOLECULAR STRUCTURE DIAGRAM LOW T

• At lower temperatures (say less than 1500 K)
  pathways that were unimportant at higher
  temperatures now become prominent
• Consider the following diagram, which is at a
  temperature of T1345 K
• The black arrows show the new pathways that now
  complement all of the high-temperature pathways,
  and there are several interesting features
• There is a strong recombination of CH3 back to
  CH4
• An alternate route from CH3 to CH2O appears
  through the intermediate production of methanol
  (CH3OH)
• CH3 radicals combine to form ethane (C2H6), a
  higher hydrocarbon than the original reactant,
  which was methane (CH4)
• The C2H6 is ultimately converted to CO (and CH2)
  through C2H4 (ethene) and C2H2 (acetylene)
• See steps 5-7 on slide 5
• The appearance of hydrocarbons higher than the
  initial reactant hydrocarbon is a common feature
  of low-temperature oxidation processes.

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CH4 MOLECULAR STRUCTURE DIAGRAM LOW T
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OXIDES OF NITROGEN (NOx) FORMATION

• Nitric oxide is an important minor species in
  combustion because of its contribution to air
  pollution
• In combustion of fuels that contain no nitrogen,
  NO is formed by three chemical mechanisms that
  involve nitrogen from the air
• Zeldovich mechanism (also called the thermal
  mechanism)
• Dominates at high temperatures over wide range of
  f
• Usually unimportant for T lt 1800 K
• Fenimore (also called the prompt mechanism)
• Important in fuel rich combustion
• N2O-intermediate mechanism
• Important in lean (f lt 0.8), low temperature
  combustion
• Mechanism is important in NO control strategies
  that involve lean premixed combustion
• Currently being explored by gas-turbine
  manufacturers

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EXTENDED ZELDOVICH MECHANISM

• This 3 reaction set is referred to as the
  extended Zeldovich mechanism
• Mechanism is coupled to the fuel chemistry
  through the O2, O, and OH species
• In process where the fuel combustion is complete
  before NO formation becomes significant, the two
  processes can be uncoupled
• If relevant time scales are sufficiently long can
  assume that the N2, O2, O, and OH concentrations
  are at their equilibrium values and the N atoms
  are in steady-state
• May also assume that the NO concentrations are
  much less than their equilibrium values, the
  reverse reactions can be neglected
• With these two assumptions (which greatly
  simplify the problem of calculating the NO
  formation), a simple rate expression results
• Note that within flame zones and in short time
  scale post-flame processes, the equilibrium
  assumption is not valid
• Compared with time scales of fuel oxidation
  process, NO is formed rather slowly by the
  thermal mechanism, thus thermal NO is generally
  considered to be formed in post-flame gases

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FENIMORE (PROMPT) MECHANISM

• Linked closely to combustion chemistry of
  hydrocarbons
• Some NO rapidly produced in the flame zone of
  laminar premixed flames long before there would
  be time to form NO by the thermal mechanism
• The general scheme of the Fenimore mechanism is
  that hydrocarbon radicals react with molecular
  nitrogen to form amines or cyano compounds, and
  the amines or cyano compounds are then converted
  to intermediate compounds that ultimately form NO
• The mechanism can be written as (ignoring the
  processes that form the CH radicals to initiate
  the mechanism)
• CH N2 ? HCN N
• C N2 ? CN N
• For f lt 1.2
• HCN O ? NCO H
• NCO H ? NH CO
• NH H ? N H2
• N OH ? NO H
• For f gt 1.2, other routes open up and the
  chemistry becomes much more complex