1. Complex Mechanisms
There are many intermediates between fuel and oxidizer
and equilibrium products
Even a mechanism of simple molecules
combustion of hydrogen
2 H2 + O2 −→ 2H2O
Is a complex interaction of many intermediates
Combustion mechanisms are complex with many intermediates, stages and hierarchies between the fuel and oxidizer and the final equilibrium
products.
Even a simple molecule such as hydrogen is a complex mechanism with many intermediates, reactions and submechanisms.

2. Hydrogen Combustion
H2 + M −→ H + H + M
H2 + O2 −→ HO2 + H
H + O2 −→ O + OH
O + H2 −→ H + OH
H2 + OH −→ H2O + H
O + H2O −→ OH + OH
H + H + M −→ H2 + M
O + O + M −→ O2 + M
H + O + M −→ OH + M
H + OH −→ H2O
H + O2 + M −→ HO2 + M
HO2 + H −→ OH + OH
HO2 + H −→ H2O + O
HO2 + O −→ H2O + OH
HO2 + HO2 −→ H2O2 + O2
HO2 + H2 −→ H2O2 + H
H2O2 + OH −→ H2O + HO2
H2O2 + H −→ H2O + OH
H2O2 + H −→ HO2 + H2
H2O2 + M −→ OH + OH + M
The hydrogen mechanism is the base of all hydrocarbon mechanisms. At the first level, the oxidation of hydrogen can be divided into two
submechanisms. The rate constants for these reactions are standardized with accepted values.
The first submechanism involves combinations of simple hydrogen and oxygen species, H, H2, H2O, O, O2, OH, and H, including some
pressure sensitive reactions.
The second submechanism involves the oxidation of H2O2 and involves the additional intermediate HO2.

3. Bonding of Hydrogen
H
H
Hydrogen Radical
Single Bonded Hydrogen
Hydrogen: Single Valence Electron
The first submechanism of the hydrogen mechanism involves essentially all combinations of simple oxygen and hydrogen radicals. To
understand them, we can look at the valence structure of each.
Hydrogen has one valence electron. The one electron is either alone making the hydrogen radical or shared to bond with another atom.

4. Bonding of Oxygen
O
O
O
Oxygen: 6 valence electrons
Double Bonded Oxygen
Single Bonded Oxygen
Oxygen Atom (singlet or triplet)
The oxygen atom has 6 valence electrons.
The electrons can be single in a radical, paired in a lone-pair or shared with another in one or two bonds.
There are 3 types of bonding that oxygen can be a part of:
- Oxygen Atom Two electron pairs and two unpaired electrons
- Single Bond A single bond, a single electron and two electron pairs.
- Double Bond Two bonds, to electron pairs.

5. Mechanism Structure
Not just a list of reactions
Structured set of submechanisms
Also different reactions are important under different
conditions
The hydrogen mechanism is not just a list of reactions. It is, in fact a structured set of submechanism and regimes that are significant under
di!erent conditions.

6. H2, O2, H, O, OH Reactions
Initiation Reactions H2 + M −→ H + H + M
(fuel and oxidizer) H2 + O2 −→ HO2 + H
Chain Reaction Steps H + O2 −→ O + OH
(increase of radicals) O + H2 −→ H + OH
H2 + OH −→ H2O + H
O + H2O −→ OH + OH
Chain Terminating Steps H + H + M −→ H2 + M
(decrease of radicals) O + O + M −→ O2 + M
H + O + M −→ OH + M
H + OH −→ H2O
Viewed as a chain branching process, we can see the di!erent types of reactions based on the number of radicals created and destroyed
between reactants and products.
The initiation reactions are those where stable species such as H2 and O2 form radicals.
The chain reaction steps are those in which one radical species in the reactants produces one or two radicals in the products. If these are
prevalent, then the hydrogen mechanism moves faster toward ignition.
The chain terminating steps are when two radicals in the reactants come together to form at least one stable species. When these reactions are
dominant, then reaction progress is slowed or stopped.

7. HO2, H2O2 Reactions
Transition to HO2 H + O2 + M −→ HO2 + M
HO2 Reactions HO2 + H −→ OH + OH
HO2 + H −→ H2O + O
HO2 + O −→ H2O + OH
Tranisition to H2O2 HO2 + HO2 −→ H2O2 + O2
HO2 + H2 −→ H2O2 + H
H2O2 Reactions H2O2 + OH −→ H2O + HO2
H2O2 + H −→ H2O + OH
H2O2 + H −→ HO2 + H2
H2O2 + M −→ OH + OH + M
The first submechanism of simple oxygen and hydrogen radicals has a transition through recombination to the mechanism with more complex
species such as HO2 and H2O2. Since HO2 is a radical, the formation of HO2 are chain branching reactions. The formation of H2O2 can be both
terminating and branching.

8. Surface Reactions
2 HO2 −→ H2O2 + O2
H2O2 −→ H2O +
1
2
O2
H, O, OH −→ destruction
To fully explain the ignition behavior of hydrogen, it was necessary to augment the reactions between intermediates with further catalytic
reactions with the walls. All these reactions tend to slow the progress toward ignition.

9. Complex Behavior
Different reactivity
in different regions
The three limits represent
three regimes
Subsets of reactions
dominate
Under di!erent regions, di!erent reactions dominate. If the terminating reactions dominate, then ignition is inhibited, but if branching
reactions dominate, ignition is promoted.
The graph shows the di!erent ‘limits’ of hydrogen ignition with respect to temperature with increasing pressure. Each regime represents a
certain subset of reactions that are dominate.

10. First Limit
The explosion limit is dependent
on the size of the vessel
The inhibitors are the destruction
of radicals, H, O, OH at the walls.
Very low pressures
The first limit is at very low pressure, even below atmospheric pressures. Experimental evidence of this is that the explosion limit is dependent
on the size of the vessel. This points to the destruction of radicals at the walls.

11. First Limit
As the pressure is going down
Something is inhibiting the chain
reaction steps
H + O2 −→ O + OH
O + H2 −→ H + OH
This means that the number of collisions to produce the chain reaction steps is low. At lower pressures, the reactions with the walls are more
prominent. A reaction at the wall acts as a termination reaction, thus lowering the explosive tendency.
Remember, if the termination reactions dominate, explosion does not occur.

12. Second Limit
Increase of Pressure inhibits the explosion
higher temperature needed to explode
not dependent on the size of the vessel
Thus not inhibited by walls
At higher pressures, the size of the vessel does not play a significant role.
But as the pressure increases, a higher temperature is needed to create an explosion.
At high pressures, three body reactions can dominate. Thus reactions with M, which are generally dependent on pressure, can be significant.
As pressure increases, the relative rates of terniary reactions increases more than binary reactions.

13. Second Limit
A gas phase reaction that is strongly dependent on pressure.
Competition between
H + O2 −→ O + OH (more radicals produced)
and
H + O2 + M −→ HO2 + M (a different radical produced)
Higher pressure is larger M concentration
HO2 less reactive This can diffuse to walls
Within the hydrogen mechanism, at the pressures above the second explosive limit, HO2 is relatively unreactive. Thus its formation can be
thought to be a termination reaction. It is able to migrate to the walls.
The inhibition of the HO2 reaction dominating above the second explosive limit, means that it requires a higher temperature to explode. The
peak pressure, which requires the highest temperature is around 300 atmospheres.

14. Third Limit
As the pressure increases, HO2 reacts with H2
HO2 + H2 −→ H2O2 + H
H2O2 + M −→ OH + OH + M
In addition, H2O, the major final
product is an excellent third body
In the third limit, at higher pressures, less temperature is needed to result in explosion. This means that the inhibiting e!ect of
the second limit, namely the HO2 inhibition, is not significant.
As the pressure increases even the inhibiting e!ect of HO2 formation is countered with the formation of H2O2 which in turn is a
significant source of hydroxyl radicals, in fact two for every H2O2 molecule.

15. CO Oxidation
CO Mechanism Propagation CO + HO2 −→ CO2 + OH
Propagation CO + OH −→ CO2 + H
H2 Mechanism Branching H + O2 −→ OH + O
Propagation H + O2 + M −→ HO2 + M
CO Mechanism Termination CO + O + M −→ CO2 + M
The following are the major steps for CO oxidation, made up of branching, propagating and termination steps.
The two propagation steps of the CO mechanism produce CO2 the final equilibrium product. But what is important is that at the same time the
number of radicals, in fact important radicals such as H and OH, in the system is not diminished (hence propagation).
Further propagation and branching are found the two important reactions in the hydrogen mechanism. H reactions with the original oxidizer,
O2, react to produce radicals important for the two CO oxidation reactions, namely, OH and HO2, one of which is a branching reaction bringing
more radicals into the system.
This complementary set of reactions substantiates the observation that H is a catalyst for CO oxidation.

16. Hierarchical Structure
CO Mechanism
H2 Mechanism
CO2
H2O
Oxidation
Hydrocarbon
Combustion mechanisms have a clear hierarchal structure. At the base is the hydrogen mechanism, where many of the critical radicals driving
the ignition are formed, such as OH and the carbonmonoxide submechanism.
On top of these mechanisms are the hydrocarbon mechanisms whose products produce precursors for these base mechanisms.

17. Carbon Bonding
C
C
C C
C
Single Bonded Carbon
Methylene: Radical Carbon
Double Bonded Carbon
Triple Bonded Carbon
(Singlet and Triplet)
C Double Bond with radical
C Triple Bond with radical
In order to understand the multitude of hydrocarbon reactions, it is useful to know at least the basic (lewis) bonding types of carbon.
Hydrocarbon mechanisms are essentially many combinations of transformations between these states.
The carbon has four electrons available for bonding (in its valence shell). The main stable carbon bonded states have either four single bonds, a
double bond with two single or a triple bond with a single bond. The simple radicals can be these same configurations, where one of the bonds
is replaced by an unbonded electron.
When it has two unbonded electrons, the carbon can be in two states, a singlet or triplet, both having two single bonds.

18. CH4 High Temperature Oxidation
Abstraction
OH,H,O
Addition of O
Loss of H
CH3
CH
4
CH
2
O
Abstraction
OH,H,O
CO
2
Aldehyde
HCO
H Loss Third Body (M, H20)
H Abstraction with HO2
CO
CO Oxidation Mechanism
Backbone of
Methane Oxidation
Mechanisms for methane oxidation can be quite large, but the main pathways will be explained in the following slides.
Methane oxidation, in detail, can be complex and with hundreds of
reaction. Many mechanisms exist in the literature, but all
have the same major features.
After an initial abstraction of hydrogen, and with the addition of an oxygen (and loss of hydrogen), an aldehyde is formed. This aldehyde
decomposes to carbon monooxide. Through the carbonmonoxide submechanism, the final equilibrium products are made.
This is the backbone of the methane oxidation.

19. CH4 High Temperature Oxidation
CH3
CH
4
CH
2
O
CO
2
CH2
CH
2
CH
(S)
OH,H Abstraction
OH Abstraction
OH Addition
Loss of H
H20 Addition
Loss of H
Side path with
CH2 Radicals
A significant side reaction to the main path is further abstractions through triplet and singlet CH2 radicals, which, with subsequent addition of
oxygen, through OH or water, form an aldehyde, within the backbone mechanism.

20. CH4 High Temperature Oxidation
CH
3
CH4
CH2O
CO2
CH
2
CH
2
CH
(S) CH
2
OH
OH Addition
H Loss
Third Body
H Loss
CH2OH
Formation
From CH3, this set of side reactions go through the CH2OH as an intermediate to the aldehyde. The CH2OH is formed through the addition of
OH radicals with the simultaneous loss of H.

21. CH4 Low Temperature Oxidation
CH
3
CH
4
CH
2
O
CO
2
CH
3
OH
CH
2
CH
2
CH
(S) CH
2
OH
Reverse
HCO
2
O
H Abstraction
H,OH
H,OH
H Abstraction
CH
3
O
Third Body
H Loss
C
2
H
4
CO
CH
2
Abstractions
Addition
3
CH
Two significant paths that appear in the low temperature ( < 1500) oxidation of methane.
One is the recombination reaction forming C2H6. This is why the propane submechanism, although a larger product, is an important to
methane oxidation.
Another submechanism involves the direct addition of OH to form CH3OH and its subsequent oxidation to aldehyde.

22. Aldehyde Decomposition
C
O
HH
X XH
H
C
O
H
X XH
H
C
O
R
C
O
HR
Aldehydes are an important intermediate from hydrocarbons to CO to CO2. This shows the electron structure of the conversion of an aldehyde
to the radical through abstraction or loss of H. The aldehyde starts as a double bond with two single bonds, one of which is to hydrogen. The
hydrogen is abstracted or lost and the aldehyde radical is produced with a doubly bonded carbon with a single electron radical and a single
bond.

23. Competition for OH
Any competition for the radicals, OH, HO2 and O
reduces efficiency
Reaction 1000K 1500K 2000K
Abs: CH4 + OH −→ CH3 + H2O
Prod: CO + OH −→ CO2 + H
Ratio Abs
P rod 4.6 9.2 13.3
In fuel rich environments, CO may remain as a product.
The main driving force of combustion reactions is the number of radicals in the system. Two of the most important of these is OH and H.
In methane combustion, in two propagating reactions, OH can be used in abstraction such as from CH4 or with CO to produce the final
equilibrium product, CO2.
When abstraction produces CH3, to oxidize to equilibrium products the mechanism has to go through the CO oxidation, which again can be
inhibited or side-tracked, inhibiting the oxidation of CO.
This competition can inhibit the oxidation of CO and, for example, in fuel rich environments, CO may be a more prominent product. Meaning
that methane combustion does not go to completion.