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MAE 5310: COMBUSTION FUNDAMENTALS

Overview of Some Important Chemical Mechanisms

October 15, 2012

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: =1.0

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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

Products AbsorbedRadicalwallk

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CARBON MONOXIDE OXIDATION• Hydrocarbon combustion can be characterized as a two-step process:

1. Breakdown of the fuel to carbon monoxide

2. 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 CnH2n+2. We will consider cases where n > 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:1. 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

2. Unsaturated olefins further oxidize to CO and H2. Essentially, all of H2 is converted to water

3. 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 -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 possible1. C3H7 + M → C3H6 + H + M2. 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

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One step mechanism

Four step mechanism

See parameters: A, Ea/R, m and n on next pageChosen to provide best fit agreement betweenExperimental and predicted flame temperatures, as well as flammability limits

See parameters: x, Ea/R, a, b, c on next pageThis 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 T=1345 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:

1. Zeldovich mechanism (also called the thermal mechanism)

• Dominates at high temperatures over wide range of • Usually unimportant for T < 1800 K

2. Fenimore (also called the prompt mechanism)

• Important in fuel rich combustion

3. N2O-intermediate mechanism

• Important in lean ( < 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 < 1.2

– HCN + O ↔ NCO + H

– NCO + H ↔ NH + CO

– NH + H ↔ N + H2

– N + OH ↔ NO + H

• For > 1.2, other routes open up and the chemistry becomes much more complex