ME 475/675 Introduction to
Combustion
Lecture 20
Chapter 5 Some Important Chemical Mechanisms, 𝐻2 − 𝑂2 , CO
oxidation, Oxidation of Hydrocarbons, Global reaction rates
Announcements
• HW 7, Due Monday, 10/12/15
• College Distinguished Lecture
• Tesla’s JB Straubel to speak about Nevada’s clean energy future
• Sunday, October 11, 2015
• 5 pm posters; 6 pm Lecture
• http://www.unr.edu/nevada-today/news/2015/jb-straubel-to-speak-at-university
HW 7 hints
• X2
• H2 / O2 reaction
• Carefully apply steady state approximations
• 4.16:
• NO formation
• you’ve already done part A
𝑑 𝑁𝑂
•
= 2𝑘1𝑓 𝑂 𝑒𝑞 𝑁2 𝑒𝑞 , so 𝑁𝑂 = 2𝑘1𝑓 𝑂 𝑒𝑞 𝑁2
𝑑𝑡
• Use equilibrium constant to find backward rate
𝑒𝑞 𝑡
• 4.18
• CO combustion
• Find characteristic time (can assume one reactant is much more plentiful than other)
• 4.19
• Use TPEQUIL to find equilibrium NO mole fraction for each conditions and compared to
the amount calculated at 10 ms. Follow directions
Chapter 5 Some Important Chemical Mechanisms
• Can be very complex, and is a topic of
current research, so changing with time
• The 𝐻2 − 𝑂2 system,
• Page 149-140 lists 20 reactions (40 when
reversed)
• Initiating, Reactions 1 and 2
• Create 𝐻 and 𝐻𝑂2 radicals
• Reactions involving O, H and OH:
• Chain reactions: 3-6 (create 𝑂𝐻, 𝑂 𝑎𝑛𝑑 𝐻)
• Chain terminating (ter-molecular) 7-10
• Reactions involving 𝐻𝑂2 (hydroperoxy) and
𝐻2 𝑂2 (hydrogen peroxide)
•
•
•
•
High pressure
Reactions 11-14 form 𝐻𝑂2 and then OH, O
Reactions 15-16 form 𝐻2 𝑂2 and O
Reactions 17-20 consume 𝐻2 𝑂2 , H and OH
Explosive limit dependence on pressure and pressure
Explosive
No Explosion, 11
destroys H atoms
Explosive
Radicals not
destroyed by walls
(reactions 3-6)
No Explosion, Walls
Destroy Radicals
CO oxidation
• This is important since hydrocarbon (𝐶𝑥 𝐻𝑦 ) combustion has 2 main steps
• 𝐶𝑥 𝐻𝑦 + 𝑂2 → 𝐶𝑂 + ⋯
• 𝐶𝑂 + 𝑂2 → 𝐶𝑂2 + ⋯
• This second step is “slow” unless 𝐻2 or 𝐻2 𝑂 are present (these molecules help produce 𝑂𝐻)
• If 𝐻2 𝑂 is the primary 𝐻 containing species
•
•
•
•
𝐶𝑂 + 𝑂2 → 𝐶𝑂2 + 𝑂
𝑂 + 𝐻2 𝑂 → 𝑂𝐻 + 𝑂𝐻
𝐶𝑂 + 𝑂𝐻 → 𝐶𝑂2 + 𝐻
𝐻 + 𝑂2 → 𝑂𝐻 + 𝑂
(slow but produces needed 𝑂, initiator of chain)
(produces needed 𝑂𝐻)
(key producer of 𝐶𝑂2 , needs 𝑂𝐻)
(produces more 𝑂𝐻)
• If 𝐻2 is present
• 𝑂 + 𝐻2 → 𝑂𝐻 + 𝐻
• 𝑂𝐻 + 𝐻2 → 𝐻2 𝑂 + 𝐻 (These lead to 𝐻𝑂2 formation, see 𝐻2 − 𝑂2 system)
• 𝐶𝑂 + 𝐻𝑂2 → 𝐶𝑂2 + 𝑂𝐻
Oxidation of Hydrocarbons
• Alkanes (or Paraffins)
• Saturated, straight-chain or branching-chain, singlebonded hydrocarbons, 𝐶𝑛 𝐻2𝑛+2
• Discuss Higher Alkanes 𝑛 ≥ 3
• Discuss 𝑛 = 1 and 2 (methane 𝐶𝐻4 , ethane 𝐶2 𝐻6 ), later
• Three step process
• 𝑂 𝑎𝑛𝑑 𝐻 attach fuel (remove H from fuel)
• Produces Alkenes (𝐶𝑛 𝐻2𝑛 , double carbon bonds) 𝐻 and water
• Alkenes oxidize
• Produces 𝐶𝑂 𝑎𝑛𝑑 𝐻2 (all is converted to Water)
• 𝐶𝑂 burns (𝐶𝑂 + 𝑂𝐻 → 𝐶𝑂2 + 𝐻 )
• Nearly all the heat release occurs in this step
Mole Fraction and Temperature versus Location (time)
• Propane 𝐶3 𝐻8 in air
• 𝐶3 𝐻8 mole fraction decreases with
distance (time)
• Alkenes (𝐶3 𝐻6 , 𝐶2 𝐻4 ), 𝐻2 and CO
increase with distance, reach a
maximum and drop off
• The temperature rises significantly at
locations where the CO is being
consumed (heat release) and 𝐶𝑂2 is
being produced
• Minor species (some oxygenated)
increase then decrease
More detailed 8-step process (I. Glassman)
1. Break C-C bond (weaker than C-H bonds)
1. 𝐶3 𝐻8 + 𝑀 → 𝐶2 𝐻5 + 𝐶𝐻3 + 𝑀
2. Create alkenes (olifins?) by H-atom abstraction
1. 𝐶2 𝐻5 + 𝑀 → 𝐶2 𝐻4 + 𝐻 + 𝑀
2.
𝐶𝐻3 + 𝑀 → 𝐶𝐻2 + 𝐻 + 𝑀
3. 𝐻 atoms help produce pool of radicals
1. 𝐻 + 𝑂2 → 𝑂𝐻 + 𝑂
4. Radicals attack fuel
1. 𝐶3 𝐻8 + 𝑂𝐻 → 𝐶3 𝐻7 + 𝐻2 𝑂
2. 𝐶3 𝐻8 + 𝐻 → 𝐶3 𝐻7 + 𝐻2
3. 𝐶3 𝐻8 + 𝑂 → 𝐶3 𝐻7 + 𝑂𝐻
5. New hydrocarbon radicals decay into alkenes via H-atom abstraction
1. 𝐶3 𝐻7 + 𝑀 → 𝐶3 𝐻6 + 𝐻 + 𝑀
2. Follows b-session rule
b-session rule
• Radical site:
• site of unpaired electron
• Strengthens the adjacent bond(s) but weakens the next bond
• The C-C or C-H bond that breaks will be one bond away from the radical site
• Two possibilities (as seen in temporal evolution two slides back):
• 𝐶3 𝐻7 + 𝑀 → 𝐶3 𝐻6 + 𝐻 + 𝑀 or
• 𝐶3 𝐻7 + 𝑀 → 𝐶2 𝐻4 + 𝐶𝐻3 + 𝑀
Continuation of 8-step process
6. O-atoms attach olefins from steps 2 and 5
1. 𝐶3 𝐻6 + 𝑂 → 𝐶2 𝐻5 + 𝐻𝐶𝑂 (formyl radical)
2. 𝐶3 𝐻6 + 𝑂 → 𝐶2 𝐻4 + 𝐻2 𝐶𝑂 (formaldehyde)
7. Methyl radicals (𝐶𝐻3 ), formaldehyde (𝐻2 𝐶𝑂) and Methylene (𝐶𝐻3 ) oxidize
1. Produces 𝐶𝑂
8. 𝐶𝑂 oxidizes to produce 𝐶𝑂2
Global and Quasi-global mechanisms rates
• Empirical
• 𝐶𝑥 𝐻𝑦 + 𝑥 +
𝑦
4
𝑂2
𝑘𝐺
𝑦
2
𝑥𝐶𝑂2 + 𝐻2 𝑂
• stoichiometric mixture with 𝑂2 , not air
•
𝑑 𝐶𝑥 𝐻𝑦
𝑑𝑡
= −𝐴𝑒𝑥𝑝
𝐸𝑎 𝑅𝑢
𝑇
𝐶𝑥 𝐻𝑦
𝑚
𝑂2
𝑛
=
𝑔𝑚𝑜𝑙𝑒
𝑐𝑚3 𝑠
• Page 157, Table 5.1: 𝐴, 𝐸𝑎 𝑅𝑢 , 𝑚 𝑎𝑛𝑑 𝑛 for different fuels
• These values are based on flame speed data fit (Ch 8)
• In Table 5.1 units for 𝐴 =
• However, we often want 𝐴
•
1 𝑔𝑚𝑜𝑙𝑒 1−𝑚−𝑛
𝑠
𝑐𝑚3
• 𝐴
1 𝑘𝑚𝑜𝑙𝑒 1−𝑚−𝑛
𝑠
𝑚3
𝑔𝑚𝑜𝑙𝑒 1−𝑚−𝑛
−
𝑚+𝑛
𝑔𝑚𝑜𝑙𝑒
𝑔𝑚𝑜𝑙𝑒
𝑐𝑚3
=
𝑐𝑚3 𝑠
𝑐𝑚3
𝑠
1−𝑚−𝑛
1 𝑘𝑚𝑜𝑙𝑒
in units of
𝑠
𝑚3
𝑘𝑚𝑜𝑙𝑒
1000 𝑔𝑚𝑜𝑙𝑒
=𝐴
1−𝑚−𝑛
100 𝑐𝑚 3
𝑚
1 𝑔𝑚𝑜𝑙𝑒 1−𝑚−𝑛
𝑠
𝑐𝑚3
=
1 𝑔𝑚𝑜𝑙𝑒 1−𝑚−𝑛
𝑠
𝑐𝑚3
Usually Want These Units
1000
1−𝑚−𝑛
=
1 𝑘𝑚𝑜𝑙𝑒 1−𝑚−𝑛
𝑠
𝑚3
10001−𝑚−𝑛 = 𝐴 𝑇𝑒𝑥𝑡𝑏𝑜𝑜𝑘 10001−𝑚−𝑛
Given in Table 5.1, p. 157
Other empirical models
• Multistep models
• Text pp. 157-8
• Fuel Surrogates
• Text pp. 158-9
Methane Combustion, 𝐶𝐻4
• 325 steps, 53 species
• Text pp. 160-7
• Two pathways
• High temperature
• Low temp (< 1500 K)
Oxides of Nitrogen Formation
• Important contribution to air pollution
• 4 mechanisms
•
•
•
•
Thermal (Zeldovich): High temperature for a range of Φ
Fenimore (or prompt): Fuel Rich Φ > 1
𝑁2 𝑂-intermediate (very lean Φ < 1 and low temperature)
NNH: new
• Thermal
•
•
•
•
𝑂 + 𝑁2 → 𝑁𝑂 + 𝑁
𝑁 + 𝑂2 → 𝑁𝑂 + 𝑂
𝑁 + 𝑂𝐻 → 𝑁𝑂 + H
Coupled with fuel combustion through 𝑂2 , 𝑂 𝑎𝑛𝑑 𝑂𝐻.
Chapter 6 Coupling Chemical and Thermal Analysis of
Reacting systems
• Four simple systems, p 184
• Constant pressure and fixed Mass
Reactor
• Constituents (reactants and products,
𝑖), 𝑖 = 1,2, … 𝑁
• P and m constant
• Find as a function of time, t
• 𝑇 (energy)
• 𝑖 (species generation)
𝑚
• 𝑉 = , 𝑛𝑒𝑒𝑑 𝜌 (state, mixture)
𝜌
• Assume we know
𝑑𝑖
𝑑𝑡
= 𝜔𝑖 = 𝑓𝑛 𝑖
• From chemical Kinetics
• Energy 𝑄 − 𝑊 =
𝑑𝑢
𝑚
𝑑𝑡