0910411 / 0910511 Combustion Class Notes - Page: Week 6 Oxidation of Hydrocarbons Text: Glassman, Ch.3

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0910411 / 0910511

Combustion

Class Notes
-

Page:
1

Week 6

Oxidation of Hydrocarbons

Text: Glassman, Ch.3


Technical Objectives:



Describe

the hierarchy of hydrocarbon oxidation kinetics:

H
2
/O
2

CO/H
2
/O
2

Aldehydes

C
2


Alkanes, Alcohols and Aromatics



Explain
qualitatively

the oxidation mechanisms for methane, alkanes, aromatics and
alcohols. For each of these syst
ems, identify the most important steps of:

Chain initiation

Chain propagation/chain branching

Chain terminating.



For a given qualitative oxidation mechanism,
assemble

a list of elementary chemical
reactions and their associated rate constants.

Motivation

B
y examining the H
2
/O
2

explosion limits, we have already determined a qualitative chemical
reaction mechanism for the H
2
/O
2

system. Having determined what elementary chemical
reactions are important, the next step is to assemble the important reactions and

acquire rate
constant parameters for each of the important reactions. (See, for example, the Appendix in
Glassman's book). Once this is done, the "chemistry part" of combustion is completely nailed
down.

But, this is just the hydrogen/oxygen system. We

have to repeat this process for all of the other
fuels. The good news is that, once we nail down the simpler systems, we can just go on up the
heirarchy.

0.

Number of Chemical Species vs. Number of Chemical Reactions

Modeling chemically reacting flow syst
ems (i.e. flames) on a computer requires the solution of
N+3 simultaneous partial differential equations, where N is the number of species required to
accurately describe a given chemical reaction mechanism:













0910411 / 0910511

Combustion

Class Notes
-

Page:
2

Week 6

Oxidation of Hydrocarbons

Text: Glassman, Ch.3


Here is the key:

Each
reaction

only
shows up as a term in the right hand side of some of the differential
equations.

Each
species
results in an entire additional differential equation that needs to be solved.

As we go up the hierarchy the number of species and number of reactions required to

accurately model a chemically reacting system increases dramatically.

Typical sizes of chemical kinetic mechanisms:

Hydrogen/Oxygen system (Kim, Yetter and Dryer, 1994):


CO/H
2
/O
2

system (Kim, Yetter and Dryer, 1994):


Methanol oxidation (Held and Dryer
, 1993):


Heptane oxidation (High temperature) (Held, Marchese and Dryer, 1997):


Heptane oxidation (Low Temperature) (Lindstedt and Maurice, 1995):


The challenge (and opportunity!) is to minimize the number of species required to accurately
model a chemi
cally reacting flow system!

With today's computational tools, the only real constraint in modeling combustion systems is the
number of species!

Current State of the Art:

1
-
Dimensional flame (droplet, premixed laminar flame):


2
-
Dimensional flame (bunsen

burner, candle, pool fire):


3
-
Dimensional flame:


3
-
Dimensional, turbulent combustion:






0910411 / 0910511

Combustion

Class Notes
-

Page:
3

Week 6

Oxidation of Hydrocarbons

Text: Glassman, Ch.3


1.

The General Hierarchy of Hydrocarbon Oxidation

For all hydrocarbon oxidation, the following general reaction scheme is basically true:








Note: All of the fue
l proceeds through CO before it goes to CO
2
. But, most of the heat release
occurs as CO is converted to CO
2
.

2.

Hydrogen/Oxygen Chemistry

From examining the hydrogen/oxygen explosion limits, we have identified the following
important chemical reactions


(1)



(2)


(3)


(4)


(5)


(6)


Note: In those reactions alone, we have already identified ___________ different chemical
species, in only 6 chemical reactions. Adding H
2
O
2

to the mix, results in only 1 more species.

The comprehensive set of chemical reacti
ons for the H
2
/O
2

system (See the Appendix in
Glassman's book) considers the same 8 species, but ___________ chemical reactions.

Recall: it is the number of
species

that is computationally difficult, adding more
reactions

is not a
big deal!




0910411 / 0910511

Combustion

Class Notes
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Page:
4

Week 6

Oxidation of Hydrocarbons

Text: Glassman, Ch.3


3. The Oxid
ation of CO/H
2
/O
2


The next level up on the hydrocarbon chemistry hierarchy is CO oxidation. The oxidation of CO
is a key step, because it has been found that:




Whereas:




So, it is the conversion of CO to CO
2

that is actually the rate
-
determining step

in most
hydrocarbon combustion systems
.


Explosion Limits of CO/O
2

Early experiments were conducted (similar to H
2
/O
2
) to determine the explosion limits of CO/O
2
.
The results looked something like this:











The problem with these experiments is that

it is almost impossible to do the experiments without
at least
some

H
2
O present. And, as it turns out, even a little (approx. 10 ppm) water will
accelerate the oxidation of CO dramatically.

Dry CO Oxidation

Consider for a minute, the situation of CO oxid
ation without any water, H
2

or H present:






0910411 / 0910511

Combustion

Class Notes
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Page:
5

Week 6

Oxidation of Hydrocarbons

Text: Glassman, Ch.3


This system is not chain branching and CO and O
2
are very stable. This reaction would take a
long, long time. Early researchers spent a lot of time trying to figure out why explosion limits
were observed at
all for this system.

Then they realized that, in all combustion systems that anyone cares about, there is enough
hydrogen around to accelerate things. We call this system, "wet" CO oxidation.

Question:

It is relatively easy to produce CO and O
2

from th
e CO
2

Martian atmosphere.
Thermodynamically, CO and O
2

should make a great fuel combination. But, without any H
2

up
there, would the reaction occur fast enough to have rocket engines or internal combustion
engines that run on CO/O
2
?






Wet CO Oxidation

On earth, there is always enough hydrogen around to accelerate the CO system. In the
presence of hydrogen:






Note: the H atom is just a catalyst it is not really consumed. But, it helps move along the CO
oxidation.

The key step in wet CO oxidation is
:




And, in fact, this step is a key step in all of hydrocarbon oxidation, since:

1.


2.

This is a weird reaction, though. A plot of the measured reaction rate constant on a log(K) vs.
1/T plot shows that it cannot be expressed in a typical Arrhenius
form:




0910411 / 0910511

Combustion

Class Notes
-

Page:
6

Week 6

Oxidation of Hydrocarbons

Text: Glassman, Ch.3


4. Crash Course in Organic Chemistry