Hello, in this video we're going to
talk about both Hydrogen and Hydrocarbon Oxidation. It'll be somewhat brief, we don't
have the bandwidth in this course to get into great deal of detail,
but just a little bit about it. Here we really delve into some aspects of
practical kinetic reaction mechanisms. Such a mechanism specifies
all the species and thermodynamic and transport properties and
all reactions and rates, and we'll start with the simplest combustion
system the oxidation of hydrogen. So, H2/O2 combustion is important for
a variety of reasons, one it can be very highly explosive,
and in some cases can be a bad thing. The classic example is the explosion of the Hindenburg blimp which was lifted with hydrogen which is lighter than air, nowadays they're all helium. And then H2/O2 combustion is important
in its own right because of rocket propulsion low emission vehicles fuel
cells, run on hydrogen oxygen combustion. And then furthermore
the H2/02 reaction mechanism is the underpinning to virtually all
hydrocarbon chemistry mechanisms. The global reaction is H2 plus one
half O2 goes to H2O, this reaction and been studied for over two centuries
starting with Sir Humphrey Davy. The general behavior was well established
by the early 20th century, and the heat of combustion of this reaction
is 141,000 kilojoules per kilogram. It's a higher heating value, which is two to three times that of other
common fuels like gasoline or kerosene. So here's an example of
an H2/O2 reaction mechanism. The references given
there you can look it up, there are a number of others, many others. And if you look at this carefully,
you will see that all of the types of reactions that
we've discussed are in here, reaction under the second
heading there H2/O2 dissociation recombination reactions show
the initiation reactions. The first three
are the dissociation of hydrogen, the second three are the dissociation
of oxygen, note that the oxygen reactions are written as three
body reactions going to the right, but that's because someone has
provided those reaction rates and remember detailed balance
lets you get both directions. The top reactions of the chain reactions, and then there are some
additional molecules that are formed HO2 and and H2/O2. And finally the termination reactions that produce water vapor reaction 8 under the second heading there
H + + M goes to H2O. That's the primary
termination reaction for H2/O2 combustion. So, this is a diagram that
was produced many years ago by workers and
it's published in the book by Louis and Vaughn ELD, and
it shows the behavior of hydrogen oxidation as a function of temperature and
pressure. So if you start at very low pressures,
not much happens, you have to get extraordinary high
initial temperatures in order for anything to happen or
to get into an explosive domain. Because at low pressure,
radicals can readily readily diffuse to the walls and
are destroyed and what makes H2O to explode are those
chain and reaction reactions. Notice that this curve line
down at the low pressure, it's a dotted line and
that means it depends in great detail on what the raw wall of the vessels
are like and people conducted experiments with different coatings
on the wall to study that. But suppose we start at a given
temperature for between 400 and 500, and we just go up vertically. Well, if you cross that
dotted line at some point, then if you initiate or
mix a stoichiometric mixture of H2 and O2 and it's at that temperature,
it will explode. And if you continue to
increase the temperature or it could increase the pressure,
however, at some point you reach what's called the second limit of behavior
in which there's no explosion. And that second limit of behavior is when
you start to form this molecule HO2, which is not particularly reactive and
so it damps down on the chain reaction. But if you keep going and increase the pressure eventually,
then you will come into a third limit where you'll have explosive behavior
again, so all this is very interesting. As I mentioned, the H2/O2 chemistry forms the basis of hydro carbon
oxidation chemistry. So this chart gives a what's called
the hierarchical structure for the oxidation of simple hydrocarbon fuels,
and the base of that is H2/O2. And then workers typically
throw in CO reactions and then formaldehyde CH2O is formaldehyde,
and that leads to methanol
alcohol type reactions. Then if you allow species
with two carbons, that generates a bunch more reactions and that ends up involving methane reactions. And ethanol alcohol reactions, and then
things begin to get really complicated. And these mechanisms can have a very
large number of reactions involved as I mentioned earlier, I work with
the reaction may hydrocarbon reaction mechanisms, mechanism that
involves 25,000 reactions. A little bit about hydrocarbon compounds,
just a refresher of your physical chemistry that most
hydrocarbons are of four types. There are alkanes which are single
bonded between carbons, alkenes which are double bonded, and
alkynes which are triple bonded. And then Arines or Aromatic Compounds
that are ring compounds, and you can form a wide variety of
hydrocarbon compounds almost an infinite variety by attaching various
different things to the carbon. So, what type of reactions do you
get in hydrocarbon reactions? Well, you can get hydrogen
abstraction reactions. These are typically what
initiation reactions, you can get what's called beta scission. It turns out that in alkanes
while in most chain molecules, the carbon-carbon bond between
the second and third carbon is the most unstable or requires the least
amount of energy to break it, and so you get a lot of these
beta scission reactions. And of course the unimolecular
reactions are various kinds. Here's an example GRIMech
is a well-known mechanism, it was designed for
natural gas combustion. It's called GRIMech because
the gas research institute funded the work to develop it. It's a mechanism with 53 species and
325 reactions, and these are just some calculations
that we're done that show. But you just start out at an initial
temperature of 1500 degrees, it's a constant pressure reactor. And if you start out at 1500 that's
a high enough temperature for initiation to take place, and
reaction and oxidation will occur with a company heat release
which causes the temperature to rise. The initial methane is destroyed and H2O is formed, Radicals are formed. One of the things that this mechanism
was designed to do is calculate Knox formation, and Knox is important
contributor to photochemical smog. And so on the right it shows
the NLmole fraction, N2O, NO2 and NH mole fractions, and
you can see a lot of NOs formed and NO is formed mainly at high temperatures. So it doesn't start to form until
the temperature starts to really rise, and it will just keep rising
as long as you let the mixture sit there at a high enough temperature
until equilibrium is reached. So, there's a large variety of reaction
mechanisms available in the literature, many of them online and
here is a list of several of them, GRIMech can be found at the website given. Lawrence Livermore National Labs has
a combustion group that produces a lot of reaction mechanisms. National Institutes of Science and
Technology in the NIST in the kinetics database lists
information about specific reactions. Cal Tech, there's a site at Cal Tech and
they provide some reaction mechanisms and then CRECK which is a group
at Milan University of Milan. They produce a number of reaction
mechanisms and there are many, many more you search online and
you'll find a lot of them. So that's it for this video, thanks for
listening and have a great day.