[email protected] (Dave Williams)
mc-chassis-design  10 May 2000

-> Nope, Carbon monoxide doesn't burn

Carbon monoxide burns quite nicely!  It's normally called something
like "producer gas", and is a byproduct of various steelmaking and
charcoal making processes.  After WWII it was common to see big boxy
contraptions strapped to vehicles with hoses running here and there;
these were charcoal reducers, with the byproduct CO used as motor fuel.

You find CO in engine exhaust because the chamber temp falls rapidly
below the 'freezing point' of CO combustion during the power stroke.
With straight CO as a fuel you don't have all the other stuff keeping
the temp down, and CO combustion can run to completion.

Ref: Glassman, "Combustion"

Glassman is a prime source for combustion theory - prime as in,
Glassman ran many of the experiments and wrote a lot of the papers that
are considered basic to the field.  Almost any book on combustion will
reference Glassman; might as well skip the intermediaries and go
straight to the source.

There is some really interesting stuff in there, like the behavior of
argon in the hydrocarbon combustion process...



[email protected] (Dave Williams)
fangle  20 Jun 2000

-> biomass gasifier unit (that is, a "producer gas" generator, also
-> called a "wood gas" generator) which is capable of providing

These are normally obsolete names for plain old carbon monoxide, which
is a reasonably practical fuel in its own right.

CO is formed in normal gasoline combustion, but the average combustion
temperature is too low for CO to react most of the time; below a certain
critical temperature the CO "freezes" and reaction stops.  A CO-only
engine doesn't have all the hydrocarbons in there hosing the reaction.

Much of the bizarre apparatus festooning most of the cars in "The Road
Warrior" was apparently supposed to be (TV imitations of) CO generators.


[email protected] (Dave Williams)
fangle  22 Jun 2000

Tom Leone was kind enough to send me a copy of "Gas Producers For
Motor Vehicles: A Historical Review" done by the government of New
Zealand.  It outlines the basic design and use of gas "producers" for
use for motor vehicles.

The gas producer is instantly familiar to anyone who has ever done
any metal casting - it's a ringer for Dave Gingery's charcoal furnace
in his metalcasting book, or any solid fuel burning furnace, for that
matter.  Rather than heating a payload of metal, the produced gas is
bled off as motor fuel.

The critical factors for thorough combustion of most fuels are time
and heat.  The solid fuel bed allows both; temperatures for gas
producers run 900 to 1000C, though a little tweaking would easily
allow them to melt iron.

How can you do this by burning animal dung, charcoal, or firewood?
The trick is the use of heat to partly burn the fuel in a "reducing",
or low-air, environment.  The rich combustion short-circuits a number
of potential reactions and leaves you with mostly carbon monoxide,
CO.  Robert Harris had lent me his copy of Glassman's "Combustion"
recently; the chemical reactions outlined in the report matched what
Glassman had stated.

The basic reactions are:

C + O2         -> CO2   =  +97   Kcal
C + 1/2 O2     -> CO    =  +29   Kcal
CO + 1/2 O2    -> CO2   =  +68   Kcal
1/2 CO + 1/2 C -> CO    =  -19.5 Kcal
CO             -> 1/2 C =  +19.5 Kcal

CO is a very nice fuel.  We normally see it as an end product in
ordinary gasoline combustion because a stoichiometric gasoline/air
mixture doesn't burn hot enough during most of the cycle to react much
of the CO produced.  When the temperature drops below 500C or so (I
don't remember exactly) the CO reactions "freeze" and combustion
stops.

If your CO percentage is high enough - as in what is made by the gas
producer - you can burn the CO directly and bypass all the
intermediate reactions.  It's a fairly decent motor fuel, resistant
to knock, with all the advantages of a gaseous fuel.  On the flip
side, combustion temperatures are high, which can cause problems in
some engines.

You can introduce water or steam into the firebox of the gas
producer.

Now for more magic:

H2O + C  -> CO + H2   = -31.2 Kcal  (1000 C)
2H2O + C -> CO2 + 2H2 = -21.4 Kcal  (500-600 C)
H2O + CO -> CO2 + H2  = +9.8  Kcal

There are some other reactions here, some of them reversible, but
the upshot is "something for nothing" - the temperatures are high
enough to dissociate water into hydrogen and oxygen.  They're
endothermic reactions, which means the dissociation takes more energy
than you can recover by recombining them... but remember, we're
talking two-stage combustion here - we're burning plenty of cheap
fuel to *make* fuel to feed a separate combustor, in this case a car
engine.

What's the deal with hydrogen?  Hydrogen, basically, can be
considered as a combustion accelerator.  Hydrazine is a good example
of a compound with lots of easily-available hydrogen - a dash added
to a slow-burning fuel like nitromethane will perk it right up.
Fuels with some amount of available hydrogen burn very well; hydrogen
exchange is the key to a whole group of potential chemical reactions.

Depending on the fuel used to charge the gas producer you can get
some other resultants, like methane, some free H2, or even ammonia.
(Yes, Virginia, you can burn ammonia.  It's loaded with hydrogen and
therefore is of very low octane, but as an additive it works much
like hydrazine as a combustion accelerant - something slow-burning
fuels like methane or CNG can benefit from)

Okay, all this stuff sounds great.  What's the catch?

Remember all the weird plumbing festooning some of the vehicles in
"The Road Warrior," or Cabbie's taxi in "Escape From New York?"
Well, now we know what all that ironmongery was supposed to be, or at
least represent.  Making producer gas isn't all that difficult, but
it requires a lot of *stuff*.

To start with you need the "producer", which is a solid fuel
furnace.  The size of the producer depends on how much gaseous fuel
it's supposed to make.  Figure anything from a trash can to an oil
drum in size.  It has to have a blower to add air.

After the gas is produced, it goes through a cooler, like an
intercooler.  This is usually just a bunch of pipe.

Next, depending on the type of fuel used, the gas has to be
filtered.  Some fuels leave abrasive ash as they burn, which is
entrained by the air flow through the fuel bed.  Fiber or centrifugal
filters or both are used to remove as much crap as possible; figure
something wastebasket-sized at least.

The very last step is the admission of the producer gas into the air
stream of the driving engine.  The NZ report showed a primitive air-
valve type carburetor, though a modern demand valve arrangement would
probably work fine.

The output of producer gas is, fortunately, largely dependent on
draw by the car's engine.  Rev the engine up, it pulls more gas from
the producer, which admits more air, etc.  There's likely a fair
amount of lag in the gas production process, but given the use of
these systems for a full century now, it seems to be manageable.

The producer is a solid fuel furnace.  When the fuel is exhausted,
you have to scrape out the clinkers and recharge it, just like a wood
burning stove.  The filter(s) must be cleaned regularly.  You have to
build up a fire before you can go anywhere - a fire very much like
the one in the charcoal grill you'd use to grill hamburgers.
Building up steam in a steam engine would be very fast in comparison,
and a lot less maintenance too.  Not to mention you have to find some
place to dump hot clinkers when you recharge the producer.  And once

<<<>>>

practical for a 2AM burrito run, but for vehicles which run all day
it works well enough.

You also have to have room for all this stuff.  You can strap the
bits all over a car or tractor, or hang them off the back of a bus,
or even package them relatively conveniently in the back of a pickup
truck, but it's still bulky.

The producers will let you run your car on anything that will burn,
just like the steam engine guys brag about, but they're a hassle.
They're only practical when gasoline or Diesel are too expensive to
afford or not available at all.

Okay, that wraps up Part I.  Part II coming up...