Wood/Coal Gas

Apr 23 02:44 UTC 2001

    I thought I read somewhere that this device was called a "gasogen",
though I'm informed that that's the name of a seltzer bottle. Anyway
what I'm discussing here is a device for adapting solid combustibles for
use in an internal combustion engine. It works by burning the fuel in
an oxygen-starved environment, producing exhaust gases that may be burned
further.

    I don't know what the rules are for wood, though I had a list of the
gas composition once when I first looked into this around 1980 or so.
However, for coal and charcoal I can make a few guesses based on the
chemistry.

    Basically, we have

    C + 0.5 O2 + 2 N2 -> CO + 2 N2                   4341 BTU/lb-C

  { CO + 2N2 } + { 0.5 O2 + 2 N2 } -> CO2 + 4 N2     4344 BTU/lb-CO
                                                  = 10136 BTU/lb-C

    This delivers, in theory, 70% of the total chemical energy into
the cylinders, yielding 780 BTUs per pound of all reactants, and
950 BTUs per pound of air used in both gas generator and engine.

    Now some preliminary guesses:

* The gas-generating step increases the molar fraction of the air by
  20% - from 2.5 units to 3 units. Consequently the pressure-volume
  product should likewise increase. Immediately upon startup the device
  should become pressurized (no pumps needed).

* This step must needs also raise the temperature of the gas. Fortunately
  the nitrogen can be expected to take up the bulk of it.

  4341 BTU/lb-C = 620 BTU/lb (1442 kJ/kg) of combustion gasses, which
  are close enough in composition to air, for our purposes, to use the
  air specific heat. This raises the temperature from, say, 300K to
  1736K (1463C, 2667F). The gasses will need to be cooled before they
  get to the intake manifold.

* Assuming that the air and combustion gasses are brought to the same
  temperature, the proper mix ratio is 6 parts coal-gas to 5 parts air
  at the engine intake. This calls for the coal-gas opening in the air
  intake passages to be 20% larger in area (10% or so larger in diameter
  if applicable) than the air passage itself.

* For the engine part, we have 321 BTUs per cubic foot of CO, which
  comes out to 58 BTUs per cubic foot of total air-fuel mix. The rating
  for hydrocarbons increases with molecular weight, but the BTUs per
  pound are all pretty close to 20000. (This is ~ 120000 BTU/gallon of
  liquid gasoline). Using hexane, we get a lower heating value of 4414
  BTU for a cubic foot of fuel vapor plus the 45.2 cubic feet of air
  required to burn it, or 95.5 BTU/ft^3. For the same intake manifold area,
  we get 60% of the power that we would get on gasoline.

* The density of carbon is given as 2500 kg/m^2 or 2.5 kg/L which is
  20.8 lb/gal, about 3.5 times the density of gasoline. One gallon of
  pure compacted carbon would yield 211000 BTUs for propulsion (300K BTUs
  total). Even given impurities and air spaces between the chunks of
  coal, a coal burner need be no larger than the gasoline tank that it
  would replace. The mass would be 3.5 times higher, of course. This
  is about the same as taking on two or three extra passengers.

    I priced coal last year. I forgot the raw cost but remember it coming
out to about 80 cents per gallon-equivalent. This is for getting *all*
the BTUs. With only the coal-gas energy recovered it comes out to around
$1.14/gal-Eq.

    The best price for charcoal that I've found was for 20 lb bags at
K-mart, at around 23 cents a pound. This is $1.91/galEq for all the
energy, and $2.72/galEq for just the coal-gas portion. Sam's Club might
have it cheaper; I'll have to check. At any rate, those 20 lb bags of
charcoal took up considerably more than a gallon of space.

    Wood is, of course, free for the taking, given a remote enough area.
Again, I don't know what the numbers are for wood, though I did find
a web site that claims that an engine would have 85% of its gasoline-
fuelled power. One of the things I wanted from the Mother Earth manual
was a figure for how much of different types of wood made up a gallon-
equivalent.


   (Trivia note: Woodgas generators make a brief appearance in the
1974 sci-fi movie _Planet Earth_.)

response 2 of 24:

Apr 23 14:13 UTC 2001

A lot of wood is sent to the dump.  Drew, how do you plan to deal with the
pollutants generated by your process?  Would it not make more sense to gasify
the coal in some stationary plant where the result can be purified?

response 4 of 24:

Apr 23 23:09 UTC 2001

Re #1:  Fallen wood is free for the taking on much public land.
However, the fuel-cost of retrieving the "free" fuel must be taken
into account.

Re #0:  I've read that power from an engine running on wood or coal gas
can be as little as 50% of the power available when running on gasoline.
Your figure showing > 50% of the net inlet volume would be used by the
fuel gas backs this up.  You'd probably need changes in spark timing to
get the most out of the fuel; for one thing, a gas mix with so much inert
gas is going to burn more slowly and work best with more spark advance.

You're not going to get a pressure bump.  You've got to have a pressure
drop through the system or you get no airflow.

Wood decomposes by destructive distillation.  I have no figures handy,
but the gas mix is going to be a lot more complex than what you get by
incomplete combustion of charcoal.  The general composition of carbohydrates
(including cellulose) is (CH2O)n; if some of it breaks down to CO + H2
you'll have a richer fuel than you'd get from charcoal.  The excess heat
you noted from 2C + O2 -> 2 CO should be more than enough to do this.

Keeping the gas generator clean is going to be a huge problem.  If you
are using coal or wood, you are going to generate a large amount of
highly toxic tars.  Coal tar is nasty crap, you don't want to mess with it.

Doing this in a fuel-injected vehicle is another issue.  You've got a
computer which controls the fuel injectors and spark; it would have to
be re-calibrated to do the spark for producer gas and you'd need some
sort of modification to shut off the fuel injectors.  The mechanics of
this vary quite a bit depending on the design of the system.

If your car has a mass-air (as opposed to speed-density) system, you've
got a possibility for going halfway.  If you can admit a fuel gas/air
mixture into the manifold through e.g. a manifold vacuum port, you would
admit less air through the throttle plate and reduce the airflow measured
at the sensor.  This would automagically cut back the gasoline flow.  You'd
also have a reduction in the fuel-gas flow when you hit the throttle,
switching you over to gasoline for acceleration.  This appears desirable.

I get the feeling that this would be much better suited for a tree-service
work truck than a personal vehicle.  You could run on sawdust, which is
certainly "free" at that point.

My take on this is that you'd have a lot of work to do to solve some of
the problems relating to particulates, tars and condensible gases in the
mix.  Unless you want to handle a lot of toxic goo (and maybe have to
dispose of it as hazmat) you are going to have to engineer something
which can recycle it internally and is self-cleaning.  Good luck; you'll
need it.

response 6 of 24:

Apr 24 15:40 UTC 2001

The cars I have to work with are (1) an old station wagon with a normal
carbeurator system, and (2) a _Grand Am_ with individual fuel injectors for
each of four cylinders (I heard this called "throttle body", but was later
informed that that was the wrong term.)

I had deemed the smaller car unsuitable because the air intake ducting was
all plastic, which I supposed would be prone to melting in spite of cooling
the exhaust gases. I'm not sure that there is a vacuum port anywhere past the
plastic that's big enough to carry enough fuelgas-air mix.

The station wagon seems suitable (might as well; it's becoming effectively
useless due to fuel costs anyway) as at least the air cleaner housing is
metal, and it's a normal carbeurator system. I'd need a valve in the fuel
line, preferably one that can be worked remotely. As for the spark timing
issue, there should be no more inert gas in the system than there would be
with gasoline; and certainly no more than if it were somehow possible to burn
the wood or coal directly inside the cylinder. (Half of the allotment goes
through the gasogen, the other half goes into the engine's air intake.)

I have an idea for cleaning the fuel-gas; not sure just how it will work. It
involves liquid filtering. The gas would go through a run of pipe with
radiator fins to cool it as much as possible, then would be bubbled through
one or more sealed 5 gallon buckets of water. What remains should be stopped
by the air filter, which might have to be changed (or cleaned) more often.

response 8 of 24:

Apr 26 12:00 UTC 2001

My grandfather reportedly had such a delivery vehicle for his bicycle business
in Germany during WWII.  I saw one on display in the Museum of Science and
Technology in Munich when I was there 15 years ago.  It looked more like a
small locomotive than it did a car.

response 10 of 24:

Apr 27 02:53 UTC 2001

Oh, and I'd use wood for this.  Don't mess with doing it with coal.  
Among the byproducts of generating coal gas are sulfuric acid and 
cyanide.  These aren't things you want in air you or your engine are 
breathing.

response 12 of 24:

May 4 14:06 UTC 2001

From the BBC, here's an article about a car that runs on a gas produced 
by fermenting vegetable matter: 
http://news.bbc.co.uk/hi/english/sci/tech/newsid_1309000/1309201.stm

Apparently it takes 220 pounds of rubbish to produce enough to power 
the car for 60 miles, though, so this doesn't seem like something 
that'll we widely useful.  City sanitation departments might be able to 
reduce their fuel costs, though...

(Heck, many landfills already have vents to release methane gas.  It's 
just allowed to vent into the atmosphere, currently, except when 
vandals light it on fire.  Why not capture it and use it as a fuel gas?)

response 14 of 24:

May 4 19:56 UTC 2001

There is a greenhouse in Ypsilanti powered by methane gas from the local dump.
Most of the workers there are deaf.  A very admirable business overall,
assuming they are still in business.  We knew a Hungarian student doing
practical training there, who would bring us their gourmet salad vegetables
and herbs, which sold well in local stores.
By powered, I mean heated when the sun is not available.  The crops were grown
hydroponically all winter.  The workers seemed rather content and were
carrying on lots of interesting conversations that I could not understand.
Not a bad place to be on a drear day in January.

response 16 of 24:

May 5 11:11 UTC 2001

My brother just sent me a little tid-bit about a sports car that runs on
rotting gabbage.  0 to 60 in under nder 6 seconds.  Interesting...  I'l

response 18 of 24:

May 28 17:49 UTC 2001

While looking for something else, I stumbled across a table of
compositions for various fuels.  Here's what it's got for a
selection of likely things:

%Vol    H2      N2      O2      CH4     CO      CO2     C2H4    C6H6

Blast furnace gas
        1.0     60.0    --      --      27.5    11.5    --      --

Blue water gas
        47.3    8.3     0.7     1.3     37.0    5.4     --      --

Carbureted water gas
        40.5    2.9     0.5     10.2    34.0    3.0     6.1     2.8

Coal gas
        54.5    5.5     0.2     24.2    10.9    3.0     1.5     1.3

Coke-oven gas
        46.5    8.1     0.8     32.1    6.3     2.2     3.5     0.5

Producer gas
        14.0    50.9    0.6     3.0     27.0    4.5     --      --

It looks like coal gas is made by destructive distillation of coal;
it's much closer to the composition of coke-oven gas than anything
else.  The output of a gasifier would probably be much closer to
producer gas, of which about 56% is non-fuel.  Producer gas would
displace a lot of air in the intake(1), leading to a lot less energy
per volume of charge(2) and low engine torque.

(1)     A molecule of octane, C8H18, requires 12.5 molecules of
        oxygen for stoichiometric combustion.  A stoichiometric 
mixture of octane and air would thus be about 1.6% fuel, balance
air.  A volume of producer gas requires about (14%/2 + 3% * 2 +
27% / 2) = 26.5% its volume of oxygen to burn, or 1.26 volumes of
air.  The total fuel-air mix would be about 44% fuel gas and 56%
air, compared to 98.4% air for octane.

(2)     Density of air @ 77 F and 14.7 PSIA = 0.0740 lbm/ft^3.  At an
        air/fuel ratio of 15.093:1 and 20770 BTU/lbm of octane, the
air/octane mixture would have an energy of 100 BTU/ft^3.  The producer
gas mixture contains three combustible species:  H2, CH4 and CO.  These
have a heat of combustion of 320 BTU/ft^3, 958 BTU/ft^3 and 310 BTU/ft^3
respectively.  Multiplying these figures by their total volume in a
44% gas/56% air mixture, the total combustion energy of the mixture
is 69 BTU/ft^3.  In practice the density differences caused by the
evaporative cooling of gasoline vs. the heat from the gas generator
would increase the difference further.

response 20 of 24:

Jun 3 19:19 UTC 2001

Re #18:
    I don't see a heading for water vapor in that table. Certainly producer
gas has *some*, even with dry wood?

    And what sort of wood are these figures for? Or is it an average?

response 22 of 24:

Jun 4 13:16 UTC 2001

There's no heading in the table for water vapor.  (I'll bet that
the raw material for producer gas is coal, not wood.)

That said, water is probably not included for several reasons.
My guesses are:

1.)     It's one of the inputs for several of the processes, and
        the amount of vapor in the product gas depends on the amount
        of excess reactant.

2.)     It's condensible (the only other potentially condensible
        compound in the table is benzene), so the amount of water
        in the final product is likely to be determined by the
        temperature; below a certain temperature the relative
        humidity will always be 100%.

response 24 of 24:

Jun 5 13:26 UTC 2001

Re #23:  I wrote #22 off-line, after I'd downloaded #20 and before
I'd seen #21.  That's my usual modus operandi.