[ExI] Cost of synfuel was Air-powered cars

hkhenson hkhenson at rogers.com
Fri Jun 6 20:37:28 UTC 2008

At 09:23 PM 6/5/2008, you wrote:
>On Thu, Jun 5, 2008 at 7:22 PM, Keith Henson 
><<mailto:hkeithhenson at gmail.com>hkeithhenson at gmail.com> wrote:
>On 6/5/08, spike <<mailto:spike66 at att.net>spike66 at att.net> wrote:
> >
> > Whenever one looks at the alternative means of hauling apes, one always
> > comes away with a new respect for good old gasoline.
>So what do we need for carbon neutral synthetic gasoline?
>At a recent conference a guy had worked out the numbers to suck carbon
>dioxide out of the air and combine it with hydrogen in a reverse
>combustion industrial operation.
>That's an interesting idea, but how exactly is this done?

Overall, nCO2 + 3n+1H2 --> H(CH2)nH + nH2O  That's where oil came 
from in the first place.

In detail, CO2 + H2 --> CO + H2O

Condense out the water, add more hydrogen and you have 
syngas.  http://en.wikipedia.org/wiki/Syngas

Feed the syngas to Fischer-Tropsch synthesis plant.


There is a detailed flow chart of how Sasol does it here


It is a relatively low temperature (350 deg C) exothermic process 
described here:


They produce about 7 million metric tons of synfuels a year.  That's 
about 0.2% of the world's oil production, but it's plenty big enough 
to get an accurate estimate of what such plants cost.

The main cost would be the hydrogen.  Ignoring the relatively low 
cost of electrolysis cells, it would be $0.40/kg at 
$0.01/kWh.  Figuring gasoline as pentane,

5CO2 + 16H2 --> C5H12 + 10H2O
5x44   + 16x2      5x12+12  10x(2+16)
220         32         72          180
252                      252

It takes 32 kg of H2 to make 72 kg of synfuel.  Since gasoline has a 
density of about 3 kg/gallon, this would be about 24 gallons.  It 
would cost (in hydrogen) 32x.40/24 or $0.53/gallon.  (This ignores 
the huge cost of the plants, but on the other hand, they are just 
front ends to existing refineries and they should last for many decades.)

I don't know how these plants would get C02 out of the air.  They 
could cook it out of limestone and let the limestone combine with CO2 
out of the air in a big lake.  Or dump calcium oxide into the ocean 
and let it combine with CO2 and settle out.  Or use scrubbers on 
regular air or feed it with anything that has reduced carbon, trash, 
old tires, or biomass.  At first they would most likely use coal, but 
*much* less coal than they would use making hydrogen from coal.  The 
first use of hydrogen from solar power satellites would probably be 
to at least double the output of the Canadian tar sands plants by 
reducing the amount of product they have to use up to make hydrogen.

I don't know if this mailing list is the place to go into details of 
chemistry since this should be fairly obvious to anyone who took high 
school chemistry.  On the other hand, I don't know how many of the 
list readers actually know chemistry.  Those who do, please check my numbers.


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