synthetic fuel

[gyronaut]

As I am an aeronautical engineer I don't know much about chemistry but I wonder if it is possible to synthetically make a hydrocarbon fuel using electricity. i.e split water and carbon dioxide and combine the hydrogen and carbon. Just want to settle a idea that I have wondered about for a long time.

Cheers
gyronaut.

 

[quark]

I too don't know much synthetic chemistry (moreover, organic chemistry was a night mare for me). But you require energy to split H and C and again you require energy to join them and this process seems to be inefficient.

However, check for Hydrogen Fuel Cells. Much work is going on them presently. A good start can be

http://www.howstuffswork.com.

Regards,

 

[phex]

it would, probably, be possible to manufacture a liquid hydrocarbon molecule from pure hydrogen and carbon, but i strongly doubt that it has any use. the process would consume incredible amounts of energy, by far more than "stored" in the hc molecule produced. it would be insanely expensive, as a process would have to be researched first of all. and, due to the huge amount of conversions included, it would by no means be an environmentally friendly process.

i do be very curious as to the nature of your idea and how you came by it. if it involves recycling automotive offgas to essentially create a "selfsustaining" car with minor loss of fuel, for example by utilising solar power to reconvert the CO2 and H2O back to usable fuel, i think it won't ever work. do elaborate on your idea, perhaps then i can say more about it.

chris

 

[25362]

To gyronaut, my blessing for your thinking.

Vegetation already combines water and carbon dioxide in a process called photosynthesis to produce carbohydrates, in the presence of chlorophyll in several steps, as in the following:

nCO₂ + nH₂O + solar energy -> (CH₂O)_n + nO₂

when n=6 we have glucose.

As for fuel (hydrocarbon) synthesis, this involves the addition of hydrogen to CO, or to CO₂, with water as a by-product.

Hydrogen can be obtained from water electrolysis, from the carbon-steam reaction, the water-gas shift reaction, and from acids reacting with metals; otherwise, from hydrocarbons by catalytic reforming, by partial oxidation, or by thermal decomposition, the last three options should be discarded as far as hydrocarbons are meant to be produced, not destroyed.

CO, as obtained from decomposition of CO₂, needs a very high activation energy of about 460 kJ/mol. However, the carbon-steam reaction mentioned above provides CO as a co-product to hydrogen by:

C + H₂O -> CO + H₂

The fuel-making reaction would be then represented by:

m CO + (2m+1) H₂ -> C_mH_2m+2 + m H₂O

CO + hydrogen can also render monohydric alcohols and glycols.

From what you said, one doesn't need CO if one has CO₂.
As for the reaction of CO₂ with hydrogen, one may obtain, for example, methane (the main component of natural gas):

CO₂ + 4H₂ -> CH₄ +2 H₂O.

In short, by splitting water with electricity as you suggest (not so cheap nor simple) you get hydrogen (and oxygen as a useful by-product). Since carbon dioxide is available, you should think of combining it with hydrogen, and voila, you'd have a fuel, with pure water as a by-product.
As a matter of fact this kind of (catalysed) reaction is being actively studied many years now, for the potential production of fuels.

 

[quark]

oops! the link is

http://www.howstuffworks.com

 

[gyronaut]

The above reply's are much enlightening. My thinking was more in the way of providing fuel for transportation. In particular aircraft as they require a high power to wieght ratio. I was considering it as a way to store energy (chemically) and I thought that making a hyrocarbon could be a good way as we already have some fairly successful means of using it (turbofans). What prompted this was that my parents have farm is solar powered and during the day once the battery's are fully charged the controller turns the solar panels off (diconnects them). For the rest of the day the panels are idle. I considered spliting water for hydrogen as a use for this excess power. In conjuction being an aero engineer I wondered just what people were going to do for fuel for big airliners when oil became scarce (a while yet). I am wondering now just how difficult and inefficient this process would be? In the case of the plane hyrocarbons are also fairly compact.

Looking on the net I have found people that reckon that they can split water for hydrogen using forms of electrolsis with an efficieny of about 90%, is this likely?

Cheers
Gyronaut

 

[moltenmetal]

The "efficiency" of water electrolysis depends on the pressure at which you generate the hydrogen. Some cells generate hydrogen at atmospheric pressure, such that you'd need to use a mechanical compressor to store it at higher pressures (a relatively lossy process). Other cells produce hydrogen at significant pressure, but these cells use energy to overcome the high activity (i.e. concentration) of product hydrogen. The end result is that you don't get this work of compression for free, and you don't get all of that work back when you expand the gas again to use it in your fuelcell, IC engine etc.

The problem with hydrogen storage is the low density: ~ 75 kg/m3 for the liquid, lower still for high pressure gas. Though the energy density is pretty good per unit of mass, it's terrible per unit of volume. People are working on all sorts of methods to increase the storage density for hydrogen, but all of them suffer from some energy efficiency loss.

Others may be better able to comment on the overall energy recovery efficiency of electrolysis w/high pressure hydrogen storage with recovery via a fuelcell and inverter, but I know that it is being considered commercially as a means to productively use off-peak electrical generation capacity. Other means of energy storage such as ultracapacitors, flywheels, and even re-charging hydroelectric reservoirs by pumping, are either being used or being studied for the same purposes.

Too bad your parents' farm isn't connected to the grid- you could sell the excess solar-generated electricity back to the utility and only pay for what you use- a practice known as "net metering".

As far as supplying hydrocarbon fuels for use in aircraft etc., if cost is no object there are far more energy-efficient ways to make these than to do so via electrolysis of carbonate. As crude oil gets rarer, more expensive (and unfortunately less CO2-efficient) sources of these hydrocarbons become more economically attractive. These include oil sands/shales, gas to liquids technolgies (Fischer-Tropsh synthesis) etc. The latter can use "synthesis gas" (i.e. a mixture of CO and H2) from gasification of stranded natural gas, coal, biomass etc. as a feedstock, so it essentially represents an inexhaustible though potentially inefficient supply of hydrocarbons for the future.