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And in the further future when humans colonize other planets, petroleum will be a uniquely Earth-based resource making it even more valuable. If you have elect

And in the further future when humans colonize other planets, petroleum will be a uniquely Earth-based resource making it even more valuable. If you have electricity, water, and a carbon source, you can manufacture the hydrocarbons currently sourced from petroleum. Small scientific colonies would import terrestrial hydrocarbons just like they would import terrestrial aluminum and glass, of course, but if you're picturing self-sustaining space industrialization and colonization, it won't be worth sending hydrocarbons from Earth to Mars. Hydrocarbon synthesis also puts an (admittedly high) upper limit on the cost of making organic chemicals on Earth. Some Peak Oilers were confused about this, imagining fossil resources as irreplaceable magic. "It's not just fuel -- pesticides, pharmaceuticals, and dyes are made from oil too!" they'd say, implying that you can't have modern pharmaceuticals without oil. But that's like saying that you can't make paper without trees. The reason that paper is overwhelmingly made from trees is because that's currently the cheapest way, not because other ways are unknown. If the abundance/cheapness of feedstocks changes, so will their share of the market. (And costs will go up too, of course. The cost of refined liquid fuels goes up nearly linearly with the cost of crude oil. The cost of drugs/dyes/pesticides rises much less, because only a small share of their total cost of production came from simple hydrocarbon feedstocks in the first place.) With the exception of nuclear fuels, which are truly "used up" during fission or fusion, all atoms are recyclable.

njarboe 9y ago
I would love to see a chart of the energy required (both theoretical and "currently best working system") to produce various hydrocarbons from CO2 and H2O feedstock. Methane, propane, heptane, etc., and then some mixtures like gasoline, diesel fuel, asphalt. I quick search did not bring anything up for me.
- philipkglass 9y ago
  Here's an analysis of methane synthesized with water, CO2, and electricity: http://www.sgc.se/ckfinder/userfiles/files/SGC284_eng.pdf http://www.sgc.se/ckfinder/userfiles/files/SGC284_eng.pdf They put it at 52% energetic efficiency from electricity. They're starting with a concentrated CO2 stream as a byproduct of a biological waste digester. If they were starting with a more dilute CO2 source like seawater or ordinary air there would also be an energy cost for concentrating the CO2. In my estimation, a reasonable rule of thumb is that it would take 2-4 megajoules of electricity to make one megajoule of common small hydrocarbons (methane, ethane, ethylene, propylene, benzene...) For example, a megawatt hour of electricity is 3600 MJ. If it were used to synthesize the hydrocarbon commodity ethylene at 1/3 efficiency, it would yield about 24 kilograms of ethylene. Bulk ethylene prices in the US in July were $645/tonne, or $15.38 for 24 kilograms. Today a megawatt hour of renewable electricity might cost $30 in a favorable location. So the electricity cost alone of renewable ethylene would be nearly double the all-in cost of petro-ethylene. Probably renewable ethylene would be closer to 4x the cost of petro-ethylene once you add other costs. But, notably, not 100x or even 10x the cost; disposable polyethylene packaging might become too expensive for routine use, absent fossils, but durable goods incorporating polyethylene parts would still be affordable. Higher-value materials synthesized starting from ethylene, like detergents and coolant fluids, would also see a relatively modest price increase. There's a lot of literature on producing liquid fuels and chemicals starting from natural gas and coal. Most schemas proceed through an intermediate called synthesis gas, ("syngas") a mixture of CO/H2 that can be converted into many different products depending on the catalysts used. To get the overall energetic efficiency you'd combine the extant huge literature on synthesis gas conversion with the assumption that synthesis gas is made from the reverse water gas reaction of CO2 with electrolytic hydrogen from water. (Not hard if you're fluent with chemistry and units conversion, otherwise may be challenging. I'd do a table of examples myself if I thought this comment would be more visible.) Here's a presentation that has a schematic view synthesizing renewable hydrocarbons without biomass -- see particularly the slide "Combining Technologies" where it shows that methanol, gasoline, diesel, and methane can all be produced from the same syngas intermediate: http://orbit.dtu.dk/files/51542760/Production_of_Green_Natural_Gas_presentation.pdf http://orbit.dtu.dk/files/51542760/Production_of_Green_Natur...
  - njarboe 9y ago
    Thanks for the info and the links. Pretty easy to get a pure CO2 stream on Mars and H2O is all over the place. Just need some nuclear generators to get cranking.

njarboe 9y ago

I would love to see a chart of the energy required (both theoretical and "currently best working system") to produce various hydrocarbons from CO2 and H2O feedstock. Methane, propane, heptane, etc., and then some mixtures like gasoline, diesel fuel, asphalt. I quick search did not bring anything up for me.

philipkglass 9y ago
Here's an analysis of methane synthesized with water, CO2, and electricity: http://www.sgc.se/ckfinder/userfiles/files/SGC284_eng.pdf http://www.sgc.se/ckfinder/userfiles/files/SGC284_eng.pdf They put it at 52% energetic efficiency from electricity. They're starting with a concentrated CO2 stream as a byproduct of a biological waste digester. If they were starting with a more dilute CO2 source like seawater or ordinary air there would also be an energy cost for concentrating the CO2. In my estimation, a reasonable rule of thumb is that it would take 2-4 megajoules of electricity to make one megajoule of common small hydrocarbons (methane, ethane, ethylene, propylene, benzene...) For example, a megawatt hour of electricity is 3600 MJ. If it were used to synthesize the hydrocarbon commodity ethylene at 1/3 efficiency, it would yield about 24 kilograms of ethylene. Bulk ethylene prices in the US in July were $645/tonne, or $15.38 for 24 kilograms. Today a megawatt hour of renewable electricity might cost $30 in a favorable location. So the electricity cost alone of renewable ethylene would be nearly double the all-in cost of petro-ethylene. Probably renewable ethylene would be closer to 4x the cost of petro-ethylene once you add other costs. But, notably, not 100x or even 10x the cost; disposable polyethylene packaging might become too expensive for routine use, absent fossils, but durable goods incorporating polyethylene parts would still be affordable. Higher-value materials synthesized starting from ethylene, like detergents and coolant fluids, would also see a relatively modest price increase. There's a lot of literature on producing liquid fuels and chemicals starting from natural gas and coal. Most schemas proceed through an intermediate called synthesis gas, ("syngas") a mixture of CO/H2 that can be converted into many different products depending on the catalysts used. To get the overall energetic efficiency you'd combine the extant huge literature on synthesis gas conversion with the assumption that synthesis gas is made from the reverse water gas reaction of CO2 with electrolytic hydrogen from water. (Not hard if you're fluent with chemistry and units conversion, otherwise may be challenging. I'd do a table of examples myself if I thought this comment would be more visible.) Here's a presentation that has a schematic view synthesizing renewable hydrocarbons without biomass -- see particularly the slide "Combining Technologies" where it shows that methanol, gasoline, diesel, and methane can all be produced from the same syngas intermediate: http://orbit.dtu.dk/files/51542760/Production_of_Green_Natural_Gas_presentation.pdf http://orbit.dtu.dk/files/51542760/Production_of_Green_Natur...
- njarboe 9y ago
  Thanks for the info and the links. Pretty easy to get a pure CO2 stream on Mars and H2O is all over the place. Just need some nuclear generators to get cranking.