Adding to my last post's mention of the Fischer-Tropsch process for producing aviation fuel without crude oil, several days ago I came across a paper from the U.S. ORNL that is excellent for quoting here:
Liquid fuels (gasoline, diesel, and jet fuel) have major advantages as transport fuels: a high energy density per unit volume and mass, ease of storage, and ease of transport. [...]
About 40% of the U.S. energy demand is met by petroleum that is converted primarily to liquid fuels. However, the world is rapidly exhausting its resources of the light crude oils used to make liquid fuels [...]
As oil becomes scarce, liquid fuels will be produced with increasing frequency from natural gas (gas to liquids) and from heavier feedstocks such as heavy oil, tar sands, oil shale, and coal. With current technology, this conversion process can be summarized as follows:
Carbon-based feedstock + Water + Oxygen (O2) -> Liquid fuels + Carbon dioxide (CO2) [...]
Alternatively, if economic hydrogen is available from non-greenhouse-emitting sources (solar, wind, nuclear, or steam reforming of fossil fuels with CO2 sequestration) and the energy for the fuel processing does not release greenhouse gases to the atmosphere, the atmospheric carbon CO2 emissions from liquid-fuel production per vehicle mile (unit of liquid fuel) can be lower than that available today from light crude oil. With nuclear hydrogen, this conversion process can become the following:
Carbon-based feedstock + Water + Nuclear energy -> Liquid fuels
Hydrogen is the primary feedstock to convert various forms of carbon into liquid fuels. [...]
There are multiple processes for the production of liquid fuels using nuclear hydrogen. The fuel production processes can be divided into three categories.
• Indirect processes. Carbon feedstocks are converted to syngas [a mixture of hydrogen and carbon monoxide (CO)] and the syngas is subsequently converted to liquid fuels.
• Direct processes. Carbon feedstocks such as coal are directly hydrogenated.
• Other. These are processes designed for a specific feedstock with specific characteristics. The best known examples are the processes that convert shale oil to liquids. [...]
Fisher-Tropsch is the most widely used indirect method for the production of liquid fuels. [...]
The first step is the production of syngas (a mixture of hydrogen, CO, and other gases) from the carbon source, water, and O2. [...]
The feedstock can be almost any carbon-containing material. Gasifiers currently operate on coal, petrocoke, garbage, natural gas, biomass, and a wide variety of other feeds. [...]
The nuclear variant involves supplying O2 for the gasification step and hydrogen to avoid the need for the water-shift reaction (reaction 4) for hydrogen production. In practice, CO2 is produced in the process, thus creating the need to recycle that CO2 back to CO by the reverse-water-shift reaction. [...]
Garbage and sewage solids. Society produces many carbon-containing wastes—many of which were originally made from fossil fuels. Ultimately, the carbon in most of these wastes is oxidized, with the CO2 released to the environment. If these feedstocks are used for liquid-fuel production, there are no additional greenhouse gas emissions beyond what would ultimately occur via the oxidation of these waste streams.
Biomass. [...] Because the CO2 used to make the biomass comes from the atmosphere, no greenhouse gas impacts result. However, only a fraction of the biomass becomes a liquid fuel. For example, the conversion of corn to ethanol results in roughly one-third of the carbon from the original corn in the ethanol, one-third in the by-product animal feed, and one-third in the form of CO2 released to the atmosphere from respiration of the yeast. Biomass is used as an energy source, with much of the energy used to make the fuel. If the biomass is directly converted into liquid fuels by Fischer-Tropsch or a similar process, all the carbon is incorporated into liquid fuels. With this option, biomass is used primarily as a carbon source, not an energy source. The quantities of liquid fuels measured in terms of energy value increase by factor of 3 or more per unit of biomass input.
Air. Liquid fuels can be made from hydrogen and CO2 extracted from (1) the atmosphere or (2) the ocean. A modified Fischer-Tropsch synthesis process is used. The hydrogen is used (1) as a feedstock to make the liquid fuels and (2) as an internal energy source to drive the process of producing the fuel. Because the CO2 is recovered from the atmosphere or seawater, no greenhouse impacts occur. About 80% of the total energy input required to produce the liquid fuel is used to produce the hydrogen. Carbon dioxide extraction from air or water is not the primary energy cost.
The direct production of liquid fuels from air and water is the ultimate option for liquid-fuel production. This option (Forsberg 2005) has been studied for both commercial liquid-fuel production and military fuel production, where a nuclear-powered tanker makes aviation and diesel fuel for naval ships and thus eliminates the logistic challenges of fueling aircraft carriers and other naval vessels. For several reasons, this is an important endpoint option for liquid-fuels production whether or not it is implemented.
• Liquid-fuel impacts. This option provides unlimited liquid fuels with no greenhouse impacts as long as the hydrogen and energy come from non-greenhouse-emitting energy sources.
• Ultraclean liquid fuel. The feedstocks contain no sulfur or heavy metals; thus, ultra clean liquid fuels are produced. [...]
From here.
The cost of producing synthetic gasoline or aviation fuel by nuclear power from atmospheric CO2 would be several dollars per gallon, e.g. with the hydrogen coming from new nuclear power plants designed for thermochemical water-splitting.* That's not too bad at all.
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* One can approximate the resulting Fischer-Tropsch reaction as
(n/2 + m)H2 + mCO -> CmHn + mH2O
The output for synthetic gasoline and other fuels is a mixture of hydrocarbons really, but consider for octane, C8H18, as an example:
m = 8, n = 18
so
17H2 + 8CO -> C8H18 + 8H2O
but suppose the CO comes from the water-gas-shift reaction, CO2 + H2 -> CO + H2O
so the consumption of hydrogen per unit mass of octane produced is suggested by, overall:
25H2 + 8CO2 -> C8H18 + 16H2O
Since hydrogen is 1.008 atomic mass units while carbon is 12.01, the molecular weights of C8H18 and H2 are are 114.22 and 2.016 respectively. So around 0.44 kg of hydrogen is used per kilogram of octane produced, with also roughly that much hydrogen consumption per kilogram of gasoline synthesized from atmospheric CO2.
While electricity could be used for hydrogen generation by electrolysis, economics are a little better for nuclear power plants designed for thermochemical water-splitting, a future cost as little as $1.42 per kilogram of hydrogen.
So synthetic gasoline produced by this process can have a hydrogen generation cost on the order of $0.62 per kilogram of gasoline. For an approximate estimate, observing as mentioned in the paper that hydrogen generation accounts for around 80% of total energy consumption, the total energy cost would be around $0.78 per kg of gasoline.
That's a nuclear energy cost for this production method on the order of $2.10 per gallon. Total costs could be somewhat more, but the energy cost should be the bulk of the total. If the hydrogen generation cost figure mentioned earlier is obtained, the net result should be no more than several dollars per gallon of gasoline.
