In the recent Nuclear Ammonia article post, ammonia was illustrated as a fuel that could propel vehicles in a zero carbon era. Despite our best efforts in developing new internal combustion engines and direct ammonia fuel cells, there will continue to be a role for carbonaceous fuels. Gasoline and jet fuel have double the volumetric energy capacity of liquid ammonia. A given fuel tank can only contain half as much ammonia combustion potential energy as gasoline combustion potential energy. Fuel tank size is very important in aircraft. Decades of engineering of airframes and turbine engines have optimized aircraft performance using diesel-like JP8 jet fuel.
CO2 emissions from burning liquid carbonaceous fuels continue to rise, exceeding even those from burning coal. Aside from the fact that CO2 emissions may cause a global warming catastrophe, and aside from the fact that the world is running out of economically recoverable oil, the US has an energy security problem.
The US produces just 35% of the 260 billion gallons used annually. We pay $400 billion per year for imported oil. The US spent $7 trillion through 2007 to maintain a US presence in the Persian Gulf.
We can and should reduce our use of liquid fuels derived from fossil petroleum, and we know we will always need some carbonaceous gasoline, diesel, and JP8 fuels. Yet there may be carbon neutral methods to recover or offset the CO2 released into the atmosphere by burning them.
Using nuclear heat and power, chemical engineers can design plants to synthesize CHx fuels from any carbon source. One source might be agriculture to harvest carbon from CO2 in the atmosphere. This is a different objective than growing corn to harvest its kernels’ sugar to be fermented into ethanol. This objective is to obtain the carbon from all the plant matter, not the potential energy from ethanol combustion. The energy source to make such synthetic fuels would really be the nuclear heat and power source.
How much agriculture might be needed? Very roughly, an acre of land can produce 3 dry weight tons of biomass per year. This is approximately the same for corn fields and forests. The mass of the carbon in biomass is about 50%, so the dry weight of carbon extracted from the atmosphere this way is about 1.5 tons per acre. The US has about one billion acres of farmland, capable in total of producing about 1.5 GT (giga tons) of carbonaceous fuels. US annual fuel consumption is about 1 GT per year. So making such fuel this way is barely conceivable, especially if we use less, perhaps substituting ammonia or battery power for most vehicles.
Project Green Freedom is conceived by Jeffrey Martin and William Kubic of Los Alamos National Laboratory. The idea is to use a nuclear power plant to provide the energy to synthesize fuel, and use the air flow of the cooling towers as a source for carbon from CO2 that makes up about 0.035% of the atmosphere. They observed that alkaline lakes absorb about 30 times the CO2 of similar size fields of switchgrass, and so conceived of trays of potassium carbonate exposed to the airflow within the nuclear plant cooling towers. The CO2 would be electrochemically removed from solution, combined with hydrogen from electrolysis of water to manufacture methanol, which is converted to gasoline. There is not yet a demonstration plant and there are some concerns about the efficacy of CO2 absorption and the number of cooling towers required. The whole fuel combustion/synthesis process would be carbon neutral, because just as much CO2 would be put into the atmosphere by burning as removed by Green Freedom.
There may be another way to implement a carbon neutral cycle for carbonaceous liquid fuels. Did you notice the “cement” line on the first illustration in this post?
The lime cycle has been used to make mortar for construction for millennia. Limestone is heated very hot to drive off CO2; it’s not really “burned”. Adding water makes calcium hydroxide used as the binding agent for mortar. Water is then given off and the setting mortar very slowly absorbs CO2 from the air to make a strong calcium carbonate cement. This idealized cycle is carbon neutral, but in the real world the process of heating the limestone is accomplished by burning large quantities of natural gas, which is why this process is the fourth largest contributor to atmospheric CO2 pollution, after natural gas, coal, and petroleum burning. In today’s construction industry, lime mortar is replaced by Portland cement, produced by a similar cycle, but with sand added to the limestone to add silicon to the chemistry, making a stronger cement. The CO2 cycle is the same.
This process is the conception of Darryl Siemer, a retired nuclear chemist from Idaho National Labs. Heat from a liquid fluoride thorium reactor (LFTR) would be transferred to the kilns to heat the sand and limestone. The molten salt might have a temperature of 800 C, so it just preheats the sand and limestone. The Portland cement process requires 1500 C, so that energy is supplied by a plasma arc powered by electricity from a LFTR. The exhaust gas contains CO2 and H2O, with the CO2 fed to a synfuel plant combining with H2 from electrolysis powered by LFTR. In this example Darryl proposed making 3 quads of carbonaceous fuel — about 8% of today’s US fuel consumption. Making that much fuel creates 300 MT (mega tonnes) of cement. The process would be carbon neutral, because the fuel synthesized and eventually burned would release CO2 into the atmosphere that would be absorbed by cement hardening as it is used in construction.
The US only uses about 106 MT per year of cement, so the rest could be exported. China uses 1800 MT of cement annually — more than half the entire world production.
So here is another source for carbon neutral carbonaceous fuels — nuclear cement.