The liquid fluoride thorium reactor (LFTR) has the potential to make electric power cheaper than from coal. Typical costs for electric power bought by US utilities average around 5-6 cents per kilowatt hour generated by coal, hydro, and natural gas sources. Government regulations are requiring utilities to buy solar- and wind-generated power at 20-30 cents/kWh. LFTR’s potential cost advantage of 3 cents/kWh is the economic incentive to stop burning CO2-emitting coal, without economically injurious carbon taxes and politically obscured feed-in tariffs. In this way LFTR can improve both the environment and the economy.
There is an additional way to benefit from LFTR’s inexpensive power — synthesizing liquid fuels to replace petroleum. The world gets 37% of its energy from petroleum, vs 21% from coal. A typical nuclear reactor power plant generates about 1 GW (1000 MW) of electric power. A large refinery produces 40 GW of power in the form of gasoline, diesel, and jet fuel. Liquid petroleum fuels contribute to global warming yet are essential to the global economy. Their high energy density and portability make them attractive energy sources for vehicles such as cars, trucks, trains, ships, and airplanes; these all carry their energy sources with them. We can use more LFTR-sourced power for more high speed electric trains and for more small short-range automobiles; we can use LFTR power plants to propel large ocean-going vessels. But we can’t electrify commercial airliners and trucks because they cannot carry heavy, bulky batteries with them.
Petroleum’s high energy density and a century of engineering experience in its use have made it essential to the US economy, and our thirst for it runs to 260 billion gallons per year, of which we import 65% at a cost of $400 billion per year. Our protective presence in the Persian Gulf is estimated to have cost over $7 trillion.
Hydrogen has been touted as the perfect fuel, burning cleanly and emitting only water vapor into the atmosphere after combustion. Hydrogen can be efficiently produced by high-temperature catalytic dissociation or high-temperature electrolysis, possible with advanced nuclear power technologies such as the high-temperature gas cooled reactor (NGNP) favored by Idaho National Labs, or LFTR with molten-salt coolant. The efficiency of conversion from thermal energy to chemical potential energy can approach 50% with the sulfur-iodine cycle if the molten salt temperature is 900 C; a slightly less efficient copper-chlorine cycle can be used at lower temperatures compatible with current nuclear-grade materials.
However hydrogen is an impractical vehicle fuel. To contain it requires either costly refrigeration at -423 F or costly compression to 5000 psi. The small molecules of H2 leak and can embrittle metals.
Nitrogen and carbon can be effective transports of the chemical potential energy of hydrogen. The liquid forms of such fuels can be readily contained in tanks at standard temperatures and modest pressures. These liquid fuel energy densities are superior to those of hydrogen, requiring smaller tanks. Methanol is a reasonable substitute for gasoline, favored by Nobel laureate George Olah; dimethyl ether can substitute for diesel fuel. Both require carbon sources, perhaps from new carbon-capture facilities at new coal plants. That carbon will be eventually released into the atmosphere when the fuel is burned; we borrowed it on the way out of the coal plant.
But what happens if we stop burning coal? Project Green Freedom proposes capturing CO2 from air, but its density is only 0.035% of air. Nitrogen is plentiful in the atmosphere (78%) and returning it to the air is nonpolluting. Consider ammonia for fuel. Ammonia is the second most common industrial chemical.
Ammonia is used to make fertilizers and even directly in farming, injecting liquid ammonia directly under the soil. Fertilizers from ammonia are responsible for enhancing agricultural production that feeds two-thirds of the global population. More than 1% of all primary energy is used to produce ammonia.
Ammonia is such a common industrial chemical that pipelines distribute it in the farm states. It is transported and contained in tanks under modest pressure, similar to propane. It is potentially hazardous to inhale; a 1% concentration inhaled for 1 hour has a 1% fatality risk. However ammonia is readily detected by its odor, and being lighter than air it rapidly dilutes in a spill. Unlike gasoline or diesel fuel, it does not catch fire in an accident; the ignition temperature is 650 C. Considering all such risks, the health hazard of ammonia is about the same as gasoline.
Ammonia has been the fuel for the record setting X-15 airplane. The University of Michigan has an ammonia-fueled truck. In Belgium in World War II ammonia fuel powered buses. Today’s flex-fuel internal combustion engines are able to run on a variety of fuels ranging from gasoline to E85 (85% ethanol, 15% gasoline). Reportedly flex-fuel engines can be adapted to run on a miscible mixture of ammonia and a small amount of dimethyl ether or ammonia mixed with reformed ammonia (NH3 -> 3/2 H2 + N2) on the way to the engine.
Fuel cells are an alternative to internal combustion engines. Hydrogen fuel cells combine with oxygen in air to generate electricity for vehicle batteries and motors. The direct-ammonia fuel cell uses ammonia directly, stripping the hydrogen from the ammonia on the hot surface of a ceramic electrolyte.
The reverse process can manufacture ammonia from streams of nitrogen separated from air and hydrogen created by dissociation powered by high-temperature process heat and electric power from LFTR electric power generators.
The hydrogen electrolysis or thermal dissociation step can be eliminated via solid-state ammonia synthesis, operating like a solid-oxide fuel cell, but in reverse. It similarly has a ceramic proton conducting membrane. It has the advantage that there is never any separated explosive hydrogen gas and it operates at low pressure. Nitrogen is obtained from the ASU (air separation unit). Water supplies the hydrogen. The ceramic membranes are tubes and the SSAS can be scaled up by using more tubes. The SSAS process is safer and cheaper than the standard Haber-Bosch process. The key cost advantage is that SSAS is projected to make ammonia at 6800 kWh per ton. With factory reactor production, LFTR electric power is projected to cost $0.03/kWh, leading to ammonia costs of about $200 per ton. This is half the cost of ammonia produced today from natural gas, and it avoids the release of carbon dioxide in that widespread manufacturing process.
The heat of combustion is the thermal energy that would be released in an internal combustion engine. Taking account of the different prices and heats of combustion of ammonia and gasoline illustrates that energy from ammonia is one-third the cost of energy from gasoline.
Ammonia fuel cost
How might this lower energy cost translate into vehicle fuel costs? The left bar chart illustrates the typical cost components of gasoline in California. Most of the cost is for the crude petroleum that provides the energy content of the gasoline. The refining costs are only about 10%, even though refineries are complex, expensive investments. We don’t really know the cost of SSAS chemical plants, but simply assume that the talented chemical engineers who built petroleum refineries can build similarly large ammonia production plants at about the same cost.
In summary, ammonia liquid fuel can replace petroleum liquid fuels for surface transport vehicles, at less cost, eliminating CO2 emissions.
This article is derived from a presentation by Robert Hargraves, Darryl Siemer, and Kirk Sorensen, entitled Nuclear Ammonia: Thorium’s Killer App, presented October 11, 2011, at the iTheo annual meeting at City College of New York.