Molten Salt Reactors and National Security

 

 ABSTRACT

The Holy Grail to environmentalist and the U.S. Navy is the production of carbon neutral transportation fuels from seawater.

The reason the Navy is interested in seawater to fuel conversion is the ability for aircraft carriers or dedicated fueling ships to produce their own fuel at sea. This would represent a major tactical advantage for the U.S. Navy that is tasked with distributing nearly 1.25 billion gallons of fuel a year worldwide. Since the seawater to jet fuel technology is still in its nascent stages, the large energy demands could only be covered in a system where you have plenty of energy to spare like a large nuclear reactor on board a large aircraft carrier or a small nuclear reactor like a molten salt reactor aboard a small littoral class ship. This U.S. Navy led program has created a technology that can do more than create jet fuel depending on the catalyst. The technology should be possible to create a variety of fuels and hydrocarbon products from seawater, like methane or even pipeline quality natural gas (the new Zumwalt Class destroyer is currently powered by compressed natural gas.)

It is doubtful though, that this process developed by the Navy, would ever be economically competitive with synthetic fuels derived from sources such as trash or coal for civilian applications but, the process does have the very likely potential to be far more cost effective than the Navy’s current bio-fuels program and fossil fuels that it uses when the transportation chain cost of the fuel is considered.

The Navy wants to greatly reduce the number of oilers in their supply line. The necessity of a destroyer or carrier to rendezvous with an oiler in the middle of a mission can be costly and a potential vulnerability for foreign attack. Independence from fossil fuels brings with it an added level of security, further insulating the Navy’s energy needs from turmoil in the Middle East.

With the Navy adopting an MSR (Molten Salt Reactor) to produce strategic synthetic fuels amongst its fleet, there is hope that the navy could view the potential of the MSR for Naval propulsion applications (such as powering larger littoral combat vessels.) Adoption by the Navy as a propulsion system may give way to civilian Naval applications.

 

The Potential of Seawater

Approximately 71 percent of the Earth’s surface is covered in water, and the oceans hold about 96.5 percent of it. As concerns regarding ocean acidification continue to encourage the exploration of carbonless and carbon neutral energy sources, research institutes, governments, and businesses are starting to look towards water as a potential alternative source of energy. Usually this conversation tends to be steered towards more conventional technologies of hydropower, hydroelectric dams, or wave generators.

The United States Navy has developed a proof-of-concept method for converting the carbon dioxide and hydrogen in ocean water into jet fuel for their fighter planes right on board their aircraft carriers. Viability of the fuel was demonstrated using a radio-controlled replica of a World War II era P-51 C Mustang powered by an off-the-shelf two-stroke engine.

Ocean as an Unconventional Source of Carbon

It is estimated that 30 – 40% of carbon dioxide (CO2) emissions produced by humans are absorbed by the Earth’s oceans. Ocean acidification can best be described as a physiochemical process driven by the gas concentration differential between the air and the sea. Henry’s Law states that when a body of water is in contact with the atmosphere, the amount of gas that dissolves into the body of water is proportional to its partial pressure.

As the concentration of CO2 in the Earth’s atmosphere increases due to human activities and other carbon intensive activities, the system as whole seeks to reach equilibrium driving the dissociation of CO2 into seawater as carbonic acid (H2CO3) and other byproducts. Most of this weak acid dissociates further into bicarbonate ions (HCO3-), carbonate ions (CO32-), and hydrogen ions. The concentration of CO2 in the oceans today is roughly 142 ppm, or approximately 100 times the concentration of CO2 in the air, making the ocean an excellent source of carbon and hydrogen for the synthesis of complex hydrocarbons.

 

Electrolytic Cation Exchange Module (E-CEM) Carbon Capture Skid

Leading the U.S. Navy’s efforts to transform seawater into jet fuel is research chemist and inventor, Dr. Heather D. Willauer. Her research in the early 2000s developed the use of surface modified iron catalysts for breaking down seawater and the porous amorphous silica alumina catalysts required for oligomerization into fuel. The E-CEM and 2 step process for hydrocarbon generation is a culmination of her research team’s efforts, and the US Navy intends to deploy some implementation of her novel jet fuel synthesis process on its Nimitz and Gerald R. Ford – Class aircraft carriers by 2020.

The U.S. Naval Research Laboratory (NRL) uses an electrolytic cation exchange module (E-CEM), Carbon Capture Skid to extract CO2 from seawater at 92 % efficiency. The ocean’s pH remains fairly constant at approximately 7.8 to 8.1 thanks in part to a complex carbonate buffer system. At these pH levels, roughly 96 % of the carbon in seawater exists as HCO3-. In order for the E-CEM to operate efficiently, it must first bring the pH of seawater down closer to 4.5 where 99% of the carbon exists as H2CO3 which can readily be degassed into CO2 and H2, which is a necessary feedstock for the production of synthetic fuel.

 

Bring the pH of Seawater below 6.5

The first step in the carbon capture process is therefore to acidify the seawater using a little electrochemistry. The E-CEM consists of a central ion exchange module sandwiched between a cathode compartment and an anode compartment separated by a cation exchange membrane. As the seawater passes through the central ion exchange compartment, an external electric current is supplied to the two electrodes facilitating the exchange of ions across the three compartments.

At the anode, deionized water or reverse osmosis permeate water is processed to produce Hydrogen ions which pass freely through the cation exchange membrane into the seawater in the central chamber. Meanwhile sodium ions migrate out of the central chamber through the cation exchange membrane closest to the cathode. The catholyte used in the cathode chamber is typically deionized or reverse osmosis permeate water that is also free of hardness ions like calcium, iron or magnesium. The overall reaction replaces sodium ions with hydrogen ions effectively lowering the pH of the seawater. The acidified seawater may then be fed into a vacuum stripper to degas and harvest the CO2 and H2.

 

Reduction and Hydrogenation of CO2 to form Light Olefins

The next step in the process is to take a feed of CO2 and H2 and contact it with a metal catalyst in a fixed bed reactor to generate light olefins (C2 – C6). The NRL uses a process similar to the standard Fischer-Tropsch hydrogenation of carbon monoxide (CO) into liquid hydrocarbons.

 

Fischer-Tropsch Process

Co-invented in 1925 by German chemists Franz Fischer and Hans Tropsch at the Kaiser Wilhelm Institute for Coal Research (present day Max Planck Institute for Coal Research), the Fischer-Tropsch process takes a carbon feedstock, converts it into CO and H2 gas via gasification, and then reacts the gas stream in a reactor to produce liquid hydrocarbons. In the gasification step, the feed must be purged of sulfur impurities which can poison the catalysts used in the reactor. The H2/CO ratio in the process is governed by the water gas shift reaction (WGSR), which describes the reversible reaction of CO and H2O to produce CO2 and H2.

Traditional Fischer-Tropsch catalysis hydrogenates CO in two steps. First, CO dissociates and forms a surface bound carbonyl complex with a metal catalyst under high temperature. Then free flowing hydrogen atoms cleave the the carbon-oxygen (C-O) bonds while free flowing carbon atoms form carbon-carbon bonds, lengthening the chains. Since CO is the species that seeds the production of longer hydrocarbon chains on the catalyst surface, CO2 is considered a waste product formed in the production of hydrogen. This is the key difference between the conventional Fisrcher-Tropsch process and the 2 step hydrogenation process developed by the NRL for hydrocarbon synthesis.

Since the NRL already has the hydrogen it needs for hydrogenation from processing seawater through the E-CEM, it is not necessary to consume carbon monoxide gas in the WGSR for hydrogen production. Since the gas stream exiting the E-CEM is composed of H2 and CO2, the NRL wants to catalyze a reverse WGSR, where by carbon dioxide dissociates into the surface of a metal catalyst, hydrogen cleaves the C-O bonds, the hydrocarbon lengthens and CO and H2O are produced as waste products. This is accomplished by modifying the surfaces of Fischer-Tropsch catalysts like cobalt, iron, and ruthenium by combining them with reverse WGSR catalysts like manganese (Mn) or ceria (CeO2), in a composite silica supported zeolite catalyst optimized for use in a fixed bed reactor.

 

Fixed Bed Reactor

A fixed bed or packed bed reactor consists of a vessel or set of tubes filled with an inert or catalytic packing material submersed in a hot liquid. The packing material (i.e. Raschig rings or porous zeolite catalyst) is shaped and dimensioned to enhance contact between two different phases within the reactor. When a gas and a liquid flow through a bed of packing material the two phases are forced into intimate contact with one another due to the dynamic and turbulent conditions within the packed bed. Furthermore a packed bed enhances heat transfer between the two phases and a catalyst, improving reaction rates and the chemical conversion efficiency of the catalyst.

The NRL uses a proprietary iron catalyst with a modified surface profile to directly process a gas feed consisting of CO2, instead of the typical CO, directly into longer chain unsaturated olefins. The NRL claims to be able to convert 60% of the CO2 in a gas feed into a target olefin and minimize the unwanted production of intermediates like methane from 97 percent to 25 percent. The end result is the synthesis of olefins that can be used as a precursor for ethylene oligomerization into jet fuel.

 

Ethylene Oligomerization into Jet Fuel

In order to convert the olefins into jet fuel, the chains need to be lengthened via a process called oligomerization. You might remember from organic chemistry class that a monomer is the smallest molecular unit in a polymer chain. An oligomer is simply what you call a finite set of monomer units. An oligomer two units long is a dimer and an oligomer three units long is a trimer. Oligomerization is simply the process of lengthening an oligomer chain by a finite degree of polymerization to achieve the desired length.

Catalyst Preparation of Nickel-Exchanged Amorphous Silica Alumina

While the oligomerization reaction also takes place within a fixed bed reactor, a different catalyst must be used to lengthen the ethylene chains. First, tetraethylorthosilicate (TEOS) and alumina are subjected to a base-catalyzed hydrolysis to form a hydrogel. The hydrogel is spread across a stainless steel sheet before baking in an oven at 110 °C to create amorphous silica alumina (ASA) pellets. The ASA pellets are then reacted with a solution of Ni(NO3)2, water and dried in an oven at 110 °C to create nickel-exchanged amorphous silica alumina (NiASA).

NiASA makes a very versatile catalyst for ethylene oligomerization. By controlling the crystallinity, pore size, and pore distribution it is theoretically possible to tailor the surface for formation of a particular hydrocarbon. In practice the degree of control was sufficient to reach the higher octane numbers of jet fuel. The NRL team packed NiASA pellets into a fixed bed reactor at 120 °C to encourage the selective oligomerization of ethylene into C8-C16 fuel fraction which can be burned as jet fuel.

 

Why a Molten Salt Reactor?

A MSR (Molten Salt Reactor) will operate at much higher temperatures than conventional Naval reactors. High enough temperatures to be able to directly crack Hydrogen from seawater rather than having to rely on much more inefficient electrolysis. Potentially, because of the high heat and small size of MSRs, and greater efficiency to produce fuel from seawater, other much smaller ships and tankers could be used to make seawater based fuels. A small and highly efficient reactor would allow the Navy greater flexibility in producing fuel for its fleet.

 

Civilian Ship Applications

A single large container ship can emit cancer and asthma-causing pollutants equivalent to that of 50 million cars. The low grade bunker fuel used by the worlds 90,000 cargo ships contains up to 2,000 times the amount of sulfur compared to diesel fuel used in automobiles. The recent boom in the global trade of manufactured goods has also resulted in a new breed of super sized container ship that consume fuel not by the gallons, but by tons per hour, and shipping now accounts for 90% of global trade by volume.

The title of world’s largest container ship is actually held by eight identical ships owned by Danish shipping line Mærsk. All eight ships are 1300ft (397.7m) long and can carry 15,200 shipping containers around the globe at a steady 25.5 knots (47.2 km/h, 29.3 mph) . The only thing limiting the size of these ships is the Suezmax standard, which is the term used to define the largest ships capable of transiting the Suez Canal fully loaded. These ships far surpass the Panamax standard (ships that can fit through the Panama Canal), which is limited to ships capable of carrying 5,000 shipping containers.

Not only are shipbuilders resetting the world record for size on a regular basis but so are the diesel engines that propel them. One of the eight longest container ships in the world, the 1,300 ft Emma Mærsk also has the world’s largest reciprocating engine. At five stories tall and weighing 2300 tonnes, this 14 cylinder turbocharged two-stroke monster puts out 84.4 MW (114,800 hp) – up to 90MW when the motor’s waste heat recovery system is taken into account. These mammoth engines consume approx 16 tons of fuel per hour or 380 tons per day while at sea.

In international waters ship emissions remains one of the least regulated parts of our global transportation system. The fuel used in ships is waste oil, basically what is left over after the crude oil refining process. It is the same as asphalt and is so thick that when cold it can be walked upon . It’s the cheapest and most polluting fuel available and the world’s 90,000 ships chew through an astonishing 7.29 million barrels of it each day, or more than 84% of all exported oil production from Saudi Arabia, the worlds largest oil exporter.

Shipping is by far the biggest transport polluter in the world. There are 760 million cars in the world today emitting approx 78,599 tons of Sulfur Oxides (SOx) annually. The world’s 90,000 vessels burn approx 370 million tons of fuel per year emitting 20 million tons of Sulfur Oxides. That equates to 260 times more Sulfur Oxides being emitted by ships than the worlds entire car fleet. One large ship alone can generate approx 5,200 tonnes of sulfur oxide pollution in a year, meaning that 15 of the largest ships now emit as much SOx as the worlds 760 million cars.

South Korea’s STX shipyard says it has designed a ship to carry 22,000 shipping containers that would be 450 meters long and there are already 3,693 new ship builds on the books for ocean going vessels over 150 meters in length due over the next three years. The amount of air pollution just these new ships will put out when launched is equal to having another 29 billion cars on the roads.

The UN’s International Maritime Organization (IMO) released a report in 2007 saying a 10% reduction in fuel burning was possible on existing ships and 30-40% possible for new ships but the technology is largely unused, as the regulations are largely voluntary.

 

Nuclear Future?

Because of their colossal engines, each as heavy as a small ship, these super-vessels use as much fuel as small power stations. But, unlike power stations or cars, they can burn the cheapest, filthiest, high-sulfur fuel: the thick residues left behind in refineries after the lighter liquids have been taken. The stuff nobody on land is allowed to use.

This fuel can best be described as ‘just waste oil, basically what is left over after all the cleaner fuels have been extracted from crude oil. It’s tar, the same as used to make asphalt. It’s the cheapest and dirtiest fuel in the world’.

 

“Bunker fuel” as it is called in the petroleum industry is thick with sulfur. IMO rules allow ships to burn fuel containing up to 4.5 per cent sulfur. That is 4,500 times more than is allowed in car fuel in the European Union and in America. The sulfur comes out of ship funnels as tiny particles, and it is these that get deep into lungs. The largest ships can each emit as much as 5,000 tons of sulfur in a year – the same as 50million typical cars.

With an estimated 800million cars driving around the planet, that means 16 super-ships can emit as much sulfur as the world fleet of cars.

Oddly enough, there is never any mention of alternative power sources such as nuclear power. Nuclear marine propulsion has been in widespread naval use for over 50 years starting in 1955. There are 150 ships in operation that use nuclear propulsion with most being submarines, although they range from icebreakers to aircraft carriers. A Nimitz class super carrier has more than twice as much power (240,000 hp, 208 MW) as the largest container ship diesel engines ever built and is capable of continuously operating for 20 years without refueling (some French Rubis-class submarines can go 30 years between refueling). The U.S. Navy has accumulated over 5,400 “reactor years” of accident-free experience, and operates more than 80 nuclear-powered ships.

Airborne pollution from these giant diesel engines has been linked to sickness in coastal residents near busy shipping lanes. Up to 60,000 premature deaths a year worldwide are claimed to be as a result of particulate matter emissions from ocean-going ship engines. The IMO, which regulates shipping for 168 member nations, last October enacted new mandatory standards for phasing in cleaner engine fuel.

By 2020, sulfur in marine fuel must be reduced by 90% although this new distilled fuel may be double the price of current low grade fuels.

MSR power plants for container ships, fueled with thorium, would be much more safe and proliferation resistant than traditional naval reactors fueled with highly enriched uranium.

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