Molten Salts for Thermal Processing
When people think of salt, the first thought that comes to mind generally is not thermal storage or materials processing. Many associate the term “salt” with simple table salt, or NaCl — just something you sprinkle over your food to bring out a little more flavor. Yet unbeknownst to most, salts not only can make your French fries taste better, but they also can serve as safe, stable — and hot — heat transfer fluids for energy generation, energy storage and chemical processing applications.
Molten salts are simply salts in the liquid state. The most common types of salts used in industry usually are:
- mixtures of different alkali
- or alkaline nitrates, nitrites, chlorides, fluorides and carbonates
Each salt has its own unique properties that allow each to be used for many different processes. But the most important feature of salts is their high-temperature thermal stability.
The term “molten salt”— especially if associated with reactive sodium or fluorine — generally carries a negative connotation, as one might think that charging a hot molten material might cause some sort of disastrous plant meltdown. In reality, molten salts are some of the most stable compounds on earth. Salts, in a way, behave almost like noble gases. Bonding an alkali metal such as potassium, to a reactive halogen such as chlorine, yields a stable and nontoxic potassium chloride salt that melts at 1,418°F (770°C) with a boiling temperature near 2,552°F (1,400°C).
Additionally, molten salts operating near their degradation temperature — or even above it — show little to almost negligible vapor pressures. Some nitrate salts, for example, will not degrade until about 1,112°F (600°C). Operating above this temperature will result in the slow formation of metal oxide crystals and the release of minimal amounts of nitrogen and oxygen gases.
These salts are well suited for applications that require very high temperatures and for engineers who do not want to worry about pressure buildup inside systems. In comparison to water-cooled systems, which only have a liquid range up to 212°F (100°C), using water at 572°F (300°C) requires equipment that can withstand 100 atmospheres of pressure. That must be accounted for in system structural design and maintenance planning. By contrast, molten salts can be used for hot bath applications that require reaction or processing temperatures near and even beyond 302°F (150°C). Salts have a high energy density per unit volume, so when molten, they will maintain temperature for a substantial period of time. Thermophysical properties — viscosity, heat capacity, thermal conductivity — of molten salts in the liquid state are identical to those of room-temperature water, so heat transfer capabilities and pumping costs are comparable to those of using a water-cooled system.
A lot of research has been conducted concerning the melting point of salts over the past few decades. One concern with molten salts is that if they are not maintained at proper temperatures, they can solidify. Combining salts of similar chemistries — for instance, mixing two or more salts together — can reduce the melting point of a salt mixture significantly. This decreases the chance of salt solidifying in valves, piping and other places where heat tracing is inadequate. Salt solidifying is not so much a concern for hot bath applications or batch processes; mainly, it must be accounted for in systems that flow the salt through piping as a heat transfer media.
Concentrated solar power has seen increasing demand with binary nitrate salts for thermal storage because of their high thermal stability, energy density and low melting point. Unfortunately, salts do not like to be in the liquid state and will cool and solidify if their temperature dips below approximately 428°F (220°C). Remelting these salts can be disastrous because they will expand significantly during the melting process. The expansion can burst piping and equipment.
Research continues into how to combine many different salts to reduce the melting point to approximately 140°F (60°C); however, trying to achieve lower melting temperatures also can result in lower operating temperatures. A salt composition with a low melting temperature (below 212°F) and high operating temperature (above 1,112°F) would be a remarkable boost for the concentrated solar energy sector. This would result in much higher steam block turbine Rankine efficiencies over the organic heat transfer fluids currently used in these systems.
Fluoride salts have been applied in molten salt nuclear reactors because they can melt near 752°F (400°C) and have 1,800°F (1,000°C) of liquid operating range before the fluid begins to decompose. Nuclear reactors are very harsh environments: Not only does the coolant chemistry have to withstand the high temperatures generated from the heat of the fission process, but they also must endure neutron and gamma ray bombardment, which can blast apart typical bonds between atoms. Salts have an advantage over organic fluids because they share ionic bonds instead of covalent bonds. In a fluid of lithium fluoride (LiF), the Li+ and F- atoms will more or less “associate” with the atom next to them. So, during this barrage from hazardous fission products, the lithium and fluorine atoms can shift and slide to a neighbor, provided there is an equal number of protons and electrons to form the ionic bond. It is this action of the fluidity and flexibility of the atoms within the coolant that allows molten salts to reach these high temperatures over conventional heat transfer fluids.
Molten salts are not perfect. As mentioned previously, they have high freezing points. As heat transfer fluids, they only can be used for applications above 392°F (200°C). Also, corrosion is a major issue. Any process that operates at 1,112°F (600°C) is going to experience severe corrosion no matter what the coolant is composed of, unless the system materials used are some sort of super-alloy such as Inconel or Hastelloy.
Nitrates are the least corrosive of the molten salts and can be used with stainless steel up to 1,112°F (600°C) with minimal corrosion. Nitrates even show low
Nitrates are the least corrosive of the molten salts and can be used with stainless steel up to 1,112°F (600°C) with minimal corrosion.
corrosion rates with plain carbon steel if temperatures are below 662°F (350°C). The low corrosivity and wide operating range (428 to 1,220°F [220 to 600°C]) of the nitrates are reasons why they are the most commonly used chemistry in most molten salt applications.
By contrast, chlorides and fluorides pose significant material engineering problems. Mentioning the word “chloride” alone to an engineer may cause them squirm, as virtually all are familiar with the corrosive potential of chlorides even in small amounts. Some high-nickel alloys fair well with chloride and fluoride coolants, but the longevity of using these materials with these salts is yet to be fully understood. These alloys also are much more expensive than traditional stainless steels, so for large-scale systems, the initial capital and maintenance costs would be immense and economically infeasible.
Corrosion inhibitors added to a fluid is a common and inexpensive practice for water- and glycol-based fluids. The inhibitors can coat the inside of piping and components without affecting the heat transfer performance of the fluid. In the harsh environments of molten salt applications, however, corrosion inhibition is challenging. Most organic compounds cannot withstand the high temperature requirements, and inorganic films dissolve readily in the hot salt melt.
Material handling of these salt chemistries is the biggest impediment to using them in energy-generation systems such as solar and nuclear. However, the technology is still in its infancy. As we have all seen throughout the advancement of science and engineering through history, something that can contain these salts without breaking the bank is out there. Market demand will drive innovation and applications for molten salts.