Cryogenic fuel

Last updated June 18, 2023

Cryogenic fuels are fuels that require storage at extremely low temperatures in order to maintain them in a liquid state. These fuels are used in machinery that operates in space (e.g. rockets and satellites) where ordinary fuel cannot be used, due to the very low temperatures often encountered in space, and the absence of an environment that supports combustion (on Earth, oxygen is abundant in the atmosphere, whereas human-explorable space is a vacuum where oxygen is virtually non-existent). Cryogenic fuels most often constitute liquefied gases such as liquid hydrogen.

Some rocket engines use regenerative cooling, the practice of circulating their cryogenic fuel around the nozzles before the fuel is pumped into the combustion chamber and ignited. This arrangement was first suggested by Eugen Sänger in the 1940s. All engines in the Saturn V rocket that sent the first crewed missions to the Moon used this design element, which is still in use today for liquid-fueled engines.

Quite often, liquid oxygen is mistakenly called cryogenic fuel, though it is actually an oxidizer and not fuel - like in any combustion engine, only the non-oxygen component of the combustion is considered "fuel", although this distinction is arbitrary.

Russian aircraft manufacturer Tupolev developed a version of its popular Tu-154 design but with a cryogenic fuel system, designated the Tu-155. Using a fuel referred to as liquefied natural gas (LNG), its first flight was in 1989.

Operation

Cryogenic fuels can be placed into two categories: inert and flammable or combustible. Both types exploit the large liquid-to-gas volume ratio that occurs when liquid transitions to gas phase. The feasibility of cryogenic fuels is associated with what is known as a high mass flow rate.^[1] With regulation, the high-density energy of cryogenic fuels is utilized to produce thrust in rockets and controllable consumption of fuel. The following sections provide further detail.

Inert

These types of fuels typically use the regulation of gas production and flow to power pistons in an engine. The large increases in pressure are controlled and directed toward the engine's pistons. The pistons move due to the mechanical power transformed from the monitored production of gaseous fuel. A notable example can be seen in Peter Dearman's liquid air vehicle. Some common inert fuels include:

Combustible

These fuels utilize the beneficial liquid cryogenic properties along with the flammable nature of the substance as a source of power. These types of fuel are well known primarily for their use in rockets. Some common combustible fuels include:

Liquid hydrogen
Liquid natural gas (LNG)
Liquid methane

Engine combustion

Combustible cryogenic fuels offer much more utility than most inert fuels can. Liquefied natural gas, as with any fuel, will only combust when properly mixed with the right amounts of air. As for LNG, the bulk majority of efficiency depends on the methane number, which is the gas equivalent of the octane number.^[2] This is determined based on the methane content of the liquefied fuel and any other dissolved gas, and varies as a result of experimental efficiencies.^[2] Maximizing efficiency in combustion engines will be a result of determining the proper fuel to air ratio and utilizing the addition other hydrocarbons for added optimal combustion.

Production efficiency

Gas liquefying processes have been improving over the past decades with the advent of better machinery and control of system heat losses. Typical techniques take advantage of the temperature of the gas dramatically cooling as the controlled pressure of a gas is released. Enough pressurization and then subsequent depressurization can liquefy most gases, as exemplified by the Joule-Thomson effect.^[3]

Liquefied natural gas

While it is cost-effective to liquefy natural gas for storage, transport, and use, roughly 10 to 15 percent of the gas gets consumed during the process.^[4] The optimal process contains four stages of propane refrigeration and two stages of ethylene refrigeration. There can be the addition of an additional refrigerant stage, but the additional costs of equipment are not economically justifiable.^{[ citation needed ]} Efficiency can be tied to the pure component cascade processes which minimize the overall source to sink temperature difference associated with refrigerant condensing. The optimized process incorporates optimized heat recovery along with the use of pure refrigerants. All process designers of liquefaction plants using proven technologies face the same challenge: to efficiently cool and condense a mixture with a pure refrigerant. In the optimized Cascade process, the mixture to be cooled and condensed is the feed gas. In the propane mixed refrigerant processes, the two mixtures requiring cooling and condensing are the feed gas and the mixed refrigerant. The chief source of inefficiency lies in the heat exchange train during the liquefaction process.^[5]

Advantages and disadvantages

Benefits

Cryogenic fuels are environmentally cleaner than gasoline or fossil fuels. Among other things, the greenhouse gas rate could potentially be reduced by 11–20% using LNG as opposed to gasoline when transporting goods.^[6]
Along with their eco-friendly nature, they have the potential to significantly decrease transportation costs of inland products because of their abundance compared to that of fossil fuels.^[6]
Cryogenic fuels have a higher mass flow rate than fossil fuels and therefore produce more thrust and power when combusted for use in an engine. This means that engines will run farther on less fuel overall than modern gas engines.^[7]
Cryogenic fuels are non-pollutants and therefore, if spilled, are no risk to the environment. There will be no need to clean up hazardous waste after a spill.^[8]

Potential drawbacks

Some cryogenic fuels, like LNG, are naturally combustible. Ignition of fuel spills could result in a large explosion. This is possible in the case of a car crash with an LNG engine.^[8]
Cryogenic storage tanks must be able to withstand high pressure. High-pressure propellant tanks require thicker walls and stronger alloys which make the vehicle tanks heavier, thereby reducing performance.
Despite non-toxic tendencies, cryogenic fuels are denser than air. As such, they can lead to asphyxiation. If leaked, the liquid will boil into a very dense, cold gas and if inhaled, could be fatal.^[9]

Related Research Articles

<span class="mw-page-title-main">Cryogenics</span> Study of the production and behaviour of materials at very low temperatures

In physics, cryogenics is the production and behaviour of materials at very low temperatures.

Liquid hydrogen (H_2(l)) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H₂ form.

<span class="mw-page-title-main">Liquid nitrogen</span> Liquid state of nitrogen

Liquid nitrogen—LN₂—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about −195.8 °C (−320 °F; 77 K). It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is widely used as a coolant.

Liquid oxygen—abbreviated LOx, LOX, LOXygen or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an application which has continued to the present.

RP-1 (alternatively, Rocket Propellant-1 or Refined Petroleum-1) is a highly refined form of kerosene outwardly similar to jet fuel, used as rocket fuel. RP-1 provides a lower specific impulse than liquid hydrogen (LH₂), but is cheaper, is stable at room temperature, and presents a lower explosion hazard. RP-1 is far denser than LH₂, giving it a higher energy density (though its specific energy is lower). RP-1 also has a fraction of the toxicity and carcinogenic hazards of hydrazine, another room-temperature liquid fuel.

The Brayton cycle is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. The original Brayton engines used a piston compressor and piston expander, but modern gas turbine engines and airbreathing jet engines also follow the Brayton cycle. Although the cycle is usually run as an open system, it is conventionally assumed for the purposes of thermodynamic analysis that the exhaust gases are reused in the intake, enabling analysis as a closed system.

A liquid-propellant rocket or liquid rocket utilizes a rocket engine that uses liquid propellants. Gaseous propellants may also be used but are not common because of their low density and difficulty with common pumping methods. Liquids are desirable because they have a reasonably high density and high specific impulse (I_sp). This allows the volume of the propellant tanks to be relatively low. The rocket propellants are usually pumped into the combustion chamber with a lightweight centrifugal turbopump, although some aerospace companies have found ways to use electric pumps with batteries, allowing the propellants to be kept under low pressure. This permits the use of low-mass propellant tanks that do not need to resist the high pressures needed to store significant amounts of gasses, resulting in a low mass ratio for the rocket.

Liquefied natural gas (LNG) is natural gas (predominantly methane, CH₄, with some mixture of ethane, C₂H₆) that has been cooled down to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the volume of natural gas in the gaseous state (at standard conditions for temperature and pressure).

Liquid air is air that has been cooled to very low temperatures, so that it has condensed into a pale blue mobile liquid. To thermally insulate it from room temperature, it is stored in specialized containers. Liquid air can absorb heat rapidly and revert to its gaseous state. It is often used for condensing other substances into liquid and/or solidifying them, and as an industrial source of nitrogen, oxygen, argon, and other inert gases through a process called air separation.

The highest specific impulse chemical rockets use liquid propellants. They can consist of a single chemical or a mix of two chemicals, called bipropellants. Bipropellants can further be divided into two categories; hypergolic propellants, which ignite when the fuel and oxidizer make contact, and non-hypergolic propellants which require an ignition source.

A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.

Industrial gases are the gaseous materials that are manufactured for use in industry. The principal gases provided are nitrogen, oxygen, carbon dioxide, argon, hydrogen, helium and acetylene, although many other gases and mixtures are also available in gas cylinders. The industry producing these gases is also known as industrial gas, which is seen as also encompassing the supply of equipment and technology to produce and use the gases. Their production is a part of the wider chemical Industry.

Mixtures of dispersed combustible materials and oxygen in the air will burn only if the fuel concentration lies within well-defined lower and upper bounds determined experimentally, referred to as flammability limits or explosive limits. Combustion can range in violence from deflagration through detonation.

A bivalent engine is an engine that can use two different types of fuel. Examples are petroleum/CNG and petroleum/LPG engines, which are widely available in the European passenger vehicle aftermarket.

<span class="mw-page-title-main">Turboexpander</span>

A turboexpander, also referred to as a turbo-expander or an expansion turbine, is a centrifugal or axial-flow turbine, through which a high-pressure gas is expanded to produce work that is often used to drive a compressor or generator.

An LNG carrier is a tank ship designed for transporting liquefied natural gas (LNG).

A liquid nitrogen vehicle is powered by liquid nitrogen, which is stored in a tank. Traditional nitrogen engine designs work by heating the liquid nitrogen in a heat exchanger, extracting heat from the ambient air and using the resulting pressurized gas to operate a piston or rotary motor. Vehicles propelled by liquid nitrogen have been demonstrated, but are not used commercially. One such vehicle, Liquid Air, was demonstrated in 1902.

<span class="mw-page-title-main">Cryogenic rocket engine</span> Type of rocket engine which uses liquid fuel stored at very low temperatures

A cryogenic rocket engine is a rocket engine that uses a cryogenic fuel and oxidizer; that is, both its fuel and oxidizer are gases which have been liquefied and are stored at very low temperatures. These highly efficient engines were first flown on the US Atlas-Centaur and were one of the main factors of NASA's success in reaching the Moon by the Saturn V rocket.

An air separation plant separates atmospheric air into its primary components, typically nitrogen and oxygen, and sometimes also argon and other rare inert gases.

A liquefied natural gas terminal is a facility for managing the import and/or export of liquefied natural gas (LNG). It comprises equipment for loading and unloading of LNG cargo to/from ocean-going tankers, for transfer across the site, liquefaction, re-gasification, processing, storage, pumping, compression, and metering of LNG. LNG as a liquid is the most efficient way to transport natural gas over long distances, usually by sea.

References

↑ Biblarz, Oscar; Sutton, George H. (2009). Rocket Propulsion Elements. New York: Wiley. p. 597. ISBN 978-0-470-08024-5.
1 2 Øyvind Buhaug (2011-09-21). "Combustion characteristics of LNG" (PDF). LNG Fuel Forum. Archived (PDF) from the original on 2012-12-22. Retrieved 2015-12-09.
↑ Oil and Gas Journal (2002-08-09). "LNG liquefaction technologies move toward greater efficiencies, lower emissions". Archived from the original on 2016-06-30. Retrieved 2015-12-09.
↑ Bill White (2012-10-02). "All you need to know about LNG". THe Oil Drum. Archived from the original on 2019-08-29. Retrieved 2015-12-09.
↑ Weldon Ransbarger (2007). "A Fresh look at LNG Process Efficiency" (PDF). LNG Industry. Archived from the original (PDF) on 2016-06-24. Retrieved 2015-12-09.
1 2 "What are the Benefits of LNG". 2015. Archived from the original on 2017-12-04. Retrieved 2015-12-02.
↑ Ramachandran, R. (2014-02-07). "Cryogenic advantage". Frontline. Archived from the original on 2014-03-29. Retrieved 2015-12-02.
1 2 Cryogenic Fuels, Inc. (1991-12-16). "Liquid Methane Fuel Characterization and Safety Assessment Report" (PDF). Metropolitan Transit Authority. Archived from the original (PDF) on 2018-10-09. Retrieved 2015-12-02.
↑ Asogekar, Nikhil. (2015-12-02). "Cryogenic Liquids-Hazards". CCOHS. Archived from the original on 2019-09-25. Retrieved 2015-12-02.

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[isbn0-470-08024-8-1] Biblarz, Oscar; Sutton, George H. (2009). Rocket Propulsion Elements. New York: Wiley. p. 597. ISBN 978-0-470-08024-5.

[Statoil_LNG_Combustion-2] 1 2 Øyvind Buhaug (2011-09-21). "Combustion characteristics of LNG" (PDF). LNG Fuel Forum. Archived (PDF) from the original on 2012-12-22. Retrieved 2015-12-09.

[LNG_Liquefaction-3] Oil and Gas Journal (2002-08-09). "LNG liquefaction technologies move toward greater efficiencies, lower emissions". Archived from the original on 2016-06-30. Retrieved 2015-12-09.

[LNG_Need_to_Know-4] Bill White (2012-10-02). "All you need to know about LNG". THe Oil Drum. Archived from the original on 2019-08-29. Retrieved 2015-12-09.

[LNG_Plant_Efficiency-5] Weldon Ransbarger (2007). "A Fresh look at LNG Process Efficiency" (PDF). LNG Industry. Archived from the original (PDF) on 2016-06-24. Retrieved 2015-12-09.

[What_are_the_Benefits_of_LNG-6] 1 2 "What are the Benefits of LNG". 2015. Archived from the original on 2017-12-04. Retrieved 2015-12-02.

[Cryo_basics-7] Ramachandran, R. (2014-02-07). "Cryogenic advantage". Frontline. Archived from the original on 2014-03-29. Retrieved 2015-12-02.

[Methane_Fuel-8] 1 2 Cryogenic Fuels, Inc. (1991-12-16). "Liquid Methane Fuel Characterization and Safety Assessment Report" (PDF). Metropolitan Transit Authority. Archived from the original (PDF) on 2018-10-09. Retrieved 2015-12-02.

[Canadian_Centre_for_Occupational_Health_&_Safety-9] Asogekar, Nikhil. (2015-12-02). "Cryogenic Liquids-Hazards". CCOHS. Archived from the original on 2019-09-25. Retrieved 2015-12-02.

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