A lifting gas, or lighter-than-air gas, is a gas that has a density lower than normal atmospheric gases and rises above them as a result, making it useful in lifting lighter-than-air aircraft. Only certain lighter-than-air gases are suitable as lifting gases. Dry air has a density of about 1.29 grams per liter (g/L) at standard conditions for temperature and pressure (STP) and an average molecular mass of 28.97 g/mol, and so lighter-than-air gases have a density lower than this.
Gases used for lifting
Hot air Heated atmospheric air is frequently used in recreational ballooning. According to the ideal gas law, an amount of gas (including mixtures of gases, such as air) at constant pressure expands as it is heated; that is, its density decreases as its temperature increases. The temperature of the hot air in the envelope will vary depending upon the ambient temperature, but the maximum continuous operating temperature for most balloons is 250 °F (120 °C).
Hydrogen Hydrogen, being the lightest existing gas (0.08988 g/L, 7% the density of air, at STP), seems to be the most appropriate gas for lifting. It can be easily produced in large quantities, for example with the water–gas shift reaction or electrolysis, but hydrogen has several disadvantages:
Hydrogen is extremely flammable. Some countries have banned the use of hydrogen as a lift gas for commercial vehicles but it is allowed for recreational free ballooning in the United States, United Kingdom and Germany. The Hindenburg disaster is frequently cited as an example of the safety risks posed by hydrogen. The extremely high cost of helium (compared to hydrogen) has led researchers to re-investigate the safety issues of using hydrogen as a lift gas, especially for vehicles not carrying passengers and being deployed away from populated areas. With good engineering and good handling practices, the risks can be significantly reduced. Because the diatomic hydrogen molecule is very small, it can easily diffuse through many materials, such as latex, so that the balloon will deflate quickly. This is one reason that many hydrogen- and helium-filled balloons are constructed out of Mylar (boPET).
Helium Helium is the second lightest gas (0.1786 g/L, 14% the density of air, at STP). For that reason, it is an attractive gas for lifting as well. A major advantage is that this gas is noncombustible. But the use of helium has some disadvantages, too:
The diffusion issue shared with hydrogen (though, as helium's molecular radius, 138 pm, is smaller, it diffuses through more materials than hydrogen). Helium is expensive. Although abundant in the universe, helium is very scarce on Earth. The only commercially viable reserves are a few natural gas wells, mostly in the US, that trapped it from the slow alpha decay of radioactive materials within Earth. By human standards, helium is a non-renewable resource that cannot be practically manufactured from other materials. When released into the atmosphere, e.g., when a helium-filled balloon leaks or bursts, helium eventually escapes into space and is lost.
Coal gas In the past, coal gas, a mixture of hydrogen, carbon monoxide, and other gases, like Blau gas, was also used in balloons. It was widely available and cheap. Disadvantages include a higher density (reducing lift), its flammability and the high toxicity of the carbon monoxide content.
Ammonia Ammonia has been used as a lifting gas in balloons, but while inexpensive, it is relatively heavy (density 0.769 g/L at STP, average molecular mass 17.03 g/mol), it is a toxic irritant, and it can damage some metals and plastics.
Methane Methane (density 0.716 g/L at STP, average molecular mass 16.04 g/mol), the main component of natural gas, is sometimes used as a lift gas when hydrogen and helium are not available. It has the advantage of not leaking through balloon walls as rapidly as the smaller molecules of hydrogen and helium. Many lighter-than-air balloons are made of aluminized plastic that limits such leakage; hydrogen and helium leak rapidly through latex balloons. However, methane is highly flammable and, like hydrogen, is not appropriate for use in passenger-carrying airships. It is also relatively dense and a potent greenhouse gas.
Combinations It is also possible to combine some of the above solutions. A well-known example is the Rozière balloon, which combines a core of helium with an outer shell of hot air.
Gases theoretically suitable for lifting
Water vapour The gaseous state of water is lighter than air (density 0.804 g/L at STP, average molecular mass 18.015 g/mol) due to water's low molar mass when compared with typical atmospheric gases such as nitrogen gas (N2). It is non-flammable and much cheaper than helium. The concept of using steam for lifting is already 200 years old. The biggest challenge has always been to make a material that can resist it. In 2003, a university team in Berlin, Germany, successfully made a balloon lifted by steam at 150 °C. However, such a design is generally impractical due to the high boiling point and condensation.
Hydrogen fluoride Hydrogen fluoride is lighter than air and could theoretically be used as a lifting gas. However, it is extremely corrosive, highly toxic, expensive, and heavier than other lifting gases, and it has a high boiling point of 19.5 °C. Its use would therefore be impractical.
Acetylene Acetylene is 10% lighter than air and could be used as a lifting gas. Its extreme flammability and low lifting power make it an unattractive choice.
Hydrogen cyanide Hydrogen cyanide, which is 7% lighter than air, is technically capable of being used as a lifting gas at temperatures above its boiling point of 25.6 °C. Its extreme toxicity, low buoyancy, and low boiling point have precluded such a use.
Neon Neon is lighter than air (density 0.900 g/L at STP, average atomic mass 20.17 g/mol) and could slowly lift a balloon. Like helium, it is non-flammable. However, it is rare on Earth and expensive, and is among the heavier lifting gases.
Nitrogen Pure nitrogen has the advantage that it is inert and abundantly available, because it is the major component of air. However, because nitrogen (as N2) is only 3% lighter than air, it is not a good choice for a lifting gas.
Ethylene Ethylene is an unsaturated hydrocarbon that's 3% less dense than air. Unlike nitrogen however, ethylene is highly flammable and far more expensive, rendering use as a lifting gas highly impractical.
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