Although oxygen is the main product targeted in air separation for gasification processes, several byproducts of air separation can be useful and valuable. These are discussed below.
Nitrogen (N2) has many applications, but, specific to gasification, nitrogen can be used to increase gas turbine efficiency as a combustion diluent. As a mostly inert gas, it can also be used for system flushing/purging. If cryogenic distillation is used for air separation, the extremely cold nitrogen stream can be used for cooling.
ASUs can also isolate argon, which accounts for slightly less than 1% of air by volume. Argon has many uses, most due to it being an inert gas (more so than N2).
Some applications of Argon include:
Argon is also used in applications for its ionization spectra, like lasers (in surgery, for example) and particle physics.
Another use is for displacing oxygen (O2):
The value of argon as a byproduct of air separation is fairly low, as its demand is limited, compared to the numerous operating cryogenic air separation units that could potentially produce argon.
While some cryogenic plants do not bother to further separate rare gases like neon, krypton, and xenon, the technology is the same, simply requiring more purification steps. Neon has a significantly lower boiling point than nitrogen (-246.08 and -195.79ºC, respectively), while krypton and xenon both boil higher than oxygen (O2: -182.95ºC; Kr: -153.22ºC; Xe: -108.12ºC). This affects the amount of cooling necessary and order of separation.
Neon, krypton, and xenon are “noble” gases that are generally unreactive (inert) and monatomic. These “rare gases,” so called because of their atmospheric scarcity (see sidebar), are valued for unique applications.
Neon is expensive because of its rarity. Like argon, neon is inert, but most applications that take advantage of that trait preferentially choose the much less expensive argon before neon (or krypton and xenon, for that matter). Neon’s low boiling temperature also means significant cryogenic cooling is necessary to fraction it off from air.
Neon emits a reddish-orange glow in electrically induced visible light discharge and is most widely known for the signs that bear its name, even if it is not the only gas that can be used. Neon is also used in a wide variety of vacuum tubes and lasers, as well as serving as an extremely low temperature refrigerant.
Like neon, krypton is rare, and hence, valuable. It has emission spectra that appears very white, meaning krypton bulbs are prized for photography. This spectra also has applications in house lighting, where, when mixed with argon, the incandescent light emits a more blue light (typically incandescent bulbs have a yellowish tint). These bulbs are more expensive, though, as krypton is approximately 100 times more expensive than argon.
Krypton is also used in laser applications, especially in the red part of the visual spectrum, as krypton has a much higher power density than neon in this range. It also plays a role in nuclear fusion energy research as part of a krypton-fluoride laser.
Like argon, neon, and krypton, xenon finds many uses in lighting. Xenon allows a filament to work easier, as it can produce the same light output with less energy. It can give off high intensity light, so it is used on airport runways, and in high-intensity discharge headlamps for cars. It has a long history of use with lasers, primarily for providing some of the energy to “start” the laser. Xenon can also be used as an anesthetic, although expensive. It has applications in wastewater treatment by ultraviolet light generation.
Please see the related links for more information on xenon and the other gas byproducts discussed on this page.