Achemical oxygen generator is a device that releasesoxygen via achemical reaction. The oxygen source is usually an inorganicsuperoxide,[1]chlorate, orperchlorate. The generators are usually ignited by afiring pin, and the chemical reaction is usuallyexothermic, making the generator a potentialfire hazard.Potassium superoxide was used as an oxygen source on early crewed missions of theSoviet space program, insubmarines for use in emergency situations, forfirefighters, and formine rescue.
Commercial aircraft provideemergency oxygen to passengers to protect them in case of loss of cabin pressure. Chemical oxygen generators are not used for the cockpit crew, who are typically supplied usingcompressed oxygen cylinders, also known as oxygen bottles. In narrow body airliners, overheadoxygen masks and oxygen generators are provided for each row of seats. In some wide-body airliners, such as theDC-10 andIL-96, oxygen generators and oxygen masks are mounted in the top portion of the seat backs, due to the increased ceiling height. If a decompression occurs, access panels are opened either by an automatic pressure switch or by a manual switch, and the masks are released. When passengers pull down on the mask, they remove the retaining pins and trigger the production of oxygen.
The oxidizer core issodium chlorate (NaClO3), which is mixed with less than 5 percentbarium peroxide (BaO2) and less than 1 percentpotassium perchlorate (KClO4). The explosives in the percussion cap are alead styphnate andtetrazene explosive mixture. The chemical reaction is exothermic and the exterior temperature of the generator will reach 260 °C (500 °F). It will produce oxygen for 12 to 22 minutes.[2][3] A two-mask generator is approximately 63 mm (2.5 in) in diameter and 223 mm (8.8 in) long. A three-mask generator is approximately 70 mm (2.8 in) in diameter and 250 mm (9.8 in) long.
On May 11, 1996, accidental activation of improperly shipped expired generators, mistakenly labeled as empty "oxy canisters",[4] caused theValuJet Flight 592 crash, killing all on board.[5] On August 10, 1986, an ATA DC-10, Flight 131, was destroyed while parked at O'Hare Airport. The cause was the accidental activation of an oxygen generator, contained in the back of a broken DC-10 seat, being shipped in the cargo compartment to a repair station. There were no fatalities or injuries as the plane contained no passengers when the fire broke out.[6][7]
A chlorate candle, or anoxygen candle, is a cylindrical chemical oxygen generator that contains a mix ofsodium chlorate andiron powder, which when ignitedsmolders at about 600 °C (1,100 °F), producingsodium chloride,iron oxide, and oxygen at a fixed rate of about 6.5 man-hours per kilogram of the mixture. The mixture has an indefinite shelf life if stored properly: candles have been stored for 20 years without decreased oxygen output.Thermal decomposition releases the oxygen. The burning iron supplies the heat. The candle must be wrapped inthermal insulation to maintain the reaction temperature and to protect surrounding equipment. The key reaction is:[8]
Potassium andlithium chlorate, andsodium,potassium andlithium perchlorates can also be used in oxygen candles.
In theVika oxygen generator used on some spacecraft, lithium perchlorate is the source ofoxygen. At 400 °C (750 °F), it releases 60% of its mass as oxygen:[9]
Advances in technology have provided industrial oxygen generator systems for use where air is available and a higher concentration of oxygen is desired.Pressure swing adsorption (PSA) incorporates amolecular sieve for gas separation. In the case of oxygen generation a zeolite-based sieve forces preferential adsorption for nitrogen.[citation needed] Clean, dry air is passed through the sieve beds on the oxygen generator, producing an oxygen-enriched gas.Nitrogen separation membrane equipment is also used.
Chemical oxygen generators are used inaircraft, breathing apparatus forfirefighters and mine rescue crews,submarines, and everywhere a compact emergency oxygen generator with long shelf life is needed. They usually contain a device for absorption ofcarbon dioxide, sometimes a filter filled withlithium hydroxide; a kilogram of LiOH absorbs about half a kilogram of CO2.