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Chemical oxygen generation (COG) is used in commercial airliners to supply emergency oxygen to passengers and crew if the plane is depressurized. Weight for weight it produces far more oxygen than compressed oxygen tanks.

I read the original paper on COG, several sites from oxygen candle manufacturers and the US Federal Aviation Administration document on oxygen equipment (riveting I assure you). However, most of the rest of my knowledge is from my own background as a physicist; I want to know how these methods compare from an engineering perspective.

Besides reduced weight, are there any other advantages of COG in aircraft? Does it have added safety benefits when compared to high pressure gaseous oxygen storage? The oxygen candle burns extremely hot when it goes off but I'd imagine high pressure gas canisters could be far more dangerous and prone to exploding if damaged.

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  • $\begingroup$ Having worked in the airline industry, I can state with confidence that if something has a weight advantage, it doesn't need any other advantages, lower weight will be sufficient. $\endgroup$ – regdoug Apr 17 '15 at 22:08
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One advantage these chemical systems may have over compressed tanks is how well they are suited for intermittent use and long-term storage (as opposed to regular, continuous operation). The US FAA Aviation Maintenance Technician Handbook points out this advantage on page 16-5:

Sodium chlorate chemical oxygen generators also have a long shelf life, making them perfect as a standby form of oxygen. They are inert below 400 °F and can remain stored with little maintenance or inspection until needed, or until their expiration date is reached.

Compressed gas cylinders, on the other hand, require regular maintenance and testing whether in use or in storage. Pressure vessels are prone to metal fatigue and corrosion over extended periods. Exposure to extreme temperatures can accelerate fatigue or cause an already-weakened cylinder to fail; COG systems, in comparison, are somewhat less vulnerable to temperature extremes.

As part of an emergency system, an oxygen cylinder would experience infrequent use and mechanical fatigue would be less of a concern than corrosion.* Corrosion is driven mainly by the reactivity and partial pressure of the stored gas and by the presence of moisture in the cylinder. Proper selection of fittings such as valves and regulators is critical to minimize the potential for a galvanic reaction between the fitting and the cylinder.

Keep in mind that pressure vessels support a difference of pressure between the inside and the outside of the vessel. During a decompression event, the outside pressure drops rapidly while the inside pressure stays constant, leading to a rapid increase in stress on the vessel. This has two implications: first, the capacity of the tank must be reduced to account for the lower atmospheric pressure at operating altitude; second, a worn tank is most likely to explode just when you need it the most—not at all a desirable quality in a safety device. (COG systems do not, to my knowledge, explode, though they can start fires when handled improperly.)

This leads to strict inspection and maintenance requirements for compressed gas cylinders. They have to be rotated in and out of service every so often to go through requalification testing and potentially reconditioning or condemnation. For some idea of how complicated these requirements are, take a look at this 2003 Aircraft Maintenance Technology article.

All of this, of course, represents a cost to the operator—service contracts, training, etc. In an industry that competes mainly on ticket prices, cost is king, and reduced maintenance costs would be an attractive selling point.

It's also nice to be able to relate the safety system cost directly to the occasions when you need to deploy the system. Going back to the initial point about suitability for intermittent, emergency applications: You're paying maintenance costs for that cylinder whether there's a depressurization or not. The cost of the oxygen itself is probably minor in comparison. So reducing the frequency of incidents by an order of magnitude may not change the cost very much. On the other hand, only paying for the COG system when you use it means the airline can reduce its costs by reducing depressurization incidents. This isn't so much an engineering concern, and I am speculating a bit here; the point is, businesses often don't make decisions in the same way that engineers do, and that's something to keep in mind when comparing design alternatives.


* You might think that pure oxygen is not a corrosive gas (perhaps because we breathe oxygen) but the definition of "corrosive" can vary a bit with context. For example, the Air Liquide Design and Safety Handbook (p. 2) uses this definition:

Corrosive Gases

These are gases that corrode material or tissue on contact, or in the presence of water. ... Due to the probability of irritation and damage to the lungs, mucous membranes and eye tissues from contact, the threshold limit values of the gas should be rigidly observed. Proper protective clothing and equipment must be used to minimize exposure to corrosive materials.

The emphasis here is on personal safety and health impacts and in that context, oxygen isn't a corrosive gas. But in the context of storing a gas in a pressurized cylinder, the emphasis is on the potential for the stored gas to react chemically with the walls or components in a way that weakens them, and in that context, pure oxygen definitely counts as "corrosive."

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  • $\begingroup$ This has been a very very helpful answer. Thanks for taking the time to write it! $\endgroup$ – Leavenotrace Apr 18 '15 at 13:56
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From personal experience of refuge systems, oxygen generating systems are preferred over medical oxygen gas tanks because of space requirements, portability and the ease of relocation.

When emergency refuge systems are designed and installed, either underground, in mines or tunnelling projects or at industrial plants like oil refineries and chemical plants, the critical aspects are:

  • the number of people that will require refuge
  • the duration of survivability, prior to being rescued
  • robustness of the refuge system
  • prevention of damage to the oxygen system from events such as: corrosive elements, explosions, cave-ins and entombment.

To supply the emergency oxygen requirements for a group of people a large number of oxygen tanks are required and the plumbing required to connect all the tanks with piping, valves and pressure regulators can be considerable. All this requires a lot of space. It also requires a lot time to connect and disconnect and reconnect when the refuge system is relocated.

Oxygen generating systems are more compact, they require less space, require less time to maintain and they are portable. Refuge system using oxygen generating devices can be repositioned quickly and with minimal effort compared. The MineARC website shows the refuge portable chambers they make, by way example.

As you state in your question weight for weight, oxygen generators produce more oxygen than gas canisters. It is for this reason that oxygen generators are also used for emergency purposes in submarines and large aircraft: they are compact, portable and they offer the occupants of such craft a longer duration of survivability which in these craft is critical.


Edit.

As an after thought, for the pilots it might be worth considering an oxygen generator that is not a candle and does not create the same amount of heat a candle would. Drager and MSA make portable oxygen supply devices for use in emergencies that allow people to pass through a hazardous atmosphere to reach a safe location.

The devices use potassium superoxide as the oxygen source. It reacts with the water vapour in a person's breathe to generate oxygen via the chemical reaction,

${4KO}_2 + {2H}_2{O} = {4KOH} + {3O}2$

The Drager Oxyboks and MSA equivalent offer an oxygen supply that will last between 8 minutes and 2 hours, depending on breathing rate. In an aircraft, this time could be expanded by having a larger supply of potassium superoxide.

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