The term “oxygen candle” refers to a solid-fuel oxygen generator, a compact device designed to produce breathable oxygen through a contained chemical reaction. This generator is a reliable, self-contained source of oxygen, often used in emergency situations where traditional compressed gas or air filtration systems are unavailable or have failed. Unlike a standard candle that consumes oxygen, the oxygen candle generates its own supply, providing a fixed amount of oxygen for a specific duration. This capability makes it indispensable in environments where oxygen levels are difficult to maintain or rapidly deplete.
The Chemical Process of Oxygen Generation
Oxygen generation within the candle is driven by thermal decomposition, a highly controlled, exothermic chemical reaction. The primary reactant is typically a compound like sodium chlorate (\(\text{NaClO}_3\)), or less commonly, potassium or lithium perchlorate. When heated, the solid compound breaks down, releasing gaseous oxygen and leaving behind a stable, solid salt residue, such as sodium chloride (\(\text{NaCl}\)) in the case of sodium chlorate.
The decomposition of sodium chlorate follows the general reaction: \(2\text{NaClO}_3 \rightarrow 2\text{NaCl} + 3\text{O}_2\). This process requires a continuous input of heat, which is supplied by a fuel component mixed into the candle’s core. Iron powder is commonly used as this internal fuel, burning at high temperatures, often around 600°C (1,112°F), to provide the necessary thermal energy for the chlorate to decompose.
The heat generated by the burning iron powder is sufficient to maintain the decomposition without needing an external power source, making the generator self-sustaining once triggered. The core mixture, which includes the chlorate, the fuel, and sometimes a catalyst like barium peroxide, ensures the reaction proceeds at a steady rate until the reactants are exhausted. Barium peroxide also scavenges trace chlorine impurities produced during the decomposition process.
Internal Components and Activation
The physical design of an oxygen candle is engineered to manage the intense heat of the reaction and ensure a predictable oxygen release. The cylindrical device contains the compacted chemical core—the mixture of the oxygen-producing compound and the iron fuel. This core is surrounded by thermal insulation to contain the heat and protect the outer housing and the surrounding environment.
Activation is initiated by a mechanism designed to deliver a high, localized burst of heat to the chemical core. This mechanism is typically an igniter, such as a percussion-activated firing pin or an electrically heated wire. The igniter strikes a small starter charge, often high in iron fuel, which rapidly generates the heat necessary to bring the main chlorate mixture up to its decomposition temperature.
Once ignited, the reaction cannot be stopped or regulated; it continues until all the reactant material is consumed. The resulting oxygen gas is then routed through a cooling and filtering system before being released. This ensures the generated oxygen is cool and free of solid particulates before it reaches the breathing environment.
Essential Applications of Oxygen Candles
Oxygen candles are primarily deployed as a reliable, long-shelf-life source of emergency oxygen in closed or inhospitable environments. Their capacity to store a large volume of oxygen in a small, dense, and stable solid form makes them valuable where space and weight are restricted.
Commercial aircraft use small, integrated oxygen candles to supply the oxygen masks that drop down during a sudden loss of cabin pressure. Pulling on the mask lanyard initiates the reaction, providing a short-term supply of breathable air until the aircraft descends to a safe altitude. Larger generators are routinely used in naval submarines and deep-sea habitats, serving as a secondary or tertiary source of oxygen should the primary air purification system fail.
In the space exploration sector, solid-fuel oxygen generators, known as Solid Fuel Oxygen Generators (SFOGs), have been utilized on crewed vehicles, including the Mir space station and the International Space Station. They provide an oxygen reserve independent of the station’s main environmental control and life support systems. They offer a simple, robust method for replenishing oxygen when resupply is difficult or primary systems are compromised.
Safety Requirements and Operational Hazards
Despite their controlled design, solid-fuel oxygen generators present hazards due to the chemical reaction. The primary safety concern is the extreme heat produced, with the candle’s internal temperature reaching hundreds of degrees Celsius. The outer casing of the generator can become hot enough, potentially up to 260°C (500°F) in some aircraft models, to cause severe burns or ignite adjacent flammable materials if mishandled or improperly mounted.
A second hazard is the rapid release of highly concentrated oxygen, which accelerates combustion. While the reaction is designed to be contained, a breach in the casing or the presence of contaminants like hydraulic oil can lead to an uncontrolled, explosive reaction. Strict operational protocols mandate that these devices be stored away from flammable liquids, oils, or grease, and that personnel wear appropriate protective gear when handling activated candles.
Because the reaction is irreversible once started, there is also a risk of oxygen enrichment in the confined space, which lowers the ignition temperature of surrounding materials. Safety guidelines require proper ventilation and positioning of the candles to ensure the oxygen is safely integrated into the environment.