A gas generator is a device for generating gas. A gas generator may create gas by a chemical reaction or from a solid or liquid source, when storing a pressurized gas is undesirable or impractical.
The term often refers to a device that uses a rocket propellant to generate large quantities of gas. The gas is typically used to drive a turbine rather than to provide thrust as in a rocket engine. Gas generators of this type are used to power turbopumps in rocket engines, and are used by some auxiliary power units to power electrical generators and hydraulic pumps.
Another common use of the term is in the Industrial gases industry, where gas generators are used to produce gaseous chemicals for sale. For example, the chemical oxygen generator, which delivers breathable oxygen at a controlled rate over a prolonged period. During World War II, portable gas generators that converted coke to producer gas were used to power vehicles as a way of alleviating petrol shortages.
Other types include the gas generator in an automobile airbag, which is designed to rapidly produce a specific quantity of inert gas.
As a power source
The V-2 rocket used hydrogen peroxide decomposed by a liquid sodium permanganate catalyst solution as a gas generator. This was used to drive a turbopump to pressurize the main LOX-ethanol propellants. In the Saturn V F-1 and Space Shuttle main engine, some of the main propellant was burned to drive the turbopump (see gas-generator cycle and staged combustion cycle). The gas generator in these designs uses a highly fuel-rich mix to keep flame temperatures relatively low.
The Space Shuttle auxiliary power unit and the F-16 emergency power unit (EPU) use hydrazine as a fuel. The gas drives a turbine which drives hydraulic pumps. In the F-16 EPU it also drives an electrical generator.
- 2 H2O2 → 2 H2O + O2
Hydrazine decomposes to nitrogen and hydrogen. The reaction is strongly exothermic and produces high volume of hot gas from small volume of liquid.
- 3 N2H4 → 4 NH3 + N2
- N2H4 → N2 + 2 H2
- 4 NH3 + N2H4 → 3 N2 + 8 H2
Many solid rocket propellant compositions can be used as gas generators.
Inflation and fire suppression
Many automobile airbags use sodium azide for inflation (as of 2003). A small pyrotechnic charge triggers its decomposition, producing nitrogen gas, which inflates the airbag in around 30 milliseconds. A typical airbag in the US might contain 130 grams of sodium azide.
Sodium azide decomposes exothermically to sodium and nitrogen.
The resulting sodium is hazardous, so other materials are added, e.g. potassium nitrate and silica, to convert it to a silicate glass.
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