Thermal runaway

Thermal runaway is the special case of the runaway (English runaway ) with respect to temperature and refers to the overheating of an exothermic chemical reaction or a complex installation due to a self-reinforcing, heat -producing process. A runaway will usually cause a destruction of the equipment due to fire or explosion.

  • 2.1 Li-ion accumulators
  • 2.2 transistor

Chemical Reaction Engineering

Exothermic chemical reactions must be controlled by cooling. The cooling must be performed in a dynamic equilibrium with the heat generation of the chemical reaction so that only as much heat is removed, that firstly the reaction does not overheat, on the other hand there is enough heat for the progress of the reaction in the system.

Thermal runaway may occur if

  • Cooling fails or has too little power.
  • The transfer of heat from the chemical mixture is too low. This can lead to the effect that the cooling achieved by a poor heat transfer from the interior of the reaction mass outer portions only. This is often the result of an insufficient mixing of the reactants.
  • The chemical reaction is accelerated by the impurities. This can, for example, the contact with the cooling liquid (water can accelerate some reactions ) or materials from the reactor wall, which sometimes can have a catalytic effect.

The original thermal runaway can be amplified in the sequence by

  • Decomposition reactions of the reactants, the products, in turn, may be reactive,
  • Defective seals and reactor jacket that lead to contact with other reactive materials.

Protection measures are

  • A generous interpretation of the cooling system,
  • Means for extinguishing a fire,
  • Protection valves, especially pressure relief valves, which lead to a Abfackelungsvorrichtung.
  • Metering devices for reaction inhibitors

Thermal stability of operating points

The risk of thermal runaway arises particularly when an unstable operating point on the reactor is operated. A reactor has i.a. two possible operating points at which the amount of heat dissipated by the cooling of the amount corresponding to that generated by the exothermic reaction.

The stable operating point at low temperature is characterized by a self-regulation, that is, that the reactor reached it independently. At temperatures below this point, more heat is generated by the reaction is removed as a result of the cooling, thereby heats the reaction mass. At temperatures above the heat dissipated is higher than the heat of reaction and the reaction mass is cooled down.

The unstable operating point is characterized in that the reactor continuously tends to leave him. At lower temperatures, the cooling is faster than the heat of reaction and the reactor is striving to return to the stable operating point. At higher temperatures, however, the cooling is no longer sufficient to dissipate the heat of reaction and the reactor threatens to go through.

The heat release curve is S-shaped, as in the high turnover is only a finite amount of heat can be released by the limited actionable mass even at arbitrarily high temperature. Above the unstable operating point so that there is another intersection between heat generation and heat dissipation curve line. This is a second stable operating point at a higher temperature and a higher specific product performance.

The choice of the desired operating point of the reactor will drop to a stable point if possible. In exceptional cases you may have the choice of an unstable operating point. The unstable operating point, however, can only be maintained by constant control interventions.

A rising inlet temperature of the coolant or a deterioration of heat transfer, eg by fouling, the heat removal line can move to the right or flatter. This moves the operating point first imperceptibly lead to slightly higher temperatures, until finally, it may happen that the intersection between heat generation and heat dissipation curve straight disappears. It is feared, because the reactor is then in a short time without any warning to the top stable operating point - the reactor passes through.

Electronics

Lithium- ion batteries

Wherever a lithium - ion battery with liquid or fixed electrolyte (lithium - polymer battery ) to a local short circuit of the internal electrodes, for example by contamination of the separator through an included foreign particles or mechanical damage, the short-circuit current can through the inner resistance to heat up the surrounding area of the damaged area so far that the surrounding areas are also affected. This process expands and is stored in the accumulator energy released in a short time. Especially gefährted are lithium batteries Cobaltdioxid. Such thermal runaways are made as the cause of the recently reinforced occurred fires in laptop batteries responsible. The trigger was probably manufacturing errors associated with variations in the operating temperature.

In more recent developments is modified battery chemistry ( LiFePO4 ), or through improvements in the cell membrane, for example, ceramic coatings ( see Li- Tec Battery ) virtually eliminated the risk of fire.

Transistor

The overheating of a transistor increases, the current permeability, which can lead to further increase in current and heat it further. This self- reinforcing dividend process can lead to self-destruction.

When a power MOSFET is increased in the ON state as the temperature increases, the drain -source on-resistance, resulting in an increasing power dissipation in the barrier layer. In case of insufficient cooling, the output in the form of heat dissipation can not be adequately dissipated, thereby reducing the on-resistance is further increased. This eventually leads to the destruction of the component.

Electrical Engineering

In oil -cooled power transformers, the risk of thermal runaway due to impurities (mostly by water in the hygroscopic cooling oils ). In this case, the dielectric loss factor, which can lead to heating and to the explosion increases.

Conflagration
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