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All about Pyrogen

Pyrogen arose out of the development, in the former Soviet Union, of a solid fuel for rockets. This fuel provided propulsion but no trailing heat or flame path to guide heat seeking missiles or alert thermal radar systems. By using a derivative of the solid rocket fuel with chemical fire fighting properties, combined with flame suppression coolant, the Soviet science laboratory, Soyuz, was able to develop a remarkable and highly effective fire suppression system.

What is Pyrogen?

Pyrogen consists of a solid aerosol-generating chemical and a solid chemical coolant.

The solid aerosol-generating chemical is a derivative of solid rocket fuels. It is a thermo-plastic mixture consisting of a solid potassium nitrate-based oxidiser, a plasticated nitrocellulose-based combustible binder and patented technological additives.

When activated, the solid-generating chemical undergoes a combustion reaction producing potassium carbonates (KHCO₃, K₂CO₃), carbon dioxide gas (CO₂(g)), nitrogen gas (N₂(g)) and water vapour (H₂O(g)). The above mixture represents the actual extinguishing agent.

As the reaction temperatures are high, potassium carbonates are formed in gaseous form, but as the vapour cools the potassium carbonates condense to a liquid and then a solid.

As solid potassium carbonates are produced by condensation, the particle size is very small, approximately one micron. Micron-sized solid particles mix with the gaseous carbon dioxide, nitrogen and water into a uniform homogeneous gas-like aerosol. This aerosol displays gas-like three-dimensional distribution, and suspension times of up to 60 minutes.

As a combustion product, Pyrogen aerosol is hot upon formation. Although Pyrogen aerosol is more effective in terms of the actual extinguishing action when in its hottest state, the negative impacts of very high temperatures are obvious. Ejection of flame at the discharge outlet and poor distribution of the aerosol within a protected enclosure is the main problem and has to be eliminated. Attempts to replicate Pyrogen by other manufacturers have been unable to overcome these negatives.

Pyrogen has a second unique formulation, the solid chemical coolant, which comes into action eliminating the problems highlighted above.

When the hot Pyrogen aerosol passes through the coolant, the coolant decomposes absorbing a great amount of heat. It is the polymer composition highly impregnated with endothermic ingredients - substances that decompose at 200-300°C without melting, generating gases and absorbing approximately 400 calories of heat per second.

How it works

Application of Pyrogen coolant enables flames to be arrested at the discharge outlet and thus it provides a uniform distribution of the aerosol within the area, which contributes to the reliability and safety of the extinguishment. Moreover, the inert gases formed during thermal decomposition of the coolant contribute to the effectiveness of the extinguishment.

Primarily, Pyrogen's extinguishing action is achieved by interfering chemically with the fire reaction. Two chemical mechanisms take place.

1) Removal of flame propagation radicals - 'chain carriers' OH, H, and O in the flame chain:

The main components of Pyrogen aerosol, potassium carbonates, are formed in the gaseous form. In the flame chain they dissociate producing potassium radicals K. Potassium radicals are very active and react with 'chain carriers' OH, H and O removing them from the fire zone, thus disrupting the fire reaction.

The chemical action of potassium radicals in Pyrogen is similar to that of bromine radicals in Halons and can be schematically represented as follows:

K + OH (r) KOH

KOH + H (r) K + H₂O

2) Recombination of flame propagation radicals - 'chain carriers' OH, H and O on aerosol particle surface:

Gaseous potassium carbonates condense to a liquid and then a solid form producing a large number of micron-sized particles. Being so small, the particles produce a large surface area, where recombination of 'chain carriers' takes place:

O + H (r) OH

H + OH (r) H₂O

Secondarily, Pyrogen extinguishing action is achieved by lowering the fire temperature to a level below which the fire reaction cannot continue (thermal cooling). Several physical mechanisms contribute to this:

i) Heat absorption via endothermic phase changes:

K₂CO₃(s) (r) K₂CO₃(l) (r) K₂CO₃(g)

ii) Heat absorption via endothermic decomposition reaction:

2KHCO₃(s) (r) K₂CO₃(s) + CO₂(g) + H₂O(g)

iii) Dilution of the fire combustion zone by the aerosol cloud (additional fuel molecules cannot participate in the combustion process): physical hydrance to flame propagation (aerosol particles slow down velocity of a flame front propagation) etc.

The extremely high surface area of the micron-sized aerosol particles increases the likelihood of radical recombination and heat absorbing reactions, thus ensuring rapid extinguishment with a small amount of agent.

Pyrogen has the lowest extinguishing concentration known among commercially available agents - flammable liquids (class B fires) are extinguished at the design factor of 100 g/m³ compared to 330 g/m³ (5%) for Halon 1301.

The high rate of aerosol discharge ensures a tremendous knockdown effect thus avoiding fire damage to assets or equipment.

Micron-size aerosol particles exhibit gas-like three-dimensional qualities that allow the agent to rapidly distribute throughout an enclosure and reach the most concealed and shielded location. Homogeneous distribution is achieved in a matter of seconds, while long suspension times all help to prevent fire re-ignition.

Applications

Pyrogen aerosol is suitable for the protection of a variety of potential fire hazards, including those involving flammable liquids, combustible solids, oils and energised electrical equipment.

Approvals

At present, Pyrogen carries Australian and New Zealand approvals by Scientific Services Laboratory (SSL), European Certification Bureau approval (ECB) and Maritime & Coastguard Agency approval (MCA). It is listed by the US Environmental Protection Agency (EPA) under its Significant New Alternatives Programme (SNAP). Most interestingly, Pyrogen is being scrutinised by the FAA, as a Halon alternative for use in commercial passenger jets. If accepted, Pyrogen will be the first Halon alternative to be approved for use in a critical and highly sensitive application.

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