Condensed aerosol fire suppression: Difference between revisions - Wikipedia

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{{missing information|oxidiser-fuel mixes for aerosol generation. It's ~~[https://www.nist.gov/system/files/documents/el/fire_research/R0000245.pdf~~ not pyrotechnic anymore], right?|date=March 2022}}

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[[Image:Nozzle, condensed aerosol fire suppression, Wiki.png|right|thumb|Nozzle of a mounted aerosol fire suppression system]]

'''Condensed aerosol fire suppression''' is a particle-based method of [[Fire suppression system|fire extinction]]. It is similar to but not identical to dry chemical fire extinction methods, using an innovative [[Pyrotechnics|pyrogenic]], condensed [[aerosol]] fire suppressant. It is a highly effective fire suppression method for [[Fire class|class A, B, C, E and F]] (as is the case for most fire-extinguishing agents, it is not applicable to metal fires —– class D).<ref name="allianz-ref">{{cite web |title=Condensed Aerosol Fire Extinguishing Systems — Allianz Tech Talk vol. 15 |url=https://www.agcs.allianz.com/content/dam/onemarketing/agcs/agcs/pdfs-risk-advisory/tech-talks/ARC-Tech-Talk-Vol-15-Condensed-Aerosol-Fire-Extinguishing-Systems-EN.pdf |publisher=Allianz Global Corporate & Specialty |access-date=15 April 15, 2023}}</ref> Some aerosol-generating compounds (e.g., [[potassium nitrate]]-based) produce a corrosive by-product that may damage electronic equipment, although later generations lower the effect.

Condensed aerosol fire suppression systems employ a fire-extinguishing agent consisting of very finely divided solid particles, suspended in an inert gas. Those superfine aerosol particles are pyrotechnically generated via the combustion of an aerosol-forming agent (AFA) which is stable at room temperature and does not need to be stored in a pressurized container.

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==Aerosol chemistry==

That fire-extinguishing medium is not stored "as is" but rather produced "on demand": the aerosol microparticles and effluent gases are generated by an [[exothermic reaction]] initiated within a condensed, solid, aerosol-forming compound.<ref name="isrmag-def">{{cite web |title=Condensed Aerosol Fire Suppression System |url=https://www.isrmag.com/condensed-aerosol-fire-suppression-system/ |website=Industrial Safety Review |access-date=15 April 15, 2023}}</ref> Typically, a fuse ignites the aerosol-forming compound to generate hot aerosol [[colloid]]s.

The hot aerosol fire suppression technology went through several generations: gen I (oil tank suppression system) and gen II ([[potassium nitrate]]-based, efficient but with corrosive byproducts) denoted as K-type systems, then gen III (boosted [[Strontium nitrate|strontium salt]]-based, less [[Hygroscopy#Deliquescence|deliquescent]] and generating less corrosive byproducts) denoted as S-type systems. All generations leverage [[alkali]]ne or [[alkaline earth metal]] [[nitrate]]s (which act as [[Oxidizing agent|oxidants]] in the hot aerosol generating process), and different kinds of [[Reducing agent|reductants]].

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Compared to gaseous suppressants (which emit only gas) and dry chemical suppression agents (which are powder-like particles of a large size – 25–150 micrometers), the [[National Fire Protection Association]] defines condensed aerosols as those that release finely divided solids of less than 10 micrometers in diameter.

The solid particulates have a considerably smaller [[Median aerodynamic diameter|mass median aerodynamic diameter]] (MMAD) than those of dry chemical suppression agents. The particulates are subject to [[Brownian motion]]: the ~~colloid’s~~colloid's high [[Molecular diffusion|diffusive]] ability and long suspension time mean the microparticles remain airborne significantly longer and leave much less residue within the protected area than alternate (dry or gas) agents. Their large combined surface area efficiently attracts free radicals through [[Adsorption|surface adsorption]].

Condensed aerosols are ''flooding'' agents. In closed premises, they are effective regardless of the location and height of the fire. This can be contrasted with dry chemical systems, which must be directly aimed at the flame. In open spaces, they are reasonably effective when targeting the top of the fire, unlike gaseous and dry agents which must be directed at the base of the flames.

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===Removal of free radicals===

As the aerosol particles (K2CO3, KHCO3) surround and come into contact with the hot flame, they absorb the flame [[heat]] energy, breaking down and releasing large concentrations of potassium radicals (K+, noted K• below —– ie. ions with an unpaired electron). Those potassium radicals bond with the hydroxide (OH+), hydrogen (H+) and oxygen (O+) free radicals that typically sustain combustion, producing harmless by-product molecules like [[potassium hydroxide]] (KOH) and water (H2O), and breaking the chain reaction required to keep combustion active.<ref name="firepro-tech">{{cite web |title=FirePro Technology |url=https://www.firepro.com/en/fire-suppression-technology |website=FirePro |access-date=15 April 15, 2023}}</ref>

Free radicals are thus removed from the chain reaction by way of the following chemical reactions:

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====Recombination of free radicals====

The microparticles provide a tremendously greater reactive surface than typically found in a combustion process, boosting the natural free ~~radical’s~~radical's recombination rate, which leads to a net ~~fire’s~~fire's energy loss:

O• + H• = OH•

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Automated fire suppression systems are widely used within the industry to protect large-scale, enclosed facilities and rooms. They can leverage hot aerosol agents, with the added benefit of low installation and maintenance costs (long shelf storage, non-pressurized storage and distribution). Whether static or portable, hot aerosol extinguishers consist of a thin metal or plastic casing (neither the aerosol-generating compound nor the resulting aerosol needs to be pressurized) and sometimes piping (although usually unnecessary), an initiation device (actuator for the exothermic reaction, either electrical or thermal), some aerosol-forming compound mixed with a reaction component and additives, a solid cooling component and some discharge port(s).

Condensed aerosol-based fire extinguishing systems (CAFES) are widespread among industries and public facilities.<ref>{{Cite journal |last1=Rohilla |first1=Meenakshi |last2=Saxena |first2=Amit |last3=Tyagi |first3=Yogesh Kumar |last4=Singh |first4=Inderpal |last5=Tanwar |first5=Rajesh Kumar |last6=Narang |first6=Rajiv |date=January 1, 2022~~-01-01~~ |title=Condensed Aerosol Based Fire Extinguishing System Covering Versatile Applications: A Review |url=https://doi.org/10.1007/s10694-021-01148-4 |journal=Fire Technology |language=en |volume=58 |issue=1 |pages=327–351 |doi=10.1007/s10694-021-01148-4 |s2cid=254514792 |issn=1572-8099}}</ref> There are two uses for applying fire extinguishing agents: as a total flooding fire protection system or as a local application fire suppression system.

To provide total flooding fire suppression, the total quantity of aerosol required to extinguish a fire inside of fixed space must be determined. The corresponding number of aerosol devices that would collectively discharge the aerosol quantity required are then mounted, typically on the ceiling or wall. Aerosol devices equipped with electric initiators are interconnected and relayed by a fire alarm control panel. Because the aerosol devices are self-contained and function as both a storage container and as a nozzle that propels the gas, no distribution network is required to transport or distribute the fire-extinguishing agent from a remote storage location, resulting in floor space savings and transportation efficiency gains.

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==Environmental footprint and hazards==

Aerosol fire suppression systems have been derived from [[pyrotechnics]] in the ~~[[1990s|~~90s]]. Discovery of [[ozone depletion]] in 1974 led to researching alternatives to conventional, [[Haloalkane|halon]]-based fire-extinguishing agents, ideally with zero ozone depletion impact and reduced [[Climate change|global warming]] potential. [[Montreal Protocol]] enforced a phased ban on halon-based products. Pyrotechnically generated aerosol extinguishing agent (PGAEA) was first suggested by Senecal in 1992.<ref>{{Cite journal |last=Senecal |first=Joseph A. |date=November 1, 1992~~-11-01~~ |title=Halon replacement chemicals: Perspectives on the alternatives |url=https://doi.org/10.1007/BF01873401 |journal=Fire Technology |language=en |volume=28 |issue=4 |pages=332–344 |doi=10.1007/BF01873401 |s2cid=109995489 |issn=1572-8099}}</ref>

The United States Environmental Protection Agency has approved condensed aerosol fire suppression systems as acceptable substitutes for [[Halon 1301]] in total flooding systems.<ref>[http://www.epa.gov/ozone/snap/fire/lists/flood.html U.S. Environmental Protection Agency], "Substitutes for Halon 1301 as a Total Flooding Agent".</ref> Condensed aerosol extinguishers are also non-ozone depleting and carry little or no global warming potential ([[Ozone depletion potential|ODP]], [[Global warming potential|GWP]] and [[Greenhouse gas#Atmospheric lifetime|ATL]] equal to zero).

[[Potassium nitrate]]-based compound produce corrosive byproducts which may damage electronic and electrical equipment, as well as degrade organic matter containing water droplets,<ref>{{Cite journal |last1=Zhu |first1=Chen-guang |last2=Wang |first2=Jun |last3=Xie |first3=Wan-xin |last4=Zheng |first4=Ting-ting |last5=Lv |first5=Chunxu |date=January 1, 2015~~-01-01~~ |title=Improving Strontium Nitrate-Based Extinguishing Aerosol by Magnesium Powder |url=https://doi.org/10.1007/s10694-013-0361-6 |journal=Fire Technology |language=en |volume=51 |issue=1 |pages=97–107 |doi=10.1007/s10694-013-0361-6 |s2cid=254499585 |issn=1572-8099}}</ref> on top of having known adverse effects on human health and impairment to environment visibility. Corrosion is related to the large numbers of corrosive byproducts such as [[oxide]]s, [[hydroxide]]s and [[carbonate]]s of [[Alkali metal|alkaline metals]] together with some acidic [[NOx]] and [[Carbon dioxide|CO2]] gases present in the hot aerosol. [[Strontium nitrate]]-based compounds seem to solve that problem: substituting alkaline metals with alkaline earth metals (like magnesium and strontium) leverages the fact that their oxides, hydroxides and carbonates are insoluble.<ref name="Zhang 707–724"/>

== Safety concerns and incidents==

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Most systems cool the generated aerosol to ensure a flameless discharge and a better (uniform) distribution of the aerosol, yet it is recommended performing a controlled discharge to avoid [[burn]]s, heat shockwaves, [[Blast injury|blasts]] or secondary fires due to hot combustion products (e.g., hot aerosol may reach or exceed 1200[[Kelvin|K]] at the discharge port). Combustion byproducts may also cause reduced visibility within an enclosed area and eye irritation.

The finer particles ([[Particulates|dp]] < 2.5μm) present in the discharged aerosol and the combustion byproducts can deposit on the [[Bronchus|bronchi walls]] in the bronchial tree, reportedly causing [[Chronic Respiratory Disease~~|chronic respiratory diseases~~]]s and [[Acute respiratory distress syndrome|acute respiratory diseases]]. Acidic [[NOx]] and [[Carbon dioxide|CO2]] gases are known respiratory pathogens. Most manufacturers recommend automating the discharge of the aerosol in emptied facilities, as well as letting byproducts settle for a few minutes after the combustion is over.

Aerosol fire suppression systems should not be used on deep-seated fires in class A (solid) materials, with [[Metal reactivity|reactive metals]] and [[Hydride|metal hydrides]], with chemicals which are quick [[Oxidizing agent|oxidizers]] (e.g., [[Gunpowder|gun powder]], [[nitrocellulose]]…), and with chemicals capable of undergoing [[Thermal decomposition|auto-thermal decomposition]].

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*[https://web.archive.org/web/20120319183225/http://www.navsea.navy.mil/teamships/Sea21_Auxiliary_Amphibious_Warfare/Sea21_Amphibious/TheSurfrider2011-1-28.pdf Dwyer, David J. 2011. Improved firefighting system Is on the way. "The Surf Rider," 14–15: 2001-01-28.]

*[https://www.nist.gov/el/fire_research/upload/R9302953.pdf Kibert, Charles J. and Dierdorf, Douglas. 1993. Encapsulated Micron Aerosol Agents (EMMA). ''Halon Alternatives Technical Conference, 1993.'' NIST. May 11–13, 1993, pp 421–435]

*[http://www.nrl.navy.mil/media/publications/spectra/ Halon Alternatives for the Ship-to-Shore Connector. ''Spectra,'' 12: 2001] {{Webarchive|url=https://web.archive.org/web/20110511021746/http://www.nrl.navy.mil/media/publications/spectra/ |date=~~2011-05-~~May 11, 2011 }}

*[https://dx.doi.org/10.6028/NIST.SP.984.4 Fleming, James W., Williams, Bradley A. and Sheinson, Ronald S. 2002. Suppression Effectiveness of Aerosols: The Effect of Size and Flame Type. NIST SP984-4. National Institute of Standards and Technology]