A very large tank just off the highway has been hit by lightning and is burning. In response, a full first alarm assignment commanded by a district chief is dispatched. Upon arrival he asks himself the following:
1.) Can we extinguish, or at least control, this fire with the resources available?
If the answer is no the only viable course of action is the evacuation of any at-risk population while protecting exposures as the fire burn itself out. If the answer is yes the next question is:
2.) How will we effect suppression and extinguishment?
Virtually all flammable liquids are lighter than water and many are miscible with it. Therefore an application of foam, a dry chemical or a mixture of the two will be required.
3.) What foam agent is needed?
4.) How much do we have, where is it kept and how soon can we get more?
Legislators and regulators have heaped another item on our chief’s plate. Now he must ask: What foam am I allowed to use? What are the potential consequences if I use it?
Welcome to the world of “political firefighting.”
Fluorine (At no.9 atomic weight 18.998) is the lightest member of the Halogen family and the most reactive. In my undergraduate days fluorine and its compounds were mostly considered laboratory curiosities. Add a small bottle of calcium fluoride (CaF2), mix that with hydrochloric (HCLaq) and sulfuric (H2SO4) acids to produces hydrogen fluoride (HF). “Hydrofluoric acid” (an aqueous solution of HF) was stored in small paraffin lined bottle buried in vermiculite.
Today this compound is a common article of commerce being shipped in tank car quantities. It is used as an alkylation catalyst in the manufacture of lead-free gasoline, refrigerants and organofluorine compounds such as Teflon (as in non stick cook ware). The manufacture of refrigerants, commonly known as “freons,” currently absorbs about sixty percent of U.S. production. Add to that quantities used to make surfactants found in a host of everyday commodities and firefighting foams. Able to dissolve most oxides, especially those of iron, hydrofluoric acid is a lso a component in products used to remove rust.
Fluorine per se is not the “bugaboo” that some would have us believe. It is a physiologically active and naturally occurring compound of the element responsible for the almost total absence of tooth decay in Deaf Smith County, Texas. It is still being added in the form of a metallic (usually stannous) fluoride to domestic water supplies and toothpastes. The problem, serious or not, is in the molecular structure of the compound.
As a chemical system, firefighting foam consists of three basic parts. First, of course, is water. Then comes is a surfactant to reduce surface tension to allow foam formation andenhance miscibility. Finally air or other gas is supplied either by chemical reaction or, more commonly, entrainment of atmospheric air as the foam is mixed and ejected from the nozzle. Other components may be necessary in special cases owing to the nature of the fire.
Using CAF (Compressed Air Foam) systems it has become possible to regulate not only the composition of the foaming agent but the gaseous component as well. Research has been done on creating foam blown with nitrogen instead of atmospheric air. Simply connect the compressor intake to a source of nitrogen rather than drawing atmospheric air. This would be applicable in fires involving pyrophoric and/or hypergolic materials such as aluminum alkyls or self oxidizing materials such as ethylene oxide (C2H4O).
Of these three entities, water and air are not usually subject to change in composition. Surfactant as the only variable in our foam formulation. A surfactant molecule has a polar end and a non-polar end. The polar end is attracted to polar (water containing) molecules while the non-polar end is attracted to the non-polar (greasy) molecules. This creates a bridge between dissimilar molecules rendering them miscible. In the process the surface tension of the liquid is reduced to the point where foam formation is possible. Early “chemical” foams were created by mixing a solution of soap (surfactant) and a base such as sodium or potassium (Na+ or K+), carbonate (Na2CO3) or bicarbonate (NaHaCO3) with an acid (usually sulfuric, H2SO4) or an acidic salt such as potassium alum (KAl(SO4)2·12H2O).
The gas (CO2) released by the reaction of the basic salt with the acid then “blows” (aerates) the foam. This was packaged as a two component formula (“A” powder and “B” powder) which was mixed and entrained by the water at the nozzle. While the foam was effective the powders were very hygroscopic and subject to caking which clogged automated foam systems. The advent of automatic foam systems using single liquid agents to produce mechanically aerated foam replaced these.
A variety of foaming agents are available. The old standby “GI” or protein is inexpensive and effective for a large number of applications. It is a good choice for small town or rural volunteer departments who want foam capability but infrequently use it, usually for motor vehicles fires. This foam probably has the lowest environmental impact of any in use. Leftover “scraps” have been carried home by personnel to apply (very lightly) to lawns and garden plots with “highly satisfactory” results.
Unfortunately, adding ethanol to motor fuel had an unintended consequence, bringingboth non polar (gasoline) and polar (ethanol) components together. Effectively extinguishing fires with these materials required foaming agents compatible with both, such as aqueous film forming foam (AFFF). However some oppose its use because it contains a fluorinated surfactant.
“Fluorinated surfactant” refers to a class of compounds used in the manufacture of products where the ability to repel water and grease is an asset, such as textiles and leather goods, cookware and electronics. And the aforementioned fire fighting foams.
We can limit our discussion to two substances: perfluoroctanoic acid (PFOA) and perfluoroctanesulfonioc acid (PFOS). These are made up of molecules having a head (polar) and a tail (non polar). Environmental and physiological impact appears to be determined by geometry as much as by chemistry. These molecular chains are composed of carbon atoms occuring in even numbers or pairs. Thus we have chains with eight carbons (designated as C8) and with six carbons (C6). The shorter the chains the less the environmental and physiological impact appears to be.
The US has banned only those compounds containing C8 chains but as research continues it is very likely that C6 chains will, eventually, be barred as well. Fluorinated surfactants persist in the human body for an extended period of time, with a half-life of two to nine years. Due to their ability to penetrate nearly anything except heavy gauge steel, glass and some plastics they remain extremely persistent in the environment.
As for adverse effects on living organisms concern exists about birth defects in laboratory animals. This has not been definitely confirmed in humans. In response the EPA has established a health advisory level of seventy parts per trillion (70 ppt).
Ninety-five percent of the population have detectable levels of PFAS within their bodies originating with any number of materials. While promulgated out of “an abundance of caution,” the health advisory all but precludes use of these compounds for anything, including fire fighting foams.
This leaves the district chief few options. He might revert back to conventional fluorine free foams (F3) such as protein, if it is compatible with the burning material. There is also the “short chain” or C6 based foaming agents which are still legal. And there are newly developed F3 agents available. However, these have major drawbacks. They are more expensive and may not be compatible with existing equipment. Reequipping the department’s apparatus would be a major undertaking.
What are the consequences of not using fluoro-surfactant based fire fighting foams. Runoff will have to be dealt with in accordance with applicable laws and regulations. However, the more effective the foam the less needed. Using foam that is half as effective will generate twice as much runoff, meaning more contaminates to be disposed.
In a tank storage fire, the best course of action is the one which engenders the least amount of adverse environmental impact or adverse effects to the human population. It may be true that fluoro-foams have a negative impact on the environment but until something better is developed we will have to continue to use these compounds for fire suppression while doing everything reasonable to prevent permanent environmental damage.
Meantime, our legislators need to be aware of the dangers of promulgating regulations without hard data to justify doing so. It is one thing for legislators to sit in some far off seat of government and listen to horror stories that the sky is falling (within the next 12 years) and quite another to stand in our district chief’s shoes as he tries to prevent a routine tank fire from becoming a major conflagration.
If these folks are sincerely convinced that we need to ban all fluoro-foams they need to finance research to develop something effective to replace them before banning their use.
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