Fact File 61. Biomass Fire Fighting and Class A Foams/Wetting Agents

Transcription

1 Fact File 61 Biomass Fire Fighting and Class A Foams/Wetting Agents

2 Biomass Fire Fighting and Class A Foams/Wetting Agents INTRODUCTION... 3 CHEMISTRY/SCIENCE HOW CLASS A FOAM AGENTS WORK... 5 ENVIRONMENTAL... 7 DELIVERY METHODS... 9 CASE HISTORIES AND CORRELATIONS...10 CONCLUSIONS of 11

3 INTRODUCTION The UK is experiencing one major fire per week involving bulk storage of biomass wood/paper and other combustible products. These occur in wood storage for power generation or in waste management sites involving a mixture of combustibles. Common features of these fires are: Large volumes of combustibles involved high fuel/fire load. Large areas of burning. Large heat output. Large emissions of toxic smoke. Large and sustained fire service resources needed to tackle each incident. Significant water quantities used. Several days needed to extinguish each fire. Extensive post fire clean up. Affected plants out of action for weeks and months. Danger to surrounding residential areas and their occupants. Fires involving wood and similar ordinary combustibles have been with us for a very long time. Historically, the major fires have been forest/wild fires, as these were the only sources of high fire loads. However, recent moves in UK and elsewhere have resulted in major electrical power generating stations converting from coal to wood firing, thereby creating a need to transport and store large quantities of wood as biomass fuel, and waste recycling becoming a legal requirement and involving major collection and storage facilities. Where the biomass is stored within silos, work done by SP Technical Research Institute of Sweden has identified both the problem of spontaneous combustion within biomass piles, and fire engineering solutions for silo protection. However, where the biomass storage is in open air piles/dumps, the firefighting challenge remains. This document sets out how the use of Class A fire fighting foams and wetting agents can be used to reduce the extent and duration of fires involving biomass storage and thereby limiting: 1. The heat and smoke produced. 2. The quantities of water needed to achieve extinguishment. 3. The resources to achieve extinguishment. 4. The overall environmental impact. It also sets out the scientific explanation of why Class A foams and wetting agents are effective on ordinary combustibles, and test evidence of foam s effectiveness on wood fires when compared with plain water. Environmental concerns are addressed, as well as case histories of Class A foam use, by fire brigades and others on these types of fire. Whilst all foams, including Class A foams, are wetting agents, not all wetting agents are foams. Both reduce surface tension a key performance characteristic for fire fighting Class A fires, as this allows the agent to penetrate the Class A combustibles. This is achieved by incorporating a suitable surface active agent (surfactant). Class A foams release their liquid slowly and provide a heat resistive blanket thus do not run off/drain away as quickly as wetting agents. For ease of reference these will be referred to in this document as Class A foam agents. 3 of 11

4 CHEMISTRY/SCIENCE HOW CLASS A FOAM AGENTS WORK INTRODUCTION For Class A fires, water remains the most common and effective extinguishing agent in use. Although by and large it works well on combustible solids, it does have some drawbacks, which can reduce its efficiency for certain applications. Particular situations where this can be relevant are those where water supplies are limited or where the fire is deep seated, as in a waste dump or heather moor. In both cases the large scale use of water is either impractical or wasteful, so there exists a real need to improve its efficiency in order to get maximum benefit from minimum application. The main drawback with water is that the very same properties which makes it a liquid in the first place, namely the intermolecular forces which hold it together, are the same ones that are responsible for its very high surface tension and subsequent poor wetting. In short, plain water isn t very wet! Not surprisingly, this somewhat limits how good it is at extinguishing fires in common solids such as plastics, wood and paper, since a significant proportion of the water applied to such fires runs off before it has had chance to take effect. However, by the judicious use of certain additives known as wetting agents, it is possible to turn water from a liquid that wets poorly into one that wets very efficiently, greatly increasing its effectiveness in fire fighting. THEORY OF CLASS A FOAM AGENTS The wetting of a solid substrate (or surface), involves the displacement of one fluid (air), by another one (water). Simply put, a Class A foam agent therefore, is any substance that increases the ability of water to displace air from a solid surface. Without getting too involved in the science behind Class A foam agent wetting, the type of solid surface determines how easily it may be wetted. So for a hard, non-granular and non-porous solid such as a plastic sheet, the degree of wetting is governed by free energy changes taking place. However, for a porous solid with a large surface area such as a textile, it is the rate of wetting which is the dominant factor. 4 of 11

5 The beneficial effects of using a Class A foam agent can be appreciated by comparing it with plain water as shown in the diagram below. On the left is a single droplet of water which, when placed on a solid surface, forms up as a bead because of its high surface tension. When a suitable surface active agent (surfactant) is added to lower the surface tension, the droplet can spread out as a thin film and so wet a much greater surface area. A rule of thumb is that if the surface tension of the Class A foam agent solution is less than the surface energy of the combustible material to which it is being applied, then wetting as a result of spreading will occur. A table of the surface energies of common combustible materials can be seen below: Material Equilibrium surface energy (mn/m) Nylon 41 Aluminium ~500 Polystyrene 33 Rubber 29 Wood Wool 34 Cotton 44 Paper Since most solutions of Class A foam agents have surface tension values below 30mN/m, it follows that on common combustibles, they will spread as a thin film and so provide better wetting than water alone (which has a surface tension of 72mN/m). Wetting can also be achieved by particular surfactants based upon organofluorine and silicone (ie aqueous film forming foams AFFFs). The advantages to be gained are generally outweighed by the increase in cost and unnecessary release of persistent chemicals into the aquatic environment. These are not considered in this Fact File. This discussion has so far assumed that the application of a liquid is to a hard, non-porous solid. However, when that isn t the case, for example a paper bale, the situation is very different. 5 of 11

6 Because of the large surface area involved, it is the rate of wetting, rather than the absolute equilibrium values of the various tensions, which will determine how effective the Class A foam agent is. It has been determined by experimentation that the wetting time is directly related to: 1. The bulk phase concentration of the surfactants in the Class A foam agent solution. 2. The rate at which they diffuse from the bulk phase to the solid/liquid boundary. Consequently, the fastest and therefore most effective wetting of porous solids is achieved by not only using a Class A foam agent solution at the upper end of its concentration range, but also in the choice of agent to begin with. Practically, this means that products specifically formulated to be Class A foam agents may be more effective than those for which wetting is a secondary function, when compared to their original purpose of foaming. Additives that promote the largest reduction in surface tension, without recourse to the use of fluorinated surfactants, and when used at a sufficiently high enough concentration, are likely to provide the best all round wetting in most circumstances for a wide range of Class A fires. 6 of 11

7 ENVIRONMENTAL INTRODUCTION Class A foam agents are highly concentrated aqueous mixtures of surfactants and organic solvents. They may also contain small amounts of additives such as buffers, corrosion inhibitors, dyes and thickeners. For the purpose of this summary, it will be assumed that only specialist Class A products are being considered and we will ignore fluorine containing Class B foams (ie AFFFs) that may occasionally be used in this role. Because Class A products can be used at as little as 0.5 parts of concentrate to 99.5 parts of water, it follows that they have very high active content. This in turn, has significant consequences for the environmental effects of any spillage of the foam concentrate, though not quite so severe for the foam solution, due to its large dilution factor with water. BIO CONSEQUENCES Class A foam agents do not persist in the environment, nor do they bioaccumulate. Hence we can focus purely on any issues associated with aquatoxicity and biodegradation. Where Class A foam agents are designed to be used in particularly environmentally sensitive areas, such as wildfire control, then applicable standards may limit the allowable toxicity of the agent. The American forestry standard USDA ( a) requires the 96 hour LC50 figure for rainbow trout (LC stands for lethal concentration ). LC values usually refer to the concentration of a chemical in the air, but in environmental studies it can also mean the concentration of a chemical in water. An LC50 represents the individual dose required to kill 50% of a population of test animals (eg rats, fish, mice, cockroaches) when exposed to the foam concentrate, shall be greater than 10mg/litre. In addition to toxic effects on aquatic organisms, Class A foam agents can have indirect effects as a consequence of their biodegradation. Like all foams, they are based on largely organic materials which are amenable to natural breakdown processes, once released into the environment. Because these processes often require oxygen to proceed, it means that oxygen depletion in a receiving body of water can occur as the foam degrades. Whether this presents a problem will depend on the total amount of oxygen required to completely degrade the foam, and the propensity and therefore rate at which it degrades. The degradation effect can also be impacted upon by the relative volumes of water and foam used. It should be stated that in general, Class A foam agents do not present a greater risk in terms of organic loading, when compared to other foam types used at a similar concentration. The Environment Agency guidance given in the environmental protection volume of the Fire Operations Manual and also the Pollution Prevention Guideline PPG18 provide best practice. In all cases, steps should be taken to contain run-off from the fire and so prevent both foam and combustion products from entering drains and water courses. The contained run-off can then be considered for treatment and disposal in cooperation with the local Environment Agency officer and the receiving sewerage undertaker. 7 of 11

8 FOAM DISPOSAL Since Class A foam agents do not contain any persistent organohalogens, it is entirely feasible that any Class A foam agent run-off resulting from an incident, once collected, could be handled by a waste water treatment plant (WWTP). Agreement should be sought with the receiving utility company and a plan drawn up of how the disposal should proceed. Certainly, large volumes of Class A foam agent waste should not be discharged directly into the foul sewer without due notice being given to the WWTP or without the consent of the Environment Agency. The effects of a sudden influx to a WWTP are twofold: 1. The transit of Class A foam agent run-off through the sewerage system will cause agitation and therefore frothing. As well as being a visual nuisance, excess Class A foam agent can interfere with flotation processes, level switches and indicators at the WWTP, leading to an unexpected loss of untreated effluent into local water courses, and hence the potential for serious pollution. This risk can be minimised by controlled release to the WWTP and in certain circumstances, by the use of defoaming agents. However, the environmental effects of any defoaming agent should be taken into consideration beforehand. 2. The influx of Class A foam agent waste may have an adverse effect on the activated sewage sludge that is used to treat the incoming waste. Class A foam agent run-off when diluted sufficiently to prevent excessive foaming at the WWTP, should not present a toxic threat to sewage bacteria, but relevant test data and a current safety data sheet, will allow an informed decision to be made, prior to any discharge. More useful guidance can be obtained from Annex F of NFPA 11 (standard for low, medium and high expansion foam). 8 of 11

9 DELIVERY METHODS Class A foam agents can be used in and with the equipment used for Class B foams. This applies to proportioning, although these foam agents can be proportioned at lower rates than Class B foams. Class A foam agents can be discharged through ordinary water hardware as they do not need to be aspirated to be effective. Thus hose streams and monitors can be used, as well as automatic fire sprinkler and deluge systems. Class A foam agents may be discharged through low or medium expansion discharge devices and compressed air foam systems to blanket ordinary combustibles and plastics. The choice of Class A foam agent delivery will depend upon the nature of the combustibles, their configuration, and the structure and layout of the hazard area. 9 of 11

10 CASE HISTORIES AND CORRELATIONS DEVON & SOMERSET A recent case study carried out by one fire and rescue service provided the following information: Between 1 April 2011 and 31 March 2012, they attended 762 vehicle fires. They assumed they used 1500 litres of water at each incident, totalling 1,143,000 litres. If you calculate the same statistic using a CAF (compressed air foam) system, using a Class A foam at 0.5%, the water used per incident would be in the region of 30 litres, totalling 22,860 litres. This produces an overall projected saving of 1,120,140 litres. The fire service in question stated that the 762 vehicle fires accounted for 14% of their overall incidents within the time period stated. The above case study shows that some fire and rescue services are using Class A foams to their advantage. The same fire service also use Class A foam agents for thatched roofs and would have no hesitation in using the product on biomass storage facilities. TYRE DUMP FIRE In a real fire fighting situation Class A foam has extinguished a tyre dump fire in a few hours after being fought unsuccessfully with just water for three days. (Source: Ansul) 10 of 11

11 CONCLUSIONS CLASS A FOAM AGENTS ARE EFFECTIVE The easiest comparison between Class A foam agents and water is from the fire test ratings of portable extinguishers. Typically, a six litre water extinguisher will achieve a 13A rating against a wood crib fire test; however, a typical six litre extinguisher charged with Class A foam agent achieves a 27A rating extinguishing a fire twice the size. Or put another way by using Class A foam agent only half the quantity of water is required to achieve extinguishment. This achieves a saving on water and fire fighting resources, as well as drastically reducing fire effluent run off and its associated containment and disposal. Class A foam agents can offer significant benefits compared to plain water when used on solid combustible fires. CLASS A FOAM AGENTS HAVE NO LONG TERM ADVERSE EFFECTS ON THE ENVIRONMENT Class A foam agents are not persistent or bioaccumulative in the environment. As with any foam, Class A foam agents should be used responsibly and all efforts made to prevent contamination of any nearby bodies of water or release into drains. DISPOSE CLASS A FOAM AGENTS TO WASTE TREATMENT PLANTS Subsequent disposal of the contained waste should be carried out only after consultation with and agreement from the Environment Agency and waste contractor. DISCLAIMER The information set out in this document is believed to be correct in the light of information currently available but it is not guaranteed and neither the Fire Industry Association nor its officers can accept any responsibility in respect of the contents or any events arising from use of the information contained within this document. Tudor House, Kingsway Business Park, Oldfield Road, Hampton, Middlesex TW12 2HD Tel: +44 (0) of 11