Fires Nature, Chemistry & Behavior

According to a research carried out by a national newspaper, fire incidents in Pakistan annually claim on average 16,500 precious lives and the total cost incurred annually remains around 400 billion which is almost 7.5% of the total estimated revenue of Budget 2017-18.

To mitigate the risk of fires we need to understand the nature, chemistry, and behavior of fires as well as the hazards that can cause fires. Generally we classify only two types of fire hazards, i.e., Ignition Sources and Fuels on the basis of reasonable practicability, because the third hazard ambient/atmospheric Oxygen cannot be controlled. Ignition Sources provide energy to cause fire. If this amount of energy is less than the Flash Point (of Fluid Fuels) or Fire Point (of Fluid and Solid Fuels), fire will not occur. But it depends on the initial condition of fuel. If the fuel is already energized (contain heat) due to exposure with any other secondary source then the Immediate Ignition sources with low energies could be able to meet the Flash or Fire Point of fuel causing fire.

Similarly understanding with nature of fire helps us control fires at initial stages. Most of us are not well aware about the difference between Flashovers and Smothered/Subdued Fires. We treat both situations with application of same techniques which are only valid for Flashovers as a result of which an oxygen deficient Smothered Fire turns into Backdraft. Flashovers are fires that occur with abundance of oxygen which turns a small growing fire into a fully developed rapid fire, it occurs in well ventilated compartments. Tongues of flame can be seen in the smoke above the fire, as rate of fire chain reaction is directly proportional to the supply of oxygen. Lowering the Oxygen Supply will reduce the rate of fire chain reaction significantly. Backdraft is a situation where a fire that was going to extinguish because of deficiency/absence of oxygen caused by complete combustion of available oxygen but due to sudden introduction of fresh oxygen it explodes and reappears. Because the combustible fire gases, partially combusted gases and fuels are there, everything is already at its fire point (i.e., filled with ignition source or heat) but due to absence of oxygen the fire is extinguishing but once fresh oxygen enters the room, it torches everything which is already at Flash or Fire Point. A subdued fire (in absence of oxygen) gives off brown or blackish smoke as Fire start releasing Carbon Monoxide and Carbon Black instead of colorless Carbon Dioxide due to unavailability of Oxygen. Backdraft is a situation that appears only in indoor fires of buildings and closed compartments, you will see the glass windows browned or blackened, a large amount of smoke with no visible flames. Smoke appearing to pulse in and out where gaps exist in doors and windows due to thermodynamic process cycle, i.e., application of gas laws like Charle’s Law & Boyle’s Law. Moreover, smoke exits from gaps causing sudden drop of pressure, temperature and volume of gases inside the room, to regain the original volume of gases to overcome vacuum in room, colorless fresh oxygen enters the room from gaps to keep the fire alive.

Simplified equation of fire for industries with direct and indirect application of chemicals and petrochemicals is mentioned below;

 

Fire Chemistry 

Fire Chain Reaction (Reduction-Oxidation – Redox Reaction - Exothermic)

Initiator     +          Oxidizer        +      Reducing Agent      à   Combustion Chain

(Ignition Source)    (Oxygen Source)    (Fuel Source)                 (Gaseous Product

                                                                                                       Fire, CO2, CO, Heat, 

                                                                                                       H2O (v), Ash)

Controls:

Making any of the above three (03) reactants as Limiting Reactant may control fire;

Sr #	Reactant	Control	Source	Detail
1.	Ignition Source	Cooling	Water or compressed inert gases at low temperatures.	Quenching/Chilling, removing the energy
2.	Oxidizer	Smothering	Sand, CO2,  AFFF, Fire Blankets, Wet Chemical	Suffocating Fire by purging with nonflammable gas to replace oxygen
3.	Fuel	Starving	Manual	Removing all unburnt or partially burnt fuels
4.	Redox Reaction	Termination	DCP	Attack on Redox Reaction

All fires occur when three type of elements come into contact simultaneously, which are Ignition sources, Oxygen and Fuel. Ignition source includes all those sources which can provide threshold energy (minimum amount of energy required to ignite a fuel) like, heat, spark, ultra violet rays, friction, electrostatic charges, intense beam of light converged at a single point, etc. Second component is oxygen or any source that can supply oxygen, such as all chemicals that are labelled as oxidizing agents, and the third component is fuel or reducing agent which includes all combustible and flammable materials regardless of their physical state as solid, liquid or gas. Encounter of all these components in right proportions causes fire. To extinguish a fire we need to disconnect any of the three components. Practically we cannot remove oxygen from open atmosphere as air contains 21% oxygen whose deficiency may cause suffocation. For open fires we can use cooling and starving methods. Cooling decreases the temperature of fire to reduce the heat below fire point of fuels. Other method is starving which is the removal of residual unignited fuels from fire scene. For indoor fires we use another technique “Smothering” to remove oxygen which acts as a supporter of combustion by means of purging with some non-flammable gas mostly Carbon Dioxide. 

It is quite relevant to discuss here that all fires possess same chain reaction which is known as oxidation-reduction (Redox) reaction. A lot of extinguishing agents are available cheaply in market which can extinguish fires by direct attack on redox reaction. The most common extinguisher with commercial name Dry Chemical Powder (DCP) is cheaply available in market to extinguish all classes of fire. As fires are generally classified in six different groups based on the physicochemical properties of fuels or burning materials. Class A fires include ordinary solid combustibles like paper, wood, cloth, rubber, plastic, etc. Class B fires include flammable and combustible liquids like organic solvents, diesel, kerosene, gasoline, lubricating oils, base oils. Class C fires include flammable gas fires like hydrogen, methane, CNG, LPG, LNG etc. Class D fires are metal fires which includes all pyrophoric metals like sodium. Class E fires are electrical fires where electricity act as ignition source, when we de-energize the supplies the burning material can be extinguished on the basis of its nature or chemistry of construction. Class F fires are kitchen based fires which involve edible Oil & Fat, Ghee etc. for all these classes of fires we have different type of extinguishers like Water, Foam, Dry Chemical Powder, Carbon Dioxide and Wet Chemical respectively.

Fire Behaviors against Extinguishers:

Water is a best extinguisher on ordinary combustible solid fires like wood, paper, card boards, cloths, rubber etc. But we cannot use water on electrical fires because water is a conductor of electricity. Further rarely people know about the thermal and electrical dissociation of water, boiling point of water is 100 Deg. Celsius but dissociation point of water is 3000 Deg. Celsius and most of the electrical fire reach a temperature of 3000 Deg. Celsius or more. Similarly Water dissociates at a potential difference of 1.23 Volts. Dissociation of water molecule causes sudden expansion of Hydrogen and Oxygen Gas at 3000 Deg. Celsius resulted in explosive fires. As nascent hydrogen is a best fuel while nascent oxygen is a best oxidizer. 01 molecule of water occupies a volume of 2.99 ×10ˉ²³ ml, which is even smaller than a needle tip. But on dissociation of same molecule of water at Standard Temperature and Pressure (i.e., 25 Deg. Celsius and 01 atm.), it occupies 67.2 dm³ or liters which is 22.48 ×10²⁶ times greater than the volume of 01 molecule of liquid water. This sudden expansion of gases cause powerful explosions. This is the reason why we do not use water to extinguish Electrical Fire, Metal Fires, Oil Fires and all fires emitting heat with calorific values near the dissociation temperature of Water Molecules. Similarly we avoid using Aqueous Film Forming Foam (AFFF) and Wet Chemical Extinguishers on Electrical Fires due to Electrical Conductivity of Water.  

We also do not use Carbon Dioxide Extinguishers on Class A (ordinary solid combustible) fires, as Class A fuels generate surface ash, this surface ash sometimes causes blanketing to disconnect Oxygen Supplies, but when we use CO2 Extinguisher on Class A fuel, it instantly removes the ash layer of burning surfaces, it is pertinent to mention here that most of the Class A fuels are made up of cellulosic material that gives off oxygen as a product of thermal decomposition, that is why smothering impact of CO2 on Class A fires remains ineffective however removal of ask brings back the flashover to subdued fire. Although we cannot use CO2 extinguisher on Class A Fire but we can use all Carbonated Soft Drinks on Class A fire. On vigorous shaking the bottle of carbonated drink, it liberates dissolved carbon dioxide that serves as expelling agent and expels the solution as extinguishing agent in mist form to extinguish fire. A 1.5 liter Carbonated Soft Drink bottle can extinguish a Fire with size equals to a large domestic dust bin. Mostly organizations refill CO2 extinguishers on yearly basis which causes a loss of huge amount, it will be revealing to you that Carbon Dioxide does not expire. Because there is no pressure gauge mounted on CO2 Extinguishers, you can check the cylinder leakages by Empty and Gross Weight of CO2 Extinguisher to find out the net weight of extinguishing agent (i.e., CO2). If the net weight is equal to the standard capacity of cylinder, there is no need to spend money on refilling of CO2 Extinguisher until the subsequent usage make it empty. We also avoid the use of CO2 Extinguishers in Confined Spaces. CO2 gas is highly compressed (around 54 atmospheric pressure) and serves as a cryogen which may cause irreversible cold burns. On discharge, it suddenly expands and displace all other gases quickly to deoxidize/smother fire. As a result of which partial pressure of CO2 increases in confined space cause asphyxiation/suffocation. If the internal volume of confined space is 5000 dm³ and we use a CO2 extinguisher of 6 liter capacity, on discharge from a pressure of 54 atm. to 01 atm. normal room pressure, it will occupy a volume of 324 dm³ and exert a partial pressure of 6.48%. However due to fire there was already sufficient CO2 and CO was present in confined space. Combined effect of existing CO2 and extinguisher’s CO2 will significantly reduce the partial pressure (21% O2) of Oxygen to cause asphyxiation/suffocation. Stratification is a phenomenon of confined spaces where gases forms different phases due to difference in densities. Light gases reach top, heavy gases settle down in bottom and gases with moderate weight/densities remain in mid height. Generally oxygen remain in mid height (breathing horizon of a man in standing position). Atomic mass of CO2 is 44 which is quite close to 32 atomic weight of Oxygen. Thus CO2 stagnate in breathing zone. Remember, deficiency of oxygen for 04 minutes causes coma and brain injuries, absence of oxygen for 04 minutes causes’ brain death. This is the reason more fatalities occur by suffocation than burns.

Dry Chemical Powder (DCP) assumed to be the best extinguisher against all types of fires because it directly attacks the Redox Reaction (Fire Chain Reaction) which is similar for all classes of fires. Chemically we can use Carbonates and Bicarbonates of Alkali and Alkaline Earth Metals to prepare Dry Chemical Powder, also we use siliconized mono-ammonium phosphate. Due to the very fine particle size (within range of PM10 to PM2.5) its dust/powder is inhalable as well as respirable. Presence of silicon make this extinguisher a source of deadly disease, i.e., Silicosis. That is why we cannot use it in confined spaces to avoid inhalation of DCP particles and kitchen fires to avoid Contamination of edible commodities. A byproduct of Cement industry i.e., Raw Meal can also be used as a best fire extinguisher of DCP Type.

Spread of Fire

Generally fires spread in 04 ways. Conduction (transfer of heat through solids), Convection (transfer of heat through fluids, i.e. liquids and gases), Radiation (transfer of heat in the form of intense energy waves that do not require any medium to propagate) and Direct Burn (direct contact with naked fires). The fourth was of spread, i.e. Direct Burn is visible by naked eye, rest of all other modes of fire spread are invisible that is why mostly people make arrangements available to control fire spread by Direct Burn. Fire travels in the form of heat through hydrant pipelining across the dwellings, it spreads in the form of heat through HVAC Ducts, Lift Ducts, and Electrical wiring connecting all rooms. Fire also spread across window glasses in the form of radiations. On exposure with heat and fire, copper and aluminum made electrical wire become heat up. This heat is directly proportional to resistance, both resistance and heat increase simultaneously, resistance in flow of current also increases the temperatures. Hot wires bring the insulation to softening point where wire insulation melts and fuses with the surface of wire cause further increase in resistance to the flow of current and consequently intense electrical arcs, and electrical fires occur. This whole process takes less than 01 minute immediately after the exposure of heat/fire with electrical wires whether concealed or exposed.

We need to spread this awareness through Electronic & Print Media, also it should be a compulsory subject of our schools curriculum at matriculation level, so as to control thousands of precious lives that are being claimed every year by such disastrous fire accidents.

Regards

Saad Abdul Wahab

Chief Executive Officer

Grow Safe (Private) Limited.