Chemistry and Physics of Fire

Chemistry and Physics of Fire To prevent fires from occurring and to extinguish them successfully after they have started, an understanding of the che

To prevent fires from occurring and to extinguish them successfully after they have started, an understanding of the chemical and physical characteristics of fire is important. The chemistry of fire involves the ways in which fires can be started and sustained at the molecular level of the fuel source. The chemical implications of the combustion process and the potential hazards of combustion by-products to workers will also be discussed. The physical aspects of fire involve its thermal properties, methods of heat transfer, and methods of extinguishment. Because fire is a chemical reaction, it is important to understand which hazardous materials pose fire hazards in the workplace and which by-products of the combustion process can often be more hazardous than the hazardous material involved in the fire. Control measures include the use of established labeling, classification systems, and handling procedures for hazardous materials. Examples of these systems include the Department of Transportation Hazardous Materials Classification Labeling System, the National Fire Protection Association (NFPA) Labeling System, and additional references on hazardous materials.

Fire Tetrahedron

For many years, it was believed that the fire process could be described by what was referred to as a fire triangle. This concept basically indicated there were three elements essential to initiate and sustain fire: oxygen, heat or some other energy source, and fuel. In more recent years, we have expanded the fire triangle into what is now referred to as the fire tetrahedron (Klinoff 2012, 97). This concept contains the three elements discussed earlier; however, it adds a fourth element, which is often referred to as the chemical chain reaction figure depicts the fire tetrahedron.

In terms of oxygen supply, air is the most common source of oxygen with, on average, 21 percent of air being oxygen. However, it should be noted that sources of oxygen may also include oxidizers. Oxidizers are substances that acquire electrons from a fuel in a chemical reaction and release oxygen during combustion (NFPA 2008, 17–125). Examples of common oxidizers include fluorine, chlorine, hydrogen peroxide, nitric acid, sulfuric acid, and hydrofluoric acid. For fire-extinguishment purposes, the fuels are classified as follows (NFPA 2008, 17–124): Class A: ordinary combustibles such as wood and paper

Class B: flammable and combustible liquids, petroleum greases, and flammable gases
Class C: fuels involving energized electrical equipment
Class D: combustible metals such as aluminum, magnesium, titanium, and zirconium
Class K: combustible cooking media

Combustion

Combustion can be defined as an exothermic chemical reaction between some substance and oxygen. The chemical reaction between the substance and an oxidizer is referred to as oxidation (NFPA 2008, 6–3). With combustion, the energy that accompanies oxidation is commonly given off as heat and light. The speed at which oxidation occurs in cases of rusting of iron and yellowing of paper varies to that of combustion. The difference between a slow oxidation like rusting and combustion is that combustion occurs so rapidly that the heat is generated faster than it is dissipated, causing a substantial temperature rise in the substance.

The development of smoke in sufficient quantities to obscure exits happens very quickly and is often the first hazard of a fire. In addition, eye irritation, which is primarily due to the concentration of irritants in the smoke, may also impact the vision of the individual trying to escape:

Unique Combustion Phenomena

Five unique combustion phenomena that may occur during a fire include explosions, deflagrations, detonations, flashovers, and back drafts. An explosion is the bursting of an enclosure due to excessive internal pressure. In a fire situation, an example of an explosion is a boiling-liquid expanding-vapor explosion (BLEVE). To illustrate a BLEVE, consider a fire contacting the surface of a flammable liquid storage tank. The last unique combustion phenomenon is referred to as a back draft. A back draft is sometimes referred to as a smoke explosion because it is a fire in an enclosed area that consumes the oxygen supply and generates CO and heat. As the oxygen is being used up, the fire tends to smoke a lot; then, if outside air is introduced, the CO will burn rapidly with explosive force

Heat versus Temperature

It is important to make a distinction between temperature and heat. Heat is a transfer of energy between two objects due to a temperature difference, while temperature is a quantity that determines when objects are in thermal equilibrium. Common units of measure for temperature are:

Celsius (C): 0°C—water freezes; 100°C—water boils
Fahrenheit (F): 32°F—water freezes; 212°F—water boils
Kelvin (K): 0°K = −273.15°C or −459.67°F (also referred to as absolute zero)

Heat of Combustion

The amount of heat released from a fire over a specific time (rate) is based on a material’s heat of combustion. The heat of combustion can be defined as the energy released by the fire per unit mass of fuel burned (Drysdale 2011,19). Obviously, this is important for fire purposes because the amount of heat released influences fire spread, fire extinguishment, and life safety. The unit for heat of combustion is kJ/g. Examples of heat of combustion of common materials include wood (16–19 kJ/g), flammable liquids (19.9–44.8 kJ/g), and flammable gases (10.1–12.1 kJ/g).

Heat Transfer

Heat energy flows due to a temperature difference, with the direction of the flow being from hot to cold or from a higher energy state to a lower energy state. The rate of heat transfer is often expressed as Btu per hour or joules per second. There are three major mechanisms of heat transfer: conduction, convection, and radiation. An understanding of heat transfer in these three modes allows for the design of buildings to limit heat transfer and correspondingly limit fire spread. Heat transfer by conduction: Heat transfer by conduction occurs when the heat is transferred in the material by molecules that are vibrating and colliding with other molecules, thereby transferring their kinetic energy through the material.

Source of Heat

There are numerous heat sources in the work environment capable of starting a fire or keeping it burning once it is started. The following is a description of heat sources follow to NFPA 1997, 1–64–67.

Physics of Combustion

In addition to the chemical properties of fire, physics plays an important role in the fire behavior of materials. Fuel in a fire can be present in one of three different states of matter: solid, liquid, or gas. Combustion usually occurs when the fuel is converted to a gaseous state, because the oxidizer occurs as a gas, and it takes both oxidizer and fuel in the gaseous state for the recombination to occur (Klinoff 2012, 99).

Fire Hazards of Materials

Some of the more common materials found in the workplace can pose potential hazards to the occupants, should these materials become involved in a fire. The most widely used materials and the most involved in fires include wood products, textiles, masonry/stone, structural steel, gypsum, and synthetic materials. Each has its own unique characteristics and composition. Each reacts differently when involved in a fire, and each generates its own unique hazards.

Hazardous Materials

The U.S. Department of Transportation (USDOT) defines a hazardous material as a substance or material capable of posing an unreasonable risk to health, safety, and property when transported in commerce (USDOT 2013a, 2). It should be noted that there is no consensus on a definition for hazardous materials, and in many cases, there is simply a listing of materials rather than a definition of the term itself, such as with the Pennsylvania Right-toKnow Law. Both the DOT and the Emergency Response Guidebook place hazardous materials into nine primary classes: explosives, gases, flammable and combustible liquids, flammable solids, oxidizers and organic peroxides, toxic and infectious substances, radioactive substances, corrosives, and miscellaneous hazardous materials (USDOT 2013c, 11). A description of each of these classes follows (USDOT 2013d, 49CFR Part 173).

Sources of Information on Hazardous Materials

There is a variety of reference sources a safety professional can use when addressing the handling, storage, and shipping of hazardous materials. The first reference is the Fire Protection Guide to Hazardous Materials published by the NFPA. This reference is useful to a safety professional because it provides recommendations for labeling, shipping, storing, and extinguishing hazardous materials.