Elements of Fire

Heat

Heat is energy in motion.

Without any form of activity, there is no heat. In other words, heat is the flow of kinetic energy from one body to another.

Heat always flows from a hotter body to a cooler one. It never moves in the other direction unless another form of energy is present during the transfer. The transfer will continue until thermal equilibrium is achieved.

This "hotness" or "coldness" is expressed as temperature. Technically speaking, the temperature of an object is the average kinetic energy of the movement of its molecules. Thermometers are used to measure temperature.

Ignition Temperature or Ignition Energy

Ignition temperature is the minimum temperature at which a substance can burn. A given material must be heated up to this temperature before ignition can begin. At this temperature, the material starts to release vapors which enables the combustion process to commence.

Sources of Heat

Heat is an essential part of fires. Without heat, no fire can start. Heat must also be continually present in the fire for it to keep burning and spreading. There are four general types of heat sources: mechanical, chemical, electrical, and nuclear. Under each type are various sources that range from daily objects to unusual ones.

Investigators must be familiar with all of these heat sources to be able to understand fires. If your knowledge in this area is weak, you will be faced with more fire incidents than you can properly resolve.

Mechanical

Mechanical heat comes from friction which is a type of force that resists motion. Commonly, we observe this in two objects rubbing against each other. Friction by itself does not automatically cause a fire. Different factors, including the properties of the materials and the nature of the motion generating the heat, should be considered.

Chemical

This is heat that comes from exothermic chemical reactions. As discussed in a previous lesson, exothermic reactions are those that include heat as one of its reactions products. Under the right circumstances, the heat released during an exothermic reaction can be sufficient to start a fire.

Electrical

Electrical sources of heat are, as the name implies, electrical devices. Under normal circumstances, appliances will only produce heat according to the constraints of their design. For instance, heat may be generated as a by-product of normal operation or an electrical appliance may be specifically made to provide heat, such as in the case of heaters. These are generally of no concern.

However, these devices can sometimes malfunction due to the poor quality of materials and/or construction. When this happens, the device produces an unexpected amount of heat that could be responsible for fires.

Electricity can come from the smallest sources, like a tiny watch battery, to the largest, such as a lightning bolt. An investigator would be remiss for not being familiar with the materials included in this wide variety.

Nuclear

As mentioned in the previous lesson, we have a special category of fuels called nuclear fuels. Initially, they were mainly utilized for their devastating potential, such as when the atom bomb was first manufactured. Today, however, nuclear fuels are being used for their energy production capabilities. Of course, the planet still has enough stores of nuclear weapons that could destroy the Earth many times over.

Spontaneous heating, or heating that occurs without an external source, can also occur biologically. Decaying matter gives off heat as they break down. If the heat is contained, the temperature of the material can keep rising to its ignition temperature. If a sufficient amount of oxygen is available, it can result in spontaneous ignition.

Heat Transfer

In sum:

• conduction: transfer of heat via direct contact between two bodies or through a heat-conducting medium
• convection: heat transfer through a circulating medium, usually air or liquids
• radiation: movement of heat via waves

Temperature Scales

• Celsius Originally known as the "centigrade scale", °C, is commonly used in scientific works
• Fahrenheit Symbolized by °F; Mostly used in English-speaking countries for purposes other than scientific works
• Kelvin K; Most commonly used thermodynamic scale with 0 K defined as the absolute zero temperature equivalent to -273.15 °C or –459.67 °F

Other notable temperature scales:

• Rankine °R: Another temperature scale that has an absolute zero at its lowest point. Unlike Kelvin, which scales with the Celsius measure, Rankine scales with the Fahrenheit measure.
• International Temperature Scale (ITS): This measure is based on the electrical resistivity, with platinum wire being used as the standard for temperature between –190°C and 660°C

Fuel

As mentioned in the previous lesson, fuels are substances that burn. In this context, it is important to remember that the term "fuel" is not limited only to what we generally imagine when someone says fuel, such as gasoline or liquified petroleum gas.

Matter generally exists in three physical states: solid, liquid, and gas. Solids dissolve to become liquids which can then vaporize into gases. Fundamentally, all fuels are converted to gas at high enough temperatures.

Each different physical state exhibits different physical and chemical properties that directly affect a fuel's combustibility. For example, gasoline as a liquid does not burn, but the vapors rising from it do.

Fuels can be sourced naturally or artificially. Examples of natural fuels are coal and natural gas. These were formed over millions of years from animal and plant remains and are now deposited underground. These are also called fossil fuels and account for about 90% of the world's energy sources. Synthetic fuels are those derived from fossil fuels. For instance, crude oil is converted to gasoline through refinement.

Most fuels release energy by burning with oxygen in the air. Others, such as chemical fuels utilized in rockets, need special oxidizers to burn. Nuclear fuels, on the other hand, release energy through fission or fusion of atoms.

Classifications of Combustible Materials

• Class A These fuels are ordinary combustible materials that are usually made from organic substances like wood and wood-based products. It is incorporated into some synthetic and organic materials like rubber, leather, and plastic.
• Class B These are fuels in the form of flammable liquids such as alcohol, acidic solutions, oil, liquid petroleum products, etc.
• Class C These are generally fire-resistant materials such as those used for electric wirings and in electric circuits.
• Class D These are combustible metallic substances like magnesium, titanium, zirconium, sodium, and potassium.
• Class K These are flammable liquids that are unique to cooking and are vegetable or animal-fat based.

Class C and Class D fuels might sound similar, but they can be distinguished from each other by their action. Class C fuels are only potentially dangerous while electricity is running through them. Class D fuels can be explosive on their own.

General Categories of Combustible Materials

• Solid Includes metallic solid materials, whether they are natural or synthetic, organic or inorganic
• Liquid Includes chemicals and all flammable liquids
• Gaseous Includes hazardous or toxic gases that are capable of ignition

Solid Fuels

The most common solid fuels are wood, paper, and cloth. Their burning rates depend upon their configurations. For instance, a pile of crumpled pieces of paper will burn faster than a stack of neatly folded sheets.

Kinds of Flammable Solids

• Pyrolyzable solid fuels are those materials that are commonly known as easy to burn, including wood and paper. The vapors released during their chemical decomposition aid in sustaining flaming combustion.
• Non-pyrolyzable solid fuels are those that are hard to ignite. A typical example would be charcoal. Chemical decomposition does not occur during the combustion of these materials, so vapors are not released during the process.

Groups of Solid Fuels

Biomass are replaceable organic matters like wood, garbage, and animal fat which can be used to produce energy. They are frequently utilized in processing plants. In the country, rice hulls are a common waste product that is transformed into power.

Effects of various factors on the combustibility of wood and wood-based products:

• physical form - the smaller the pieces, the easier/faster they burn
• moisture/water content - as shown in a video in a previous lesson, dry wood burns faster than fresh wood
• heat conductivity - the higher the conductivity, the faster the material burns
• rate and period of heating - less-flammable materials do not readily ignite
• rate of combustion - fuel is consumed faster if the combustion is more intense
• ignition temperature - the lower the number, the faster it will be reached, thus the fire can start sooner

Fabrics and Textiles

Fibers are very delicate and thin strands of material. Fabrics are twisted or woven fibers. While textiles are machine-woven or knitted fabrics.

Classifications of Fabrics

• Natural Fibers come from animals (e.g. wool, silk), plants (e.g. abaca, pineapple, coconut), and minerals (e.g. asbestos).
• Synthetic or Artificial Fibers are manmade and synthesized from various chemicals. Examples include acetate, fiberglass, and steel.

Effect of various factors on the combustibility of fibers

• chemical composition - Natural and artificial fibers are generally highly combustible, especially when they're dry. Mineral and artificial inorganic fibers are usually fire-resistant.
• fiber finish or coating merged with organic fibers supports the continued burning of the fabric
• fabric weight - heavier fabrics resist ignition
• tightness of weave - tighter woven cloth take more time to ignite
• flame retardant treatment - as the name implies, treated cloth has higher ignition resistance

Fabric Ignition

Limiting Oxygen Index (LOI) is a measure of the tendency of a fabric to continue burning after the initial ignition source is removed. Fabrics with higher LOI and ignition temperature are safer for clothing and furniture because they are likely to cease burning once the initial flame is removed.

Plastics are categorized as Class A fuels except for cellulose nitrate or those that contain it. It is a chemical powder used in bombs and is also called pyroxylin.

Coal is a black, combustible, solid mineral that is produced when matter undergoes partial decomposition due to exposure to varying temperatures. It is generally used to heat buildings and energize machinery.

Coal comes in the following forms: lignite or brown coal, sub-bituminous coal, bituminous coal, and anthracite.

Bituminous coal contains more carbon and produces more heat than either lignite or sub-bituminous coal. Thus, it is considered the most important type of coal in industry. Bituminous coal also appears most abundantly in nature.

Anthracite, while it contains more carbon and produces more heat than the other types, is not used as much since it is the toughest type of coal making it difficult to ignite and slow-burning. Additionally, anthracite is rare compared to the others.

Peat is partially decayed plant matter found in swamps and bogs. This is used as fuel in regions where oil and coal are scarce such as Ireland and Scotland. In these places, they cut, form into blocks, then dry the peat, which is then used to heat homes.

Liquid Fuels

As the name implies, these are combustible materials that are in liquid form.

Liquids, in general, have the following characteristics:

• assumes the shape of its container
• has a definite volume, but no definite shape
• are slightly compressible because their molecules are more loosely-packed compared to that of liquids

Liquid fuels also share the aforementioned characteristics. Additionally, just as the case for solids, sufficient heat must be available to turn liquid fuels into gas for combustion to occur.

Types of Liquid Fuels

Gaseous Fuels

Finally, we have gaseous fuels, or those fuels that exist in gaseous form.

Molecules of gases are in relative free motion. Just like liquids, gases have no definite shape. Unlike liquids that have a definite volume, however, gases have no definite volume. They can expand to fill the room or container.

Since this type of fuel is already in gas form, heat is no longer required to transform it. Gaseous fuels can readily ignite under the proper conditions,

Note, however, that not all gases burn. Some of them, like carbon dioxide, are even used to extinguish fires.

Classifications of Gaseous Fuels

According to Source

• Natural Gas is used to heat buildings, cook food, and provide energy. Methane, the chief component of natural gas, is a colorless and odorless gas. Hence, foul-smelling substances, like sulfur, are usually added to them to help detect leaks. Common examples are butane and propane. They are easily contained and distributed when pressurized into liquid form. They change back into gas when pressure is removed.
• Manufactured Gas is used when other types are scarce. Coal, petroleum, and biomass can be converted to gas through various chemical processes.

According to Physical Properties

• Compressed Gas Exist as a gas in its container even under normal temperatures
• Liquified Gas Partly liquid and partly gaseous in its container
• Cryogenic Gas Liquified gas which exist in its container at a temperature far below normal atmospheric temperature, usually slightly above its boiling point, and low to moderate pressure.

According to Usage

• Fuel For use in generating heat, power, light, and comfort
• Industrial For use in processes as welding and cutting, refrigeration, water treatment, etc.
• Medical For use in treatments such as anesthesia and respiratory therapy

Special Classifications of Fuels

• Chemical Fuels Used chiefly in rocket engines, chemical fuels, which can be in solid or liquid form, produce great amounts of heat a power. They are made up of a fuel and an oxidizer, such as nitrogen tetroxide. Hydrazine is common rocket fuel. (The skeletal formula of this substance is shown here.) Chemical fuels are also used in racing cars.
• Nuclear Fuels These provide energy through the fission or fusion of atoms. The most commonly used nuclear fuel is uranium, but plutonium may also be used. They are mainly used to generate electricity, but they can also be used to power some submarines and ships.

Oxygen

As previously discussed, oxygen is a chemical element with the chemical symbol O. As a gas, it exists as a colorless, odorless, tasteless substance in the form of O₂. It occurs freely in the air and combines easily with other elements.

← Chapter 1: Fundamentals of Fire Science

Chapter 3: Behavior of Fire →