2.1 Chemistry and Physics of Fire

2.1 Chemistry and Physics of Fire

Fire protection engineering requires a rigorous foundation in the chemical and thermodynamic principles of fire. At its core, fire is an exothermic chemical reaction involving the rapid oxidation of a fuel (reducing agent) by an oxidizing agent, typically atmospheric oxygen. This reaction releases thermal energy, light, and various combustion products.

The Fire Triangle and Fire Tetrahedron Models

Historically, fire behavior was explained using the Fire Triangle, which comprises three components: fuel, oxidizing agent, and heat. Removing any of these elements ceases combustion. This model is sufficient for describing non-flaming combustion (smoldering or glowing combustion).

For flaming combustion, fire scientists introduced the Fire Tetrahedron, which adds a fourth dimension: the Self-Sustained Chemical Chain Reaction. The difference between these models lies in the reaction phase:

• Non-Flaming/Smoldering Combustion: This is a heterogeneous surface-phase reaction where oxidation occurs directly on the surface of a solid fuel. Gaseous intermediates are not required. Examples include glowing charcoal and smoldering upholstery. Because it occurs at the solid-gas interface, it is relatively slow, operates at lower temperatures, and produces high concentrations of carbon monoxide.
• Flaming Combustion: This is a homogeneous gas-phase reaction. Solid or liquid fuels must first undergo phase changes to generate flammable gases. Once these gases mix with oxygen and are heated, they ignite. This flame is maintained by highly reactive, short-lived chemical intermediates known as free radicals (primarily hydrogen $H^\bullet$, oxygen $O^\bullet$, and hydroxyl $OH^\bullet$ radicals). These radicals propagate the reaction through chain-branching steps: $H^\bullet + O_2 \rightarrow OH^\bullet + O^\bullet$ $O^\bullet + H_2 \rightarrow OH^\bullet + H^\bullet$ This rapid multiplication of radicals drives combustion. If these free radicals are scavenged, the chain reaction is broken, and the flame is extinguished. Clean gaseous agents (e.g., FM-200, Novec 1230) act as chemical chain breakers.

Modes of Combustion: Flaming vs. Smoldering

Flaming combustion is categorized by how the fuel and oxidizer mix:

• Premixed Flames: The fuel gas and oxidizing gas are fully mixed before ignition. An example is a Bunsen burner. If ignited in an enclosed space, premixed mixtures can cause rapid, explosive deflagrations because the entire volume is within the flammable range and ready to react.
• Diffusion Flames: The fuel gas and oxidizing gas are initially separate, reacting at the interface where they diffuse into each other. A candle flame or wood fire are examples. The rate of combustion is limited by the physical mixing of the reactants. Most structural fires are dominated by diffusion flames.

Physical States of Fuel and Phase Changes

Except for smoldering, flaming combustion only occurs when the fuel is in a gaseous state. Solid and liquid fuels must undergo phase changes to produce ignitable vapors:

Liquid Fuels and Vaporization

Liquid fuels must vaporize to burn. The ease of vaporization depends on vapor pressure, which increases exponentially with temperature. Key properties include:

• Flash Point: The minimum temperature at which a liquid produces sufficient vapor to form an ignitable mixture in air near its surface, requiring an external pilot source to ignite.
• Fire Point: The lowest temperature at which a liquid produces enough vapor to sustain continuous combustion (at least 5 seconds) after piloted ignition. It is typically a few degrees higher than the flash point.
• Vapor Density: The ratio of the weight of a vapor to the weight of an equal volume of dry air. Most flammable liquid vapors are heavier than air (vapor density > 1.0) and accumulate in low-lying areas like basements, trenches, and sump pits, creating explosion hazards.

Solid Fuels and Pyrolysis

Solid fuels must undergo pyrolysis—the irreversible chemical decomposition of organic material due to thermal exposure in the absence of, or prior to, oxidation. Using wood (cellulose) as an example:

• Drying Stage (up to 200°C): Absorbed moisture is driven off. The wood undergoes minor endothermic decomposition, releasing water vapor but no ignitable volatiles.
• Active Pyrolysis Stage (200°C to 500°C): The chemical structure breaks down. Active decomposition produces charcoal, tar vapors, carbon monoxide, methane, and methanol. These gases are highly flammable and will ignite once their concentration in air reaches the lower flammability limit.
• Char Oxidation Stage (above 500°C): After volatiles are driven off, the remaining carbonaceous residue (charcoal) undergoes direct surface oxidation (smoldering combustion).

Stoichiometric Ratios and Flammability Limits

For combustion to occur, fuel vapor and oxygen must mix in specific proportions. The ideal chemical mixture is the stoichiometric ratio, where reactants are present in exactly the correct proportions for complete combustion: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

• Fuel-Lean Mixtures: The fuel concentration is below the stoichiometric ratio (excess oxygen). The lowest concentration of fuel vapor in air that can support flame propagation is the Lower Flammability Limit (LFL) or Lower Explosive Limit (LEL). Below this, the mixture is too lean to burn.
• Fuel-Rich Mixtures: The fuel concentration is above the stoichiometric ratio. Combustion is incomplete, yielding carbon monoxide, soot, and unburned pyrolyzates. The highest concentration that supports flame propagation is the Upper Flammability Limit (UFL) or Upper Explosive Limit (UEL).
• Flammable Range: The range between LFL and UFL. Gasoline has a narrow range (1.4% to 7.6%), while hydrogen has an extremely wide range (4.0% to 75.0%).

Increasing the temperature lowers the LFL and raises the UFL, widening the flammable range. Increasing pressure also generally widens the limits.

Ignition Sources and Processes

Ignition is the initiation of self-sustained combustion. There are two primary mechanisms:

1. Piloted Ignition: The ignition of a flammable fuel-air mixture by a localized, high-energy external source, such as a spark, electric arc, or pilot flame.
2. Autoignition: The initiation of self-sustained combustion without an external spark or flame. Autoignition occurs when the entire fuel-air mixture is heated to its Autoignition Temperature (AIT), where internal chemical heat generation exceeds heat loss. The AIT of a substance is always significantly higher than its flash point (e.g., gasoline has a flash point of $-43°C$ but an AIT of $280°C$).

Two critical parameters are:

• Minimum Ignition Energy (MIE): The minimum capacitive spark energy required to initiate piloted ignition. Methane has an MIE of 0.28 mJ, while hydrogen has an extremely low MIE of 0.017 mJ.
• Ignition Delay: The time lag between thermal exposure and the onset of flaming combustion. It is governed by heat transfer, fuel thermal properties, and pyrolysis chemical kinetics.