SEC Unit-5: Cases Involving Arson and Explosives

Definition of Fire

Fire is an exothermic, self-sustaining reaction involving the rapid oxidation of solid, liquid, or gas-phase fuel.

Combustion Process

Combustion is a chemical reaction where combustible material combines with an oxidizing agent (typically oxygen), producing heat, light, and further chemical reactions.

Characteristics of Combustion

The process is exothermic, releasing heat and light, which supports continuous chemical reactions in the presence of fuel and an oxidizing agent.

Key Requirements for Fire

Four essential elements are required for fire:

1. Fuel

2. Oxidizing Agent(usually oxygen)

3. Heat energy

4. Uninhibited Chain Reaction

Fire Tetrahedron

The fire tetrahedron visually represents the four elements essential for fire: fuel, oxidizing agent, heat energy, and uninhibited chain reaction.

Role of Heat in Combustion

When fuel and oxidizer reach a certain temperature, exothermic reactions begin, generating additional heat that maintains the fire.

Fuel

Fuel is a flammable or combustible material that initiates combustion. It can be a solid, liquid, or gas but only burns in the vapour phase.

Vapour Phase Requirement for Combustion

For combustion to occur, fuel must reach a vapour phase by being heated above its flashpoint. This allows the fuel to produce vapour pressure, which ignites in air and sustains combustion.

Types of Fuel

Common fuel types include:

• Solid (e.g., wood)

• Liquid (e.g., gasoline)

• Gas (e.g., propane)

Each type must achieve a vapour state to combust.

Wood as Fuel

Wood is a common solid fuel with high molecular weight compounds like lignins and cellulose. It needs to undergo pyrolysis (thermal decomposition) to produce combustible gases.

Pyrolysis Process in Wood

Pyrolysis is the process where wood, when heated past its flashpoint, decomposes cellulose and lignins into small molecules that enter the gas phase, allowing combustion to begin once they reach ignition temperature.

Role of Heat Energy in Combustion

Heat energy is necessary to ignite fuel and bring it to its ignition temperature, the minimum temperature for self-sustaining combustion.

Ignition Temperature

The ignition temperature is the minimum heat required for fuel to begin burning and continue combustion on its own, making the process self-sustaining.

Heat Produced During Combustion

Combustion is an exothermic reaction, releasing heat when chemical bonds are broken and formed during chemical reactions.

Self-Perpetuating Nature of Fire

Because combustion releases more heat than required, the fire is self-perpetuating and continues without additional energy input.

Why Fire is Hot

Fire generates high temperatures due to the ongoing exothermic reactions that release excess heat energy, making it both self-sustaining and hot.

Definition of Oxidising Agent

An oxidising agent is required to support burning by reacting with the fuel. The most common oxidiser is oxygen in the air.

Role of Oxygen in Combustion

Oxygen reacts with volatile gases released by the fuel. Once these gases reach their ignition temperature, they break apart and combine with oxygen to form water vapour, carbon dioxide, and release more heat.

Oxidation in Combustion

Oxidation is the process where fuel molecules react with oxygen, producing combustion products like water vapour and carbon dioxide, while releasing heat. This reaction is observed as burning and smoke.

Combustion Products of Oxidation

Oxidation results in the formation of water vapour, carbon dioxide, and other combustion products along with the release of heat.

Definition of Uninhibited Chain Reaction

An uninhibited chain reaction is the self-perpetuating process of combustion, where continual reactions between fuel and oxygen generate heat, keeping the fire sustained.

Role of Heat in Chain Reaction

The surplus heat from combustion keeps the fuel at its ignition temperature, ensuring the fire remains hot enough to continue burning.

Conditions for Sustaining Fire

A fire will persist as long as there is sufficient fuel and oxygen to maintain the chain reaction. It ceases when these are exhausted.

Importance in Fire Tetrahedron

The uninhibited chain reaction completes the fire tetrahedron by enabling the ongoing interaction between fuel, heat, and oxygen, ensuring the fire’s continuity.