Nuts and Bolts and Fire Chemistry Re-Visited

 

            Fire is a chemical process that results in a change in the state of matter by the process of oxidation, resulting in the production of heat and light. Not all oxidation produces heat or light, as is the case of rusting of metals. Rust is a slow process, fire is faster, and explosions are the most rapid form of oxidation. Fire is typically represented as a triangle, one leg being a heat source, another leg the source of fuel and the third a source of oxygen. This model remained the standard for many years. The Fire triangle now represents smoldering combustion which produces less heat and light. The flaming combustion is now defined as the fire triangle with the addition of an uninhibited chemical chain reaction, better known as the “Fire Tetrahedron ” .

            When you examine the various parts of the process of the production of flaming combustion you should consider the fuel portion first. It is the fuel that will dictate how much energy will be required to produce fire. A solid ,such as a common 2x4 board, will not burn.  In the solid state of matter, it requires more energy (heat) to change the material from a solid to a gaseous state. The process is known as pyrolysis, where  the application of heat produces gaseous by-products that burn. The next state of matter is liquid, which still requires energy to change the liquid to a gas, but at a lesser amount than the solid state of matter. The gas or vapor state uses the least amount of energy to produce flaming combustion and thus is the most dangerous state of matter for a combustible product to be in. It is said that you can place a burning match into the full gasoline tank of a vehicle and it would go out. The liquid state is less volatile in comparison to the vapor state of the same product which has been known to produce rapid combustion aka an explosion.

            The next leg of the triangle to examine is the heat source. Heat is most often produced by electric arcing, friction, and/or a chemical reaction. The arc may be from lightning or an accumulation of a static charge or the movement of electrons in a circuit. Friction is best defined as the interface between two surfaces, The best example is when the brakes of a tractor-trailer overheat on a long downgrade. The heat from a chemical reaction is best represented by the common match. There are chemical reactions that create heat, like when metallic sodium is placed in water. Chemical reactions may be exothermic-heat producing or endothermic- heat absorbing reactions.

            The next leg of the triangle is the oxidizer. The atmosphere is made of roughly 21 percent oxygen with the majority being nitrogen and some trace amounts of noble gases. When it comes to combustion and fire behavior, the oxidizer is the one element that we can influence the most. We take actions to reduce the effect of oxygen by smothering the fire, or somehow reducing the amount of air that is available to the combustion process.

Fire Behavior and Control

            In the modern study of fire behavior, the impact of the available air supply to a fire, has become the focus of discussion and debate. This is one leg of the triangle that we can control. The heat and fuel are typically not as easy to manage as they are present in the environment and are not easily manipulated. Controlling or limiting the air in the container is much easier than managing heat production or the fuel package.

            The term ventilation limited fire is tossed around the dinner table often times with little or no real understanding of the process. It has now become standard practice to limit the air flow to a fire burning in a compartment. With limited air the fire will starve and often remain controllable with limited extension or growth. This has given rise to the concepts of coordinated ventilation and fire attack techniques. It was once common practice to break windows and remove doors from buildings all prior to having a fire attack line in place and working on containing the fire. This practice led to uncontrolled fire growth and increased risks to civilians and firefighters alike as well as increased property damage and losses.

            When we examine fire behavior, we look at the components of the fire triangle: the volume and type of fuel, the heat source and the amount of available oxygen in the container. Fires create high pressure systems, as hot gases rise and fill the box. The size of the container or box has an impact on fire behavior. A small space will fill up and the heat, smoke and fire will leave the space sooner. A large space will take longer to fill before the fire attempts to leave the original space. When the box can no longer absorb any more energy or pressure, then that energy will be looking for an escape route from the box. When we create a path to a low pressure area; the next room, out a window, a void space or over our heads at the front door, the fire and products of combustion are going to move to that location in a rapid fashion. This may lead to a hostile fire event.

            Such events, flashovers, back drafts or smoke explosions always lead to more fire, heat and smoke damage and unfortunately often to firefighter injuries or deaths. Coordinating the application of water and a controlled release of the energy, will allow for a decrease in the negative impact that a fire has on the space and the people in the space. The other factors in the modern fire environment include building layouts, construction materials and techniques and the fuel packages of the modern home.

            I often hear that today’s fires are different from the fires 20 years ago. Fire remains the chemical process that we reviewed in the beginning of the paper. What is different is the fuel package. The typical home is full of hydrocarbon based products that have a much greater heat release rate than the legacy fuel packages of natural fibers and wood products. It does not take as much energy for the modern material to off-gas flammable vapors thus more energy is available in the form of heat, leading to the concept that the fires are hotter. Fires burn at the same temperatures; they just get to those temps at an accelerated rate compared to fires burning legacy fuels.

          Again, it is imperative that the modern firefighter understands the need to limit the air available to the fire, while coordinating the application of water into the space. The basic concepts of breaking a leg of the fire triangle remain valid, The art and science of firefighting requires a balance of understanding the process of combustion; the interaction of fuel, heat, air and how those elements impact and are impacted by the size of the box. The actions of the firefighters working to manage the incident dictate the outcome and impact that the emergency has on the public and our staff. Well practiced and coordinated fire ground operations will benefit the firefighters, officers and the community.