3.1: Gaseous Fuels and Flammability
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Nearly all fuels need to be converted to the gaseous state, a “state of matter that has neither a fixed volume nor a fixed shape" in order to burn.1 This conversion will be discussed in the following chapters. Because gaseous fuels are already in a gaseous state, these fuels often require the least amount of energy to ignite, and can result in explosive reactions due to the speed at which the reaction occurs.
Flammability Limits
The upper and lower concentration limit at a specified temperature and pressure of a flammable gas or vapor of an ignitable liquid in air, expressed as a percentage of fuel by volume that can be ignited.2 The upper and lower flammability limits can also be referred to as the explosive limits.
The upper flammability limit (UFL) or the upper explosive limit (UEL) refers to the highest concentration of fuel, or the highest ratio of fuel in air that will support combustion. Concentrations above this limit are considered too rich to burn and will not ignite even if exposed to an ignition source such as an open flame.
The lower flammability limit (LFL) or lower explosive limit (LEL) refers to the lowest concentration of fuel, or the lowest ratio of fuel in air that will support combustion. Concentrations below this limit are considered too lean to burn and do not have enough fuel to ignite or support combustion.
Combustion will only occur within the flammable range as depicted in the illustration below.
Stochiometric Reactions
A balanced chemical equation not only describes some of the chemical properties of substances—by showing us what substances react with what other substances to make what products—but also shows numerical relationships between the reactants and the products. The study of these numerical relationships is called stoichiometry.3
In a balanced or stoichiometric reaction, all of the products that are involved in the reaction are converted into the new product. For example, you can think of the reaction as a recipe for grilled cheese. The recipe would be 2 slices of bread, 1 slice of cheese, and 1 tablespoon of butter. If you had each of these ingredients in the exact quantities and combined them they would equal 1 grilled cheese sandwich. However, if you had a loaf of bread with 20 slices of bread, a package of cheese with 24 slices, and 8 tablespoons of butter, you could combine them to make 8 grilled cheese sandwiches, however you would have 12 slices of bread and 16 slices of cheese left over. This is not a stoichiometric reaction and much more representative of a fire. Fires rarely occur as stochiometric reactions, however, we can characterize some fires as more stochiometric than others, meaning they utilize more of the fuel in a complete combustion reaction. For example, if we compare the campfire to that of the blowtorch, we can see that the campfire is far less stoichiometric, resulting in smoke and soot being left over or not being utilized completely in the reaction.
Vapor Density
Also termed specific gravity of a gas, vapor density is the ratio of the average molecular weight of a gas or vapor to the average molecular weight of air.4
Air is considered to have a molecular weight of 1. Therefore, gases with a vapor density of less than 1 will rise in air, and gases with a vapor density of less than one will sink in air. Knowing the vapor density of a gas can be helpful in understanding where vapors will accumulate. For example, propane, which has a vapor density of 1.5 will accumulate in the lower floors of a structure such as the basement, or in low levels of terrain.
References
1. Adapted from Brainard, J. (2014). 4.2 Behavior of gases. In CK-12 Physical Science For Middle School. https://www.ck12.org/book/ck-12-physical-science-for-middle-school/r2/section/4.2/. Licensed CC BY-NC.
2. National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024
3. Ball, D. W., Hill, J. W., & Scott, R. J. (2011). 5.2 Chemical equations. In Introduction to Chemistry: General, Organic, and Biological. https://2012books.lardbucket.org/boo...equations.html. Licensed CC BY-NC-SA 3.0.
4. National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024