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Oxygen Consumption and Energy Release
That the energy released by combustion is related to the oxygen consumed in the reaction is not a new idea: “The door should be kept shut while the water is being brought, and the air excluded as much as possible, as the fire burns exactly in proportion to the quantity of air which it receives” (Braidwood, 1866, p. 64). For the time, James Braidwood, first Chief of the City of London Fire Brigade had a remarkable understanding of combustion. Despite this practical understanding of oxygen and release of energy through combustion, it wasn’t until 50 years later that this relationship was quantified. In 1917, British scientist W.M. Thornton discovered that while the heat of combustion of various types of organic (carbon based) fuel varies widely, the amount of oxygen required for release of a given amount of energy remains remarkably consistent (Thornton, 1917).
While the heat release of 13.1 MJ/kg (13.1 kJ/g) of oxygen consumed during combustion is often referred to as Thornton’s Rule, discovery of this concept and quantification of this value under a variety of conditions was the work of a number of individuals. For example, in the 1970’s, researchers at the National Bureau of Standards (now the National Institute of Standards and Technology, NIST) independently discovered the same thing and extended this work to include many other types of organic materials and examined both complete and incomplete combustion (Parker, 1977; Huggett, 1980).
Heat release during combustion is dependent on oxygen. However, the atmosphere is comprised of only 21% oxygen. Examining the relationship between consumption of atmospheric oxygen and energy release requires adaptation of Thornton’s Rule based on oxygen concentration. Multiplying 13.1 MJ/kg of oxygen by 21% gives a value of 2.751 MJ/kg of air. The Society of Fire Protection Engineering (SFPE) Handbook of Fire Protection Engineering (SFPE, 2002) rounds this value to 3.0 MJ/kg of air. While it is easy to understand that air has mass, it may be a bit more difficult to visualize a kilo of air! The density of dry air at sea level and at a temperature of 20o C is 1.2 kg/m3 (0.075 lbs./ft3). Air density decreases as temperature or moisture content of the air increases, but this provides a starting point for visualizing the relationship between volume and mass at normal temperature and pressure.
As illustrated in Figure 1, multiplying the mass of a cubic meter of air (1.2 kg) by the energy released per unit mass of air (3.0 KJ/kg) provides an approximation of the energy released when the oxygen in one cubic meter of air is consumed in a combustion reaction.
Figure 2. Energy Release per Cubic Meter of Dry Air
Oxygen to support energy release resulting from combustion occurring within a closed compartment is substantially (but not entirely) limited to the mass of air in the compartment. Normal air exchange between the interior and exterior of a building is expressed as the number of complete air exchanges (by volume) per hour and varies depending on the purpose and function of the space. In residential structures, the air in the building is completely exchanged approximately four times per hour. In commercial and industrial buildings this rate may be significantly higher, depending on use.
Designed air exchange and leakage provide additional oxygen that can support ongoing combustion, but this is generally not a major factor in buildings where the windows and doors are closed and intact.
Oxygen Concentration and Ventilation Controlled Fires
Energy release as a result of combustion is directly proportional to the oxygen consumed in the reaction. However, when a fire is burning in an oxygen limited environment such as an enclosed space, not all of the oxygen can be used to support flaming combustion. As observed by Mowrer, “A diffusion flame immersed in a vitiated [oxygen limited] atmosphere will extinguish before consuming all the available oxygen from the atmosphere” (McGrattan, Hostikka, Floyd, Baum, & Rehm, 2008, p. 85).
As oxygen within a compartment is consumed, fire growth becomes limited by ventilation (inclusive of the air within the compartment at ignition and the ongoing air exchange). Ventilation becomes the dominant factor in fire development when the oxygen concentration is between 14 and 16 %.
Oxygen Concentration and Flaming Combustion
As temperature increases, the oxygen concentration required to support flaming combustion decreases. Figure 3 illustrates the relationship between gas temperature and the concentration of oxygen required to support flaming combustion. Keeping in mind that temperature within involved and adjacent compartments can vary considerably, flaming combustion may be possible in some areas and not in others.
Figure 3. Oxygen Concentration Required for Flaming Combustion
Note: Adapted from Fire Dynamics Simulator (Version 5) Technical Reference Guide(p. 25), by K. McGrattan, S. Hostikka, J. Floyd, H Baum, & R. Rehm, 2008, National Institute of Standards and Technology.
Oxygen concentration required to support flaming combustion varies over a wide range based on temperature. However, in examining fire development in a single compartment or a residential structure, it is reasonable to use the value of 10.5% as the concentration required to support flaming combustion based on the fairly consistent temperatures of between 500o C and 600o C developed prior to ventilation (window failure, opening a door for access, or tactical ventilation operations). This assumption is based on analysis of the data from full scale residential fire tests conducted by Underwriters Laboratories in representative legacy and contemporary structures (Kerber, 2011).
