Physics, Earth & Space
By Design, Earth Is a Planet Fit for Fire
Editor’s note: We are pleased to present a series adapted from biologist Michael Denton’s book, Fire-Maker: How Humans Were Designed to Harness Fire and Transform Our Planet, from Discovery Institute Press. Find the whole series here. Dr. Denton’s forthcoming book, The Miracle of the Cell, will be published in September.
Earth’s size is just about right — not so small that its gravity was too weak to hold the atmosphere and not so large that its gravity would hold too much atmosphere including harmful gases.Frank Press and Raymond Siever, Earth (New York, W.H. Freeman, 1986), 4.
As we have seen so far in this series, fire was an absolutely crucial component in humanity’s rise to civilization and an advanced technology. Indeed, it is difficult to imagine any path to technology that does not begin and continue with fire. That path was not of man’s own making, but was facilitated by a remarkable fitness in the nature of things, witnessed in the utility of metals, the availability of their ores, and the fact that temperatures sufficient to smelt metals from their ores are attainable in charcoal fires.
But nature has provided another vitally important contribution to humankind’s harnessing of fire: It has maintained an atmosphere on the Earth that has just the right properties for both fire-making and the fire-maker.
The Right Atmosphere
A planet fit for fire and its utilization by beings like ourselves must have an atmosphere that supports both respiration and fire. Although it is not widely appreciated, the atmospheric conditions necessary for respiration and those for combustion are different. It is quite possible for a planet to have an atmosphere that supports fire but not respiration (e.g., altitudes above the summit of Mount Everest), one that supports respiration butnot combustion, or one that supports neither.
The critical point is that fire “spread” (sustainability) is determined by different atmospheric factors from those that ensure oxygen uptake in the lungs. In a paper sponsored by NASA, the authors comment:
The mechanism of flame spread comprises a very complex interplay of diffusion, heat transfer, and chemical processes in the fuel material and in the ambient gas phase… flame spread rates over the surface of combustible solids are reduced by the presence of an inert dilutent in the atmosphere… The rate of flame spread can be correlated with the specific heat of the atmosphere per mole of oxygen… the apparent ignition energy is strongly dependent on the thermal dissipative characteristics of the atmosphere….1
In another NASA-sponsored paper entitled pointedly, “Habitable Atmospheres which Do Not Support Combustion,” Edward McHale comments:
It was discovered that if the heat capacity of the atmosphere could be raised to ~50 cal/°C mole 02, the atmospheres would not support combustion of any ordinary material. Many properties of the environment determine the rate of flame spread, and the simple correlation with heat capacity obtains because the agents being considered are inert and only act physically to suppress combustion… combustion depends on the feedback of energy on the flame zone to the unburned fuel in order to bring it to the combustion temperature. Inert gases act as heat sinks for the combustion energy, cooling the flame and interfering with this feedback process and, at sufficiently high concentrations, quenching combustion.
However, the atmosphere plays a different role in sustaining life than in supporting combustion. The life support function requires a partial pressure (~2.5 psi [130mm mg] or greater) of oxygen sufficient to maintain the necessary oxygen tension in the blood. Dilutent gases, if they are physiologically inert [like nitrogen], have only a minor effect on this process. Hence, by selection of a proper additive it should be possible to prepare an atmosphere of high heat capacity that is also physiologically inert. This would comprise a habitable atmosphere that would not support combustion.2
Because the factors that influence oxygen uptake in the lungs (including partial pressure of oxygen in the atmosphere, currently 160 mm Hg) and the factors that influence fire spread (including the percentage of oxygen, currently 21 percent, and the presence of dilutents in the atmosphere) are quite different, it is possible to engineer atmospheres capable of sustaining oxygen uptake in the lungs but not fire. Douglas Drysdale points out:
It is possible to create an atmosphere that will support life but not flame. If the thermal capacity of the atmosphere per mole of oxygen is increased to more than c. 275J/K (corresponding to 12% O2 in N2), the flame cannot exist under normal ambient conditions. A level of oxygen as low as 12% will not support normal human activity [except for races acclimatized to living at high altitudes] but if this atmosphere is pressurized to 1.7 bar, the oxygen partial pressure will be increased to 160mm Hg, equivalent to that in a normal atmosphere and therefore perfectly habitable — although incapable of supporting combustion.3
Such atmospheres have been considered for use in various confined spaces such as space ships. Nitrogen is an effective dilutent and tests by the U.S. Navy showed that if the oxygen/nitrogen mix is changed by the addition of more nitrogen to an atmosphere, the fire may be quenched even though the partial pressure of the oxygen is still 160 mm Hg and perfectly capable of supporting human respiration.4
A Fact of Enormous Consequence
Because the process of combustion differs fundamentally from oxygen uptake in the lungs, the fact that there is an atmosphere that supports both is of enormous consequence. It was this primal coincidence that allowed mankind to utilize fire in the first place and set out on his technological journey from the Stone Age to the 21st century.
It is worth noting the additional fortunate fact that nitrogen does not have a specific heat capacity much lower than it does or fire might be difficult to tame in ambient conditions. Because nitrogen is essential to confer density to the atmosphere and necessary to keep the oceans from evaporating — no other candidate is available — its specific heat capacity is another element of fitness in nature which has enabled the control of fire by humans.
In sum, the current atmosphere is fit — but for different reasons — both for sustaining fire and for supporting human oxidative metabolism. On the one hand, the overall atmospheric pressure (currently 760 mm mg) cannot be much increased or the work of breathing would be significantly increased,5 as would the risk of fire.6 On the other hand, it cannot be radically less or the oceans would have long ago evaporated, although recent work suggests that at times in the distant past it may have been less than half its current level.7
Tomorrow, “Why Do We Not Spontaneously Combust?”
- USAF School of Aerospace Medicine, The Combustibility of Materials in Oxygen–Helium and Oxygen—Nitrogen Atmospheres, Clayton Huggett, Guenther Von Elbe, Wilburt Haggerty, SAM-TR-66-85, Brooks Air Force Base, 1966, 7 and 14, accessed on April 1, 2016, http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/4600/SAM_66_85.pdf?sequence=1.
- NASA, Habitable Atmospheres which Do Not Support Combustion, Edward McHale, 30, Alexandria, Atlantic Research (1972), https://archive.org/details/nasa_techdoc_19720014620.
- Drysdale, Douglas, An Introduction to Fire Dynamics, 3rd ed. (Chichester, West Sussex: Wiley, 2011), 377.
- Drysdale ibid; Clayton Hugget, “Habitable Atmospheres which do not support combustion,” Flame and Combustion 20, no. 1 (1972): 140-142; Vytenis Babrauskas, and Stephen J. Grayson, Heat Release in Fires, (London: Interscience Communications, 2009). On page 316 the authors comment: “The effects of pressure on the burning of combustibles has become of great interest to the U.S. Navy as a means of extinguishing fires… In a landmark paper entitled Habitable Atmospheres Which do not Support Combustion… Huggett explained that for survival humans depend on there bing a minimum partial pressure of oxygen, and a minimum concentration. By contrast, the combustion process requires a minimum flame temperature to avoid extinction. This minimum fame temperature can be related to a minimum heat capacity per mole of O2, this being about 170 to 210J/C per mole of O2. Thus, if the total pressure of the atmosphere is increased by the forced injection of an inert gas into a sealed atmosphere, it may be possible to extinguish a fire without injuring persons… In small scale pool fire tests extinction was typically achieved when the nitrogen dilutent raised the total pressure to about 1.6 atmospheres. Subsequent, engineering details have been pursued in an ambitious program of large scale tests.”
- Richard Maynard Case and D. E. Evans, eds., Variations in Human Physiology (Manchester, UK: Manchester University Press, 1985). The work of breathing increases with the density (pressure) of the atmosphere. As the authors comment on pages 105: “The maximum voluntary ventilation is approximately proportional to the reciprocal of the square root of the density. This means at a depth of 30 m (4 bar) the maximum voluntary ventilation is only 50% of that at sea level. … [The work of breathing is increased for another reason] as the density of the air breathed increases, so the flow of air in the airways becomes more turbulent, resulting in an increase in airway resistance. In addition, an increased density of gases hinders their intra-alveolar diffusion … As a result of these factors, whereas maximum work capacity at sea level is normally limited by cardiovascular transport of oxygen, the limitations [of increased pressure/density] are largely ventilatory.”
- Increasing atmospheric pressure much above the current level of 760 mm mg (1 bar) at sea level (keeping the composition the same i.e., 21 percent oxygen and 79 percent nitrogen) i.e., hyperbaric conditions also increases the danger of fire. As reported in the US National Oceanic and Atmospheric Administration (NOAA) hand book section 6.5.2: “The burning rate when the pressure is equivalent to [3 bar] is twice that of sea level air, and is 2.5 times as fast at [6 bar].” (National Oceanic and Atmospheric Administration, Department of Commerce, NOAA Diving Manual: Diving for Science and Technology, 1991, section 6-14.) In effect hyperbaric atmospheres have a similar effect as increasing the percentage of oxygen and render the control of fire highly problematical.
- Sanjoy M. Som, Roger Buick, James W. Hagadorn, Tim S. Blake, John M. Perreault, Jelte P. Harnmeijer, and David C. Catling, “Earth’s Air Pressure 2.7 Billion Years Ago Constrained to Less than Half of Modern Levels,” Nature Geoscience (2016). doi:10.1038/ngeo2713.