Serpentinite Slurries against Forest Fire
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Schuiling, R.D1)., Ooij, J.van2), Sickmann, K2)., Warnar, M2).
1)Institute of Geosciences, Utrecht University
2)Junior College, Utrecht University
Keywords: forest fires, serpentinite slurries, calcination, endothermic reaction, CO2 capture
Silicates that weather fast and are abundantly available are olivine and serpentine. If serpentine is calcined, it weathers even faster, but calcination requires a lot of heat, which makes it counterproductive to produce calcined serpentine for CO2 capture. In cases, however, where heat is a problem, like in forest fires, one can extinguish them to greater advantage by using serpentinite slurries instead of plain water. The calcined residue that is deposited as a cake on the burning material prevents the escape of inflammable gases, and the calcination itself withdraws large quantities of heat from the fire. After the fire is extinguished, the calcined material in contact with the atmosphere will rapidly weather and capture CO2. This compensates part of the CO2 that was produced by the fire. Moreover, the material will deliver magnesium to the soil, leading to healthier new forests. In tests, where the efficacy of quenching fires with serpentine slurries was compared to the effect of water, it turned out that serpentinite slurries performed far better. The approach may also be useful in tunnel fires.
Rising CO2 levels in the atmosphere are considered by many to cause a climate change. Their role in causing ocean acidification is unambiguous. The Earth is continuously degassing, and CO2 is emitted by volcanoes and by the dissociation of limestones carried to great depth in subduction zones. There must exist efficient feedback systems, otherwise the Earth atmosphere and climatic conditions would be like on Venus, which has an atmosphere of 85 bars CO2 pressure, and a surface temperature of 460 0C. The main feedback mechanism is the weathering of basic silicates, which can take place on Earth thanks to the presence of liquid water, which is lacking on Venus.
The natural choice for the best method to counteract climate change and ocean acidification is mineral carbonation . During the reaction of basic silicates with carbonic acid and water (a process known as weathering), CO2 is transformed into bicarbonate ions. The bicarbonate solutions are carried by rivers to sea, where they form carbonate sediments (limestones and dolomites). These carbonates represent the ultimate and sustainable storage of CO2 (fig.1).
Fig.1: The cliffs of Dover are one of the places where Nature has stored its CO2
During the entire geological history the Earth has emitted CO2, produced by volcanoes or by the dissociation of deeply subducted limestones. If there had been no feedback mechanism by which this CO2 is taken out of the atmosphere again and stored in a stable form, our atmosphere would be like on Venus. The CO2 pressure of the atmosphere on Venus is 85 bars, and its surface temperature is 460 oC. The presence of liquid water on Earth, which makes weathering possible, has saved us from extreme greenhouse conditions like on our sister planet.
This weathering has kept the CO2 levels of the atmosphere within reasonable and livable bounds. There have been fluctuations, depending on the rate of geological processes like mountain building and volcanism , but the advantage of weathering is that it has an inbuilt negative feedback. The higher the CO2 pressure in the atmosphere, the more acid the waters in equilibrium with this atmosphere, and the faster the rate of weathering by which atmospheric CO2 levels are reduced. At present, by burning in a few hundred years the fossil fuels that have taken hundreds of millions of years to form, the CO2 level of the atmosphere is rapidly rising, and the oceans are acidifying, because the weathering process cannot keep pace with the greatly increased CO2 emission, which is 30 to 60 times larger than the natural emission of CO2.
By selecting materials that weather easily and are abundantly available, mining and milling these, and spreading them over land, shallow sea and beaches, the process of weathering can be enhanced, leading to a new balance between emission and capture . Next to these major applications, minerals like olivine or serpentine can serve the same purpose in a number of niche applications. One such niche application is the use of serpentine slurries to quench fires, while the residual solids weather fast. This compensates for the CO2 that was produced during the fire. It is very much an environmental technology, as the environment provides not only the material to be used, but also imposes the conditions under which it will be applied .
If one knows the heat capacity of water and serpentine as a function of temperature , it is straightforward to calculate the total heat that is withdrawn from a fire when a certain volume of a mixture of serpentine and water is sprayed over it. One needs, of course, also take into account the heats of vaporization of the water, and the heat of dissociation of the serpentine. The last can be calculated if the mineralogical composition of the end-product is known. Equilibrium phases would be olivine, enstatite and water vapor, but in reality this equilibrium is usually not reached in the short period in the fire, but rather some more or less amorphous or poorly crystalline intermediate materials. This means that the calculated heat of dissociation should be increased by an unknown, but rather modest amount (difference in heat content between crystalline compounds and their amorphous equivalents), making serpentine slightly more efficient than would follow from the equilibrium calculation, where data for crystalline materials and pure water are used. Mixes of 40 wt% of serpentine and 60 % of water are still quite liquid and easy to pump. The total heat withdrawn by this mixture when heated from room temperature to 900 K was compared to that of pure water over the same temperature interval. It turns out that the serpentine slurry withdraws 27% more heat than an equivalent volume of water.
Several tests were carried out at the test site of Brandbeveiliging (Fire Protection) BV in Wijchen to observe the effects of quenching fires with serpentine slurries, and compare these to quenches with water. There was no doubt that fires were considerably faster quenched with serpentinite than with water. With serpentine slurries, the effect was quasi-immediate. Moreover, a fire quenched with water had the tendency to repeatedly start flaming again after the spraying was stopped, whereas the fires that were sprayed with serpentinite slurries were quenched permanently. For this experiment, serpentine powder from a serpentinite quarry near Leoben (Austria) was used. A previous quenching test was carried out with a serpentine-rich residue from the PASEK olivine mine in NW Spain, with similar results.
It is not clear whether the difference in behavior of fires when quenched with either water of serpentinite slurries is (entirely) due to their different heat content. It was observed that the serpentinite covers the burning material with a thin baked layer. This skin prevents the escape of inflammable gases, and this effect may be at least as important as the larger heat withdrawal.
Possibly the best use of serpentinite slurries would be in forest or heather fires, both on the ground as from the air. Dropping a load of serpentinite slurries from a plane or helicopter has the advantage that the slurry can have a higher percentage of serpentine than when it must be pumped, because for dropping a higher viscosity is permitted, as even a more viscous mud can be scooped out of a basin. In countries with a warm and dry season, and frequent forest fires in summer, small permanent basins (ponds) should be constructed near large forests, where serpentine slurries can be kept ready for emergencies. Another type of fire where the application of serpentine slurries might come in useful is in tunnels.
After the fire
Once the fire is quenched, a baked serpentine residue is left. This is very reactive to mixtures of CO2 and water. Experiments with calcined serpentine that was alternately shaken in a closed bottle, and left standing open to the air showed that the reaction with water, and the absorption of CO2 was much more intense than the same treatment with fine-grained olivine. In fact, when calcined serpentine powder was first brought in contact with water, the pH rose within 5 minutes to 9.6 (fig.2).
This is another major advantage of the use of serpentine slurries, they compensate rapidly a significant part of the CO2 that was emitted by the fire. An additional beneficial effect may be the release of magnesium, increasing the soil pH and helping trees to grow faster.
Fig.2 pH variation of suspensions of calcined serpentine and fine grained olivine.
Serpentine slurries provide a cheap and effective environmental technology to quench widespread fires. Likely applications are in forest fires or tunnel fires. After the fire is ended, the baked residue of the serpentine reacts fast with CO2 and water, by which the greenhouse gas CO2 is converted to, and stored as innocent bicarbonate solutions. This compensates the CO2 that was emitted by the fire.
We wish to thank Javier Martinez Rubio from the PASEK mine (Spain), and Jan Koller from the Isomag serpentinite quarry (Austria) for making serpentine samples available.
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