Olivine: a supergreen fuel
© Schuiling; licensee Springer. 2013
Received: 26 August 2013
Accepted: 26 August 2013
Published: 2 September 2013
The hydration and carbonation of olivine, the most common mineral on Earth, produce a large amount of heat. Unfortunately, the reaction is too slow for normal technological applications, but when thermally well isolated, most of this heat can be recovered, not only for space heating but even for the production of high-pressure steam. During the reaction, CO2 is sustainably sequestered. In this paper, a number of potential applications are described. Using the hydration and carbonation of olivine not only increases the energy production but also sequesters at the same time large volumes of CO2 that would otherwise be emitted, or would have to be removed by expensive technologies. The term “supergreen fuel” refers to the fact that this energy production is not associated with CO2 production, but quite the contrary, it even sequesters CO2 while producing energy.
By this and similar weathering reactions throughout the history of the Earth, CO2 was removed from the atmosphere. The resulting magnesium bicarbonate solutions are carried by rivers to the sea, where marine organisms (corals, shellfish, and plankton) convert them to carbonate rocks. This is the way by which 99.94% of all the CO2 that has leaked out of the planet has been sustainably captured in rocks . This has saved us from the fate of our sister planet Venus, where weathering is impossible because Venus has no liquid water. All the CO2 that was ever emitted by Venus has stayed in the atmosphere, leading to a CO2 pressure of 85 bars and a surface temperature of 460°C.
At present, the amount of CO2 produced by the burning of fossil fuels is so large that this weathering process cannot keep pace, so the CO2 content of the atmosphere is rising, causing probably a climate change and certainly an ocean acidification. Increasing the rate of weathering as well to reach a new balance is the straightforward answer to the problem of rising CO2 levels in the atmosphere .
Exergy production from carbonation and hydration
Thermodynamic properties of substances in the reaction
Molar weight (grams)
Standard enthalpy change of formation (kJ/mol)
CO2 (ideal gas)
The enthalpy of the reaction is −169.1 kJ/mol CO2 or 0.47 kJ/gram of the stoichiometric mixture. As long as the water stays in the liquid state, the changes in the enthalpy of reaction as a function of temperature are minimal. With an average value for the specific heat of a gram of the stoichiometric mixture of 1.6 J/g/K (value for basalt), the system, when perfectly isolated, would reach a temperature of 593 K when it starts at 298 K. If water would be available as steam at the site, a higher end temperature could be reached. No allowance is made for the contribution of the iron end-member in this calculation. The reaction is different (see ‘Methane production’ section) because during the transformation, the divalent iron oxidizes to magnetite.
How can this heat be used?
It should be stressed again that the reaction proceeds slowly at low temperatures, so it is imperative that the system should have a large volume and be isolated very well to make sure that the heat that is slowly produced is not conducted or radiated away. Rocks are excellent isolating systems, so in the examples that follow, thick layers of olivine-rich rocks will be used for thermal isolation. The applications can roughly be divided into methane production, space heating, and power production.
Part of the CO2 in the biogas is transformed into bicarbonate. The bicarbonate dissolves in the digestate, making the biogas richer.
The absolute amount of produced methane increases as a consequence of reaction (3).
The biodigester does not smell any more. This is also a consequence of the iron part of the olivine which reacts with the H2S and precipitates as iron sulfides.
So, adding olivine powder to the biodigesters produces a richer and cleaner biogas and captures CO2. Because the fayalite is part of the mixed crystal of olivine, it must weather exactly at the same slow rate as the magnesium end-member. The olivine reaction will not lead to any disruption of the digester system, and in the experiments, no heating effect has been observed.
Salts (not only NaCl but also potassium and magnesium salts) are often mined by solution mining. This way of mining leaves, in the end large subterranean solution, holes filled with brine. These holes must remain filled with saturated brine and permanently pressurized to avoid subsidence or even collapse of the cavity. As a first step, one must install two pipes into the brine, one to pump in olivine sand, the other to remove brine. At the end, the cavity is filled with olivine sand. It will probably require some mechanical way to spread the olivine evenly in the cavity. The remaining free volume for the brine shrinks when the cavity is filled with olivine, and the displaced brine can be recovered. The cavity filled with olivine with pore water of brine forms a well-isolated system, in which one can pump warm CO2 from a nearby power station. The reaction starts slowly, but as the temperature rises due to the exothermic reaction taking place, the reaction goes faster, and one can recover hot (saline) water with which a warm swimming pool can be filled, or hot fresh water if the water is passed through a heat exchanger in the cavity. This hot water can even be used for space heating. When the temperature becomes too low, the water is pumped back into the cavity. The reaction products (serpentine + magnesite) of the olivine take up a larger volume than the loose olivine sand, so the pores between the grains gradually close and form a rock. This new serpentine-magnesite rock can support the cavity, making permanent monitoring and pressurizing superfluous.
Greenhouses also require heating. In NE Greece, there are a number of geothermal wells used for heating greenhouses. The geothermal fluids here are rich in CO2, which is emitted to the atmosphere. If one covers the well head with a hill of olivine sand from the tailings of a nearby magnesite mine, this olivine will react with the hot geothermal fluid. The heat of this exothermal reaction is added to the geothermal heat, thus increasing the amount of heat available for the greenhouses, and the CO2 in the geothermal fluid is no longer emitted to the atmosphere but is sequestered by the olivine reaction and sustainably stored.
A potentially attractive application for tourists could be the following. In the sea around the island of Milos (and for a part on the island itself), large volumes of hot CO2 are bubbling from the floor of the shallow sea .
Until now, olivine was only considered as a means to sequester CO2 by its reaction with CO2 and water. This process is known as enhanced weathering. The fact that this reaction also produces a considerable amount of energy has not yet been considered, because this heat is released too slowly for use in conventional technologies. It, therefore, does not fit into any classical system of power production. Olivine can become a supergreen fuel, however, if one fulfills the required conditions of good thermal isolation and large size of the reactant volume.
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