At some time in the future, the production of oil and natural gas will come to an end, and Oman is therefore looking for alternatives to diversify its industrial base. The largest, but as yet barely touched natural commodity is olivine, as Oman’s major mountain chain is the world’s largest ophiolite complex. In the struggle against climate change proposals for direct injection of CO2 into the olivine rich rocks in Oman have already been published. CO2, together with water, will react with olivine and form stable carbonates. By this method large volumes of CO2 can be sustainably captured. The rate of reaction will increase in response to the release of the considerable heat of reaction, which will heat the rock. It is expected that this uneven heating may crack the rocks further, making them more accessible to fluid injection.The focus of this contribution will be on some possible ex situ uses, both in Oman and elsewhere. As our starting point was diversification of the industrial base, only applications with an economic perspective in the near or the far future are considered.
The Oman ophiolites bear spectacular evidence that they can capture CO2 without human intervention, but the rate of capture is insufficient to balance the huge amounts of CO2 that are pumped in the atmosphere by the burning of fossil fuels. This imbalance might be redressed if we find ways to increase the rate of reaction.
Role of olivine
For all the olivine applications mentioned below, the prime driver is its capacity to weather faster than every other common silicate, capturing CO2 in the process. For any moderately humid climate, the most important weathering reaction is
(1) Mg2SiO4 + 4 CO2 + 4 H2O à 2 Mg2+ + 4 HCO3– + H4SiO4
In dry climates like in Oman, it is likely that the weathering reaction is more commonly
(2) 2 Mg2SiO4 + CO2 + 2 H2O à MgCO3 + Mg3Si2O5(OH)4
Whatever the climate-dependent weathering reaction, its final result is the sustainable capture of CO2
Some potential applications
- Export of olivine in crushed forms is the simplest commercial application. In the countries of destination, the use of olivine is not restricted to combat climate change, but can also involve its traditional uses in the steel industry for slag conditioning, as foundry sand or in sandblasting.
- A special case may be export of olivine to the Maldives. The Maldive islands will disappear if the expected sea level rise as a consequence of global warming becomes reality. By covering the islands with a layer of olivine, and surrounding them by a rim of olivine blocks mixed with olivine sand, they will be better able to withstand waves and currents. All marine constructions with olivine help to counteract ocean acidification. A special application would be to build so-called olivine hills on the atolls, constructed on a slightly inclined impermeable bottom in the shape of a very wide gutter. Rainwater falling on the hill will react with the olivine on passing, turning into a healthy magnesium bicarbonate drinking water that can be collected at the lowest point of the impermeable bottom. In this way the hill will capture CO2 while providing at the same time safe drinking water to the local population in normal times, but can serve as a refuge in times of storm and inundation.
- Marine constructions with olivine. Compared to other common materials used for marine constructions, olivine has the advantage that its hardness is (almost) equal to that of quartz, but that it is significantly heavier (3350 kg/m3 against 2700 kg/m3), making it more resistant to erosion. Its major advantage, however, may be that marine constructions with olivine are probably self-cementing. All shallow seawaters are slightly supersaturated with calcium carbonate, but this leads almost never to real precipitation of calcite. If seawater fills the pores between the olivine grains, and if this water is not too quickly replaced, its pH will go up due to the reaction with sea water. This pH rise, through its effect on the carbonate equilibria will strongly increase the calcite supersaturation, and calcite will start to precipitate between the olivine grains and on the olivine blocks, cementing them into a solid structure. For whatever purpose those reefs are constructed (protection of harbors, preventing erosion of coasts, or to serve as a substrate for corals and shell-fish), this self-cementing property could come in usefully.
- When the gas production declines, it can be partly replaced by biogas. Present-day operation of biodigesters, treating organic wastes (agricultural residues, sewage sludge, slaughterhouse wastes) produces in general a biogas consisting for about 2/3 of methane and 1/3 of CO2. To make it a richer gas, it would be an advantage to reduce the CO2-content. If one adds olivine powder to the digester, part of the produced CO2 is removed from the gas phase to the liquid digestate as bicarbonate. When the biodigester is operated at a higher pressure, this effect will be even stronger. It was found that there are two additional advantages when olivine powder is added. The digester loses its bad smell. This is caused by the fact that the fayalite part (fayalite is Fe2SiO4) of the olivine reacts with H2S to solid iron sulfides, thereby removing the smell. Another result is even more surprising. It was found that the absolute amount of produced methane also increased by the addition of olivine powder. This is again due to the Fe-endmember in the olivine. When fayalite weathers under anaerobic conditions, the FeO that is released reacts with water and CO2 to methane and magnetite, according to
(3) 6 Fe2SiO4 + CO2 + 14 H2O à 6 H4SiO4 + CH4 + 4 Fe3O4
This way the addition of olivine powder can increase the production of green energy.
- Silica in solution is always produced by the weathering of olivine. Silica is an essential nutrient for plants, particularly “wet” grasses (rice, reeds, bamboo). In marine waters as well as in fresh waters most diatom species require silica for their exoskeleton. Diatoms grow very quickly, but they are often limited by the availability of silica. Diatoms are very rich in lipids (appr.50% of their body weight), and are an excellent raw material for the production of biodiesel. By combining a beach covered with olivine between the high tide and the low tide line, by which the olivine sand on the beach is in turn wetted and drained, and after constructing an artificial lagoon in front of this beach, one can use this as a diatom farm. During ebb tide, when the silica-rich water flows out of the lagoon, carrying the diatoms with it, these must be retained on a plankton net at the outlet, and can be harvested. The advantages of biofuel from diatoms over biofuel produced from land based biofuel crops (corn, soja, palmoil and the like) are evident. One doesn’t use farmland that should be used for world food production, it doesn’t use up scarce irrigation water, and to provide the macronutrients nitrate and phosphate a bit of effluent from a wastewater treatment plant is probably sufficient.
These last two applications, biogas production and biodiesel production would constitute a green follow-up for Oman’s oil and gas history.
- And to end with a very futuristic idea, but which may be realized in the future thanks to the exciting developments in genetic modification:
Some plants are hyperaccumulators for nickel, and even when they grow on normal peridotites without any nickel mineralisation the Ni-content of their ashes may be 5% or more, making it a rich nickel ore. Clues have been found as to the determining factor for this property. This opens the exciting, but still distant possibility of growing salt tolerant plants on olivine beds in salt marshes, after the nickel hyperaccumulator factor has been implanted in transgenic halophytic plants. From the ash of these plants nickel can be recovered in an environmentally friendly and potentially economic way, while the olivine in which this nickel was incorporated is weathering quickly in warm salty water and captures CO2.
Conclusions
It is certainly not true that the olivine option to counteract climate change and ocean acidification can only earn money by carbon credits, once it is certified. Certification is complicated, because in several applications of the olivine option one can not always measure exactly the amount of CO2 that is captured. Fortunately for many other applications the effect can be measured quite well by comparing the compositions of ingoing and outgoing waters in systems where olivine is added. In reality, certification would likely progress in the same way that certification for emitting CO2 has progressed. In some cases emissions are directly measured, in many other cases emissions are sampled and modeled to an accepted standard. Regulatory developments would do well to track any developments in testing and demonstrating olivine applications; and there are many existing standards and processes that would be relevant for the above applications.
Olivine can become a major export product of Oman. The country has the largest olivine-rich rock massifs of the world. Its position along the Indian Ocean, and the possession of a modern port (Sohar) make the handling and shipping of olivine relatively simple. For applications within the country, one should look primarily for applications that require no fresh water. There are several such options, interestingly leading to biogas and biodiesel production, so in a sense a substitute for Oman’s present major economic base.
Olivine carries the promise to be the cheapest large-scale option to halt the rise of the CO2 levels of the atmosphere, and eventually reduce them if necessary. The pre-industrial CO2 level of 270 ppm is in no way a holy number to which we must return at all cost.