The Carbonate Solution, Part 1: Brute Force
- June 21, 2016
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This week I want to expand on the potential role of carbonate minerals for fighting rising CO₂ levels in the atmosphere. Previously, in Treasure of the Sierra Nevada and Cruising to Vegas I had explained how moving some of the stranded products of chemical weathering of the Sierras from the Great Basin to the Pacific ocean could offset some of the fossil carbon we’re pouring into the atmosphere. It could even help to reduce atmospheric CO₂ levels once we’ve brought our carbon emissions under control. The key was in the chemistry of soluble carbonates present in the alkaline clay and mineral deposits formed from weathered rock. But there are other sources for carbonate minerals and other ways of using some of them. Could they help?
The Great Basin is hardly the only place where rich deposits of carbonate minerals reside. Indeed, there are vast deposits of calcium carbonate — the predominant ingredient in chalk and limestone — all around the world. They are found wherever land that was once shallow seabed has been uplifted. Many of those deposits are close to existing shorelines or still under shallow seabeds not yet uplifted. Couldn’t they be used? They’d certainly be easier to access and transport to the ocean. Limestone has a carbonate mass fraction that’s several times higher than the alkaline clays of the Great Basin, so in addition to being closer to the ocean, less material should be needed.
The answer is complex. What distinguishes the carbonates in the Great Basin is that a good fraction of them are soluble evaporites. Solubility is important, because it’s the conversion of a dissolved carbonate ion, CO₃²⁻, to a bicarbonate ion, HCO₃⁻, that makes the parent solution more alkaline and enables it to absorb more CO₂. Calcium carbonate, however, is not soluble in plain seawater. In that sense, the answer would be “no, they can’t be used” — at least not in the same way as the soluble carbonates from the Great Basin deposits.
If one is willing to look beyond the issue of easy solubility, however, there turn out to be several ways that the ubiquitous nature and high concentration of carbonate in chalk and limestone could be exploited for CCS.
In all of what follows, bear in mind that the real issues are economics and scalability. It doesn’t matter if an approach is technically feasible if it can’t be implemented at an acceptable cost or scaled to a useful level. Indeed, it’s the limited scalability of the transport leg of the Great Basin project that leads me to consider other options. The economic feasibility of transporting clay from the Great Basin to the Pacific depends on collateral benefits of the proposed canal: pumped hydro energy storage, recreation and environmental benefits, and tourism. I think those benefits saturate at near the scale of what I proposed. A larger canal, able to transport more material, wouldn’t bring in more tourist dollars or satisfy a market for energy storage beyond what the smaller canal would already supply.
With that in mind, let’s look at some of the ways other natural carbonates might be used. But first, a short digression about the “storage” element of CCS.
In all discussions of CCS, the issue of how and where to safely store captured CO₂ invariably arises. Most petroleum geologists insist that there’s adequate capacity in depleted oil and gas fields. The existence of oil and gas in these fields has already proved that they have impermeable cap layers that have held over millions of years. But even accepting that, if storage were limited to depleted oil and gas fields the cost would be high. We’d need a huge network of long-distance CO₂ pipelines to transport captured CO₂ from power plants in parts of the country that don’t happen to be near any depleted oil or gas fields. That’s why there’s interest in deep saline aquifers. They’re widespread, easing the transport problem, but their safety is also more controversial.
There are three approaches that I know of for storing carbon dioxide whose safety, scalability, and long term security are not in question. One is mineralization. I’ll say more about that in a minute. Another is injection into offshore sediment beds at depths of several hundred meters. The ocean temperature there is low enough and the pressure high enough that any CO₂ diffusing upward through the sediment bed would combine with water to form clathrate ice. The clathrate ice would fill the pores of the sediment and serve as a permanent, self-healing cap layer for gas further down. The third approach stores CO₂ as dissolved inorganic carbon (DIC) in the oceans. That’s what the soluble carbonates from the Great Basin are all about.
Mineralization is nice because CO₂ becomes chemically bound into solid carbonate minerals. It forms minimum energy mineral states that are very stable. It’s “nature’s way” of removing CO₂ from the atmosphere over the long term, but it’s a slow process. Early studies of how it might be accelerated considered giant reactor vessels in which crushed silicate rocks would be reacted with water and concentrated CO₂ at elevated temperatures to form carbonate minerals. That works, in principle; suitable silicate rock is very common. However, the energy, equipment, and material handling requirements were deemed too high for economic feasibility.
There’s a shortcut to mineralization that has been theorized and recently tested. If pressurized CO₂ is mixed with water and injected into the porous basalt of old lava flows, the acidic solution of water and CO₂ will react with base minerals in the basalt. The CO₂ content of the injected solution will be mineralized in a period of weeks to months. That was confirmed in a field trial recently conducted in Iceland.
That sounds well and good; there’s no shortage of old lava flows that should serve. But how to capture the CO₂ and distribute it to the injection sites remains problematic. It’s hard to work up much enthusiasm for any of the point source capture methods currently available. The equipment is costly, and its operation takes a heavy toll on power plant output. And aside from pipeline construction contractors, nobody likes the thought of all the CO₂ pipelines that would need to be laid. On top of that, there’s the fact that large point sources (like power plants) account for less than half of fossil carbon emissions that need to be curtailed. If it could be done economically, air capture — and especially capture via enhanced ocean uptake — would certainly be preferable.
Brute force approach
There are several ways that carbonate chemistry could be exploited to use calcium carbonate in chalk and limestone for CCS. One is what I’ll term the “brute force approach”. It is not subtle and not efficient in terms of energy expended per tonne of CO₂ captured and stored, but it’s simple and relatively “bulletproof”.
The brute force approach is an indirect air capture method. It uses enhanced alkalinity in ocean surface waters to counter ocean acidification and increase uptake of atmospheric CO₂. The “brute force” aspect come into play in how it creates the enhanced alkalinity. It does it via large scale calcination of calcium carbonate — the primary constituent of limestones.
Calcination of limestone is a very old technology. Lime kilns were built and used in early civilizations to make quicklime for plaster and for stabilizing mud as a building material. Today the largest use for calcined limestone is in production of portland cement. The fossil fuels burned to fire production kilns plus the CO₂ released from limestone in the process are estimated to account for 5% of all anthropogenic carbon emissions. Developing alternatives to portland cement with lower carbon footprints is a thriving category of the green technology movement. So how could calcination of limestone over and above the needs of portland cement production possibly help?
The problem with production of portland cement is that all the CO₂ from firing the kiln, along with the CO₂ evolved from the thermal decomposition of CaCO₃, are released into the atmosphere. That’s by far the cheapest way, so long as capture of CO₂ is not rewarded and dumping to the atmosphere is permitted. But if the CO₂ were not dumped, then portland cement, along with plaster and other products made with calcined limestone, would be very green. They ultimately absorb as much CO₂ from the atmosphere as was evolved in producing the quicklime that went into them. Hence calcination of limestone is a handy way to get a nearly pure, “sequestration ready” stream of CO₂ right at the injection site, while producing a product that will absorb CO₂ from the atmosphere.
The heat source used for calcining limestone is irrelevant to the process itself. For existing installations, it’s almost always combustion of wood, coal, or natural gas. However, it could be anything capable of delivering the required temperatures of at least 850 °C. Highly concentrated solar energy could be used, or even electrical resistance heating if electricity were super-cheap. But for large-scale operations of the sort needed to seriously address CO₂ emissions, the ideal heat source would be a small nuclear reactor. It would need to be one of the high temperature designs, using molten salt or lead. But it would only need to produce heat, not power, so it would be much simpler than a power reactor.
If this approach were used to sequester CO₂ at the current 9.8 gigatonne (GT) rate of fossil carbon emissions (40 GT CO₂), the production rate for limestone calcination would need to be roughly 50 GT of CaO annually from just over 90 GT of calcium carbonate. Large as those numbers are, they’re not utterly impossible. Availability of limestone is not a limiting factor; there are millions of gigatonnes of accessible deposits around the globe.
The big hurdle is thermal energy. 50 GT of CaO is roughly 40 times more than the cement industry consumes annually, and the energy needed to produce it would nearly double the world’s primary energy consumption. However, it’s thermal energy, not electricity. If it could be supplied by a new generation of cheap nuclear reactors that consumed 100% or their uranium or thorium fuels, it would not put a noticeable dent in the world supply of those elements. But how could so much caustic CaO be economically distributed and used to pull CO₂ from the atmosphere? That’s where enhanced alkalinity of ocean surface waters comes in.
Enhanced alkalinity of ocean surface waters is the same mechanism that would be used for carbon mitigation in the project I wrote about in “Treasure of the Sierra Nevada” and “Cruising to Vegas”. The procedure is simply to load freighters with alkaline material and send them out on long looping courses to dispense alkalinity into the sea. The freighters would be equipped with filtered input ports to suck in seawater (and no fish). The filtered seawater would be used to dissolve controlled amounts of alkaline material, The resulting alkaline seawater would then be pumped through spray nozzles at the stern of the ship. The nozzles would function like giant lawn sprinklers, spraying a rain of alkaline seawater over a 100 meter wide swath of ocean in the ship’s wake.
The extreme dilution of the alkaline droplets upon hitting the ocean surface would be sufficient to insure that the pH of ocean water behind the ship remained safe for sea life. The pH would of course be raised slightly — that being the whole point of the operation. But the rise from a single pass of a single ship would be minute. For CCS, that doesn’t matter; concentration of alkalinity is largely irrelevant to the amount of atmospheric CO₂ that can taken up. To a first approximation, only the total amount of alkalinity added matters. The more uniformly the added alkalinity is spread, the more closely the approximation holds.
That’s not to say there are no potential environmental consequences to this approach that would need to be understood and addressed before it it could be implemented at scale. In particular, it would be nearly impossible to ensure that nothing but alkalinity were added to the seawater. Limestone is far from pure calcium carbonate, and the products of calcining it are far from pure CaO. The alkaline solution produced aboard the freighter would inevitably include colloidal particles of silica and iron-aluminum silicates of the sort found in common clays.
That would probably be good. Those minerals are normally supplied to the ocean from dust blown high into the atmosphere and carried thousands of miles. Their scarcity in ocean waters is a limiting factor in bio-productivity. Increasing their availability as a byproduct of CO₂ mitigation efforts would be a boon to both calcareous and siliceous phytoplankton. That, in turn, should produce consequent benefits on up the food chain, for the health and productivity of the ocean ecosystem as a whole. But it’s not guaranteed. Carbon sequestration by this method is full scale geo-engineering, and unintended consequences are possible. The method would need to be studied and approached gingerly, working up from small tests.
As I said, the brute force approach of limestone calcination is not energy efficient. Energy efficiency, per se, may not be as important as economic efficiency, and the fact that calcination is simple and requires only thermal energy, rather than electricity, does matter. But given a cheap source of high grade thermal energy, it’s not that much harder to produce electricity. So even if cheap nuclear technology of the sort that would enable the brute force CCS approach is developed, its development would also reduce the volume of emissions needing to be captured in the first place.
That level of nuclear technology would even reduce fossil carbon emissions from liquid fuels in the transportation sector. Cheap, reliable electricity would make electrification of transport more attractive, while simultaneously making synthesis of fuels from CO₂ and hydrogen competitive with fossil hydrocarbons. Hence calcination of limestone — the brute force approach to CCS — is unlikely to ever expand far beyond its current market for making portland cement. It could rise to a few gigatonnes per year as part of efforts to roll back CO₂ levels once fossil carbon emissions have been curtailed, but is unlikely ever to become our front line of defense to hold back global warming.
Now suppose that cheap nuclear technology is not successfully developed any time soon. Where would that leave us? We’d be forced to depend on conservation, energy efficiency, and diffuse and irregular renewables to cut fossil carbon emissions. Some feel that that would not be a bad thing at all. But what of rolling back the disastrously high atmospheric CO₂ levels we’re certain to be stuck with before we can get to zero on the fashionable RE pathway?
The Great Basin project that I wrote about in my last two posts might deliver a few hundred megatons of CO₂ capture capacity per year. It probably doesn’t scale well beyond that. But there are energy-efficient ways to exploit carbonate chemistry for CCS that are worth exploring. I had intended to write about them here, but I find I’ve used up my allotted schedule time and word count already. So that discussion will be deferred until next week.
In the meantime, patient readers, happy pondering!