Energy Convers. Mgmt Vol. 33, No. 5-8, pp. 565-572, 1992 0196-8904/92 $5.00+0.00 Printed in Great Britain Pergamon Press Lid

A GEOLOGICAL PERSPECTIVE ON GLOBAL WARMING AND THE POSSIBILITY OF CARBON DIOXIDE REMOVAL AS CALCIUM CARBONATE MINERAL

H.E. DUNSMORE

Geological Survey of Canada 601 Booth Street

Ottawa, Ontario KIA 0E8 Canada

ABSTRACT

Nature removes carbon dioxide from the atmosphere through photosyn- thesis, and by forming carbonate minerals. Following Nature's example, carbon dioxide should not be regarded as a waste, but as a resource from which useful products can be made. Highly concentrated, calcium- rich brines are commonly found associated with subsurface salt deposits. By bringing together the energy and chemical industries, it may be possible to use these brines to lock up carbon dioxide, while at the same time producing calcium carbonate, hydrochloric acid and a variety of other chemical-industrial commodities.

KEYWORDS

Global warming, carbon dioxide capture, precipitated calcium carbonate, calcium-rich brines, chemicals from brines

INTRODUCTION

Of the many environmental issues facing industrial society today, the gravest of all is the prospect of global warming. If this occurs, it will be caused in large part by the release of CO 2 to the atmosphere from the burning of fossil fuels. If, as seems likely, steps must be taken to moderate this enhanced greenhouse effect, it is crucial that the nature of the problem be clearly and fully understood so that appropriate solutions are put in place.

This paper will address two main issues. The causes and consequences of possible anthropogenic global warming will be discussed from a geological perspective, emphasizing how the Earth's atmothermal balance has been maintained throughout geological time. Based on this evidence, a technique for capturing CO 2 will be presented, one which should be applicable in many parts of the world.

UNDERSTANDING THE PROBLEM

Life continues to flourish on Earth because surface temperatures have always been maintained within tolerable limits. This is accomplished by controlling the atmospheric content of greenhouse gases, including carbon dioxide. We shall begin this discussion by considering how carbon is distributed on the planet.

Distribution of Carbon

The Earth's carbon can be divided into two reservoirs of quite unequal size (Fig. i). The larger of the two, containing something like 99.94% of the total, is made up of "dead" carbon locked away in the lithosphere,

565

566 DUNSMORE: CO~ REMOVAL AS CALCIUM CARBONATE MINERAL

Calcium Carbonate 35,000 *

Ca-Mg-Carbonate 25,000

Sedimentary Carbon 15,000 (K©rogen & Graphite) Recoverable Fossil Fuels 4

46.64% "I 1-79.99%--I

33.31% "J |

Oceanic HCO3" - CO3- 42 0.056%

Dead Surficial Carbon 3 0,0040% (Humus and Peat) Atmosphcric CO2 0.72 0.00095% --

All life 0.56 0.00074% *K S x 1015

0.06%

Fig. I. Estimates of the relative distribution of carbon on Earth, in kg and % (after Berner and Lasaga, 1989).

that is, in the rocks of the Earth's crust. Some 80% of this fossil carbon is fixed in the oxidized state as geochemically stable carbonate minerals in limestone and dolomite deposits (Fig. i). The remaining 20%, consisting of the altered remains of organisms, occurs as kerogen and graphite disseminated throughout sedimentary and meta-sedimentary rocks. A tiny fraction of this carbon has been maturated and con- centrated as recoverable fossil fuels.

The smaller of the two reservoirs, containing a mere 0.06% of the global total, is made up of the "live" carbon actively being cycled within the atmosphere-hydrosphere-biosphere. Over 90% of this carbon is in solution in the world's oceans. The remainder, in decreasing order of abundance, is found in dead surficial material, the atmosphere, and living organisms (Fig. i).

One is struck by the overwhelming dominance of the lithosphere as a store of carbon and the comparative insignificance, and implied fragility, of the atmosphere and life systems. Within the lithosphere, four times more carbon is held in the oxidized form, as carbonate minerals, than in the reduced form, as organic remains.

Earth's Biogeochemical Thermostat

The basic configuration of the Earth's carbon-based biogeochemical thermostat is illustrated in Fig. 2. Two versions are shown, one depicting the situation prior to industrialization in the early 1800's, and the other after industrialization. Both consist of the "live" and "dead" reservoirs linked by various fluxes.

Throughout geological time, carbon has been continuously cycled back and forth between the two reservoirs (Fig. 2a). It is released from the lithosphere by geological processes powered by plate tectonics; these include the emission of CO2-rich volcanic fluids, and the weathering and erosion of carbon-bearing rocks following uplift. The return flux combines both biological and geological processes; organisms produce the carbonate minerals and organic matter, primarily within the surface layers and on the margins of the world's oceans, but geological processes are responsible for ensuring that this material is successfully trans- ferred back into the "dead" carbon reservoir.

Carbonate minerals are stable in the surficial environmemt, so in most instances the transition is complete when the mineral is formed. Organic carbon, on the other hand, can only be judged to have crossed the line when the host sediment has been buried and lithified.

DUNSMORE: CO 2 REMOVAL AS CALCIUM CARBONATE MINERAL 567

We~rlng Carbonate

F].g. 2a ~ . ~

Weathering

\

Fig. 2b

- ~ , ~ , , ~ CARBON

/MCaicining ~ \ Carbonate

ANTHRO- i,~EN~C \ ~mO~EOLC~ICAL

Fig. 2. Schematic representation of Earth's carbon-based biogeochemical thermostat, prior to industrialization in the early 1800's (2a), and post-industrialization (2b).

To maintain atmothermal balance, changes in flux on the left side of the loop (Fig. 2a) must be matched by a corresponding change on the right. If not, carbon will either accumulate or become depleted in the minute "live" carbon reservoir, including the atmosphere. For example, if the left side becomes dominant, CO 2 will build up in the atmosphere and a permanent, runaway greenhouse effect will result, as on our sister planet Venus. On the other hand, if the right side of the loop becomes dominant, the atmosphere will lose most of its CO 2 and the planet will freeze up, as occurred on Mars. Over time and within limits, a balance must be maintained between these two fluxes; otherwise, liquid water will disappear from the Earth's surface, and with it, life as we know it.

Since the early 1800's, our industrial society has been effectively short-circuiting the thermostat (Fig. 2b). We have been digging up "dead" carbon as quickly as we can find it--carbon which Nature thought had been safely locked away--and, after use, are simply dumping it into the "live" reservoir. This CO 2 is not only coming from fossil fuels, but also from the use of limestone for cement production, metals smelting, and so on. Because we have not provided a mechanism to refossilize this carbon, it is accumulating in the atmosphere-hydrosphere. biosphere, a system which already has its own anthropogenic problems and its own pool of carbon to contend with.

Conceptually at least, the solution to the CO 2 problem is straight- forward: any anthropogenic release of carbon from the lithosphere must be matched by an equal and opposite return. In other words, we must either eliminate the anthropogenic flux depicted in Fig. 2b, or generate a flux of equal magnitude in the opposite direction.

The amount of carbon that our civilization is defossilizing has reached astounding proportions. Estimates put it at i0 to 15 times the natural flux of carbon out of the lithosphere (Turner, 1981, Franqois and Walker, 1992). This may be hard tobelieve, until one visualizes the amount of fossil fuel consumed each day. Recently, the world's oil industry has been producing about sixty-six million barrels of petroleum daily. If that number of oil drums was stood side by side in a row, the line would stretch for one complete revolution around the circum- ference of the Earth. To this must be added the oil equivalent of the natural gas and coal combusted each day, as well as the CO 2 produced from calcining limestone.

568 DUNSMORE: CO:, REMOVAL AS CALCIUM CARBONATE MINERAL

The anthropogenic flux of carbon out of the lithosphere is still very small compared to the magnitude of the annual organic production and decay cycles, that is, the massive and rapid cycling of carbon within the "live" reservoir. Perhaps it is this fact which has made the problem seem less urgent than it may well be. If the analysis depicted in Fig. 2 is correct, it is imbalances in the rates of defossilization- refossilization of carbon which, in the long term, will determine whether the planet's surface warms or cools.

It is crucial to our understanding of the CO 2 problem to appreciate that the various carbon fluxes operate on very different time scales. Carbon within the atmosphere-hydrosphere-biosphere is cycled relatively quickly, anywhere from minutes to several centuries. The same is true of our exploitation of fossil carbon as an energy source and industrial commodity. On the other hand, geological and biogeological processes are very much more ponderous, operating within time frames of hundreds of thousands or even millions of years. Thus, the processes responsible for returning carbon to the safety of the lithosphere simply cannot respond quickly enough to our sudden interference to avoid a buildup of atmospheric CO 2.

Since the industrial revolution two centuries ago, the CO 2 content of the atmosphere has been rising exponentially. About half of the anthro- pogenic carbon remains in the atmosphere (Holland, 1978); the remainder, for now at least, has been quickly cycled into biomass and the oceans. There has simply not been enough time for much of this polluting carbon to have been transferred back into the lithosphere.

Given geological time, natural processes will eventually restore the CO 2 balance, as they always have done, but the lengths of time involved are daunting. Some 57 million years ago, a C02-induced greenhouse warming event took place due to the abrupt release of volcanic gases accompanying reorganization of tectonic plates in the North Atlantic; the geological record indicates that four million years were required for the climate to return to normal (McGowran, 1989). Putting this in perspective, our species has only been in existence for about a quarter of a million years.

FOLLOWING NATURE'S EXAMPLE

When it comes to removing CO 2 from the atmosphere and disposing of it safely, we can probably do no better than attempt to duplicate the approach and processes which Nature has used for billions of years. Carbon control and disposal has been so crucial to life on Earth that all options have no doubt been tried, and only the most effective and efficient retained.

CO? is a Resource, Not a Waste

Before returning carbon to the lithosphere, the natural world uses it to make things, such as living organisms and carbonate shells, spines and skeletons. Thus, CO 2 quickly became an essential resource, not a waste to be disposed of. We too should begin to think of CO 2 in terms of useful products that might be made from it.

This approach is analogous to energy-from-waste and recycling programmes for our municipal and industrial garbage. The advantage is that a financial return is realized while at the same time the environmental impact is reduced.

Carbonate Minerals~ the Preferred Disposal Medium

Four times out of five, Nature refossilizes carbon as calcium or calcium-magnesium carbonates (Fig. i). Because these materials are stable in most surficial and geological environments, CO 2 is effectively stored for periods of time that are measured in millennia, not minutes,

DUNSMORE: CO 2 REMOVAL AS CALCIUM CARBONATE MINERAL 569

months or years. Only one time out of five is the organic carbon route chosen.

The problem with the organic option is clearly one of preservation. Highly unstable organic matter must be prevented from releasing green- house gases, methane as well as CO 2, for hundreds of thousands of years. Attempts to refossilize carbon by artificially increasing the organic productivity of the oceans, or with massive afforestation programmes on land, will only achieve their intended objective if the material produced is successfully transferred from the biosphere to the litho- sphere. Using coal as an example, we would have to bury, at least as securely as radioactive wastes, a quantity of plant material which will, in time, generate coal deposits equivalent in carbon content to the ones we are currently exploiting. It would make more sense to go back to burning wood and leave the coal in the ground.

There are many valid reasons for planting trees, but doing so to justify the continued use of fossil fuels does not appear to be one of them. Reforestation is an excellent solution to deforestation. Enhancing organic productivity in one region of the ocean is the best way to compensate for reduced productivity somewhere else. These are biological solutions to biological problems, but a geological source of carbon must be matched by a geological sink. Following Nature's example, the best geological solution appears to be the precipitation of massive quantities of stable carbonate minerals.

SOURCE OF CALCIUM AND MAGNESIUM

To remove CO 2 as carbonate minerals would require a massive supply of easily obtained calcium and magnesium in close proximity to the deposits of fossil fuels. The traditional source of these metals has been carbonate rocks, but that's part of the problem. Obtaining them from other types of rocks, such as calcium sulphates or silicates, or from sea water would be prohibitively expensive. There is, however, another source which should be able to meet the requirements.

Subsurface Evaporitic Brines

Sedimentary basins not only host coal, oil and natural gas, but also extensive deposits of evaporitic salts and brines. These were produced by solar evaporation of sea water in a restricted environment, resulting in the orderly precipitation of carbonate, sulphate and chloride salts. As well, large volumes of highly concentrated sea water in the form of residual brines were also generated. These reactive chloride solutions become highly enriched in a variety of elements, including calcium, magnesium, bromine, iodine, boron and lithium.

Like fossil fuels, salts and brines represent fossilized solar energy in chemical form; one is the end-product of photosynthesis, the other of evaporitic differentiation. This fact was not lost on the inorganic chemical industry which for many years has consumed large amounts of salts and brines as feedstock for a worldwide industry. In cold climates, we exploit this chemical potential energy in a samll way when we spread sodium chloride on our streets and walks to melt the ice.

A number of evaporite deposits and associated brines are found within the large sedimentary basin underlying the western Canadian provinces of Alberta, Saskatchewan and Manitoba. This basin also contains almost all of Canada's oil and gas, a major share of its coal as well as the vast tar sands deposits. One of the best developed evaporite deposits is the Prairie Evaporite of Middle Devonian age within the Elk Point Basin (Fig. 3). In the Saskatchewan portion of the basin, this unit hosts one of the richest and most extensive potash (KC1) deposits in the world.

The distribution of known brine occurrences in the Elk Point Basin, both calcium and magnesium-rich, are illustrated in Fig. 3. Some of

570 DUNSMORE: CO 2 REMOVAL AS CALCIUM CARBONATE MINERAL

I ALBERTA "~.~a ~

Mg~ Ca BRINE SHOWING ~ /~ OR PRODUCTION

LIMIT OF PRAIRIE EVAPORITE PRAIRIE EVAPORITE ISOPACH, FEET

ELK POINT BASIN WESTERN CANADA

, I 400 KM I ,

SASKATCHEWAN, 3o0 Mt

U.S.A.

Fig. 3. Distribution of subsurface brine occurrences in western Canada.

these brines are being produced commercially, some are seeping into potash mines, and some were recovered during exploration for oil and gas. Both types are chloride solutions with specific gravities as high as 1.36. The magnesium brines contain up to 7% magnesium by weight and appear to be restricted to the potash horizons, both being the product of extreme solar evaporation. The calcium-rich brines typically contain 10% calcium (compared to 40% for limestone), 25% chloride and 39% total dissolved solids. They are much more widely distributed and are hosted by several different formations. There has been no systematic search for these brines, most known occurrences having been discovered by chance, but indications are that the resource is very large indeed.

Brines of these types are, of course, not restricted to western Canada. Basins in central and eastern Canada also contain evaporites and similar calcium solutions. Brines such as these have been exploited commercially in the United States for many years. A large portion of western and central Europe is underlain by thick salt deposits, the Zechstein evaporites; calcium and magnesium-rich brines are probably to be found here as well.

In summary, subsurface brines are the only source of calcium and magnesium which could be extracted from the crust at a cost and on a scale, to match the withdrawal of fossil fuels. Fortunately, the poison and potential antidote are spatially correlated, both being found within major sedimentary basins around the world.

HOW COULD CO 2 REMOVAL BE ACHIEVED?

The Concept

Capturing CO 2 as stable carbonate minerals would require a merging of the resource, energy and chemical industries in a relationship analogous to symbiosis in biology.

Carbon dioxide produced from the combustion of fossil fuels would be passed from the power plant or oil refinery to the chemical plant. Here calcium-rich brine from the subsurface would be used to produce calcium carbonate and hydrochloric acid, which would in turn be passed

DUNSMORE: CO 2 REMOVAL AS CALCIUM CARBONATE MINERAL 571

to secondary industries. Waste brine and spent acid would be injected back into the subsurface. Surplus calcium carbonate could be returned to the pit from which the coal, for example, was taken; the space formerly occupied by the coal could accommodate about 70% of the total volume of calcium carbonate produced. The chemical plant would also be expected to produce a variety of other commodities using more traditional feedstocks.

The concept is, of course, not without its share of problems. The two principal difficulties are that precipitation of calcium carbonate does not proceed on its own accord and, secondly, that enormous amounts of hydrochloric acid would be produced.

As can be seen in Fig. 4, calcium carbonate precipitation is the result of four simultaneous equilibria. The rate-limiting step is apparently reaction 2. Organisms long ago got over this obstacle by inventing a powerful enzyme, carbonic anhydrase, which increases the reaction rate

Overall Reaction:

Ca++(aq) + CO2(g) + H20 ~ CaCO3(s) + 2H+(aq)

Four Simultaneous Equilibria:

I. C02(g) ~ C02(aq)

2. CO2(aq) + H20 --~ H+(~) + HCO3"(~)

3. HCO3"(~) -~ H+(~) + CO3-(aq)

4. Ca++(aq) + COf'(aq) ~ CaCO3(s)

Fig. 4. Hydration of carbon dioxide and precipitation of calcium carbonate.

by I00 000 times. As a result, a weakly saline solution like our blood becomes a highly efficient CO 2 absorber and transfer medium. It can also be seen that acid would have to be removed from the system at steps 2 and 3.

The amount of acid produced would seem a more insurmountable problem. A simple calculation shows that two tonnes of HCI would be produced for each tonne of coal (33% C) burned. The question is asked why the oceans are not acidic considering the amounts of limestone that have been precipitated throughout geological time. It may be that clay minerals carried to the oceans from the continents have neutralized the acid (H.J. Abercrombie, pets. comm.). If so, non-marine clays and other silicate minerals might be used to consume acid while at the same time extracting useful resources from rocks and minerals. If the chemical route to advanced ceramics dominates the next century, large amounts of acid might be required.

Possible Methods

Technology to precipitate calcium carbonate using CO 2 and calcium chloride solutions has never been developed. This may be because the problem has not been given much thought. As long as naturally occurring limestone could be used, there was no incentive to do so.

The most exotic route would be to duplicate biological processes, using membranes and powerful catalysts like carbonic anhydrase. Some of the most primitive organisms perfected the process over three billion years ago. Recent advances in biotechnology make this less far-fetched than

572 DUNSMORE: CO 2 REMOVAL AS CALCIUM CARBONATE MINERAL

it once was.

It would be technically feasible to capture CO 2 as sodium or potassium carbonate using the Solvay process. These materials could then be com- bined in solution with calcium chloride to precipitate calcium carbonate. It may be technically feasible, but is unikely to economically viable.

Another method which has been suggested uses heat to produce magnesium oxide and hydrochloric acid from magnesium chloride and water (Lipinsky, 1991). The magnesium oxide would then be reacted with CO 2 to generate magnesium carbonate.

Spray technology has also been proposed. Calcium chloride brine would be atomized in a stream of hot CO2, and the hydrochloric acid separated as a gas on the basis of differential boiling points.

CONCLUSIONS

If our industrial society wishes to continue to burn fossil fuels in ever-increasingly amounts, it must also be prepared to take the steps necessary to clean up the resulting pollution. From a geological perspective, the only secure and lasting solution is to return the CO 2 to the lithosphere. It must not simply be dumped into the atmosphere, hydrosphere or biosphere.

Nature's preferred solution is to form carbonate minerals using the calcium and magnesium present in the world's oceans. The organisms which carry out this biomineralization are repaid for their considerable efforts by the useful commodities they have learned to produce, such as shells and skeletons. We can do no better than follow Nature's example.

Nature has not only shown us the way, but has also done the work of concentrating the required calcium and magnesium. These elements are conveniently stored in brines in close proximity to many of the world's fossil fuel deposits. By merging the energy and chemical industries, it should be possible to release and harness the full chemical potential of both the fossil fuel,s and the brines and salts.

What has become a serious problem may in fact present an opportunity, if only we are smart enough to figure it out.

REFERENCES

Berner, R.A. and A.C. Lasaga (1989). Modeling the geochemical carbon cycle. Scientific Amer., Mar. 1989, 74-81.

Franqois, L.M. and J.C.G. Walker (1992). Modelling the Phanerozoic carbon cycle and climate: constraints from the 87Sr/86Sr isotopic ratio of seawater. Am. J. Sci.,292, 81-135.

Holland, H.D. (1978). The Chemistry of the Atmosphere and Oceans. Wiley, New York.

Lipinsky, E.S. (1991). R&D Status of Carbon Dioxide Separation, Disposal and Utilization Technologies. Unpubl. Report.

McGowran, B. (1989). Silica burp in the Eocene ocean. Geoloqy, I_/7, 857-860.

Turner, G. (1981). The development of the atmosphere. In: The Evolving Earth (L.R.M. Cocks, ed.), Chap. 8, pp.121-136. Brit. Museum & Cambridge Press, London & Cambridge.