Sequestration of C02

In the future, C02 might be removed chemically from the exhaust gases of major point sources that would otherwise release it into the atmosphere, such as power plants that burn fossil fuels and that collectively are responsible for one-quarter to one-third of total emissions. The carbon dioxide gas so recovered would then be sequestered—i.e., deposited in an underground or ocean location that would prevent its release into the air. For example, the C02 could be sequestered by burial in the deep oceans, where it would dissolve, or in very deep aquifers under land or the seas, or in empty oil and natural gas wells or coal seams. The total amount of carbon dioxide that will be produced by fossil-fuel combustion over this century will amount to more than one trillion tonnes, so vast amounts of storage would be required (Problem 7-6).

PROBLEM 7-6

Calculate the volume, and from it the length in kilometers, of each side of a cube of liquid or solid carbon dioxide whose density is the same as that of water, i.e., 1 g/cm3, and whose mass is a trillion (1012) tonnes.

Since it is not economically feasible to transport and store emission gases from power plants, the dilute carbon dioxide (usually 12-14% by volume) in the gas mixture must be captured and concentrated. The energy input required for the C02-concentrating phase of carbon sequestration schemes would represent a substantial fraction, from one-third to one-half, of the output of the power plant to which it is connected; therefore more fuel would be combusted and more air pollutants produced. The equipment required to capture and concentrate COz is very large, since huge volumes of air are involved. The capture/concentration of the gas accounts for about three-quarters of the cost of the entire sequestration process and currently is estimated to cost about $100 per tonne of carbon. Nevertheless, some observers feel it would still be cheaper to sequester carbon from fossil fuels than to convert to a renewable-fuel economy.

The capture of carbon dioxide is generally accomplished by passing the cooled (to about 50°C) emission gas through an aqueous solvent containing 15-30% by mass of an imime, R2NH (or RNH2), which combines with the C02 to produce an anion, in a process analogous to that between the gas and water:

Over time, the amine solution becomes saturated with the gas since all the amine molecules become tied up with COz. Heat is then used to reverse this reaction (at about 120°C), thereby producing a concentrated stream of carbon dioxide for later disposal and regenerating the amine solution after it has been cooled sufficiently. The amines commonly employed in this chemical absorption technology are monoethanolamine and chethanolamine, since they can absorb high loads of the gas and require relatively little heating to release it later. With these amines, over 95% recovery of C02 can be achieved. A strong base such as sodium hydroxide could be used to absorb the C02, but the bicarbonate salt it produces requires much more heat to later decompose it and release its carbon dioxide. Current research centers on finding a solvent that would bind the C02 efficiently but less tightly and therefore require less energy to decompose it later when concentrated.

Nitrogen and sulfur oxides must be removed from the emission gas before it interacts with the solvent, since otherwise they would react with and degrade the amine. Heating the solution to liberate the C02 and regenerate the amine solvent and compressing the gas once it is produced are the most energy-intensive steps of the process. In addition, enormous amounts of the solution must be transported from place to place. The decomposition of some of the amine, producing ammonia and salts, inevitably occurs during the «purification cycles.

Carbon dioxide can also be chemically absorbed by certain metal oxides, which will release the gas when heated. For example, calcium oxide, CaO, can quickly remove C02 from hot emission gases by formation of calcium carbonate, CaC03:

Subsequent heating of the solid to about 900°C, once it has been largely converted to the carbonate, reverses the reaction and produces concentrated C02 and regenerates CaO. Unfortunately, calcium oxide deactivates relatively quickly over many absorption/deabsorption cycles, so fresh oxide must constantly be added to maintain the absorptive activity of the solid.

Three other techniques are available by which carbon dioxide can be stripped from exhaust gases:

• Membrane separation Polymeric membranes that allow C02 to pass through them, while the other gases are excluded, can be employed to recover about 85% of the carbon dioxide. This technology has been used for many decades in the oil industry, since it is more economical than chemical absorption when the concentration of carbon dioxide in the source gas is relatively high. There is currently much research and development under way in devising membranes that will be efficient and economical for power plant emissions.

* Physical adsorption Certain solids, such as some zeolites and activated carbon, that have large surface areas will adsorb CO, from the gas mixture and later release it upon heating. Methanol and glycols are also used as solvents to capture carbon dioxide from concentrated emission gases.

• Cryogenic separation Since C02 has a higher condensation temperature than nitrogen or oxygen, it can be isolated as a liquid by condensing the gas mixture at a very low temperature under high pressure. However, the energy requirement for this technique is approximately double that of chemical absorption using amines.

One way to circumvent the high expense and energy consumption required to isolate and concentrate the C02 from conventional power plants is by oxycombustion. In this technique, currently under development, the fossil fuel is burned not in air but rather in oxygen gas, 02. If the stoichiometric amount of oxygen is supplied, the exhaust gas from oxycombustion will consist entirely of carbon dioxide and require no isolation step. (By contrast, since air is only 19% oxygen by volume, the maximum level of C02 when air is used for combustion is also only 19%.) Of course, the original isolation of oxygen from air requires energy, and the combustion facilities must be redesigned to be able to use pure oxygen. In practice, since combustion in pure 02 produces a flame too hot (3500°C) for power plant materials, it is diluted with some of the CO, from the combustion to reduce the flame temperature. The output gas from oxycombustion is compressed and dried of the water vapor produced during combustion. It can then be transported by pipeline as a dense supercritical fluid (see Chapter 6).

Another scheme proposed for the future involves the conversion of a fossil fuel, either coal or natural gas, to hydrogen gas, H2, which would be employed as the fuel, either in a power plant or in a vehicle, in a reaction that generates no additional carbon dioxide. In essence, the fuel value of the coal or natural gas is transferred to hydrogen by the gasification process. Such techniques for generating H2 for use as a fuel are described in detail in Chapter 8; in general, the process corresponds to the reaction carbon-hydrogen fuel + water * C02 -F H2

The high concentration of pressurized C02 in the gas mixture (in principle, 50% by volume) allows for a more economical isolation of the gas, in this case using a liquid glycol solvent, than its capture from emission gases in a conventional power plant. Alternatively, a membrane that allows only hydrogen to pass through it could be employed to produce a gas stream largely composed of C02. Prototype power plants in which methane is first converted to hydrogen and carbon dioxide, with the latter extracted and pumped into an underground oil field, are planned for Scotland and California. The Future' Gen project of the U.S. Department of Energy involves the construction of a "zero-emission" coal-gasification power plant that will capture and store all the carbon dioxide it produces.

The transfer of the energy value from a fossil fuel to hydrogen eliminates the impractical task of isolating the carbon dioxide and collecting it when the fuel itself is used to power vehicles and to heat or cool buildings, applications that currently account for more than two-thirds of its emissions. Other industrial processes in which carbon dioxide at relatively high concentration can be separated by membrane techniques include natural gas purification and fermentation plants. The concentration of C02 in emission gases from cement plants, which produce the gas by heating calcium carbonate to release calcium oxide, reaches 15-30% and should be susceptible to more economical capture methods than those used for the more diluted emissions from power plants.

A number of different methods and locations for storing carbon dioxide have been proposed and are under current investigation, as discussed in the following sections.

Deep Ocean Disposal of C02

Various schemes for delivering carbon dioxide in massive amounts to the seas and depositing it there as such are labeled ocean acidic in Figure 7-8 and have

Annual emission

Mineral carbonates

Mineral carbonates nderground tion

nderground tion

Woadv\ carbon biomass

100 1000 10,000 100,000 1,000,000 Carbon storage capacity (Gt)

FIGURE 7-8 Capacities and storage times for various C02 sequestration technologies. [Source: Adapted from K. S. Lackner, "A Guide to C02 Sequestration," Science 300 (2003): 1677.]

the potential to store many hundreds of gigatonnes of carbon dioxide for many hundreds of years. The schemes are referred to as acidic because dissolving carbon dioxide gas directly in seawater produces carbonic acid, H2CO3, a weak acid that would increase the acidity of the ocean in the near vicinity:

Adding large amounts of carbon dioxide would lower the pH of ocean water by tenths of a unit, although much larger decreases of several pH units would occur near the points of injection.

Carbon dioxide destined for ocean storage could be transported by a pipeline, originating either from the shore or from a ship stationed above the disposal site, extending to the depth required (Figure 7-9). Even relatively shallow injection of the gas in the ocean, at 200-400-m depth, would produce a satisfactory result, provided that the seafloor there is slanted sufficiently to allow the dense, C02-rich water to. be transported by gravity to greater depths. Simulations show that most of the gas would return to the surface and enter the atmosphere within a few decades if the C02-rich water was simply diluted by mixing with surrounding water, rather than sinking. Over a

FIGURE 7-9 Potential sequestration sites for carbon dioxide. [Source: Redrawn from Scientific American (Feb. 2000): 72-79.]

period of centuries, excess carbon dioxide would eventually return to the atmosphere, but presumably by that time alternative energy sources would have replaced fossil fuels and the atmospheric C02 problem would then be less serious.

A phase diagram for C02 is illustrated in Figure 7-10a. Below about 500 m deep, water pressure would force pure carbon dioxide to be a compressible liquid, which above 2700 m is less dense than water and would float upward. Below that depth, it is denser than water and would sink.

However, since ocean temperatures are less than 9°C, the liquid or concentrated gas below 500 m could combine with water to form a solid, ice-like clathrate hydrate, C02 • 6 H20, that, if fully formed, would be denser than seawater and would sink to the deep ocean. Thus lakes of liquid and/or clathrate carbon dioxide could form on the seafloor. Figure 7-10b illustrates an experiment in which a beaker of liquid carbon dioxide was placed almost 4 km deep off Monterey Bay, California.

Direct disposal of C02 to the deep ocean would require a pipeline to penetrate to a depth of3000 to 5000 m, producing a pool of liquified carbon dioxide, denser than seawater at this depth (see Figure 7-10b). Some—perhaps just the surface—or all of the liquid carbon dioxide would convert to the solid clathrate. The pool of liquid carbon dioxide would, probably over centuries, dissolve into the surrounding water. Unfortunately, sea life under

FIGURE 7-10 (a) Phase diagram for carbon dioxide. The green line shows the phase boundary between gaseous and liquid C02. The shaded area indicates the conditions under which the hydrate is stable if sufficient C02 is present, (b) Liquid carbon dioxide overflowing from a beaker placed on the seafloor at 3650-m depth. A mass of transparent hydrate formed at the upper interface, sank to the bottom of the beaker, and pushed out some of the liquid C02. ISource: P. G. Brewer et al., "Direct Experiments on (he Ocean Disposal of Fossil Fuel C02," Science 284 (1999): 943.]

FIGURE 7-10 (a) Phase diagram for carbon dioxide. The green line shows the phase boundary between gaseous and liquid C02. The shaded area indicates the conditions under which the hydrate is stable if sufficient C02 is present, (b) Liquid carbon dioxide overflowing from a beaker placed on the seafloor at 3650-m depth. A mass of transparent hydrate formed at the upper interface, sank to the bottom of the beaker, and pushed out some of the liquid C02. ISource: P. G. Brewer et al., "Direct Experiments on (he Ocean Disposal of Fossil Fuel C02," Science 284 (1999): 943.]

this pool would be exterminated. There is also some fear that earthquakes or asteroid impact could destabilize the pool, resulting in the release of massive amounts of carbon dioxide gas into the air above.

Near the seafloor, dissolved carbon dioxide could eventually react with the solid calcium carbonate, CaC03, in sediments formed from seashells etc. to produce soluble calcium bicarbonate, Ca(HC03)2:

(This reaction is discussed in detail in Chapter 11.) For practical purposes, the CO? •» now chemically trapped in the bicarbonate form, would remain indefinitely in the dissolved state.

In an alternative scheme, labeled ocean neutral in Figure 7-8, calcium carbonate or some other suitable substance such as calcium silicate (a cheap, abundant mineral) would be reacted with carbon dioxide to transform it to solid silicon dioxide, Si02, and aqueous calcium bicarbonate, which could be drained into ocean depths:

Acidity problems associated with direct carbon dioxide dissolution in sea-water are avoided in this way.

Huge amounts of limestone or calcium silicate would be required for this form of sequestration, but the potential for C02 storage by this method is very great and the storage time of the gas is many thousands of years (Figure 7-8). In addition, it may be possible to react power plant emissions directly with a mineral, thereby avoiding the energy-expensive step of extracting and concentrating the carbon dioxide.

PROBLEM 7-7

Calculate the mass, in tonnes, of calcium carbonate that is required to react with each tonne of carbon dioxide.

Alternatively, surface rocks containing alkaline silicates could be crushed and then reacted with carbon dioxide to produce insoluble solid carbonates that could simply be buried in the ground. Unfortunately, direct carbonation reactions involving C02 are slow unless the mineral is heated, a step that is costly in money and energy. In one indirect scheme, magnesium silicate rock is reacted with hydrochloric acid, HC1, to produce silicon dioxide and magnesium chloride, MgCl2. Reaction of this salt with carbonic acid produces insoluble magnesium carbonate, MgC03, and re-forms the hydrogen chloride, which, in principle, can be recycled:

There are still energy costs and additional C02 production associated with such procedures, however.

An alternative scheme proposed recently for carbon dioxide storage is to inject it into sediments under the seafloor. Because it would be under high pressure and at low temperature, it would exist there as a dense liquid or would combine with the water in the sediments to form the solid hydrate. Although expensive, this injection process could be useful for power plants in coastal locations.

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Guide to Alternative Fuels

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