Underground Storage of CO2

At the core of the idea of CCS technology is the subterranean, permanent storage of CO2 to prevent it entering the atmosphere. The CO2 is injected below imperme able cap rock which prevents the CO2 from escaping to the Earth ' s surface. This is modeled on natural gas which, in such conditions, has been fixed in subterranean deposits across geological periods. The subsoil, with its structure, offers various options that can serve as storage sites for CO2.

Located below the impermeable cap rock are numerous porous rock layers with cavities filled with various media. The pores contain natural gas or oil, for example. One obvious possibility for storing CO 2 ' therefore, is to use depleted gas or oil fields. These two alternatives have the additional merit that the geology is very well investigated. Even gas or oil fields that are still in operation can be considered for storing carbon. Here, another benefit emerges, since the injection of CO2 presses out more oil or gas from the deposit, thus increasing the yield. These techniques are called enhanced oil recovery (EOR' and enhanced gas recovery (EGR' resp. Here, CO2 storage even generates economic added value. In the case of oil, this has been practiced in the USA ever since the 1970s. Today, the USA injects some 35 million tons of CO2 for EOR annually into the underground. EGR, by contrast, is not yet state of the art. The problem here is a possible mixing of gas and CO2 which drastically impairs the quality of the natural gas.

For many countries or plants in which carbon can be captured, the CO 2 storage capacities of the gas or oil fields within reach are so low, however, that they cannot make any significant contribution to a broad-based application of CCS. In such cases, recourse is had to another option for storing CO 2 - In what are by far the largest areas, the porous rock beneath impermeable cap rock is filled with salt water. These areas are called saline formations or saline aquifers. The salt water makes it clear that there is no contact with the groundwater.

The salt water that fills the pores of the saline formations must be displaced when the CO2 is injected. This can only be done to a very low degree, of course, since otherwise the pressure in the aquifer would be inadmissibly increased. Hence, only less than 1% of the pore volume is available for storing carbon. The precise value must be established for each particular storage site. All the same, the global capacities for storing CO2 in saline formations are very large. It is these capacities that form the basis for CCS's crucial contribution to climate protection.

Storing CO2 in saline aquifers, too, is already being practiced. In Norway, for example, CO2 is separated, using amine scrubbing, from natural gas extracted from the Sleipner gas field. The carbon is then returned to the area of the gas field and injected into an adjacent saline aquifer. Every year, approx. 1 million tons of CO2 has been stored in this way since 1996.

The suitability of a storage site for injecting and storing CO2 depends on various factors. In addition to the intake capacity for CO2, the speed with which the carbon dioxide can be injected into the storage site, too, plays a major role. To establish the pertinent, physical parameters, like porosity and permeability, injection tests ultimately have to be made. Only on this basis is a final characterization and assessment of a storage facility possible.

The CO2 that is injected into a saline aquifer is prevented from escaping by several mechanisms. The impermeable cap rock under which the CO2 accumu-

Figure 11.7 Three options for underground C02 storage in geological formations.

lates due to its lower density relative to the salt water is, of course, the main factor and the most important criterion for the storage site being considered. Beyond this, however, the carbon is trapped in a saline aquifer in several additional ways. Once the C02 has reached its position in the aquifer after injection, the surrounding salt water encloses the C02 in the pores. The capillary forces keep the water in the channels and prevent the C02 from displacing the water and from migrating. In a second mechanism, some of the C02 is dissolved in the salt water. Another part forms carbonates. These additional mechanisms ultimately mean that most of the C02 can no longer move freely. Besides the cap, which is actually already sufficiently leak-tight, this ensures that the C02 remains permanently in the depository. The different options for underground C02 storage can be viewed in Figure 11.7.

There are potential C02 storage sites both onshore and offshore below the ocean. Onshore storage sites have the advantage that they can be reached with much lower outlays, although the issue here tends to be one of public acceptance. Accessing offshore C02 storage sites usually requires immense material and energetic outlays.

The storage of C02 means that the C02 must fulfill certain purity requirements. Hence, when the C02 capture process is defined and designed, the possible impurities of the C02 must be precisely analyzed and, if necessary, removed in further treatment steps.

Carbonation

One alternative to subterranean C02 storage as a method for permanently trapping C02 is carbonation. In the present context, this means artificially imitating the weathering of rocks. Reaction partners for CO2 are silicates like serpentine (magnesium silicate), and the process produces a carbonate like magnesite; CO 2 is permanently bound, the products are chemically stable, and can be deposited without any problem. Silicates are distributed around the world in such quantities that they would suffice for all known coal deposits or, rather, for the CO2 quantities emerging from them, to form carbonate.

Despite its initially attractive properties, carbonation is only feasible at best for niche applications. The problems here lie in the logistics and in energy consumption. Each ton of CO2 needs about 7 tons of rock as reaction partner. This means that, for each ton of coal, an additional 10 to 20 tons of silicate rock must be taken to the power plant and also hauled off again as carbonate rock, which is impossible to handle in logistical terms. In nature, the carbonation reaction is very slow, because of the low concentrations of CO2 in the air, the small specific contact surface between silicate rock and CO2 and the activation energy for the reaction. To obtain an acceptable reaction speed, various measures must be taken. Separating the CO2 produces a high CO2, concentration. In addition, the CO2 is pressurized. The silicate rock is fine ^rain milled, so that the reaction surface is greatly enlarged, and heat is added to the reactor to accelerate the reaction. Despite all these measures the carbonation reaction still takes hours, that is, it is not suitable for technical deployment involving large quantities. Moreover, the measures are so energy- i ntensive that the efficiency of a power station is halved.

This being so, carbonation is a process that can only be used where marginal conditions are extremely favorable, that is, where transport routes for the material are short and where the product can be marketed. It is no alternative to storing CO2 below ground.

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