Читать книгу Carbon Dioxide Emission Management in Power Generation - Prof. Lars O. Nord - Страница 17

There are many examples of natural emissions of CO₂ to the atmosphere. Forest fires and grass fires are natural phenomena, which release large amounts of CO₂ to the atmosphere. On the other hand, after such fires, new trees and grass will grow and eventually bind more or less the same amount of CO₂ as was released. One can say that as long as the total amount of biomass on a global basis is constant over a longer period of time, the combustion, decay, and growth of biomass is balanced with respect to CO₂. In this respect, the use of biomass for energy purposes can be regarded as CO₂ neutral. Consequently, if the amount of biomass is increasing through an expansion of the area covered by forests, this contributes to reducing the amount of CO₂ in the atmosphere. In reality, there is a significant ongoing reduction in biomass globally by man-made forest fires and wood cutting, which contributes to about 20% of the annual increase in greenhouse gas emissions.

Figure 1.1 Vostok ice core data for 420 000 years. Atmospheric CO₂ concentrations from trapped gas bubbles.

Source: Petit et al. (1999). Reproduced with permission of Springer Nature.

There are also a number of examples of seepage of CO₂ from the ground. From many of the ground seepages, the flux is so small that it is hardly noticeable. Others have a high flux so that the ground-level concentration can cause death of plants through ‘root anoxia’ and even pose risks to human health and safety. At concentrations above about 2%, CO₂ has a strong effect on respiratory physiology, and at concentrations above 7–10%, it can cause unconsciousness and death. Exposure studies have not revealed any adverse health effect from chronic exposure to concentrations below 1% (Fleming et al. 1992).

One important aspect to note about CO₂ is that it has a higher molecular weight than air, meaning that it is denser than air. As a consequence, the release of CO₂ at a low point in the terrain, with little or no wind, can cause the concentration at that point to increase to a very high level, as the CO₂ will not be transported vertically by buoyancy.

Seepage of CO₂ from the ground has a relevance to CO₂ capture and storage because stored CO₂ may leak out to the atmosphere. The impact on human health from releases of CO₂ can be severe. Some examples are Lake Monoun, Lake Nyos, and Lake Kivu, which are the only three lakes in the world known to be saturated with CO₂ (Clarke 2001; IEA-GHG 2006). The former two are located quite close to each other in the Oku Volcanic Field in Cameroon, whereas the latter is in Rwanda. In these lakes, the content of CO₂ per volume of water increases with depth because the solubility of CO₂ in water depends on the pressure, as can be seen in Figure 4.17.

On 15 August 1984, Lake Monoun exploded in a limnic eruption probably caused by an earthquake, which resulted in the release of a large amount of CO₂. As a direct consequence of the release, 37 people were killed. A gas cloud came up from a crater in the eastern part of the lake at night. The 37 people who died were residents in a low-lying area close to the lake. Survivors reported that the whitish, smoke-like cloud smelled bitter and acidic. The vegetation was flattened around the eastern part of the lake, probably by a water wave caused by the gas cloud.

On 21 August 1986, Lake Nyos suddenly emitted a large cloud of CO₂ estimated as over 1 Mt, which travelled more than 10 km and suffocated 1700 people and 3500 livestock in the area near the lake. The lower levels of the deep lake had become saturated by CO₂ coming from a magma chamber beneath the region. The magma chamber is an abundant source of CO₂, which seeps up through the lake bed, charging the waters of Lake Nyos with an estimated 90 Mt of CO₂. It is thought that high rainfall just before the incident, and possibly a landslide, had displaced the CO₂-rich water at the bottom, releasing a massive bubble of CO₂ gas from the lake, in a natural phenomenon now referred to as ‘lake overturn’. The heavy gas then sank to the ground and rolled in a cloud several tens of metres deep across the surrounding countryside.

Pipes have now been put in place in Lake Nyos and Lake Monoun to siphon water from the lower layers up to the surface and allow the CO₂ at the bottom of the lake to slowly bubble out. Events such as those in Lake Monoun and Lake Nyos can take place only in lakes that do not overturn annually, and where the water becomes stratified with very high concentrations of CO₂ at large depths. This can happen in deep tropical lakes.

In the Mammoth Mountain area in California, USA, CO₂ is leaking out of the ground at more than 1200 t/d (Farrar et al. 1995; Sorey et al. 1998). The concentration of CO₂ near the ground has been measured at over 50%. Three people are known to have died because of CO₂, and there is an area where the trees are dead.

There exist a few volcanoes with crater lakes. These lakes can be rather deep and still, with stratification and deep layers containing large amounts of CO₂, similar to Lake Nyos.

Geysers emit CO₂ to the atmosphere. Water charged with CO₂ rising from deep in the ground is released periodically in an explosive manner. Hot springs are similar to geysers but release CO₂-rich water at a continuous rate.

In 1979, at the Dieng volcano complex in Indonesia, a release estimated at about 200 kt of CO₂ took place in a rather short time before a major eruption. The CO₂ flowed from the volcano and down to a plain where 142 people were killed by suffocation.

There are a number of examples of CO₂ emissions from sedimentary basins. These emissions are characterised by being smaller and more stable over time compared to those previously given examples from volcanic areas. One example is the Southeast Basin in France where there are several small CO₂ fields in the ground leaking to the surface. The CO₂ is dissolved in the groundwater and comes out of the ground in springs as carbonated water. This has become the basis for a mineral water industry, such as Vichy and Perrier.

At some places, CO₂ is leaking from the seabed. Just outside the Aeolian Islands in the south of Italy, about 25 kt CO₂/yr is leaking over an area covering several square kilometres. Most of the CO₂ is being dissolved in the seawater.

A distinction should be made between natural emissions of CO₂ in volcanic areas and emissions in sedimentary areas. In volcanic areas, the emission of CO₂ is often characterised by a sudden release of CO₂, often caused by unstable seismic activities. High temperature and steam are often present, which builds up high pressure and severe emission with a very high concentration of CO₂. Emissions from sedimentary basins are characterised as more diffuse and definitely not sudden. Most sedimentary basins are located in tectonically stable regions with less or no seismic activity. They typically contain porous rocks or sandstone, which is gas permeable. In some areas, there are also impermeable rock layers that act as seals so that gas cannot go through and reach the surface. Oil and natural gas reservoirs in sedimentary basins have proved that such structures can hold gases locked in the ground for millions of years.

There are also natural accumulations of CO₂ in the ground (IPCC-CCS 2005). One example of such a sedimentary basin is the McElmo Dome in Colorado, USA. It contains about 1.6 Gt of CO₂ (98% purity) and is sealed by a 700 m-thick impermeable layer. Similar amounts of CO₂ are trapped in other fields in the United States: the St Johns Field in Arizona, the Bravo Dome in New Mexico, the Sheep Mountain in Colorado, the Jackson Dome, and the Pisgah Anticline in Mississippi. All fields mentioned above produce commercially traded CO₂, most of which is used to enhance oil production (refer to Section 2.1.2). The largest CO₂ accumulation known is the Natuna D Alpha field in Indonesia containing more than 9 Gt CO₂. In the Natuna field, there is also a substantial amount of natural gas, more than 700 Mt. Similar to the Natuna field, although smaller, is the La Barge field in Wyoming, USA. In general, there are many natural gas fields that contain a substantial amount of CO₂ and in some cases also some H₂S.

From some of these fields, there is a measurable leakage, whereas others appear to have no leakage. The mechanisms for these leakages are very well understood, so that one can tell with high probability which structures can hold CO₂ trapped for a long period of time.