Carbon Dioxide Emission Management in Power Generation. Prof. Lars O. Nord

concentration GWP (100 yr time horizon) Atmospheric lifetime (yr) Carbon dioxide CO₂ ≈386 ppm 1 100 Methane CH₄ ≈1800 ppb 25 12 Nitrous oxide N₂O ≈320 ppb 298 114 Ozone O₃ ≈34 ppb N/A h/d CFC-11 CCl₃F ≈240 ppt 4750 45 CFC-12 CCl₂F₂ ≈536 ppt 10 900 100 CFC-113 CCl₂FFClF₂ ≈76 ppt 6 130 85 HCFC-22 CHClF₂ ≈200 ppt 1 810 12 HCFC-141b CH₃CCl₂F ≈20 ppt 725 9 HCFC-142b CH₃CClF₂ ≈20 ppt 2 310 18 Halon 1211 CBrCIF₂ ≈4 ppt 1 890 16 Halon 1301 CBrCIF₃ ≈3 ppt 7 140 65 HFC-134a CH₂FCF₃ ≈52 ppt 1 430 14 Carbon tetrachloride CCL₄ ≈88 ppt 1 400 26 Methyl chloroform CH₃CCl₃ ≈10 ppt 146 5 Sulfur hexafluoride SF₆ ≈7 ppt 22 800 3200

For a much longer time frame, data from the Vostok ice core provide an insight into the variations in CO₂ levels for the past four glacial–interglacial cycles. In January 1998, the collaborative ice-drilling project between Russia, the United States, and France at the Russian Vostok station in East Antarctica yielded the deepest ice core ever recovered, reaching a depth of 3623 m (Petit et al. 1999). Data from this project are displayed in Figure 1.1. One conclusion from the authors was that present-day atmospheric burdens of CO₂ seem to have been unprecedented during the past 420 000 years. Note that the mean resolution for the CO₂ measurements was about 1500 years.

1.3 Natural Accumulations and Emissions of CO₂

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