Sequestration of Carbon Dioxide
3. Store in deep oceans
The storage underground could be part of enhanced oil recovery (EOR). At present 80% of recovered carbon dioxide is used in EOR and about 70 oil fields use this worldwide, sequestering some 31 million m3 of carbon dioxide per day. The retention times and capacities for carbon sequestration are given in Table 3.6.
The storage in the deep oceans has a number of possibilities, as the oceans con- tain 40,000 Gt carbon compared with 780 Gt in the atmosphere. Thus, the oceans are an immense carbon sink where captured carbon dioxide could be stored, and the options to put it in the oceans are as follows (Grimston et al., 2001):
Dry ice released into the sea from ships.
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Liquid carbon dioxide injected at depth of 1000 m from a ship or ocean-bottom
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manifold forming a rising droplet plume.
A dense carbon dioxide–seawater mixture formed at a depth of 500–1000 m
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forming a sinking current.
Liquid carbon dioxide introduced into a sea bed depression forming a stable lake
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at a depth of below 4000 m.
Forestry
All plants fix carbon dioxide during photosynthesis, which is released again when the plant dies and the plant material is degraded by microorganisms. The carbon dioxide fixed is used to synthesize storage compounds such as starch and oils, and cellular struc- tural components such as cellulose and lignin. It is the structural components that are the slowest to degrade when the plant dies and these are the highest in woody plants.
It has been estimated that the amount of carbon taken up by vegetation was 3.2 Gt C/year, and 1.7 Gt C/year is lost mainly through deforestation, which gives a net increase of 1.5 Gt C/year. The carbon emissions from fossil fuels are 6.4 Gt C/year so that without any mitigation measures, some 23% of the carbon dioxide is removed.
However, afforestation does not remove all the carbon dioxide produced and the atmospheric levels are still rising, but it does show the potential of biomass to seques- ter carbon dioxide. Table 3.7 indicates the potential of carbon dioxide sequestration by planting new forests, managing existing forests, managing crops, etc.
Woody plants have a carbon content of 0.54 kg carbon per kilogram of dry wood (Cook and Beyea, 2000). If tree growth is linear in the early years, the carbon dioxide removed can be calculated. The estimates for carbon dioxide sequestered by maize, switchgrass, short-rotation coppice willow and standing forest wood are shown in Table 3.8. Some of the carbon dioxide is only sequestered on a temporary basis as the switchgrass and short-rotation coppice will be burnt as a fuel, and other parts of the crops will be returned to the soil where they will be degraded. However, if the wood is used as construction material, the carbon dioxide will be locked up for considerably
Table 3.7. Potential for carbon dioxide sequestration by forests. (From Cannell, 2003.) Region Potential
Carbon storage capacity (Gt C)
World 50–100
Europe 5–10
UK 0.3–0.5
Carbon sequestration rates 50–100 years (Mt C/year)
World 1,000–2,000
Europe 50–100
UK 1–2
Table 3.6. Global capacity and residence time for the various carbon sinks.
(Adapted from Grimston et al., 2001.)
Sink Capacity (GtC) Retention time (years)
Oceans 1,000–10,000 Up to 1,000
Forestry 60–90 50
Agriculture 45–120 50–100
Enhanced oil recovery 20–65 10–50
Coal beds 80–260 >100,000
Oil and gas reservoirs 130–500 >100,000 Deep aquifers 30–650 >100,000
longer. The carbon dioxide sequestered in forest material is less because growth is slower and forest regeneration, once harvested, is not always certain. The world is losing areas of dense forests either to building or agriculture, and reversal of this trend would help to reduce carbon dioxide levels in the short term.
Microalgal sequestration
The use of microalgae to sequester carbon dioxide was proposed in the past by a number of authors (Benemann, 1997; Sheehan et al., 1998; Chisti, 2007; Skjanes et al., 2007).
Microalgae have been proposed as systems for the sequestration of carbon diox- ide (Sawayama et al., 1995; Zeiler et al., 1995; Ono and Cuello, 2006; Cheng et al., 2006; de Morais and Costa, 2007a,b) and the production of biofuels (Chisti, 2007).
The biofuels include biogas (methane) by anaerobic digestion of the biomass (Spolaore et al., 2006), biodiesel from microalgal oils (Nagle and Lemke, 1990; Minowa et al., 1995; Sawayama et al., 1995; Miao and Wu, 2006; Xu et al., 2006; Chisti, 2007), and biohydrogen (Fedorov et al., 2005), and the direct use of algae in emulsion fuels (Scragg et al., 2003).
Microalgae should be considered to have the following features:
Higher photosynthetic efficiency than terrestrial plants.
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Rapid growth rate, doubling times of 8–24 h.
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High lipid content 20–70%.
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Direct capture of carbon dioxide, 100 t algae fix ~183 t carbon dioxide.
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Can be grown on a large scale.
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Will not compete with terrestrial plants in food production.
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Produce valuable products.
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Freshwater and marine species.
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Have a much better yield of oil per hectare: oil palm 5000 t/ha, algae 58,700 t/ha
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(Chisti, 2007).
Figure 3.12 shows a possible system for carbon dioxide sequestration and biofuel production using carbon dioxide from a stationary carbon dioxide source such as a power station. Microalgae can fix large quantities of carbon dioxide but it is likely that only a proportion of the carbon dioxide in the flue gases will be removed. Also the flue gases from power stations contain other gases which may affect the growth of microalgae. A number of studies have been carried out on the effect of flue gases on microalgae. Nannochloris sp. was shown to grow in the presence of 100 ppm Table 3.8. Estimated carbon dioxide sequestration by various crops. (Adapted from Cook and Beyea, 2000.)
Carbon CO2 CO2
content sequestered Yield reduction
Biomass type (kg/kg; dry) (kg/kg) (t/ha) (t/ha/year)
Maize 0.4 0.3 15–20 5.4
Switchgrass 0.4 0.4 15–20 7.4
Short-rotation coppice 3-year rotation 0.54 0.55 10–15 7.4 Forest wood 100-year rotation 0.54 0.14 – –
nitric oxide (NO) (Yoshihara et al., 1996). Tetraselmis sp. could grow in flue gas containing 185 ppm sulfur oxides, 125 ppm nitrogen oxides and 14.1% carbon dioxide (Matsumoto et al., 1995). A Chlorella sp. was also found to grow in the pres- ence of various combinations of sulfur and nitrogen oxides (Maeda et al., 1995).
Chemical sequestration
Chemical sequestration involves the conversion of carbon dioxide to inorganic car- bonates, and the use of carbon dioxide in the production of urea and plastics.
Agricultural practices
Agriculture uses a very large area of land and because of this size it is responsible for the emission of large quantities of greenhouse gases. Agriculture has been regarded as responsible for 25% of carbon dioxide, 50% of methane and 70% of the nitrous oxide released (Hutchinson et al., 2007). The World Resources Institute (2006) gives the values of 27% carbon dioxide, 53% methane and 75% nitrous oxide. The green- house gas emissions from agricultural sources are given in Table 3.9. These emissions arise from the following:
Fossil fuel use in cultivation, harvesting, etc.
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Nitrogen fertilizer use.
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Heat CO2
Algal pond
Biocoil
Algal biomass
Biodiesel
Fig. 3.12. Possible sequestration of carbon dioxide from a power station and use of algal biomass to produce biodiesel.
Rice production.
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Deforestation.
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Livestock (enteric fermentation).
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Manure.
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Mitigation of greenhouse gases, sometimes known as stabilization, may require meth- ods that are expensive, therefore low-cost options have been investigated. Agriculture offers a number of low-cost technologies that include:
Altering land use.
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Changing cultivation methods.
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Better management of livestock.
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Altering crop mix and fertilization methods.
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Expanding the production of biofuels.
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An example of the changes that can be made to carbon dioxide sequestration is shown in Table 3.10. A number of agricultural methods have been shown to increase the retention of carbon in soils for Canadian Prairies (Hutchinson et al., 2007).
Another example of changes in agricultural methods that reduce greenhouse gas emis- sions is the anaerobic treatment of slurry and liquid manure. Rather than placing liquid manure on the land or retaining it in lagoons where methane and nitrous oxide are produced, it can be anaerobically digested. Slurry or liquid manure anaerobically digested produces a mixture of methane (CH4) and carbon dioxide (CO2) which can be use as a gaseous fuel (Clemens et al., 2006).