Studies regarding the C[O.sub.2] recovery from the atmosphere.

Tokar, Adriana ; Negoitescu, Arina ; Mihon, Liviu 等

1. INTRODUCTION

The sun is the Earth's primary energy source, providing heat from over 150 million km away. Its rays enter our atmosphere and shower upon on our planet. About one third of this solar energy is reflected back into the universe by shimmering glaciers, water and other bright surfaces. Two thirds, however, are absorbed by the Earth, warming the land, oceans, and atmosphere. Much of this heat radiates back out into space, but some of it is stored in the atmosphere, process called the greenhouse effect. Without it, the Earth's average temperature would be a chilling -18 C[degrees], even despite the sun's constant energy supply.

Studies indicate that until some billion years ago, there was so much carbon dioxide (C[O.sub.2]) and methane in our atmosphere that average temperatures on Earth were as high as 70 C[degrees]. But bacteria and plants slowly turned C[O.sub.2] into oxygen and the concentration of C[O.sub.2] in our current atmosphere dropped to just about 383 parts per million (ppm), a unit of measurement used for very low concentrations of gases that has become a kind of currency in climate change debates (Easterling, 1997).

2. SOLUTIONS FOR C[O.sub.2] REDUCTION

Investments in research on global warming have resulted in different solutions on neutralization of C[O.sub.2] emissions. Each inhabitant of the planet can contribute financially to the projects development for C[O.sub.2] emissions reduction. Drivers and air flights passengers can help to neutralize carbon emissions by paying a fee for each ton of C[O.sub.2] emitted depending on the mileage. The largest sources are vehicles and non-nuclear power stations--that burn coal, oil or gas, otherwise known as fossil fuels. In Figure 1 are presented the main sectors which generate C[O.sub.2]. Natural gas vehicles are one alternative mode of transportation that is helping to reduce C[O.sub.2] emissions, protect public health and the environment as is presented in Figure 2.

World C[O.sub.2] emissions are expected to increase by 1.9% annually between 2001 - 2025. Developing countries emissions are expected to grow above the world average at 2.7% annually between 2001 - 2025 and surpass emissions of industrialized countries around 2018 as it is shown in Figure 3.

Hybrid Cars, are helping to bridge the gap between conventional gasoline engines and the cleaner hydrogen vehicles of the future.

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Hybrids clearly cut fuel consumption, particularly in city traffic. With only a few hundred thousand vehicles, the proportion of hybrid vehicles in the global market is still quite small. By 2015, hybrids could account for up to 2 percent of all cars in Europe-rapid growth from a modest starting point (www.knowlidge.allianz.com, 2003).

3. METHODS FOR C[O.sub.2] REDUCTION AND STORAGE

To prevent the carbon dioxide building up in the atmosphere (possibly causing global warming and definitely causing ocean acidification), we can catch the C[O.sub.2], and store it. The best place to capture CO2 is at the major sources of emissions.

The power-plant emissions, called flue gases, are typically only 10% to 15% CO2 in a coal-burning plant and about 5% when natural gas is the fuel. For the CO2 to be stored efficiently, it first has to be separated from the other flue gases.

There are three strategies: separate the CO2 after combustion, extract carbon of the fuel before combustion so that only hydrogen burns and produce only water, burn fossil fuels in oxygen rather than air, resulting in concentrated C[O.sub.2] (http://eia.doe.gov, 2005).

Chemical solutions can be used to dissolve the CO2 while passing the other gases to the atmosphere. The technique most widely used today employs a group of compounds called amines. They absorb CO2by forming chemical bonds, particularly at high pressure and low temperature. The resulting chemical solution is later heated and the pressure reduced, releasing concentrated CO2. Other solvents are used to dissolve CO2 without chemical bonding. In this physical absorption process the CO2 dissolves under pressure and is later removed from the solvent by reducing the pressure. The solvent may then be reused. Another strategy for capturing CO2 is to cool the flue gases to the point where the CO2 becomes liquid. This process requires considerable energy for refrigeration. An advantage is that the liquid can be easily transported by truck or ship. It is also possible to separate gases by using thin films called membranes. Some gases will pass through a membrane faster than others. This allows the different gases to be separated from one another.

Getting the Carbon Out before Combustion. Methane in natural gas produces by burning both C[O.sub.2] and water ([H.sub.2]O). If carbon can be taken out before combustion it will result hydrogen, which produces only water during burning. So, carbon monoxide (CO) and hydrogen will result from fuel with oxygen and/or steam reaction. Than the carbon monoxide reacts with more steam to produce C[O.sub.2] and more hydrogen. Finally, the C[O.sub.2] is separated and the hydrogen is used as fuel in a gas turbine (http://co2capturestorage.inf, 2009).

Once concentrated C[O.sub.2] has been captured, the next step is to store it somewhere, Figure 2. The options are as it follows:

--Geological Formations. Storage in geological formations is currently the most promising solution for widespread, long-term sequestration of CO2. Some projects are already under way. In order to reduce greenhouse gases and global warming, stored CO2 must be kept out of the atmosphere for hundreds or thousands of years. Oil and natural-gas reservoirs, deep saltwater aquifers, and coal seams have existed for millions of years with only very gradual changes. There is strong evidence that if properly managed, these formations could provide for long-term storage of CO2.

--Depleted Oil and Natural-Gas Reservoirs. Oil and natural gas are found these in permeable and porous rocks such as sandstone. These rocks contain microscopic spaces, called pores, which fill with fluids. The fluids may be water, oil, or natural gas. An oil or natural-gas reservoir is more like a sponge than a bottle. Once an oil or natural-gas field has been productive for a period of time, a good portion of the hydrocarbons has been removed. There is space available to store CO2. The porous and permeable rock layer that contains these fluids is covered by an impermeable cap rock that does not let them pass through. Normally, oil and natural gas will tend to migrate upward through permeable rock because they are lighter than the water that is also found in such rock formations. The cap rock traps them. Since oil and natural gas have been sequestered in such formations for millions of years, there is good reason to believe that CO2 will remain there as well.

--Enhanced Oil Recovery. Much of the technology needed to store C[O.sub.2] in oil fields is already being used for a process known as enhanced oil recovery (EOR).

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When a reservoir is newly tapped, the oil is typically under pressure and flows freely to the surface. As oil is removed, pressure drops and pumping is needed to recover more.

At some point recovery becomes uneconomical and is stopped, or additional techniques are used to extract more oil. One approach is to pump C[O.sub.2] into the reservoir. This increases pressure so the oil flows more readily. Also, the C[O.sub.2] dissolves in the oil and causes it to become less viscous and flow more easily. It expands in volume as well, further increasing pressure. C[O.sub.2] is pumped into the reservoir through an "injection well." This forces the oil toward a "production well," where it rises to the surface (Aresta, 2003).

4. CONCLUSION

While we are still far from seeing major concentrations of C[O.sub.2] in our atmosphere, slight changes already alter the way our celestial heating system works. Measurements of carbon dioxide amounts show that C[O.sub.2] has increased from about 313 ppm in 1960 to about 383 ppm at present. That means for every million particles in our atmosphere, there are now 70 C[O.sub.2] particles more than in 1960. Even if this does not seem like much, scientists say this increase, most probably caused by human activities, is mainly responsible for rising global temperatures throughout the last decades. Figure 5 shows that at the end of the century, temperatures would increase by up to 3 [degrees]C in Europe.

Three different types of technologies for capturing C[O.sub.2] exist: post-combustion, pre-combustion, and oxyfuel combustion. These can be applied to large point sources, such as large fossil fuel or biomass energy facilities, major C[O.sub.2] emitting industries, natural gas production, synthetic fuel plants and fossil fuel-based hydrogen production plants. Although in Romania there is no industrial applicable system for reducing and capturing the C[O.sub.2] emissions yet, romanian researchers must become interested in these technologies implementation and development.

5. REFERENCES

Aresta, M. (2003). Carbon Dioxide Recovery and Utilization, Kluwer Academic Publishers, ISBN 1-4020-1409-0, Netherland

Easterling, D.R., et al. (1997). Maximum and minimum temperature trends for the globe. Science, 277, pp. 364-367

*** (2009) http://co2capturestorage.inf--IEA Greenhouse Gas R&D Programme, Accesed on: 2009-04-13

*** (2005) http://eia.doe.gov--Energy Information Administration (EIA), Accesed on: 2009-02-14

*** (2003) www.knowlidge.allianz.com--Allianz Center for Technology in Germany, Accesed on: 2009-03-24

Fig. 1. Global Carbon Dioxide Production by Sector

Industry 22%
Transport 23%
Electricity Generation 39%
Agriculture 2%
Other 4%
Residential 10%

Source: Energy Information Ad
http://eia.doe.gov

Note: Table made from pie chart.