CARBON DIOXIDE CAPTURE AND STORAGE

Approximately one third of all CO₂ emissions due to human activity come from fossil fuels used for generating electricity, with each power plant capable of emitting several million tones of CO₂ annually. A variety of other industrial processes also emit large amounts of CO₂ from each plant, for example oil refineries, cement works, and iron and steel production. These emissions could be reduced substantially, without major changes to the basic process, by capturing and storing the CO₂. Other sources of emissions, such as transport and domestic buildings, cannot be tackled in the same way because of the large number of small sources of CO₂.

  Carbon capture and storage (CCS) is an approach to minimize global warming by capturing carbon dioxide (CO₂) from large point sources such as fossil fuel power plants and storing it instead of releasing it into the atmosphere  CCS applied to a modern conventional power plant could reduce CO₂ emissions to the atmosphere by approximately 80-90% compared to a plant without CCS.

1.   INTRODUCTION

Carbon dioxide (CO₂) is a greenhouse gas that occurs naturally in the atmosphere. Human activities are increasing the concentration of CO₂ in the atmosphere thus contributing to Earth’s global warming. CO₂ is emitted when fuel is burnt – be it in large power plants, in car engines, or in heating systems. It can also be emitted by some other industrial processes, for instance when resources are extracted and processed, or when forests are burnt.Currently, 30 Gt per year of CO₂ is emitted due to human activities.The increase in concentration of carbon in the past two hundred years is shown in the Fig

2.   THE CHARACTERISTICS OF CCS

Capture of CO2 can be applied to large point sources. The CO2 would then be compressed and transported for storage in geological formations, in the ocean, in mineral carbonates2, or for use in industrial processes. Large point sources of CO2 include large fossil fuel or biomass energy facilities, major CO2-emitting industries, natural gas production, synthetic fuel plants and fossil fuel-based hydrogen production plants (see Table 3.1).

Potential technical storage methods are: geological storage (in geological formations, such as oil and gas fields, unminable coal beds and deep saline formations3), ocean storage (direct release into the ocean water column or onto the deep seafloor) and industrial fixation of CO2 into inorganic carbonates. This report also discusses industrial uses of CO2, but this is not expected to contribute much to the reduction of CO2emissions.  

3.    SOURCES OF CO₂ EMISSIONS SUITABLE FOR CAPTURE AND STORAGE

Several factors determine whether carbon dioxide capture is a viable option for a particular emission source:

•     The size of the emission source,
•     Whether it is stationary or mobile,
•     How near it is to potential storage sites, and
•     How concentrated its co₂ emissions are.

            Carbon dioxide could be captured from a large stationary emission sources such as a power plants or industrial facilities that produce large amounts of carbon dioxide. If such facilities are located near potential storage sites, for example suitable geological formations, they are possible candidates for the early implementation of CO₂ capture  and storage (CCS).

Small or mobile emission sources in homes, businesses or transportation are not being considered at this stage because they are not suitable for capture and storage

Process Number of sources Emissions (MtCO2 yr-1) Fossil fuels Power 4,942 10,539 Cement production 1,175 932 Refineries 638 798 Iron and steel industry 269 646 Petrochemical industry 470 379 Oil and gas processing N/A 50 Other sources 90 33 Biomass Bioethanol and bioenergy 303 91 Total 7,887 13,466 Table 4.1 Profile by process or industrial activity of worldwide large stationary CO2 sources with emissions of more than 0.1 MtCO2 per year.

In 2000, close to 60% of the CO₂ emissions due to the use of fossil fuels were  produced by large stationary emission sources, such as power plants and oil and gas extraction or processing industries (see Table 3.1).

Four major clusters of emissions from such stationary emission sources are: the Midwest and eastern USA, the northwestern part of Europe, the eastern coast of China and the Indian subcontinent (see Figure 4.2).

4.     CO₂ CAPTURE 

The purpose of CO₂ capture is to produce a concentrated stream of CO₂ at high pressure that can readily be transported to a storage site. Although, in principle, the entire gas stream containing low concentrations of CO₂ could be transported and injected underground, energy costs and other associated costs generally make this approach impractical. It is therefore necessary to produce a nearly pure CO₂ stream for transport and storage. Applications separating CO₂ in large industrial plants, including natural gas treatment plants and ammonia production facilities, are already in operation today. Currently, CO₂ is typically removed to purify other industrial gas streams. Removal has been used for storage purposes in only a few cases; in most cases, the CO₂ is emitted to the atmosphere. Capture processes also have been used to obtain commercially useful amounts of CO₂ from flue gas streams generated by the combustion of coal or natural gas. However, there have been no applications of CO₂ capture at large (e.g., 500 MW) power plants.

Three systems are available for power plants: post-combustion, pre-combustion, and oxy fuel combustion systems. The captured CO₂ must then be purified and compressed for transport and storage.

CONCLUSION

            Large reductions in emissions of CO₂ to the atmosphere are likely to be needed to avoid major climate change. Capture and storage ofCO₂, in combination with other CO₂ abatement techniques, could enable these large reductions to be achieved with least impact on the global energy infrastructure and the economy.  Capture and storage is particularly well suited to use in central power generation and many energy-intensive industrial processes. CO₂ capture and storage technology also provides a means of introducing hydrogen as an energy carrier for distributed and mobile energy users.

 

            For power stations, the cost of capture and storage is about $50/t ofCO₂ avoided. This compares favorably with the cost of many other options considered for achieving large reductions in emissions. Use of this technique would allow continued provision of large-scale energy supplies using the established energy infrastructure.  There is considerable scope for new ideas to reduce energy consumption and costs of CO₂ capture and storage which would accelerate the development and introduction of this technology