Carbon dioxide capture

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Capture and storage removes carbon dioxide from the gas streams that would otherwise be emitted to the atmosphere, and transfers it for indefinite long term storage in geological reservoirs, such as depleted oil and gas fields or deep saline aquifers. In the energy sector, candidates for carbon dioxide capture and storage undertakings include large stationary sources such as power stations and natural gas sweetening units. This chapter deals only with CO2 capture associated with combustion activities, particularly those relative to power plants. Fugitive emissions arising from the transfer of carbon dioxide from the point of capture to the geological storage, and emissions from the storage site itself, are covered in Chapter 5 of this Volume. Other possibilities also exist in industry to capture CO2 from process streams. These are covered in Volume 3.

There are three main approaches for capturing CO2 arising from the combustion of fossil fuels and/or biomass (Figure 2.5). Post-combustion capture refers to the removal of CO2 from flue gases produced by combustion of a fuel (oil, coal, natural gas or biomass) in air. Pre-combustion capture involves the production of synthesis gas (syngas), namely the mixture of carbon monoxide and hydrogen, by reacting energy feedstocks with steam and/or oxygen or air. The resulting carbon monoxide is reacted with steam by the shift reaction to produce CO2 and more hydrogen. The stream leaving the shift reactor is separated into a high purity CO2 stream and H2-rich fuel that can be used in many applications, such as boilers, gas turbines and fuel cells.

Oxy-fuel combustion uses either almost pure oxygen or a mixture of almost pure oxygen and a CO2-rich recycled flue gas instead of air for fuel combustion. The flue gas contains mainly H2O and CO2 with excess oxygen required to ensure complete combustion of the fuel. It will also contain any other components in the fuel, any diluents in the oxygen stream supplied, any inert matter in the fuel and from air leakage into the system from the atmosphere. The net flue gas, after cooling to condense water vapour, contains from about 80 to 98 percent CO2 depending on the fuel used and the particular oxy-fuel combustion process.

Figure 2.5 CO2 capture systems from stationary combustion sources

Oil Coal Gas Biomass

Oil Coal Gas Biomass

Power & Heat

Power & Heat

CO2 Separation

Post-combustion

Compression Dehydration

Gas - Light hydrocarbons

Oil Coal Gas Biomass

Oil Coal Gas Biomass

Gas - Light hydrocarbons

Oil Coal Gas Biomass

Reforming

Air/O2 Steam

Air/O2 Steam

Reforming

Partial Oxidation / Gasification

Syngas

Syngas

(CO2)

Power & Heat

02

Air Separation

n2

Compression Dehydration

Partial Oxidation / Gasification

Compression Dehydration

H2-rich fuel

Shift

Syngas

reactor

CO2 Separation

H2-rich fuel

Power & Heat

Compression Dehydration

Pre-combustion

Power & Heat

Compression Dehydration

Oxyfuel combustion

Carbon dioxide capture has some energy requirements with a corresponding increase in fossil fuel consumption. Also the capture process is less than 100 percent efficient, so a fraction of CO2 will still be emitted from the gas stream. Chapter 3 of the IPCC Special Report on CO2 Capture and Storage (Thambimuthu et al, 2005) provides a thorough overview of the current and emerging technologies for capturing CO2 from different streams arising in the energy and the industrial processes sectors.

The general scheme concerning the carbon flows in the three approaches for capturing CO2 from streams arising in combustion processes is depicted in Figure 2.6. The system boundary considered in this chapter includes the power plant or other process of interest, the CO2 removal unit and compression/dehydration of the captured CO2 but does not include CO2 transport and storage systems. This general scheme also contemplates the possibility that pre-combustion capture systems can also be applied to multi-product plants (also known as polygeneration plants). The type of polygeneration plant considered in this chapter employs fossil fuel feedstocks to produce electricity and/or heat plus a variety of co-products such as hydrogen, chemicals and liquid fuels. In those processes associated with post-combustion and oxyfuel combustion capture systems, no carbonaceous co-products are typically produced.

Figure 2.6

Carbon flows in and out of the system boundary for a CO2 capture system associated with stationary combustion processes

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