In order to sequester carbon dioxide it first has to be extracted from the flue gases of power stations, cement works and refineries in a process known as ‘carbon capture and storage’. Carbon dioxide capture requires a relatively pure carbon dioxide stream for transport and storage. In some cases the carbon dioxide needs to be compressed, as mineral carbonization requires high pressures. There are three main methods of capturing carbon dioxide.

The first is post-combustion or process removal using absorption, adsorption, cryo­genic and membrane technologies. Adsorption uses amine-based solvents or cold metha­nol. Carbon dioxide can also be removed by passing over activated charcoal or through special membranes (Feron and Jansen, 1995; Grimston et al., 2001; Gronkvist et al, 2006). Advanced methods are adsorption on to zeolites or activated carbon fibres.

The second is pre-combustion gasification of coal in a power station which pro­duces a mixture of carbon monoxide and hydrogen. If this mixture is treated with steam, the carbon monoxide is converted to carbon dioxide and hydrogen. The hydrogen on combustion in the power station forms water and the carbon dioxide and water can easily be separated:

Gasification = CO + H2 (3.3)

Water shift reaction CO + H2 + H2O = H2 + CO2 (3.4)

The third method is to use oxygen in the combustion process, which produces a flue gas of carbon dioxide and water. An alternative to oxygen addition is to use a metal oxide as an oxygen source, known as chemical looping.

Some examples of carbon dioxide sources suitable for carbon capture and storage are given in Fig. 3.11. Cement production is one example where carbon dioxide can be sequestered, and should be as cement production contributes 6-7% of the total global carbon dioxide released into the atmosphere. Cement factories are stationary and hence suitable for retrofitting of carbon dioxide sequestration. In cement produc­tion, calcium carbonate (limestone) is converted to lime (CaO) in a rotary kiln, releas­ing carbon dioxide. The lime is then heated at around 1450°C to form clinker, which when mixed with 5% gypsum forms cement (Gronkvist et al., 2006). One tonne of cement releases about 500 kg carbon dioxide in its production. In addition, carbon dioxide is released from the fuel used to heat the kiln and has been estimated to be 275 kg carbon dioxide per tonne of lime, giving a total of 775 kg carbon dioxide produced per tonne of cement.

Whatever the process producing carbon dioxide, once the carbon dioxide has been separated it needs to be stored to keep it from reaching the atmosphere. The carbon dioxide can be stored using one of the following systems (Fig. 3.11):

1. Store underground in oil and gas reservoirs, deep saline aquifers, coal beds, active and uneconomical oil and gas reservoirs.

2. Hold in deep non-mineable coal formations and coal bed methane formations.

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 con­tain 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.

• Liquid carbon dioxide injected at depth of 1000 m from a ship or ocean-bottom manifold forming a rising droplet plume.

• A dense carbon dioxide-seawater mixture formed at a depth of 500-1000 m forming a sinking current.

• Liquid carbon dioxide introduced into a sea bed depression forming a stable lake at a depth of below 4000 m.