Carbon capture and storage.

Abstract

Human activities are now responsible for the annual emission of some 31.6 billion tonnes of dioxide, which contains 8.6 billion tonnes of carbon, causing the total atmospheric burden of C[H.sub.2] to reach 400 ppm, above the Arctic, and 395 ppm globally (1). In order to mitigate the rise in C[H.sub.2] concentration and ideally to reduce it, various methods for capture and storage (2) (CCS) have been proposed, also known as carbon capture and sequestration. To this amount of C[H.sub.2] entirely would require phenomenal levels of engineering since around 87 million tonnes per day of C[H.sub.2] would need to be captured. There are essentially two methods to remove from fuel: post-combustion and precombustion. In post-combustion treatment, the flue-gas of a power station is passed through a liquid amine (e.g. ethanolamine and its derivatives) which dissolves the C[O.sub.2] from it. In the pre-combustion approach, the fuel (coal, gas, biomass) is processed into a mixture of C[O.sub.2] + [H.sub.2] and the C[O.sub.2] is removed: initially, a mixture of CO + [H.sub.2]O is produced [Eqn (1)] but the CO is converted to C[O.sub.2] by reaction with [H.sub.2]O, squeezing-out another molecule of [H.sub.2] in the process [water-gas shift reaction; Eqn (2)]. Following either method, the C[O.sub.2] must be stored somewhere (Figure 1), for which strategies include pumping it into rocky formations (such as depleted oil and gas wells) at a pressure of 100 atmospheres, or even piping it in liquid form under pressure onto the sea-floor where it is cold enough and the pressure high enough that it is hoped the material will stay there, assisted by the formation of C[O.sub.2]-hydrate. These and other aspects of CCS are now elaborated upon. [FIGURE 1 OMITTED] Capture While the extraction of C[O.sub.2] from the air is technically possible, the gas is most readily captured at point sources (2) e.g. large fossil fuel or biomass power plants, industries with major C[O.sub.2] emissions such as cement factories, natural gas processing plants, and hydrogen production plants which generate syngas by steam-reforming [Eqn (1)] and convert the CO to C[O.sub.2] using the water-gas shift reaction [Eqn (2)]: C[H.sub.4] + 2[H.sub.2]0 [right arrow] CO + 3[H.sub.2] (1) CO + [H.sub.2]O [right arrow] C[O.sub.2] + [H.sub.2] (2) As the distance from the point source increases, the concentration of C[O.sub.2] falls rapidly which necessitates an increase in the amount of air that must be processed per unit mass of C[O.sub.2] captured. The combustion of coal in oxygen produces a relatively pure, concentrated stream of C[O.sub.2], which might be processed directly. Organisms that produce ethanol by fermentation generate cool, and practically pure, C[O.sub.2] which is suitable for underground storage. World ethanol production in 2008 was close to 16 billion US gallons, which at a density of 789.00 kg [m.sup.-3] amounts to 61,000,000 [m.sup.3] or 48 million tonnes of the material (3). There are essentially three principal methods for capturing C[O.sub.2]: post-combustion, pre-combustion, and oxyfuel combustion: * In post-combustion capture, the C[O.sub.2] is removed following combustion of the fossil fuel, i.e. the scheme that would be used to reduce emissions from fossil-fuel fired power plants. The technology is well understood and is used in other industrial applications, although some considerable scale-up would be required to deal with the C[O.sub.2] output from a standard (e.g. I GW) power station, which might burn 3.5 million tonnes of coal per year. * Pre-combustion capture might be accomplished using the kind of technology that is used to make (ammonia for) fertiliser, and fuels ([H.sub.2], C[H.sub.4]),and for power production (4).The reactions described in Eqns (1) and (2) prevail, and the final C[O.sub.2] can be extracted from a relatively pure exhaust stream, leaving [H. …