Carbon sequestration

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If carbon capture is to provide a successful strategy for removing carbon dioxide from coal-burning power plants then a means of storing the resultant gas must be found. The amount of carbon dioxide produced by power plants across the globe is approximately 10 billion tonnes each year (Science Daily 2007). The USA alone produces 1.4 billion tonnes, equivalent to approximately one-third of the natural gas piped around the USA annually (Science Daily 2007). As with carbon capture, the technology to transport and store carbon dioxide is already available and has been tested in three pilot-scale projects. These include injection of carbon dioxide from a coal gasification plant in the USA into an oilfield at Weyburn, Canada where it is used to force additional oil from the field (enhanced oil recovery), injection into a sandstone reservoir in the Saharan region of Algeria and sequestration of carbon dioxide separated from natural gas from the Norwegian Sleipner gas field into a brine aquifer under the North Sea. This latter was a response to Norway's carbon tax. None of these projects stores the quantity of carbon dioxide that would be produced by a base load coal-fired power plant and the technology has yet to be proved at this scale.

If a significant proportion of the carbon dioxide from coal combustion is to be sequestered, then very large stores will have to be identified. The most promising and cheapest of these available today are exhausted oil and gas wells. Such sites will provide cheap and convenient places to test the viability of carbon dioxide sequestration but they cannot, alone, provide anything like the capacity needed if sequestration is to have a major impact on global emissions (Ansolabehere et al. 2007). Fortunately, there are many other types of geological formation and these, between them, should be able to accommodate all the carbon dioxide that is likely to be sequestered over the next 50 years.

Sequestration, however, is only part of the problem. The sequestered carbon dioxide must remain isolated indefinitely if the capture and storage strategy is to be effective. This means that all sequestration sites will have to be monitored for decades, probably longer. While these sites will almost certainly be operated initially by private sector companies, the responsibility for their security is likely, eventually, to fall to national governments. It is important that this should be understood from the outset if the strategy is to be effective.

5.3.4 Costs

While the efficiency gains with supercritical and ultra-supercritical coal-fired plants tend to balance the increased cost of the high-technology boiler systems, the introduction of capture and sequestration will have a significant effect on the cost of electricity generated from coal. The loss of efficiency, even without the costs associated with running a capture plant and the transportation and storage of carbon dioxide, will push prices up by approximately 25 per cent.

The capital cost of a new subcritical coal-fired plant without carbon capture is $1323 per kW, according to a study carried out jointly by the US New Energy Technology Laboratory and Parsons (Ciferno et al. 2006). The same plant with carbon capture would cost $2358 per kW. Meanwhile, a new supercritical coal plant without capture would cost $1355 per kW and with capture, $2365 per kW. The same study found a new IGCC plant without capture would cost $1557 per kW and with capture, $1950 per kW. Meanwhile, MIT reported the cost of a new ultra-supercritical coal-fired plant to be $1360 per kW, rising to $2090 per kW with capture (Ansolabehere et al. 2007). The same study put the cost of an oxy-fuel plant to be $1900 per kW.

Table 5.4 presents estimates of Ansolabehere et al. (2007) for the cost of electricity from these various power plant configurations. Figures from a further study by NETL-Parsons, while not quoted here, are broadly consistent with them for the configurations studied. For the plants without capture, the cost of electricity for the conventional coal-burning plants varies from $0.0478 to $0.0496 per kWh or just under 4 per cent, which is probably insignificant. The IGCC plant with an estimated generating cost of $0.0513 per kWh is higher. When carbon capture is

Table 5.4 Cost of electricity from different coal-fired power plant configurations

Type of plant

Without carbon capture ($ per kWh)

With carbon capture ($ per kWh)

Subcritical

0.0484

0.0816

Supercritical

0.0478

0.0769

Ultra-supercritical

0.0496

0.0734

IGCC

0.0513

0.0652

Oxy-fuel combustion

-

0.0698

Source: Ansolabehere et al. (2007).

Source: Ansolabehere et al. (2007).

added, however, the IGCC plant becomes the cheapest generator with a cost of $0.0652 per kWh, an increase of 27 per cent over the cost without capture. Oxy-fuel combustion also looks competitive on this estimate, at $0.0698 per kWh, while of the conventional coal-burning plants with carbon capture the ultra-supercritical plant offers the most cost-effective means of generation, producing electricity for $0.0734 per kWh, 48 per cent higher than the cost without capture. The most expensive configuration with capture is the subcritical plant with a cost of $0.0816 per kWh, 69 per cent more than the same plant without capture.

The cost of transportation and storage must be added to the costs in Table 5.4. This will depend on the type of storage and the distance of the storage site from the power plant. Typical estimates put the cost at between $1 and $8 per tCO2 (Breeze 2006).

When evaluating these figures with a view to forming future strategy, there are two further points to bear in mind. First, while the IGCC plant appears to offer the cheapest source of electricity with carbon capture, this type of plant is still relatively new and so far its reliability has proved lower than that of the more conventional boiler-based plants (Ansolabehere et al. 2007). Second, supercritical and ultra-supercritical plants built today will be able to be retrofitted with carbon capture technologies at a later date. This makes less sense for an IGCC plant as a result of the tight integration between the different plant components necessary at the time of construction and could make the economics of the ultra-supercritical plant the most favourable.

These figures can be placed in perspective by comparing the estimated cost of power from that of various other technologies. The cost of electricity from a new nuclear power plant is likely to be between $0.030 and $0.067 per kWh. A new large hydropower plant can generate for $0.040-0.080 per kWh and an onshore wind plant might be expected to produce power for between $0.060 and $0.090 per kWh (Breeze 2007b).

All these figures suggest that alternatives to coal-fired power generation with carbon capture might be more cost-effective. But, as has already been stressed, these alternatives cannot replace coal-fired generation in the short or medium term. So, while a shift to coal-fired generation with carbon capture may well offer future economic opportunities for a range of other technologies, this shift is still necessary in the interests of the planet.

There is one final question: how is this shift to be achieved? Two things are required. The first is investment, primarily from Western governments, to develop the technologies for carbon capture and storage to a state where they can be deployed economically on a wide scale. The second is the introduction of global carbon emission limits, with cost penalties for emitting carbon that are sufficiently stringent to persuade generators across the globe to build plants based on these technologies. Both are within grasp, but, as the arguments at the UN conference in Bali in December 2007 showed, there are some hard bargains to be struck if consensus is to be reached. If, indeed, such a consensus can be achieved then there is no reason why carbon emissions from coal combustion should not fall significantly by 2050 even while the amount of coal burnt continues to rise. That may not be the solution sought by many environmentalists but it does provide a realistic route towards a carbon-free energy economy. If that can prevent catastrophic global warming, then there is no obvious reason why it should not be pursued.

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