Carbon capture and sequestration
Mar 07 2013
Given the global-warming problem caused by CO2 accumulation, one promising option is to is to use carbon capture and storage (CCS). CCS refers to the process of capturing carbon dioxide (CO2) from large-scale emission point sources, for example, fossil fuel-powered electricity-generating stations or industrial plants, and depositing the gas in geological formations or the deep ocean for long-term storage. Thus, it involves three processes: separating and capturing the CO2, transporting it to a suitable storage location, and permanently storing it.
The CCS technology appears to be promising indeed. Firstly, fossil fuels could be consumed on a large scale while emitting far fewer greenhouse gases to the atmosphere. Second, the CCS technology can be integrated with existing electricity generation and distribution infrastructure. Third, the CCS technology has already been demonstrated to work in some applications. However, the CCS technology has also posed a number of challenges.
Firstly, the technology has yet to be deployed on a commercial basis with fossil fuel power stations. Second, it is likely to be relatively expensive and is unlikely to be adopted on a large scale unless a significant CO2 cost penalty is introduced, particularly in relation to retrofitting existing plants. Third, it is a technology that has a relatively long time horizon before it could make a large contribution to mitigation efforts. Fourth, the location of existing fossil fuel plants is often a considerable distance from suitable storage sites. Finally, the environmental risks of storing large quantities of CO2 in geological and ocean reservoirs have yet to be fully assessed.
CO2 capture can be done either prior to combustion or by separation from the industrial gas stream after combustion. Pre-combustion removal is possible with some processes (for example, hydrogen production facilities) and with new, advanced coal-based generating technologies (for example, integrated gasification combined cycle technologies — IGCC). Post-combustion removal requires separation of CO2 from other waste gases (mainly nitrogen), as CO2 usually only accounts for 5-15 per cent of the flue gas. Both pre- and post- combustion processes can remove about 85-95 per cent CO2 before it is released into the atmosphere; however, the removal process is energy intensive and thus costly at present. To a ball park estimate, a standard coal-based power plant will consume 20-40 per cent more energy per unit of electrical output than a similar plant without the CCS. Based on the most recent IPCC analysis, the cost per unit of electricity generated from a standard coal-based plant fitted with CCS technology is expected to be 40-85 per cent higher than the same plant without CCS (This equates to a cost of $25-50 for each tonne of CO2 avoided). Once the CO2 has been captured it needs to be transported to its storage site. There is a large variability in this cost depending upon the distance of the storage site from a power plant. For plants that are located adjacent to a suitable storage site, the costs would be small, generally less than $1/tonne. For distances of up to 1,000–1,500 km, pipelines are expected to be the most cost-effective option, costing approximately $10–15/tonne of CO2 transported. Rail or road transport is technically possible, but would prove to be much more expensive than the pipeline option.
The third and final stage in the CCS is the actual long-term storage at a suitable site. The three principal storage options are: (1) injection into suitable geological reservoirs (cost: $1-8 per tonne of CO2); (2) storage in the deep ocean (cost: $6-30 per tonne of CO2 and still to be tried commercially); and, (3) conversion of CO2 into stable mineral carbonates (cost: $50-100 per tonne of CO2). Thus, currently, the first option appears to be the most feasible.
What about the leakage and environmental risks? The risks of leakage from well-managed geological storage sites are considered low in probability, probably less than one pre cent over periods of up to 1,000 years where the cost of inspection and verification can be as low as $1 per tonne of CO2 stored. However, the risks of leakage from ocean storage are much more significant. Over time periods spanning several centuries, deep ocean waters mix with surface waters. If CO2-rich deep-ocean water were to reach the surface, stored CO2 would be released to the atmosphere. The IPCC has calculated that for CO2 injected at a depth of 1,500 m, 25 per cent of the stored CO2 could subsequently be released to the atmosphere within two centuries, and as much as 50 per cent in four centuries. At 3,000 m, the leakage rate is estimated to be 20 per cent over four centuries.
CO2 is not flammable and only becomes a health risk (from asphyxiation) when air concentrations exceed 10 per cent by volume. However, there could be local environmental hazards associated with sudden large-scale releases, particularly from ruptured pipelines or well blow-outs. Again, with ocean storage, there are potential adverse ecological impacts. CO2 is a slightly acidic gas, and injecting large volumes into the oceans would result in an increase in ocean acidity, which could adversely affect the marine life.
(The writer is on the faculty of Indian Institute of Technology, Mandi, India)