Carbon capture and storage technologies can be used to capture 90% of the emissions from existing fossil fuel-fired power plants, enabling existing plants to continue generating electricity to meet increased international demand for decarbonised energy, says Alstom carbon dioxide (CO₂) systems business development director Dr Philippe Paelinck.

Worldwide, the power generation industry accounts for about 40% of global CO₂ emissions, while transport accounts for about 20% of global emissions. This means the power sector is the primary target for dealing with CO₂ emissions.

“To reduce the CO₂ emissions from power generation, we can take three main approaches. The first is to increase decarbonised technologies, like hydro, wind, solar and nuclear. The second is increased efficiency of fossil fuel power gene- ration. We must do more with less.

“However, new fossil fuel installations are continuously being built to meet increasing energy demand, which means that carbon capture and storage (CCS) is needed. This is the third approach – decarbonising current power generation.”

Storage of the 99% pure CO₂ is typically done at depths of between 2 000 m and 3 000 m in sedimentary basins and saline formations. These are porous rock layers into which the CO₂ is injected. The liquid CO₂ is trapped in these formations and dissolves in any saline water present.

“Eventually, the CO₂ will mineralise over time, so the longer it stays there, the safer the storage becomes. About 50% of CO₂ is permanently trapped after only ten years in the porous structure.”

However, Paelinck warns that studies on suitable storage sites need to be conducted by countries and validated before carbon capturing begins.

Alstom’s CCS technologies that can be retrofitted take two main forms, namely post- combustion CO₂ capture and storage and oxycombustion CO₂ capture and storage.

In postcombustion, CO2 is removed from the flue gas by washing the gas with a solvent. Advanced amine solutions have been piloted and demonstrated in large-scale applications by Alstom and other companies and are well known in the natural gas industry as a useful solution.

Futher, Alstom is also developing a chilled ammonia solution, which uses a simple reaction of ammonium carbonate to ammonium bicarbonate to wash out the CO₂ from the flue gas. This requires less energy than the advanced amine technology.

“Postcombustion has many advantages because one can adapt it to any power plant, or to the industry, and keep the systems on your site. You are not forced to take all the flue gas and can take a slipstream and handle a part of it. The technology is quite flexible for existing plants.”

The company’s second carbon capture solution is oxycombustion, which combusts the fuel in an oxy- gen-rich atmosphere to create a CO₂-rich flue gas that can then easily be captured and stored.

“The trick here is to control the temperature in the boiler because pure oxygen leads to very high temperatures. What we do to combat this is to recycle a part of the flue gas stream, mix it with the incoming oxygen and then combust the fuel in this mixture.”

A key issue for this solution is to produce oxygen affordably and with the least expense of energy. He notes that the gas industry and other large industries should be able to help with this part of the equation.

“This is a very reliable and robust solution that can potentially be applied to new power plants. We have tested retrofitting of this technology and it is possible to retrofit a boiler to run on an oxycombustion cycle, but the key concerns here are to reassess the boiler’s integrity under such high temperatures and to reassess the metallurgy of the boilers to cope with the new atmosphere, which is mainly CO₂ and water vapour,” explains Paelinck.

“CCS must be considered as a transition measure in South Africa until other energy sources can gradually replace high-carbon fuels,” he says.

Meanwhile, CCS will add about 50% to the levelised costs of electri- city production over the long term, but this has to be compared to other decarbonised solutions, such as renewable energies and nuclear power.

A publicly available study, commissioned by Alstom, indicates that the cost of implementing CCS on existing and new installations is similar to the cost for other decarbonised energy-generation technologies, which means that it is financially viable, explains Paelinck.

CCS solutions require energy to capture and compress the gas and a CCS solution will come at an energy penalty of about 20% to power plants to reduce their CO₂ emissions by 90%.

Retrofitted or greenfield CCS technologies together can help to account for about 20% of emission reductions by 2035, he notes.

State-owned power utility Eskom research manager Barry McColl says the Medupi power station, which is under construction, is not CCS ready but the Kusile power station, also under construction, is CCS ready.

At Kusile, provision has been made for the CCS plant and for reducing the particulate emissions from the station, while sulphur oxide emissions will also be reduced. Sulphur dioxide causes problems with the catalyst involved in the reaction, he said

“Geotechnical studies are being done and there is a centre in South Africa called the Centre for Carbon Capture and Storage. The centre, along with other partners, Eskom and Sasol included, has helped to develop the Carbon Capture and Storage Act, which was produced last year and has identified certain areas in South Africa that are reasonably good for carbon storage. We are exploring those areas at the moment. We also have the Council for Geosciences on board and national oil company PetroSA looking at the rock formations and the ground with the intention to have an injection experiment by 2016,” concludes McColl.

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