Why is CO2 generated in the production of energy ?
CO2, carbon dioxide, is generated by the combustion of carbon-containing fuels. The reaction C+O2 -> CO2 accounts for a substantial fraction of the energy released when burning hydrocarbons (natural gas, oil and coal, biomass). In the case of fossil fuels, this releases carbon which was trapped for several million years. It is therefore an intrinsic product of combustion and, unlike many other pollutants, no combustion technique can reduce its formation.
Why capture and sequestrate CO2?
There is strong scientific evidence, most notably from the reports of the Intergovernmental Panel on Climate Change (IPCC) that man-made emissions of CO2 contribute to the increase in the greenhouse gas concentrations in the atmosphere.
The CO2 concentration in the atmosphere has increased by more than 30% from the pre-industrial era. The use of fossil fuels for energy applications accounts for more than 70% of this, and CO2 given its concentration, is the most potent greenhouse gas, after water vapour.
There is also evidence that greenhouse gas may cause an increase in the average temperature of the earth, which could result in dramatic changes in climate and the ecosystem.
Current predictions indicate that even with great emphasis on the development of renewable energy sources, our dependence on fossil fuels is likely to remain for the foreseeable future. The use of fossil fuels with reduced CO2 emissions is therefore an integral part of the transition towards a sustainable energy future.
The Kyoto Protocol
The Kyoto Protocol results from a series of Conferences of the Parties (COP) of the UNFCCC (United Nations Framework Convention on Climate Change). Annex I countries (i.e. developed countries) are committed to an average overall reduction of about 5.2% of all greenhouse gas emissions in the period 2008-2012 compared to 1990.
The EU as a whole must achieve an 8% reduction to comply with the Protocol, and its Member States have reached a "burden sharing" agreement, which takes different countries' diverse situations into account.
Overview of the technologies
Because the technologies for CO2 capture and storage are still in their infancy, many possible routes still exist. The options for CO2 capture can be split into three broad categories. For storage, there are also several options. All these technologies address the problem of CO2 emission by large stationary sources.
Pre-combustion capture is the transformation of the fuel into a mixture of CO and H2 through gasification, shift reaction or partial oxidation.
The CO and H2 mixture can be used as fuel in a turbine cycle, giving rise to a high CO2 concentration stream, so that simple separation techniques can be applied.
Work is needed on the system integration of all the necessary components.
- Post combustion capture refers to the extraction of CO2 from the flue gas.
- This is usually envisaged through absorption, with a solvent which is regenerated.
- It is a mature technology, but it is not yet developed for large-scale power generation. Work is also needed on innovative solvents and cost reductions.
- Another way to capture CO2 from flue gas is by adsorption, that is the fixation of CO2 molecules on a surface. The adsorbing material undergoes a series of pressure or temperature variations to store/release CO2 as required. Material and process developments are needed.
- Cryogenics is another way to separate gases. However, the low concentration of CO2 in the flue gas makes this uneconomical. Nevertheless this could be a route worth developing for cycles which increase the CO2 concentration in the flue gas.
- Membranes are another way to achieve gas separation. Work is required on developing the membranes themselves, on their optimisation for large-scale generation conditions, and on minimising the energy required for separation.
The basic idea behind oxygen combustion capture is to conduct the combustion in almost pure oxygen (without the nitrogen of the air), to achieve higher CO2 concentrations in the flue gas.
Work is needed on air separation units (often cryogenics) to improve and integrate them into the rest of the system.
Work is needed on oxygen combustion.
Developments are also needed for the system for oxygen combustion, since all working fluids are different.
Novel and emerging concepts.
- A number of innovative approaches could also be used to capture CO2. Ideas such as novel cycles, chemical looping, biological fixation, or hydrate formation have been considered.
There are many more options for storage of CO2 including in:
- CO2 can be injected at depths where is it heavier than water, to form CO2 lakes on sea bottoms.
- Research is required on the legal issues, public acceptability and ecological consequences of the technique.
Depleted oil or gas fields, with enhanced oil/gas recovery (EOR)
- CO2 can be used to displace additional oil or gas from a depleted reservoir.
- Studies are required on topics such as process, stability, public acceptance, injection and dispersion techniques.
- CO2 can be injected into deep unminable coal beds, to displace coal bed methane, which can be captured and used.
- Research topics are similar to those of enhanced oil and gas recovery.
Deep sub-sea or on-shore saline aquifers
- CO2 can be dissolved in huge quantities in deep saline aquifers, which can be widespread, of no value, and stable.
- Interesting issues are the long-term stability, safety aspects, public acceptability, and energy consumption.
Chemical compounds, mainly Al, Si, Mg based
- CO2 can be stored in a chemically-bound component.
- Studies are mainly required on the process kinetics, and public acceptability of the technology.
Ocean fertilisation of phytoplankton in surface waters
Non-minable coal beds, with enhanced coal bed methane production (ECBM)
Terrestrial ecosystems (forests, soils)
Estimated Reservoir Capacities
Deep saline reservoirs
|Disused oil and gas fields||900|
|Unminable coal beds||> 15|