Carbon capture and storage
Carbon capture, use and storage (CCUS) is an integrated suite of technologies that can capture up to 90% of the CO2 emissions produced from the use of fossil fuels in electricity generation and industrial processes, preventing the CO2 from entering the atmosphere.
Capture technologies allow the separation of CO2 from gases produced in electricity generation and industrial processes by one of three methods:
- Pre-combustion capture
- Post-combustion capture
- Oxyfuel combustion
CO2 is then transported for safe use or storage. Millions of tonnes of CO2 are already transported annually for commercial purposes by pipelines, ships and road tanker. The US has four decades of experience of transporting CO2 by pipeline for enhanced oil recovery projects.
Pre combustion co2 capture
Pre-combustion capture refers to removing CO2 from fossil fuels before combustion is completed. For example, in gasification processes a feedstock (such as coal) is partially oxidized in steam and oxygen/air under high temperature and pressure to form synthesis gas. This synthesis gas, or syngas, is a mixture of hydrogen, carbon monoxide, CO2, and smaller amounts of other gaseous components, such as methane. The syngas can then undergo the water-gas shift reaction to convert CO and water (H2O) to H2 and CO2, producing a H2 and CO2-rich gas mixture. The concentration of CO2 in this mixture can range from 15-50%. The CO2 can then be captured and separated, transported, and ultimately sequestered, and the H2-rich fuel combusted.
Compared to post-combustion technology, which removes dilute CO2 (~5-15% CO2 concentration) from flue gas streams and is at low pressure, the shifted synthesis gas stream is rich in CO2 and at higher pressure, which allows for easier removal before the H2 is combusted. Due to the more concentrated CO2, pre-combustion capture typically is more efficient but the capital costs of the base gasification process are often more expensive than traditional pulverized coal power plants.
Today’s commercially available pre-combustion carbon capture technologies generally use physical or chemical adsorption processes, and will cost around $60/tonne to capture CO2 generated by an integrated gasification combined cycle (IGCC) power plant.
Post combustion co2 capture
CO2 can be captured from the exhaust of a combustion process by absorbing it in a suitable solvent. This is called post-combustion capture. The absorbed CO2 is liberated from the solvent and is compressed for transportation and storage. Other methods for separating CO2 include high pressure membrane filtration, adsorption/desorption processes and cryogenic separation
Oxy fuel combustion
In the process of oxy-fuel combustion the oxygen required is separated from air prior to combustion and the fuel is combusted in oxygen diluted with recycled flue-gas rather than by air. This oxygen-rich, nitrogen-free atmosphere results in final flue-gases consisting mainly of CO2 and H2O (water), so producing a more concentrated CO2 stream for easier purification.
Carbon Sequestration is capturing and securely storing carbon dioxide emitted from the global energy system.
Importantly, carbon sequestration is both a natural and artificial process by which carbon dioxide is removed from the Earth’s atmosphere and then stored in liquid or solid form. It is a process of capture and deliberate, whether natural or artificial, storage of CO2 over a long period of time. The initial purpose of doing this is to delay global warming and avoid extreme climate change. It is also important to note that other forms of carbon, not just CO2 are stored during this sequestration process.
A more scientific explanation (and example) is; the removal and storage of carbon from the atmosphere to sinks – oceans, soil, forests – through physical means and the natural process best known as photosynthesis. There is one positive trend of carbon sequestration. While large areas of forests have been cleared over the years, today humankind are still making concerted efforts to grow more forests to invigorate carbon sequestration.
In other words, carbon sequestration means capturing the carbon dioxide (CO2) produced from new and old coal powered power plants and large industrial sources before it is released in the atmosphere. Once captured, the CO2 is put into long term storage either by storing it in carbon sinks (such as oceans, forests or soils) or underground injection and geologic sequestration into deep underground rock formations.
- Carbon dioxide (CO2) capture and sequestration (CCS) is therefore a 3 step process that involves:
- Capturing the CO2 from coal or natural based powered plants
- Transporting the stored carbon usually through pipelines
- Storage of CO2 into deep non-porous layers of rock that trap it and prevent it from migrating upward.
Types of Sequestration:
There are number of technologies under investigation for sequestering carbon from the atmosphere. These can be discussed under three main categories:
- Ocean Sequestration: Carbon stored in oceans through direct injection or fertilization.
- Geologic Sequestration: Natural pore spaces in geologic formations serve as reservoirs for long-term carbon dioxide storage.
- Terrestrial Sequestration: A large amount of carbon is stored in soils and vegetation, which are our natural carbon sinks. Increasing carbon fixation through photosynthesis, slowing down or reducing decomposition of organic matter, and changing land use practices can enhance carbon uptake in these natural sinks.
- Geologic Sequestration is thought to have the largest potential for near-term application.
Geologic Sequestration Trapping Mechanisms
- Hydrodynamic Trapping: Carbon dioxide can be trapped as a gas under low-permeability cap rock (much like natural gas is stored in gas reservoirs).
- Solubility Trapping: Carbon dioxide can be dissolved into a liquid, such as water or oil.
- Mineral Carbonation: Carbon dioxide can react with the minerals, fluids, and organic matter in a geologic formation to form stable compounds/minerals; largely calcium, iron, and magnesium carbonates.
While research continues on mineral carbonation, early results indicate reaction times to be too slow for this technology to have near-term widespread applicability. Carbon dioxide can be effectively stored in the earth’s subsurface by hydrodynamic trapping and solubility trapping – usually a combination of the two is most effective.