Carbon Capture and Storage (CSS)
To better understand carbon capture and storage it helps to have a brief introduction to carbon and the carbon cycle.
Carbon is the fourth most abundant element in the universe and the fifteenth most abundant element on earth. Carbon’s special polymer-forming abilities make it the chemical basis for all life on earth.
The Carbon Cycle
The carbon cycle involves the movement of carbon about the earth’s geological and biological systems. The geological carbon cycle is the exchange of carbon, mainly through tectonic activity, between earth’s, rocks, water systems and air. The biological carbon cycle, which exchanges about 1000 times more carbon than the geological carbon cycle, is the exchange of carbon, mainly through photosynthesis and respiration, between earth’s life forms and earth’s air, land and water systems.
The carbon cycle in its natural form is dynamic and of fundamental importance to life on earth. However, since the industrial revolution, human consumption of fossil fuels and the destruction of important forest areas has created an unnatural increase to the amount of CO2 in the atmosphere. Because this change has serious implications for the future of life on earth there have been global political initiatives to control human intervention in the carbon cycle, including developing carbon capture techniques.
Carbon capture is divided into two main areas:
Carbon Capture and Storage (CSS)
CSS is the process of capturing and storing CO2 emissions from power plants and industrial manufacturing plants. There are three methods for CSS, which are:
- Oxy-fuel combustion
Post-combustion involves capturing waste gases by adding a filter to the smoke stack of power and manufacturing plants. The captured gases (called flue gases) are processed to extract CO2, which then gets shipped to a storage area. This process recovers 80 to 90 percent of the waste CO2. The advantages include being able to retrofit older styled plants with this system. The disadvantages include that the final CO2 extraction and preparation for storage process is energy intensive.
Pre-combustion, as the name implies, involves capturing CO2 before a fossil fuel is burned. This process involves extracting CO2 by partially oxidizing the fuel in a gasifier, which results in the separation of CO2 from CO (carbon monoxide) and H2 (Hydrogen). The hydrogen can then be used as fuel. Approximately 80 to 90 percent of CO2 is captured this way. The advantages include that the process is relatively inexpensive. The disadvantages include that the process cannot be retrofitted to older plants.
Oxy-fuel combustion involves burning fossil fuels in pure oxygen instead of air, which creates CO2 and water vapor. The water vapor is condensed leaving almost pure CO2, which can then be transported to a storage area. This process captures about 90 percent of the CO2. The advantages include an effective method of carbon capture. The disadvantages include the cost of supplying pure oxygen.
All three of the above carbon capture processes require that the captured CO2 be stored. Carbon storage is an area that is still under development and full of questions. There are two main areas for CO2 storage: underground and under water.
Underground storage includes injecting CO2 into:
- Oil fields
- Gas fields
- Saline formations
- Basalt formations
- Sandstone reservoirs
Although the technology exists to accomplish CO2 storage in these types of geological formations there remain many unknowns, e.g., the long term effects of stored CO2 on these areas and the potential for leakage. In most cases storing CO2 involves unrecoverable costs.
On the other hand, there is plenty of space for storage. It has been estimated that in the U.S. alone there is enough underground CO2 storage area to last 500 years.
Under water CO2 storage techniques include:
- Injecting CO2 to depths greater than 1000 m, where the CO2 dissolves
- Injecting CO2 to depths below 3000 m where a CO2 “lake” forms due to the CO2 being heavier than water
- Storing CO2 as clathrate hydrates
- Under high pressure and low temperature, e.g., the deep ocean floor, CO2 becomes a negatively buoyant icy compound called clathrate hydrate
Techniques for storing CO2 under water are less well understood than those for underground storage and pose many questions including:
- How long the CO2 will remain stored
- Effects on marine wildlife, eco-systems and overall environment
- Increased acidification of the oceans
Regarding costs, it is expected that deploying CCS systems would involve increases of 50% for the retail/residential sectors and 100% for the industrial sector. This includes increased energy usage ranging from 11% to 40%, depending on the type of plant involved and the CCS method used.
Bio carbon capture falls under the definition of “geo-engineering” and involves using organic compounds to convert captured greenhouse gases into useable materials. The principle behind this is very sound due to the importance of CO2 in organic systems. A good example is Bio CCS Algal Synthesis, which involves injecting greenhouse gases into membranes containing waste water and select strains of algae, which, together with sunlight or UV light, causes the oil rich biomass to double in mass every 24 hours. From this biomass, plastics, fuels and livestock feed can be created. A project that is currently testing this type of system is MBD Energy Limited in Australia.
Advantages to Bio CSS include that most processes are relatively inexpensive and costs are recovered from the useable products created. Disadvantages include that much of the Bio CSS processes are currently still under development.