Here is a term paper on the top four techniques used for storing carbon dioxide. The techniques are: 1. Geo-Sequestration 2. New Technology to Capture Carbon in Aluminum Industry 3. Carbon Sequestration by Algae 4. Carbon Dioxide Absorption by New Metal-Organic Crystals.
Term Paper # 1. Geo-Sequestration:
Geo-sequestration is the process of removing carbon from the atmosphere and depositing it in a reservoir. When carried out deliberately, this may also be referred to as carbon dioxide removals, which is a form of geo-engineering. Carbon dioxide is naturally captured from the atmosphere through biological, chemical or physical processes. The natural processes include the capture of atmospheric carbon dioxide in a solid material (such as growing trees, other vegetation, and soils) or a carbon sink through biological or physical processes, such as photosynthesis.
The fossil fuels such as, coal, oil and natural gas, currently supply, around 85% of the world’s energy needs. The International Energy Agency predicts that fossil fuels will continue to be heavily used for many years to come. The burning of fossil fuels is a major source of excess CO2, the gas that has most contributed to the increased concentration of greenhouse gases in the atmosphere. The geo-sequestration involves the capture and long-term underground storage of carbon dioxide.
Carbon dioxide produced by coal- fired power generation plants and other industrial sources is compressed to form a liquid and injected into deep underground geological formations, such as, saline aquifers, coal seams, and used oil and gas reservoirs. The geo-sequestration enables the long- term geological storage of CO2.
Carbon Capture & Storage (CCS) Technology:
The coal based power generating plants with long life (50 years and beyond) and their growing numbers are becoming major source of carbon dioxide emission. To prevent the carbon dioxide to escape in the atmosphere, the technology to ‘capture’ carbon and ‘storing’ elsewhere, has been developed.
The stages in the process of geo-sequestration of carbon dioxide from the combustion products are as follows:
i. Enrichment of carbon dioxide, in the flue gas.
ii. Separation or capture of carbon dioxide from the flue gas.
iii. Compression of carbon dioxide to a lower volume for ease of transportation and storage.
iv. Storage of compressed carbon dioxide in leak proof areas.
Carbon Dioxide Enrichment of Flue Gas in Power Plants:
The main source for bulk capture of CO2 is electricity generation from fossil fuel sources. Industrial processes such as, natural-gas processing, ammonia production, and cement manufacture, produce relatively small quantities of CO2 and thus are not suitable for bulk storage. A far larger source of CO2, accounting for one-third of total CO2 emissions in Australia, is fossil-fuelled electricity generation.
The capture of CO2 from a stationary source, such as power plant, involves separating and purifying CO2 from the bulk of the flue gas stream before geological storage. Due to the presence of large quantities of nitrogen, the partial pressure of the carbon dioxide in the flue gas is very low in the fossil fuel based power plants.
The direct recovery of the carbon dioxide from such flue gas, known as “flue gas approach”, is not a techno-economically viable proposition. Therefore, alternate strategies involving enrichment of carbon dioxide in the flue gases are used in capturing CO2 from electricity generation plants.
The processes for CO2 enrichment in flue gas include the followings:
A. Post-Combustion, Using Air for Combustion:
Pulverized coal using the flue gas approach presents the largest economic hurdle to CO2 sequestration. The flue gas approaches in use today require clean-up of the NOx and SO2 prior to CO2 separation. If the sinks are tolerant to NO2 and SO2, it is possible to eliminate separate control steps and sequester the NOx and SO2 along with the CO2, resulting in a zero emissions power plant.
B. The “hydrogen” or “syngas” approach or pre-combustion or hydrogen route, generally referred to as Integrated Gasification Combined Cycle or IGCC, Integrated coal gasification combined cycle (IGCC) plant is an example of the hydrogen route. Coal is gasified to form synthetic gas (syngas) of CO and H2.
The gas then undergoes the water- gas shift, where the CO is reacted with steam to form CO2 and H2. The CO2 is then removed along with the hydrogen for sending to a gas turbine combined cycle. A similar process is available for natural gas, where the syngas is formed by steam reforming of methane.
The hydrogen route opens up opportunities for “polygeneration”, where besides electricity and CO2, additional products are produced. For example, instead of sending hydrogen to a turbine, it can be used to fuel a “hydrogen economy”. In addition, syngas is an excellent feedstock for many chemical processes.
C. Pre-Combustion, Oxyfuels, the “Oxygen” Approach:
The major component of flue gas is nitrogen, present as 75% of air feed, without which CO2 capture from flue gas would be greatly simplified. Therefore use of oxygen instead of air for combustion would lead to easier separation of CO2 from flue gas.
However, combustion with oxygen produces too high a temperature for today’s materials, so some flue gas must be recycled to moderate the temperature. Applying this process is easier for steam turbine plants than gas turbine plants. In the former, relatively straightforward boiler modifications are required, for the latter, much more.
Separation and Capturing of Carbon Dioxide:
The separation and capture of carbon dioxide operations take place at the power plant site, including compression. After separation, CO2 is generally compressed to the order of 100 atm, for easy transportation.
The idea of separating and capturing CO2 from the flue gas of power plants did not start with concern about the greenhouse effect. Rather, it gained attention as a possible economic source of CO2, especially for use in enhanced oil recovery (EOR) operations where CO2 is injected into oil reservoirs to increase the mobility of the oil and, therefore, the productivity of the reservoir.
Several commercial CO2 capture plants were constructed in the late 1970s and early 1980s in the US. Also, this process is used to produce CO2 for carbonation of brine. Several more CO2 capture plants were subsequently built to produce CO2 for commercial applications. Some of these plants took advantage of the economic incentives in the Public Utility Regulatory Policies Act (PURPA) of 1978 for “qualifying facilities”.
Four main techniques for separation of carbon dioxide from flue gas are as follows:
a. Absorption, where CO2 is selectively absorbed into liquid solvents,
b. Membranes, where CO2 is separated by semi permeable plastic or ceramic membranes,
c. Adsorption, where CO2 is separated using specially designed solid particles, and
d. Low Temperature Processes, where separation is achieved by chilling and/or freezing the gas stream.
Plants based on membrane separation, cryogenic fractionation, and adsorptions using molecular sieves are less energy efficient and more expensive than chemical absorption. The primary difference in capturing CO2 for commercial markets versus capturing CO2 for sequestration is the role of energy. In the former case, price is sole consideration not the quantum of energy used.
In the latter case, minimum quantity of energy is to be used to avoid causing more CO2 emissions. Therefore, capturing CO2 for purposes of sequestration requires more emphasis on reducing energy inputs than the traditional commercial process. Virtually all commercial processes for CO2 separation for sequestration are based on absorption in liquid solvents.
The solvents used may be categorized into two types:
a. Chemical solvents, such as aqueous solutions of monoethanolamine (MEA) or potassium carbonate, where the mechanism of absorption is via a reversible chemical reaction. The absorption rate of CO2 in unpromoted hot potassium carbonate solutions is vastly improved in solutions promoted with diethanolamine (DEA) or with sterically hindered amines.
b. Physical solvents, such as, methanol used in Rectisol or dimethyl ethers of polyethylene glycols used in Selexoll, where the absorption of gases occurs without chemical reactions.
The hydrogen route for carbon enrichment allows for a CO2 removal process by a physical solvent process like Selexsol, which is much less energy intensive than the MEA process, because capture takes place from the high pressure syngas as opposed to the atmospheric pressure flue gas.
The most widely used method till date for capturing carbon dioxide is based on chemical absorption with a monoethanolamine (MEA) solvent. MEA was developed over 60 years ago as a general, non-selective solvent to remove acid gases, such as CO2 and H2S, from natural gas streams. The process allows flue gas to contact an MEA solution in the absorber. The MEA selectively absorbs the CO2 and is then sent to a stripper.
In the stripper, the CO2-rich MEA solution is heated to release almost pure CO2. The lean MEA solution is then recycled to the absorber. The technique has been in use for more than four decades in producing endothermic protective atmospheric gas (4%CO + N2), by burning ATS grade, low sulfur kerosene. The protective endothermic gas is used in continuous annealing furnace for bright annealing of cold rolled steel strips.
Sequestration of Carbon Dioxide:
While the capture of CO2 for geo-sequestration is a relatively new concept, CO2 capture for commercial markets has been practiced in Australia and overseas for many years. CO2 is captured from natural gas wells in south-east South Australia, near Mt Gambier and in southern Victoria, near Port Campbell. The CO2 is then used for various commercial processes including carbonation of beverages and dry-ice production.
After capturing, CO2 can be stored in leak proof areas for a long period, such as, oil and gas fields, which have preserved oil and gas for million years. Also can be used as, excess gas for producing oil by forcing from ground by injected CO2. British Geological survey says there is an estimated 6.2-8.5 Gt of storage capacity in the UK’s on- and off shore oil and gas fields-which could hold in entire CO2 emissions for 10-15 years.
UK is expected to build first CCS demonstrator plant by 2014. In USA the first coal based power project to capture carbon dioxide (pumping to underground storage), based on CCS technology, and due to come up in Illinois, has been shelved till 2015.
In the United States, CO2 capture at power plants using chemical absorption solvent has been practiced since the late 1970s, with the captured CO2 being used for enhanced oil recovery. There are plans in the United States to build the world’s first integrated gasification combined cycle plant, known as Future Gen, that will not only produce electricity but also hydrogen fuel, with the CO2 generated in the process being captured and sequestered.
Following capture, CO2 is usually transported from the source, such as a power station, to the geological storage site in a compressed form via a pipeline. It can also be transported by truck, rail or ships depending on the location of both the source and the geological storage site and injected via pipeline deep underground.
Comparing Offshore Geological Storage of CO2 to Ocean Storage of CO2:
In both offshore geological and ocean storage of CO2, include, capturing the gas from a stationary emissions source such as a power plant or other industrial facility and then transporting the highly compressed CO2 offshore via a sub-sea pipeline or ocean tanker. There is, however, a major difference between offshore geological sequestration and ocean sequestration in the way in which the CO2 is stored.
Offshore geological storage involves the CO2 being injected into a geological formation deep beneath the sea bed where it will be stored for thousands of years, isolated from the ocean water. In the case of ocean storage, the CO2 is injected directly into the water column either at mid-depth (1500 to 3000 metres), where it dissolves in the ocean waters, or at greater depths (below 3000 metres), where it forms a deep CO2 lake.
Offshore geological storage has been successfully demonstrated at Statoil’s Sleipner field in the North Sea (about 250 km off the coast of Norway). At Sleipner, CO2 is separated from produced natural gas and stored in a deep saline formation about 1000 metres beneath the seabed. No ocean sequestration demonstration projects exist so far.
Term Paper # 2. New Technology to Capture Carbon in Aluminum Industry:
In 2007, ALCOA, multinational aluminum major, launched a new ‘carbon capture’ technology to its Kwinana aluminum refinery in Western Australia. The process for capturing includes mixing of carbon dioxide with bauxite residue, a byproduct of aluminum metal extraction process.
This locks up large amount of the greenhouse gas generated by the aluminium extraction plant, which would otherwise be released to atmosphere. The Kwinana carbonation plant will lock up 70,000 tons of CO2 a year, the equivalent of eliminating the emissions of 17,500 automobiles.
The new technology has the potential to deliver significant global greenhouse benefits and will contribute to a reduction in the aluminum industry’s environmental footprint. By mixing carbon dioxide with bauxite residue, the compound formed has similar alkalinity level as found in alkaline soil. The new mixture can be used as additive to improve soil, for road foundation and building material.
In addition to its greenhouse benefits, ALCOA’s residue carbon capture process delivers other economic, environmental and social benefits. A major benefit includes biological removal of sodium oxalate through biodegradation by natural bacterial activities in the carbonate residue. Normal process for removal is heating in kiln, an energy consuming thermal process.
Term Paper # 3. Carbon Sequestration by Algae:
Engineers have designed a simple, sustainable and natural carbon sequestration solution using algae. A team at Ohio University created a photo bioreactor that uses photosynthesis to grow algae, passing carbon dioxide over large membranes, placed vertically to save space.
The carbon dioxide produced by the algae is harvested by dissolving into the surrounding water. The algae can be harvested and made into biodiesel fuel and feed for animals. A reactor with 1.25 million square meters of algae screens could be up and running by 2010.
Term Paper # 4. Carbon Dioxide Absorption by New Metal-Organic Crystals:
Scientists at University of California have made metal-organic crystals, named as zeolite imidazolate, or ZIF, capable of soaking up carbon dioxide gas like a sponge. The crystals are non-toxic and would require little extra energy from a power plant, making them an ideal alternative to current methods of CO2 filtering.
The porous structures can be heated to high temperatures without decomposing and can be boiled in water or solvents for a week and remain stable, making them suitable for use in hot, energy-producing environments like power plants.
The highly porous crystals have “extraordinary- capacity for storing CO2“; one litre of the crystals could store about 83 litres of CO2. Estimates from United Nation’s energy and climate experts have pegged the cost of capturing CO2 between $25 US and $60 US a tonne for conventional coal-fired plants.
The coal industry uses the term “clean coal” to describe technologies designed to enhance both the efficiency and the environmental acceptability of coal extraction, preparation and use, with no specific quantitative limits on any emissions, particularly of carbon dioxide.
Solar Radiation Management:
The techniques include, creating stratospheric sulfur aerosols to scatter or reflect solar radiation, using pale-colored roofing and paving materials, cloud reflectivity enhancement by using fine salt water spray to whiten clouds and increase cloud reflectivity, and space sunshade by obstructing solar radiation with space-based mirrors or other structures Limited trials have been conducted on cool roof and stratoscopic sulfur aerosol spray.
Regulation of Geo-Engineering:
According to Ken Caldeira, a professor of climate science at Stanford University, reducing greenhouse gases will cost around 2 percent of the gross domestic product, while geo-engineering (by putting reflective aerosols into the upper atmosphere) will cost about one-thousandth of that.
However, geo-engineering needs to be regulated in order to prevent adverse effects; that there should be independent assessment of the impacts of geo-engineering research proposals before actual use of the technologies.
These are views expressed by the scientists at a recent conference at Asilomar, and also echoed by scientific communities, including Royal Society and the Academy of Sciences for the Developing World. However, large-scale geo-engineering projects (excepting ALCOA process) are yet to be commercialized.