(Online Course) GS Concepts : Pollution & Environment - Carbon Capture and Storage (CCS)

Topic: Carbon Capture and Storage (CCS)

CARBON CAPTURE AND STORAGE (CCS), alternatively referred to as carbon capture and sequestration, is a technology that will attempt to prevent large quantities of CO₂ from being released into the atmosphere from the use of fossil fuels in power generation and other industries. It is often regarded as a means of mitigating the contribution of fossil fuel emissions to global warming. The process is based on capturing carbon dioxide (CO2) from large point sources, such as fossil fuel power plants, and storing it in such a way that it does not enter the atmosphere. It can also be used to describe the scrubbing of CO2 from ambient air as a geo engineering technique. Although CO2 has been injected into geological formations for various purposes, the long term storage of CO2 is a relatively new concept. The first commercial example was Weyburn in 2000.
An integrated pilot-scale CCS power plant was to begin operating in September 2008 in the eastern German power plant Schwarze Pumpe run by utility Vattenfall, in the hope of answering questions about technological feasibility and economic efficiency. CCS applied to a modern conventional power plant could reduce CO2 emissions to the atmosphere by approximately 80-90% compared to a plant without CCS. The IPCC estimates that the economic potential of CCS could be between 10% and 55% of the total carbon mitigation effort until year 2100.

Capturing and compressing CO2 requires much energy and would increase the fuel needs of a coal-fired plant with CCS by 25%-40%. These and other system costs are estimated to increase the cost of energy from a new power plant with CCS by 21-91%. These estimates apply to purpose-built plants near a storage location; applying the technology to preexisting plants or plants far from a storage location would be more expensive. Recent industry reports suggest that with successful research, development and deployment (RD&D), sequestered coal-based electricity generation in 2025 will cost less than unsequestered coal-based electricity generation today.

Storage of the CO2 is envisaged either in deep geological formations, in deep ocean masses, or in the form of mineral carbonates. In the case of deep ocean storage, there is a risk of greatly increasing the problem of ocean acidification, an issue that also stems from the excess of carbon dioxide already in the atmosphere and oceans. Geological formations are currently considered the most promising sequestration sites. The National Energy Technology Laboratory (NETL) reported that North America has enough storage capacity at its current rate of production for more than 900 years worth of carbon dioxide.[6] A general problem is that long term predictions about submarine or underground storage security are very difficult and uncertain, and there is still the risk that CO2might leak from the storage into the atmosphere.

Sequestration

Various forms have been conceived for permanent storage of CO2. These forms include gaseous storage in various deep geological formations (including saline formations and exhausted gas fields), liquid storage in the ocean, and solid storage by reaction of CO2 with metal oxides to produce stable carbonates.

Geological Storage

Also known as geo-sequestration, this method involves injecting carbon dioxide, generally in supercritical form, directly into underground geological formations. Oil fields, gas fields, saline formations, unmineable coal seams, and saline-filled basalt formations have been suggested as storage sites. Various physical (e.g., highly impermeable caprock) and geochemical trapping mechanisms would prevent the CO2 from escaping to the surface.

CO² is sometimes injected into declining oil fields to increase oil recovery. Approximately 30 to 50 million metric tonnes of CO2 are injected annually in the United States into declining oil fields. This option is attractive because the geology of hydrocarbon reservoirs is generally well understood and storage costs may be partly offset by the sale of additional oil that is recovered. Disadvantages of old oil fields are their geographic distribution and their limited capacity, as well as the fact that subsequent burning of the additional oil so recovered will offset much or all of the reduction in CO2emissions. Unmineable coal seams can be used to store CO2 because the CO2 molecules attach to the surface of coal. The technical feasibility, however, depends on the permeability of the coal bed. In the process of absorption the coal releases previously absorbed methane, and the methane can be recovered (enhanced coal bed methane recovery). The sale of the methane can be used to offset a portion of the cost of the CO2 storage. Burning the resultant methane, however, would produce CO2, which would negate some of the benefit of sequestering the original CO2. Saline formations contain highly mineralized brines, and have so far been considered of no benefit to humans. Saline aquifers have been used for storage of chemical waste in a few cases. The main advantage of saline aquifers is their large potential storage volume and their common occurrence. The major disadvantage of saline aquifers is that relatively little is known about them, especially compared to oil fields. To keep the cost of storage acceptable, the geophysical exploration may be limited, resulting in larger uncertainty about the aquifer structure. Unlike storage in oil fields or coal beds, no side product will offset the storage cost. Leakage of CO2 back into the atmosphere may be a problem in saline aquifer storage. Current research shows, however, that several trapping mechanisms immobilize the CO2 underground, reducing the risk of leakage.

For well-selected, designed and managed geological storage sites, the IPCC estimates that CO2 could be trapped for millions of years, and the sites are likely to retain over 99% of the injected CO2over 1,000 years. In 2009 it was reported that scientists had mapped 6,000 square miles (16,000 km2) of rock formations in the U.S. that could be used to store 500 years’ worth of U.S. carbon dioxide emissions.

Ocean Storage

Another proposed form of carbon storage is in the oceans. Several concepts have been proposed:

• ‘Dissolution’ injects CO2 by ship or pipeline into the ocean water column at depths of 1000 – 3000 m, forming an upward-plume, and the CO2 subse-quently dissolves in seawater.

• Through ‘lake’ deposits, by injecting CO2 directly into the sea at depths greater than 3000 m, where high-pressure liquefies CO2, making it denser than water, and forms a downward-plume that may accumulate on the sea floor as a ‘lake’, and is expected to delay dissolution of CO2 into the ocean and atmosphere, possibly for millennia.

• Use a chemical reaction to combine CO2 with a carbonate mineral (such as limestone) to form bicarbonate(s), for example: CO2 + CaCO3 + H2O’! Ca(HCO3)2 (aq). However, the aqueous bicarbonate solution must not be allowed to dry out, or else the reaction will reverse.

• Store the CO2 in solid clathrate hydrates already existing on the ocean floor, [23][24] or growing more solid clathrate.

The environmental effects of oceanic storage are generally negative, and poorly understood. Large concentrations of CO2 could kill ocean organisms, but another problem is that dissolved CO2 would eventually equilibrate with the atmosphere, so the storage would not be permanent. In addition, as part of the CO2 reacts with the water to form carbonic acid, H2CO3, the acidity of the ocean water increases. The resulting environmental effects on benthic life forms of the bathypelagic, abyssopelagic and hadopelagic zones are poorly understood. Even though life appears to be rather sparse in the deep ocean basins, energy and chemical effects in these deep basins could have far reaching implications. Much more work is needed to define the extent of the potential problems.

The time it takes water in the deeper oceans to circulate to the surface has been estimated to be approximately 1600 years, depending on currents and other changing conditions. Costs for deep ocean disposal of liquid CO2 are estimated at US$40"80/tonne of CO2 (2002 USD). This figure covers the cost of sequestration at the power plant and naval transport to the disposal site.

The bicarbonate approach would reduce the pH effects and enhance the retention of CO2 in the ocean, but this would also increase the costs and other environmental effects.
An additional method of long term ocean based sequestration is to gather crop residue such as corn stalks or excess hay into large weighted bales of biomass and deposit it in the alluvial fan areas of the deep ocean basin. Dropping these residues in alluvial fans would cause the residues to be quickly buried in silt on the sea floor, sequestering the biomass for very long time spans. Alluvial fans exist in all of the world’s oceans and seas where river deltas fall off the edge of the continental shelf, such as the Mississippi alluvial fan in the Gulf of Mexico and the Nile alluvial fan in the Mediterranean Sea.

Unfortunately, biomass and crop residues form an extremely important and valuable component of topsoil and sustainable agriculture. Removing them from the terrestrial equation is fraught with problems. [citation needed] If fertilized crops were used, it would exacerbate nutrient depletion and increase dependence on chemical fertilizers and, therefore, petro-chemicals, thus defeating the original intentions of reducing CO2 in the atmosphere. However it is more likely that less-expensive cellulosic energy-crops would be used, and these are typically unfertilized; although, it is likely that petrochemicals would still be used for harvesting and transport.

Mineral Storage

In this process, CO2 is exothermically reacted with available metal oxides, which in turn produces stable carbonates. This process occurs naturally over many years and is responsible for a great amount of surface limestone. The idea of using Olivine has been promoted by the geochemist Prof. Schuiling. The reaction rate can be made faster, for example by reacting at higher temperatures and/or pressures, or by pre-treatment of the minerals, although this method can require additional energy. The IPCC estimates that a power plant equipped with CCS using mineral storage will need 60-180% more energy than a power plant without CCS.
The economics of mineral carbonation at scale are now being tested in a world first pilot plant project based in Newcastle, Australia. New techniques for mineral activation and reaction have been developed the GreenMag Group and the University of Newcastle and funded by the New South Wales and Australian Governments to be operational by 2013.

A study on mineral sequestration in the US states: Carbon sequestration by reacting naturally occurring Mg and Ca containing minerals with CO2 to form carbonates has many unique advantages. Most notabl[e] is the fact that carbonates have a lower energy state than CO2, which is why mineral carbonation is thermodynamically favorable and occurs naturally (e.g., the weathering of rock over geologic time periods). Secondly, the raw materials such as magnesium based minerals are abundant. Finally, the produced carbonates are unarguably stable and thus re-release of CO2 into the atmosphere is not an issue. However, conventional carbonation pathways are slow under ambient temperatures and pressures. The significant challenge being addressed by this effort is to identify an industrially and environmentally viable carbonation route that will allow mineral sequestration to be implemented with acceptable economics.

Limitations of CCS for Power Stations

One limitation of CCS is its energy penalty. The technology is expected to use between 10 and 40 percent of the energy produced by a power station. Wide-scale adoption of CCS may erase efficiency gains of the last 50 years, and increase resource consumption by one third. Even taking the fuel penalty into account, however, overall levels of CO2 abatement would remain high at approximately 80-90%, compared to a plant without CCS.[93] It is theoretically possible for CCS, when combined with combustion of biomass, to result in net negative emissions, but this is not currently feasible given the lack of development of CCS technologies and the limitations of biomass production.

The use of CCS can reduce CO2 emissions from the stacks of coal power plants by 85-90% or more, but it has no effect on CO2 emissions due to the mining and transport of coal. It will actually “increase such emissions and of air pollutants per unit of net delivered power and will increase all ecological, land-use, air-pollution, and water-pollution impacts from coal mining, transport, and processing, because the CCS system requires 25% more energy, thus 25% more coal combustion, than does a system without CCS”.

Another concern regards the permanence of storage schemes. It is claimed that safe and permanent storage of CO2 cannot be guaranteed and that even very low leakage rates could undermine any climate mitigation effect. The IPCC concludes, however,, that the proportion of CO2 retained in appropriately selected and managed geological reservoirs is very likely to exceed 99% over 100 years and is likely to exceed 99% over 1,000 years.
Finally, there is the issue of cost. Greenpeace claims that CCS could lead to a doubling of plant costs. CCS though may remain economically attractive in comparison to other forms of low carbon electricity generation. It is also claimed by opponents to CCS that money spent on CCS will divert investments away from other solutions to climate change.

Environmental Effects

The theoretical merit of CCS systems is the reduction of CO2 emissions by up to 90%, depending on plant type. Generally, environmental effects from use of CCS arise during power production, CO2capture, transport, and storage. Issues relating to storage are discussed in those sections.

Additional energy is required for CO2 capture, and this means that substan-tially more fuel has to be used, depending on the plant type. For new super-critical pulverized coal (PC) plants using current technology, the extra energy requirements range from 24-40%, while for natural gas combined cycle (NGCC) plants the range is 11-22% and for coal-based gasification combined cycle (IGCC) systems it is 14-25% [IPCC, 2005]. Obviously, fuel use and environ-mental problems arising from mining and extraction of coal or gas increase accordingly. Plants equipped with flue gas desulfurization (FGD) systems for sulfur dioxide control require proportionally greater amounts of limestone, and systems equipped with selective catalytic reduction systems for nitrogen oxides produced during combustion require proportionally greater amounts of ammonia.
IPCC has provided estimates of air emissions from various CCS plant designs (see table below). While CO2 is drastically reduced though never completely captured, emissions of air pollutants increase significantly, generally due to the energy penalty of capture. Hence, the use of CCS entails a reduction in air quality.

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