The National Mining Association (NMA) is sponsoring a website and advertising campaign promoting the concept of capturing carbon emissions and storing it.
They claim:
“As America and the world move toward policies to stabilize and reduce carbon emissions, carbon capture and storage (CCS) technologies present one of the most promising and effective options for large-scale reductions in CO2 emissions from energy use.
CCS is the latest in a portfolio of clean coal technologies that have successfully managed emissions from coal-based generation.
There are three primary methods of capturing CO2. Pre-combustion, or separating CO2 from gasified coal prior to combustion; post-combustion, which captures CO2 from the flue gas stream after the coal is burned; and oxy coal combustion, where the combustion of coal takes place in an oxygen-rich environment, resulting in flue gas more ready for sequestration.
Pre-combustion
In pre-combustion carbon capture, coal is gasified by applying heat and steam in a high-pressure, controlled-oxygen environment. The resulting syngas consists primarily of hydrogen (H2) and carbon monoxide (CO) gases. By processing the CO in a water-gas-shift reactor, the addition of water produces CO2 and additional H2 gases. The highly concentrated CO2 can be separated and stored, while the hydrogen may be cleanly combusted or, as under a project being developed under the Department of Energy (DOE) Fuel Cell Program, used in hydrogen fuel cells. Due to the increased concentration of CO2 in the pre-combustion syngas, pre-combustion carbon capture technologies are extremely efficient compared to in post-combustion flue gas. By using pre-combustion processes, CO2 emissions may be reduced by 90 to 95 percent.
Pre CombustionIntegrated gasification combined cycle (IGCC) coal-based plants are prime candidates for pre-combustion gasification. In the near-term, CO2 gas will likely be separated from the syngas streams via physical or chemical solvents such as glycol-based Selexol and amine-based gas removal systems. Work is underway on the development of membrane separation units to selectively separate H2 gas from CO and CO2 gases.
Post-combustion
Post CombustionAfter combustion in a pulverized coal plant, CO2 may be removed from the resulting flue gas. This approach may be retrofitted to existing coal-based power plants without significant modifications to the plant, but is more challenging than pre-combustion methods due to the low pressure and diluted nature of the post-combustion gases. However, properly deployed, post-combustion technologies can capture 80 to 90 percent of CO2 emissions.
Currently in post-combustion capture, CO2 is captured from flue gas which (largely comprised of nitrogen gas and CO2) through the use of chemical solvents such as amines (nitrogen-based organic compounds). These technologies have been deployed in limited slipstream applications. Future opportunities and improved performance may be found in membranes, sorbents or cryogenic isolation, which are currently being researched.
Oxy-coal combustion
Post CombustionThe oxy-coal process creates a oxygen-rich environment for coal combustion, resulting in a more complete combustion and a nearly pure CO2 and water vapor exhaust stream. When cooled, CO2 is easily separated from the water in this process. Oxy-coal combustion may be retrofitted to existing coal plants, however the current process of separating oxygen from air cryogenically is energy-intensive and requires an input of approximately 15 percent of a plant’s annual energy output. A new technology, called chemical looping combustion, in which oxygen is separated from the air via the oxidation of a metallic compound, may reduce costs in the future. Oxy-coal combustion can remove 90 percent of CO2 from emissions.”
But, what will be done with the captured emissions? How can it safely be transported? How long will it stay stored?
The NMA’s answers:
“Carbon Storage
Once CO2 is captured from a power plant, it can be transported via pipeline or truck to locations where appropriate geologic conditions will allow for safe storage.
Depleted Oil and Gas Reservoirs
Oil and gas reservoirs are formations that held crude oil and natural gas. Generally, they consist of a layer of porous rock with a dome-shaped layer of non-porous rock above. The dome shape, which historically trapped oil or gas, has the potential to act as a carbon trap once oil and gas drilling is completed. According to the Department of Energy’s (DOE) National Energy Technology Laboratory (NETL), “more than 88 billion metric tons of geologic storage potential exists in 9,667 oil and gas reservoirs distributed over 27 states and 3 [Canadian] provinces.”
In addition, CO2 injected into an oil reservoir may dissolve into oil trapped in the porous rocks of the formation, thus reducing the oil’s viscosity. That, in turn, allows an additional 10 to 15 percent of the oil to be recovered from the well. This process of enhanced oil recovery has been in use in the U.S. since the 1970s.
Unmineable Coal Seams
Some coal seams are either too deep or too thin to be mined economically. However, all coal seams contain methane, and wells may be drilled to collect the methane for use in energy applications. Once the initial stores of methane are recovered, CO2 may be pumped into the wells, where it is preferentially stored in the coal, releasing additional methane. According to NETL, between three and 13 molecules of CO2 are absorbed for each molecule of methane, making coal seams an excellent storage location for CO2. “More than 180 billion metric tons of CO2 sequestration potential exists in unmineable coal seams…distributed over 24 states and 3 provinces,” according to NETL.
Saline formations
Less understood but very promising is the storage potential of deep saline aquifers, or brine-saturated rock formations that occur deep underground or under the ocean. An analysis by the Massachusetts Institute of Technology (MIT) in 2006 showed that wells deep underground consisting of porous rock, such as limestone or sandstone, saturated with saltwater would form an effective trap for injected CO2. Geologically, over time, some CO2 would react with rock minerals to form solid carbonates, further immobilizing it. Deep saline aquifers could potentially store between 3,300 to more than 12,200 billion metric tons of CO2, according to NETL.
The Sleipner project off the coast of Norway has been using deep saline storage since 1996 as part of their natural gas drilling efforts. CO2 from the project is injected into the Utsira formation, a sandy reservoir 800 meters beneath the North Sea. Twenty thousand tons of CO2 are added to storage each week. After more than 10 years the project continues to be successful.
Additional pilot programs for deep saline aquifer storage are currently under development all over the world.”
Does this seem like a reasonable plan to anyone?
