Carbon Reduction Methods
By: Seth Vaughn
Carbon sequestration is capturing the carbon emissions from burning fossil fuels and storing them safely away from the atmosphere. In pre-industrial times, every million molecules of air contained about 280 molecules of carbon dioxide. Today that proportion exceeds 380 molecules per million, and it continues to climb. A chemical system for capturing carbon dioxide is already used at some facilities for commercial purposes, such as beverage carbonation and dry ice manufacture. The same approach could be adapted for coal-burning electric power plants, where smokestacks could be replaced with absorption towers. One tower would contain chemicals that isolate carbon dioxide from the other gases (nitrogen and water vapor) that escape into the air and absorb it. A second tower would separate the carbon dioxide from the absorbing chemicals, allowing them to be returned to the first tower for reuse. Another method is burning coal in pure oxygen rather than ordinary air. That would make separating the carbon dioxide from the exhaust much easier, as it would be mixed only with water vapor, and not with nitrogen.
Several underground possibilities have been investigated. Some places include old gas and oil fields. Storage in abandoned oil fields, for example, offers an important economic advantage — the carbon dioxide interacts with the remaining oil to make it easier to remove. Injecting carbon dioxide dislodges oil trapped in the pores of underground rock, and carbon dioxide’s presence reduces the friction disabling the flow of oil through the rock to wells. Depleted oil and gas fields do not, however, have the capacity to store the amounts of carbon dioxide that eventually will need to be sequestered. By some estimates, the world will need reservoirs capable of containing a trillion tons of carbon dioxide by the end of the century. Concerns about leaks suggest to some experts that the best strategy might be literally deep-sixing carbon dioxide, by injecting it into sediments beneath the ocean floor. High pressure from above would keep the carbon dioxide in the sediments and out of the ocean itself. It might cost more to implement than other methods, but it would be free from worries about leaks. And in the case of some coastal sites of carbon dioxide production, ocean sequestration might be a more attractive strategy than transporting it to far-off sedimentary basins. It is also possible that engineers will be able to develop new techniques for sequestering carbon dioxide that are based upon natural processes. For example, when atmospheric concentrations of carbon dioxide increased in geologic times to a certain unknown threshold, it went into the ocean and combined with positively charged calcium ions to form calcium carbonate – limestone. Similarly, engineers might devise ways of pumping carbon dioxide into the ocean in ways that would lock it eternally into rock, which makes the carbon dioxide harmless to the atmosphere and organic life. Capture and sequestration technologies are currently available and can dramatically reduce (by 80-90%) CO2 emissions from power plants that burn fossil fuels. Applied to a 500 megawatt coal-fired power plant, which emits roughly 3 million tons of CO2 per year, the amount of GHG emissions avoided (with a 90% reduction efficiency) would be equivalent to:
- Planting more than 62 million trees, and waiting at least 10 years for them to grow.
- Avoiding annual electricity-related emissions from more than 300,000 homes.
After capture, carbon dioxide (CO2) is compressed and then transported to a site where it is injected underground for permanent storage (also known as “sequestration”). CO2 is commonly transported by pipeline, but it can also be transported by train, truck, or ship.
The U.S. Department of Energy estimates that anywhere from 1,800 to 20,000 billion metric tons of CO2 could be stored underground in the United States. That is equivalent to 600 to 6,700 years of current level emissions from large stationary sources in the United States. the compressed CO2 is injected deep underground into solid, but porous rock, such as sandstone, shale, dolomite, basalt, or deep coal seams. Suitable formations for CO2 sequestration are located under one or more layers of cap rock, which trap the CO2 and prevent upward migration. These sites are then rigorously monitored to ensure that the CO2 remains permanently underground because safety is a main priority for the Environmental Protection Agency.