The Need for Carbon Capture & Sequestration
There are three certainties that lend value to carbon capture and sequestration (CCS):
- Combustion of fossil fuels results in carbon dioxide (CO 2) formation. If CO2 formation is caused by humans, it is termed anthropogenic CO 2 formation.
- Concentration of CO 2 in the Earth’s atmosphere has been increasing since the beginning of the Industrial Age.
CO2 is a so-called greenhouse gas, trapping heat normally reflected from the Earth’s surface back into Space. This trapping of heat elevates temperatures at the Earth’s surface. While less certain, a majority of climate scientists hypothesize that the increase in atmospheric CO2 concentration is causally linked to anthropogenic CO2 formation.
The third certainty lending value to CCS is that injection of CO2 enhances recovery of oil from mature oil fields while sequestering CO2. These three certainties make CCS important to consider.
The two principal sources of anthropogenic CO2 , both in the U.S. (see figure below) and in the world as a whole, are the electrical power-generating and transportation sectors (Wilcox, 2012). There has been much emphasis on capturing CO2 from fossil fuel-fired power generation plants. They are stationary sources of CO2 and relatively few in number, and therefore amenable to carbon capture. The transportation sector is currently less amenable due to its mobile and much more numerous CO2 sources.
General Stages of Carbon Capture and Sequestration
The figure below, from a paper by R. Stuart Haszeldine (see For more information for complete reference), illustrates the general stages of carbon capture and sequestration (CCS). Anthropogenic CO 2, formed as a consequence of fossil fuel combustion in electrical power generation (2 in figure), must be captured; there are several approaches CO 2 capture, with the post-combustion capture approach most advanced in its development. Captured CO 2 is compressed and transported by pipeline (3 in figure) to the site of sequestration. CO 2 is injected into sequestration sites (4 in figure), with the two principal possibilities being deep saline aquifers and depleted oil and gas fields. An alternative to these two options is a mature gas field, in which case injected CO 2 enhances oil recovery (1 in figure). Another section of this webpage addresses enhanced oil recovery (EOR).
Prior to sequestration, captured CO 2 must be compressed at high temperature and pressure to its supercritical phase. In this phase, CO 2 has properties of both a liquid and a gas and is therefore denser. This is economically desirable for CO 2 transportation, which is typically accomplished via pipeline.
There are two principal options for the injection of CO 2: deep saline aquifers and depleted oil and gas fields. Both options have advantages and disadvantages. Deep saline aquifers are estimated to have a sequestering capacity of 1000-10,000 gigatonnes of CO 2, whereas depleted oil and gas fields are estimated to have a 675- to 900-gigatonne capacity for sequestration. An additional option is to inject captured CO 2 into producing, so-called mature, oil and gas fields. This approach, which is employed in the Permian basin of Texas and the Weyburn-Midale basin of Saskatchewan, enhances oil production from mature oil fields and results in significant (approximately 50%) of injected CO 2 being sequestered underground during the initial injection. . CO 2 that returns to the surface with produced oil is separated from the oil and re-injected (and sequestered). Another section of this webpage will address this approach, termed enhanced oil recovery.
In evaluating a geological formation for its sequestering potential, there are several important criteria:
- porosity. Greater porosity (in a depleted oil and gas field) permits greater volumes of CO 2 to be stored.
- prevention of CO 2 leakage. As the figure below illustrates (from Kaldi and colleagues; see For more information for complete reference), structural/stratigraphic (depleted oil and gas fields) and solubility (deep saline aquifers) mechanisms are initially responsible for trapping CO 2. The structural and stratigraphic mechanism is actually that responsible for confining oil and gas prior to their extraction. In the long term, mineral trapping assumes a more significant role. Mineral trapping is the incorporation of CO 2 into carbonate minerals. This mechanism is especially secure in that it renders CO 2 immobile.
- depth of the geological formation for sequestration. Deep sequestration is required to prevent interference with relatively shallow groundwater aquifers. If CO 2 leaks into aquifers, acidity of groundwater increases and this increased acidity can mobilize metals such as arsenic and lead. This would have dire public health consequences. Such consequences dictate extensive monitoring, both direct and with remote sensing, of CO 2 movement underground.
Enhanced Oil Recovery
Production from an oil or gas well, while high initially, declines rapidly over the well’s first year or two of life as illustrated in the figure below. The intent of CO 2 injection into an oil reservoir is to attenuate this decline; that is, shift the production curve up and to the right. Enhanced oil recovery (EOR) by CO 2 injection has been used since the 1970s. In the Permian basin of Texas, naturally-occurring CO 2 is used; in the Weyburn-Midale basin of Saskatchewan, anthropogenic CO 2 from a coal gasification plant in North Dakota is used for EOR. Additional oil production due to CO 2 injection is termed incremental production.
CO 2 injection EOR is most appropriate for light, condensate and volatile oil reservoirs, and for carbonate formations that exhibit low porosity. Injected CO 2 and oil are miscible, meaning that they mix in all proportions. Oil becomes less viscous upon mixing with CO 2 and flows more easily to the wellbore.
Whether a combination of carbon capture and EOR is economic depends on the costs of capturing anthropogenic CO 2, compressing and transporting it, as well as those of EOR. These costs are offset by revenue from the incremental oil production and, potentially, the cost of carbon (i.e., a credit if a carbon tax or emissions trading program is in place). The figure below shows that if the prices of oil and carbon are sufficiently high (as in August, 2009), costs of carbon capture and EOR are offset; on the other hand, lower oil and carbon prices (oil price, $40/barrel; carbon price, $15/tonne; February of 2009) result in the process being uneconomic.
CCS/EOR Projects in North America
- Boundary Dam in Saskatchewan: The coal-fired Boundary Dam Unit 3 operated by SaskPower, with a nameplate capacity of 140 megawatts (MW), is the world’s first commercial-scale CCS project. Carbon capture, using a post-combustion amine technology, began in October, 2014 and reduces CO2 emissions by 90% (as well as emissions of other pollutants such as SO2). Parasitic load of the amine technology reduces Unit 3 nameplate capacity to 110 MW. Captured CO2 is piped 40 miles to the Weyburn Oil Unit for EOR. Injected CO2 in Weyburn is sequestered at a depth of approximately one mile. In July, 2016, Boundary Dam Unit 3 surpassed the 1 million-ton captured CO2 milestone.
- Petra Nova in Texas: Petra Nova, otherwise known as Unit 8 of the W.A. Parrish power station operated by NRG Energy southwest of Houston, began operation in January, 2017. Petra Nova has a nameplate capacity of 240 MW; unlike Boundary Dam 3, there is no parasitic load because a gas-fired unit at the Parrish station powers the post-combustion amine technology for carbon capture. Similar to Boundary Dam 3, CO2 emissions are reduced 90%, with captured CO2 piped to the West Ranch Oil Field. Interestingly, NRG Energy built and owns the pipeline, as well as a stake in West Ranch, and therefore acts as its own customer for CO2.
- Kemper in Mississippi: The Kemper County Energy Facility of Southern Company, while not yet operational, will employ CCS. The project has been beset by problems, principally cost overruns (the facility’s cost has increased from less than $3 billion to more than $7 billion) and construction delays (it was originally scheduled to be operating by May, 2014). The technology to be employed at Kemper differs from that used at Boundary Dam and Petra Nova in that pre-combustion carbon capture will be used. This technology is described in detail in the article authored by Rubin and colleagues noted under Additional Resources below. Briefly, coal is initially reacted with steam and oxygen to yield syngas, a mixture of carbon monoxide and hydrogen. The carbon monoxide is then converted to CO2 via reaction with steam; CO2 is subsequently captured. Pre-combustion carbon capture is more efficient because CO2 concentration in the CO2/hydrogen gas mixture is higher than that in flue gas on which post-combustion carbon capture is performed. As explained in the Rubin article, capture improves with increasing CO2 concentration. Similar to the Boundary Dam and Petra Nova projects, captured CO2 will be transported via pipeline to oil fields in Mississippi for EOR. At Kemper, hydrogen, the other component of syngas, will combusted using combined cycle technology that is more commonly associated with natural gas combustion. Nameplate capacity of the facility will exceed 500 MW. In mid-June 2017, the Mississippi Public Service Commission ordered Southern Company to operate the Kemper Facility’s coal-fired units that were to employ pre-combustion carbon capture instead as natural gas-fired units. This order was followed shortly thereafter by a Southern Company decision to suspend work on carbon capture portions of its project. This plant’s history illustrates the challenges of deploying new technologies.
WIEB Briefing Paper [pdf]
P. Jaramillo Journal Article [pdf]
Links to more information
- Hazeldine, RS. Carbon capture and storage: How green can black be?
Science. 325:1647-1652, 2009
This is one in a series of artices from a Special Section of the top-flight journal Science. A particular strength of this article is its section on technical challenges for the major stages of CCS, including capture, transportation and injection/sequestration. The article has outstanding figures, one of which is used in this webpage.
- Rubin ES, Mantripragada H, Marks A, Versteeg P, Kichin J.
The outlook for improved carbon capture technology
Progress in Energy and Combustion Science. 38:630-671, 2012
This review article’s initial sections, which cover the three major approaches to carbon capture, provide excellent explanations of these technologies. Accompanying figures are very helpful for understanding the technologies. There are also interesting sections on the energy penalty for and costs of carbon capture. The article then becomes more technical in its coverage of the various approaches to carbon capture and their outlooks.
- Plains CO2 Reduction Partnership (PCO2R)
This website, which is maintained by the University of North Dakota Energy & Environmental Research Center, reflects work of the Plains CO 2 Reduction (PCOR) Partnership. PCOR is one of the seven regional partnerships funded by the Regional Carbon Sequestration Partnership Initiative of the U.S. Department of Energy. PCO 2R produces the PCO 2R Partnership Atlas, a very professional publication that uses maps and schematic figures to explain CCS.