It has been more than 10 years since the first commercial carbon dioxide (CO2) capture and sequestration system motivated by greenhouse gas reduction was placed into service. In 1991, Norway became the world's first country to impose a tax on CO2 emissions from point sources, to the tune of $55/ton. Five years later, Statoil began injecting CO2 beneath the bottom of the North Sea to avoid the stiff carbon tax. Today, the state-owned firm is injecting about a million tons of CO2 per year, in the process saving about $55 million a year in taxes. That's a pretty good ongoing return on an $80 million investment.
In North America, EnCana's Weyburn, Sask., field tertiary oil-recovery project dwarfs all similar sequestration projects. The CO2 by-product of the Great Plains Synfuels Plant in North Dakota is collected and transported north to an oil field in Saskatchewan through a 200-mile pipeline and injected underground, extending the field's productive life (Figure 1). The CO2 acts much like a solvent, removing oil trapped in cracks of reservoir rock. In Saskatchewan, the results have been impressive: a two-thirds increase in oil production from the field since CO2 flooding began in 2000. The project is expected to permanently store 20 million tons of CO2 over its lifetime.
The differences between these two projects go well beyond their technical details and original motivations. The current U.S. plan for carbon capture and sequestration lies somewhere between Norway's top-down regulatory approach and the free-market partnership between Weyburn and Great Plains Synfuels. Washington envisions meeting the carbon challenge with government-industry partnerships seeded by federal money that industry would match.
In the U.S., there are seven regional carbon sequestration partnerships spanning 40 states (Table 1), and they are poised to scale up their research and pilot plant operations as the third phase of a multidecade effort. The partnerships spent much of 2003–2005 characterizing the regional opportunities for capture and storage of CO2 in North America and publishing the National Carbon Sequestration Atlas and Geographic Information System and other materials. Phase 2 began in 2005 with field evaluations that will continue through 2009. Phase 3 is the deployment phase; several high-volume (up to 1 million tons/year) sequestration pilot projects are scheduled to be built in North America between now and 2016.
The U.S. Department of Energy (DOE) sequestration program is funding a diverse portfolio of around 70 different R&D projects with a projected 2007 budget of around $74 million. Many of the projects enjoy strong industry support; the private sector is providing 39% of their funding, on average. U.S. investment in the sequestration R&D program to date is on the order of $260 million.
First things first
Over the past 160 years, atmospheric levels of CO2 have risen from around 280 ppm to 360 ppm. The increase has been caused primarily by skyrocketing growth in the combustion of fossil fuels by vehicles, factories, and power plants. Predictions of global energy use this century suggest continued increases in carbon emissions and atmospheric concentrations of CO2 unless major changes are made in the ways we produce and use energy, and in how we manage carbon.
In the U.S., power generation accounts for about one-third of national, man-made CO2 emissions. Creating a power plant that emits no carbon (the goal of DOE's FutureGen effort) will require the simultaneous development of carbon capture and sequestration technologies. Sequestration projects such as the ones in Norway and the U.S. described above will be only marginally useful unless the tonnage of CO2 emitted by power plants can be reduced considerably. Likewise, a CO2-capturing integrated gasification combined-cycle (IGCC) plant without a place to safely store the gas will accomplish just as little.
State-of-the-art conventional generation technologies are still growing in thermal efficiency, and we could see a further improvement of 4% to 5% over the next decade or so (Table 2). New alloys being developed for ultrasupercritical boilers, and steam turbines may push the efficiency of plants based on them to 50% to 52% by 2020, and to 52% to 55% by 2050. Table 2 also lists the range of CO2 emissions for each of the power generation technologies considered.
Many CO2 capture options
Power engineers would be wise to gain a understanding of the growing role that coal gasification in general, and IGCC in particular, will play in clean electricity production worldwide over the next decade. At present, it appears that the carbon-capture portion of future IGCC plants will be based on one of four general technological approaches:
- Postcombustion CO2 capture
- Oxy-fuel combustion
- Precombustion decarbonization
- A potpourri of novel concepts that resist categorization
Each technology has advantages and disadvantages. Some have been proven in the chemicals production industry; others, though holding much future promise, are still in the laboratory development stage. The remainder of this article explores these four categories in greater detail.
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