In an intriguing footnote to their historic climate deal this month, Chinese President Xi Jinping and U.S. President Barack Obama called for demonstration of a hitherto obscure tweak to carbon capture and storage (CCS) technology that could simultaneously increase its carbon storage capacity and reduce its thirst for water. Such an upgrade to CCS holds obvious attraction for China, which is the world's top carbon polluter and also faces severe water deficits, especially in the coal-rich north and west.
As the Union of Concerned Scientists puts it in its The Equation blog, “Cracking this nut … could be a huge issue for China.”
Obama and Xi's deal pledges joint funding for a project that would inject about 1 million tons per year of captured carbon dioxide deep underground and, in the process, produce approximately 1.4 million cubic meters of water annually. One potential target is is GreenGen, an advanced coal-fired generating plant in Tianjin that was explicitly designed as a CCS test bed.
Such ‘enhanced water recovery’ can be understood as an extension of the CCS-based enhanced oil recovery that is financing installation of carbon capture equipment at several North American coal-fired power plants. These include the upgraded coal generator in Estevan, Saskatchewan, that recently became the first coal plant to capture its CO2. Canadian utility SaskPower sells 1 million tons (1 megaton) of compressed CO2 to an aging oilfield nearby where it is pumped down into the oil-bearing formation to accelerate the upward flow of petroleum.
If enhancing oil production is a revenue option for CCS, producing water with CCS is primarily about easing the storage of captured CO2 in deep saline aquifers. Geologists see carbonating saline aquifers as the most likely storage target for CCS if it becomes a universal aspect of fossil fuel power generation. Bringing up briny water in the process is not as lucrative as oil production, but offers some potentially significant benefits, starting with decreasing the pressure of the aquifer.
High pressures could fracture overlying rock layers and release injected CO2—a threat highlighted by unforeseen surface deformation detected five years ago atop a CO2 injection site in Algeria. None of the 3.85 megatons of CO2 stored in the 2-kilometer-deep reservoir escaped, but operators prematurely terminated CO2 injection, and anxiety over leakage risks paralyzed Germany’s leading CCS project. In May, the lead company on the German project, European power giant Vattenfall, terminated its CCS research and development activities.
Lowering pressure can reduce risk and/or enable faster and more extensive injection of CO2. A 2012 report on water removal from the International Energy Agency in Paris found that one CCS megaproject in Australia (Chevron's nearly-completed Gorgon liquified natural gas project) could boost CO2 storage capacity from 97.3 megatons to over 600 megatons by producing water.
The displaced water, meanwhile, can be put to good use—assuming it can be desalinated. One needs look no further than the coal-fired power plants that release most of China's CO2. Those plants use lots of water for cooling, and adding carbon capture is likely to make them consume even more.
In July, at one of the bilateral meetings that laid the groundwork for this month's presidential deal, the U.S. and China's joint Clean Energy Research Consortium commissioned a feasibility study for enhanced water recovery near the GreenGen plant in Tianjin. Their findings, presented in September at a U.S.-China CO2 emissions symposium in Hangzhou, were an unreserved endorsement of this technique, according to Bill Bourcier, a geochemist at Lawrence Livermore National Lab in California and a member of the study team.
GreenGen uses integrated gasification combined cycle (IGCC) technology that is supposed to facilitate carbon capture. According to the feasibility study, GreenGen is currently selling 0.1 MT of captured CO2 per year to beverage producers, and is preparing to increase the amount it captures to between 1 and 2 MT per year. While the China Huaneng Group, which is GreenGen’s majority stakeholder, has previously said it would sell its CO2 for use in enhanced oil recovery, the U.S. and Chinese research team was tasked with quickly evaluating whether CO2 could be safely stored in saline aquifers around Tianjin that are bounded by numerous geological faults.
The study found that removing water would substantially lower the risk of CO2 leakage and simultaneously provide an economically viable source of industrial water.
The report conservatively estimates the cost of purifying the relatively dilute water at RMB14-18 ($2.28-2.93) per cubic meter. That figure is twice the cost of desalinating seawater in California, but Bourcier says the real cost is likely much lower, perhaps as low as 50 cents per cubic meter. Either way, he says, it's “small potatoes” within the total operating budget of a coal-fired power station.
The optimal CO2 injection site Bourcier et al identified is 8 kilometers from the Tianjin plant site. According to their plan, before injection would begin in the 2-kilometer-deep aquifer, water would be extracted for six months. Thereafter, CO2 injection at that site would be accompanied by ongoing water removal via additional wells a few kilometers away (see image above). Bourcier says the binational team has been asked to deliver more precise numbers by mid-December; the operation could be running within a year if it gets a green light.
Chinese researchers, meanwhile, are evaluating alternate sites for an enhanced water recovery demonstration. Qi Li, a professor at the Chinese Academy of Sciences' Institute of Rock and Soil Mechanics in Wuhan, recently completed a national assessment of Chinese CCS potential. Qi told me via e-mail earlier this year that he is seeking funds for a demonstration in Western China's Xinjiang Province.
Xinjiang is not just where Li found the greatest capacity for CO2 storage in saline aquifers (44,680 megatons) and the most potential for enhanced water recovery (355 billion gallons). Xinjiang also has China's second largest coal reserves and a dearth of potable water supplies. The province is a target for coal-based chemical and liquid fuels production, which consumes roughly 10 tons of water for every ton of product. But demand for potable water there is already several times greater than its sustainable supply. Qi makes the case for Xinjiang's predicament/opportunity in this 2013 presentation.
Bourcier, meanwhile, is looking for opportunities to demonstrate enhanced water recovery in the United States. His group is interested in a saline aquifer near Bakersfield, California, that has a risk profile similar to the Chinese site. As Bourcier puts it, there would be no shortage of takers for the recovered water: “It is an opportune time to have a freshwater by-product in California.”