The current approach to carbon capture and sequestration (CCS) from, for example, pulverized coal-fired power plants, is not economically feasible without either large subsidies or a very high price on carbon. Current schemes require roughly 1/3rd of a power plant’s energy for CO2 capture and pressurization, and neither merchant nor regulated utilities can accommodate this magnitude of added cost.
The University of Texas at Austin is proposing an approach that would combine two technologies, introduce a third, and could reduce the cost of CCS to a point that CCS would survive in a competitive market environment without subsidies or a price on carbon.
Specifically, the production of energy from geothermal aquifers and the sequestration of carbon dioxide and other greenhouse gases in deep, saline aquifers has evolved as separate, independent technologies. We are proposing a game changing new idea that combines these two technologies and adds another: dissolution of carbon dioxide into extracted brine which is then re-injected. Additional elements are the production of energy from the extracted brine to help offset the cost of capture, pressurization, and injection and the subsequent injection of brine containing carbon dioxide back into the aquifer.
Production of energy from extracted brine helps to offset the cost of capture, pressurization, and injection.
The conventional vehicle for carbon (CO2) capture and pressurization from flue gas requires upwards of 30 percent of a power plant’s energy. This cost is prohibitive, and cannot be significantly reduced because of the thermodynamic limit (~12%) for conventional capture methods and pressurization requirements. In addition, there are some fundamental problems with current carbon capture, pressurization, and sequestration methods (CCS). These include, but are not limited to, the need to pressurize sufficiently to overcome aquifer pressure for injection; rapid increase in back pressure resulting from limited CO2 solubility and diffusivity in the aquifer; and the “pure cost” of CCS with no offsetting benefit [in the absence of a cost for CO2 emission (e.g., cap and trade or carbon tax)].
We are changing the manner of CO2 injection, taking advantage of both dissolved methane and heat content of the saline aquifer water. Instead of injecting CO2 into the aquifer, the saline water is pumped to the surface, and the CO2 captured from the flue gas is injected under modest pressure (~1,000 psi) into the water. This immediately reduces the cost of CO2 pressurization. Further, when CO2 contacts water with dissolved methane, the methane is expelled from solution resulting in a wave front of methane that can be captured, and then either sold commercially or used to generate the lost electrical energy through CO2 capture. The production of methane under such conditions has actually already been observed in the field. Further, the saline water comes to the surface from original reservoir temperatures of the order of 300o F. This heat can be used to assist the energy required for CO2 capture, with preliminary estimates suggesting offsets comparable to the value of the released methane. Pressurization is required to return the saline water with injected CO2 into the aquifer, but injection is aided by gravity and less costly energetically than pumping the same amount of CO2 directly into the aquifer. Realistic simulations would optimize the surface CO2 injection pressure and the saline water (containing the CO2) injection pressure into the aquifer. Finally, the proposed operation takes saline solution from a different portion of the aquifer than the returned saline water. This provides a much more robust permanence for CO2 storage.
The current simulations are for idealized methane-saturated saline aquifers, but with realistic properties and values of dissolved methane measured from actual test wells. These simulations indicate that the combined value of the methane and heat energy from the produced saline water is of the same order as the cost of separating, pressurizing and injecting the CO2.
Geologic studies have shown that there is a huge amount of methane dissolved in aquifers near and along the Texas, Mississippi, and Louisiana coasts.
The University of Texas at Austin is proposing ways to offset the cost of CCS to a point where it becomes commercially viable without subsidies or a price on carbon. The global interest in CCS could result in U.S. technological leadership, with job creation and economic advantage.
Written by Raymond Orbach, Energy Institute at the University of Texas
