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Although post-combustion carbon dioxide capture can’t be considered proven at the utility power plant scale, some capture processes have been demonstrated at large pilot or pre-commercial scale facilities, leading suppliers to say it’s ready for full-scale application at natural gas power plants. This information was among the findings and conclusions of an assessment of carbon dioxide capture and storage for California natural gas combined cycle plants discussed at an April 16 California Energy Commission workshop. It’s expected that post-combustion capture would be the easiest approach to integrate with most existing natural gas-powered plants, according to the study. However, the study also states that special consideration must be given to conditions at many California locations, such as high summer temperatures and limited availability of water. Another factor is the requirement for dry cooling of plants built in California. The study was completed for the Energy Commission by Colorado-based energy infrastructure services company CB&I. “The purpose of this (study) was to actually develop in effect a proposal that PG&E or one of the utilities in California would submit for a demonstration,” Mike Gravely, the Energy Commission’s Energy Systems Research Office manager, said. In the current market and regulatory environment, the projected life-cycle costs for a natural gas-fired plant with carbon capture and storage exceed the current cost of a conventional natural gas plant with the purchase of CO 2 emission allowances from California’s cap-and-trade market. However, allowance prices are expected to increase over time and carbon capture technology research and development requires significant lead time. Thus, the Energy Commission has said it’s in the public interest for early capture and storage projects in California, in order to expand the portfolio of economic greenhouse gas reduction options. “For California to reach its long-term 2030 and 2050 goals, we’re going to have to start to seriously consider adding capture of CO 2 for power plants and big producers,” Gravely said. The study found that the requirement for dry cooling, which is common for new natural gas plants built in California, leads to the use of expensive CO 2 capture system cooling water refrigeration when attempting to reach a typical design criterion of 90 percent capture on all but the hottest summer days. An initial evaluation assumed that the capture system cooling water would be chilled in order to maintain the absorption process in the temperature range required to attain a 90 percent capture rate on a hot summer day. The study also found that for dry-cooled systems, substantial savings resulted from relaxing the capture effectiveness criterion to 90 percent CO 2 capture on an average day rather than on a hot summer day. Although such a capture system would then remove less than 90 percent of the CO 2 on hot days, the study says that would be acceptable with a flexible regulatory structure. Lowering the bar on hot days also would help make capture technology suitable for applications where the CO 2 is used for beneficial uses, such as enhanced oil recovery. In situations where modest amounts of cooling water is available, such as reclaimed wastewater or other gray water, the CO 2 capture process unit would have its best performance and lowest cost. The study also examined recirculation of a portion of the flue gas back to the combustion turbine inlet to increase the CO 2 concentration and reduce the flue gas treatment flow. Findings suggested the approach could yield savings in CO 2 capture system capital costs and energy requirements. An “oxy-combustion” approach discussed during the workshop uses chemical looping with oxygen. These approaches reduce the volume of combustion gases that must be treated for CO 2 separation. “Oxy-combustion appears to offer future promise if unique equipment can be successfully and economically scaled up and if the high cost and auxiliary power requirements of oxygen production can be addressed,” the study states. “Considerable development efforts are under way, particularly for units at the small end of utility scale that could be sited in or near oilfields where the separated CO 2 can be used for enhanced oil recovery operations.” Next steps toward using carbon capture and storage at natural gas plants in California were also discussed during the workshop. Among those listed were: CO 2 absorption solvents and advanced capture technologies that improve the functionality of CO 2 capture systems at high ambient temperatures without undue cooling requirements. Optimization studies utilizing CO 2 capture process simulation models in conjunction with technical-economic assessments of alternative cooling approaches for CO 2 capture and compression systems incorporating the unique aspects of California natural gas plants. Private/public investment in a pilot project to evaluate CO 2 capture technologies tailored specifically to California design conditions. The pilot facility might also test technology improvements, such as flue gas recirculation and high temperature solvents. Developments designed specifically for oxy-fuel combustion and pre-combustion CO 2 capture systems. —Mark Edward Nero

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