Combined-Cycle Approach Holds Potential for Offshore-Facility Carbon Capture

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 225466, “Combined Cycle and Carbon Capture on Offshore Installations: History, Status, and Future,” by Pål Kloster, CTO Energy and Techouse AS. The paper has not been peer-reviewed. _ Floating thermal power plants implementing CO2 capture and storage (CCS) provide clean power and enhanced oil recovery to the offshore oil and gas industry. Some have used combined-cycle technology, in which gas turbines (GTs) and steam turbines (STs) are used together to convert fuel into electricity. The complete paper describes the potential of combined-cycle technology in CCS applications for offshore facilities. Features of Combined Cycle With Exhaust-Gas Recycling (EGR) and CCS Most offshore installations use GTs to power the facility. The process-heat requirement normally is covered by heat recovery from the GT exhaust. The GTs exhibit between 30 and 38% power efficiency and represent approximately 85–95% of CO2 emissions from offshore oil and gas installations. Introducing combined cycles where pressurized steam is produced by heat recovery of the GT exhaust and expanded over an ST will reduce the fuel gas requirements and CO2 emissions by approximately 25%. Probably the most-studied technology to clean CO2 from GT exhaust gas is by using an absorption process based on amines. The size and, consequently, the cost of such a cleaning process is highly dependent on the volume flow of exhaust gas and the percentage of CO2 removed. Before entering the amine absorber, the exhaust gas must be cooled to approximately 35°C. The cooling is performed in direct-contact coolers (DCCs), components normally requiring space separate from the amine plant. A way to reduce the volume flow to the absorber is to recycle cooled exhaust gas to the air intake of the GTs, a process known as EGR. This is possible because GTs run with large amounts of excess air, leaving approximately 13 vol% of oxygen in the exhaust gas. Recycling of a portion of the exhaust gas will replace some of the inlet air flow to the GTs, increase CO2 concentrations, and reduce exhaust flow to the CCS plant. This will reduce the size of the amine absorber and the CCS plant. Designing and arranging the EGR system efficiently will yield a net reduction in investment cost, size, weight, and space requirement. Quantitatively, the EGR flow will be between 40 and 50% of the exhaust gas flow. The result is an approximate doubling of CO2 concentration and 50% reduced flow to the amine absorber. The flow area and the overall weight of the absorber is reduced by 40–50%. History and Current Status Combined Cycles Offshore. The first combined-cycle plants on offshore oil and gas installations were installed in the North Sea in the late 1990s, but they are now operating globally. However, to achieve a 20–25% reduction in fuel gas consumption and CO2 emissions using combined-cycle technology, several hurdles must be overcome for the concept to fully penetrate the offshore market.