| Title | Using CO2 from an IGCC Plant as a Heat Transfer Fluid for the Extraction of Geothermal Energy for Power Generation from EGS |
|---|---|
| Authors | Arun RAM MOHAN, UDAY Turaga, Vishakha SHEMBEKAR, SUBBARAMAN Viswanathan, Derek ELSWORTH, Sarma PISUPATI |
| Year | 2013 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Enhanced Geothermal Systems, CO2, IGCC, Aspen plus, Modeling |
| Abstract | The greatest geothermal heat potential in the United States underlies arid regions where the scarcity of potable water makes using water as a heat transfer fluid problematic. There is a necessity to replace water with a cheap and abundant fluid. Carbon dioxide is one such fluid with 5.6 gigatons emitted from the energy and the industrial sector in the United States in 2010. In this work, we explore the possibility of using carbon dioxide (CO2) as a heat-transfer fluid in enhanced geothermal systems (EGS) by organic Rankine cycle (ORC). This addresses both the dual issues of water scarcity and CO2 sequestration in a symbiotic manner. As CO2 is available for sequestration at high pressures from an Integrated Gasification Combined Cycle (IGCC) plant, this idea is examined by pairing an IGCC plant with appropriate EGS plant to facilitate both the simultaneous extraction of geothermal heat and sequestration of CO2 as well as power generation from EGS. The ORC portion of EGS was modeled by ASPEN Plus version 7.3. A 315 MWe IGCC plant using Pittsburgh no 8 coal is used as a basis for the study. It emits 68 kg per sec of CO2 which is compressed to 15 MPa at 60 °C for circulation in the EGS reservoir. Similar to water loss at other EGS demonstration projects, we assume that 10% of the total CO2 injected into the geothermal reservoir is lost from the reservoir and is sequestered on some time scale. The mass circulation rate of CO2 in the reservoir is therefore 680 kg/s. Two deep (6 km) geothermal sources are considered, at temperatures of 300 °C and 200 °C, respectively with circulation divided between multiple wells to retain individual injectors at less than 100 kg /s (10 wells at 68 kg/s each). Well simulation studies are conducted in ASPEN Plus version 7.3 with pipe borehole diameters decreasing from the surface to 6 km depth. The pressure and the temperature profile of CO2 along the injection well and the production well obtained from the well simulation suggests that the pressure of CO2 leaving the production well is substantially higher than the injection pressure. The temperature of CO2 leaving the production well is lower than the geothermal source temperature. The potential of the CO2 leaving the production well to generate power is studied by modifying an organic Rankine cycle. The CO2 leaving the production well expands through a high pressure turbine to 15 MPa and then transfers its thermal energy to the working fluid in the ORC so that it is discharged at 15 MPa and 30 °C and can then be recirculated to the reservoir via the injection well. The power output from the ORC is studied as a function of ambient temperature, geothermal source temperature and working fluids. Performance of various ORC working fluids such as neopentane, ammonia, n-butane, isobutane and isopentane was compared. Pairing IGCC with EGS, in addition to producing electricity from the organic Rankine cycle, recovers the work done to compress the CO2 from IGCC for sequestration. Finally, the second law efficiency of such a system is compared with an organic flash |