Technical and Economic Feasibility Of Polygeneration: A Case Study Of Combining CO2–EGS With IGCC

There is growing interest in polygeneration due to a number of factors.  First is the growing supply of natural gas from shale reservoirs that has depressed both natural gas as well as coal pricing.  Second is the interest in integration to achieve various environmental goals.  For example, inclusion of biomass in the feedstock could help reduce the carbon intensity of an otherwise fossil-fueled process, while the efficient use of CO2 from effluent streams could be a solution to expensive sequestration.  Finally, advances in technology are allowing the conceptualization of a number of polygeneration approaches.  Even so, each of these ideas has to be evaluated using rigorous technical and economic assessments.  We demonstrate such an assessment for one such polygeneration concept that seeks to combine IGCC and EGS plants and use CO₂ as a heat transfer fluid.

Utilizing CO₂ as a heat transfer fluid for the extraction of geothermal heat energy for the production of electricity in enhanced geothermal systems (EGS) is attractive especially in the arid regions. CO₂ from an integrated gasification combined cycle (IGCC) plant is more suitable for this purpose as it is discharged at high pressures ready for injection. The combination of pairing an IGCC plant with EGS facilitates the simultaneous extraction of geothermal heat energy and sequestration of CO₂. In this study, the processes of CO₂ flow through both the injection and production wells as well as the power generation using an organic Rankine cycle (ORC) were modeled. Well simulations evaluate the pressure and temperature profile of CO2 in the injection and the production well. Based on the energy of CO₂ leaving the production well, the ORC is modified to maximize the amount of power generated so that the CO₂ can be recirculated to the injection well for heat extraction. The power generated from the ORC was studied as a function of working fluids in ORC and two different geothermal source temperatures 200 °C and 300 °C. Finally, we present preliminary estimates of the lifecycle greenhouse gas emissions and Levelized cost of electricity (LCOE) from such a combined process.