(271b) Coal Oxycombustion Flowsheet Optimization

Increasing atmospheric concentrations of carbon dioxide have unquestionable sparked scientific interest in developing fuel sources with reduced carbon footprints. Many researchers have begun developing technologies to enable carbon capture, utilization and sequestration with traditional fossil fuels. All of these new powerplant flowsheets require some separation to purify CO2 to high concentrations for utilization or sequestration at high pressures. In a coal oxycombustion process the additional separation happens at beginning. Oxygen is concentrated from air, and the coal is burned in a very nitrogen light environment. Thus the flue gas leaving the boiler contains mainly water and CO2. Unlike some other coal carbon capture schemes, large separation units are not required post-combustion. There is also some potential to retrofit existing coal power plants for oxycombustion.

Coal oxycombustion on a large scale is a very immature technology; heuristic methods tailored to design near high performance oxycombustion plants do not exist. Furthermore the oxyfuel combustion flowsheet is highly integrated; changing one part of the flowsheet can dramatically impact the performance of other units. This makes design and optimization especially challenging. However this also means the oxycombustion design problem an excellent test-bed to develop new flowsheet optimization technologies.

An example oxycombustion design dilemma is centered on the massive compressor train used to compress CO2 to 150+ atm for sequestration. Some members of the oxycombustion research community are advocating for adiabatic CO2 compression with only a few stages. Although this strategy requires additional mechanical work, it also heats the high pressure CO2 stream. Some of this energy can then be recovered through heat integration with the steam cycle. Other members of the community advocate for using streams in the cryogenic air separation unit for interstage compressor cooling, thus reducing the mechanical work for compression. Due to the integrated nature of the flowsheet both approaches have the potential to impact the design of every major unit. Without a framework to optimize the flowsheet, selecting the best compression strategy remains difficult and un-systematic task.

In this talk we present our efforts to answer the compressor design question through use of advance flowsheet optimization techniques. More specifically we employ a fully equation oriented optimization approach. All model equations, including thermodynamics, and their derivatives are open to the optimization algorithm. This allows us to utilize start of the art large scale nonlinear programming solvers. We present our models for each unit, along with our experiences incorporating cubic equations of state (non-ideal thermodynamics) and heat integration into the optimization problem. We conclude by presenting our plans to extend these developments to the entire oxycombustion flowsheet.