Numerical optimization of steam cycles and steam generators designs for coal to FT plants

In this work, a recently developed optimization methodology for the design of heat recovery steam cycles (HRSCs) and steam generators (HRSGs) is applied to the design of steam cycles for two interesting coal to Fischer-Tropsch (FT) fuels plants: a FT synthesis process with high recycle fraction of the unconverted FT gases (CTL-RC-CCS) to maximize the production of liquid fuels, and a FT synthesis process with once-through reactor (CTL-OT-CCS). The analysis reveals that designing efficient HRSCs for the once-through FT plant is relatively straightforward, while designing the HRSC for plant CTL-RC-CCS is very challenging because the recoverable thermal power is concentrated at low temperatures (i.e., below 260 °C) and only a small fraction can be used to superheat steam. In addition, it is necessary to establish whether it is advantageous to burn the FT off-gases in a boiler or in a gas-steam turbine combined cycle. For this reason, a detailed boiler model is developed and included into the optimization methodology. As a consequence of the improved heat integration, the electric efficiency of the two plants is increased by about 2 percentage points with respect to the solutions previously published.