Process optimization and working fluid mixture design for organic Rankine cycles (ORCs) recovering compression heat in oxy-combustion power plants

In this study, an Organic Rankine Cycle (ORC) is proposed to be integrated with the flue gas pre-compression process to reduce the energy cost resulting from Carbon Capture and Storage (CCS). An equation-based flowsheet optimization model is developed considering the mixture working fluid design, ORC operating conditions and the compression process simultaneously. The optimal number of stages of CO2 compression, the working fluid composition and the optimal operating conditions of ORCs and the compression train can be determined simultaneously using the proposed mathematical model. Proper heat integration can boost the power output of the ORC system significantly. The heat integration model considering variable process streams is extended to the integrated ORC and flue gas compression train process. The results show that the optimal number of stages is 4 and a pure working fluid could perform better than a mixture working fluid if operating conditions are chosen properly. The integration of ORCs can reduce the energy penalty by 7.9% compared with the original optimal design that did not include ORCs. In addition, one compressor stage is avoided.