Mapping the waste heat recovery potential of CO2 intercooling compression via ORC

Abstract

The goal of the present work is the systematic mapping of the electricity savings potential of waste heat recovery (WHR) from CO2 compression intercoolers via Organic Rankine Cycle (ORC) technology. Α generalized approach covering a broad range of target CO2 discharge pressures from 50 to 500 bar for 3 to up to 8 compression stages is followed. The maximum, theoretical electricity savings based on the exergy of the heat transfer fluid (HTF) used for intercooling range between 10 % and 24 %. Considering the exergy efficiency of optimized air-cooled ORCs operating with alkanes and hydrofluoroolefins for HTF temperatures between 70 °C and 130 °C, the maximum electricity savings ratio (i.e., the electricity savings compared to the total electricity consumption of the compression unit) is about 5 %, while the installed power of the WHR-ORC ranges from about 3 to 23 kWe per kg/s of compressed CO2. Because of the intense variability of CO2 specific heat capacity over a wide range of pressures, there is a very strong dependence of the waste heat potential on the final discharge pressure and number of stages. Through optimized mixing of the HTF streams, the performance of the WHR-ORC can be improved for particular discharge pressure ranges. Proper sizing of the ORC air-cooled condenser for minimizing air-side pressure drop is critical to ensure the feasibility of WHR-ORCs. According to a techno-economic analysis, WHR-ORCs can result in discounted payback periods from 6 to 12 years and 2.5 to 4 years for 4000 and 8000 operating hours, respectively.