Effects of process-thermal configuration on energy, exergy, and thermo-economic performance of solar driven supercritical water gasification

Abstract An investigation of the effect of process-thermal configuration on the thermodynamic and economic performance of a solar driven supercritical water gasification (SCWG) of microalgae biomass has been presented in this study. Traditional and intrinsic exergy analysis (transiting and utilizable exergy) of the base SCWG model (Configuration I, four heat exchangers) were used to assess other process-thermal configuration options for optimal performance. Two alternative process configurations (II and III; three and two heat exchangers) were re-produced from the first SCWG configuration and analysed. The two new configurations were modelled by considering the inlet temperature of the water separator units, flow exergy, heat transfer optimization, and point of entry of recycled hot water. The results show that higher SCWG gasification temperature resulted in increased energy efficiency, carbon efficiency, and exergy efficiency with Configuration III having the best performance at 600–610 °C and 1000–1100 °C for optimal gasification and reformer temperature, respectively. While energy efficiency increased with microalgae concentration, exergy efficiency declined due to losses at the heat exchangers, thermodynamic irreversibilities associated with increased char formation and higher system transiting exergy. Lower inlet exergy in the high-temperature separator unit (HT-SEP) resulted in higher transiting (unaccounted) exergy and less utilizable exergy for improved process conversion and overall system efficiency. Increased utilizable exergy was achieved through minimum heat recovery by reducing the number process streams and heat exchangers leading to higher exergy potential for the HT-SEP and minimal process heat duty. The intrinsic exergy efficiencies of the HT-SEP unit, for instance were 58.1%, 72.5% and 82.4% compared to traditional exergy efficiencies at 44%, 23.8%, and 15.1%, for the three configurations, respectively. The best performing SCWG configuration had overall energy efficiency, thermal energy savings, intrinsic exergy efficiency, and carbon efficiency at 81.26%, 56.84 MW, 75.44%, 98.68%. The findings of the exergy assessment in this study establishes intrinsic exergy analysis, as opposed to traditional exergy analysis, as a more realistic assessment and understanding of process efficiency and performance for SCWG processes. Configuration III with a solar PTC-natural gas energy integration resulted in the lowest MFSP (∼49 AUD/GJ) and the closest to the target price for renewable energy-based syngas which emphasizes the benefits of process-energy configuration trade-offs for improved techno-economic performance.