Exergetic Assessment of a Syngas-Redox-Based IGCC Plant for Generating Electricity

Carbon capture from advanced integrated gasification combined-cycle (IGCC) processes should outperform conventional coal combustion with subsequent CO₂ separation in terms of efficiency and CO₂ capture rates. This paper provides a thermodynamic assessment, using an exergy analysis of a syngas redox (SGR) process for generating electricity. The power island of the proposed process uses syngas produced by coal gasification and is then cleaned through a high-temperature gas desulfurization (HGD) process. Hematite (Fe₂O₃) is used as an oxygen carrier to oxidize the syngas. To achieve a closed-cycle operation, the reduced iron particles are first partially re-oxidized with steam and then fully re-oxidized with pressurized air. One advantage of this design is that the resulting hydrogen (using steam in the re-oxidation section) can be utilized within the same plant or be sold as a secondary product. In the proposed process, diluted hydrogen is combusted in a gas turbine. Heat integration is central to the design. Thus far, the SGR process and the HGD unit are not commercially availiable. To establish a benchmark, the rate of exergy destruction within the SGR process was compared to a coal-fed Shell gasification IGCC design with Selexol-based precombustion carbon capture. Some thermodynamic inefficiencies were found to shift from the gas turbine to the steam cycle and redox system, while the net efficiency remained almost the same. A process simulation was undertaken, using Aspen Plus and the engineering equation solver (EES).