Exergy analysis of IGCC with CO₂ precombustion capture system

The high energy penalty for CO₂ capture is one of the largest barriers for implementing IGCC technology with CCS. Based on the first IGCC plant-Tianjin GREENGEN project in China, this paper is to analyze the constitutions and inner causes of the energy penalty for CO₂ capture in IGCC through exergy analysis and to investigate the energy penalty decrease potential through energy level analysis. Results show that the energy penalty for CO₂ capture is around 8.1 percentage points when the capture rate is 90%. Direct energy penalty which is caused by separation and compression of CO₂ takes about 35~40% of the total and indirect energy penalty including the steam consumption and the exergy losses of fuel caused by syngas shift reaction takes the rest. By optimization of the separation process and system heat integration, we can reduce the energy penalty for CO₂ capture to about 4.0~4.5 percentage points under 90% capture rate. Besides, as the CO₂ capture rate goes up, the direct energy penalty decreases with the increasing CO₂ purity before separation but the indirect energy penalty increases with the greater exergy losses of syngas and steam consumption for shift reaction. Thus, there is an optimal CO₂ capture rate that makes the energy penalty for unit CO₂ capture minimum.