Fuel Gas Expansion
Texaco and others have proposed the use of a quench IGCC design in which the gasification is conducted at a higher pressure of about 70 bar (e. g., Allen and Griffiths 1990). Additional power can then be generated by expansion of the clean fuel gas from 70 to about 20 bar followed by gas reheat and firing in the gas turbine. The gas cooling by an expander can also be used to provide much of the refrigeration demand of a physical absorption desulfurization system (Zwiefelhofer and Holtmann 1996). Fuel gas expansion is being used in two of the heavy oil IGCC plants at Falconara and Priolo in Italy, which also increases the power generated in these plants.
Efficiencies
The efficiency of gasification is at best about 80%, which, assuming 60% for the CC, implies that the overall efficiency of an IGCC will not be much higher than 80×60/100=48%. However, for virtually all gasifiers an oxygen plant (ASU) is required and the gas treating also requires energy, and hence the efficiencies of the best first-generation plants are all below 45%. And even the lower figures that are reached require a fair amount of integration.
For a further understanding of the cycle, a Sankey diagram of a refinery residue — based IGCC is shown in Figure 7-19.