Heat-recovery steam generators: Understand the basics

Gas turbines with heat-recovery steam generators (HRSGs) can be found in virtually every chemical process industries (CPI) plant. They can be operated in either the cogeneration mode or the combined-cycle mode. In the cogeneration mode, steam produced from the HRSG is mainly used for process applications, whereas in the combined-cycle mode, power is generated via a steam turbine generator. Recent trends in HRSG design include multiple-pressure units for maximum energy recovery, the use of high-temperature superheaters or reheaters in combined-cycle plants, and auxiliary firing for efficient steam generation. In addition, furnace firing is often employed in small capacity units when the exhaust gas is raised to temperatures of 2,400--3,000 F to maximize steam generation and thus improve fuel utilization. This article highlights some of the basic facts about gas turbine HRSGs. This information can help plant engineers, consultants, and those planning cogeneration projects make important decisions about the system and performance related aspects.