Improvements in efficiency and reliability have made gas turbines a major factor for expanding power plant capacity. And, adding a heat recovery steam generator (HRSG) and Rankine steam cycle can increase overall combined-cycle thermal efficiency to as high as 85 percent on a lower heating volume (LHV) basis.
In addition to being highly efficient systems, gas-fired turbine combined-cycle HRSG technologies, also offer such advantages over conventional fossil-fueled plants as cycling and peaking service capability, low operating costs, and reduced greenhouse gas emissions.
HRSGs are specialized waste heat recovery boilers used with gas turbines and designed for large gas volumes with minimum pressure drop so the impact on gas-turbine efficiency is minimized. Using an HRSG with auxiliary or supplemental fuel firing in a duct burner can increase steam production, control steam superheat temperature, or meet process steam requirements. HRSG designs can also directly incorporate selective catalytic reduction (SCR) technology for NOx control.
Originally designed to produce steam at one pressure level, today's HRSGs may have three pressure levels with superheat and reheat; and they may be once-through, or combined drum/once-through systems. In addition, HRSG manufacturers offer many different variations within these basic designs.
However, as their designs have become increasingly complex over the last 10 years or so, HRSG systems have also had their share of numerous failures and operating problems with thermal cycling being the key factor contributing to such system failures. Cumulative fatigue damage from cycling, unfortunately, cannot be reversed.
Waste Heat Recovery System
According to a recent EPRI study, the most predominant HRSG failure mechanisms are corrosion fatigue and thermal fatigue, followed by flow-accelerated corrosion and pitting. An analysis of EPRI's study indicates that chemistry cycles are responsible for over 70 percent of lost generation costs, much higher than in conventional fossil plants.
Failures that have been occurring for many years in conventional power plants, such as under-deposit corrosion (hydrogen damage, acid phosphate corrosion, and caustic gouging) are predominant in HRSGs. The major additional high temperature failure mechanism for HRSGs, according to the EPRI study, is flow-accelerated corrosion of evaporator circuits.
Therefore, the successful operation of combined-cycle power plants requires operators to initially consider and subsequently properly optimize cycle chemistry - while maintaining full control and knowing what to do in emergency situations.
Ensuring HRSG efficiency
Operations and maintenance of HRSGs play a crucial role in ensuring HRSG efficiency, for new as well as for existing power plants.
With existing HRSGs, operators should explore system modifications that can boost the reliability and extend the service life of HRSGs. At a minimum, operators should implement a monitoring program to gage the effects of cycling.
A typical combined-cycle HRSG produces "main steam" at three pressure levels, plus "reheat steam." Supplemental firing through a duct burner often is installed as well to increase the steam-production capacity. Most combined-cycle plants also have a feedwater heater to increase the heat recovery and reduce the deaerating load of the integral deaerator.
Heat-transfer sections of the HRSG are located in series along the exhaust-gas path to optimize the heat recovery. It is not uncommon to have 15 to 20 different heat-transfer sections-superheaters, reheaters, evaporators, and economizers-at various locations along the gas path.
And, emissions-control regulations often require the addition of a selective catalytic reduction system (SCR) for NOx control and, in some cases, a separate catalytic converter for CO control. Some HRSGs may also be equipped with an exhaust-gas bypass damper, to enable simple-cycle gas-turbine operation.