HRSG and gas path
Heat Recovery Steam Generator
Also known as HRSG, heat-recovery steam generator.
A Heat Recovery Steam Generator (HRSG) recovers heat from the exhaust of a gas turbine to generate steam — the second cycle of a combined-cycle gas turbine (CCGT) power plant. HRSGs raise overall plant efficiency from the ~38% of a simple-cycle gas turbine to 55–62% of a modern combined-cycle plant.
HRSG layout
A typical HRSG contains multiple finned-tube tube banks arranged in series along the gas-path direction: superheaters, evaporators, economisers, and (on units with SCR) the catalyst layers. Modern HRSGs operate at three pressure levels (HP, IP, LP) to maximise energy recovery from the cooling exhaust gas.
Fouling
HRSG fouling is generally lighter than coal-fired boiler fouling because gas-turbine exhaust contains far less particulate. The dominant fouling mechanisms are:
- Ammonium bisulphate (ABS) on units with SCR — slipped ammonia + SO₃ from fuel sulphur condenses on finned tubes
- Fine ash deposition on finned-tube banks reducing heat transfer
- Duct-burner-driven particulate on units with supplementary firing
- Cold-end corrosion below the acid dew point on sulphur-bearing fuels
Cleaning
Sonic horns installed across the gas path are increasingly common on HRSG maintenance plans, particularly for keeping SCR catalyst layers and cold-end finned tubes clear of ABS without the need for offline water-wash campaigns.
Related terms
Related terms
- Combined-cycle gas turbineA CCGT plant combines a gas turbine with a steam turbine driven by an HRSG recovering exhaust heat. Plant efficiency reaches 55–62% LHV; HRSG cleanliness is critical.
- Finned tube and harp tubeFinned tubes carry helically-wound fins to multiply gas-side surface area in HRSGs. Harp tubes are the vertical bundle configuration. Fin geometry is particularly fouling-sensitive.
- Duct burnerA duct burner is an auxiliary gas burner installed in the HRSG inlet duct to add heat to the gas-turbine exhaust. Used for steam-flow boosting and cogeneration peak shaping.
- Ammonium bisulphateAmmonium bisulphate is a sticky low-melting deposit formed when slipped ammonia reacts with SO3 in cooling flue gas. The dominant cold-end fouling species on SCR-equipped boilers.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.