Heat Recovery Steam Generator (HRSG) - convert gas-turbine exhaust to steam

A Heat Recovery Steam Generator (HRSG) recovers heat from gas-turbine exhaust and uses it to generate steam. It is the heat-recovery bridge between the gas turbine and the steam cycle in a combined-cycle plant, and it is also used in cogeneration where steam is supplied to an industrial process.

Layout

Modern HRSGs usually contain multiple pressure levels: high-pressure, intermediate-pressure and low-pressure economisers, evaporators and superheaters. Tubes are commonly arranged as finned-tube harps across a large rectangular gas path. Some units include duct burners, SCR catalyst, CO catalyst, bypass stacks and diverter dampers.

Operating features

Gas-turbine exhaust is oxygen-rich and far cleaner than coal-boiler flue gas, but it still carries rust, insulation fibres, catalyst fines, oil-derived particulate, ambient dust and products from backup fuel or duct firing. HRSGs also experience rapid cycling, thermal fatigue and condensate-related corrosion when plants start and stop frequently.

Fouling mechanisms

The main fouling risks are fine particulate trapped in fin packs, ammonium bisulphate deposits from ammonia slip and sulphur, corrosion products from upstream ductwork, and oil or liquid-fuel ash during backup firing. Fouling raises gas-side pressure drop, lowers steam production, increases stack temperature and can mask catalyst surfaces.

Maintenance and cleaning

Offline water washing, dry cleaning, inspection access and catalyst vacuuming are common, but they require downtime and careful drying. Sonic horns provide online preventive cleaning for finned tubes, catalyst faces and cold-end regions where deposits are still friable. Horn placement should consider gas-flow direction, harp spacing, casing attenuation and access for diaphragm maintenance.

Inspection notes

HRSG inspection should focus on areas where gas slows, turns or cools through sticky ranges: inlet transitions, catalyst faces, finned economisers, cold-end modules and duct-burner downstream zones. Cycling units also need checks for drainability, thermal fatigue and casing leaks because air in-leakage and condensation can worsen deposits. Acoustic horns should be accessible from platforms, isolated from water-wash paths where possible and protected from thermal expansion loads at the nozzle.

Layout and operating variables

An HRSG is built from evaporator, economiser, superheater and sometimes reheater sections arranged behind one or more gas turbines or process exhaust sources. It may be unfired or fitted with duct burners. Important design variables include exhaust mass flow, inlet temperature, pressure levels, pinch point, approach temperature, supplementary firing rate, tube fin geometry, casing expansion, bypass stack arrangement, SCR catalyst location and allowable pressure drop.

Unlike a coal boiler, an HRSG usually sees cleaner gas, but it can still foul. Gas turbines burning liquid fuel, refinery gas, syngas or contaminated fuel can introduce ash, sulphur, alkali, vanadium or catalyst fines. Duct burners can create hot streaks or incomplete combustion. SCR systems can introduce ammonium-bisulphate risk where ammonia slip, sulphur and temperature align. Cycling service adds thermal fatigue, drain-management problems and casing seal stress.

Failure modes and maintenance checks

Common HRSG problems include fin fouling, tube leaks, flow-accelerated corrosion, under-deposit corrosion, fatigue at headers and tube penetrations, failed expansion joints, bypass-damper leakage, catalyst plugging, silencer damage and stack emissions excursions. Operators track gas-side pressure drop, stack temperature, steam temperatures, approach and pinch trends, duct-burner flame quality, drain performance and SCR differential pressure.

Acoustic cleaning can be relevant in HRSGs that accumulate dry ash or catalyst fines on finned tubes, catalyst faces or hoppers. Horn placement has to respect casing access, external noise, gas temperature and the tight spacing of finned bundles. If the deposit is oily, wet, chemically sticky or caused by an upstream fuel problem, acoustic cleaning may reduce symptoms but will not remove the underlying cause. Outage inspection remains important because finned tube banks can hide severe local plugging behind a normal average pressure-drop trend.

Commissioning and cycling context

Commissioning should establish baseline pressure drop, stack temperature, steam temperatures, drain response and catalyst condition before fouling or cycling damage accumulates. Cycling HRSGs need special attention to warm-up rates, attemperator use, drum-level control and condensate removal because fatigue damage can grow even when the gas side remains clean. Cleaning systems should therefore be evaluated alongside thermal-stress controls, not as a separate accessory.

The baseline should be repeated after major fuel, turbine or duct-burner changes.

Heat Recovery Steam Generator