Combined-cycle gas turbine (CCGT) - gas turbine plus HRSG and steam turbine

A combined-cycle gas turbine (CCGT) plant combines a gas turbine with a steam turbine driven by an HRSG that recovers heat from the gas-turbine exhaust. The arrangement raises overall plant efficiency from ~38% LHV for a simple-cycle gas turbine to 55-62% LHV for modern CCGT, with the latest H-class machines pushing 64%+.

Why HRSG cleanliness matters

CCGT plants are economically dispatched ahead of coal in most markets, but margins per MWh are tight and competition from renewables intensifies the focus on heat rate. Every 0.5% efficiency loss from HRSG fouling translates directly to fuel cost. Sonic horns on the HRSG gas path are increasingly part of the standard maintenance toolkit on modern combined-cycle plants.

Cycling adds complication

Modern CCGT plants increasingly two-shift - running daytime and shutting overnight when renewable supply meets demand. Frequent start-stop cycling worsens HRSG fouling because cold metal surfaces during shutdowns condense moisture that bonds dust into a harder deposit. Continuous sonic-horn cleaning helps offset cycling-driven fouling acceleration.

Plant arrangement

A CCGT may be configured as one gas turbine and one steam turbine, two gas turbines feeding one steam turbine, or larger multi-shaft arrangements. The gas turbine provides fast power, while the HRSG captures exhaust heat to make steam for additional output. Duct burners can add heat in the HRSG when extra steam production is required.

Because the gas turbine exhaust is relatively clean compared with coal or waste combustion, CCGT fouling is often slower and more subtle. The main gas-path concerns are HRSG tube fouling, ammonia slip and ammonium-salt formation around SCR systems, duct-burner deposits, corrosion during cycling, and debris left from construction or maintenance.

Operating modes and failure modes

Baseload operation keeps the HRSG hot and stable. Cycling operation creates repeated condensation, thermal fatigue and lay-up challenges. Low-load operation can also place SCR catalyst below its preferred temperature range, increasing ammonia slip and downstream deposition risk.

Operators monitor gas-turbine exhaust temperature, HRSG pressure levels, steam temperature, duct-burner performance, SCR inlet temperature, ammonia slip, stack NOx and HRSG pressure drop. A small increase in gas-side pressure drop can reduce gas-turbine output because the turbine sees higher backpressure.

Acoustic-cleaning context

Sonic horns are used selectively in CCGT plants, mainly on HRSG tube banks, SCR catalyst faces and duct areas where fine deposits accumulate. The cleaning case is strongest for units that cycle, burn oil backup fuel, use duct firing, or have SCR ammonium-salt problems. The goal is to preserve heat-transfer margin and pressure-drop margin without intrusive online washing or frequent outages.

Field checks

CCGT operation is tracked through gas-turbine load, exhaust temperature, HRSG pressure levels, steam temperatures, duct burner firing, condenser performance and start frequency. The HRSG is especially sensitive to cycling because tube bundles, headers and casing seals see repeated thermal movement. Even light fouling can matter if it increases backpressure on the gas turbine or reduces heat recovery enough to affect steam-turbine output.

Maintenance checks focus on gas-turbine compressor cleanliness, inlet filtration, HRSG tube condition, attemperators, drains, expansion joints, diverter dampers and stack dampers. Sites with duct burners or supplementary firing can see ash, sulphate or corrosion products deposit in the HRSG even when the main fuel is clean. Acoustic cleaning is relevant mainly in HRSG sections that accumulate dry deposits, especially where offline washing is disruptive. It is not normally the first tool for clean natural-gas service, but it can be justified when deposits measurably raise backpressure or reduce heat transfer.

Combined-cycle gas turbine