HRSG and gas path
Combined-cycle gas turbine
Also known as CCGT, combined cycle, combined-cycle plant.
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.
Related terms
Related terms
- Heat Recovery Steam GeneratorAn HRSG recovers heat from a gas turbine's exhaust to generate steam, the second cycle of a combined-cycle plant. Finned-tube ash deposition and ABS fouling are the main cleaning concerns.
- 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.
- Selective Catalytic ReductionSCR is the dominant NOx-control technology on industrial combustion plant. Ammonia is injected upstream of a catalyst that converts NOx to nitrogen and water.
- Heat rateHeat rate is the fuel energy required to produce one unit of electrical output, measured in BTU/kWh or kJ/kWh. Fouling on convective surfaces directly degrades heat rate.