HRSG and gas path
Gas-air heater and gas-gas heater
Also known as GAH, GGH, gas-gas heater.
A gas-air heater (GAH) transfers heat from hot flue gas to cooler combustion air — functionally the same as the boiler air heater, with the term GAH used more often in cement-plant and metallurgical contexts. A gas-gas heater (GGH) transfers heat between two flue-gas streams, most commonly used in FGD installations to reheat scrubbed (cooled) flue gas before stack discharge so plume buoyancy and dispersion meet permit requirements.
Configurations
- Regenerative GAH/GGH — rotating matrix like a Ljungström
- Recuperative GAH/GGH — fixed tube bundle
- Heat-pipe GAH/GGH — sealed two-phase fluid in tubes, no moving parts
Fouling and cleaning
GAH and GGH baskets and tubes foul with ash and (on units downstream of FGD) calcium-rich sulphite or sulphate deposits. Cleaning options follow the same pattern as for the boiler air heater: steam sootblowing, sonic horns, and periodic water washing during major outages.
Related terms
Related terms
- Air heaterAn air heater (also air preheater, APH) recovers low-grade heat from flue gas to preheat combustion air. Cold-end fouling and corrosion are the dominant operational challenges.
- Ljungström air preheaterA Ljungström air preheater uses a rotating matrix of heat-exchange baskets that cycle between the flue-gas and combustion-air sides. The dominant utility APH design worldwide.
- Electrostatic precipitatorAn ESP removes particulate from flue gas by charging dust and collecting it on plate electrodes. Sonic horns are widely used to dislodge ash from plates and to keep hoppers from bridging.
- 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.