Duct burner

A duct burner is an auxiliary burner installed in the inlet duct of a Heat Recovery Steam Generator. It fires fuel into the oxygen-rich gas-turbine exhaust before the gas reaches the first tube banks, raising gas temperature and increasing steam production beyond unfired HRSG output.

Why plants use it

Duct burners are common in cogeneration plants, district-heating schemes, refineries and process sites where steam demand does not exactly follow gas-turbine load. They provide peak process steam, support steam-temperature control during transients and allow a combined-cycle plant to trade some efficiency for flexible steam or power output. Natural gas is the usual fuel; distillate or other backup fuels appear on some older or industrial units.

Operating mechanisms

A duct-burner system includes burner elements, fuel trains, flame scanners, igniters, combustion-air assumptions based on turbine exhaust oxygen, and HRSG protection logic. Firing raises local gas temperature and changes the heat split through superheater, evaporator and economiser sections. The HRSG must be designed for the resulting temperature profile, pressure drop, tube-metal temperatures and casing expansion.

Fouling and failure modes

Gas-only duct firing is relatively clean, but any oil firing, poor atomisation, flame impingement or carryover from upstream systems can increase particulate deposition on finned tubes. Common issues include uneven temperature distribution, damaged burner elements, flame instability, high CO, high NOx, hot streaking, tube overheating and faster fouling in the first harp rows. Supplementary firing can also increase the severity of ammonium bisulphate fouling where SCR ammonia slip and sulphur are present.

Cleaning implications

An HRSG that runs heavily fired has a different cleaning duty from an unfired gas-only unit. Deposits form where temperature falls through sticky ranges and where fin spacing traps fine solids. Sonic horns are most useful on downstream finned-tube banks, SCR catalyst faces and cold-end sections where online vibration of the gas volume can prevent fine ash and ammonium-salt deposits from consolidating.

Specification and troubleshooting notes

Duct-burner reviews should include fuel composition, turndown, flame length, expected oxygen in turbine exhaust, temperature distribution at the first harp and emissions guarantees. Uneven firing can be more damaging than high average firing because local hot streaks accelerate tube-metal ageing and deposit formation. If fouling appears after a change in supplementary firing pattern, the investigation should compare duct-burner hours, fuel changes, flame scanner trips, CO, NOx, ammonia slip and HRSG pressure drop before assuming the tube bank itself changed.

Operating and safety context

Duct burners are usually controlled by heat input demand, steam temperature, HRSG pressure, emissions limits and gas-turbine exhaust conditions. Design variables include burner grid arrangement, flame length, duct residence time, supplemental fuel pressure, oxygen content, liner protection, flame detection, purge volume and the allowable temperature profile entering downstream tube banks or catalyst. Poor mixing can create hot streaks that damage superheater tubes, SCR catalyst or expansion joints.

Maintenance checks cover fuel-valve tightness, pilots, igniters, scanners, flame stabilisers, refractory, casing hot spots and trip testing. Start-up and restart logic is safety-critical because unburned fuel in a duct can create an explosion risk. Fouling relevance is indirect: higher firing can shift ash stickiness, accelerate ammonium-bisulphate deposition in downstream SCR or air-heater zones, and change where acoustic cleaners are needed to keep heat-transfer or catalyst passages open.