Supplemental duct firing (duct burner) is the combustion of auxiliary fuel (typically natural gas or refinery off-gas) directly within the gas turbine exhaust gas stream upstream of the HRSG heat transfer tube bundles. Because gas turbine exhaust still contains 12% to 15% oxygen by volume at 500°C to 600°C, it serves as preheated vitiated combustion air, allowing high thermal efficiency without requiring external fresh air blowers.

Duct burners burn fuel in an oxygen-depleted (vitiated) atmosphere. If supplemental firing is pushed too high, exhaust gas oxygen drops below the flame stability limit (typically 10.5% to 11.0% O2 by volume wet). Below this threshold, combustion kinetics slow drastically, leading to unburned hydrocarbons, severe carbon monoxide (CO) spikes, flame detachment, and dangerous acoustic pressure oscillations in the duct.

Duct firing raises the flue gas temperature entering the superheater and evaporator sections from ~540°C up to 750°C–900°C. This massive enthalpy boost increases the log-mean temperature difference (LMTD) across evaporator tubes, enabling up to a 50% to 100% boost in high-pressure steam production to meet peak electrical dispatch or industrial process steam demands.

Standard uncooled duct burner grids and HRSG carbon-steel or low-alloy duct walls typically limit firing temperatures to 760°C–870°C (1400°F–1600°F). Exceeding ~900°C requires high-nickel alloy casing liners, stainless-steel internal insulation pins, refractory ceramic fiber blankets, and steam-cooled or insulated front-row superheater tubes to prevent metallurgical burnout.

Duct burner flame length depends on runner fuel gas jet momentum, vitiated gas cross-flow velocity (typically 12 to 18 m/s), and heat release per runner unit length. Adequate firing duct clearance (typically 3.0 to 5.5 meters between the burner grid and the first tube bundle) must be maintained to prevent direct flame impingement, which causes localized tube dryout, overheating, and catastrophic burst failure.