Excess air in the gas turbine exhaust allows the combustion of additional fuel via supplemental firing in the HRSG. By producing higher gas temperatures due to combustion, supplemental firing increases steam production. The amount of increased steam production will vary depending on the degree of fuel input in the burners and the supplemental firing requirement. Some burners are available only to counter GT degradation at high ambient temperatures; others are intended to generate significant quantities of additional steam, thereby providing the capability for significant additional power output or export steam production.
There are two main types of duct burners:
- “Standard”(typically round) gas burners
- Linear grid type burners
Standard burners are sometimes mounted in the HRSG walls, particularly in smaller HRSGs, but are more usually mounted in the gas stream. Most often, combined cycle plants use the linear (grid) burners that span the duct. These provide even heating of the gas stream . Burner elements are generally located either in the transition duct between the GT exhaust and the lead row of the HRSG or inter-stage upstream of the initial superheat or reheat assemblies.
Inspection should include the burner runners internal to the HRSG. The inspection should look for:
- General condition of burner runners
- Damage to flow control elements (“bat wings”)
- Damage or binding of supports
- Excessive sag in runners > 2 to 3 inches at midpoint (5 to 8cm)
- Damage or binding in expansion sleeves
- General conditions of pilot elements and igniters
- General conditions of fuel orifices
- Consequential damage to adjacent casing liners
- Evidence of over-firing such as deposits on bat-wings
- Evidence of flame impingement on downstream tube surfaces
Figure 1 shows a runner that was improperly installed at the time of construction. This buckled after less than 10000 hours of operation.
Figure 1: Buckled Active Runner on Grid Burner
 Jackson, P. Moelling, D. Malloy, J. Taylor, M. HRSG Inspection Planning Guide - Second Edition, 2006. ISBN 0-9719616-2-X