While both Supplementary or Duct Firing and Turbine Inlet Air Chilling (TIAC) are solutions to offset the megawatt output degradation of gas turbines when ambient temperatures rise, the two technologies take very different approaches. With TIAC, the combustion gas turbine inlet air is chilled. In the case of duct firing, injection of fuel is utilized to increase the temperature and mass flow rate of the exhaust gases.
Rather than competing, the two technologies – duct firing and turbine inlet cooling – can actually complement each other when used correctly.
For maximum power output, power plant owners can utilizing the reliable power augmentation provided by TIAC, and balance the requirements with duct firing. This scenario allows them to produce the required power at the lowest possible heat rate.
However, the combination of TIAC with Duct Firing is rare – most owners choose one solution over the other. Let’s look at how Duct Firing works.
Supplementary (Duct) firing may be used in combined cycles in the HRSG – raising the exhaust temperatures from about 1112F (600 °C) at GT exhaust to about 1472F (800°C) or even 1832F (1000 °C). Using supplementary firing will however not raise the combined cycle efficiency for most combined cycles. Supplemental firing is often used to improve peak power production of the unit, or to enable higher steam production to compensate for failure of a second unit.
Maximum supplementary firing refers to the maximum fuel that can be fired with the oxygen available in the gas turbine exhaust. The steam cycle is conventional with reheat and regeneration. Hot gas turbine exhaust is used as the combustion air. Regenerative air preheater is not required. A fresh air fan which makes it possible to operate the steam plant even when the gas turbine is not in operation, increases the availability of the unit.
The use of large supplementary firing in Combined Cycle Systems with high gas turbine inlet temperatures causes the efficiency to drop. For this reason the Combined Cycle Plants with maximum supplementary firing are only of minimal importance nowadays, in comparison to simple Combined Cycle installations. However, they have two advantages that is 1) coal can be burned in the steam generator as the supplementary fuel, and 2) very good part load efficiency.
The HRSG can be designed with supplementary firing of fuel after the gas turbine in order to increase the quantity or temperature of the steam generated. Without supplementary firing, the efficiency of the combined cycle power plant is higher, but supplementary firing lets the plant respond to fluctuations of electrical load. Supplementary burners are also called duct burners.
More fuel is sometimes added to the turbine’s exhaust. This is possible because the turbine exhaust gas (flue gas) still contains some oxygen. Temperature limits at the gas turbine inlet force the turbine to use excess air, above the optimal stoichiometric ratio to burn the fuel. Often in gas turbine designs part of the compressed air flow bypasses the burner and is used to cool the turbine blades.
Supplementary firing raises the temperature of the exhaust gas from 800 to 900 degree Celsius. Relatively high flue gas temperature raises the condition of steam (84 bar, 525 degree Celsius), thereby improving the efficiency of steam cycle.
How to Boost?
Perhaps the biggest factor for power plant owners looking to reclaim lost megawatts, especially during the high demand summer season, is that it’s virtually impossible to retrofit an existing plant with Duct Firing. On the other hand, a Turbine Inlet Air Chilling system retrofit is easily done during a planned plant outage.
For new plant design and construction, the following should be considered when deciding if Supplementary or Duct Firing is the right power augmentation choice:
Duct Firing Strengths
- Low Capex (compared to TIAC)
- Heavy Duct Firing can provide significant power output increase, more than the steam cycle output, assuming the combined cycle is designed properly.
- Can respond well to load fluctuation (Peaks shaving)
- Different types of fuel can be utilized, such as Gas, Diesel, Coal, etc. and it can be similar or different from the Gas Turbine fuel
Duct Firing Limitations
- Supplementary firing hurts the heat rate significantly. (Almost double the effect of TIAC)
- Not possible or at least not an easy solution for existing plants which were not designed to work with duct firing (contrary to TIAC)
- Challenging and limited effect when water is not available (air cooled) due to the back pressure limitation on the Steam Turbine.
- Increases the NOx level – Negative effect on the environment (another effect of the higher heat rate)