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Publication Date: 2008-Sep-30
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USE OF supplementary firing to improve THE efficiency & reliability of a power plant burning A LOW LHV fuel


  • It is well known in the power generation field that:


    • Combined cycle power plants (i.e Gas Turbine associated with Heat Recovery Steam Generator) represent a more efficient solution (Efficiency > 55% on a natural gas base)  than Conventional Steam Power Plants ( Efficiency range 30 to 45%)
    • The efficiency of the Gas turbine without HRSG is typically in the range of 30% to 40%, and the efficiency of the steam cycle downstream the GT is in the range of 20 to 30%
    • Combined cycle power plants have a much lower availability (due to both longer maintenance periods, and more frequent unscheduled shut-downs) than Conventional Steam Power Plants (difference in availability above 5%)
    • Supplementary Firing  (SF), i.e. the combustion of fuel at the inlet of the HRSG in the gas turbine exhausts, increases the power produced by a combined cycle.


  • It is also known in processes based on gasification such as Corex™/Finex™ that the supply of oxygen in a very reliable way is critical. Thus the compressors of the Air Separation Unit (ASU) feeding oxygen to those processes must use a very reliable source of energy, which is in the existing state of the art either electrical power (with possible back up from the grid), or steam generated by a conventional steam generator. In the current state of the art, to our knowledge, there is no drive of ASU by steam coming (at least for the major part) from a HRSG equipped with Supplementary Firing for the following reasons:


    • Availability (overall availability of the Combined Cycle is much lower than availability of Steam Generator)
    • Efficiency reasons (efficiency of HRSG steam cycle is lower than efficiency of Steam Generator steam cycle)

·         Surprisingly, it has been found that in the case of a power plant including at least one gas turbine and where the fuel has a LHV lower than 10 000 kJ/Nm3, it is preferable for both reliability and efficiency to burn at least one part of said fuel not consumed by the GT, in a HRSG with supplementary firing rather than in a conventional steam generator, provided that:

o     The temperature after supplementary firing is set below 900°C (setting it above would require the implementation of a very expensive HRSG technology)

o     An Auto Flying Take Over system is implemented (this system allows to go on using the HRSG with fresh air w...