Final Exam (Dec.): Combined Cycle (with solution)

  • Due No Due Date
  • Points 12
  • Submitting on paper
  • Available after Dec 19, 2017 at 8:55am

A combined cycle operates with a tripple-pressure HRSG, without reheat but with supplementary firing. Steam is generated and superheated in a high-pressure line (HP), middle-pressure line (MP) and a low-pressure line (LP); and expanded in the same steam turbine driving an electrical generator.  

There are no steam extractions and no feedwater preheaters in the steam cycle, also disregarding the feedwater tank.

The gas turbine's main specifications are given in the following table:

Gas Turbine type

El. Output
(MW)

El. Efficiency
(%)

Gas flow (kg/s)

Exhaust temp. (°C)

GE MS5002

31

35

101

510

 

Other necessary parameters and assumptions are listed here below:

Gas content in flue gas throughout the HRSG         x = 0.4
Gas temperature after supplementary firing            Tgas,in = 600 °C

High pressure (HP) steam production at                 70 bar, 450 °C
Middle pressure (MP) steam production at             15 bar, 300 °C
Low pressure (LP) steam production at                  5 bar, 210 °C   
Pinch point temperature difference                         8.16 °C
Steam mass flow fraction in HP line                       50 % (HP share of total steam flow)
Steam mass flow fraction in MP line                       30 % (MP share of total steam flow)
Pressure in the steam condenser                            0.2 bar
Mechanical efficiency of steam turbine shaft           95 %
Efficiency of the el. generator for steam turbine      95 %

 

Neglect the pump work. Assume no pressure losses and no heat losses anywhere. Assume that the feedwater entering the LP drum is saturated.

The isentropic efficiency of the steam turbine is not given. However, the steam production values in the HRSG define the steam expansion process in the steam turbine, which can be continued further to condenser pressure in the h-s diagram and thus graphically finding the real end of expansion!

Gas enthalpy values are applied as per the table in the help file. The type of fuel is not important here.

 

The list of partial answers below shows the necessary calculation steps:

 

Sub-task

  Units

  Points

 1)

Enthalpy of high-pressure (HP) superheated steam   

  kJ/kg

   0.5

 2)

Enthalpy of mid-pressure (MP) superheated steam  

  kJ/kg

   0.5

 3)

Enthalpy of low-pressure (LP) superheated steam     

  kJ/kg

   0.5

 4)

Enthalpy of steam at the end of real expansion          

  kJ/kg

   1.5

 5)

Enthalpy of feedwater at the pinch point                   

  kJ/kg

   0.5

 6)

Gas enthalpy at entry to HRSG  

  kJ/kg

   0.5

 7)

Gas enthalpy at pinch point

  kJ/kg

   0.5

 8)

Total steam mass flow in all three lines summed         

  kg/s

    2.25

 9)

Net electrical power output from the steam turbine       

  MW

    2.25

 10)

Total fuel energy input rate

  MW

    1.5

 11)

Combined Cycle electrical efficiency, in percent           

  %

    1.5

 Final Exam solution (Dec.)- Combined Cycle_Page_1.jpg

Final Exam solution (Dec.)- Combined Cycle_Page_2.jpg

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