Mid-term Exam: Combined Cycle (with solution)

Q4.  Combined Cycle  (12 points)

 A combined cycle operates with a tripple-pressure HRSG, without supplementary firing and without reheat. 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 listed in the following table:

Other necessary parameters and assumptions are listed here below:

Gas content in flue gas throughout the HRSG       x = 0.3
High pressure (HP) steam production at              100 bar, 500 °C
Middle pressure (MP) steam production at           20 bar, 340 °C
Low pressure (LP) steam production at                2 bar, 160 °C
Pinch point temperature difference                       19.8 °C
Steam mass flow fraction in HP line                     30 % (HP share of total steam flow)
Steam mass flow fraction in MP line                    40 % (MP share of total steam flow)
Pressure in the steam condenser                         0.1 bar
Mechanical efficiency of steam turbine                  96 %
Efficiency of the el. generator for steam turbine    93 %

Neglect the pump work and 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 can be applied as per the gas table in the help file. The type of fuel is not important here.