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Q9. A steam power plant consists of turbine, condenser, feed pump and boiler. Steam enters the turbine at a pressure of 50 bar and a temperature of 400°C, and expands to 0.2 bar, dryness fraction 0.9. The steam is then fully condensed without undercooling. Feed pump work may be disregarded. The boiler has an efficiency of 87%. The fuel used has a calorific value of 38 MJ/kg and contains 85% carbon by mass.

It is proposed that the plant be modified to incorporate “carbon capture”, which involves separating the CO₂ from the exhaust gas and pumping it to a storage facility. It is estimated that separating the CO₂ from the flue gases will require 19% of the turbine power output, and pumping the CO₂ will require 40 kJ per kg of CO₂.

Determine the overall thermal efficiency of the plant:

(a) Before modification; (8)

(b) After modification. (8)

Note: Relative atomic masses: C = 12; O = 16