Lesson test: Combined cycles and cogeneration

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Combined cycles and cogeneration

1.[2p]
Why do a gas turbine and a steam cycle work so well in series?

Their temperature ranges are complementary, so between them they cover flame to river

Correct

Both cycles use the same working fluid, so no heat exchanger is needed

The answer is: Their temperature ranges are complementary, so between them they cover flame to river

The steam cycle can drive the gas turbine's compressor

The answer is: Their temperature ranges are complementary, so between them they cover flame to river

Two cycles of the same efficiency multiply to a higher one

The answer is: Their temperature ranges are complementary, so between them they cover flame to river

2.[3p]
A topping cycle has $η_{g} = 0.431$ , $80$ per cent of its rejected heat reaches a bottoming cycle of $η_{s} = 0.38$ . What is the combined efficiency?

CorrectNot quite: 0.603

3.[2p]
A cheaper plant has $η_{g} = 0.36$ , $f = 0.75$ and $η_{s} = 0.35$ . What is its combined efficiency?

CorrectNot quite: 0.528

4.[2p]
A gas turbine passing $600$ kg/s produces $373.7$ kJ/kg net on $866.7$ kJ/kg of fuel heat. What is its output, in MW?

CorrectNot quite: 224

5.[2p]
The pinch point in a heat recovery steam generator limits how much heat can be recovered, because raising the steam pressure raises the boiling temperature.

True

Correct

False

The answer is: True

6.[3p]
What sets a floor under the stack temperature of a heat recovery steam generator?

Sulphuric acid in the flue gas condenses on cold surfaces and destroys them

Correct

The steam would freeze in the tubes below that temperature

The answer is: Sulphuric acid in the flue gas condenses on cold surfaces and destroys them

The gas turbine cannot exhaust below atmospheric pressure

The answer is: Sulphuric acid in the flue gas condenses on cold surfaces and destroys them

The pinch point moves to the superheater

The answer is: Sulphuric acid in the flue gas condenses on cold surfaces and destroys them

7.[3p]
A back-pressure plant exhausting at $200$ kPa produces $557.1$ kJ/kg of work and delivers $2051.3$ kJ/kg of useful heat on $2608.4$ kJ/kg of fuel heat. What is its electrical efficiency?

CorrectNot quite: 0.214

8.[3p]
Why is a cogeneration plant's utilisation factor a misleading single figure?

Most of the output is low-grade heat, which is worth far less per joule than electricity

Correct

It counts the pump work twice

The answer is: Most of the output is low-grade heat, which is worth far less per joule than electricity

It ignores the heat rejected to the surroundings

The answer is: Most of the output is low-grade heat, which is worth far less per joule than electricity

It cannot exceed the plant's thermal efficiency

The answer is: Most of the output is low-grade heat, which is worth far less per joule than electricity

9.[2p]
Match each component to what it does in a combined-cycle plant.

Gas turbine
Heat recovery steam generator
Steam turbine
Stack

the topping cycle
carries the unrecovered heat away
the bottoming cycle
an unfired boiler

Show the answer

Gas turbine: the topping cycle Heat recovery steam generator: an unfired boiler Steam turbine: the bottoming cycle Stack: carries the unrecovered heat away

1.[2p] Why do a gas turbine and a steam cycle work so well in series?

2.[3p] A topping cycle has ηg=0.431, 80 per cent of its rejected heat reaches a bottoming cycle of ηs=0.38. What is the combined efficiency?

3.[2p] A cheaper plant has ηg=0.36, f=0.75 and ηs=0.35. What is its combined efficiency?

4.[2p] A gas turbine passing 600 kg/s produces 373.7 kJ/kg net on 866.7 kJ/kg of fuel heat. What is its output, in MW?

5.[2p] The pinch point in a heat recovery steam generator limits how much heat can be recovered, because raising the steam pressure raises the boiling temperature.

6.[3p] What sets a floor under the stack temperature of a heat recovery steam generator?

7.[3p] A back-pressure plant exhausting at 200 kPa produces 557.1 kJ/kg of work and delivers 2051.3 kJ/kg of useful heat on 2608.4 kJ/kg of fuel heat. What is its electrical efficiency?

8.[3p] Why is a cogeneration plant's utilisation factor a misleading single figure?

9.[2p] Match each component to what it does in a combined-cycle plant.

1.[2p]
Why do a gas turbine and a steam cycle work so well in series?

2.[3p]
A topping cycle has $η_{g} = 0.431$ , $80$ per cent of its rejected heat reaches a bottoming cycle of $η_{s} = 0.38$ . What is the combined efficiency?

3.[2p]
A cheaper plant has $η_{g} = 0.36$ , $f = 0.75$ and $η_{s} = 0.35$ . What is its combined efficiency?

4.[2p]
A gas turbine passing $600$ kg/s produces $373.7$ kJ/kg net on $866.7$ kJ/kg of fuel heat. What is its output, in MW?

5.[2p]
The pinch point in a heat recovery steam generator limits how much heat can be recovered, because raising the steam pressure raises the boiling temperature.

6.[3p]
What sets a floor under the stack temperature of a heat recovery steam generator?

7.[3p]
A back-pressure plant exhausting at $200$ kPa produces $557.1$ kJ/kg of work and delivers $2051.3$ kJ/kg of useful heat on $2608.4$ kJ/kg of fuel heat. What is its electrical efficiency?

8.[3p]
Why is a cogeneration plant's utilisation factor a misleading single figure?

9.[2p]
Match each component to what it does in a combined-cycle plant.