Combined Power Plant

A combined cycle power plant joins a gas turbine (Brayton) cycle as the topping cycle and a steam turbine (Rankine) cycle as the bottoming cycle. The hot gas turbine exhaust (about 500-600 C) raises steam in a heat recovery steam generator (HRSG).

Basic components

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

• Gas turbine unit: compressor (C), combustion chamber, gas turbine (GT), generator G1.
• HRSG / waste heat boiler: economiser, evaporator and superheater heated by GT exhaust (sometimes with supplementary firing).
• Steam unit: steam turbine (ST), generator G2, condenser, feed pump.

T-s diagram

  T
  ^              3
  |             /|
  |            / |   Gas cycle 1-2-3-4
  |           /  |   (Brayton, top)
  |          /   |
  |         2    |
  |         |    4
  |         |   /   4-1: hot exhaust
  |         |  /    gives heat to HRSG
  |    a    | /
  |   /|    1/
  |  d |
  |  | |     Steam cycle c-d-a-b
  |  c-b     (Rankine, bottom)
  +--------------------------> s
  (gas and steam drawn per kg of
   their own fluid, schematic only)

The heat rejected by the gas cycle (4-1) is used as heat input to the steam cycle, so the overall efficiency (about 50-60%) is higher than either cycle alone.

A combined cycle plant uses the hot exhaust of a gas turbine to generate steam for a steam turbine, so the same fuel produces power in two cycles.

Advantages

• High overall efficiency, 50-60%, against about 35-40% for steam and 30-35% for simple gas turbine plants.
• Lower fuel cost and lower CO2 per kWh.
• Short construction time; gas turbine part can run first (phased construction).
• Low capital cost per kW and less space than a coal plant.
• Quick start-up; good for intermediate and peak loads.
• Less cooling water (only the steam part, about one-third of output, needs a condenser).
• Lower emissions (natural gas fuel).

Working principle

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

1. Air is compressed in the compressor (1-2) and fuel is burnt in the combustion chamber (2-3).
2. Hot gas expands in the gas turbine (3-4) and drives generator G1.
3. The exhaust gas (about 500-600 C) passes through the HRSG where it heats feed water, evaporates it and superheats the steam; gas leaves through the stack at about 100-150 C (4-5).
4. Steam expands in the steam turbine (a-b) and drives generator G2; it is condensed (b-c) and pumped back to the HRSG (c-d).

The gas cycle adds heat at a very high temperature (about 1200-1400 C) and the steam cycle rejects heat at a low temperature (about 35-45 C). Using the gas cycle's waste heat as the steam cycle's input gives

$$\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST}$$

For example, $\eta_{GT} = 0.35$ and $\eta_{ST} = 0.30$ give $\eta_{cc} = 0.35 + 0.30 - 0.105 = 0.545$, i.e. about 54.5%.

In a combined plant the waste heat of the topping cycle (mainly gas turbine exhaust at 450-600 C, still containing about 15% oxygen) is recovered to produce more power or useful heat.

Common methods of utilising the gas turbine exhaust (waste) heat:

1. Heat recovery steam generator (HRSG), unfired: exhaust passes through an economiser, evaporator and superheater and raises steam for a steam turbine. Simplest and most common; steam turbine gives about one-third of the plant output.
2. Supplementary fired HRSG: extra fuel is burnt in a duct burner in the exhaust (it still contains about 15-16% O2) to raise gas temperature and steam output/temperature; increases output and flexibility.
3. Exhaust-fired (fully fired) boiler: GT exhaust is used as hot combustion air for a conventional fired steam boiler, replacing the air preheater.
4. Feed water heating: exhaust heats the boiler feed water in a gas-to-water heat exchanger (economiser), replacing some or all bled-steam feed heaters of a steam plant.
5. Pressurised (supercharged) boiler: combustion takes place under pressure in the boiler furnace; after raising steam, the gases expand in a gas turbine that drives the compressor.
6. Cogeneration / process heat: exhaust or exhaust-raised steam is used for industrial process heating, district heating or absorption cooling.
7. Regenerator / recuperator: exhaust preheats the compressed air of the gas turbine itself (regenerative gas turbine).

Methods of utilising waste heat

Common methods of utilising the gas turbine exhaust (waste) heat:

1. Heat recovery steam generator (HRSG), unfired: exhaust passes through an economiser, evaporator and superheater and raises steam for a steam turbine. Simplest and most common; steam turbine gives about one-third of the plant output.
2. Supplementary fired HRSG: extra fuel is burnt in a duct burner in the exhaust (it still contains about 15-16% O2) to raise gas temperature and steam output/temperature; increases output and flexibility.
3. Exhaust-fired (fully fired) boiler: GT exhaust is used as hot combustion air for a conventional fired steam boiler, replacing the air preheater.
4. Feed water heating: exhaust heats the boiler feed water in a gas-to-water heat exchanger (economiser), replacing some or all bled-steam feed heaters of a steam plant.
5. Pressurised (supercharged) boiler: combustion takes place under pressure in the boiler furnace; after raising steam, the gases expand in a gas turbine that drives the compressor.
6. Cogeneration / process heat: exhaust or exhaust-raised steam is used for industrial process heating, district heating or absorption cooling.
7. Regenerator / recuperator: exhaust preheats the compressed air of the gas turbine itself (regenerative gas turbine).

Layout: gas turbine exhaust used to heat feed water

  air       fuel
   | 1        |
   v          v
  [C]--2-->[ CC ]--3-->[GT]==> G1
   ||                   |  4  exhaust gas
   ++=====shaft=========+  v
          +--------------------------+
 stack <-5| Feed water heater (gas   |
          | to water heat exchanger) |
          +--------------------------+
             ^ d             | e  hot feed water
             |               v
          +------+     +----------+  a  +----+
          | Pump |     |  Steam   |---->| ST |==> G2
          +------+     |  boiler  |     +----+
             ^ c       +----------+       | b
             |              ^ fuel        v
             |           +-----------------+
             +-----------|    Condenser    |
                         +-----------------+

Working:

1. Air is compressed (1-2), burnt with fuel in the combustion chamber (2-3) and expands in the gas turbine (3-4), driving the compressor and generator G1.
2. The hot exhaust (about 450-550 C) passes through the feed water heater (an economiser-type gas-to-water heat exchanger) and leaves to the stack at 5.
3. Condensate from the steam plant is pumped (c-d) and heated by the exhaust gas (d-e) nearly up to the boiling temperature. This replaces the bled-steam feed heaters, so steam that would have been extracted now expands fully in the turbine and produces extra power.
4. The hot feed water enters the conventional fired steam boiler, where fuel supplies the latent heat and superheat (e-a).
5. Steam expands in the steam turbine (a-b) driving G2, is condensed (b-c) and the cycle repeats.

The steam boiler is still fully fired, so this scheme gives a modest gain (about 3-5% points) and is often used to repower an existing steam plant by adding a gas turbine.

T-s diagram

  T
  ^          3
  |         /|
  |        / |     gas cycle 1-2-3-4,
  |       2  |     exhaust 4-5 heats
  |       |  4     feed water d-e
  |       |   \
  |       1    5
  |                    a
  |       f---------g /|
  |      e             |
  |     /              |
  |    d               |
  |    c---------------b
  +---------------------------> s
  c-d pump, d-e heating by GT exhaust,
  e-f-g-a heating in fired boiler,
  a-b steam turbine, b-c condenser

Working of combined power plant

A combined power plant (combined cycle plant) uses two thermodynamic cycles in series, so that the heat rejected by the first (topping) cycle becomes the heat input of the second (bottoming) cycle. The most common is the gas turbine - steam turbine combined cycle: a gas turbine (Brayton cycle, heat added at 1100-1500 C) exhausts at 450-600 C, and this exhaust raises steam for a steam turbine (Rankine cycle, heat rejected at 30-45 C). Overall efficiency of 50-60% is achieved, compared with 30-40% for each plant alone.

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

Working:

1. Air is compressed in the compressor (1-2), burnt with fuel in the combustion chamber (2-3) and expanded in the gas turbine (3-4), which drives the compressor and generator G1.
2. Exhaust gas at about 500-600 C enters the HRSG. Here it heats the feed water in the economiser, evaporates it in the evaporator and superheats it in the superheater (d-a). The cooled gas goes to the stack at about 100-150 C (4-5).
3. Steam expands in the steam turbine (a-b) and drives generator G2.
4. Exhaust steam is condensed (b-c) and pumped back to the HRSG (c-d).
5. Optionally, a duct burner before the HRSG burns extra fuel (supplementary firing) to raise steam output.

Heat added at high temperature in the gas cycle and rejected at low temperature in the steam condenser gives a high overall efficiency.

Determining performance

Performance of a combined plant is judged by its overall thermal efficiency (or heat rate):

$$\eta_{cc} = \frac{W_{GT} + W_{ST}}{Q_{GT} + Q_{sf}}$$

where $W_{GT}$, $W_{ST}$ are net outputs of gas and steam units, $Q_{GT}$ is fuel heat to the gas turbine and $Q_{sf}$ is any supplementary fuel heat.

For an unfired HRSG with no losses, all heat rejected by the gas turbine, $Q_{GT}(1-\eta_{GT})$, goes to the steam cycle, so

$$\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\,\eta_{ST}$$

Example: $\eta_{GT} = 0.35$, $\eta_{ST} = 0.32$ gives $\eta_{cc} = 0.35 + 0.32 - 0.112 = 0.558$ (55.8%).

If the HRSG (heat recovery) effectiveness is $\eta_{HR}$ (part of exhaust heat lost to stack), $\eta_{cc} = \eta_{GT} + \eta_{HR}\,\eta_{ST}(1 - \eta_{GT})$.

Other performance figures: heat rate = 3600 / $\eta_{cc}$ kJ/kWh (about 6000-7000 kJ/kWh), power ratio $W_{ST}/W_{GT}$ (about 0.5), part-load efficiency and stack temperature.

Introduction

A combined power plant (combined cycle plant) uses two thermodynamic cycles in series, so that the heat rejected by the first (topping) cycle becomes the heat input of the second (bottoming) cycle. The most common is the gas turbine - steam turbine combined cycle: a gas turbine (Brayton cycle, heat added at 1100-1500 C) exhausts at 450-600 C, and this exhaust raises steam for a steam turbine (Rankine cycle, heat rejected at 30-45 C). Overall efficiency of 50-60% is achieved, compared with 30-40% for each plant alone.

Main parts: compressor, combustion chamber, gas turbine, heat recovery boiler or fired boiler, steam turbine, condenser, feed pump and two generators. The arrangement depends on how the exhaust heat is used; the three popular designs are shown below.

Design 1: Unfired (or supplementary fired) heat recovery boiler (HRSG)

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

The GT exhaust raises all the steam in a heat recovery boiler. A duct burner may add supplementary fuel. This is the most common modern design (efficiency 50-60%).

Design 2: Exhaust used as combustion air in a fired boiler

  air      fuel
   |1        |
   v         v
  [C]--2-->[CC]--3-->[GT]==> G1
                      | 4 hot exhaust (~15% O2)
                      v   used as combustion air
         fuel --> +-----------------+  a  +----+
                  | Fired steam     |---->| ST |==> G2
       stack <----| boiler          |     +----+
                  +-----------------+       | b
                     ^ d                    v
                     |  +------+  c  +-----------+
                     +--| Pump |<----| Condenser |
                        +------+     +-----------+

The hot, oxygen-rich exhaust replaces the air preheater and forced draft air of a conventional fired boiler. Steam turbine output is large compared to gas turbine output.

Design 3: Exhaust heats the feed water

  air       fuel
   | 1        |
   v          v
  [C]--2-->[ CC ]--3-->[GT]==> G1
   ||                   |  4  exhaust gas
   ++=====shaft=========+  v
          +--------------------------+
 stack <-5| Feed water heater (gas   |
          | to water heat exchanger) |
          +--------------------------+
             ^ d             | e  hot feed water
             |               v
          +------+     +----------+  a  +----+
          | Pump |     |  Steam   |---->| ST |==> G2
          +------+     |  boiler  |     +----+
             ^ c       +----------+       | b
             |              ^ fuel        v
             |           +-----------------+
             +-----------|    Condenser    |
                         +-----------------+

The exhaust heats the condensate/feed water of a conventional steam plant, replacing bled-steam feed heaters; the boiler supplies the rest of the heat.

The three popular ways of combining a gas turbine with a steam cycle are: (1) unfired/supplementary fired heat recovery boiler, (2) exhaust used as combustion air in a fired boiler, and (3) exhaust used for feed water heating.

Design 1: Unfired (or supplementary fired) heat recovery boiler (HRSG)

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

The GT exhaust raises all the steam in a heat recovery boiler. A duct burner may add supplementary fuel. This is the most common modern design (efficiency 50-60%).

Design 2: Exhaust used as combustion air in a fired boiler

  air      fuel
   |1        |
   v         v
  [C]--2-->[CC]--3-->[GT]==> G1
                      | 4 hot exhaust (~15% O2)
                      v   used as combustion air
         fuel --> +-----------------+  a  +----+
                  | Fired steam     |---->| ST |==> G2
       stack <----| boiler          |     +----+
                  +-----------------+       | b
                     ^ d                    v
                     |  +------+  c  +-----------+
                     +--| Pump |<----| Condenser |
                        +------+     +-----------+

The hot, oxygen-rich exhaust replaces the air preheater and forced draft air of a conventional fired boiler. Steam turbine output is large compared to gas turbine output.

Design 3: Exhaust heats the feed water

  air       fuel
   | 1        |
   v          v
  [C]--2-->[ CC ]--3-->[GT]==> G1
   ||                   |  4  exhaust gas
   ++=====shaft=========+  v
          +--------------------------+
 stack <-5| Feed water heater (gas   |
          | to water heat exchanger) |
          +--------------------------+
             ^ d             | e  hot feed water
             |               v
          +------+     +----------+  a  +----+
          | Pump |     |  Steam   |---->| ST |==> G2
          +------+     |  boiler  |     +----+
             ^ c       +----------+       | b
             |              ^ fuel        v
             |           +-----------------+
             +-----------|    Condenser    |
                         +-----------------+

The exhaust heats the condensate/feed water of a conventional steam plant, replacing bled-steam feed heaters; the boiler supplies the rest of the heat.

Explanation of design 1 (heat recovery boiler) with T-s diagram

Working:

1. Air is compressed in the compressor (1-2), burnt with fuel in the combustion chamber (2-3) and expanded in the gas turbine (3-4), which drives the compressor and generator G1.
2. Exhaust gas at about 500-600 C enters the HRSG. Here it heats the feed water in the economiser, evaporates it in the evaporator and superheats it in the superheater (d-a). The cooled gas goes to the stack at about 100-150 C (4-5).
3. Steam expands in the steam turbine (a-b) and drives generator G2.
4. Exhaust steam is condensed (b-c) and pumped back to the HRSG (c-d).
5. Optionally, a duct burner before the HRSG burns extra fuel (supplementary firing) to raise steam output.

Heat added at high temperature in the gas cycle and rejected at low temperature in the steam condenser gives a high overall efficiency.

  T
  ^              3
  |             /|
  |            / |   Gas cycle 1-2-3-4
  |           /  |   (Brayton, top)
  |          /   |
  |         2    |
  |         |    4
  |         |   /   4-1: hot exhaust
  |         |  /    gives heat to HRSG
  |    a    | /
  |   /|    1/
  |  d |
  |  | |     Steam cycle c-d-a-b
  |  c-b     (Rankine, bottom)
  +--------------------------> s
  (gas and steam drawn per kg of
   their own fluid, schematic only)

Possible combinations

A combined cycle couples a high-temperature topping cycle with a low-temperature bottoming cycle. Possible combinations:

1. Gas turbine - steam turbine (Brayton-Rankine), the most common.
2. Diesel/gas engine - steam turbine (engine exhaust raises steam).
3. MHD (magnetohydrodynamic) generator - steam plant.
4. Thermionic - steam and thermoelectric - steam plants.
5. Nuclear (gas-cooled reactor) - steam / gas turbine plant.
6. Binary vapour cycle, e.g. mercury - steam.
7. Steam - organic Rankine cycle (low-temperature bottoming).

Advantages of combined cycles

• High overall efficiency (50-60%), so lower fuel cost and lower CO2 per kWh.
• Low capital cost per kW and short construction time.
• Quick start and good load-following.
• Less cooling water and less space than a steam plant of the same rating.
• Lower emissions (clean fuels) and flexibility for cogeneration.

What is a combined power plant

A combined power plant (combined cycle plant) uses two thermodynamic cycles in series, so that the heat rejected by the first (topping) cycle becomes the heat input of the second (bottoming) cycle. The most common is the gas turbine - steam turbine combined cycle: a gas turbine (Brayton cycle, heat added at 1100-1500 C) exhausts at 450-600 C, and this exhaust raises steam for a steam turbine (Rankine cycle, heat rejected at 30-45 C). Overall efficiency of 50-60% is achieved, compared with 30-40% for each plant alone.

Layouts of various types

Design 1: Unfired (or supplementary fired) heat recovery boiler (HRSG)

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

The GT exhaust raises all the steam in a heat recovery boiler. A duct burner may add supplementary fuel. This is the most common modern design (efficiency 50-60%).

Design 2: Exhaust used as combustion air in a fired boiler

  air      fuel
   |1        |
   v         v
  [C]--2-->[CC]--3-->[GT]==> G1
                      | 4 hot exhaust (~15% O2)
                      v   used as combustion air
         fuel --> +-----------------+  a  +----+
                  | Fired steam     |---->| ST |==> G2
       stack <----| boiler          |     +----+
                  +-----------------+       | b
                     ^ d                    v
                     |  +------+  c  +-----------+
                     +--| Pump |<----| Condenser |
                        +------+     +-----------+

The hot, oxygen-rich exhaust replaces the air preheater and forced draft air of a conventional fired boiler. Steam turbine output is large compared to gas turbine output.

Design 3: Exhaust heats the feed water

  air       fuel
   | 1        |
   v          v
  [C]--2-->[ CC ]--3-->[GT]==> G1
   ||                   |  4  exhaust gas
   ++=====shaft=========+  v
          +--------------------------+
 stack <-5| Feed water heater (gas   |
          | to water heat exchanger) |
          +--------------------------+
             ^ d             | e  hot feed water
             |               v
          +------+     +----------+  a  +----+
          | Pump |     |  Steam   |---->| ST |==> G2
          +------+     |  boiler  |     +----+
             ^ c       +----------+       | b
             |              ^ fuel        v
             |           +-----------------+
             +-----------|    Condenser    |
                         +-----------------+

The exhaust heats the condensate/feed water of a conventional steam plant, replacing bled-steam feed heaters; the boiler supplies the rest of the heat.

Performance and economics

Performance of a combined plant is judged by its overall thermal efficiency (or heat rate):

$$\eta_{cc} = \frac{W_{GT} + W_{ST}}{Q_{GT} + Q_{sf}}$$

where $W_{GT}$, $W_{ST}$ are net outputs of gas and steam units, $Q_{GT}$ is fuel heat to the gas turbine and $Q_{sf}$ is any supplementary fuel heat.

For an unfired HRSG with no losses, all heat rejected by the gas turbine, $Q_{GT}(1-\eta_{GT})$, goes to the steam cycle, so

$$\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\,\eta_{ST}$$

Example: $\eta_{GT} = 0.35$, $\eta_{ST} = 0.32$ gives $\eta_{cc} = 0.35 + 0.32 - 0.112 = 0.558$ (55.8%).

If the HRSG (heat recovery) effectiveness is $\eta_{HR}$ (part of exhaust heat lost to stack), $\eta_{cc} = \eta_{GT} + \eta_{HR}\,\eta_{ST}(1 - \eta_{GT})$.

Other performance figures: heat rate = 3600 / $\eta_{cc}$ kJ/kWh (about 6000-7000 kJ/kWh), power ratio $W_{ST}/W_{GT}$ (about 0.5), part-load efficiency and stack temperature.

Economics:

• Capital cost per kW is lower than coal or nuclear plants (gas turbine is factory made, HRSG is simple, no coal/ash handling); about 60-70% of a coal plant.
• Construction time 2-3 years; phased construction earns revenue early.
• Fuel cost is the largest cost item; high efficiency reduces it, but gas/oil price decides the cost of energy.
• Operating and maintenance cost is moderate; fewer staff, but GT hot parts need regular overhaul.
• Cost of electricity is lowest for base and intermediate load where cheap natural gas is available; supplementary firing gives cheap peaking capacity.

A combined power plant (combined cycle plant) uses two thermodynamic cycles in series, so that the heat rejected by the first (topping) cycle becomes the heat input of the second (bottoming) cycle. The most common is the gas turbine - steam turbine combined cycle: a gas turbine (Brayton cycle, heat added at 1100-1500 C) exhausts at 450-600 C, and this exhaust raises steam for a steam turbine (Rankine cycle, heat rejected at 30-45 C). Overall efficiency of 50-60% is achieved, compared with 30-40% for each plant alone.

Layout

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

Components

• Gas turbine unit: axial compressor, combustion chamber and gas turbine on one shaft with generator G1.
• Heat recovery steam generator (HRSG): banks of economiser, evaporator (with drum) and superheater placed in the exhaust duct; may have a duct burner for supplementary firing.
• Steam unit: steam turbine with generator G2, condenser, condensate and feed pumps, deaerator.

Working

Working:

1. Air is compressed in the compressor (1-2), burnt with fuel in the combustion chamber (2-3) and expanded in the gas turbine (3-4), which drives the compressor and generator G1.
2. Exhaust gas at about 500-600 C enters the HRSG. Here it heats the feed water in the economiser, evaporates it in the evaporator and superheats it in the superheater (d-a). The cooled gas goes to the stack at about 100-150 C (4-5).
3. Steam expands in the steam turbine (a-b) and drives generator G2.
4. Exhaust steam is condensed (b-c) and pumped back to the HRSG (c-d).
5. Optionally, a duct burner before the HRSG burns extra fuel (supplementary firing) to raise steam output.

Heat added at high temperature in the gas cycle and rejected at low temperature in the steam condenser gives a high overall efficiency.

T-s diagram

  T
  ^              3
  |             /|
  |            / |   Gas cycle 1-2-3-4
  |           /  |   (Brayton, top)
  |          /   |
  |         2    |
  |         |    4
  |         |   /   4-1: hot exhaust
  |         |  /    gives heat to HRSG
  |    a    | /
  |   /|    1/
  |  d |
  |  | |     Steam cycle c-d-a-b
  |  c-b     (Rankine, bottom)
  +--------------------------> s
  (gas and steam drawn per kg of
   their own fluid, schematic only)

Process summary:

Efficiency

$$\eta_{cc} = \frac{W_{GT} + W_{ST}}{Q_{fuel}} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST}$$

With $\eta_{GT} = 0.35$ and $\eta_{ST} = 0.30$: $\eta_{cc} = 0.35 + 0.30 - 0.105 = 0.545$, about 54.5%. Typical modern plants reach 55-60%.

A gas and steam combined cycle uses the exhaust heat of a gas turbine to generate steam for a steam turbine, so overall efficiency becomes 50-60%.

Advantages

• High overall efficiency (50-60%) against 35-40% for steam and 30-38% for gas turbine plants, so lower fuel cost per kWh.
• Lower capital cost per kW than coal or nuclear plants; the gas turbine part is compact and factory built.
• Short construction time (2-3 years); can be built in phases (gas turbine runs first in open cycle).
• Quick start-up and good load-following, so useful for intermediate and peak loads as well as base load.
• Less cooling water needed, since only the steam part (about one-third of output) has a condenser.
• Lower emissions of CO2, SO2 and particulates per kWh (usually natural gas fuel); no ash handling.
• Less space and smaller staff; high reliability and availability.
• Flexible: supplementary firing, cogeneration (process steam), repowering of old steam plants.

Disadvantages

• Needs clean, costly fuel (natural gas or distillate oil); not suited to coal unless gasified.
• Output and efficiency fall with high ambient temperature and altitude (gas turbine takes less mass of air).
• More complex plant and controls; two kinds of machines to maintain.
• Gas turbine hot parts need frequent inspection; high-temperature materials are costly.
• Unit size limited by available gas turbine sizes.
• Part-load efficiency falls when gas turbines are throttled.

Popular designs of combination cycles

Design 1: Unfired (or supplementary fired) heat recovery boiler (HRSG)

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

The GT exhaust raises all the steam in a heat recovery boiler. A duct burner may add supplementary fuel. This is the most common modern design (efficiency 50-60%).

Design 2: Exhaust used as combustion air in a fired boiler

  air      fuel
   |1        |
   v         v
  [C]--2-->[CC]--3-->[GT]==> G1
                      | 4 hot exhaust (~15% O2)
                      v   used as combustion air
         fuel --> +-----------------+  a  +----+
                  | Fired steam     |---->| ST |==> G2
       stack <----| boiler          |     +----+
                  +-----------------+       | b
                     ^ d                    v
                     |  +------+  c  +-----------+
                     +--| Pump |<----| Condenser |
                        +------+     +-----------+

The hot, oxygen-rich exhaust replaces the air preheater and forced draft air of a conventional fired boiler. Steam turbine output is large compared to gas turbine output.

Design 3: Exhaust heats the feed water

  air       fuel
   | 1        |
   v          v
  [C]--2-->[ CC ]--3-->[GT]==> G1
   ||                   |  4  exhaust gas
   ++=====shaft=========+  v
          +--------------------------+
 stack <-5| Feed water heater (gas   |
          | to water heat exchanger) |
          +--------------------------+
             ^ d             | e  hot feed water
             |               v
          +------+     +----------+  a  +----+
          | Pump |     |  Steam   |---->| ST |==> G2
          +------+     |  boiler  |     +----+
             ^ c       +----------+       | b
             |              ^ fuel        v
             |           +-----------------+
             +-----------|    Condenser    |
                         +-----------------+

The exhaust heats the condensate/feed water of a conventional steam plant, replacing bled-steam feed heaters; the boiler supplies the rest of the heat.

A gas and steam combined cycle joins a gas turbine (topping) cycle with a steam (bottoming) cycle. The GT exhaust heat produces steam, so the same fuel produces more power.

Advantages

• High overall efficiency (50-60%) against 35-40% for steam and 30-38% for gas turbine plants, so lower fuel cost per kWh.
• Lower capital cost per kW than coal or nuclear plants; the gas turbine part is compact and factory built.
• Short construction time (2-3 years); can be built in phases (gas turbine runs first in open cycle).
• Quick start-up and good load-following, so useful for intermediate and peak loads as well as base load.
• Less cooling water needed, since only the steam part (about one-third of output) has a condenser.
• Lower emissions of CO2, SO2 and particulates per kWh (usually natural gas fuel); no ash handling.
• Less space and smaller staff; high reliability and availability.
• Flexible: supplementary firing, cogeneration (process steam), repowering of old steam plants.

Three popular designs

Design 1: Unfired (or supplementary fired) heat recovery boiler (HRSG)

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

The GT exhaust raises all the steam in a heat recovery boiler. A duct burner may add supplementary fuel. This is the most common modern design (efficiency 50-60%).

Design 2: Exhaust used as combustion air in a fired boiler

  air      fuel
   |1        |
   v         v
  [C]--2-->[CC]--3-->[GT]==> G1
                      | 4 hot exhaust (~15% O2)
                      v   used as combustion air
         fuel --> +-----------------+  a  +----+
                  | Fired steam     |---->| ST |==> G2
       stack <----| boiler          |     +----+
                  +-----------------+       | b
                     ^ d                    v
                     |  +------+  c  +-----------+
                     +--| Pump |<----| Condenser |
                        +------+     +-----------+

The hot, oxygen-rich exhaust replaces the air preheater and forced draft air of a conventional fired boiler. Steam turbine output is large compared to gas turbine output.

Design 3: Exhaust heats the feed water

  air       fuel
   | 1        |
   v          v
  [C]--2-->[ CC ]--3-->[GT]==> G1
   ||                   |  4  exhaust gas
   ++=====shaft=========+  v
          +--------------------------+
 stack <-5| Feed water heater (gas   |
          | to water heat exchanger) |
          +--------------------------+
             ^ d             | e  hot feed water
             |               v
          +------+     +----------+  a  +----+
          | Pump |     |  Steam   |---->| ST |==> G2
          +------+     |  boiler  |     +----+
             ^ c       +----------+       | b
             |              ^ fuel        v
             |           +-----------------+
             +-----------|    Condenser    |
                         +-----------------+

The exhaust heats the condensate/feed water of a conventional steam plant, replacing bled-steam feed heaters; the boiler supplies the rest of the heat.

Comparison

A combined power plant uses the waste heat of a gas turbine exhaust to raise steam for a steam turbine. Its main advantages:

• High overall efficiency (50-60%) against 35-40% for steam and 30-38% for gas turbine plants, so lower fuel cost per kWh.
• Lower capital cost per kW than coal or nuclear plants; the gas turbine part is compact and factory built.
• Short construction time (2-3 years); can be built in phases (gas turbine runs first in open cycle).
• Quick start-up and good load-following, so useful for intermediate and peak loads as well as base load.
• Less cooling water needed, since only the steam part (about one-third of output) has a condenser.
• Lower emissions of CO2, SO2 and particulates per kWh (usually natural gas fuel); no ash handling.
• Less space and smaller staff; high reliability and availability.
• Flexible: supplementary firing, cogeneration (process steam), repowering of old steam plants.

$$\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST}$$

For example, $\eta_{GT} = 0.35$ and $\eta_{ST} = 0.30$ give $\eta_{cc} = 0.545$ (54.5%), far higher than either plant alone.

Circuit: heating feed water with exhaust gases

The gas turbine is regenerative: its exhaust first preheats the compressed air in a regenerator, and the remaining exhaust heat heats the boiler feed water.

  air 1     fuel
   v         v
  [C]-2->[ Regen ]-x->[ CC ]-3->[GT]==> G1
           ^   |                  | 4
           +---|------------------+
               v y exhaust
   stack <-- +------------------+
             | Feed water heater|<--d--+
             +------------------+      |
                   | e hot water   +------+
                   v               | Pump |
  fuel --> +--------------+   a    +------+
           | Steam boiler |--+       ^ c
           +--------------+  v       |
                           [ST]==>G2 |
                             | b     |
                             v       |
                        [Condenser]--+

1. Compressed air (2) is preheated by the turbine exhaust in the regenerator (x), heated in the combustion chamber (3) and expanded in the gas turbine (4).
2. Exhaust leaving the regenerator (y), still at about 300-400 C, flows through the feed water heater and then to the stack.
3. Condensate pumped from the condenser (d) is heated by this gas to state e and enters the fired steam boiler, which produces superheated steam (a) for the steam turbine.
4. Since bled-steam heaters are no longer needed, more steam expands fully in the turbine, increasing steam plant output with the same boiler fuel.

Reasons for building a combined cycle plant

• Gas turbine exhaust (450-600 C) carries 60-70% of fuel energy; using it gives overall efficiency of 50-60%.
• Lower fuel cost and CO2 per kWh; lower emissions with natural gas.
• Lower capital cost and shorter construction time than coal or nuclear plants; phased installation possible.
• Fast start and load-following, useful for varying demand.
• Less cooling water and space than a steam plant of the same output.
• Old steam plants can be repowered by adding a gas turbine.

In this scheme a gas turbine is added to a conventional steam plant and its exhaust is used to heat the boiler feed water, replacing some or all regenerative (bled-steam) feed water heaters.

Layout

  air       fuel
   | 1        |
   v          v
  [C]--2-->[ CC ]--3-->[GT]==> G1
   ||                   |  4  exhaust gas
   ++=====shaft=========+  v
          +--------------------------+
 stack <-5| Feed water heater (gas   |
          | to water heat exchanger) |
          +--------------------------+
             ^ d             | e  hot feed water
             |               v
          +------+     +----------+  a  +----+
          | Pump |     |  Steam   |---->| ST |==> G2
          +------+     |  boiler  |     +----+
             ^ c       +----------+       | b
             |              ^ fuel        v
             |           +-----------------+
             +-----------|    Condenser    |
                         +-----------------+

Working:

1. Air is compressed (1-2), burnt with fuel in the combustion chamber (2-3) and expands in the gas turbine (3-4), driving the compressor and generator G1.
2. The hot exhaust (about 450-550 C) passes through the feed water heater (an economiser-type gas-to-water heat exchanger) and leaves to the stack at 5.
3. Condensate from the steam plant is pumped (c-d) and heated by the exhaust gas (d-e) nearly up to the boiling temperature. This replaces the bled-steam feed heaters, so steam that would have been extracted now expands fully in the turbine and produces extra power.
4. The hot feed water enters the conventional fired steam boiler, where fuel supplies the latent heat and superheat (e-a).
5. Steam expands in the steam turbine (a-b) driving G2, is condensed (b-c) and the cycle repeats.

The steam boiler is still fully fired, so this scheme gives a modest gain (about 3-5% points) and is often used to repower an existing steam plant by adding a gas turbine.

T-s diagram

  T
  ^          3
  |         /|
  |        / |     gas cycle 1-2-3-4,
  |       2  |     exhaust 4-5 heats
  |       |  4     feed water d-e
  |       |   \
  |       1    5
  |                    a
  |       f---------g /|
  |      e             |
  |     /              |
  |    d               |
  |    c---------------b
  +---------------------------> s
  c-d pump, d-e heating by GT exhaust,
  e-f-g-a heating in fired boiler,
  a-b steam turbine, b-c condenser

The heat given by the gas, $\dot m_g c_p (T_4 - T_5)$, equals the heat taken by the water, $\dot m_w (h_e - h_d)$.

Basic components of a gas and steam turbine combined cycle

  air                   fuel
   |                     |
   v    +----+      +---------+      +-----+
  [C]===|    |----->| Comb.   |----->| GT  |===> G1
   |    +----+  2   | chamber | 3    +-----+
   +--------------------------------------|
                                          | 4 hot exhaust
                                          v  (~500-600 C)
   stack <---- 5 ----+------------------------+
                     |  HRSG (economiser,     |
                     |  evaporator, S/H)      |
                     +------------------------+
                        ^ water         | steam a
                        | d             v
                    +------+       +-----+
                    | Pump |       | ST  |===> G2
                    +------+       +-----+
                        ^  c            | b
                        |   +-----------+---+
                        +---| Condenser     |
                            +---------------+

• Gas turbine plant: compressor (C), combustion chamber, gas turbine (GT), generator G1.
• Heat recovery steam generator (HRSG): economiser, evaporator and superheater in the exhaust duct; optional duct burner.
• Steam plant: steam turbine (ST), generator G2, condenser, feed pump.

How the waste heat is utilised

1. The gas turbine exhaust leaves at about 500-600 C and still contains 15-16% oxygen; in a simple gas turbine plant this heat (about 60-65% of fuel energy) would go to the atmosphere.
2. In the HRSG the exhaust gives up this heat in stages: first superheating the steam, then evaporating water, and finally preheating feed water in the economiser. Gas leaves the stack at about 100-150 C.
3. The steam produced drives the steam turbine and generates about one-third of the total plant output without any extra fuel.
4. If more steam is needed, extra fuel is burnt in the exhaust (supplementary firing), using its remaining oxygen.

Thus the heat rejected by the gas turbine cycle becomes the heat input of the steam cycle, and the overall efficiency rises to about 50-60%:

$$\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST}$$