What is a gas-steam combined cycle?

Short Answer:

A gas-steam combined cycle is a modern power generation system that combines a gas turbine (Brayton cycle) and a steam turbine (Rankine cycle) to produce electricity more efficiently. The exhaust heat from the gas turbine is used to produce steam for the steam turbine, thus generating additional power without extra fuel consumption.

In simple words, a gas-steam combined cycle makes use of the waste heat from a gas turbine to run a steam turbine. This process improves overall efficiency, reduces fuel usage, and lowers emissions compared to single-cycle power plants, making it one of the most efficient energy systems used today.

Detailed Explanation :

Gas-Steam Combined Cycle

A gas-steam combined cycle is a highly efficient and advanced power generation system that combines two thermodynamic cycles — the gas turbine cycle (Brayton cycle) and the steam turbine cycle (Rankine cycle) — in a single plant. The key concept of this system is waste heat recovery, which means using the hot exhaust gases from the gas turbine to generate steam for the steam turbine.

In a simple gas turbine plant, a large portion of the fuel energy (around 60%) is lost as heat in the exhaust gases. The gas-steam combined cycle captures this waste heat and uses it to produce additional power, improving overall efficiency to 55–65%. This combination of two cycles allows better fuel utilization, reduced emissions, and cost-effective electricity generation.

The gas-steam combined cycle is widely used in modern thermal power plants and combined heat and power (CHP) plants due to its high performance and environmental advantages.

Working of Gas-Steam Combined Cycle

The working of a gas-steam combined cycle involves the integration of two cycles:

  1. Gas turbine cycle (Brayton cycle) – for primary power generation.
  2. Steam turbine cycle (Rankine cycle) – for secondary power generation using waste heat.

These two cycles work together as follows:

  1. Gas Turbine (Brayton Cycle) Operation

The first part of the combined cycle is the gas turbine system, which operates on the Brayton cycle.

Steps involved:

  • Compression: Air from the atmosphere is compressed to high pressure in a compressor.
  • Combustion: The compressed air enters the combustion chamber, where fuel (usually natural gas) is injected and burned at constant pressure.
  • Expansion: The high-temperature and high-pressure gases expand through the gas turbine, driving its blades to produce mechanical energy, which is converted into electrical power using a generator.
  • Exhaust: The gases leaving the turbine are still at high temperature (500°C–600°C) and contain large amounts of energy.

Instead of releasing these hot gases directly to the atmosphere, they are directed to the heat recovery steam generator (HRSG) for use in the steam cycle.

  1. Steam Turbine (Rankine Cycle) Operation

The second part of the combined cycle is the steam turbine system, which operates on the Rankine cycle.

Steps involved:

  • Heat Recovery: The hot exhaust gases from the gas turbine pass through the Heat Recovery Steam Generator (HRSG).
  • Steam Generation: The HRSG uses the heat from exhaust gases to convert feedwater into steam without using any additional fuel.
  • Expansion: The high-pressure steam produced is sent to the steam turbine, where it expands and produces additional mechanical power.
  • Condensation: After expansion, the exhaust steam is condensed into water in a condenser and pumped back to the HRSG for reuse.

This process completes the Rankine cycle, adding extra power output without additional fuel consumption.

Integration of the Two Cycles

In a gas-steam combined cycle, the gas turbine and steam turbine are connected in such a way that:

  • The gas turbine produces power directly.
  • The exhaust heat from the gas turbine is used in the HRSG to produce steam for the steam turbine.
  • The steam turbine generates additional power using this recovered heat energy.

Therefore, the combined output of both turbines gives a much higher total power output compared to a single gas or steam turbine plant using the same amount of fuel.

Thermodynamic Representation

  • The gas turbine works on the Brayton cycle, which includes isentropic compression, constant pressure heat addition, and isentropic expansion.
  • The steam turbine works on the Rankine cycle, which includes isentropic expansion, condensation, and constant pressure heat addition.
  • The exhaust gases from the Brayton cycle serve as the heat source for the Rankine cycle through the HRSG.

By combining the two, the plant achieves higher thermal efficiency and better utilization of energy.

Efficiency of Gas-Steam Combined Cycle

The overall efficiency of a gas-steam combined cycle is calculated as:

Where:

  •  = efficiency of the gas turbine (Brayton cycle)
  •  = efficiency of the steam turbine (Rankine cycle)

Typically,

  • Gas turbine efficiency ≈ 35–40%
  • Steam turbine efficiency ≈ 25–30%
  • Combined efficiency ≈ 55–65%

This high efficiency is due to the reuse of waste heat that would otherwise be lost to the atmosphere.

Advantages of Gas-Steam Combined Cycle

  1. High Thermal Efficiency:
    • Efficiency up to 60–65% due to utilization of waste heat from the gas turbine.
  2. Lower Fuel Consumption:
    • Produces more electricity per unit of fuel, reducing operating cost.
  3. Reduced Environmental Pollution:
    • Lower CO₂ and NOₓ emissions because of efficient combustion and less fuel burning.
  4. Compact and Lightweight:
    • Combined cycle plants are smaller and easier to install than conventional coal-fired plants.
  5. Quick Start and Load Response:
    • Gas turbines start quickly, making the plant suitable for both base-load and peak-load power supply.
  6. Better Reliability and Low Maintenance:
    • Fewer moving parts and modern materials ensure longer operational life and low maintenance.
  7. Cogeneration Capability:
    • Waste heat from the steam turbine can be used for heating, improving total energy utilization.

Limitations of Gas-Steam Combined Cycle

  1. High Initial Cost:
    • Installation of HRSG, dual turbines, and control systems increases capital investment.
  2. Complex System:
    • Integration of two different cycles requires sophisticated control and skilled operation.
  3. Fuel Quality Requirement:
    • Generally operates best on clean fuels like natural gas; impure fuels can damage turbine blades.
  4. Cooling Water Requirement:
    • Steam condenser needs a large quantity of cooling water.

Despite these limitations, the high efficiency and low fuel usage make it one of the most preferred technologies for modern power plants.

Applications of Gas-Steam Combined Cycle

  • Power generation plants for industrial and utility purposes.
  • Cogeneration plants (combined heat and power) for industries like refineries and paper mills.
  • Marine propulsion systems in large ships.
  • Aviation ground power units.
  • Integrated renewable systems, where the combined cycle works with solar or biomass systems.
Conclusion

In conclusion, a gas-steam combined cycle is an advanced power generation system that combines the Brayton (gas turbine) and Rankine (steam turbine) cycles to maximize energy efficiency. The exhaust heat from the gas turbine is reused in the HRSG to generate steam for the steam turbine, resulting in higher efficiency, lower fuel consumption, and reduced emissions. Though the system has a high initial cost and complexity, its superior performance, low environmental impact, and fuel economy make it one of the most efficient and sustainable methods for modern electricity generation.