Skip to content

Combined Cycle Power Plant

July 11, 2020

The coal-fired power plant has an efficiency of under 30%. In other words, there is a high waste in the latent energy of the fuel. Combined cycle power plant or Combined cycle gas turbine is a combination of both gas and steam power production technologies able to achieve efficiency by up to 60%.

It is a type of natural gas power plant which consists of a simple cycle gas plant in combination with a second steam engine to generate the electricity.

As the hot gases exhaust from the initial gas turbine sent to the steam engine to produces steam. This steam expands through a turbine to generate additional electricity through a Rankine cycle.

And this process increases the plant’s overall efficiency which can be as great as 55%.

We should know that this combination doesn’t apply to only gas turbines as we could use it with steam turbines.

Combined heat and power plant

Combined heat and power plant or Cogeneration plant integrates the production of both usable heat (thermal energy) and electricity in one single.

It is a highly efficient process because there are nearly two-thirds of the energy produced by conventional plants wasted in heat discharged to the atmosphere and also energy waster during the distribution of electricity to customers.

And the CHP technology is typically located where there is a need for both thermal energy and electricity where it can be used for space heating, cooling, domestic hot water and industrial processes.

In this plant, we can use a variety of fuels, both fossil and renewable-based.

And we can deploy combined heat and power plant technology quickly, with few geographic limitations, and cost-effectively.

There are some benefits to this technology as:

  • The costs of energy reduced.
  • It reduces emissions.
  • The efficiency is higher than in a traditional thermal power generation or in an open_cycle gas turbine generation.

How does a combined cycle power plant work?

The working principle of a combined power plant doesn’t have a great difference from the coal-fired plant.

As in the first stage, we burn gas to rotate a gas turbine; coupled to an electrical generator that rotates to produce the electric power.

Hence in the second stage, the hot gasses leaving the gas turbine passes into a heat recovery steam generator to produce steam which is used to rotate a steam turbine in addition to a generator that also rotates to produce the electricity.

Then the steam condenses and we recycle the system as in the steam power plant.

This means that we have two generators, one driven by the gas turbine to produce two-thirds of the plant output electricity and the other driven by the steam turbine to produce one-third of the plant output electricity.

Also, the gas turbine and the steam turbine are joined to a single generator used for the startup operation of the gas turbine alone.

And we can disconnect the steam turbine by a hydraulic clutch.

Combined cycle power plant construction:

There are two most common systems of the combined power plant which are:

Combustion turbine ( reciprocating engine) with a heat recovery unit

In this system, we burn fuels like natural gas, oil, or biogas to turn the generators to produce electricity.

And we use the heat recovery device to capture the heat from the turbine to be converted into useful thermal energy (steam or hot water).

Steam boiler with a steam turbine

In this system, we begin by producing the steam in the boiler then use it to turn a turbine connected to a generator which then turns to produce electricity. We can also use the steam leaving the turbine to produce useful thermal energy.

In this system, we can use a variety of fuels as oil, natural gas, biomass, or coal.

Also, the combined cycle plant may include:

Single shaft configuration: which consists of one gas turbine, one steam turbine, one generator, and one heat recovery steam generator.
And the gas turbine and the steam turbine are fused to the single generator.

Multi-shaft configuration: which consists of one or more gas turbine generators and a heat recovery steam generator that supplies steam through a common header to a separate single steam turbine generator. But the overall investment is about 5% higher in costs in this configuration.

Combined cycle power plant efficiency

It’s sometimes difficult to calculate or predict the overall performance of the combined cycle plant. So, we only should know that the combined heat power plants can reach efficiency ratings in excess of 60%.

And there are types of combined plants capable of reaching the full power in less than 30 minutes; and also offers flexibility for companies integrating renewable generation into systems.

Also, the combined cycle units reduce the amount of fuel needed to do the same amount of work, reduce site CO2 emission by 65 %, reduce SOx emission by 95%, remove up 90% of the nitrous oxides, and also cut particulate emissions to zero.

Advantages of combined cycle power plant:

Combined cycle plant has great advantages as:

  • It’s highly reliable, flexible, and available.
  • The size and weight are small enough to be suitable for ships, aircraft engines, and locomotives gas turbines.
  • We can use natural gas, which is very suitable fuel.
  • A lower initial cost compared to an equivalent steam plant.
  • The cooling water required is less than for the steam plant of the same capacity output.
  • It can start-up and shut-down quickly with fewer start-up losses.
  • Maintenance and installation time is less compared to a thermal power plant.
  • We can locate these plants near to load centers so transmission costs and losses reduced.
  • This plant can run off any fuel like oil, gas, biogas, or methane gas.
  • The high overall efficiency exceeding 50% and low emission levels of pollutants make it suitable for use in heavily populated regions.
  • It’s generally operated fully automatically so it’s suitable for use where operating staff is less experienced.
  • It’s easy to convert simple gas turbine units to combined cycle operational power plants using gasified coal and replacing the turbine burners to accomplish the fuel conversion.

Disadvantages of a combined cycle power plant:

There are some disadvantages of the combined power plant as:

  • The technologies needed are more expensive and complex, so initial investments for building a plant is high.
  • The Maintenance cost is high.
  • It isn’t suitable as a peak load plant.
  • The natural gas used is a non-renewable and highly flammable source.
  • The efficiency of part-load demand is poor.
  • It requires special metals to maintain the unit operated at high temperature and pressure.

Applications of combined heat power plant:

It is used in over 4400 facilities including:

Industry: as chemical plants, sawmills, refineries, laundries, food industry, plastic molding industry, farming, and greenhouses because combined plants provide a stable electrical power supply which we can isolate them from the local electricity grid in needing.

Hospitals: we all know that hospitals need high electrical and thermal energy for critical reliability so they use a 2 MW combined plant to enhance the quality of power by assuring smooth operation of the clinical devices.

Commercial: as office buildings, airports, shopping malls, hotels, health clubs, universities, and so on where they need to reduce the size and the capital investment in production equipment.

Institutions: like prisons, military bases, schools.

Manufacturers: chemical, refining, ethanol, pulp and paper, and glass manufacturing.