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Combined Cycle Power Plant

1. Introduction

A combined-cycle power plant uses both a gas and a steam turbine together to produce up to 50 percent more electricity from the same fuel than a traditional simple-cycle plant. The gas turbine operates according to the Brayton cycle and the steam system operates according to the Rankine cycle. The waste heat from the gas turbine is routed to the nearby steam turbine, which generates extra power. Improve performance with digital.

The main components of a combined cycle power plant are

  • Gas Turbine (GT)

  • HRSG or heat recovery steam generator

  • Steam Turbine and

  • Other accessories or associated items.

2. Gas Turbine (GT)

A gas turbine, also called a combustion turbine, is a type of continuous and internal combustion engine that converts natural gas or fluid into mechanical energy.

The main elements common to all gas turbine engines are an upstream rotating gas compressor, a combustor, a downstream turbine on the same shaft as the compressor. As Bryan's cycle principle is the working principle of GT, it is simple. The turbines draw in air at the front of the unit and then compress; mix with fuel and ignite the mixture at high pressure. The hot gas released expands through the turbine blades connected to the turbine shaft. The shaft turns thus developing mechanical energy which is converted into electrical /energy by the generator.

3.Heat Recovery Steam Generator

HRSG, Heat recovery steam generator is an energy recovery heat exchanger that recovers the exhaust gases (that can be used in cogeneration or combined cycle) from the GT discharge. It has also the characteristics of a boiler because there are steam drums, where the generated steam is separated from boiling water before entering the superheaters.

The four major components of HRSG:

  • Economizer

  • Evaporator

  • Superheater

  • Water preheater

To meet the operating requirements all the components of HRSGs are put together.

Categories of HRSG

1. On the basis of the flow of exhaust gases

  • Vertical HRSG

  • Horizontal HRSG

2. On the basis of pressure levels

  • Single pressure level HRSG

  • Multi pressure HRSG

5. Steam Turbine

A steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible expansion process. The shaft of the turbine rotates with the energy produced from the steam. The amount of energy produced is based on the enthalpy drop across the machine. Enthalpy is a measure of the total energy (heat) in the steam by reference to a reference state or reference temperature normally chosen to be 0 Degree Celsius.

Operation Control Modes

· Fixed pressure mode – The steam turbine will be operated in fixed pressure mode when the load is below 50%. In this mode of operation, pressure from the steam generator remains constant and is controlled by the main control, if the steam turbine is not taking all produced steam, the pressure of a steam generator is controlled by the bypass valves.

· Sliding pressure mode – When the 50% load is reached the main control valve is fully open. With increasing gas turbine loads the steam turbine will be operated in sliding pressure mode. In this case, the live steam pressure varies proportionally with the steam flow.

· Load control– when the generator is synchronized to the grid, its frequency is governed by the grid. The turbine controller maintains the baseload by adjusting the steam flow

Steam Turbine

6. Air Cooled Condenser

Air Cooled Condenser (ACCs) are environmentally preferable to the traditional water-cooled condensers for rejecting heat in combined-cycle power plants (CCPPs). An ACC is a direct dry cooling system where steam is condensed inside air-cooled finned tubes. The cool ambient airflow outside the finned tubes is what removes heat and defines the functionality of an ACC. In thermal power plants, the steam from the turbine exhaust flows into the ACC where condensation occurs. Then the condensate returns to the boiler in a closed loop. Since the steam coming from the turbine is at low pressure, the ACC works at a pressure close to a vacuum, and non-condensable gases are removed continuously by an air evacuation unit. Pressure transmitters protect the ACC in case of overpressure. The control system modulates the number of fans into operation and fan speed and steam isolating valve position to meet the back pressure set point. Temperature transmitters in the main steam duct protect the condenser against overheating.

Air Cooled Condenser

6. Types of Condensing System

The selection of condensing systems varies based on environmental conditions. They are classified into the following categories:-

· Water-cooled surface condensers and wet condensing system

· Air-cooled condensers

· Alternative condensing systems

7. Air Extraction System

The non–condensable have to be evacuated from the condenser before steam can be introduced at start-up (hogging process) and should be continuously removed during normal operation (holding process)

· Hogging Process – For the hogging process, the requirements are to lower the pressure as quickly as possible from the atmospheric pressure (946 mbar (a)) to 250 mbar (a)) within 30 minutes.

· Holding Process – Once the vacuum is established and during normal operation, the hogging extraction skid is shut down and only one holding vacuum set continuously removes the non-condensable.

8. Bypass Stack and Diverter

When electric power generation is mandatory, it is advisable to run the gas turbine in the open cycle and exhaust the flue gas into the atmosphere rather than sending it to the HRSG. A bypass stack and a diverter are required to close the path to the HRSG and opens it into the atmosphere through the bypass stack. The diverter is connected to the GT exhaust duct before the diverging cone of the HRSG, and this implies that the GT has to meet the plant emissions limits, as any SCR in the HRSG is also bypassed. Throttling by the diverter could also be used to control steam generation in the HRSG. This configuration is rare. The most important characteristic of a well-designed diverter is its ability to completely switch the flue gas from the bypass stack to HRSG, under all operating conditions.

9. Auxiliary Systems

i. Boiler Feed Water Pump

The LP drum can be used to feed the boiler feed water (BFW) pumps on level control as explained in the three elements control system. If there are HP and IP sections, the BFW pumps can be multiple-stage centrifugal pumps with an intermediate discharge for the IP section. Automatic minimum flow bypass, a Three-way Yarway valve, on the HP discharge nozzle of the pump is used for minimum flow protection.

ii. Bypass System

The superheated steam to the steam turbine is bypassed to the condenser during the start-up, ST shutdown, and load rejection. Each bypass requires a pressure reduction and desuperheating with boiler feedwater or condensate to meet downstream condenser conditions.

The bypass arrangement includes:-

  • HP bypass from HP header to IP header (cold reheat side if reheating is implemented)

  • IP bypass from IP header (hot reheat side if reheating is implemented) to the condenser.

  • LP by-pass from LP header to the condenser.

iii. Blow Down Tank

The HP and IP steam drums drain into a pressure blow-down tank in large boilers. Additionally, an atmospheric blow-off tank is also provided to receive the water from the blow-down tank plus drains from the LP drum and the blow-off from the HP and IP drums. To keep the required steam purity, a small percentage (1–3%) of the water in the steam drums is discharged to continuous blow-down.

iv. Demineralization Plant

The demineralization plant is usually controlled by its own PLC, which is interfaced with the DCS, but sometimes is controlled directly by the plant DCS system. This consideration can be the basis for sizing the demineralized water storage tank. The water needed for filling the HRSG and as make-up water during normal operation is generated in a demineralization plant. It is stored in a tank that should be sized sufficiently large to provide water in case of disruption in production.

v. Closed Circuit Cooling Water

The users of the Closed Circuit Cooling Water are turbine generators, condensate and feed water pumps, sampling systems, etc. If an air condenser is used, the closed-circuit cooling water system becomes much smaller, because the amount of water needed in the rest of the plant is a relatively small percentage of that needed for the water condenser.

**The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations, and write-ups are only for reference and educational purposes.

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