Gas turbine:

A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Gas turbine may also refer to just the turbine component. Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the combustor, combustion of the fuel increases the temperature. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning the turbine which powers the compressor and, for some turbines, drives their mechanical output. The energy given up to the turbine comes from the reduction in the temperature of the exhaust gas.

                                                Figure: Gas turbine chamber.

Gas turbine works on the basis of Bray ton cycle. Brayton cycle is called the backbone of the gas turbine. Here the T-S and P-V diagram is shown in below:

All four processes of the Brayton cycle are executed in steady flow devices so they should be analyzed as steady-flow processes.

When the changes in kinetic and potential energies are neglected, the energy balance for a steady-flow process can be express, on a unit-mass basis, as −

                       

                                                                                               

           Figure: 1^st stage blade of GT

                                        Figure: T-S and P-V diagram of Brayton cycle.

(Q _in –Q _out) + (W_in –W _out) = H _exit – H _inlet

Therefore, heat transfers to and form the working fluid are

                        Q _in = H₃ – H₂ = C_p (T₃- T₂)

            And, Q _out = H₄ – H₁ = C_p (T₄ – T₁)

Then the thermal efficiency of the ideal Brayton cycle is-

 Brayton efficiency = 1 – (Q _out / Q _in).

           

                       

Turbine blades:

The energy conversion takes place through the turbine blades. A turbine consists of alternate rows of blades. This blades convert the chemical or thermal energy of working fluid into kinetic energy and then from kinetic energy to mechanical energy as rotation of the shaft.

 There are two types of blade, fixed and moving blade. Moving blade is also two types.

One is impulse blade and another reaction blade.

Fixed blade:

A fixed blade assembly is very important for turbine blading. It is also known as diaphragm. The shape of the blade is the key to the energy conversion process. Since the fixed blades have a conversing nozzle shape, it is also called nozzles. When steam is passed over the fixed blades, they increase the velocity of steam as an operation of nozzles. Here blades are converted the thermal energy of steam into kinetic energy by causing the steam to speed up and gain velocity.

                                                                                          

Moving blade:

Moving blade can be shaped in either of two ways: reaction shaped or impulse shaped. The shape of the blade determines how the energy is actually converted. Either type of moving blades or a combination of both can be attached to the shaft of the rotor on dices, called wheels as shown in the figure. Along the outer rim of the blades is a metal band, called shrouding which ties the blades together. The moving blades convert the kinetic energy in the moving speed into the mechanical energy as rotor rotation.

                      

Steam turbine:

Steam turbine is such type of turbine where steam is used as working fluid. When steam is injected over the blades it rotates at a certain speed. Since steam is used for rotation it is called steam turbine. Generally it is used at steam turbine power station.

In most power station, steam turbines are used for steam turbine units. All the turbines are manufactured by BBC (Germany).

                                                 Figure: Steam turbine (case opened).

Main parts of steam turbine:                

Ø      Rotor

Ø      Blades (fixed and moving)

Ø      Bearings (thrust and journal)

Ø      Turbine casing

Ø      Valves (main stop valve, control valve etc).

  

Turbine:

A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work. The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and impart rotational energy to the rotor.

Types of turbine:

1.      Steam turbine

2.      Gas turbine

3.      Water turbine

4.      Wind turbine

5.      Transonic turbine

6.      Ceramic turbine

Benefits of Waste Heat Recovery:

Benefits of ‘waste heat recovery’ can be broadly classified in two categories:

Direct Benefits:

Recovery of waste heat has a direct effect on the efficiency of the process. This is reflected by reduction in the utility consumption & costs, and process cost.

Indirect Benefits:

a) Reduction in pollution: A number of toxic combustible wastes such as carbon monoxide gas, sour gas, carbon black off gases, oil sludge and other plastic chemicals etc, releasing to atmosphere when burnt in the incinerators serves dual purpose i.e. recovers heat and reduces the environmental pollution levels.

b) Reduction in equipment sizes: Waste heat recovery reduces the fuel consumption, which leads to reduction in the flue gas produced. This results in reduction in equipment sizes of all flue gas handling equipments such as fans, stacks, ducts, burners, etc.

c) Reduction in auxiliary energy consumption: Reduction in equipment sizes gives additional benefits in the form of reduction in auxiliary energy consumption like electricity for fans, pumps etc.

Waste Heat Recovery Unit (WHRU):

After combustion some flue gases are produced having temperature about 500⁰C which exits through the chimney. This will be great loss if the flue gases are not used properly. For this a combine cycle plant is introduced which use the flue gas as working fluid. Here damper is used to recover the waste heat.  

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                            Figure: Flow diagram of Waste Heat Recovery Unit (WHRU)

Damper blocks the chimney and passes the flue gas to boiler and produce steam. This steam is used to generate electricity at steam turbine. Since both of steam and gas turbines are used it is called combine cycle power plant (CCPP).

                            Figure: Waste Heat Recovery Unit