POWER PLANT ENGINEERING (UNIT-2)

UNIT – 2


INTERNAL COMBUSTION AND GAS TURBINE POWER PLANTS

DIESEL POWER PLANT: Plant layout with auxiliaries – fuel supply system, air starting equipment, super charging.
GAS TURBINE PLANT: Introduction,classification-construction layout with auxiliaries,combined cycle power plants and comparison.


Diesel Power Plant Layout:





Image result for diesel power plant

Applications of diesel power plant:


  • Diesel power plant is in the range of 2 to 50 MW capacity. They are used as central station for small or medium power supplies.
  • They can be used as stand-by plants to hydro-electric power plants and steam power plants for emergency services.
  • They can be used as peak load plants in combinations with thermal or hydro-plants.
  • They are quite suitable for mobile power generation and are widely used in transportation systems such as automobiles, railways, air planes and ships.
  • Now-a-days power cut has become a regular feature for industries. The only solution to tide over this difficulty is to install diesel generating sets.

Diesel engine:

Diesel engines or compression ignition engines as they are called are generally classified as two stroke engine and four stroke engines. In diesel engine, air admitted into the cylinder is compressed, the compression ratio being 12 to 20. At the end of compression stroke, fuel is injected. It burns and the burning gases expand and do work on the position. The engine is directly coupled to the generator. The gases are then exhausted from the cylinder to atmosphere.


Engine starting system:

This includes air compressor and starting air tank. The function of this system is to start the engine from cold supplying compressed air.


Fuel system:

Pump draws diesel from storage tank and supplies it to the small day tank through the filter. Day tank supplies the daily fuel need of engine. The day tan is usually placed high so that diesel flows to engine under gravity. Diesel is again filtered before being injected into the engine by the fuel injection pump. The fuel is supplied to the engine according to the load on the plant.


Air intake system:

Air filters are used to remove dust from the incoming air. Air filters may be dry type, which is made up of felt, wool or cloth. In oil bath type filters, the sir is swept over a bath of oil so that dust particles get coated.


Exhaust system:

In the exhaust system, silencer (muffler) is provide to reduce the noise.


Engine cooling system:

The temperature of burning gases in the engine cylinder is the order of 1500 to 2000’C. to keep the temperature at the reasonable level, water is circulated inside the engine in water jackets which are passage around the cylinder, piston, combustion chamber etc. hot water leaving the jacket is sent to heat ex-changer. Raw water is made to flow through the heat ex-changer, where it takes up the heat of jacket water. It is then cooled in the cooling tower and recirculates again.


Engine lubrication system:

It includes lubricating oil tank, oil pump and cooler. Lubrication is essential to reduce friction and wear of engine parts such as cylinder walls and piston. Lubricating oil which gets heated due to friction of moving parts is cooled before re-circulation. The cooling water used in the engine is used for cooling the lubricant also.


Advantages of diesel power plant:

  1. Plant layout is simple. Hence it can be quickly installed and commissioned, while the erection and starting of a steam power plant or hydro-plant takes a fairly long time.
  2. Quick starting and easy pick-up of loads are possible in a very short time.
  3. Location of the plant is near the load center.
  4. The load operation is easy and requires minimum labors.
  5. Efficiency at part loads does not fall so much as that of a steam plant.
  6. Fuel handling is easier and no problem of ash disposal exists.
  7. The plant is smaller in size than steam power plant for same capacity.
  8. Diesel plants operate at high overall efficiency than steam.

Disadvantages of diesel power plant:

  1. Plant capacity is limited to about 50 MW of power.
  2. Diesel fuel is much more expensive than coal.
  3. The maintenance and lubrication costs are high.
  4. Diesel engines are not guaranteed for operation under continuous, while steam can work under 25% of overload continuously.
Methods of supercharging:

1.Compressor driven by engine

2.Compressor driven by turbine

3.Compressor driven by external source of power

4.Compressor driven by engine shaft and  free turbine

5.Direct coupling  between engine, compressor and turbine


1.Compressor driven by engine:



2.Compressor driven by turbine:




3.Compressor driven by external source of power:



4.Compressor driven by engine shaft and  free turbine:



5.Direct coupling  between engine, compressor and turbine:




Difficulty in Starting Of Diesel Engine Power plants:

Diesel engines are equipped with things called glow plugs.Glow plugs are like spark plugs, but instead of igniting a fuel-air mixture, they warm the inside of the cylinder during startup.  Since diesels rely on the spontaneous combustion of a compressed, and thus heated, fuel-air mixture, the dense metal of the cylinders and pistons will conduct the heat of the compression easily and thus an external heat source is required at start.   So, when it is colder than usual out, the entire block is colder.  The rest of the block conducts the heat from the cylinders that the glow plugs provided, therefore they aren't as effective.  In cold weather areas, it is wise to order your diesel with an engine block heater.  This heater works like a glow plug for the entire block to achieve a more neutral thermal disparity between it and the cylinders.  Glow plugs require the vehicle's electrical system to be on for a short period of time to heat the cylinders.  On older vehicles this can be up to a minute.  On newer ones, just a few seconds.  For vehicles in a cold climate without a block heater, they may need to be "primed" longer to achieve the same effects.  This should only apply to cold starts.  If the engine is still hot, the plugs won't usually need to be primed long and a heater won't be needed. In summary, the block heater and glow plugs essentially perform the same task: to heat the engine to foster spontaneous combustion by hindering heat transfer from the compressed air-fuel mixture to the metal components of the engine.  Gasoline engines do not require this because combustion is assisted by the open spark of a spark plug.

Gas Turbine Power Plants:

The gas turbine power plants which are used in electric power industry are classified into two groups as per the cycle of operation.
(a) Open cycle gas turbine.
(b) Closed cycle gas turbine.



Open cycle gas turbine



In the open cycle gas turbine air is drawn in to the compressor from the atmosphere. The compressed air is heated by using the burner; the air must be burned directly. In the combustion chamber the fuel in the air inside maintains at constant pressure. From the combustion chamber the high pressure hot gases drive the turbine. The power must be developed when the turbine shaft rotates. There is no self-starting in the Gas turbines. The starting motor drives the compressor till the fuel is injected in the combustion chamber. If the turbine frights gain speed then the starting motor is disconnected. The power established by the gas turbine is used to initiative the compressor and the remaining is used to initiative other machinery or generator. In the open cycle gas turbine the system and the working fluid are replaced continuously and gases are drained into the atmosphere. Then the total flow derives from the atmosphere and again returns to the surrounding.


Closed cycle gas turbine

According to the closed cycle gas turbine, the compressed air from the surroundings is heated by using the heat ex-changer (air heater). At constant pressure from the external source the heat is additionally given to the heat ex-changer. High pressure working fluid increases over the turbine and then the power is developed. The exhaust working fluid must be cooled in a pre-cooler. Same fluid is sent into the compressor before the process is done. In the turbine same working fluid is always distributed. From an external source the fuel is required for adding heat so the fuel ranges from kerosene and then to the heavy oil.  Straightly the fertilizer and coal slurry enter the turbine without reducing the efficiency.


Gas Turbine power plant - components:

Gas Turbine:A gas turbine is also known as combustion turbine. It is an internal type of combustion engine. In the downstream turbine upstream rotating compressor is coupled in-between combustion chamber also.It is defined as the principal prime mover which is of the turbine type and the medium of working is permanent gas”.
Components present in the gas turbine power plant:
  • Compressor
  • Inter-cooler
  • Re-generator
  • Combustion chamber
  • Gas turbine
  • Reheating unit
Compressor
In most of the cases centrifugal and axial turbines are used in the gas turbine power plants. Two compressors are used in the gas turbine power plant. In that one of them is low pressure compressor and the other one is high pressure compressor. Through the filer the atmospheric air must be drawn into the compressor by using the low pressure compressor. The developed power must be used to run the compressor.  It uses nearly 66% of the power source. With the help of the inter-cooler low pressure air, and is moved into the high pressure compressor. Then immediately the high pressure air must be moves in to the re-generator.

Inter-cooler:

The main aim of the intercooler is to reduce the compressor work and it is to be placed between the low pressure and high pressure compressors. Where the pressure ratio must be high then intercoolers are used. The energy required to compress the air must be proportional to the inlet air pressure. The compressed air cooling in the inter-cooler is complete by water.

Re-generator:

When the air is entering to the re-generator then preheating is done to reduce the fuel consumption in the combustion chamber, where the efficiency needs to be increased. This is done by heat of hot exhaust gases coming out of the turbine.

Combustion chamber:

From the re-generator hot air must be allowed to flow into the combustion chamber. The fuel like natural gas, coal and kerosene are inserted into the combustion chamber. So the high temperature and high pressure products of combustion must be passes through the turbine.
Working of gas turbine power plant:
From the atmosphere the air must be drawn into the low presser compressor with the help of the air filter and then it must be compressed.The compressed low pressure air must come out into the compressor which contains high pressure through the inter-cooler. So the heat compressed air must be removed. Then the compressed air which consists of high presser must be allowed to go into the combustion chamber with the help of the re-generator. The fuel must be added to the compressor air with the help of the combustion chamber, and then the fuel combustion needs to takes place. So the products obtained from the combustion must be allowed into the high pressure turbine. The exhaust present the high pressure turbine must be enters in to another combustion chamber. In which the additional fuel must be added then it moves through the low pressure turbine. In the low pressure turbine after completion of expansion the exhaust must be used as heat the sir coming out with high pressure. Then it moves to the combustion chamber followed by the re-generator. Then the exhaust must be released in to the atmosphere.




Advantage of gas turbine power plant:

  • In gas turbine power plant natural gas is suitable as the fuel. Depends up the available of the natural gas turbine must be installed.
  • Gas turbine power plants can effort carefully for small organization hours.
  • Loading of fuel needs less capacity and handling is stress-free.
  • Gas turbine power plant is minor and close in size when compared with the steam power plants
  • It can be started fast and can be place on load in an identical small interval time
  • The maintenance cost is less
  • It is simple in construction compare to the remaining power plants
  • In case of the steam power plants there is no requirement for boiler, condenser
  • The gas turbine can function at high speed meanwhile there is no reciprocating part
  • Cheaper fuel such as paraffin, kerosene, powdered coal and benzene can be used
  • Gas turbine power plants can be used in water shortage areas
  • Less pollution is noticed
  • A smaller amount of water is necessary for the method
Disadvantage of gas turbine power plant:
  • To effort the compressor 66% of the developed source is recycled
  • The gas turbine unit has a small thermal efficiency
  • The running speed of the gas turbine is in the range from 40,000 to 1,00,000 rpm
  • The working temperature is as great as 2000 C, for this reason different alloys and metals are used for the several components of the turbine
  • Special cooling techniques are necessary for cooling the turbine blades
  • It is hard to start a gas turbine as associated to a diesel engine in a diesel power plant
  • The life span of a gas turbine plant is up to 10 years, after which its efficiency declines to less than 10 percent

Application of gas turbine power plant:

  • To drive generators and supply loads in steam, diesel or hydro plants
  • To work with conventional steam boilers as combination plants
  • Thermal process industries
  • Petro-chemical industries
  • Power generation in aircraft and ships for their propulsion.
  • They are not suitable for cars because of their high speeds
Combined cycle power plants and comparison:
This is how a combined-cycle plant works to produce electricity and captures waste heat from the gas turbine to increase efficiency and electrical output.




1.Gas turbine burns fuel

  • The gas turbine compresses air and mixes it with fuel that is heated to a very high temperature. The hot air-fuel mixture moves through the gas turbine blades, making them spin.
  • The fast-spinning turbine drives a generator that converts a portion of the spinning energy into electricity.
2.Heat recovery system captures exhaust




  • A Heat Recovery Steam Generator (HRSG) captures exhaust heat from the gas turbine that would otherwise escape through the exhaust stack.

    • The HRSG creates steam from the gas turbine exhaust heat and delivers it to the steam turbine.
    3.Steam turbine delivers additional electricity:

    • The steam turbine sends its energy to the generator drive shaft, where it is converted into additional electricity.

    Advantages of combine cycle gas turbine power plants:

    • The efficiency of the combined cycle plant is better or higher than the turbine cycle or steam cycle plant. The efficiency of combined cycle power plant will be of the order of about 45 to 50%.
    • The capital cost of combined cycle power plant with supplementary firing is slightly higher than the simple gas turbine plant and much below those of the classical steam turbine power plant of the same capacity. This is one of the major advantage of combined cycle power plant.
    • The combined cycle power plant is more suitable for rapid start and shutdown than the steam power plant. Therefore these plants accepts load variations quickly and helps in maintaining the stability in the electrical grid.
    • The cooling water requirement of the combined cycle plant is much lower than the normal steam turbine power plant having same capacity output. In these type of plants almost 2/3rd of the power is generated by the gas turbine plant which requires very little or no cooling water. Conventional cooling water system (once through) or recycle system is adopted. In case of lack of sufficient water at the site, dry cooling system can also be provided for cooling. Less water requirement for combine cycle plants is also major advantage while selecting the site of the plant.
    • The combined system offers self-sustained feature. For example if the plant is shut down due to any fault or maintenance, no start up power supply can be required to start the unit. Gas turbine offers to start the plant and supply power to plant auxiliary loads. Therefore combined cycle plants has advantage of self sustaining feature.
    • Simple gas turbine units can be converted to combined cycle operational power plants using gasified coal. The two phase development requires short installation time for peaking power plus the future capability for efficient operation for a base load generation. Only the burners on the gas turbine need to be replaced to accomplish the fuel conversion.
    • Combined cycle plants have the advantage of high ratio of power output to the area occupied. Therefore for designing a combined cycle plant space requirement is not a concern
    • It has the advantage of low operational personnel.
    • Maintenance duration (down time) for combined cycle power plant is significantly low. This improves the plant availability feature
    • When the natural gasified coal is used as the fuel, combined cycle greatly reduces CO2 emissions. Also the amount of heat rejected into the surrounding atmosphere (environment) is less than for completing system, because of the higher overall efficiency. When the coal gasification is employed, up to 98% of the sulphur in the coal is recovered. Thus SO2 emissions are quite lesser than the conventional coal fired power plant.
    • The amount of solid waste from coal gasification combined cycle is less compared to conventional steam turbine plant.
    • The superior efficiency of the combine cycle power plants over conventional reheat cycle plants not only conserves the fossil fuels but also drastically reduces the emissions and waste per unit of electricity generated.

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