26. Explain Reheat cycle?
If the dryness fraction of steam leaving the turbine is less than 0.88, then, corrosion and erosion of turbine blades occur. To avoid this situation, reheat is used.
In the reheat cycle, the expansion of steam takes place in one (or) more turbines. Steam is expanded in the HP turbine first, and then it is reheated. The reheated steam is again expanded in. the LP turbine.
27. What are the important advantages of Re heating?
- Due to reheating, network done increases
- Heat supply increases
- Thermal efficiency increases
- Due to reheating, the turbine exit dryness fraction increases so moisture decreases so blade erosion becomes minimum so life of the turbine will be increased.
28. Name different methods of reheating?
- Gas Reheating
- Live steam reheating
- Combined gas live steam reheater.
29. Define bleeding in steam power plant?
Assume I kg of steam is expanded in the turbine. Before complete amount of steam is expanded, some amount of steam (m kg) is extracted ‘ Extracting the steam in the turbine before exhaust is called bleeding. This bled steam is used to heat the feed water.
30. Explain the term Regeneration?
Regeneration means heating the feed water by steam taken from the turbine. The steam is exhausted (bled) from the turbine at several locations before exhaust and is supplied to regenerator (feed water heater) to heat the feed water.
31. State some advantages of Regeneration cycle?
- Heat supplied to boiler becomes reduced
- Thermal efficiency is increased since the average temperature of heat addition to the cycle is increased.
- Due to bleeding in the turbine, erosion of turbine due to moisture is reduced.
32. Name different methods used to extract steam for heating the feed water?
- Direct contact heater
- Drain pump method
- All drains to hot well
- Cascade system
33. Define the term waste heat recovery?
Waste heat is the heat which is not at all used and exhausted out as a waste product. Waste heat is normally available from the industry in the form of process steam and water at high temperature. Also, the waste heat is discharged with the exhaust gases in so many industries. This heat can be recovered for useful purpose. This process is known as waste heat recovery.
34. What are the waste materials, which can be used for fuel for power generation?
- Municipal waste
- Industrial waste
- Paper waste
- Rubber waste.
35 Write about waste heat boilers?
The waste heat boilers use the waste heat in gases coming out of diesel engines and gas turbines at high temperature (or) use the waste as a fuel in the incineration.
Some boilers use the industrial dirty gases for power generation.
36. Write about fluidised bed boilers?
When the high velocity gas is passed through a packed bed of finely divided solid particles, the particles become suspended in the gas stream and the packed bed becomes a fluidised bed. When the gas velocity is very high, the fluidised bed become turbulent and rapid mixing of particles occurs. Ultimately, the, behaviour of mixture solid particles and gas become a fluid. Burning of a fuel in such a state is known as Fluidised Bed Combustion. The boiler plant using this fluidised bed combustion is known as fluidised bed boilers.
37. State some advantages of fluidised bed boilers?
- Any type of fuel solid, liquid (or) gaseous fuel (or) domestic and industrial waste can be used in FBC system. Any type of combustible matter can be burned by adjusting the factors as size, air velocity and rate of feed.
- High heat transfer rate is possible to the surfaces immersed in the bed, because solid mixing is extremely possible.
- High combustion efficiency.
- The solid fuel need not be pulverised in fluidised bed boilers.
38. Name the two types of coal handling?
- Out plant handling
- In plant handling.
39. Write about out plant handling?
Out plant handling includes the handling of coalmine to the thermal power plant. These handlings are outside the plant in the following ways.
- Transportation by sea (or) river
- Transportation by rail
- Transportation by road
- Transportation of coal by pipeline.
40. Write about inplant handling of coal?
In order to handle large quantity of coal inside the plant, some mechanical handling systems are provided f6r smooth, easy and better controlled operation. The inplant coal handling is divided, into following categories.
- Coal unloading
- Coal preparation
- Coal transfer
- Coal storage
41. Why the preparation of coal is necessary?
The coal from coal nines cannot be directly fed into the furnace. Proper preparation of coal should be done before feeding the coal to the furnace. In the coal preparation, the coal passes through the different equipments like 1. Crushers 2. Sizers 3. Driers and Magnetic Separators.
42. Name the different types of coal transforming equipments?
1. Belt conveyors 2. Screw conveyors 3. Bucket elevators 4. Grab bucket elevators 5. Skip hoists 6. Flight conveyors.
The coal transfer starts by carrying of coal from unloading point to the storage site.
43. What is the use of belt conveyors?
Belt conveyors are mostly used for transporting coal over long distance with large quantity. An endless belt is made to run over a pair of end drums and pulleys and supported by series of roller at regular intervals.
44. Write about screw conveyor and bucket conveyors?
In screw conveyor, an endless helicoid screw is fitted to the 9haft. On one end of the shaft, the driving mechanism is fitted and the other end of the shaft is supported on a ball bearing. While the screw is rotating, the coal is transferred` from one end to the other end.
Bucket conveyors are used as vertical lifts. The coal is loaded at the bottom and unloaded at the top in the bucket conveyors.
45. Define draught, what is the use of draught in thermal power plants?
Draught is defined as a small pressure difference required between the fuel bed (furnace) and outside air to maintain constant flow of air and to discharge the gases through chimney to the atmosphere. Draught can be obtained by chimney, fan, steam jet (or) air jet (or) combination of these.
The uses are
- To supply required quantity of air to the furnace for combustion of fuel.
- To draw the combustion products through the system.
- To remove burnt products from the system
46. Write about classification of draught?
Draught is classified as
1. Natural draught
2. Artificial draught
The artificial draught is further classified as
(a) Steam jet draught
(b) Mechanical draught
(c) Induced draught
(d) Forced draught
47. Define the term Natural draught and what are the advantages of natural draught system?
The tall chimney creates the natural draught by the temperature difference between hot gases in the chimney and cold atmospheric air outside the chimney.
The advantages are
- No external power is required
- Air pollution is less since gases are discharged at high level.
- No maintenance cost
- Capital cost is less than artificial draught.
48. Write about artificial draught?
In modem power plants, the draught should be flexible to meet the fluctuating loads and it should be independent of atmospheric conditions. To achieve this, the aid of draft fans becomes must and by employing the draft fans, the height of the chimney would be reduced.
49. Write about forced draught system?
In this system, the blower (forced draft fan) is located at the base of the boiler near the grate. Air is forced to the furnace by forced fan and the flue gases are forced to chimney through economiser and air preheater.
50. What are the advantages of forced draught system
- Since the fan handles cold air, the fan size and the power required are less.
- No need of water cooled bearings because the air being handled is cold air,
- Pressure throughout the system is above atmospheric pressure so the air leakage into the furnace is reduced.