Explain the FBC (Fluidized Bed combustion) and neat diagram

• Principal of FBC :

1. A fluidized bed may be defined as the bed of solid particles behaving as a fluid.
2.  When a gas is passed through a packet bed of finely divided solid particles, it experiences a pressure drop across the bed.
3. At low gas velocities, this pressure drop is small and does not disturb the particles. But if the gas velocity is increased further, a stage is reached, when particles are suspended in the gas stream and the packet bed becomes a 'fluidized bed'.
4. With further increase in gas velocity, the bed becomes turbulent and rapid mixing of particles occurs.
5. In general, the behavior of this mixture of solid particles and gas is like a fluid. Burning of a fuel in such a state is known as fluidized bed combustion (FBC).

• Working :

1. Fig. given below shows the arrangement of the FBC system.
2. The fuel and inert material dolomite are fed on the distributor plate and from its bottom air is supplied.
3. The high velocity of air keeps the solid feed material in suspending condition during burning.
4. The generated heat is rapidly transferred to the water passing through the tubes immersed in the bed and generated steam is taken out.
5.  During the burning sulphur dioxide formed is absorbed by the dolomite and prevents its escape with the exhaust gases. The molten slag is tapped from the top surface of the bed.
6. The primary object of using the inert material is to control the bed temperature, it accounts for 90% of the bed volume.

• Advantages :

1. As a result of better heat transfer, the unit size and hence the capital costs are reduced.
2. Since combustion temperatures are low, the fouling and corrosion of tubes is reduced considerably.
3. Pollution is controlled and combustion of high-sulphur coal is possible.

• Disadvantages :

1. The major drawback is that the air has to be supplied at a high pressure so as to support the bed, the fan power is increased sufficiently so controls are difficult and plant life is low. 
2. 
   Fluidized bed systems are generally smaller for a given thermal output than conventional equipment.

what do you mean by "supercritical boiler" and "Super charged boiler" ?

A.  Supercritical Boiler :

1. The steam generator in which steam is produced above critical pressure of 221.2 Bar are know as super critical boiler.
2. It operates on Rankine cycle and drum less boiler. 
3. Usually , a sub-critical boiler consist of three distinct section an preheat (economiser), evaporator and super heater.






4. The condensate water from the condenser is compressed from the condenser pressure to super critical pressure in the feed pump. The operation theoretically is shown by process 1-2.
5.  this water is heated at super critical pressure shown by process 2-3. It may be observed that water is in sub-cooled state at point 2. Its temperature goes on increasing with heat addition.
6. No distinction is observed between the liquid and gaseous state.
7. The liquid water become gas after its temperature is rise above critical temperature of 374.15 degree Celcius.
8. thus it is a continuous tube which is heated along its length with water going in at one end super heated steam leaving at the other end.
9. Due to large frictional resistance the feed pump pressure is about 40% higher then boiler pressure.

B.  Supercharged boiler :

1. In a super charged boiler , the combustion is carried out under pressure in the combustion chamber by supplying the compressed air. 
2. The exhaust gases from the combustion chamber are used to run the gas turbine as they are exhaust to higher pressure.
3. The gas turbine runs the air compressor to supply the compressed air to the combustion chamber.
4. Followings are the advantage of the supercharged boiler : 

a. Owing the very high overall heat transfer coefficient the heat transfer surface required to hardly 20-25% of the heat transfer surface of a conventional boiler. 

b. The part of the gas turbine output can be used to drive other auxiliaries.

c. The small heat capacity storage of the boiler gives better response to control.

d. Rapid start of the boiler is possible.

e. The comparatively less number of operators is required.
 

 

Working of Bobcock and wilcox Boiler

 working principal

D = Drum                                  PG = pressure gauge 

     DTH = Down take header              ST =  super heater tube 

       WT = water tube                          SV =  Safety valve 

       BP  = Baffle plates                    MSV =  Main stop valve

          D = Doors                                APP =  Anti priming Pipe

          G = Grate                                      L = Lower junction box

       FD = Fire Door                               U = Upper junction box

      MC = Mud collector                     FV = Feed valve 

     WLI = Water level indicator 

1.  Fig.. shown the Bobcock and Wilcox  water tube boiler. It concist of a large number of parallel tubes inclined at angle which varies from (5 to 15 degree) to the horizontal.
2.  These tubes connect the up take header with the down take header.
3.  Both of these are connected to the shell having a substantial quantity of water in it.
4.  The up take header is connected to the shell through a short tube.
5.  A long tube is employed is connected the down take header with shell.
6.  The coal is fed through the fir hole on to the chain grate stoker.
7.  The velocity of the chain is so adjusted as to ensure complete combustion of coal by the time it reaches the other end of the great.
8.  The flue gasses first rise up then move down and once again rises up due to presence of baffles.
9.  The hot water and steam moisture rise up through the up take header into the boiler shell.
10.  The steam separates from water in the shell and collects in the steam space
11.  The cold water flows down into the tubes through the down take header.
12.  Thus a continuous circulation of water is maintained by the connection current set up.

 

 

Layout of a steam power plant and explain it

The layout of steam power plant comprises of the following four circuits: 

• a. Coal and Ash circuit: 

1. Coal arrives at storage yard and after necessary handling, passes on to the furnaces through the fuel feeding device.
2. Ash resulting from combustion of coal collect at back of the boiler and is and is removed to the ash storage yard through ash handling equipment.

• b. Air and gas circuit:

1. Air is taken from the atmosphere through the action of a forced or induced format fan and passes on the furnace through  the air preheat, where it has been heated by heat of flu gases which pass to the chimney via the preheat .

• c. Feed water and Flow circuit:

1. In the water and steam circuit condensate leaving the condenser is first heated in a closed feed water  heater through extracted steam from the lowest pressure extraction point of turbine.
2. It then passes through the and a few more water heaters before going into the boiler through economizer

• d. Cooling water Circuit:

1. The cooling water supply to the condenser helps in maintaining a low pressure in it.
2. The water may be taken from a natural source such as river, Lake or Sea or the same water may be cooled and circulated over again.