New Text DocumentAdvantages of Fluidised bed combustion over conventional firing systems

• Fuel flexibility,• Reduced emission of noxious pollutants such as SOx and NOx,• Compact boiler design and • Higher combustion efficiency.

Various methods available to control the excess air are:

• Portable oxygen analysers and draft gauges - Excess air reduction up to 20% is feasible.• Continuous oxygen analyzer with a local readout mounted draft gauge, by which the operator can adjust air flow. A further reduction of 10–15% can be achieved.• Damper control by continuous oxygen analyzer. This enables an operator to remotely control a number of firing systems simultaneously.• The automatic fan speed control from O2 analyser feed, It’s cost is really justified only for large systems.

Basic Process and Components

The thermodynamic cycle for the steam turbine is the Rankine cycle. The cycle isthe basis for conventional power generating stations and consists of a heat source (boiler) that converts water to high-pressure steam. In the steam cycle, water is first pumped to medium to high pressure. It is then heated to the boiling temperature corresponding to the pressure, boiled (heated from liquid tovapor), and then most frequently superheated (heated to a temperature above that of boiling). A multistage turbine expands the pressurized steam to lower pressure and the steam is then exhausted either to a condenser at vacuum conditions or into an intermediate temperature steam distribution system that delivers the steam to the industrial or commercial application. The condensate from the condenser or from the steam utilization system returns to the feedwater pump for continuation of the cycle. Figure 3 shows the primary components of a boiler/steamturbine system.

Fluidized Bed Combustion (FBC) systems:

Due to the escalation in oil and gas prices continuously the world over,the power engineers were trying to develop new combustion system with coal as fuel since it is available in abundance in India. In the earlier designs of furnaces using Pulverized coal were required to be operated upon in the temperature range of 1400-1700˚C in order to get higher combustion efficiencies. But, at such higher temperature it forms slag on superheater tubes, it evaporatesalkali metal in ash and its deposition on heat transfer surfaces causes fouling, corrosion and reduced heat transfer rates. It forms pollutants NO in large amounts since the temperature maintained are considerably high.Sulphur x content in the coal forms SO which in turn forms acids with moisture on condensation 2 of steam. The capital cost for removal of SO from the gas is high. Another problem is 2 that the furnaces designed for a particular type of coal cannot burn with every other type of coal with same combustion efficiency and safety. The difficulties experienced by the engineers as enumerated above has led to thedvelopment of fluidized bed combustion (FBC) systems in which any type of coal could be burnt efficiently in the range of 800-1000˚C. Practically at these lower temperatures the production of pollutant like NO is lower since the fixation of x atmospheric nitrogen is difficult to occur below the temperature of 1100˚C. Also, it reduces the problem of slag formation. Principle of operation of Fluidized Bed Combustion system:

Fig shows the simple fluidized bed which has finely divided particles offuel half filled in the column. At its bottom a distributor plate is fitted through which the air is passed in upward direction with some low velocity. The pressure drop across the combustion chamber can be measuredwith the help of manometer.The velocity of air is increased gradually and the logarithmic plot of pressure drop and velocity is shown in fig.Upto the point A the bed is stable. Between the point A to B the bed is unstable and the particles

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New Text Documenttend to adjust their position to prevent as little resistance to flow as

Advantages of FBC:

1. Coal used is 2 to 3% of bed volume. Reminder space is occupied by the inert material like sintered ash, fused alumina, sand etc. These materials help in controlling the bed temperature in the range of 800-1000˚C. 2. The low temperature of combustion in FBC allows ash to remain soft andless abrasive. 3. SO emissions are much less due to addition of dolomite. 4. NOx emissions are low since the amount of excess air used is less and temperatures are maintained are low in FBC in the range of 800-1000˚C compared to pulverized coal combustion system. It is so, since the fixation of free nitrogen at low temperature does not occur. 5. Their initial and operational costs are low. 6. The rates of heat transfer are much higher since the system behaves like violently burning liquid. 7. Combustion efficiencies obtained are as high as 99.5%. 8. Coal with high content of ash can be used. 9. Volatilization of alkali compound does not occur, therefore, their deposit on boiler tubes is minimized. 10. Size of plant and its cost is considerably reduced.

Disadvantages of FBC:

1. The coal feeding and ash handling systems is complex. 2. It requires crushed coal of proper size. 3. It needs auxiliary starting system like an oil or gas burner. 4. Large variation in load cannot be met with. 5. Difficult to control carbon carryover with the flue gases. Classification of Fluidized Bed Combustion Boilers:

Fluidized bed combustion boilers are classified as follows:

A. Circulating Fluidized Bed Combustion Boilers (CFBCB) B. Pressurized Fluidized Bed Combustion Boilers (PFBCB)

Circulating Fluidized Bed Combustion Boilers (CFBCB):

The combustion chamber of circulating fluidized bed combustion boilers operates under atmospheric pressure due to which these are also called as atmospheric fluidized bed combustion boilers.These types of boiler are presently in developedstate and being used in many countries. These boilers are further classified as underfeed and overfeed boilers depending upon whether the loading of fuel and supply of air is from the bottom of the bed or on the top of the bed respectively.The underfeed atmospheric fluidized bed combustion boilers are compact in design; however, they have high operational costs. Whereas, the overfeed FBCB are simple in operation with less operational cost but these give lower heat transfer rates per m2 of area of bed. A CFBCB is as shown in fig.These boilers are called as circulating type of FBCB since a good portion of particles exit the combustion chamber with the flue gases. The particles are thenseparated from the exhaust and reinserted into the fluidized bed system.The fluidized air flows through the grid plate from the air plenum chamber into the bed where the combustion of coal occurs. The coal is crushed between 6mm to 22mm size before supplied to the combustion chamber with lime stone by the feed hopper.The mixture of fuel and air becomes a swirling mixture in combustion chamber and the rapid combustion takes place at temperature of 800-900˚C. Heat istransferred to submerged tube bank. Lime stone allows the sulphur to remain in the bed and formation of the H2SO4 is prevented. The lower combustion temperatures prevent the formation of NOx, thus the inferior quality of coal can be used and no pulverizer is needed.The products of combustion leaving the bed contain large portion of unburned carbon particles that are collected in cyclone separator. It separates the carbon, particles from dust particles by centrifugal action. The carbon particlesare returned back to combustion chamber to complete their combustion.The boiler water tubes

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New Text Documentare located in the furnace and the steam is generated at a rapid rate.

Advantages

1. Pulverized coal is not needed.2. NOx formation does not take place.3. Desulphurization of coal take place at the bed due to addition of lime stone, therefore the exhaust gases can be cooled to lower temperature before discharged through the chimney.4. Slagging problem is reduced due to combustion at low temperature.5. Low grade coal can be used.6. Cheaper alloys can be used for its construction due to low combustion temperature of fuel.7. Heat release rates are 10 to 15 times higher compared to conventional boiler.8. Size of plant is considerably reduced. ized Fluidized Bed Combustion Boilers (PFBCB):

Pressurized Fluidized Bed Combustion Boilers (PFBCB):

Pressurized FBCB is the latest system used for generation of power in which air is supplied under pressure of at 10 bar approximately with the help of air compressor driven by a gas turbine as shown in fig. the supply of air under pressure results into better rates of heat transfer compared to circulating FBC boiler.The mixture of fuel and lime stone is supplied under pressure to the fluidized bed having a part of evaporator tubes immersed in the bed and remaindertubes as free board tubes above the bed. The ash is collected at the bottom.The flue gases along with unburnt carbon leaving the combustion chamber are passed through a cyclone separator where the unburnt carbon particles are separated fromthe gas due to cntrifugal action on the mixture. Separated carbon particles are returned to the bed to complete their combustion.The flue gases from separator are now passed through a dust separator. The cleaned flue gas is then supplied to the gas turbine where it expands upto the atmospheric pressure. The power of gas turbine is used to run an air compressor for supplying the compressed air to the combustion chamber.FBC systems have various design e.g. FBC with gas turbine, FBC with steam turbine, FBC with SO2 recovery system.

Advantages:

1. High rate of coal loading with high rate of combustion is achieved.2. Load variation on plant can be met easily by varying the coal loading on plant.3. Considerable reduction in volume of furnace, hence size of plant is reduced.4. High steam generation rates are achieved.5. Low NOX emissions.6. Reduced formation of SO2.7. The cost of plant is reduced.8. Requires reduced air velocity compared to CFBCB.

Disadvantages:

1. Combustion rates cannot be controlled.2. Life of plant is low compared with conventional boiler.3. Erosion of walls of furnace.4. Complexity of operation.

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