24-09-2014, 02:25 PM
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications. When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream � the bed is called �fluidized�. With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid � �bubbling fluidized bed�. Fluidization depends largely on the particle size and the air velocity.
1. ABSTRACT OF THE PROJECT DONE
NAME OF THE PROJECT : TO STUDY THE INCREASE EFFICIENCY OF BOILER:
Aboileris a closedvesselin whichwateror otherfluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications. When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized”. With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid – “bubbling fluidized bed”. Fluidization depends largely on the particle size and the air velocity.
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AKASH JASROTIA
100761132992
2. PROJECT UNDERTAKEN
2.1 TO STUDY THE INCREASE EFFICIENCY OF BOILER:
Aboileris a closedvesselin whichwateror otherfluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.
Fluidized bed boilers
2.2 MECHANISM OF FLUIDIZED BED COMBUSTION
When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity.
As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized”.
With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid – “bubbling fluidized bed”.
At higher velocities, bubbles disappear, and particles are blown out of the bed.
Therefore, some amounts of particles have to be recirculated to maintain a stable system – “circulating fluidized bed”.
This principle of fluidization is illustrated in Figure 6.1.
Fluidization depends largely on the particle size and the air velocity. The mean solids velocity increases at a slower rate than does the gas velocity, as illustrated in Figure 6.2. The difference between the mean solid velocity and mean gas velocity is called as slip velocity.
Maximum slip velocity between the solids and the gas is desirable for good heat transfer and intimate contact.
If sand particles in a fluidized state are heated to the ignition temperatures of coal, and coal is injected continuously into the bed, the coal will burn rapidly and bed attains a uniform temperature. The fluidized bed combustion (FBC) takes place at about 840OC to 950OC.
Since this temperature is much below the ash fusion temperature, melting of ash and associated problems are avoided.
The lower combustion temperature is achieved because of high coefficient of heat transfer due to rapid mixing in the fluidized bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. The gas velocity is maintained between minimum fluidization velocity and particle entrainment velocity. This ensures stable operation of the bed and avoids particle entrainment in the gas stream.
Combustion process requires the three “T”s that is Time, Temperature and Turbulence. In
FBC, turbulence is promoted by fluidization. Improved mixing generates evenly distributed heat at lower temperature. Residence time is many times greater than conventional grate firing. Thus an FBC system releases heat more efficiently at lower temperatures.
Since limestone is used as particle bed, control of sulfur dioxide and nitrogen oxide emissions in the combustion chamber is achieved without any additional control equipment.
This is one of the major advantages over conventional boilers.
2.3 TYPES OF FLUIDISED BED COMBUSTION BOILERS:
There are three basic types of fluidized bed combustion boilers: 1. Atmospheric classic Fluidized Bed Combustion System (AFBC)
2. Atmospheric circulating (fast) Fluidized Bed Combustion system(CFBC)
3. Pressurized Fluidized Bed Combustion System (PFBC).
2.4 AFBC / BUBBLING BED
In AFBC, coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type of fuel feed and fed into the combustion chamber. The atmospheric air, which acts as both the fluidization air and combustion air, is delivered at a pressure and flows through the bed after being preheated by the exhaust flue gases. The velocity of fluidizing air is in the range of 1.2to 3.7 m /sec. The rate at which air is blown through the bed determines the amount of fuel that can be reacted.
Almost all AFBC/ bubbling bed boilers use in-bed evaporator tubes in the bed of limestone, sand and fuel for extracting the heat from the bed to maintain the bed temperature.
The bed depth is usually 0.9 m to 1.5 m deep and the pressure drop averages about 1 inch of water per inch of bed depth. Very little material leaves the bubbling bed – only about 2 to 4kg of solids is recycled per ton of fuel burned. Typical fluidized bed combustors of this type are shown in Figures 6.3 and 6.4.
Bubbling bed boiler
Bubbling boiler -2
The combustion gases pass over the super heater sections of the boiler, flow past the economizer, the dust collectors and the air preheaters before being exhausted to atmosphere.
The main special feature of atmospheric fluidized bed combustion is the constraint imposed by the relatively narrow temperature range within which the bed must be operated.
With coal, there is risk of clinker formation in the bed if the temperature exceeds 950oC and loss of combustion efficiency if the temperature falls below 800oC. For efficient sulphur retention, the temperature should be in the range of 800oC to 850oC.
2.5 GENERAL ARRANGEMENTS OF AFBC BOILER
AFBC boilers comprise of following systems:
i) Fuel feeding system
ii) Air Distributor
iii) Bed & In-bed heat transfer surface
iv) Ash handling system
Many of these are common to all types of FBC boilers
1. Fuel feeding system
For feeding fuel, sorbents like limestone or dolomite, usually two methods are followed: under bed pneumatic feeding and over-bed feeding.
Under Bed Pneumatic Feeding
If the fuel is coal, it is crushed to 1-6 mm size and pneumatically transported from feed hopper to the combustor through a feed pipe piercing the distributor. Based on the capacity of the boiler, the number of feed points is increased, as it is necessary to distribute the fuel into the bed uniformly.
Over-Bed Feeding
The crushed coal, 6-10 mm size is conveyed from coal bunker to a spreader by a screw conveyor. The spreader distributes the coal over the surface of the bed uniformly. This type of fuel feeding system accepts over size fuel also and eliminates transport lines, when compared to under-bed feeding system.
2. Air Distributor
The purpose of the distributor is to introduce the fluidizing air evenly through the bed cross-section thereby keeping the solid particles in constant motion, and preventing the formation of defluidization zones within the bed. The distributor, which forms the furnace floor, is normally constructed from metal plate with a number of perforations in a definite geometric pattern. The perforations may be located in simple nozzles or nozzles with bubble caps, which serve to prevent solid particles from flowing back into the space below the distributor.
The distributor plate is protected from high temperature of the furnace by:
i) Refractory Lining
ii) A Static Layer of the Bed Material or
iii) Water Cooled Tubes.
3. Bed & In-Bed Heat Transfer Surface: a) Bed
The bed material can be sand, ash, crushed refractory or limestone, with an average size of about 1 mm. Depending on the bed height these are of two types: shallow bed and deep bed.
At the same fluidizing velocity, the two ends fluidize differently, thus affecting the heat transfer to an immersed heat transfer surfaces. A shallow bed offers a lower bed resistance and hence a lower pressure drop and lower fan power consumption. In the case of deep bed, the pressure drop is more and this increases the effective gas velocity and also the fan power.
b) In-Bed Heat Transfer Surface
In a fluidized in-bed heat transfer process, it is necessary to transfer heat between the bed material and an immersed surface, which could be that of a tube bundle, or a coil. The heat exchanger orientation can be horizontal, vertical or inclined. From a pressure drop point of view, a horizontal bundle in a shallow bed is more attractive than a vertical bundle in a deep bed. Also, the heat transfer in the bed depends on number of parameters like (i) bed pressure
(ii) Bed temperature (iii) superficial gas velocity (IV) particle size (v) Heat exchanger design and (VI) gas distributor plate design.
4. Ash Handling System
a) Bottom ash removal
In the FBC boilers, the bottom ash constitutes roughly 30 - 40 % of the total ash, the rest being the fly ash. The bed ash is removed by continuous over flow to maintain bed height and also by intermittent flow from the bottom to remove over size particles, avoid accumulation and consequent defluidization. While firing high ash coal such as washery rejects, the bed ash overflow drain quantity is considerable so special care has to be taken.
b) Fly ash removal The amount of fly ash to be handled in FBC boiler is relatively very high, when compared to conventional boilers. This is due to elutriation of particles at high velocities. Fly ash carried away by the flue gas is removed in number of stages; firstly in convection section, then from the bottom of air preheater/economizer and finally a major portion is removed in dust collectors. The types of dust collectors used are cyclone, bag filters, electrostatic precipitators (ESP’s) or some combination of all of these. To increase the combustion efficiency, recycling of fly ash is practiced in some of the units.
Circulating Fluidized Bed Combustion (CFBC)
Circulating Fluidized Bed Combustion (CFBC) technology has evolved from conventional bubbling bed combustion as a means to overcome some of the drawbacks associated with conventional bubbling bed combustion (see Figure 6.5). This CFBC technology utilizes the fluidized bed principle in which crushed (6 –12 mm size) fuel and limestone are injected into the furnace or combustor. The particles are suspended in a stream of upwardly flowing air (60-70% of the total air), which enters the bottom of the furnace through air distribution nozzles. The fluidizing velocity in circulating beds ranges from 3.7 to 9 m/sec. The balance of combustion air is admitted above the bottom of the furnace as secondary air. The combustion takes place at 840-900oC, and the fine particles (<450 microns) are elutriated out of the furnace with flue gas velocity of 4-6 m/s.
The particles are then collected by the solids separators and circulated back into the furnace. Solid recycle is about 50 to 100 kg per kg of fuel burnt. There are no steam generation tubes immersed in the bed. The circulating bed is designed to move a lot more solids out of the furnace area and to achieve most of the heat transfer outside the combustion zone - convection section, water walls, and at the exit of the riser. Some circulating bed units even have external heat exchanges. The particles circulation provides efficient heat transfer to the furnace walls and longer residence time for carbon and limestone utilization. Similar to Pulverized Coal (PC) firing, the controlling parameters in the CFB combustion process are temperature, residence time and turbulence. For large units, the taller furnace characteristics of CFBC boiler offers better space utilization, greater fuel particle and sorbent residence time for efficient combustion and SO2 capture, and easier application of staged combustion techniques for NOx control than AFBC generators. CFBC boilers are said to achieve better calcium to sulphur utilization – 1.5 to 1 vs. 3.2 to 1 for the AFBC boilers, although the furnace temperatures are almost the same.
CFBC boilers are generally claimed to be more economical than AFBC boilers for industrial application requiring more than 75 – 100 T/hr of steam
CFBC requires huge mechanical cyclones to capture and recycle the large amount of bed material, which requires a tall boiler.
6. FBC Boilers
A CFBC could be good choice if the following conditions are met.
Capacity of boiler is large to medium
Sulphur emission and NOx control is important
The boiler is required to fire low-grade fuel or fuel with highly fluctuating fuel quality.
Major performance features of the circulating bed system are as follows:
a) It has a high processing capacity because of the high gas velocity through the system.
b) The temperature of about 870oC is reasonably constant throughout the process because of the high turbulence and circulation of solids. The low combustion temperature also results in minimal NOx formation.
c) Sulfur present in the fuel is retained in the circulating solids in the form of calcium sulphate and removed in solid form. The use of limestone or dolomite sorbents allows
a higher sulfur retention rate, and limestone requirements have been demonstrated to be substantially less than with bubbling bed combustor.
d) The combustion air is supplied at 1.5 to 2 psig rather than 3-5 psig as required by bubbling bed combustors.
e) It has high combustion efficiency.
f) It has a better turndown ratio than bubbling bed systems.
g) Erosion of the heat transfer surface in the combustion chamber is reduced, since the surface is parallel to the flow. In a bubbling bed system, the surface generally is perpendicular to the flow.
Pressurized Fluid Bed Combustion
Pressurized Fluidized Bed Combustion (PFBC) is a variation of fluid bed technology that is meant for large-scale coal burning applications. In PFBC, the bed vessel is operated at pressure up to 16 ata ( 16 kg/cm2).
The off-gas from the fluidized bed combustor drives the gas turbine. The steam turbine is driven by steam raised in tubes immersed in the fluidized bed. The condensate from the steam turbine is pre-heated using waste heat from gas turbine exhaust and is then taken as feed water for steam generation.
The PFBC system can be used for cogeneration or combined cycle power generation. By combining the gas and steam turbines in this way, electricity is generated more efficiently than in conventional system. The overall conversion efficiency is higher by 5% to 8%. .
Advantages of Fluidised Bed Combustion Boilers
1. High Efficiency
FBC boilers can burn fuel with a combustion efficiency of over 95% irrespective of ash content. FBC boilers can operate with overall efficiency of 84% (plus or minus 2%).
2. Reduction in Boiler Size
High heat transfer rate over a small heat transfer area immersed in the bed result in overall size reduction of the boiler.
3. Fuel Flexibility
FBC boilers can be operated efficiently with a variety of fuels. Even fuels like flotation slimes, washer rejects, agro waste can be burnt efficiently. These can be fed either independently or in combination with coal into the same furnace.
4. Ability to Burn Low Grade Fuel
FBC boilers would give the rated output even with inferior quality fuel. The boilers can fire coals with ash content as high as 62% and having calorific value as low as 2,500 kcal/kg. Even carbon content of only 1% by weight can sustain the fluidized bed combustion.
5. Ability to Burn Fines
Coal containing fines below 6 mm can be burnt efficiently in FBC boiler, which is very difficult to achieve in conventional firing system.
6. Pollution Control
SO2 formation can be greatly minimized by addition of limestone or dolomite for high sulphur coals. 3% limestone is required for every 1% sulphur in the coal feed. Low combustion temperature eliminates NOx formation.
7. Low Corrosion and Erosion
The corrosion and erosion effects are less due to lower combustion temperature, softness of ash and low particle velocity (of the order of 1 m/sec).
8. Easier Ash Removal – No Clinker Formation
Since the temperature of the furnace is in the range of 750 – 900o C in FBC boilers, even coal of low ash fusion temperature can be burnt without clinker formation. Ash removal is easier as the ash flows like liquid from the combustion chamber. Hence less manpower is required for ash handling.
2.6 BOILER FITTINGS AND ACCESSORIES
§ Safety valve:It is used to relieve pressure and prevent possibleexplosion of a boiler.
§ Water level indicators:They show the operator the level of fluid in the boiler, also known as a sight glass,water gauge