Fluidized Bed Combustion (FBC) Boiler
● It is a advance method of combustion used to burn solid fuel particles are suspended in a bol. bubbling fluidity bed of ash and other particulate materials like sand, limestone etc., through which jets of air are blown to provide the oxygen required for combustion
● This result fast and complete mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed.
● FBC plants are capable of burning a variety of low gerade solid fuels, such as coal and wood, biomass, at high efficiency and without any expensive preparation of fuel such as pulverization of coal, etc.
● Also, FBCS are smaller than the equivalent conventional furnace, so may offer significant advantages over the latter in terms of cost and flexibility,
● FBC reduces the amount of sulphur emitted in the form of SO× emissions. Limestone is used to precipitate out sulphate during combustion, which also allows more efficient heat transfer from the boiler to the apparatus used to capture the heat energy (usually water tubes). The heated released comes in direct contact with the tubes and transfer heat by conduction which increases the efficiency.
● FBC allows thermal power plants to burn coal at lower temperatures, releasing less amount of nitrous oxide (NO×) whereas, burning at low temperatures also causes increased polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the lower temperatures of combustion 800°C. having other added benefits.
Working principle of FBC system :
● When a gas is passes through a packed bed of finely divided solid particles, it experiences a pressure drop across the bed. A fluidized bed is composed of fuel like coal, coke, biomass, etc. and bed material ash, sand, and/or sorbent contained within an atmospheric or pressurized vessel
● The bed becomes fluidized when air or other gas flows upward at a velocity sufficient to expand the bed. The arrangement of FBC is illustrated....
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| Fluidized Bed Combustion |
● The fuel and the inert material dolomite are supply to the distributor plate and air is passes from the bottom of the distributor plate. The high velocity air keep the solid feed material in a suspended form during combustion. At low fluidizing velocities (0.9 to 3 m/s) relatively high solids densities are maintained in the bed and only a small fraction of the solids are entrained from the bed. The heat of combustion transfer rapidly to the water flowing through the tube which are immersed in the bed. During combustion SO2 is form is absorbed by dolomite limestone and prevent its escape with the exhaust gases. The molten slag is trapped from the top surface of the bed.
● The primary objective of inert material is to control the bed temperature at about 90% of the bed volume. The heat released by combustion is first used for inert material and balance the absorbed by the heat transfer surfaces. The inert material should be resistance to heat and disintegrate and have similar density as coal. Also it should not disintegrate to coal and bed material. Sintered ash, fuesed alumina, sand, mullite and zirconia are the inner material for FBC. The 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.
Need of Fluidized Bed Combustion :
● The maximum percentage of the coal available in India is of low grade quality containing high ash having low calorific value. The traditional grate fuel firing systems have some limitations and are technically economically unavailable to meet the challenges of future.
● FBC has provides various alternative and has significant advantages over conventional firing system and having multiple benefits compact boiler design, fuel flexibility, higher combustion efficiency and reduced emission of noxious pollutants such as SOx and NOx. The fuels burnt in these boilers include coal, washery rejects, rice husk, bagasse and other agricultural wastes. The fluidized bed boilers have a wide capacity range- 0.5 T/hr to over 100 T/hr.
Types of Fluidized Bed Combustion :
There are two basic types of fluidized bed combustion boilers:
1. Atmospheric classic Fluidized Bed Combustion System (AFBC):
Atmospheric fluidized beds use limestone or dolomite to capture sulfur released by the combustion of coal. Jets of air suspend the mixture of sorbent and burning coal during combustion, converting the mixture into a suspension of red-hot particles that flow like a fluid. These boilers operate at atmospheric pressure Again these are subdivided in to
a) Bubbling fluidized bed combustion (BFB)
b) Circulating (fast) Fluidized Bed Combustion system (CFBC)
2. Pressurized Fluidized Bed Combustion System (PFBC) :
The first-generation PFBC system also uses a sorbent and jets of air to suspend the mixture of sorbent and burning coal during combustion. However, these systems operate at elevated pressures and produce a high-pressure gas stream at temperatures that can drive a gas turbine. Steam generated from the heat in the fluidized bed is sent to a steam turbine, creating a highly efficient combined cycle system.
Various Arrangement:
1. Atmospheric classic Fluidized Bed Combustion System
In AFBC, coal is crushed to a size of 1-10 mm depending on the rank of coal, type of fuel feed and into the combustion chamber. The bed consisting of about 97% limestone or inert material and 3% burning fuel The atmospheric air, which acts as both the fluidization air and combustion air, is delivered at a pressure the bottom of the combustion chamber, and flows through the bed after being preheated by the exhaust flue gases.
The velocity of fluidizing air is in the range of 1.2 to 3.7 m/sec. The rate at which air is blown through the bed determines the amount of fuel that can be reacted, The bed temperature is controlled by heat transfer tubes immersed in the bed and by varying the quantity of coal in the bed. As the coal particle size reduces, result in combustion or attrition, the particles are elutriated from the bed and carried out the combustor. A portion of the particles elutriated from the bed are collected by a cyclone or multi-cylone collector down-stream of the convection pass and returned to the bed to improve combustion efficiency.
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. 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 this 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 950°C and loss of combustion efficiency if the temperature falls below 800°C.
Atmospheric circulating bed combustor:
● In a ACFB, primary air is introduced into the lower portion of the combustor, where the heavy bed material is fluidized and retained. The upper portion of the combustor contains the less dense material that is entrained from the bed. Secondary air typically is introduced at higher levels in the combustor to ensure complete combustion and to reduce NOx emissions.
● The burn gases generated in the combustion furnace moves upward with a some particles of solids inventory entrained. These entrained solids are separated from the combustion or burn gas in hot cyclone - separator or in mechanical particle separators, and are continuously returned back to the combustion chamber.
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| Atmospheric circulating bed combustor |
● The combustion chamber of a CFB unit consists of membrane-type welded water walls to provide most of the evaporative boiler surface. The lower third of the combustor is refractory lined to protect the water walls from erosion in the high-velocity dense bed region.
● Various CFB design uses external heat exchangers, which are unfired dense BFB units that extract heat from the solids collected by the dust collectors before it is returned to the combustor. The external heat exchangers are used to provide additional evaporative heat transfer surface as well as superheat and reheat surface.
The flue gas, after removal of more than 99% of the entrained solids in the eyclone or particle separator, exists the yolone or separator to a convection pass. The flue gases passes to economizer, superheat, and reheat surface as required by the application.
Pressurized Fluidized Bed Combustion
● Before coal can be burned in an FBC, it is necessary to heat the inert bed material to about 600°C (the ignition point of coal) using an auxiliary heating system.
● Two common methods are
(1) combustion of auxiliary fuel as a flame above the bed fluidized with air:
(2) passing hot gas through the bed.
● A pressurized FBC of this type, which has the advantages of FBC and combined steam-gas turbine cycle operations. A The coal and limestone (dolomite) are treated and fed to the furnace. The combustion (pressurizing) air is delivered by the gas turbine compressors. The hot, pressurized combustion gases, after transferring most of their heat to the boiler, are then passed through cyclones and a granulator bed filter, to drive the gas turbine and its generator. The steam produced in the boiler goes to the steam turbines and condenser in a conventional power plant cycle.
● Improved fluidization quality for a PFBC boiler occurs because the higher pressure brings about a reduction in bubble size that is especially important for beds composed of fine particles. With this design the hot gases must be expanded through a turbine, so the boiler can't be evaluated by itself. Performance measurement includes the gas turbine operation.
● Another approach for a PFBC design uses compressed air as the cooling medium for the combustion chamber, which is then mixed with the pressurized products of combustion (after they have been cleaned). The mix then expands through the turbine, and any left over heat energy passes to a waste heat boiler. In this cycle, about 60% of the power is from the gas turbine and the remainder from the steam turbine.
● One AFBC plant operating at a steam pressure of 17,000 kPa and 540°C has a wind box divided into nine independently controlled compartments to allow separate fluidization of the bed for startup and for load reduction with either underbed or overbed firing.
Control System of FBC:
The rate of heat released per unit area is a linear function of velocity of gas in a bed. Higher the velocity largest amount of heat released. As size of fuel particle increased then velocity is to be increased so that particles will not be entrained in the gas and as size increased heat transfer rate reduces.
The Combustion bed temperature control consist of a thermocouple in a bed which gives signal simultaneously temperature controller provided with a servo motorized valve and variable speed control motor. The servo valve control the air supply valve in the pipeline changes the position and control the excess of air supply to combustion bed improving efficiency. The coal feeder motor characteristics is match main air supply and maintain the ratio of air fuel supply constant.
The steam pressure and water outlet temperature is control by heat input to the primary heating surface. This is archive by controlling the air supply to heating zone. The characteristics of shallow bed that can control heat transfer 10 immersed tube from maximum down to negligible by controlling the velocity of fluidization and hence bed expansion, foe steam unit is build up to control the feed water to maintain level of water down.
Advantages and Disadvantages of Fluidized Bed Combustion :
Advantages:
The advantages of FBC in comparison to conventional pulverized coal fueled units are as follows : i
1. Pollution Control : SO2 can be removed in the combustion process by adding limestone to the fluidized bed, eliminating the need for an external desulfurization process. Because of the reduced combustion temperature, NOx emissions are inherently low.
2. Fuel Flexibility : Fluidized bed boilers are inherently fuel flexible and, with proper design provision, can burn a variety of fuels.
3. No Slagging in the Furnace : Combustion FBC units takes place at temperatures below the ash fusion temperature of most fuels. Consequently, tendencies for slagging and fouling are reduced with FBC.
4. High Reliability : The absence of moving parts in the combustion zone results in a high degree of reliability and low maintenance costs.
5. Reduction in Boiler Size : The heat transfer rate is high over a small heat transfer area immersed in the bed result in overall size reduction of the boiler.
6. Low Corrosion and Erosion : The corrosion and erosion effects are less because of lower combustion temperature, softness of ash and low particle velocity
7. Quick Start-Up : Due to high turbulence of the bed facilitates quick start up and shut down.
8. Smaller Plant Size : The plant is smaller due to the high heat transfer rate in the furnace and the absence of wet exhaust gas scrubbing equipment
Disadvantages :
1. High Power Requirements for Combustion Air :
For a given boiler output, an FD fan on an FBC unit might require 225 kW versus 75 kW for a stoker fired system
2. Carry over:
Gradual loss of the fluidized bed, including the fuel (carbon) particles, resulting in lower efficieney and higher cost dust collectors.
