INTRODUCTION 2 In 1921, the first fluidized bed being used successfully in Germany. Fluidized bed combustion has emerged as a feasible alternative and has significant advantages over conventional firing system and offers 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, wood chips & other agricultural wastes. The fluidized bed boilers have a wide capacity range- 0.5 T/hr to over 100 T/hr.
3MECHANISMFluidisation of solids Sand particles resting on a mesh (left) become fluidised when air is blown through (right) and take on the appearance and some of the properties of a boiling fluid. Granular solids remain in layers when one is poured on to another (left), but rapid mixing occurs on fluidisation (right). A bed of stationary particles supports objects whatever their density (left). On fluidisation, an object of lower density (the green ball) floats while the higher density (red ball) sinks. In a bed of stationary particles (left), heat is transferred slowly and there are big differences in temperature. In a fluidised bed (right), rapid mixing ensures uniformity of temperature.
CHARACTERISTICS OF FLUDIZED BED 4 Fluidization depends largely on the particle size and the air velocity. Combustion process requires the three “T”s that is Time, Temperature and Turbulence. In FBC, turbulence is promoted by fluidisation. If sand particles in a fluidized state is heated to the ignition temperatures of coal, 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. Limestone is used as particle bed, control of sulphur dioxide and nitrogen oxide emissions in the combustion chamber.
Types of Fluidised Bed Combustion Boilers 5There are three basic types of fluidised bedcombustion boilers:1. Atmospheric classic Fluidised Bed Combustion System(AFBC)2. Atmospheric circulating (fast) Fluidised Bed Combustionsystem(CFBC)3. Pressurised Fluidised Bed Combustion System (PFBC).
61.AFBC / Bubbling BedCoal is crushed to a size of 1 – 10 mm depending on the rank ofcoal.The atmospheric air, which acts as both the fluidization airand combustion air.The velocity of fluidising air is ranges from 1.2 to 3.7 m /sec.Almost all AFBC/ bubbling bed boilers usein-bed evaporator for extracting the heatfrom the bed to maintain the bed temperature.The bed depth is 0.9 m to 1.5 m deep. Air Distributor
7General Arrangements of AFBC BoilerAFBC boilers comprise of followingsystems:i) Fuel feeding systemii) Air Distributoriii) Bed & In-bed heat transfersurfaceiv) Ash handling system
1. FUEL FEEDING SYSTEM 8For feeding fuel, sorbents like limestone ,usually two methods are followed:under bed pneumatic feeding and over-bed feeding.Under Bed Pneumatic FeedingIf the fuel is coal, it is crushed to 1-6 mm sizeand pneumatically transported from feed hopperto the combustor through a feed pipe piercing thedistributor. feed pipe
9Over-Bed Feeding The crushed coal, 6-10 mm size is conveyedfrom coal bunker to a spreader by a screw conveyor.The spreader distributes the coal over the surfaceof the bed uniformly. This type of fuel feeding system accepts over sizefuel also and eliminates transport lines,when compared to under-bed feeding system coal bunker screw conveyor
10 2. Air Distributor Purpose is to introduce the fluidizing air through the bed cross section, keeping the solid particles in constant motion. The distributor, is constructed from metal plate having a number nozzles or nozzles with bubble caps. The distributor plate is protected from high temperature of the furnace by: i) Refractory Lining ii) A Static Layer of the Bed Material iii) Water Cooled Tubes.
11 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. 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.
124. Ash Handling Systema) Bottom ash removal In the FBC boilers, the bottom ash constitutes roughly 30 - 40 % of the total ash, the rest being the fly ash. Ash from the boiler furnace outlet falls into the crusher, where large ash particle is crushed to small size. Finally ash is carried by high velocity air to delivery point.
13b) Fly ash removal The amount of fly ash to be handled in FBC boiler is relatively very high, when compared to conventional boilers. 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) . To diminish the SOx, recycling of fly ash is practiced in some of the units.
2. Circulating Fluidised Bed Combustion (CFBC) 14 Taller boiler structure Coal is crushed to a size of 6 –12 mm depending on the rank of coal. The fluidising velocity in circulating beds ranges from 3.7 to 9 m/sec. Combustion efficiency as high as 99.5%. 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. There are no steam generation tubes immersed in the bed.
Advantage over bubbling bed boiler 15 Higher combustion efficiency. Higher sulphur retention degree. Better limestone utilization. Lower emission level of NOx and SOx. More economic produces 75 – 100 T/hr of steam . Capacity range is about 400-500 MW & that of FBC boiler is10-300 MW. No. of feeding point less, because lateral mixing is done by high velocity fluidizing air.
163.Pressurised Fluid Bed CombustionCombined cycle i.e Rankine & Brayton cycle using steam & gas turbine.Operating temp. & pressure is 860OC & 16-18 bars.The fuel is fed along with the sorbent and is maintained in fluidised condition in the pressurized combustion chamber.The pressurized flue gases are cleaned off & are expandedinto a gas turbine.In addition, the excess air requirements of the boiler aremet by the gas turbine compressor.Power generated by steam cycle and that generatedby gas turbine which is of the order of 80:20
17Advantages Improved Cycle Efficiency & is estimated to be 4-5 % more than conventional steam plant. Lower fluidising velocities (around 1m/sec) which reduce the risk of erosion for immersed heat transfer tubes. Reduced Emissions & Improved Combustion. Reduced Boiler Size.
Advantages of FBC boiler over Conventional boiler1. High Efficiency. Combustion efficiency of over 95% Overall operating efficiency is 84%2. Fuel Flexibility.3. Ability to Burn Low Grade Fuel.4.Pollution Control. SOx formation is minimised by addition of limestone for high sulphur coals. CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) + CO2 (gas) SO3 + CaCO3 = CaSO3 + CO3 Low combustion temperature eliminates NOx formation. NOx formation takes place around1500o C5. Easier Ash Removal – No Clinker Formation
6. Simple Operation, Quick Start-Up7. No Slagging in the Furnace-No Soot Blowing8. Provisions of Automatic Coal and Ash Handling System9. Provision of Automatic Ignition System10.High Reliability and low maintenance costs.
Disadvantages of FBC boiler over Conventional boiler1. The only disadvantage is that, a large amount of power will be required to lift up the silica surface. So motor of force draft fan will b twice bigger than that of other conventional system.