FLUIDISATION BED COMBUSTION (FBC) BOILERS BY ISAAC JOHN GUIDE PROF : XYZ  

INTRODUCTION Viable alternative Multiple benefits compact boiler design, fuel flexibility, higher combustion efficiency and reduced emission of pollutants SOx and NOx. Fuels coal, washery rejects, rice husk, bagasse & other agricultural wastes. Capacity range- 0.5 T/hr to over 100 T/hr.

HISTORY: A NEW WAY TO BURN 0ne afternoon more than a decade ago, a British engineer named Douglas Elliott tapped a bit of coal, ground to crumbs in a kitchen blender, into a tin can mounted on the wall. Below the can was an ordinary small open fireplace – ordinary, that is, at a casual glance.   For instead of burning coals, a red- hot bed of sand gently undulated like simmering lava. As the coal trickled out of the can onto the glowing bed below, each crumb exploded into flame with a pop and a blue-white flare. It burned only about two pounds of coal an hour, but the little fireplace glowed fiercely, delivering much more warmth than its size suggested.

FLUIDISATION The operation through which fine solids are transformed into a fluid like state through contact with either a gas or a liquid. A process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state. This process occurs when a fluid (liquid or gas) is passed up through the granular material.

MECHANISM OF FLUIDISED BED COMBUSTION

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)

ATMOSPHERIC FLUIDISED BED COMBUSTION SYSTEM (AFBC) Crushed coal size 1 – 10 mm Velocity of fluidising air – 1.2 to 3.7 m /sec. Narrow temperature range - 800oC to 850oC. Unit Size-25MWe Thermal Efficiency-30%

GENERAL ARRANGEMENTS OF AFBC BOILERS AFBC boilers comprise of following systems: Fuel feeding system Under bed pneumatic feeding Over bed feeding ii) Air Distributor iii) Bed & In-bed heat transfer surface iv) Ash handling system Bottom ash removal Fly ash removal Many of these are common to all types of FBC boilers

CIRCULATING FLUIDISED BED COMBUSTION SYSTEM (CFBC) Crushed coal size 6 – 12 mm Velocity of fluidising air – 3.7 to 9 m/sec Solid recycle – 50 to 100 kg per kg of fuel burnt Unit Size - 250-300MWe Thermal Efficiency-40%

PRESSURISED FLUIDISED BED COMBUSTION SYSTEM (PFBC) Temperature – 860 C Pressure- 16-18bars Flue gases expanded in gas turbine Used for cogeneration Unit Size-80MWe Thermal Efficiency-40%

RETROFITTING OF FBC SYSTEMS TO CONVENTIONAL BOILERS Retrofitting fluidized bed coal fired combustion systems to conventional boilers has been Carried out successfully both in India and abroad. The important aspects to be considered in retrofit projects are: a) Water/steam circulation design b) Furnace bottom-grate clearance c) Type of particulate control device d) Fan capacity e) Availability of space.

COMPARISON BETWEEN STOKER AND FLUIDISED BED COMBUSTION   COMPARISON BETWEEN STOKER AND FLUIDISED BED COMBUSTION S.No STROKER FBC 1 TRAVELLING/FIXED GRATE   FLUIDISED BED WITH AIR BOX SYSTEM 2 BED EVAPORATER IS NOT APPLICABLE BED EVAPORATER WILL BE INTRODUCED 3 BED SUPER HEATER IS NOT APPLICABLE BED SUPER HEATER SHALL BE INTRODUCED 4 ONLY OVER BED FEEDING SYSTEM UNDER/OVER BED FEEDING SYSTEM IS POSSIBLE 5 BED ASH HOPPER WITH SUBMERGE BELT CONVEYOR BED ASH DRAIN WITH COOLER AND PNEUMATIC / MECHANICAL CONVEYING 6 PRIMARY AIR SYSTEM PRIMARY AIR SYSTEM IS REQUIRED FOR FUEL TRANSPORTION IN TO BED

ADVANTAGES OF FBC BOILERS 1. High Efficiency 2. Reduction in Boiler Size 3. Fuel Flexibility 4. Ability to Burn Low Grade Fuel 5. Ability to Burn Fines 6. Pollution Control 7. Easier Ash Removal – No Clinker Formation 8. No Slagging in the Furnace-No Soot Blowing 9. Provisions of Automatic Coal and Ash Handling System 10.High Efficiency of Power Generation

CONCLUSIONS Decreasing electricity prices and improving environmental protection Clean energy More flexible than conventional plants Reduced emissions and Improved combustion Research and development is required in order to improve the FBC technology.

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