Fluidized Bed Combustion System

1. Fluidized Bed Combustion System Fluidized Bed Combustion System www.powerpointpresentationon.blogspot.com

2. FLUDIZED BED COAL FIRED BOILERS • INTRODUCTION • When a air or gas is passed through an inert bed of solid • particles (supported on mesh) will initially sleek upward • through sand. • With Further increase in velocity the air starts bubbling • through bed and Particles attain a state of high turbulence. • Under such conditions Bed attains the appearance of fluid • and exhibit the properties of Fluid.

3. PRINCIPLE OF FBC

4. MECHANISM OF FLUIDIZED BED COMBUSTION • Temperature of bed should be at least equal to ignition temperature of coal. • Bed temperature must not increase melting point of Ash. • Equilibrium temperature achieved through transfer tubes immersed in bed and walls of combustor. • Gas velocity must be maintained between fluidization velocity and the particle entrainment velocity.

5. FIXING, BUBBLING AND FAST FLUIDIZED BEDS As the velocity of a gas flowing through a bed of particles increases, a value is reaches when the bed fluidizes and bubbles form as in a boiling liquid. At higher velocities the bubbles disappear; and the solids are rapidly blown out of the bed and must be recycled to maintain a stable system.

6. TYPES OF FBC • AFBC: Atmospheric Fluidized Bed Combustion • Bubbling fluidized bed combustion. • Circulating fluidized bed combustion. • PFBC : Pressurized fluidized bed combustion.

7. BUBBLING FLUIDIZED BED COMBUSTION

8. FEATURES OF B.F.B • Distribution plate through which air is blown for fluidizing • Immersed steam-raising or water heating tubes which extract heat directly from the bed. • Tubes above the bed which extract heat from hot combustion gas before it enters the flue duct.

9. CIRCULATING FLUIDIZED BED REACTOR

10. FEATURES OF CIRCULATING F.B.C • At high fluidizing gas velocities in which a fast recycling bed of fine material is superimposed on a bubbling bed of larger particles . • The combustion temperature is controlled by rate of recycling of fine material. • Hot fine material is separated from the flue gas by a cyclone and is partially cooled in a separate low velocity fluidized bed heat exchanger, where the heat is given up to the steam.

11. PRESSURIZED FLUIDIZED BED COMBUSTION

12. IMPORTANT POINTS • Coal is crushed to a size of 6mm • The air is blown inside by a high pressure fan • The velocity of air is 3-10 (ft/s) • The ignition takes place ,solid densities are reduced • A temperature of about 1500-1600f is produced

13. IMPORTANT POINTS • Heat energy is utilized by water in the water walls and gets converted into steam • Flue gases are collected by a cyclone separator. • It separates the mixture into pure flue gas, ash & unburnt coal particles. • The unburnt coal particles are again re-circulated.

14. HORIZONTAL FIRE-TUBE PACKAGE *Integral water cooled rectangular furnace is used *A carbon reinjection is mounted ,which entraps Large particles ad re-circulate `

15. WATER TUBE BOILER FBC • Has a water cooled • Membrane wall panel • Flue gas pass through a • Super heater ad a • Convective Bank • A grit arrestor placed • In the flue gas path • Collects the particulate • Matter for recirculation

16. VERTICAL FIRE TUBE PACKAGE • It has three pass multi- • tubular construction. • Cooling water tubes are • used within the bed to • Obtain Equilibrium.

17. CFBC: CIRCULATING FLUIDIZED BED COBUSTION A FLUIDIZED BED THAT IS OPERATED AT VELOCITIES IN THE RANGE OF 13 – 22 FEET PER SECOND IS REFERRED TO AS CFBC.

18. CFBC: CIRCULATING FLUIDIZED BED COBUSTION

19. Primary air is introduced into the lower portion of combustor. • Secondary air is introduced at higher levels in the combustor to ensure complete combustion and to reduce NOx emissions. • combustion chamber of a CFB unit consists of membrane type welded water walls.

20. FUEL FEED SYSTEM • Gravimetric feeders with rotary lock valves. • Provision of negative pressure in the combustor at the fuel feed points to eliminate the rotary lock valves. • Pneumatic fuel feed system can also be used.

21. FUEL REQUIREMENTS • Low volatile fuels such as Anthracite must be crushed to • smaller top size than higher volatile fuels • ( approx 1/6 inch or less) • Larger fuel size are acceptable for higher volatile fuels. • SORBENT REQUIREMENTS: • Lime stone is crushed to a top size of 1000 microns • with average size 150 microns.

22. TECHNICAL PERFORMANCE. # CFBC boiler can exhibit high efficiency. # CFBC boilers are simple to operate and maintain. # CFBC boilers offer better dynamic response.

23. ENVIRONMENTAL PERFORMANCE # Emissions * In case of CFBC technology NOx production is reduced substantially. * Production of SOx can be controlled.

24. PFBC:PRESSURIZED FLUIDIZED BED COBUSTOR UTILITY Its utility began in 1960 to express purpose of Increasing cycle efficiency in coal fueled plants.

25. GENERAL CONFIGRATION Classification done in 2 types I. First generation units. • Turbocharged units. • Combined cycle units. II. Second generation units.

26. PFBC TURBOCHARGED • Combustion gas from PFBC boiler is reduced to 750 F. • Little net Gas Turbine output. • Major part of electricity is produced by Steam Turbine.

27. PFBC COMBINED CYCLE UNIT • Combustion gas at 1500F-1600F is fed to gas turbine fro PFBC. • 20% of plant electrical output is provided by Gas Turbine. • Increase of 2%-3% of thermal efficiency compared to turbocharged unit.

28. SECOND GENERATION UNIT • Low btu-gas produced from pyrolyzed coal in carbonizer • and gas produced from char are mixed in Topping combustor. • Combustion of these gases in topping combustor increases • temperature of gas up-to 2300 F. • Higher input gas temperature leads to increase of gas turbine • efficiency. • Heat is recovered from gas turbine exhaust in a heat recovery • steam generation called as HRSG.

29. SECOND GENERATION UNIT • Power generated by S.T • is approximately equal to G.T. • Thermal Efficiency • up-to 45% & more is achieved.

30. PFBC # PFBC combustor can either use * Circulating bed combustor * Bubbling bed combustor # Fluidizing velocity is kept up-to 3 feet/sec. It is kept low to minimize combustor erosion. # Fuel and Sorbent can be send to combustor in 2 forms: * Dry form * Pump able paste

31. PFBC # Coal in dry form: • Coal is dried and grounded to an average size of 50 microns. • Carried to combustor or carbonize through lock hoppers. # Sorbent in dry form • Limestone is used as sorbent. • Grounded to an average size of 100 microns. • Usually feed with coal or can feed in separate stream.

32. PFBC # Paste Feed System • Coal is crushed to ¼ inch top size and mixed with water to about 25 % of water. • Paste is pumped into combustor through positive displacement pup. # Sorbent • Sorbent can be mixed with coal or can be feed separately as with dry feed system.

33. PFBC PERFORMANCE # Overall performance of PFBC units in terms of: * Heat transfer * Combustion efficiency * Emission Is better than as compared to AFBC.

34. PFBC # Higher operating pressure ( 12 to 13 atm) of PFBC units results in higher heat transfer rate and combustion. # NOx emissions are reduced considerably as compared to AFBC due to pressurization.

35. DESIGN,OPERATION AND MAINTENANCE OF FBC BOILERS • FUEL FLEXIBILITY FBC boilers can be operated efficiently with a variety of fuels. • COAL QUALITY Coals with high moisture, high ash can be efficiently burnt.

36. DESIGN,OPERATION AND MAINTENANCE OF FBC BOILERS • FUEL INJECTION The coal is evenly distributed by judicious location of the feed points • FREE BOARD Due consideration is given for provision of adequate free board space and design of the convective bank

37. DESIGN,OPERATION AND MAINTENANCE OF FBC BOILERS • PRESSURE DROP AND BED HEIGHT A shallower fuel bed allows over bed feeding, whereas a deeper fuel bed allows bottom feeding • LOAD CONTROL Steam o/p is controlled by manipulating the principal bed parameters: • Height • Temperature • Inventory and • Fluidization velocity

38. DESIGN,OPERATION AND MAINTAINANCE OF FBC BOILERS • Bed temperature: FBC normally operates at a lower temperature range i.e. 750 to 950ºc • Heat transfer: The overall heat transfer coefficient decreases with increase in particle size and increases with increase in fluidization velocity

39. ADVANTAGE OF FBC BOILERS • Considerable reduction in boiler size is possible due to high heat transfer rate. • Low combustion temp. of the order of 750-900 C facilitates burning of coal with low ash fusion temp. ,prevent NOx- formation, reduces high temp. corrosion and erosion. • High Sulphur coals can be burnt efficiently without much generation of Sox by feeding limestone continuously. • The unit can be designed to burn a variety of fuels.

40. High turbulence of the bed facilitates quick startup and shut down. • Efficiency of 80% and above can be achieved. • Lower coal crushing cost due to higher particle size. • High thermal inertia helps to overcome the problem of flame stability.

41. DISADVANTAGES OF FBC • The need for fuel particle size to be less than 300mm. • A relatively high pressure-drop is required to fluidize a bed of • granular particles. The pressure drop is proportional to the weight • of the bed, but after incipient fluidization rises only slowly with • the gas velocity. • The flue gas carries a high dust load. • Although the operation of a bed is basically stable and the evolution • Oftemperature is slow the problem of fluidized bed regulation and • control is notstraightforward.

42. The possibility of sintering of bed material limits the maximum • operatingtemperature, generally to a value of 850-950 °C, but • sometimes more. • The operating experience with fluidized bed combustors is still • limited. Wearup on submerged surfaces, the occurrence of attrition • and elutriation upon bed particles, the evolution of the particle size • distribution and of the composition of the bed material cannot be • predicted with confidence.

43. CURRENT RESEARCH AND TRENDS • Most current research aims to quantify and explain the behavior of the phase interactions in the bed. • Specific research topics include particle size distributions, various transfer coefficients, phase interactions, velocity and pressure effects, and computer modeling. • The aim of this research is to produce more accurate models of the inner movements and phenomena of the bed.

44. Thank You