Fluidized Bed Combustion System

Fluidized Bed Combustion System

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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 upwardthrough sand.With Further increase in velocity the air starts bubblingthrough bed and Particles attain a state of high turbulence. Under such conditions Bed attains the appearance of fluid and exhibit the properties of Fluid.

PRINCIPLE OF FBC

MECHANISM OF FLUIDIZED BED COMBUSTIONTemperature 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.

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.

FIXING, BUBBLING AND FAST FLUIDIZED BEDS

TYPES OF FBC

I. AFBC : Atmospheric Fluidized Bed Combustion

Bubbling fluidized bed combustion. Circulating fluidized bed combustion.

II. PFBC : Pressurized fluidized bed combustion.

BUBBLING FLUIDIZED BED COMBUSTION

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.

CIRCULATING FLUIDIZED BED REACTOR

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.

PRESSURIZED FLUIDIZED BED COMBUSTION

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

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.

HORIZONTAL FIRE-TUBE PACKAGE

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*Integral water cooled rectangular furnace is used

*A carbon reinjection is mounted ,which entraps Large particles ad re-circulate

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

VERTICAL FIRE TUBE PACKAGE

• It has three pass multi-tubular construction.

• Cooling water tubes are used within the bed to Obtain Equilibrium.

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.

CFBC: CIRCULATING FLUIDIZED BED COBUSTION

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.

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.

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.

TECHNICAL PERFORMANCE.

# CFBC boiler can exhibit high efficiency.

# CFBC boilers are simple to operate and maintain.

# CFBC boilers offer better dynamic response.

ENVIRONMENTAL PERFORMANCE

# Emissions

* In case of CFBC technology NOx production is reduced substantially.

* Production of SOx can be controlled.

PFBC: PRESSURIZED FLUIDIZED BED COBUSTOR

UTILITY

Its utility began in 1960 to express purpose of Increasing cycle efficiency in coal fueled plants.

GENERAL CONFIGRATION

Classification done in 2 types

I. First generation units.

Turbocharged units. Combined cycle units.

II. Second generation units.

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.

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.

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.

SECOND GENERATION UNIT

• Power generated by S.T is approximately equal to G.T.

• Thermal Efficiency up-to 45% & more is achieved.

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

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.

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.

PFBC PERFORMANCE

# Overall performance of PFBC units in terms of:

* Heat transfer * Combustion efficiency * Emission

Is better than as compared to AFBC.

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.

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.

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

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

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

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.

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.

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 Of temperature is slow the problem of fluidized bed regulation and control is not straightforward.

• The possibility of sintering of bed material limits the maximum operating temperature, generally to a value of 850-950 °C, but sometimes more.

• The operating experience with fluidized bed combustors is still limited. Wear up 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.

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.

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