Key benefits are fuel flexibility, reduced emission and lower bed temperature.

Different Versions Industrial boilers ore developing with gradual change in their design of different

parts and to optimize the thermal efficiency, fuel multiplicity and to sustain with

environmental norms, with cost effectiveness for installation operation and

maintenance.

In our country early fifties and sixties small multi fuel boilers were used for process

industries and high pressure coal based boilers were used for power generation.

With decreasing trend of coal heat value boiler was constantly modified to reach the

present status of CFBC boiler from previous version of stationary furnaces type

chain grate type, spreader stoker type, PF type, AFBC type and CFBC type. CFBC

is a new generation coal fired fluidized bed boiler developed since 1950 has the

advantage of high efficiency low Sox and Nox. emission and higher turn down ratio.

CFBC BOILER Assume a container with an air supply plenum at the bottom , A distributer plate

that promotes even air flow through the bed and an upper chamber filled with

sand. If a small quantity of air flow through the distributer plate in to the bed , If

it will pass through the voids of an immobile mass of sand. For low velocity the

air does not exert much force on sand and they remain in place. This condition

is called fixed bed or static bed. By increasing a flow rate ,the air exert greater

force on the sand and thereby reduces the contact force between the sand

particle caused by gravity. By increasing the air flow further ,a point is reached

where the drag force on particle counter balance the gravity force and sand

particle become suspended in up flowing air steam. The point where the bed

start to behave as a fluid is called the minimum fluidization condition.

CFBC BOILER As the air increased further , the bed become less uniform bubbles of air start

to form and the bed becomes violent . At this condition bed is in expanded

condition, This is called bubbling fluidized bed and this occurs at

atmospheric condition. Hence, It is called as Atmospheric Fluidized Bed. As

the velocity of air further increased (around 10 m/sec.) a condition is

reached where in whole of bed material is in circulation from furnace to a

collection chamber (U beam hopper) and back to furnace. Thus resulting in

uniform temperature profile throughout the furnace. Because the whole bed

is in continuous circulation internally, This type of boiler is called IR-

CFBC. (Internal Re-circulating fluidized bed combustion).

CFBC BOILER TECHNOLOGY OFFERS

High combustion efficiency

Compact and economical design.

Higher reliability and availability.

Lower maintenance cost.

Reduced erosion.

Fuel flexibility.

Gas Velocities

CFBC - 4 - 6 m/sec.

AFBC - 2 - 3 mtr./sec.

PF - 8 - 10 mtr./sec.

IMPROVED PERFORMANCE AT REDUCED COST

CFBC boiler design as – Two stage particle separation system.

First - U beam impact separators.

Second – Mechanical dust collector or multi-cyclone dust collector.

DESCRIPTION OF CFBC BOILER CFBC boilers are compact natural circulation ,Top

supported ,Single drum unit. The primary and secondary particle separator is an integral part of boiler / furnace and connected to the particle return system.

THE BOILER IS COMPRISED OF- CFB Furnace

U-Beam (Particle separator)

Particle Return System

Convection pass and back end equipment.

Internal Recirculation - Circulating Fluidized Bed Boilers

CFBC FURNACE The CFBC furnace construction is of all welded

membrane tube panels. Functionally the furnace is divided into three distinct zone .

A -Primary zones

B - Secondary zone

C - Free board zone

PRIMARY ZONE In the primary zone the fuel,sand,recycled material

and 50% to 60% of the air for combustion are introduced.

SECONDARY AND FREE BOARD ZONE

In secondary zone the balance of combustion air is introduced.

Combustion is completed in free board zone

CFBC BOILER

• The combustion air split correspond to the furnace zones . i.e. Primary , lower secondary

and upper secondary air flow zone.

lower secondary zone – Burner

upper secondary zone – OF (Over Fired)

• Primary air effects the actual fluidization of bed material and is supplied through a

distributor plate at the bottom of the furnace. lower secondary and upper secondary air are

supplied through nozzle located on both the front or rear furnace walls at two separate

elevations.

• The air distribution patterns in the furnace can be adjusted to provide for the best control

of Nox (Nitrous Oxide) formation.

• In addition to the enclosure walls , water cooled division walls for the entire height of the

furnace provides additional heating surface.

U-BEAM Flue gas and solids leaving the furnace passes through a high temperature

primary particle collector where nearly all of the solids are separated from

the flue gas. A unique feature of boiler is a particle separator made of

labyrinth type array of beam having a shape similar to the letter “U”. The U-

beam made from stainless steel alloy. Or fiber material for less weight &

cost and less erosion. The collector is designed to remove solids from dense

gas–solids mixture .the solids impacting the first two rows of beams fall by

gravity back down the rear of the furnace .Solids impacting the remaining

beams fall by gravity in two particle collector hopper located directly

beneath the array of beams and enters the rear furnace through particle

return system especially designed for this application.

Hot Cyclone CFBC

BOILER CONVECTION PASS The flue gas leaving the particle separator flows in to the

boiler convection pass which includes the superheated and

economizer banks . The gas velocity in these tube banks is

designed specifically to minimize the erosion . Gases

enters the air pre heater and then to the secondary particle

separator . Gas leaving air heater enters the ESP and then

to the stack through ID Fans.

U-Beam separators

© 2003 The Babcock & Wilcox Company, all rights reserved

U-Beam separators41

2

3Flue

Gas

&

Solid

Flow

Flue

Gas

1. Sidewallmembrane panel

2. U-beam - SS309H/ SS310H/RA253MA

3. Seal baffle

4. Refractory

U-Beams

Internal Recirculation - Circulating Fluidized Bed Boilers

Internal Recirculation Circulating Fluidized Bed Boilers

A simplified approach to improved flexibility and reliability

Design Features

High combustion efficiency

Compact, economical design

Higher reliability and availability

Lower maintenance costs

Reduced erosion

Fuel flexibility

Low emissions

Why CFBC Boiler ?

CFBC is a Fuel Flexible Technology

Accepts wide range of fuels

Volatile matter - >15 %

Ash - UP TO 60%

Heating value - > 2150 Kcal/kg

Moisture - < 10%

Use of lower rank fuels reduces fuel costs

Fuel flexibility - minimizes fuel supply uncertainties

Ability to burn low cost and waste fuels

Steam Flow Circuit Riser tubes are used to direct the steam water mixture from the outlet headers of each

water circuit to the steam drum.

The water steam mixture in drum is separated by cyclone separators and the primary and

secondary scrubber arrangement.

The saturated steam leaves the steam drum and enters primary super heater in let header

through supply pipes.Steam passes through the primary super heater coils and exits the

outlet header.Then steam enters the attemperator header for desuperheating.

The attemperator uses water from the high pressure feed water heater discharge to

control the secondary super heater outlet temperature.

From the attemperator the steam flow to the secondary super heater inlet header and to

the counter flow secondary super heater bank. Steam leaves the coils and exits from the

secondary super heater outlet header. Steam enters the turbine through main steam stop

valve and NRV.

Flue Gas Path CFBC boilers are balanced draft combustors. The pressure balance point is at the top

of the furnace, ahead of the front row of U beams, The flue gas flow path through is

as follows :

Gas carrying with large quantity of bed solids, flow into the in bed U beams, above

hopper U beams hot particle collector. The U beams removes most of the solids

entrained in gas. The flue gas leaves the U beams and flows through the secondary

super heater zone, primary super heater zone, Economizer, primary & secondary air

heaters. At the bottom of the air heaters gas enters secondary particle U beams

separators which collects the majorities of the remaining solids entrained in gas.

The cleaned gas then passes through ESP for final gas cleaning and stack through ID

fans. One stands by ID fanes is provided. Dampers are provided before and after the

ID fans for maintenance of fans. With the exception of the U beam and secondary

particle collection system below APH, The flue path is identical to what is found

conventional Unit.

CFBC BOILER

1st Pass

Fuel

2nd Pass

SH - 1

U Beam Section SH - 2

Bed Ash

3rd Pass

Economizer

APH

ESP

Fans

Ash

Recycle

Particle Transfer Hopper

PA air

IR-CFBC Boiler Reheat

IR-CFBC Advantages High Upper Furnace Heat Transfer Rate and Precise

furnace temperature control Result of high efficiency

solids collection and solids recycle control from the MDC

(Mechanical Dust Collection)

Extended turndown ratio (no oil/gas)

100% to 20% MCR (Maximum Continuous Rating)

Low auxiliary power requirements

Due to Low Flue Gas ΔP and no fluidizing blower

IR-CFBC Advantages

Fast shut down / cool down

80%+ of Bed material is drained during cooldown. Can

shutdown / cool / enter / re-start within a 24 hour period

compared to 2 to 3 times longer for competitor CFB designs.

Low Maintenance Costs

Less refractory or hot expansion joints, ‘RDZ’ at lower furnace

refractory interface, no fluidized sealing system, low furnace exit

velocity, low gas velocity in convective heating surfaces,...

The B&W IR-CFBC Two-Staged Solids Separation System

Benefits High overall solid collection efficiency ~ More than

99.7%

Precise furnace temperature control ~ By controlling solid recycle rate from the secondary collector

Extended turndown ratio without use of auxiliary fuel (oil/gas) ~ 100% to 20% MCR

Low auxiliary power requirements compared to competitor cyclone-based CFB technologies ~ 50-100 mm w.c.

Thin Cooled Refractory

75% less refractory in IR-CFB compared to hot cyclone CFB

Difference Between CFBC & AFBC CFBC boiler Flue gas velocity-3.7 to 4.3 m/sec Over bed feeding U beam Technology No bed coil UBC-Less than 2% Efficiency high (88%) Clinker formation chances low Tube erosion high as compared

to AFBC Ash recycle (unburned heavy

particle) system. Fuel consumption is less as

compared to AFBC

AFBC boiler Flue gas velocity-1.5 to 2.0 m/sec Under bed feeding No U beam bed coil UBC-Less than 3 to 4% Efficiency low (84%) Clinker formation chances high Tube erosion low as compared to

CFBC No Ash recycle (unburned heavy

particle) system. Fuel consumption is more as

compared to CFBC

Conclusion With the changing environmental condition of Raw material

availability cost and sustainability of boilers are still under

modification stage with the effort of optimizing their efficiency and

reducing troubles.

Presently different versions of boiler are being installed by various

venders like IJT Foster Wheeler (USA) design with compact separator

model, Thyssen Krupp AG Germany Design for cold cyclone model.

CVL with water cooled “hot loop” cyclone. Songhai electric Alstom

design for cyclone & V type seal, Greensol Hungzhon, Wuxi etc with

steam cooled cyclone. In view of operation troubles all boiler designers’

are on the way to compete each other, with best possible designs and

constant innovation to become the leader of the market.