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Babcock & Wilcox 1

 J.F. CroninBabcock & Wilcox

Barberton, Ohio, U.S.A.

From Liquor to Sludge—Conversion of a RecoveryBoiler to a Bubbling Fluid Bed

BR-1675

IntroductionWhen a mill no longer needs an existing recovery boiler itis faced with the problem of what to do with the boiler in the

future. The mill may have a new recovery of a larger size or

may have changed from Kraft to recycle product. The sludge

that once was landfilled is now a valuable fuel to generate steam

and can displace other more expensive fossil fuels. A U.S. mill,

having semi-retired a vintage 1964 Combustion Engineering

(C-E) recovery with a larger recovery boiler, wanted to replace

older power boilers with a power boiler capable of firing

papermill sludge; tire derived fuel and wood waste. This paper

discusses the modifications required to convert this recovery

boiler to a power boiler and the considerations that paper mills

need to understand when converting equipment to solid fuel

firing.

BackgroundThis project converted the recovery boiler to a power boiler

and increased steam capacity from 34.5 to 56.7 kg/sec (274,000

to 450,000 lbs/hr) while maintaining steam outlet conditions

at 58.6 bar and 441C (850 psig/825 F). The converted boiler

was designed as a bubbling fluid bed (BFB) boiler capable of 

burning wood waste, sludge and tires. The project was done on

an EPC basis (Engineering, Procurement and Construction). In

order to provide our customer with the most technically sound

and cost-effective proposal for this project, B&W hired an en-gineering construction company who provided balance of plant

design, procurement, and construction.

The existing unit is a 1964 vintage C-E 600 TPD two drum

recovery boiler. The furnace is 7.5 m (24.7 ft) wide by 7.13 m

(23.4 ft) deep. The wall construction uses 5.08 cm (2 in.) OD

tubes on 5.16 cm (2 1/32-in.) centers. The 168 cm (66 in.)

steam drum and 122 cm (48 in.) mud drums are on 7 m (23 ft)

centers. For about two years prior to it’s conversion, the boiler

was maintained in a “hot standby” condition, using the steam

coil air heater to heat air to keep the boiler and precipitator

warm. This was for a corrosion concern. The boiler was oper-

ated, at least annually, whenever the newer recovery boiler had

an outage.

Economic and environmental issues justified conversion of the chemical recovery unit to a bubbling fluid bed. About 60

percent of mill sludge was being sent to the mill’s on-site land-

fill, based on hauling 24 hours a day. The converted boiler can

burn all mill sludge 450 tonne (500 ton)/day, thereby essen-

tially eliminating sludge landfill. This will extend the useful

life of the on-site landfill. The mill was able to retire two older

bark boilers and reduce coal usage on their remaining power

boilers. Conversion to a BFB unit allowed existing assets to

be fully utilized, such as the upper furnace, superheater, boiler

bank, precipitator, building, etc. Also, the cost to maintain the

recovery unit in ‘hot standby’ was eliminated.

Presented to:1999 TAPPI Engineering ConferenceSeptember 12-16, 1999Anaheim, California, U.S.A.

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History of B&W BFBB&W has extensive experience with boiler conversions and

modifications, including almost total replacement of older re-

covery boilers with newer recovery units while reusing exist-

ing buildings and ancillary equipment. B&W has designed,

manufactured, and constructed complete new facilities with

fluid bed boilers, including circulating fluid bed and bubbling

fluid bed, and has converted other boilers to BFB.

B&W uses the original open bottom design as illustrated in

Fig. 1. This BFB design feature eliminates the problems asso-

ciated with bed draining, the size of debris a drain can remove,

and the need for water-cooled conveyors. The furnace hopper

is cool and does not require insulation. Further, B&W designs

the hoppers to be bottom-supported while the boiler remains

top supported. This design concept is similar to the water seals

used on many solid fuel top-supported power boilers designed

by most boiler manufacturers. The primary benefit to this de-

sign is that the weight (load) of the fluidizing air system, bed

(sand and fuel) and hoppers is not part of the overall boiler

loading. The loading to top supports, and top steel, is consid-

erably reduced. The furnace header-to-hopper seal is based on

similar seals used on water-cooled stokers and similar BFB

installations.

Type and quality of fuel will effect the bubbling bed opera-

tion; temperature, combustion staging and air heater require-

ments. Historically a tubular air heater has been required. In

the case of this project, the fuel combination did not require

heated air to the bubbling bed. This allowed for the elimina-

tion of any tubular air heater and the addition of more econo-

mizer surface. Flue gas recirculation into the bubbling bed air

system is used to control bed temperature while staging com-

bustion.

Boiler PerformanceThe boiler maximum continuous rating (MCR) is 56.7 kg.sec

(450,000 lb/hr) steam at 441C (825F) and 58.6 bar (850 psig)

using the specified fuels, which include hog fuel (bark, wood

chips, etc), paper mill sludge, and chipped tires (or tire-de-

rived fuel, TDF). The bark moisture can vary between 45-55%,

and sludge moisture can vary between 50-60%. Woodwaste is

both purchased and self generated. The boiler is designed to

burn natural gas using the four start-up burners and two loadburners, and produce 34 kg/sec (270,000 lb/hr) steam at rated

conditions. In operation the boiler will fire a wide range of 

fuel quality, affected by fuel sourcing and weather conditions.

This BFB is designed to process both good and poor fuels.

The predicted boiler performance is summarized in Table

1. Table 2 provides the fuel specification for each of the four

fuels. Within Table 1, Case 1 presents boiler performance at

MCR with bark, TDF and sludge. The sludge rate is equiva-

lent to 450 tonne (500 ton)/day of sludge, which is the nominal

mill sludge output at this time. Case 2 shows boiler perfor-

mance at MCR with bark and sludge only, and the sludge rate

is equivalent to 725 tonne (800 ton)/day of sludge, which rep-

resents a future sludge condition. This boiler has been oper-

ated with 800 ton/day sludge. Finally, Case 3 is the low load

condition with 25.2 kg/sec (200,000 lb/hr) steam flow while

firing bark and 450 tonne (500 ton)/day sludge.

Illustration 1 shows the BFB retrofit boiler side view, with

only the front half of the economizer is depicted. There is also

a second bank of economizer not shown.

Table 3 summarizes the overall scope of supply for this

project.

Bubbling Fluidized Bed (BFB) Boiler The new lower furnace utilizes a fully membraned wall con-

struction to provide a completely welded, gas-tight enclosure,

stiffened by buckstays, and covered by insulation and externallagging (see Figs. 2 and 3). The previous furnace wall enclo-

sure was tangent tube.

Lower Furnace and Fluid Bed BottomThe membrane furnace walls were constructed using 7.62

cm (3 in.) OD by 4.57 mm (0.180 in.) minimum wall SA178A

tubes on 10.32 cm (4 1/16 in.) centers. The 10.32 cm (4 1/16-

in.) side spacing allowed the new 7.62 cm (3 in.) OD tubes to

be swaged and welded directly to every other existing 5.08 cm

(2 in.) OD tube. The remaining 5.08 cm (2 in.) OD tubes were

routed into a new transition header located near the weld line.

These new headers on all four walls provide the transition be-tween the new 7.62 cm (3 in.) membrane-welded lower furnace

tubes and the existing 5.08 cm (2 in.) tangent tubes.

To achieve the desired steaming capacity of 56.7 kg.sec

(450,000 lb/hr) the lower furnace combustion zone was enlarged

from the main furnace shaft to provide a deeper combustion

zone. The bubbling fluid bed plan area requirements were set

to achieve the desired bed velocity and bed temperature. The

new lower furnace depth was increased from 7.09 to 10.21 m

(23 ft 4 in. to 33 ft 6 in.). The furnace width remained the

same as original design at 7.52 m (24 ft 8 in.). This arrange-

ment resulted in both adequate furnace liberation rate and ad-Figure 1 B&W’s open bottom bed design.

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Table 1

Predicted Performance Summary

MCR – bark, MCR – bark, Reduced loadTDF, sludge sludge 450/(500) TPD

 sludge, bark

Steam Leaving SH, 103 kg/s (lb/hr) 0.06 (450) 0.06 (450) 0.03 (200)Excess air, % 30 30 40

Blowdown, % 2 2 2

Bark Rate, 103 kg/s (lb/hr) 0.02 (131.5) 0.02 (135.5) 6.38x10-3 (50.6)

Sludge Rate, 103 kg/s (lb/hr) 5.25x10-3 (41.7) 8.40x10-3 (66.7) 5.25x10-3 (41.7)

Tire Rate, 103 kg/s (lb/hr) 5.29x10-4 (4.2) 0.0 0.0

Heat Available Fuel, W (MBtu/hr) 2.19x108 (748) 2.26x108 (772) 9.87x107 (337)

Flue Gas Leaving ECN, 103 kg/s (lb/hr) 0.12 (927.5) 0.11 (909.7) 0.06 (478.8)

Flue Gas Temp. Lvg ECN, C (F) 177 (350) 177 (350) 160 (320)Flue Gas Recirculation, 103 kg/s (lb/hr) 7.06x10-3 (56.0) 0.0 5.67x10-3 (45.0)

Steam Press, at SH outlet, Bar (psi) 59 (860) 59 (860) 59 (855)Steam Temp. Leaving SH, C (F) 441 (825) 441 (825) 441 (825)

Air to Furnace, 103 kg/s (lb/hr) 0.09 (691.4) 0.09 (694.7) 0.04 (329.8)Air Temp. Entering WCAH, C (F) 27 (80) 27 (80) 27 (80)Air Temp. Leaving WCAH, C (F) 107 (225) 107 (225) 99 (210)

Table 2

Predicted Performance Fuel Specification

% By Weight Bark Sludge Tire Natural Gas

 C 25.0 14.62 83.87 69.27

 H2 3.0 1.93 7.09 22.65

 Cl 0.0 0.0 0.0

H2O 50.0 60.00 0.62

 N2 0.1 0.17 0.24 8.08

 O2

20.2 9.75 2.17

 S 0.0 0.05 1.23 < 1 ppm

 Ash 1.7 10.23 4.78

Unknown(2)

3.25

 kJ/kg (Btu/lb) 10,000 (4,300) 6,600 (2,841) 36,000 (15,500) 50,600 (21,800)

Min. Density, kg/m3 (lb/ft3) 272(17) 400 (25) 609 (38) NA

Max. Size, cm (in.) 7.62 (3) 7.62 (3) 7.62 (3) NA

Min. Size, cm (in.) 10% < 0.32 (0.125) 10% < 0.32 (0.125) 10% < 0.63 (0.25) NA

Wire Content 0 0 10% belt, 1% bead NA

(2)Required for analysis to add up to 100%, considered as ash.

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Table 3

General Scope of Work

Convert the boiler from a chemical recovery unit to a BFBpower boiler, and increase capacity

Boiler modificationsNew gas burners (start-up and load burners)New motor driven fluidizing air fan and motor driven

secondary air fan

DuctsNew horizontal economizer Water coil air heater Flues and hoppers, from boiler generating bank outlet to

economizer to existing precipitator inlet damper Flues from precipitator outlet, including new outlet

nozzles, to ID fanTurbine driven ID fanFlues to stacks (2 stacks)Sand feed systemBoiler bed drain system and sand reclaim systemPneumatic ash removal from economizer hoppers and

precipitator silosInsulation, lagging and refractorySolid fuel feed system, including dedicated bark reclaimer,

dedicated sludge reclaimer, belt, magnet, disk screen,belt to boiler building, belt inside boiler building

Wood bins on north and south sides (boile r right and leftwalls)

Wood screw conveyors, to transport material from southside to north wood bin

Wood chutes and wood spoutsNew DCS system and new instrumentationNew transformer New electrical room for large motor MCCs and DCS

equipmentReuse electrical room, install new MCCsCable and trayPiping modificationsPlatform modifications, including concrete and gratingCivil and structural

 Asbestos removalDemolitionConstruction

Commissioning

equate residence time for complete combustion. The new lower

furnace is refractory-lined to control bed temperature and pro-

tect the tube walls from the reducing (oxygen deficient) atmo-

sphere. The refractory extends up to the overfire air elevation.

The new construction of the lower furnace enlargement uses

panels fore and aft of the main side wall panels. Each panel

contains 15 tubes and connects to upper and lower wall head-

ers. These panels are directly supplied with water from new

downcomer sections.

The fluid bed bottom uses an open hopper design with one

manual slide gate valve and one pneumatically actuated spherivalve per hopper for the removal of rocks which enter with the

fuels. There are 12 hoppers (3 wide x 4 deep). To simplify

fieldwork, the hoppers were factory assembled into four ship

units (see Fig. 4). Each ship unit consisted of three hoppers,

fluidizing air pipes and horizontal support steel. The bed de-

bris removal cycle is typically once every 12 hours, requiring

only a small portion of sand to be removed. The bed hoppers

are equipped with internal tables to prevent funneling of ma-

terial. The hoppers do not require insulation and, therefore,

were furnished with a high-temperature primer and finish coat

of paint. Figure 3 Bubbling fluidized bed—3D view.

Figure 2 Bubbling fluidized bed boiler—side view.

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The eight large diameter fluidizing air ducts are an integral

part of the hoppers and supply air to the bubble caps to fluidize

the bed. The bubble caps are arranged on a staggered 10.16 by

10.16 cm (4 in. by 4 in.) matrix. Twelve thermowells are pro-

vided in the bed and four thermowells are insta lled on the lower

water walls to monitor bed temperature.

Solid fuel is introduced through six airswept distributor

spouts with three located on the north wall and three located

on the south wall utilizing an interlaced pattern. This arrange-

ment was engineered to provide the proper fuel distribution

required to achieve uniform combustion and uniform flue gas

temperatures leaving the furnace and entering the convection

pass.

Bed Support

The existing furnace and boiler pressure parts are top-sup-ported and allow for downward expansion during heating from

ambient to operating temperature. The fluid bed combustor is

operated with all of the lower furnace hoppers and the volume

of the lower furnace to a point about 76.2 cm (30 in.) above the

top of the bubble caps completely full of bed material. The

lower furnace hoppers are bottom-supported on dedicated struc-

tural steel from grade. A seal system with fabric expansion

 joint provides for downward expansion of the boiler.

Buckstays/Rear Wall SupportThree new buckstay elevations were supplied for the new

lower furnace. Constant load hangers support the rear wall and

attach to a buckstay below water drum; front wall and sidewallsare supported by existing upper furnace tubes and existing top

support rods.

Bed Start-Up Burners and Load BurnersFour overbed natural gas start-up burners are provided to

heat the bed from a cold condition to the temperature where

solid fuel combustion occurs. Two burners are located on each

of the front and rear walls. Each burner is rated at 4.22x107 kJ

(40 MBtu) per hour heat input. Each pair of burners operates as

a single burner.

Figure 4 Hoppers are factory assembled to simplify fieldwork.

Two natural gas load burners are provided on the front wall,

3.5 m (11.5 ft) above the overfire air nozzles. These load burn-

ers are B&W’s low-NOx type XCL-S design, each rated for

1.58x108 kJ (150 MBtu) operation.

Combustion Air SystemThe combustion air system consists of one motor driven flu-

idizing fan, one motor driven secondary air fan (which suppliesair to all burners and OFA nozzles), and one water coil air heater

for the secondary air system. This two-fan arrangement, along

with the flue gas recirculation, provides the flexibility neces-

sary for the specified fuel combinations for bark, sludge, and

tires, as well as the given fuel moisture content ranges. The bed

fluidizing air system uses high static air supplied to the bubble

caps to fluidize the bed material. The overfire air system con-

tains two levels of air nozzles on each of the front and rear walls.

Flue gas recirculation to the bubbling bed is required with

the specified fuel combination and moisture ranges listed in the

project specifications. The recirculated flue gas is taken from

the ID fan outlet and introduced into the fluidizing air system at

the FD fan inlet. The introduction of the flue gas into the fluid-

izing air system will allow complete bed fluidization while stag-

ing sufficient combustion in the overfire air zone to maintain

the bed in the desired 760-871C (1400-1600F) temperature range

throughout the various combinations of fuels and moisture con-

tents.

Furnace ScreenA portion of the original recovery furnace screen platens

were removed to increase gas temperature entering the super-

heater.

Superheater ArrangementThe existing superheater was reused as much as possible,but slightly modified, to provide the desired temperature (441C

 / 825F) and pressure (58.6 bar / 850 psig) at the main steam

header. The modifications required were: install six missing

platens in the first two banks, completely remove the third bank,

and provide new jumper tubes to connect the second bank (in

direction of gas flow) to the existing inlet header of the first

superheater bank.

A new, larger interstage attemperator and main steam piping

was required to reduce the steam side pressure drop at the in-

creased steaming rate of 56.7 kg/sec (450,000 lb/hr) and to

maintain 58.6 bar (850 psig) at the main steam header. Spray

water is supplied from the existing water line.

Generating Bank and DrumsNo changes were required to the existing generating bank.

Economizer The existing vertical cross-flow economizer was removed

and replaced by a horizontal, bare tube design. The new, con-

tinuous in-line, bare tube economizer is constructed of 5.08 cm

(2 in.) OD tubes. The modules are arranged in two gas passes,

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each pass utilizing a counterflow design. The flue gas path is

from boiler outlet to the first pass with flue gases flowing vert i-

cally down, gases flow above economizer hoppers to the sec-

ond gas pass, then flue gases f low vertically upward. The econo-

mizer gas outlet connects to the existing precipitator inlet damp-

ers.

Water Coil Air Heater A water coil air heater is installed in the overfire air system.Feedwater is routed from the new economizer to the new water

coil air heater and then to the steam drum. The new water coil

air heater reduces the temperature of the feedwater entering the

steam drum to maintain the necessary margin of subcooled

feedwater to the drum.

Safety ValvesThe existing steam drum and superheater safety valves had

sufficient relieving capacity for the 56.7 kg/sec (450,000 lb/hr) of 

steam rate.

Fuel Handling SystemThe bark reclaimer is a six-wide drag chain reclaimer rated

to discharge 91 tonnes (100 tons) per hour of hogged wood

waste. Material is discharged onto the reclaimer by either a belt

conveyor from the woodyard or by front-end loader. The dis-

charge rate can be varied by controlling the speed of the vari-

able frequency drive unit.

The second reclaimer is identical to the bark reclaimer in

design, but is dedicated to sludge. Sludge is presently loaded

using a front-end loader, but a dedicated sludge conveyor will

be added in the future.

Bark and sludge from the above reclaimers discharge to the

#1 belt conveyor, which is 1.2 m (48 in.) wide by approximately

58 m (190 ft) long. Tramp iron is removed by a self-cleaning

electromagnet, suspended above the conveyor.

A disc screen is provided at the discharge of the #1 belt con-

veyor. Material less than nominal 7.6 cm (3 in.) will pass through

the screen and onto the #2 belt conveyor, which is 1.2 m (48

in.) wide and approximately 106.7 m (350 ft) long, and termi-

nates inside the boiler house on the seventh floor.

Material discharged from the #2 belt conveyor drops to the

#3 belt conveyor. This is a 1.2 m (48 in.) wide by approximately

18.3 m (60 ft) long horizontal belt conveyor on the sixth floor.

The material discharged from the #3 belt conveyor is nor-

mally directed to the #1 screw conveyor and metered to both

the north and south bark bins. In the event of a failure of the #1

screw conveyor, a bypass gate can be manually positioned todivert material directly into the south bark bin in the event that

a problem prevented the operation of the #1 screw conveyor or

the north bark bin.

The north and south fuel bins are of identical construction,

each consisting of nine 45.7 cm (18 in.) diameter variable pitch

screws to form a continuous live bottom bin. Each bin is made

up of three separate feed zones, each feeding one of the three

windswept spouts located on that side of the boiler. Each speed

zone has a constant torque, variable frequency drive system.

B&W used dual bark bins to provide better fuel distribution to

the bed, more uniform bed temperature, more uniform heat dis-

tribution to the convection pass, and better control of boiler

emissions.

Ash Handling and Sand Reclaim SystemsBottom ash is removed from the bubbling bed utilizing five

heavy-duty, round link type drag chains. Four drag conveyors

receive ash from the bed drains (or hopper bottom ash outlets)and discharge into the fifth conveyor, which is a collection con-

veyor. The collection conveyor transports materials into the sand

reclaim system.

The sand reclaim system separates the sand from rocks, ag-

glomerated sand and debris brought in with the fuels and wire

from the TDF. The sand is returned to the sand storage silo.

Fly ash collected from the economizer and electrostatic pre-

cipitator is conveyed by a pneumatic ash system. The pneumatic

conveying system is designed to convey 9.1 tonnes (10 tons)/ 

hour of fly ash on a continuous basis. The fly ash is conveyed

to a filter/receiver which discharges into the ash silo. An ash

mixer is located at the silo outlet to condition the ash with wa-

ter before material is dumped into trucks.

NOx Reduction SystemA Selective Non-Catalytic Reduction (SCNR) system was

provided to reduce the NOx emissions from the boiler. SNCR

uses urea or ammonia injected within a temperature zone of the

furnace to recombine with NOx  to form N

2  and water vapor.

There is no catalyst used in an SNCR. The SNCR is a series of 

pre-engineered modularized component assemblies, which are

combined to produce a complete urea based chemical delivery

system. The urea can be injected through 20 injection lances.

The injection lances are located at three different boiler zones.

The following are equipment descriptions of the SNCR system:

1. Storage tank: Designed to store 75,700 L (20,000 gal) of 

bulk chemical. The storage tank is of fiberglass construction

and includes a heating package to maintain 27C (80F).

2. Circulation Module: This module is used in conjunction

with the storage tank to keep the chemical fluid warm and cir-

culating. It also assures that a proper supply of chemical is

delivered to the metering/mixing module.

3. Metering/Mixing Module: This module provides com-

plete metering and control systems for the chemical and in-

cludes an electrically driven, hydraulically actuated diaphragm

pump with steady-flow output. The module is designed for in-

dependent level control, which permits a biasing of the chemi-

cal to each level of injection. The water supply is adjusted, via

a regulator, to a set pressure that will allow for proper flow toeach distribution module.

4. Distribution Module: This module provides complete

flow to individual injection lances. The modules are placed

 just prior to the injectors and are used as a guide and check for

proper injector performance. Air for atomization and cooling

is introduced through this module. One distribution module

controls up to 8 injectors.

5. Injection Assemblies: Injection lances are used to spray

the chemical into the boiler furnace. The injectors are 1.9 cm

(1.14 in.), 316 L stainless steel tubes, with an inner atomiza-

tion tube. The standard length is 0.76 m (2.5 ft).

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Special ConcernsThe project included several unique features and challenges.

Boiler 1. This is the first B&W “open bottom” bubbling fluid bed

that has bottom supported hoppers connected to top supported

pressure parts. B&W’s other domestic open bottom designed

BFB units are bottom supported.

2. The seal between furnace (top supported) and hoppers

(bottom supported) is similar to other designs, but accommo-

dates 10.8 cm (4.25 in.) of vertical growth down and 2.54 cm (1

in.) of horizontal growth.

3. The sand hoppers were pre-assembled (4 ship units, 3

hoppers each unit). This included fluidizing air pipes (with

bubble caps) welded to the hoppers.

4. Rear wall support: The existing recovery boiler used con-

stant load hangers at the furnace floor to support the floor and

rearwall. A new constant load hanger system was needed for

the rearwall when the furnace floor was removed and replaced

with the fluid bed.

5. The new membraned lower furnace (with 7.6 cm / 3 in.

OD tubes on 10.32 cm / 4-1/16 in. centers) was connected to

the existing tangent tube furnace (with 5.08 cm / 2 in. OD tubes

on 5.16 cm / 2-1/32 in. centers), and new inlet headers recon-nect every other existing tube (at cut line) to downcomer circuits.

Auxiliary Equipment and Construction1. The bark and sludge reclaimers are located in the area

previously used by the mill for bark storage, about 137 m (450

ft) from the boiler building. The #2 belt conveyor is routed un-

der an existing conveyor (behind the boiler house) and the #2

belt conveyor penetrates the building wall on the seventh floor,

southwest corner, at an obtuse angle to building wall (and steel).

2. Inside the building, two woodbins were installed, one on

north wall and one on south wall. Fuel is transferred from south

side to north via four wood screws.

3. Finding locations for two FD fans was challenging, but

solved.

4. The vertical clearance between concrete at grade and the

bottom of platform steel established maximum heights for cer-

tain equipment, such as economizer modules and urea storage

tank.5. The new economizer was shipped in 6 modules, weigh-

ing between 43,090 and 65,770 kg (95,000 and 145,000 lb) each.

6. New foundations for fans, ash silo and other equipment

had to consider underground conditions (such as piping) and

unknown subsoil conditions.

7. The only entry into building was at grade, on north wall,

adjacent to an alley.

8. Space had to be found for a new electrical room (MCC

and DCS cabinets) and new transformer.

9. Construction work was performed inside a building, which

had other operating equipment and boilers, without causing ac-cidental trip(s), while maintaining safe working conditions.

10. Hazardous material, mainly asbestos, was identified and

removed (customer removed hazardous material).

11. Safe demolition of existing equipment and material in-

cluded:

- Electrical equipment

- Cables

- Piping

- Cascade evaporator

- Economizers

- SCAH

- Platforms, including concrete and steel

- Dissolving tank, liquor pumps, etc.

- Old control room (previously abandoned)

12. After completion of above demolition by the demolition

contractor, the boiler erector demolished boiler and superheater

components, and then installed new equipment and material.

13. Coordination of outages was required, for tie-ins between

new equipment (electrical and piping) to existing equipment.

14. Relocation of cables and piping was needed, either for

access or to create space for new equipment.

15. Planning and execution of economizer module erection

included:

- Transportation to site and to the alley.

- The modules were floated inside the building (used air pads).

- The two largest modules, weighing 65,770 kg (145,000

lb) each, measured 2.9 m (9.5 ft) high x 5.56 m (18.25 ft) wide

x 9.14 m (30 ft) long.

- Modules were assembled at grade by first moving in the

top module, middle module, lower module and support steel,

and then the assembly (welded together) was raised to eleva-

tion (two lifts, 159 tonne / 175 ton each).

OperationThe conversion project has achieved its major goals, and all

steam and fuel conditions have been achieved. Bark, sludge,

tire and natural gas have been fired at all required conditions

between steam flows of 25.2 and 56.7 kg/sec (200,00 and

450,000 lb/hr). Main steam temperature is about 17 C (30 F)

low at MCR conditions, but this low temperature will be cor-

rected later this year by removing additional furnace screen platens.Sustained operation at MCR and 726 tonne (800 ton)/day

(future condition) has been limited due to a capacity problem in

the pneumatic ash system. The ash discharge rate from filter/ 

receiver to ash silo was a constriction point, but modifications

will be performed to correct this problem.

During initial operation, we corrected an acoustic standing

wave in the front economizer by installing two plate baffles.All boiler emission predictions have been achieved. Some

typical emissions at MCR are:

Particulate less than 0.045 kg (0.1 lb)/MBtu at stack 

(with existing precipitator)

NOx

less than 50.8 kg (112 lb)/hr over 24 hours

SO2 less than 50.8 kg (112 lb)/hr over 24 hours

CO less than 31.75 kg (70 lb)/hr over 24 hours

VOC less than 1.36 kg (3 lb)/hr over 24 hours

ConclusionThis paper has been written to give an indication of needed

modifications to convert an existing recovery boiler to a BFB

power boiler. This modification continues to gain in popularityas new recovery capacity is developed or considered. The re-

covery boiler is typically sized conservatively when applied to

burning solid fuel such as sludge and woodwaste. This retrofit

resulted in a steam flow increase of 66%.

The modifications normally entail getting the new fuel to

the boiler, providing a combustion zone with air system and

removing the ash and by products. B&W considers resusing as

much of the existing boiler as is possible. Even the particulate

removal used by the recovery boiler maybe reused.

As noted in this paper, a good design results in a well gener-

ating boiler producing low emissions.

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8 Babcock & Wilcox

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