Babcock & Wilcox 1

M. MaryamchikD.L. Wietzke

Babcock & WilcoxBarberton, Ohio, U.S.A.

Presented to:POWER-GEN International ’99November 30-December 2, 1999New Orleans, Louisiana, U.S.A.

B&W IR-CFB Boiler Operating ExperienceUpdate and Design

BR-1691

AbstractThis paper presents an annual update of experience with the

B&W IR-CFB boiler product. Included in the update is experi-ence from Ebensburg Power, Southern Illinois University, andKanoria Chemical.

Additionally, the IR-CFB boiler process and design is de-scribed in this paper. Process features included in this revieware the two-stage solids separation system, furnace density andtemperature control, boiler turndown ratio, auxiliary power con-sumption, space requirements, etc. The paper provides a CFBtechnology comparison of those design features and applica-tions to various fuel and repowering situations.

BackgroundA unique and distinct feature of B&W’s CFB boiler is a two-

stage solids separation system. The primary stage is an impact-

type solids separator arranged as an array of U-shaped beams(U-beams) located at the furnace exit. It is followed by the sec-ondary separation stage located after the convective superheaterin the lower gas temperature region (varies from 150C to 500Cby project) and employing either a mechanical dust collector(MDC) or the first field(s) of an electrostatic precipitator (ESP).While the bulk of circulating solids collection is carried out bythe U-beams, the secondary separator is used for collecting thefinest fraction of those solids. The evolution of B&W CFB sol-ids separation system design and corresponding typical solidsbalances is shown in Figure 1.

The first generation of B&W CFB technology was repre-sented by the wood-fired boilers featuring a U-beam separatorinstalled externally to the furnace. Solids collected by the U-beams were returned to the furnace through non-mechanical flowcontrollable L-valves.

100 5.0 5.0 0.5

Fly Ash

MulticloneDust Collector

Cumulative EfficiencyComponent Collection

Efficiency

95%95%

99.5%90.0%

4.595 Solids

FlowControl

A. First Generation

SolidsStorage Hopper

100 3.0 3.030

70

0.3

Fly Ash

MulticloneDust Collector

Cumulative EfficiencyComponent Collection

Efficiency

97.0%90.0%

70%70%

99.7%90.0%

2.7

27 SolidsFlow

Control

B. Second Generation

SolidsStorage Hopper

Note: Illustrated values are based on 100 units of solids exiting the furnace shaft.

100 3.0 3.030

70

0.3

Fly Ash

MechanicalDust Collector

Cumulative EfficiencyComponent Collection

Efficiency

97.0%90.0%

70%70%

99.7%90.0%

2.7

27 SolidsTransferHopper

SolidsStorageHopper

SolidsFlow

Control

C. Third Generation

Figure 1 B&W CFB solids circulation schematics.

2 Babcock & Wilcox

Figure 2 Ebensburg CFB boiler.

Figure 3 IR-CFB primary particle collection system.

In the second-generation coal-fired CFB boilers (Figure 2),the first two rows of U-beams were installed inside the furnace(in-furnace U-beams). Solids collected by those rows fall downalong the furnace rear wall. Particles collected by the followingrows are still recycled externally through the L-valves thoughthe rate of this recycle was reduced several times as comparedto the first generation L-valve.

In the third (current) generation of B&W CFB boilers allsolids collected in the U-beams are I nternally Recycled withinthe furnace, thus the name “IR -CFB”. The IR-CFB U-beamdesign is shown in Figure 3.

The in-furnace U-beams collect from 60 to 75% of all enter-ing solids. Particles collected by the U-beam rows external tothe furnace, located after the furnace exit plane (three to fourrows), go through a particle transfer hopper that drains solids tothe furnace through discharge ports in the upper furnace wall.This second set of U-beams collects from 75 to 90% of the sol-

ids that pass the in-furnace U-beams. Therefore, in-furnace andexternal U-beams collect 90 – 97% of all entering solids.

The fine solids fractions passing the U-beams are collectedin the secondary stage of the solids separation system (MDC orthe first fields of ESP). This stage captures typically from 90 to95+% of solids passing the U-beams resulting in the overallefficiency of the two-stage solids separation system of up to 99.8%.

Solids collected at the secondary separation stage are recycledto the furnace at a controlled rate with a variable speed screw orrotary valve. The recycle rate is set to maintain furnace solidsinventory and upper furnace solids density. Achieving the de-sired furnace solids density profile thus results in maintainingthe target bed / furnace temperature. Material being collectedin excess of the recycle rate is purged from the system. Whilethe overall collection efficiency of the solids separation systemis affected by the purge rate, it normally remains in the range of99.3% to 99.7%. The range of overall grade efficiency of theB&W CFB solids collection system is shown in Figure 4. Thesystem effectively captures and recycles all particles coarserthan 80 micron.

Figure 4 Range of overall grade efficiency of B&W CFB sol-ids collection system.

Furnace Roof

U-Beam Support

In-Furnace U-Beams

External U-Beams

Solids Transfer Hopper

In-Furnace U-Beams

External U-Beams

Solids Transfer Hopper

Gas Flow

Furnace

Babcock & Wilcox 3

Table 1CFB Boiler Comparison

B&W IR-CFB Hot-Cyclone CFB FW “Compact” Cold-Cyclone CFB

Solids Separation System Two-stage Single-stage Single-stage Single-stage(100% efficiency (100% efficiency (100% efficiency (100% efficiencyfor particles of for particles of for particles of for particles ofd>80 micron*) d>100 micron) d>100 micron) d>100 micron)

*Recycling finer particles increases furnace heat transfer rate, improves combustin efficiency and limestone utilization.

Upper Furnace Density, 0.7-1.0 0.5-0.7 0.5-0.7 0.3-0.5lb/ft3 (kg/m3) (11-16) (8-11) (8-11) (5-8)

Furnace Temperature Desired temperature Temperature is pre- Temperture is pre- Lower bed temperatureControl can be maintained determined by determined by is controlled by

within +/-5C interval furnace and heat furnace and heat adjusting cold cyclonefor wide range of exchanger design exchanger design ash recycle rate.fuels and operating along with fuel and along with fuel and Temperature spanconditions by limestone limestone across furnace heightadjusting secondary properties/sizing. properties/sizing. is up to 100C.recycle rate.

Boiler Turndown Without 5 : 1 3.5 : 1 3.5 : 1 3.5 : 1Auxiliary Fuel

Refractory: Thickness, in. (mm) 0.6-2.0 (15-50) ~3 (~75) ~3 (~75) ~2 (~50)

Covered Areas Lower furnace, Lower furnace, Lower furnace, Entire furnace,U-beam zone cyclone, recycle cyclone, recycle cyclone (3-4 timesenclosure walls loop (5-10 times loop (3-5 times more than @ B&W

more than @ B&W more than @ B&W CFB)CFB) CFB)

Hot-Temperature None 3-5 per cyclone Number varies with NoneExpansion Joints arrangement

Furnace Shaft Velocity, 16-24 16-18 16-18 13-15ft/s (m/s) (4.9-7.3) (4.9-5.5) (4.9-5.5) (4.0-4.5)

Furnace Exit Velocity, 21-32 75-85 75-85 NAft/s (m/s) (6.4-9.8) (22-26) (22-26)

High-Pressure Air Not required Required for Required for Required for siphonsJ-valves J-valves

Total Pressure Drop 4 (1.0) 6-8 (1.5-2.0) ~6 (~1.5) 4-6 (1.0-1.5)Across Solids Separator(s), (U-beams + MDC)in. wc (kPa)

Aux. Power Consumption Lower Higher Higher Moderate

CFB Boiler Technology Design & Perfor-mance Comparison

A comparison of B&W IR-CFB design and performance fea-tures with those of other major commercial CFB combustiontechnologies is shown in Table 1.

Experience UpdateThere are currently two IR-CFB boilers in operation. The

first one, shown in Figure 5, is located at Southern Illinois Uni-versity (SIU) in Carbondale, Illinois, U.S.A. and is designedfor 35 MWt output for cogeneration application, utilizing high-sulfur, low-ash Illinois coal. The second boiler (see Figure 6) is

located at Kanoria Chemicals & Industries Ltd. (KCIL) inRenukoot, India and is designed for 81 MWt output for captivepower requirement, firing high-ash, low-sulfur coal. This boilerwas supplied by Thermax B&W Ltd., a joint venture companyof B&W and Thermax of India. Other B&W IR-CFB projectsranging up to 125 MWe with reheat are currently in various stagesof contracting and design.

While sharing all IR-CFB design features, the SIU and KCILboilers have the following major differences:

• SIU boiler features secondary solids recycle from MDC,and on the KCIL unit these solids are recycled from the ESP.

• Bottom ash removal/cooling is carried out with a water-cooled screw at SIU and with fluid-bed coolers at KCIL.

4 Babcock & Wilcox

Figure 5 Southern Illinois University IR-CFB boiler. Figure 6 Kanoria IR-CFB boiler.

• While the KCIL boiler includes a tubular air heater, lowfeed water temperature at SIU resulted in not needing an air heater.

Both the SIU and KCIL boilers were put in commercial op-eration in 1997. Their performance and availability data areshown in Table 2 and Figures 7 and 8.

Long-term B&W CFB performance can be illustrated by theoperational data from the boiler at Ebensburg, Pennsylvania,firing waste bituminous coal (Figure 2). This “second-genera-tion” CFB unit, originally designed for 55 MWe capacity (211 t/hr steam flow), has operated at ~10% overload (61 MWe; 239 t/hr) due to increased load demand. The Ebensburg boiler perfor-mance and availability are shown in Figure 9 and Table 3, re-spectively.

While typical furnace full load velocity at B&W CFB boil-ers is 6 m/s, it ranges from 4.9 m/s for the SIU boiler (per cus-tomer specification) to an overload velocity of 7.3 m/s atEbensburg. The existing operating experience justifies use ofany furnace velocity from this range to better suit the require-ments of a given project.

Design ApplicationsThe wide range of furnace design velocity when combined

with the two-stage solids separation system provides exceptionalflexibility with the B&W CFB boiler design.

In order to accommodate project-specific space requirementswithout sacrificing boiler performance, the furnace height and/or plan area can be adjusted along with the furnace velocity toprovide sufficient residence time for combustion and sulfur capture.

When replacing an existing PC-fired boiler, the B&W CFBboiler can utilize available plant space while meeting or exceed-ing the capacity of the unit and burning much lower grade fuelsthan the original plant.

For most of the fuels, a cost-optimized IR-CFB design wouldutilize a higher-temperature MDC. The higher temperature MDCis located upstream of the economizer in the gas temperature

zone of 400 to 500C (as opposed to the 200 to 300C zone, down-stream of the economizer). While cost of the MDC increasesslightly due to higher gas volume and possible use of low al-loys instead of carbon steel, it provides overall cost benefits forthe following reasons:

• Lower cost of economizer because of lower heat duty andhigher possible gas velocity (both resulting from reduced sol-ids loading)

• Fewer rows of U-beams (total 5 instead of 6) with in-creased MDC solids collection

• Inclined screws, utilized as a feeding and metering de-vice for secondary recycle solids, also serve to convey solids tothe furnace thus eliminating the need for a separate conveyingsystem, like air-assisted conveyors.

Those design features are illustrated in Figure 10 showing aboiler arrangement for one of the new projects. This boiler alsofeatures extensive in-furnace superheating surface providingeffective heat transfer due to high solids bulk density in the upperfurnace. A cavity downstream of U-beams is provided for selectivenon-catalytic reduction of NOx by spraying ammonia into the gas flow.

An alternative arrangement may be used when firing fuelslike oil shale featuring such properties as:

• High amount of solids generated per unit heat release• High reactivity• High internal Ca/S molar ratio• Extreme fouling tendency of the fine ash fractions.Massive generation of bed material when firing oil shale al-

lows sufficient furnace solids inventory at reduced solids re-cycle ratio. High reactivity of the fuel permits the reduced re-cycle ratio without sacrificing the combustion efficiency. Highinternal Ca/S ratio provides excellent sulfur capture without us-ing sorbent, e.g. limestone, thus eliminating concerns about theefficiency of the sorbent utilization at the reduced solids recycle ratio.

On the other hand, ash fouling properties would demand self-cleaning operation of the boiler convection pass in order to avoidhigh-maintenance cleaning of those heating surfaces. Such self-

Babcock & Wilcox 5

Table 2IR-CFB Boiler Performance @ 100% MCR

KCIL SIUDesign Test Data Design Test Data

Steam Flow, kg/hr (klb/hr) 105,000 (231) 103,000 (227) 46,000 (101.5) 46,000 (101.5)Steam Pressure, MPa (psig) 6.4 (913) 6.2 (884) 4.7 (675) 4.4 (640)Steam Temperature, C (F) 485 (905) 483 (901) 399 (750) 399 (750)FW Temperature, C (F) 180 (356) 180 (356) 109 (228) 109 (228)Steam Temperature Control

Range, % MCR 60-100 60-100 50-100 40-100Turndown 3.5:1 5:1 4:1 5:1Flue Gas Temperature

Leaving Airheater, C (F) 140 (284) 130-140 (266-284) — —Flue Gas Temperature

Leaving Economizer, C (F) — — 149 (300) 155 (311)Coal Flow Rate, kg/hr (klb/hr) 25,760 (56.7) 21,760 (47.9) 5400 (11.9) 5400 (11.9)Furnace Bed Temperature, C (F) 860 (1580) 865-880 (1589-1616) 865 (1589) 870 (1598)Upper Furnace Temperature, C (F) 878 (1612) 865-880 (1589-1616) 875 (1607) 880 (1616)Furnace Bottom ∆P, mmwc (in. wc) 610 (24.0) 600-680 (23.6-26.8) 610 (24.0) 610 (24.0)Furnace Upper ∆P, mmwc (in. wc) 340 (13.4) 300-380 (12.0-15.0) 254 (10.0) 260 (10.2)Boiler Efficiency (on Higher Heating

Value Basis), % 87.9 88.8 86.6 86.6Excess Air, % 20 16-20 20 19Ca/S Ratio — — 2.3 2.3Performance Coal AnalysisProximate Analysis, % by wtAsh 45.0 37.40 8.50 12.23Moisture 10.0 9.40 11.30 7.45Sulfur 0.4 0.22 3.10 2.71Volatile Matter 18.0 25.70 34.00 33.64Fixed Carbon 24.0 27.28 46.20 46.68Ultimate Analysis, % by wtCarbon 32.00 40.00 65.13 64.75Hydrogen 2.10 3.20 4.50 4.52Oxygen 9.82 8.83 5.96 7.06Sulfur 0.40 0.22 3.10 2.71Nitrogen 0.68 0.91 1.51 1.28Moisture 10.00 9.40 11.30 7.45Ash 45.00 37.40 8.50 12.23Higher Heating Value, kCal/kg (Btu/lb) 3500 (6300) 3910 (7038) 6492 (11,686) 6505 (11,709)Coal Size, mm (in.) 6.4 x 0 (1/4 x 0) 6.4 x 0 (1/4 x 0) 12.7 x 0 (1/2 x 0) 20 x 0 (3/4 x 0)Mid Size (d50), mm (in.) 0.75 (0.03) 1.2 (0.05) 3 (1/8) 9 (3/8)Limestone Size, micron (mesh) — — 1180 x 0 (16-) 1180 x 0 (16-)EmissionsNOx, ppm (lb/106 Btu) 100 (0.16) <75 (<0.12) <170 (<0.25) 90-100 (0.13-0.15)SO2 w/o Limestone, mg/Nm3 (lb/106 Btu) <1600 (<1.27) <800 (<0.63) — —SO2, % removal — — 90 90CO, ppm (lb/106 Btu) — — 200 (0.18) 150-200 (0.14-0.18)

100

95

90

85

80

11.010.3

01997

(July 15-Dec. 31)1998

Boi

ler A

vaila

bilit

y, %

Forced Outage

Commissioning Outage

Planned Outage

Boiler Available

89.089.7

18.4

81.1

0.5*

1999(Jan.-July)

* not boiler-related reason

Figure 7 SIU boiler availability.

100

95

90

85

80

8.2 7.5

01997 1998

Boi

ler A

vaila

bilit

y, %

Forced Outage

Planned Outage

Boiler Available

85.0

88.3

7.7

90.6

1999(Jan.-July)

6.8

4.2 1.7

Figure 8 KCIL boiler availability.

6 Babcock & Wilcox

Table 3Ebensburg Operating Data

Operating Steam Flow, t/hr (klb/hr) 234 (516)

Steam Flow @ MCR, t/hr (klb/hr) 211 (465)

Steam Temperature, C (F) 512 (953)

Steam Pressure, MPa (psig) 10.6 (1540)

SH Steam Temperature Control Range, % 30-110

Load Turndown Ratio Without Auxiliary Fuel 5:1

EmissionsNOx, ppm (lb/106 Btu) <100 (<0.14)SO2, ppm (lb/106 Btu) <300 (<0.60)CO, ppm (lb/106 Btu) <230 (<0.25)

Ca/S Molar Ratio 2.1-2.4

cleaning can be provided by “medium-size” particles flowingthrough the convection pass.

Boiler arrangement for firing oil shale is shown in Figure11. Only in-furnace U-beams are utilized as a primary separa-tor allowing coarser solids to the convection pass. No externalU-beams or recycle are needed. The MDC is located downstreamof the economizer thus providing self-cleaning of the convec-tion pass heating surfaces by “medium-size” particles. Similarto that of the primary separator, MDC collection efficiency andrecycle can be substantially reduced.

ConclusionA two-stage solids separation system with controllable sec-

ondary recycle provides efficient boiler operation with preciseprocess control. Coupled with internal recycle of the bulk ofcirculating solids and a wide range of furnace velocity, it al-lows a flexible, compact, cost effective and high performanceCFB boiler design suitable for multiple fuels in retrofit andgreenfield applications.

Figure 9 Ebensburg boiler availability.

Figure 10 125 MWe IR-CFB boiler with reheat.

Bo

iler

Ava

ilab

ility

, P

erc

en

t100

95

90

85

80

0

Forced Outage Planned Outage Boiler Availability

1991 1992 1993 1994 1995 1996 1997 1998 1999(May-Dec.) (Jan.-July)

1.9

8.4

89.7

1.2

9.4

89.4

3.9

4.5

91.6

5.3

5.6

89.1

2.6

6.6

90.8

2.2

2.8

95.0

1.4

5.6

93.0

1.8

4.2

94.0

1.5

2.9

95.6

Babcock & Wilcox 7

Copyright © 1999 by The Babcock & Wilcox Company,a McDermott company.

All rights reserved.

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Disclaimer

Although the information presented in this work is believed to be reliable, this work is published with the understanding that The Babcock & WilcoxCompany and the authors are supplying general information and are not attempting to render or provide engineering or professional services.Neither The Babcock & Wilcox Company nor any of its employees make any warranty, guarantee, or representation, whether expressed or implied,with respect to the accuracy, completeness or usefulness of any information, product, process or apparatus discussed in this work; and neither TheBabcock & Wilcox Company nor any of its employees shall be liable for any losses or damages with respect to or resulting from the use of, or theinability to use, any information, product, process or apparatus discussed in this work.

Figure 11 IR-CFB boiler for oil shale firing.