Design Considerations of B&W Internal Circulation CFB Boilers S. Kavidass Babcock & Wilcox Barberton, Ohio, U.S.A. K.C. Alexander Babcock & Wilcox Barberton, Ohio, U.S.A. Presented to: Power-Gen Americas ‘95 December 5-7,1995 Anaheim, California, U.S.A. BR-1602 Abstract Introduction Worldwide, the useof Circulating Fluidized-Bed (CFB) boiler technology is rapidly increasing due to the ability to burn low grade fuels while meeting the required NOX, SO,, CO, VOC, and particulate emissions requirements. The CFB boiler can produce steam economically for pro- cessand electric power generation. B&W has more than 35 years in fluidized-bed boiler technology and has developed four major product lines: atmospheric bubbling fluidized beds, atmospheric circu- lating fluidized beds,fast internal circulating bedsthrough a technical agreement with Austrian Energy & Environ- ment, and pressurized bubbling fluidized beds. This paper discusses various aspects of Babcock & Wilcox (B&W) internal recirculation circulating fluidized- bed (IR-CFB) boiler design including fuel, boiler process parameters,and emissions. The B&W CFB boiler is unique in design. It utilizes proven impact-type particle separa- tors (U-beams) with in-furnace solids recirculation. The paper describes the methodology for setting up process parameters, heat duty and boiler design, including auxil- iary equipment selection and advantages. To date, B&W and its joint venture and licensee com- panies have sold more than 36 fluidized-bed projects worldwide of which 10 are atmospheric circulating fluid- ized beds (see Table 1). In 1984, B&W licensed CFB technology from Studsvik A.B. of Sweden. B&W developed a boiler design based on Studsvik’s technology, then improved the original de- sign concept. B&W now has worldwide ownership of this technology, including all patents. Table 1 Circulating Fluidized-Bed Boiler Experience (B&W, AE&E, B&W JV) Customer Plant Location Ultrapower West Enfield, Maine, USA Ultrapower Jonesboro, Maine, USA Sithe Energy Marysville, California, USA Lauhoff Grain Co. Danville, Illinois, USA Ebensburg Power Co. Ebensburg, Pennsylvania, USA Pusan Dyeing Co. Pusan, Republic of Korea Thai Petrochemical Ind. Rayong, Thailand Kanoria Chemicals & Industries Ltd. Renukooot, India Southern Illinois University Carbondale, Illinois, USA Los Angeles County Sanitation District (3 boilers) Carson, California, USA Capacity lblhr (t/hr) 220,000 (100) 220,000 (100) 164,000 (74.3) 225,800 (102.4) 465,000 (211) 176,370 (80) 286,600 (130) Fuel Wood Wastes &Wood Chips Wood Wastes &Wood Chips Wood Wastes Bituminous Coal High Ash Waste Coal Coal & Heavy 011 Coal, Lignite, 011 & Gas Start-Up Date 1986 1986 1986 1989 1990 1991 1994 231,480 (105) High Ash Coal 1995 120,000 (54.4) Coal, Petroleum Coke & Natural Gas 1996 48,000 (21.8) Sewage Sludge -
Transcript
Design Considerations of B&W Internal Circulation CFB Boilers
S. Kavidass Babcock & Wilcox
Barberton, Ohio, U.S.A.
K.C. Alexander Babcock & Wilcox
Barberton, Ohio, U.S.A.
Presented to: Power-Gen Americas ‘95 December 5-7,1995 Anaheim, California, U.S.A.
BR-1602
Abstract Introduction Worldwide, the use of Circulating Fluidized-Bed (CFB)
boiler technology is rapidly increasing due to the ability to burn low grade fuels while meeting the required NOX, SO,, CO, VOC, and particulate emissions requirements. The CFB boiler can produce steam economically for pro- cess and electric power generation.
B&W has more than 35 years in fluidized-bed boiler technology and has developed four major product lines: atmospheric bubbling fluidized beds, atmospheric circu- lating fluidized beds, fast internal circulating beds through a technical agreement with Austrian Energy & Environ- ment, and pressurized bubbling fluidized beds.
This paper discusses various aspects of Babcock & Wilcox (B&W) internal recirculation circulating fluidized- bed (IR-CFB) boiler design including fuel, boiler process parameters, and emissions. The B&W CFB boiler is unique in design. It utilizes proven impact-type particle separa- tors (U-beams) with in-furnace solids recirculation. The paper describes the methodology for setting up process parameters, heat duty and boiler design, including auxil- iary equipment selection and advantages.
To date, B&W and its joint venture and licensee com- panies have sold more than 36 fluidized-bed projects worldwide of which 10 are atmospheric circulating fluid- ized beds (see Table 1).
In 1984, B&W licensed CFB technology from Studsvik A.B. of Sweden. B&W developed a boiler design based on Studsvik’s technology, then improved the original de- sign concept. B&W now has worldwide ownership of this technology, including all patents.
Kanoria Chemicals & Industries Ltd. Renukooot, India
Southern Illinois University Carbondale, Illinois, USA
Los Angeles County Sanitation District (3 boilers) Carson, California, USA
Capacity lblhr (t/hr)
220,000 (100)
220,000 (100)
164,000 (74.3)
225,800 (102.4)
465,000 (211)
176,370 (80)
286,600 (130)
Fuel
Wood Wastes &Wood Chips
Wood Wastes &Wood Chips
Wood Wastes
Bituminous Coal
High Ash Waste Coal
Coal & Heavy 011
Coal, Lignite, 011 & Gas
Start-Up Date
1986
1986
1986
1989
1990
1991
1994
231,480 (105) High Ash Coal 1995
120,000 (54.4) Coal, Petroleum Coke & Natural Gas 1996
48,000 (21.8) Sewage Sludge -
B&W has built several CFB pilot units at its Alliance Research Center to investigate the process and character- ize performance of various fuels and sorbents. This in- cludes a 0.7 x 0.7 x 7 m cold CFB model, 0.23 x 0.23 x 10 m (0.28 MW,) CFB combustor and a 0.7 x 0.7 x 23 m (2.5 MW,) CFB combustor. All of these facilities are used to evaluate innovative concepts for components or process changes. A unique 2.5 MWt CFB facility provides results representative of commercial-size unit performance.
B&W has progressed through three CFB boiler design generations in the commercial market (Figure 1). In the first generation design, the U-beam separators are located outside the furnace, and all the separated solids recircu- late via L-valves to the lower furnace. In the second gen-
Note: Illustrated values are based on 100 units of solids exiting the furnace shaft.
Figure 1 B&W CFB solids circulation schematics.
eration design, patented in-furnace U-beams were added. These in-furnace U-beams separate about 75% of the sol- ids for circulation within the furnace. The remaining sol- ids are separated by the external U-beams and recirculate through L-valves to the lower furnace.
B&W now offers a third generation IR-CFB boiler de- sign based on operating experience from the two coal- fired second generation CFB boilers and results from the 2.5 MW, CFB test facility. In this design, all solids col- lected by U-beams are internally recirculated by way of the upper furnace. B&W is the pioneer and leader in de- veloping the internal circulation CFB boiler. Also, design improvements have been made in several areas for higher availability and lower maintenance. Currently two of these all internal recirculation (IR-CFB) boilers have been sold and units are being proposed up to 150 MWe while pursu- ing units 200 MW, reheat and above. This paper focus on non-reheat IR-CFB boiler design considerations.
Economic Advantages of a CFB Boiler The primary objective of selecting a CFB boiler is to
reduce capital and operating costs. CFB boilers provide the economic viability for burning low grade fuels with superior environmental performance. The economic ad- vantages of a CFB boiler are mainly due to the following:
l Accepts low quality, less costly fuels. l Offers greater fuel flexibility (within the specified range)
as compared to pulverized coal (PC)-fired boilers. l Reduces the fuel crushing (coarser feed size) cost. l Lower capital cost (no expensive pollution control
equipment) and lower operating cost.
B&W IR-CFB Boiler In the B&W internal recirculation circulating fluidized-
bed boiler, a portion of combustion air (say 55%-70% - fuel based) is introduced through the bottom of the bed. The bed material consists of char, lime, spent sorbent, sand (no limestone is used), and ash. The bottom of the bed is supported by water-cooled membrane walls with air nozzle for air distribution. The fuel and limestone are fed into the lower bed. In the presence of fluidizing air, the fuel and limestone quickly and uniformly mix under the tur- bulent environment and behave like a fluid. Carbon par- ticles in the fuel are exposed to the combustion air at flu- idized-bed temperatures of 843 to 899C (1550-1650F).
The balance of combustion air is admitted as overfire air at two levels at the top of lower furnace, both furnace front and rear walls, through special nozzles for staged combustion. This staged combustion limits the formation of nitrogen oxides (NOJ. The fluidizing air velocity is greater than the terminal velocity of most of the particles in the bed, thus the fluidizing air entrains the solid par- ticles and carries them through the combustor shaft. The solids distribution in a CFB furnace is such that a high density region exists in the bottom of the furnace (dense bed) and a lower density region exists in the shaft of the furnace (dilute bed). The transition between these two regions is gradual. The entrained solids and gas mixture enters the two rows of in-furnace U-beam separators where about 75% of solids, including unburned carbon and unutilized calcium oxide (CaO), are separated and returned to the furnace internally. The remaining 25% of the solids
2 Babcock & Wilcox
is separated by the external four rows of U-beams and transferred to the furnace just below the in-furnace U- beams. All collected solids return to the lower furnace, falling as a curtain along the rear wall.
The lines collected by the secondary separator (multi- cyclone dust collector, MDC) are also recirculated to the lower furnace to minimize carbon loss and increase the limestone utilization. Solids refluxing inside the furnace provide intensive solids mixing and longer particle resi- dence time to enhance fuel burnout and improve sorbent utilization. It may be necessary to provide supplemental inert solid bed material such as sand to maintain furnace inventory when a fuel with low ash and low sulfur content is used. Bed drain solids are cooled with or without re- covery of the solids’ sensible heat. The fines classified by the fluidized-bed cooler, if used, are reinjected to the fur- nace.
B&W IR-CFB boilers operate at relatively high solids densities in the upper furnace compared with some other CFB units. This provides a high rate of gas-solids reac- tion for combustion and good sulfur capture, and enhances heat transfer between the bed and the furnace walls.
cluding the methodology for determining heating surfaces:
Data Required for Boiler Design Fuel: Fuel type and range of fuel properties, proximate
and ultimate analysis, HHV or LHV, fuel ash chemical analysis to determine fouling and agglomeration and at- trition characteristics.
For significantly new fuels, bench scale combustion tests are recommended to determine fuel characteristics.
Limestone: Chemical analysis, reactivity and attrition characteristics.
Sand: Chemical composition, shape factor and hardness. Steam: Steam flow, superheater (SH) outlet tempera-
ture and pressure, feed water temperature at MCR and peak load conditions. Additional data on desuperheating spray water temperature, superheater steam temperature control range, and boiler turndown.
Auxiliary Fuel: Fuel analysis for start-up and load carrying.
Site Data: Site and ambient conditions. Emission: Emission requirements such as NOX, SO,,
CO, VOC and particulate.
Heat and Material Balance Including Boiler Efficiency
B&W has established CFB functional methods, based on pilot facility test data for various fuels and commer- cial operating boilers data base, to select the process pa- rameters. The fuel (ash, moisture content, fixed carbon (FC), volatile matter (VM), total sulfur), limestone, and emissions data are carefully analyzed to run a computer- ized heat and material balance and fuel efficiency calcu- lation program.
The key process parameters used to establish the heat and material balance are steam conditions, furnace tem- perature, calcium to sulfur molar ratio (Ca/S ratio) for the specified sulfur capture efficiency, combustion efficiency,
excess air, bottom/fly ash split for ash, inert/sorbent, MDC or electrostatic precipitator (ESP) solids recycle rate, U- beams and MDC solid collection efficiencies, primary (PA) to secondary air (SA) ratio, and flue gas temperature entering and leaving the air heater. A special feature of B&W’s CFB heat and material balance is that it incorpo- rates two step solids separation and recirculation. Also, fly ash may leave the boiler as a MDC or ESP first-pass (in the case of ash recycle from ESP first-pass) purge in addition to the ash disposal to the baghouse or ESP.
The fuel efficiency is predicted based on the perfor- mance test code PTC 4.1 heat loss method and the Ameri- can Boiler Manufacturers’ Association recommendation for sorbent sulfation and calcination and radiation loss. Bed drain cooler solids outlet heat loss is accounted for in the efficiency calculation. B&W IR-CFB boiler radia- tion loss is close to that of a conventional PC-fired boiler. Some of the typical output data established from the heat and material balance program are fuel flow, sorbent flow, sand flow, primary and secondary air flow, flue gas flows, boiler efficiency, heat input and output, bed drain solids rate, convective solids loading, MDC solids output, and flue gas and particle stream compositions.
Furnace Design Procedure The success of any CFB boiler design and operation
are vested on the furnace design. The important aspects of the furnace design are the furnace temperature, solids inventory and distribution, limestone and fuel particle size, gas residence time, furnace depth and furnace heating surface.
The output data of the heat and material balance pro- gram are used as input to the furnace design program. Also, some additional process parameters are required to run the computer program: furnace height with effective heat- ing surface in various zones, including division (evapora- tive surface) walls and wing (superheater surface) walls if applicable, heat release rate in various zones, lower fur- nace velocity 3.7 to 4.3 m/s (12-14 ft/s) and upper fur- nace superficial velocity 5.5 to 6.7 m/s (18-22 ft/s), fur- nace depth 3.7 to 4.6 m (12-15 ft), particle density and size for circulating solids, refractory height/thickness/ conductivity, solids mass flux at the furnace outlet, U- beam cavity heating surface, etc.
Predicted furnace performance includes the furnace vertical temperature profile, heat transfer rates to the wa- ter wall at different zones and cavities, solids bulk densi- ties and furnace pressure drop profile, and furnace exit gas/solids temperature.
In setting furnace surface, the furnace plan area is set by velocity and furnace height is set by residence time, with internal surface added as required to control bed tem- perature and satisfy heat absorption requirements.
The furnace temperature is one of the key parameters in the CFB boiler design as it influences sorbent utiliza- tion to meet required SO, emissions, NOX emissions, com- bustion efficiency, and heat transferto the furnace walls. The furnace temperature is selected based on fuel proper- ties (including fixed carbon to volatile matter ratio and ash composition) and emission control consideration.
B&W has spent considerable effort establishing the heat transfer correlations and furnace design procedures for
Babcock & Wilcox 3
CFB boilers. The furnace is typically divided, for heat transfer calculations, into as many as 8 to 10 zones de- pending upon the height of the furnace. A typical overall heat transfer coefficient (U,) of the furnace is in the range of 153.3 to 199 W/m*K (27-35 Btu/hr ft* F).
The furnace evaporative heat duty and heat transfer is predicted using correlations that take into consideration radiation and convection (gas-wall and particle-wall) heat transfer for dense and dilute beds.
The B&W CFB boiler design featuring MDC ash re- cycle to the furnace utilizes a high solids recirculation rate, thus establishing relatively high furnace inventory. IR-CFB upper furnace inventory is around 0.04 to 0.05 kPa (0.16 to 0.20 in. wg) per 0.305 m (foot) of furnace height. Typical temperature and density profiles are shown in Figures 2 & 3. A careful evaluation is done to select the optimum furnace with consideration to the perfor- mance requirement, capital cost and auxiliary power con- sumption.
Convective Heating Surface Design The output data of furnace performance, and heat and
material balance programs are transferred to the convec- tive heating surface program to design the superheater, economizer and air heater. Other input data are typical for the convection surface design but have specifics for CFB conditions.
The B&W IR-CFB boiler uses a pendant superheater design arrangement with the superheater located after the U-beam separator as shown in Figure 4. The flue gas ve- locity through the superheater is as low as 7.6 to 8.5 m/s (usually 25 to 28 ftis) and is very uniform across the su- perheater. As a result, the potential for erosion is greatly reduced.
The economizer is designed for flue gas velocity of 7.6 to 10.7 m/s (25 to 35 ft/s), depending on the fuel and ash characteristics. The economizer is located in the second pass of the boiler. The economizer enclosure is made up of externally insulated 6.3 mm (0.25 in.) carbon steel plate.
100 -4 3OF k-
90
80
70 .c
6 60 al
‘, 50 8 5 40
IL
30
1450 1500 1550 1600 1650
Furnace Temperature, F
Figure 2 IR-CFB furnace predicted temperature profile. Figure 4 Third generation CFB boiler.
4 Babcock &Wilcox
:irculating Gidized~
Bed Furnace
Frel
- - - - - - - - -
Disen- I 9;$l~eLl i I Tra
Level of Secondary Air Injection I I
- - - - - - OO
I I \. J
A;r hndbox Bulk Density, lb/ft3 (kg/d)
Figure 3 Typical atmospheric pressure circulating-bed furnace density profile.
The economizer tube spacing and in-line arrangement minimize tube erosion and fouling potential. The econo- mizer flue gas outlet temperature is selected considering the feedwater temperature plus 42 to 56C (75 to 100F) for the optimum heat absorption split between the economizer and air heater. The economizer feed water outlet tempera- ture is normally limited to 42 C (75F) less than saturation temperature.
The tubular air heater is designed to recover the re- maining heat in the flue gas to meet the boiler efficiency requirement. B&W current design practice is to design the air heater with flue gas outside the tubes. The tubes’ spacing and arrangement are in-line, as in the economizer, to minimize fouling potential and erosion. The air heater
is typically located after the MDC. Typical flue gas ve- locities used to design an air heater are 9 to 12 m/s (30- 40 ft/s). Air temperature entering the air heater is selected to prevent cold-end tube corrosion and is dependent on fuel properties and flue gas end temperature. Dewpoint corrosion potential from SO, is greatly reduced with in- furnace sulfur capture.
The typical values of combustion efficiency, excess air and furnace operating temperatures for an IR-CFB are given in Table 2.
Boiler Heat Duty Distribution The boiler heat duty distribution is optimized by using
the outputs from the furnace and convection pass calcula- tions. The following heat balance equations must be sat- isfied:
Heat Input - Heat Losses = Heat Output
Heat Output = Qfurn + QsH + QECo + QAH
Q qevap furn = + %H-INFURN
4 evap = UT x sc x (Tf”r”-Tsat)
q SH = QSH - %H-INFURN
%H-INFURN = MSF ’ [hSH-INF”RO’T - hSAl’]
%H = MSF ’ thSHOT - hSH-INF”ROT)
Q EC0 = MFV,’ ’ (hFV40 - hFWI)
Qm = Mars x chFGE - hFGL)
A typical IR-CFB boiler heat duty distribution is given in Figure 5. The final superheat temperature and the con- trol range are set by the customer. Economic consider- ations are used to determine the superheater split and lo- cation for primary and secondary superheater surfaces, in- furnace and convection surfaces and the amount of attemperator spray. Units producing saturated steam or with low superheater temperature may require a boiler bank to meet the additional evaporative heat duty. (See Figure 6).
The economizer and air heater heat duty split requires a careful evaluation. The economizer heat transfer coeffi- cient of 56.8 to 65.1 W/m’K (lo-12 Btu/hr ft* F) is 2.5 to 3.0 times higher than the air heater heat transfer coeffi-
i+Subcooled--Evaporation-Superheat+
I I I I I I I I I I I 0 10 20 30 40 50 60 70 80 90 100
Actual Distribution, %
Figure 5 Typical IR-CFB boiler heat duty distribution (water and steam).
cient. To take advantage of this fact, it is desirable to maxi- mize economizer heat absorption, except for cases when the air preheat must be maximized for combustion of high moisture fuels.
Based on the computer performance analysis and opti- mized heat duty distribution, the boiler configuration is established. A typical B&W design configuration is shown in Figure 4. The furnace depth selection dictates the U- beam length and furnace width decides the number of U- beams. The superheaters, both primary and secondary, are located after the U-beams. The MDC is behind the econo- mizer for secondary particle separation. The objective of placing the MDC after the economizer is to reduce the capital cost.
IR-CFB Boiler Design Description B&W’s IR-CFB boiler design has incorporated the fol-
lowing advanced features based on operating experience with coal and wood-fired CFB boilers:
l Improved fuel feed system l Simplified MDC ash recycle system l All pendant superheater l Full internal solids recirculation system from the pri-
mary separator l Reduced number of U-beam rows l Furnace inventory control through adjustable MDC
recycle rate
Table 2 IR-CFB Process Parameters
Fuel Type
Waste Wood
Lignite
Sub-Bituminous Coal
Bituminous Coal - High Volatile
Bituminous Coal - Low Volatile
Bituminous Waste - High Ash Coal (Low to Medium Volatile)
Delayed Petroleum Coke
Anthracite (6-8% Volatile Matter)
Anthracite (4-6% Volatile Matter)
Combustion Efficiency Excess Air
>99.5% 20%
99.0-99.5% 15%
98.5-99.0% 20%
98.0-99.0% 20%
97.5-98.0% 20%
96.5-97.0% 20%
98.0-99.0% 20%
96.5-97.5% 25%
96.0-96.5% 25%
Furnace Temperature
843C 155OF
843C 1550F
857C 1575F
857C 1575F
871C 16OOF
871C. 16OOF
885C 16OOF
8850 1625F
885C 1625F
Babcock & Wilcox 5
----I- Superheater
7efractory II-IF I Economizer
Figure 6 Southern Illinois University CFB boiler.
The IR-CFB boiler design consists of the following major systems, shown in Figure 4. The main boiler de- sign components are:
l Boiler furnace l Primary air nozzles (bubble caps) l U-beam solids separators and recirculation system l Secondary solids separator and recirculation system l Bed drain solids coolers l Superheater and economizer l Tubular air heater l Fuel/limestone feed system
Boiler Furnace The furnace cross section dimensions are selected based
on flue gas superficial velocity. Currently, B&W uses 3.7 and 4.6 m (12 and 15 ft) deep furnace. The furnace enclo- sure is made of gas-tight membraned water cooled walls 76 mm tube diameter on 102 mm centers (3 in. on 4 in. centers). The bottom, or primary zone, furnace cross sec- tion is reduced to provide good solids mixing and pro- mote solids entrainment at low load. The auxiliary start- up burners and flyash reinjection points from the second- ary separator are also located in this region.
In addition to the enclosure walls, internal heat trans- fer surfaces such as division and wing walls are used to
achieve the desired furnace temperature. The division walls 76 mm tube diameter on 102 mm centers (3 in. on 4 in. centers) span about 60% of the furnace depth and full furnace height. The wing walls 51 mm tube diameters on 63.5 mm centers (2 in. on 2.5 in. centers) are located in the upper furnace and extend down from the furnace roof near the front wall (See Figure 7). Length and quantity of steam-cooled wing walls are varied depending on final steam conditions.
Steam Cooled Wing Wall
r 2 Ii, in 3 ‘I4 in.
1
Pin Studs (Membrane)
Division Wall Cross Section at Refractory Zone
Figure 7 IR-CFB wing wail.
6 Babcock & Wilcox
A thin layer of refractory is applied to all lower fur- nace wall surfaces (including division walls) to protect against corrosion and erosion. B&W normally uses 16 mm (0.625 in.) thick refractory over a dense pin studded pat- tern. Refractory thickness selection sometimes may vary depending upon the fuel properties. An ultra high strength, abrasion-resistant low cement alumina refractory is used for the lower furnace. Refractory is also installed on the furnace roof tubes and each wing wall nose.
B&W uses carefully designed overfire air nozzles to admit secondary air to complete the combustion through good flue gas/air mixing. Good mixing is achieved by using variable diameter high velocity nozzles in the front and rear furnace walls.
Fuel/Limestone Feed System Fuel handling (crushing, storing and feeding into the
furnace) is one of the major challenges in the CFB boiler operation, especially with “opportunity” fuels. The feed system should be designed to ensure reliable fuel feed over the CFB boiler lifetime with low maintenance. Fuel char- acteristics, including the flow properties, abrasiveness, moisture content, fuel size, etc., have to be investigated to select the proper feed system. Fuel is fed to the boiler front wall through a series of gravity feed chutes (see Figure 8). The fuel chute with stand pipe should have at least a 60 to 65 degree angle from horizontal. This system
Lower Furnace Frontwall
also provides the seal between furnace operating pressure and atmosphere. Primary air is used to sweep the fuel into the furnace and as seal air to the feeder.
The number of feed points is set to achieve fuel burn- out in the furnace. Lower volatile fuels require fewer feed points. To reduce the number of coal feeders, bidirectional screws or a pair of drag chain conveyors can be used to connect the discharge of each fuel feeder to the individual fuel chutes.
Limestone crushing, storing and feeding are relatively easy compared to the fuel when limestone moisture con- tent is kept within an acceptable range. The limestone size distribution is important for sulfur capture, limestone uti- lization and inventory control. Limestone is fed either pneumatically or mechanically into the CFB boiler. In a mechanical system, the limestone is fed into the discharge end of the fuel feeder via rotary seal feeders. The lime- stone falls by gravity down the fuel feed chute with fuel into the furnace. The pneumatic system feeds the lime- stone directly in the furnace through furnace openings in the front and rear walls. The location and number of feed points is dependent on fuel properties and sulfur capture requirements.
Primary Air Nozzles The windbox or air plenum is made completely of wa-
ter-cooled panels except at the rear wall. Bubble caps are fitted on the distributor floor panel as shown in Figure 9. The B&W bubble cap pressure drop at full load is about 4 kPa (16 in. wg). The bubble caps are designed to dis- tribute the air uniformly, prevent the back sifting of sol-
ids at low load operation, and create good turbulence and fuel/sorbent mixing in the primary zone. The bubble cap material is SS 304 and the nominal spacing between the caps is 102 mm x 102 mm (4 in. x 4 in.).
U-Beam Solids Separators The solids separation system is a key element to any
CFB boiler design, influencing both capital and operat- ing costs. B&W has invested heavily in research and de- velopment to produce viable and economical U-beam separators as well as a functional mechanical collector. The U-beam separator is shown in Figure 10. The boiler has two stages of primary solids separators: in-furnace U-beam separators and external U-beam separators.
The in-furnace U-beams (two rows) are able to collect more than 75% of the solids entering the primary separa- tor. The flue gas velocity across the U-beams is approxi- mately 8 m/s (26 ft/s), providing high collection efficiency and limiting the gas-side pressure drop to 0.25 kPa (usu- ally cl.0 in. wg) as compared with cyclone-type separa- tors’ pressure drop of 1.25 to 1.5 kPa (5 to 6 in. wg). The material used for U-beams is either TP 309 H/TP 310 H or TP 253 MA depending upon the furnace temperature.
Four rows of U-beams are installed externally to the furnace and most of the remaining solids are collected by these U-beams. A particle transfer hopper is located at the bottom of the U-beams. The separated solids are dis- charged from this hopper directly into the furnace. An important technical feature associated with internal sol- ids recycle system design is providing a pressure seal be- tween the furnace and solids transfer hopper. To confirm validity of the internal recycle design, the concept was extensively tested at B&W’s Alliance Research Center. The test results from the 2.5 MW, test facility are given in Table 3.
To prevent flue gas flowing from the furnace into the transfer hopper, the discharge opening and hopper internals are designed to operate with a column of falling solids
1. Sidewall Membrane Panel 2. U-Beam 3. Seal Baffle
Figure 10a U-Beam separators - plan view.
8
which forms a pressure seal. Providing the pressure seal in this design is an easier task as compared with the sol- ids return to the lower furnace for two reasons. First, the pressure differential is much smaller at 0.5 kPa (typically < 2 in. wg), versus about 8 kPa (32 in. wg) at the lower furnace. Second, the pressure differential quickly de- creases with load reduction while in the lower furnace the pressure remains essentially the same.
Secondary Solids Separation and Recirculation System
The multicyclone (secondary dust collector) is located at the bottom of the convection pass after the economizer and has a top inlet and side outlet. The multicyclone inlet velocity is chosen for 21.3 m/s (70 ft/s). The tube diam- eter is normally 229 mm (9 in.) arranged over the second pass entire cross-section. The MDC provides outstanding retainment of fine particles (size less than 60 pm) as com- pared to large-diameter “hot” cyclones used in cyclone- type CFBs. MDC collector tubes and spin vanes have high hardness (up to 550 BHN), designed for several years of useful life. The MDC tubes can be easily replaced during a planned boiler outage.
The collected fines are stored in the MDC hopper. Vari- able speed rotary feeders are used to control the ash re- cycle flow from the hopper to the furnace. The ash is dropped onto an air-assisted conveyor which transports the solids for reinjection. By adjusting the rotary feeder speed the amount of solids transferred to the furnace can be changed for furnace temperature control.
In some developing countries, MDC cost and availability are a major concern. To meet the CFB boiler requirements for these countries, electrostatic precipitator (ESP) first pass ash recycle is envisaged (see Figure 11). The ESP first-pass ash is relatively coarse and average particle size is around 70 pm. Collected ash is stored in a separate ash hopper, with controlled recycle in the furnace.
Bed Drain Solids Coolers The purpose of draining the bed material from the fur-
nace is to control the bed solids inventory and remove oversized material accumulated during operation. 203 mm (8 in.) diameter bed drain pipes are used to drain the ma- terial. The number of bed drain points is selected based on the furnace plan area and the fuel. The drained mate- rial is at bed temperature and carries a considerable amount of sensible heat. The material is cooled to an acceptable temperature before disposal into the ash system.
Water-cooled screws or fluidized-bed ash coolers can be used for the bed drain cooling. The type of ash cooler depends on fuel properties, plant economics, heat utiliza- tion, and the need for bed material classification for rein- jection of fines particles.
Superheater and Economizer Superheater heating surfaces may consist of surface
(primary and secondary) in the convection pass as well as
Figure 11 Kanoria CFB boiler.
superheater wing walls located in the furnace. The wing wall surface may be located before the primary superheater or between the primary and secondary superheaters. An attemperator is used to control the final steam tempera- ture over the design load range taking into account poten- tial upsets in the furnace heat absorption.
The economizer is designed with tubes running front to back. The top row of economizer tubes is protected with full-length tube shields. No sootblowers are used for the pendant superheater or economizer.
Air Heater The air heater design requires special attention to high
solids loading, and ash erosivity. For the air heater lo- cated after the MDC, 12 gauge (0.109, in. thick) tubes are used. In the case of ash recycle from the ESP first-pass, a tube thickness of 11 gauge (0.120 in.) is recommended. A hopper may be provided at the bottom of the air heater. When the air heater is installed after the MDC, the ash collected in the hopper is purged to the ash disposal sys- tem. With ash recycle from the ESP first-pass, the ash collected in the hopper is recycled to the furnace. A steam coil air heater (SCAH) is used to protect the cold end of the air heater. If the air inlet temperature is greater than 66 C (150 F), a SCAH is not used.
Advantages of the B&W IR-CFB Boiler l The boiler is compact with primary U-beam separa-
tors and internal solids recycle.
Babcock & Wilcox 9
l The boiler has a smaller footprint (typically 20-25% less building volume compared to a cyclone-based CFB boiler).
l Boiler design is especially suitable for repowering of PC-fired boilers where space is limited.
l Two stage solids separation provides for high carbon burn-up efficiencies, better limestone utilization and higher solids residence time.
l Dynamic load change response is achieved due to the absence of massive refractory and the ability to con- trol furnace inventory using variable ash recycle from the MDC.
Control of NO, Emissions NOX present in flue gas generally comes from two
sources: the oxidation of nitrogen compounds in the fuel (fuel NOX), and the reaction between the nitrogen and oxygen in the combustion air (thermal NO,).
l Wide turndown ratio (5:l) without auxiliary fuel is possible due to the selection of furnace velocity and controllable solids recycle.
l Less refractory in the boiler provides for quick start- up (< 6 hours) and less maintenance (hot cyclone has 4 times the amount of refractory).
l Higher boiler reliability and lower maintenance are achieved.
As a result of the low temperatures at which a fluid- ized bed operates, thermal NO, makes a minor contribu- tion to overall emissions. Circulating fluidized-bed boil- ers are also designed to suppress the amount of the fuel NOX formed. This is accomplished by supplying less than the theoretical amount of combustion air as primary air. As a result of this staged air admission process, some of the fuel nitrogen compounds released in the lower fur- nace decompose into molecular nitrogen rather than form- ing fuel NOX.
CFB Boiler Emissions Control
The degree of air staging (or primary-to-secondary air ratio) is a function of the fuel. It is established when the boiler is designed to provide for both good combustion efficiency and low NO, formation. In practice, the pri- mary-to-secondary air ratio may be adjusted, to a limited degree, to arrive at the best combination of fuel burnout and NOX emissions.
Environmental regulations impose limits on emissions from boilers and combustion processes. The emissions lim- its vary, but the pollutants controlled are generally the same. These are sulfur dioxide (SO,), nitrogen oxide (NO,), car- bon monoxide (CO), hydrocarbons and particulate mat- ter. Circulating fluidized-bed boilers are designed to burn solid fuels while controlling many of these emissions.
Control of CO and Hydrocarbons Emissions
Control of SO, Emissions
A CFB boiler is designed to maximize combustion effi- ciency by minimizing unburned carbon, and the quantity of CO and hydrocarbons in the flue gas. This is done by choosing the bed temperature, primary-to-secondary air split, proper number of fuel feed points, proper design of the overfire air system, and providing sufficient furnace residence time for mixing and maximum fuel burnout.
When sulfur bearing fuels are burned, most of the sul- fur is oxidized to SO,, which becomes one of the con- stituents of the flue gas. When limestone is added to the bed in the temperature range of 8 16- 899C ( 1500- 1650F), it undergoes a transformation called calcination to form lime (CaO) by endothermic reaction:
Since some of these factors also influence SO, capture and NOX emissions, a compromise is usually sought dur- ing boiler design and tuning to achieve the optimal over- all performance.
CaCO, + + CaO + CO, - 766 Btu/lb of CaCO,
Once formed, solid CaO reacts with gaseous SO, and oxygen to form CaSO, according to the following exo- thermic reaction:
Typical NO, and CO values are shown in Table 4. Ad- ditional NO, reduction (40% to 60% of CFB process NO,) can be achieved by injecting ammonia (NH,) either in the upper furnace or after the U-beams. In both cases a suffi- cient residence time (not less than 0.5 set) shall be pro- vided for NO, reduction reactions before gases enter U- beams or the superheater surface, respectively.
SO, + ‘/* 0, + CaO + CaSO, (s) + 6733 Btu/lb of S
The resulting calcium sulfate is a chemically stable solid at fluidized-bed operating temperatures and is re- moved from the system for disposal.
Control of Particulate Emissions To meet the particulate emission requirements, a final
dust collector is required. Often there is a question whether
Sulfur dioxide reductions of 90% are typically achieved in a circulating fluidized-bed boiler with calcium to sul- fur (Ca/S) mole ratios of 2 to 2.5, depending on the sulfur content of the fuel and the reactivity of the limestone. In general, the lower the sulfur concentration in the fuel, the greater the calcium to sulfur mole ratio must be for a given SO, removal efficiency. For removal requirements greater than 90%, the amount of limestone needed must be in- creased. Limestone utilization is also dependent on the bed temperature, decreasing quickly when the tempera- ture is outside of a 8 16 to 899C (1500- 1600F) range.
The physical properties and reactivity of limestone vary significantly and it may be necessary to try several differ- ent limestones before the best one is found.
Table 4
Typical IR-CFB Emissions (Ib/MBtu)
Fuel NO, Bituminous Coal
Low Volatile 0.05 - 0.10
High Volatile 0.10 - 0.15
Lignite and Sub-Bituminous Coal 0.10 - 0.20
Waste Wood 0.15 - 0.25
Petroleum Coke 0.10 - 0.20
Anthracite 0.10 - 0.15
co
10.20
10.10
10.10
50.06
10.20
10.25
10 Babcock &Wilcox
an ESP or a baghouse should be used. B&W usually se- lects a baghouse when limestone is used for sulfur cap- ture. A baghouse is less sensitive to excursions in dust loading and ash content variation in the fuel. A baghouse has a high gas-side pressure drop of 1.5 to 2.0 kPa (6-8 in. wg) and requires occasional bag replacement, giving higher O&M costs than an ESP. An ESP can be selected as an alternate particulate control device where there is no limestone used for sulfur capture. The concern is that the presence of CaO and CaSO, affects the resistivity and may reduce the ionization potential. The ESP should have enough fields to accommodate excursions and fuel ash variation. ESP gas-side pressure drop is less than 0.5 kPa (2 in. wg).
IR-CFB Boiler Existing Contracts Two projects utilizing the B&W IR-CFB boiler design
are in progress:
Southern Illinois University IR-CFB Boiler Contract An IR-CFB boiler for Southern Illinois University
(Figure 6) will generate 12.8 kg/s of 4.5 MPa/399C (101,500 lb/hr of 675 psig/750F) steam. This boiler will utilize Illinois bituminous coal and, as an alternate fuel, petroleum coke. The unit is also capable of carrying 67% load while firing natural gas. Boiler erection is in progress and start-up of the unit is scheduled for June 1996.
Kanoria Chemicals IR-CFB Boiler Contract Another IR-CFB boiler design is being manufactured
by Thermax Babcock & Wilcox Ltd. (TBW), one of B&W’s joint venture companies, which is located in Pune, India. TBW is supplying this boiler for Kanoria Chemi- cals & Industries Ltd. in Renukoot, India. The boiler, shown in Figure 11, will generate 29.2 kg/s (231,480 lb/ hr) of steam with parameters of 6.3 MPa and 485 C (938 psig and 905 F). The fuel is high-ash low-calorific value subbituminous coal. Boiler erection is nearing comple- tion and start-up of the unit is scheduled for December 1995.
Conclusion B&W has established CFB boiler design guidelines
based on 35 years of fluidized-bed experience plus the knowledge gained from the test facilities and data from commercial operating units. This information and knowl- edge is integrated with sophisticated computer programs to provide the tools needed to design CFB boilers for spe- cific fuels and other boiler requirements.
CFB boiler designers use detailed information on fuel, sorbent, steam conditions, stack temperature and emis- sion requirements for setting process parameters and boiler configurations providing an economic CFB boiler design.
B&W CFB boiler design experience was utilized to develop an advanced design of CFB boilers with internal
solids recirculation. B&W’s IR-CFB boiler features a simple and compact design providing lower capital and operating costs and high reliability.
Nomenclature Q rum - Furnace heat duty, Btu/hr
U, - Overall furnace heat transfer coefficient, Btu/hr ft’ F
SC - Effective furnace heat transfer surface, ft2
T rum - Furnace temperature, deg F
T,,, - Saturation temperature at boiler operating pressure, deg F
D.E., “Operating Experience with High Ash Waste Coal in a B&W CFB Boiler,” Power-Gen Asia ‘94, Hong Kong, August 23-25, 1994.
2. Belin, F., Maryamchik, M., Fuller, T.A., and Perna, M.A., “CFB Combustor with Internal Solids Recircula- tion - Pilot Testing and Design Applications,” 13th Inter- national Conference on Fluidized-Bed Combustion, Orlando, Florida, May 7-10, 1995.
3. Jones, C.S., Alexander, K.C., Belin, F., “CFB Boil- ers for Ukrainian Low Grade Coals,” Power-Gen Ameri- cas ‘94 Conference, Orlando, Florida, December 7-9, 1994.
