wet fgd system overview and operation
TRANSCRIPT
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Wet FGD SystemOverview and Operation
Ray Gansley
WPCA Wet FGD SeminarPower Gen International
December 1, 2008
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Agenda
• Introduction
• Major Process Equipment
• Balance of Plant Equipment
• Controls
• Summary
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MAKE-UP WATER TANK
CHLORIDE PURGE
HYDROCLONE
BELT FILTER
SPRAY TOWER ABSORBER
ELECTROSTATIC PRECIPITATOR
STACKWATER
AIR
LIMESTONE
BALL MILL
TO BALL MILL
FROM MW TANK
GYPSUM
REAGENT PREPARATION
ABSORBER
DEWATERING
Process Flow Diagram
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Agenda
• Introduction
• Major Process Equipment
• Balance of Plant Equipment
• Controls
• Summary
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Absorber Island
Absorber
Oxidation Compressors
Spray Pumps
Switchgear
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Jet Bubbling ReactorTray Tower
Fountain TowerOpen Spray Tower
Absorber Designs
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Absorber Unit Operations
Unit Operations
• Inlet – gas distribution, humidification
• Spray zone – gas/liquid contact
• Mist eliminator zone –liquid/gas separation
• Reaction tank – oxidation, dissolution, crystallization
Design Goals
• Lowest lifecycle cost− Capital− O&M
• High reliability
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Absorber Inlet
• Highly corrosive environment− SO3 condensation− Deposits/splash-back
• Design/construction− Solid 3/16” thick C-276− 10º slope toward absorber− Extends 10-15 ft. from shell− Internal overhead deflector
and side shields− Test ports and emergency
quench nozzles− Gas velocity – 3,000-3,600
ft/min
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Absorber Shell
Straight-SideFlared-Side
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• Multiple spray levels with dedicated pumps allows efficient turndown
• Gas velocity – 13-14 ft/sec
Contact Zone
• Spray flow
– Typical afternoon thunderstorm – 1-2 in/hr
– Spray tower – 10 ft/min
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• Design Approach− Intimate gas/liquid contact− No gas sneakage
• Multiple spray stages depending on SO2 inlet and removal efficiency
• Overall spray coverage (>200% at 3-feet) and nozzle distribution to provide even distribution and prevent gas sneakage.
• Wall rings to prevent gas sneakage near side walls.
• Pressure drop vs. pumping power trade-off
Contact Zone
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Spray Nozzles
• Multiple counter-current spray stages
• Nozzle type− Top stage—single orifice hollow
cone− Lower stages—dual orifice
hollow cone• Sauter mean droplet diameter of
~2,000 micron at 8 psig
• Silicon carbide construction
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Spray Zone
Mist Eliminator
Mist Eliminator
Two stages of vertical-flow chevron vanesConstruction
– Polypropylene– Polysulfone– FRP– Alloy/stainless steel
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Gas Flow
5 10 15 20 25C
arry
ove r
Gas Velocity (ft/sec)
Mist Eliminator Operation
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Mist Eliminator Wash System
• Fresh water wash on intermittent basis− First stage washed from
below (1.5 gpm/ft2) and above (0.75 gpm/ft2)
− Second stage washed from below (0.75 gpm/ft2)
• Construction− Piping – FRP, polypropylene,
alloy/SS− Nozzles – polypropylene,
alloy/SS
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Typical size: 50,000-75,000 gpm at 55-85 TDHPump Efficiency: 87-90+%1,000-2,500 hpHorizontal, centrifugalRubber lined casingImpeller
– Hard metal– Rubber-covered
Absorber Recycle Pumps
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Spray Pump Capacity
10,000
20,000
30,000
40,000
50,000
60,000
70,000
1970 1980 1990 2000
Flow
(gpm
)
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Recycle Pump Curve
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• Integral to absorber
• Functions− Limestone dissolution− Oxidation− Gypsum crystallization
• Design considerations− Solids and liquid residence time− Foaming/freeboard
• Side entry agitators with air lances
Reaction Tank
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Materials of Construction
ShellRubber-lined carbon steelFlakeglass-lined carbon steelStainless steel (317LMN)Duplex stainless steel (2205, 255)Nickel-based alloyRoll-clad alloyLined concrete (tile, polypropylene)
External Spray PipingFRPRubber-lined carbon steelSS/alloy
Mist EliminatorsFRPPolypropylenePolysulfoneSS/alloy
HeadersRubber-lined carbon steelFRPStainless steel (317LMN)Duplex stainless steel (2205, 255)Nickel-based alloy
InletC-276
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Reagent Preparation Island
Day Bin
Ball MillFeed Tank
Classifier
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BALL MILL
FROM MW TANK
Limestone Grinding System
WPCA Wet FGD Seminar - December 1, 2008
BALL MILLRECYCLE
TANK
REAGENTFEEDTANK
CLASSIFIER
REAGENT SILO
TOABSORBER
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• Limestone grinding− Horizontal/vertical
wet ball mills− On-site vs. off-site
preparation• Product ground to
90-95% < 44 μ; 30% solids• Rubber-lined with
hardened steel balls
Limestone Preparation
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Pre-Ground Limestone Injection
• Dry, pre-ground limestone feed options− Pneumatic injection into reaction tank below slurry level− Pre-slurry in small tank
• Advantages− Low capital cost (i.e. no ball mills, auxiliary equipment,
buildings, etc.)− Less equipment to maintain
• Disadvantages− Higher delivered cost
• Lifecycle cost evaluation on case-by-case basis
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Dewatering Island
Hydroclone
Belt Filter
Vacuum Pump
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Primary Dewatering
• Initial solid/liquid separation step− Feed: 15-20% TSS− Underflow: 50-55% TSS− Overflow: 3-5% TSS
• Overflow enriched with CaCO3 due to particle size difference; improved reagent utilization
• Steel casing with rubber, urethane, and ceramic internals
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Secondary Dewatering
• Horizontal belt filter− 8-10% moisture in gypsum− Optimal for cake washing
• Rotary drum filter− 10-15% moisture− Smaller footprint
• Other options− Gypsum stacking− Centrifuge− Thickener
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• Typical specification:− >95% CaSO4•2H2O− <0.5–1.0% CaSO3•½H2O− <100 ppm Cl− <10% moisture− pH 6–8− 30–40µ MMD
• Requires:− High purity limestone (95–
96%)− High efficiency ESP/FF− 99+% oxidation− Belt filter− Cake washing
Wallboard Gypsum
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Agenda
• Introduction
• Major Process Equipment
• Balance of Plant Equipment
• Controls
• Summary
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• Truck, rail, or barge delivery
• ¾” x 0” (20 x 0 mm) crushed limestone
• Outdoor long-term storage feasible in most areas
• Indoor/covered storage in harsh/wet climates
Limestone Delivery/Storage
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Limestone Silo
• Provide buffer between limestone pile and ball mill operation
• 16-24 hr capacity
• Carbon steel construction with polymer or stainless steel hopper lining
• Vibrating bottom/mechanicalactivation
• Vent filter for fugitive dust control
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• Covered storage options− Direct discharge to bunker/
manual reclaim− Convey to dome/manual
reclaim− Convey to building or silo/
automated reclaim• Open pile
− Feasible− Possible issues: moisture,
fugitive dust, leaching• Selection depends on:
− On-site storage requirement− Gypsum contract
Gypsum Storage/Handling
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Fan Arrangement – Retrofit
ELECTROSTATIC PRECIPITATOR
STACK
ID FAN
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Fan Arrangement – Retrofit
ABSORBER
ELECTROSTATIC PRECIPITATOR
STACK
ID FAN
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Fan Arrangement – Retrofit
ABSORBER
ELECTROSTATIC PRECIPITATOR
STACK
ID FAN BOOSTER FAN
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Fan Arrangement – Retrofit
ABSORBER
ELECTROSTATIC PRECIPITATOR
STACK
ID FAN
BOOSTER FAN
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• Purge stream required to control chlorides/inert solid for wallboard gypsum
• Purge stream treatment strategy is site-specific; depends on:− Discharge limits− Receiving body− FGD process design− Fuel, limestone, make-up water
• Treatment options:− Direct discharge− Volume reduction (evaporation,…)− Physical/chemical treatment
• Suspended solids• pH adjustment• Heavy metals removal• COD removal
Wastewater Treatment
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Agenda
• Introduction
• Major Process Equipment
• Balance of Plant Equipment
• Controls
• Summary
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WFGD Controls
• Distributed Control Systems− Overview− Redundancy
• Primary WFGD Control Loops & Instrumentation− Absorber Area− Flue Gas System
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Distributed Control Systems (DCS)– Control and automation of WFGD
processes and components– Interface with remaining power
plant controls– Monitor key process parameters– Process trends, reports, historical
data loggingLocal Control Panels (LCP)Programmable Logic Controllers (PLC)Main FGD Control Area– Dedicated WFGD Control Station– Centralized WFGD Control with
Boiler/Power Plant
WFGD Control Systems
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Redundant Data Highway
OperatorStation Laser Printer
LocalI/O
OperatorStation
RedundantProcessController
EngineeringStation
Line Printer Line Printer
RemoteI/O
SerialDevice
SerialDevice
LocalI/O
RedundantProcessController
RemoteI/O
SerialDevice
SerialDevice
LocalI/O
RedundantProcessController
DCS CONFIGURATION DIAGRAM
Distributed Control System
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Major Control Loops - Absorber
Make-Up Water
L
Solids / Gypsum Bleed
D
pH
Reagent Feed
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Scrubbing Liquor pH– Most important process variable
to be controlled in WFGD →continuous, in-line monitoring, daily lab verification
– Indication of the amount of CaCO3 in scrubbing solution
– Scrubbing liquor below desired range → lower SO2 removal efficiency
– Scrubber liquor above desired range → higher SO2 removal efficiency, excess CaCO3 and reduced gypsum byproduct purity
0 7 14
5.3 – 5.8 Scrubbing Liquor pH
Neutral H2O
acid base
pH / Reagent Feed Control
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Limestone slurry
pH
pH
Reaction Tank
Absorber
FE
pH Controller
LS Flow Controller
SO2 Boiler Load (MW)
pH / Reagent Feed Control
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pH PROBES
TRANSMITTERS
LOCAL DISPLAY PANELS
PROCESS PIPING
pH Instrumentation
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Flue Gas Water Vapor
Mist Eliminator
Wash Water
Limestone Slurry
Oxidation Air Sat. Water
Absorber Bleed Stream
Flue GasWater VaporScrubber Slurry Solids
– Important WFGD process variable, verified daily with laboratory samples
– Scrubber slurry solids: gypsum, limestone, flyash, inerts
– Numerous inputs affecting water balance and solids concentration
– Control solids concentration within narrow range: 14-16 wt.%
Solids Control– Solids too low: potential for
gypsum scaling– Solids too high: increased
potential for wear of equipment– Equipment sizing
Dewatering ReturnMake-up Water
Gypsum Bleed / Solids Control
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Secondary Dewatering
DE
Reaction Tank
Absorber
Return to Absorber
Primary Dewatering Hydroclones
DENSITY
L
LEVEL
L
ABSORBER BLEED PUMPS
Gypsum Solids / Density Control
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Density Measurement
Nuclear Density Meter
• Nuclear Density Meters− Source emits gamma
radiation through process− Detector measures energy
of radiation − Capable of detecting
process variations within desired narrow range
− Safe− Accurate− Simple calibration− Verify with laboratory
analyses
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PROCESS SLURRY
DENSITY METER
LOCAL DISPLAY PANEL
Density Instrumentation
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DE
Reaction Tank
Absorber
L
L
DENSITY
Make-up water
DY
Level Controller
f(x)
f(x)
OX AIR
Level / Make-Up Water Control
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SPRAY TOWER ABSORBER
EXISTING STACK
Booster Fan Control
BOOSTER FANS
ID FANS
P
F
ZY
VANE PITCH CONTROL
NEW STACK
STACK REVERSE FLOW
DUCT PRESSURE
ID Fan Load, Boiler MW
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SPRAY TOWER ABSORBER
EXISTING STACK
Booster Fan Control
BOOSTER FANS
ID FANS
P
ID FAN LOAD, BOILER (MW)
ZY
VANE PITCH CONTROL
UNTREATED FLUE GAS
(BYPASS)
DUCT PRESSURE
SO2TREATED FLUE
GAS
ID FANS
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SPRAY TOWER ABSORBER
Emergency Quench
TY
T
T
INLET FLUE GAS
OUTLET (CLEAN) FLUE GAS
QUENCH WATER
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Agenda
• Introduction
• Major Process Equipment
• Balance of Plant Equipment
• Controls
• Summary
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WFGD Operations Staff
• O&M staffing varies; depends on:− Owner staffing philosophy/
practices− Plant layout/design− Reagent/byproduct− Labor bargaining agreements− Other equipment at plant
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O&M Staff for 2 x 700 MW WFGD
Position Description No. CoverageFGD Supervisor 1 1 shift, 5 days/week FGD Operator 1 3 shifts, 7 days/week FGD Mechanic 1 1 shift, 5 days/week FGD Computer Tech 1 1 shift, 5days/week FGD Specialist 1 1 shift, 5 days/week FGD Material Handling 1 3 shifts, 7 days/week
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Lifecycle Cost Comparison
WPCA Wet FGD Seminar - December 1, 2008
High Sulfur Low SulfurTotal Plant Capital Cost $ 150,000,000 120,000,000
$/kW 250 200Plant Capacity (per unit) M W e 600 600No. Units 1 1Plant Capacity Factor % 85 85Equivalent Full-Load Hour hr 7,446 7,446Escalation Rate % 3.0 3.0NPV Discount Rate % 10.0 10.0SO2 Removal % 98 98SO2 Production Rate lb/hr 45,000 12,000
lb/M M Btu 7.50 2.00ID Fan Flue Gas Flow acfm 2,100,000 2,220,000
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Lifecycle Cost Comparison
WPCA Wet FGD Seminar - December 1, 2008
High Sulfur Low SulfurNo. O&M Personnel/Shift 5 5O&M Labor Rate $/hr 50.00 50.00AQCS Power kW 9,000 4,500Fan Power Consumption kW 2,464 1,628Total Power kW 11,464 6,128Power Cost $/kW -hr 0.03 0.03Limestone Consumption lb/hr 74,709 19,922Limestone Cost $/ton 20.00 20.00Gypsum Production lb/hr 138,618 36,965Gypsum Price $/ton (5.00) (5.00)Repair/M aintenance % of TPC 1.5 1.5
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Lifecycle Cost Comparison
WPCA Wet FGD Seminar - December 1, 2008
Technology High Sulfur Low SulfurO&M Labor $/yr 2,190,000 2,190,000Auxiliary Power $/yr 2,560,776 1,368,838Limestone $/yr 5,562,823 1,483,420W W TS Reagents $/yr 1,500,000 500,000Gypsum $/yr (2,580,382) (688,102)Repair/M aintenance $/yr 2,250,000 1,800,000Total $/yr 11,483,218 6,654,156
$/M W -hr 2.57 1.49
Total Plant Capital Cost $ 150,000,000 120,000,000O&M Cost (25-Yr NPV) $ 138,240,000 69,540,000Lifecycle Cost $ 288,240,000 189,540,000
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Summary
• WFGD system design is mature; operation is routine
• Major process islands− Absorber− Reagent− Dewatering
• Key balance of plant equipment− Flue gas handling− Material handling− Wastewater treatment− Electrical system
WPCA Wet FGD Seminar - December 1, 2008
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WPCA Wet FGD Seminar - December 1, 2008
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