Challenges and Possibilities in Char Bed Controlwhitty/.../5.2.Saviharju-Char_Bed_Control.pdf ·...
Transcript of Challenges and Possibilities in Char Bed Controlwhitty/.../5.2.Saviharju-Char_Bed_Control.pdf ·...
Challenges and Possibilities in Char Bed Control
Colloquium on Black Liquor Combustion and Gasification, May 13-16, 2003
Kari Saviharju, Jarmo Kaila
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Challenges and Possibilities in Char Bed ControlSlide No. 2
Impact of Char Bed Management
Boiler operabilityMaximum capacityReduction rateSmelt flowsGreen liquor suspended solidsEmissionsFouling Corrosion
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Challenges and Possibilities in Char Bed ControlSlide No. 3
Recover Boiler Operability
After pyrolysis approximately half of the combustible material in black liquor has been converted to char• This should combust in the lower furnace below the liquor gun level• Small droplets can escape with vertical flows but with proper air
system and set-up will rapidly combust in flightLarge portion of the char combusts in flight• Large droplets can have aerodynamic properties that allow them to
impact the furnace wallsIn the hot lower furnace reduction reactions won’t need long residence time• Need is for good char coverage, not for large bed volume
Bed must be stable and not allowed to climb up the walls. Liquorspraying on the walls must also be avoided• Sticks to walls and occasionally collapses down• Uneven smelt flow, even smelt rushes or explosions
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Challenges and Possibilities in Char Bed ControlSlide No. 4
Recover Boiler OperabilityImpact of Liquor Properties
Variation in various liquor properties has impact on liquor spraying and lower furnace operations and thus impacts bed behavior
Variation should be minimized or at least identified/anticipated:• Cook • Dry solids• Soap• Viscosity• Ash mixing and variations in it – evaporator wash etc• Make-up chemical feed
The different departments of the mill have to work together and share information for better char bed control
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Challenges and Possibilities in Char Bed ControlSlide No. 5
The Key Factors for Char Bed Control
Liquor spraying, swelling and other physical properties• Liquor Gun locations and elevations• Spray geometry, velocities (pressure) and direction• Average droplet size and size distribution• Mass flows with the directional distribution and coverage
Liquor combustion properties• Liquor dry solids• Inorganics in liquor (ash mixing)• Swelling• Time needed for drying, pyrolysis and combustion
Air Feeds into the Lower Furnace and induced flow fields• Temperature Distributions in the Lower Furnace
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Challenges and Possibilities in Char Bed ControlSlide No. 6
Liquor A Liquor B
Liquor Droplet Swelling (Åbo Akademi)
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Challenges and Possibilities in Char Bed ControlSlide No. 7
Combustion in Flight (Figure from Mika Järvinen’s thesis)
0
2.5
5
7.5
10
12.5
0 2.5 5 7.5 10 12.5Distance from the gun, m
Dis
tanc
e fro
m th
e flo
or, m
0.5
12
4
0.5 1
0.5 1
Flight trajactories for 0.5, 1, 2 and 4 mm droplets, w0 =13 m/s, 5o tilt, 27-fold swelling, wup =6 m/s
95 % dried95 % pyrolysedComplete char combustion
Roughly:•Relative velocity w = w0 . exp[-L. SW2/3/D ]
•L=Flight distance
•SW=Swelling index = droplet volume/original volume
Example:
•1 mm droplet, which SW=30 looses its velocity in 2 m
•6 mm droplet, with SW=12 (typical for high D.s.), looses 1/3 of the velocity in 5 m, and 10 m is needed to reduce velocity by 60 %. In practice furnace flows and liquor gun positions are needed in steering the flight
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Challenges and Possibilities in Char Bed ControlSlide No. 8
Flow Fields in the Furnace
Flow fields in the furnace are complex, and gradients can be strong• Flow Fields have a strong effect on droplet’s flight • Droplet should be dry, pyrolysed and char combusted when entering
strong flows close to the walls• Wet droplets do not follow the gas flows, but can fly straight to the
walls• This is often difficult to completely prevent in small furnaces• Too heavy, although swollen, droplets impact the wall in strong
rotational firing concepts
Positioning and type of the liquor guns is important• Liquor gun positions in relation to lower furnace gas flow fields• Liquor gun type and operational parameters
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Challenges and Possibilities in Char Bed ControlSlide No. 9
Velocity vectors and particle concentrations, Vertical Air™ System in a 3000 t D.S./d boiler
Black liquor concentration and velocity vectors in the secondary air levels
Lowest secondary air level Second secondary air level Highest secondary air level
Concentration = 0 kg/m3 (blue) - 0.1 kg/m3 (red). Velocity has been shown between 0 m/s - 80 m/s.
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Challenges and Possibilities in Char Bed ControlSlide No. 10
Reduction
Smelt temperature is typically approximately 830-870 °C , lower furnace temperature is higher at 1000-1200 °C• Temperature quickly drops a couple of hundred °C inside the bed
Good reduction requires:• Good char coverage• Sufficiently high temperature
With modern high dry solids and reasonable liquor heating values large bed volume is not needed for good reduction• The increase in temperature from 850 °C to 1000-1100 °C makes
reduction kinetics with carbon 20 fold higher• With long contact time high reduction can also be achieved with lower
solidsSmelt pools in contact with air jets must be eliminated
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Challenges and Possibilities in Char Bed ControlSlide No. 11
Example of Bed Management by Boiler Tuning
In the next example there are four photos of a rapidly changing bed
Starting case:
• Bed is “normal”, low and elongated in shape• Bed reaches half way between primary and
secondary air ports.• Low char covered region surrounds the bed
on all four walls• Just slight spraying into one corner,
otherwise corners are “clean”
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Challenges and Possibilities in Char Bed ControlSlide No. 12
Altitude Contours of a Char Bed
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Challenges and Possibilities in Char Bed ControlSlide No. 13
Bed at 2000 t/d BLS LoadPicture from the Front Wall
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Challenges and Possibilities in Char Bed ControlSlide No. 14
Bed at 2000 t/d BLS LoadPicture from the Side Wall
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Challenges and Possibilities in Char Bed ControlSlide No. 15
Example of Bed Management by Boiler Tuning
Change in liquor quality
Sudden bed growth on the front wall next to gun opening
• Forms rapidly a massive ridge when liquor spray hits the top of bed and doesn’t fly over it
• Growing material is very light, swollen liquor• This growth can become compacted into dense inorganic material if
allowed to remain• Slight adjustment on the gun angle and liquor temperature brought
the bed rapidly back to normal
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Challenges and Possibilities in Char Bed ControlSlide No. 16
Spraying Straight into the Bed
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Challenges and Possibilities in Char Bed ControlSlide No. 17
Rapid Changes in Bed Height
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Challenges and Possibilities in Char Bed ControlSlide No. 18
Example of Bed Management by Boiler Tuning
Lessons learned
• Good bed cameras in proper locations are important
• Important to monitor lower furnace temperatures
• Pro-active attitude to char bed management is needed, changes can happen quickly
• Slight fouling of spray guns can alter their characteristics• Keep them clean
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Challenges and Possibilities in Char Bed ControlSlide No. 19
Smelt FlowsA Case Before Boiler Trimming
AVG TEMP
70 69 68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 4871 4772 4673 4574 4475 4376 C 4277 4178 4079 3980 38
AVG TEMP 81 37 AVG TEMP1078 82 36 1060
83 3584 3485 3386 3287 3188 3089 2990 2891 2792 2693 2594 24
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
AVG TEMP
1075
1075
1025
1048
1170 930
1120
1140
1130
1085
980
940
940
1015
1105 1075
1135 980
1090 1115
1065 1100
940 1045
1030
1072
960
LOWER FURNACE AVG1080 1140
1080 1125
1180
1125 1035
1040 1150
1160
1155
1160
1115
1102
1135
1080
1070
1050
1000
1020
RELATIVE SMELT FLOWS AND SMELT TEMPERATURE AT THE SPOUTS
30% 40% 100% 120% 100%
811 ºC 835 ºC 849 ºC 868 ºC 815 ºC
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Challenges and Possibilities in Char Bed ControlSlide No. 20
Summary
Char bed (and lower furnace) operation has major impact on recovery boiler operations• Operatability, capacity• Reduction rate, smelt flows, green liquor suspended solids• Emissions, fouling, corrosion
Char Bed Behavior and control is a function of multiple inter-related parameters• Liquor properties• Liquor spraying parameters• Lower furnace flow fields
Boiler operators need good tools and training to adjust boiler for changing operating conditions• Good bed cameras in proper locations essential• Willingness and knowledge to change operating parameters