Challenges and Possibilities in Char Bed Controlwhitty/.../5.2.Saviharju-Char_Bed_Control.pdf ·...

$Page 1: Challenges and Possibilities in Char Bed Controlwhitty/.../5.2.Saviharju-Char_Bed_Control.pdf · \RB\Papers\2003 BL Colloquium\BLCFinal.ppt Challenges and Possibilities in Char Bed$
Challenges and Possibilities in Char Bed Control

Colloquium on Black Liquor Combustion and Gasification, May 13-16, 2003

Kari Saviharju, Jarmo Kaila

\RB\Papers\2003 BL Colloquium\BLCFinal.ppt

Challenges and Possibilities in Char Bed ControlSlide No. 2

Impact of Char Bed Management

Boiler operabilityMaximum capacityReduction rateSmelt flowsGreen liquor suspended solidsEmissionsFouling Corrosion



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



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



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



Liquor A Liquor B

Liquor Droplet Swelling (Åbo Akademi)



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



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



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.



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



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”



Altitude Contours of a Char Bed



Bed at 2000 t/d BLS LoadPicture from the Front Wall



Bed at 2000 t/d BLS LoadPicture from the Side Wall




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



Spraying Straight into the Bed



Rapid Changes in Bed Height




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



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



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

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