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The progressive kiln process.

Simulation, quality, energy and drying cost

considerations

Jarl-Gunnar Salin

SP Technical Research Institute of Sweden

Wood Technology

Stockholm, Sweden

Introduction

• Progressive kilns for sawn timber have a

semi-continuous working mode and have been very popular

in some countries such as Finland, Sweden, Russia etc.

• In Finland and Sweden about half of the total

production is dried in progressive kilns

• This kiln was earlier considered suitable only for low

quality bulk timber

• There are three different types of these kilns

• A tool, TorksimLC, that simulates the drying process has

been used for some theoretical results presented in the

following.

(2)

Single zone progressive kiln

Traditional two-zone

progressive kiln

(3)

Two-zone OTC-type

progressive kiln

Simulation model features for

TorksimLC

• Both single zone and two-zone kilns.

• Drying described as a continuous process (not

stepwise) in order to keep it simple.

• Simulation is done for a single piece with average

properties. Drying of pure heartwood/sapwood in

the climate established is also included.

• Layout is similar to the batch kiln software

TORKSIM and TorksimGlobal, that are familiar

to many kiln operators in Sweden and some in

Norway.

(4)

Software features,

(cont.)

Data output

1. Climate in the length direction

2. Average MC

-”-3. MC profile

-”-4. Wood temperature

-”-5. Stress development

-”-6. Slicing test gap

-”-7. Energy consumption

8. Drying costs (fixed costs + energy)

Simulation model verification

Traditional two-zone kiln

55 60 65 70 75 80 0 10 20 30 40 50 60 70 80 Time, h T e m p e ra tu re , C DBT measured WBT measured DBT simulated WBT simulated

(5)

Model verification

OTC kiln

60 65 70 75 80 85 0 20 40 60 80 100 Time, h T e m p e ra tu re , C DBT measured WBT measured DBT simulated WBT simulated

Drying time as a function of temperature in

a traditional progressive kiln with

47x100 mm

2

Norway spruce

0 20 40 60 80 100 120 140 160 180 200 50 60 70 80 90

Entering air temperature, oC

D ry in g ti m e , h

(6)

Drying time as a function of temperature in an

OTC kiln with 47x100 mm

2

Norway spruce

0 20 40 60 80 100 120 140 160 50 60 70 80 90 Air temperature, C Dr y in g t im e , h

Drying time as a function of final MC in a

traditional progressive kiln with

47x100 mm

2

Norway spruce

0 10 20 30 40 50 60 70 10 12 14 16 18 20

Final moisture content, %

D ry in g ti m e , h

(7)

Drying time as a function of final MC in an

OTC kiln with 47x100 mm

2

Norway spruce

0 10 20 30 40 50 60 70 10 12 14 16 18 20

Final moisture content, %

D ry in g ti m e , h

Experimental comparison

traditional progressive - batch

Batch

Progr.

Batch

Progr.

5,8/4,9

7,5/5,9

Twist mm/3m

1,8

1,6

1,2

0,4

Check length, %

1,5/0,5

2,0/0,38

0,91/0,45

1,29/0,5

Slicing test, mm

15,5/2,1

13,5/0,6

17,6/1,6

16,1/1,5

Final MC %

84

81

91

84,3

Drying time, h

75

75

(75 ?)

72

Max. temp.,

o

C

50 x 125

50 x 125

Pine

Batch

Progr.

Batch

Progr.

5,8/4,9

7,5/5,9

Twist mm/3m

1,8

1,6

1,2

0,4

Check length, %

1,5/0,5

2,0/0,38

0,91/0,45

1,29/0,5

Slicing test, mm

15,5/2,1

13,5/0,6

17,6/1,6

16,1/1,5

Final MC %

84

81

91

84,3

Drying time, h

75

75

(75 ?)

72

Max. temp.,

o

C

50 x 125

50 x 125

Pine

(8)

Experimental comparison OTC - batch

Batch

OTC

Batch

OTC

2,6/3,2

3,0/3,1

Twist mm/3m

0,9

2,2

3,4

6,8

Check length, %

1,18/0,4

1,4/0,4

1,22/0,52

1,1/0,4

Slicing test, mm

14,2/1,4

12,3/1,3

15,8/2,1

14,9/1,7

Final MC %

108

99

107

96

Drying time, h

75

80

75

77

Max. temp.,

o

C

50 x 125

50 x 200

Pine

Batch

OTC

Batch

OTC

2,6/3,2

3,0/3,1

Twist mm/3m

0,9

2,2

3,4

6,8

Check length, %

1,18/0,4

1,4/0,4

1,22/0,52

1,1/0,4

Slicing test, mm

14,2/1,4

12,3/1,3

15,8/2,1

14,9/1,7

Final MC %

108

99

107

96

Drying time, h

75

80

75

77

Max. temp.,

o

C

50 x 125

50 x 200

Pine

Notation: average/standard deviation

Cost optimised single zone kilns for

47x100 mm

2

Norway spruce

Stacks 18

Drying time, h 70

Energy, kWh/m

3

217

Cost, €/m

3

9,7

Stacks 15

Drying time, h 51

Energy, kWh/m

3

205

Cost, €/m

3

8,9

80

Stacks 20

Drying time, h 153

Energy, kWh/m

3

219

Cost, €/m

3

12,9

Stacks 15

Drying time, h 96

Energy, kWh/m

3

205

Cost, €/m

3

11,3

70

Target MC 12 %

Target MC 16 %

Temp.,

o

C

Stacks 18

Drying time, h 70

Energy, kWh/m

3

217

Cost, €/m

3

9,7

Stacks 15

Drying time, h 51

Energy, kWh/m

3

205

Cost, €/m

3

8,9

80

Stacks 20

Drying time, h 153

Energy, kWh/m

3

219

Cost, €/m

3

12,9

Stacks 15

Drying time, h 96

Energy, kWh/m

3

205

Cost, €/m

3

11,3

70

Target MC 12 %

Target MC 16 %

Temp.,

o

C

(9)

Cost optimised kilns for 47x100 mm

2

Norway spruce to 16% MC with max 80

o

C

10,5 232 39 5 Batch kiln constant WBT, increasing DBT 10,1 229 37 5 Batch kiln constant DBT, decreasing WBT 8,1 206 50 5+17

OTC 2-zone kiln

8,3 204 54 9+12 Traditional 2-zone 8,9 205 51 15 Single zone Cost €/m3 Energy kWh/m3 Drying time, h Kiln stacks Kiln type 10,5 232 39 5 Batch kiln constant WBT, increasing DBT 10,1 229 37 5 Batch kiln constant DBT, decreasing WBT 8,1 206 50 5+17

OTC 2-zone kiln

8,3 204 54 9+12 Traditional 2-zone 8,9 205 51 15 Single zone Cost €/m3 Energy kWh/m3 Drying time, h Kiln stacks Kiln type

Cost optimised kilns for 47x100 mm

2

Norway spruce to

12% MC

with max 80

o

C

12,5 246 54 5 Batch kiln constant WBT, increasing DBT 11,6 248 46 5 Batch kiln constant DBT, decreasing WBT 8,9 219 70 4+21

OTC 2-zone kiln

9,3 217 71 11+11 Traditional 2-zone 9,7 217 70 18 Single zone Cost €/m3 Energy kWh/m3 Drying time, h Kiln stacks Kiln type 12,5 246 54 5 Batch kiln constant WBT, increasing DBT 11,6 248 46 5 Batch kiln constant DBT, decreasing WBT 8,9 219 70 4+21

OTC 2-zone kiln

9,3 217 71 11+11 Traditional 2-zone 9,7 217 70 18 Single zone Cost €/m3 Energy kWh/m3 Drying time, h Kiln stacks Kiln type

(10)

Conclusions

• A higher temperature level has a strong positive

influence on progressive kiln efficiency

• Two-zone progressive kilns are more efficient

than single zone kilns

• The energy consumption and drying costs are

clearly lower than in batch kilns

• No simple way to achieve a final timber

equalisation/conditioning in a progressive kiln

• Preferably the same dimension should be dried all

the time, i.e. relatively big sawmills

References

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