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.
Single zone progressive kiln Traditional two-zone progressive kiln
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.
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 DBT measured 80 WBT measured 75 DBT simulated Temperature, C WBT simulated 70 65 60 55 0 10 20 30 40 50 60 70 80 Time, h
Model verification OTC kiln DBT measured 85 WBT measured DBT simulated Temperature, C 80 WBT simulated 75 70 65 60 0 20 40 60 80 100 Time, h Drying time as a function of temperature in a traditional progressive kiln with 47x100 mm 2 Norway spruce 200 180 160 Drying time, h 140 120 100 80 60 40 20 0 50 60 70 80 90 Entering air temperature, o C
Drying time as a function of temperature in an OTC kiln with 47x100 mm 2 Norway spruce 160 140 120 Drying time, h 100 80 60 40 20 0 50 60 70 80 90 Air temperature, C Drying time as a function of final MC in a traditional progressive kiln with 47x100 mm 2 Norway spruce 70 60 Drying time, h 50 40 30 20 10 0 10 12 14 16 18 20 Final moisture content, %
Drying time as a function of final MC in an OTC kiln with 47x100 mm 2 Norway spruce 70 60 50 Drying time, h 40 30 20 10 0 10 12 14 16 18 20 Final moisture content, % Experimental comparison traditional progressive - batch Progr. Progr. Batch Batch Progr. Progr. Batch Batch Pine Pine 50 x 125 50 x 125 50 x 125 50 x 125 Max. temp., o C Max. temp., o C 72 72 (75 ?) (75 ?) 75 75 75 75 Drying time, h Drying time, h 84,3 84,3 91 91 81 81 84 84 Final MC % Final MC % 16,1/1,5 16,1/1,5 17,6/1,6 17,6/1,6 13,5/0,6 13,5/0,6 15,5/2,1 15,5/2,1 Slicing test, mm Slicing test, mm 1,29/0,5 1,29/0,5 0,91/0,45 0,91/0,45 2,0/0,38 2,0/0,38 1,5/0,5 1,5/0,5 Check length, % Check length, % 0,4 0,4 1,2 1,2 1,6 1,6 1,8 1,8 Twist mm/3m Twist mm/3m 7,5/5,9 7,5/5,9 5,8/4,9 5,8/4,9 Notation: average/standard deviation
Experimental comparison OTC - batch OTC OTC Batch Batch OTC OTC Batch Batch Pine Pine 50 x 200 50 x 200 50 x 125 50 x 125 Max. temp., o C Max. temp., o C 77 77 75 75 80 80 75 75 Drying time, h Drying time, h 96 96 107 107 99 99 108 108 Final MC % Final MC % 14,9/1,7 14,9/1,7 15,8/2,1 15,8/2,1 12,3/1,3 12,3/1,3 14,2/1,4 14,2/1,4 Slicing test, mm Slicing test, mm 1,1/0,4 1,1/0,4 1,22/0,52 1,22/0,52 1,4/0,4 1,4/0,4 1,18/0,4 1,18/0,4 Check length, % Check length, % 6,8 6,8 3,4 3,4 2,2 2,2 0,9 0,9 Twist mm/3m Twist mm/3m 3,0/3,1 3,0/3,1 2,6/3,2 2,6/3,2 Notation: average/standard deviation Cost optimised single zone kilns for 47x100 mm 2 Norway spruce Temp., Temp., Target MC 16 % Target MC 16 % Target MC 12 % Target MC 12 % o C o C 70 70 Stacks 15 Stacks 15 Stacks 20 Stacks 20 Drying time, h 96 Drying time, h 96 Drying time, h 153 Drying time, h 153 Energy, kWh/m 3 205 Energy, kWh/m 3 205 Energy, kWh/m 3 219 Energy, kWh/m 3 219 Cost, € /m 3 Cost, € /m 3 Cost, € /m 3 Cost, € /m 3 11,3 11,3 12,9 12,9 80 80 Stacks 15 Stacks 15 Stacks 18 Stacks 18 Drying time, h 51 Drying time, h 51 Drying time, h 70 Drying time, h 70 Energy, kWh/m 3 205 Energy, kWh/m 3 205 Energy, kWh/m 3 217 Energy, kWh/m 3 217 Cost, € /m 3 Cost, € /m 3 Cost, € /m 3 Cost, € /m 3 8,9 8,9 9,7 9,7
Cost optimised kilns for 47x100 mm 2 Norway spruce to 16% MC with max 80 o C Kiln type Kiln type Kiln Kiln Drying Drying Energy Energy Cost Cost stacks stacks time, h time, h € /m 3 € /m 3 kWh/m 3 kWh/m 3 Single zone Single zone 15 15 51 51 205 205 8,9 8,9 Traditional 2-zone Traditional 2-zone 9+12 9+12 54 54 204 204 8,3 8,3 OTC 2-zone kiln OTC 2-zone kiln 5+17 5+17 50 50 206 206 8,1 8,1 Batch kiln Batch kiln 5 5 37 37 229 229 10,1 10,1 constant DBT, constant DBT, decreasing WBT decreasing WBT Batch kiln Batch kiln 5 5 39 39 232 232 10,5 10,5 constant WBT, constant WBT, increasing DBT increasing DBT Cost optimised kilns for 47x100 mm 2 Norway spruce to 12% MC with max 80 o C Kiln type Kiln type Kiln Kiln Drying Drying Energy Energy Cost Cost stacks stacks time, h time, h € /m 3 € /m 3 kWh/m 3 kWh/m 3 Single zone Single zone 18 18 70 70 217 217 9,7 9,7 Traditional 2-zone Traditional 2-zone 11+11 11+11 71 71 217 217 9,3 9,3 OTC 2-zone kiln OTC 2-zone kiln 4+21 4+21 70 70 219 219 8,9 8,9 Batch kiln Batch kiln 5 5 46 46 248 248 11,6 11,6 constant DBT, constant DBT, decreasing WBT decreasing WBT Batch kiln Batch kiln 5 5 54 54 246 246 12,5 12,5 constant WBT, constant WBT, increasing DBT increasing DBT
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 Further information: jarlgunnar.salin@sp.se
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