Annex X Energy Efficient Drying and Dewatering Technologies · 2018. 4. 11. · Murlidhar Gupta, Jin Yang and Christian Roy measured the density and the porosity for individual bark

International Energy Agency

Industrial Energy-related Technologies and Systems (IETS)

Annex X

Energy Efficient Drying and Dewatering Technologies

Technical report 2 Dewatering of bark

Compiled by Stig Stenström Lund University, Sweden



Sweden



About the IEA

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About the IETS

The Industrial Energy-related Technologies and Systems (IETS) is one of IEA’s over 40

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program focuses on energy use in a broad range of industry sectors, uniting IEA activities

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Mexico, Portugal, Sweden, USA, the Netherlands and Belgium.

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Contents

1. Background and objective ............................................................................................. 1 2. Dewatering of bark ........................................................................................................ 1 3. Some data for bark ......................................................................................................... 3 4. Some typical bark dewatering installations ................................................................... 4

5 Experiments performed for dewatering of bark ............................................................. 6 6 Some concluding remarks ............................................................................................ 11 7 Literature ...................................................................................................................... 14



- 1 -

1. Background and objective

Increased used of biofuels as renewable energy sources such as bark, sawdust and GROT

(Swedish for branches and the top part of the tree) will increase the demand for energy

efficient dewatering and drying of these materials. A lower water content results in a higher

thermal value for the product, smaller boilers, more efficient combustion, improved

possibilities to control the combustion process and lower costs for transportation and

storage. Also a low water content is normally required when these biofuels are used as raw

materials in a gasification process.

The traditional use of bark has been to burn it at the pulp and paper mills but increased

energy efficiency for these processes gives opportunities for external deliveries of bark also

to municipal power boilers. Estimates for Sweden show that about 3.1 TWh thermal energy

was used 2006 for drying of these products and since the demand for these products

increases it is reasonable to assume that the interest for mechanical dewatering and drying of

these products will also increase.

The objective in this part is to summarize the available results about mechanical dewa-tering

of bark and point the way for some methods with improved dewatering for this material.

This summary is based on a project performed by Håkansson and Stenström [1] in Sweden

during 2007, „‟Efficient dewatering of bark in heated presses. Survey and pilot-scale trials‟‟.

It was financed by Värmeforsk and the report is available in Swedish at their homepage

www.varmeforsk.se.

2. Dewatering of bark

Bark is normally dewatered in rotating bark presses and during the summer months dry

matter contents between 40 and 45 % are achieved but the dry matter content can be

significantly lower during the cold winter months when the bark is cold and sometimes

frozen. In Sweden about 30 bark presses have been installed by the Finnish company

Saalasti, see Figure 1.

Normally the perforated drum is not heated but techniques are available which will make

this an opportunity with improved dewatering. However none of the sold bark presses in

Sweden or Finland have so far been equipped with this possibility.



- 2 -

Figure 1. Dewatering of bark in a heated press from Saalasti.

The dry matter content of the bark varies of the year, some data from Finnsih mill is shown

in Figure 2. (repijä = from the cutter, puristin = from the press, kasalle = to the bark

storage). When the ingoing dry matter content is at 30 % the outgoing dry matter content is

increased to about 45 % but only limited dewatering is achieved when the ingoing dry

matter content reaches over 40 %. Some bark from birch can reach dry matter contents

above 50 % and there is no need for pressing of this product.

Figure 2. Variation in ingoing and outgoing dry matter content at a Finnish mill.

Very limited data about dewatering of bark is available in the scientific literature. The most

detailed data are given in the previously mentioned report by Håkansson and Stenström [1]

but some scattered data can also be found in the Master thesis by Askaner [2] from 1976

and the report from VTT [3]. Obviously there is a need for more work in this important

area.



- 3 -

3. Some data for bark

SEM pictures of bark from spruce and pine are shown in Figures 3 and 4 at a magni-

fycation of 250X. The conclusion based on these pictures is that the bark from the pine tree

has a more open structure than the bark from the spruce tree.

Figure 3. Bark from spruce. Figure 4. Bark from pine.

One important parameter when evaluating dewatering capabilities is the water retention

value (WRV) but no data have been available in the open literature. It was thus deter-

mined by STFI-Packforsk in Stockholm using the standard method SCAN C-62:00. The

analysis gave a WRV value for bark from pine and spuce between 1.6 and 1.7

corresponding to a dry matter content between 37 and 38.5 %.

Murlidhar Gupta, Jin Yang and Christian Roy measured the density and the porosity for

individual bark particles [4]. The bark used consisted of 31% Abies balsamea, 55% white

spruce Picea glausa and 14% black spruce Picea maiana. The density and the porosity

reported by the authors were 482 kg/m3 and 0.71 respectively. In their second publication

[5], the specific heat and the thermal conductivity were measured for dry softwood bark

with the same composition as in their previous publication. According to the authors the

specific heat increases linearly from 1364 J/kgºC at 40 ºC to 1777 J/kgºC at 140 ºC. Also

the thermal conductivity increases from 0.205 W/mºC at 37ºC to 0.231 W/mºC at 75ºC. In

this publication the absolute porosity for bark was given as 0.68.

Koch [6] presents data for the specific heat for bark from fast and slowly growing trees

from the species Pinus glabra. According to Koech the specific heat for both the fast and

the slowly growing species can be described by the following equation:

2p T0215.0T0803.11.1390C

with T given in degrees Celsius in the interval 60 to 140 °C. In his publication Koech

compares his data with other literature data indicating that the specific heat for dry bark



- 4 -

from American spruce, pine and birch to be 1338.9 J/kg°C at 22°C and that the specific

heat for bark from pine with short needles to be 1376.5 J/kg°C at 56°C.

4. Some typical bark dewatering installations

Debarking of the logs can be performed in wet or in a dry drum. If the dry process is used

the bark is normally softened by spraying the logs with hot water or with steam. After the

barking drum, the bark is separated in a coarse and in a fine fraction and the coarse fraction

sent to a bark grinder or cutter. On some mills the bark is stored outside and sometimes

mixed with other biofuels such as sawdust or sludges before the mixture is sent to the bark

press. The bark press is driven by an electrical motor with a power of about 50 kW to

dewater 60 m3 of loose bark per hour. Further 5 kW is needed to operate the feed screw to

the press.

In the following section a number of typical bark dewatering installations at Swedish mills

will be described.

At the Stora Enso mill at Skoghall, the pulpwood (approximately 70 spruce and 30 % pine)

goes directly to the bark drum and is flushed with water with a temperature of about 45 °C.

After the drum the bark is fed to a Rotom cutter and then transported to the bark-press

where the dry matter content is increased from about 22 to 40-45 % dry matter content, It

is then fed to the bark boiler while the water from the bark press is concentrated in the

evaporator before the organic components are burnt in the recovery boiler. The principal

process flow sheet is shown in Figure 5.

Figure 5. The bark process at the Stora Enso mill at Skoghall.

The bark press and the perforated press drum at the installation at Stora Enso Hylte are

shown in Figures 6 and 7.

Bark drum Bark cutter Bark press Pulpwood

Boiler

~22% DM

40-45%DM

Water ~45°C

Evaporator



- 5 -

Figure 6. The bark press at Stora Enso Hylte. Figure 7. The perforated press drum.

At the Mondi Packaging mill at Dynäs, the pulpwood is flushed with water before it is fed

to the bark drum where debarking takes place without water. After the drum the bark is cut

and stored outside together with dewatered sludge from the wastewater treatment plant and

small pieces from the chipping house. When needed it is the transported to the power

boiler. Since the bark has a low pH and contains some sand and gravel, the wear on the

holes in the bark press was extensive resulting in a failure of the drum in the bark press. To

eliminate this problem the drum material was changed from ordinary to stainless steel. The

water is circulated in a closed system from the bark press, to the sludge sedimentation and

the clarified water pumped to the thawing of the pulpwood. The principal process flow

sheet is shown in Figure 8.

At the Södra pulp mill at Värö the pulp wood consists of about 70 % spruce and 30 % pine,

the flowsheet is shown in Figure 9. Before the bark drum the logs are flushed with water,

the temperature of the water varies with the season. The bark drum is operated dry, the

bark is then crushed and stored outside for a couple of weeks. During the storage biological

processes start which heats up the bark, a process which results in some drying of the bark.

However during the cold winter months ice formed in the storage can lead to problems in

the bark press. From the storage the bark is separated in three fractions, the small fraction

goes directly to the bark press, the medium fraction goes to a hammer mill and the largest

fraction to another bark cutter. The water from the bark press is pumped to the internal

waste water treatment plant and the pressed bark to the power boiler and the gasifier.

Figure 8. The bark process at the Mondi Packaging mill at Dynäs.

Thawing Bark cutter

Bark storage

Pulpwood

Water 30°C summer 40-45°C winter

Bark drum

Screw transport Bark press Boiler

Chipping house Sedimentation

Water from the bark press

Dewatered sludge



- 6 -

Figure 9. The bark process at the Södra mill at Värö.

5 Experiments performed for dewatering of bark

Experiments were performed at STFI-Packforsk to study how parameters such as pressure,

residence time and temperature affect the dewatering of the bark. The experiments were

performed in a MTS-press, see Figure 10. A new heated press head was constructed that

allowed bringing the bark into contact with a head which could be heated to up to 150 °C.

The thickness of the bark bed was 30 mm. The design of the cell and the press head is

shown in Figure 11.

Thawing

Hammer mill

Bark press

Pulpwood

Recirculated water and fresh water

Boiler

Bark drum

Waste water Press water ~41%DM

~38%DM

Bark crush

Screen

Transport

Bark cutter

Bark storage



- 7 -

Figure 10. MTS-press at STFI-Packforsk.

O-ring

Press feltPerforated plate

Cylinder

Press

piston

Heated press head

ThermocoupleBark bed

Figure 11. The bark dewatering cell and the press head.

The experiments were performed with ground bark from the SCA mill in Obbola which for

these tests were humidified so that the initial dry matter content was 28 %. About 60 gram

of bark was used for each experiments resulting in an initial bed thickness of 25 mm. After

the pressing experiments the thickness ranged between 10 and 13 mm depending on the

pressure used.

The results for different pressures between 2.5 and 20 bar and different temperatures

between 20 and 150 °C and a pressing time of 30 seconds are shown in Figure 12.



- 8 -

4 6 8 10 12 14 16 18 203

4

5

6

7

8

9

10

11

12

Tryck [bar]

Torr

halt [

%T

S]

Torrhalt vid olika tryck och temperaturer, 30s

4 6 8 10 12 14 16 18 2028

30

32

34

36

38

40

Tryck [bar]

Torr

halt [

%T

S]

Torrhalt efter pressning vid olika tryck och temperaturer, 30s

20ºC

50ºC

100ºC

150ºC

20ºC

50ºC

100ºC

150ºC

Figure 12. Dry matter content (Swed. torrhalt) for different pressures

(Swed. tryck) at 30 s.

With a cold press head the maximum dry matter content was about 35 % and increasing the

press head temperature to 150 °C resulted in a further dewatering to 38 % DM. Increasing

the pressing time to 45 seconds did not result in significant changes, see Figure 13.

4 6 8 10 12 14 16 18 203

4

5

6

7

8

9

10

11

12

Tryck [bar]

Torr

halt [

%T

S]

Torrhalt vid olika tryck och temperaturer, 45s

4 6 8 10 12 14 16 18 2028

30

32

34

36

38

40

Tryck [bar]

Torr

halt [

%T

S]

Torrhalt efter pressning vid olika tryck och temperaturer, 45s

20ºC

50ºC

100ºC

150ºC

20ºC

50ºC

100ºC

150ºC

Figure 13. Dry matter content (Swed. torrhalt) for different pressures

(Swed. tryck) at 45 s.

Experiments were also performed with different fractions of the bark, see Figures 14 - 16.

The pressing time was 30 seconds.

The results indicate a higher dry matter content for the smaller fractions. for the fraction 3-

5 mm the dry matter content reached 40.8 % at 10 bar and 150 °C which represents a 39 %

reduction of the initial water content in the bark.



- 9 -

3 4 5 6 7 8 9 103

4

5

6

7

8

9

10

11

12

Tryck [bar]

Torr

halt [

%T

S]

Torrhalt vid olika tryck och temperaturer, 30s. fraktion 3-5mm

3 4 5 6 7 8 9 1028

30

32

34

36

38

40

Tryck [bar]

Torr

halt [

%T

S]

Torrhalt efter pressning vid olika tryck och temperaturer, 30s. fraktion 3-5mm

20ºC

50ºC

100ºC

150ºC

20ºC

50ºC

100ºC

150ºC

Figure 14. Dry matter content for the fraction 3-5 mm.

3 4 5 6 7 8 9 103

4

5

6

7

8

9

10

11

12

Tryck [bar]

Torr

halt [

%T

S]


3 4 5 6 7 8 9 1028

30

32

34

36

38

40

Tryck [bar]

Torr

halt [

%T

S]


20ºC

50ºC

100ºC

150ºC

20ºC

50ºC

100ºC

150ºC




- 10 -

3 4 5 6 7 8 9 103

4

5

6

7

8

9

10

11

12

Tryck [bar]

Torr

halt [

%T

S]


3 4 5 6 7 8 9 1028

30

32

34

36

38

40

Tryck [bar]

Torr

halt [

%T

S]


20ºC

50ºC

100ºC

150ºC

20ºC

50ºC

100ºC

150ºC


Experiments were also performed to measure the moisture content in different positions in

the bed by separating a number of layers with a porous fabric. The experiments were

performed with the fraction 3-5 mm, a pressure of 10 bar, a temperature of 100 °C and a

pressing time of 30 seconds. The result was plotted as moisture ratio (kg water/kg dry

substance) for different dry matter content and is shown in Figure 17.

0.5 1 1.5 2 2.5 3 3.5 41

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

Massa torrsubstans per ytenhet från perforeradplatta [kgTS/m2]

Fuktk

vot

[kgH

20/k

gT

S]

Fuktkvotsprofil i barkkaka efter pressning

Figure 17. Moisture ratio (Swed. fuktkvot) for different positions in the bark bed.

Clearly the gradient in the bark bed is large, at the perforated plate (at the water outlet) the

moisture ratio was 1.05 or a dry matter content of 49 % and at the pressing head only a

moisture ratio of 1.85 or a dry matter content of 35 %.The results are in good agreement

with the industrial experience at Saalasti that pressing in two stages with remixing of the

bark between the stages results in higher outgoing dry matter contents. Another method to

increase the dry matter content is to use double sided dewatering, similar to what is used in

the press section of the paper machine.



- 11 -

The results showed that heating of the bark is beneficial for high outgoing dry matter

contents of the bark. Temperature measurements clearly indicated that conductive heating

of the bark bed from the heated pressing head is a slow process. Calculations for a pressing

head temperature of 150 °C and pressing times between 15 and 45 seconds are shown in

Figure 18.

0 0.002 0.004 0.006 0.008 0.01 0.0120

20

40

60

80

100

120

140

160Temparaturprofil

Avstånd från perforerad platta [m]

Tem

pera

tur

[ºC

]

Temperaturprofil

Figure 18. Bed temperatures for different distances from

the heated pressing head (Swed. avstånd från perforerad platta).

After 45 seconds the temperature profile has only penetrated about 5 mm showing that

either the bark should be remixed during the passage through the bark press or other

methods should be used for heating the bark. One possibility could be to use steaming of

the bark in a vessel ahead of the bark press such as in the feed screw.

6 Some concluding remarks

The economics for a bark pressing process where 30 ton of bark with an ingoing dry matter

content of 35 % was evaluated. The amount of dry substance to be treated is 2.92 kg/s and

corresponds to the amount of bark which is produced at a pulp mill with the capacity 400

000 tons of pulp per year. With existing technology an outgoing dry matter content of 40

% can be reached and by heating the bark an increased dry matter content can be achieved,

in this example calculations were performed for outgoing dry matter contents between 42

and 46 %. The effective heating value of the bark will be increased which can be used

either for increased steam production, the amount of external fuels reduced or the

excessive amount of bark sold externally. In this example, some of the bark has been used

for heating the wet bark and the remaining part sold externally. The following calculations

estimate the economics for such an alternative.

The effective heating value for a wet material can be calculated according to the following

equation:

2.5F - )F1(H H barkeff



- 12 -

The heating value for dry bark has been taken as 19 MJ/kg of dry bark. Additionally the

following data have been used for the calculations:

Operating time for the bark press 4250 h per year

Thermal efficiency of the bark boiler 87 %

-value for the back pressure turbine 0.25

Value of produced electricity 0.40 SEK/kWh

Value of obtained electricity certificates 0.20 SEK /kWh

Value of externally sold bark 0.20 SEK /kWh

It should be mentioned that in Sweden electricity certificates are obtained for the electri-

city that is produced from renewable energy sources such as biofuels and wind. They

account for a substantial part of the economic value of the produced electricity.

The steam required to heat the bark (dry substance and water) has been estimated to be 543

kW. Based on the increased steam production and the -value of 0.25, the electricity

production will increase by 136 kW. The total power needed in the bark boiler will thus be

(543+136)/0.87 = 780 kW. With these results the following revenues can be calculated for

different dry matter contents between 40 and 46 %.

Outgoing dry matter content, % 40 % 42 % 44 % 46 %

Wet bark after the bark press, kg/s 7.30 6.95 6.64 6.35

Heff, MJ/kg wet fuel 6.10 6.53 6.96 7.39

Bark power in the boiler, MW 44.53 45.38 46.21 46.93

Increased value for the bark, kSEK 0 723 1428 2040

Bark for preheating steam, kSEK -663 -663 -663 -663

Increased electricity production, kSEK 231 231 231 231

Electrical certificates, kSEK 116 116 116 116

Total revenues for the process, kSEK -316 407 1112 1724

Figure 19 shows how the total revenues increase with an increased dry matter content after

the bark press. Since the costs for preheating the bark are 663 kSEK, a dry matter content

of about 41 % is needed in order to make the process profitable.



- 13 -

-500

0

500

1000

1500

2000

2500

40 41 42 43 44 45 46 47 48

Revenues, kSEK/year

% DM after the bark press

Figure 19 Total revenues for a preheated bark press.

In these figures the investment costs for preheating the bark have not been included. A

rough estimate from Saalasti gives 40 000 Euro for a heated press roll. Also insulated pipes

for steam have to be added and it is belied that these extra costs will be below

100 000 Euro.

The results from the project indicate that it is possible to achieve an increase in dry matter

content of 2-4 % even if the heating of the bark is not fully optimised. With an optimised

process it seems reasonable that a dry matter increase in the upper range can be achieved.

The conclusion from these calculations is that heating of the bark and the bark press is a

profitable investment.

Increased dry matter content of the bark will normally also result in a more efficient

combustion in the bark boiler which is of the same economical magnitude as the increase

in the effective heating value.

The operating cost for the steam is not negligible and other methods for heating the bark

should be investigated such as using low value waste heat such as flue gases from the

boilers or hot water from the bleaching stages.



- 14 -

7 Literature

[1] Håkansson, M., Stenström, S., Effektivare avvattning av bark i värmda presser.

Problemkartering samt försök i pilotskala, Värmeforsk report 1041 (Summary in English,

report in Swedish), 2007.

[2] Askaner, A., Barkpressning, Examensarbete vid institutionen för Kemisk apparatteknik,

Chalmers Tekniska Högskola, 1976.

[3] Impola, R., Kuoren käsittely polttoaineeksi – PUUT07 (Processing of bark into fuel).

Yearbook on Wood Energy Technology Programme 2000, VTT Symposium 205, Pp. 217 –

231, 2000.

[4] Gupta, M., Yang, J. and Roy, C., Density of softwood bark and softwood char: procedural

calibration and measurement by water soaking and kerosene immersion method. Department

of Chemical Engineering, Université Laval, Sainte-Foy, Québec, Canada, 2001.

[5] Gupta, M., Yang, J. and Roy, C., Specific heat and thermal conductivity of softwood bark and

softwood char particles. Department of Chemical Engineering, Université Laval, Sainte-Foy,

Québec, Canada, 2002.

[6] Koch P.. Specific Heat of Ovendry Spruce Pine Wood and Bark. Wood Science Vol. 1, No. 4,

1969.

Documents

Annex X Energy Efficient Drying and Dewatering Technologies · 2018. 4. 11. · Murlidhar Gupta, Jin Yang and Christian Roy measured the density and the porosity for individual bark