1

ENERGY CONSEQUENCES IN A MINIMUM EFFLUENTMARKET KRAFT PULP MILL

Ulrika Wising Thore BerntssonPh.D Student Professor

Department of Heat and Power TechnologyChalmers University of Technology412 96 GöteborgSweden

Anders ÅsbladMs.Sc. Chem. Eng.CIT Industriell Energianalys AB412 88 GöteborgSweden

Abstract

Many environmental objectives in pulp and paper mills focus on water closure; i.e.,

many efforts are being made to close plants' water loops. In order to close the water

loops, the problem of accumulation of Non Process Elements (NPE) has to be

controlled. There are many ways of closing the water loops and there are several

processes either available or under development to remove NPE. The combined effects

of water loop closure as well as introduction of new processes have on the energy

consumption in the plant is evaluated here by looking at four different model mills with

different closures and processes used to remove NPE. The processes used to remove

NPE evaluated are pre-evaporation of bleach plant effluents, chip pre-treatment and

combinations thereof. They are part of the group of process discussed today for this

purpose, and they have a significant impact on the energy demand. Earlier studies show

that if the secondary heat system is designed differently from common practice today

and uses the excess heat made available for evaporation, the total heat demand can be

reduced by almost 15%. The same approach is used here. First, a well process integrated

2

mill is evaluated for excess heat at 70-100°C; this excess heat is made available by

redesigning the secondary heat system. By allowing process modifications the

evaporation plant is redesigned and the excess heat made available is used there. This

results in a net reduction in the total live steam demand for the plant. When doing this

for the four minimum effluent mills, the result shows that closure of the mill can be

achieved without increasing the live steam demand, even when including pre-

evaporation (a large energy consumer). The approximate cost for making these changes

is evaluated; the payback period for making these changes in the different minimum

effluent mills vary between 3 and 5 years.

Introduction

Many pulp and paper plants are closing their water loops and/or building effluent

treatment plants in order to satisfy environmental regulations. When closing the water

loops there is an accumulation of Non Process Elements (NPE). These NPE can cause

problems in the system, scaling being one example. In order to stop the accumulation of

NPE, different processes to remove NPE needs to be introduced to the system. These

processes that removes NPE are referred to as kidneys. There are several kidneys

developed and under development today for this purpose. Many kidneys for NPE

involve evaporation, which is an energy intensive process. Shown here are suggestions

how to integrate them without significantly affecting the live steam demand.

In a pulp mill necessary cooling is performed while producing warm and hot water. In

modern mills there is usually a surplus of warm and hot water. Earlier work shows that

if designing the secondary heat system differently, such that only necessary warm and

hot water is produced, excess heat can be made available and using the excess heat for

3

evaporation can reduce the steam demand for the mill. This also reduces the cooling

need. This has been evaluated in earlier work for two different model mills, one

Reference model mill referred to as the base case, and one model mill with a new type

of dryer [1]. For the base case, which is a �state of the art� market kraft pulp mill, the

total steam demand can be reduced by 14% compared to a mill having a conventional

secondary heat system. This is of interest if evaporation is used as a kidney; the extra

evaporation needed could then be satisfied without increasing the total heat demand for

the mill. Therefore the same method has been applied here for four different minimum

effluent kraft pulp model mills.

In order to evaluate the energy consequences of closing the water loops, four different

minimum effluent mills with different concepts for closure are investigated here from

an energy perspective. These mills are energy integrated and the benefits of integration

are shown. A novel design of the secondary heat system is herein presented and

compared to a reference secondary heat system. How the heat is made available, by

designing the secondary heat system differently is also shown. In a cost analysis the

total investment cost for the novel system making heat available and used is compared

with the investment cost for the reference system. The result in steam demand is shown

and conclusions are presented.

The work presented in this paper is part of the Swedish National Program �The Eco

Cyclic Pulp Mill�, financed by MISTRA, the Swedish Foundation for Strategic

Environmental Research and the Swedish Energy Administration [2]. The vision of this

program is an eco-cyclic kraft pulp mill producing high quality products, using as much

as possible of the energy and biomass potential in the raw material entering the mill.

There is a sub project called Energy Potential, which comprises this work, whose aim is

4

to identify efficient energy systems in the minimum impact mill that are economically

and technically attractive.

Aim

The aim of this project is to technically and economically evaluate the effect that

closing a mill�s water loop has on the energy consumption in a market pulp mill and

how the total steam demand can be minimized with reasonable economic conditions.

Four different minimum effluent mills with varying amounts of effluent and different

kidneys are evaluated here.

Methods

Material and energy balances have been simulated for the four different model mills.

The result from these simulations is the base for a process integration study.

For the process integration study Pinch Analysis [3] has been used. This is a well-

established tool used for improving energy efficiency mainly in the petrochemical and

chemical industries. Its use in the pulp and paper industry has been less frequent, but

recently the interest in this tool has increased [4-6]. In this paper Pro Pi [7], an Excel

based program, has been used as a Pinch Analysis Tool. The process integration study is

presented more thoroughly by Wising et al. [1]. In this work it has not been evaluated

how to execute the actual process integration; rather, it is discussed how to make excess

heat available and used, after process integration is completed, to reach even further

energy savings.

The Grand Composite Curves (GCC�s) from the process integration study are evaluated

and the potential for process modification is identified through those curves.

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The secondary heat system is designed to make the excess heat identified in the process

integration study available. Earlier work shows that this excess heat made available is

suitable for evaporation and it is favorable to have the excess heat at the highest

possible temperature in order to save the most live steam [8, 9].

In order to use the excess heat in the evaporation plant, the plant has to be designed in a

new fashion. When using excess heat in the evaporation plant, the excess heat is

introduced at one or more intermediate temperature levels and cascaded through the

effects below this temperature. This method is described more thoroughly by Algehed et

al. [8, 9]. Depending on the temperature of the excess heat and the design of the

evaporation plant the excess heat can be used in one or more effects.

The minimum effluent mills

The bleach plant is the largest contributor to wastewater in a pulp mill. There are

several different water reuse models for reducing wastewater from a bleach plant. One

method is to reuse the filtrate in the upstream stages. The problem with reusing the

filtrate is the accumulation of NPE. The main source for s is the wood entering the mill,

and this differs depending on the origin of the wood [2]. In a mill with enough effluents

the s will leave the plant with the wastewater. When closing the water loops, however,

kidneys might be needed to remove s; several different kidneys are in use today and

even more are under development. Here pre-evaporation of bleach plant effluent, pre-

treatment of the chips entering the plant, chloride kidney for the solution of recovery

boiler ashes and combinations of the kidneys are investigated from an energy

perspective. There are several other kidneys for removal of s, for example different

membranes, electro-dialysis, etc., but pre-evaporation and chip pre-treatment are two

6

kidneys with significant impact on the steam demand, thus they are evaluated here. We

are not studying the capacity or efficiency of the kidneys but merely taking the result

from other parts of the Eco-Cyclic Pulp Mill research program, and assuming that the

quantity and quality of the pulp can be maintained.

The minimum effluent mills are closed versions of the base case as described

thoroughly by Wising et al. [1] The main alternatives that will be investigated here are:

Mill A - 7 m3/ADMT bleach filtrate to pre-evaporationMill B - 7 m3/ADMT bleach filtrate to chip pre-treatment and

5.8 m3/ADMT from the chip pre-treatment to pre-evaporationMill C - 4.7 m3/ADMT bleach filtrate to chip pre-treatment and

3.5 m3/ADMT from the chip pre-treatment to pre-evaporationMill D - 3.7 m3/ADMT bleach filtrate to chip pre-treatment and

2.5 m3/ADMT from the chip pre-treatment to pre-evaporation.

The bleach plant effluent volume is 11 m3/ADMT for the base case; due to the problems

with NPE the amount of effluent from the bleach plant has only been reduced to

7 m3/ADMT for Mills A and B. For Mills C and D the amount has been reduced farther

and represents future situations with a different water reuse model in the bleach plant

(see �The bleach plant�, below), 4.7 m3/ADMT for Mill C and 3.7 m3/ADMT for

Mill D. The chip pre-treatment makes such a closure possible. In the closing of the mills

water loops, both the amount and temperature of the effluent is limited. For all the

above alternatives the concentrated effluent will go to the recovery boiler for

destruction and there will be a chloride kidney for the solution of recovery boiler ashes.

The condensates will be stripped and reused in the mill or discarded as clean effluent.

The cost for chemicals has increased marginally compared to the base case and the

amount of purchased lime increases marginally for Mill A but decreases marginally for

Mills B, C and D [2]. For the mills with chip pre-treatment the evaporation demand for

black liquor has increased because the liquid/wood ratio has increased.

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The bleach plant

The reuse of water in the bleach plant is different for Mills A and B (Figure 1)

compared to Mills C and D (Figure 2). This is mainly in order to be able to reuse more

effluents in Mills C and D, which represent future mills. There is a positive effect on the

energy consumption for Mills C and D because of this large reuse of water. In Mills C

and D the spillage has been eliminated and the dilution factors have been reduced in

order to lower the amount of effluents.

Q 1 O P Q 2 P ODF ro m th e h e a d b o xC o n d e n s a te

T o c h ip p re -tre a tm e n t o r ev a p o ra tio n

S p ill0 .2 5

S p ill0 .2 5

S p ill0 .2 5

S p ill0 .2 5

D Q1

Q 15

7 T o O 2-w as h

Q 1 O P Q 2 P ODQ 1 O P Q 2 P ODF ro m th e h e a d b o xC o n d e n s a te

T o c h ip p re -tre a tm e n t o r ev a p o ra tio n

S p ill0 .2 5

S p ill0 .2 5

S p ill0 .2 5

S p ill0 .2 5

D Q1

Q 15

7 T o O 2-w as h

Figure 1: Reuse of water in the bleach plant for Mills A and B

Q 1 O P Q 2 P ODF ro m th e h e a d b o xC o n d e n s a te

T o O 2-w as hT o c h ip p re -tre a tm e n t

3 .7 - 4 .7

Q 1 O P Q 2 P ODQ 1 O P Q 2 P ODF ro m th e h e a d b o xC o n d e n s a te

T o O 2-w as hT o c h ip p re -tre a tm e n t

3 .7 - 4 .7

Figure 2: Reuse of water in the bleach plant for Mills C and D

8

Pre-evaporation

For all the mills in this study including the reference case, the pre-evaporation is placed

below the sixth effect in the black liquor evaporation plant between 55°C and 40°C.

This has already been implemented in a few mills today. Optionally, Mechanical Vapor

Recompression (MVR) could also be used for pre-evaporation, but that has not been

evaluated here. Previous work shows that MVR can achieve favorable results [10]. One

reference pre-evaporation plant has been designed for each of the different model mills,

to be compared to the evaporation design where excess heat is used. The live steam

demand for the different mills increases compared to the base case, depending on the

amount of effluent pre-evaporated.

Chip pre-treatment

One interesting alternative to treatment of the bleach filtrate is an acid pre-treatment of

wood before the cooking stage. Most of the NPEs originate from the wood itself. If

these can be removed before cooking, there might not be a need for treatment of the

bleach plant filtrate in order to reuse it. Studies have shown that the amount of Ba, Ca,

K and Mn in the wood chips can be reduced between 50-80% even with small volumes

of acid [2, 11]. The studies also show that the temperature in the chip pre-treatment

cannot exceed 100°C, for above that temperature the pulp yield and pulp quality will

decrease. One option with the pre-treatment of wood chips is to use the filtrate from the

acid stages in the bleach plant as pre-treatment (Figure 3), while evaporating the

effluents from the pre-treatment in the evaporation plant and burning the concentrated

effluents in the recovery boiler. In Figure 3 the chip pre-treatment is positioned before

the steaming vessel, referred to as Configuration II, but having the pre-treatment after

the steaming vessel is investigated as well, Configuration I. So for the mills with chip

9

pre-treatment (B, C, D), there are two configurations. The performance of the chip pre-

treatment and the quality of the pulp is not considerably affected by the position of the

steaming vessel [12].

Chip pre-treatment

Steaming vesselChip bin Digester

Bleach plant effluent

To pre-evaporation

Wood chips

Flash steam

Flash steam

Chip pre-treatment

Steaming vesselChip bin Digester

Bleach plant effluent

To pre-evaporation

Wood chips

Flash steam

Flash steam

Figure 3: Chip pre-treatment, Configuration I; chip pre-treatment before the steaming vessel.

The chloride kidney

The chloride kidney is assumed to be an evaporative crystallization of recovery boiler

ashes that has been marketed by several companies [13-15]. The energy consequences

for the chloride kidney are assumed to be negligible but will be investigated further in

future work.

Process integration

The live steam demand for the four mills before process integration including the

kidneys can be seen in Table 1.

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Table 1: Total live steam demand before process integration

Base case(GJ/ADMT)

Mill A(GJ/ADMT)

Mill B(GJ/ADMT)

Mill C(GJ/ADMT)

Mill D(GJ/ADMT)

Total live steam demandConfiguration IConfiguration II

10.4 11.9--

11.711.9

10.710.9

10.410.6

For the process integration study certain restrictions have been applied; not all streams

are included because of the present design of the individual processes as defined in the

research program [1]. As a result the GCC�s do not represent all the true streams in the

system but rather the defined equipment. This is discussed more thoroughly by Wising

et al. [1]. With these restrictions, the lowest possible energy demand using economically

viable temperature differences between the heat-exchanged streams for the four mills

are shown in Table 2. There is a slight difference between Configurations I and II both

before and after process integration, caused by the temperature limit in the pre-

treatment. All of the four mills have a potential for process integration above

1 GJ/ADMT.

Table 2: Total live steam demand after process integration

Base case(GJ/ADMT)

Mill A(GJ/ADMT)

Mill B(GJ/ADMT)

Mill C(GJ/ADMT)

Mill D(GJ/ADMT)

Total live steam demandConfiguration IConfiguration II

9.3 10.4--

10.910.7

9.99.7

9.69.4

The GCC�s for the different mills can be seen in Figures 4, 5 and 6. When evaluating

the GCC for the different cases there is a large cooling demand below the pinch

temperature. This cooling demand can be made into usable excess heat if process

modifications are allowed. As earlier studies have shown there is a large potential to use

this excess heat below the pinch temperature for evaporation if the evaporation plant is

re-designed [8, 9].

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In a market pulp plant, all the heat below the pinch temperature is used for production

of warm and hot water. As a result, heat at a medium temperature (between 70-100°C)

is rejected as waste heat, since the plant does not have the need for all the warm and hot

water produced. Therefore it is very important to design the secondary heat system so

that only the necessary warm and hot water is produced without causing any operational

problems. The remaining excess heat below the pinch temperature is then available for

use elsewhere in the plant.

0

50

100

150

200

0 2 4 6 8 10 12

Q (GJ/ADMT)

T (°

C)

Mill A

Figure 4: GCC for Mill A

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100

150

200

0 2 4 6 8 10 12

Q (GJ/ADMT)

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Mill CMill B

Figure 5: GCC for Mills B, C and D, Configuration I

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0

50

100

150

200

0 2 4 6 8 10 12

Q (GJ/ADMT)

T (C

)

������������������Mill DMill CMill B

Figure 6: GCC for Mills B, C and D, Configuration II

For Mill A the excess heat below the pinch temperature above 80°C is 2.3 GJ/ADMT.

For Mills B, C and D the excess heat below the pinch temperature is the same but can

vary depending on whether the steaming vessel is positioned before or after the chip

13

pre-treatment. With Configuration I the excess heat below the pinch temperature above

80°C is larger (3.2 GJ/ADMT compared to 2.3 GJ/ADMT) for Configuration II.

Secondary heat system

As mentioned above, in order to make the excess heat available the secondary heat

system needs to be designed so that only the necessary warm and hot water is produced.

Since the excess heat is going to be used in the evaporation plant, the higher the

temperatures on the excess heat the better. The aim when designing the secondary heat

system has therefore been to make excess heat available at the highest possible

temperature. For each novel secondary heat system a reference system has been

designed as well, where all the heat below the pinch temperature is cooled (Figure 7 and

Figure 8). As can be seen in all the figures showing the secondary heat systems, Figure

7-Figure 10, the pinch temperature is 95°C and all the excess heat available is below

this temperature.

For all the novel secondary heat systems a distribution system has to be designed as

well. This distribution system transform the excess heat made available into usable

steam, i.e. both heat exchangers and a steam reformer are included in the distribution

system.

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S tea m fro m e v ap o ra tio n

C o o lin g d ilu tio n a nd w a sh filter

C o nd e nsin g o f the re lie f v ap o rs

F la sh ste a m fro m th ird flash

S tea m fro m sm e lt-d isso lv in g ta nk

W aste w ate r

M ak eup b o ile r fe ed w a ter

H o t w ate r

W arm w ate r

4 0

9 0

1 00

8 5

8 5

4 5

4 0

9 6

1 00

8 5

1 00

7 3

1 0 9 5

5 0 9 0

1 0 5 0

9 5

3 5 6 3

3 5

5 0 9 0

1 0 5 0H o t w ate r

W arm w ate r4 3

S tea m fro m e v ap o ra tio n

C o o lin g d ilu tio n a nd w a sh filter

C o nd e nsin g o f the re lie f v ap o rs

F la sh ste a m fro m th ird flash

S tea m fro m sm e lt-d isso lv in g ta nk

W aste w ate r

M ak eup b o ile r fe ed w a ter

H o t w ate r

W arm w ate r

4 0

9 0

1 00

8 5

8 5

4 5

4 0

9 6

1 00

8 5

1 00

7 3

1 0 9 5

5 0 9 0

1 0 5 0

9 5

3 5 6 3

3 5

5 0 9 0

1 0 5 0H o t w ate r

W arm w ate r4 3

Figure 7: Reference heat exchanger network for Mills B, C and D; Configuration I, with alltemperatures shown in Celcius

S tea m fro m e v ap o ra tio n

C o o lin g d ilu tio n a nd w a sh filter

C o nd e nsin g o f the re lie f v ap o rs

F la sh ste a m fro m th ird flash

S tea m fro m sm e lt-d isso lv in g ta nk

W aste w ate r

M ak eup b o ile r fe ed w a ter

H o t w ate r

W arm w ate r

4 0

9 0

1 00

8 5

8 5

4 5

4 0

9 6

1 00

8 5

1 00

7 3

1 0 9 5

5 0 9 0

1 0 5 0

9 5

3 5 6 3

3 5

5 0 9 0

1 0 5 0H o t w ate r

W arm w ate r

S tea m fro m e v ap o ra tio n

C o o lin g d ilu tio n a nd w a sh filter

C o nd e nsin g o f the re lie f v ap o rs

F la sh ste a m fro m th ird flash

S tea m fro m sm e lt-d isso lv in g ta nk

W aste w ate r

M ak eup b o ile r fe ed w a ter

H o t w ate r

W arm w ate r

4 0

9 0

1 00

8 5

8 5

4 5

4 0

9 6

1 00

8 5

1 00

7 3

1 0 9 5

5 0 9 0

1 0 5 0

9 5

3 5 6 3

3 5

5 0 9 0

1 0 5 0H o t w ate r

W arm w ate r

Figure 8: Reference heat exchanger network for Mills B, C and D; Configuration II, with alltemperatures shown in Celcius

In the novel networks (Figure 9 and Figure 10), only the necessary warm and hot water

is produced, which means that not all the heat below the pinch temperature is cooled

and the flow through the heat exchanger network is smaller. Instead, the medium

15

temperature heat is left for use in the evaporation plant, finally ending up in the surface

condenser. Only the necessary warm and hot water is produced, and considered together

with the low temperature in the surface condenser (40°C), the need for external cooling

of the wastewater before released to the effluent treatment plant in the novel system can

be thus minimized as well as the cooling of the circulated water in the system.

S tea m fro m e v ap o ra tio n

C o o lin g d ilu tio n a nd w a sh filter

C o nd e nsin g o f the re lie f v ap o rs

F la sh ste a m fro m th ird flash

S tea m fro m sm e lt-d isso lv in g ta nk

W aste w ate r

M ak eup b o ile r fe ed w a ter

H o t w ate r

W arm w ate r

4 0

9 0

1 00

8 5

8 5

4 5

4 0

9 6

1 00

8 5

1 00

7 3

1 0 9 5

5 0 9 0

1 0 5 0

9 5

3 5 6 3

3 5

S tea m fro m e v ap o ra tio n

C o o lin g d ilu tio n a nd w a sh filter

C o nd e nsin g o f the re lie f v ap o rs

F la sh ste a m fro m th ird flash

S tea m fro m sm e lt-d isso lv in g ta nk

W aste w ate r

M ak eup b o ile r fe ed w a ter

H o t w ate r

W arm w ate r

S tea m fro m e v ap o ra tio n

C o o lin g d ilu tio n a nd w a sh filter

C o nd e nsin g o f the re lie f v ap o rs

F la sh ste a m fro m th ird flash

S tea m fro m sm e lt-d isso lv in g ta nk

W aste w ate r

M ak eup b o ile r fe ed w a ter

H o t w ate r

W arm w ate r

4 0

9 0

1 00

8 5

8 5

4 5

4 0

9 6

1 00

8 5

1 00

7 3

1 0 9 5

5 0 9 0

1 0 5 0

4 0

9 0

1 00

8 5

8 5

4 5

4 0

9 6

1 00

8 5

1 00

7 3

1 0 9 5

5 0 9 0

1 0 5 0

9 5

3 5 6 3

3 5

Figure 9: Novel heat exchanger network for Mills B, C and D; Configuration I and II, with alltemperatures shown in Celcius

S tea m fro m e v ap o ra tio n

C o o lin g d ilu tio n a nd w a sh filter

C o nd e nsin g o f the re lie f v ap o rs

F la sh ste a m fro m th ird flash

S tea m fro m sm e lt-d isso lv in g ta nk

W aste w ate r

M ak eup b o ile r fe ed w a ter

H o t w ate r

W arm w ate r

4 0

9 0

1 00

8 5

8 5

4 0

9 6

1 00

8 5

1 00

4 5 7 3

1 0 9 5

5 0 9 0

1 0 5 0

9 5

2 9

3 5

5 5

8 5 9 3

W ash liq uo r to b o tto m o f the d ige ste r

S tea m fro m e v ap o ra tio n

C o o lin g d ilu tio n a nd w a sh filter

C o nd e nsin g o f the re lie f v ap o rs

F la sh ste a m fro m th ird flash

S tea m fro m sm e lt-d isso lv in g ta nk

W aste w ate r

M ak eup b o ile r fe ed w a ter

H o t w ate r

W arm w ate r

4 0

9 0

1 00

8 5

8 5

4 0

9 6

1 00

8 5

1 00

4 5 7 34 5 7 3

1 0 9 5

5 0 9 0

1 0 5 0

9 5

2 9

3 5

5 5

8 5 9 38 5 9 3

W ash liq uo r to b o tto m o f the d ige ste r

Figure 10: Novel heat exchanger network for Mill A, with all temperatures shown in Celcius

16

Mills with chip pre-treatment

For Mills B, C and D there are two novel secondary heat systems; one for each

configuration that differs only in heat load, the same streams are heat exchanged

(Figure 9). They are the same for Mills B, C and D because the only difference below

the pinch temperature for the three mills is the amount of excess heat to the surface

condenser. This can be released directly to the effluent treatment plant because of the

low temperature, therefore not affecting the secondary heat system [1]. There is a slight

difference between Mill B compared to Mills C and D due to the different reuse of

filtrates in the bleach plant but that difference is insignificant below the pinch

temperature.

The number of heat exchanger units in the two novel secondary heat systems is seven

including the heat exchanger units in the distribution system. For the reference

secondary heat system the number of heat exchanger units are nine for Configuration I

and eight for Configuration II. The medium temperature excess heat after heat

exchanging both in the reference system and the novel systems can be seen in Figure 11

and Figure 12. These are not part of a GCC so the temperatures are actual temperatures.

The figures show that the medium temperature heat available differs remarkably

between the novel and reference secondary heat systems. The novel systems each leave

usable heat above 80°C of 1.9 GJ/ADMT for Configuration II and 2.6 GJ/ADMT for

Configuration I. This is more than 80% of the cooling demand between 80-100°C

according to the GCC.

17

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8

Q (GJ/ADMT)

T (°

C)

Reference networkNovel network

Figure 11: Excess heat for Mills B, C and D, Configuration I

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7

Q (GJ/ADMT)

T (°

C)

Reference networkNovel network

Figure 12: Excess heat for Mills B, C and D, Configuration II

Mill with only pre-evaporation

For Mill A the novel secondary heat system consists of 7 heat exchanger units including

the heat exchanger units in the distribution system; the reference system consists of 8

18

heat exchanger units. The medium temperature excess heat after heat exchanging both

in the reference system and the novel system can be seen in Figure 13. As for Mills B, C

and D there is a large difference for the usable excess heat between the novel and

reference system. The novel system leaves 1.8 GJ/ADMT of usable excess heat above

80°C, which is approximately 80% of the cooling demand between 80-100°C according

to the GCC.

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8 9

Q (GJ/ADMT)

T (°

C)

Novel networkReference network

Figure 13: Excess heat for Mill A

In these model mills the flue gases have not been accounted for. This is for two reasons:

first there is no need to clean the flue gases further with a scrubber; second, it is difficult

and/or expensive to use the heat in the flue gases above 60°C. In the base case there is a

surplus of heat around 60°C, thus presenting no need for the flue gas heat. In the models

with minimum effluent there is not a surplus of heat at 60°C and as can be seen in the

heat-exchanging networks (Figure 9 and Figure 10), warm water at 50°C is produced by

96°C heat. If instead a scrubber was installed and the heat from the flue gases taken into

19

consideration, the warm water could be produced by 60°C heat and not 96°C heat. This

would leave the 96°C heat to be used elsewhere, and reduce the total steam demand. As

an example 1 GJ/ADMT of 60°C heat has been added to Configuration II, the network

has been constructed, and the resulting excess heat can be seen in Figure 14. In

Figure 14 there is a surplus of heat at 60°C because not all the heat from the added

1 GJ/ADMT could replace a heat source of higher temperature. Only the heating of

warm water by the 96°C heat could be replaced, thus there would be no positive gain to

further increase the heat source at 60°C. Compared to the novel network there is an

extra 0.23 GJ/ADMT of heat at 96°. For Mills A and B, the extra available heat would

be 0.27 GJ/ADMT, the higher value for Mills A and B is created by the different water

reuse model in the bleach plant.

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8

Q (GJ/ADMT)

T (°

C)

Reference networkNovel network with scrubberNovel network

Figure 14: Excess heat for Mills B, C and D; Configuration II with heat from scrubber included

20

Evaporation design for use of excess heat

When using excess heat in the evaporation plant the excess heat is introduced at one or

more intermediate temperature levels and cascaded through the effects below this

temperature. This method is described more thoroughly by Algehed et al. [8, 9].

Depending on the temperature of the excess heat and the design of the evaporation

plant, the excess heat can be used in one or more effects. For all the mills in this study

the evaporation plant is a seven-effect evaporation train. This is the most economical

design for these conditions [8, 9]. When there is excess heat available it is supplied in

effect four and cascaded through four effects.

Economy

As well known, it is important in the planning of a process not only to know if it is

technically feasible but also to know the cost for implementing it. It is very difficult to

say exactly how much it is going to cost to build the secondary heat system and use the

excess heat in the plant. Here, approximate investment cost for the new system where

excess heat is made available and used in the evaporation plant compared to the

investment cost for a reference secondary system is shown. They are not retrofit

situations but green field plants. Included in the investment cost for the new system are:

• The novel secondary heat system

• The collection of excess heat

• Transformation of the excess heat to usable heat

• The evaporation plant

21

This is compared to the investment cost for the reference secondary heat system and the

reference evaporation plant.

There is a need for a cooling tower in the reference system and there is a need for a

smaller cooling tower in the novel system. The cost for cooling tower is site specific,

depending on where the plant is located; it has therefore been excluded in this analysis.

The cost should be larger for the reference system than the new system because of the

smaller cooling need for the new system. Since the same streams are heat exchanged in

both the reference system and the novel system the piping and tubing are assumed to be

the same and therefore excluded in the investment cost. The considerations for

investment costs in this analysis are budget costs only, which include heat exchange

area cost of $400/m2.

Table 3: Conditions for the economic evaluation

Annuity factor 0.1Cost for electricity $20-$30/MWhBoiler efficiency 0.80 based on LVHPower to heat ratio 0.28Value of biomass $7/MWh

When building the new system the plant uses less live steam than the reference system,

but when reducing the steam demand the electricity production is reduced as well. In

order to perform an economic evaluation, the cost for buying that electricity back is

included; this could also be the reduced revenue from selling the electricity. An average

value for buying electricity in the Swedish pulp and paper industry today is $20/MWh

were a higher value is more representative of the conditions in the North American pulp

and paper industry. When estimating the changes in electricity production due to the

reduced steam demand the turbine is assumed to have capacity both for increased and

decreased electricity production, thus the investment cost for the turbine is not affected

22

here. The conditions for those calculations are presented in Table 3. In this plant the

saved fuel is biomass because it is already energy efficient and the value for biomass is

set to $7/MWh, which by some is considered low. The biomass consists of both bark

and lignin; in order to be able to sell lignin there has to be a process available for its

precipitation. Today there is ongoing research in this field, as well as in the Eco-cyclic

Pulp Mill research program [16, 17]. If the biomass had been processed the value when

selling it would of course be greater. Since this is a green field plant an annuity factor of

0.1 is applied even for the energy investments, but 0.2 is usually used for such

investments in retrofit situations.

Secondary heat system

The investment cost for the novel and reference secondary heat system for the two

models can be seen in Table 4.

Table 4: Investment cost for the different secondary heat systems

Number of heatexchanger units

Area requirements(m2)

Area cost(M$)

Reference network, Mill A 8 3000 1.2Novel network, Mill A 6 1900 0.8Reference network, Mills B, C andD Configuration I 8 3900 1.6Novel network, Mills B, C and DConfiguration I 6 2300 0.9Reference network, Mills B, C andD Configuration II 9 3300 1.3Novel network, Mills B, C and DConfiguration II 6 2300 0.9

Distribution system

The distribution system includes heat exchangers to transfer the excess heat to hot water

and a steam reformer. There is an optimization between the investment cost for the

distribution system and the extra investment cost in the evaporation plant as well as

23

running costs. Shown here in Table 5 is the optimized investment cost for the

distribution system.

Table 5: Investment costs for the distribution system

Number of heatexchanger units

Area requirements(m2)

Area cost(M$)

Mill ADistribution systemSteam reformer

11

20002700

0.81.1

Mills B, C and D; Configuration IDistribution systemSteam reformer

11

45005300

1.52.0

Mills B, C and D; Configuration IIDistribution systemSteam reformer

11

16001800

0.60.7

Evaporation plant

The cost for using this excess heat in the evaporation plant is shown (Table 6) and

compared to the cost for the reference evaporation plant where no excess heat is used

[8, 9].

24

Table 6: Investment cost for the evaporation plant

Live steam demand forevaporation (GJ/ADMT)

Area requirements(m2)

Investment cost(M$)

Mill ANo excess heatExcess heat

5.44.2

6100068000

35.338.6

Mill B, Configuration INo excess heatExcess heat

5.33.7

6100071000

34.940.0

Mill B, Configuration IINo excess heatExcess heat

5.44.1

6100068000

35.038.7

Mill C, Configuration INo excess heatExcess heat

4.62.9

5500069000

32.338.9

Mill C, Configuration IINo excess heatExcess heat

4.73.4

5500064000

32.436.4

Mill D, Configuration INo excess heatExcess heat

4.32.7

5300068000

31.338.4

Mill D, Configuration IINo excess heatExcess heat

4.33.1

5300062000

31.435.8

Summary of the Results

The result of this study can be seen in Table 7, where the numbers in parenthesis

represents the values corresponding to the higher electricity price. Since the cost for the

chip pre-treatment is not included in this study the payback period for Mills B, C and D

has not been estimated. Even without the chip pre-treatment the payback period would

be approximately the same as for Mill A or significantly larger.

25

Table 7: Reduction of live steam demand for the four model mills compared to the reference system

Reductionof total live

steamdemand

Yearlyrevenuefor soldbiomass(M$/year)

Extra annualinvestmentcost for thenew system(M$/year)

Cost1 forbuying

electricity(M$/year)

Net profit(M$/year)

Pay-backperiod2

(years)

Mill A 12% 2.8 0.5 1.3 (1.9) 0.8 (0.2) 3.6 (7.3)Mill B

Configuration IConfiguration II

16%12%

3.72.8

0.80.5

1.8 (2.8)1.4 (2.0)

1.0 (0.1)0.9 (0.2)

--

Mill CConfiguration IConfiguration II

18%12%

3.72.7

0.90.5

1.9 (2.8)1.3 (2.0)

0.9 (0)0.8 (0.2)

--

Mill DConfiguration IConfiguration II

18%13%

3.62.6

1.00.5

1.8 (2.6)1.3 (2.0)

0.8 (0)0.8 (0.1)

--

1 Cost or reduction in revenue2 A payback period is not calculated for Mills B, C and D since not all costs are included

With the economic conditions discussed above, there is an economic gain in designing

the secondary heat system differently from today and using the excess heat made

available in the plant. When closing the water loops there is usually an increase in steam

demand because there is a larger evaporation demand. A comparison to the base case, a

mill without reduced effluent can be seen in Table 8 [1].

26

Table 8: Total heat demand before and after process integration and redesign of secondary heatsystem

Before processintegration(GJ/ADMT)

After processintegration(GJ/ADMT)

After redesign of thesecondary heat

system (GJ/ADMT)

Saved heatdemand

(GJ/ADMT)

Base case 10.4 9.3 7.8 2.6Mill A 11.9 10.4 9.2 2.7Mill B

Configuration IConfiguration II

11.911.7

10.710.9

9.19.6

2.82.1

Mill CConfiguration IConfiguration II

10.910.7

9.79.9

8.08.6

2.92.1

Mill DConfiguration IConfiguration II

10.610.4

9.49.6

7.88.4

2.82.0

Discussion and conclusions

The payback period for Mill A is lower compared to the base case, presented more

thoroughly by Wising et al. [1]. For the base case it is 6.7 years and for Mill A it is 3.6.

This is mainly due to the fact that for the base case there were more heat exchanger

units in the distribution system, thus a larger investment cost. For Mills B, C and D the

cost for the actual chip pre-treatment is not included in the cost, which is why the profit

for these cases will be lowered if included. For that reason the most profitable solution

is to pre-evaporate without a chip pre-treatment not considering the positive effects the

chip pre-treatment might have on the running of the plant.

When closing the water loops in the mills presented here, the best-case scenario results

in the same live steam demand as for the base case. This is because all the mills have

pre-evaporation as a kidney, which is very energy intensive. The case where we can

achieve the same live steam demand as the base case is where the mills� water loops are

closed very tightly as in Mill D, thus having the lowest pre-evaporation demand. The

benefits in running the mill while having the chip pre-treatment is not taken into

27

account here except for the fact that we can close the water loops as tightly as is done in

Mills C and D because of the chip pre-treatment. Thus the chip pre-treatment makes it

possible to close the mill without increasing the live steam demand. There are other

methods of closing the mill that have not been evaluated here that might be more

favorable from an energy perspective, for example different membranes or heat

pumping.

References

[1] Wising U., Berntsson T. and Åsblad A., "Usable excess heat in future Kraft pulpmills", 2001

[2] KAM, Eco-cyclic Pulp Mill. Final report KAM 1, 1996-1999, Report A32,STFI, Stockholm, 2000

[3] Linnhoff B., et al., User guide on process integration for the efficient use ofenergy, IchemE, Rugby, UK, 1982

[4] Cripps H. R., et al., "Pinch integration achieves minimum energy evaporationcapacity", 1996 Engineering conference, TAPPI Press, 1996

[5] Retsina T., Cripps H. R. and Whitmire L., "Unpinch mill steam restrictions:systematic solutions to perennial problems", 1997 Engineering andpapermakers: forming bonds for better papermaking, TAPPI Press, 1997

[6] Stromberg J., Berglin N. and Berntsson T., "Using process integration toapproach the minimum impact pulp mill", 1997 TAPPI Environmentalconference and exhibit, TAPPI Press, 1997

[7] CIT Industriell Energianalys AB, http://www.cit.chalmers.se

[8] Algehed J., et al., "Opportunities for process integrated evaporation in kraft pulpmills", 2000 TAPPI Engineering Conference, TAPPI press, 2000

[9] Algehed J. and Berntsson T., "Evaporation of black liquor and wastewater usingexcess heat at medium high temperature; simulation and economic evaluation ofvarious options", Submitted for publication in Nord. Pulp Pap. Res. J., 2001

[10] Algehed J., Stromberg J. and Berntsson T., "Energy-efficient pre-evaporation ofbleach plant filtrates: an economic evaluation of various options (ExtendedAbstract)", Tappi Journal, no. 9, p. 55, 2000

28

[11] Brelid H., Friberg T. and Simonson R., "TCF bleaching of softwood kraft pulp.Part 4. Removal of manganese from wood shavings prior to cooking", Nord.Pulp Pap. Res. J., no. 1, p. 50-56, 1998

[12] Theliander H., Professor at the Department of Forest Products and ChemicalEngineering at Chalmers University of Technology, Personal communication,2001

[13] Koskiniemi J., et al., "Removal of chlorides from chemical circulation in thekraft pulp mill", Appita, no. 6, p. 460-463, 1999

[14] Earl P. and Lawless D., "Recovery of sodium chloride from a kraft pulp mill forre-use in bleaching chemical manufacture", International Pulp BleachingConference, KCL Finnish Pulp and Paper Research Institute, 1998

[15] Straton S. C. and Ferguson M., "Progress report on the BFR technologydemonstration: December 1996", Pulp Pap. Can., no. 3, p. 45-47, 1998

[16] Sundin J. and Hartler N., "Precipitation of kraft lignin by metal cations inalkaline solutions", Nord. Pulp Pap. Res. J., no. 4, p. 306-312, 2000

[17] Lora J. H. , Abacherli A. and Doppenberg F., "Debottlenecking the recoverysystem of soda pulp mills by lignin recovery and wet oxidation: application tonon-wood fibers black liquors", 2000 Pulping/process and product qualityconference, TAPPI Press, 2000