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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.
5
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.
7
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.
10
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].
11
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
12
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����������������
0
50
100
150
200
0 2 4 6 8 10 12
Q (GJ/ADMT)
T (C
)����������������� Mill D
Mill CMill B
Figure 5: GCC for Mills B, C and D, Configuration I
������������������
����������������������������������
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�������������������
�����������������������������������������������������������������������������������������������������������������������������������������������������
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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.
14
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