Energy system consequences when installing an advanced delignification process.
Dans l'industrie des pates et papiers lors de projets de modernisation importants, les consequences globales des systemes d'energie ne sont pas estimees en detail; en general, seules les consequences locales sont prises en compte lors de l'evaluation de projets possibles. Dans cet article, on presente les effets sur le systeme d'energie global de l'installation d'un procede de delignification avance (soient un digesteur de pulpe de conception actuelle et un procede de delignification a l'oxygene). De meme, on analyse l'effet d'une telle installation sur le potentiel pour une plus grande integration des energies. En outre, on presente les consequences en terme de C[O.sub.2] liees a l'installation d'un tel systeme et a l'integration energetique de l'usine.
Keywords: pulp and paper, process integration, C[O.sub.2] emissions, digester, oxygen delignification process
When investing in new equipment in a mill, the effect on the pulp quality is often thoroughly investigated, usually through modelling. Even possible scaling effects and other process oriented consequences are evaluated. The energy demand for the new process is also taken into account in the decision making process. Seldom is the entire energy system evaluated or considered, even though installing new equipment not only affects the local system where the new equipment is installed but rather the entire energy system. In this work an example of how the entire energy system is affected when installing a new oxygen delignification unit and/or a new digester is presented.
Not only do major process changes affect the entire energy system in a pulp mill, also the potential for process integration changes. Even in a very energy efficient mill with negligible pinch violations, such as the model mills evaluated in this work, it has been shown that further process integration is possible if allowing for process modifications (Wising et al., 2002).
The opportunities for further process integration are related to the warm and hot water production system (Ahtila and Svinhufvud, 1998). In a kraft pulp mill it is common that excess heat sources are used to produce a surplus of warm and hot water that is overflowed from the tanks and cooled in a cooling tower/cooling system, i.e., producing a surplus of warm and hot water cools the excess heat sources in the process. Earlier work has shown that these heat sources that are being cooled can be made available if changes to the warm and hot water production system are allowed (Wising et al., 2002). The heat sources can then be used for evaporation in order to minimize fresh live steam use if the evaporation plant is redesigned (Algehed et al., 2002). Other uses for the heat sources can be for example, heat pumping or district heating. The potential for making the heat sources available is different in different mills due to variations in equipment.
Reducing the energy consumption in a mill, either through replacing older equipment for more energy efficient ones or by integrating the processes, usually leads to a reduction of fossil fuel use in the mill or in society (Francis et al., 2002). Since C[O.sub.2] emissions are becoming more important because of the ratification of the Kyoto Protocol and it coming into effect, the C[O.sub.2] consequences for the different model mills are presented in this work. Two future scenarios for marginal electrical power production are used in order to give an illustration of the C[O.sub.2] consequences dependent on the electricity grid.
The work that we present in this paper is part of the Swedish national Eco-Cyclic Pulp Mill research program (KAM, 2003). The vision of this program is an eco-cyclic and energy efficient system for high-quality pulp and paper products. The aim of the Energy Potential sub-project, which includes the work presented in this paper, is to identify energy systems in and around the pulp and paper mill that are economically, technically and environmentally attractive.
The aim of this work is to illustrate the effects replacing major process equipment can have on the overall energy system of a kraft pulp mill. Not only the obvious energy savings by replacing old equipment by new, more energy efficient equipment are evaluated, but also the effects it can have on the potential for process integration.
THE REFERENCE MODEL MILL
In the Eco-Cyclic Pulp Mill research program a theoretical kraft pulp model mill has been developed, called the Reference Mill (KAM, 2003). This model mill consists of "state-of-the-art" technology built and run in Swedish and Finnish pulp and paper industries today. It represents a new mill, as it would be built if built today. Future developments have been illustrated in the research program as future model mills, resembling the Reference Mill where one or more processes within the mill have not yet been commercially tested.
The Reference Mill produces 2000 air-dried metric tonnes of bleached softwood pulp per day. It uses 70% round wood that is debarked in a dry debarker and then chipped, and 30% sawmill chips. The chips are defrosted/heated up in a chip bin followed by a continuous pre-steaming vessel. The chips are then fed into a continuous, low temperature, two-vessel digester with hot black liquor impregnation and high alkali in co- and countercurrent stages. This digester is working in principle as the Compact Cooking[TM] concept marketed by Kvaerner Pulping (Lundgren and Andtbacka, 2000). The cooking temperature is 148[degrees]C and the heat of reaction is sufficient, meaning that the circulations in the digester do not need to be heated, as the temperature profile is still fairly constant. Also, due to the low temperature there is only one flash of the black liquor going to the evaporation plant. The flash steam is used in the chip bin and Low Pressure (LP) steam at 0.45 MPa is needed for the pre-steaming vessel. The Kappa number after the digester is 27. Due to the lower temperature/pressure in the digester, cheaper construction material can be used, resulting in a lower investment cost compared to higher temperature/pressure digesters.
The digester is followed by a two-stage low temperature oxygen delignification process at 95[degrees]C and after this process the Kappa number is 10. This is followed by a 4-stage ECF bleach plant, producing a pulp with a brightness of ISO = 89%. All the washes in the oxygen delignification process and the bleach plant are wash presses with a high efficiency (Displacement Ratio (DR) = 0.50 and discharge consistency = 35%). After the bleach plant, the pulp is dried in a conventional cylinder pulp dryer with a shoe press. The mill has a total yield (pulp out/ chips in) of 43.7% and is very energy efficient with a total steam demand of 10.8 GJ/air-dried metric tones (GJ/t). Due to it being so energy efficient, it uses no fossil fuels, plus it exports half of its bark from the debarker; the other half is gasified and used as fuel in the lime kiln.
The Reference Mill has a conventional counter-current falling film six-effect evaporation plant and concentrators where the black liquor is concentrated from 16% to 80% using a mix of LP steam and Medium Pressure (MP) steam (1.2 MPa). The steam from the sixth effect is condensed in a condenser referred to as the surface condenser. The black liquor is fired in a recovery boiler (8.0 MPa, 490[degrees]C), where the steam for the process is produced. Before the steam is used in the process, it is expanded in a backpressure turbine to 0.45 MPa (LP-steam) with two extractions, one at 3.0 MPa for soot blowing and one at 1.2 MPa (MP-steam), producing all the electricity needed in the mill. Even though the Reference Mill does not have a power boiler or a bark boiler, it still has a steam surplus due to it being so energy efficient. The steam surplus is expanded in a condensing turbine producing a surplus of electricity sold to the grid. The recausticizing plant and lime kiln are of conventional design, producing white liquor with 35% sulphidity.
There are two cooling towers in the Reference Mill, one for the condensing turbine/surface condenser, and one for wastewater before the wastewater treatment plant since it has a temperature limit.
In this work, three different model mills have been evaluated: Model Mills 1, 2 and 3. The differences between them are summarized in Table 1. All processes except for the digester and oxygen delignification process are the same for the three model mills.
The first model mill evaluated in this work, Model Mill 1, shares all properties and components of the Reference Mill save two: in Model Mill 1, both the digester and oxygen delignification from an earlier model mill in the same research program have been substituted. This digester is a continuous, two-vessel digester with black liquor impregnation with an even temperature profile throughout the digester (Iso-Thermal). The cooking temperature is 160[degrees]C, the digester has three circulations of which two are heated indirectly with MP-steam in order to keep the even temperature profile and the Kappa number after the digester is 20. The black liquor to the evaporation plant from the digester is flashed in two stages; the flash steam from flash 1 is used in the steaming vessel and the flash steam from flash 2 is used in the chip bin. It is working in principle as the BLI[TM]-ITC[TM]-digester by Kvaerner Pulping (Backstrom and Jensen, 2000).
The digester is followed by a two-stage oxygen delignification process at 100[degrees]C and after the oxygen delignification process the Kappa number is 9. This is followed by a 4-stage ECF bleach plant producing a pulp with the same brightness (ISO = 89%) as the Reference Mill. Model Mill 1 has a slightly lower total yield (42.9%) compared to the Reference Mill, which is mostly due to the conditions in the digester. It is almost as energy efficient as the Reference Mill, with a total steam demand of 11.7 GJ/t.
The second model mill evaluated in this work, Model Mill 2, shares all properties and components of the Reference Mill save one: the digester from the same earlier model mill has been substituted into Model Mill 2 with the same total yield (42.9%) as Model Mill 1.
The third model mill evaluated in this work, Model Mill 3, is identical to the Reference Mill. The reason why the earlier model mill in the same research program is not directly compared to the Reference Mill is because we want to show the effects if only the digester and/or oxygen delignification are upgraded. Additional differences (not related to the digester or oxygen delignification processes) between the two mills such as water consumption and spill handling render such comparisons either prohibitively difficult or otherwise ill-suited for yielding useful results.
For Model Mill 3 the Kappa number after the digester is 27 instead of 20, since more of the delignification occurs in the oxygen delignification plant compared to Model Mills 1 and 2. The temperature of the oxygen delignification process is reduced compared to Model Mill 1 and may lead to an increased investment cost as well as an extra cost for chemicals. It is not necessary to reduce the temperature in the oxygen delignification process as it is correlated to retention time, amount of chemicals and pulp concentration; it is solely done in order to reduce the energy consumption. It is assumed that the energy consumption needed to produce the increased amount of chemicals is negligible. For Model Mill 3, the Kappa number after the oxygen delignification is 10 compared to 9 for Model Mill 1 and is followed by the same bleach plant as mentioned above. The difference in Kappa number of the pulp entering the bleach plant will not result in increased steam demand in the bleach plant, rather, the size of the towers and/or the retention time might have to be increased incrementally.
Because of the differences in total yield between Model Mills 1 and 2 compared to Model Mill 3, the load on the evaporation plant and recovery boiler is lower for Model Mill 3 resulting in a reduced steam production. Since there is a steam surplus this will only marginally affect the electricity production.
PROCESS INTEGRATION AND EXCESS HEAT
In a pulp and paper mill, heat sources (hot streams) that need to be cooled are used for the production of warm and hot water. A mill with low water consumption usually has a surplus of warm and hot water. This surplus typically overflows from the tanks and is cooled in a cooling tower or other cooling system. Earlier work has shown that the hot streams used today for the production of a surplus of warm and hot water can be made available if changes to the warm and hot water production system are made (Wising et al., 2002). The hot streams that can be made available are referred to as excess heat. When redesigning the warm and hot water production system, the hot streams with the lowest possible temperature have been used for warm and hot water heating, leaving the remaining hot streams with a higher temperature for other uses.
In particular, excess heat can be used in the evaporation plant to minimize live steam use if the evaporation plant is redesigned as well (Algehed, 2002). Other potential uses for excess heat include heat pumping and/or district heating. When reducing the live steam demand the mill can reduce its own use of fuel, typically fossil fuels. If the mill is already energy efficient and does not use any fossil fuels, a surplus of electrical power can be produced or lignin can be precipitated and sold as a biomass fuel (Algehed et al., 2003). The potential for making excess heat available is of course different in different mills due to the difference in equipment. A thorough techno-economic study is thus essential to correctly address the long-term water and energy use reduction program for every mill.
Even when there are no pinch violations left in a mill, there is still a potential for energy savings from using excess heat; this is the case for the model mills evaluated in this work. Since there are no pinch violations in these model mills, pinch analysis has only been used in order to identify the amount and temperature of the excess heat that can be made available in a mill.
When making excess heat available for use, there are several system consequences. In this work the excess heat will be used for evaporation and as a consequence the evaporation plant needs to be redesigned. It will also affect the cooling need of the process since by making these changes the steam consumption of the process will be reduced and consequently the cooling requirements. Also, in order to make the excess heat available, the warm and hot water production system needs to be redesigned.
In this work both a conventional evaporation plant and a non-conventional evaporation plant have been designed for each of the three model mills. The conventional evaporation plant is a six effect evaporation train, operating counter-current, with an integrated stripper column between effects 1 and 2 where 50% of all condensate is stripped (Figure 1). The final concentration of the black liquor is 80% and the condensing temperature of the steam from the last effect (effect 6) is 55[degrees]C.
[FIGURE 1 OMITTED]
Non-conventional evaporation as defined in the Eco-Cyclic Research Program is designed differently compared to the conventionally designed evaporation in two main ways: first, excess heat is supplied at one or more temperature levels and secondly, the excess heat from the steam from the sixth evaporator effect is utilized more efficiently by installing one or more effects between 55 and 40[degrees]C, (Figure 2) (Algehed, 2002). The excess heat from the process has been turned into steam at a temperature that is lower than the temperature of LP-steam before it is fed to the evaporation plant. The final concentration of the black liquor is the same, 80%, and 50% of the condensate is still stripped.
[FIGURE 2 OMITTED]
The evaporation has been designed using a Microsoft Excel based tool, developed by the heat and power technology group at the department of chemical and biological engineering in cooperation with AF-IPK and Kvaerner Pulping. This tool takes into account factors such as size of individual evaporator effects, boiling point rise, viscosity and heat transfer coefficients at different temperature levels and scaling conditions giving the optimal number of effects and their steam and area requirements (Algehed, 2002).
Warm and Hot Water Production System
Since cooling of processes in the mill produces the warm and hot water needed in the mill, it is both a utility and process water; this makes the warm and hot water production system complex, and it is not apparent how to reduce the warm and hot water production. Often when designing a heat exchanger network (HEN) to supply the mill with both warm and hot water as well as cooling utility the aim is to satisfy the need for warm and hot water at a minimum cost while minimizing the need for an external cooling tower. The model mills in this work already have heat exchanger networks (HENs) with no pinch violations and the excess heat (cooling need) from these HENs is at the lowest possible temperature, minimizing the cooling tower need. Since the cooling need is the excess heat that could be used for other purposes, the aim when designing new HENs is to have the excess heat at the highest possible temperature. When designing the new HENs, lower temperature heat sources have been chosen, when possible, to heat the warm and hot water in order to make higher temperature heat sources available for use elsewhere (Linnhoff et al., 1992). In order to design the HENs, the model mills have been simulated to get stream data and a pinch analysis tool has been used (Pro Pi[TM]).
When using the excess heat for evaporation, the evaporation plant needs to be redesigned. A consequence of that redesign in this work is a lower temperature in the steam condenser after the sixth evaporation effect, i.e., the surface condenser. Traditionally, the surface condenser is the main heat source for the production of warm water. Since the surface condenser temperature is lowered, the warm and hot water production needs to be fulfilled without depending on the relatively high temperature (55[degrees]C) of the surface condenser. The warm and hot water production system for each of the three model mills has been designed in order to minimize the need for using the heat from the surface condenser. It is only used in the wintertime to heat incoming fresh water to 18[degrees]C.
When making changes such as these to a system, which lead to steam savings, the cooling load decreases since less heat is transferred through the system. When redesigning the warm and hot water production system, the heat in the wastewater is utilized more efficiently and there is no need to cool the wastewater before it reaches the wastewater treatment plant. One of the two cooling towers' main uses is cooling of wastewater. Theoretically this cooling tower can be removed, but since the wastewater treatment plant is very sensitive with regard to temperature, there may be a need to retain the cooling tower for control purposes. Another major difference is that there is a smaller surplus of hot water that needs to be cooled, since the heat sources used for the surplus hot water production are used for evaporation. Consequently, the excess heat has been transferred from excess hot water that needs to be cooled to an increased cooling load for the surface condenser in the evaporation plant.
When saving live steam in the pulp and paper industry, C[O.sub.2] emissions in the mill can decrease. Depending on the efficiency of the mill, i.e., if fossil fuel is used and what is done with the saved live steam, different consequences can be calculated for the C[O.sub.2] emissions. In the model mills presented here, there are no fossil fuels needed today. Half of the bark is gasified and used in the lime kiln and the rest is sold. In order to export more biomass than the bark, lignin has to be precipitated. In earlier studies within the research program, it has been shown that this is the best option for these model mills from a C[O.sub.2] perspective (Algehed et al., 2003). However, it is a very complex process and it affects the entire mill. In this work it has been assumed that the saved live steam is used for power production in a condensing turbine.
The extra power produced is sold and in order to compare different measures for reduced C[O.sub.2] emissions, it is important to know how the power it replaces was produced. This is, of course, an impossible task, and as a compromise, the power it replaces on the market is assumed to be the marginal power production in the power production system where the mill is an actor.
In Sweden, there is an unregulated power market. The Swedish National Board of Energy Administration (STEM) has evaluated that market and made some estimates for the marginal power production and its C[O.sub.2] emissions. Currently, the marginal power production is coal-condensing (CC) power imported from Denmark and its C[O.sub.2] emissions is 885 kg C[O.sub.2] per MWh power produced. In the future (2008-2012) the probable marginal power production will be natural gas fired combined cycle (NGCC) power imported from Norway with C[O.sub.2] emissions of 396 kg C[O.sub.2] per MWh power produced (STEM, 2002). The C[O.sub.2] emissions include both average losses associated with the distribution of power as well as the power plant's own power use. Both scenarios have been evaluated here.
In order to identify the amount and temperature of the excess heat available, pinch analysis has been used as a tool. Since these mills are very energy efficient and have no pinch violations, the results from the pinch analysis give the amount of excess heat below the pinch temperature directly in a Grand Composite Curve (GCC) (Figure 3). Included in Figure 3 is an enlargement of a part of the GCC, just below the pinch temperature and above 60[degrees]C, where the excess heat is shown in more detail. It does not give the true temperature of the excess heat since the temperatures in the GCC are adjusted, but the streams can be identified and thereby their true temperature. As much as possible of the excess heat above 80[degrees]C is made available by designing the new HEN following the guidelines given for the design of the warm and hot water production system. The amount of excess heat available is shown in Table 2.
[FIGURE 3 OMITTED]
The excess heat sources identified for Model Mill 1 are available in the digester department and the bleach plant comprising the black liquor cooler (BL) and flash steam from an MC-tower in the bleach plant (BP) (Table 2). Instead of indirectly using the excess heat from the black liquor cooler like in the original HEN, the black liquor is fed directly to the evaporation plant without cooling. The black liquor might need to be stored above atmospheric pressures at times in order to accomplish this.
Table 2 shows the difference in the amount of excess heat available for the three model mills: 1.2 GJ/t for Model Mill 1, 1.0 GJ/t for Model Mill 2 and 0.5 GJ/t for Model Mill 3. There are several reasons for the differences in the amount of excess heat available. For Model Mill 3, the main reason is the lower temperature in the new digester, 148[degrees]C, compared to 160[degrees]C for the digester in Model Mills 1 and 2. As a result of that lower temperature, less steam is used in the digester, but also less flash steam is produced as well as a smaller amount of vent gases. Since flash steam and the vent gases are two major sources of excess heat that could be used elsewhere in the process, this reduces the potential for process integration when installing the new digester. Also, black liquor from the new digester is at a lower temperature compared to Model Mills 1 and 2, resulting in smaller savings in the evaporation plant when using the black liquor directly without cooling.
Another reason for the reduction in excess heat is the lower temperature in the oxygen delignification process. For Model Mill 1, the temperature in the second tower is 100[degrees]C and the filtrate from the tower has a temperature of 97[degrees]C. The filtrate from the process needs to be cooled before it is pumped and reused since there is a temperature limitation on the MC-pumps. This results in a high temperature heat source. In the new oxygen delignification plant (Model Mills 2 and 3), the temperature in the second tower is lower, 95[degrees]C. This leads to a decreased cooling demand of the filtrate by at least half and the excess heat from the oxygen delignification plant is reduced by the same amount.
In Table 3, the live steam demand for evaporation is shown. As mentioned earlier, when changing the digester, the total yield for the mill increases. This reduces the amount of black liquor that needs to be evaporated, reducing the live steam demand for the original six-effect evaporation for Model Mill 3 relative to Model Mills 1 and 2.
When using excess heat, the live steam demand for evaporation is reduced considerably for Model Mill 1 as well as for the other two model mills. The differences are, naturally, caused by the larger amount of excess heat available in Model Mill 1 compared to the two other model mills.
When using excess heat for evaporation, the distribution of the area in between the effects changes. Earlier work shows that when designing a non-conventional evaporation plant, the area demand increases (Algehed, 2002). This is due to the fact that part of the steam to the evaporation plant is at a lower temperature than the LP-steam and not all the steam goes through all the effects, and the cascade effect is reduced. Since the amount of excess heat available is smaller compared to earlier work, the result here differs. Two factors counteract each other, resulting in very small or no increased area demand. First, compared to the original evaporation plant, less live steam is used in the first three effects, requiring less area. Second, in the lower effects (nos. 4-7) the area demand is higher and more steam is used in these effects compared to the original evaporation plant. This is also the case for other model mills presented in earlier work, except for the ratio between the increased/decreased area demands. For these three model mills this ratio is close to one, whereas for other model mills with more excess heat available this ratio is much higher than one.
When using excess heat for evaporation, one extra unit is added in the cascade resulting in a total of seven instead of six units. This does not influence the area demand, but it does affect the investment cost for evaporation.
Total Live Steam Demand
The total live steam demand for the different model mills can be seen in Table 4. The reduction in live steam demand when replacing the digester and the oxygen delignification process is significant. When comparing these different measures (Table 5), the most efficient measure is replacement of the digester and oxygen delignification process (Model Mill 3) combined with process integration.
C[O.sub.2] Calculation Results
Decreasing the live steam demand in a mill, which results in an increased power production, has negative C[O.sub.2] emissions when it replaces power produced by fossil fuels, i.e., off-site. The two cases presented here are coal condensing power and natural gas .red combined cycles power. The measure that is most effective for reduction in live steam demand is replacing the digester and the oxygen delignification process as well as process integrating the mill (process integration of Model Mill 3). This leads to a reduction in C[O.sub.2] emissions in the short term of 96 kg/t (Table 6). This is equivalent to a 67 000 tons of C[O.sub.2] reduction per year for this specific model mill. By energy integration of Model Mill 1 the C[O.sub.2] reduction per year is equivalent to 53 000 tons per year.
If this had been a typical mill, it might have been using fossil fuels for supplementary firing; the C[O.sub.2] consequences would then have been on-site rather than off-site. Instead of increasing electricity production, the mill would just reduce its use of fossil fuels. This might also be more favourable from an economic perspective.
Replacing the oxygen delignification process or replacing both the digester and the oxygen delignification process are usually necessary in order to improve quality, yield, capacity, etc. Energy integration investments are not always necessary with regard to pulp production. Earlier work has shown that under most economic conditions there is an investment opportunity concerning investments tied to the saved live steam/increased electricity production in conjunction with the installation of a new evaporation plant (Algehed, 2002).
First of all, the investment costs for making these kinds of changes are very site- and mill- specific, making it possible to draw only general conclusions. Secondly, in earlier work investment costs for the mills have been calculated as follows: The investment costs for the digester and oxygen delignification process have been estimated by AF-IPK within the Eco-Cyclic Pulp Mill Research Program (KAM, 2003). The excess heat sources used for evaporation are the same as those earlier used for warm and hot water production. The heat exchangers are then already in place, albeit used for different purposes. There is only an extra investment cost if there is a need for new heat exchangers or extra heat exchanging area. Piping costs are assumed to be the same (Wising et al., 2002). Finally, the investment cost in earlier work for the evaporation plant has been estimated together with Kvaerner Pulping and AF-IPK (Algehed, 2002).
When combining the results from earlier works with the results from this work, there are some general economic consequences that need to be addressed: The new digester has a lower investment cost compared to the original digester (KAM, 2003). This is mainly due to a lower temperature and pressure in the digester, and thereby lower pressure classified material can be used, which is less expensive. When changing the digester and the oxygen delignification process, the steam consumption is reduced by 0.9 GJ/t, which is economically favourable since more electricity can be produced. Also, the cost of raw material is reduced from 159 to 155 USD/t corresponding to 2.5 MUSD per year for a pulp production of 2000 t/day. The reduction in raw material cost with the new digester and oxygen delignification process is due to the increased total yield for the mill. For the investment cost of the oxygen delignification process, when lowering the temperature, there might be an increased cost due to a longer retention time or larger unit, but this is dependent on the pulp quality and brightness from the digester.
The area cost for evaporation is approximately the same for all the mills, with a slight reduction for the mill where the digester and oxygen delignification process have been replaced due to a smaller evaporation load. There is a cost for an extra unit that is placed below the original evaporation train when using excess heat for evaporation. Since the changes in the evaporation plant are small, it may be feasible to retrofit the evaporation process instead of replacing it. This needs to be investigated further and is dependent on the status of the evaporation plant at the specific mill. Since earlier work shows that it is economically favourable to change the warm and hot water production system when replacing the evaporation plant, it could be even more favourable if only a retrofit of the evaporation plant is needed (Wising, 2003). Also, when retrofitting the warm and hot water production system, the number of heat exchangers is reduced by one. The reduction in the numbers of heat exchangers is due to the fact that the black liquor is not cooled before it is used in the evaporation plant.
The cost for investing in a retrofit of the warm and hot water production system as well as the retrofit/new investment in the evaporation plant will be lower if the digester and oxygen delignification process are replaced at the same time. This is due to less excess heat being available when changing the digester and oxygen delignification, such that less excess heat is transferred to the evaporation plant resulting in a lower cost for both the warm and hot water production system and the evaporation plant.
DISCUSSION AND CONCLUSIONS
In this paper we have shown the energy consequences and potential when making a significant change to a mill by replacing the oxygen delignification process and/or digester. It is important to evaluate the mill in this respect before making any investment decisions, since changes made to the mill have system consequences that are not always obvious.
If the evaporation plant is the bottleneck for the mill, there is a large potential for saving energy when retrofitting or installing a new evaporation plant. For Model Mill 1 the savings were 1.5 GJ/t or 13%.
When changing the digester and oxygen delignification process (Model Mill 3), the steam demand is lowered by 0.9 GJ/t, because the new processes are more energy efficient mostly due to lower temperature levels. The reduced temperatures and pressures also makes it considerably less expensive compared to the original digester. The digester and oxygen delignification process will not likely be replaced because of the energy saving potential; the reduction of energy consumption is an added benefit when the mill is replacing those processes for other reasons. If one changes/ retrofits the warm and hot water production system and the evaporation plant in conjunction with the replacement of the digester and oxygen delignification, another 8% reduction of the energy demand can be achieved. This is the largest energy saving, 1.9 GJ/t or 16% (process integration of Model Mill 3). When comparing the two cases, the largest energy savings by process integration (13%) were achieved for Model Mill 1, and
when process integrating Model Mill 3, an 8% reduction was achieved. These measures are not cumulative; some of the possible gains from the process integration are lost when replacing the digester and oxygen delignification. Still, since the energy demand is lower for the new equipment, the largest total energy reduction is achieved. Since the measures are not cumulative, if the digester is to be replaced and the mill is already process integrated, the effect from process integrating the mill will probably be reduced once the new digester is in place. These are of course important issues to take into consideration in the strategic planning of a mill.
The C[O.sub.2] consequences when installing new more energy efficient processes and/or integrating processes in order to save energy are dependent on the fuel the mill uses today, the off-site electricity production it can replace and how much energy that one can save. Here we have shown that in the Scandinavian electricity market for both the short and long term, C[O.sub.2] emissions can be reduced considerably. For the mill with the new digester and oxygen delignification process (Model Mill 3) the reduction of C[O.sub.2] emissions corresponds to 67 000 tons C[O.sub.2] per year when at the same time process integrating the mill and replacing off-site produced coal condensing power.
To make these changes to a mill is economically favourable under most economic conditions, but can vary considerably between mills. It is important to make a more thorough techno-economic analysis including mill specific factors before knowing the profitability of such an investment for a specific mill.
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Wising, U., "Process Integration in Model Kraft Pulp Mills--Technical, Economic and Environmental Implications," PhD Thesis, Department of Chemical Engineering and Environmental Science, Chalmers University of Technology, Gothenburg, Sweden (2003).
Wising, U., T. Berntsson and A. Asblad. "Usable Excess Heat in Future Kraft Pulp Mills," TAPPI J., 27-29 (Nov. 2002).
Manuscript received November 27, 2003; revised manuscript received June 24, 2005; accepted for publication September 8, 2005.
Ulrika Wising (1 *), Thore Berntsson (2) and Anders Asblad (3)
(1.) Department of Chemical Engineering, NSERC Environmental Design Engineering Chair-Process Integration in the Pulp and Paper Industry, Ecole Polytechnique de Montreal, Montreal QC, Canada H3T 1J7
(2.) Heat and Power Technology, Department of Chemical and Biological Engineering, Chalmers University of Technology, 412 96 Goteborg, Sweden
(3.) CIT Industriell Energianalys AB, 412 88 Goteborg, Sweden
* Author to whom correspondence may be addressed.
E-mail address: firstname.lastname@example.org
Table 1. Differences between the Model Mills 1, 2 and 3 Model Mill 1 Model Mill 2 Model Mill 3/ The Reference Mill Iso-thermal digester Iso-thermal digester Low temperature at 160[degrees]C, at 160[degrees]C, digester at Kappa = 20 Kappa = 20 148[degrees]C, Kappa = 27 Two-stage oxygen Two-stage oxygen Two-stage oxygen delignification at delignification at delignification at 100[degrees]C to 95[degrees]C to 95[degrees]C to Kappa = 9 Kappa = 10 Kappa = 10 Table 2. Excess heat available for the different model mills Model Mill 1 Model Mill 2 Model Mill 3 0.5 GJ/t at 0.3 GJ/t at 0.50 GJ/t between 95[degrees]C from 90[degrees]C from 97-85[degrees]C (BL) bleach plant bleach plant 0.7 GJ/t 0.7 GJ/t between 101-85[degrees]C (BL) 101-85[degrees]C (BL) Table 3. Live steam demand for evaporation for the different model mills Model Mill 1 Model Mill 2 Model Mill 3 Reference 6-effect 4.5 GJ/t 4.5 GJ/t 3.9 GJ/t evaporation Evaporation using 3.0 GJ/t 3.2 GJ/t 3.0 GJ/t excess heat Percent decrease 32% 30% 23% Table 4. Total live steam demand for the different model mills Model Mill 1 Model Mill 2 Model Mill 3 Original design 11.7 GJ/t 11.5 GJ/t 10.8 GJ/t Energy integrated 10.2 GJ/t 10.2 GJ/t 9.8 GJ/t using excess heat Reduction in total 13% 12% 9% live steam demand Table 5. Total live steam reduction when applying different measures compared to Model Mill 1 Process integration of Model Mill 1 13% Replacing the oxygen delignification process 2% (Model Mill 2) Process integration of Model Mill 2 13% Replacing the digester and the oxygen 8% delignification process (Model Mill 3) Process integration of Model Mill 3 16% Table 6. Increased electricity production compared to Model Mill 1 and the negative C[O.sub.2] emissions for replacing off-site electricity production. Different measures compared Extra Reduction of to Model Mill 1 electrical C[O.sub.2] emissions power depending on type of produced marginal power (kWh/t) production (kg/t) CC NGCC Energy integration of Model Mill 1 85 76 34 Model Mill 2 11 9 4 Energy integration of Model Mill 2 89 79 35 Model Mill 3 53 47 21 Energy integration of Model Mill 3 108 96 43
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|Author:||Wising, Ulrika; Berntsson, Thore; Asblad, Anders|
|Publication:||Canadian Journal of Chemical Engineering|
|Date:||Feb 1, 2006|
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