Optimization of Paper Production at Ahlstrom-Munksj¨ o
Analysis based on energy audit
Optimering av pappersproduktionen hos Ahlstrom-Munkj¨o En analys med energikartl¨aggning som grund
Wilhelm Sahl´en
Faculty of Health, Science and Technology
Degree Project for Master of Science in Engineering, Mechanical Engineering Credit: 30 HP
Supervisor: Gunnel Fredriksson Examiner: Jens Bergstr¨om 2019-08-02
Abstract
Ahlstrom-Munksj¨o is a global company within the paper industry. Their mill in Billingsfors was in need of an energy audit, localizing energy flows within the mill. An energy audit is a good tool for any company to get a good overview picture of their energy usage of the examined system, it is also seen as the first stage for an energy-saving strategy. On the Billingsfors mill, the energy audit is included in the continuous work of increasing their energy efficiency in order to keep up with the new regulations and advancements in this industry sector.
The energy audit was done on the whole mill with focus on the paper production and the district heating system. More detailed information about the paper production, a comparative study of two paper machines, PM2 and PM6, is also presented. The reason for the comparative study on PM2 and PM6 was due to an unexpectedly high recorded gas consumption from PM2. The two paper machines were believed to be similar enough to compare them without any deeper knowledge of the machines. They were also compared to a reference machine, representing an average machine with similar setup. The comparisons showed that PM2 does in fact have a higher gas consumption than PM6. To identify the reason for the higher consumption the dry solid contents of PM2 needs to be examined. The two machines do not have the exact same process when producing paper and a more detailed comparative study should be done for a better understanding of the high gas consumption.
However, the two machines have a lower gas consumption than the reference machine, this meaning they both have a relatively low gas consumption.
Keywords: Paper mill, Paper production, Energy audit, Energy efficiency, Yankee drying section
Sammanfattning
Ahlstrom-Munksj¨o ¨ar ett globalt f¨oretag inom pappersindustrin. Deras bruk i Billingsfors beh¨ovde en energikartl¨aggning som identifierar energifl¨oden inom bruket. En energikartl¨aggning
¨
ar ett bra verktyg f¨or f¨oretag f¨or att f˚a en bra ¨overblick ¨over deras energianv¨andning p˚a det unders¨okta systemet, det ses ocks˚a som den f¨orsta etappen f¨or en strategi inom energibesparing.
Energikartl¨aggningen hos Billingsfors bruk ing˚ar i det kontinuerliga arbetet med att ¨oka sin energieffektivitet f¨or att f¨olja med i framstegen och de nya reglerna inom denna industrisektor.
Energikartl¨aggningen gjordes p˚a hela fabriken med fokus p˚a pappersproduktionen och fj¨arrv¨armesystemet. Mer detaljerad information om pappersproduktionen, en j¨amf¨orelsestudie av tv˚a pappermaskiner, PM2 och PM6, presenteras ocks˚a. Anledningen till j¨amf¨orelsestudien p˚a PM2 och PM6 berodde p˚a en ov¨antat h¨og gasf¨orbrukning hos PM2. De tv˚a pappersmaskinerna antogs ha tillr¨ackligt lika processer f¨or att j¨amf¨ora dem utan n˚agon djupare kunskap om maskinerna.
De j¨amf¨ordes ocks˚a mot en referensmaskin, som representerar en genomsnittlig maskin med liknande process. J¨amf¨orelserna visade att PM2 faktiskt har en h¨ogre gasf¨orbrukning ¨an PM6.
F¨or att identifiera orsaken till den h¨ogre f¨orbrukningen m˚aste torrhalterna hos PM2’s processteg unders¨okas. De tv˚a maskinerna har inte exakt samma process vid produktion av papper och en mer detaljerad j¨amf¨orelsestudie b¨or g¨oras f¨or att b¨attre f¨orst˚a den h¨oga gasf¨orbrukningen. De tv˚a maskinerna har dock en l¨agre gasf¨orbrukning ¨an referensmaskinen, vilket inneb¨ar att de b˚ada har en relativt l˚ag gasf¨orbrukning.
Nyckelord: Pappersbruk, Pappersproduktion, Energikartl¨aggning, Energieffektivisering, Yankee torkning
Table of contents
Abstract I
Sammanfattning II
List of Figures V
List of Tables VI
1 Introduction 1
1.1 Problem formulation . . . 1
1.2 Purpose . . . 2
1.3 Goal . . . 2
1.4 Delimitations . . . 2
2 Theory 3 2.1 Paper production . . . 3
2.1.1 Stock preparation . . . 3
2.1.2 Wire section . . . 4
2.1.3 Press section . . . 5
2.1.4 Dryer section . . . 5
2.1.5 Reeling and sheet cutting . . . 7
2.2 District heating . . . 8
2.2.1 Heat Exchanger . . . 8
2.2.2 Condenser . . . 9
3 Framework 10 3.1 Pulp production . . . 10
3.2 Paper production . . . 10
3.2.1 PM2 and PM6 . . . 10
3.2.2 PM5 . . . 11
3.2.3 VPD, Pulp Processor . . . 12
3.3 District heating . . . 12
4 Methods 14 4.1 Energy audits . . . 14
4.1.1 Billingsfors mill, model 1 . . . 15
4.1.2 Paper production, model 2 . . . 16
4.1.3 District heating, model 3 . . . 18
4.2 Comparative study of PM2 and PM6 . . . 19
4.2.1 Yankee cylinder . . . 20
4.3 Energy calculation . . . 20
4.4 Assumptions . . . 21
5 Results 22 5.1 Energy audit . . . 22
5.1.1 Billingsfors Mill . . . 22
5.1.2 Paper production . . . 23 5.1.3 District heating . . . 26 5.2 Comparison of PM2 and PM6 . . . 27
6 Discussion 31
6.1 Comparative study of PM2 and PM6 . . . 32 6.1.1 Representative machine comparison . . . 33
7 Conclusions 35
7.1 Future work . . . 35
8 Acknowledgements 36
References 37
Appendix 39
A Process map over the district heating system 39
List of Figures
1 Box diagram over a paper production. . . 3
2 A two-way plate heat exchanger, heat medium inserted at the top (red arrow), carrier being heated inserted at the bottom (blue arrow) [16], with permission. . . 8
3 Process picture over a condenser, with permission from OKG AB [20]. . . 9
4 PM2 seen from the side, the process starts from the right. . . 11
5 PM6 seen from the side, the process starts from the right. . . 11
6 PM5 seen from the side, the process starts from the right. . . 12
7 The district heating system at Billingsfors Mill, each box represent heat exchangers. 13 8 Flow chart showing how an energy audit is usable in an industry and how to take actions from it, reworked from Energimyndigheten [4]. . . 14
9 Block diagram of the mill, model 1, with its energy flow. . . 16
10 A representation of how the models are audited with different levels. . . 17
11 Block diagram of the paper production, model 2, VPD and after-treatment in the mill. 17 12 Block diagram of the district heating system, model 3, in the mill. The first letter of the flow is categorized. . . 18
13 Flow schedule over the drying part of a paper machine, simplified. . . 19
14 Simplified overview over a Yankee cylinder and it’s drying hood. . . 20
15 Block diagram over the billingsfors mill with values over each energy flow, values are given in GWh per year. . . 22
16 Block diagram over the paper production at Billingsfors mill with values over each energy flow, values are given in GWh per year. . . 24
17 Block diagram over the district heating system at billingsfors mill with values over each energy flow, values are given in GWh. . . 26
18 Flow schedule over PM6, where water exits at the bottom and the dry solid content percentages of the paper is shown after each section. . . 27
19 A simplified figure over the drying process, without the wire section, and its corresponding dry solid content for PM6, the same dry solid contents are assumed for PM2. . . 28
20 Flow schedule over PM2, where water exits at the bottom and the dry solid content of the paper is shown after each section. . . 28
21 The energy distribution of steam and gas of PM2, PM6 and a reference machine (data from [25]). . . 33
A.1 Process map over the district heating system . . . 39
List of Tables
1 Each flow with their respective forms of energy . . . 16 2 Fuel and energy content of each flow within the paper production . . . 18 3 Fuel and energy content of the flows within the district heating. The first letter of
the flow is categorized. . . 19 4 The gas consumption per year within the paper production model, distributed over
the processes . . . 25 5 Steam consumption per year within the paper production model, distributed over
the processes . . . 25 6 Fresh pulp feed distribution per year over the three paper machines . . . 25 7 Electricity consumption of PM2, PM6 and PM5 per year . . . 25 8 PM2’s proportion to PM6 of each energy flow and the width of PM2 and PM6 . . . 26 9 The dry solid content and mass flow during each step of the drying process for PM6 27 10 The dry solid content and mass flow during each step of the drying process for PM2 29 11 Ingoing energy from the energy audit over the Yankee system at PM2, per year . . . 29 12 Outgoing energy from the energy audit over the Yankee system at PM2, per year . . 29 13 Ingoing energy from the energy audit over the Yankee system at PM6, per year . . . 30 14 Outgoing energy from the energy audit over the Yankee system at PM6, per year . . 30 15 Gas consumption per square meter paper produced and amount of paper produced
per kg gas for PM6 and PM2 . . . 30
1 Introduction
The company Ahlstrom-Munksj¨o produces fiber-based materials such as electrotechnical paper, decor paper, food packaging and labeling, medical fiber materials and more. The fibers originate from trees. The paper production at Ahlstr¨om-Munksj¨o’s mill in Billingsfors produces liner for metal packaging, electrical paper and decor paper [1].
Ahlstrom-Munksj¨o is a global company with an annual net sales of around 3 billion EUR, 8000 employees at 45 different sites in 14 different countries supplying customers worldwide. On April 1, 2017, a merger between the Ahlstrom Corporation and Munksj¨o Oyj took place, the merger was the birth of the company Ahlstrom-Munkjs¨o. Their share is listed at Nasdaq in both Helsinki and Stockholm [1].
The company have set a number of goals with regards to the use of energy and resources for their sites. One of the goals is that almost all of their sites (95% of them) should have their environmental management system audited by a third-party by the end of year 2020.
Ahlstrom-Munksj¨o strives for an annual reduction of specific energy consumption, water, and COD (Chemical Oxygen Demand) emissions per gross ton product from 2018 and forward [2]. The company feels an obligation to promote and apply a more environmental efficient performance throughout the process since the industry is a resource demanding one. The emissions coming from the production are waste to landfills, noise and emissions to air and water. There are specific regulations concerning environmental emissions that needs to be fulfilled. Difficulty in satisfying these regulations, or difficulty in improvement of the resource management, will have a negative effect on Ahlstrom-Munksj¨o and on the environment [3].
An energy audit is favorable for the company since it gives a good overall picture of the energy usage of the examined site. The energy audit will also make it possible to easier connect the sites production processes to their respective energy usage. A good way to present an energy audit is through graphics, such as block diagrams [4]. With the facts discovered from the energy audit the company may develop economical and more environmentally sustainable action proposals to decrease the flaws. The main goal of the audit is to find the biggest energy consumers within the mills. This will help the company to get a broader understanding of their energy consumption.
The audit will provide better conditions for a more effective operation and maintenance and the utilization of technical installations can be optimized [5]. According to Kong et al., Backlund and Thollander and Boharb et al. [6–9] the first stage of an energy-saving strategy is to perform an energy audit. An energy audit is used to analyze and to improve the energy consumption within an industry. The focus of the audit should be to improve the energy efficiency without affecting the output from the mill in a negative way [6]. A study on how an energy audit program affects small to medium sized firms in Sweden showed that all 241 firms evaluated had an annual energy saving potential between 860 to 1270 MWh [7].
1.1 Problem formulation
Ahlstrom-Munkj¨o’s mill in Billingsfors is in need of a revised energy audit. The company has done an audit that showed flaws and is not complete. The site needs continuous work with energy efficiency to keep up with new regulations and technical advancements.
1.2 Purpose
The identification of positions with a potential for improvement is a step in Ahlstrom-Munksj¨o’s work to modernize the mill. The purpose of this study is to identify energy flows at Ahlstrom-Munkj¨o’s mill in Billingsfors. The study will function as a tool in the work with continuous improvements.
1.3 Goal
The Goal of this thesis is to present an energy audit for the mill, based on the previous energy audit. If possible, the thesis will propose possible improvements based on the energy audit.
1.4 Delimitations
Ahlstrom-Munksj¨o’s mill in Billingsfors produces both pulp and paper. This thesis will not include the pulp production on a deeper level. This meaning the internal flows within the pulp production will not be included but the external flows from the pulp production will be in the model since they are affecting the other systems. The pulp factory is under reconstruction and is planned to be finished by September 2019, and therefore there is no use for an energy audit on today’s setup.
A risk analysis is planned for the district heating system, which may lead to changes to the system.
A detailed energy audit will be done after this risk analysis and is thus not included in this thesis.
The study do not take seasonal changes in account. Since the thesis do not make a detailed study of the district heating system, this simplification is deemed to a minor effect on the results.
The report do not evaluate the economical aspect of the energy audit outcome.
Paper machine, PM5, will not be included in any comparative studies of the paper machines. This is due to the differences of the paper machine setups in the mill.
2 Theory
In the theory chapter the reader gets an insight in how paper is produced with a modern set up. The different kind of fuel and energy usages are mentioned. A district heating system is explained, there is one located and operating at the Billingsfors mill. The pulp manufacturing is not explained.
2.1 Paper production
The mill in Billingsfors is an integrated mill. This means that they produce their own pulp for the paper machines. The other type of mills are a non-integrated mills, where only pulp or paper is produced. A mill might have to buy the right kind of pulp, matching the right criterion, to be able to fulfill the specific recipe for a quality [10].
The pulp goes through various sections within the paper machine before it comes out as a finished paper product. As illustrated in Figure 1 the steps are;
• Stock preparation
• Wire section
• Press section
• Dryer section
• Reeling and sheet cutting
The main ingoing and outgoing energy flows are included, each step is explained more detailed below [11].
Figure 1: Box diagram over a paper production.
2.1.1 Stock preparation
Stock is the name for fibres solved in water, the fibres are obtained from trees in different methods.
Paper production uses different types of pulps depending on the end product. The most common types of pulp are chemical and mechanical pulp. They are either bleached or unbleached. Unbleached chemical pulp is mainly used for wrapping paper, kraft liner, sack paper and other strong papers.
Bleached chemical pulp is mainly used for high quality tissue paper, paperboard, copying and printing paper [11].
Before the stock reaches the beginning of the paper machine it has to go through the short circulation where the stock is diluted with the white water extracted from the wire section of the paper manufacturing process. The stock is diluted to a concentration of around 1% fibres. The stock is cleaned through several steps to remove impurities that can create problems with the paper later on in the process. Impurities can for example be sand, bark or coarse pieces of fibers. The cleaning of the stock is done with a centri-cleaner and strained several times for a higher cleanliness [11]. The centri-cleaner hurls the stock into the bottom part of a cone forcing a high rotation of the stock against the cone wall where the reject is exiting from the top of the cone and the cleaned stock exits from the base of the cone [12].
Before the stock reaches the paper machine and the headbox it is pumped through a tank with a low pressure forcing air bubbles out from the stock. Air bubbles in the stock can give flaws in the paper production in the form of small holes in the paper and affect the drainage negatively [11].
2.1.2 Wire section
The wire section is the first section of the paper machine. In the wire section the web of fibres that make up the paper is formed from the stock by removing water to a solid dry content of around 20%. The stock is applied on the wire with the help of a headbox. A headbox is a component that is designed to produce a jet stream of the stock that is directed onto the wire. The stream has to be homogeneous and have an even thickness and velocity over the width of the wire. The wire is traveling with about the same speed as the jet stream in order to get a good forming process of the stock on the wire. The angle of the jet onto the wire is also chosen thoroughly. It is vital that the jet is homogeneous and the first layer of stock is distributed evenly over the wire, without waves or swirls disturbing the layer [11].
The wire is made from thin plastic threads forming an endless cloth and the length of the cloth is individually determined to match the specific paper machine. The cloth is often made in polyester and the design of the cloth is determined from the dewatering characteristics, the strength and stability wanted. The design of the cloth will therefore affect the characteristics of the paper as well [12].
When the first layer of stock hits the wire, the dewatering begins which forms the first layer of fibers.
The first layer behaves like a filter for the dewatering to come, where it all have to flow through the filter. The filter is thereby gaining more and more fibers slowing down the draining process.
It is therefore encouraged to disturb the growth and creation of the filter. This is done by having suction pulses produced on the underside of the wire, a large number of suction pulses are preferred for good drainage. The suction pulses are affecting many different factors, it is the dewatering capacity, the formation of the paper, the distribution of filler and fine fraction in z-direction and all the properties of the paper. The suction pulses are created by having foils located under the wire but in contact with the wire, creating first a pressure pulse then a suction pulse, using the gravitational force to extract the water. The foil is also able to get water out of the stock by scraping on the wire and thereby creating the pressure pulse. The wire also ends up vibrating after the foils, improving formation of the stock layer [11].
The foils are often placed inside suction boxes, which have a lowered pressure inside creating vacuum, directed onto the wire, aiding the dewatering further. The lowered pressure is caused by vacuum pumps. There are often two sections of suction boxes, consisting of wet suction boxes and dry suction boxes. The difference between the two are that the dry suction boxes also suck air
through the web while the wet ones only suck out water. There is a visible (in some machines) boarder line on the web where one can see where the dry suction boxes begins, the line should be straight across, smooth and stable. The wire ends by passing the suction couch roll, which is a perforated shell cylinder with a suction box inside. The suction box is sucking out water leaving the web, as mentioned earlier, to a 20% dry solid content [11].
2.1.3 Press section
To raise the dry solid content as much as possible before the drying section the web is pressed several times. Often around two or three pressings are used and the dry solid content is raised to around 50-55%. In the pressings there are often pressing blankets or felts on one or both sides of the web to be able to extract the water. In the pressing the water is forced into the felts, in the first press there are two felts, one on each side of the web. If the felts were not there, the water could damage the web by building up a hydraulic pressure. There are also holes or channels in the cylinders pressing the web where excess water can leave the felts. One felt is often used during the second and third pressing, the other side is a smooth cylinder without any holes or channels for the water to flow through. The water flows then from the smooth cylinder through the web to the felt on the other side [11].
In modern paper production lines the speed of the produced paper is higher than it used to be.
This demands more effective components to maintain the quality of the paper. In the press section, the increased speed means that the paper is exposed to the press for a shorter time. In order to have a successful press there are two things that should be obtained, enough pressure to be able to compress the web consisting of wet fibres and enough time for the water to flow out from the web to the felt. Since the time that the paper is pressed has been reduced due to the increased speed, the efficiency of the press has been decreased. To achieve a better press a longer pressing zone is favourable [11]. This is done from, a so called, shoe press where one of the pressing cylinders have an elastic felt on the exterior. This elastic felt makes it possible for a concave pressure on the web and around the other cylinder [12]. The shoe press can give up to five times longer dwell time for the paper passing through [11].
The desire to maximize the efficiency of the dewatering in the press section is great since it is considerably cheaper to drain the paper web by pressing it in the wet section than dry it in the dryer section. The outgoing dry solid content from the pressing section therefore affects the economy and energy consumption of the paper production process greatly. In the pressing section, energy is often applied using pressure achieved mechanically or hydraulically. To achieve even higher efficiency many mills are heating up the web since the increased temperature lowers the viscosity of the water and softens the fibres making it easier to extract the water. There are however limits for the temperature since it affects the components and the paper properties [12].
2.1.4 Dryer section
The dryer section is the last step of dewatering the paper, to a dry solid content of around 90%.
This step is necessary to be able to reach that kind of percentage. There are often two bigger types of drying processes that is used, the Yankee cylinder dryer and a multicylinder section [11].
Both processes uses some sort of drying cylinders in order to dry the paper web. A drying cylinder is
heating up the paper web making the water in the web evaporate. The cylinders get their heat from steam, which is pumped into the inside of the cylinders, condensation appears on the inner wall.
The condensate gathers in the bottom of the cylinder and is being forced out from the cylinder by the continuous pressure withheld from the steam pumped in. The pressure of the incoming steam must therefore always be higher than the pressure needed for the condensate to exit [11].
In order to optimize the drying process there are certain factors that needs to interact with each other in the best possible way. The factors affect heat flow, and therefore the drying process, are the following:
• Wall thickness of the drying cylinder and thermal conductivity.
The design and material choice of the cylinder is crucial. It is desirable to have a good thermal conductivity and a thin wall in order to get maximum heat to the paper web. But if the wall is too thin it wont be able to manage the inner pressure from the steam. Usually gray cast iron is used [12].
• Contact between paper web and cylinder.
A good contact between the paper web and cylinder is desirable to get maximum heat flow.
A plastic wire is used to press the paper web against the cylinders, forcing any air film or bubbles out of the way. The contact zone should be as large as possible to take as much advantage as possible of the heat from the cylinders. The good contact on a Yankee cylinder stems from a press pressing the paper web onto the Yankee cylinder[12].
• Temperature and moisture content of the paper web.
The heat flow will be greater if the temperature of the paper web is low. The heat flow will also benefit from a higher moisture content of the paper web[12]. Paper have a better isolation ability than water [13].
• Thickness of the condensate film on the inside of the drying cylinder.
As pointed out above, thermal conductivity plays a vital role, condensate, or water, have a much lower thermal conductivity than cast iron [13, 14], which the drying cylinder is made of. The condensate film is therefore considered an obstacle for the heat flow. It is desirable to minimize this film to get a better heat flow [12].
• Ventilation in the dryer section
The air in the drying section becomes very humid, the humidity retards the evaporation of the paper web. By having a good ventilation the humidity is reduced and the evaporation rate is increased. With the increased evaporation the temperature of the paper is reduced and, as mentioned before, the heat flow is increased[12].
• Steam pressure and number of drying cylinders.
If the other factors have been optimized and the desired drying capacity is still not enough, the steam pressure inside the drying cylinders can be increased. This is to a limit where either the economical or technical restriction is reached. Another, more viable, alternative is to increase the number of drying cylinders in the section. For Yankee cylinders the diameter can be increased but just to a technical limit where manufacture and transport of the cylinder becomes to difficult and expensive [12].
The multicylinder system uses many drying cylinders placed close to each other. The cylinders have a diameter of 1,5 to 1,8 m and there can be around 25 to 90 of them in one section. The placement of the cylinders is often in two rows over each other in such a way that the surface being dried always changes when it is transferred to the next cylinder. The cylinders are separated
into different sections with separate steam pressures to be able to optimize the drying further.
The ventilation of the multicylinder system plays a vital role. The multicylinder section are often enclosed by an isolated drying hood, the leakage, both in and out of the hood, should be minimal.
The most important task of the ventilation hood is to extract steam, steam originated from the paper web [12].
The Yankee cylinder uses the same principle as an ordinary drying cylinder, the difference is that it is usually much bigger, a diameter of 4-6m compared to 1,5-1,8m. The Yankee cylinder is often a cast iron cylinder with a good surface finish. The first step for the paper web before it gets into contact with the Yankee cylinder is a pressing in one or two steps with heat applied. The presses are done by cylinders with a rubber cover to get that elasticity to form a shoe pressing motion. This kind of pressing against the Yankee cylinder gives a great heat flow to the paper web due to the fact that the paper web sticks to the Yankee surface through surface tension of the water in the web. A greater heat flow is reached with this setup than with a multicylinder setup where a plastic wire is used to press the web against the surface [12].
In order to increase the drying capacity of a Yankee cylinder a drying hood is placed over the cylinder. The drying hood dries the paper web even further by blowing hot air through holes inside the hood and perpendicular onto the paper. The raised temperature helps the air to carry more water and so be able to transport away more evaporated water from the web. The hot air can be produced from high pressurized steam, up to 200◦C with 30 bar steam, and with fuels like gas or oil it can reach higher temperatures [12]. Since the air flowing inside the hood can reach temperatures up to 700◦C it is important to design the hood for the thermal expansions [15]. Factors influencing the drying hoods performance are temperature, humidity and velocity of the air, size of the holes where the hot air is blown through and the distance from hood wall to cylinder surface. The energy for drying the paper at this stage is expensive, both economically and environmentally since using gas and steam. Logically the best way to reduce costs and emissions is to maximize the dry solid content from the wire and the press section before entering the dryer section. To get a higher efficiency on the drying hood the hot air is recycled, by dividing the hood in to two parts where air outlet of the first parts is heating up the second parts air inlet. Each part have their own gas burners and fans that can be optimized to achieve desired air temperature and dry solid content [12]. The first parts is often called wet-end section and the second part dry-end section. To have an optimal running process of the Yankee hood, no humid air from the machine room should enter the hood nor should any hot air from the hood leak out to the machine room [15].
2.1.5 Reeling and sheet cutting
After the drying process is done and the paper have reached its final dry solid content of around 90%, it gets rolled up on a big reel. When a certain amount of paper have been rolled up, the web is cut and the web continues onto a new reel. From these big reels the paper is then cut into the right diameter, matching the, for example, after-treatment machines. The parts where the quality of the paper does not live up to standard is cut away as well as the damaged parts. The finished reel of paper must have a homogeneous hardness and must be round in shape, otherwise it might get problematic in the after treatment process. Sometimes, depending on the paper, product and customer, the mill itself can cut the product from the reels instead of delivering reels to another site, a converting site [11].
2.2 District heating
The main purpose of district heating is the transportation of thermal energy from a heat source to a customer. This is done with pipes holding a heat carrier, often pressurized water. The pipes should be as short as possible and have isolation to minimize the heat loss. The consumer can then retrieve the heat from the carrier and use it for example for heating up buildings [16].
For an industrial site to get more efficient and environmental friendly they can install a district heating system to recover heat that would otherwise become waste heat [16]. By recovering the waste heat from an industrial site the site can increase its efficiency greatly. The temperature of this waste heat from the industries is playing a vital role, the temperature needs to be high enough so that the customer can extract any heat from it. This can often be controlled at the heat source site with different heat exchangers and by monitoring the heat transfer [17].
2.2.1 Heat Exchanger
The heat exchange from the heat carrier to the fluid for the district heating takes place with a heat exchanger. There are many different kinds of heat exchangers but the most common ones are plate heat exchangers. Plate heat exchangers have commonly two fluids in them, one is the heat carrier and the other one the fluid being heated. The fluids flow in opposite direction of each other. The fluids flow through very thin plates in thin channels, every other channel allows the heat carrier to pass through and every other the other fluid. The plates are corrugated to force turbulence in the fluids causing a larger area for heat transferring and therefore higher heat transfer rate. The cool fluid will exit hot and the heat carrier will exit cool, as an outcome of this heat transfer. An illustration of this is seen in Figure 2. Between the thin plates there are rubber gaskets making it able for a tight fit and to direct and create the channels for the fluids so they will not mix. There are several reasons not to mix the fluids, they are individual for each system but an example is where you have hot oil that needs to be cooled with water. The amount of plates and channels can vary due to the restrictions and expectations of the heat exchanger [16–18].
Figure 2: A two-way plate heat exchanger, heat medium inserted at the top (red arrow), carrier being heated inserted at the bottom (blue arrow) [16], with permission.
2.2.2 Condenser
A condenser is a thermal heat exchanger where the heat source is steam that is put into a vessel.
In this vessel there are tubes of the fluid being heated from the steam, the tubes prevent the two substances to mix. Condensers can be used in both cooling system and heating systems. The steam is then cooled down when in contact with the cold fluid in the tubes and condenses. The change of phase of the steam causes a decrease of volume[19]. This decrease of volume causes a natural low pressure in the vessel, a lower pressure gives a better efficiency since the energy content of the steam is used better, an overview over a condenser is shown in Figure 3 [20].
Figure 3: Process picture over a condenser, with permission from OKG AB [20].
3 Framework
This thesis will treat the systems at Ahlstrom-Munksj¨o’s Mill in Billingsfors. This chapter explains in short the setup of the mill’s pulp and paper manufacturing processes and shows schematic pictures of the paper machines. The district heating system of the mill is explained as well.
3.1 Pulp production
The pulp mill also produces steam for other parts of the mill. The mill uses high pressure steam and low pressure steam. The high pressure is at 10 bar (over the atmosphere pressure of 1 bar [21]) and the low pressure is at 4 bar (over the atmosphere). When the steam is produced the pressure is up to 40 bar and to lower the pressure to 10 bar the mill have an integrated steam turbine and generator. The possibility to blow the excess steam pressure into the atmosphere instead of direct it through a turbine is also an option to lower the pressure. The turbine is a way for the company to extract more energy out of the process instead of raising the waste energy of the mill by blowing it out to the atmosphere. The turbine and the generator makes it possible for the mill to produce their own electricity while lowering the pressure of the steam to match the rest of the system. This information was extracted from personal encounters with Christian Malmberg at Ahlstrom-Munksj¨o.
3.2 Paper production
The Ahlstrom-Munksj¨o mill in Billingsfors uses three different paper machines and one pulp processing machine (VPD). The three paper machines are named PM2, PM5 and PM6. PM2 and PM6 use the same process, but PM6 is wider (5.1m vs 3.2m) and can therefore produce more paper. The process and products of each paper machine and pulp processing machine are described below. Information and figures of the paper production comes from contact with Johan Franz´en from H˚afrestr¨oms F¨oretagspark AB, currently working at Ahlstrom-Munksj¨o mill in Billingsfors.
3.2.1 PM2 and PM6
As mentioned earlier PM2 and PM6 uses the same process for paper manufacture. They can produce the same type of paper but at different widths. The paper is manufactured with different qualities.
Different qualities are created with corresponding recipes. A recipe uses a specific combination of long and short tree fibers, a specific manufacturing method of the pulp and different surface weights.
The process starts with a headbox producing a jet stream to the wire section where foils and suction boxes are positioned. The dry solid content after the wire section varies with the type of paper quality produced. After the wire section the press section takes place on the Yankee cylinder with a shoe press mechanism. The Yankee cylinder and the Yankee drying hood dries the paper further to the desired dry solid content. PM2 also have an electrified infrared heating system and an infrared heating operated on gas. The infrared systems are placed after the Yankee and used
when needed, the gas infrared is rarely used. The machines are seen from the side in Figure 4 and 5. The machines produce different types of paper but the most common one is steel liner for packaging of steel.
Figure 4: PM2 seen from the side, the process starts from the right.
Figure 5: PM6 seen from the side, the process starts from the right.
3.2.2 PM5
Paper machine PM5 uses a combination of a yankee cylinder and a multicylinder system as drying section, see Figure 6.
Figure 6: PM5 seen from the side, the process starts from the right.
The stock comes from a headbox going through a wire and press section where they have foils and suction boxes similar to PM2 and PM6, this section is not seen in Figure 6. The press section is composed of large cylinders that press on the web which force the water out of the stock. After the wire and the press sections the web enters the multicylinder system where the drying process starts. The web then enters on to the Yankee cylinder. PM5 is used to create a different type of paper, compared to PM2 and PM6. The paper is used for other products and demands a different type of paper making process.
3.2.3 VPD, Pulp Processor
The VPD (Valspress version D) only produces pulp, the pulp is transported to other mills that produce paper out of the pulp. The pulp has a dry solid content of around 50%, this percentage is reached through only a pressing section.
3.3 District heating
The district heating system is a way for the mill to use its waste heat from the paper process to get a higher efficiency and less waste. During the summer, or when the outside temperature is raised, the customer is in less need of the district heating. This leading to a larger , unwanted, waste of energy at the mill. The information about the district heating at Ahlstrom-Munksj¨o in Billingsfors is extracted from personal encounters with Christian Malmberg from Ahlstrom-Munksj¨o. The district heating water is to be delivered to the customer at a temperature of 90◦C and is returned to the mill at a temperature of 50◦C. The flow of the district heating water to the customer, and in the system, is estimated to an average of 35 m3/h. The district heating system is heated up in four steps, seen in Figure 7.
Figure 7: The district heating system at Billingsfors Mill, each box represent heat exchangers.
Each box in Figure 7 represent heat exchangers heating up the district heating carrier. The four steps of heating the district heating carrier, water, is:
• Stripper Condensate
This type of condensate is the most filthy of the condensate in the mill. It originates from a certain condensate that is pumped into a stripping plant where non-condensable gases are removed from the liquid, the left over is named the stripper condensate. However the Stripper Condensate have been too filthy for the heat exchanger, clogging the heat exchanger, therefore the use of it is negligible. The heat exchanger is a plate heat exchanger.
• Evaporation Condensate
Evaporation condensate originates from the washing liquid from the the pulp wash, it is heated up and the water evaporated is cooled down and changes phase to condensate. For the evaporation condensate heat transfer a plate heat exchanger is used. The condensate is heating up the district heating water to a temperature of around 55◦C (average temperature).
• Hot water
The hot water heat transfer also takes place with the help of a plate heat exchanger, the temperature after the heat exchanger is around 70◦C (average temperature).
• Steam
The last step of heating up the district heating water is a heat exchanger where steam heats up the water. The steam is used to get 90◦C after the exchanger. The heat exchanger is a condenser where the district heating water runs inside the tubes and the steam is inside the vessel surrounding the tubes.
4 Methods
In this section it is explained how the mill’s major processes are divided into different sub-models and the detailed level of each model. It is also explained how the energy audit is presented. A comparative study between PM2 and PM6 is explained as well as the assumptions for the thesis.
The first step was to create a model (model 1) over the whole mill where an energy audit was carried out to get an overview over the mill and its energy and fuel usage. Model 1 includes the three largest processes at the mill; the pulp, steam and electricity production, the paper production and the district heating. Two more detailed models were created (model 2 and 3), each model corresponds to the latter two of the three processes in the first model. One process (pulp, steam and electricity production) were left out to delimit the thesis. The pulp production is currently under a reconstruction and an energy audit over the current setup would lose its value once the new system is up and running. Energy audits were made on model 2 and 3, showing energy distribution within the processes, paper manufacturing (model 2) and district heating (model 3). A risk analysis is planned for the district heating system, see section 1.4, and it is therefore unnecessary to perform a more detailed audit on model 3. From results of the energy audit of model 2, conclusions were drawn to focus on PM2 and PM6 within the paper production and at their gas consumption, the paper machines are the largest consumers of energy within the paper production. The two paper machines, PM2 and PM6, were believed to have similar processes and their data to be comparable with each other. The focus on the gas consumption is relevant due to the fact that it is the most expensive part of the drying section, both economically and for the environment. A comparative study of the two paper machines was done as well as a more detailed energy audit over the Yankee cylinder and its drying hood for each paper machine.
4.1 Energy audits
In order to find the processes with the largest optimization potentials, an energy audit was performed.
The steps recommended by Energimyndigheten [4] are (see Figure 8):
Figure 8: Flow chart showing how an energy audit is usable in an industry and how to take actions from it, reworked from Energimyndigheten [4].
The energy audit started by taking the whole factory into account (model 1). The overview of energy audit had a system limit around the mill and within the system three other sub-systems:
• Pulp-, Steam- and Electricity production
• Paper production
• District heating
The energy audit was divided into steps, the first step, over model 1, showed the energy distribution over these three systems. The second step was over model 2 and 3 showing the energy distribution over the paper production and the district heating system respectively. The results from the energy audit over the paper production (model 2) showed a supposedly high gas consumption for PM2, this conclusion was made by comparing its data with the data for PM6. As mentioned earlier this comparison between the two machines seemed simple and an easy way to draw the conclusion that PM2’s gas consumption was high, it could however also mean that PM6’s gas consumption was rather low instead. To evaluate the conclusion a comparison between PM2 and PM6 was done.
The comparison included component or section differences and energy and fuel consumption over the Yankee system, see section 4.2.
The audit shows the energy flow from the beginning of the process (raw goods and bought electricity) to the end (district heating outcome, waste heat and finished product). The results were presented in block diagrams since this is a good way to get an overview picture of the audit that is easy to understand [4]. The input data used in the audit was obtained through personal encounters with staff at the mill, data logs from the mill and some data were calculated from the data logs. Through the personal encounters with the staff at the mill several data values were estimated as well, the software Microsoft Excel was used to compile the audit. The energy audit in this thesis is based on the energy audit provided by Ahlstrom-Munksj¨o. The energy audit from Ahlstrom-Munksj¨o over Billingsfors mill contained different data on values and flows within the mill. The audit contained some flaws and needed to be structured in a better way for an easier understanding and a more correct audit. The main focus of their audit were the in and out flows of the whole mill in total and not so much on a more detailed level.
All input data were recomputed to the same unit, Watt hours, to be able to compare the different fuels and energy forms. The mill uses a variation of fuels to operate, and the fuels were transformed into different energy types.
The audit was designed so that the input value or values of one process should be equal to the output value or values. All energy going into a process should exit it. The first law of thermodynamics states that energy cannot be created nor destroyed within a closed system. Each model is seen as a closed system where this implies. The energy form of input and output can be different and needs to be determined and distributed for each process the audit deals with. The results will present the calculated and measured values and the results will be compared to the theoretically correct audit, where input equals output.
4.1.1 Billingsfors mill, model 1
The first audit was done with the system limit outside the whole mill, the audit was rough and did not go into a deep level within the mill. The audit is presented through a block diagram shown in Figure 9 where the mill has been divided into three main systems with different energy flows.
Figure 9: Block diagram of the mill, model 1, with its energy flow.
Each energy flow has been named for an ease of presentation. Each flow contains the sum of every energy form flowing from or to that process, the energy forms for each flow is presented in Table 1.
Table 1: Each flow with their respective forms of energy
IN BB CC DD EE FF GG HH JJ KK LL OUT
Oil Gas Electricity Water Wood chips Lye Sediment Paper Bark
Electricity Lye Sediment Paper Bark Oil Water
Electricity Water
Electricity Gas
Pulp Steam Electricity
Steam Water
Waste heat Turpentin
Condensate Paper
Waste heat Waste water Paper Pulp
Water Water Condensate
Water Steam Waste heat Waste water
The distribution of the different energy forms and flows used in the mill and in the processes was made with the help of the staff of the mill together with information about the paper making process in literature [10–12, 15].
4.1.2 Paper production, model 2
One level deeper into the energy audit the paper production was audited, model 2. The system limits are around the paper machines, the VPD and the after-treatment machines. The system is shown in Figure 11 as a block diagram. This is a more detailed description of the paper production block in Figure 9, the flows on the outside in Figure 11 is therefore the same flows on the paper production block in Figure 9. A representation of the levels of the models is shown in Figure 10 where the paper production and district heating audit is within the same audit as the whole mill.
This Figure (10) is for an easier understanding of the usage of different levels of the energy audits.
Figure 10: A representation of how the models are audited with different levels.
Figure 11: Block diagram of the paper production, model 2, VPD and after-treatment in the mill.
Each flow inside the paper production was categorized into four groups as they have the same content but goes into/from different machines. The categorization is shown in Table 2.
Table 2: Fuel and energy content of each flow within the paper production D-Flows E-Flows H-Flows J-Flows
Electricity Gas
Electricity Pulp Steam
Paper Condensate
Waste heat Paper Pulp
4.1.3 District heating, model 3
The district heating system is shown in Figure 12 where the system limit is within the district heating box from Figure 9. Figure 10 is also applied here where the district heating is one level deeper within the first block diagram in Figure 9. The block diagram in Figure 12 was reworked from Figure A.1 in Appendix A, the reworked model contains the the different heat exchangers and energy flows as Figure 12 shows. Figure A.1 was given from the mill.
Figure 12: Block diagram of the district heating system, model 3, in the mill. The first letter of the flow is categorized.
The first three steps in the district heating system are ordinary plate heat exchangers with, stripper condensate, condensate from the evaporation stage and hot water as heating medium. The last step is a condenser where steam is heating up the district heating carrier. Each step increases the temperature of the heat carrier until a final temperature is reached. This temperature is determined by the provider (the mill) and the customer (the municipal) together, see section 3.3.
The flows within the district heating and their respective fuels and energy content are shown in Table 3.
Table 3: Fuel and energy content of the flows within the district heating. The first letter of the flow is categorized.
C and K -Flows F-Flows L-Flows Water
Condensate Steam Water
Water Condensate
4.2 Comparative study of PM2 and PM6
Both PM2 and PM6 produces the same type of paper and quality, the difference is the width of the machines. PM6 is 5.1 m wide and PM2 is 3.2 m wide, consequently PM2 is 37.25% less wide than PM6. The difference in energy consumption between the two paper machines was thought to be quite predictable, where PM6 should use more energy than PM2 due to the larger width.
The prediction was that PM2 should be consuming around 62.75% of PM6 energy consumption.
To compare these two machines the process was first assumed to be similar, see section 3.2.1. Any differences, in the paper manufacturing process, between the two paper machines were determined through contact with the staff of the mill.
In order to make the comparison, values had to be converted to comparable units and quantities.
This was done by using information like, speed, width, gas and steam usage, paper produced and dry solid content percentages for each machine. All information could not be retrieved. Some assumptions were made, see section 4.4. First the amount of paper exiting the paper machine was calculated with known data as speed, width, operating hours, dry solid content of finished paper and the surface weight of the paper. The distribution between pulp and water was then calculated through the dry solid content. It was assumed that the fiber content of the paper web is constant during the process. Mass, water and pulp content was calculated for each step for which the dry solid content was known (constant pulp content over the whole process). The drying part of a paper machine consists of three main sections, see Figure 13. The dry solid content and the outgoing water was shown for each section.
Figure 13: Flow schedule over the drying part of a paper machine, simplified.
The gas usage was calculated to get a usage per square meter of paper for each machine for easy comparison.
The two machines were also compared component wise, do they have the same components? Each component in the sections affects the paper process and also all the different usages of fuels, dewatering and evaporation of the paper web.
4.2.1 Yankee cylinder
The different sections of a Yankee cylinder is shown in Figure 14 and this Figure is used as reference to divide different stages in the energy balance.
Figure 14: Simplified overview over a Yankee cylinder and it’s drying hood.
The energy flow through the different sections is through water in the press section and through evaporation and thermal energy at the Yankee cylinder and hood. A smaller model on the Yankee cylinder and its hood was done using Figure 14. The model included only energy going into the system and out. Energy in the form of gas, steam and air was put into the system and energy in the form of evaporated water, condensate and waste heat comes out of the system in the audit.
4.3 Energy calculation
There are many different resources that are processed in the pulp and paper mill, these resources all have different energy content. The different energy forms found in the pulp and paper mill are electricity and thermal energy. The energy is expressed in Watt hours or Joule, a conversion factor of 3.6 GJ/MWh is used to get the wanted unit.
The energy content of water and saturated steam were taken from Thermodynamics: an engineering approach [22]. Other energy content information for fuels like oil, tall oil, wood chips and paper were taken from The Swedish Environmental Protection Agency [23]. The energy content of the gas used in the mill was given from the staff of the mill, a mixture of propane and butane gas is used.
4.4 Assumptions
The operation time for the mill was determined to 340 days on a year and 230 days for the VPD.
This was determined together with the staff of the mill. The study does not take any seasons into account. The seasonal change affects mostly the outside temperature changing affecting different thermal flows within the mill. The operating data have been retrieved from 2018, which also means that the results and all calculations will be based on data from 2018.
Both PM2 and PM6 follow a schedule of what paper quality to produce, they produce several types of paper and an assumption was therefore made that they only produce one type of paper during a year. This was done by checking which paper quality was produced the most during 2018, time wise. The quality that was produced the most was a steel liner with a surface weight of 35g/m2 and a dry solid content of 93.5%. Other input data such as gas, steam usage and speed was also extracted during the production of this quality. Mean values of the speed, gas and steam usage were calculated. These mean values were assumed to be the operating data, of mentioned categories, over a whole year.
15% had to be excluded from the numbers on pulp flow reaching the paper machines to get the fresh pulp, fresh pulp from the pulp manufacturing. The 15% excluded are recycled paper from the paper machines, or waste paper that is put back into the cycle. The number was estimated by staff on the mill.
For the sake of the energy audit of the Yankee drying cylinder and its hood on PM2, the dry solid content percentages are assumed to be the same for PM2 as for PM6. The values for PM6 are known, but for PM2 they are not. This assumption also includes the assumption that the both machines looks the same, have the same components and process the paper in the same way. This is a very rough assumption but is mainly used for the audit of the Yankee.
The own made electricity from the pulp, steam and electricity production was assumed to be distributed to the paper production. The own made electricity was not distributed equally over all processes but making some of the processes operate on 100% on the own made electricity and some on only bought electricity.
5 Results
The results begin with the first model over the whole mill and work its way down to a more detailed level to the comparison between PM2 and PM6.
5.1 Energy audit
Several energy audits were made, one for the whole mill and two on a deeper level as well as one on the Yankee drying section at PM2 and PM6. The results are presented using the block diagrams showed in section 4, Figure 9, 11, 12 and 14. The audits show the input and output flows of each process, some flows connect the processes together. Each block diagram present an overview over the system, showing the largest energy flows.
5.1.1 Billingsfors Mill
The first energy audit of the mill is presented in Figure 15 where each energy flow has a given value representing the energy content of each flow. The energy content is given in GWh per year.
Figure 15: Block diagram over the billingsfors mill with values over each energy flow, values are given in GWh per year.
The inflows identified in the audit are one to each process, flow BB, CC and DD. They all come from the IN flow, outside from the mill. These flows include energies within fuels, bought electricity and water. The pulp, steam and electricity production have two more ingoing energy flows, one coming from the district heating system, flow LL, containing condensate used in the district heating system.
The second one from the paper production, flow HH, containing waste paper and condensate from the drying process. The outgoing flows from the pulp, steam and electricity production are three, one going to the district heating system, flow FF, including hot water, condensate and steam. The second flow goes to the paper production, flow EE, containing pulp, steam and own made electricity.
The third flow, flow GG, are waste energy, in the form of thermal energy, and turpentine, turpentine being sold and the waste energy released to the surroundings.
The ingoing flows for the district heating system are flow CC, containing electricity and the district heating carrier, water. The second ingoing flow is flow FF from the pulp, steam and electricity production. The outgoing flows are flow LL going back to the pulp, steam and electricity production and flow KK containing heated up water.
Ingoing flows for the paper production are flow DD and EE. DD containing electricity and gas. The second ingoing flow, flow EE, is from the pulp, steam and electricity production, the same EE flow as mentioned above. The outgoing flows are flow HH back to the pulp, steam and electricity production and flow JJ, containing water from the drying process and fumes from the gas combustion in the Yankee drying hood.
The largest flows are flow BB, GG, EE and JJ. It is the pulp, steam and electricity production that is the largest energy consumer according to the audit. The second largest, is the paper production and the district heating system is the process consuming the least energy. In this study, the largest energy saving potential was determined to be at the paper production since the pulp, steam and electricity production are under a ongoing reconstruction, see section 1.4.
The outgoing flow, flow OUT, is the sum of the outgoing flows from each process. The value of this flow should however be equal to the ingoing flow , flow IN, for the model to be correct. The difference can be explained by assuming it to be waste energy, energy transformed to, for example, sound, heat or processes not taking advantage of the the fuel/energy to its fullest.
5.1.2 Paper production
The paper production block in model 1 was divided into five processes, three paper machines, PM2, PM5 and PM6, one pulp processor, VPD, and the after-treatment machines, T&B (Press and Coating). Model 1 can be seen in Figure 16.
Figure 16: Block diagram over the paper production at Billingsfors mill with values over each energy flow, values are given in GWh per year.
Flow DD is distributed over the different processes within the paper production system. The three paper machines have the same type of energy content in their flows but different quantities.
The flows DA, DB, DC and DF contains electricity and gas. The Pulp processor, VPD, does not consume any gas, only electricity is distributed, through flow DE, to this process.
The other ingoing flow, EE, containing pulp, own made electricity and steam is distributed over all five processes. The three paper machines are receiving the same type of flow but at different quantities through flow EA, EB and EC. The content of the flows are steam, electricity and pulp.
The VPD is receiving pulp and electricity through flow ED and the after-treatment process is receiving electricity through flow EF.
The outgoing flows from the paper machines, flow JA, JB and JC, contain paper, fumes and water. The water is included in the outgoing flow JJ. The data collected of water outlet were not distributed over the three machines but a total of them all. Outgoing flow JD from VPD contains water and pulp (dry solid content of 50 % ). From the after-treatment processes the outgoing flow, JE, contains fumes from gas combustion.
The outgoing flow returning to the pulp, steam and electricity production, flow HH, comes from the paper machines through flow HA, HB and HC containing condensate and waste paper. The value of HH is seen as correct but the distribution of HH over HA, HB and HC can be questioned. The flow HH was measured and is seen as correct, but the flows HA, HB and HC have been measured only once or twice by staff of the mill.
The energy consumption and usages for the different processes within the paper production are shown in Tables 4-7. In these tables the gas, steam and electricity consumption is presented as well as the fresh pulp feed.
Table 4: The gas consumption per year within the paper production model, distributed over the processes
Gas consumption ton MWh
PM2 1 396 17 974
PM6 1 600 20 600
PM5 273 3 515
T&B 147 1 893
Total 3 416 43 982
Table 5: Steam consumption per year within the paper production model, distributed over the processes
Steam consumption GJ MWh
PM2 58 943 16 373
PM6 88 080 24 467
PM5 76 531 21 259
After-treatment 4 560 1 267
Other 75 109 20 864
Total 303 223 84 230
Table 6: Fresh pulp feed distribution per year over the three paper machines
Fresh pulp m3 MWh
PM2 361 643 17 456
PM6 587 202 28 344
PM5 317 646 15 347
Total 1 266 791 61 147
Table 7: Electricity consumption of PM2, PM6 and PM5 per year Electricity MWh
PM2 14 821
PM6 26 202
PM5 12 979
Total 54 002
As seen in table 4, the gas consumption for PM2 is relatively large when compared to PM6. PM2 is ∼37 % narrower than PM6 hence expected to consume less gas, corresponding to ∼63 % of PM6 consumption, 12 925 MWh instead of the measured value of 17 974 MWh, a 28 % difference.
When looking at the steam consumption in Table 5 this statement is plausible. The calculated steam consumption for PM2 resulted in 15 351 MWh and differs 2 622 MWh or 14.6 % compared with the measured value for PM2.
The statement is also valid for the fresh pulp feed, when analyzing the values in Table 6, 63 % of PM6 feed, 28 344 MWh, is equal to 17 784 MWh compared to the measured value for PM2 of 17 456 MWh, only 1.8 % difference.
The electricity consumption also follows this trend, by evaluating the values in Table 7 the theoretical value (63 % of PM6 consumption) is equal to 16 440 MWh, comparing this to the measured value for PM2 of 14 821 MWh it differs 10.9 %. The gas consumption of PM2 is therefore much higher than the rest of its energy usage when compared to PM6. The results are presented in Table 8. The differences of gas, steam, pulp and electricity should match the difference of the width, 62.25%.
Table 8: PM2’s proportion to PM6 of each energy flow and the width of PM2 and PM6 PM2 PM6 PM2’s proportion to PM6
Width [m] 3.2 5.1 62.75
Gas [MWh] 17 974 20 600 87.25 Steam [MWh] 16 373 24 467 66.99 Pulp [MWh] 17 456 28 344 61.59 Electricity [MWh] 14 821 26 202 56.56
PM5 produces different kind of paper qualities compared to PM2 and PM6. The paper making process is also different since PM5 uses a combination of a multicylinder system and a Yankee system. PM5 is therefore not included in the comparative study.
5.1.3 District heating
The district heating block from model 1 is shown in Figure 17. The district heating system contains four heat exchangers, heating up the district heating carrier. The carrier is then pumped out to the nearby municipality where they extract heat from the carrier and the carrier is returned with less energy and then heated up again.
Figure 17: Block diagram over the district heating system at billingsfors mill with values over each energy flow, values are given in GWh.
The ingoing flow CC contains the heat carrier in the district heating system. It is the return from the municipal and contains water of 50◦C. The heat carrier is heated through four heat exchangers. The ingoing flow, FF, heating up the carrier originate from the pulp, steam and electricity production.
The first two heat exchangers are using condensate, the third hot water and the last step to ensure
the right temperature of the heat carrier utilizes steam. The flows CAA, CAB, CAC and CAD are showing the effect of each heat exchanger on the heat carrier. The energy content after each heat exchanger is raised. The first heat exchanger, Stripper heat exchanger, is not receiving an energy flow due to the filthy condensate in this process. The filthy condensate is clogging the heat exchanger interfering the heat transfer in the heat exchanger.
The two outgoing flows are the heat carrier for the municipal at an elevated temperature of 90◦C, flow KK. The second outgoing flow, flow LL, is condensate and hot water, both at a lower temperature than when ingoing.
5.2 Comparison of PM2 and PM6
The flow schedule for PM6 is presented in Figure 18, the dry solid content percentages after each section is known, the water outlet is calculated.
Figure 18: Flow schedule over PM6, where water exits at the bottom and the dry solid content percentages of the paper is shown after each section.
The water outlet is calculated by knowing the width, operating time and speed of the paper machine as well as the surface weight of the paper quality produced. This gives a total of 34 292.62 ton produced paper during one year, with a dry solid content of 93.5%. The fiber content is assumed to be constant during the paper making process, causing only water to be removed from the paper web. The water removed in each step can thereby be calculated with the dry solid content and the knowledge of the fiber content, fiber content is equal to 93.5% of the total tonnage of produced paper, 32 063.6 ton. The mass content of the paper web during the paper making process is presented in Table 9.
Table 9: The dry solid content and mass flow during each step of the drying process for PM6 Drying process Headbox Wire section Press Yankee
Dry solid content % 3.5 12 37 93.5
Mass [ton] 916 103 267 197 86 658 34293
Fiber [ton] 32 064 32 064 32 064 32 064
Water [ton] 884 039 235 133 54 595 2229.02
Water outlet [ton] - 648 906 180 538 52 366
The drying process is illustrated in a simplified figure, Figure 19, where the dry solid content is written showing where in the process it corresponds.
Figure 19: A simplified figure over the drying process, without the wire section, and its corresponding dry solid content for PM6, the same dry solid contents are assumed for PM2.
For PM2 the dry solid contents are assumed to be the same as for PM6 to be able to make the same kind of audits and to make a comparison between the two machines. The flow schedule for PM2 is shown in Figure 20 where the water outlet and dry solid content after each section is given.
Figure 20: Flow schedule over PM2, where water exits at the bottom and the dry solid content of the paper is shown after each section.
The water outlet is calculated the same way as for PM6, known width, speed, operating time and amount produced paper, 19854.91 ton. The paper quality is the same as for PM6, as well as assumed constant fibre content. The mass content of the paper web during the process is presented in Table 10.
Table 10: The dry solid content and mass flow during each step of the drying process for PM2 Drying process Headbox Wire section Press Yankee
Dry solid content % 3.5 12 37 93.5
Mass [ton] 530 410 154 703 50 174 19 855
Fibre [ton] 18 564 18 564 18 564 18 564
Water [ton] 511 845 136 139 31 610 1 291
Water outlet [ton] - 375 707 104 529 30 319
An energy audit over the Yankee drying system, where the dry solid content changes from 37% to 93.5% is presented in Table 11. The input energy forms are gas and air for the Yankee hood and steam for the Yankee cylinder. The outgoing energy is assumed to be condensate, waste heat to the machine room, waste heat through the chimney and the amount of required energy to evaporate the amount of water exiting from the paper web through this step. The outgoing energies are shown in Table 12.
Table 11: Ingoing energy from the energy audit over the Yankee system at PM2, per year
Yankee energy audit ton GJ MWh
Gas 1 524 70 628 19 619
Steam 24 861 68 919 19 144
Air 173 795 16 511 4 586
Total input energy 200 181 156 058 43 349
Table 12: Outgoing energy from the energy audit over the Yankee system at PM2, per year
Yankee energy audit ton GJ MWh
Evaporated water 30 319 68 400 19 000
Chimney, hot air 173 795 87 183 24 218
Waste heat to machine room
Condensate 140◦C 19 752 11 637 3 233 Total outgoing energy 223 866 167 220 46 451
The data presented in Table 11 and 12 are the mean values for gas and steam consumption and collected data from the chimney, outgoing from the Yankee drying hood. The total input energy and outgoing energy values does not match, see explanation in section 6. There is waste heat to the machine room in the real case, the reason for the empty row in Table 12 is that the waste heat is equal to the difference between input energy and output energy. In this case output energy is larger than input energy which would give a negative value on the waste heat to the machine room, this is not reasonable.
This energy audit over the Yankee system was also done for PM6. Data at the chimney from the Yankee drying hood is missing. The mill does not possess the right instruments to investigate the flows from the chimney and the measurement process needs to be outsourced. The incomplete energy audit is presented in Table 13 and 14.
