Patent Application
20120325370
The present invention concerns a method for the production and processing of wood chips for pulp and paper production in accordance with the claims.
Methods for producing wood chips for pulp production or alternatively bio-energy are previously known. Chipping is normally done by some type of wood chipper such as a disc or drum chipper. Common for these chippers are that they contain a number of chipping tools that cut the wood into chips. The chipping tools consist normally of knives or the like. The characteristics and properties of the chips are affected by the geometry of the chipping tools, but also by the cutting angle in relation to the fibre's direction in the wood.
The angles that affect the chipping process and the chip properties are shown in
The properties the chips receive during chipping affect the subsequent unit processes. As an example in production of sulphate (kraft) or sulphite pulp it has traditionally been seen as a benefit for the subsequent processes that the chips have as little compression damage as possible. Chipping for pulp production is therefore done using a technology to minimize such compression damage.
Compression damage is caused by the compressive stresses acting on the edge of the chip that is in contact with the chipping tool (knife) during chipping. The shape and cutting angles of the chipping tool (knife) will result in such compression damage to different extents. It has been shown that the compression damage in the chips is minimized at a spout angle (ε) approaching 30°. A spout angle close to 30° is therefore used during the chipping process according to the present state of the art. This angle has been seen as the most beneficial for the fibre properties for chemical pulps.
Two major problems for production of pulp and paper is high energy consumption and high investment costs in process equipment. The energy consumption of the wood chipping process is a minor (insignificant) part of the total energy consumption. For production of mechanical pulp such as thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP) the energy consumption is high, often in the range 1000-3000 kWh/t. The most energy demanding process equipments are the refiners in which the defibration of chips into pulp fibres and further fibrillation and development of these fibres are performed. These refiners consumes up to 90% of the electrical energy used in the pulp production. The current high price of energy and the ongoing concerns about greenhouse gases motivates reductions in the energy consumption during pulp production. More specifically there exists a need to reduce energy consumption during the extremely energy consuming process of turning chips into pulp. In addition there is a need to increase the production capacity during production of both mechanical and chemical pulps without new capital investments.
A number of methods to reduce the consumption of electrical energy in the refining stage have been developed in the past. For example several pre-treatment stages for chips before refining have been developed. Trials have shown that pre-treatment of chips has the potential of reducing the specific energy consumption [kWh/t] in the subsequent refining stages.
A number of different equipment types have been developed to compress wood chips after chipping in order to reduce energy consumption during refining. For example the chips may be subjected to compression in a compression screw (plug screw). The drawbacks of compression screws are that they increase the capital cost of the plant and the complexity of the process. The present method is also principally different in the respect that in a screw the chips are compressed in a random direction whereas in the present method the compression is oriented in the fibre direction. The energy consumption for compression screw pre-treatment of chips is in the range of 20-40 kWh/t.
The company Andritz has developed an equipment which is marketed as RT Pressafiner. Using the RT Pressafiner the chips are compressed by the action of an advanced compression screw. The RT Pressafiner has the disadvantages of adding to the process complexity. Further the chips are not only compressed in the fibre direction. This equipment also requires a lot of space and may thus be difficult to install in an existing process.
Further it is known that energy consumption may be reduced by compressing chips in roll nip (between at least two rolls). The design essentially hinders the chips from being compressed in the direction of the fibres resulting in the compression of the chips at a right angle in relation to the fibre direction. This method is therefore significantly different from the method described in the present patent application.
It has also been shown that energy consumption during refining of chips may be decreased by a chemical pre-treatment of the chips between chipping and refining. One such method is described in an article by Hill, Sabourin, Aichinger and Johansson as presented at IMPC Sundsvall 2009.
It has unexpectedly been shown that it is possible to achieve a large reduction in electrical energy consumption during chip refining without introducing new process stages. This can be accomplished by applying different load angles 4 according to what is described in the present method. The method according to the present invention goes against the established knowledge in the field which declares that minimizing the compression damage in the fibre direction is the best alternative. It has been shown that this established knowledge is not correct when the purpose is to produce at least the equivalent quality of printing and board paper grades with a reduced total energy consumption. The present method creates only a slight increase in energy consumption during chipping (verification experiment 3).
The main purpose of the present invention is to create a method for chipping that results in significantly reduced energy consumption during defibration and development of wood into single fibres in subsequent process steps. This occurs by an opening of the wood structure by the compressive loads that arise during chipping. This shall be achieved without any significant increase in energy consumption during chipping. An additional purpose of the present invention is to create a method for chipping that may be combined with at least one additional process step to reduce the energy consumption in at least one subsequent process step in the paper pulp production process. Another purpose of the present invention is to ease impregnation of the chips by chemicals or water and allow the impregnation chemicals to come into contact with a larger surface area upon which the chemicals can react. Another purpose of the present invention is to increase production capacity without new investments in the process steps after chipping.
The invention will be described in greater detail below with reference to the accompanying schematic drawings that in an exemplifying purpose show the current preferred embodiments of the invention.
With reference to
The wood chipper utilized in the chipping process consists of a previously known type of wood chipper with one or several chipping tools 3, 14 in which chipping occurs according to the present chipping method. The present invention is applicable for chippers of the drum chipper, disc chipper as well as the reduction chipper types.
It has unexpectedly been shown that energy consumption in the subsequent refining stages is substantially reduced when wood chipping is performed with all of the chipper's chipping tools at the angle 4, in the range from 75° to 105° and preferably 80° to 100° between the wood fibre direction and the side of the chipping tool facing the chip.
Due to the wedge shape of the chipping tool loading angles in the range 75° to 105°, according to the present invention, will cause so large compressive forces in the fibre direction that a considerable cracking of the wood structure will happen. These compressive forces are directed in the fibre direction which is most beneficial. The process will in the following be called Directed Chipping, DC. The main requirement for this type of chipping is the application of loading angles 4 in the interval 75° to 105°. In currently utilized wood chippers, the loading angles are mostly around 115°.
In alternative embodiments of the present invention an adjustment of the chip length is done according to the wood raw material type and/or fibre length. The optimal chip length is different for different wood species. The adjustment of the chip length may occur within a considerable interval. For practical reasons such as the performance of subsequent feeding screws, the chip length should however remain in the interval 10-40 mm. It is however conceivable that other chip lengths than 10-40 mm may be used in alternative processes.
In an alternative embodiment of the present method the temperature of the wood logs is controlled in a pre-treatment stage 5 before chipping. The temperature of the wood logs is controlled to a desired temperature within the interval −10 to 130° C. The control of the log temperature may occur in a temperature controlled process zone or similar. The temperature may also be controlled in the choice of storage conditions for the logs. Storage may occur in water of different temperatures or alternatively in a conventional wood yard before debarking and chipping. Logs may for example be intermediately stored in hot process water after debarking which allows for high log temperatures before chipping. As the mechanical properties of wood depend strongly on the temperature, the degree of cracking of the chips during chipping will also depend on the temperature. In this alternative embodiment of the present method it is possible to choose a pre-treatment temperature so that optimal chip cracking occurs.
In an alternative embodiment of the present method the directional chipping is combined with a control of the solids content of the wood logs within the interval 30-70% solids content. This happens in a pre-treatment stage 5. The mechanical properties of wood are strongly influenced by the solids content and the loading angle's effect on chip cracking may be optimized by controlling the solids content. The solid content of wood may be adjusted and kept under control by a well organized logistics chain from logging through intermediate storage to the pulp mill wood yard, barking and chipping. The choice of storage conditions, e.g. in water, land storage with water irrigation or without irrigation will affect the solids content. This alternative embodiment of the present method optimizes the solids content so that an optimal chip cracking can occur.
In an alternative embodiment of the present method, the directional chipping in 6 is combined with a control of the cutting speed within the interval 15 to 40 m/s. Wood generally behaves as a visco-elastic material meaning that the cutting speed will have an influence on the cracking of the chips and that this speed may be optimized to achieve maximum cracking. Such speed control may be done by controlling the revolution speed of the motor of the chipper.
With reference to
In an alternative embodiment of the present method, the directional chipping in 6 is combined with chip impregnation with water, chemicals or enzymes in the next process step 7. The increased cracking of the wood that is achieved by using an adequate load angle will ease liquid diffusion into the chips and increase the specific surface area where liquids etc. may react in a beneficial manner with wood.
In mechanical pulping processes such as thermomechanical pulping or chemithermomechanical pulping, chemicals are often used to improve the fibre/pulp properties for specific end products (such as printing paper, board, tissue and fluff pulp). These chemicals can e.g. be added in the different process stages; chipping, chip impregnation, chip preheating 7 or during chip refining 8. For mechanical pulp production different types of sulphites, peroxides, caustic solutions (lye), complex binders and as of late also different types of enzymes are used to improve pulp properties. It has been shown that these types of chemicals together with the present invention significantly improve pulp characteristics compared to what can be achieved with conventional chipping technology.
For chemical pulping processes such as the sulphate (kraft) and sulphite processes both continuous and batch based cooking are used. Here the chemical impregnation is significantly improved by the present invention and the cooking (reaction) time is also significantly shortened. This will improve the production capacity in existing mills.
In the detailed description of the present invention design details and methods may have been omitted that are obvious to professionals in the field. Such obvious design details are included to the extent that is necessary so that the intended function of the present invention is achieved.
Even though certain preferred embodiments of the present invention have been described in detail, other variations and modifications within the scope of the invention may be realised by professionals in the field. All such variations and modifications are also considered to be within the scope of the following claims.
Results from experimental trials have shown that the present invention has unexpected technical effects. It has been verified that greater compression damage in chips has a beneficial effect on the pulping process. This new knowledge contradicts the well established knowledge in the industry that compression damage in chips should be avoided. In the following text, the results from three trials performed using the directional chipping of the present method are presented.
In the trial chips were produced using three different load angles 94°, 104° and 114° where 114° corresponds to conventional technology, see angle γ in
In the second trial the effect of the load angle on production of thermomechanical and two different chemi-thermomechanical pulps was investigated. The chipping was done as in trial 1 but only with the load angles 94° and 114°. The pulp refining in this trial was performed in two stages unlike trial 1.
In
An important property, particularly for printing paper, is the tensile strength, here given as the tensile strength index. In
Another important property of printing paper is its opacity which depends on the light scattering properties of the paper.
During paper production it is beneficial if the pulp's dewatering properties (CSF) can be controlled so that the predetermined end use properties in the paper such as tensile strength and opacity are as optimal as possible. The dependency of tensile stiffness index on the dewatering property CSF is shown in
The pulp that was produced from chips chipped using a load angle of 94° and with NaHSO3 in the dilution water had the best combination of properties for printing paper and in addition the lowest specific energy consumption.
When chemithermomechanical (CTMP) pulps are produced for board, tissue and fluff pulp qualities, high bulk (low density) and absorption properties are important, however opacity is not important. These types of CTMP are produced by impregnating the chips with an alkaline sulphite solution (Na2SO3) in an impregnation vessel after which the chips are pre-heated so that the sulphite has time to react with the wood before the chips reach the refiner. The fibres ability to give a high bulk depends on how large a fraction of intact fibres that have been produced in the refiner. This is limited by the demand for very low shives content in the pulp. It has been shown that CTMP produced from chips chipped at a load angle of 94° has a considerably lower specific energy consumption to reach a certain low shive level compared to CTMP produced at a load angle of 114°.
In summary it can be concluded (among other things) that (
If the energy that is saved in a later stage in the refining process is larger than the increase in energy consumption during chipping with the load angle at 94° (compared to 114°), the proposed method according to the present invention is highly valuable. To investigate the energy consumption during chipping for the load angles 114° and 94° the trials described below were conducted.
At the load angle 114° and a chip length of 25 mm the chipper was adjusted to a speed of 400 rpm which corresponds to a speed of 20 m/s for the chipping tool. When this speed was reached, the energy supply was turned off for the electric engine driving the chipper. Then the number of chips lengths produced by the stored rotational energy in the system was measured. This was done so that the length of the wood log with cross sectional dimensions of 50 mm×100 mm, that was chipped before the chipper stopped completely was measured and divided by the chip length of 25 mm. For the load angle 114° the number of chip lengths was 134 and for 94° the number of chip lengths was 120. The moment of inertia of the rotating system is 142 kgm2 so the stored rotational energy could be calculated to 1.25*105 J shortly before the chipping started. The energy consumption per chip length for the two load angles is then respectively 0.90 kJ for 114° and 0.94 for 94°. Assuming a density of 350 kg/m3 for dry Norway spruce and that each chip length produced a volume of 0.025×0.05×0.1 m3=1.24*10−4 m3 and thus the weight 0.043 kg this meaning that 5.8 kWh is consumed to produce one metric ton of chips at the load angle 114° while 6 kWh are consumed at a 94° load angle. This must be compared to a total energy consumption of 1500-2000 kWh/t pulp.
By use of a chipping method according to the present invention a number of benefits are achieved. The most prominent benefit is the increased energy efficient refining of the chips when they are produced in accordance with the method of the present invention. This is achieved thanks to that the chipping method according to the present patent application causes a beneficial cracking between the fibres in the chips so that they are more easily separated.
The more open structure of the chips also provides the benefit that chemicals such as sulphite solutions, peroxide solutions, alkali and others in addition to enzymes get better access to a larger reaction surface. This increases reaction speed, improves reaction evenness and reduces the chemical consumption to reach a certain pulp property. The chip refining is made more efficient by the more even impregnation of the chips and thus less problems occur with parts of the chips not being treated by the chemicals. An ineffective reaction between chips and chemicals cause more formation of shives during refining and in addition the added chemicals are less efficiently used which is a major problem in pulping.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1000210-3 | Mar 2010 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE2011/000042 | 3/2/2011 | WO | 00 | 8/30/2012 |
