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On the wood chipping process: a study on basic mechanisms in order to optimize chip properties for pulping
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In both the chemical and mechanical pulping process, the logs are cut into wood chips by a disc chipper before fibre separation. To make the wood chipping process more efficient, one have to investigate in detail the coupling between process parameters and the quality of the chips. One objective of this thesis was to obtain an understanding of the fundamental mechanisms behind the creation of wood chips. Another objective with the thesis was to investigate whether it was possible to, in a way tailor the chipping process so as to reduce the energy consumption in a following mechanical refining process.

Both experimental and analytical/numerical approaches have been taken in this work. The first part of the experimental investigations, were performed with an in-house developed chipping device and a digital speckle photography equipment.

The results from the experimental investigation showed that the friction between the log and chipping tool is probably one crucial factor for the chip formation. Further more it was found that the indentation process is approximately self-similar, and that the stress field over the entire crack-plane is critical for chip creation.

The developed analytical model predicts the normal and shear strain distribution and to be more specific, the model can predict the compressive stresses parallel to the fibre direction for an assumed linear elastic and orthotropic material. The analytical distributions were found to be in reasonable agreement with the corresponding distributions obtained from a finite element analysis.

To be able to study the chipping process under realistic conditions, which for example means to use chipping rates representative for a real wood chipper, a laboratory chipper was developed. Details regarding the chipper and how to evaluate the force measurements are given together with an example of how the force on the cutting tool (the knife) varies with time during cutting.

To investigate the influence of a certain chipping process parameter, the chips were after production in the laboratory chipper, refined in a pilot refiner during conditions optimized for TMP (thermomechanical pulp) and CTMP (chemithermomechanical pulp) processes. It was concluded that the details concerning the chip process had a large impact on e.g. the energy consumption in both first stage and second stage refining. Results showing this are given in this thesis.

Abstract [sv]

För både kemisk och mekanisk pappersmassa så tillverkas flis av trädstockar med hjälp av en skivhugg innan fibrerna separeras. För att göra flisningsprocessen mer effektiv, måste kopplingen mellan processparametrar och fliskvalitet studeras. Ett mål med denna avhandling är att ge fundamental kunskap om mekanismerna bakom bildandet av träflis.

Både experimentella och analytiska/numeriska metoder har använts i detta arbete. De experimentella undersökningarna har gjorts med hjälp av egen utvecklad utrustning.

Resultaten från den experimentella undersökningen visar att friktionen mellan stammen och flisningsverktyget har betydelse vid flisning. Vidare observerades det att inträngnings processen är approximativt självlik (self similar) och att det är spänningsfältet över hela sprickplanet som är kritiskt för bildandet av en flis.

Den utvecklade analytiska modellen förutsäger normal- och skjuvspänningsfördelningen över sprickplanet och kan mer specifikt förutsäga den kompressiva belastning som verkar parallellt fiberriktningen i ett linjärt elastiskt och ortotropt material (trä). De analytiskt bestämda fördelningarna stämmer relativt väl överens med motsvarande fördelningar beräknad med finit element analys.

För att kunna studera flisningsprocessen under realistiska förhållanden, vilket bl.a. betyder att skärhastigheter som är representativa för en verklig process skall användas, så utvecklades inom ramen för avhandlingsarbetet, en laboratoriesflishugg. Detaljer rörande flishuggen samt hur uppmätta lastsignaler skall utvärderas ges tillsammans med ett exmpel på hur kraften på skärverktyget (kniven) varierar under ett skärförlopp.

Inverkan av en viss flisningsprocessparameter undersöktes genom att flis tillverkades i laboratorieflishuggen varefter de raffinerades i en pilotraffinör under förhållanden som var optimerade för TMP (termomekanisk massa) och CTMP (kemitermomekanisk massa) processerna. Det konstaterades att detaljer i flisningsprocessen hade stor inverkan på t.ex. energiåtgången i både första stegs – och andrastegsraffinering. Resultat som verifierar detta ges i avhandlingen.

 

Place, publisher, year, edition, pages
Sundsvall: Mittuniversitetet , 2010. , p. 36
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 83
Keywords [en]
Wood chipping, Chip formation, Digital Speckle Photography, Friction, Fracture Processes, Analytical model, Laboratory Wood Chipper, Force Measurement, Vibration Synthesis, Energy Efficiency, Chip Damage, TMP, CTMP
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:miun:diva-11911ISBN: 978-91-86073-72-5 (print)OAI: oai:DiVA.org:miun-11911DiVA, id: diva2:343868
Public defence
2010-05-21, O102, Holmgatan 10, Sundsvall, 10:15 (Swedish)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted. Paper 5: Submitted.

Available from: 2010-08-16 Created: 2010-08-16 Last updated: 2015-11-04Bibliographically approved
List of papers
1. A Method for Experimental Investigation of the Wood Chipping Process
Open this publication in new window or tab >>A Method for Experimental Investigation of the Wood Chipping Process
2008 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 23, no 3, p. 339-342Article in journal (Refereed) Published
Abstract [en]

SUMMARY: To be able to formulate criteria’s for determining the onset of the creation of a wood chip; it is desirable to be able to study the deformation fields in a vicinity of the edge of the chipping tool. To that end, an experimental setup has been developed in which the chipping can be performed under very well defined conditions. In this setup it is possible to control the rate of indentation of the chipping tool and also to measure the force on the tool. The setup admits also that the angle of the wood specimen with respect to the cutting plane can be varied in both a horizontal and a vertical plane. To determine the deformations, a Digital Speckle Photography (DSP) equipment is used, which together with image processing software makes it possible to determine the strain field on the surface of the wood specimen. One observation that can be made in these studies is that prior to the creation of a chip, there is a localization of strains in a thin region starting from the edge of the tool and directed parallel to the grain. Another observation made during the experimental work is that there exist different types of fracture processes, each giving different chip thicknesses.

Keywords
Fracture Processes
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:miun:diva-6847 (URN)10.3183/NPPRJ-2008-23-03-p339-342 (DOI)000260397100013 ()2-s2.0-53849111967 (Scopus ID)6046 (Local ID)6046 (Archive number)6046 (OAI)
Projects
Flisningsprocessen
Available from: 2008-11-30 Created: 2008-11-02 Last updated: 2017-12-14Bibliographically approved
2. An Analytical and Numerical Study of some aspects of the Wood Chipping Process
Open this publication in new window or tab >>An Analytical and Numerical Study of some aspects of the Wood Chipping Process
2009 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 24, no 2, p. 225-230Article in journal (Refereed) Published
Abstract [en]

SUMMARY: In order to model the wood chipping process, the primary process parameters have been identified and their first order interaction studied. The model is analytical and incorporates, in particular, the influence of sliding friction between the wood chipping tool and the log. To estimate the accuracy of the analytical model, a Finite Element (FE) analysis of the problem considered was also performed. The analytical model and the FE analysis are both restricted to small deformations and linear elastic orthotropic material behaviour. The most severe limitation with both the analytical and the FE model is the assumption of linearly elastic material. On the other hand, it is felt that existing models of anisotropic plasticity in metals are lacking too much of physical relevance, if applied to wood. The analytical model predicts the normal and shear strain distribution in the crack-plane prior to crack initiation. The analytical distributions are in reasonable agreement with the corresponding distribution of the FE analysis. Based on experimental findings, it is suggested that the stress field over the entire crack-plane, in conjunction with the stress field close to the tip of the chipping tool, are critical for chip creation, rather than just the latter.

Keywords
Wood chipping; Chip formation; Analytical model; Friction
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:miun:diva-7127 (URN)10.3183/NPPRJ-2009-24-02-p225-230 (DOI)000268735100015 ()2-s2.0-68049117085 (Scopus ID)
Projects
Flisningsprocessen
Available from: 2008-12-10 Created: 2008-11-21 Last updated: 2017-12-14Bibliographically approved
3. A laboratory wood chipper for chipping under realistic conditions
Open this publication in new window or tab >>A laboratory wood chipper for chipping under realistic conditions
2011 (English)In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 51, no 8, p. 1309-1316Article in journal (Refereed) Published
Abstract [en]

In order to be able to study the damage mechanisms and in general the mechanisms active when a wood chip is created during the wood chipping process, it is crucial to have access to an experimental equipment in which chips can be produced under realistic conditions. In this paper is presented a laboratory chipper, which has been developed to admit chipping at rates that can be varied in a large interval i.e. at rates ranging from zero to 50 m/s. The knife used to cut the chips is mounted in a knife holder, which is instrumented in such a way that forces in three orthogonal directions can be measured. Since the actual force and the measured force differs due to inertia effects, a simple mathematical model is developed and used to evaluate the force acting on the knife. Some results are shown from the force measurements and it is concluded that the laboratory chipper is a versatile tool in the process of increasing the understanding of the chipping process.

Keywords
Wood chipping, laboratory chipper, force measurement, vibration synthesis
National Category
Materials Engineering
Identifiers
urn:nbn:se:miun:diva-11915 (URN)10.1007/s11340-010-9452-1 (DOI)000294801600007 ()2-s2.0-80052444327 (Scopus ID)
Available from: 2010-08-16 Created: 2010-08-16 Last updated: 2017-12-12Bibliographically approved
4. Properties of wood chips for thermomechanical pulp (TMP) production as a function of spout angle
Open this publication in new window or tab >>Properties of wood chips for thermomechanical pulp (TMP) production as a function of spout angle
2011 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 65, no 6, p. 805-809Article in journal (Refereed) Published
Abstract [en]

Spruce woodchipswere produced under well-controlled conditions in a laboratory woodchipper at spout angles of 30°, 40°, and 50° at a cutting rate of 20 m s-1 and with a nominalchip length of 25 mm. Thechips were then refined under thermomechanical pulp (TMP) conditions in a pilot refiner plant. The pulpproperties such as freeness, average fiber length, and shives content were determined and evaluated as a function of specific energy consumption. For a first stage refining and for a freeness value of 350 ml, a decrease in specific electrical energy consumption could be achieved by performing thewood chipping at a spout angle of 50° as compared to 30° which is the spout angle commonly used. A patent application regarding this method has been filed and is pending. It is realized that a freeness value is not directly indicative of any quality measure, such as, for example tensile index and light scattering coefficient but the obtained results can be interpreted to be promising. Further studies are needed regarding the impact of the modified chipping process.

Place, publisher, year, edition, pages
Walter de Gruyter, 2011
Keywords
Chip damage, Energy efficiency, TMP, Wood chipping
National Category
Other Chemical Engineering Mechanical Engineering
Identifiers
urn:nbn:se:miun:diva-11916 (URN)10.1515/HF.2011.087 (DOI)000297527600004 ()2-s2.0-80053577151 (Scopus ID)
Available from: 2010-08-16 Created: 2010-08-16 Last updated: 2017-12-12Bibliographically approved
5. Energy Efficiency Improvement Potential in TMP and CTMP by Axial Precompression of Wood During Chipping
Open this publication in new window or tab >>Energy Efficiency Improvement Potential in TMP and CTMP by Axial Precompression of Wood During Chipping
(English)Manuscript (preprint) (Other academic)
Abstract [en]

It has recently been shown that there is a possibillity to improve the energy efficiency during first stage TMP (thermomechanical pulp) refining by performing the wood chipping so as to increase the compressive damage in the chips and hence breaking up the wood structure. The parameter that was varied was the spout angle i.e. the angle between the fibre direction of the wood specimen and the cutting plane. In this article is investigated how the refining of chips produced at two different spout angles i.e. 30 and 50, influenced the properties of CTMP (chemithermomechanical pulp). It was found that the specific energyinput for a certain CSF (Canadian Standard Freeness) value was lower for chips produced at the spout angle of 50 but that the addition of chemicals (NaHSO3) to the dilution water had no influence on the specific energy value for a given CSF value. However, the tensile index and specific light scattering coefficient was substantially higher for handsheets made of the pulp refined from chips produced at the spout angle 50 and with NaHSO3 added compared to handsheets from pulp made from 30 and 50 chips without chemicals added.

Keywords
Wood chipping, TMP, CTMP, Energy efficiency
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-11917 (URN)
Note

Manuscriptet är publicerat i J-FOR, Journal of science and technology for forest and processes, 2012 vol 2 no 3 med titeln "Evaluation of Collimated Chipping Technology in Mechanical Pulping"

Available from: 2010-08-16 Created: 2010-08-16 Last updated: 2012-12-20Bibliographically approved

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Hellström, Lisbeth

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