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Improved energy efficiency in mill scale production of mechanical pulp by increased wood softening and refining intensity
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. Holmen AB.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the electric energy efficiency of single stage double disc refining for production of printing grade mechanical pulp from Norway spruce wood chips. The thesis is based on the hypothesis, that more energy efficiency refining can be attained by balanced increases of wood softening and refining intensity. Five mill scale trials were performed where wood softening and refining intensity was varied by applying or changing the following process parameters and variables:

  • Chip pretreatment/impregnation with water
  • Low dosages of sodium sulfite (Na2SO3) added to impregnation
  • Temperature and retention time in the atmospheric preheater bin
  • Refining temperature (housing pressure)
  • Feeding segment design combined with increased production rate

By combining suitable increases in wood softening and refining intensity, it was possible to reduce the specific electric energy consumption in refining by 15% (~290 kWh per bone dry ton (bdt)) while preserving important pulp properties within ±5%, compared to the standard double disc refining process. This was done by combining chip impregnation, using an addition of 0.36% (on bone dry basis) sodium sulfite, with a new feeding segment design which enabled 25% higher production rate.

When using the new feeding segment design at an increased production rate at unchanged wood softening, it led to reduced fiber length and increased sheet light scattering coefficient at certain tensile index, compared with the standard segment design at normal production rate. This is consistent with the effects normally seen when the refining intensity is increasing. The specific electric energy consumption was 8% lower at a tensile index of 43.5 Nm/g (on Rapid Köthen laboratory sheets) compared to refining at lower intensity using the standard segment design at normal production rate.

Mechanical chip pretreatment with subsequent water impregnation showed a reduction in specific electric energy consumption of 6% (~120 kWh/bdt). When chip impregnation was applied in a later trial with a milder chip compression, it led to increased wood softening seen as better preserved fiber length and reduced light scattering coefficient. This resulted in a reduction in tensile index at certain specific electric energy consumption when applied with the standard refining condition but to an increase in tensile index when applied with refining at higher intensity using the feeding segment design at higher production rate.

An addition of 1.2% sodium sulfite during impregnation led to a sulfonate content of pulps of ~0.28% (as Na2SO3 equivalents, including post sulfonation) and an average increase in tensile index of about 8.3 Nm/g, when compared to unsulfonated pulps at certain specific electric energy consumption. The increase in tensile index correlated with increased delamination and internal fibrillation of fibers (measured by Simon’s staining), which indicate that the increase in tensile index for sulfonated pulps was a result of improved fiber flexibility and collapsibility. The reduction in disc gap at certain specific electric energy consumption in refining due to an increased wood softening after sulfonation may explain the increase in delamination and internal fibrillation for sulfonated pulps. The smaller disc gap probably led to a more intense refining, i.e. loading at higher deformation rates due to a higher degree of deformation in bar crossings.

Different temperatures (80 vs. 97°C) and retention times (6 vs. 9 min.) in the atmospheric preheater bin were studied. This showed that the lower temperature and shorter retention time was beneficial for the tensile strength and light scattering of pulp when applying low dosage sodium sulfite pretreatment. This was most likely a result of too high degree of wood softening prior to defibration in the breaker bar zone when combining low dosage sodium sulfite pretreatment with the higher preheating bin temperature at longer retention time.

Different refining temperatures (4.6 and 6.4 bar(g) refiner housing pressure) were evaluated both without and with low additions (0.6% and 1.2%) of sodium sulfite. Raising the refining temperature increased tensile index by 3.2 Nm/g and the addition of 1.2% sodium sulfite by 8.6 Nm/g. The combined increase (~12 Nm/g) was similar to the effect of increasing the specific electric energy consumption by 380 kWh/bdt, when comparing pulps at equal tensile index. However, the pulps produced with increased refining temperature and sodium sulfite addition had lower light scattering coefficient at certain tensile index. The combination of increased refining temperature and addition of 0.6% sodium sulfite was interesting and resulted in pulp with higher tensile index, light scattering coefficient and brightness together with lower shives content at certain specific electric energy consumption, compared with pulp produced at the lower refining temperature without addition of sodium sulfite.

Finally, an implementation of the technology presented here is discussed in relation to the Braviken mill (Holmen Paper AB, Norrköping, Sweden) concerning reduction in electric energy consumption and steam recovery. The technology has potential to reduce the electrical energy use by ~100 GWh/year at the Braviken paper mill, where this study was performed.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2016. , 124 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 242
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:miun:diva-27322ISBN: 978-91-88025-59-3 (print)OAI: oai:DiVA.org:miun-27322DiVA: diva2:913831
Public defence
2016-04-21, O102, Holmgatan 10, Sunsvall, 10:00 (English)
Opponent
Supervisors
Note

Vid tidpunkten för disputationen var följande delarbeten opublicerade: delarbete 3 och 4 inskickat.

At the time of the doctoral defence the following papers were unpublished: paper 3 and 4 submitted.

Available from: 2016-03-24 Created: 2016-03-22 Last updated: 2016-03-24Bibliographically approved
List of papers
1. Pressurised compressive chip pre-treatment of Norway spruce with a mill scale Impressafiner
Open this publication in new window or tab >>Pressurised compressive chip pre-treatment of Norway spruce with a mill scale Impressafiner
2012 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, ISSN 0283-2631, Vol. 27, no 1, 056-062 p.Article in journal (Refereed) Published
Abstract [en]

Mill scale trials were performed to evaluate pressurised compressive chip pre-treatment with the Impressafiner installed in one of the thermomechanical pulp lines at Braviken paper mill (Holmen Paper AB). The aim of the study was to determine if earlier reported effects of the Impressafiner pre-treatment on spruce chips from pilot scale trials (i.e. energy reduction and extractives removal) could also be attained with the mill scale Impressafiner.The mill scale Impressafiner pre-treatment resulted in partial disintegration of chips into a material consisting of fragmented chips with cracks running along the longitudinal fibre axis. Splits or evidence for weaknesses were observed between the primary and secondary fibre walls of pre-treated chips. An increase in water uptake for pre-treated chips was also observed. The extractive content was reduced by up to 24% for pulps produced with pre-treated chips compared to pulps from untreated chips. Pulp produced from pre-treated chips had higher tensile- and tear indices, elongation and light scattering and lower freeness compared to pulps from untreated chips produced with the same total specific energy consumption. The total specific energy needed to reach a tensile index of 47 Nm/g was reduced by 120 kWh/bone dry ton (6%) with Impressafiner pre-treatment. A smaller refiner plate gap was needed to reach the same specific energy consumption for pre-treated chips compared to untreated chips.

Keyword
Chip pre-treatment, Energy reduction, Extractives, Fibre microstructure, Impressafiner, Norway spruce, Thermomechanical pulp
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-21421 (URN)10.3183/NPPRJ-2012-27-01-p056-062 (DOI)
Available from: 2014-02-25 Created: 2014-02-25 Last updated: 2017-12-05Bibliographically approved
2. Low dosage sulfite pretreatment in a modern TMP-line
Open this publication in new window or tab >>Low dosage sulfite pretreatment in a modern TMP-line
Show others...
2015 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 30, no 4, 591-598 p.Article in journal (Refereed) Published
Abstract [en]

The effects of low dosage sulfite pretreatment combined with modern high consistency double disc refining were evaluated for production of thermomechanical pulp in a mill scale trial using Norway spruce wood at the Braviken paper mill (Holmen Paper AB, Sweden). Spruce wood chips were mechanically pretreated in an Impressafiner before impregnation with different dosages (0-1.2%) of sodium sulfite (Na2SO3) at pH 9. Approximately 23% of the added sulfite was converted to sulfonate groups in pulp, resulting in a sulfonate content of 0-0.28% (as Na2SO3). The low dosage sulfite addition increased tensile index, elongation, density, brightness and decreased shive content, light scattering and light absorption coefficients when compared at equal specific energy consumption (SEC). The increase in tensile index was proportional to dosage of sulfite. Further analyses showed that low dosage sulfite addition did not affect the distribution of the Bauer-McNett fractions nor the fibre length for pulps refined with equal SEC. However, the low dosage sulfite addition increased fibre delamination/internal fibrillation (D/IF). With the addition of 1.2% Na2SO3, it was possible to produce pulp with a tensile index of 47 Nm/g using ~320 kWh/bdt (~15%) lower refining energy, compared with pulps produced without sulfite addition.

Keyword
Chip pretreatment, double disc refining, energy reduction, Impressafiner, low dosage, Norway spruce, mill scale, pulp properties, sulfite, TMP
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-26284 (URN)10.3183/NPPRJ-2015-30-04-p591-598 (DOI)000366040300006 ()2-s2.0-84951857094 (Scopus ID)
Available from: 2015-11-22 Created: 2015-11-22 Last updated: 2017-08-10Bibliographically approved
3. Low dosage sulfite pretreatment at different refining temperatures in mill scale TMP production
Open this publication in new window or tab >>Low dosage sulfite pretreatment at different refining temperatures in mill scale TMP production
Show others...
2017 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 32, no 1, 59-69 p.Article in journal (Refereed) Published
Abstract [en]

The effects of low dosage sodium sulfite ( Na2SO3) pretreatment (0, 0.6 and 1.2% on bone dry wood, pH 9) at two different refining temperatures (4.6 and 6.4 bar(g) refiner housing pressure) were evaluated for production of thermomechanical pulp with a double disc refiner in mill scale using Norway spruce wood at Braviken paper mill (Holmen Paper AB, Sweden). The sulfonate content of the pulps was not affected by the different refining temperatures and was 0.29% (as Na2SO3 equivalents) for the highest sulfite addition.

Tensile index at constant SEC was increased by 3.2 Nm/g when the refining temperature was increased, and by 8.6 Nm/g when 1.2% sodium sulfite was added. The effects were additive and led to an increase in tensile index of similar to 12 Nm/g at constant SEC when combined and would enable a reduction in SEC of 380 kWh/bdt (similar to 20%) to similar tensile index. The degree of delamination and internal fibrillation of the fibers was increased by both increased refining temperature and sulfite addition.

Pulp brightness was slightly reduced (<= 0.4% ISO) by increased refining temperature and increased (2-3% ISO), when sodium sulfite was added. Spectra of reflectance factors (360-740 nm) were used to study the optical properties of produced pulps.

Keyword
Double disc refining, Energy reduction, Mill scale, Norway spruce, Pulp properties, Refining temperature, Sulfite pretreatment, TMP
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-27332 (URN)10.3183/NPPRJ-2017-32-01-p059-069 (DOI)000398384400008 ()2-s2.0-85016434323 (Scopus ID)
Available from: 2016-03-24 Created: 2016-03-24 Last updated: 2017-06-09Bibliographically approved
4. Substantially improved energy efficiency in mechanical pulping by new feeding segment design and low dosage sulfonation in mill scale
Open this publication in new window or tab >>Substantially improved energy efficiency in mechanical pulping by new feeding segment design and low dosage sulfonation in mill scale
Show others...
(English)Article in journal (Refereed) Submitted
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-27333 (URN)
Available from: 2016-03-24 Created: 2016-03-24 Last updated: 2016-12-09Bibliographically approved
5. Optical properties of thermomechanical pulp (TMP) obtained from sulfite-pretreated Norway spruce with focus on two-photon spectral imaging (TPSI)
Open this publication in new window or tab >>Optical properties of thermomechanical pulp (TMP) obtained from sulfite-pretreated Norway spruce with focus on two-photon spectral imaging (TPSI)
2012 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, ISSN 0018-3830, Vol. 66, no 7, 817-824 p.Article in journal (Refereed) Published
Abstract [en]

Chips of Norway spruce have been impregnated with Na2SO3 and refined at two specific energy consumptions levels at full mill scale. The optical properties of thermomechanical pulps (TMPs) obtained were analyzed in terms of brightness, light scattering, opacity, and autofluorescence by spectral imaging. Even at low sulfite dosage (0.24% sulfite by dry weight) light absorption was reduced, and the brightness was elevated, and a clear dose-response effect was observed. Two-photon spectral imaging (TPSI) showed that sulfonation, impregnation, and refining affect the fluorescence properties differently. Compared to native wood, both processed wood chips and pulp fibers revealed blue-shifted fluorescence maxima, a characteristic of shortened conjugated systems. Two subpopulations of fibers with different optical properties were observed, and the fluorescence of one fiber population was red shifted.

Keyword
microscopy; optical properties; pulp; refining, spectral imaging; sulfonation; two-photon excitation; wood
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-21422 (URN)10.1515/hf-2011-0184 (DOI)
Available from: 2014-02-25 Created: 2014-02-25 Last updated: 2017-12-05Bibliographically approved

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