Filling the Gap

Save favourite 24 Mar March 2016

Improved fibre quality, energy efficiency and production stability through increased knowledge on what happens in the refiner gap.


Today numerous variants of refining techniques are available that give different combinations of pulp and paper properties and electric energy efficiency. The refining technique is, to a very large extent, crucial for how efficient the defibration and the fibrillation can be performed. Today’s biggest refiners for TMP and CTMP production have potential to produce 900 ton of pulp per day with a main motor power up to 40 MW. This is performed in 300-600 μm wide refining gaps at peripheral speeds up to 200 m/s and at 140 – 200°C temperature, where wood chips are broken down to fibre and fines with retention times of 0.5 – 2 s. 

There are today proposals for hypotheses that explain the differences between the refining techniques or the ways to run the refiners, but the development is still mainly based on trial and error as long as the measurement systems are not developed enough to be able to test the hypotheses. This means that the development regarding both the end product properties and energy efficiency improvement is relative slow. 

Annually the production of mechanical pulps in Sweden is about 3 Mton, consuming about 6 TWh of which between 50 and 70% (3-4.5 TWh) is reused as heat/steam in drying of paper or pulp. The development of more effective solutions that only means smaller modifications in existing refining technology and thereby can be implemented without big investments is of special interest.

There are ideas and proposals to relatively big changes in design of refining segments and feeding techniques to existing big refiners, but in practice these ideas and proposals are very difficult to implement because the knowledge about the mechanisms in the refiner gap itself is still inadequate.

Researchers at FSCN-Mid Sweden University and CIT-Chalmers have in tight cooperation with the forest industry companies and with support from primarily the Knowledge Foundation (KK-stiftelsen) and the Swedish Energy Agency (Energimyndigheten) during a longer period of time increased the level of knowledge regarding wood and fibre mechanics and tribology at different temperatures and loading modes and also created calculation models of how mechanical energy is transformed into heat and creation of fracture surfaces. With help of these investigations and investigations done by other research organisations it is possible to construct hypotheses regarding the mechanisms of defibration and fibrillation in refiners. The problem is that it is not possible to test these hypotheses without access to information about how the fibre material and the load on this material are distributed over the refining zone.

The purpose of this project is to solve the described problem in order to essentially make it possible to speed up the development of refining techniques that are electric energy efficient; partly by developing new measurement techniques that describe the fibre distribution in refining gaps on-line and partly by improving earlier developed techniques to measure load distribution, temperature profiles and refiner gap width.


The potential for future improved efficiency in existing refiners is estimated to about 25% regarding fibre properties and energy consumption. Both the number of stops caused by refiners and the wear of the refiner segments can be reduced by more than 50% due to improvements in production stability. In order to achieve the main objectives the project comprises four subprojects, each with an own project information that includes results and ongoing activities.

Researchers in the area

Per Engstrand, professor and project leader




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