Production of mechanical pulps requires high specific electrical energy compared to many other attrition processes. In Scandinavia, the lowest specific refining energy for production of thermomechanical pulp is around 1800 kWh/t for newsprint quality, which is roughly 60 times higher than for crushing of stone to a similar size distribution. The high specific energy demand for refining has naturally motivated large efforts in the search for improved efficiency. It is always practical to be able to quantify improvements in efficiency for comparison of process designs and of different machine types. However, there is no commonly accepted definition of efficiency for mechanical pulping processes. In published work within mechanical pulping, energy efficiency has been presented in different ways. In this paper, we discuss definitions of energy efficiency and aspects that ought to be considered when energy efficiency is presented. Although focus of this work is on energy efficiency for refiner processes, the principles can be applied to other types of mechanical pulping processes such as stone groundwood.
Funding source: Stora Enso
Funding statement: This work was supported by Holmen and Stora Enso.
The authors appreciate valuable comments on the manuscript by Jan-Erik Berg and the linguistic revision by Ester Quintana.
Conflict of interest: The authors declare no conflicts of interest.
Data shown in this study has been collected from mill processes in three printing paper mills in Sweden. Raw material in all mills were Norway spruce. Data for the processes are summarized in Table A1 and in the text below.
Brief descriptions of the experimental procedures for the trials not in referred literature are given below:
TMP line in the Holmen, Hallsta mill, Sweden. Figure 8B.
Raw material: Mix of roundwood and sawmill chips of Norway spruce.
For Figure 8B, pulp samples were taken from one of the four parallel main lines. The sampled line has an RTS refiner (SB170, Andritz) as primary stage and a Jylhä SD65 (Valmet) refiner as second stage. Samples were taken from the blow lines of the primary RTS refiner at two levels of production rate (14 and 17 adt/h) and two levels of disc gap at each production rate (Motor power 11–15 MW).
Pulp from two RTS-lines and one 2-stage Jylhä SD65 lines are mixed and refined in two stages with LC refiners (TwinFlo 52, Andritz) equipped with 58” segments. For Figure 10, pulp samples were taken from the chest before the LC refiners and after first and second LC refiners. The TMP line is also described by Ferritsius et al. (2020).
Pulp samples were hot disintegrated according to ISO 5263-3 and handsheets were made according to a modified version of ISO 5269–1:2005. Tensile index was measured on handsheets according to ISO 1924-2.
RGP68 DD StoraEnso Kvarnsveden Mill, Sweden. Figure 8A.
Raw material: Roundwood of Norway spruce.
Pulp samples were taken from the blow-line of one of the single-stage RGP68 DD refiners at two levels of production rate (14 and 17 adt/h) and three levels of disc gap at each production rate. Pulps were hot disintegrated before testing according to ISO 5263-3:2004. Laboratory sheets were made according to ISO 5270:2012, but using 20 strips for tensile testing instead of 10 strips. Since all samples were tested twice, tensile index values are based on 40 strips for each point.
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