Energies 2021 , 14 , 1161
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Table 2. Specific electricity and heat consumption of the pulp processes in pulp and paper production.
Specific Heat Consumption (GJ/t ADpulp )
Specific Electricity Consumption (kWh/t ADpulp 1 )
Reference
Process
Mechanical Pulp 1650.0 (1100–2200) 1972.0 (1600–3000) 2041.0 (1800–3600)
Groundwood Pulp
0.0 0.0 0.0 0.0
[15,17] [15,17] [15–17]
Refiner-Mechanical Pulp Thermomechanical Pulp Washing and Screening
0.9 (0.03–0.93)
50.0
[11] [11]
Bleaching
100.0
Chemical Pulp
4.4 (2.43–5.64) 2 4.2 (4.11–6.48)
Kraft Digester
406.0
[15,16] [15,16]
sulphite Digester
572.0 (226.5–1358)
Washing and Screening Oxygen Delignification 3
30.0 75.0
0.0 0.5
[20] [20]
Bleaching 4 Pulp Drying 5
159 (60–185) 155.0 (90–160)
4.3 (0.4–4.3)
[15,18,21]
4.5 6
[15,18]
Chemical Recovery Process
Liquor Evaporators
25.0
4.4 (2.2–5.4)
[15,18]
Recovery Boiler
58
1.1
[15]
Kiln and Recausticizing
50.0
1.19 (0.77–2.69)
[20,22]
Recycled Fibers Pulp 392.0 (256.1–428.6)
Recycled Fibers Pulp
0.0 0.0 0.0
[15,16]
Screening De-inking
50.0 80.0 40.0 30.0
[11] [11] [11] [11]
Concentration and Dispersion
0.54
Bleaching
0.0
1 . t ADPulp : Air Dry ton of Pulp [17] 2 . Depends on the continuous or batch digester. 3 . Oxygen delignification is applied only to kraft pulp, not sulfite pulp. 4 . The energy demand varies depending on the number and type of bleaching grades [21]. 5 . The values of energy consumption for drying of pulp are assumed to be the same for all pulp types due to missing information in the literature. 6 . The energy requirement for pulp drying is about 25% of the total thermal energy and 15–20% of the total electrical energy consumption for pulp production. It might be at a lower rate (2.5–3.5 GJ/t pulp ) [17].
2.1.3. Paper Production After pulping, the bleached or unbleached pulp is transferred to the papermaking process. In most paper mills, more than one type of pulp is used to obtain paper with the desired properties. The pulp is delivered in the form of a fiber suspension to integrated mills or in dry form (10% moisture content) to non-integrated mills. First, the pulp undergoes preparation steps to ensure that it is delivered to the paper machine in the best condition, such as fiber slurry (dissolving in a suspension), blending of different pulps, or refining (to change the morphology of the fibers). Other processes such as cleaning or screening can be used to achieve a better paper quality, depending on the pulp’s conditions. Paper fillers may also be added to the pulp. Mineral fillers are used to reduce pulp consumption and change the properties of the paper produced, or chemical additives are added to improve quality and optical properties, such as texture and brightness. After the stock preparation steps, the pulp is fed into the paper machine through the headbox. In the paper machine, the paper is formed, and most of the paper’s properties are established. The paper machine is a large dewatering device consisting of a headbox, a wire section, a press section, and a dryer section [17]. The headbox aims to produce a homogeneous distribution of the fibers over the papermaking process’s entire width to achieve uniform paper formation. In the wire section (forming section), pulp with a fiber content of about 0.5% to 1% [17,23] is introduced and supported by a single/twin-wire, then the water is removed from the bottom side by gravity or other elements, and the pulp leaves the section with a fiber content of 15–20% [17]. After that, the formed paper, which is still very week, enters the press section. In this section, the paper web is passed through a set of rollers to remove water and increase the fiber content to 45–50% [17,23].
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