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Immense potential for energy-efficient mechanical pulping

Outstanding progress has been made in reducing the energy demand of mechanical pulping and in improving its energy recovery efficiency. A better understanding of the interactions between thermal, mechanical, chemical, and recently, the enzymatic pretreatment of wood chips have led to an improved energy-to-quality ratio. Different morphologies of softwood fibres (long, coarse, higher lignin content) and hardwood fibres (short, slender, lower lignin content) have led to specific process developments. Softwoods: In the RTS-TMP process, (Fig. 1) the retention (R) time of the chips at high temperature (T)/pressure (above the lignin softening temperature), and the refiner operational speed (S) are optimised. Typically, the refiner rotational speed is increased from standard 1,500/1,800 r/m to 2,100/2,300 r/m. Compared to conventional TMP, energy savings in the range of 15 to 20 % are possible. RT-Pretreatment (Fig. 2) compresses and shears chips after pre-steaming (retention time (R) 10 to 20 s and temperature (T) approx. 125 °C). This creates favourable fracture zones between the cellulose rich S1 and S2 cell wall layers. Energy savings of 100 to 180 kWh/t for the overall process have been achieved. A further development extends the RTPretreatment process with subsequent disc defiberiser treatment (Fig. 3). This is known as RTF-Pretreament, and allows TMP for news 1. Standard TMP 2. State-of-the-art 3. State-of-the-art 4. Latest Pretreatment Primary refiner SD RTS RTF ­ RTS ACTMP

Fig. 1: RTS-TMP process

for still higher refining intensity with a reduced energy demand. A combined RTF-RTS-TMP process requires 25 to 30 % less energy than a conventional TMP process at comparable strength properties. The most recent breakthrough is the Advanced Chemi-Thermomechanical Pulp (ATMP) process. After liberating the S2 cell wall layer of the individual fibers with RTF-Pretreatment, the fibre surface is modified by a low dosage of acid bisulfite, which enhances the fibre bonding properties and the brightness, and significantly improves the quality-toenergy ratio of the refining process. Another further development is lowconsistency (LC) refining right after a first or second TMP stage and/or after HC reject refining. Key process parameters are: 1) very low specific edge load (0.4 to 0.6 Ws/m), 2) high stock temperature (about 85 °C or higher), and 3) very uniform feeding and controlled flow conditions (pressure, consistency, flow). The Reject Refiner SD SD SD SD ­ LC Elec. Energy kWh/admt 2,100 1,700 1,500 1,300

application of 100 to 200 kWh/t in LC refining can replace 200 to 400 kWh/t of HC refining at comparable freeness and tensile strength. Table 1 shows how the specific energy demand for newsprint TMP from spruce has been reduced in recent years. Hardwoods: Hardwoods, with their much shorter fibres, much higher number of slender fibres per unit of weight, and their lower lignin content, lend themselves to combined alkaline chemical and mechanical treatment. Compared to the traditional BCTMP process, the P-RC APMP process (Fig. 4) enjoys growing acceptance as the state-of-the-art. Chips are macerated in heavy screw press-type devices and impregnated with sodium hydroxide and hydrogen peroxide before and in-between refining stages. Sodium hydroxide softens the lignin, resulting in easier fibre separation and reduced energy demand. Hydrogen peroxide prevents yellowing, bleaches the already destructured chips, and carboxylates the lignin (increasing the bonding potential). High chemical dosages (4 to 7 % alkali and 3 to 5 % peroxide) give high strength and brightness at acceptable opacity/ light scattering. As with softwoods, the use of LC refining of pre-refined pulp reduces the overall electric energy demand. LC refiners are installed as the second refining stage and in the reject refining position. The

Secondary refiner SD SD SD LC

Tab. 1: Reduction of energy requirements for newsprint TMP (spruce) (SD = Single Disc Refiner, LC = Low Consistency Refining)

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Fig. 3: The RTF-RTS process

Fig. 2: RT-Pretreatment

Fig. 4: P-RC APMP process

chemically softened fibres can withstand the higher intensity treatment even at reduced temperatures. The process developments above have not only been applied to new installations, but have also led to major savings with existing systems, since the number of refining stages can be reduced. The last stage in two- and three-stage HC TMP plants can be converted either to dedicated reject refining or can be replaced by an LC refining stage. Papierfabrik Perlen in Switzerland is a good example for producing up to 400 t/d single-stage newsprint TMP from spruce in one TC 66 RTS refiner. Part of the pulp is upgraded by fractionation and separate refining for LWC grades. The Braviken Mill of Holmen Paper has installed LC Twin Flo type refiners after all of its four TMP lines. In fact, LC refiners which provide the necessary large surface area (i. e. low specific edge load) have been installed for capacities up to 1,000 t/d ­ with an applied power of 5 MW. This replaces about 8 MW of HC refining.

The "direct blow concept" has also largely simplified the equipment train of multi-stage TMP plants. The interstage pressurised cyclone and conveying system is eliminated by simply blowing the pulp to an upgraded Fibre Centrifuge Feeder (FCF) of the subsequent refining stage, improving steam recovery efficiency. Similar benefits can be obtained by replacing the pressurised cyclone with a Fiber Centrifuge (FC) due to the improved fibre /steam separation effect and the elimination of cyclone quench water. Existing first-stage TMP refiners have been converted to higher than standard speed (i. e. >1500 rpm). This results not only in a capacity increase, but also in significant energy savings. Advanced process control systems like the registered Refiner ACE (Advanced Control Expert) ensure fast start-ups and on-quality pulp. In addition, these systems monitor all refiner and process relevant parameters. Refiner loadability is improved and load variations are reduced, contributing to reduced energy requirements.

TMP plants have also benefited from the dramatic development in refiner plate design. Energy-saving capabilities of SEnergy and Fibre MaxX plates improve refining efficiency by as much as 20 %. Unlike conventional low-energy plates which operate at reduced fibre retention time and narrower gaps, new design LE-Gator and Compression plates use the energy-saving effect of high "filling" and compression of the material in the refining zone. Close to 30 % reduction in the specific energy demand at comparable pulp quality has been achieved on hardwood. The amount of electric energy that could be saved in TMP plants around the world by using modern techniques is immense. Conservative estimates are that 70 to 75 % of all plants are still using conventional technology. A simple 15 % reduction in electric energy consumption would save about 750 MW. This corresponds to the production of 750 modern windmills or of power plants consuming about 5 million t of biomass annually.

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