Mechanical Pulping

The primary goals of pulping are to free fibers in wood from the lignin that binds these fibers together, and then to suspend the fibers in water into a slurry suitable for paper making. As the oldest form of pulping, mechanical pulping uses mechanical energy to weaken and separate fibers from wood via a grinding action. The advantage to mechanical pulping is that it produces much higher yields than chemical pulping processes (up to 97% - BREF, 2010. p. 422). However, because this process does not dissolve lignin and because mechanical grinding produces shorter fibers, the fiber strength and age resistance of the resulting pulp are low. Consequently, most mechanical pulp is used for lower grade papers such as newsprint, magazines, and catalogues. Mechanical pulping also requires more raw materials screening to remove contaminants such as dirt, and knots than chemical pulping processes. There are four main types of mechanical pulping (Kramer, et al., 2009. p. 15):

  • Stone groundwood (SGW) pulping: In the SGW process, small logs are ground against artificial bonded stones made of silicon carbide or aluminum oxide grits. The process gives a high yield, but the fibers produced can be very short and often must be combined with expensive chemical fibers to be strong enough to pass through the paper machine and subsequent coating and printing processes.
  • Refiner mechanical pulping (RMP): In RMP wood feedstock is ground between two grooved discs. The process keeps the high yield advantages of the SGW process, while producing somewhat longer fibers with greater strength. This permits lighter weight paper to be used for printing and result in more print media per ton of feedstock. The RMP process can use wood feedstock other than logs, such as wood scraps and sawdust from lumber mills.
  • Thermomechanical pulping (TMP): In TMP wood chips are first steamed to soften them before being ground in the same manner as the RMP process. The TMP process generates the highest grade mechanical pulp but is also a high energy intensity process due to its steam use. This process can also produce a darker pulp that is more costly to bleach. Despite these drawbacks, TMP is the most common mechanical process in use today.
  • Chemi-thermomechanical pulping (CTMP): CTMP involves the application of chemicals to wood chips prior to refiner pulping. The chemical pre-treatment of wood chips allows for less destructive separation of fibers from the feedstock, resulting in longer fibers, higher fiber content, and far fewer shives. The CTMP process also produces more flexible fibers (which provide higher sheet density, burst strength, and tensile strength) and higher pulp brightness than the TMP process. Its primary drawback, like TMP, is that it is a high energy intensity process (Kramer et al., 2009. p.15).

The specific energy consumption in mechanical pulping is dependent on the particular pulping process, the properties of the raw material and, to a large extent, the quality demands on the pulp set by the end product (the freeness value). A substantial part (>90%) of the energy input to the mechanical pulping processes is converted into heat, some of which can be recovered (BREF, 2010. p.423).

Mechanical PulpingSchematic

Mechanical PulpingTechnologies & Measures

Technology or MeasureEnergy Savings PotentialCO2 Emission Reduction Potential Based on LiteratureCostsDevelopment Status
RTS PulpingA study estimates RTS pulp can be manufactured with about 15% lower specific energy requirements than pulp produced with conventional refining system. Another study reports specific energy of RTS pulping process 20% lower than Thermo-mechanical pulping. Increasing rotational speed on TMP refiners are estimated to reduce energy use between 15 to 30%, depending on plate type and refiner mode (Kramer et al., 2009. p.99). Savings are estimated at 306 kWh/t-pulp (Martin et al., 2000. p. 24US flag Emission reduction potential is estimated at 128 kg CO2/t-pulp (Martin et al., 2000. p. 21)

US flag Implementation costs are estimated around $50/t-pulp (estimation from the year 2000) (Martin et al., 2000. p. 24)

For a typical mill with 1000 ton per day capacity, a reduction of 20% in motor power requirement translates into $5 million annual savings (Sabourin, 2006).
Refiner Improvements

Canadian flag

 A newspaper mills applied refiner control strategy to minimize variations in the freeness of ultra high yield sulfite pulp saved 51.3 kWh/t product.

By decreasing consistency of pulping from 50% to 30%, 7 to 15% electricity savings possible in Thermo-mechnical and refiner mechanical pulping.

Savings due to mechanical improvements are estimated at 11%. (Kramer, et al., 2009. p.96)

EU flag Double disc refiners used in some Swedish and Norwegian mills consume aroun 300 kWh/t less energy than single disc refiners (BREF, 2010. p. 471).

Potential electricity savings of 11% may be achieved at the capital cost of around $7.7/ton (2000 dollars) of pulp production (Kramer, et al., 2009. p.96).

ThermopulpingStudies suggest that this technique can reduce specific energy consumption compared to thermo-mechanical pulping by up to 20% (Kramer et al., 2009. p.98).Commercial
Heat Recovery in Thermo-mechanical Pulping

One study estimates heat recovery systems for pressurized refiners can generate 1.1 to 1.9 tons of clean steam at dryer pressure per ton of pulp (Kramer et al., 2009. p.99).

Japanese flag For a 150 tpd plant, recovery of 695 GJ of energy in the form of steam and 1175 GJ of energy in the form of hot water is reported (NEDO, 2008. p.166)

Average installation costs have been estimated to be $21/t-pulp (2000 dollars), with significant increases in operation and maintenance costs. Payback periods vary widely depending on capital costs, but can be as low as a few months (Kramer et al., 2009. p.99) The payback time for the investment is usually around 1 year (BREF, 2010. p. 462)Commercial
Pressurized GroundwoodElectricity savings of 20 to 36 percent comparing to mechanical pulping processes at atmospheric pressure are reported in literature (Kramer, et al., 2009. p.96). Commercial
Installation of High Efficiency Refiners

The power saving of about 290 kW had been achieved.

Total installation cost of central refining system is estimated to be Rs. 5.60 Crores.

New Energy Efficient TMP ProcessThe conventional TMP process consumes electricity in the range 1600 - 3200 kWh/ADt. The new efficient process promises the energy consumption reduction by 200 - 450 kWh/t.Energy savings must be balanced with investment. Commercial
Low Consistency Refining (LCR)The application of 100 to 200 kWh/t in LC refining can replace 200 to 400 kWh/t of high consistency refining at comparable freeness and tensile strenth (IPW, 2010. p. 24) Energy savings by 142 kWh/t thermomechanical pulp can be achieved.CO2 savings by 16.3 Kg/t is estimated.Commercial
Improvements in Chemi-Thermomechanical Pulping (CTMP)

Energy savings of about 283 kWh/t is estimated if the first refiner speed is increased and pH of the sulphite liquor is decreased (Martin et al., 2000. p.26).

Emissions saving by 128 kg-CO2/t is estimated (Martin et al., 2000. p.20).

Retrofit cost of implementing the improvement is estimated to be $300/t (Martin et al., 2000. p.26).

Advanced Thermo Mechanical Pulping (ATMP)A 300-800 kWh/t energy saving for a given pulp tensile strength has been confirmed at mills running the combination of chemicals and mechanical treatment alone. Commercial

Mechanical Pulping Publications

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