Chemical 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. In chemical pulping lignin is separated from the rest of the wood in a digester under pressure and with the use of cooking chemicals. In the most widely used Kraft process, an aqueous solution of sodium hydroxide and sodium sulfide, known as white liquor, selectively dissolve the lignin and make it soluble in the cooking liquid. After 2 to 4 hours, the mixture of pulp, spent pulping chemicals and wood waste is discharged from the digester. The pulp is washed to separate it from the black liquor - the pulping chemicals and wood waste. The pulp, called brownstock at this point in the process, is ready to be bleached. Softwood pulp from a conventional cooking process contains about 4.5% lignin. This lignin will be removed and the pulp will be brightened during the bleaching process (Reach for Unbleached Foundation, 2012). 

Kraft pulping is a low yield process, converting on 45% of the wood into pulp. However, the pulp has better strength and brightness properties (Paper Online, 2012).

Chemical PulpingTechnologies & Measures

Technology or MeasureEnergy Savings PotentialCO2 Emission Reduction Potential Based on LiteratureCostsDevelopment Status
Use of Pulping Aids

US flag Energy saving of 0.131 GJ/t-processed wood was reported in one study using a pulping aid named ChemStone OAE-11.

US flag

 With the use of Phosphonate as a pulping aid energy savings of 8 to 10% were reported (Kramer et al., 2009. p.90).

US flag

 Some studies have suggested savings of around $20 per ton of bleached pulp after the cost of chemicals have been considered (Kramer et al., 2009. p.90). 
Optimize Dilution Factor in Brownstock WashingIn a US based plant, the measure helped reduce natural gas usage by 327 000 GJ/y and saved 45.6 m3 of water per hour. (Kramer et al., 2009. p.91)Corresponding cost savings of $580,000/year were realized (Kramer et al., 2009. p.91)Commercial
Continuous Digester Control System

US flag Initial implementation of one computer model resulted in 1% energy savings for the pulping process.

Digester Blow/Flash Heat RecoveryIn one study, for a plant producing more than 650 000 tons of paper, improved heat recovery has been found to have the potential to save 991,700 GJ of of fuel and 743,775 GJ of steam, annually (US DOE, 2003)The investment and annual savings for the blow hear recovery system are estimated to be $2 250 000 and $2 235 000, respectively – giving a payback time less than one year (US DOE, 2003)Commercial
Directed Green Liquor Utilization PulpingEnergy reduction by 25% is achieveable in the pulping process.Demonstration
Steam Cycle Washer for Unbleached PulpReduction by 21% in electricity consumption and up to 40% in fuel use for unbleached pulp production.Not availableResearch
Continuous DigesterFor Broadleaf trees, steam savings can be by 0.40 t-steam/t-pulp compared to the batch digestor. For Conifers, steam savings can be as high as 0.70 t-steam/t-pulp.Equipment cost of digester is about ¥3,700 million. Construction cost is estimated as ¥1,000 million approximately. If the elimination of the need for downstream process equipment for recovery of foreign matter and operating energy for such equipment, the payback time of the investment may become less than 10 years.Commercial
Batch Digester ModificationsEnergy savings of indirect heating may amount to 3.17 GJ/t (Kramer et al., 2009. p.91). Energy savings of around 2.3 GJ/ADT pulp is estimated with the use of cold blow systems (NCASI, 2001. p. 62).CO2 emissions saving by 105.6 kg/t is estimated (Martin et al., 2000. p.21).Indirect heating increases operational and maintenance costs. For a 1000 tpd plant, reduced energy consumption enabled by cold blow system is estimated to result in savings of $2 million/y. However, capital costs for additional equipment (i.e., additional pumps and accumulators for the recovered black liquor) are quite high for this measure.(Kramer et al., 2009. p.91). Commercial
Alcohol Based Solvent PulpingAdditional fuel input amounts to 0.9 GJ/t pulp. Electricity savings are estimated to be 273 kWh/t (Kramer et al., 2009. p.37).A 142 ktonne test plant constructed in 1994 had the installation cost of $21.1/t paper (Kramer et al., 2009. p.37).Demonstration
Dynamic Simulation Model for Continuous DigestersThe technique can save 0.132 GJ per ton of processed wood chips. The technique reduces CO2 emissions in lime kiln. Demonstration
Lo Solids Continuous Cooking SystemIn an Indian mill, switching from conventional batch digester cooking to downflow Lo solids continuous cooking reduced steam consumption from 1.48 t-steam/t-unbleached pulp to 0.66 t-steam/t-unbleached pulp (CII, 2008. p. 52). The equipment and installation costs are around Rs. 5000 Lakhs. Commercial
SuperBatch Displacement Cooking

Indian flag In an Indian mill, with the introduction of SuperBatch Cooking medium pressure steam consumption for the cooking process is reduced to 560 kg/BD MT (Bone dry metric tons) of pulp. Low-pressure steam consumption is 180 kg/BD MT of pulp (CII, 2008. p. 53)

The equipment and installation costs are around Rs. 6000 Lakhs. Commercial