Bleaching

The bleaching process removes color from the pulp (due to residual lignin) by adding chemicals to the pulp in varying combinations, depending on the end use of the product. The same bleaching processes can be used for any of the pulping process categories. The most common bleaching chemicals are chlorine, chlorine dioxide, hydrogen peroxide, oxygen, caustic,and sodium hypochlorite. Concerns over chlorinated compounds such as dioxins, furans, and chloroform have resulted in a shift away from the use of chlorinated compounds in the bleaching process. Bleaching chemicals are added to the pulp in stages in the bleaching towers. Spent bleaching chemicals are removed between each stage in the washers. Washer effluent is collected in the seal tanks and either re-used in other stages as wash water or sent to wastewater treatment (US EPA, 2010. p. 5).

BleachingTechnologies & Measures

Technology or MeasureEnergy Savings PotentialCO2 Emission Reduction Potential Based on LiteratureCostsDevelopment Status
Improved Brownstock WashingEstimated energy savings can be as much as 001 GJ/ton of production. Electricity savings may be around 12 kWh/ton of production (Kramer et al., 2009. p.93).Commercial
Heat Recovery from Bleach Plant Effluent

US flag In one US based plant, energy savings are estimated to be 939 000 GJ/year (Kramer, et al., 2009. p.92).

In the same US based mill, corresponding annual cost savings are $2.4 million (2003 dollars). At the capital investment of $1.6 million, the payback period is estimated as 0.7 years (Kramer, et al., 2009. p.92).Commercial
Secondary Heat Use for Chlorine Dioxide (ClO2) Pre-heating

US flag In one study, for a mill producing 1 000 ADT of bleached pulp, where Kappa value to bleaching is 30, where a ClO2 concentration of 9.5 g/l is used, and where ClO2 is heated from 2.8 to 43.3 C, 0.59 GJ/ADT of energy savings were estimated by using secondary heat in preheating (NCASI, 2001. p. 77).

US flag For the same mill studied a net CO2 reduction of 12 625 t-CO2/year has been estimated.

US flag For the plant corresponding to 0.59 GJ/ADT saving, annual cost savings of $430 900 (2001 dollars) were estimated. Capital costs are largely due to the cost of heat exchanger and piping (NCASI, 2001. p.77).

US flag

 For another plant, savings in fuel, electricity, and steam were estimated at $61,000, while capital costs were estimated at $124,000 (2003 dollars) ((Kramer et al., 2009. p.93). 

Commercial
Washing PressesSavings from reduced steam consumption are estimated as 0.38 GJ/t pulp (Martin et al., 2000. p.30). Operation cost savings from lower chemical use will be approximately $0.53/t pulp. Capital cost with pre- and post-lignification process equipment has been estimated as $6 million for a plant of capacity 900 tpd (Martin et al., 2000. p.30).Commercial