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A range of techniques are available to prepare raw materials so less energy is required during processing.
Energy can also be saved by increasing the percentage of recycled fibres in the papermaking process, which reduces the need for raw material preparation.
Chemical pulping separates wood chips into cellulose fibres through a cooking process involving chemical solutions and elevated temperature and pressure.
There are numerous strategies and technologies to improve the energy efficiency of the chemical recovery process which recycles black liquor for use as boiler fuel and cooking liquor.
It is possible to save energy in the paper pressing process through the use of shoe (extended nip) press and gap formers, while optimising the paper machine's vacuum system.
Significant energy savings can be achieved through investing in better dryer control, heat recovery technologies and system optimisation.
In paper mills, steam generation and distribution using boilers tends to be responsible for as much as 80% of all fuel used, with motor driven systems typically using over 80% of all electricity.
New methods of debarking can reduce the energy required to remove bark, increase wood recovery rates and reduce transportation costs.
A belt conveyor operating at 1 kWh/tonne can replace an 18.2 kWh/tonne pneumatic conveyor, resulting in a savings of 17,200 kWh/day, or around US$100,000-$200,000/yr in electricity costs, depending on the size of the conveyor.
Automatic chip handling and screening for thickness provides downstream benefits by reducing steam consumption in the digester and evaporator. The return on investment is 5–8 years.
Bar-type chip screens use less energy than other types of screens, but have very similar upfront capital costs. Energy savings are estimated to be 0.34 GJ/tonne chemical pulp.
Digesting is one of the major steam consumers in the pulp mill.
Chemical pulping aids can be added to the pulping process to increase liquor penetration and promote more even cooking. This can reduce energy consumption by 0.125 GJ/tonne (8-10%), as well as reduce pulp rejects, whilst increasing yield 2 to 4% per tonne of wood.
Pulp is washed after the digestion step to remove cooking liquor chemicals and organic compounds dissolved from the wood chips.
Heat exchangers can be used to recover the large amounts of heat in the bleach plant effluent. An audit at one facility showed that the heat from the bleach plant effluent could be used to generate hot water for the paper machine.
Use of recycled fibres from recycled paper saves energy because:
Black liquor concentrators increase the solids content of black liquor prior to combustion in a recovery furnace. This means less water has to be evaporated in the recovery furnace, which can increase the efficiency of steam generation substantially.
Undertaking delignification prior to bleaching greatly reduces the energy required in the bleaching process.
Chemical recovery furnaces consist of tubes that circulate pressurised water to permit steam generation. These tubes are normally made out of carbon steel.
Gap formers are a more productive alternative to the Fourdrinier paper machine due to the significantly reduced time it takes for paper formation.
Lawrence Berkeley National Laboratories researched opportunities for heat recovery in the paper drying process: