In pulp and 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. The majority of motor-driven electricity use is used for:
- pumps and fans
- compressed air
- materials processing
Energy efficiency opportunities are available through upgrading and optimising the efficient use of these systems. Examples of such opportunities include:
- insulating pipework
- replacing lighting
- improving equipment reliability
- process rationalisation
- effective shut down procedures
Other examples of energy efficiency opportunities within the various manufacturing stages are outlined below.
Raw materials preparation
A range of techniques are available to prepare raw materials so less energy is required during processing.
A cradle debarker can remove bark from logs using only one-third of the energy consumption of traditional debarking methods.
Cradle debarking also causes less damage to logs, for a greater wood recovery rate. Transportation costs can also be reduced by eliminating the need for off-site debarking.
Chip handling, screening and conditioning
Energy-efficiency gains can be made in the process of converting logs to wood chips. They include:
- replacing multiple, less efficient chippers with large, single units
- replacing pneumatic chip conveyors with more efficient belt conveyors
- incorporating automatic chip handling and thickness screening. This can give downstream benefits by reducing the steam needed in the digester and evaporator, and increasing digester yield by up to 10%
- using chip screen systems tailored for the application, delivering greater efficiency but with very similar upfront capital costs
Energy can also be saved by increasing the percentage of recycled fibres in the papermaking process, which reduces the need for raw material preparation. Recycled inputs have increased substantially in recent years as paper recovery rates in Australia have improved.
Chemical pulping separates wood chips into cellulose fibres through a cooking process involving chemical solutions and elevated temperature and pressure. More efficient heat and chemical recovery can save significant amounts of energy. Pulp washing processes can also reduce the amount of energy required in the evaporation process.
Digesting is one of the major steam consumers in the pulp mill.
Modern displacement batch digesters and/or continuous digesters use about half of the steam required in conventional batch digesters. As a part of the pulping process, steam is produced when hot pulp and cooking liquor is reduced to atmospheric pressure at the end of the cooking cycle.
The steam is condensed and stored as hot water in an accumulator tank in batch digesters. It’s possible to recover heat from either batch digesters or continuous digesters for other parts of the paper-making process. This includes steam generation, water heating or black liquor evaporation. The newer systems also produce a more uniform pulp quality, which in turn allows yields to be increased.
Captured thermal energy from digester processes can offset the need for steam generated by a boiler.
Chemical aids can be added to the pulping process to increase liquor penetration and promote more even cooking. This can reduce energy consumption by 10%, as well as reducing pulp rejects. It can also increase yield by up to 4% per tonne of wood.
Dilution in pulp washing
Pulp is washed after the digestion step to remove cooking liquor chemicals and organic compounds dissolved from the wood chips.
Optimising the dilution factor lowers the amount of water that must be evaporated from weak black liquor, thereby reducing steam consumption in the evaporators.
Utilising pressure diffusion or wash presses for brown stock washing instead of conventional vacuum pressure units, can reduce electricity and steam use further while also reducing chemical use.
Heat recovery from bleaching
Heat exchangers can recover the large amounts of heat in bleach plant effluent. This recovered heat can be used to generate hot water for the paper machine.
Use of recycled fibres from recycled paper saves energy because:
- it means less raw material needs to be prepared
- it takes less energy to pulp recycled paper products than wood chips
Recycled pulp does however produce sludge that can present a disposal difficulty, although new techniques for recycling and reuse are being developed.
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.
Black liquor solids concentrators
Black liquor concentrators increase the solids content prior to combustion in a recovery furnace. This means less water is evaporated in the recovery furnace, which can increase the efficiency of steam generation substantially.
Extended delignification and oxygen delignification
Undertaking delignification prior to bleaching greatly reduces the energy needed in the bleaching process.
Extended delignification and oxygen delignification are established technologies that can remove as much as 70% of the lignin before the bleaching process. This can reduce the electrical consumption of the bleach plant by around 30%. Extended delignification can also reduce cooking liquor consumption by up to 10%.
Composite tubes for recovery furnaces
Chemical recovery furnaces consist of tubes that circulate pressurised water to permit steam generation. These tubes are normally made of carbon steel.
Severe corrosion, thinning and occasional tube failure has led to improvements including weld overlay and co-extruded tubing alloys. Replacing carbon steel tubes in the recovery furnace with composite alloy tubes allows the use of black liquor with higher dry solids content. This increases thermal efficiency of the recovery furnace and reduces furnace shutdowns.
It’s possible to save energy in the paper pressing process, for example through the use of shoe (extended nip) press and gap-formers.
Some examples of opportunities in this area are outlined below.
Extended nip presses
Pressing normally occurs with two felt liners pressed between two rotating cylinders.
Extended nip presses use a large concave shoe instead of one with rotating cylinders. The additional pressing area adds dwell time in the nip and allows for greater water extraction (about 7% more water removal) up to a level of 50% dryness. Greater water extraction reduces energy requirements in the dryer.
Paper machine vacuum systems
Vacuum pumps and systems use significant amounts of energy. Inefficiencies in these systems can, however, increase the energy required in the water removal stage.
Gap-formers are a more productive alternative to the Fourdrinier paper machine due to the significantly reduced time it takes for paper formation.
Combining gap-formers with an improvement in the drying capacity may increase production capacity by as much as 30%, with electricity savings of around 40 kWh/t of paper.
Significant energy savings can be achieved with better dryer control, heat recovery technologies and system optimisation. Traditional air-to-air heat recovery systems typically recover about 15% of the energy contained in the hood exhaust air, but this could be increased to 70%.
Heat recovery by reducing air requirements
15% of the energy contained in hood exhaust air tends to be recovered in traditional air-to-air heat recovery systems. This could be increased to 70% by:
- retrofitting paper machines with enclosed hoods (50% energy saving compared to canopy hoods)
- enclosing the paper machine, which lowers the volume of air needed to be heated and reduces requirements in the exhaust fan
- optimising the ventilation system by ensuring ventilation systems run at the minimum temperature required
Advanced dryer controls
Advanced dryer control systems enable the control of set points and process parameters to reduce steam use and improve productivity.
New sensor and control systems
Sensors and control systems allow access to real-time data for optimisation of production systems and decisions, better cost and quality control, and reduction of waste.
See the Industry 4.0 guide for more information.
Alternatives to fossil fuel can reduce costs and emissions. For example, greater use of biomass wastes, electrification of steam driven processes, and integration of renewable process heat (heat pumps and solar heating), are increasingly being adopted.
See the Renewables guide for more information.
National Pulp and Paper Sustainability Report 2018 (PDF 6.3MB) Australian Forest Products Association
Pulp and Paper Manufacturing Australian Industry and Skills Committee
Tracking report: Pulp and paper International Energy Agency