Pulp and paper opportunities to save

Improve the efficiency of raw materials preparation

A range of techniques are available to prepare raw materials so less energy is required during processing.

Some examples of opportunities in this area are outlined below.

Use cradle debarking

Newer methods of debarking can reduce the energy required to remove bark, increase wood recovery rates and reduce transportation costs.

A cradle debarker can remove bark from logs in a manner that reduces energy consumption by as much as a third compared with traditional debarking methods. 

Cradle debarking is also reported to result in less damage to logs, leading to a greater wood recovery rate. Transportation costs can also be reduced by eliminating the need for off-site debarking.

Improve efficiency of chip handling, screening and conditioning

Energy-efficiency gains can be made in the process steps which convert 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 which can provide downstream benefits by reducing the steam required in the digester and evaporator, and increasing digester yield by 5-10%
  • using modern chip screen systems which are tailored for the application, delivering greater efficiency but with very similar upfront capital costs

Increase use of recycled inputs

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 over the last decade as paper recovery rates in Australia have improved.

Increase the efficiency of chemical pulping

Chemical pulping separates wood chips into cellulose fibres through a cooking process involving chemical solutions and elevated temperature and pressure. A range of energy efficiency opportunities involving more efficient heat and chemical recovery can save significant amounts of energy. Pulp washing processes can also be used which reduce the amount of energy required in the evaporation process.

Some examples of opportunities in this area are outlined below.

Improve digester efficiency

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 stored as hot water in an accumulator tank in batch digesters. It is possible to recover heat from either batch digesters or continuous digesters to use for other parts of the paper making process such as 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 be used to offset the need for steam generated by a boiler.

Use pulping aids

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 0.125GJ/tonne (8-10%), as well as reduce pulp rejects, whilst increasing yield 2 to 4% per tonne of wood.

Optimise the control of the dilution factor 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.

At one plant, it was estimated that these improvements could reduce gas consumption by 315,000GJ per year.

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. Electricity savings are estimated at approximately 12kWh/t of production.

Recover heat from the bleaching processes

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. Energy savings were estimated to be around 900,000GJ per year.

Use a higher percentage of recycled fibres

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. 

Implement efficiency improvements in chemical recovery

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.

Some examples of opportunities in this area are outlined below.

Install black liquor solids concentrators

Black liquor concentrators increase the solids content 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.

One study shows a 900t per day pulp and paper mill that installed a liquor concentrator increased its solids content from 73% to 80% and reduced annual energy usage by about 115,500GJ.

Utilise extended delignification and oxygen delignification

Undertaking delignification prior to bleaching greatly reduces the energy required 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%. 

Improve composite tubes for recovery furnaces

Chemical recovery furnaces consist of tubes that circulate pressurised water to permit steam generation. These tubes are normally made out of carbon steel.

Severe corrosion, thinning and occasional tube failure has led to the development of more advanced tube alloys, 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 the thermal efficiency of the recovery furnace and decreases the number of furnace shutdowns.

Enhance the efficiency of paper pressing

It is 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.

Use 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.

Optimise 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.

Audits of a Canadian operation with 14 paper machines, found a potential for 3.5MW in demand through modifications, operational changes, and the right sizing of the system.

Use gap formers

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.

Improve techniques for paper drying

Significant energy savings can be achieved through investing in 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 60-70%.

Invest in heat recovery from the paper drying process

Lawrence Berkeley National Laboratories researched opportunities for heat recovery in the paper drying process.

One mill replaced the dryers with stationary siphons in their paper machine and was able to achieve energy savings of 0.89GJ/t from improved drying efficiency.

A second system used mechanical vapour recompression in a pilot facility to reuse superheated steam in the drying process. Steam savings for this approach were up to 5GJ/t (50%).

A third system effectively used heat pump systems to recover waste heat in the drying section. 

Improve 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 reduces the volume of air needed to be heated and reduces requirements in the exhaust fan
  • optimising the ventilation system by ensuring that ventilation systems run at the minimum temperature required

Use advanced dryer controls

Advanced dryer control systems enable the control of dryer system set points and process parameters to reduce steam use and improve productivity.

One US study of the use of dryer control software showed significant reductions in steam requirements. Savings were estimated at US$360,000. The company also reported significant improvement with product quality and throughput. 

Upgrade and optimise technology

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.

The majority of motor-driven systems’ electricity use is used to drive pumps, fans, materials processing, conveyors and compressed air systems. Additional energy efficiency opportunities are available through upgrading and optimising the efficient use of these systems.

Paper and pulp mill energy efficiency can be further improved by plant wide opportunities, such as insulating pipework, lighting replacement, equipment reliability improvements, and process rationalisation. Ensuring there are effective shut down procedures to reduce unnecessary energy overheads when production is stopped is also important.