Opportunities – Pulp and Paper Manufacturing

Improve the efficiency of raw materials preparation

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.

Some examples of opportunities in this area are outlined below.

Use cradle debarking

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

A U.S. Department of Energy study reported that using a cradle debarker can remove bark from logs in a manner that reduces energy consumption by as much as 33% per log compared with traditional debarking methods. ¹ This could save a paper mill as much as US$30 per ton of wood in debarking costs. ²

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.³

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 pneumatic chip conveyors operating at 18.2 kWh/tonne with more efficient belt conveyors operating at 1 kWh/tonne. This can result in savings of 17,200 kWh/day, or around US$100,000-$200,000/yr in electricity costs, depending on the size of the conveyor.¹
• 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%. The return on investment (ROI) is estimated at 5–8 years.²
• Using bar-type chip screens which 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.

For more information

• Energy Efficiency Improvement and Cost Saving Opportunities for the Pulp and Paper Industry 2009 (Opens in a new window)
  • Ernest Orlando Lawrence Berkeley National Laboratory
  • PDF 2.3 MB

  This guide developed by Lawrence Berkeley Laboratory in the U.S discusses energy efficiency practices and energy-efficient technologies that can be implemented at the component, process, facility, and organizational levels of pulp and paper companies. Many measure descriptions include expected savings in energy and energy-related costs, which are based on case study data from real-world applications. Typical payback periods and references to further information in the technical literature are also provided, where available.

• Available and Emerging Technologies for Reducing Greenhouse Gas Emissions from the Pulp and Paper Manufacturing Industry 2010 (Opens in a new window)
  • US Environmental Protection Agency
  • PDF 421 KB

  This report from the U.S Environmental Protection Agency provides an overview of control techniques and measures that are available to mitigate greenhouse gas emissions in the pulp and paper manufacturing sector. It includes a summary of key opportunities within key technologies in the various stages of the manufacturing process.

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.¹ 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. In continuous digesters, extracted black liquor flows to a tank where it is flashed.² It is possible to recover heat from either batch digesters or continuous digesters to use for other parts of the papermaking process such as steam generation, water heating or black liquor evaporation.³ The newer systems also produce a more uniform pulp quality, which in turn allows yields to be increased.

For black liquor evaporation it is also possible to use flash steam from batch digester blow (created by flashing from the hot water accumulator) or black liquor flash from a continuous digester in a multi-stage evaporator. Captured thermal energy from this process can be used to offset the need for steam generated by a boiler.⁴

Cost savings from investing in heat recovery can be as high as US$1-2 million/yr for a 1,000-tonne/day mill.⁵

For more information

Use pulping aids

Chemical pulping aids can be added to the pulping process to increase liquor penetration and promote more even cooking.¹  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. ²

For more information

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.¹

Brown stock can be used in this washing step, resulting in a higher level of chemical recovery and minimising dilution of black liquor. 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 natural gas usage by 315,000 GJ/yr, resulting in costs savings of around AUD$400,000-AUD$600,000/yr. ²

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 12 kWh/tonne of production. ³

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 900,000 GJ/yr, with annual cost savings of $1.5-2.4 million (2003 dollars). After a capital investment of US$1.6 million, the estimated payback period was between 0.5 and 1.4 years.

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.¹

Recycled pulp does however produce sludge that can present a disposal difficulty. ² There are also limitations in the amount of recycled fibre that can be used in any given product ³. Despite this, life cycle assessments by the US Environmental Protection Agency ⁴ and emission reduction analysis by the European Commission⁵ has identified paper recycling as an important greenhouse gas emissions reduction option.

For more information

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 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.

One study shows a 900 tonne 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 115,500 GJ. Cost savings for the mill were about US$900,000 per year, with an estimated payback period of 4–7 years.

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.¹ This can reduce the electrical consumption of the bleach plant by around 28%². Extended delignification can also reduce cooking liquor consumption by 5-10%. ³

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 through the use of shoe (extended nip) press and gap formers, while optimising the paper machine’s vacuum system.

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 5 to 7% more water removal) to a level of 35 to 50% dryness. Greater water extraction reduces energy requirements in the dryer.¹

For more information

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 stage¹.

Audits of such systems, such as a Canadian operation with 14 paper machines, found a potential for 3.5 MW in electricity savings through modifications, operational changes, and the right sizing of the system. This resulted in US$400,000 in savings per annum.²

For more information

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 formation¹.

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/tonne of paper. ²

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%.

Some examples of opportunities in this area are outlined below.

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 0.89 GJ/t from improved drying efficiency, with a cost savings of US$25,000.¹
• 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 5 GJ/t (50%).²
• A third system used heat pump systems to recover waste heat in the drying section. ³ Estimated energy savings of around 0.45 GJ/tonne of paper are achievable through paper machine heat recovery⁴, with installation costs of around US$18 per tonne of paper.⁵

For more information

Improve heat recovery by reducing air requirements

Fifteen per cent 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 60 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.¹

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 productivity¹.

One US study of dryer control software reported use of the technology led to significant reductions in steam requirements. Savings were estimated at US$360,000 annually with a 3–4 year return on investment. The company also reported significant improvement with product quality and throughput. ²

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, handling equipment (e.g. conveyors) and compressed air systems. Additional energy efficiency opportunities are available through upgrading and optimising the efficient use of these systems.¹

International studies show that combining technology specific and process specific opportunities could achieve significant energy efficiency improvements for existing or new paper and pulp mills.²