Archived: Code of Practice for management of air emissions from pulp and paper mills: chapter 3

3 Recommended Environmental Protection Practices

• 3.1 General Practices
• 3.2 Chemical Mill
  • 3.2.1 Wood handling and preparation
  • 3.2.2 Pulping and delignification
  • 3.2.3 Screening and washing of unbleached pulp
  • 3.2.4 Bleaching
  • 3.2.5 Drying
  • 3.2.6 Recovery boilers
  • 3.2.7 Lime kiln
  • 3.2.8 Preparation of chemicals
  • 3.2.2 Boiler
• 3.3 Mechanical Mill
  • 3.3.1 Chip handling and preparation
  • 3.3.2 Heating wood chips
  • 3.3.3 Impregnation
  • 3.3.4 Separation of fibres
  • 3.3.5 Screening and cleaning
  • 3.3.6 Thickening
  • 3.3.7 Bleaching
  • 3.3.8 Boiler

This section presents various methods and measures to limit SO₂ and TPM air emissions from pulp and paper mills. It is not intended to limit the use of other technologies and practices that could provide equivalent or superior environmental protection. Each recommendation’s applicability also needs to be assessed based on the individual conditions and concerns of each mill.

3.1 General Practices

Under this Code of Practice, the term “general practices” means the activities, actions, processes and procedures that, aside from legal and technical requirements, help reduce as much as possible the harmful effects of mills on the surrounding area. The effective development and implementation of these practices will also facilitate the continuous improvement of overall environmental performance.

Recommendations:

• Each mill should establish and maintain operating procedures and ensure that all staff members are adequately trained on any equipment that generates air emissions, and on the monitoring systems for such equipment.
  
• Each mill should develop and implement a maintenance and efficiency audit program for emissions monitoring devices, including documented:
  • maintenance and audit procedures;
  • inspection schedule for each equipment related to air emissions;
  • procedures to communicate discrepancies to mill management.
• Each mill should develop, implement and maintain an environment management system that complies with a known national standard, such as ISO 14000 standards.^Footnote1

3.2 Chemical Mill

The kraft process is most often used in manufacturing chemical pulp. This process dissolves the lignin that binds the fibres together under the action of the pulping liquor (a sodium hydroxide [NaOH] and sodium sulphide [Na₂S] solution) and high temperature. Part of the hemicellulose is also dissolved during pulping.

For sulphite pulping, the cooking liquor is a solution that can contain sulphurous acid (H₂SO₃), sulphite and bisulphite salts of calcium (Ca), magnesium (Mg), sodium (Na) or ammonium (NH₄), depending on the base used. Pulping and delignification are carried out by sulphonation and hydrolysis reactions that form soluble lignosulphonates.

Dissolving pulp can be produced from kraft or sulphite pulp, by adding a step to extract the hemicellulose.

The following is a description of the main steps involved in manufacturing chemical pulp, with the related concerns and recommendations regarding SO₂ and TPM air emissions.

3.2.1 Wood handling and preparation

The fibre is generally received directly in the form of logs, wood chips or sawdust as by-products of the wood products industry, particularly sawmills.

When the fibre is received as a log, it contains bark and must be debarked before being used in the pulping process. The bark is sent to a pile to be used as an energy source. For chemical pulping, the log will be reduced to same-sized chips by a chipper. The chips do not have bark and can be used after washing and screening.

Concern: None related to TPM and SO₂.

3.2.2 Pulping and delignification

The fibres are released from the wood matrix by dissolving the lignin and part of the hemicellulose in a pulping solution that contains sodium hydroxide and sodium sulphide (kraft process) or an acidic solution of bisulphite and sulphite salts (sulphite process). Pulping is done in digesters in batches or continuously.

Concern: The gases from the digester can contain reduced sulphur compounds and volatile organic compounds (VOCs). These gases can also come from chip steaming vessels, the blow pit or the steam recovery system. Pulping does not represent a significant source of particulate emissions.

Recommendation:

• The malodorous gases from pulp and paper mills can be collected and treated based on their concentration and volume. Treatment methods for these malodorous gases are incineration and scrubbing in alkaline conditions. There are several locations where incineration can be done: lime kilns, thermal oxidizers or recovery boilers.
  • When low-volume, high-concentration non-condensable gases are incinerated in the lime kiln, the sulphur is partially absorbed by the lime product. Total reduced sulphur (TRS) compound emissions from the kiln are managed by effectively controlling the lime mud washing and combustion conditions. Incinerating concentrated malodorous gases in a thermal oxidizer transforms the TRS emissions into SO₂ emissions. That thermal oxidizer should therefore always be equipped with a wet scrubber using alkaline solutions.
    
  • The incineration of concentrated gas in the recovery boiler burning black liquor with high dry solids contents will yield lower sulphur emissions, both in terms of SO₂ and TRS.

3.2.3 Screening and washing of unbleached pulp

After leaving the digester, the pulp contains fibres and residual liquor. Depending on the degree of delignification achieved and the type of process, nearly 50% of the wood is dissolved during the pulping process. The residual liquor contains inorganic compounds from the wood, as well as a large proportion of organic compounds. When the brown stock is washed, the residual liquor is removed from the pulp and sent to the recovery system, where the cooking agents will be recovered.

Before being sent to the bleach plant, the brown stock is screened to remove the knots and fibre bundles from the pulp. Rejects from screening and deknotting can be sent back to the digester, burned in boilers, or disposed of in a landfill.

Concern: None related to TPM or SO₂.

3.2.4 Bleaching

The objective of bleaching is to remove or oxidize the residual lignin and impurities from the pulp to achieve the desired level and stability of brightness as well as to meet certain cleanness and strength quality criteria. Bleaching is done in stages, using different chemicals such as chlorine dioxide, oxygen, hydrogen peroxide and sodium hydroxide, depending on the process used and the desired characteristics of the pulp.

Concern: None related to TPM or SO₂.

3.2.5 Drying

In an integrated pulp and paper mill, the bleached pulp is sent in a wet state at a 3-4% consistency to the stock preparation plant for papermaking.

For non-integrated pulp mills where the pulp is not used for papermaking at the same site, the pulp is treated to facilitate handling. The pulp is first squeezed, then pressed, then dried to obtain the desired dryness. The pulp is then cut into sheets, and bales are formed for shipment.

Concern: None related to TPM or SO₂.

3.2.6 Recovery boilers

The primary objectives of recovery boilers are to initiate the recovery of the inorganic pulping chemicals present in the black liquor and to burn the organic matter in the liquor to supply a significant portion of the energy (steam) needed by the process.

Concern: The recovery boiler is a significant source of TPM and SO₂ emissions into the atmosphere. Another significant source of particulate and TRS emissions is the dissolving tank vent.

Recommendations:

• TRS emissions from the recovery boiler can be reduced through effective combustion control. Carbon monoxide (CO) monitoring can be used to assess combustion efficiency.
  
• SO₂ emissions from the recovery boiler can be limited by burning black liquor with high concentrations of dry solids. Increased concentrations of dry solids in the black liquor help reduce sulphur emissions from the recovery boiler because a higher quantity of sodium is vaporized and reacts with the sulphur. Increased concentrations of dry solids in the black liquor also result in increased recovery boiler energy generation and capacity. However, this measure could increase nitrogen oxide (NOx) emissions from the recovery boiler.
  
• It is best to install a wet scrubber with alkaline solutions for the recovery boiler combustion gases when a new boiler is installed, even though it is possible, but often more costly, to equip existing boilers with scrubbers. Recovery boilers that burn black liquor with high concentration of dry solids tends to produce fewer but variable sulphur emissions. Consequently, SO₂ control (even with alkaline scrubbers) can be inefficient and may not always be an attractive option.
  
• Dust emissions from recovery boilers can be reduced by using efficient electrostatic precipitators.
  
• Sulphur emissions from recovery boilers can be reduced by controlling the combustion process parameters, including temperature, air intake, black liquor distribution in the boiler and boiler load.
  
• TRS emissions from the dissolving tank vent can be reduced by installing a wet scrubber. An alkaline solution containing sodium hydroxide will remove some of the TRS, and the mechanical action in the scrubber will remove the particulates. TPM emissions can also be controlled by wet venturi scrubbers followed by mist eliminators.

3.2.7 Lime kiln

This equipment is part of the pulping liquor regeneration cycle, transforming the calcium carbonate (CaCO₃) into lime (CaO) and carbon dioxide (CO₂). It generally uses natural gas or fossil fuels as a source of heat for this reaction.

Concern: Depending on the fuel used, the lime kiln could be a significant source of SO₂ and TPM emissions into the atmosphere. Typical fuels used in lime kilns are fuel oils and natural gas.

Recommendations:

• SO₂ emissions from lime kilns can be reduced using low-sulphur fuels. To minimize the formation of SO₂, either reduce the overall amount of sulphur input (low-sulphur content fuels) to the kiln or, when burning non-condensable gases, eliminate the acidic sulphur compounds contained in these gases by scrubbing them before they are incinerated in the lime kiln.
  
• Dust emissions from the lime kiln can be reduced using efficient electrostatic precipitators and wet scrubbers. Kilns equipped with wet scrubbers for TPM emission control can obtain additional SO₂ capture in the scrubber with alkaline solutions.

3.2.8 Preparation of chemicals

The most common chemicals for bleaching used in the kraft process are chlorine dioxide, ozone, oxygen and peroxyde.

There are a number of ways to create chlorine dioxide. Generally, in Canada, it is obtained from sodium chlorate. To transform the chlorate ion into chlorine dioxide, a reducing agent, such as a chloride ion, hydrogen peroxide, sulphur dioxide or methanol is used.

Ozone should be produced on site because of its instability. It is made by placing oxygen between two high-voltage electrodes.

Concern: None related to TPM or SO₂.

3.2.9 Boiler

Boilers consist of a combustion chamber, which is designed to capture the thermal energy produced by burning fuel and transfer this energy into a heat transfer fluid (such as water or oil). The energy is typically used to produce steam which can serve various purposes, such as electricity production and process heating.

Contrary to a recovery boiler, the black liquor is not burned there. Two main types of boilers are generally used:

1. Fossil fuel boiler
2. Biofuel boiler

 The fossil fuel boiler uses natural gas, fuel oil or a combination of the two.

The biofuel boiler can use a number of fuel types, such as waste wood, bark or wastewater treatment biosolids.

Concern: Depending on the fuel used, the boiler could be a significant source of SO₂ and TPM emissions into the atmosphere.

Recommendations:

• Emissions can be limited by balancing cogeneration of heat and energy depending on the need for each.
  
• TPM emissions can be reduced by using an electrostatic precipitator or efficient bag filters.
  
• SO₂ emissions from the boiler can be reduced by switching to low sulphur fuel or add biomass fuel. When cofiring with biomass, a significant fraction of SO₂ formed during combustion can be captured by the wood ash.
  
• SO₂ emissions from the boiler using biomass fuel can be reduced by installing a wet scrubber.

3.3 Mechanical Mill

The mechanical and chemi-mechanical pulping processes require a mechanical action using very few chemicals, or none at all, to separate the fibres. In the chemi-mechanical process, the wood chips are slightly chemically treated before refining.

There are several methods for producing mechanical pulp, depending on whether chemical treatment took place, the type of fibre-separation process and the pressure used, such as:

• stone groundwood pulping (SGW) or pressure groundwood pulping (PGW): the debarked logs of wood are defibred on an abrasive stone
• refiner mechanical pulping (RMP): the chips are defibred between two rotating discs
• pressure refiner mechanical pulping (PRMP): the chips are defibred between two rotating discs, under pressure
• thermo-mechanical pulping (TMP): the chips are steamed then defibred between two rotating discs, generally under pressure
• chemi-mechanical pulping (CMP): the chips are impregnated with a chemical product (sodium sulphite [Na₂SO₃], sodium hydroxide [NaOH], sodium carbonate [Na₂CO₃]) then defibred with one of the refining processes
• repulping: dry pulp is moistened and mashed by a mechanical agitator until the fibres separate and the desired consistency is obtained

The following is a description of the main steps to make mechanical pulp, along with the related concerns and recommendations regarding SO₂ and TPM atmospheric emissions.

3.3.1 Chip handling and preparation

The fibre is generally received in the form of chips, a by-product of the wood products industry, particularly sawmills. The chips are screened to remove chips that are too large and sawdust. The large chips can be recut to the desired size. The screened chips are then washed to remove all debris that could damage the refiners.

Concern: None related to TPM or SO₂.

3.3.2 Heating wood chips

In the TMP process, wood chips are heated with steam under pressure for a few minutes before they are refined.

Concern: None related to TPM or SO₂.

3.3.3 Impregnation

This step is mainly used in the chemi-mechanical process. The wood chips are impregnated with a chemical product (Na₂SO₃, NaOH, Na₂CO₃) before they are refined using one of the mechanical pulping processes.

Concern: None related to TPM or SO₂.

3.3.4 Separation of fibres

In the stone groundwood pulping process, debarked logs of wood are pressed against a grinding wheel.

In the refining process, wood chips and water are forced between two rotating discs a millimetre or less apart. On the surface of each disc, slits and bars compress and shear the wood chips in order to defibre them. Part of the energy used by the refiner transforms the water into steam.

Concern: During refining, the steam produced releases pollutants, such as particulate matter.

Recommendation:

• Using a scrubber on the dirty steam produced or using a steam recovery system substantially reduces the TPM emitted and improves the facility's energy efficiency.

3.3.5 Screening and cleaning

Pressure screens and hydrocyclones are used to clean the pulp. Baskets with slits or holes are used for screening. Hydrocyclones are used with the screens. Rejects are sent to the next stage in a cascade arrangement or directly to the reject refining stage.

Concern: None related to TPM or SO₂.

3.3.6 Thickening

A disc filter or a screw press is used to thicken the final pulp product. This stage is essential to eliminating part of the dissolved matter in the water--matter that could affect the paper machine--and to maximizing the pulp storage tank’s capacity.

Concern: None related to TPM or SO₂.

3.3.7 Bleaching

Hydrogen peroxide or sodium hydrosulphite is generally used in mechanical pulp bleaching. There may be one or two bleaching stages, depending on the brightness sought.

Concern: None related to TPM or SO₂.

3.3.8 Boiler

Boilers provide the energy required for the process. The same concerns and recommendations apply as those regarding the boiler in the chemical pulping process (Section 3.2.9).