Code of practice for the management of air emissions from pulp and paper facilities: chapter 3

3 Recommended environmental protection practices

This section presents various methods and measures to limit SO2 and TPM air emissions from pulp and paper facilities. 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 facility.

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 minimize facility emissions. The effective development and implementation of these practices will also facilitate the continuous improvement of overall environmental performance.


3.2 Chemical facility

The kraft process is the most common process used in chemical facilities. 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.

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 also carried out at high temperature.

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

3.2.1 HVLC and LVHC non-condensable gases

In chemical facilities, production-process-based emissions of gaseous sulphuric compounds are generated from multiple locations in the pulping and chemical recovery areas. They can be low-volume, high concentration (LVHC) non-condensable gases (NCGs) or high-volume, low concentration (HVLC) non-condensable gases. See Definitions.

Concern:  Sources of SO2


3.2.2 Wood handling and preparation

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 and the log is chipped.

Concern:  None related to TPM and SO2

3.2.3 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 carried out in digesters, either as part of a batch or continuous process.

Concern: The digester process, which includes wood chip pre-steaming vessels, blow tanks and relief steam condensers, generates a mixture of sulphur containing gases.

Non-condensable gases from the digester area contain high levels of TRS. The reduced sulphur compounds are transformed into SO2 if these gases are burned in lime kilns, boilers, recovery boilers or stand-alone thermal oxidizers.

Combustion is a common practice used to eliminate NCGs.

Pulping does not represent a significant source of particulate emissions.


Lime kilns, boilers, and thermal oxidizers are typically used to burn LVHC gases, while HVLC gases can be combusted in boilers and recovery boilers given that these units can handle larger gas volumes. Provided the associated SO2 emissions are relatively stable, they can be reduced by installing a wet scrubber using alkali in the scrubbing solution.

3.2.4 Screening and washing of unbleached pulp

The product obtained from the pulping process is a mixture of wood fibre (pulp) and spent cooking chemical (liquor). Depending on the degree of delignification achieved and the type of process involved, nearly 50% of the wood is chemically dissolved during pulping. These wood-based organic and inorganic constituents are contained in the spent cooking liquor. The spent pulping liquor is separated from the pulp (brown stock) in the washing process and sent to the chemical recovery process.

The washed pulp (brown stock) is then screened to remove 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. Brown stock washers generate gases which arise from the residual spent cooking chemical in the pulp. These gases are a source of odorous reduced sulphur gases, including SO2.


To minimize the release of SO2 emissions from vents of the pulp washing process, vents can be piped to collect the emissions in the HVLC collection system. See section 3.2.1.

3.2.5 Bleaching

The objective of bleaching is to remove or oxidize the residual lignin and impurities in 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

3.2.6 Drying

In an integrated pulp and paper facility, 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 facilities 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 and 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

3.2.7 Concentration of spent pulping liquor

The weak (10-20% solids) spent pulping liquor is concentrated in the evaporator prior to combustion in the chemical recovery boiler.  Concentration of spent pulping liquor is usually achieved by using steam-heated, multiple-effect evaporators and indirectly heated concentrators.  Gases released from multiple-effect evaporators consist mainly of TRS and VOCs.

Concern: These gases are commonly collected in the HVLC system and combusted, resulting in SO2 emissions.

Recommendations: See section 3.2.1

3.2.8 Recovery boilers

The primary purpose of recovery boilers is to initiate the recovery of the inorganic pulping chemicals present in the spent pulping liquor and to burn the organic matter in the liquor in order to obtain a significant portion of the energy (steam) needed by the process. Collected LVHC and HVLC gases are commonly sent to the recovery boiler to be combusted.

Concern: The recovery boiler is a significant source of TPM and SO2 emissions to the atmosphere.


3.2.9 Smelt dissolving tank

The dissolving tank receives the smelt from the recovery boiler. The dissolving tank vents gases containing high amounts of TRS and particulates. It generates lower levels of SO2 emissions. Vent gases have a high moisture content.

Molten smelt is drained from the chemical recovery boiler into the dissolving tank, where it is diluted with “weak wash” to form green liquor.  The dissolving tank is not a significant source of SO2 emissions but the moisture-laden dissolving tank emissions contain TRS and particulate matter.

Concern: The smelt dissolving tank emissions are a significant source of TRS and particulate matter. The dissolving tank vent gases are commonly collected in the LVHC and sent to combustion. The combustion of concentrated TRS emissions transforms the reduced sulphur into SO2 emissions.


3.2.10 Lime kiln

The Lime kiln is part of the pulping liquor regeneration cycle, transforming the calcium carbonate (CaCO3) into lime (CaO) and carbon dioxide (CO2). It generally uses fossil fuels to supply heat for mud drying and chemical reaction. The gases exiting the kiln are laden with particulate matter (lime dust) and may also contain sulphur compounds originating from liquor carryover in the lime mud.

Lime kilns can also be used to incinerate LVHC and HVLC non-condensable gases, thereby oxidizing the odorous reduced sulphur compounds into SO2.

TPM emissions primarily depend on fuel type, combustion technology and emission control device, while SO2 emissions mostly depend on the fuel’s sulphur content (e.g., fuel oil, petroleum coke, NCGs or SOGs) and, sometimes, post-combustion controls (e.g., gas desulphurization).  A relatively smaller portion of the sulphur comes with lime mud.


TPM and SO2 are significant emissions from lime kilns because

  1. a large portion of the sulphur coming into the kiln with the fuel is oxidized to SO2 under normal conditions; and
  2. high levels of particulate matter are picked up by combustion gases as they pass through the kiln.


To reduce TPM emissions, ESPs or bag filters can be used as end-of-pipe controls.

3.2.11  Preparation of chemicals

The most common bleaching chemicals used in the Kraft process are chlorine dioxide, ozone, oxygen, and peroxide.

There are a number of ways to generate chlorine dioxide. In Canada, it is generally 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

3.2.12  Boilers

Boilers are typically used to produce steam which can serve various purposes, such as process heating and electricity production.

Boilers may use only one type of fuel (e.g., gaseous), switch between different fuels, or use multiple fuels simultaneously or in alternation (e.g., natural gases and heavy fuel oil). Combustion of solid fuels, such as biomass, generates emissions laden with particulate matter.  Combustion of sulphur containing fossil fuels and/or combustion of LVHC/HVLC non-condensable gases can lead to SO2 generation.

Concern: Boilers can be used to combust fuel and waste streams which can carry various pollutants. The boiler could be a significant source of SO2 and TPM emissions to the atmosphere.


3.3 Mechanical facility

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

The following is a description of the main steps involved in making mechanical pulp, along with the 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 sawdust and pieces that are too large. 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 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 that is produced carries pollutants such as particulate matter.


Using a scrubber on the dirty steam produced or using a steam recovery system substantially reduces TPM emissions 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. 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 in order to remove part of the dissolved matter in the water—matter that could affect the paper machine—and to maximize 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 with the boiler in the chemical facility process (section 3.2.12).

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