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.

Recommendations:

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

Recommendations:

• Collection and treatment of these gases is beneficial as it reduces the discharge of odorous gases to the atmosphere and allows for some energy and chemical recovery. 
• The combustion oxidizes TRS gases to SO₂ during incineration. HVLC and LVHC gases can be treated to reduce the sulphur compounds contained in them before incineration, and/or emissions from combustion can be treated in a wet scrubber using alkali in the scrubbing solution. HVLC and LVHC gases can be collected in separate system.

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.

Recommendation:

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.

Recommendations:

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.

Recommendations:

• TPM emissions from recovery boilers can be reduced by using electrostatic precipitators (ESP).
• A wet scrubber with alkaline solution can be used on a recovery boiler to minimize sulphur emissions. Given the variability of the level of SO₂ emissions from recovery boilers, removal efficiency can vary but it is typically greater than 90%. 
• To minimize SO₂ emissions from the combustion of the collected gases, the collected gases can be treated by an alkaline scrubber prior to being sent to the recovery boiler.
• Increased concentrations of dry solids in the spent pulping liquor help reduce sulphur emissions from the recovery boiler because a higher quantity of sodium is vaporized to react with the sulphur. A new Kraft recovery boiler can have a percentage of dry solids in the spent pulping liquor greater than 80%. However, this could increase TPM and nitrogen oxide (NO_x) emissions from the recovery boiler if no countermeasures are in place.
• Sulphur emissions from recovery boilers can be minimized by controlling the combustion process parameters, including temperature, air intake, spent pulping liquor distribution in the recovery boiler, and boiler load. In a Kraft recovery boiler, the creation of conditions for a hotter lower furnace zone results in higher relative amounts of volatilized sodium compounds (from the molten char formed both within the char bed and within the falling liquor droplets), which then react with SO₂ to produce sodium sulphate, thereby lowering SO₂ emissions.
• For sulphite facilities, recovery boilers can be paired with scrubbers/washers with alkaline solution to capture the sulphur and chemicals from the burned spent liquor. To control the emission release during the flushing and cleaning of the deposits in the scrubbers and washer, a bank of scrubbers/washers can be use in order to have the flue gases treated by one during the cleaning of the other.
• To further control these emissions, all the emission control devices (multicyclone or electrostatic precipitator, multistage scrubbers) must have the proper layout and operating set-up, which is site-specific.

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.

Recommendations:

• Gaseous emissions from the dissolving tank can be reduced by treatment in a wet scrubber using an alkaline solution such as weak wash.
• Particulate emissions from the dissolving tank can also be treated in a wet scrubber and then a mist eliminator to removed entrained droplets.
• Incineration of vent gases can be done in lime kilns, thermal oxidizers, boilers or in more recently built, recovery boilers or those modified for the incineration of dissolving tank vent gases.
• Dissolving tank gases that are to be incinerated in a recovery boiler must first be conditioned by passing them through a direct contact condenser, a mist eliminator, and a reheater. With this approach, the recovery boiler is the unit that ultimately handles the load of TPM, TRS and SO₂ from the dissolving tank.

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.

Concern:

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.

Recommendations:

• For SO₂ emission reduction from lime kilns, wet scrubbers with alkali solutions may be a possible solution.
  When sulphur-containing fuels are incinerated in a lime kiln, a portion of the sulphur can be absorbed by the lime product up to a certain total sulphur input. Beyond this point, SO₂ emissions can be reduced by limiting the overall amount of sulphur input to the kiln via fossil fuels and non-condensable gases (NCGs), including low volume, high concentration of (LVHC) and stripper off-gases (SOGs).
• When burning LVHC or SOGs in the lime kiln, facilities could use a wet scrubber to reduce the sulphur compounds contained in these gases before incineration.
• With effective mud washing and filtration, the soluble sulphide in the lime mud can be reduced prior to entering the lime kiln, thereby reducing the amount of sulphur that is oxidized to SO₂ during incineration.

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.

Recommendations:

• TPM emissions can be reduced by using an electrostatic precipitator and/or efficient bag filters. Both ESPs and bag filters can be highly efficient particulate removal devices with design efficiencies in excess of 99%.
• SO₂ emissions can be reduced by switching to lower sulphur content fuel or by installing a wet scrubber with alkaline solution.

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:

• stone groundwood pulping (SGW) or pressure groundwood pulping (PGW): the debarked logs are defibred by pressing them against 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 by 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 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.

Recommendation:

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).