2.3 Techniques to consider in the determination of BAT

In this paragraph all relevant techniques currently commercially available for prevention or reduction of emissions/waste and reducing consumption of energy and raw materials, both for new and existing installations are given. This list of techniques to consider in the determination of BAT is not exhaustive and may be continued when reviewing this document. These techniques cover in-process measures and end-of-pipe technology as well and stands for possibilities for improvement to achieve integrated prevention and control of pollution.

In Table 2.31 an overview of techniques for reduction of emissions from kraft pulping are given. In the rows the single available techniques are compiled. It was tried to give possible cross-media effects of every technique in the same table. It can be concluded that it is not an easy task to describe cross-media effects in a way that would not cause any dispute. There are a lot of "grey-zones" in the assessment of cross-media effects. Furthermore, they may depend on other techniques, which may be linked to a given measure, the reference to which a technique is compared with and the system boundary taken into account as well. Therefore, the qualitative assessment of cross-media effects should only be taken as help for operators or inspectors which side-effects a measure may possibly occur and is not much more than a starting-point when considering possible shifts of pollution. The results of the assessment should not be considered as imperative. Furthermore, prevention and control measures can avoid some of them. Besides other things, the cross-media effect will also depend on the specific conditions of every mill. Therefore, a general statement can hardly be given. However, the table may indicate at which environmental media (water, air, waste, and energy) a measure is aiming at. The corresponding paragraphs under the detailed discussion of every technique give further explanations.

In the columns the tendency of effects caused by different techniques on emissions, raw material consumption and the mill performance are indicated qualitatively by using arrows up "?" and down "?". The arrows down "?" indicate savings of raw materials or energy and a reduction of emissions into the different media water, air and soil. Arrows up "?" indicate an increase of emissions, consumption and impacts on the production process. Some of the measures for pollution prevention and control presented in this section concern more than one environmental medium (as water, air or soil) at a time. Some techniques may have a positive and/or negative impact on other environmental media or on raw material and energy consumption (cross-media-effects). The given effects will be noted by using the arrows. An arrow in brackets "(?)" means only a slight - often negligible - increase of energy consumption, raw material use or emission to environment when implementing a certain measure. The economic aspects (investments, operational costs) are not included in the table but are explained in the text. Data on economics can only give a rough picture and indicate the levels of costs. They will vary with the design of the whole plant and depend among others on the size of the mill and how a measure fits together with other mill equipment.

Each technique is provided with a reference mark, which helps to identify the according section in the text where each single technique is discussed in detail. In the following the use of Table 2.31 is explained by means of the brief discussion of one technique (see Section 2.3.1 "dry debarking"):

In a transfer from wet debarking to dry debarking there is no effect on chemical consumption. Energy consumption in debarking increases due to the operation of the debarking drum in dry debarking mode. On the other hand, substantial amount of energy may be gained, if the bark is used as an auxiliary fuel at lower water content. Dry debarking results in a significant decrease of emissions to water, e.g. flow reduction in the range of 5 - 10 m3/t, TSS reduction in the range of 2 - 10 kg/t and total COD loading can be reduced up to 10%. There are no effects on air emissions and on solid waste generation.

In this Chapter the main technical options with the best environmental and economical performance are highlighted. It has been tried to discuss the single measures following a uniform structure to have a similar presentation of all techniques to consider in the determination of BAT. Of each technique a description, discussion of applicability, main achieved environmental performance, monitoring of emissions, cross-media effects, operational experiences, data on economics, driving force for implementing this technique, reference plants and literature are given. Most of the techniques can be found in numerous plants in Europe whereas a few of them are only implemented in some mills.

The right combination out of this list of techniques offers the opportunity to the mills to achieve both good environmental and economic performance at the same time. However, it is up to the mill, which of the possible techniques is applied to achieve the resulting consumption and emission levels given in the next chapter "Best Available Techniques". The description of each technique includes an outline of advantages, drawbacks and consequences of the implementation of a certain technique.

Pulp and paper mills are characterised by a relatively high degree of integration so that most of the opportunities for improvement are process-integrated measures. But a few end-of-pipe techniques that are considered as BAT are described as well. One has to bear in mind, that there is a certain overlapping between the further down described process-internal solutions and external control measures.

Table 2.31: Overview of available techniques in kraft pulping and their impact on the environment and the mill performance respectively

2.3.1 Dry debarking

Description of the technique: In woodhandling the discharges of organic compounds and suspended solids can be reduced with dry debarking. Wet debarkers rotate logs in a pool of water and remove bark by knocking the log against the side of a drum by using large volumes of water. The water used in this process is recycled but a certain amount is lost as overflow to carry away the removed bark. In wet debarking 3 to 10 m3 of water per tonne of pulp are discharged. Organic compounds like resin acids, fatty acids etc. and highly coloured materials leach out of the bark and into this wastewater stream.

In recent years dry debarking has been installed in many mills. Process water is used only for log washing and de-icing (in cold climates water or steam is used for thawing of wood) and is recirculated effectively with minimum generation of wastewater and water pollutants. Dry debarking creates bark with a lower water content, which will result in a better energy balance for the mill. Less water is needed in the debarking and the dissolved amount of organic substances is reduced.

Raw effluents from a debarking plant are toxic to aquatic life. Biological treatment has proven to be very efficient in eliminating toxicity.

Applicability and characterisation of the measure: Process integrated technique. The dry debarking or one with low wastewater discharges can be applied at both new and existing mills. Dry debarkers already dominate the industry, and wet systems are in the process of being phased out. New mills almost exclusively and an increasing number of existing mills are using dry debarking.

Main achieved environmental performance: With dry debarking, the wood handling wastewater volume is usually in the range of 0.5-2.5 m3/ADt. Decrease in wastewater amount is obtained by increased internal water circulation. By changing from wet debarking to dry debarking, the wastewater amount would decrease often by 5-10 m3/ADt. With dry debarking the total COD loading can be reduced up to 10 %. Typical discharges of dry debarking are as follows:

Table 2.32: Pollution load of wet and dry debarking effluent before biological treatment
[Finnish Report, 1997]; BOD7 has been converted into BOD5 by use of the formula BOD7 /1.16 = BOD5 proposed within the same report; kg pollutant/m3 wood has been converted into kg pollutant/t of pulp by assuming a typical pulp wood consumption of 5 m3 wood/t of pulp

The higher bark dryness in the boiler feed improves the energy efficiency, meaning that emissions per the amount of produced energy will drop.

Cross media effects: Energy consumption in debarking may increase due to the operation of the debarking drum in dry debarking mode. On the other hand, substantial amount of energy may be gained, if the bark is used as an auxiliary fuel at lower water content.

Operational experiences: Where wet debarking is used, improved water recirculation coupled with grit and solids removal systems for water has been applied with success. Dry debarking usually requires fresh wood in order to obtain good debarking results.

Economical aspects: The costs of dry drum debarkers should not differ significantly from a wet system. Typical investment cost of a completely new dry debarking system is about 15 MEuros for a capacity of 1500 ADt/d pulp.

The conversion of an existing wet debarking system to a dry debarking system costs 4-6 MEuros. These costs include equipment and installation. Possible site-specific need for new buildings, special noise abatement costs or similar are not included but they may further increase the costs. Operating costs are 250000– 350000 Euro/a but in both cases it may involve considerable investments.

Driving force for implementing this technique: The dry debarking decreases TSS, BOD and COD load as well as organic compounds like resin acids, fatty acids leaching out of the bark and into this wastewater stream. Some of these substances are regarded as toxic to aquatic life. The measure also increases energy yield.

Reference plants: Numerous plants in Europe and North America.

Literature
[J. Pöyry, 1997a], [SEPA-Report 4712-4, 1997], [Finnish BAT Report, 1997]

2.3.2 Extended modified cooking (batch or continuous) to a low kappa

Description of the technique: Delignification before the bleach plant is done in digesters and, at many mills, also by use of oxygen delignification. Therefore, measure 2.3.2 "extended modified cooking" and 2.3.4 "oxygen delignification" should be considered as a unit because what is essential from an environmental point of view is the total degree of delignification achieved as a result of delignification in the stages before the pulp enters the bleach plant. There should be a balance between kappa reduction in cooking and in oxygen delignification since the selectivity is much higher in the latter system (see 2.3.4). For the mill it is important to control the various processes so that yield and strength characteristics are optimum for a given lignin content.

In order to decrease the lignin content (lower kappa numbers) in the pulp entering the bleach plant so as to reduce the use of the expensive bleaching chemicals, extended delignification (or modified kraft cooking) was introduced in the end of 1970s and at the beginning of 1980s. The reduction in lignin content will reduce the amount of pollutants discharged while increasing the amount of organic substances going to the recovery boiler. Several modified kraft processes in both continuous and batch systems have been developed and applied commercially.

Continuous cooking
In the continuous system, Modified Continuous Cook (MCC), Extended Modified Continuous Cook (EMCC) and Isothermal Cooking (ITC) represent the three alternatives. In the MCC process, the cooking zone in the digester is divided into two zones, namely an initial co-current zone and a subsequent counter-current zone. The charge of the white liquor is split between the two zones. The purpose of the modification has been to decrease the initial alkali concentration, keep an even concentration of alkali during the cooking process and a low concentration of the dissolved lignin in the final part of the cook. EMCC process was developed on the basis of the MCC process. The difference is that in the EMCC process, white liquor is also charged to the washing zone in order to extend further the delignification in the digester.

The newest development within cooking technology is Iso Thermal Cooking (ITC) which is a further improvement of MCC. In ITC the whole digester vessel is used for delignification which means milder conditions (lower cooking temperature) is achieved and consequently the strength of pulp maintained. Depending on the target kappa number the charge of cooking chemicals is either unchanged or slightly higher. Since ITC means a lower cooking temperature there is no increase in steam usage and the impact on yield is limited.

Batch cooking
In the discontinuous system, there are three commercially applied processes: Rapid Displacement Heating (RDH), SuperBatch and EnerBatch. In the RDH and Superbatch processes, a pretreatment (impregnation) with black liquor is carried out in order to decrease the heat consumption and at the same time to increase the initial sulphide concentration and decrease the effective alkali charge. In the Enerbatch process, a pretreatment with white liquor followed by a pretreatment with black liquor is performed. All these displacement cooking processes show a substantial energy saving and an improved pulp quality.

The lignin content is usually measured as the kappa number with a standardised method. Conventional cooking has its limits, regarding how low the kappa number can be brought without deterioration of pulp quality (This kappa number is around 30-32 for softwood and 18-20 for hardwood). By use of several cooking modifications the kappa from the cooking of softwood can be reduced to a level of 18-22 for softwood and 14-16 for hardwood, while the yield and strength properties are still maintained. The kappa reduction depends beside others on the modified cooking technology applied and whether a retrofitted or new installation is used. As an example the Kappa trends in Finnish kraft pulp mills are shown in Figure 2.8.

Figure 2.8: Kappa trends in Finnish kraft pulp mills
[Finnish BAT Report, 1997]

Applicability and characterisation of the measure: Process integrated technique. The measure can be adopted in new kraft mills and to a limited extent in existing mills.

A number of continuous digesters have been retrofitted to ITC without having to sacrifice production. However, this possibility has to be evaluated in each individual case (this has to do with the dimensions of the pressure vessel in relation to the capacity). Some other continuous digesters have been rebuilt to MCC. To achieve this, white liquor is pumped into the digester at several points. With MCC and ITC it is possible to cook the pulp to lower Kappa number, without losses in quality (Kappa number 20 - 24 for softwoods and 14 - 18 for hardwoods). In continuous cooking systems the capacity of the plant would decrease with extended cooking and imply higher cost burden to the pulp mill.
In batch cooking, extended delignification is carried out by means of displacement and black liquor recycling techniques. The process is possible to install as retrofit in conventional plants, if the digester capacity is large enough. In a new installation the kappa number from the cook may be kept at 15-16 for softwoods and at about 12 for hardwoods. In practice, the modifications of an existing batch cooking system are possible to carry out with additional batch digesters and additional investment costs without losing the capacity of the cooking plant.

Main achieved environmental performance: Lower lignin content means fewer discharges from the bleach plant of not only organic substances but also nutrients (e.g.). One kappa unit corresponds roughly to 0.15 % lignin in the pulp. If the kappa number of pulp from cooking or oxygen delignification (see 2.1.6) can be lowered by one unit the COD released in the bleach plant would reduce by approximately 2 kg/ADt (COD from TCF-bleaching can be as high as about 3 kg COD/kappa number). However, to get a figure on the total discharge from the bleach plant one has to add the amount of pollutants which has not been washed out in the closed part of the process (see 2.3.10).

Cross media effects: Extended cooking affects several elements in the kraft process:

• the consumption of active alkali (NaOH + Na2S) may slightly increase
• the amount of dissolved substances going to the recovery system increases
• heat generation in the recovery boiler increases
• the demand for bleaching chemicals decreases
• there is a lower pollutant load in the wastewater from bleaching
• in modified batch cooking the energy consumption and blow steam amount decrease in the cooking, but the steam consumption in the evaporation of the black liquor may increase.

The impact of extended cooking on production is very site specific.

Operational experiences: Kappa number reductions of 6-7 units for softwood and 4 - 5 units for hardwood have been accomplished without loss of strength properties.

Economics: Investment costs for modification of existing conventional cooking system for extended delignification are 4-5 MEuros at the mill producing 1500 ADT/d. In order to utilise BOD- and COD-reduction, the washing efficiency should be increased as well. This would cost 2-4 MEuros more. However, with ITC there is no need to add more washing equipment if the retrofit is made at constant capacity. With continuous digester or when the recovery boiler is already operating with full capacity, the production loss can be 4-8 %. In such cases ways to accommodate for the increase in solids load are to be found as additional evaporation stages to increase the dry solids concentration in the black liquor, anthraquinone addition in pulping or adding of incremental boiler capacity.

Generally, the impact of extended cooking on production is very site specific. If the chemical recovery system is a bottleneck at a mill then introduction of extended delignification risks a loss in production due to the increased demand on that part of the system (see above).

Driving force for implementing this technique: The reduction of emissions to water is the main reason to implement this technique. A beneficial effect is also a reduction of the consumption of expensive bleaching chemicals. This reduction should be compared to the possible loss of yield and increased wood consumption, in order to judge possible net cost savings case by case.

Reference plants: Numerous plants in Europe.

Literature:
[J. Pöyry, 1997a], [J. Pöyry, 1997b], [Finnish BAT Report, 1997], [Bowen, 1990]

2.3.3 Closed screening

Description of the technique: The water system in the brown stock screening plant can be completely closed which is reality in most European mills. With modern wood handling and cooking down less than 0.5 % knots and shives are left in the pulp after cooking. The closing contributes to the reduction of organic compounds in the effluents and they are then recovered and incinerated in the recovery boiler. The idea is to bring the clean counter-currently through the fibreline, which gradually increases the dry solid content of the liquor.

Applicability and characterisation of the measure: Process integrated technique. The measure can be adopted in new and existing kraft mills.
The closing of the washing and screening may require supplementation or replacement of existing equipment with new units to reach lower wash water consumption and to have better materials to resist corrosion. In a few existing mills the capacity of the evaporation plant or the recovery boiler may need to be increased to cope with the improved closure of the washing and screening departments.

Main achieved environmental performance: The closing contributes to the significant reduction of organic compounds in the effluents. They are then recovered and incinerated in the recovery boiler. Thus, the screening plant has no discharges to water.

Cross media effects: Energy consumption increases due to increased need for evaporation.

Operational experiences: The measure has been applied since 1980s with good experiences. For instance, in Finland closed screening and brown stock washing is reality in almost all mills.

Economics: Investment costs for closed screening case are typically 4-6 Metros with new mills and 6-8 MEuros at existing mills. Operating costs are 0.3-0.5 MEuros/a for a capacity of 1500 ADT/d.
An important development step in screening is that today it is possible to screen at higher pulp consistencies than before. Consequences are lower investment costs and consumption of electrical energy.

Driving force for implementing this technique: The reduction of emissions to water is the main reason to implement the technique.

Reference plants: Numerous plants in Europe.

Literature
[J. Pöyry, 1997a], [J. Pöyry, 1997b], [SSVL 1974]

2.3.4 Oxygen delignification

Description of the technique: After cooking the fibres still contain some lignin which must be removed before final bleaching. To preserve pulp strength, the lignin must be removed in a selective way with minimum damage to the cellulosic part of the fibres and with minimum yield loss. About half of the remaining lignin in the brown stock can be removed and recovered by adding oxygen to an alkaline fibre suspension. In oxygen delignification oxygen, oxidised white liquor and magnesium sulphate is mixed with the pulp at either high consistency (25-30 %) or medium consistency (10-15 %) in a reactor. An oxygen delignification stage has also been called oxygen bleaching (oxygen stages are nowadays also used in the bleach plant and in this report oxygen delignification is used for the processing of unbleached pulp). In order to maintain the sodium balance of the mill, the oxygen stage normally uses the oxidised cooking liquor, where sodium hydroxide is the main alkaline chemical and sodium sulphide has been oxidised to thiosulphate. The delignification reactor is pressurised and the temperature is elevated to about 100OC.

The oxygen delignification takes place in one or two stages after the cooking and prior to the bleaching and can achieve a delignification efficiency of 40 to 60%. An efficiency over 40% normally requires two-stage installations. The waste liquor is sent counter-currently to the chemical recovery system. In Figure 2.9 and Figure 2.10 examples for a modern process lay out of one-stage and two-stage delignification are given.

Figure 2.9: One stage oxygen delignification

Figure 2.10: Two-stage oxygen delignification

Applicability and characterisation of the measure: The measure can be adopted in new and existing kraft mills but not in the same way and at the same costs.

The installation of oxygen delignification phase in the existing kraft mill may decreases the fibre line production, if there is not enough spare capacity in the whole recovery system.

The additional evaporator steam requirements are from 0 - 4 % for high consistency system and from 4 - 10 % for medium consistency system. The total additional solids load is about 70 kg/t for softwood and 45 kg/t with hardwood. The steam generation of the excess solids is about 1.5 - 2.5 % less than the increasing of solids load because of a lower heating value of the black liquor from oxygen stage.

Main achieved environmental performance: The major benefits of oxygen delignification are decrease of the amount of chemicals in final bleaching and total costs for bleaching chemicals and decrease of pollution load from the bleaching plant (COD and chlorinated organic compounds from final bleaching in case of ECF bleaching). Modern mills are always designed for a combination of modified cooking and oxygen delignification and for the effect on the environment (discharges of COD and AOX) both techniques have to be considered together.

Table 2.33 summarises kappa numbers currently achieved with different delignification technologies and gives a rough comparison of the effluent loads to be expected with and without extended delignification.

Table 2.33: Kappa numbers currently achieved with different delignification technologies and comparison of the calculated effluent COD without considering the washing losses

The reduction of kappa, organic substances and the consumption of chemicals in oxygen delignification are strongly related to the efficiency of washing between stages. The mentioned environmental performance is not reached without efficient washing (see 2.3.10).

Cross media effects: In energy consumption the measure means slightly increased energy recovery from dissolved organic substances but also decreased heat value of the black liquor from the inorganic compounds.
An increased recovery could contribute to increased NOx emissions from the recovery boiler.

Operational experiences: The strength properties of oxygen bleached pulp and conventionally bleached pulp are very similar although oxygen bleached pulp has lower average viscosity. No significant differences are seen in burst factor and tear factor at given breaking length.

According some mill experiences an oxygen stage before softwood bleaching sequence results in more particles and shives.

Economics: Investment cost for an oxygen delignification system is typically 35 - 40 MEuros for 1500 ADt/d bleached pulp production. Its operating costs are 2.5-3.0 MEuro/a. However, the oxygen delignification will decrease the chemical consumption in bleaching. The net effect is a cost saving which depends on the wood species. At existing mills, additional dry solids loads to recovery boiler have been reported up to 10 % and more general it is at least 4-6 % additionally, and 4-6 % more capacity would be required in recaustising and lime kiln. Should this capacity not be readily available, it normally results to a corresponding loss in production capacity of the whole mill.

Driving force for implementing this technique: The reduction of emissions to water (effluent treatment plant and recipient) is major reason to implement the method.
Reference plants: Numerous plants in Europe and in America.

Literature:
[J. Pöyry, 1997b], [Finnish BAT Report, 1997], [J. Pöyry, 1997a], [Bowen, 1990]

2.3.5 Ozone bleaching

Description of the technique: Ozone bleaching is related to the production of ECF and TCF pulps. The main purpose of using ozone is to provide more delignification power. Ozone activates the fibres towards peroxide and this results in higher brightness and lower peroxide consumption.

Ozone is generated by means of silent electrical discharges in a stream of oxygen gas. Ozone-bleaching (O3) has very high investment costs due to the high costs of ozone generators and auxiliary equipment for ozone generation. Since the ozone concentration will be only about 14-16 % in oxygen, fairly large volumes of oxygen are required. Thus, the operating cost is rather high due to a relatively high cost of oxygen (needed for ozone generation) as well as to the high power consumption. A modern ozone generator may consume 10-15 kWh/kg ozone when feeding it with oxygen.

Applicability and characterisation of the measure: Process integrated measure. The measure can be adopted in new and existing kraft mills.

Main achieved environmental performance: In ECF bleaching replacement for chlorine dioxide further reduces the discharges of AOX ("ECF light"). In TCF bleaching ozone is a common bleaching stage. In TCF mills the use of ozone and other chlorine free bleaching chemicals makes is less complicated to close up the filtrate streams from washing stages (see 2.3.8). A pressurised (PO)-stage at the end of the bleaching sequence is another option to reduce the charge of chlorine dioxide. In TCF pulp mills a PO stage is relatively frequent.

Cross media effects: No major cross media effects.

Operational experiences: Ozone with ECF bleaching plant normally results in pulp with the same papermaking properties.

Economics: Investment costs for a 1500 Adt/d ozone bleaching system are 12-15 MEuro. Corresponding operating costs are 1.8-2.1 MEuro/a.

Driving force for implementing this technique: The reduction of emissions to water (AOX) is the main reason to apply this method.

Reference plants: About 16 plants since 1992 of which 13 plants are for kraft pulp mills.

Literature:
[J. Pöyry, 1997b], [Finnish BAT report, 1997], [J. Pöyry, 1997a], [Fuhrmann, 1998]

2.3.6 ECF bleaching technique

Description of the technique: ECF bleaching (Elemental Chlorine Free) is a bleaching sequence without the use of elemental chlorine (chlorine gas, Cl2). In ECF chlorine dioxide is usually the main bleaching agent. The lignin removal by bleaching is carried out in several stages, the first two stages primarily releasing and extracting lignin and the subsequent stages stand for removing the lignin residues and finishing the product. A bleach plant consists of a sequence of separate bleaching stages with different chemicals or combination of chemicals added.

The elemental chlorine can be replaced with chlorine dioxide in the first bleaching stage, because the chlorine dioxide per chlorine atom has a fivefold oxidation power compared with chlorine and it has practically the same selective lignin removal properties. Reinforcing the alkaline extraction stages in bleaching with oxygen and/or hydrogen peroxide results in an enhanced oxidising bleaching effect, which reduces the residual lignin content of the pulp before the final chlorine dioxide bleaching stages.

Increasing the degree of chlorine dioxide substitution decreases the formation of chlorinated organic substances and eliminates the formation of dioxins, which are considered to have adverse environmental effects in the receiving waters.

The increased substitution of chlorine by chlorine dioxide requires generally modifications in the bleaching process and also expansion of the on-site chlorine dioxide plant.

Various technical solutions have been tested and some of them have been proven to be more appropriate for full-scale production. The ECF-bleaching is different for softwood and hardwood, and in existing mills the possible ECF-concept is tied with the current bleaching process. Generally, to reach a certain brightness target hardwood requires fewer chemicals than softwood, which usually means that the number of bleaching stages can be shorter. Table 2.34 is listing a number of options out of the big amount of possible variations. Examples for light ECF sequences are (DZ)(EOP)D, (DQ)(PO), D(EOP)D(PO) and these can be applied for both hardwood and softwood depending on the brightness target.

Table 2.34: Bleaching sequences in ECF softwood (SW) and hardwood (HW) kraft processes

Chlorine dioxide has the highest selectivity among technical bleaching chemicals. Bleaching with only chlorine dioxide in the first bleaching stage means that the total charge of effective chlorine has to be increased and oxygen and hydrogen peroxide are more extensively used in the extraction stages compared to a conventional C or C/D stage.

Example of the chlorine dioxide stage (D) characteristics:
Pulp consistency:10 %; Reaction time: 30 min; Temperature: 60?C; Final-pH: 3.5
Example of alkaline extraction stage characteristics reinforced with oxygen and peroxide (EOP):
Pulp consistency: 12 %; Reaction time: 60 min; Temperature: 60 - 70?C; Alkali charge: 10 - 20 kg/ADt; Oxygen charge: 3 - 6 kg/ADt; Hydrogen peroxide charge: 2 - 4 kg/ADt

Peroxide can be applied in several positions or several different ways:

• Reinforcement of a mild initial oxygen stage (low or moderate charge)
• Reinforcement of alkaline extraction stages (low charge)
• Final brightness adjustment in high density storage towers (low charge)
• Separate delignification/bleaching stage (high charge)
• Separate pressurised delignification/bleaching stage (high charge, PO). The positive impact of having a (PO)-stage in the end of the bleach plant is that ECF pulp can be produced in an existing mill without having to invest in additional chlorine dioxide capacity.

Applicability and characterisation of the measure: The measure can be adopted in new and existing kraft mills. Conversion of existing mill to ECF mill has been possible but require often considerable modifications in the fibre line and chlorine dioxide production: Chlorine dioxide generators have to be upgraded to meet the increased demand of this bleaching chemical. Existing bleach plants have to be retrofitted with different chemical mixing etc. systems. Bleaching chemical cost will increase.

Main achieved environmental performance: Eliminates 2,3,7,8-TCDD and 2,3,7,8-TCDF to non-detectable levels. However, the complete elimination of dioxins in ECF bleached effluents is a question of kappa-number and purity of ClO2. With high kappa and impure ClO2 (i.e. high concentration of Cl2) the probability of forming dioxins increase. Eliminates the priority chlorophenols proposed by the U.S. Environmental Protection Agency (EPA) for regulation to non-detectable levels. Decreases chloroform formation. Decreases chlorinated organic compound (AOX) formation to a level of 0.2-1.0 kg/ADt prior to external effluent treatment. Usually AOX levels < 0.3 kg AOX/ADT can easily be achieved by ECF bleaching.

Cross media effects: Implementation of ECF has required the pulp and paper industry to increase the use of substituting bleaching chemicals which require considerable amounts of energy in manufacturing of chlorine dioxide, oxygen and hydrogen peroxide.

Operational experiences: The production of ECF has been tested and practised in full-scale pulp lines for several years.

Economics: The investment costs for a 1500 ADt/d ECF bleaching system are 8-10 MEuros at new mills and 3-5 MEuros at existing mills. The operating costs are 10-12 MEuro/a. These costs are based on the assumption that an existing bleach plant can be used and the investment costs include then the necessary increase in chlorine dioxide production. The operating costs also contain thus the additional cost of using chlorine dioxide instead of elementary chlorine for bleaching.

Driving force for implementing this technique: The reduction of AOX emissions to water (effluent treatment plant and recipient) is the main reason to implement the method.

Reference plants: Several plants in Europe, North and South America and South Africa.

Literature:
[J. Pöyry, 1997b], [Finnish BAT report, 1997], [J. Pöyry, 1997a], [ECF, 1997], [SEPA-Report 4713-2, 1997]

2.3.7 TCF bleaching technique

Description of the technique: Totally Chlorine Free (TCF) bleaching is a bleaching process carried out without any chlorine containing chemicals. TCF bleaching has been developing rapidly, even if its application has required commonly several modifications in the pulping process. In TCF-bleaching hydrogen peroxide together with ozone (Z) or peracetic acid (PA) are the most commonly used chemicals. Provided that the pulp has a low enough kappa number after extended cooking and oxygen delignification and that transition metals (e.g. Mn2+) have been removed in the necessary chelating stages (Q-stages), it is possible to attain full market brightness with peroxide as a the sole bleaching chemical. However, the dose-response curve for brightness versus peroxide consumption is quite shallow at top brightness, which means that even small disturbances in the incoming kappa number can cause rather high bleaching costs and downgrading of the pulp because of low brightness.

One possible option of reducing the hydrogen peroxide consumption is to introduce an ozone stage into the sequence in a position before the peroxide stage (ZQP). Ozone is very efficient to reduce the amount of peroxide required to obtain even very high brightness levels. A drawback with ozone is that in larger charges it has a tendency to attack the cellulose chains.

Peracids have now become commercially available in the form of e.g. peracetic acid (PA). This bleaching chemical is a valuable complement in a stage preceding hydrogen peroxide where it can replace ozone. Full brightness can be achieved even when the unbleached pulp has a slightly higher kappa number than the very lowest. The drawback with peracetic is its still rather high cost.

Examples for different TCF bleaching sequences are listed in Table 2.35.

Table 2.35: Bleaching sequences for TCF softwood and hardwood (HW) kraft pulping

Example of Q-stage characteristics:
EDTA: 1-2 kg/ADt; pH: 5.7 - 6.2; Pulp consistency: 10 %; Reaction time: 60 min; Temperature: 90?C

Example of E-stage (alkaline extraction stage) characteristics reinforced with oxygen and peroxide (EOP):
NaOH: 10 - 20 kg/ADt; Oxygen: 3 - 6 kg/ADt; H2O2: 2 - 4 kg/ADt; pH: 11; Reaction time: 60 min; Temperature: 60 - 70?C;

Example of P-stage characteristics :
H2O2: 20-40 kg/ADt; pH: 11 - 11.5; Retention time: 4 h; Temperature: 90?C

The first TCF bleaching sequence was based on peroxide under alkaline conditions and an extensive use of hydrogen peroxide is still the main feature of all TCF bleaching sequences. Decomposition of peroxide is catalysed by certain metal ions which have to be removed in an acidic stage before the peroxide stage.

Peroxide can be applied in several positions or several different ways:

• Reinforcement of a mild initial oxygen stage (low or moderate charge): OP
• Reinforcement of alkaline extraction stages (low charge): EP
• Final brightness adjustment in high density storage towers (low charge): P
• Separate delignification/bleaching stage (high charge): P
• Separate pressurised delignification/bleaching stage (high charge): PO

Pre-treatment of the pulp with a suitable electrophilic agent before peroxide bleaching may “activate” the fibres and improve their response to peroxide. Ozone can promote this kind of effects.

Ozone has become the most common complement to peroxide in TCF bleaching sequences. The main purpose of using ozone is to provide more delignification power. Ozone activates the fibres towards hydrogen peroxide and these results in a higher brightness and a somewhat lower hydrogen peroxide consumption. On the other hand, the selectivity of ozone is poor. Excessive application, too high temperature or other unsuitable treatment may lead to serious cellulose degradation. Ozone should preferably be applied under acidic conditions (pH ?2-3). Too high temperature (>70?C) impairs the selectivity. High pressure increases the solubility of ozone in the aqueous phase during bleaching (dissolved ozone is claimed to be less aggressive to the carbohydrates than ozone in the gas phase). Pulp consistency is an important parameter in ozone bleaching. Installations of ozone bleaching are operating under medium consistency (8-15 %) or high consistency (>30 %) conditions.

If ozone is applied, a new unit of on-site chemical manufacturing is necessary because of the rapid decomposition of ozone in transportation or storage. The operating costs of TCF pulping are usually somewhat higher than those of ECF pulping.

Applicability and characterisation of the measure: The measure can be adopted in new and existing kraft mills.

In existing mills, using chelating stage a new oxygen stage and washer is usually needed to convert the ECF bleaching sequence to TCF. If hydrogen oxide or ozone stages are used, two new bleaching towers are used and reconstruction of bleaching filters. Ozone bleaching needs ozone generators and reactor. For peracetic acid one bleaching tower is needed.

In new greenfield mills, less modifications and investment costs are required but operating costs are likely to be the same order of magnitude.

Main achieved environmental performance: In TCF bleaching the formation of AOX is zero.

Cross media effects: Today there are no significant differences in chemical and energy consumption, when comparing an ECF and TCF alternative..

Operational experiences: TCF-bleaching is now a well established technology. Many mills in Europe have created a possibility of produce TCF pulp in separate campaigns instead of ECF pulp depending on the market demand. A few mills are manufacturing only TCF pulp. Slightly higher production costs and no improvement in the product quality have, however limited the demand and the share of TCF-pulps has not been increasing in the last couple of years.

Economics: The investment costs for peroxide bleaching at new mills with 1500 ADt/d production rate are 7-8 MEuros with existing pulp mills the costs are 2-5 MEuros depending on the materials of the existing bleaching equipment. If the materials tolerate hydrogen peroxide the costs are 2-3 MEuros. Operating costs with peroxide bleaching are considerably higher, 18-21 MEuro/a, than with ECF bleaching due to the higher chemical costs.

If both ozone and peroxide bleaching are applied, the investment costs are higher (see also Section 2.3.5 ozone bleaching).

Driving force for implementing this technique: The AOX emissions to the water are reduced and chloro-organic compounds are not formed in the TCF bleaching.

Reference plants: Many plants in Europe and some plants elsewhere in the world.

Literature:
[J. Pöyry, 1997b], [Finnish BAT report, 1997], [J. Pöyry, 1997a], [ECF, 1997], [SEPA-Report 4713-2, 1997]

2.3.8 Partial closure of the bleach plant

Description of the technique: There are limited discharges of pollutants to water before the bleaching stage. If the bleaching stage can be wholly or partly closed, this would result in substantial further reductions in discharges to water of organic substances, nutrients and metals.

The foremost prerequisite when closing the bleach plant, by which is here meant the recycling of the filtrates to the chemical recovery, is to reduce the volumetric flow through the bleach plant. This can be achieved by leading the liquids counter-currently from the last bleach stage through the sequence via the oxygen stage washing apparatus to the brown stock washer as shown by use of example in Figure 2.11 [Alfthan, 1996].

Figure 2.11: Principle of a possible water system in closed up bleach plants
F1 - F6 = Washing filters in the bleach plant; PR1 + PR2 = Washing presses in oxygen delignification; UW = horizontal washer as last stage of brown stock washing; the tanks provide the required storage capacity for internal waters. The water system presented in this figure has only been operated over a period of a few months. Today it is partially re-opened

As shown in Figure 2.11 to increase the closure of the bleach plant extra storage capacity for internal waters and a rebuilt of the water distribution system is necessary.

It is inevitable that dissolved organic substances and reaction products build up in the filtrate circulating around the peroxide stages. The accumulation of dissolved solids causes a considerable extra increase in the consumption of bleaching chemicals. It can even be difficult to reach full brightness at all. A further complication with counter-current washing is that pH-adjustments with sulphuric acid and caustic soda will be costly because of the considerable buffer capacity of the pulp. The sodium-sulphur balance of the mill may therefore be disrupted. The conclusion is that a really closed bleach plant is not an available technique up to now. But it is possible to carry the filtrates in two counter-current streams, one acidic and one alkaline. The alkaline water may be used for washing the pulp in the unbleached part of the process. This will result in a significant reduction of flows and discharges from the bleach plant compared to conventional systems.

It has to be considered that calcium is present in the pulp and during all oxidative bleaching a substantial amount of oxalic acid is formed. When an alkaline filtrate is used as wash water on a pulp coming from an acidic stage or is mixed with an acidic filtrate where calcium is dissolved, there is a risk that solid calcium oxalate precipitates. The tendency for calcium oxalate to precipitate becomes stronger the higher the concentrations of calcium and oxalate. In other words: the more the system is closed or the filtrates are recycled, the higher is the risk for precipitation or scaling. This problem still awaits a solution. It is hardly possible to reduce the discharge from the bleach plant further than down to 5 m3/t, the currently lowest achieved value. More research is therefore needed to find "kidneys" (e.g. ion exchange or precipitation) to purge the system either of calcium or oxalate or probably both. These kidneys are also needed to avoid the build-up of other unwanted species (e.g. so-called non-process-elements) that may impair the bleaching process, the technical equipment or the product.

Finally, increased contents of chlorides may cause corrosion in the process equipment if wastewater from the bleach plant is recycled to the closed system. Therefore, preferably TCF or ECF mills using modest amounts of chlorine dioxide try to increase the degree of closure of the bleach plant.

Applicability and characterisation of the measure: Generally, the reduction of fresh water consumption in the bleach plant can be installed in both existing or new mills. But for existing mills the investments for white water storage, piping system and the implementation of a control system for water management are relatively higher. Newer mills usually have already less water to handle because of more efficient equipment. A prerequisite for these measures is a sufficient capacity in evaporators and recovery boiler. It should be noted that the evaporation of bleach plant effluents is easier to apply in case of TCF bleaching. For safety reasons in ECF bleaching there is an elevated risk of chloride corrosion in the recovery boiler with today’s knowledge.

Main achieved environmental performance: This pollution prevention measure results in reduction in COD loads and flows as well. Typical figures for wastewater quantities from a bleach plant are 20 - 40 m3 water/ADt. 20 - 25 m3/ADt should suffice in a modern filter bleachery. It has been reported that partly closure of the bleachery may reach a volume reduction down to 10 m3/ADt and a corresponding COD discharge of about 6 kg/ADt. One mill reported a flow reduction from the bleach plant down to only 5 m3/t and a corresponding COD reduction from 30 kg COD/t down to 14 kg COD/t including a reduction of toxicity in the effluents at the same time.

Cross-media effects: The P and D bleaching stages and alkaline extraction stage as well benefits from the higher temperatures in the water system. Dissolved organic substances from the bleach plant effluents are lead via evaporation plant to the recovery boiler. This requires additional capacity and energy consumption in the evaporation plant. On the other hand, energy and space for external treatment may be saved and less excess sludge may be generated.

Operational experiences: Trials with the closing up of the water systems of a birch line in a Swedish kraft mill started already in 1993. Installations to allow for regular full scale production without bleach plant effluent have been in operation for some month. As a consequence of scaling and clogging problems (precipitation of calcium-oxalate) the water system has been partly re-opened in the first acid stage to remove the filtrate with the highest calcium concentration. After trimming of the process conditions the mill is now running a partly closed up bleach plant. A few other mills in Europe and North America have partly closed up the bleach plant.

Economics: Considerable investments have to be taken: A total rebuild of the water distribution system in the bleach plant including extra storage for internal waters is necessary. A control strategy for the water management in the plant has to be developed and implemented. Reliable data on costs were not available.

Driving force for implementing this technique: Reduction of discharges of BOD and COD. Pulp mills operating without external biological treatment were the pioneers in reducing the effluents from the bleach plant by process-internal measures.

Reference plants: Some mills in Sweden, Finland and North America

Literature:
[Annergren, 1996], [Alfthan, 1996], [Höök, 1997]

2.3.9 Collection of almost all spillages

Description of the technique: Chemical pulp mills need to carry out in-plant measures to minimise discharges of process effluents. The external effluent treatment, especially when comprising of biological treatment, can be severely upset due to accidental discharges from chemical pulping.

Pulping liquor is lost from seals on brown stock washers, pumps and valves in liquor service, from knotters and screens, sewered evaporator boil-out solutions, and other intentional liquor diversions during maintenance, start-ups and shut downs. Liquor is also lost in spills resulting from process upsets, tank overflows, mechanical breakdowns, operator errors and construction activities.

Chemical pulp mills should be designed around the following concepts:

• Collection of diverted or spilled liquor at the highest possible liquor solids concentration.
• Return of collected liquor and fibre to the process at appropriate locations
• Curbing of isolate critical process areas (including tall oil and turpentine) to avoid concentrated or harmful streams entering the external effluent treatment or contaminating clean water sewers.
• Conductivity or pH monitoring at strategic locations to detect losses and spills (spill control system which makes possible prevention of spillage and monitoring).

Of the process liquors, both the weak liquor coming from the unbleached pulp washing as well as the strong liquor produced out of it in an evaporation plant has special importance for efficient collection. These liquors cause unnecessary loading and occasionally upsets in the external effluent treatment.

The general concept for efficient recovery also requires to arrange the contaminated effluent sewers so that most of the spills, contaminated sealing waters or floor washing in key areas - pulp cooking, washing and screening, used liquor storage and evaporation, cooking liquor preparation - are collected in sumps and pumped either directly or via an intermediate tank into an appropriate liquor storage tank. The need of such recovery must, typically due to economic reasons, be limited to areas where the mixed spill concentration is at least 2-3 % dry dissolved solids. Floor washing and sealing water in these collection areas dilute the recoverable stream so care must be taken not to dilute process liquors too much.

Hot condensates from pulp cooking and liquor evaporation must also be stored prior to their reuse in the pulp production. These condensates are divided according to their cleanliness to clean, semi-contaminated and contaminated fraction. Especially the last-mentioned may find fewer uses and despite of sufficient storage volume is sewered causing some organic load and increase in total effluent temperature. Contaminated and semi-contaminated condensate bleedout is decreased simply by improved usage by replacing with them fresh water. One way, also for environmental reasons, is to feed them through steam stripping to remove reduced sulphur and volatile organic compounds and make them fairly clean hot water for a wider variety of process uses.

Inside product areas, such as tall oil plant and turpentine recovery, care shall be taken to avoid spills going to the external treatment. Soap and turpentine contain substances that may cause toxic effects in the biological treatment.

A single line kraft mill would need up to five collection sumps equipped with conductivity actuated recovery pumps. Moderately complex mills would require up to 9 and complex mills up to 12 sumps.
Besides technical aspects covered above training of staff can be a very effective way of reducing discharges of harmful substances.

Applicability and characterisation of the measure: Process integrated measure. Applicable to both new and existing mills. However, the effective spill control system is easier to install when designing and building new installation rather than to retrofit in old mills.
In existing mills the solution to efficient process stream containment lies also in the key process equipment itself. Thus building of spill containment should be done in connection with evaluation of other cost effective improvements, especially in pulp washing and screening, evaporation and liquor filtering.

Main achieved environmental performance: This pollution central measure is connected to BAT 2.3.12 “Use of sufficiently large buffer tanks etc.” The achieved environmental performance requires a combination of both BAT´s.

A general analysis has indicated that with good process management and properly designed spill containment, recovery and having 5-10 % extra capacity in the evaporation plant a reduction in effluent load of 3-8 kg COD/ADt can be reached in comparison to mills with no or inefficient spill recovery and poor process stability. In total spills can be less than 2 kg COD/ADt.

Additionally the risk of upsets in an external treatment plant is reduced, when accidental discharges with high organic and sometimes toxic load or continuously high or low pH of the incoming stream can be avoided.

Cross-media effects: In order to handle the collected spills, 5-10 % more evaporation capacity would be needed. This would also consume 5-10% more steam and power. However, collection of spills mean recovering of energy and chemicals when it is burned in the recovery boiler.

Operational experiences: Liquor spill and overflow containment in the process and establishment of relevant management policy in the chemical production has been found beneficial and necessary both on the economical and environmental point of view. This matter has been solved efficiently in many mills with fairly simple methods. The limitations to efficient implementation of these measures come from mill-specific process bottlenecks, typically in pulp washing and screening or evaporation.
Economics: The investment cost for spill liquor handling systems at a kraft mill producing 1500 ADt/d pulp mill is 0.8-1.5 MEuro. If evaporation plant needs to be expanded with 0.8 m3/ADt an additional cost of 4-6 MEuros will be required. The operating costs of the system are estimated to be 100000 - 400000 Euro/a, but can vary considerably between existing and new mills. With new mills there is generally more excess heat and the operating costs are in the lower part of the range.

Driving force for implementing this technique: Reduction of discharges of BOD and COD.

Reference plants: Numerous mills around the world.

Literature:
[Tappi Proceedings, 1996], [SEPA-Report 4713-2, 1997]

2.3.10 Efficient washing and process control

Description of the technique: The objective of brown stock washing is to separate the pulp fibres as completely as possible from dissolved organic and inorganic chemicals before the pulp leaves the pulping department thereby recovering as much as possible of the cooking chemicals and of dissolved organic substances.

The washing stage consists of a combination of successive dilution and thickening or displacement. In practice, each combination of washing equipment makes use these methods at different proportions. There is a variety of pulp washing equipment the most typical being vacuum or pressurised drum filters, fourdrinier type washers, atmospheric and pressurised diffuser washers and wash presses. Out of these, the washer press and pressurised drum or diffuser washers represent the best performance. With increased consistency of the pulp, the quantity of contaminated water going with the pulp is reduced.

As the washing is never 100% efficient a certain amount of chemicals and pollutants (carry-over) is transported with the pulp into the bleach plant where it consumes bleaching chemicals and is usually discharged to the sewer.

Applicability and characterisation of the measure: The measure can be adopted in new and existing kraft mills.

In practice, at existing mills the modifications of a washing system may be outweighed also due to practical reasons by the complete set of new washing equipment.

A closed washing system increases the importance of collection of temporary spills. The outgoing washing liquor contains both organic and inorganic substances. Washing waters withdrawn from the process are disposed through the external treatment.

Main achieved environmental performance: The washing loss after a conventional drum washing system could be around 5-8 kg COD/ADt, in comparison to 2-4 kg COD/ADt obtained in a modern washer system comprising a press washer. The remaining substances are adsorbed on or enclosed in fibres. In the latter system, the outgoing pulp consistency increases from roughly 10-15 % to 25-35 % and the water content decreases from 6-10 m3/ADt to 2-3 m3/ADt. Washing stages in series can reach up to 96-98 % recovery efficiency of black liquor.

When efficient washing takes place before an oxygen delignification stage, there will be reduction in the oxygen consumption. If there is an efficient washing system prior to the first bleaching stage, the carry-over of organics to bleaching will drop, resulting in reduced AOX, BOD5 and COD discharge to the mill sewer.
Monitoring: Standardised methods exist for measuring this carry-over which is often denoted "washing loss". Washing losses were originally measured as the sodium sulphate content in the pulp. As this loss of make-up chemical has become less important, the washing loss is nowadays normally measured as COD.

Cross media effects: The inorganics that stay in the pulp result in increased make-up chemical requirements.

Operational experiences: A basic concept is several years old. The washing technology has been developed over the years and now solutions have been tested and proven on operation.

Economics: Investment costs are typically 4-6 MEuros at new mills and approximately 2-4 MEuros for existing mills. No major additional operating costs are involved.

Driving force for implementing this technique: The reduction of emissions to water is the main reason to implement the method.

Reference plants: Numerous plants in Europe.

Literature:
[J. Pöyry, 1997a], [J. Pöyry, 1997b], [Tappi Environmental Conference, 1992],

2.3.11 Stripping of the most concentrated contaminated condensates and re-use of most condensates in the process

Description of the technique: The aim of the stripping of contaminated concentrated condensates is to reduce fresh water consumption of the mill to reduce organic pollution load to the wastewater treatment plant and to reduce TRS emissions. The stripping and reuse of condensates can reduce the COD load to the effluent treatment plant significantly.

Condensates can be classified as:

• primary condensates - live-steam condensates that are normally clean enough to be reused as boiler feed water (after polishing)
• secondary condensates - contaminated steam condensates that are flashed from black liquor, pulp suspensions, etc.

Condensates originate from the process vapours from digesters and the evaporation plant. In total about 8 - 10 m3/ADt of total condensates are formed with a COD load of about 20 - 30 kg/t and 7-10 kg/ADt of BOD5. Normally about 1 m3/ADT is heavily polluted, 4 m3/ADt medium and 4 m3/ADt low contaminated.

The COD is mainly methanol (5-10 kg/ADt) with some ethanol and a number of organic sulphuric compounds (1-2 kg/ADt TRS), 1-2 kg turpentine and inorganic nitrous compounds. Foul condensate contains furthermore ketones, terpenes, phenolics, resin and fatty acids and various dissolved gases. A large proportion of nitrogen discharged from a Kraft pulp mill is contained in condensates.

About 1 m3 of condensate per tonne of pulp has a COD concentration of 10 - 20 kg COD/m3. The level is higher in condensates from hardwood pulp than from softwood. These strong condensates are normally treated in a stripper where the removal efficiency for most compounds is over 90% depending on the pH. Stripping systems usually remove malodorous gases (TRS) and COD contributing substances at the same time. Stripped condensates after treatment can be 1 - 1.5 kg COD/m3 of condensate. The stripped gases are either incinerated in a dedicated burner with subsequent SO2 scrubbing or burnt in the lime kiln. The latter option may cause problems, which affect the capacity to absorb sulphurous compounds of the lime kiln.

About 7 - 9 m3 of weaker condensates (medium and low contaminated) are formed with COD ranging from 0.5 to 2 kg COD/m3 containing a total of about 8 - 12 kg of COD/t of pulp.

Alternatively, moderately contaminated condensates can be stripped in a system linked to the evaporation plant thereby effecting treatment without any substantial additional use of energy. In this way the total COD load before any reuse is reduced to about 5 kg/t, a reduction of about 50% compared to only treating the most contaminated condensates.

The stripping column can be a separate equipment or it can be integrated part of the evaporation plant. The condensates are fed to the top of the stripping column. Steam or vaporised condensate rises from the bottom of the column in a counter-current manner to the foul condensate. The overhead vapour from the stripping column is sent to a reflux condenser where it is partly condensed. The purpose of the reflux condenser is to condense some of the water in the gases and to increase the concentration of volatile material in the gases leaving the condenser. The non-condensable gases from the condenser contain the majority of the volatile compounds that are stripped in the stripping column. They are led to incineration where the organic and TRS compounds are destroyed by thermal oxidation.

Cleaned condensates are free of metals and therefore particularly useful for washing in the bleach plant when aiming at closing up this part of the process. They can also be reused in brown stock washing, in the causticizing area (mud washing and dilution, mud filter showers), as TRS-scrubbing liquor for lime kilns or as white liquor make-up water. This means that some condensates will be used in closed parts of the process and will not be discharged to waste. Other condensates will be used in open parts, e.g. the bleach plant, and end up in the effluent together with those condensates, which are not reused but discharged directly to waste.

Applicability and characterisation of the measure: Steam stripping is a viable in-plant treatment method for reducing COD and odour from kraft mill foul condensates. The process can be applied at both existing and new kraft mills. The condensate stripping column can be separate or it can be integrated to the evaporation plant. In the former case, live steam would be required whereas in the latter case secondary steam from evaporator effects can be used. However, thermal oxidation of the vapours from stripper system is needed. Lime kilns, power boilers and separate TRS incinerators can be used for this purpose.

Main achieved environmental performance: The best place to reuse the condensates is pulp washing either on the last washer or on the decker in a mill with a closed screen-room water system. The typical wash water demand is 10 - 13 m3/ADt. The evaporator-area and digester- area condensate available for reuse can amount to 6 - 9 m3/ADt, which is the amount of potential water savings. In total stripping of only the heavily polluted condensates would result in 4-6 kg COD/ADT while with stripping of the medium contaminated condensates about 3-5 kg COD/ADT can be achieved. However, condensates discharged to effluent treatment is mostly readily biodegradable. TRS removal is about 97 % from the condensate, methanol removal about 92%.

Cross media effects: When steam stripping is used, the non-condensable gases (NCGs) have to be incinerated separately in order to avoid release of concentrated TRS gases into the atmosphere. This has been discussed in more detail in connection with sections 2.3.18 and 2.3.19.

When stripping of concentrated contaminated condensates is applied, the load to the wastewater plant will be reduced and if there is difficulties to meet the permit new investments in the effluent treatment plant may be avoided. This also means that less energy is needed for aeration and less energy and chemicals in the sludge treatment.
When combining the recovery of clean condensates and stripped condensates, fresh water consumption may be decreased up to 6 m3/ADt. Because the condensates are hot, part of the energy used in the stripping column can be saved.

Fugitive TRS emissions from wastewater treatment plant can be reduced by steam stripping of condensates that removes TRS compounds from foul condensates.

As the stripper off-gases contain 8-12 kg/ADt methanol, there is potential to save fuel oil or natural gas, provided that the stripper gas can replace the fuel.

The stripping of condensates reduces the low level emissions of TRS compounds from foul condensates. The TRS compounds include hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide. These emissions are partially responsible for foul odours from a kraft mill.

Operational experiences: The stripping of contaminated condensate has been used many years at modern mills. When, the stripping system is used for high methanol removals, the condensates from the stripping column is relatively clean and can be re-used in the pulp mill for application s such as brown stock washing.

The basic for the design should be the minimising the flow to the stripping system by segregating the condensates to reduced investments. In the evaporation plant, the first liquor vapour condensate can be split into two fractions. Surface condenser can be split into two units or two condensing steps. The blow vapour from a batch digester can be condensed in two steps. Secondary steam can be used as the main steam source to the stripping column.

Economics: The investment costs for the stripper system at a 1500 ADt/d kraft pulp mill are about 2.0 – 2.5 MEuro. Additional investments may be required to increase the capacity of the mill’s evaporation plant, but this depends very much on the existing evaporation plant configuration. Retrofitting costs can vary between 1-4 MEuros.

The operating costs of condensate stripping consist mainly of the cost of steam used in stripping and maintenance. If the stripper is operated separate from the evaporation plant, the operating costs are significantly higher due to the demand of fresh steam. The costs are about 0.6 – 0.7 MEuros/a. If the stripper is connected between the evaporation stages, the operating costs are lower. The operating costs in the latter case are 0.3-0.4 MEuro/a.

Driving force for implementing this technique: Kraft mills may face COD discharge problem. It may have inadequately sized or operating wastewater treatment system or have new more stringent limits or a mill expansion or process modification which increase in the COD load to the wastewater treatment plant. Thus there is a need to re-use condensate in the pulp mill processes. The reuse of foul condensates without treatment has adverse effects on pulp quality.

Reference plants: Numerous mills in Europe.

Literature:
[Sebbas, 1988], [Zunich, 1993]

2.3.12 Use of sufficiently large buffer tanks for storage of concentrated or hot liquids from the process

Description of the technique: Chemical pulp mills need to carry out careful in-mill measures to minimise discharge of concentrated or hot process streams out as effluents. First, the external effluent treatment, especially when comprising of biological treatment, can be severely upset due to accidental discharges from chemical pulping. Second, some process liquor streams have economical importance due to their fuel value or the chemicals they contain.

This pollution control measure is connected to BAT 2.3.9 "Collection of almost all spillages". For spill control system please refer to this paragraph. For prevention of unnecessary loading and occasionally upsets in the external effluent treatment process cooking and recovery liquors and dirty condensates should have storage capacity exceeding normal operating volumes by at least 30 % extra. Clean streams are diverted from potential spill areas to avoid dilution of recovered process liquors.

The volumes available to control these weak and strong liquors from sulphate or sulphite production especially in start-up, shutdown or upset conditions are crucial. A basic demand for storage volumes is caused by the liquor concentration, measured as dry solids content. For instance, in an old kraft pulp mill the weak black liquor can be at 8 % and strong liquor at 60 % concentration opposed with 16 % and 75 % of a modern mill. This means that proportionally the storage volumes must be considerably larger in mills with low-efficiency washing of unbleached pulp or standard evaporation plant without liquor concentrator units.

The spare volume required above the normal condition must be able to contain peak process flows of a few hours due to operational disturbances. The spare volume must further make sure that enough weak liquor can be stored that the evaporation plant can operate normally despite of a short shutdown in cooking and washing or that a part of a multi-line or a single-line evaporation plant can be shut down for short maintenance. The storage volume must also be large enough to store enough strong liquor so that problems or short-lived production cut in recovery boiler can be solved without decreasing evaporation throughput or that the evaporation plant can be shut down for short maintenance.

Existing mills considering the implementation of oxygen delignification must assess their weak and strong liquor storage and evaporation plant capacity, because this process step is likely to increase the amount of water going through evaporation.

Liquors after the recovery boiler and with additional process steps reformed to fresh cooking liquor are free from organics but have very high pH-value. These liquor tanks equally need buffer capacity for short-lived peak flows or for instance shutdowns in connected process units such as liquor filters. These liquors, if sewered out to the effluent treatment plant cause pH shocks which, if poorly controlled before biological treatment, result in upsets.

Applicability and characterisation of the measure: Process integrated measure. Optimisation of necessary buffer storage capacity for hot or concentrated streams is applicable for both existing and new mills and is in many ways a must. These measures do not only help to maintain valuable process chemicals in the production and improve process economy, but they considerably affect the environmental performance of the mill. In existing mills the solution to efficient process stream containment lies also in the key process equipment itself. Thus building of buffer storage and spill containment should be done in junction with evaluation of other cost effective improvements, especially in pulp washing and screening, evaporation and liquor filtering.

Main achieved environmental performance: This pollution control measure is connected to BAT 2.3.9 “Collection of almost all spillages”. The achieved environmental performance requires a combination of both BAT´s.

The risk of upsets in an external treatment plant is reduced, when accidental discharges with high organic and sometimes toxic load or continuously high or low pH of the incoming stream can be avoided. The effect on hydraulic loading is not as pronounced except in mills where washing and screening water system is open.
Monitoring: Conductivity probes are suitable for detecting and evaluating mill liquor spills, because in many cases a conductivity-liquor concentration correlation for each specific stream can be established. In low-concentration streams with varying pH, an on-line pH probe is standard.
Hot streams needing specific monitoring in the sewer can be readily monitored with on-line temperature probes

Cross-media effects: Changes in liquor and hot liquid tanks and control often require changes or improvements in other equipment, especially in pulp washing or evaporation. The handling of hot liquids requires 5-10 % more energy in the evaporation plant

Operational experiences: These are many ways to solve liquor spill and overflow containment in the process and establishment of relevant management policy in the chemical. The limitations to efficient implementation of these measures come from mill-specific process bottlenecks, typically in pulp washing and screening or evaporation.

Economics: Investment costs for 1500 ADt/d pulp production about 0.8 - 1.0 MEuros for two 3000 m3 storage tanks including necessary piping, insulation and pumps with electrification and process control.

Driving force for implementing this technique: These measures are promoted by environmental and process safety reasons.

Reference plants: Numerous modern mills around the word.

2.3.13 Secondary or Biological Treatment - Aerobic Methods

Description of the technique: Secondary or biological treatment is carried out for the removal of organic matter (organic substances), which is achieved by biological degradation. Prior to the secondary treatment there is commonly a set of primary treatment stages including solids removal, neutralisation, cooling and equalisation. These primary stages aim at protecting the secondary treatment from excessive loads and shocks and overall at providing more cost-efficient purification of effluents. In most cases pulp and paper mill effluents are treated with aerobic methods. The most common aerobic treatment methods used in pulp and paper industry are aerated lagoon and activated sludge process. The former achieves less reduction of pollutants but is cheaper.

Aerated lagoon
An aerated lagoon has a large volume with residence times for the effluent within 3-20 days. The micro-organisms grow in suspension in the bulk of liquid, reaching in the lagoon relatively low solids concentration, 100-300 mg/l. The growth of microbes requires oxygen, which is provided almost exclusively by mechanical aeration equipment. Surface turbine aerators are the most common aeration units, but in deep lagoons also bottom aerators with self-induced or compressed air feed are also used. Aeration equipment provides also mixing required to keep solids in suspension and enhance microbial action.
Aerated lagoons are due to the large area and volume they require earth basins and can be constructed with or without a settling zone. In the first case the end of the lagoon is left without aeration and mixing, thus allowing solids to settle. In the latter case this settling is carried out in a separate pond. The biological process does not involve recirculation of biomass from the end to the beginning of the basin. The settled sludge is removed seldom, once in 1-10 years.

The use of aerated lagoons has recently become less common for many reasons, one important being its lower removal efficiency of effluent contaminants in comparison with the activated sludge process.
Activated sludge process
The activated sludge plant consists of two main units, the aeration basin and the secondary clarifier (sedimentation basin). In the first stage, the aeration basin, the effluent is treated with a culture of micro-organisms (the activated sludge), which is present in a high concentration. Activated sludge plants at kraft pulp mills have a retention time of about 15-48 h with the higher values in recent installations.

The sludge is separated from the water in the clarifier. The main part of the sludge is recycled to the aeration basin, which is necessary for keeping the high sludge concentration. A small part of the sludge, corresponding to the net growth, is removed from the system as the excess sludge.

Oxygen and mixing is provided to the aeration basin by mechanical aeration equipment. Various types of aerators are in use, such as surface aerators, submerged turbine aerators, fine bubble aerators and jet aerators. The three last mentioned types require compressed air from blowers or compressors.

A large number of different process and plant designs exist for the activated sludge process. These alternatives may be different regarding design of the aeration basin, the clarifier, the aeration equipment, as well as the sludge recycling. One special process design is the pure oxygen activated sludge, where pure oxygen or oxygen-enriched air is used instead of regular air.

Applicability and characterisation of the measure: End-of-pipe technique.

Aerated lagoon
The aerated lagoon can be applied at both existing and new kraft pulp mills. However, its use is in decline, mainly due to its low to moderate removal of effluent contaminants, the large land area and basin volume it requires, high energy requirement for and poor energy efficiency in aeration and mixing. In addition effluent foam and smell problems are sometimes encountered. The settled sludge removal and disposal can also generate problems. It is doubtful whether it can be still considered as BAT.

Activated sludge process
The process can be applied at both existing and new kraft pulp mills. In the existing mills some kind of water consumption reduction measures should preferably be carried out to reduce the investment costs. The activated sludge process is often used, when high or very high treatment efficiencies are required. In the latter case, however, a two-stage biological process is an optional choice.

Activated sludge plants are used widely in the pulp and paper industry. As rough estimate the activated sludge process is used in 60-75% of all the biological effluent treatment plants in this pulp and paper industry. This is also the most common process used in recently built plants.

Advantages of the activated sludge process are the potential of high or very high treatment efficiencies, the possibilities to control the process (particularly the oxygen consumption), and the relatively low space demand.
Disadvantages are the relatively high vulnerability to disturbances and consequently a risk of operational instability without any protective measures, such as an equalisation basin, high production of biological waste sludge and the high operating costs.

Alternatives to activated sludge systems exist which are more compact and less expensive. The experience from such installations is more limited but they are claimed to have equivalent reduction levels to activated sludge systems.

Main achieved environmental performance:

Aerated lagoon
Treatment efficiencies vary widely depending on the type of effluent, design of the treatment plant and operating conditions. Typical removal efficiencies are 40-85 % for BOD5, 30-60 % for COD and 20 - 45 % for AOX being in the higher end of the range after the high residence time, 15-30 days, lagoons and when the effluent temperature or the contaminants do not effectively inhibit the growth. There is no nitrogen removal and less phosphorus removal being in the range of 0 - 15 %.

The removal of solids is very case specific and in some instances the outlet effluent contains more solids than the inlet stream. The incoming solids are adequately removed in a lagoon with a settling zone, but the microbial growth produces biosolids with poor settling characteristics. The discharge of solids is lower from high residence time lagoons and settling zones.

In comparison to the activated sludge process there can be also adverse effects, such as occasional heavy mist in humid and cold weather and odour from anaerobic or anoxic basin areas.

Activated sludge process
Treatment efficiencies vary and depend on effluent type, plant design and operating conditions. Typical values are within the ranges of 85-98 % BOD5 removal and 60-85 % of COD removal.
AOX-reduction is in the range of 40-65 %, phosphorus and nitrogen is reduced by 40-85 and 20-50% respectively. The overall efficiency of TSS-removal of primary and secondary treatment is about 85-90 %.

Typical concentrations after activated sludge treatment are given in Table 2.36. At well designed and run plants the easily degradable part of the organic substances, measured as BOD5 can be brought down to about 20 - 40 mg/l and the amount of Total Suspended Solids (TSS) to about the same level. Concentrations of phosphorous and nitrogen are to a high degree correlated to TSS as the amount of dissolved nutrients can be reduced to very low levels by running the plant properly. For COD, which is a measure of total load of organic substances, the concentration after treatment depends on the content of heavily degradable matter.

Table 2.36: Typical concentrations in mg/l in effluents of kraft pulp mills after biological treatment (activated sludge plants) assuming well designed and run facilities

Cross media effects:

Aerated lagoon
The lagoon process requires large land areas, which are not always available next or inside the mill area. The lagoon treatment produces less sludge than the activated sludge process, but the sludge is difficult to dewater efficiently. Thus, the sludge disposal by burning requires typically more dewatering chemicals and support fuel per dry weight than the sludge generated by the activated sludge process.

Activated sludge process
The effluent treatment plant produces sludge which after dewatering can be burned, providing in some cases net positive heat value. The treated wastewater is clean enough for reuse in some points of the production process. The problem is that usually the wastewater treatment plant is located so far from the mill that recycling is not economical.
Operational experiences:

Aerated lagoon
Aerated lagoons have been used for a long time in many pulp and paper mills to reach medium level removal of effluent contaminants. However, currently many of the existing lagoons in the industry have been closed down or retrofitted into a high-efficiency activated sludge process or supplementary effluent treatment basins.

Activated sludge process
Activated sludge plants have been used for many years in all types of kraft mills with good results.

Aerated lagoon
The cost of this process is very dependent on where and how the aerated lagoon can be built. If it can be built by reclaiming a bay in the receiving waters and closing its open side with an earth damn the cost is substantially lower than when the lagoon must be built on dry land.
Thus the investment costs vary a lot, being in the range of 16- 20 MEuros for a 1500 ADT/d kraft pulp mill. This cost covers also the primary treatment and sludge dewatering. The corresponding operating costs are 1.3-1.7 MEuros/a, comprising mostly of the cost of electrical power required for aeration and mixing in the lagoon.

Activated sludge process
The investment costs for a completely new activated sludge treatment plant are approximately 19-24 MEuros for a kraft pulp mill with 1500 ADt/d production, being at the lower end of the cost range for an unbleached kraft pulp mill. These costs include also the necessary primary treatment and sludge handling. The corresponding operating costs are 2.0-2.6 MEuros/a.

Driving force for implementing this technique: The aerated lagoon can be used, when low to medium level removal of organics, contained by the effluent, is sufficient. The activated sludge process is preferably used, when high or very high treatment efficiencies are required.
Reference plants: Numerous plants all over the world for all types of effluents.

Literature:
[J. Pöyry, 1997b], [Finnish BAT Report, 1997], [SEPA-Report 4713-2, 1997]

2.3.14 Tertiary treatment of wastewater with chemical precipitation

Description of the technique: In some cases it is necessary to extend the effluent treatment with a tertiary treatment. In most cases the tertiary treatment is simply chemical precipitation. The dissolved organic substances are separated by precipitation and subsequent filtration or clarification.

The chemicals used for precipitation are usually the following:

- aluminium salts - Al2(SO4)3 and Aln(OH)mCl3n-m
- ferric salts (Fe(III)) - FeCl3 and Fe2(SO4)3
- ferro salts (Fe(II)) - FeSO4
- lime

To optimise the flocculation, polyelectrolytes are used in the mixing phase.

Applicability and characterisation of the measure: Applicable to both and existing mills.

Main achieved environmental performance: Tertiary treatment with chemical precipitation is mainly applied to reduce nutrient especially phosphorus. Results from Swedish pilot plant studies on biological treatment combined with chemical precipitation have shown that the following reduction rates can be achieved: Phosphorus 80-90%, nitrogen 30-60%, COD 80-90%, AOX 80-90 % [PARCOM, 1994].

Cross media effects: The precipitation of organic material in the effluent with inorganic chemicals results in a great quantity of a slimy sludge, very difficult to dewater and dump as landfill. The chemical cost is substantial and the purification is selective; neutral substances cannot be captured as efficiently as dissociated ions.

Operational experiences: No specific problems are known which are linked to the application to this technique.

Economics: The investment costs amount to 2.6 MEuros for a kraft pulp mill with a capacity of 250000t/a and 3.8 MEuros for a mill with 500000 t/a capacity respectively. The investment costs for chemical precipitation includes equalisation tank, chemical dissolving equipment, chemical dosing equipment, precipitation and flocculation unit and tertiary clarifier. Operating costs amount to about 50000 Euros being somewhat lower for the mill with less capacity.

Driving force for implementing this technique: Usually tertiary treatment of wastewater is only regarded as necessary when the nutrient concentrations in the effluent have to be lowered i.e. if the mill discharges to very sensitive recipients.

Reference plants: Biological treatment plus chemical precipitation of wastewater from the manufacturing of bleached kraft pulp is used on commercial scale e.g. in Sweden (Skoghall, integrated kraft and CTMP pulp mill.

Literature:
[PARCOM, 1994], [SEPA-Report 4713-2, 1997]

2.3.15 Increase in the dry solids content of black liquor

Description of the technique: In the recovery boiler the inorganic substances are reduced and separated as a smelt (mainly as Na2S and Na2CO3) from the bottom and the organic substance oxidised and thereby generating heat. In a conventional recovery boiler there is an oxidising zone in the upper part and a reducing part in the lower part. The strong black liquor is introduced through one or several liquor nozzles into the reducing zone (see Figure 2.3). Combustion air is usually supplied at three different levels as primary secondary and tertiary air (from the bottom up-wards).

Emissions from the recovery boiler consist mainly of particulates, nitrogen oxides and sulphur dioxide. The emission levels are kept as low as possible by optimising the combustion parameters such as temperature, air supply, black liquor dry solids content and the chemical balance.
The aim of enhanced evaporation is to achieve as high content of dry solids (DS) as possible in the strong black liquor. After a conventional evaporation the DS content in the strong black liquor is about 65 %. By installing a superconcentrator, DS content up to 80 % can be achieved. However, the achievable DS-content depends on the wood species. A target for optimal dry solid content of thick liquor in a balanced mill could be 72-73% after evaporation but measured before the recovery boiler mixer.

Applicability and characterisation of the measure: Process integrated technique. The process can be applied at both new and existing kraft mills. A superconcentrator can be implemented as a separate phase also to existing evaporation plants.
However, the maximum DS content is limited by the increase of the viscosity and scaling tendency of the strong black liquor. This depends on the wood species and temperature. In practise, with eucalyptus and some other hardwood species it is difficult to achieve higher than 70 % DS content.

Main achieved environmental performance: The sulphur emission from the recovery boiler is down to 5-50 mg S/Nm3 or 0.1-0.3 kg S/ADt of pulp or sometimes almost down to zero because more sodium will vaporise and react with sulphur.

Cross media effects: The reduction of sulphur emissions by high DS-content increases the emissions of particulates prior to flue gas cleaning. To compensate this, more efficient and expensive electrostatic precipitator has to be installed.

At high dry solid content (TS >80%) there is a considerable release of sulphur compounds from the last evaporator stage, which have to be collected and incinerated.

Increasing dry solids content of black liquor have the tendency to increase NOx emissions from the recovery boiler if no counter-measure is taken (see 2.3.22).

Operational experiences : This measure has been tested in several pulp mills in Northern Europe. It is in operation in full scale in Northern and Southern Europe.

Economics: In existing mills the cost of improved evaporation and concentration of strong black liquor is tied to the target concentration. At existing mills with 1500 ADt/d kraft pulp production the investment costs for increase in black liquor concentration from 63 % upward are as follows:

Concentration from 63 % to 70 %, 1.7-2.0 MEuro
Concentration from 63 % to 75 %, 3.5-4.0 MEuro
Concentration from 63 % to 80 %, 8.0-9.0 MEuro

The operating costs of the improvements are not significant because of the increase in energy economy (this being 1-7 %) and gain in recovery boiler capacity. The increase of dry solids into the recovery boiler may even result in some net savings.

Driving force for implementing this technique: Kraft mills may face sulphur dioxide emission problem and these emissions in the recovery boiler can be reduced by increasing the dry solids content of black liquor. Another case by case achievable result is the increase of the capacity of the recovery boiler (4-7 %).

Alternatively, flue gas scrubbers can be installed for the same purpose (see 2.3.16).

Reference plants: Numerous plants in Scandinavia and at least one in Spain.

Literature
[Finnish BAT Report, 1997]

2.3.16 Installation of scrubbers on the recovery boiler

This measure can be applied alternatively to the BAT under 2.3.15.

Description of the technique: In order to decrease the emissions of sulphur dioxide from the recovery boiler, it can be equipped with a flue gas scrubber. A kraft recovery boiler scrubber of the wet type may include three process stages (from the bottom up, see Figure 2.12).

Figure 2.12: Flue gas scrubber for recovery boilers
[SEPA-Report 4008, 1992]

Chloride is absorbed by cold water introduced in the flue gas inlet. The chloride efficiency is normally 60-70%. In the washing zone, SO2 and particulates are removed. Scrubbing takes places at a pH of 6-7. The pH value is controlled with addition of sodium hydroxide, weak liquor or oxidised white liquor. SO2 reacts with the scrubbing liquor and Na2SO3 and also some Na2SO4 is formed. TRS in the form of H2S can be removed together with SO2. However, to remove hydrogen sulphide from the flue gases, a high pH of the scrubbing liquor would be required. At such a high pH, also carbon dioxide would be absorbed, which is unrealistic due to the relatively large amounts of carbon dioxide being formed in the combustion.
Surplus liquor from the scrubber is recycled to the process, normally to the white liquor preparation.

Applicability and characterisation of the measure: Installation of a scrubber is preferably done at the same time as a new boiler is installed, although at much higher cost also existing boilers can be equipped with scrubbers. Recovery boilers burning high dry solids black liquor normally give rise to low sulphur emission which makes the installation of scrubber less interesting.

Main achieved environmental performance: The removal efficiency for SO2 is typically > 90%. A scrubber on the recovery boiler can reduce the sulphur emissions from 0.5 - 2 kg/ADt down to 0.1 - 0.3 kg S/ADt or concentrations from 50-200 mg/Nm3 down to 10 - 50 mg/Nm3.

Monitoring of emissions: Continuous SO2 measurement prior to and after the scrubber is needed to control the operation of the scrubber.

Cross media effects: By introduction of fresh water in top of the scrubber, hot water can be produced (if there is a need). The water is normally clean enough to be used as wash water in the bleach plant. The scrubber needs alkali in the form of oxidised white liquor, weak liquor or sodium hydroxide, which can increase the capacity demands on the recovery department.

Operational experiences: Scrubbers on recovery boilers can be operated without problems.

Economics: The equipment comes usually as a package from the supplier. The investment costs for a bleached kraft mill with a production capacity of 250000 and 500000 t/a amount to 7.2 MEuros and 10.4 MEuros respectively. They include scrubber, scrubber liquor pumps, circulation pumps, electrification and instrumentation. Operating cost amount to 580000 Euro/a and 920000 Euros respectively.

Driving force for implementing this technique: Reduction of SO2 emissions. Heat recovery. With high dry solids the primary SO2 emission can be substantially reduced and for such mills the driving force is small. The internal energy situation of the mill might or might not motivate warm water production in the scrubber. In a modern kraft recovery boiler, especially if it operates on high dry solids, the H2S is normally not a problem that needs scrubbing to resolve.

Reference plants: Scrubbers have been installed on numerous recovery boilers in the last decades.

Literature:
[SEPA-Report 4713-2, 1997], [SEPA-Report 4008, 1992]

2.3.17 Collection of weak gases for incineration in recovery boiler

Description of the technique: Control of TRS emissions can be divided to treatment of concentrated malodorous non-condensable gases (NCGs) which contain about 4 kg TRS/t (measured as S) and diluted or lean malodorous gases which contain about 0.5 kg TRS/t (measured as S). The treatment of concentrated NCGs is generally carried out by collection NCGs from the cooking and evaporation departments (see 2.3.11) and their disposal by incineration. There are different options available.

The incineration of concentrated malodorous gases in the recovery boiler is on possible option., There are a few mills in Europe and about 4 mills in the world burning strong malodorous gases in the recovery boiler.

High volume and low concentration gases are formed in black and white liquor handling, pulp washing and floor channels with black or white liquor residues. The actual composition will vary greatly case by case.

The collection is carried out with gas pipelines and blowers for gas transfer. The collected lean malodorous gases can be incinerated as secondary or tertiary air of the recovery boiler. Depending on the volume of diluted NCGs and the lay out of the pulp mill there can be several TRS-destruction systems for different departments. An alternative for incineration is alkaline scrubber or oxidising scrubber. The recovery boiler is able to destroy diluted malodorous gases. However, the recovery boiler is only one alternative for incineration of diluted gases.

Normal amount of diluted NCGs at a 1000 tonnes/d mill is about 50000 - 100000 m3 /h. The amount of the gases depends on the mill concept; with continuous cooking and diffuser washing the volumes are lower than with batch cooking and pressure washers.

Applicability and characterisation of the measure: The measure can be adopted in new and existing kraft mills.

In the existing pulp mills may be very difficult to retrofit a collection and treatment of diluted NCGs. This is due to the limitations of the lay out and long distances between sources of malodorous gases and the recovery boiler.

Main achieved environmental performance: The TRS emissions of the high volume (low concentration) gases can be reduced almost totally by collecting and burning.

Cross media effects: No cross-media effects.

Operational experiences: The methods have been used at reference mills several years without problems.

The tertiary air flow rate into a recovery boiler is limited and other burning alternatives may be needed, as well.

Economics: The investment costs for the weak malodorous gas collection and their disposal in the recovery boiler are 3.6-4.5 MEuros for a kraft mill with 1500 ADt/d production. However, because much of the costs are for piping the costs can be considerably higher at mills, which spread on a large area. The operating costs for the system are 0.3-0.5 MEuro/a.

Driving force for implementing this technique: The more efficient reduction of TRS emissions of the kraft mill is the main reason to implement this technique.

Reference plants: Numerous pulp mills.

Literature:
[TAPPI, Proceedings, 1997], [TAPPI, Proceedings, 1994], [J. Pöyry, 1997a], [J. Pöyry, 1997b], [SEPA-Report 4713-2]

2.3.18 Collection and incineration of odorous gases (strong and weak gases) in the lime kiln

Description of the technique: Control of malodorous gas caused primarily by Total Reduced Sulphur (TRS) emissions can be divided to treatment of concentrated non-condensable gases (NCG) and diluted or lean malodorous gases. The treatment of concentrated NCGs is generally carried out by collection and incineration of NCGs from cooking and evaporation departments.
Incineration of concentrated NCG can be carried out in the lime kiln or in a separate NCG-incinerator equipped with an SO2-scrubber. The concentrated NCGs contain over 90 % of all TRS-compounds generated in the cooking of pulp.

High concentrated and low volume gases are formed in turpentine recovery system, continuous digester flash steam condensers, foul condensate storage tanks, evaporator non-condensable gas relief and hotwells, and in batch cooking blow heat recovery system instead of continuous digester flash steam condensates. The actual composition will vary greatly from case by case.

The main sources of lean malodorous gases are washing and screening equipment of unbleached pulp, several tanks of pulp and washing liquor in the washing and screening, storage tanks of black liquor in the evaporation plant and storage tanks of white liquor in the recausticising plant.

The collection is carried out with gas pipelines, ejectors and blowers for gas transfer. The collected lean malodorous gases can be incinerated as secondary air of lime kiln or in a separate NCG-incinerator, in a bark boiler or other auxiliary boiler or as secondary or tertiary air of the recovery boiler. Depending on the volume of diluted NCGs and the lay out of the pulp mill there can be several TRS-destruction systems for different departments.
Applicability and characterisation of the measure: The measures can be adopted in new and existing kraft mills. In the existing pulp mills it may be difficult to retrofit a collection and treatment of diluted NCGs.

Main achieved environmental performance: The TRS emissions of the kraft mill can be reduced over 90 % by only collecting and burning the concentrated TRS-compounds.

Cross media effects: The advantage of burning the malodorous gas in the lime kiln is that no extra furnace is needed. In addition, the sulphur in the gas can be absorbed in the lime, which decrease the emission of sulphur dioxide. However, only a limited amount of sulphur can be absorbed in the lime kiln by gaseous sodium forming sodium sulphate. The main sulphur absorbing compound is thus the sodium carbonate (Na2CO3) in the lime mud. When this capacity is exhausted, SO2 is released. This effect is enhanced when malodorous non-condensable gases are incinerated in a kiln. Therefore, SO2 emissions are usually a clear function of the amount of malodorous gas flow. To minimise the formation of SO2 either the sulphur content in the fuel can be reduced or if malodorous non-condensable gases (NCGs) are to be burnt in the lime kiln, sulphur compounds can be scrubbed out of these gases prior to burning in the lime kiln.

TRS-control can reduce also the malodorous components released in the wastewater treatment.

An average 10-15 % of the fuel used in a lime kiln can be replaced by the heat value of the concentrated malodorous gases. However, the variation of the amount of energy of the gas may make it difficult to hold a lime of good and uniform quality. Condensation of methanol after the stripper column can minimise the problem with varying gas quality, but require additional investment costs.
Operational experiences: The measure is widely used. Some problems have appeared in the modern lime kiln because of using low excess oxygen level.

Economics: Investment costs of collection and incineration of both strong and weak gases are typically 4-5 MEuros at new mills and 5-8 MEuros at existing mills with a capacity of 1500 ADt/d. No major increase in operating cost, if the heat value of recovered methanol can be utilised. Otherwise, an increase of 0.3-0.5 MEuros/a is anticipated.

Driving force for implementing this technique: The reduction of TRS emissions of the kraft mill is a major reason to implement this technique

Reference plants: Numerous plants in Europe and North America.

Literature:
[TAPPI, Proceedings, 1997]

2.3.19 Collection and incineration of odorous gases (strong and weak gases) by use of a separate furnace equipped with scrubbers for SO2

Description of the technique: Incineration of odorous gases (see 2.3.17 and 2.3.18) can also be carried out in a separate NCG incinerator equipped with a SO2 scrubber. A separate furnace can take care of the heat value in a boiler.

Applicability and characterisation of the measure: The measure can be adopted in new and existing kraft mills.

In the existing pulp mills may be difficult to retrofit a collection and treatment of diluted NCGs.
Main achieved environmental performance: The TRS emissions of the kraft mill can be reduced over 90 % by only collecting and burning the concentrated NCGs.

Cross media effects: Incineration of odorous gases in a separate furnace has the tendency to increase NOx emissions if no counter-measure is taken.

Operational experiences: The technology is used at many mills several years without problems.

Economics: Investment costs are typically 7-8 MEuros at new mills and 8-11 MEuros at existing mills with 1500 ADt/d production capacity. Operating costs usually increase with 0.3-0.5 MEuros/a because the heat value of recovered methanol cannot be utilised.

Driving force for implementing this technique: The reduction of TRS emissions of the kraft mill is a major reason to implement this technique.

Reference plants: Numerous plants in Europe.

Literature:
[TAPPI, Proceedings 1997]

2.3.20 Installation of low NOx technology in auxiliary boilers (bark, oil, coal) and the lime kiln

Description of the technique: In chemical pulp mills, a variety of regenerative or fossil fuels - bark, coal, lignite, oil or natural gas - may be used for supplemental steam production, typically coupled with turbines for electric power production. In burning of these fuels environmentally sound incineration techniques are called for to minimise particulate, SO2 and also NOx emissions.

Low NOx technology applied to burning of solid fuels and pulp and paper mill wastes with fluidised bed boilers is discussed in 6.3.12. Coal and lignite suit well to be burned as major or support fuel in fluidised bed systems, which by careful operation control promotes low NOx formation.

In conventional oil or natural gas fuelled boilers, the burners feeding the fuel-air mixture, must apply designs that maintain low NOx burning conditions. Also coal or peat is often burned as finely ground dust in conventional boilers, fed through burners that with proper designs provide low NOx burning.

The primary burning air is brought through the burner in the fuel-air mix. Secondary and tertiary air is fed in separately to maintain an appropriate primary:secondary:tertiary air balance in the flame area to maintain low NOx combustion. Some air may still be fed, if necessary above the main flame area to complete the fuel combustion.

The purpose of the multi-phase air feed is to burn the fuel without excess air and actually even under reducing conditions, meaning that

• there is not enough oxygen to promote strong NOx formation.
• the flame temperature is lower than in conventional burners which further decreases NOx formation.

Part of the NOx formed will reduce back to elementary nitrogen for instance when a residual amount of the fuel is burned in the outer flame area or outside it.
Applicability and characterisation of the measure: Low NOx burners can be used both in existing and new boilers.
When powdered solid fuels, such as coal or peat, are used it is important that the if they have high humidity they are pre-dried to support fast and efficient burning. Additionally they require that the burning air is preheated to assure quick ignition and complete burning.

Main achieved environmental performance: Generally, emissions vary with the fuel. In comparison to conventional burners with 250-500 mg/MJ NOx emissions the low NOx burners can reach 120-140 mg/MJ level in stack emissions.

Monitoring of emissions: Emission monitoring with online NOx meters can be carried out. Also oxygen meters can help to determine that low NOx burning conditions are maintained. For accurate measurements in-field sampling and lab analysis is required.

Cross-media effects: No major effects.

Operational experiences: Low NOx burners have been applied successfully in the retrofit of existing boilers and construction of new ones.

Economics: The investment costs are typically 0.5 - 0.8 MEuros. No major increase in the operating costs is anticipated.

Driving force for implementing this technique: Low NOx burners are mainly used to reduce NOx emissions from auxiliary boilers.

Reference plants: Numerous mills in Northern and Western Europe.

Literature:
[Rentz, 1996], [J. Pöyry, 1997 b], [Finnish BAT Report, 1997], [Ministry of Education, 1994]

2.3.21 SNCR on bark boilers

Description of the technique: Due to the low combustion temperature bark boilers give relatively low NOx emissions. Emissions are typically 70-100 mg NOx/MJ when only bark is fired. At times when oil is used in the bark boiler an increase of NOx to about 100 - 150 mg NOx/MJ can be determined. Excess oxygen effects the NOx formation and should therefore be avoided. Too low excess oxygen increase however the risk for emissions of CO and VOC.

Primary NO is formed in furnaces either through reaction with nitrogen in air (thermal NO) or through oxidation of nitrogen in fuel (fuel NO). Formation of thermal NO increases with increasing temperature of the flame. A part of the NO is further oxidised to NO2.

In the SNCR process, NO is reduced by urea to nitrogen, carbon dioxide and water according to the conceptual reaction 2 NO + (NH2)2CO + 1/2 O2 ? 2 N2 + CO2 + 2 H2O.

The reaction occurs around 1000oC.

Applicability and characterisation of the measure: Equipment to inject urea (or ammonia) can be installed in both existing and new boilers. The optimal reaction conditions can be difficult to obtain in existing boilers, thus reducing the potential NOx reduction to about 40 %.

Main achieved environmental performance: The total NOx-reduction achievable in a bark boiler is about 30-50% by making changing in the combustions techniques (see 2.3.20) and/or by applying an SCNR process. The NOx emissions would then amount to 40-60 mg/MJ equal to about 100-200 mg/Nm3. Emissions of gaseous sulphur are low or about 10-20 mg/MJ when burning bark.

Monitoring of emissions: Continuous NOx measurement can be installed and experience shows reliable results.

Cross media effects: Depending on the stochiometry the urea is added a slight increase of ammonia (slip) may be determined.

Operational experiences: Installations of the technique have been in operation since the early 1990ies. Good availability is normally reported, but a number of incidents have occurred where the injection of urea solution has caused damages inside the boiler. As this is both, a safety risk and a cost installations have to be made with case and the operation properly monitored.

SNCR technique is difficult to control because of relatively fast changes of load might happen in bark boilers. This results in variations in NOx reductions achieved by these techniques.

The process can be a potential source of emission of N2O or NH3 but measurements demonstrate the risk to be marginal. At least in Sweden SNCR on bark boiler is an established technique.

Economics: The investment costs for adding SNCR to the bark boiler for a bleached kraft mill with a production capacity of 250000 and 500000 t/a amount to 690000 Euros and 1.15 MEuros respectively. The investment costs include injection equipment, pipes, pumps, tanks and rebuild/adoption of the boiler. The operating costs are mainly urea. About 1-2 kg urea is required per kg NOx removed.

Driving force for implementing this technique: NOx has an acidifying potential and may increase eutrophication. In some sensitive lake areas in Europe a further reduction of NOx emissions by secondary measures as SNCR technique is therefore regarded as necessary. A fee on NOx emissions in Sweden may also give an incentive for further NOx reduction.

Reference plants: Some mills in Sweden

Literature:
[SEPA-Report 4008], [SEPA-Report 4713-2, 1997], [Personal information from a Swedish mill]

2.3.22 Over Fire Air Technique (OFA) on recovery boilers

Description of the technique: The kraft recovery boiler operates with a reducing atmosphere in the bottom. Accordingly the NOx formation in the recovery boiler is lower than that in other furnaces. However, modifications to the air feed system have proved successful with respect to NOx reductions. Thermal NOx by fixation of nitrogen in the combustion air can be reduced by limiting the amount of air in the combustion zone. In purpose designed systems air injection ports are installed at quartiary level. A reduced NOx formation can be achieved in a kraft recovery boiler through modifying the air feed system such as introducing a fourth air inlet in the upper part of the boiler. The reduction of NOx emissions attributable to the use of this technique is variable, dependent on the boiler type and design and the method of OFA application, and will normally be 10-25%.

Applicability and characterisation of the measure: Applicable to both and existing mills.

Main achieved environmental performance: The achieved NOx-reduction seems to be different from recovery boiler to recovery boiler. In some Swedish kraft pulp mills the following experiences have been reported:
Case 1: Installation and use of OFA-technique on an existing recovery boiler and operation since 1990: 30% NOx-reduction achieved.
Case 2: Installation of the OFA technique on an existing recovery boiler. The new air feed system is not any more used because of the increase of temperature in the overheater.
Case 3: Installation of the OFA technique on an existing recovery boiler in 1995: 20% NOx-reduction achieved and in operation since the beginning of 1997.
Case 4: First new recovery boiler with OFA technique in 1996: The reduction of NOx emissions of the latter are summarised in the table below:

Table 2.37: Reduction of NOx emissions by use of over fire air technique in a new recovery boiler

Cross-media effect: No cross-media effects occur.

Operational experiences: The reduction of NOx emissions attributable to the use of this technique is variable, dependent on the boiler type and design and the method of OFA application. It has to be adapted to the specific conditions of recovery boilers. The application of this technique - which is widely used in other combustion processes - may result in increases in carbon monoxide and unburned carbon emissions if not well controlled.

Economics: The investment costs for modifying the air introduction to the recovery for a bleached kraft mill with a production capacity of 250000 and 500000 t/a amount to 1.7 MEuros and 2.3 MEuros respectively. The investment costs include new air inlets to the recovery boiler, instrumentation, pipes and fans. There is no change in operating costs.

Driving force for implementing the technique: NOx has an acidifying potential and may increase eutrophication. In some sensitive lake areas in Europe a further reduction of NOx emissions by secondary measures is therefore regarded as necessary.

Reference plants: A few mills in Sweden

Literature:
[SEPA-Report 4713-2, 1997]

2.3.23 Installation of improved washing of lime mud in recausticizing

Description of the technique: Lime (CaO) is used to causticize green liquor (Na2S + Na2CO3) into white liquor (Na2S + NaOH). After causticizing, lime mud (CaCO3) is formed. Normally lime mud is recycled in a lime kiln, where lime mud is burned and new lime is created. Before the lime is sent to the kiln, it must be washed in order to remove residual sodium hydroxide, sodium sulphide and other sodium salts, and then dewatered.

The equipment used for lime mud washing is usually either clarifiers or press filters. In the past, two-stage mud washers were in widespread use but more recently single-stage mud washing in a unit-type clarifier with storage or in a pressure filter has become dominant.

Improved lime mud washing can reduce the residual content of white liquor in the mud from 100 mg/dm3 to 0-30 mg/dm3 in modern filters. The lime mud dryness can also be increased from 50-60 % to 70-80 %. More efficient washing reduces the concentration of sulphide in the lime mud, thus reducing the formation of hydrogen sulphide in the lime kiln during the reburning process.

Applicability and characterisation of the measure: Applicable to both and existing mills.

Main achieved environmental performance: Possible reduction of H2S (TRS) in the lime kiln, which depends mainly on the availability of sodium in the lime and the sulphur content of all fuels fed to the lime kiln. At the lowest sulphur input a small reduction can be achieved but with higher sulphur inputs the effect can be non-existent or detrimental.

Cross-media effects: If washed to a too low sodium content, the emissions of TRS and also particulate emissions from the lime kiln tend to increase.

Operational experiences: Improved washing of lime mud has been practised over 10 years at pulp mills in Europe. Monitoring of residual sodium (NaOH) is required to avoid damming of the lime kiln.

Economics: Investment costs are typically 1 -1.5 MEuros.

Driving force for implementing this technique: Reduction of H2S (TRS) and odours from the flue gases of the lime kiln.

Reference plants: Numerous plants in Europe.

Literature
[SEPA-Report 4713-2, 1997]

2.3.24 Electrostatic precipitator for dust reduction in bark boiler and lime kiln

Description of the technique:
Bark boiler
Wood residues (from bark and wood waste) are burnt to a large extent for steam production. Steam from boilers is often used for power generation through back-pressure turbines. This heat and power generation are necessary to reduce emissions of fossil fuels. With incineration, fewer wastes are produced for disposal. Nowadays, a new incineration plant always requires a sophisticated incineration system combined with advanced flue gas cleaning system.

The main emission from waste wood boilers is particulate matter. The particles consist of ash and a residue of unburned material. Normally bark boilers have cyclones for dust collection (85% efficiency). Today, electrostatic precipitators with cleaning efficiency above 95% are also used more and more.

Lime kiln
A proper design of the lime kiln will minimise the dust formation. The sodium vaporisation mechanism is related to the amount of sodium in the kiln and the high temperature of calcination section of the kiln. The extent of vaporisation can up to a certain level be controlled by a proper adjustment of flame shape and position.

The use of different fuels also effects the dust emission. An oil flame at the hot end of the kiln will, due to its good radiation properties, give a high extent of sodium vaporisation from the lime producing sodium sulphate with sulphur dioxide. In order to minimise CaO, Na2SO4 and Na2CO3 particles, an electrostatic precipitator can be installed. The electrostatic precipitator has better dust removal properties than a scrubber. If there also are scrubbing facilities, the electrostatic precipitator should be situated prior to the scrubber. The electrostatic precipitator requires regular maintenance and monitoring. Overloading or uneven loading may cause clogging of the chambers in the precipitator.

Applicability and characterisation of the measure: The measure can be adopted in new and existing kraft mills.

Main achieved environmental performance: The main achieved environmental performance with bark boilers is reduction of particulates from a level of 250-500 mg/m3n to a level of 100-150 mg/m3n when using cyclones. An electrostatic precipitator for the flue gas from bark boiler can achieve cleaning efficiency above 95% corresponding to dust emissions of about 20-40 mg/Nm3 (at 10% O2 and dry gas)

The dust emissions from lime kiln will be 20 - 100 mg/m3n after the electrostatic precipitator.

The major part of dust leaving the kiln with the flue gas is CaO. It mostly escapes from the feed end of the kiln. The amount of dust coming from the hot end of the kiln is significantly lower. The main components of the dust emission of the stack are fine Na2SO4 and Na2CO3 particles, because CaO particles are more efficiently captured in flue gas cleaning equipment

Cross media effects: No major effects.

Operational experiences: The measure has been applied in a number of pulp mills.

Economics: In a 1500 ADt/d kraft mill, the investment costs for an electrostatic precipitator at the bark boiler are about 3-4 MEuros and at the lime kiln 5-6 MEuros. The operating costs are increased with less than 0.3 MEuros/a in both cases.

Driving force for implementing this technique: The reduction of dust emissions to air is major reason to implement the method.

Reference plants: Numerous plants in Europe.

Literature:
[J. Pöyry, 1997a], [J. Pöyry, 1997b], [SEPA-Report 4713-2, 1997]