EFFECTS OF NON PROCESS ELEMENTS IN THE CHEMICAL RECOVERY SYSTEM OF A KRAFT PULP MILL AND INTEGRATED PULP AND PAPER MILLS ORIGINATING FROM THE
INCINERATION OF BIOLOGICAL SLUDGE IN THE RECOVERY BOILER
Presented at Vitória 2006, 9-12 April 2006, Brazil
Effluent treatment of pulp and paper mills generates a biological sludge that according to EU-directives from 2005 has to be taken care of by incineration or by production of soil for
agriculture and/or ground cover. An alternative way to take care of the sludge is evaporation and incineration in the recovery boiler.
The biological sludge, however, contains non process elements, NPE, which increase the risks of incrustation in the recovery system and increases the need of make up lime.
The effect of evaporation and incineration of biological sludge in the recovery boiler has been investigated for a pulp mill that has used this method for two years. Experience shows that
the system works very well and the mill has not seen any negative effects from the increased intake of NPEs.
Integrated pulp and paper mills generate biological sludge which will contain higher amounts of NPEs. The prospects of evaporating and incinerating biological sludge in the recovery boiler
of integrated pulp and paper mills have been investigated. Analysis of biological sludge from three different pulp and paper mills were used to simulate the effects of NPE when incinerating the biological sludge in
the recovery boiler. The results of the simulations show that the method is also applicable for integrated pulp and paper mills, with the drawback that two of the investigated mills required addition of magnesium to
the black liquor.
Biological sludge, evaporation, incineration, NPE, scaling
The increased requirements for environmentally friendly pulp and paper mills have brought about an increasing installation of effluent treatment plants. Micro organisms are used for
biological decomposition of organic material in the effluent. The micro organism growth produces a surplus of biological sludge which must be removed from the system. According to EU-directives, from 2005 the
surplus sludge has to be taken care of by incineration or by production of soil for agriculture and/ground cover.
The surplus sludge production has been minimised by process developments, but there is still a large surplus that has to be taken care of. Possible methods are:
- Landfill, no longer allowed according to EU-directives;
- Composting to produce industrial soil;
- Dewatering and incineration in the bark fuel boiler;
- Evaporation and incineration in the recovery boiler.
Composting incurs costs for transport and handling of the sludge. Dewatering and incineration in the bark fuel boiler creates handling difficulties and the low dry solids content means that
the sludge incineration consumes heat in the boiler. Many pulp mills also shut down the bark fuel boiler in summer time.
The technology to evaporate the biological sludge and incinerate it in the recovery boiler has been in operation for a couple of years in two Finnish pulp and paper mills and in three
Swedish pulp mills. This method has the advantage that the sludge is handled in a sealed system and that the combustion of the sludge gives a positive heat input to the boiler. The increased load on the evaporation
and recovery boiler is marginal, about 0.5%.
The disadvantage of incinerating the biological sludge in the recovery boiler is the introduction of non process elements, NPE, into the liquor cycle. By NPE is meant all elements that are
not a part of the lignin or carbohydrates and do not have any function during the cooking or in the recovery system. NPE may cause scaling in the evaporation plant and increased dead load in the lime cycle,
increasing the limestone make-up.
INCINERATION OF BIOLOGICAL SLUDGE IN THE RECOVERY BOILER OF SKUTSKÄR MILL
Different alternatives for disposing of the biological sludge were discussed during the definition study for a new effluent treatment plant in the Skutskär mill in 1999. The alternative with
incineration in the bark fuel boiler was not attractive since the mill has a steam surplus in summer time. A continued incineration in the bark boiler would also have required substantial investment in modernisation
of the equipment. The alternative with incineration in the recovery boiler seemed very promising in this situation.
The effluent treatment plant in Skutskär was taken into operation in October 2000. The low loaded activated sludge plant comprises pre-sedimentation, anoxic zone (for chlorate removal),
aerated selector, aerated basin and two secondary sedimentation basins. The incoming water to the anoxic basin is cooled in a cooling tower. The extraction of biological sludge to evaporation and incineration in the
recovery boiler was started in August 2001. The biological sludge is mainly micro organisms. It is difficult to dewater and may cause deposits on the evaporator surfaces. The cell walls of the micro organism need to
be hydrolysed (decomposed) with alkali at 80 – 90oC for about an hour to facilitate the evaporation.
The sludge for incineration is pumped to a buffer tank where polymers are added. The sludge is then pumped to a centrifuge and concentrated to 7 – 10% dryness. The sludge falls from
the centrifuge to a hydrolysis tank where it is mixed with intermediate black liquor. The mixture of hydrolysed sludge and black liquor is pumped to the evaporation plant after about an hour reaction time. The
concentration of various NPE in the sludge is shown in Table 1. The last column shows how much of the total NPE input to the mill that comes from the recycled biological sludge.
Table 1. The content of NPEs in the biological sludge and the input of NPEs per ADt and also as a part of the total input to the recovery system. The quantity of biological sludge to the
recovery system is for Skutskär 4.5 kg DS/ADt.
OPERATIONAL EXPERIENCE OF THE INCINERATION OF BIOLOGICAL SLUDGE IN THE RECOVERY BOILER
The biological sludge to the chemical recovery was about 4.5 kg DS/ADt during the test period (1). The evaporation and incineration in the recovery boiler increase the evaporation
load with about 0.06 t/ADt and the load on the recovery boiler with 0.3%.
The operational experience from the system for hydrolysis of the biological sludge and evaporation/incineration in the recovery boiler is very good. The handling of the sludge takes
place in a sealed system that demands little supervision and maintenance. Overall, the mill has not seen any negative effects that can be explained by the increased intake of NPEs to the chemical recovery system.
Aluminium can led to troublesome scaling of sodium-aluminium-silicates on the heat surfaces in the evaporation plant. Effective elimination of aluminium by the green liquor dreg is
obtained with the double salt hydrotalcite if the quotient Mg/Al is kept higher than 4-5 in the black liquor. In the Skutskär mill MgSO4 is used in the oxygen stage and introduces enough
Mg for effective precipitation.
The need for make-up lime is increased due to the build-up of phosphorus in the lime. Depending on the level of make-up lime the requirements will increase 2-5 kg/ADt. If a higher
level of phosphorus is accepted, instead of increasing the lime make-up, the running costs will be somewhat higher, roughly € 0.1 per ADt due to increased ballast.
The NOx in the flue gases from the recovery boiler has not increased since the start-up of incineration of biological sludge in the recovery boiler. A possible explanation may be that
the nitrogen in the biological sludge exists in a state that gives less formation of NOx compared to nitrogen in the black liquor (2, 3, 4).
For the other NPE as silicon, calcium, chloride, iron, manganese, potassium and copper the contribution from the sludge is low. It will not give any problems for the chemical recovery
system at the Skutskär mill.
SIMULATION OF EFFECTS WHEN INCINERATING BIOLOGICAL SLUDGE IN THE RECOVERY BOILER OF INTEGRATED PULP AND PAPER MILLS
The situation in integrated pulp and paper mills is lightly different due to additional input of NPE from chemicals and recycled paper used in the paper mill.
To evaluate the possibilities of incinerating biological sludge in the recovery boiler in integrated pulp and paper mills, we have made simulations of NPE for a couple of integrated
mills in Sweden (5).
The simulation comprises the following NPE: aluminium, silicon, phosphorus, chloride, potassium, manganese, magnesium, iron, copper, barium and calcium. The simulation program
WinGEMS 5.0 (Pacific Simulation) was used in the study to predict the levels of NPE in the recovery system of the kraft mills.
Three different integrated pulp and paper mills were selected for the study, Billerus Skärblacka, ASSIDomän Frövi and SCA Obbola. The mills are different concerning raw
materials, process, degree of own produced pulp and paper products. All three mills have effluent treatment and use the method of activated sludge that generates a surplus of
biological sludge. The sludge composition shown in Table 2 is an example of sludge from an integrated pulp and paper mill.
Table 2. The content of NPEs in the biological sludge, the input of NPEs per ADt and as a part of the total input to the recovery system. The quantity of biological sludge surplus for
Skärblacka is 4.5 kg DS/ADt.
Billerus Skärblacka AB produces bleached and unbleached draft pulp and semi-chemical NSSC. The pulp is used in four paper machines which produce sack paper, MG-paper and fluting.
Bleached pulp which is not used for paper making is dried and sold as market pulp.
Frövi is an integrated pulp and board mill producing packaging and liquid board. The board's top and bottom plies are made of bleached and unbleached kraft pulp, with an intermediate
ply of purchased CTMP. The bleached pulp is both softwood and hardwood TCF.
Obbola produces linerboard from kraft and recycled pulp. About 60% of the furnish is unbleached kraft pulp from the pulp mill.
The results of the study proved that for Skärblacka mill the method is applicable.
For Obbola the prerequisites are somewhat more complicated; the effluent treatment generates a considerably higher amount of sludge per ton pulp produced due to the
treatment technique and use of waste paper (40%) for the paper machine. The high input of aluminium with the sludge will give troublesome scaling of sodium-aluminium-silicate on the
heat surfaces in the evaporation plant. To eliminate the aluminium from the system magnesium can be added to the black liquor which will precipitate as the double salt
hydrotalcite in the green liquor and be rejected by the green liquor dregs. For Frövi mill the situation is somewhat more favourable compared to Obbola but a smaller amount of
magnesium still has to be added to avoid incrustation of sodium-aluminium-silicate in the evaporation plant.
The need of make-up lime will increase due to the content of phosphorus in the biological sludge which otherwise will build up in the lime. The increased need of make-up lime was
estimated to be 2 kg/ADt for Skärblacka and Frövi while Obbola will need about 2-3 kg/ADt.
Experience from mills that employ incineration of biological sludge in the recovery boiler shows that NOx in the flue gas has not increased. For Skärblacka and Frövi the increase of
nitrogen by the biological sludge will be moderate and will probably not give any increased formation of NOx in the flue gas. For Obbola the amount of nitrogen by the sludge will be
higher which increases the risks of having higher levels of NOx in the flue gas.
The other NPEs as chlorides, potassium, calcium, manganese, iron, copper, barium and calcium will not give any problems for the recovery system.
For pulp mills the method of evaporation and incineration of the biological sludge in the recovery boiler gives the possibility of handling the sludge in a sealed system which is easy
to operate. The method is also applicable to integrated pulp and paper mills. Disadvantages are a somewhat increased need of make-up lime and also for some mills the necessity of
adding magnesium to the recovery system, depending on the process and which type of product is produced.
The method should be compared with the benefits and drawbacks of other methods that are available. The costs of the methods must be calculated based on the actual mill.
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liquor, Journal of Pulp and Paper Science, 27 (2001) 3, 75-81.
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, Language Swedish.
Author's contact details:
ÅF-Process, PO Box 8309, S-104 20 Stockholm, Sweden