Energy consumption in the pulp and paper industry - Model mills 2010 Integrated fine paper mill

Transcription

1 REPORT Date Version No 1 Energy consumption in the pulp and paper industry - Model mills 2010 Integrated fine paper mill ÅF-ENGINEERING AB Market Area Forest Industry ÅForsk Reference: ÅF-Engineering AB Frösundaleden 2, SE Stockholm, Sweden. Phone Fax VAT No SE Registered office Stockholm. paper/fine paper final.docx

2 Contents Page 1 INTRODUCTION 5 2 MODEL MILL - OVERVIEW General Design Criteria Mill production and capacity Energy systems and balances 9 3 MODEL MILL PROCESS DESCRIPTION Wood Supply Woodyard Digester Brownstock deknotting and screening Oxygen delignification Pulp washing Bleaching System closure and degree of bleach plant filtrate recovery Chlorine dioxide generation Evaporation Handling of condensates Handling of non-condensable gases Tall oil recovery Recovery boiler Causticizing Lime kiln Paper Mill Capacity Stock preparation Bleached kraft supply Broke system Mixing/machine chest Filler supply Short circulation Paper machine Fresh water system White water system and buffer volumes Energy aspects of the paper machine Power boiler Steam turbines and steam distribution Cooling and recovery of low-temperature heat Effluent treatment Spill handling system Water supply and treatment 36 4 MODEL MILL - ENERGY BALANCE 38

3 5 COMPARISON OF MODEL MILL AND TYPICAL MILL Type mill process description Digester Oxygen stage Pulp washing Bleaching Paper machine Evaporation Recovery boiler Lime kiln Power boiler Steam turbines and steam distribution Energy balance comparison Model mill vs type mill 48 6 REFERENCES 53

4 Appendices Appendix 1 Model mill - Mass balance block diagrams Appendix 2 Model mill - Energy balances Appendix 3 Model mill Secondary heat balance

5 Integrated fine paper mill Page 5 1 Introduction The purpose of this ÅForsk financed study is to update the hypothetical reference mills developed in the 2005 FRAM project to reflect the technical changes that have occurred in recent years. The main emphasis in this study is on the technical changes which have affected energy consumption and production. Four different types of pulp and paper mills are considered: Bleached market kraft pulp mills one softwood mill (pine), and two hardwood mills (birch and eucalyptus) Integrated fine paper mill, with the pulp mill producing softwood and hardwood pulp in campaigns Kraftliner mill Magazine paper mill, bleached super calendered (SC) TMP There was no eucalyptus kraft pulp mill in the FRAM project, but such a mill has been included in this study. Each of the reference mills from the FRAM project has been reviewed by ÅF. The kraft pulp mills have also been reviewed by Innventia. Modifications made in this study are based on ÅF/Innventia experience with existing mills, and in some cases data from the major mill equipment suppliers. Material and energy balances have been calculated for the 2010 model mill using spreadsheet models developed by ÅF. The FRAM project also included type mills which represented typical, existing Nordic mills. To help highlight potential energy improvements in existing mills, the type mill is included here for comparison to the model mill. The type mills in this study are identical to the type mills from the FRAM project, In the FRAM project the energy consumption of the type mills considered data from a survey of energy consumption and production in the Swedish pulp and paper industry which was conducted in This survey was updated in 2007, and indicated that the main development in the existing Nordic pulp and paper mills between 2000 and 2007 has been an increase in cogenerated power, and an increase in biofuel usage.

6 Integrated fine paper mill Page 6 2 Model mill - overview 2.1 General Design Criteria The integrated fine paper mill produces softwood and hardwood pulp in campaigns in the pulp mill. The pulp mill is similar to the bleached kraft market pulp mills in this study except for the dryer and paper machine parts. The mill design is based on best available and commercially proven technology in the Nordic countries. The design of the mill considers: high, consistent paper quality which is competitived on the international market the product is elemental chlorine free (ECF) low specific consumptions of wood, chemicals and water high energy efficiency maximized production of bio-energy, and minimal usage of fossil fuels low environmental emissions; on the level of newer modern mills cost-effective solutions Different suppliers offer different process equipment. The model mill is not based on equipment from any one supplier. In general the key process data used in the balances in this study are conservative and should not exclude any of the major pulp mill equipment suppliers. 2.2 Mill production and capacity The pulp mill produces bleached softwood and hardwood pulp in compaigns. The needed capacities of the various departments in the pulp mill are different when producing softwood and hardwood pulp. The main difference between hardwood and softwood is that hardwood has a higher yield. This means that the black liquor dry solids per ton of pulp is higher for softwood than for hardwood, and consequently the required capacity of the chemical recovery line

7 Integrated fine paper mill Page 7 is greater for a softwood mill compared to a hardwood mill with the same pulp produciton capacity. The pulp mill has a maximum continuous rate (MCR) of 2000 ADt/d for softwood and 2500 ADt/d for hardwood. At these production rates the load on the recovery boiler is approximately constant. Mass balances have been prepared for the pulp mill at mill MCR conditions to determine the capacity requirements for the main mill areas. The balances cover mainly wood/fibre-, dry solids-, evaporation-, causticizing and lime. Block diagrams which summarize the mass balances for both softwood and hardwood operation are included in Appendix 1. Table 2-1 summarizes the key operating and dimensioning data and for the pulp mill. The fine paper mill has two paper machines with the same design and production. The total production is 3130 t/d at pulp mill MCR. The paper machine furnish consists of 19% bleached softwood pulp, 56% bleached hardwood pulp and 25% filler. The paper is surface sized with starch to improve strength properties of the paper. Table 2-2 summarizes the key operating data for the paper machines.

8 Integrated fine paper mill Page 8 Table 2-1. Summary of pulp mill key operating data. Softwood Birch Pulp production ADt/24 h Wood yard Wood to digester t/24 h Bark and wood waste t/24 h Digester Plant Cont Cont Kappa number Unscreened deknotted digester yield % Alkali charge on wood as effective alkali NaOH,% 20, Sulphidity (white liquor) mole-% Oxygen Stage Kappa number after oxygen stage Alkali charge as NaOH kg/adt Oxygen charge kg/adt Washing Department Dilution factor in the last stage m 3 /ADt unbl Evaporation Plant Weak black liquor to evaporation, excl.spill t/h ditto dry solids content % ,7 Strong black liquor, dry solids content incl. ash % Total evaporation, including spill t/h Recovery Boiler Estimated higher heating value of virgin DS MJ/kg ,8 Strong liquor virgin solids to mixing tank t/24 h Net useful heat from liquor, virgin solids MJ/kg DS Net useful heat from liquor MW Causticizing and Lime Kiln Causticizing efficiency mole-% Total white liquor production m 3 /24 h Lime kiln load t/24 h Active CaO in lime % Lime kiln fuel Bark / wood waste

9 Integrated fine paper mill Page 9 Table 2-2. Summary of paper mill key operating data. Speed at pope m/min Width on pope M 9 Grammage g/m 2 80 (75-160) Production on pope (100% eff.) t/h 73.1 Paper dryness % 93 PM furnish composition -Hard wood % 56 -Soft wood % 19 -Filler % 25 -Surface size of paper (starch) % 3 Paper mill efficiency % 82 Operating days per year Days 355 Paper production net (PM1 + PM2), Kraft mill MCR t/day Paper production net (PM1 + PM2) t/a Bleached hardwood consumption ADt/a Bleached softwood consumption ADt/a Filler consumption t 100 /a Starch consumption t 100 /a Energy systems and balances The fine paper model mill is very energy efficient and the black liquor alone produces enough steam to satisfy the process steam consumption of the mill during softwood campaigns. During hardwood campaigns the steam from the recovery boiler is not sufficient for the mill s requirement, and additional steam from the power boiler is required. The lime kiln is fired with bark powder, or gasified bark, and the remaining bark from the woodyard and chip screening is burned in the power boiler. When all available falling bark is burned in the power boiler there is an excess of steam which is utilized in a condensing turbine to produce in power. In both softwood and hardwood campaigns the power produced is still not sufficient to meet the mill s demand, and additional power is purchased.

10 Integrated fine paper mill Page 10 Some key items which have been changed in the model mill compared to the reference mill in the FRAM study include: HP steam data 100 bar(g), 505 o C (increased from 80 bar(g) and 490 o C in the FRAM project) Feed water preheating to 175 o C to increase HP steam generation (increased from 146 o C in the FRAM project) Recovery boiler flue gas cooler to reduce LP steam consumed in air preheating Top preheating of all recovery boiler combustion air to 205 o C (85% of combustion air heated to 165 o C in the FRAM project) Latest technology for pulp digesting which has a lower cooking temperature than other systems 7 effect evaporation plant (6 effect evaporation plant in the FRAM project) Digester steam consumption has increased increased slightly with the new liquor extraction Steam consumption in the bleach plant is reduced; more chlorine dioxide and less hydrogen peroxide allow a lower bleaching temperature Dryness the papermachine press section to the dryer has been increased from 50% in the FRAM project to about 52%, based on mill experience Paper machine power consumption has been reduced from 600 kwh/t to 550 kwh/t, based on mill experience A net reduction in mill steam demand compared to the FRAM study makes a condensing turbine a feasible option. Additional factors (which were also relevant in the FRAM project) which make the model mill energy efficient include: Recovery boiler sootblowing steam is extracted at 25 bar(g) from the turbine instead of using HP steam Low pressure steam used in the paper,machine Pressurized condensate system High temperature of hot water, o C, and maximum use of hot water instead of steam in the bleach plant, and paper machine Bark press for bark to the power boiler Table 2-4 compares the overall steam and power balances for the 2010 model mill and the FRAM reference mill.

11 Integrated fine paper mill Page 11 Table 2-3. Summary of steam and power balances FRAM reference. Softwood Hardwood STEAM BALANCE GJ/ADt pulp GJ/t paper GJ/ADt pulp GJ/t paper Generation Recovery boiler Power boiler Secondary heat Total steam generation Consumption Process steam Back pressure turbine Condensing turbine Total steam consumption POWER BALANCE kwh/adt pulp kwh/t paper kwh/adt pulp kwh/t paper Generation Back pressure power Condensing power Purchased power Total power generation Consumption Total power consumption

12 Integrated fine paper mill Page 12 Table 2-4. Summary of steam and power balances- Model mills Softwood Hardwood STEAM BALANCE GJ/ADt pulp GJ/t paper GJ/ADt pulp GJ/t paper Generation Recovery boiler Power boiler Secondary heat Total steam generation Consumption Process steam Back pressure turbine Condensing turbine Total steam consumption POWER BALANCE kwh/adt pulp kwh/t paper kwh/adt pulp kwh/t paper Generation Back pressure power Condensing power Purchased power Total power generation Consumption Total power consumption

13 Integrated fine paper mill Page 13 3 Model mill process description 3.1 Wood Supply The softwood raw material consists of 50% pine (Pinus sylvestris) and 50% spruce (Picea abies). The relation between roundwood with bark and sawmill chips is 70% roundwood and 30% sawmill chips. The birch is mainly Betula spp. with about 10% other hardwoods, mainly aspen. The supply is 100% as roundwood, with bark. 3.2 Woodyard The debarking is performed in dry debarking drums which are designed for a barking efficiency of 95%. There is a closed re-circulation of sprinkling and deicing water. The de-icing water is heated by the means of heat exchanging with surplus hot water. The effluent is collected together with the press water from the bark presses in a sedimentation basin for re-circulation. The sludge from the sedimentation basin is burned in the power boiler. A portion of the bark is utilized as fuel for the lime kiln; the rest is burned in the power boiler. After debarking the logs are transported to a metal detector and a water stone trap. In the chipper, logs are cut to chips. Consistent chip thickness is important for uniform cooking and a low pins fraction is important for the runnability of the digester. The chips are therefore screened to get an optimal chip size. Accepted chips are transported to a chip silo. Over-thick, over-sized chips are taken to a re-chipper and then back to chip screening. Fines are stored and burned in the power boiler. 3.3 Digester Either continuous or batch digesting can be used, and both alternatives have pros and cons. Continuous digesters are the dominant technology for both existing and new mills. Also, in general the batch processes, as marketed today have higher steam consumption than the continuous processes. Thus the continuous cooking process has been selected for this study. The Metso Compact Cooking concept, see Figure 3-1, is one example of a modern cooking system. Chips are presteamed and impregnated with white

14 Integrated fine paper mill Page 14 liquor and black liquor at atmospheric conditions in a vessel, and the cooking is performed at a relatively high alkalinity with co-current liquor flow at relatively low temperature. The cooking temperature is about 143 o C for softwood, and 138 o C for hardwood. Black liquor is extracted for evaporation via a single stage flash tank from the impregnation vessel, the transfer circulation between the impregnation vessel and the digester. Andritz DownFlow LoSolids cooking system without or with pressurized impregnation vessel is another example of a modern cooking system. Figure 3-1. Example continuous cooking system (Metso Compact Cooking). Table 3-1. Digester key figures. SW HW Kappa number, digester blowline Deknotted digester yield % White liquor AA concentration NaOH, g/l Alkali charge on wood as effective alkali NaOH,% Sulphidity, white liquor % Extracted turpentine kg/adtdig 2 0 In order to improve yield and fibre strength the kappa number after cooking could be increased by some units. This should however be balanced with the delignification in the oxygen stage.

15 Integrated fine paper mill Page Brownstock deknotting and screening There are several important quality parameters for pulp. One of them is very high cleanliness, i.e. a low content of shives and coloured spots originating from the pulpwood (resin and bark) as well as foreign materials such as sand, plastic, rubber and rust. Pressurized deknotting separates knots from the pulp. After deknotting the pulp is screened at 3-4% consistency by barrier (slotted) screens in three or four stages. The knots are recooked. Screen rejects from the last screening stage end up as effluent treatment sludge which is burned in the power boiler. 3.5 Oxygen delignification Oxygen delignification is done in two stages without intermediate washing to a kappa number of 12 for softwood and 10 for hardwood. Oxidized white liquor is the primary alkali source. To optimise the delignification in the initial and final phases the reaction time is approximately 10 minutes in the 1 st stage and approximately 60 minutes in the 2 nd stage. Table 3-2. Oxygen delignification key figures. SW HW Kappa number after oxygen stage Dissolved DS (yield losses) % MgSO4 charge 2,3 1.0 Alkali charge oxidised WL, as NaOH kg/adt O Oxygen charge kg/adt O Temperature ºC 95/98 95/ Pulp washing The brown stock wash consists of: Two stages of pre-oxygen washing for hardwood and three stages for softwood. Either wash presses or drum displacement (DD) filters can be used. Post oxygen washing with one 2-stage DD washer before the oxygen bleached storage tower. Alternatively two wash presses could be used. These wash presses may both be placed after the oxygen bleached

16 Integrated fine paper mill Page 16 storage tower or, one of the presses could be before the tower and one after (pre bleach press). Figure 3-2 shows one alternative for brownstock washing. The brownstock washing dilution factor is 2.5 m 3 /ADt. The carryover of COD from the oxygen delignification to the bleach plant is calculated to be approx 5 kg COD/ADt, excluding the bleach plant filtrate recirculated to brown stock washing. Figure 3-2. One example of a typical brownstock washing system (Metso). 3.7 Bleaching Both the softwood and birch pulps are bleached to a final brightness of 90% ISO. The bleach plant is designed with four bleaching stages. For softwood pulp the first stage is operated as a conventional D-stage, and the sequence is D(EPO)DP. For hardwood pulp the first stage is operated as a D hot -stage, and the sequence is D hot (EPO)DP. Wash presses are used for all washing in the bleach plant. The main reasons for selecting a hot first D stage for hardwood pulp are that a lower charge of ClO 2 is required to attain the required pulp brightness and less brightness reversion of the fully bleached pulp. These benefits are, however, not attained on softwood pulps as they contain considerably less hexenuronic acids than hardwood pulps. Hexenuronic acids are effectively removed in D hot -stages.

17 Integrated fine paper mill Page 17 The last bleaching stage could be a D-stage instead of a P-stage. This is partly an economic decision which depends on the prices of chlorine dioxide, hydrogen peroxide and sodium hydroxide. A final P-stage in place of a final D- stage may also decrease brightness reversion of the pulp. The expected bleach plant chemical charges and conditions are summarized in Table 3-3, and Table 3-4. Table 3-3. Expected chemical charges for the SW kraft pulp with the sequence D(EPO)DP to 90%ISO brightness ( kg/adt). ClO 2 as ClO 2 and not as active Cl. Kappa number of pulp to bleaching: 12. Temp (C) Time SO 2 or (min) ph ClO 2 O 2 H 2O 2 NaOH H 2SO 4 NaHSO 3 as SO 2 Stage D ~2,5 9 4 (EPO) D P ~ (a) (a) After P-stage Table 3-4. Expected chemical charges for the birch kraft pulp with the sequence D hot (EPO)DP to 90%ISO brightness ( kg/adt). ClO 2 as ClO 2 and not as active Cl. Kappa number of pulp to bleaching: 10. Temp (C) Time SO 2 or (min) ph ClO 2 O 2 H 2O 2 NaOH H 2SO 4 NaHSO 3 as SO 2 Stage D hot ~3 7 6 (EPO) D P ~ (a) (a) After P-stage For softwood the bleaching sequence results in a yield of 98%, which corresponds to a total yield of about 44%. For hardwood the bleaching sequence results in a yield of about 97.5% and a total yield of about 49% System closure and degree of bleach plant filtrate recovery A high degree of system closure can create problems with scale formation within the bleach and evaporation plants, high bleaching chemical consumption, corrosion and plugging problems in the recovery boiler and problems controlling the Na/S balance of the mill. Bleach plant liquors must be handled in an optimal manner; for example mixing should be performed within critical temperature and ph limits, where the risk for scaling is the lowest.

18 Integrated fine paper mill Page 18 Based on experience a relatively conservative approach regarding system closure has been adopted to ensure sustained trouble free operation with good economics. The bleach plant is designed to release t/adt of effluent. This range includes an allowance for up to 5 t/adt of fresh water. This extra intake of fresh water can be used for dilution at any position in the bleach plant where there is a risk for precipitation. The extra intake of fresh water also makes it possible to bleed out metals and Cl - -ions. Additionally, 6 t/adt of effluent is discharged from the paper machines. Figure 3-2 shows the approximate liquor flows in the bleach plant. Hot water is used as wash liquor on the wash press after the P-stage. The filtrate from this wash press is then used as wash liquor on the 2 nd D stage wash press. Fresh water is used as wash liquor on the (EPO) stage wash press and condensate is used as wash liquor on the 1 st D-stage press. The filtrate from the (EPO) wash press is then transferred as wash liquor to the 2 nd wash press after the oxygen stage. Hot/cold water ~5 t/adt Clean condensate Hot water 4.1 t/adt 4.1 t/adt Chemicals ~2 t/adt Hot water xx t/adt HD D EPO D P O 2 To 1 st O2 washer To treatment To treatment 4.5 t/adt ~10t/ADt ~5 t/adt Figure 3-3. The approximate liquor flows (t/adt) of the ECF bleach plant. The dilution factor is about 2 t/adt. 3.8 Chlorine dioxide generation The selection of the chlorine dioxide process (R8 or R10) is mainly based on the millwide sodium/sulphur balance. (R8 and R10 are the trade names from Erco. Eka (Akzo Nobel) has similar processes called SVP.)

19 Integrated fine paper mill Page 19 In both the R8 and R10 processes purchased sodium chlorate reacts with sulphuric acid, with methanol as the reducing agent, to produce chlorine dioxide and the by-product Na 3 H(SO 4 ) 2. The R10 process however has an additional step where Na 3 H(SO 4 ) 2 is split into Na 2 SO 4 and H 2 SO 4, and the H 2 SO 4 is returned to the ClO 2 generation process. In the softwood mill the R8 process is selected. The (Na 3 H(SO 4 ) 2 ) by product, is used to partially replace sulphuric acid used for soap splitting. Since there is no soap splitting and an excess of sulphur in the hardwood mill the R10 process is selected to minimize the amount of excess sulphur (which is purged as recovery boiler precipitator ash). 3.9 Evaporation The evaporation plant is a conventional 7-effect system utilising LP and MP steam (Figure 3-4). It is designed to produce 80% dry solids liquor (including recovery boiler ash). All evaporator bodies are of the falling-film type, and the seven effects are designed to operate in counter-current fashion, i.e. with live steam being fed to the first effect and weak liquor to the seventh effect. Tanks are installed for weak, intermediate and strong black liquors well as for soap, spills and condensates. The firing liquor storage tank is pressurized due to the high dry solids content. Ash mixing is done before the first effect. Firing liquor at 80% DS is produced in the first effect. The first effect is divided into three bodies connected in series on the liquor side. Washing of the first effect is done one body at a time using weak black liquor. To avoid upsets in firing liquor concentration when washing, and to facilitate ash mixing, a strong black liquor storage tank is placed after the second effect. The body operating with the strongest liquor in the first effect is heated by intermediate pressure steam from a steam ejector driven by MP steam and compressing LP steam. The other two bodies in the first effect are heated by LP steam only. Sludge from the biological treatment is mixed into the black liquor in an intermediate storage tank.

20 Integrated fine paper mill Page Handling of condensates A stripping system for foul condensates from the digester and evaporation systems is included. The stripping column is integrated within the evaporation plant to reduce the live steam consumption (Figure 3-4). A methanol rectification column with turpentine decanter and foul methanol liquid storage is also included. The methanol is incinerated in the recovery boiler. Evaporation condensates are divided depending on contamination degree and distributed to different consumers within the mill. Approximately 4.5 m 3 /ADt of the cleanest condensate (approximately 200 mg/l COD; 80 o C) is used as wash liquor in the bleach plant. Approximately 3.5 m 3 /ADt intermediate condensate (approximately 1000 mg/l COD, 65 o C) is used in the causticizing plant. The remaining condensate is also clean condensate, and is discharged as effluent. The surface condenser is designed for a warm water temperature of 50 C and to give condensate separation in principle as for the evaporators. Figure 3-4. Evaporation plant including condensate stripper Handling of non-condensable gases Non-condensable gases (NCGs) are collected throughout the mill. Both strong gases and weak gases are burned in the recovery boiler. In mills which have a large excess of sulphur, an alternative is to incinerate the gases in a dedicated boiler.

21 Integrated fine paper mill Page Tall oil recovery Soap that separates in the weak and intermediate black liquor tanks is decanted to a soap decanter and then led to a separate storage tank. The soap is then pumped to the tall oil plant, where it is upgraded to crude tall oil. The amount of soap depends on the wood used for pulping. With 50% pine and 50% spruce, the tall oil production is assumed to be 35 kg/adt for softwood. There is no soap from hardwood. The most common type of tall oil plant uses sulphuric acid for soap splitting, and sulphur is thus added to the recovery cycle. Mills that use an R8 process for chlorine dioxide generation can use the sodium sesquisulphate (Na 3 H(SO 4 ) 2 ) by-product to partly replace H 2 SO 4 that would otherwise have been used for soap splitting. Some mills use carbon dioxide to pre-treat the soap. The product after this pretreatment is a mixture of soap and tall oil (soap oil), and a water phase containing sodium bicarbonate. The water phase is separated from the soap oil, and then the soap oil is treated with sulphuric acid as in a traditional tall oil plant. Pre-treatment with carbon dioxide however reduces the sulphuric acid requirement by about 40% Recovery boiler The optimum recovery boiler steam pressure and temperature is not the same in different regions. In Sweden the majority of existing recovery boilers were designed when electricity prices were low. These boilers were in general designed for 60 bar steam pressure and corresponding temperatures. At higher steam temperatures more expensive metallurgy is required for the superheater, which means a sharp increase in investment and maintenance costs. Higher steam pressure and temperature cannot be economically justified with low electricity prices. In contrast, Finland, for example, has had higher electricity prices, and the majority of recovery boilers operate at bar. Newer boilers are often designed for greater than 100 bar pressure to maximize power generation. With increasing electricity prices three new recovery boilers in Sweden have been designed for higher steam pressure and temperature. In this study the recovery boiler is designed to produce high pressure steam at 100 bar(g) and 505C.

22 Integrated fine paper mill Page 22 Some of the key factors in the recovery boiler design which are related to maximizing power generation include: Feedwater preheating which increases steam generation and consequently the power generation. One drawback of feedwater preheating is increased flue gas temperature after the economiser which increases the flue gas loss and increases the cost of the precipitator. Flue gas cooling after the precipitator the heat uptake in the cooler will typically replace LP steam for combustion air preheating. The LP steam can instead be sent to a condensing turbine to produce power. Also reduces the negative impact of increased flue gas temperature due to feedwater preheating. Top preheating heating of all combustion air to about 205 o C to increase power generation. Sootblowing steam is extracted from the turbine instead of using high pressure steam from the recovery boiler. Sootblowing steam 25 bar (g) HP steam 100 bar(g), 505 C FW heater Boiler feed water Flue gas FW heater Top preheated Combustion air Black liquor 80 %DS (incl. dust and biosludge) Weak wash Electrostatic Precipitator Flue gas cooler Dust purge Green liquor Smelt Dissolver NCGs Dust recycle (mixed with b.l. in evap. plant)

23 Integrated fine paper mill Page 23 Figure 3-5. Recovery boiler and smelt dissolver. The high liquor concentration contributes to a high bed temperature, which leads to low sulphur emissions from the bed. Combustion air is distributed on multiple levels to facilitate complete combustion and minimize NO x formation. Dust that is not captured in the economizer section is removed in the electrostatic precipitator (ESP). Most of the dust is recycled and mixed with the black liquor in the evaporation plant, as described in section 3.9. A fraction of the dust is purged, mainly to control sulphur and sodium, with the additional benefit of reducing potassium and chloride concentrations in the liquor cycle. With increased recovery boiler temperature and pressure the tolerance for potassium and chloride in the black liquor is reduced. At the design pressure and temperatures for this boiler the maximum chloride concentration in the liquor is about 0.3 wt% and about 2.0% for potassium. Exceeding these concentrations increases the risk for recovery boiler corrosion and plugging problems. Softwood and birch have relatively low levels of chloride and potassium, so the limits for chloride and potassium can be met by purging a small amount of precipitator dust (which is anyways necessary to maintain the sulphur and sodium balance) Causticizing The mill is equipped with conventional causticizing with both green liquor and white liquor filtration. The green liquor is filtered in two parallel green liquor filter units. The dregs are washed and dewatered in a filter press before being discharged. Condensate from the evaporation plant is used for dregs washing. Dregs and grits are combined and sent to landfill. Green liquor from the storage tank is cooled in a flash-type green liquor cooler before the lime slaker-classifier. Slaking and causticizing is performed in a single line with causticizing vessels in series. The causticized liquor is filtered in a pressure disc filter. The main advantage of a disc filters over other types of white liquor filters is the low liquor content of the discharged lime mud which eliminates the need for a separate lime mud washing stage. Alternatively the causticized liquor could be filtered using tube filters followed by another set of tube filters for the weak wash.

24 Integrated fine paper mill Page 24 Green liquor Green liquor dregs Lime kiln Condensate Slaker Causticizer Lime mud removal White liquor filter Lime mud filter Weak wash to smelt dissolver tank Disk filter Clarified white liquor Figure 3-6. White liquor preparation (white liquor disc filter option). Lime mud from the lime mud vessel is pumped to the agitated lime mud storage tank. The lime mud is washed and dewatered on a lime mud disc or drum filter. Condensate from the evaporation plant is used for lime mud dilution and hot water for the lime mud filter wash showers. Spills are reclaimed from two spill sumps and pumped to the weak wash storage tank Lime kiln The lime kiln is equipped with an external lime mud dryer and modern product coolers. The external lime mud dryer consists of a vertical flue gas duct where the lime mud is dried and preheated by the hot flue gases. The dry mud is separated from the flue gases in a cyclone and then introduced to the kiln. This arrangement allows a shorter kiln compared to a conventional kiln where the lime mud is dried inside the kiln. External lime mud dryers are incorporated in the majority of new kilns today, and since the late 1980 s a large number of existing kilns have been equipped with an external dryer to increase kiln capacity. Modern types of product coolers demand less space and have lower radiation heat losses than conventional planetary coolers.

25 Integrated fine paper mill Page 25 Dust is removed from the flue gases by means of an electrostatic precipitator. The ID fan is installed downstream the precipitator. A fraction of the lime mud is purged, primarily to control phosphate levels. Limestone is used for make-up. To save on oil consumption, a number of European mills use bark or wood residues as fuel for the lime kiln. The biomass is either pulverized and fired directly, or gasified and then fired in the kiln. There are many differences between these two processes, however, in terms of the overall mill energy balance they are similar, and either can be used. A detailed review of the biomass fuel is not in the scope of this project; however the main fuel for the lime kiln is bark or wood residue Paper Mill Capacity To match the capacity of the kraft pulp mill, the the paper mill has two identical paper machines. Both PM1 and PM2 produce uncoated fine paper from softwood and hardwood. Each paper machine has an annual production of t/a. The furnish composition is shown below. PM furnish composition Filler 25% Fibre 75% - Bleached softwood 19% - Bleached hardwood 56% The corresponding furnish requirements for each paper machine are: Bleached hardwood ADt/a Bleached softwood ADt/a Filler t 100 /a Starch t 100 /a A block diagram for PM1 and PM2 is shown in Figure 3-7.

26 Integrated fine paper mill Page 26 White water to bleaching plant Bleached soft wood Bleached hard wood Intermediate pulp Filler Bl. soft wood pulp chest Bl. hard wood pulp chest Intermediate pulp chest Refiner Refiner Soft wood dosing chest Hard wood dosing chest Mixing chest Broke dosing chest Screen Filler Storage tower Filter Machine chest Broke tower deaeration Screen and deaeration Wire silo Headbox Wire section Pulper White water tank Disc filter White water tower Press Dryer Sizer After dryer Pulper Pulper Pulper Pulper Calender Pulper Reel/Finishing Figure 3-7. Process concept of the fine paper machines

27 Integrated fine paper mill Page Stock preparation Bleached kraft supply The pulp mill produces softwood and hardwood in campaigns of 30 h and 70 h respectively. There are three MC-storage towers of m 3 each for hardwood and three MC-storage towers of m 3 each for softwood. Since there is not a perfect plug-flow through the pulp mill, there will be some intermediate pulp produced when changing from hardwood to softwood. This intermediate pulp is stored in a m 3 MC-tower. From the MC-storage towers, pulp is diluted and pumped to their respective pulp chest. From the pulp chests, the pulp is diluted and pumped via refiners to the hardwood and softwood dosing chests. In this mill hardwood and softwood are refined separately to optimise their properties. Intermediate pulp is pumped via the hardwood refiner to the hardwood dosing chest. From the dosing chests, pulp is diluted and proportioned to the mixing chest Broke system Broke from all the pulpers on the machine is pumped via the couch pit to the broke storage tower (3500 m 3 ). From the broke tower, the pulp is dewatered on a thickener and taken to the broke dosing chest at about 4% consistency. Broke, which is proportioned to the paper machine, is pumped via a deflaker to the mixing chest and some of the pulp is re-circulated to the tower to increase the consistency in the tower Mixing/machine chest After the mixing chest there is a final consistency control and the thick stock is screened with slotted barrier screens. The barrier screening system is installed between the mixing chest and the machine chest and in this position will screen both the virgin pulp supplied to the paper machine as well as the broke Filler supply Filler is dissolved and stored in a m 3 storage tower at 40% concentration. Filler is added to the short circulation of the paper machine.

28 Integrated fine paper mill Page Short circulation As a consequence of the barrier screening, it is possible to reduce the power consumption by eliminating the hydrocyclone system in the short circulation. The short circulation then consists of the wire silo, de-aeration equipment, fan pump and machine screening in two stages. The headbox is of dilution control type and the system includes two speed-controlled pumps, de-aeration and a pressure screen. Filler is charged ahead of the head box pump. The charge is controlled by the QCS-system to give a constant filler level in the paper independent of the amount of broke added. Retention aids are added before the machine screen and after the machine screen, or alternatively only after the machine screen Paper machine The paper machine is based on a concept to allow for a high quality fine paper production at a high machine efficiency and high speed. The paper machine is dimensioned for 1850 m/min. The paper machine headbox is of a cross profile dilution type, which means that dilution water is added via special control valves across the machine width in the headbox. Each valve setting is based on information from the measuring frame in the dry end, the QCS-system. Since this correction is not made by the slice lip, but through local changes in stock consistency, fibre orientation is not influenced. The wire section is a modern twin wire section to give the best paper uniformity with regard to formation, basis weight profile, ash profiles and sheet structure. The press section is designed for optimum runnability of the machine by means of a closed web run from the wire section to the dryer section. A high dryness content of the web leaving the press section as well as an equal-sidedness are other important factors the press section has to perform. The press concept is two straight shoe presses following each other giving a final dryness after the press section of approx. 52%. The dryer section consists of a pre-dryer section and an after dryer section. The pre-dryer section is a combination of drying cylinders in an upper row and vacuum assisted rolls in a lower row integrated with an air handling system including web stabilising equipment for increased runnability and minimum energy consumption.

29 Integrated fine paper mill Page 29 The sizer after the pre-dryer section adds surface size to both sides of the web by means of an application roll system to increase strength properties of the paper. The sizer is followed by the after dryer section which is a combination of dryer cylinders and vacuum rolls in the first part followed by a conventional dryer section with drying cylinders both in top and bottom position. This arrangement is for curl control of cutsize papers which need special consideration regarding flatness and runnability in copying machines etc. The calender is of soft calender type in a tandem arrangement to give the optimum surface properties. To build a high quality parent roll with a diameter of m the nip load as well as the parent roll torque has to be controlled and this is the case with all modern reel system today. This way of building a parent roll will give the best conditions when handling the roll in the winder. Winding fine paper is not so critical as winding coated paper and for this reason a centre winder is not needed. For finished rolls with a diameter of m a common two drum winder is sufficient, but with a roll diameter of m a winder type giving a lower linear nip load between finished roll and supporting winder rolls is needed. Main data for the papermachines are presented in Table 3-5. Table 3-5. Main data for papermachines Speed design m/min Speed at pope m/min Width on pope m 9 Grammage g/m 2 80 (75-160) Production on pope (100% eff.) t 93 /h 73.1 Paper dryness % 93 PM furnish composition -Hardwood % 56 -Softwood % 19 -Filler % 25 Surface size of paper (starch) % 3 Paper mill efficiency % 82 Operating days per year days 355 Paper production net, annual average t 93 /day 1 439

30 Integrated fine paper mill Page 30 Paper production net, Kraft mill MCR t 93 /day Paper production net t 93 /year Fresh water system The warm water system is the main fresh water consumer in the paper mill. Warm water is mainly used for high pressure cleaning showers in the wire- and press sections and for dilution of different chemicals. The process related fresh water consumption is about 6 m 3 /t of paper. Warm water is received from the kraft mill. Used cooling waters and other uncontaminated process waters are collected separately and re-circulated via a cooling tower to the fresh water systems White water system and buffer volumes The paper machine white water system consists mainly of a white water tank for paper machine excess water connected to a disc filter save-all. Clear filtrate from the disc filter is used for showers in the wet end and is also stored in a white water storage tower (5 500 m 3 ) to be used for consistency control and for broke dissolving. The surplus clear filtrate is pumped to the bleach plant. Accidental discharges are avoided with a dimensioning of broke, pulp and white water storage buffer volumes in balance, which means that the white water storage towers in the system should have a volume corresponding to the total sum of all pulp storage towers. To be correct it is not the physical volumes that should be equal, it is the used buffer volume that is important. A white water tower which is always filled provides no buffer volume. A correct dimensioning and use of the storage buffer volumes also means minimal variations in the flow of waste water to the external treatment plant which should result in higher treatment efficiency and lower investment and operation costs for the external treatment plant Energy aspects of the paper machine The main input of energy to the paper machines is steam for drying of the paper. About 3.0 GJ/t evaporated water is needed for drying. The efficiency of the paper machines (need for re-drying of broke) and the dryness of the paper

31 Integrated fine paper mill Page 31 after the press section are of great importance for the steam consumption. With a press dryness of 52%, a final dryness of 93%, 3% surface size and 10% redrying, the heat consumption for drying is 3.77 GJ/t paper. Steam is also used for heating purposes on the paper machine. In the press section a 3.5 bar steam box heats the web to increase dryness and improve dryness profile. The air to the blow boxes must also be heated with steam. The total consumption of steam on the paper machine is about 4.23 GJ/t paper. The total consumption of electric energy for the paper machine is about 550 kwh/t. The main part of this power consumption is in motors for pumps, screens, drives and refiners in the paper mill. Most of this energy is going into the process flow as thermal energy and contributes to keeping the system temperature on a high level. The desired level is somewhere in the range o C. A high temperature improves the dewatering on the wet end and minimises bacteriological and slime problems. On the wire section, the process water loses about 10 MW of heat to the surrounding air by evaporation. The high dryness of the pulp from the pulp mill means that only a small amount of thermal energy is transferred with the pulp from the pulp mill. To maintain the desired white water temperature, heat is transferred from the heat recovery system of the drying section to the paper machine white water. Table 3-6. Paper machine energy consumption data Dryness to dryer % 52 Paper dryness % 93 Evaporated t/t paper 0.82 Evaporated sizing t/t paper 0.32 Sum evaporated t/t paper 1.14 Redrying etc % 10 Total evaporation t/t paper 1.26 Heat consumption GJ/t evap. 3.0 Heat consumption drying GJ/t paper 3.77 Heat consumption miscellaneous GJ/t paper 0.46 Total heat consumption paper mill GJ/t paper 4.23 Power consumption incl. refining kwh/t paper 550 Water consumption m 3 /t paper 6.0

32 Integrated fine paper mill Page Power boiler The recovery boiler alone does not produce enough steam to meet the demand of the integrated fine paper mill. A power boiler is therefore used to produce the additional steam. The power boiler is fuelled with wood residues from the woodyard and chip screening areas, plus sludge from the effluent treatment plant. The power boiler is designed to provide steam for mill start-up and shut downs, and there is no need for an additional fossil fuel fired boiler dedicated for startups and shut downs. The power boiler is designed with a bubbling fluidised bed (BFB). HP steam 100 bar(g), 505 C Sootblowing steam 25 bar(g) Boiler feed water Flue gas Primary Sludge Sec. Biosludge Falling Bark Electrostatic Precipitator Combustion air Figure 3-8. Bubbling fluidised bed power boiler

33 Integrated fine paper mill Page Steam turbines and steam distribution Steam is reduced through a backpressure steam turbine to 3.5 bar(g). This pressure has been selected to facilitate maximum electric power production without requiring unnecessary large evaporator bodies or heat surfaces in the paper machines. Intermediate pressure steam of 25 bar(g) is extracted for soot blowing and 9 bar(g) steam is extracted to the MP-steam system. MP steam and LP steam are de-superheated with boiler feedwater before distribution. HP steam not required in the process is utilised in a condensing steam turbine for further electric power generation. Table 3-7. Steam data. C bar(g) HP steam IP steam, extracted for sootblowing MP steam, desuperheated LP steam, desuperheated Cooling and recovery of low-temperature heat In addition to normal heat losses of different kinds, approximately one third of all the energy that is introduced with the fuel to the system will have to be cooled away by a cooling system. The secondary energy system comprises the recovery of heat that is generated from steam and electricity and that is finally withdrawn from the system by cooling. In principle, the system can be divided into two parts: one where heat is recovered for the production of warm and hot water, another part where excess heat is cooled by the means of a cooling tower. The design of the model mill is conventional, except for the very low fresh water consumption. Low-temperature heat is recovered from a number of sources in the kraft mill, e.g., the surface condenser of the evaporation plant, the smelt dissolver vapour condenser, and the turpentine condenser. The heat is used for hot water production and for boiler feedwater heating. Condensate from the evaporation plant is used in the pulp washing and in the lime mud wash. The cooling water system is integrated with the process water system. Cooling is carried out in cooling towers. See Appendix 3 for the secondary heat balance.

34 Integrated fine paper mill Page Effluent treatment Pulp is produced in campaigns with softwood 25% of the time and hardwood 75% of the time. Effluent treatment is designed for hardwood campaigns, and discharges are calculated as long term mean averages. Effluent treatment consists of pre-treatment, (cooling equipment and neutralisation), primary treatment and biological treatment. In the pre-treatment there is a primary clarifier to remove fibre sludge. The estimated suspended solids content of the effluent from the mill pulp and paper mill is about 100 mg/l. After the clarifier the suspended solids content is about 50 mg/l. The primary sludge is dewatered in a centrifuge and incinerated in the power boiler (alternatively the primary sludge could be sold to a fluting mill or similar, depending on the price). After the primary clarifier the effluent is screened, cooled to about 37 o C with heat exchangers or cooling towers, and the ph is adjusted to about 7. Table 3-8. Inlet data to biological treatment. Softwood campaigns Hardwood campaigns Total effluent m 3 /d COD mg/l kg/d SS mg/l kg/d Temperature C ~37 ~37 ph ~7 ~7 Primary sludge kg DS/d For biological treatment there is a bio-film reactor with suspended carriers followed by an activated sludge system. The activated sludge system is comprised of an aeration basin and secondary clarifier. The system is designed for low bio-sludge production and low nutrient discharges. COD reduction is estimated to be about 65-70% for softwood and about 70-75% for the hardwood mill. Suspended solids out from the secondary clarifier are about 50 mg/l. The biological sludge is dewatered to about 10% in a centrifuge and mixed with intermediate black liquor in the evaporation plant, before firing in the recovery boiler.