Environmentally Friendly Production of Pulp and Paper (Bajpai/Pulp and Paper Production) || Cleaner Production Measures in Pulp and Paper Processing

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  • P1: OTA/XYZ P2: ABCc05 JWBS031-Bajpai June 9, 2010 12:21 Printer Name: Sheridan

    Chapter 5

    Cleaner Production Measuresin Pulp and Paper Processing

    5.1 KRAFT PULPING

    In recent years, the technological trends in pulp and paper industry, in general,have been directed toward developing and adopting cleaner production technologies/techniques for resource conservation and to reduce the pollution loads at source soas to make the paper industry environmentally friendly and sustainable. The environ-mental impacts of kraft pulp production mainly result from the chemicals used for bothcooking and bleaching, such as sulfur, chlorine, chlorinated compounds, remainingorganic and inorganic materials, lignin and undissolved fiber material, typically mixedup in the black liquor, and eventually Na and Mg salts, chelating agents and heavymetals such as manganese or iron. Thus, the major environmental problems of kraftpulp production are water consumption, wastewater loads, and gaseous emissions,including malodorous sulfur compounds and CO2 (European Commission, 2001).In the last few years, the major cleaner production technological developments havebeen directed toward reducing pollution parameters specially color and adsorbableorganic halides (AOX) through pulping and bleaching modifications, improved pulpwashing, as well as end-of-pipe treatment methods. In air emissions, the major focushas been on reduction/elimination of noncondensable gases (NCGs) to reduce odorproblem associated with kraft pulping process. With likelihood of ban on landfillingof solid wastes, options are being explored for its incineration or utilization.

    The key production and control practices that will lead to pollution preventionand waste minimization in the kraft pulp mills are presented as follows (EuropeanCommission, 2001; Gavrilescu, 2005; Hitchens et al., 2001; Nilsson et al., 2007;RPDC, 2004; Suhr, 2000; Vasara et al., 2001; Woodman, 1993):

    Optimized wood handling Dry debarking Increased delignification before the bleach plant by extended or modified

    cooking

    Environmentally Friendly Production of Pulp and Paper, by Pratima BajpaiCopyright C 2010 John Wiley & Sons, Inc.

    110

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    5.1 Kraft Pulping 111

    Highly efficient brown stock washing and closed-cycle brown stock screening Oxygen delignification Ozone bleaching Elemental chlorine-free (ECF) and totally chlorine-free (TCF) bleaching The use of oxygen and/or hydrogen peroxide in the alkaline extraction stage Removal of hexenuronic acids by acidic hydrolysis and hot dioxide stage Partial closure of the bleach plant Collection of spillages Improved pulp washing Stripping and reuse of concentrated, contaminated condensates Biological wastewater treatment Tertiary treatment of wastewater Increase in the dry solids (DS) content of black liquor Installation of scrubbers on the recovery boiler Collection and treatment of odorous NCGs for incineration in the recovery

    boiler Incineration of odorous gases in the limekiln Installation of low-NOx technology in auxiliary boilers and the limekiln Selective noncatalytic reduction (SNCR) on bark boilers Over-fire air (OFA) technique on recovery boilers Installation of improved washing of lime mud in recausticizing Recycling/reuse of wastewater Solid waste management/utilization

    Optimized Wood Handling

    Wood is transported to the mills from the felling site by truck, rail, or barge, and maybe received either as logs or chips depending on wood age. Logs are mainly deliveredwith the bark on it and have to be debarked before further processing. Chips arenormally free of bark and can be used after screening and possibly washing. Somechemical pulp mills store wood chips in piles for 45 days or more during which timethere is some degradation of resinous compounds (extractives) within the wood byoxidative and enzymatic mechanisms. The pile of wood chips can become quite warmduring this maturation period. Storage for longer periods may reduce the amount ofpulp obtainable from wood (yield) and pulp strength.

    Debarked wood logs are reduced to chips in a chipper. Chip uniformity is ex-tremely important for proper circulation and penetration of the pulping chemicals;hence, considerable attention is paid to operational control and maintenance of thechipper. Chips between 10 and 30 mm in length, and 2 and 5 mm in thickness, are

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    112 Chapter 5 Cleaner Production Measures in Pulp and Paper Processing

    generally considered acceptable for pulping. The chipped wood is passed over avibrating screen that removes undersized particles (fines) and routes oversized chipsfor rechipping. Normally, fines are burned with bark as hogged fuel, although theymay also be pulped separately in specialized sawdust digesters. In most mills,chips are segregated only according to chip length. Chip thickness screening has be-come important as mills realize the need to extend delignification and reduce bleachplant chemical demands. Both absolute chip thickness and thickness uniformity havea significant impact on delignification, because the kraft cooking liquor can onlypenetrate the chip to a certain thickness (Tikka et al., 1992). Thin chips are easierto cook to lower kappa numbers. Uncooked cores from overthick chips will lowerthe average kappa reduction of a cook and contribute to higher bleaching chemicaldemands. To improve thickness uniformity, many mills are now adopting screeningequipment that separates chips according to thickness (Strakes and Bielgus, 1992).Oversized chips may be reprocessed in a crusher or rechipper. Sawdust and fines arenormally burned in a power boiler but can also be cooked together with chips (ifthe digester has sufficient volume) or separately in a sawdust digester. Depending onthe quality of chips from the chipper, the chips are often screened for thickness, asthis is an important parameter in kraft pulping. The overall optimum can sometimesbe reached by sacrificing some raw material to secure stable processing conditions,which, in turn, promote better pulp quality and less pollution. The material removedin the screening operation may be sold for other purposes or burned in a power boilerwith heat recovery (US EPA, 1992).

    Dry Debarking

    The bark of the tree comprises about 10% of the weight of the tree trunk. Bark doesnot yield good papermaking material because it is resistant to pulping, contains a highpercentage of extractives, and retains dirt. In most pulping processes, bark is removedfrom the logs before they undergo chipping. The most common debarking mechanismused for pulpwood is the debarking drum, which removes bark by tumbling the logstogether in a large cylinder. Slots in the outside the drum allow the removed bark tofall through. The bark collected from these operations is usually fed to the hoggedfuel boiler and used to generate process heat or steam. Wet debarkers rotate logsin a pool of water and remove bark by knocking the log against the side of a drumby using large volumes of water. The water used in this process is recycled, but acertain amount is lost as overflow to carry away the removed bark. In wet debarking,310 m3 of water per tonne of pulp is discharged. Organic compounds such as resinacids, fatty acids, and highly colored materials leach out of the bark and into thiswastewater stream. This effluent stream is collected and routed to the wastewatertreatment system, where the pollutants are normally removed quite effectively bythe biological processes that take place there. Dry debarking methods such as drydrum debarkers eliminate the water stream and the pollutants associated with it. Drydebarkers already dominate the industry, and wet systems have been in the process ofbeing phased out since the 1970s (Smook, 1992). Process water is used only for log

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    5.1 Kraft Pulping 113

    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 waterpollutants. Dry debarking creates bark with a lower water content, which will resultin a better energy balance for the mill. Less water is needed in the debarking, andthe dissolved amount of organic substances is reduced (Finnish BAT Report, 1997;Poyry, 1997; SEPA-Report 4712-4, 1997a).

    Raw effluents from a debarking plant are toxic to aquatic life. Biological treat-ment has proved to be very efficient in eliminating toxicity. The dry debarking orone with low wastewater discharges can be applied at both new and existing mills.New mills almost exclusively and an increasing number of existing mills are usingdry debarking.

    With dry debarking, the wood handling wastewater volume is usually in therange of 0.52.5 m3/ADt. Decrease in wastewater amount is obtained by increasedinternal water circulation. By changing from wet debarking to dry debarking, thewastewater amount would decrease often by 510 m3/ADt. With dry debarking thetotal chemical oxygen demand (COD) loading can be reduced up to 10% (Salo, 1999).The higher bark dryness in the boiler feed improves the energy efficiency, meaningthat emissions per the amount of produced energy will drop. Energy consumption indebarking may increase due to the operation of the debarking drum in dry debarkingmode. On the other hand, substantial amount of energy may be gained if the bark isused as an auxiliary fuel at lower water content.

    Where wet debarking is used, improved water recirculation coupled with grit andsolid removal systems for water has been applied with success. Dry debarking usuallyrequires fresh wood to obtain good debarking results. The costs of dry drum debarkersshould not differ significantly from a wet system. Typical investment cost of a com-pletely new dry debarking system is about 15 million for a capacity of 1500 ADt/day pulp.

    The conversion of an existing wet debarking system to a dry debarking systemcosts 46 million. These costs include equipment and installation. Driving forcefor implementing this technique is that dry debarking decreases total suspendedsolids (TSS), biological oxygen demand (BOD), and COD load as well as organiccompounds such as resin acids, fatty acids, leaching out of the bark and into thiswastewater stream. Some of these substances are regarded as toxic to aquatic life.The measure also increases energy yield. Dry debarking is being used in several millsaround the world.

    Increased Delignification before the Bleach Plantby Extended or Modified Cooking

    Extended delignification is done before the bleach plant to decrease the lignin contentin the pulp entering the bleach plant to reduce the use of the bleaching chemicals(Bowen and Hsu, 1990). Reduction in the use of bleaching chemicals will reduce theamount of pollutants discharged while increasing the amount of organic substancesgoing to the recovery boiler. In the recent years, new cooking methods have been

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    developed that allow more selective delignification and provide mills with new pulp-ing options. Mills can cook to lower kappa numbers, allowing less bleach chemicalusage or permitting alternative bleaching methods. They can also maintain kappanumbers while improving yield and strength properties.

    Continuous System

    In the continuous system, modified continuous cooking (MCC), extended modifiedcontinuous cooking (EMCC), and isothermal cooking (ITC) represent the main alter-natives (Andtbacka, 1991; Bowen and Hsu, 1990; Headley, 1996). The MCC processis based on two main principles. As compared with conventional cooking, it startsat a reduced concentration of effective alkali and ends at a higher concentration ofeffective alkali. The lignin concentration is reduced at the end of the cooking. Thisis accomplished by splitting the alkali charge into different insertion points and byending the cooking in the countercurrent zone, where the liquor flows in an oppo-site direction to the chips. In a typical MCC digester, cooking is divided into twozones. The impregnation vessel has a 30-minute retention. The concurrent zone hasa 60-minute retention, and the countercurrent zone has a 60-minute retention. Thewhite liquor charge is split into three zones. Approximately 65% is added to the im-pregnation vessel, 15% to the transfer circulation line, and 20% to the countercurrentzone. Two benefits are gained by adding white liquor to the countercurrent zone.The initial hydroxide concentration is lowered. Some cooking is performed in thecountercurrent zone, where the dissolved solids are lower.

    EMCC is a further development in extended cooking. A fourth white liquoraddition point is added to the high heat circulation, and the temperature in the high-heat zone is increased from approximately 290 to 300F. The EMCC process furtherdecreases the initial hydroxide concentration and increases the amount of cooking inthe countercurrent zone.

    Isothermal cooking (ITC) is developed by Kvaerner and expands on EMCC withan additional circulation loop and the fifth white liquor addition point (Engstrom andHjort, 1996). The ITC circulation is a very high volume (2100 gal/b.d.st) and requires aspecial type of screen to handle the flow. The screens are equipped with backflushingvalves that reduce blinding. The high circulation rates allow even more cookingin the countercurrent zone. As compared with EMCC, the initial hydroxide is reduced.The temperature in the digester is lower and nearly uniform throughout the digester.The amount of cooking in the countercurrent zone is increased. Existing digesterscan be retrofitted to ITC provided an upflow exists in the digester. The downtimerequired is 1014 days. The cost of a retrofit is approximately US$3 million. To date,several digesters in Europe and Japan have been converted to ITC. One retrofit hasbeen installed in North America to a southern kraft mill.

    Black liquor impregnation, also developed by Kvaerner, takes extraction liquorfrom the digester back to the impregnation vessel. This liquor has a relatively low hy-droxide concentration and comparatively high hydrogen sulfide concentration. Blackliquor impregnation is only applicable for new digester installations or when replacingthe impregnation vessel due to corrosion or other problems. Existing impregnation

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    5.1 Kraft Pulping 115

    vessels cannot be converted to black liquor impregnation unless operated well belowdesign capacity. The retention time required is approximately 40 minutes comparedwith 30 minutes for a conventional impregnation vessel. The main benefit observedwith black liquor impregnation is a 10% increase in tear.

    Batch System

    In the batch system, there are thre...

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