Recovery Boiler Capability to Accommodate Alternative Kraft Mill Processes

P I

John I.. Clement Manage6 Pulp and Paper lndus try Marketing The Babcock & Wilcox Company Barberton, Ohio

he Technical Association of the Pulp and The kraft process starts with feeding wood Paper Industry (TAPPI) sponsored a Paper In- chips to the digester. Chips are cooked under pres- Tdustry Research Needs workshop at the State sure in a steam heated aqueous solution of sodium University of New York, College of Environmental hydroxide (NaOH) and sodium sulfide (Na2S) Science and Forestry in Syracuse, New York, May known as white liquor, or cooking liquor. Cooking 26-28,1992, The Pulping and Bleaching Processes can take place in continuous or batch digesters. Panel identified the major need in kraft black liquor After cooking, pulp is separated from the residual recovery, namely, an affordable, incremental, liquor in a process known as brown stock washing. chemical recovery capacity increase to remove the The most common method in use features a bottleneck in the recovery boiler operation. This countercurrent series of vacuum drum washers to paper will explor? kraft mill process factors that displace the liquor with minimum dilution. cause a pulp producer to require more capacity. Modern continuous digester installations incor- Approaches to removing the bottleneck and porate a brown stock washing stage in the lower providing increased black liquor processing part of the digester body. Following washing, the capability include retrofitting the existing recovery pulp is screened and cleaned to remove knots and boiler to increase its capacity. Retrofit case histories shives and to produce pulp for use in the final pulp were selected from an extensive list of retrofit ex- and paper products. Currently, industry is focused perience to represent a spectrum of possible ap- on maximizing washing efficiency to recover proaches. chemicals, thereby reducing the chemical demand in the bleach plant. Kraft Process and the The black liquor rinsed from the pulp in the Recovery Boiler washers is an aqueous solution containing wood lignins, organic ' material, and inorganic com- The kraft process diagram (Fig. 1) shows the typical pounds oxidized during the cooking process. Typi- relationship of the recovery boiler to the overall cally, the combined organic and inorganic pulp mill operation. The primary function of a compounds are present at a 13 to 17 percent con- recovery boiler is to combust the black liquor centration of solids in weak black liquor. The kraft produced in the pulping operation for the purpose cycle processes this black liquor through a series of recovering the .inorganic chemicals in the operations, including evaporation, combustion reduced form required for recycling to the digester. organic materials, reduction of the spent inorgan1 The heat content of the black liquor is recovered as compounds, and reconstitution of the white liqu steam for process heat and cogeneration electrical The recovery boiler furnace (Fig. 2) production. Electrical production is a secondary developed to combust the black liquor organ1 function, but an important economic considera- material while reducing the oxidized inorgan1 tion. material in a pile, or bed, on the furnace floor. fi

66 '. I J.L. CLEMENT

Figure 1.Yraft process diagram. molten inorganic chemicals, or smelt, are dis- tario Ministry of the Environment, which describes charged to a tank in which they are dissolved to the impact on the recovery boiler throughput of al- form green liquor. The active chemicals in green ternative kraft mill processes (N. McCubbin Con- liquor are sodium carbonate (Na2C03) and Na2S. sultants, 1992). Several processes have been Green liquor is clarified to remove insoluble developed to extend delignification of the kraft material, then reacted with lime (CaO) in a caus- pulps beyond the conventional levels. This exten- ticiting plant to convert Na2CO3 to active NaOH sion is environmentally desirable in circumstances in the product white liquor. in which the pulp will be bleached in subsequent Energy is released as the black liquor organic processes. "Extended delignification'' is also known compounds are combusted, paralleling the reduc- as "extended cooking." Delignification by oxygen, tion of sulfur compounds to form smelt in the extended cooking, and improved brown stock recovery furnace. This . combustion energy washing and screening can produce up to 12 per- produces steam, which can be introduced to a tur- cent additional black liquor solids in a mill that has bine generator to supply a large portion of the ener- relatively high losses of black liquor solids in the gy demand of the pulp and paper mill. Steam sewer. In mills that already have good washing, extracted from the turbine at low pressure is reused defined as sodium losses under 10 kg of sodium in the process for cooking wood chips, evapora- sulfate (Na2S04) per ton of pulp, the additional tion, recovery furnace air heating, and drying the recovery load would be about 6 percent. pulp or paper products. If the alternative processes are considered inde- pendently, oxygen delignification increases the Impact of Alternative solid flow to the recovery boiler about 4.4 percent, Manufacturing Processes which represents a 3.3 percent increase in heat input. Applying extended cooking would increase Alternative kraft mill processes are increasing the the solids to the recovery boiler approximately 3 quantity of organic and inorganic material in the percent when the bleached yield of pulp from black liquor that must be processed in the recovery wood is maintained. The alternative of combining boiler. Therefore, additional incremental capacity both oxygen delignification and digesterextended is required to recover the inorganic chemicals. In delignification, as well as efficient washing, could many mills, this capability is limited by the installed result in a drop in the yield of bleached pulp from equipment. wood, with corresponding increases in wood con- Values for the increased capacity requirements sumption and steam production. The combined are quoted from a 1992 reDort DreDared for the On- changes to a mill operating with high losses could Figure 2.-Typical modern recovery boiler.

result in a 15 percent increase in solids to the only about 25 percent of the by-product acid can recovery boiler and a 16 percent increase in'heat be used. The excess must be neutralized before input. being discharged to the sewer; this effect is geheral- Chlorine dioxide (C102) is used increasingly to ly accomplished with lime. replace the traditional chlorine in the first stage of the bleach plant. C102 substitution is a simple approach to reducing organochlorines discharged Recovery Boiler Capacity from the mill. C102 is always manufactured on the kraft mill's site since it is impractical to transport There are several ways to increase the recovery significant quantities. The C102 plant is linked with boiler capacity by 6 to 15 percent generally re- the recovery process since the manufacturing quired by the combined application of oxygen process produces significant quantities of sodium- delignification, extended cooking, efficient wah- I based by-product acid. It is general practice to ing, and similar processes. Incremental capacity in- i blend the effluent acid from the C102 generator into creases can also be measured in several ways. It weak black liquor at a rate equivalent to the total can be an increase in the time between shutdowns $1 sodium losses in the mill. In a modern mill with low to waterwash the recovery boiler, or, the capability 'I sodium losses and practicing 100 percent substitu- to burn more black liquor without increasing the 1 tion to produce a molecular chlorine free pulp, time (run campaign) between outages. 1.L. CLEMENT

The most significant limiting factor to increased must be totally removed and the boiler cooled. recovery capacity is ash buildup on the boiler heat Total smelt bed removal precludes a possibility of transfer surfaces (Fig. 3). Low melting point, inor- water contacting molten chemical in the bed and ganic ash compounds formed in combustion of producing a smelt water reaction. Interruption in black liquor are borne upward by the gases in the operation for one to two days for waterwashing furnace and stick to the surface of the tubes. As the represents a potential loss in pulp production, or al- black liquor throughput is increased, the increased ternatively, continuing pulp mill production by temperature and velocity of the combustion gases shipping black liquor to an alternate mill site for increase the propensity of ash to stick and block the burning, and purchasing the lost electrical genera- gas passages. Ash is cleaned from tube surfaces tion from the utility. Downtime is an expensive using jets of steam introduced through sootblowers. proposition! Even with sootblowers in constant use to clean Therefore, the operator requiring additional surfaces, the operator has no alternative but to shut capacity to process black liquor solids has two down the boiler periodically and waterwash the basic approaches to consider. The first is to add an tube surfaces. To accomplish this, the smelt bed additional recovery equipment line to process the solids. Alternatives to adding more lines are the focus of development projects. One alternative, a black liquor gasification unit installed at Frovifors in Sweden, is reported to have operated commercially for half a year (Bostrom and Hillstrom, 1992). The purpose of the instal- lation was to increase the recovery boiler capacity by 75 to 100 tons of dry solids per day, corresponding approximately to an 8 to 10 percent increase in the capacity of the existing recovery boiler. Another solution to adding more lines is being developed by MTCl and ThermoChem and is based on pulse-assisted fluid bed gasification technology for indirect gasification of black liquor (Mansour et al. 1992). A design is reported to be under way of a 72 to 100 wet ton per day gasifier for demonstration testing at a southeastern mill. An incremental increase in capacity can be accomplished with a more traditional approach to combustion of black liquor by installing a modular unit similar to that installed at Millar Western Pulp Company in Meadow Lake, Saskatchewan (see Fig. 4). The boiler is designed to process 225 tons per day (or 496,000 Ib per day) of dry solids. The second way to achieve the required increase in black Figure 3.4ecovery boiler ash deposits. liquor throughput is the retrofit- - 69 t

Alternative and Emerging Technologies - Pulping

ting of a mill’s existing recovery boiler. It is general- Retrofit Case Histories ly possible to retrofit a recovery boiler to increase capacity; howeve;, in Some mills, the boiler is al- An innovative project to increase recovery boiler ready operating at its limit. capacity was completed in 1991 by Georgia-

Figure 4.4ffluent recovery boiler. J.L. CLEMENT

Pacific at their Cedar Springs, Georgia, mill lower heat value than was used in the original (Pedroso and Edwards, 19911. Two 1960-vintage design. Nevertheless, they are indicative of the in- recovery boilers were replaced with one drum cremental increases that can be achieved through modern boilers using the existing space, heel, and various programs. foundations to the maximum extent. The new boiler configuration provided a 22 percent increase in capacity. Elimination of the direct contact, black Case History No. 1 liquor evaporator accomplished a reduction in total The capacity of the recovery boiler at Stone Con- reduced sulfur (TRS) emissions. Using a thoroughly tainer Corporation in Florence, South Carolina, was planned construction approach, a 75-day outage limited by buildup of ash deposits in the (black liquor out to liquor in) was achieved for Unit economizer. Frequent outages were required to 1, and 65 days for Unit 2. Figure 5 shows the boiler waterwash the economizer. The operator acquired and equipment configuration before and after the an increase of 10 percent in liquor solids capacity upgrade. and conversion of the recovery unit to low odor by eliminating the direct contact evaporator. Babcock and Wilcox provided engineering, material and construction to modify the boiler to achieve the increase in solids throughput. Modernization included a state-of-the-art, three-level air system; modified liquor firing system; additional superheater surface to maintain the temperature of steam; and an extension of the building to accommodate the new long flow economizer. For maintenance purposes, the lower furnace and Before Modification boiler bank were replaced. The lower furnace was replaced with modern composite tube construction n and the boiler bank was retubed. In addition, the larger superheater required a new screen. The configuration before and after modifications is shown in Figure 6. The boiler was started in 1991, and the retrofit proved successfu I.

Case History No. 2 Potlatch Corporation in McGehee, Arkansas, ob- tained a 63 percent increase in solids processing capability when Babcock & Wilcox enlarged the furnace and added a new three-level air system as shown in Figure 7. The furnace depth was increased five feet for the entire height of the furnace. v After Modification The increase in furnace size resulted in a solids increase from 680 tons per day (1.5 million Ib per Figure 5.4eorgia-Pacific. day) to 1,111 tons per day (2.45 million Ib per day). Steam flow increased correspondingly by 54 per- An incremental capacity increase can be cent. The steam flow increase resulted in part from achieved with a more modest approach to a retrofit the customer's decision not to modify the super- program. Capacity improvements are presented as heater surface, offset by a lower than rated solids black liquor solids rate, but, it should be under- heating value. The unit operates at a lower steam stood that recovery capacity is rated in heat input. temperature, decreased from the initial design Some of the increases in black liquor solids temperature of 440'C (825'F) to 392'C (737'F). The at capacity increase was completed in early 1992. throughput may be the result of operating- a 71 Alternative and Emerging Technologies - Pulping

A t?er Modification Figure &-Stone container.

Case History No. 3 The success of this project encouraged additional modifications to achieve a significant over- Gaylord Container Corporation at Bogalusa, load at 1,451 tons per day (3.2 million Ib per day) Louisiana, wanted to increase the time between of solids. The upgrade invoived changing the su- six-week waterwash intervals on an overloaded ' perheater and removing every other row of generat- recovery boiler while also reducing the TRS emis- ing bank tubes (see Fig. 8). The unit proved capable sions from the stack. The boiler, rated at 1,043 tons of operating at a significant overload of 28 percent per day (2.3 million Ib per day) of black liquor over the original heat input rate for six months be- solids, was operating at 1,270 tons per day (2.8 mil- tween waterwash ings. lion Ib per day). Final air flow modifications completed in June 1988 resulted in reduced TRS emissions coinciding with extending the interval Case History No. 4 for watenvashing to beyond six weeks. The final air The retrofitting of the boiler at James River's st. system modification introduced secondary air from Francisville, Louisiana, mill was driven by the State furnace sidewalls only and tertiary air from the of Louisiana's requirement to bring boiler operation front and rear walls. Babcock and Wilcox's port design for interlaced air jets was maintained at both levels. I.L. CLEMENT

. 1

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Front Wall After Modification nModification

/I New . I Composite i I Lower I I Furnace ii 'i I h iI :j i7 - jI 'I ! ri I I1 Before Modification After Modification

Figure 7.4otlatch. solids processing capacity. The unit was originally ports supplied with high static ambient air and in- designed to process solids at a rate of 81 6 tons per terlaced secondary air ports was also incorporated. day (1.8 million Ib per day) and routinely operated Other modifications included a new superheater to at 998 tons per day (2.2 million Ib per day). To restore the original steam temperature; additional achieve 1,224 tons per day (2.7 million Ib per day) black liquor burners for operating flexibility; and of dry solids processing capability, the furnace vol- additional sootblowers arranged to efficiently ume was increased to that required to process the remove ash deposits from the heat transfer surfaces. desired solids by adding seven feet to the furnace The odor compliance was achieved by replacing depth and nine feet to the height (Fig. 9). A new the direct contact evaporator with a long flow combustion air system with interlaced tertiary air economizer to cool the gases prior to discharge.

73 Alternative and Emerging Technologies - Pulping

Two years of planning went into the capacity budget. The retrofit relieved a significant capacity increase and low odor conversion. A team effort by bottleneck while satisfying emission requirements. lames River with Babcock & Wilcox resulted in a fully integrated project program with detailed en- Conclusion gineering governed by feasibility of construction. The intensive effort resulted in the entire project Alternative kraft mill processes are incrementally being completed in a @-day outage, Seven days increasing the quantity of black liquor solids that ahead of the forecasted downtime, and within must be processed in the recovery boiler. Although

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Cyclone Evaporator

Before Modification After Modification Figure 9.4ames River. a number of approaches to increasing a mill's black References liquor processing capability exist, no one solution fits every situation and each must be studied to as- Bostriim, G. and R. Hillstrijm. 1992. Status report from the Chemrec recovery booster at Frtivifors. Page 451 in Proc. certain the most economical approach. The new 1992 Int. Chem. Recovery Conf. Can. Pulp. Pap. Ass. Tech. technologies should be investigated as an alterna- Ass. Pulp. Pap. Indus. Press. Seattle, WA. tive to processing the increased solids in the exist- Mansour, M.N., K. Dural-Swamy, W.G. Steedman, and R.E. ing recovery boiler or boilers. While the retrofitting Kazares. 1992. Chemical and energy recovery from black of an existing recovery boiler is an attractive option, liquor by steam reforming. Page 473 in Proc. 1992 Int. feasibility and cost need to determined. many Chem. Recovery Conf. Can. Pulp. Pap. Ass. Tech. Ass. be In Pulp. Pap. Indus. Press. Seattle, WA. cases, the retrofitting of a recovery boiler can N. McCubbin Consultants, Inc. Best Available Technol- achieve a capacity increase exceeding the require- 1992. ' ogy for [he Ontario Pulp and Paper Industry. Rep. PlBS ments imposed by alternative processes. 1847. Ontario Ministry Environ., Water Resour. Branch. Toronto, Canada. Pedroso, P.P. and E.I. Edwards. 1991. Replacing 1960 vintage recovery boilers on an accelerated construction schedule at Cedar Springs, Georgia. Page 55 in Proc. I991 Eng. Conf. Tech. Ass. Pulp. Pap. Indus. Press. Nashville, TN.