Liquor Cycle Chloride Control Restores Recovery Boiler Availability

Transcription

1 Liquor Cycle Chloride Control Restores Recovery Boiler Availability L.A. Hiner Babcock & Wilcox Barberton, Ohio, U.S.A. M.A. Blair Babcock & Wilcox Birmingham, Alabama, U.S.A. S.C. Moyer D. Wiggins Alabama River Pulp Co. Perdue Hill, Alabama, U.S.A. Presented to: TAPPI Engineering Conference September 17-21, 2000 Atlanta, Georgia, U.S.A. BR-1704 Abstract Kraft recovery boilers operating today are experiencing higher fouling rates due to increased chloride content within the liquor cycle. It has been shown that excessive chloride content in recovery boiler ash results in increased fouling rates on the boiler s convective surfaces. (1, 2) Tighter liquor cycles and more efficient precipitator ash collection have resulted in higher chloride content in the liquor being sent to recovery boilers. By purging a small portion of the precipitator ash, the Alabama River Pulp and Alabama Pine Pulp Companies of Claiborne, Alabama have taken dramatic steps to restore the availability of their recovery boilers to design levels. Their liquor chloride control strategy has allowed both boilers to run between planned outages without unscheduled cleanings or loss of steam temperature. The following examination of this strategy will describe operation at the mills before and after changes were made in the liquor cycles, and will compare the impacts of ash dumping to operational cost savings. Introduction Liquor and ash chemistry is critical within the Kraft process for maintaining recovery boiler availability. As such, it has been the subject of intensive study within the pulp and paper industry. Studies have shown that ash fouling and gas path plugging within recovery boilers are directly related to the melting properties of the ash in the boiler. (1,2) It has also been shown that chloride and potassium concentrations within the liquor cycle are the most significant factors affecting ash melting points. Therefore, by reducing the chloride and potassium levels within the liquor cycle, ash melting properties are made less conducive to fouling. The result is improved cleanliness within the boiler, which translates into greater availability with less performance degradation. In most cases, efforts to maintain boiler performance by reducing ash chloride levels have been undertaken on units with very high ash chloride concentrations. High chloride concentrations are the result of high chloride sources such as wood supply or makeup chemicals. Environmental improvements within the last decade that have led to reduced loss of process chemicals have also resulted in an increase in non-pulping deadload chemicals, including chlorides, within the liquor stream. After experiencing increased fouling and plugging within their recovery boilers, Alabama River Pulp (ARP) and Alabama Pine Pulp (APP) undertook a joint program to reduce their liquor chloride levels and thereby restore recovery boiler performance. By adopting a strategy of periodically purging precipitator ash and by switching to lower chloride caustic, ash chloride levels were reduced from around 4.5% to 1.5% or less by weight (8.8% mole to 2.8% mole Cl/(Na+K)). This change has allowed both of the mills to restore operation from an eight week run between cleanings to the scheduled annual outages without steam temperature degradation or draft loss. This has allowed the mills to reduced the downtime on the units from five unscheduled outages to no unscheduled outages. Babcock & Wilcox 1

2 Temperature, Deg. C Radical Deformation Sticky K/(K + Na) = 5 mole % Slagging First 500 Melting Cl/(Na + K) mole % Figure 1 Effect of chloride on the deposit sticky temperature zone (Tran). Ash Generation, Deposit Formation, and Ash Recycling Ash generated in recovery boilers is typically composed of three types: carryover particles, aerosol particles (ejecta), and sub-micron fume. (3,4) Carryover is smelt or partially burned black liquor particles entrained within the flue gas leaving the furnace. Aerosol particles are ejected from black liquor droplets as the droplets dry and burn within the gas stream or on the char bed. Fume particles form when inorganic chemicals volatized in the lower furnace condense as the flue gas cools in the upper furnace and boiler banks. Fume composition varies depending on furnace operation and the inorganic composition of the black liquor. Ash deposits form when particles generated in the furnace accumulate and stick on the boiler s heat transfer surfaces. The proportional makeup of ash accumulations varies in different regions of the boiler. Carryover and larger ejecta tend to deposit mostly in the screen and superheat areas by impacting onto the surface or deposits. The relative ratios of these larger particles then decrease as the flue gas makes its way aft through the boiler while the proportions of small ejecta and fume particles increase. H. N. Tran and his associates have studied in great depth the effects of ash chemical composition on fouling within recovery boilers. Their work strongly suggests that the main factor determining ash deposition rates is the ash stickiness. Their studies also state that ash stickiness is a function of the amount of liquid phase present in the ash, which in turn is dependent upon the ash chloride content and temperature. A simplified relationship between the ash chloride content and the sticky temperature can be shown by the Tran graph as in Figure 1. (1) This graph may be used to illustrate the anticipated effects of elevated ash chloride levels in general terms. Temperature, Deg. F The conditions of this particular graph are at 5% potassium molar ratio, conditions similar to those at the ARP/APP mills. Here it can be seen that the sticky band on this graph covers the widest temperature range from 5 to 10% molar chloride levels and would be in a narrow range at higher and lower levels of chloride. Ash accumulations on heat transfer surfaces create an insulating barrier that reduces heat transfer into the boiler tubes. Consequently, as the ash deposits build and heat transfer decreases, steam outlet temperatures decay and the flue gasses retain higher temperatures as they pass through the boiler banks. Higher flue gas temperatures lead to more ash in its molten phase being carried further aft in the boiler where it adds to already present accumulations. As ash deposits grow, they also begin restricting gas flow through the boiler, plugging the gas passes, and eventually increasing the furnace draft loss to inoperable levels. Eventually the boiler must be taken off line and water washed to restore furnace draft and boiler efficiency. Unscheduled outages for washing often significantly affect pulp production due to the loss of steam and recovery capacity. Ash that does not deposit and stick on the boiler convection surfaces is collected in convection pass hoppers or within electrostatic precipitators. This ash is then recycled into the liquor stream to increase the recovery of pulping chemicals. Typically, recycled ash at ARP/APP makes up 6 to 8% of the total virgin dry black liquor solids processed. Pulp Mill Description The Alabama River Pulp / Alabama Pine Pulp Complex is one of the largest pulping sites in the world. Elemental Chlorine Free (ECF) pulp and Chlorine bleached pulp are produced at this complex, using the latest environmental and process technologies. (5) The ARP facility is a hardwood mill that started operation in March of The APP mill is a softwood mill that began operating in December The pulping liquor produced from these mills is burned in two Babcock & Wilcox (B&W) recovery boilers. A variable portion of the liquor from both mills is shared between the two recovery processes. Each boiler is sized for its respective mill capacity. Table 1 provides a brief description of each boiler. Operational History Prior to startup of the APP mill in late 1991, the ARP recovery boiler could operate on an approximate yearly basis between water washes. Subsequent changes in both mills reduced chemical losses and lowered emissions. These changes also affected liquor chemistry and, consequently, recovery boiler performance. The effects of these changes eventually reduced the run time of the ARP boiler to 80 days and limited the run time on the APP boiler to 60 days far short of their design capability. Table 1 Babcock & Wilcox Recovery Boiler Descriptions Alabama River Pulp (ARP) Alabama Pine Pulp (APP) Year of Startup Unit Size, mtpd Operating Pressure, kpa (psig) 8720 (1250) 8720 (1250) Steam Temperature, C (F) 482 (900) 482 (900) Solids 62% 70% Recovery Boiler Design 2 drum low odor 1 drum low odor 2 Babcock & Wilcox

3 Figure 2 Virgin liquor chloride content. A- APP startup. B- new precipitator on ARP. The first change affecting black liquor chemistry was the startup in late 1991 of the APP recovery boiler, a modern, single drum boiler with high efficiency precipitators. The second change was the replacement in 1995 of the ARP precipitators with high efficiency units. Ash collected by recovery boiler precipitators is a combination of sodium- and potassium-based salts of sulfate, carbonate, and chlorides. Of these compounds, sodium and potassium chloride are physically the smallest and are therefore the most difficult to collect. High efficiency precipitators more readily capture these compounds and thereby increase the amounts of chlorides that are re-introduced into the liquor via ash recycling. Historical black liquor sampling at ARP/APP indicated a significant increase in liquor chloride levels immediately after the startup of the APP boiler in late 1991 (Figure 2). The installation of the new precipitators on the ARP unit in 1995 further increased chloride levels while reducing chemical losses. As liquor chloride levels increased, both units experienced increased water wash frequency, reduced availability, and rapid degradation of superheated steam temperatures. Initially, the APP boiler experienced higher fouling rates than the ARP unit. Several factors contributed to this phenomenon, the most significant of these was likely the higher efficiency of the APP precipitator. In addition, the APP unit initially operated with higher solids (72% vs. 66% on ARP). The higher solids firing and larger furnace size of APP would likely result in higher furnace temperatures and increased chloride content in the ash. Since the higher furnace temperatures would volatize more chlorides from the liquor, the additional chloride fume then condensed in the convection passes and enriched the chloride levels in the ash recycled from those regions. High efficiency precipitators would capture more of the chloride compounds that would have been lost through the stack, and returning them to the liquor through ash recycling. The installation of new precipitators in 1995 on the ARP unit eliminated the differences in the precipitator performance and resulted in further degradation of the ARP availability. In addition to causing performance degradation within the boilers, increased chlorides may also have been responsible for accelerated deterioration of boiler pressure parts. Accelerated corrosion within the secondary superheater of the APP boiler was attributed to higher chloride levels within the boiler. Subsequently, steam outlet temperatures were reduced from 482C to 468C to reduce corrosion rates in the secondary superheater. Increased chloride levels were also documented as potential contributors to a floor tube failure in the ARP unit in (6) Chloride Reduction Program In order to restore recovery boiler availability and efficiency, the mills undertook a multi-step approach to reduce liquor chloride levels. A target level for liquor chlorides was determined from historical data and then the mills began taking steps to reduce chloride within the liquor cycles. Based on the period when the ARP boiler could operate between outages without unscheduled water washes, liquor sampling records were used to establish a target liquor chloride level less than 0.3% by weight of the dry black liquor solids. Typical enrichment factors for precipitator ash chlorides versus black liquor chlorides were then used to calculate a target value of 1.5% by weight (2.8% Cl/(Na+K) mole %) for the ash chloride level. Typical ash chloride levels before purge and the target chloride levels are shown plotted on Tran s curve in Figure 3. The ash chlorides are measured by the ARP/APP labs to monitor progress on a daily basis. Studies performed by Tran (1,2) suggested that chloride levels are more concentrated in recovery boiler ash than elsewhere in the pulp mill. Research performed by ARP and APP personnel further substantiated this data. Because of this, the mills began purging a portion of their precipitator ash rather than recycling all of the ash back into the liquor stream, removing chlorides at their most concentrated point. Although new technologies had been developed to remove chlorides from recycle ash on recovery boilers, (6,7,8) the capital and operating costs precluded application in this case. Use of this process requires an ash slurry to facilitate chloride removal. This would have required an increase in evaporator loading to maintain black liquor solids at current firing levels. Evaluation by mill personnel determined that the process costs of a chloride removal system versus an ash purging strategy had little justification in operating cost and would require a capital investment that would not be justified by the savings within a reasonable time frame. In addition, an initial trial period was necessary to determine a target level of chlorides in the ash, the level of chloride removal necessary to maintain the target value, and the effect of process changes on the chlorides coming into the liquor cycle. ARP and APP management therefore chose to purge ash and to accept a somewhat higher makeup chemical requirement in lieu of a large capital expenditure. Another consideration involved the plan to convert bleaching technologies of the mills to all ECF bleaching. The conversion will result in increased sulfur makeup, and would likely Temperature, Deg. C A Sticky First Melting 1.5% By Weight B K/(K + Na) = 5 mole % Radical Deformation Slagging Figure 3 ARP/APP chloride levels. A target Cl levels; B prepurge Cl levels. Typical Pre-Purge 4.5% By Weight Cl Cl/(Na + K) mole % Temperature, Deg. F Babcock & Wilcox 3

4 by weight of the total precipitator ash collected. The mills have since automated the purge process so that the conveyor is operated in the purge mode for an adjustable period on an hourly basis. Currently, the mills periodically check liquor sulfidity and ash chloride content to adjust the purge rate as required. Figure 4 ARP/APP chloride sources. require purging of salt cake to maintain sulfidity levels in the liquor cycle. The amount of ash purged to control sulfidity would be equal to or greater than that required to control chlorides. The chloride purge would then have a diminishing economic impact as the mills move toward total ECF pulp bleaching. To reduce the chlorides coming into the liquor cycle, the mills examined the possible chloride sources and their relative chloride contribution. The mills findings are shown in Figure 4. One major source for chloride introduction was the caustic makeup chemical (24%). Low chloride caustic was available, but at a cost premium. After an economic evaluation of maintaining a high purge rate for precipitator ash versus reducing the purge rate and switching to low chloride caustic, the mills opted to reduce purge rates and switch caustic. Switching to low chloride caustic reduced incoming chloride from the makeup from 1700ppm to less than 50ppm. Sesquisalt was also identified as a major source of chloride introduction (18%). This is used as the makeup sulfur stream to the pulp mills and used to acidify tall oil soap in the APP mill. Subsequently, the mills were able to enact changes in bleach plant operations that reduced chloride levels in the sesquisalt but these reductions have not been quantified. Chloride Purge System The precipitator ash purge was accomplished through one of the precipitator ash transfer conveyors on each of the recovery boilers. An ash chute with a rotary valve was installed from the back end of one of the conveyors to a new ash sluice tank at ground level. When the mills choose to purge ash, that ash conveyor is reversed and the ash falls through the chute and is collected in the sluice tank. Initially the mills chose to continuously purge all the ash flow from one of the APP precipitator conveyors to rapidly reduce liquor chloride levels within that mill s liquor cycle. Although the chloride levels were indeed rapidly reduced, mill sulfidity also dropped into the teens and smelt volatility increased for a period until sulfidity control was re-established by lowering the purge rates. At lower purge rates, sulfidity has been easily controlled and the mills have experienced no further operational issues as a result of the purge. Once the ash chloride target of 1.5% was achieved, it was found that it could be maintained with a purge rate of 10 to 12% Results Reduction of the liquor chloride content at the ARP/APP mills has led to dramatic steps in restoring performance and availability of both recovery boilers to their design capabilities. Since the ash chloride target of 1.5% has been attained, steam temperature decay and draft loss due to ash fouling have not been a concern. Operating availability has been extended from 60 and 80 days to well beyond six-month scheduled outages. Operations have exceeded 220 days without cleaning and without notable signs of fouling or plugging. APP mill operating run times are shown in Figure 5, noting scheduled outages separate from forced outages for cleaning. Costs incurred due to ash chemical loss through purging were actually lower than anticipated. The quantities of ash that had to be purged were significantly reduced as a result of bleach plant sesquisalt chloride reduction and by switching to low chloride caustic makeup. Final steam temperature and attemperator spray flow are plotted for the APP mill in Figure 6. This plot shows the performance prior to purging as well as performance since purging. ARP and APP have realized several areas of cost savings since achieving their ash chloride level target. By reducing ash chlorides, superheater corrosion rates have been reduced. This has allowed the mills to raise the superheater outlet temperature back to 482C, which has resulted in improved efficiency of the steam turbine. The savings realized from this have been used successfully to justify continued use of low chloride makeup caustic. Another significant saving has been seen in the ability to reduce sootblower steam consumption. Sootblower operation has been reduced from running four blowers at a time to running two blowers at a time. This has resulted in a 50% savings in sootblower steam equal to 22 tonnes per hour (50 kpph) of superheated steam. This steam savings has also allowed the mills to reduce auxiliary fuel consumption and sootblower maintenance and sootblower steam condensate losses. Since draft loss due to plugging has not been of concern, a savings has also been realized in ID fan power consumption. In addition to these direct savings, the mills have also reported that the time required to water wash the boilers prior to scheduled outages has been reduced by two-thirds (24 hours to 8 hours). This is a result of the units being cleaner at the beginning of the wash and of the ash being more friable and more easily removed. Although the benefit has not been calculated, liquor chloride reduction has reduced deadload in each mill s cycle by more than 1%. This will decrease steam usage in the evaporators as well as reducing the power consumption of the pumps used to transport the liquor throughout the mills. Summary The changes implemented at the ARP/APP mills to reduce liquor chlorides have proven to be cost effective steps resulting in restored boiler availability and performance while having had low capital costs and low operating costs. After two years of 4 Babcock & Wilcox

5 Forced Water Wash Outage Scheduled Outage 150 Days Apr 1992 APP Startup Pre Ash Purging Figure 5 APP operating run times before and after mill chloride control. Dec 1998 Ash Purging Present Babcock & Wilcox 5

6 Aug-96 Sep-96 Oct-96 Nov-96 Dec-96 Jan-97 Feb-97 Mar-97 Oct-99 Nov-99 Dec-99 Jan-00 Feb-00 Mar-00 Apr-00 May-00 Jun-00 Jul-00 Figure 6 Steam temperature and attemperator performance Steam Temperature, Deg. C Water Wash Water Wash Water Wash Annual Scheduled Outage Attemperator Flow, kg/s Steam Temperature Steam Temperature Attemperator Flow Attemperator Flow 6 Babcock & Wilcox

7 operation at reduced chloride levels, these mills continue to operate with high availability and have had no unscheduled outages due to fouling or plugging. Benefits: Increased boiler availability (unscheduled outages due to plugging have been eliminated). Reduced corrosion within the recovery boilers and pulp mills. Reduced sootblower steam consumption and reduced sootblower maintenance. Sustained superheated steam temperature (improved steam turbine performance). Reduced deadload in the liquor cycle (reduced evaporator steam consumption). Low capital cost for implementation. The method is compatible with future conversion to ECF bleaching. System requires little or no maintenance. Easily adjustable purge rate to adjust ash chloride levels or sulfidity. One third reduction in water wash time prior to planned outages. Drawbacks: Slightly increased quantities of makeup chemical are required. Higher cost of low chloride caustic. High initial purge rates may be required to attain initial chloride reduction. References 1. Tran H.N., Kraft Recovery Boiler Plugging and Prevention, 1992 Tappi Kraft Recovery Short Course Notes p , Orlando, FL, Jan Tran, H.N., How Does a Recovery Boiler Become Plugged? 1988 Kraft Recovery Operations Seminar, January Verrill C.L., Inorganic Fume Formation During Pyrolosis of Black Liquor Droplet Combustion, Ph.D. thesis in Progress, Atlanta, GA, The Institute of Paper Science and Technology, Verrill C.L., Nichols K.M., Fume Formation During Black Liquor Droplet Combustion: The Importance of Sodium Release During Devolatilization, 1995 International Chemical Recovery Conference, June Jaye P. H., The History of Alabama River Pulp Company and The Claiborne Mill Complex: Alabama River Pulp marks 21 years in Operation, March 2000 Internal announcement at APP/ARP. 6. Ferguson, Kelly H., Corrosion-Related Floor Tube Failures Point to More Harsh Boiler Conditions, January 1995 Pulp & Paper Magazine. 7. Brown, C.J., Sheedy, M, Paleologou, M., Novel Ion Exchange Kidneys for Mill Closure, Emerging Technologies Symposium, Orlando, FL October Shenassa R., Reeve, D.W., Dick, P.D., Costa, M.L. Chloride and Potassium Control in Closed Kraft Mill Liquor Cycles, 1995 International Chemical Recovery Conference, June Nunes,W.L., Puig, F., Lindman N., Purging of ESP-Ash to Control Steam Temperature, 1995 International Chemical Recovery Conference, June Copyright 2000 by The Babcock & Wilcox Company, a McDermott company. All rights reserved. No part of this work may be published, translated or reproduced in any form or by any means, or incorporated into any information retrieval system, without the written permission of the copyright holder. Permission requests should be addressed to: Market Communications, The Babcock & Wilcox Company, P.O. Box 351, Barberton, Ohio, U.S.A Disclaimer Although the information presented in this work is believed to be reliable, this work is published with the understanding that The Babcock & Wilcox Company and the authors are supplying general information and are not attempting to render or provide engineering or professional services. Neither The Babcock & Wilcox Company nor any of its employees make any warranty, guarantee, or representation, whether expressed or implied, with respect to the accuracy, completeness or usefulness of any information, product, process or apparatus discussed in this work; and neither The Babcock & Wilcox Company nor any of its employees shall be liable for any losses or damages with respect to or resulting from the use of, or the inability to use, any information, product, process or apparatus discussed in this work. Babcock & Wilcox 7